Plk2 promotes synaptic destabilization through disruption of N-cadherin adhesion complexes during homeostatic adaptation to hyperexcitation.

Synaptogenesis in the brain is highly organized and orchestrated by synaptic cellular adhesion molecules (CAMs) such as N-cadherin and amyloid precursor protein (APP) that contribute to the stabilization and structure of synapses. Although N-cadherin plays an integral role in synapse formation and synaptic plasticity, its function in synapse dismantling is not as well understood. Synapse weakening and loss are prominent features of neurodegenerative diseases, and can also be observed during homeostatic compensation to neuronal hyperexcitation. Previously, we have shown that during homeostatic synaptic plasticity, APP is a target for cleavage triggered by phosphorylation by Polo-like kinase 2 (Plk2). Here, we found that Plk2 directly phosphorylates N-cadherin, and during neuronal hyperexcitation Plk2 promotes N-cadherin proteolytic processing, degradation, and disruption of complexes with APP. We further examined the molecular mechanisms underlying N-cadherin degradation. Loss of N-cadherin adhesive function destabilizes excitatory synapses and promotes their structural dismantling as a prerequisite to eventual synapse elimination. This pathway, which may normally help to homeostatically restrain excitability, could also shed light on the dysregulated synapse loss that occurs in cognitive disorders.

ACh Transfers: Homeostatic Plasticity of Cholinergic Synapses.

The field of homeostatic plasticity continues to advance rapidly, highlighting the importance of stabilizing neuronal activity within functional limits in the context of numerous fundamental processes such as development, learning, and memory. Most homeostatic plasticity studies have been focused on glutamatergic synapses, while the rules that govern homeostatic regulation of other synapse types are less understood. While cholinergic synapses have emerged as a critical component in the etiology of mammalian neurodegenerative disease mechanisms, relatively few studies have been conducted on the homeostatic plasticity of such synapses, particularly in the mammalian nervous system. An exploration of homeostatic mechanisms at the cholinergic synapse may illuminate potential therapeutic targets for disease management and treatment. We will review cholinergic homeostatic plasticity in the mammalian neuromuscular junction, the autonomic nervous system, central synapses, and in relation to pathological conditions including Alzheimer disease and DYT1 dystonia. This work provides a historical context for the field of cholinergic homeostatic regulation by examining common themes, unique features, and outstanding questions associated with these distinct cholinergic synapse types and aims to inform future research in the field.

Central Cholinergic Synapse Formation in Optimized Primary Septal-Hippocampal Co-cultures.

Septal innervation of basal forebrain cholinergic neurons to the hippocampus is critical for normal learning and memory and is severely degenerated in Alzheimer's disease. To understand the molecular events underlying physiological cholinergic synaptogenesis and remodeling, as well as pathological loss, we developed an optimized primary septal-hippocampal co-culture system. Hippocampal and septal tissue were harvested from embryonic Sprague-Dawley rat brain and cultured together at varying densities, cell ratios, and in the presence of different growth factors. We identified conditions that produced robust septal-hippocampal synapse formation. We used confocal microscopy with primary antibodies and fluorescent ligands to validate that this system was capable of generating developmentally mature cholinergic synapses. Such synapses were comprised of physiological synaptic partners and mimicked the molecular composition of in vivo counterparts. This co-culture system will facilitate the study of the formation, plasticity, and dysfunction of central mammalian cholinergic synapses.

Activation of nicotinic acetylcholine receptors induces potentiation and synchronization within in vitro hippocampal networks.

Nicotinic acetylcholine receptors (nAChRs) are known to play a role in cognitive functions of the hippocampus, such as memory consolidation. Given that they conduct Ca2+ and are capable of regulating the release of glutamate and γ-aminobutyric acid (GABA) within the hippocampus, thereby shifting the excitatory-inhibitory ratio, we hypothesized that the activation of nAChRs will result in the potentiation of hippocampal networks and alter synchronization. We used nicotine as a tool to investigate the impact of activation of nAChRs on neuronal network dynamics in primary embryonic rat hippocampal cultures prepared from timed-pregnant Sprague-Dawley rats. We perturbed cultured hippocampal networks with increasing concentrations of bath-applied nicotine and performed network extracellular recordings of action potentials using a microelectrode array. We found that nicotine modulated network dynamics in a concentration-dependent manner; it enhanced firing of action potentials as well as facilitated bursting activity. In addition, we used pharmacological agents to determine the contributions of discrete nAChR subtypes to the observed network dynamics. We found that ß4-containing nAChRs are necessary for the observed increases in spiking, bursting, and synchrony, while the activation of α7 nAChRs augments nicotine-mediated network potentiation but is not necessary for its manifestation. We also observed that antagonists of N-methyl-D-aspartate receptors (NMDARs) and group I metabotropic glutamate receptors (mGluRs) partially blocked the effects of nicotine. Furthermore, nicotine exposure promoted autophosphorylation of Ca2+ /calmodulin-dependent kinase II (CaMKII) and serine 831 phosphorylation of the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) subunit GluA1. These results suggest that nicotinic receptors induce potentiation and synchronization of hippocampal networks and glutamatergic synaptic transmission. Findings from this work highlight the impact of cholinergic signaling in generating network-wide potentiation in the form of enhanced spiking and bursting dynamics that coincide with molecular correlates of memory such as increased phosphorylation of CaMKII and GluA1. OPEN SCIENCE BADGES: This article has received a badge for *Open Materials* because it provided all relevant information to reproduce the study in the manuscript. More information about the Open Practices badges can be found at https://cos.io/our-services/open-science-badges/.