Postsynaptic density (PSD) is a remarkable subcellular feature of chemical synapses and dynamically responds to neuronal activities. The excitatory and inhibitory synapses can be distinguished by the size and shape of their PSDs, either contain mesh-like PSDs or uniform thin sheet-like ones. Recently, our lab reconstituted excitatory PSD (ePSD)-like assemblies by purified scaffold proteins via phase separation in vitro. In sharp contrast to excitatory synapses, our knowledge of the formation and dynamic regulations of inhibitory synapses is very scarce. The field is still debating whether inhibitory postsynaptic density (iPSD) even exists, let alone the mechanism of how iPSDs might form. Sadly, almost every form of psychiatric disorder is linked to the malfunctioning of inhibitory synap...[ Read more ]

Postsynaptic density (PSD) is a remarkable subcellular feature of chemical synapses and dynamically responds to neuronal activities. The excitatory and inhibitory synapses can be distinguished by the size and shape of their PSDs, either contain mesh-like PSDs or uniform thin sheet-like ones. Recently, our lab reconstituted excitatory PSD (ePSD)-like assemblies by purified scaffold proteins via phase separation in vitro. In sharp contrast to excitatory synapses, our knowledge of the formation and dynamic regulations of inhibitory synapses is very scarce. The field is still debating whether inhibitory postsynaptic density (iPSD) even exists, let alone the mechanism of how iPSDs might form. Sadly, almost every form of psychiatric disorder is linked to the malfunctioning of inhibitory synapses. Thus, there is an enormous gap between the functional importance of inhibitory synapses and our knowledge of their action mechanisms.

To elucidate the organization mechanism of iPSD, I explored the iPSD complex, identified several pairs of interactions among scaffold proteins, and set up a multivalent interaction network. I also found that iPSD proteins could phase-separate into liquid condensates either with their binding partners or induced by the signal molecule-calcium. Part of this work published on Cell Research provided comprehensive evidence showing that iPSDs can form by glycine or GABA receptor binding-induced phase separation of gephyrin. The formation of this highly condensed phase enriched scaffold proteins and clustered receptors both in solution and on supported membrane bilayers. Importantly, this study further revealed that iPSD condensates formed by phase separation are very different from the ePSD condensates. iPSD is thin and has a sheet-like structure. In contrast, ePSD is thick and disc-like shaped. The thin and sheet-like structure of iPSD has escaped electron microscope detection in the past few decades.

Additionally, the phase separation of the iPSD complex could be regulated by synaptic activities, via changes of the intracellular electrical potential or post-translational modifications. In sum, the work has answered one of the most fundamental and decades-long questions in neuroscience and also opened a new avenue for the field to study the function of inhibitory synapses for many years to come.

In another ongoing project, I tried to answer a few most fundamental scientific questions concerning the functions and disease mechanisms related to IQSECs in our brain. Combining integrated approaches of biochemistry, structural biology, cell biology, and animal models, our unpublished results reveal a novel autoinhibition and calcium-dependent activation mechanism of IQSECs and shed light on why mutations of IQSECs can cause various brain disorders in humans. Transgenic mice carrying pathogenic variants of iqsecs recapitulated key aspects of human phenotypes, and further suggest the critical neuronal function of IQSECs. This piece of work will have immediate benefit to the scientific community in terms of understanding the normal physiological functions of IQSECs, particularly IQSEC2, in our nerve system. The knowledge derived from this research will also be valuable for understanding the scientific basis of genetic disorders caused by mutations of IQSECs.