Abstract:
Bdnf/TrkB signaling plays an essential role in excitatory and inhibitory synapse stabilization and plasticity, and its disruption is closely linked to the pathophysiology of various neuropsychiatric and neurodevelopmental disorders. Further understanding these mechanisms may provide novel insights into therapeutic strategies for treating synaptic dysfunctions in these conditions. Over the past decade, TrkB has emerged in its role in inhibitory synapse regulation through its impact on gephyrin (Gphn), the key scaffolding protein at the inhibitory postsynapse. Clustering of gephyrin is a direct correlate for inhibitory transmission, and aberrations in TrkB function are associated with decreased Gphn clusters. This study aimed to elucidate how TrkB exerts its control over gephyrin, particularly dissecting the individual contributions of cognate Shc- and PLCγ-dependent pathways to provide a more profound knowledge of the molecular mechanisms underlying TrkB-dependent regulation of GABAergic synaptic plasticity.
To dissect the individual contribution of TrkB signaling pathways in neurons, mutants of the rat TrkB receptor-deficient in autophosphorylation or the induction of PLCγ- or Shc-dependent signaling pathways were developed. By lentiviral overexpression in primary hippocampal neurons, their contribution to the regulation of inhibitory synaptic protein accumulation was assessed by immunocytochemistry and confocal laser scan microscopy with a specific focus on gephyrin clustering. Furthermore, the functional consequences of aberrant synaptic connectivity were evaluated by analyzing patterns of intracellular calcium transients of neurons with aberrant TrkB signaling. The results revealed that TrkB cognate signaling pathways specifically account for the regulation of Gphn by limiting its clustering in size and density (Shc-dependent pathways) and the localization of Gphn at synaptic sites (PLCγ-dependent pathways) at the somata of primary hippocampal neurons, therefore maintaining the E/I balance in neurons. To further address the contribution of TrkB to inhibitory synaptic plasticity, inhibitory long-term potentiation was chemically induced through a moderate increase in intracellular calcium concentration. This revealed a novel role for TrkB in the calcium-dependent potentiation of inhibitory synapses since the catalytic activity of TrkB was a prerequisite for the associated increase in Gphn accumulation and density of inhibitory synapses. This potentiation was exerted by both signaling pathway strands, including MAPK/Erk1/2 and CaMKII signaling.
The findings presented in this study support the notion of TrkB as a crucial regulator of GABAergic synapses through its modulation of Gphn clustering. Furthermore, by extending the role of TrkB in calcium-dependent synaptic plasticity by iLTP, they suggest TrkB as an upstream cross-talking mediator of activity-dependent homeostatic heterosynaptic plasticity, crucial for maintaining and adjusting E/I balance.
Neuroscience