Investigating how CSTB affects the activity of cysteine cathepsin in the AD‐DS, EOAD, and human cellular models of trisomy Hsa21.

Background: People with Down syndrome (DS) have a high genetic risk of developing Alzheimer's disease (AD) ‐related dementia and pathology. Triplication of a human chromosome 21 (Hsa21)‐located gene APP play an important role in AD pathology. Wiseman et al. demonstrated that triplication of a gene or genes on Hsa21, other than APP exacerbates amyloid‐β pathology in an APP transgenic mouse model. Multiple lines of evidence indicate that cysteine cathepsin activity influences APP cleavage and Aβ accumulation. Cystine cathepsins are predominantly expressed in microglia in the brain and are involved in key neuroinflammation pathways. Hsa21‐located gene CYSTATIN B (CSTB) is an endogenous inhibitor of cathepsin proteases. We hypothesise that three copies of CSTB inhibit cathepsin activity and that this could contribute to altered neuroinflammation in people with DS. Method: Human fibroblasts from individuals with DS and euploid controls, temporal cortex samples from individuals who had AD‐DS and Braak‐stage matched euploid cases of early‐onset AD (EOAD) or healthy aging and human iPSC‐derived microglia‐like cells (iMLCs: T21 and an isogenic control) were used. Western blot and immunocytochemistry were conducted to determine the abundance and subcellular localisation of CSTB. Cathepsin B (CatB) activity in human fibroblasts and temporal cortex samples was measured using an enzymatic assay (Abcam) and/or a pan‐cysteine cathepsin quenched activity‐based probe (BMV109). Result: Western blotting confirmed that trisomy of Hsa21 resulted in an increase in the protein level of CSTB, that endogenous CSTB is predominantly localised in the cytoplasm and that its subcellular localisation is not altered by trisomy of Hsa21. Significantly lower CatB activity occurs in the temporal cortex of people who had AD‐DS compared with individuals who had EOAD. However, CatB enzymatic activity was not significantly altered in Hsa21 trisomic fibroblasts. Conclusion: Next, we will determine in iMLCs, whether trisomy of chromosome 21 alters CatB activity and which genes have altered expression at baseline and after neuroinflammatory challenge. [ABSTRACT FROM AUTHOR]