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Parvalbumin (PV)-expressing basket interneurons in the prefrontal cortex (PFC) regulate pyramidal cell firing, synchrony, and network oscillations. Yet, it is unclear how their perisomatic inputs to pyramidal neurons are integrated into neural circuitry and adjusted postnatally. Neural cell adhesion molecule NCAM is expressed in a variety of cells in the PFC and cooperates with EphrinA/EphAs to regulate inhibitory synapse density. Here, analysis of a novel parvalbumin (PV)-Cre: NCAM F/F mouse mutant revealed that NCAM functions presynaptically in PV+ basket interneurons to regulate postnatal elimination of perisomatic synapses. Mutant mice exhibited an increased density of PV+ perisomatic puncta in PFC layer 2/3, while live imaging in mutant brain slices revealed fewer puncta that were dynamically eliminated. Furthermore, EphrinA5-induced growth cone collapse in PV+ interneurons in culture depended on NCAM expression. Electrophysiological recording from layer 2/3 pyramidal cells in mutant PFC slices showed a slower rise time of inhibitory synaptic currents. PV-Cre: NCAM F/F mice exhibited impairments in working memory and social behavior that may be impacted by altered PFC circuitry. These findings suggest that the density of perisomatic synapses of PV+ basket interneurons is regulated postnatally by NCAM, likely through EphrinA-dependent elimination, which is important for appropriate PFC network function and behavior., (© The Author(s) 2020. Published by Oxford University Press. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.)

Neuron-glial related cell adhesion molecule NrCAM is a newly identified negative regulator of spine density that genetically interacts with Semaphorin3F (Sema3F), and is implicated in autism spectrum disorders (ASD). To investigate a role for NrCAM in spine pruning during the critical adolescent period when networks are established, we generated novel conditional, inducible NrCAM mutant mice (Nex1Cre-ERT2: NrCAMflox/flox). We demonstrate that NrCAM functions cell autonomously during adolescence in pyramidal neurons to restrict spine density in the visual (V1) and medial frontal cortex (MFC). Guided by molecular modeling, we found that NrCAM promoted clustering of the Sema3F holoreceptor complex by interfacing with Neuropilin-2 (Npn2) and PDZ scaffold protein SAP102. NrCAM-induced receptor clustering stimulated the Rap-GAP activity of PlexinA3 (PlexA3) within the holoreceptor complex, which in turn, inhibited Rap1-GTPase and inactivated adhesive β1 integrins, essential for Sema3F-induced spine pruning. These results define a developmental function for NrCAM in Sema3F receptor signaling that limits dendritic spine density on cortical pyramidal neurons during adolescence.

Though aberrant neuronal migration in response to maternal thyroid hormone (TH) deficiency before the onset of fetal thyroid function (embryonic day [E] 17.5) in rat cerebral cortex has been described, molecular events mediating morphogenic actions have remained elusive. To investigate the effect of maternal TH deficiency on neocortical development, rat dams were maintained on methimazole from gestational day 6 until sacrifice. Decreased number and length of radial glia, loss of neuronal bipolarity, and impaired neuronal migration were correctible with early (E13-15) TH replacement. Reelin downregulation under hypothyroidism is neither due to enhanced apoptosis in Cajal-Retzius cells nor mediated through brain-derived neurotrophic factor-tyrosine receptor kinase B alterations. Results based on gel shift and chromatin immunoprecipitation assays show the transcriptional control of reelin by TH through the presence of intronic TH response element. Furthermore, hypothyroidism significantly increased TH receptor α1 with decreased reelin, apolipoprotein E receptor 2, very low-density lipoprotein receptor expression, and activation of cytosolic adapter protein disabled 1 that compromised the reelin signaling. Integrins (α(v) and β₁) are significantly decreased without alteration of α₃ indicating intact neuroglial recognition but disrupted adhesion and glial end-feet attachment. Results provide mechanistic basis of essentiality of adequate maternal TH levels to ensue proper fetal neocortical cytoarchitecture and importance of early thyroxine replacement.

Understanding of how maternal thyroid inadequacy during early gestation poses a risk for developmental outcomes is still a challenge for the neuroendocrine community. Early neocortical neurogenesis is accompanied by maternal thyroid hormone (TH) transfer to fetal brain, appearance of TH receptors, and absence of antineurogenesis signals, followed by optimization of neuronal numbers through apoptosis. However, the effects of TH deprivation on neurogenesis and neuronal cell death before the onset of fetal thyroid are still not clear. We show that maternal TH deficiency during early gestational period causes massive premature elevation in the expression of neuronal nitric oxide synthase (nNOS) with an associated neuronal death in embryonic rat neocortex. Maternal hypothyroidism was induced by feeding methimazole (0.025% wt/vol) in the drinking water to pregnant Sprague Dawley rats from embryonic d 6. Cerebral cortices from fetuses were harvested at different embryonic stages (embryonic d 14, 16, and 18) of hypothyroid and euthyroid groups. Immunoblotting and real-time PCR results showed that both protein and RNA levels of nNOS were prematurely increased under maternal hypothyroidism, and showed reversibility upon T4 administration. Immunohistochemistry revealed an increased nNOS immunoreactivity in both the cortical plate and proliferative zone of neocortex along with a corroborative decrease in the microtubule associated protein-2 positive neurons under maternal TH insufficiency. Results combined, put forth nNOS as a novel target of maternal TH action in embryonic neocortex, and underscore the importance of prenatal screening and timely rectification of maternal TH insufficiency, even of a moderate degree.