PICALM, a clathrin adaptor protein, plays important roles in clathrin-mediated endocytosis in all cell types. Recently, genome-wide association studies identified single nucleotide polymorphisms in PICALM gene as genetic risk factors for late-onset Alzheimer disease (LOAD). We analysed by western blotting with several anti-PICALM antibodies the pattern of expression of PICALM in human brain extracts. We found that PICALM was abnormally cleaved in AD samples and that the level of the uncleaved 65-75 kDa full-length PICALM species was significantly decreased in AD brains. Cleavage of human PICALM after activation of endogenous calpain or caspase was demonstrated in vitro. Immunohistochemistry revealed that PICALM was associated in situ with neurofibrillary tangles, co-localising with conformationally abnormal and hyperphosphorylated tau in LOAD, familial AD and Down syndrome cases. PHF-tau proteins co-immunoprecipitated with PICALM. PICALM was highly expressed in microglia in LOAD. These observations suggest that PICALM is associated with the development of AD tau pathology. PICALM cleavage could contribute to endocytic dysfunction in AD.

Enlarged early endosomes have been observed in neurons and fibroblasts in Down syndrome (DS). These endosome abnormalities have been implicated in the early development of Alzheimer's disease (AD) pathology in these subjects. Here, we show the presence of enlarged endosomes in blood mononuclear cells and lymphoblastoid cell lines (LCLs) from individuals with DS using immunofluorescence and confocal microscopy. Genotype-phenotype correlations in LCLs carrying partial trisomies 21 revealed that triplication of a 2.56 Mb locus in 21q22.11 is associated with the endosomal abnormalities. This locus contains the gene encoding the phosphoinositide phosphatase synaptojanin 1 (SYNJ1), a key regulator of the signalling phospholipid phosphatidylinositol-4,5-biphosphate that has been shown to regulate clathrin-mediated endocytosis. We found that SYNJ1 transcripts are increased in LCLs from individuals with DS and that overexpression of SYNJ1 in a neuroblastoma cell line as well as in transgenic mice leads to enlarged endosomes. Moreover, the proportion of enlarged endosomes in fibroblasts from an individual with DS was reduced after silencing SYNJ1 expression with RNA interference. In LCLs carrying amyloid precursor protein (APP) microduplications causing autosomal dominant early-onset AD, enlarged endosomes were absent, suggesting that APP overexpression alone is not involved in the modification of early endosomes in this cell type. These findings provide new insights into the contribution of SYNJ1 overexpression to the endosomal changes observed in DS and suggest an attractive new target for rescuing endocytic dysfunction and lipid metabolism in DS and in AD.