Calreticulin

Calreticulin (CALR) is a calcium-binding molecular chaperone residing in the endoplasmic reticulum that facilitates protein folding, oligomeric assembly, and quality control through the calreticulin/calnexin cycle (UniProt: P27797). It transiently associates with monoglucosylated glycoproteins synthesized in the ER and also regulates nuclear export of certain transcription factors. The protein plays roles in oocyte maturation and the cortical reaction, though these functions are better characterized outside the nervous system.

In Alzheimer's Disease, calreticulin shows increased abundance in post-mortem AD brain tissue compared to age-matched controls, with a mean log2 fold-change of +0.8159 across two subcellular fractions (Chaparral AD proteomics). This upregulation was detected via tandem mass tag (TMT) quantitative proteomics across multiple subcellular fractions (P2, P3, S2, S3), suggesting a potential compensatory or stress response in AD pathology. The elevation is consistent with ER stress and proteostatic dysfunction observed in Alzheimer's neuropathology, where enhanced chaperone activity may reflect attempts to manage accumulation of misfolded proteins.