TRKB interaction with PSD95 is associated with latency of fluoxetine and 2R,6R-hydroxynorketamine.

Brain derived neurotrophic factor (BDNF) and its receptor tropomyosin kinase receptor B (TRKB) are key regulators of activity-dependent plasticity in the brain. TRKB is the target for both slow- and rapid-acting antidepressants and BDNF-TRKB system mediates the plasticity-inducing effects of antidepressants through their downstream targets. Particularly, the protein complexes that regulate the trafficking and synapse recruitment of TRKB receptors might be crucial in this process. In the present study, we investigated the interaction of TRKB with the postsynaptic density protein 95 (PSD95). We found that antidepressants increase the TRKB:PSD95 interaction in adult mouse hippocampus. Fluoxetine, a slow-acting antidepressant, increases this interaction only after a long-term (7 days) treatment, while (2R,6R)-hydroxynorketamine (RHNK), an active metabolite of rapid-acting antidepressant ketamine, achieves this within a short treatment regimen (3 days). Moreover, the drug-induced changes of TRKB:PSD95 interaction correlate with drug latency in behaviour, observed in mice subjected to an object location memory test (OLM). While silencing of PSD95 by viral delivery of shRNA in hippocampus abolished the RHNK-induced plasticity in mice in OLM, overexpression of PSD95 shortened the fluoxetine latency. In summary, changes in the TRKB:PSD95 interaction contribute to differences observed in drug latency. This study sheds a light on a novel mechanism of action of different classes of antidepressants.

An opposing function of paralogs in balancing developmental synapse maturation.

The disc-large (DLG)-membrane-associated guanylate kinase (MAGUK) family of proteins forms a central signaling hub of the glutamate receptor complex. Among this family, some proteins regulate developmental maturation of glutamatergic synapses, a process vulnerable to aberrations, which may lead to neurodevelopmental disorders. As is typical for paralogs, the DLG-MAGUK proteins postsynaptic density (PSD)-95 and PSD-93 share similar functional domains and were previously thought to regulate glutamatergic synapses similarly. Here, we show that they play opposing roles in glutamatergic synapse maturation. Specifically, PSD-95 promoted, whereas PSD-93 inhibited maturation of immature α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid-type glutamate receptor (AMPAR)-silent synapses in mouse cortex during development. Furthermore, through experience-dependent regulation of its protein levels, PSD-93 directly inhibited PSD-95's promoting effect on silent synapse maturation in the visual cortex. The concerted function of these two paralogs governed the critical period of juvenile ocular dominance plasticity (jODP), and fine-tuned visual perception during development. In contrast to the silent synapse-based mechanism of adjusting visual perception, visual acuity improved by different mechanisms. Thus, by controlling the pace of silent synapse maturation, the opposing but properly balanced actions of PSD-93 and PSD-95 are essential for fine-tuning cortical networks for receptive field integration during developmental critical periods, and imply aberrations in either direction of this process as potential causes for neurodevelopmental disorders.