VIEWS & REVIEWS OPEN ACCESS Chitinases and chitinase-like proteins as biomarkers in neurologic disorders Rucsanda Pinteac, MSc, Xavier Montalban, MD, PhD, and Manuel Comabella, MD, PhD Correspondence Dr. Comabella Neurol Neuroimmunol Neuroinflamm 2021;8:e921. doi:10.1212/NXI.0000000000000921 manuel.comabella@vhir.org Abstract Chitinases are hydrolytic enzymes widely distributed in nature. Despite their physiologic and pathophysiologic roles are not well understood, chitinases are emerging as biomarkers in a broad range of neurologic disorders, where in many cases, protein levels measured in the CSF have been shown to correlate with disease activity and progression. In this review, we will summarize the structural features of human chitinases and chitinase-like proteins and their potential physiologic and pathologic functions in the CNS. We will also review existing evi- dence for the role of chitinases and chitinase-like proteins as diagnostic and prognostic bio- markers in inflammatory, neurodegenerative diseases, and psychiatric disorders. Finally, we will comment on future perspectives of chitinase studies in neurologic conditions. From the Servei de Neurologia-Neuroimmunologia, Centre d’Esclerosi Múltiple de Catalunya (Cemcat), Institut de Recerca Vall d’Hebron (VHIR), Hospital Universitari Vall d’Hebron, Universitat Autònoma de Barcelona, Spain. Go to Neurology.org/NN for full disclosures. Funding information is provided at the end of the article. The Article Processing Charge was funded by the authors. This is an open access article distributed under the terms of the Creative Commons Attribution-NonCommercial-NoDerivatives License 4.0 (CC BY-NC-ND), which permits downloading and sharing the work provided it is properly cited. The work cannot be changed in any way or used commercially without permission from the journal. Copyright © 2020 The Author(s). Published by Wolters Kluwer Health, Inc. on behalf of the American Academy of Neurology. 1 Glossary Aβ = β-amyloid; AMCase = acidic mammalian chitinase; CHI3L1 = chitinase-3-like 1; CHI3L2 = chitinase-3-like 2; CHIT1 = chitotriosidase; CIS = clinically isolated syndrome; CJD = Creutzfeldt-Jakob disease; CLP = chitinase-like protein; CRTH2 = prostaglandin D2 receptor 2; EAE = experimental autoimmune encephalomyelitis; Erk = extracellular signal-regulated kinase; FAK = focal adhesion kinase; FTD = frontotemporal dementia; GH18 = 18 glycosyl hydrolase; IL-13Rα2 = interleukin-13 receptor α2; IFN = interferon; MAPK = mitogen-activated protein kinase; MCI = mild cognitive impairment; NF-κB = nuclear factor kappa B; OVGP1 = oviductin-specific glycoprotein; PCS = post-concussion syndrome; PD = Parkinson disease; PPMS = primary progressive MS; RRMS = relapsing-remitting MS; SI-CLP = stabilin-1–interacting CLP; SPMS = secondary progressive MS; TGF-β = transforming growth factor β; Th2 = T-helper cell type 2; TIM = triosephosphate isomerase; TMEM219 = transmembrane protein 219; TNFα = tumor necrosis factor alpha. Biomarkers are important in neurologic disorders to aid in the including arthropods, protozoan parasites, nematodes, bac- distinction between diseases with similar clinical phenotypes teria, and fungi.1 Chitin synthetases and chitinases are re- or shared pathophysiologic mechanisms, to monitor and sponsible for chitin metabolism in chitin-containing stratify patients according to disease activity and progression, organisms.1 Despite the absence of endogenous chitin or and to predict the response to therapies. Biomarkers can also chitin synthases genes, mammals express true chitinases with be useful for a better understanding of disease etiopatho- enzymatic activity and homologous structurally related genesis and, in some instances, may become therapeutic tar- chitinase-like proteins (CLPs) that lack enzymatic activity but gets for particular diseases. In neurologic conditions, they can still can bind to chitin with high affinity.2 be roughly divided into biomarkers of inflammation and neurodegeneration. It is well known that neuroinflammation Because of their amino acid sequence similarities, human is not only an important component of more classical in- chitinases are classified into the 18 glycosyl hydrolase (GH18) flammatory disorders like MS but it is also an essential family.2–4 The human protein family encodes 2 true chiti- mechanism in the pathophysiology of more purely neurode- nases, chitotriosidase (CHIT1), and acidic mammalian chi- generative disorders such as Alzheimer disease or amyo- tinase (AMCase), and several CLPs: chitinase-3-like 1 trophic lateral sclerosis, among others. Despite their precise (CHI3L1), chitinase-3-like 2 (CHI3L2), oviductin-specific function is still unknown, chitinases are emerging as bio- glycoprotein (OVGP1), and stabilin-1–interacting CLP (SI- markers in inflammatory and neurodegenerative disorders CLP).5 Proteins of the GH18 family are composed of the where levels measured in body fluids may tag the neuro- following structural components: a signal peptide, no/several inflammatory component and overall reflect the degree of serine-threonine–rich linkers, GH18 catalytic domains, and astrocyte or microglial participation in the pathophysiology of no/several cysteine-rich carbohydrate or chitin-binding do- the different neurologic conditions. This review will summa- mains.4 The triosephosphate isomerase (TIM) barrel domain rize existing evidence about the role of chitinases as bio- is the catalytical domain of the GH18 superfamily.6 Enzy- markers in a number of inflammatory and neurodegenerative matically active chitinases have a glutamate residue that acts as diseases and their potential implication in the CNS physio- a proton donor in their catalytic center and an aspartic acid pathology. References for the review were identified by that stabilizes the substrate.7 Because of the substitution of searches of the PubMed database literature from 1974 until critical amino acids in the catalytic center, CLPs have lost their September 2020 using the search terms “chitinase” or “chi- chitinolytic activity but still conserve the TIM barrel and the totriosidase” together with the neurologic diseases of the CNS chitin-binding domain.8 Molecular phylogenetic studies sug- and psychiatric disorders included in the review: “multiple gest that a first duplication event allowed the specialization of sclerosis,” “Alzheimer’s disease,” “amyotrophic lateral sclero- CHIT1 and AMCase. The subsequent evolution of CLPs is a sis,” “stroke,” “traumatic brain injury,” “Creutzfeldt-Jakob product of posterior gene duplications and loss of function disease,” “Parkinson’s disease,” “HIV encephalitis,” “schizo- mutations.3,8 Except for the SI-CLP gene that is located on phrenia,” or “bipolar disorder.” Only articles published in chromosome 11, all human GH18 members are located on English were reviewed. The final reference list was obtained chromosome 1.8 mainly based on originality and relevance to the topic of the review. Table 1 summarizes main properties and function of chiti- nases and CLPs. CHIT1 and AMCase are true chitinases that can degrade chitin. The main physiologic role of human true Human chitinases: genes, family, chitinases is the defense and clearance of chitin and chitinous organisms.9 CHIT1 was the first human chitinase discovered structure, and function and characterized in macrophages from patients with Gaucher Chitin is the second most abundant polysaccharide in nature disease.10,11 CHIT1 is expressed in activated macrophages on and an essential structural component in numerous organisms diverse triggering stimuli,12–14 neutrophils granules,15 and 2 Neurology: Neuroimmunology & Neuroinflammation | Volume 8, Number 1 | January 2021 Neurology.org/NN Table 1 Human chitinases and CLPs Gene Catalytic Subcellular Name symbol activity Glycoprotein Cellular source location Function Chitotriosidase CHIT1 + + Macrophages, Extracellular Degrades chitin, chitotriose, and neutrophils, and region or chitobiose; may participate in the defense microglia secreted; against nematodes and other chitinous lysosome pathogens; and plays a role in reactive gliosis Acidic mammalian CHIA + + Macrophages Extracellular Degrades chitin and chitotriose; may chitinase region or participate in the defense against secreted nematodes, fungi, and other pathogens; and plays a role in T-helper cell type 2 (Th2) immune response Chitinase-3–like CHI3L1 − + Monocytes/ Extracellular May play a role in tissue remodeling and protein 1/YKL-40 macrophages, region or in the capacity of cells to respond to and chondrocytes, secreted cope with changes in their environment; synovial cells, positive regulation of angiogenesis; plays osteoclasts, and a role in Th2 inflammatory response and astrocytes interleukin (IL)-13–induced inflammation, regulating allergen sensitization, inflammatory cell apoptosis, dendritic cell accumulation, and M2 macrophage differentiation; facilitates invasion of pathogenic enteric bacteria into colonic mucosa and lymphoid organs; mediates activation of AKT1 signaling pathway and IL8 production in colonic epithelial cells; regulates antibacterial responses in the lung by contributing to macrophage bacterial killing, controlling bacterial dissemination, and augmenting host tolerance; regulates hyperoxia-induced injury, inflammation, and epithelial apoptosis in the lung; and plays a role in reactive gliosis Chitinase-3–like CHI3L2 − − Cartilage Extracellular Growth/differentiation factor for articular protein 2/YKL-39 chondrocytes region or chondrocytes and may play a role in secreted immune response and tissue remodeling Oviductal OVGP1 − + Oviductal epithelial Extracellular Binds to oocyte zona pellucida in vivo and glycoprotein 1 cells region or may play a role in the fertilization process secreted and/or early embryonic development Stabilin-interacting CHID1 − + Macrophages Extracellular Binds to stabilin-1; saccharide- and LPS- chitinase-like region or binding protein with possible roles in protein, chitinase secreted; pathogen sensing and endotoxin domain-containing lysosome neutralization protein 1 cells that exhibit phagocytic activity such as monocyte-derived the secretory epithelial cells and type 2 alveolar cells.23 Be- cells, Kupffer cells, osteoclasts, and dendritic cells.16 Elevated sides, AMCase has been pointed out as a critical regulator of plasma levels of CHIT1 are found in disorders that involve pulmonary T-helper cell type 2 (Th2) inflammation in mu- macrophage activation such as atherosclerosis,17 sarcoido- rine models.24,25 sis,18 MS,19 and infectious diseases like malaria20 or fungal infections. Existing data suggest that CHIT1 plays a role in CLPs can bind to chitin but not to degrade it. CLPs exhibit inflammation not only as a defense against chitinous patho- lectin properties and can bind to extracellular matrix com- gens but also as a mediator of the innate immune response ponents such as hyaluronan or proteoglycans and cell surface (table 1). glycosaminoglycans.26 The physiologic functions of CLPs are not entirely understood, but they appear to be involved in a AMCase was the second chitinase to be discovered and is plethora of biological processes, including tissue remodeling, mainly expressed in the stomach and to a lesser extent in the inflammation, and regulation of the immune response. lung,21 which are exposed to exogenous chitin through food ingestion and air inhalation, respectively. AMCase is stable at CHI3L1 or YKL-40 is by far the most described CLP because extremely acid pH and may be involved in processing the of its implication in diverse pathologic conditions. CHI3L1 is exogenous chitin that enters the digestive tract.22 In the lung, a 40-kD glycoprotein that was first discovered as an abundant AMCase is expressed and secreted into the airway lumen by molecule in media from cultured chondrocytes and synovial Neurology.org/NN Neurology: Neuroimmunology & Neuroinflammation | Volume 8, Number 1 | January 2021 3 Table 2 Summary of CHI3L1 studies in neurologic disorders Disease Comments: findings Correlationsa CNS expression MS High levels in early phases are associated with ↑ risk for MS and NFL/CXCL13/ Astrocytes and macrophage/microglial cells neurologic disability (CSF)e15,e23–e25 MMP-9 from chronic active lesions with high and low inflammatory activity Levels correlate with clinical and radiologic disease activity (CSF)e26 Levels are increased in progressive forms of the disease (CSF and blood)e15,e27,e28 Predictor of progression to an SPMS phase of the disease (CSF)e28 Associated with brain volume loss (CSF)e23,e29 and the therapeutic response to IFNβ (blood)e32 Alzheimer Levels are ↑ in preclinical, prodromal, and demential cases of AD Tau (total and In a subset of astrocytes from the frontal cortex disease (AD) vs CNC (CSF)e38–e41 phospho-tau)/ β-amyloid High levels are associated with ↑ risk of AD in CNCs or patients with MCI (CSF)e37,e39,e42 Levels increase over time in patients with AD and MCI (CSF)e42 High levels are associated with cortical thinning in parietal and temporal areas (CSF)e39,e43 Controversial results as diagnostic biomarker to distinguish AD from FTD (CSF)e37,e38,e40,e41,e44 Levels ↑ in AD vs α-synucleinopathies (CSF)e38,e45 Amyotrophic Levels are ↑ in ALS vs ALS mimics, healthy controls, and other NFL/pNFH Astrocytes from the spinal and frontal cortex lateral disease controls (CSF)e18,e53,e55–e57 sclerosis (ALS) Controversial results as prognostic biomarkere18,e53,e55,e57,e59,e60 Levels are longitudinally stable or slightly increase over timee18,e53,e56,e57 Associated with cognitive impairment (CSF)e18 Stroke Levels correlate with infarct volume, stroke severity, and Transient expression in astrocytes proximal to functional outcome (blood)e66 the injury in the acute stage Independent predictor of the clinical outcome of the LAA stroke (blood)e67 High baseline levels are associated with an increased risk of stroke (blood)e68–e71 Traumatic Levels increase at day 1 postinjury, peak at day 4, and decrease by IL-1β, TNFα, and Transient expression in astrocytes in the brain injury day 6 (CSF)e75 CRP contusional and pericontusional areab (TBI) May be predictor of worse survival outcome (CSF)e75 Levels correlate with volume of intracranial lesion (blood)e76 Associated with level of consciousness after TBI (blood)e76 Levels are ↑ in patients with PCS vs healthy controls and correlate with symptom severity and lifetime number of contusions (CSF) e77 Creutzfeld- Levels are ↑ in patients with CJD vs patients with Alzheimer Tau protein Reactive protoplasmic and perivascular Jakob disease disease, FTD, and also controls (CSF)e49,e80 astrocytes and fibrillar astrocytes in the WM (CJD) Levels are ↑ in patients with CJD vs patients with neurodegenerative dementias, neurologic controls, and healthy controls (blood)e81 Parkinson Levels are ↑ in patients with PD after 2 y of follow-up compared disease (PD) with baseline (CSF)e85 Levels correlate with cognitive impairment (CSF)e85 Continued 4 Neurology: Neuroimmunology & Neuroinflammation | Volume 8, Number 1 | January 2021 Neurology.org/NN Table 2 Summary of CHI3L1 studies in neurologic disorders (continued) Disease Comments: findings Correlationsa CNS expression HIV- Levels are ↑ in patients with HAD compared with asymptomatic NFL associated neurocognitive patients with HIV, patients with HIV receiving dementia antiretroviral treatment, and HIV-negative individuals (CSF)e90 (HAD) Abbreviations: CHI3L2 = chitinase-3–like 2; CNC = cognitively normal control; CRP = C-reactive protein; FTD = frontotemporal dementia; IFNβ = interferon- beta; IL-1β = interleukin 1β; LAA = large-artery atherosclerotic stroke; MCI = mild cognitive impairment; NFL = neurofilament light subunit; PCS = post- concussion syndrome; pNFH = phosphorylated neurofilament heavy chain protein; SPMS = secondary progressive MS; TNFα = tumor necrosis factor alpha; WM = white matter. Blood refers to studies conducted in serum or plasma. a Correlations between CHI3L1 and other biomarkers. b Rat model of TBI. cells.27 Despite CHI3L1 is expressed in a wide variety of CHI3L1 is an acute-phase reactant, secreted by immune cells, including macrophages,28 osteoclasts,29 microglia, cells in response to proinflammatory stimuli such as IL6 and and astrocytes,30 its ligands and signaling have only re- interferon (IFN)-γ, IL-1β, or tumor necrosis factor alpha cently started to be unraveled. CHI3L1 signaling is com- (TNFα).40 CHI3L1 is expressed during the differentiation plex and depends on the cellular context (summarized in and maturation of dendritic cells in a time-dependent figure 1). Recently, hyaluronan was described as the manner.29 Surprisingly, increased expression of CHI3L1 has physiologic ligand of CHI3L1.26 Several studies showed been observed in both M1- and M2-polarized macro- physical interaction of CHI3L1 with the interleukin-13 phages,33 suggesting pleiotropic functions during in- receptor α2 (IL-13Rα2) and transmembrane protein 219 flammation. CHI3L1 seems to have a role regulating the (TMEM219).31–33 The multimeric complex formed by Th1/Th2 inflammatory balance.41 Upregulation of CHI3L1 CHI3L1, IL-13Rα2, and TMEM219 was found to activate is a shared biomarker of numerous inflammatory non- mitogen-activated protein kinase (MAPK), protein kinase neurologic conditions such as inflammatory bowel disease, B/AKT and Wnt/β-catenin signaling, and mediate a wide cardiovascular disease, rheumatoid arthritis, diabetes, range of CHI3L1 effector responses including, among chronic obstructive pulmonary disease, atherosclerosis, and others, regulation of lung oxidant injury response and ap- cancer.42–45 CHI3L1 expression has been demonstrated in optosis, macrophage pyroptosis and inflammasome acti- many types of neoplastic cells and is associated with tumor vation, transforming growth factor β1 (TGF-β1) induction, progression46 and epithelial-mesenchymal transition.37 and melanoma metastasis.31,32 In addition, M2 Moreover, CHI3L1 is a potent angiogenic factor that pro- macrophage-secreted CHI3L1 promoted gastric and breast motes tumor angiogenesis.47 Despite the apparent patho- cancer metastasis via IL-13Rα2 by activating the MAPK physiologic implication of CHI3L1 in both inflammatory signaling pathway and leading to an upregulation of matrix and reparative responses, there are many unknowns. metalloproteinase genes, thus facilitating extracellular ma- CHI3L1 has the potential to be a therapeutic target in trix degradation.33 CHI3L1 has also been reported to in- conditions characterized by chronic inflammation, fibrosis, teract with the prostaglandin D2 receptor 2 (CRTH2)34 and tissue remodeling. and with galectin-335 leading to an exaggerated fibroproli- ferative response and dysregulation of apoptosis in CHI3L2 or YKL-39 is closely related to CHI3L1 in size and Hermansky-Pudlak syndrome. Moreover, CHI3L1 has sequence, although has several structural differences.48 In been reported to bind to the side chain molecules of contrast to CHI3L1, CHI3L2 is not a glycoprotein. syndecan-1 and integrin αvβ5 activating downstream focal CHI3L2 was discovered as a 39-kD protein from the con- adhesion kinase (FAK) and MAPK and inducing tumor ditioned medium of human articular cartilage chondrocyte angiogenesis.36 CHI3L1 interaction with CD44v3 has been primary cultures. CHI3L1 and CHI3L2 have been shown to shown to induce epithelial-to-mesenchymal transition activate ERK1/2 in 293 and U373 cells leading to different through β-catenin/extracellular signal–regulated kinase outcomes: CHI3L1 activates cell proliferation, whereas (Erk)/Akt signaling in gastric cancer.37 CHI3L1 was also CHI3L2 decreases it.49 CHI3L2 is a recognized biomarker described as a RAGE receptor (receptor for advanced of the osteoarthritic condition and is thought to be a novel glycation end products) binding partner activating STAT3, growth/differentiation factor for articular cartilage chon- β-catenin, and nuclear factor kappa B (NF-κB) pathways drocytes.50 However, how CHI3L2 regulates cell pro- and stimulating the proliferation of intestinal epithelial liferation and differentiation is not yet understood due to a cells.38 In summary, the binding of CHI3L1 to its receptors lack of knowledge about its receptors and intracellular activates multiple and heterogeneous signaling pathways signaling pathways. CHI3L2 is strongly expressed in including ERK1/2, PI3K, Wnt/β-catenin, FAK861/397, or tumor-associated macrophages.51 In vitro studies showed MAPK leading to different cellular outcomes.39 that TGF-β and IL-4 induce the expression of CHI3L2 in Neurology.org/NN Neurology: Neuroimmunology & Neuroinflammation | Volume 8, Number 1 | January 2021 5 M2-polarized macrophages and that CHI3L2 acts as a alternative activation markers arginase 1 and mannose re- proangiogenic and monocyte recruiting factor.52 Similar to ceptor type C 1/CD206 were significantly increased.58 The CHI3L1, CHI3L2 function is also associated with immune findings provided by this study suggest that CHIT1 may response and tissue remodeling. exert a protective role through the regulation of microglial polarization. In addition, CHIT1 was reported to increase Finally, OVGP1 and SI-CLP are the less described CLPs. OVGP1 TGF-β1–induced Aβ phagocytosis by acting as a cofactor is a 120-kD glycoprotein, which is specifically secreted into the of TGF-β1 in the N9 microglial cell line.59 Of interest, oviductal lumen and may be implicated in the fertilization pro- CHIT1 did not affect Aβ uptake by itself, but enhanced the cess.53 SI-CLP, the most recent identified CLP, was first discov- TGF-β1–induced expression of TGF-β receptor 1 and ac- ered as a novel interacting partner of stabilin-1, and it appears to be tivation of Smad signaling.59 Despite these studies, addi- involved in macrophage alternative activation.54 tional research addressing CHIT1 expression, role, and signaling in the CNS is still required. CHI3L1 source in the human CNS has been mostly associ- Role of chitinases and CLPs in ated with reactive astrocytes in distinct neuroinflammatory contexts.60,e1,e2 Besides, CHI3L1 expression was reported the CNS both during in vitro human astrocytes differentiation from For many years, the CNS was thought to be an immunopri- neural progenitors and during postnatal brain development in vileged site. Nowadays, it is accepted that the immune re- mouse, coincident with astrocyte differentiation.e3 sponse takes place in the brain involving peripheral and local components. Neuroinflammation plays a role not only in There are several in vitro studies that explore CHI3L1 effects primarily inflammatory diseases such as MS but also in non- and signaling on different cellular types of the CNS (figure 2). immunologic disorders like Alzheimer disease, amyotrophic Treatment with the proinflammatory cytokines IL-1β and lateral sclerosis, acute CNS injury, and psychiatric disorders.55 TNFα induced CHI3L1 expression in primary human astro- Neuroinflammation is a physiologic process triggered by a cytese2 and in hiPSC-derived astrocytes.e4 The IL-1β and wide range of harmful stimuli and involves a complex in- TNFα–induced CHI3L1 transcription was completely terplay of distinct cell types, including microglia, astrocytes, inhibited by IL-1R and TNFαR antagonists and partially by other glial cells, and peripheral components. It is necessary for NF-κB inhibitors.e2 CHI3L1 has been described as a migra- tissue repair, recovery, and regeneration. Nevertheless, it can tion factor for primary human astrocytes,e2,e3 and human as- be a double-edged sword exerting detrimental effects when trocytes stably transfected with CHI3L1 exhibited radiation dysregulated or chronic. In this regard, chitinases and CLPs resistance and increased invasion in vitro of glioma cells.e5 have been extensively described as markers of neuro- Moreover, CHI3L1 expression is induced in glioblastoma inflammation and reactive gliosis in a wide variety of neuro- tumor-associated astrocytes promoting proliferation and mi- logic disorders. For some of these disorders, chitinases and gration through IL-13Rα2 signaling.e6 CLPs have also been proposed as prognostic biomarkers and potential therapeutic targets. Notwithstanding, their specific Several groups reported CHI3L1 effects on oligodendro- role in neuroinflammation remains to be elucidated, and it genesis. For instance, in a cellular model of Alexander disease, appears to be disease or context specific. a leukodystrophy caused by mutations in the GFAP gene, increased astrocyte-secreted CHI3L1 mediated the inhibition Despite CHIT1 implication in diverse neurologic diseases, of oligodendrocyte progenitor cell proliferation and myeli- few studies addressing its function and signaling in the CNS nation through CRTH2 signaling.e7 On the other hand, are available. CHIT1 expression in the CNS was mainly at- Starossom et al. described that CHI3L1 promoted in vitro tributed to microglia and infiltrating peripheral macro- oligodendrogenesis in human neural stem cells.e8 phages.56 CHIT1 immunoreactivity seems to be limited to microglial cells in mixed mouse primary glial cultures.57 Regarding CHI3L1 effects on neurons, 2 independent groups Moreover, exposure of the cultured microglial cells to CHIT1 reported a dose-dependent neurotoxic effect on mouse pri- caused morphologic transformations indicative of an activated mary neuronal cultures.30,e9 The mechanisms driving the status.57 However, M1/M2 polarization was not investigated CHI3L1-induced neuronal dysfunction and death are still in their study. Oligomeric β-amyloid (Aβ) pretreatment in- unknown. duced CHIT1 expression in a cultured N9 microglial cell line.58 A significant reduction in Aβ oligomers was reported in CHI3L1 knockout mice exhibited worse outcomes and ex- the D-galactose and aluminum chloride–induced Alzheimer acerbated gliosis both in strokee10 and traumatic brain disease animal models after treatment with CHIT1, suggest- injurye11 models suggesting a protective role of CHI3L1 in ing that the Aβ oligomer deposition is influenced by CHIT1 acute neuroinflammation. In the stroke model of middle ce- activity.58 In addition, the proinflammatory cytokines TNFα rebral arterial occlusion, CHI3L1 KO mice display accelerated and IL-1β were significantly decreased in the brain of CHIT1- stroke development through enhancement of neuro- treated animals and N9 microglial cells, whereas the M2 inflammation because of decreased STAT6-dependent 6 Neurology: Neuroimmunology & Neuroinflammation | Volume 8, Number 1 | January 2021 Neurology.org/NN Table 3 Summary of CHIT1 and CHI3L2 studies in neurologic disorders Disease Comments: findings MS CHI3L2 High CSF levels in early phases were associated with ↑ risk for MS in 1 study,e16 but another study in CSF and serum failed to find such associatione15 Levels are ↑ in patients with RRMS vs SPMSe15 CHIT1 Correlation of CSF levels with long-term disease activitye34 Alzheimer disease (AD) CHIT1 Both CSF levels and enzymatic activity are ↑ in patients with AD compared with no dementia controlse46–e48 Amyotrophic lateral CHI3L2 sclerosis (ALS) Levels are ↑ in patients with ALS vs ALS mimics and healthy controlse18,e53 Correlation of CSF levels with disease progression ratee53 Upregulation in the motor cortex of patients with sporadic ALS e19 CHIT1 Levels are ↑ in patients with ALS vs ALS mimics, disease controls, and healthy controls56,e18,e53–e58 Controversial results as prognostic biomarker Expressed in the corticospinal tract of patients with ALS and colocalized with microglial and phagocytic markers56 Stroke CHIT1 Patients with acute stroke exhibit higher baseline plasma chitinolytic activity compared with healthy controls but with no prognostic value in long-term clinical outcomee64 Association of low levels of plasma chitinolytic activity with short-term clinical improvemente64 Correlation of plasma chitinolytic activity with stroke severitye65 Traumatic brain injury CHIT1 Upregulated in the contusional and pericontusional areae78 Creutzfeld-Jakob disease CHIT1 (CJD) Levels are ↑ in patients with CJD vs controlse80 Abbreviations: CHI3L2 = chitinase-3–like 2; CHIT1 = chitotriosidase; RRMS = relapsing-remitting MS; SPMS = secondary progressive MS. microglial activation.e10 Similarly, CHI3L1 KO mice pre- is needed to elucidate CHI3L1 signaling in the CNS and its sented more severe pathology and more pronounced reactive disease-specific roles. gliosis after cortical controlled impact than their WT littermates, thus supporting a role of CHI3L1 modulating astrocytic and Little is known about CHI3L2 physiologic and pathologic role microglial reactive gliosis.e11 There are discrepant results re- in the CNS. Increased CHI3L2 expression or CSF levels have garding experimental autoimmune encephalomyelitis (EAE) been reported in various neuroinflammatory conditions.e15– e19 model in CHI3L1 KO mice. Although Bonneh-Barkay et al. However, CHI3L2 role in the CNS pathology remains described an exacerbation of clinical EAE, inflammation, gliosis, obscure. and demyelination in CHI3L1 KO mice,e12 Cantó et al. reported no differences of EAE course and gliosis between CHI3L1 KO mice and their WT littermates.e13 Furthermore, treatment of Aβ1- Chitinases and CLP as biomarkers of 42–infused mice, a model of AD, with K284-6111, a CHI3L1 inhibitor, resulted in decreased neuroinflammation and a mem- neurologic and psychiatric disorders ory recovery effect.e14 MS MS is a neurodegenerative immune-mediated disorder of the All these data taken along underline the complexity of the CNS and one of the leading causes of nontraumatic neuro- CHI3L1 signaling networks in the CNS. Further investigation logic disability among young adults.e20,e21 Considering the Neurology.org/NN Neurology: Neuroimmunology & Neuroinflammation | Volume 8, Number 1 | January 2021 7 high degree of heterogeneity that characterizes the disease, association between high CSF or serum CHI3L2 levels and there is a strong need for biomarkers that reliably capture the later conversion to MS.e15 In contrast to CHI3L1, CSF different aspects of MS heterogeneity and help in the de- CHI3L2 levels were found to be significantly increased in velopment of a personalized medicine based on a better un- patients with RRMS compared with patients with progressive derstanding of disease diagnosis, stratification, and prediction MS.e15 In a recent study, CSF CHIT1 levels were found to of disease course and response to therapies.e22 correlate with long-term disease activity.e34 The majority of chitinase studies in patients with MS mea- Alzheimer disease sured CHI3L1 and were conducted in the CSF, although few Alzheimer disease is the most common cause of dementia in studies also determined CHI3L1 levels in peripheral blood the Western world and causes a progressive impairment of samples. Studies conducted at the time of the first neurologic cognitive functions.e35 Neuroinflammation can also be event suggestive of CNS demyelination or clinically isolated detected in the brain from the early stages of the disease, and a syndrome (CIS) have pointed to a prognostic role of CHI3L1 number of inflammatory biomarkers have been investigated in in the early phases of the disease. In this context, high CSF patients with Alzheimer disease.e36 levels at the time of the CIS have been associated with a higher risk of conversion to MSe15,e23–e25 and also with an increased Similar to MS, the majority of chitinase studies in patients risk for the development of neurologic disability.e23,e25 with Alzheimer disease measured CHI3L1 levels and were conducted in CSF samples from patients. Overall, these In patients with relapsing-remitting MS (RRMS), CSF studies reflect the utility of CHI3L1 as a diagnostic and CHI3L1 levels were found to correlate with disease activity, prognostic biomarker in Alzheimer disease. In an initial pro- and levels were significantly increased in patients at the teomic study using 2 dimensional difference gel electropho- time of clinical relapses compared with patients in re- resis and liquid chromatography tandem mass spectrometry, mission and also correlated with a higher number of CSF CHI3L1 was first proposed as a potential biomarker in contrast-enhancing lesions in the brain MRI.e26 Compari- patients with Alzheimer disease.e37 Regarding its prognostic son of CHI3L1 levels between patients with different role, CSF CHI3L1 levels are usually found increased in pre- clinical forms revealed significantly higher levels in patients clinical, prodromal, and demential cases of Alzheimer disease with progressive forms of the disease (secondary and pri- compared with cognitively normal individuals.e38–e41 CHI3L1 mary progressive MS—SPMS and PPMS, respectively) in also behaves as a predictive biomarker of Alzheimer disease, relation to patients with RRMS and CIS, and these findings insomuch as high CSF CHI3L1 levels were found to be as- were observed both in samples from the CSF and periph- sociated with an increased risk of future development of eral blood (serum and plasma).e15,e27,e28 In patients with Alzheimer disease in cognitively unimpaired individuals or RRMS, high CSF CHI3L1 levels were an independent patients with mild cognitive impairment (MCI) compared predictor of neurologic disability worsening and also of with patients with stable MCI.e37,e39,e42 CSF CHI3L1 levels progression to an SPMS phase of the disease.e28 In patients have been reported to increase longitudinally during the with CIS and RRMS, levels of CSF CHI3L1 were signifi- stages of MCI and Alzheimer disease, but not in cognitively cantly associated with brain volume loss evaluated by the normal individuals, suggesting that this biomarker might also brain parenchymal fraction change during follow-up.e23,e29 be useful for disease monitoring.e42 High levels of CSF CSF CHI3L1 levels can also be modified by MS therapies, CHI3L1 have also been associated with brain MRI changes and for instance levels were significantly reduced by the such as cortical thinning in parietal areas including precuneus, effect of natalizumab and fingolimod,e30,e31 and in another posterior cingulate, and superior parietal corticese39 and also study serum CHI3L1 levels were associated with the re- in temporal areas.e43 sponse to IFNβ,e32 CSF CHI3L1 levels were found to correlate with CSF levels of other neurodegenerative and Regarding the role of CHI3L1 as a diagnostic biomarker, inflammatory biomarkers such as the neurofilament light several studies aimed to distinguish Alzheimer disease from chain, CXCL13, or MMP-9.e28,e33 other dementing illnesses, with controversial results for the frontotemporal dementia (FTD). Whereas some studies CHI3L1 expression was observed in chronic active lesions reported no differences between patients with Alzheimer from patients with MS, but it was absent in brain tissue from disease and FTD,e38,e40 others found either increased non-neurologic controls. Within CNS lesions, CHI3L1 ex- CHI3L1 levels in patients with FTD compared with patients pression was present both in macrophages/microglial cells with Alzheimer diseasee37 or increased levels of CHI3L1 in and astrocytes, with an important contribution of astrocytes patients with Alzheimer disease vs FTD.e41 However, classi- to CHI3L1 expression in those chronic active lesions classi- fication of patients with FTD into pathologic phenotypes fied as having high inflammatory activity.e33 revealed higher CSF CHI3L1 levels in the tau-positive FTD subtype compared with patients with Alzheimer disease.e44 Regarding other chitinases, in 1 study, high CSF CHI3L2 Comparisons of CHI3L1 levels between Alzheimer disease levels were associated with an increased risk of MS in patients and α-synucleinopathies revealed significantly higher CHI3L1 with CIS,e16 although another study failed to find an levels in patients with Alzheimer disease compared with 8 Neurology: Neuroimmunology & Neuroinflammation | Volume 8, Number 1 | January 2021 Neurology.org/NN Figure 1 CHI3L1 reported signaling pathways The interaction of CHI3L1 with IL-13Rα2 and TMEM219 activates several intracellular signaling pathways such as MAPK, protein kinase B/Akt, and Wnt/ β-catenin, leading to diverse cellular outcomes including regulation of lung oxidant injury response, apoptosis, tumor metastasis, and allergic inflammation. CHI3L1 binding to CRTH2 mediates a fibroproliferative response and dysregulation of apoptosis in Hermansky-Pudlak syndrome and an inhibition of OPCs proliferation in a human cellular model. CHI3L1 can induce angiogenesis by binding to the side chain molecules of syndecan-1, then inducing coordination with integrin αvβ5 and activating downstream FAK and MAPK. CHI3L1 interaction with RAGE activates STAT6, Wnt/β-catenin and NF-κB and promotes proliferation of intestinal epithelial cells. Last, CHI3L1 interacts with CD44v3 and IL-13Rα2 inducing epithelial to mesenchymal transition through Erk1/2, Wnt/ β-catenin, and Akt signaling. CHI3L1 = chitinase-3-like 1; CRTH2 = prostaglandin D2 receptor 2; Erk = extracellular signal–regulated kinase; FAK = focal adhesion kinase; IL-13Rα2 = interleukin-13 receptor α2; MAPK = mitogen-activated protein kinase; OPC = oligodendrocyte progenitor cell; TMEM219 = transmembrane protein 219. patients with Parkinson disease (PD) dementia or dementia Amyotrophic lateral sclerosis with Lewy bodies.e38,e45 Amyotrophic lateral sclerosis (ALS) is a fatal neurode- generative disease characterized by the selective neuro- CSF CHI3L1 levels have been found to significantly cor- degeneration and death of the upper and lower motor relate with other CSF biomarkers in patients with Alz- neurons, usually leading to death within 3–5 years from the heimer disease. In this context, strong correlations were symptom onset.e51 Although the mechanisms are not yet observed between CSF CHI3L1 levels and biomarkers of understood, activation of a neuroinflammatory response neuronal degeneration such as phospho-tau or total and reactive gliosis are prominent features of ALS and are tau.e39,e43 Similarly, CHI3L1 was found to correlate with related to disease progression.e52 There is growing interest β-amyloid.e38 No differences have been observed in in the modulation of neuroinflammation as a potential CHI3L1 levels between APOEe4 allele carriers and therapeutic target in ALS. Therefore, biomarkers that track noncarriers.e38,e45 neuroinflammatory activity are necessary to stratify pa- tients for clinical trials and monitoring treatment response. Studies measuring CHIT1 revealed significantly higher CSF CHIT1 levels in patients with Alzheimer disease Chitinases have been shown to play roles as diagnostic and compared with cognitively healthy controls.e46 Similarly, prognostic biomarkers in ALS. As diagnostic biomarkers, enzymatic activity of CHIT1 was also found increased in studies using proteomice53,e54 and immune- the CSF of patients with Alzheimer disease compared with targeted56,e18,e55–57 approaches reported significantly in- no dementia controls.e47,e48 creased CSF CHIT1, CHI3L1, and CHI3L2 levels in patients with ALS compared with healthy controls, ALS mimics (pa- In the CNS, CHI3L1 expression is mainly found in a subset of tients without ALS with motor weakness in whom ALS was astrocytes in the frontal cortex from patients with Alzheimer considered in the differential diagnosis), or other disease disease, sometimes in close apposition to amyloid plaques but controls. Diagnostic performance of CHIT1, CHI3L1, and also independent of plaque pathology.e1,e49,e50 CHI3L2 distinguishing patients with ALS from ALS mimics Neurology.org/NN Neurology: Neuroimmunology & Neuroinflammation | Volume 8, Number 1 | January 2021 9 has also been acceptable,e18,e53 although with discrepant Stroke results.e56 Ischemic stroke is a leading cause of disability and death in the world.e62 Once the cerebral arteria is occluded, a cascade of Several pieces of evidence support a prognostic value for pathologic events starts. Necroptotic or apoptotic cell death, CHIT1, CHI3L1, and CHI3L2 in ALS. A number of studies oxidative stress, reactive gliosis, and migration of peripheral showed that CHIT1,e53,e57,e58 CHI3L1,e53,e57,e59,e60 and immune cells into the brain parenchyma are features of is- CHI3L2e53 CSF levels correlated with disease progression chemic brain damage.e62 Ischemic stroke is often related to rates, and higher CSF baseline levels of CHIT1 and CHI3L1 atherosclerosis, an inflammatory condition in which mono- were associated with shorter survival time.56,e53,e56,e60 How- cytes infiltrate into the vessel wall and then differentiate into ever, other studies failed to find a prognostic value for CHIT1 lipid-laden macrophages. Serum CHIT1 activity has been and CHI3L1.e18,e55 proposed as a biomarker of atherosclerosis, and its activity was increased in patients with atherosclerosis and correlated with Studies evaluating longitudinal profiles of CSF chitinase levels the extension of the atherosclerotic lesion.17,e63 Patients with obtained by repeated lumbar punctures revealed either con- acute stroke were reported to have significantly higher base- stant CSF CHIT1 or CHI3L1 levels over time78,117 or slightly line plasma CHIT1 enzymatic activity compared with healthy increased CSF CHI3L1 levels during the disease.e53,e56 controls, and low levels of plasma chitinolytic activity were associated with short-term clinical improvement.e64 However, Cognitive and behavioral impairment is present in up to 30% the latter had no prognostic value in long-term clinical out- of patients with ALS.e61 CHI3L1, but not CHIT1 or CHI3L2, comes.e64 Besides, CHIT1 plasmatic activity was reported to has been associated with cognitive dysfunction in patients correlate directly with stroke severity, independent of preex- with ALS.e18 isting inflammatory conditions.e65 CSF CHI3L1 levels were found to strongly correlate with Similarly, serum CHI3L1 levels have been found to cor- biomarkers of neurodegeneration such as the neurofilament relate with infarct volume, stroke severity, and functional light chain and the phosphorylated neurofilament heavy chain outcome in patients with acute ischemic stroke with a protein.e18,e56,e57 CHI3L1 peak on the second day after symptom onset.e66 Moreover, serum CHI3L1 has been described as an in- CNS expression studies showed significant upregulation of dependent biomarker to predict the clinical outcome of CHI3L1 and CHI3L2 in the motor cortex of patients with large-artery atherosclerotic stroke, and high CHI3L1 levels sporadic ALS compared with neurologically healthy con- were associated with poor clinical outcome.e67 Several trols, and their expression levels were correlated with the prospective epidemiologic large-scale studies reported that survival time from date of onset.e19 Other studies showed elevated plasma baseline CHI3L1 levels are associated with significantly increased expression of CHI3L1 in the ante- an increased risk of ischemic stroke.e68–e71 Nevertheless, rior horn of the spinal cord in patients with sporadic further investigation is needed to elucidate whether circu- ALS.e59 CHI3L1 immunoreactivity was restricted to lating CHI3L1 has a causative role in ischemic stroke or it is GFAP-positive astrocytes in the frontal cortex and spinal a nonspecific marker of inflammation. cord of patients with ALS, indicating that reactive astro- cytes are the main CNS cell source of CHI3L1 expressio- Comparison of CHI3L1 transcription in acute, subacute, and n.e57,e59 CHIT1 immunostaining was found to colocalize chronic infarcted human brain samples revealed a transient with Iba1 (microglial marker) and CD68 (phagocytic ac- expression at the acute stage (3–5 days poststroke) mostly in tivity marker)-positive cells in the postmortem spinal astrocytes proximal to the injury site.60 It would thus appear cord of patients with ALS.56 CHIT1-immunoreactive cells that acute inflammation induces CHI3L1 expression, and were present mainly in the corticospinal tract of patients when inflammation resolves, CHI3L1 expression diminishes. with ALS but not healthy controls, Creutzfeldt-Jakob disease (CJD), or Alzheimer disease patients, thereby suggesting a disease-specific CHIT1 expression pattern in Traumatic brain injury ALS.56 Traumatic brain injury is one of the leading causes of dis- ability and death worldwide.e72 The primary mechanical All these data taken together suggest that CHI3L1 seems to insult is followed by acute neuroinflammation that leads to be a promising biomarker of pathogenic glial activation in astrocyte and microglial activation and increased secretion ALS, and it may be useful as a surrogate for future treat- of immune mediators.e73,e74 Temporal profiles of CSF ments targeting neuroinflammation. Regarding CHIT1, the CHI3L1 levels following traumatic brain injury showed fact that homozygous carriers of a CHIT1 polymorphism, significantly elevated CHI3L1 levels after the first day which leads to diminished CHIT1 expression, do not ex- compared with healthy controls, and levels reached a peak hibit decreased ALS severity supports the hypothesis that at 4 days posttraumatic brain injury declining subsequently CHIT1 is rather a neuroinflammatory marker than an ac- at days 5 and 6 after injury. CHI3L1 levels were strongly tive contributor to ALS pathogenesis.e58 correlated with levels of proinflammatory cytokines in the 10 Neurology: Neuroimmunology & Neuroinflammation | Volume 8, Number 1 | January 2021 Neurology.org/NN Figure 2 Summary of reported CHI3L1 effects and signaling on CNS cellular types Astrocytes are the main source of CHI3L1 in the CNS. Once secreted, CHI3L1 can induce multiple responses depending on the cellular type and context. In vitro studies reported that (1) CHI3L1 increases migration and radiation resistance of cultured human astrocytes; (2) CHI3L1 expression is induced in glioblastoma- associated astrocytes promoting tumor proliferation and migration through IL-13Rα2 signaling; (3) CHI3L1 inhibits proliferation and myelination in OPCs and (4) enhances oligodendrogenesis of NSCs; and (5) CHI3L1 is neurotoxic in mouse primary cultured neurons. In vivo study: CHI3L1 KO mice exhibit decreased STAT6 microglial activation after CCI. CCI = cortical controlled impact; CHI3L1 = chitinase-3–like 1; CRTH2 = prostaglandin D2 receptor 2; IL-13Rα2 = interleukin- 13 receptor α2; NSC = neural stem cell; OPC = oligodendrocyte progenitor cell. CSF such as IL-1β, TNFα, and C-reactive protein.e75 Of In patients with persistent post-concussion syndrome (PCS) note, patients who died due to traumatic brain injury ten- secondary to repetitive concussive traumatic brain injury, CSF ded to have elevated CSF CHI3L1 levels compared with CHI3L1 levels were significantly increased compared with patients who survived, suggesting that increased CHI3L1 controls and correlated with symptom severity and the life- levels may be predictive of worse survival outcomes.e75 time number of concussions.e77 These findings suggest that PCS may be associated with biomarker evidence of astroglial Serum levels of CHI3L1 together with serum amyloid A1, activation. C-reactive protein, and S100β showed the most robust association with level of consciousness following trau- Proteomic analysis of craniotomy and postmortem human matic brain injury. e76 Furthermore, levels correlated brain samples revealed that both CHI3L1 and CHIT1 were significantly with the extent of traumatic intracranial le- upregulated in the contusion and pericontusional area.e78 sions and had good potential to identify bleeding in In this study, additional immunohistochemistries to iden- several intracranial compartments, and for instance, tify the chitinase cellular source of CNS expression were CHI3L1 was the best biomarker for subdural blood col- not performed; however, in a rat model of traumatic brain lection detection. e76 Altogether, these findings point to injury, CHI3L1 expression was found to be associated with CHI3L1 as a promising biomarker to determine the reactive astrocytes in the pericontusional cortex starting 1 neurologic severity of patients with traumatic brain day after the insult and remaining stable for several days, injury. thus paralleling the findings of temporal profiles of CSF Neurology.org/NN Neurology: Neuroimmunology & Neuroinflammation | Volume 8, Number 1 | January 2021 11 CHI3L1 levels observed in patients with traumatic brain controlse45,e53 or lower CHI3L1 levels in patients vs con- injury.e75 trols.e86 In this context, more studies of chitinases are needed in patients with PD to better understand their po- Creutzfeldt-Jakob disease tential role in disease pathogenesis. CJD is a fatal neurodegenerative disease caused by misfolded transmissible proteinaceous particles named prionse79. Stud- HIV-associated dementia ies of chitinases in CJD evaluated their potential as diagnostic Cognitive impairment in patients with HIV can range from biomarkers distinguishing CJD from other neurodegenerative asymptomatic neurocognitive impairment to HIV-associated disorders. In a recent study measuring glial markers (CHI3L1, dementia.e87 Astrocytes are known to play a role in the CHIT1, and GFAP) in neurodegenerative dementias char- pathogenesis of HIV-associated dementia.e88 In 1 study, CSF acterized by protein misfolding and aggregation, CSF CHI3L1 levels were found to be significantly increased in CHI3L1 levels were significantly elevated in patients with patients with HIV-associated dementia compared with un- sporadic CJD compared with patients with Alzheimer disease, treated neuroasymptomatic patients with HIV irrespective of FTD, and controls without clinical or neuroradiologic evi- CD4+ T cell counts, patients with HIV under antiretroviral dence of CNS disease.e80 In contrast, although CSF CHIT1 treatment, and HIV-negative individuals.e89 Furthermore, and GFAP levels were elevated in patients with CJD vs CSF CHI3L1 levels correlated with CSF neurofilament light controls, they could not discriminate between different chain.e89 Similar to PD, more chitinase studies are necessary neurodegenerative dementias.e80 Similar findings were ob- in patients with HIV-associated dementia, although it should served in another study, where CSF CHI3L1 levels were be taken into account that the prevalence of this condition has reported significantly increased in patients with sporadic CJD been drastically reduced in the post-antiretroviral treatment compared with neurologic controls and patients with other era.e87 neurodegenerative dementias, and levels significantly corre- lated with tau, a marker of axonal degeneration.e49 Studies in Psychiatric diseases peripheral blood also revealed significantly increased plasma Schizophrenia is a complex major mental disorder affecting CHI3L1 levels in patients with CJD compared with other 1% of the population worldwide.e90 Schizophrenia etiology neurodegenerative dementias, neurologic controls, and is multifactorial, involving both genetic and environmental healthy controls.e81 All these findings underscore the pres- factors.e91 A first genetic association study provided evi- ence of an important neuroinflammatory response in patients dence of an association between functional variants at the with prion disease. CHI3L1 locus and schizophrenia genetic risk.e92 The ge- netic variations of CHI3L1 may influence pathologic pro- In CNS expression studies, CHI3L1 was reported to be highly cesses taking place in schizophrenia development, thus upregulated in sporadic CJD frontal cortex and cerebellum, being involved in the disease susceptibility. Compelling and it was positively correlated with GFAP expression.e49 evidence sets a role for innate immune activation in schiz- Immunohistochemistry analysis revealed a specific pattern of ophrenia.e93 First-episode psychosis patients showed a CHI3L1 distribution in sporadic CJD: although in control higher number of blood monocytes and increased plasma brains, CHI3L1 immunoreactivity was mainly restricted to levels of CHI3L1 and monocyte chemoattractant protein 1 fibrillar astrocytes in the white matter, the sporadic CJD compared with controls.e94 However, caution must be ap- brains showed CHI3L1 expression in reactive protoplasmic plied to interpret these results owing to the small sample and perivascular astrocytes and also an increased signal in size. Gene expression analysis of postmortem brain tissue fibrillar astrocytes in the white matter.e49 revealed an upregulation of CHI3L1 in the hippocampuse95 and prefrontal cortexe96 of patients with schizophrenia Parkinson disease compared with controls suggesting a neuroinflammatory PD is a common neurodegenerative condition mainly response. Nevertheless, these studies are limited to differ- characterized by the death of dopaminergic neurons in the ential gene expression and do not assess the CHI3L1 source substantia nigra and the presence of intracellular fibrillar in the schizophrenic brain. aggregates of α-synuclein.e82 Neuroinflammation seems also to play a role in disease pathogenesis and contribute to Bipolar disorder is a chronic psychiatric disease character- neurodegeneration.e83,e84 Studies of chitinases in patients ized by recurrent episodes of depression and mania that with PD are scarce. In a longitudinal study with lumbar affects more than 1% of worldwide population.e97 Systemic puncture performed at baseline and after 2 years of follow- inflammation and neuroinflammation are gaining attention up, CSF CHI3L1 levels were significantly increased over as key players in bipolar disorder pathogenesis.e98 In this time in patients with both short and long disease duration.e85 regard, both CSF and serum CHI3L1 levels were reported Furthermore, the increase in CHI3L1 levels significantly to be significantly higher in patients with bipolar disorder correlated with worsening of cognitive function.e85 Other than controls.e99 Moreover, CSF CHI3L1 levels have been studies revealed either no differences in CSF CHI3L1, associated with cognitive impairment in patients with CHI3L2, and CHIT1 levels between patients with PD and euthymic bipolar disorder.e100 In a study investigating the 12 Neurology: Neuroimmunology & Neuroinflammation | Volume 8, Number 1 | January 2021 Neurology.org/NN relationship between baseline CSF CHI3L1 levels and long- neurologic disorders (unlike the neurofilament light chain). term clinical outcomes, CHI3L1 was found to be negatively (5) Finally, chitinases may be not only CSF secreted bio- associated with maniac/hypomaniac or psychotic epi- markers that reflect the degree of astrocyte activation or sodes.e101 Although the association survived correction for injury but also they have the potential to become therapeutic multiple testing, this result should be interpreted with targets that set the rationale for the design of new therapies. caution because of a possible collinearity issue reported by For instance, CHI3L1 has shown to be neurotoxic in a dose- the authors. dependent manner.30,e9 In conclusion, although there are still several unknowns in Future perspectives of chitinase and the function of chitinases and CLPs in the CNS, they are promising candidates that are consolidating their role as CLP studies in neurologic disorders diagnostic and prognostic biomarkers in a broad spectrum Based on the studies outlined in the previous section, chi- of inflammatory and neurodegenerative diseases. The po- tinases and CLPs, particularly CHI3L1, are playing roles as tential for chitinases to become therapeutic targets cer- biomarkers in a wide range of neurologic disorders in which tainly opens new avenues for future research in the field of there is a neuroinflammatory component as part of their biomarkers for neurologic disorders. pathophysiology. This inflammatory component will result in astrocyte activation, which ultimately seems to be the cell involved in chitinase secretion, although a contribution of Study funding microglia cannot be ruled out as some studies also pointed No targeted funding reported. to a microglial source of chitinases. A number of consider- ations in relation to chitinases and CLPs studies are the Disclosure following: (1) Depending on the neurologic disease and the No relevant disclosures. Go to Neurology.org/NN for full outcome analyzed, there are sometimes discrepancies in the disclosures. results. This has been and is still the rule in many biomarker studies, and chitinases are not an exception, and most likely Publication history reflects the complexity and heterogeneity of the neurologic Received by Neurology: Neuroimmunology & Neuroinflammation conditions that are being evaluated. However, other factors August 7, 2020. Accepted in final form September 23, 2020. such as preanalytical variability, insufficient sample size, or the use of different criteria to classify patients may also contribute to the discrepancies. (2) Although statistically significant, differences among groups in some studies are small and indicate important overlaps of CSF or blood Appendix Authors chitinase levels between patients belonging to different Name Location Contribution categories. This aspect, which may also lead to discrepancies Rucsanda Centre d’Esclerosi Múltiple Drafting/revising the across studies, may be a limitation for the use of chitinases as Pinteac, de Catalunya (Cemcat), manuscript for content, diagnostic biomarkers. However, chitinase data may com- MSc Institut de Recerca, including medical writing Hospital Vall d’Hebron, for content, and study plement clinical and radiologic information or may be Barcelona, Spain concept combined with other biomarkers to increase diagnostic ac- Xavier Centre d’Esclerosi Múltiple Drafting/revising the curacy. (3) Despite more studies are needed in this di- Montalban, de Catalunya (Cemcat), manuscript for content, rection, chitinases probably have their greatest potential as MD, PhD Hospital Vall d’Hebron, including medical writing Barcelona, Spain for content, and study prognostic biomarkers in neurologic disorders. In this con- concept text, the different studies outlined in the review have shown Manuel Centre d’Esclerosi Múltiple Drafting/revising the a more solid role for chitinases as prognostic biomarkers Comabella, de Catalunya (Cemcat), manuscript for content, compared with their diagnostic potential, most likely MD, PhD Institut de Recerca, including medical writing Hospital Vall d’Hebron, for content, and study reflecting the usually detrimental contribution of astrocytes Barcelona, Spain concept to the pathophysiology of neurologic diseases. (4) In con- trast to other biomarkers whose expression is more CNS specific, for instance neurofilaments, tau or GFAP, chitinases References are secreted by a broad range of cells from peripheral 1. Komi DEA, Sharma L, Dela Cruz CS. Chitin effect on inflammation and immune compartments. In this context, there is an important con- response. Clin Rev Allergy Immunol 2018;54:213–223. 2. Guan SP, Mok YK, Koo KN, Chu KL, Wong WS. 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