Disease-Elevated CHI3L1/CHIT1 Chitinases Are Biomarkers and Partial Compensators for Senescent Microglial Phagocytic Failure

CHI3L1 (YKL-40), CHIT1 (chitotriosidase), and CHI3L2 are among the most consistently elevated proteins in ALS biofluids and tissue, making them strong candidate disease biomarkers. In sporadic ALS CSF, CHIT1 shows strong elevation and was among the first candidate biomarkers identified for the disease (PMID 24295388). Consistent with this, CSF CHIT1, CHI3L1, and CHI3L2 correlate with neuroinflammatory disease state across patient cohorts (PMID 31123140). At the tissue level, CHI3L1 and CHI3L2 are overexpressed in motor cortex and spinal cord from sporadic ALS patients, indicating disease-linked glial activation in affected CNS regions (PMID 28989002). Chitotriosidase activity specifically functions as a biomarker of microglial activation in ALS, reflecting the prominent innate immune component of the disease (PMID 30134252).\n\nThe hypothesis proposes that these chitinases are not merely markers but function as partial compensators for senescent microglial phagocytic failure. In this view, disease-elevated chitinases partially compensate for diminished clearance capacity of aging or stressed microglia, and CHIT1 can accentuate motor-neuron neurodegeneration through neuroinflammatory signaling, challenging the idea that disease-elevated chitinases are mainly compensatory (PMID 32762702). Notably, neurodegenerative biomarkers outperformed neuroinflammatory biomarkers in ALS, limiting the practical dominance of CHI3L1/CHIT1 as mechanistic indicators (PMID 37789557). The biomarker role is firmly established across five independent studies; however, whether the observed elevation reflects a protective compensatory response, a pathogenic amplification loop, or both remains unresolved.