Nanoscale synaptic heterogeneity, loss, and plasticity in Alzheimer's disease - PROJECT SUMMARY/ABSTRACT Cognitive decline in Alzheimer's disease (AD) is closely linked to the loss of dendritic spines. By binding to spines, neurotoxic Amyloid-beta (A), a pathological hallmark of AD, impairs synaptic function and plasticity. Still, how molecularly and structurally diverse synapses in the brain respond to A is not understood. Indeed, while the function of many spines declines, some spines are thought to increase in size as a way to compensate for synaptic failure in neighboring spines as suggested by the presence of large surviving spines in human patients and AD mouse models. Understanding the molecular and cellular mechanisms of local compensation, if present, might provide significant insights for restoring synaptic function in AD. Leveraging super-resolution Stimulating Emission Depletion (STED) microscopy in living and fixed neurons, we aim to investigate how synapses with diverse nano-architecture respond to A pathology. We have previously demonstrated that dendritic spines consist of aligned nanomodules of pre- and post-synaptic proteins – PSD-95, Bassoon, Synaptotagmin-1 (SYT1), and AMPAR and NMDAR, forming structurally diverse synaptic substructures. To determine the role of spine nano-architecture in AD, we tested the effects of A on the organization of Bassoon (active zone) and PSD-95 (PSDs) nanomodules in cultured neurons and 5xFAD mice. Our preliminary data indicate that A destabilizes PSD-95 in the smallest spines, while large spines with multiple PSD-95 nanomodules remain structurally unperturbed, with some spines increasing in size. Based on our preliminary data, we hypothesize that Aβ-dependent synaptic loss is intricately connected to local compensatory mechanisms, and this relationship is shaped by diverse molecular signaling defined by the nano-architecture of small and large spines. In our first aim, we will determine the role of acute Aβ on synaptic loss and compensatory plasticity. Live-cell STED imaging will assess the effects of Aβ on PSD-95 stability in spines with different numbers of PSD-95 nanomodules. Using CRISPR DNA editing to visualize endogenous PSD-95 and AMPARs, we will determine the link between synaptic loss and local compensation and a potential role of LTP-like structural plasticity. For our second aim, we will investigate the impact of Aβ on the structure and function of diverse synapses in the auditory cortex. Employing slice electrophysiology and complementary multi-color STED imaging, we aim to identify which synapses are vulnerable to Aβ, which synapses compensate, and whether AMPAR nanodomain reorganization underlies these responses. In our third aim, we will identify how Aβ destabilizes PSD-95 nano-organization in small spines. By undertaking quantal Ca2+ imaging and STED imaging in fixed and living cells, will test the hypothesis that Aβ destabilizes PSD-95 in small spines downstream of NMDARs that lead to ERK1/2 dependent phosphorylation of ephrin-B3 S332, known to regulate PSD-95 stability in spines. Our proposed experiments will define the intricate relationships between Aβ and the diverse nano-architecture of AMPAR and NMDARs and will provide valuable insights into potential therapeutic strategies in AD.
