NARSAD Fellowship

alpha-actinin regulates postsynaptic AMPAR targeting by anchoring PSD-95

01/05/2016 to 14/09/2016

Autism Spectrum Disorders (ASDs) are neurodevelopmental disorders that often constitute an enormous burden for everybody involved. Recently, a number of mutations have been identified that unequivocally link ASDs to malfunctions of glutamatergic synapses. Glutamate is the major excitatory neurotransmitter in the brain. Major ASD-associated genes include PSD-95, Neuroligin, and Shank. These proteins are indispensable components of the postsynaptic structure of excitatory synapses. PSD-95 in particular is the central structural element of these synapses. It links Neuroligin to Shank and is required for the postsynaptic localization of AMPA- and NMDA-type glutamate receptors. Loss of AMPA receptors and the associated reduction in the number of AMPA-containing excitatory synapses lead to an imbalance between excitatory and inhibitory neurotransmission which constitutes a hallmark of ASDs. Additionally, synaptic plasticity, which underlies learning and memory, is typically mediated by changes in the number of postsynaptic AMPA receptors. Important for treatments of ASDs is that the loss of AMPA receptors, which is often the result of malfunction of PSD-95 and its associated proteins, is emerging as a drug target.
Although PSD-95 is key to postsynaptic structure and function, we do not know at all how it is anchored to F-actin, the cytoskeletal element determining postsynaptic structure and size. I hypothesize that α-actinin, primarily known as an F-actin cross-linking factor, is responsible for the postsynaptic localization of PSD-95 and thereby AMPAR. So far, my data show that α-actinin binds to PSD-95. Experimentally removing α-actinin from cultured neurons leads to a loss of postsynaptically located PSD-95 and the number of excitatory synapses. Based on my current data, I hypothesize that α-actinin anchors PSD-95 and therefore AMPAR at the postsynaptic site.
To prove this overarching hypothesis I propose to introduce a minimal mutation into PSD-95 to inhibit the interaction with α-actinin while preserving all other functions (Aim 1). Mutated PSD-95 will subsequently be used to replace endogenous PSD-95 in cultured neurons (Aim 2). Changes in the number and content of synapses in cells expressing mutant PSD-95 will be analyzed to test the hypothesis, that the interaction between PSD-95 and α-actinin is responsible for the synaptic localization of PSD-95 and AMPAR.
Defining how PSD-95 is anchored at postsynaptic sites will fill a crucial gap in our understanding of synaptic structure, which is critical for guiding the future development of therapeutics for the treatment of ASDs, which could involve organic compounds that foster protein interactions as well as gene replacement therapies.