Maturation of cortical circuits requires Semaphorin 7A.

Abnormal cortical circuits underlie some cognitive and psychiatric disorders, yet the molecular signals that generate normal cortical networks remain poorly understood. Semaphorin 7A (Sema7A) is an atypical member of the semaphorin family that is GPI-linked, expressed principally postnatally, and enriched in sensory cortex. Significantly, SEMA7A is deleted in individuals with 15q24 microdeletion syndrome, characterized by developmental delay, autism, and sensory perceptual deficits. We studied the role that Sema7A plays in establishing functional cortical circuitry in mouse somatosensory barrel cortex. We found that Sema7A is expressed in spiny stellate cells and GABAergic interneurons and that its absence disrupts barrel cytoarchitecture, reduces asymmetrical orientation of spiny stellate cell dendrites, and functionally impairs thalamocortically evoked synaptic responses, with reduced feed-forward GABAergic inhibition. These data identify Sema7A as a regulator of thalamocortical and local circuit development in layer 4 and provide a molecular handle that can be used to explore the coordinated generation of excitatory and inhibitory cortical circuits.

N-cadherin regulates molecular organization of excitatory and inhibitory synaptic circuits in adult hippocampus in vivo.

N-Cadherin and ß-catenin form a transsynaptic adhesion complex required for spine and synapse development. In adulthood, N-cadherin mediates persistent synaptic plasticity, but whether the role of N-cadherin at mature synapses is similar to that at developing synapses is unclear. To address this, we conditionally ablated N-cadherin from excitatory forebrain synapses in mice starting in late postnatal life and examined hippocampal structure and function in adulthood. In the absence of N-cadherin, ß-catenin levels were reduced, but numbers of excitatory synapses were unchanged, and there was no impact on number or shape of dendrites or spines. However, the composition of synaptic molecules was altered. Levels of GluA1 and its scaffolding protein PSD95 were diminished and the density of immunolabeled puncta was decreased, without effects on other glutamate receptors and their scaffolding proteins. Additionally, loss of N-cadherin at excitatory synapses triggered increases in the density of markers for inhibitory synapses and decreased severity of hippocampal seizures. Finally, adult mutant mice were profoundly impaired in hippocampal-dependent memory for spatial episodes. These results demonstrate a novel function for the N-cadherin/ß-catenin complex in regulating ionotropic receptor composition of excitatory synapses, an appropriate balance of excitatory and inhibitory synaptic proteins and the maintenance of neural circuitry necessary to generate flexible yet persistent cognitive and synaptic function.

Compensatory redistribution of neuroligins and N-cadherin following deletion of synaptic ß1-integrin.

ß1-containing integrins are required for persistent synaptic potentiation in hippocampus and regulate hippocampal-dependent learning. Based largely on indirect evidence, there is a prevailing assumption that ß1-integrins are localized at synapses, where they contribute to synapse adhesion and signaling, but this has not been examined directly. Here we investigate the fine localization of ß1-integrin in adult mouse hippocampus using high-resolution immunogold labeling, with a particular emphasis on synaptic labeling patterns. We find that ß1-integrins localize to synapses in CA1 and are concentrated postsynaptically. At the postsynaptic membrane, ß1-integrins are found more commonly clustered near active zone centers rather than at the peripheral edges. In mice harboring a conditional deletion of ß1-integrins, labeling for N-cadherin and neuroligins increases. Western blots show increased levels of N-cadherin in total lysates and neuroligins increase selectively in synaptosomes. These data suggest there is a dynamic, compensatory adjustment of synaptic adhesion. Such adjustment is specific only for certain cell adhesion molecules (CAMs), because labeling for SynCAM is unchanged. Together, our findings demonstrate unequivocally that ß1-integrin is an integral synaptic adhesion protein, and suggest that adhesive function at the synapse reflects a cooperative and dynamic network of multiple CAM families.

Synaptic loss and retention of different classic cadherins with LTP-associated synaptic structural remodeling in vivo.

Cadherins are synaptic cell adhesion molecules that contribute to persistently enhanced synaptic strength characteristic of long-term potentiation (LTP). What is relatively unexplored is how synaptic activity of the kind that induces LTP-associated remodeling of synapse structure affects localization of cadherins, particularly in mature animals in vivo, details which could offer insight into how different cadherins contribute to synaptic plasticity. Here, we use a well-described in vivo LTP induction protocol that produces robust synaptic morphological remodeling in dentate gyrus of adult rats in combination with confocal and immunogold electron microscopy to localize cadherin-8 and N-cadherin at remodeled synapses. We find that the density and size of cadherin-8 puncta are significantly diminished in the potentiated middle molecular layer (MML) while concurrently, N-cadherin remains tightly clustered at remodeled synapses. These changes are specific to the potentiated MML, and occur without any change in density or size of synaptophysin puncta. Thus, the loss of cadherin-8 probably represents selective removal from synapses rather than overall loss of synaptic junctions. Together, these findings suggest that activity-regulated loss and retention of different synaptic cadherins could contribute to dual demands of both flexibility and stability in synapse structure that may be important for synaptic morphological remodeling that accompanies long-lasting plasticity.

Regeneration of axons in injured spinal cord by activation of bone morphogenetic protein/Smad1 signaling pathway in adult neurons.

Axon growth potential is highest in young neurons but diminishes with age, thus becoming a significant obstacle to axonal regeneration after injury in maturity. The mechanism for the decline is incompletely understood, and no effective clinical treatment is available to rekindle innate growth capability. Here, we show that Smad1-dependent bone morphogenetic protein (BMP) signaling is developmentally regulated and governs axonal growth in dorsal root ganglion (DRG) neurons. Down-regulation of the pathway contributes to the age-related decline of the axon growth potential. Reactivating Smad1 selectively in adult DRG neurons results in sensory axon regeneration in a mouse model of spinal cord injury (SCI). Smad1 signaling can be effectively manipulated by an adeno-associated virus (AAV) vector encoding BMP4 delivered by a clinically applicable and minimally invasive technique, an approach devoid of unwanted abnormalities in mechanosensation or pain perception. Importantly, transected axons are able to regenerate even when the AAV treatment is delivered after SCI, thus mimicking a clinically relevant scenario. Together, our results identify a therapeutic target to promote axonal regeneration after SCI.

Neuropathic pain- and glial derived neurotrophic factor-associated regulation of cadherins in spinal circuits of the dorsal horn.

Neuropathic pain is associated with reorganization of spinal synaptic circuits, implying that adhesion proteins that normally build and modify synapses must be involved. The adhesion proteins E- and N-cadherin delineate different synapses furnished by nociceptive primary afferents, but dynamic aspects of cadherin localization in relationship to onset, maintenance or reversibility of neuropathic pain are uncharacterized. Here, we find very different responses of these cadherins to L5 spinal nerve transection (SNT)-induced mechanical allodynia and to intrathecal glial derived neurotrophic factor (GDNF), which has potent analgesic effects in this pain model. In L5, E-cadherin is rapidly eliminated in patches within lamina IIi contemporaneously with the onset of mechanical allodynia. Intrathecal GDNF in conjunction with, or at 7 days after, L5 SNT prevents or reverses both the loss of E-cadherin and abnormal pain sensation. In contrast, N-cadherin undergoes a delayed and transient increase uniformly across lamina I-II that is insensitive to GDNF. Some N-cadherin-labeled profiles codistribute with GAP-43, suggesting a role in axon sprouting. Patterns of immunolabeling for GDNF receptor components GFRα1, NCAM, and RET after L5 SNT suggest that GFRα1 and NCAM are the principal receptors operative in this model. In addition, GFRα1 codistributes with E-cadherin, but not N-cadherin, profiles. Together, these data indicate strikingly divergent patterns of temporal and molecular regulation of different cadherins at distinct nociceptive circuits in response to spinal nerve injury, suggesting that the two cadherins and the circuits with which they are affiliated participate in different aspects of synaptic and circuit reorganization associated with neuropathic pain.