Neuropilin 2 Signaling Mediates Corticostriatal Transmission, Spine Maintenance, and Goal-Directed Learning in Mice

Dysregulation of Neuropilin-2 Expression in Inhibitory Neurons Impairs Hippocampal Circuit Development Leading to Autism-Epilepsy Phenotype

Dysregulation of development, migration, and function of interneurons, collectively termed interneuronopathies, have been proposed as a shared mechanism for autism spectrum disorders (ASDs) and childhood epilepsy. Neuropilin-2 (Nrp2), a candidate ASD gene, is a critical regulator of interneuron migration from the median ganglionic eminence (MGE) to the pallium, including the hippocampus. While clinical studies have identified Nrp2 polymorphisms in patients with ASD, whether dysregulation of Nrp2-dependent interneuron migration contributes to pathogenesis of ASD and epilepsy has not been tested. We tested the hypothesis that the lack of Nrp2 in MGE-derived interneuron precursors disrupts the excitation/inhibition balance in hippocampal circuits, thus predisposing the network to seizures and behavioral patterns associated with ASD. Embryonic deletion of Nrp2 during the developmental period for migration of MGE derived interneuron precursors (iCKO) significantly reduced parvalbumin, neuropeptide Y, and somatostatin positive neurons in the hippocampal CA1. Consequently, when compared to controls, the frequency of inhibitory synaptic currents in CA1 pyramidal cells was reduced while frequency of excitatory synaptic currents was increased in iCKO mice. Although passive and active membrane properties of CA1 pyramidal cells were unchanged, iCKO mice showed enhanced susceptibility to chemically evoked seizures. Moreover, iCKO mice exhibited selective behavioral deficits in both preference for social novelty and goal-directed learning, which are consistent with ASD-like phenotype. Together, our findings show that disruption of developmental Nrp2 regulation of interneuron circuit establishment, produces ASD-like behaviors and enhanced risk for epilepsy. These results support the developmental interneuronopathy hypothesis of ASD epilepsy comorbidity.

Training-induced circuit-specific excitatory synaptogenesis in mice is required for effort control

Synaptogenesis is essential for circuit development; however, it is unknown whether it is critical for the establishment and performance of goal-directed voluntary behaviors. Here, we show that operant conditioning via lever-press for food reward training in mice induces excitatory synapse formation onto a subset of anterior cingulate cortex neurons projecting to the dorsomedial striatum (ACC→DMS). Training-induced synaptogenesis is controlled by the Gabapentin/Thrombospondin receptor α2δ-1, which is an essential neuronal protein for proper intracortical excitatory synaptogenesis. Using germline and conditional knockout mice, we found that deletion of α2δ-1 in the adult ACC→DMS circuit diminishes training-induced excitatory synaptogenesis. Surprisingly, this manipulation does not impact learning but results in a significant increase in effort exertion without affecting sensitivity to reward value or changing contingencies. Bidirectional optogenetic manipulation of ACC→DMS neurons rescues or phenocopies the behaviors of the α2δ-1 cKO mice, highlighting the importance of synaptogenesis within this cortico-striatal circuit in regulating effort exertion.

Mass Spectrometry as a Quantitative Proteomic Analysis Tool for the Search for Temporal Lobe Epilepsy Biomarkers: A Systematic Review

Temporal lobe epilepsy (TLE) is the most common form of epilepsy in adults. Tissue reorganization at the site of the epileptogenic focus is accompanied by changes in the expression patterns of protein molecules. The study of mRNA and its corresponding proteins is crucial for understanding the pathogenesis of the disease. Protein expression profiles do not always directly correlate with the levels of their transcripts; therefore, it is protein profiling that is no less important for understanding the molecular mechanisms and biological processes of TLE. The study and annotation of proteins that are statistically significantly different in patients with TLE is an approach to search for biomarkers of this disease, various stages of its development, as well as a method for searching for specific targets for the development of a further therapeutic strategy. When writing a systematic review, the following aggregators of scientific journals were used: MDPI, PubMed, ScienceDirect, Springer, and Web of Science. Scientific articles were searched using the following keywords: "proteomic", "mass-spectrometry", "protein expression", "temporal lobe epilepsy", and "biomarkers". Publications from 2003 to the present have been analyzed. Studies of brain tissues, experimental models of epilepsy, as well as biological fluids, were analyzed. For each of the groups, aberrantly expressed proteins found in various studies were isolated. Most of the studies omitted important characteristics of the studied patients, such as: duration of illness, type and response to therapy, gender, etc. Proteins that overlap across different tissue types and different studies have been highlighted: DPYSL, SYT1, STMN1, APOE, NME1, and others. The most common biological processes for them were the positive regulation of neurofibrillary tangle assembly, the regulation of amyloid fibril formation, lipoprotein catabolic process, the positive regulation of vesicle fusion, the positive regulation of oxidative stress-induced intrinsic apoptotic signaling pathway, removal of superoxide radicals, axon extension, and the regulation of actin filament depolymerization. MS-based proteomic profiling for a relevant study must accept a number of limitations, the most important of which is the need to compare different types of neurological and, in particular, epileptic disorders. Such a criterion could increase the specificity of the search work and, in the future, lead to the discovery of biomarkers for a particular disease.

Induced pluripotent stem cell-derived astrocytes from patients with schizophrenia exhibit an inflammatory phenotype that affects vascularization

Molecular and functional abnormalities of astrocytes have been implicated in the etiology and pathogenesis of schizophrenia (SCZ). In this study, we examined the proteome, inflammatory responses, and secretome effects on vascularization of human induced pluripotent stem cell (hiPSC)-derived astrocytes from patients with SCZ. Proteomic analysis revealed alterations in proteins related to immune function and vascularization. Reduced expression of the nuclear factor kappa B (NF-κB) p65 subunit was observed in these astrocytes, with no incremental secretion of cytokines after tumor necrosis factor alpha (TNF-α) stimulation. Among inflammatory cytokines, secretion of interleukin (IL)-8 was particularly elevated in SCZ-patient-derived-astrocyte-conditioned medium (A_SCZCM). In a chicken chorioallantoic membrane (CAM) assay, A_SCZCM reduced the diameter of newly grown vessels. This effect could be mimicked with exogenous addition of IL-8. Taken together, our results suggest that SCZ astrocytes are immunologically dysfunctional and may consequently affect vascularization through secreted factors.

Semaphorin signaling restricts neuronal regeneration in C. elegans

Extracellular signaling proteins serve as neuronal growth cone guidance molecules during development and are well positioned to be involved in neuronal regeneration and recovery from injury. Semaphorins and their receptors, the plexins, are a family of conserved proteins involved in development that, in the nervous system, are axonal guidance cues mediating axon pathfinding and synapse formation. The Caenorhabditis elegans genome encodes for three semaphorins and two plexin receptors: the transmembrane semaphorins, SMP-1 and SMP-2, signal through their receptor, PLX-1, while the secreted semaphorin, MAB-20, signals through PLX-2. Here, we evaluate the locomotion behavior of knockout animals missing each of the semaphorins and plexins and the neuronal morphology of plexin knockout animals; we described the cellular expression pattern of the promoters of all plexins in the nervous system of C. elegans; and we evaluated their effect on the regrowth and reconnection of motoneuron neurites and the recovery of locomotion behavior following precise laser microsurgery. Regrowth and reconnection were more prevalent in the absence of each plexin, while recovery of locomotion surpassed regeneration in all genotypes.

Electroacupuncture of Baihui and Shenting ameliorates cognitive deficits via Pten/Akt pathway in a rat cerebral ischemia injury model

Cerebral ischemic stroke is a huge threat to the health and life of many people. Electroacupuncture (EA) at Baihui (GV20) and Shenting (GV24) acupoints can notably alleviate cerebral ischemia/reperfusion injury (CIRI). However, the molecular basis underlying the effectiveness of EA at the GV20 and GV24 acupoints for CIRI remains largely unknown. Our present study demonstrated that EA treatment at the GV20 and GV24 acupoints markedly alleviated middle cerebral artery occlusion/reperfusion (MCAO/R)-induced cognitive deficits and cerebral infarction in rats. Proteomics analysis revealed that 195 and 218 proteins were dysregulated in rat hippocampal tissues in the MCAO/R vs. sham group and thhhe EA vs. MCAO/R group, respectively. Moreover, 62 proteins with converse alteration trends in MCAO/R vs. sham and EA vs. MCAO/R groups were identified. These proteins might be implicated in the EA-mediated protective effect against MCAO/R-induced cerebral injury. GO enrichment analysis showed that 39 dysregulated proteins in the MCAO/R vs. sham group and 40 dysregulated proteins in the EA vs. MCAO/R group were related to brain and nerve development. Protein–protein interaction analysis of the abovementioned dysregulated proteins associated with brain and nerve development suggested that Pten/Akt pathway-related proteins might play major roles in regulating EA-mediated protective effects against MCAO/R-induced brain and nerve injury. Western blot assays demonstrated that Pak4, Akt3, and Efnb2 were expressed at low levels in the MCAO/R group vs. the sham group but at high levels in the EA group vs. the MCAO/R group. In conclusion, multiple proteins related to the protective effect of EA at the GV20 and GV24 acupoints against CIRI were identified in our study.

A Deficiency of the Psychiatric Risk Gene DLG2/PSD-93 Causes Excitatory Synaptic Deficits in the Dorsolateral Striatum

Genetic variations resulting in the loss of function of the discs large homologs (DLG2)/postsynaptic density protein-93 (PSD-93) gene have been implicated in the increased risk for schizophrenia, intellectual disability, and autism spectrum disorders (ASDs). Previously, we have reported that mice lacking exon 14 of the Dlg2 gene (Dlg2^–/– mice) display autistic-like behaviors, including social deficits and increased repetitive behaviors, as well as suppressed spontaneous excitatory postsynaptic currents in the striatum. However, the neural substrate underpinning such aberrant synaptic network activity remains unclear. Here, we found that the corticostriatal synaptic transmission was significantly impaired in Dlg2^–/– mice, which did not seem attributed to defects in presynaptic releases of cortical neurons, but to the reduced number of functional synapses in the striatum, as manifested in the suppressed frequency of miniature excitatory postsynaptic currents in spiny projection neurons (SPNs). Using transmission electron microscopy, we found that both the density of postsynaptic densities and the fraction of perforated synapses were significantly decreased in the Dlg2^–/– dorsolateral striatum. The density of dendritic spines was significantly reduced in striatal SPNs, but notably, not in the cortical pyramidal neurons of Dlg2^–/– mice. Furthermore, a DLG2/PSD-93 deficiency resulted in the compensatory increases of DLG4/PSD-95 and decreases in the expression of TrkA in the striatum, but not particularly in the cortex. These results suggest that striatal dysfunction might play a role in the pathology of psychiatric disorders that are associated with a disruption of the Dlg2 gene.

The role of ciliopathy-associated type 3 adenylyl cyclase in infanticidal behavior in virgin adult male mice

Virgin adult male mice often display killing of alien newborns, defined as infanticide, and this behavior is dependent on olfactory signaling. Olfactory perception is achieved by the main olfactory system (MOS) or vomeronasal system (VNS). Although it has been established that the VNS is crucial for infanticide in male mice, the role of the MOS in infanticide remains unknown. Herein, by producing lesions via ZnSO₄ perfusion and N-methyl-D-aspartic acid stereotactic injection, we demonstrated that the main olfactory epithelium (MOE), anterior olfactory nucleus (AON), or ventromedial hypothalamus (VMH) is crucial for infanticide in adult males. By using CRISPR-Cas9 coupled with adeno-associated viruses to induce specific knockdown of type 3 adenylyl cyclase (AC3) in these tissues, we further demonstrated that AC3, a ciliopathy-associated protein, in the MOE and the expression of related proteins in the AON or VMH are necessary for infanticidal behavior in virgin adult male mice.

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MOE lesions and knockdown of AC3 in the MOE result in abnormal infanticidal behavior

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The infanticidal behavior of male mice is impaired by lesioning of the AON or VMH

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AC3 knockdown in the AON or VMH affects the infanticidal behavior of male mice

The HDAC inhibitor CI-994 acts as a molecular memory aid by facilitating synaptic and intracellular communication after learning

Memory formation relies on a plethora of functions, including epigenetic modifications. Over recent years, multiple studies have indicated the potential of HDAC inhibitors (HDACis) as cognitive enhancers, but their mode of action is not fully understood. Here, we tested whether HDACi treatment improves memory formation via “cognitive epigenetic priming,” stipulating that HDACis—without inherent target specificity—specifically enhance naturally occurring plasticity processes. We found that combining HDACis with fear learning, but not either treatment alone, enhances synaptic plasticity as well as memory-promoting transcriptional signaling in the hippocampus, a brain area recruited by fear learning, but not in unrelated areas. These results lend experimental support to the theory of cognitive epigenetic priming.

Long-term memory formation relies on synaptic plasticity, neuronal activity-dependent gene transcription, and epigenetic modifications. Multiple studies have shown that HDAC inhibitor (HDACi) treatments can enhance individual aspects of these processes and thereby act as putative cognitive enhancers. However, their mode of action is not fully understood. In particular, it is unclear how systemic application of HDACis, which are devoid of substrate specificity, can target pathways that promote memory formation. In this study, we explore the electrophysiological, transcriptional, and epigenetic responses that are induced by CI-994, a class I HDACi, combined with contextual fear conditioning (CFC) in mice. We show that CI-994–mediated improvement of memory formation is accompanied by enhanced long-term potentiation in the hippocampus, a brain region recruited by CFC, but not in the striatum, a brain region not primarily implicated in fear learning. Furthermore, using a combination of bulk and single-cell RNA-sequencing, we find that, when paired with CFC, HDACi treatment engages synaptic plasticity-promoting gene expression more strongly in the hippocampus, specifically in the dentate gyrus (DG). Finally, using chromatin immunoprecipitation-sequencing (ChIP-seq) of DG neurons, we show that the combined action of HDACi application and conditioning is required to elicit enhancer histone acetylation in pathways that underlie improved memory performance. Together, these results indicate that systemic HDACi administration amplifies brain region-specific processes that are naturally induced by learning.

Reduced hippocampal inhibition and enhanced autism-epilepsy comorbidity in mice lacking neuropilin 2

The neuropilin receptors and their secreted semaphorin ligands play key roles in brain circuit development by regulating numerous crucial neuronal processes, including the maturation of synapses and migration of GABAergic interneurons. Consistent with its developmental roles, the neuropilin 2 (Nrp2) locus contains polymorphisms in patients with autism spectrum disorder (ASD). Nrp2-deficient mice show autism-like behavioral deficits and propensity to develop seizures. In order to determine the pathophysiology in Nrp2 deficiency, we examined the hippocampal numbers of interneuron subtypes and inhibitory regulation of hippocampal CA1 pyramidal neurons in mice lacking one or both copies of Nrp2. Immunostaining for interneuron subtypes revealed that Nrp2^-/- mice have a reduced number of parvalbumin, somatostatin, and neuropeptide Y cells, mainly in CA1. Whole-cell recordings identified reduced firing and hyperpolarized shift in resting membrane potential in CA1 pyramidal neurons from Nrp2^+/- and Nrp2^-/- mice compared to age-matched wild-type controls indicating decrease in intrinsic excitability. Simultaneously, the frequency and amplitude of spontaneous inhibitory postsynaptic currents (sIPSCs) are reduced in Nrp2-deficient mice. A convulsive dose of kainic acid evoked electrographic and behavioral seizures with significantly shorter latency, longer duration, and higher severity in Nrp2^-/- compared to Nrp2^+/+ animals. Finally, Nrp2^+/- and Nrp2^-/- but not Nrp2^+/+, mice have impaired cognitive flexibility demonstrated by reward-based reversal learning, a task associated with hippocampal circuit function. Together these data demonstrate a broad reduction in interneuron subtypes and compromised inhibition in CA1 of Nrp2^-/- mice, which could contribute to the heightened seizure susceptibility and behavioral deficits consistent with an ASD/epilepsy phenotype.

Loss of Tsc1 from striatal direct pathway neurons impairs endocannabinoid-LTD and enhances motor routine learning

Tuberous sclerosis complex (TSC) is a neurodevelopmental disorder that often presents with psychiatric conditions, including autism spectrum disorder (ASD). ASD is characterized by restricted, repetitive, and inflexible behaviors, which may result from abnormal activity in striatal circuits that mediate motor learning and action selection. To test whether altered striatal activity contributes to aberrant motor behaviors in the context of TSC, we conditionally deleted Tsc1 from direct or indirect pathway striatal projection neurons (dSPNs or iSPNs, respectively). We find that dSPN-specific loss of Tsc1 impairs endocannabinoid-mediated long-term depression (eCB-LTD) at cortico-dSPN synapses and strongly enhances corticostriatal synaptic drive, which is not observed in iSPNs. dSPN-Tsc1 KO, but not iSPN-Tsc1 KO, mice show enhanced motor learning, a phenotype observed in several mouse models of ASD. These findings demonstrate that dSPNs are particularly sensitive to Tsc1 loss and suggest that enhanced corticostriatal activation may contribute to altered motor behaviors in TSC.

Semaphorins in Adult Nervous System Plasticity and Disease

Semaphorins, originally discovered as guidance cues for developing axons, are involved in many processes that shape the nervous system during development, from neuronal proliferation and migration to neuritogenesis and synapse formation. Interestingly, the expression of many Semaphorins persists after development. For instance, Semaphorin 3A is a component of perineuronal nets, the extracellular matrix structures enwrapping certain types of neurons in the adult CNS, which contribute to the closure of the critical period for plasticity. Semaphorin 3G and 4C play a crucial role in the control of adult hippocampal connectivity and memory processes, and Semaphorin 5A and 7A regulate adult neurogenesis. This evidence points to a role of Semaphorins in the regulation of adult neuronal plasticity. In this review, we address the distribution of Semaphorins in the adult nervous system and we discuss their function in physiological and pathological processes.

Impaired instrumental learning in Spred1-/- mice, a model for a rare RASopathy

RASopathies are neuro-cardio-facio-cutaneous disorders stemming from mutations in genes regulating the RAS-MAPK pathway. Legius syndrome is a rare RASopathy disorder caused by mutations in the SPRED1 gene. SPRED1 protein negatively regulates activation of Ras by inhibiting RAS/RAF and by its interaction with neurofibromin, a Ras GTPase-activating protein (RAS-GAP). Cognitive impairments have been reported in Legius syndrome as well as in other RASopathy disorders. Modelling these cognitive deficits in a Spred1 mouse model for Legius syndrome has demonstrated spatial learning and memory deficits, but other cognitive domains remained unexplored. Here, we attempted to utilize a cognitive touchscreen battery to investigate if Spred1^-/- mice exhibit deficits in other cognitive domains. We show that Spred1^-/- mice had heterogeneous performance in instrumental operant learning, with a large subgroup (n = 9/20) failing to reach the standard criterion on touchscreen operant pretraining, precluding further cognitive testing. To examine whether targeting the RAS-MAPK signalling pathway could rescue these cognitive impairments, Spred1^-/- mice were acutely treated with the clinically relevant mitogen-activated protein kinase (MEK) inhibitor PD325901. However, MEK inhibition did not improve their instrumental learning. We conclude that Spred1^-/- mice can model severe cognitive impairments that cannot be reversed in adulthood.

Synaptic recognition molecules in development and disease

Synaptic connectivity patterns underlie brain functions. How recognition molecules control where and when neurons form synapses with each other, therefore, is a fundamental question of cellular neuroscience. This chapter delineates adhesion and signaling complexes as well as secreted factors that contribute to synaptic partner recognition in the vertebrate brain. The sections follow a developmental perspective and discuss how recognition molecules (1) guide initial synaptic wiring, (2) provide for the rejection of incorrect partner choices, (3) contribute to synapse specification, and (4) support the removal of inappropriate synapses once formed. These processes involve a rich repertoire of molecular players and key protein families are described, notably the Cadherin and immunoglobulin superfamilies, Semaphorins/Plexins, Leucine-rich repeat containing proteins, and Neurexins and their binding partners. Molecular themes that diversify these recognition systems are defined and highlighted throughout the text, including the neuron-type specific expression and combinatorial action of recognition factors, alternative splicing, and post-translational modifications. Methodological innovations advancing the field such as proteomic approaches and single cell expression studies are additionally described. Further, the chapter highlights the importance of choosing an appropriate brain region to analyze synaptic recognition factors and the advantages offered by laminated structures like the hippocampus or retina. In a concluding section, the profound disease relevance of aberrant synaptic recognition for neurodevelopmental and psychiatric disorders is discussed. Based on the current progress, an outlook is presented on research goals that can further advance insights into how recognition molecules provide for the astounding precision and diversity of synaptic connections.

Repurposing developmental mechanisms in the adult nervous system

Molecules and cellular processes important for nervous system development can be repurposed in adulthood for the regulation of adult neurogenesis, synaptic plasticity, and neural regeneration following injury or degeneration. Efforts to recapitulate neural development in order to ameliorate injury or disease are promising, but these often fall short of functional restoration due in part to our incomplete understanding of how these damaged circuits initially developed. Despite these limitations, such strategies provide hope that harnessing developmental mechanisms can restore nervous system functions following damage or disease.