Glial S100A6 Degrades ß-amyloid Aggregation through Targeting Competition with Zinc Ions.

Evidence has been accumulating that zinc ions can trigger ß-amyloid (Aß) deposition and senile plaque formation in the brain, a pathological hallmark of Alzheimer's disease (AD). Chelating zinc inhibits Aß aggregation and may hold promise as a therapeutic strategy for AD. S100A6 is an acidic Ca2+/Zn2+-binding protein found only in a small number of astrocytes in the normal brain. However, in the AD brain, S100A6 is highly expressed in astrocytes around Aß plaques. The role of the astrocytic S100A6 upregulation in AD is unknown. In the present study, we examined the effects of S100A6 on Aß plaques and intracellular zinc levels in a mouse model of AD. Chronic exposure to zinc increased Aß deposition and S100A6 expression, both reversible by the zinc chelator clioquinol, in the brains of amyloid precursor protein/presenilin 1 (APP/PS1) transgenic mice. To examine whether exogenous S100A6 could induce Aß plaque disaggregation through competition for zinc in vitro, we incubated APP/PS1 mouse brain sections with recombinant human S100A6 protein or co-incubated them with human S100A6-expressing cells. Both treatments efficiently reduced the Aß plaque burden in situ. In addition, treatment with exogenous S100A6 protected cultured COS-7 cells against zinc toxicity. Our results show for the first time that increased S100A6 levels correlate with both Aß disaggregation and decrease of Aß plaque-associated zinc contents in brain sections with AD-like pathology. Astrocytic S100A6 in AD may protect from Aß deposition through zinc sequestration.

A homolog of glyceraldehyde-3-phosphate dehydrogenase from Riemerella anatipestifer is an extracellular protein and exhibits biological activity.

Riemerella anatipestifer is the causative agent of septicemia anserum exsudativa in ducks. Its pathogenesis and virulence factors are still unclear. The glycolytic enzyme, glyceraldehyde-3-phosphate dehydrogenase (GAPDH), an anchorless and multifunctional protein on the surface of several pathogenic microorganisms, is involved in virulence and adhesion. Whether homologs of GAPDH exist, and display similar characteristics in R. anatipestifer (RaGAPDH) has not been determined. In our research, the RaGAPDH activity from various R. anatipestifer isolates was confirmed. Twenty-two gapdh genes from genomic DNA of R. anatipestifer isolates were cloned and sequenced for phylogenetic analysis. The distribution of RaGAPDH in R. anatipestifer CZ2 strain was confirmed by antisera to recombinant RaGAPDH. The ability of purified RaGAPDH to bind host proteins was analyzed by solid-phase ligand-binding assay. Results revealed that all R. anatipestifer isolates showed different levels of GAPDH activity except four strains, which contained a gapdh-like gene. The gapdh of R. anatipestifer, which is located phylogenetically in the same branch as enterohemorrhagic Escherichia coli (EHEC), belonged to class I GAPDH, and encoded a 36.7-kDa protein. All RaGAPDH-encoding gene sequences from field isolates of R. anatipestifer displayed 100% homology. The RaGAPDH localized on the extracellular membrane of several R. anatipestifer strains. Further, it was released into the culture medium, and exhibited GAPDH enzyme activity. We also confirmed the binding of RaGAPDH to plasminogen and fibrinogen. These results demonstrated that GAPDH was present in R. anatipestifer, although not in all strains, and that RaGAPDH might contribute to the microorganism's virulence.

Decreased expression of cathepsin D in monocytes is related to the defective degradation of amyloid-ß in Alzheimer's disease.

Alzheimer's disease (AD) is a progressive neurodegenerative dementia characterized by pathological senile plaques composed of amyloid-ß (Aß) in the cerebral cortex and hippocampus. Bone marrow-derived monocytes of patients with AD migrate across the blood-brain barrier into the brain, but are defective at clearing Aß in the neuritic plaques. However, the underlying mechanisms remain unclear. Here, in patients with AD, we found that cathepsin D, a major lysosomal aspartic protease, was underexpressed in monocytes, resulting in the defective degradation of Aß by monocytes/macrophages. Further, downregulation of cathepsin D in THP-1 cells significantly reduced the clearance of amyloid plaques in the brain sections of AßPP/PS1 mice. The clearance ability was recovered by the overexpression of cathepsin D in AD monocytes. These results suggest that decreased expression of cathepsin D in the peripheral monocytes is a potential signature of AD, and that this decreased expression is involved in Aß degradation and AD pathogenesis.