Constitutive neuronal expression and disease-associated upregulation of chitinases in amyotrophic lateral sclerosis

Chitinases are hydrolytic enzymes responsible for degrading chitin and have been evolutionarily conserved across various species. Although their signaling pathways are not fully understood, the chitinases are considered active immunomodulators across several cell types. Specific isoforms, including Chitotriosidase-1 (CHIT1), Chitinase-3-like protein 1 (CHI3L1), and human-specific Chitinase-3-like protein 2 (CHI3L2), have emerged as markers of inflammation across the neurodegenerative spectrum, including amyotrophic lateral sclerosis (ALS). ALS is a fatal neuromuscular condition, and therapeutic development has been severely hindered by phenotypic heterogeneity and an incomplete understanding of etiology. Although several overlapping disease mechanisms can contribute to neuronal death, inflammation can exacerbate pathology. Prior studies have reported that CHIT1, CHI3L1, and CHI3L2 levels are elevated in the cerebrospinal fluid (CSF) of ALS patients and associated with disease aggressiveness. Nevertheless, several open questions critical to our understanding of the chitinases' role in ALS disease burden remain: namely, 1) which cell types in the central nervous system (CNS) are chitinase sources under physiological conditions, 2) which of these display chitinase upregulation in ALS, and 3) what is the diagnostic utility of the chitinases relative to established biomarkers. Here, we utilize pre-clinical models and post-mortem human tissue to demonstrate at both the transcriptomic and protein level that neurons are a primary source of chitinases; furthermore, neuronal chitinase expression is conserved across species. Under physiological conditions, CHI3L1 is more abundant and widely expressed across various cell types, whereas CHIT1 is predominantly expressed in neurons. Additionally, utilizing symptomatic mice from three familial ALS models, we demonstrate isoform-specific expression profiles, with astroglial and microglial upregulation of CHI3L1, and neuronal and microglial upregulation of CHIT1. Differing expression dynamics and diagnostic utility were also noted in our clinical cohort: CSF CHIT1 and CHI3L2 levels had more discriminatory power when distinguishing between ALS vs. non-ALS controls, while CHI3L1 was more closely associated with inflammation and aging across the neurodegenerative spectrum. Although the chitinases did not diagnostically outperform the neurofilament proteins as biomarkers, we propose that appreciating their expression patterns can aid in optimizing biomarker-guided trial design. Taken together, we demonstrate that chitinase upregulation in ALS is evident in various CNS cell types and that its neuronal expression may provide new insights into its role in disease activity.