The Functional Epididymal Amyloid Cystatin‐Related Epididymal Spermatogenic ( CRES ) is a Component of the Mammalian Brain Extracellular Matrix

ABSTRACT CRES is the defining member of a reproductive subgroup of family 2 cystatins of cysteine protease inhibitors. We previously showed that CRES and other subgroup members are part of a highly plastic amyloid‐containing extracellular matrix (ECM) with host defense functions in the mouse epididymal lumen. Based on parallels between the epididymis and the brain, we hypothesized that CRES and CRES amyloids might also function within the brain including the ECM. Here we show that CRES is produced by hippocampal neurons and astrocytes in the male and female mouse and human brain. Further, approximately 50% of hippocampal astrocytes from aged mice, like the aged human donor samples, had significantly reduced levels of CRES compared to younger mice, suggesting an age‐related decline in CRES could contribute to altered brain function. Immunofluorescence experiments showed CRES colocalized with the ECM markers phosphacan and wisteria floribunda agglutinin indicating that CRES is part of the ECM. CRES monomer and high molecular weight SDS‐resistant forms were found in insoluble fractions of the hippocampus, cortex, cerebellum, and midbrain and bound to the protein aggregation disease (PAD) ligand, which preferentially binds amyloids but not protein monomers, suggesting a population of CRES normally exists in the brain as an amyloid structure. Collectively, our studies demonstrate that CRES/CRES amyloid is present in the mammalian brain and may contribute to ECM structure and function.