The proteome of synaptic vesicles

Functional characterisation of SV31, a Zn2+-binding synaptic vesicle protein Synaptic vesicles are key organelles in chemical signal transmission allowing neurons to communicate with each other and neighboring cells. The numerous tasks of synaptic vesicle are governed by a unique set of proteins. We recently identified in collaboration with the Karas lab the complete proteinaceous inventory of synaptic vesicles by immunoisolation. About 45 of a total of 185 proteins were novel proteins. One of them is a synaptic vesicle membrane protein of 31 kDa, which we named SV31. Based on its amino acid sequence similarity to a prokaryotic heavy metal ion transporter we analyzed its metal ion-binding properties and demonstrated that both in vivo and in vitro SV31 is capable to bind Zn2+. Studies performed by Lisa Waberer focus on a detailed functionally characterization of SV31.

The physiological function of APP at the hippocampal presynaptic active zone

Dementia is associated with a decline in mental ability, impairments in memory and changes in personality. The most frequent form of dementia worldwide is the sporadic Alzheimer´s disease (AD). Alzheimer´s disease is mainly associated with hyperphosphorylated tau as well as intra- and extracellular deposits of amyloid β (Aβ) oligomers. More than 20 years ago the amyloid precursor protein (APP) was identified as the precursor protein of Aβ peptide, the main component of senile plaques in brains affected by Alzheimer´s disease. The pathophysiology of AD, characterized by a massive loss of synapses, cognitive decline and behavioral changes, was in principal attributed to the accumulation of Aβ. Within the last decades, much effort has gone into understanding the molecular basis of the progression of Alzheimer´s disease. However, little is known about the actual physiological function of amyloid precursor proteins.

In early work of our project we successfully established a method that allows us to highly purify the native presynaptic active zone (PAZ) from individual total mouse brain. This newly developed straightforward experimental approach comprises in principal two consecutive steps: subcellular fractionation steps and immunopurification (Weingarten et al. 2014). Employing this method we could previously demonstrate that APP and its mammalian family members the amyloid precursor like proteins 1 and 2 (APLP1, APLP2) are highly accumulated at neurotransmitter release sites (Lassek et al. 2013) and that deletion of APP results in severe downregulation of prominent synaptic vesicle proteins like SV2, synaptotagmin-1 and synaptophysin (Lassek et al. 2014).

Our next discovery provides significant differences in protein abundance within the PAZ proteome derived from individual mouse brain regions – like olfactory bulb, hippocampus and cerebellum. These differences point to a specific adaption of these regions and furthermore, emphasize their ability to undergo structural and functional changes also in the adult CNS.

Currently, our research topic focuses on the impact of APP deletion at the hippocampal PAZ proteome derived from adult mice. Using a state-of-the-art proteomic approach (Tandem Mass Tag, TMT) we identified approximately 1000 proteins including significant differences in the abundance of proteins involved in calcium homeostasis and intracellular signaling. It is of note that these alterations already occurred in younger adults, before the onset of phenotype specific impairments in learning and memory that occur in elderly mice.

Our future perspective is to build up a network of APP interaction partners within the hippocampal presynaptic active zone. Furthermore, we intend to unravel how deletion of APP affects this network during space and time leading to impairments in learning and memory. These alterations may provide a molecular link to the pathogenesis of Alzheimer´s disease and open new strategies for therapeutic approaches.

People working on this project: Melanie Laßek and Jens Weingarten