Dual roles of PirB on pre- and postsynapse in hippocampal asymmetry

Left-right asymmetry is a fundamental feature of higher-order brain structure and function. Within the mouse hippocampus, properties of synapses between pyramidal cells depend on the hemispheric origin of presynaptic inputs. In the CA3-CA1 connection, right-input synapses in stratum radiatum have larger postsynaptic density (PSD) and higher perforation ratio, expressing lower NR2B subunit density, than left-input synapses. The major histocompatibility complex class I (MHCI) and Paired immunoglobulin-like receptor B (PirB) contribute to this asymmetry, suggesting potential trans-synaptic signaling via MHCI and PirB.Our investigation aimed to discern PirB's role on the input or target side through ultrastructural analysis utilizing resolution of electron microscopy. We performed AAV- mediated conditional knockout (cKO) of PirB in either the CA3 or CA1 cells. Surprisingly, our ultrastructural analysis revealed that cKO in either pre- or postsynaptic neurons abolished morphological asymmetry. Both left- and right-input synapses displayed a low perforation ratio in both hemispheres of CA1, similar to that in the wild-type left-input synapses. Immunofluorescence of PirB revealed robust labeling in the CA1 area of the wild-type and complete absence in the PirB KO mice. In conclusion, our findings reveal the critical role of PirB on both sides of the synapse in the hippocampal asymmetry formation and maintenance. Hypothetically, this may be attributed to specialized functions of PirB on each side of the synapse. Further investigation to visualize PirB in a high resolution will provide insights into how the PirB and MHCI work in concert for the generation of hippocampal asymmetry in the pre- and postsynaptic sites.