Semaphorin4D Induces Inhibitory Synapse Formation by Rapid Stabilization of Presynaptic Boutons via MET Coactivation

Plexin-B1 and Plexin-B2 play non-redundant roles in GABAergic synapse formation

Synapse formation in the mammalian brain is a complex and dynamic process requiring coordinated function of dozens of molecular families such as cell adhesion molecules (CAMs) and ligand-receptor pairs (Ephs/Ephrins, Neuroligins/Neurexins, Semaphorins/Plexins). Due to the large number of molecular players and possible functional redundancies within gene families, it is challenging to determine the precise synaptogenic roles of individual molecules, which is key to understanding the consequences of mutations in these genes for brain function. Furthermore, few molecules are known to exclusively regulate either GABAergic or glutamatergic synapses, and cell and molecular mechanisms underlying GABAergic synapse formation in particular are not thoroughly understood. We previously demonstrated that Semaphorin-4D (Sema4D) regulates GABAergic synapse development in the mammalian hippocampus while having no effect on glutamatergic synapse development, and this effect occurs through binding to its high affinity receptor, Plexin-B1. In addition, we demonstrated that RNAi-mediated Plexin-B2 knock-down decreases GABAergic synapse density suggesting that both receptors function in this process. Here, we perform a structure-function study of the Plexin-B1 and Plexin-B2 receptors to identify the protein domains in each receptor which are required for its synaptogenic function. Further, we examine whether Plexin-B2 is required in the presynaptic neuron, the postsynaptic neuron, or both to regulate GABAergic synapse formation. Our data reveal that Plexin-B1 and Plexin-B2 function non-redundantly to regulate GABAergic synapse formation and suggest that the transmembrane domain may underlie functional distinctions. We also provide evidence that Plexin-B2 expression in presynaptic GABAergic interneurons, as well as postsynaptic pyramidal cells, regulates GABAergic synapse formation in hippocampus. These findings lay the groundwork for future investigations into the precise signaling pathways required for synapse formation downstream of Plexin-B receptor signaling.

Plexin-B1 and Plexin-B2 play non-redundant roles in GABAergic synapse formation

Synapse formation in the mammalian brain is a complex and dynamic process requiring coordinated function of dozens of molecular families such as cell adhesion molecules (CAMs) and ligand-receptor pairs (Ephs/Ephrins, Neuroligins/Neurexins, Semaphorins/Plexins). Due to the large number of molecular players and possible functional redundancies within gene families, it is challenging to determine the precise synaptogenic roles of individual molecules, which is key to understanding the consequences of mutations in these genes for brain function. Furthermore, few molecules are known to exclusively regulate either GABAergic or glutamatergic synapses, and cell and molecular mechanisms underlying GABAergic synapse formation in particular are not thoroughly understood. However, we previously demonstrated that Semaphorin-4D (Sema4D) regulates GABAergic synapse development in the mammalian hippocampus while having no effect on glutamatergic synapse development, and this effect occurs through binding to its high affinity receptor, Plexin-B1. Furthermore, Plexin-B2 contributes to GABAergic synapse formation as well but is not required for GABAergic synapse formation induced by binding to Sema4D. Here, we perform a structure-function study of the Plexin-B1 and Plexin-B2 receptors to identify the protein domains in each receptor that are required for its synaptogenic function. We also provide evidence that Plexin-B2 expression in presynaptic parvalbumin-positive interneurons is required for formation of GABAergic synapses onto excitatory pyramidal neurons in CA1. Our data reveal that Plexin-B1 and Plexin-B2 function non-redundantly to regulate GABAergic synapse formation and suggest that the transmembrane domain may underlie these functional distinctions. These findings lay the groundwork for future investigations into the precise signaling pathways required for synapse formation downstream of Plexin-B receptor signaling.

Semaphorin 4D induced inhibitory synaptogenesis decreases epileptiform activity and alters progression to Status Epilepticus in mice

Previously we demonstrated that intra-hippocampal infusion of purified, Semaphorin 4D (Sema4D) extracellular domain (ECD) into the mouse hippocampus rapidly promotes formation of GABAergic synapses and decreases seizure susceptibility in mice. Given the relatively fast action of Sema4D treatment revealed by these studies, we sought to determine the time course of Sema4D treatment on hippocampal network activity using an acute hippocampal slice preparation. We performed long-term extracellular recordings from area CA1 encompassing a 2-hour application of Sema4D and found that hippocampal excitation is suppressed for hours following treatment. We also asked if Sema4D treatment could ameliorate seizures in an acute seizure model: the kainic acid (KA) mouse model. We demonstrate that Sema4D treatment delays and suppresses ictal activity, delays the transition to Status Epilepticus (SE), and lessens the severity of SE. Lastly, we sought to explore alternative methods of Sema4D delivery to hippocampus and thus created an Adeno Associated Virus expressing the ECD of Sema4D. Our data reveal that virally delivered, chronically overexpressed Sema4D-ECD promotes GABAergic synapse formation and suppresses ictal activity and progression to SE. These results provide proof of concept that viral delivery of Sema4D is an efficacious and promising delivery method to abate epileptiform activity and progression to SE.

Circuit-selective cell-autonomous regulation of inhibition in pyramidal neurons by Ste20-like kinase

Maintaining an appropriate balance between excitation and inhibition is critical for neuronal information processing. Cortical neurons can cell-autonomously adjust the inhibition they receive to individual levels of excitatory input, but the underlying mechanisms are unclear. We describe that Ste20-like kinase (SLK) mediates cell-autonomous regulation of excitation-inhibition balance in the thalamocortical feedforward circuit, but not in the feedback circuit. This effect is due to regulation of inhibition originating from parvalbumin-expressing interneurons, while inhibition via somatostatin-expressing interneurons is unaffected. Computational modeling shows that this mechanism promotes stable excitatory-inhibitory ratios across pyramidal cells and ensures robust and sparse coding. Patch-clamp RNA sequencing yields genes differentially regulated by SLK knockdown, as well as genes associated with excitation-inhibition balance participating in transsynaptic communication and cytoskeletal dynamics. These data identify a mechanism for cell-autonomous regulation of a specific inhibitory circuit that is critical to ensure that a majority of cortical pyramidal cells participate in information coding.

An uncommon neuronal class conveys visual signals from rods and cones to retinal ganglion cells

Neurons in the central nervous system (CNS) are distinguished by the neurotransmitter types they release, their synaptic connections, morphology, and genetic profiles. To fully understand how the CNS works, it is critical to identify all neuronal classes and reveal their synaptic connections. The retina has been extensively used to study neuronal development and circuit formation. Here, we describe a previously unidentified interneuron in mammalian retina. This interneuron shares some morphological, physiological, and molecular features with retinal bipolar cells, such as receiving input from photoreceptors and relaying visual signals to retinal ganglion cells. It also shares some features with amacrine cells (ACs), particularly Aii-ACs, such as their neurite morphology in the inner plexiform layer, the expression of some AC-specific markers, and possibly the release of the inhibitory neurotransmitter glycine. Thus, we unveil an uncommon interneuron, which may play an atypical role in vision.

Axonal CB1 Receptors Mediate Inhibitory Bouton Formation via cAMP Increase and PKA

Experience-dependent formation and removal of inhibitory synapses are essential throughout life. For instance, GABAergic synapses are removed to facilitate learning, and strong excitatory activity is accompanied by the formation of inhibitory synapses to maintain coordination between excitation and inhibition. We recently discovered that active dendrites trigger the growth of inhibitory synapses via CB1 receptor-mediated endocannabinoid signaling, but the underlying mechanism remained unclear. Using two-photon microscopy to monitor the formation of individual inhibitory boutons in hippocampal organotypic slices from mice (both sexes), we found that CB1 receptor activation mediated the formation of inhibitory boutons and promoted their subsequent stabilization. Inhibitory bouton formation did not require neuronal activity and was independent of G_i/o-protein signaling, but was directly induced by elevating cAMP levels using forskolin and by activating G_s-proteins using DREADDs. Blocking PKA activity prevented CB1 receptor-mediated inhibitory bouton formation. Our findings reveal that axonal CB1 receptors signal via unconventional downstream pathways and that inhibitory bouton formation is triggered by an increase in axonal cAMP levels. Our results demonstrate an unexpected role for axonal CB1 receptors in axon-specific, and context-dependent, inhibitory synapse formation.SIGNIFICANCE STATEMENT Coordination between excitation and inhibition is required for proper brain function throughout life. It was previously shown that new inhibitory synapses can be formed in response to strong excitation to maintain this coordination, and this was mediated by endocannabinoid signaling via CB1 receptors. As activation of CB1 receptors generally results in the suppression of synaptic transmission, it remained unclear how CB1 receptors can mediate the formation of inhibitory synapses. Here we show that CB1 receptors on inhibitory axons signal via unconventional intracellular pathways and that inhibitory bouton formation is triggered by an increase in axonal cAMP levels and requires PKA activity. Our findings point to a central role for axonal cAMP signaling in activity-dependent inhibitory synapse formation.

Reduction of Dendritic Inhibition in CA1 Pyramidal Neurons in Amyloidosis Models of Early Alzheimer's Disease

Background:

In an early stage of Alzheimer’s disease (AD), before the formation of amyloid plaques, neuronal network hyperactivity has been reported in both patients and animal models. This suggests an underlying disturbance of the balance between excitation and inhibition. Several studies have highlighted the role of somatic inhibition in early AD, while less is known about dendritic inhibition.

Objective:

In this study we investigated how inhibitory synaptic currents are affected by elevated Aβ levels.

Methods:

We performed whole-cell patch clamp recordings of CA1 pyramidal neurons in organotypic hippocampal slice cultures after treatment with Aβ-oligomers and in hippocampal brain slices from App^NL-F-G mice (APP-KI).

Results:

We found a reduction of spontaneous inhibitory postsynaptic currents (sIPSCs) in CA1 pyramidal neurons in organotypic slices after 24 h Aβ treatment. sIPSCs with slow rise times were reduced, suggesting a specific loss of dendritic inhibitory inputs. As miniature IPSCs and synaptic density were unaffected, these results suggest a decrease in activity-dependent transmission after Aβ treatment. We observed a similar, although weaker, reduction in sIPSCs in CA1 pyramidal neurons from APP-KI mice compared to control. When separated by sex, the strongest reduction in sIPSC frequency was found in slices from male APP-KI mice. Consistent with hyperexcitability in pyramidal cells, dendritically targeting interneurons received slightly more excitatory input. GABAergic action potentials had faster kinetics in APP-KI slices.

Conclusion:

Our results show that Aβ affects dendritic inhibition via impaired action potential driven release, possibly due to altered kinetics of GABAergic action potentials. Reduced dendritic inhibition may contribute to neuronal hyperactivity in early AD.

Modelling and Refining Neuronal Circuits with Guidance Cues: Involvement of Semaphorins

The establishment of neuronal circuits requires neurons to develop and maintain appropriate connections with cellular partners in and out the central nervous system. These phenomena include elaboration of dendritic arborization and formation of synaptic contacts, initially made in excess. Subsequently, refinement occurs, and pruning takes places both at axonal and synaptic level, defining a homeostatic balance maintained throughout the lifespan. All these events require genetic regulations which happens cell-autonomously and are strongly influenced by environmental factors. This review aims to discuss the involvement of guidance cues from the Semaphorin family.

Autism Spectrum Disorder Risk Factor Met Regulates the Organization of Inhibitory Synapses

A common hypothesis explains autism spectrum disorder (ASD) as a neurodevelopmental disorder linked to excitatory/inhibitory (E/I) imbalance in neuronal network connectivity. Mutation of genes including Met and downstream signaling components, e.g., PTEN, Tsc2 and, Rheb are involved in the control of synapse formation and stabilization and were all considered as risk genes for ASD. While the impact of Met on glutamatergic synapses was widely appreciated, its contribution to the stability of inhibitory, GABAergic synapses is poorly understood. The stabilization of GABAergic synapses depends on clustering of the postsynaptic scaffolding protein gephyrin. Here, we show in vivo and in vitro that Met is necessary and sufficient for the stabilization of GABAergic synapses via induction of gephyrin clustering. Likewise, we provide evidence for Met-dependent gephyrin clustering via activation of mTOR. Our results support the notion that deficient GABAergic signaling represents a pathomechanism for ASD.

Experience-Dependent Inhibitory Plasticity Is Mediated by CCK+ Basket Cells in the Developing Dentate Gyrus

Early postnatal experience shapes both inhibitory and excitatory networks in the hippocampus. However, the underlying circuit plasticity is unclear. Using an enriched environment (EE) paradigm during the preweaning period in mice of either sex, we assessed the circuit plasticity of inhibitory cell types in the hippocampus. We found that cholecystokinin (CCK)-expressing basket cells strongly increased somatic inhibition on the excitatory granular cells (GCs) following EE, whereas another pivotal inhibitory cell type, parvalbumin (PV)-expressing cells, did not show changes. Using electrophysiological analysis and the use of cannabinoid receptor 1 (CB1R) agonist WIN 55 212-2, we demonstrate that the change in somatic inhibition from CCK+ neurons increases CB1R-mediated inhibition in the circuit. By inhibiting activity of the entorhinal cortex (EC) using a chemogenetic approach, we further demonstrate that the activity of the projections from the EC mediates the developmental assembly of CCK+ basket cell network. Altogether, our study places the experience-dependent remodeling of CCK+ basket cell innervation as a central process to adjust inhibition in the dentate gyrus and shows that cortical inputs to the hippocampus play an instructional role in controlling the refinement of the synaptic connections during the preweaning period.SIGNIFICANCE STATEMENT Brain plasticity is triggered by experience during postnatal brain development and shapes the maturing neural circuits. In humans, altered experience-dependent plasticity can have long-lasting detrimental effects on circuit function and lead to psychiatric disorders. Yet, the cellular mechanisms governing how early experience fine-tunes the maturing synaptic network is not fully understood. Here, taking advantage of an enrichment-housing paradigm, we unravel a new plasticity mechanism involved in the maintenance of the inhibitory to excitatory balance in the hippocampus. Our findings demonstrate that cortical activity instructs the assembly of the CCK+ basket cell network. Considering the importance of this specific cell type for learning and memory, experience-dependent remodeling of CCK+ cells may be a critical determinant for establishing appropriate neural networks.

Amyloid-β Oligomers Induce Only Mild Changes to Inhibitory Bouton Dynamics

The amyloid-β protein precursor is highly expressed in a subset of inhibitory neuron in the hippocampus, and inhibitory neurons have been suggested to play an important role in early Alzheimer’s disease plaque load. Here we investigated bouton dynamics in axons of hippocampal interneurons in two independent amyloidosis models. Short-term (24 h) amyloid-β (Aβ)-oligomer application to organotypic hippocampal slices slightly increased inhibitory bouton dynamics, but bouton density and dynamics were unchanged in hippocampus slices of young-adult App^{NL - F - G}-mice, in which Aβ levels are chronically elevated. These results indicate that loss or defective adaptation of inhibitory synapses are not a major contribution to Aβ-induced hyperexcitability.

The Role of Semaphorins in Metabolic Disorders

Semaphorins are a family originally identified as axonal guidance molecules. They are also involved in tumor growth, angiogenesis, immune regulation, as well as other biological and pathological processes. Recent studies have shown that semaphorins play a role in metabolic diseases including obesity, adipose inflammation, and diabetic complications, including diabetic retinopathy, diabetic nephropathy, diabetic neuropathy, diabetic wound healing, and diabetic osteoporosis. Evidence provides mechanistic insights regarding the role of semaphorins in metabolic diseases by regulating adipogenesis, hypothalamic melanocortin circuit, immune responses, and angiogenesis. In this review, we summarize recent progress regarding the role of semaphorins in obesity, adipose inflammation, and diabetic complications.

Endocannabinoid Signaling Mediates Local Dendritic Coordination between Excitatory and Inhibitory Synapses

Dendritic inhibitory synapses are most efficient in modulating excitatory inputs localized on the same dendrite, but it is unknown whether their location is random or regulated. Here, we show that the formation of inhibitory synapses can be directed by excitatory synaptic activity on the same dendrite. We stimulated dendritic spines close to a GABAergic axon crossing by pairing two-photon glutamate uncaging with postsynaptic depolarization in CA1 pyramidal cells. We found that repeated spine stimulation promoted growth of a GABAergic bouton onto the same dendrite. The dendritic feedback signal required postsynaptic activation of DAGL, which produces the endocannabinoid 2-AG, and was mediated by CB1 receptors. We could also induce inhibitory bouton growth by local, brief applications of 2-AG. Our findings reveal a dendritic signaling mechanism to trigger growth of an inhibitory bouton at dendritic locations with strong excitatory synaptic activity, and this mechanism may serve to ensure inhibitory control over clustered excitatory inputs.