p75 Neurotrophin Receptor Activation Regulates the Timing of the Maturation of Cortical Parvalbumin Interneuron Connectivity and Promotes Juvenile-like Plasticity in Adult Visual Cortex.

By virtue of their extensive axonal arborization and perisomatic synaptic targeting, cortical inhibitory parvalbumin (PV) cells strongly regulate principal cell output and plasticity and modulate experience-dependent refinement of cortical circuits during development. An interesting aspect of PV cell connectivity is its prolonged maturation time course, which is completed only by end of adolescence. The p75 neurotrophin receptor (p75NTR) regulates numerous cellular functions; however, its role on cortical circuit development and plasticity remains elusive, mainly because localizing p75NTR expression with cellular and temporal resolution has been challenging. By using RNAscope and a modified version of the proximity ligation assay, we found that p75NTR expression in PV cells decreases between the second and fourth postnatal week, at a time when PV cell synapse numbers increase dramatically. Conditional knockout of p75NTR in single PV neurons in vitro and in PV cell networks in vivo causes precocious formation of PV cell perisomatic innervation and perineural nets around PV cell somata, therefore suggesting that p75NTR expression modulates the timing of maturation of PV cell connectivity in the adolescent cortex. Remarkably, we found that PV cells still express p75NTR in adult mouse cortex of both sexes and that its activation is sufficient to destabilize PV cell connectivity and to restore cortical plasticity following monocular deprivation in vivo Together, our results show that p75NTR activation dynamically regulates PV cell connectivity, and represent a novel tool to foster brain plasticity in adults.SIGNIFICANCE STATEMENT In the cortex, inhibitory, GABA-releasing neurons control the output and plasticity of excitatory neurons. Within this diverse group, parvalbumin-expressing (PV) cells form the larger inhibitory system. PV cell connectivity develops slowly, reaching maturity only at the end of adolescence; however, the mechanisms controlling the timing of its maturation are not well understood. We discovered that the expression of the neurotrophin receptor p75NTR in PV cells inhibits the maturation of their connectivity in a cell-autonomous fashion, both in vitro and in vivo, and that p75NTR activation in adult PV cells promotes their remodeling and restores cortical plasticity. These results reveal a new p75NTR function in the regulation of the time course of PV cell maturation and in limiting cortical plasticity.

Complex regulation of Gephyrin splicing is a determinant of inhibitory postsynaptic diversity.

Gephyrin (GPHN) regulates the clustering of postsynaptic components at inhibitory synapses and is involved in pathophysiology of neuropsychiatric disorders. Here, we uncover an extensive diversity of GPHN transcripts that are tightly controlled by splicing during mouse and human brain development. Proteomic analysis reveals at least a hundred isoforms of GPHN incorporated at inhibitory Glycine and gamma-aminobutyric acid A receptors containing synapses. They exhibit different localization and postsynaptic clustering properties, and altering the expression level of one isoform is sufficient to affect the number, size, and density of inhibitory synapses in cerebellar Purkinje cells. Furthermore, we discovered that splicing defects reported in neuropsychiatric disorders are carried by multiple alternative GPHN transcripts, demonstrating the need for a thorough analysis of the GPHN transcriptome in patients. Overall, we show that alternative splicing of GPHN is an important genetic variation to consider in neurological diseases and a determinant of the diversity of postsynaptic inhibitory synapses.

Excitatory granule neuron precursors orchestrate laminar localization and differentiation of cerebellar inhibitory interneuron subtypes.

GABAergic interneurons migrate long distances through stereotyped migration programs toward specific laminar positions. During their migration, GABAergic interneurons are morphologically alike but then differentiate into a rich array of interneuron subtypes critical for brain function. How interneuron subtypes acquire their final phenotypic traits remains largely unknown. Here, we show that cerebellar molecular layer GABAergic interneurons, derived from the same progenitor pool, use separate migration paths to reach their laminar position and differentiate into distinct basket cell (BC) and stellate cell (SC) GABAergic interneuron subtypes. Using two-photon live imaging, we find that SC final laminar position requires an extra step of tangential migration supported by a subpopulation of glutamatergic granule cells (GCs). Conditional depletion of GCs affects SC differentiation but does not affect BCs. Our results reveal how timely feedforward control of inhibitory interneuron migration path regulates their terminal differentiation and, thus, establishment of the local inhibitory circuit assembly.

D1-mGlu5 heteromers mediate noncanonical dopamine signaling in Parkinson's disease.

Dopamine receptor D1 modulates glutamatergic transmission in cortico-basal ganglia circuits and represents a major target of L-DOPA therapy in Parkinson's disease. Here we show that D1 and metabotropic glutamate type 5 (mGlu5) receptors can form previously unknown heteromeric entities with distinctive functional properties. Interacting with Gq proteins, cell-surface D1-mGlu5 heteromers exacerbated PLC signaling and intracellular calcium release in response to either glutamate or dopamine. In rodent models of Parkinson's disease, D1-mGlu5 nanocomplexes were strongly upregulated in the dopamine-denervated striatum, resulting in a synergistic activation of PLC signaling by D1 and mGlu5 receptor agonists. In turn, D1-mGlu5-dependent PLC signaling was causally linked with excessive activation of extracellular signal-regulated kinases in striatal neurons, leading to dyskinesia in animals treated with L-DOPA or D1 receptor agonists. The discovery of D1-mGlu5 functional heteromers mediating maladaptive molecular and motor responses in the dopamine-denervated striatum may prompt the development of new therapeutic principles for Parkinson's disease.

Dual Function of NRP1 in Axon Guidance and Subcellular Target Recognition in Cerebellum.

Subcellular target recognition in the CNS is the culmination of a multiple-step program including axon guidance, target recognition, and synaptogenesis. In cerebellum, basket cells (BCs) innervate the soma and axon initial segment (AIS) of Purkinje cells (PCs) to form the pinceau synapse, but the underlying mechanisms remain incompletely understood. Here, we demonstrate that neuropilin-1 (NRP1), a Semaphorin receptor expressed in BCs, controls both axonal guidance and subcellular target recognition. We show that loss of Semaphorin 3A function or specific deletion of NRP1 in BCs alters the stereotyped organization of BC axon and impairs pinceau synapse formation. Further, we identified NRP1 as a trans-synaptic binding partner of the cell adhesion molecule neurofascin-186 (NF186) expressed in the PC AIS during pinceau synapse formation. These findings identify a dual function of NRP1 in both axon guidance and subcellular target recognition in the construction of GABAergic circuitry.

SEMA3A signaling controls layer-specific interneuron branching in the cerebellum.

BACKGROUND: GABAergic interneurons regulate the balance and dynamics of neural circuits, in part, by elaborating their strategically placed axon branches that innervate specific cellular and subcellular targets. However, the molecular mechanisms that regulate target-directed GABAergic axon branching are not well understood. RESULTS: Here we show that the secreted axon guidance molecule, SEMA3A, expressed locally by Purkinje cells, regulates cerebellar basket cell axon branching through its cognate receptor Neuropilin-1 (NRP1). SEMA3A was specifically localized and enriched in the Purkinje cell layer (PCL). In sema3A(-/-) and nrp1(sema-/sema-) mice lacking SEMA3A-binding domains, basket axon branching in PCL was reduced. We demonstrate that SEMA3A-induced axon branching was dependent on local recruitment of soluble guanylyl cyclase (sGC) to the plasma membrane of basket cells, and sGC subcellular trafficking was regulated by the Src kinase FYN. In fyn-deficient mice, basket axon terminal branching was reduced in PCL, but not in the molecular layer. CONCLUSIONS: These results demonstrate a critical role of local SEMA3A signaling in layer-specific axonal branching, which contributes to target innervation.