Background Chitotriosidase (Chit-1) and chitinase-3-like protein 1 (CHI3L1) protein levels are increased in the cerebrospinal fluid (CSF) of neurodegenerative diseases, including amyotrophic lateral sclerosis (ALS), frontotemporal dementia (FTD), and Alzheimer’s disease (AD). Few studies have examined the spatial expression of chitinase expressing cells with respect to neuropathologic hallmarks of disease.

Methods RNA-sequencing was used to examine Chit-1 and CHI3L1 gene expression in the spinal cord and motor cortex. Immunohistochemistry was used to characterize the distribution of Chit-1 and CHI3L1 expressing cells in ALS, C9-ALS, FTLD, AD, and non-neurologic disease controls. Immunofluorescence confocal microscopy was used to correlate distribution of Chit-1 and CHI3L1 expressing cells to TDP pathology.

Results Chit-1 gene expression was increased in the spinal cord, and CHI3L1 expression was increased in both the spinal cord and motor cortex of sALS and C9-ALS patients when compared to controls. Highest levels of Chit-1+ glia were in cortical regions that contain hallmark neuropathology for each neurodegenerative disease. CHI3L1+ glia were only significantly increased in sALS. Neither Chit-1+ nor CHI3L1+ glia were in close proximity to pTDP containing neurons in the motor cortex gray matter; however, there was a significant co-localization of glial pTDP with Chit-1 and CHI3L1 in the motor cortex white matter.

Conclusions Chit-1 and CHI3L1 expressing cells were most abundant in the white matter of cortical regions affected by each neurodegenerative disease and the spinal cord. Chit-1 or CHI3L1 expressing cells in the white matter also contained phosphorylated TDP-43. We also observed correlations between levels of Chit-1 or CHI3L1 expressing cells in the white matter to disease duration.

What is already known on this topic Prior studies identified elevated levels of Chit-1 and CHI3L1 proteins in the CSF of various neurodegenerative conditions, though few studies examined levels of Chit-1 and CHI3L1 expressing cells both spatially and in relation to disease pathology.

What this study adds We performed an extensive spatial characterization of Chit-1 and CHI3L1 protein levels across multiple regions and neurodegenerative conditions. This study also correlates Chit-1 and CHI3L1 expression to TDP pathology and other clinical parameters of disease duration.

How this study might affect research, practice or policy Our findings indicate that the majority of Chit-1 and CHI3L1 expressing glia are located in the cortical subpial layer and the white matter, suggesting a role for chitinases in modulating neuroinflammatory mechanisms or reparative/regenerative responses in the white matter of ALS and other neurodegenerative diseases. This study suggests new therapeutic opportunities for targeting chitinase expressing cells in neurodegenerative diseases.

RB reports receiving consulting fees from Amylyx, Alector LLC, AcuraStem, Clene Therapeutics, NeuroSense Therapeutics, and BrainStorm, and is a co-founder and has stock options in nVector, Inc.

Funding support was provided by the Barrow Neurological Foundation and Target ALS grant CF-2025-PMT to RB.

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IRB of Barrow Neurological Institute gave ethical approval for this work (IRB Protocol #: PHXB-13BN080).

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