Aggregation of a-synuclein (aS) is involved in the pathogenesis of Parkinson's disease (PD) and a variety of related neurodegenerative disorders. The physiological function of aS is largely unknown. We demonstrate with in vitro vesicle fusion experiments that aS has an inhibitory function on membrane fusion. Upon increased expression in cultured cells and in Caenorhabditis elegans, aS binds to mitochondria and leads to mitochondrial fragmentation. In C. elegans age-dependent fragmentation of mitochondria is enhanced and shifted to an earlier time point upon expression of exogenous aS. In contrast, siRNA-mediated downregulation of aS results in elongated mitochondria in cell culture. aS can act independently of mitochondrial fusion and fission proteins in shifting the dynamic morphologic equilibrium of mitochondria towards reduced fusion. Upon cellular fusion, aS prevents fusion of differently labelled mitochondrial populations. Thus, aS inhibits fusion due to its unique membrane interaction. Finally, mitochondrial fragmentation induced by expression of aS is rescued by coexpression of PINK1, parkin or DJ-1 but not the PD-associated mutations PINK1 G309D and parkin D1-79 or by DJ-1 C106A.

Coding variants in the triggering receptor expressed on myeloid cells 2 (TREM2) are associated with late onset Alzheimer’s disease (AD). We demonstrate that amyloid plaque seeding is increased in the absence of functional Trem2. Increased seeding is accompanied by decreased microglial clustering around newly seeded plaques and reduced plaque associated Apolipoprotein E (ApoE). Reduced ApoE deposition in plaques is also observed in brains of AD patients carrying TREM2 coding variants. Proteomic analyses and microglia depletion experiments revealed microglia as one origin of plaque associated ApoE. Longitudinal amyloid small animal positron emission tomography demonstrates accelerated amyloidogenesis in Trem2 loss of function mutants at early stages, which progressed at a lower rate with aging. These findings suggest that in the absence of functional Trem2 early amyloidogenesis is accelerated due to reduced phagocytic clearance of amyloid seeds despite reduced plaque associated ApoE.

Alzheimer disease (AD) is characterized by the accumulation of amyloid plaques, which are predominantly composed of amyloid-β peptide. Two principal physiological pathways either prevent or promote amyloid-β generation from its precursor, β-amyloid precursor protein (APP), in a competitive manner. Although APP processing has been studied in great detail, unknown proteolytic events seem to hinder stoichiometric analyses of APP metabolism in vivo. Here we describe a new physiological APP processing pathway, which generates proteolytic fragments capable of inhibiting neuronal activity within the hippocampus. We identify higher molecular mass carboxy-terminal fragments (CTFs) of APP, termed CTF-η, in addition to the long-known CTF-α and CTF-β fragments generated by the α- and β-secretases ADAM10 (a disintegrin and metalloproteinase 10) and BACE1 (β-site APP cleaving enzyme 1), respectively. CTF-η generation is mediated in part by membrane-bound matrix metalloproteinases such as MT5-MMP, referred to as η-secretase activity. η-Secretase cleavage occurs primarily at amino acids 504-505 of APP695, releasing a truncated ectodomain. After shedding of this ectodomain, CTF-η is further processed by ADAM10 and BACE1 to release long and short Aη peptides (termed Aη-α and Aη-β). CTFs produced by η-secretase are enriched in dystrophic neurites in an AD mouse model and in human AD brains. Genetic and pharmacological inhibition of BACE1 activity results in robust accumulation of CTF-η and Aη-α. In mice treated with a potent BACE1 inhibitor, hippocampal long-term potentiation was reduced. Notably, when recombinant or synthetic Aη-α was applied on hippocampal slices ex vivo, long-term potentiation was lowered. Furthermore, in vivo single-cell two-photon calcium imaging showed that hippocampal neuronal activity was attenuated by Aη-α. These findings not only demonstrate a major functionally relevant APP processing pathway, but may also indicate potential translational relevance for therapeutic strategies targeting APP processing.

A number of neurodegenerative disorders, including Parkinson's disease, dementia with Lewy bodies, and multiple system atrophy, are characterized by the intracellular deposition of fibrillar aggregates that contain a high proportion of ␣-synuclein (␣S). The interaction with the membrane-water interface strongly modulates folding and aggregation of the protein. The present study investigates the lipid binding and the coil-helix transition of ␣S, using titration calorimetry, differential scanning calorimetry, and circular dichroism spectroscopy. Titration of the protein with small unilamellar vesicles composed of zwitterionic phospholipids below the chain melting temperature of the lipids yielded exceptionally large exothermic heat values. The sigmoidal titration curves were evaluated in terms of a simple model that assumes saturable binding sites at the vesicle surface. The cumulative heat release and the ellipticity were linearly correlated as a result of simultaneous binding and helix folding. There was no heat release and folding of ␣S in the presence of large unilamellar vesicles, indicating that a small radius of curvature is necessary for the ␣S-membrane interaction. The heat release and the negative heat capacity of the proteinvesicle interaction could not be attributed to the coilhelix transition of the protein alone. We speculate that binding and helix folding of ␣S depends on the presence of defect structures in the membrane-water interface, which in turn results in lipid ordering in the highly curved vesicular membranes. This will be discussed with regard to a possible role of the protein for the stabilization of synaptic vesicle membranes. ␣-Synuclein (␣S)1 is a protein of 140 amino acids that has been identified as a major component of the intracytoplasmic fibrillar deposits (Lewy bodies) associated with idiopathic and inherited forms of Parkinson's disease (1). The majority of cases are idiopathic, whereas mutations in the ␣S gene are known to be responsible for rare inherited, early onset variants of Parkinson's disease (2-4). Although the molecular mode of action of ␣S and of its homologs is as yet unknown, it was assumed that the protein modulates the dopamine neurotransmission by regulating synaptic vesicle (SV) mobilization from the presynaptic reserve pool (5, 6) or directly regulating the dopamine metabolism (7-9). However, attempts to identify a specific SV-binding protein, e.g. by protein cross-linking, have been unsuccessful so far.At a certain threshold concentration, ␣S tends to aggregate into amyloid fibrils (10), whereas the homolog ␤S, which lacks a stretch of amino acids within the central portion of ␣S, has a much lower fibrillization propensity (11, 12) and may even inhibit ␣S fibrillization (13-15), indicating that the hydrophobic central part of ␣S is essential for its fibrillar aggregation (16). A recent study on the structure of mature fibrillar aggregates, using site-directed spin labeling, indicates that the N terminus of ␣S is less ordered than the central portion and that t...