Brain disorders constitute a significant cause of disability compared to any other category of illness in contemporary societies, highlighting the need to understand and identify therapeutic interventions. Synapses connect neurons into circuits, enabling and simultaneously processing the transfer of information. Almost all functional brain disorders have a direct or indirect link to synapses including autism spectrum disorder (ASD) and epilepsy, two highly comorbid conditions. Neurexins are presynaptic adhesion molecules that have been increasingly implicated in ASD and epilepsy, as evidenced by genetic mutations in the clinical population. Neurexins function as context-dependent specifiers of synaptic properties and critical modulators in excitatory and inhibitory transmission. Disruptions in this modulation have long been recognized as a hallmark of ASD and epilepsy, making neurexins excellent starting points for understanding the etiology underlying these disorders.

The overall goal of this dissertation is to understand how, neurexin-2 (Nrxn2) mutations can contribute to impairments in synaptic transmission, and behavioral dysregulation at the circuit level. First, I introduce the fundamental principles underlying ASD and epilepsy, review the current understanding of Nrxn2, and identify gaps in the field (Chapter 1). Second, I used a conditional knock-out approach to delete neurexin 2 (Nrxn2 cKO) in the hippocampus and cortex and identified (i) an overall increase in hippocampal CA3→CA1 network activity, (ii) behavioral abnormalities, such as reduced social preference and increased nestlet shredding behavior, core ASD-like phenotypes and (iii) unprovoked spontaneous reoccurring electrographic and behavioral seizures, a core epilepsy-like phenotype (Chapter 2). Next, I demonstrated the potential of Nrxn2 cKO as a valuable genetic tool to investigate the biological pathways that link ASD and epilepsy by investigating mobility and immobility as correlates of sleep (Chapter 3). I show that Nrxn2 cKO mice exhibit fragmented mobility/immobility bouts and irregular NREM sleep distribution. Further, when sleep was modulated by inducing sleep with zolpidem, I could rescue the social deficits observed in Nrxn2 cKO mice in males, but not in females. Finally, I use bulk RNA sequencing to conduct a preliminary investigation of the transcriptional signature of Nrxn2 cKO mice at baseline and under sleep-deprived conditions (Chapter 4). My preliminary results show differentially expressed genes and broad gene ontology categories influenced by Nrxn2. The findings in this dissertation offer novel and valuable insights about Neurexin2, as well as provide insights for generating hypotheses to understand and improve the ASD and epilepsy prognosis.

Brain-derived neurotrophic factor (BDNF) is an extremely potent, positive modulator of theta burst induced long-term potentiation (LTP) in the adult hippocampus. The present studies tested whether the neurotrophin exerts its effects by facilitating cytoskeletal changes in dendritic spines. BDNF caused no changes in phalloidin labeling of filamentous actin (F-actin) when applied alone to rat hippocampal slices but markedly enhanced the number of densely labeled spines produced by a threshold level of theta burst stimulation. Conversely, the BDNF scavenger TrkB-Fc completely blocked increases in spine F-actin produced by suprathreshold levels of theta stimulation. TrkB-Fc also blocked LTP consolidation when applied 1-2 min, but not 10 min, after theta trains. Additional experiments confirmed that p21 activated kinase and cofilin, two actin-regulatory proteins implicated in spine morphogenesis, are concentrated in spines in mature hippocampus and further showed that both undergo rapid, dose-dependent phosphorylation after infusion of BDNF. These results demonstrate that the influence of BDNF on the actin cytoskeleton is retained into adulthood in which it serves to positively modulate the time-dependent LTP consolidation process.

Stabilization of long-term potentiation (LTP) is commonly proposed to involve changes in synaptic morphology and reorganization of the spine cytoskeleton. Here we tested whether, as predicted from this hypothesis, induction of LTP by theta-burst stimulation activates an actin regulatory pathway and alters synapse morphology within the same dendritic spines. TBS increased severalfold the numbers of spines containing phosphorylated (p) p21-activated kinase (PAK) or its downstream target cofilin; the latter regulates actin filament assembly. The PAK/cofilin phosphoproteins were increased at 2 min but not 30 s post-TBS, peaked at 7 min, and then declined. Double immunostaining for the postsynaptic density protein PSD95 revealed that spines with high pPAK or pCofilin levels had larger synapses (+60-70%) with a more normal size frequency distribution than did neighboring spines. Based on these results and simulations of shape changes to synapse-like objects, we propose that theta stimulation markedly increases the probability that a spine will enter a state characterized by a large, ovoid synapse and that this morphology is important for expression and later stabilization of LTP.