A Schizophrenia-Related Deletion Leads to KCNQ2-Dependent Abnormal Dopaminergic Modulation of Prefrontal Cortical Interneuron Activity.

Altered prefrontal cortex function is implicated in schizophrenia (SCZ) pathophysiology and could arise from imbalance between excitation and inhibition (E/I) in local circuits. It remains unclear whether and how such imbalances relate to genetic etiologies. We used a mouse model of the SCZ-predisposing 22q11.2 deletion (Df(16)A+/- mice) to evaluate how this genetic lesion affects the excitability of layer V prefrontal pyramidal neurons and its modulation by dopamine (DA). Df(16)A+/- mice have normal balance between E/I at baseline but are unable to maintain it upon dopaminergic challenge. Specifically, in wild-type mice, D1 receptor (D1R) activation enhances excitability of layer V prefrontal pyramidal neurons and D2 receptor (D2R) activation reduces it. Whereas the excitatory effect upon D1R activation is enhanced in Df(16)A+/- mice, the inhibitory effect upon D2R activation is reduced. The latter is partly due to the inability of mutant mice to activate GABAergic parvalbumin (PV)+ interneurons through D2Rs. We further demonstrate that reduced KCNQ2 channel function in PV+ interneurons in Df(16)A+/- mice renders them less capable of inhibiting pyramidal neurons upon D2 modulation. Thus, DA modulation of PV+ interneurons and control of E/I are altered in Df(16)A+/- mice with a higher excitation and lower inhibition during dopaminergic modulation.

Alteration of Neuronal Excitability and Short-Term Synaptic Plasticity in the Prefrontal Cortex of a Mouse Model of Mental Illness.

Using a genetic mouse model that faithfully recapitulates a DISC1 genetic alteration strongly associated with schizophrenia and other psychiatric disorders, we examined the impact of this mutation within the prefrontal cortex. Although cortical layering, cytoarchitecture, and proteome were found to be largely unaffected, electrophysiological examination of the mPFC revealed both neuronal hyperexcitability and alterations in short-term synaptic plasticity consistent with enhanced neurotransmitter release. Increased excitability of layer II/III pyramidal neurons was accompanied by consistent reductions in voltage-activated potassium currents near the action potential threshold as well as by enhanced recruitment of inputs arising from superficial layers to layer V. We further observed reductions in both the paired-pulse ratios and the enhanced short-term depression of layer V synapses arising from superficial layers consistent with enhanced neurotransmitter release at these synapses. Recordings from layer II/III pyramidal neurons revealed action potential widening that could account for enhanced neurotransmitter release. Significantly, we found that reduced functional expression of the voltage-dependent potassium channel subunit Kv1.1 substantially contributes to both the excitability and short-term plasticity alterations that we observed. The underlying dysregulation of Kv1.1 expression was attributable to cAMP elevations in the PFC secondary to reduced phosphodiesterase 4 activity present in Disc1 deficiency and was rescued by pharmacological blockade of adenylate cyclase. Our results demonstrate a potentially devastating impact of Disc1 deficiency on neural circuit function, partly due to Kv1.1 dysregulation that leads to a dual dysfunction consisting of enhanced neuronal excitability and altered short-term synaptic plasticity.SIGNIFICANCE STATEMENT Schizophrenia is a profoundly disabling psychiatric illness with a devastating impact not only upon the afflicted but also upon their families and the broader society. Although the underlying causes of schizophrenia remain poorly understood, a growing body of studies has identified and strongly implicated various specific risk genes in schizophrenia pathogenesis. Here, using a genetic mouse model, we explored the impact of one of the most highly penetrant schizophrenia risk genes, DISC1, upon the medial prefrontal cortex, the region believed to be most prominently dysfunctional in schizophrenia. We found substantial derangements in both neuronal excitability and short-term synaptic plasticity-parameters that critically govern neural circuit information processing-suggesting that similar changes may critically, and more broadly, underlie the neural computational dysfunction prototypical of schizophrenia.

Mice with a naturally occurring DISC1 mutation display a broad spectrum of behaviors associated to psychiatric disorders.

Disrupted in schizophrenia-1 (DISC1) gene is associated with several neuropsychiatric disorders as it is disrupted by a balanced translocation involving chromosomes 1 and 11 in a large Scottish pedigree with high prevalence of schizophrenia, bipolar disorder and major depression. Since its identification, several mouse models with DISC1 genetic modifications have been generated using different approaches. Interestingly, a natural deletion of 25bp in the 129 mouse strain alters the DISC1 gene reading frame leading to a premature stop codon very close to the gene breakpoint in the mutant allele of the Scottish family. In the present study we confirmed that the 129DISC1(Del) mutation results in reduced level of full length DISC1 in hippocampus of heterozygous mice and we have characterized the behavioral consequences of heterozygous 129DISC1(Del) mutation in a mixed B6129 genetic background. We found alterations in spontaneous locomotor activity (hyperactivity in males and hypoactivity in females), deficits in pre-pulse inhibition (PPI) and also increased despair behavior in heterozygous 129DISC1(Del) mice, thus reproducing typical behaviors associated to psychiatric disorders. Since this mouse strain is widely and commercially available, we propose it as an amenable tool to study DISC1-related biochemical alterations and psychiatric behaviors.

Altered axonal targeting and short-term plasticity in the hippocampus of Disc1 mutant mice.

Carefully designed animal models of genetic risk factors are likely to aid our understanding of the pathogenesis of schizophrenia. Here, we study a mouse strain with a truncating lesion in the endogenous Disc1 ortholog designed to model the effects of a schizophrenia-predisposing mutation and offer a detailed account of the consequences that this mutation has on the development and function of a hippocampal circuit. We uncover widespread and cumulative cytoarchitectural alterations in the dentate gyrus during neonatal and adult neurogenesis, which include errors in axonal targeting and are accompanied by changes in short-term plasticity at the mossy fiber/CA3 circuit. We also provide evidence that cAMP levels are elevated as a result of the Disc1 mutation, leading to altered axonal targeting and dendritic growth. The identified structural alterations are, for the most part, not consistent with the growth-promoting and premature maturation effects inferred from previous RNAi-based Disc1 knockdown. Our results provide support to the notion that modest disturbances of neuronal connectivity and accompanying deficits in short-term synaptic dynamics is a general feature of schizophrenia-predisposing mutations.

Newly generated cells are increased in hippocampus of adult mice lacking a serine protease inhibitor.

BACKGROUND: Neurogenesis in the hippocampal dentate gyrus and the subventricular zone occurs throughout the life of mammals and newly generated neurons can integrate functionally into established neuronal circuits. Neurogenesis levels in the dentate gyrus are modulated by changes in the environment (enrichment, exercise), hippocampal-dependent tasks, NMDA receptor (NMDAR) activity, sonic hedgehog (SHH) and/or other factors. RESULTS: previously, we showed that Protease Nexin-1 (PN-1), a potent serine protease inhibitor, regulates the NMDAR availability and activity as well as SHH signaling. Compared with wild-type (WT), we detected a significant increase in BrdU-labeled cells in the dentate gyrus of mice lacking PN-1 (PN-1 -/-) both in controls and after running exercise. Patched homologue 1 (Ptc1) and Gli1 mRNA levels were higher and Gli3 down-regulated in mutant mice under standard conditions and to a lesser extent after running exercise. However, the number of surviving BrdU-positive cells did not differ between WT and PN-1 -/- animals. NMDAR availability was altered in the hippocampus of mutant animals after exercise. CONCLUSION: All together our results indicate that PN-1 controls progenitors proliferation through an effect on the SHH pathway and suggest an influence of the serpin on the survival of newly generated neurons through modulation of NMDAR availability.

Molecules, signaling, and schizophrenia.

Schizophrenia is one of the most common psychiatric disorders, but despite some progress in identifying the genetic factors implicated in its development, the molecular mechanisms underlying its etiology and pathogenesis remain poorly understood. However, accumulating evidence suggests that regardless of the underlying genetic complexity, the mechanisms of the disease may impact a small number of common signaling pathways. In this review, we discuss the evidence for a role of schizophrenia susceptibility genes in intracellular signaling cascades by focusing on three prominent candidate genes: AKT, PPP3CC (calcineurin), and DISC1. We describe the regulation of a number of signaling cascades by AKT and calcineurin through protein phosphorylation and dephosphorylation, and the recently uncovered functions of DISC1 in cAMP and GSK3beta signaling. In addition, we present independent evidence for the involvement of their downstream signaling pathways in schizophrenia. Finally, we discuss evidence supporting an impact of these susceptibility genes on common intracellular signaling pathways and the convergence of their effects on neuronal processes implicated in schizophrenia.

A mutation in mouse Disc1 that models a schizophrenia risk allele leads to specific alterations in neuronal architecture and cognition.

DISC1 is a strong candidate susceptibility gene for schizophrenia, bipolar disorder, and depression. Using a mouse strain carrying an endogenous Disc1 orthologue engineered to model the putative effects of the disease-associated chromosomal translocation we demonstrate that impaired Disc1 function results in region-specific morphological alterations, including alterations in the organization of newly born and mature neurons of the dentate gyrus. Field recordings at CA3/CA1 synapses revealed a deficit in short-term plasticity. Using a battery of cognitive tests we found a selective impairment in working memory (WM), which may relate to deficits in WM and executive function observed in individuals with schizophrenia. Our results implicate malfunction of neural circuits within the hippocampus and medial prefrontal cortex and selective deficits in WM as contributing to the genetic risk conferred by this gene.

Mice lacking protease nexin-1 show delayed structural and functional recovery after sciatic nerve crush.

Multiple molecular mechanisms influence nerve regeneration. Because serine proteases were shown to affect peripheral nerve regeneration, we performed nerve crush experiments to study synapse reinnervation in adult mice lacking the serpin protease nexin-1 (PN-1). PN-1 is a potent endogenous inhibitor of thrombin, trypsin, tissue plasminogen activators (tPAs), and urokinase plasminogen activators. Compared with the wild type, a significant delay in synapse reinnervation was detected in PN-1 knock-out (KO) animals, which was associated with both reduced proliferation and increased apoptosis of Schwann cells. Various factors known to affect Schwann cells were also altered. Fibrin deposits, tPA activity, mature BDNF, and the low-affinity p75 neurotrophin receptor were increased in injured sciatic nerves of mutant mice. To test whether the absence of PN-1 in Schwann cells or in the axon caused delay in reinnervation, PN-1 was overexpressed exclusively in the nerves of PN-1 KO mice. Neuronal PN-1 expression did not rescue the delayed reinnervation. The results suggest that Schwann cell-derived PN-1 is crucial for proper reinnervation through its contribution to the autocrine control of proliferation and survival. Thus, the precise balance between distinct proteases and serpins such as PN-1 can modulate the overall impact on the kinetics of recovery.

Disc1 is mutated in the 129S6/SvEv strain and modulates working memory in mice.

Disrupted-In-Schizophrenia (DISC1) is a leading candidate schizophrenia susceptibility gene. Here, we describe a deletion variant in mDisc1 specific to the 129S6/SvEv strain that introduces a termination codon at exon 7, abolishes production of the full-length protein, and impairs working memory performance when transferred to the C57BL/6J genetic background. Our findings provide insights into how DISC1 variation contributes to schizophrenia susceptibility in humans and the behavioral divergence between 129S6/SvEv and C57BL/6J mouse strains and have implications for modeling psychiatric diseases in mice.

Regulation of brain proteolytic activity is necessary for the in vivo function of NMDA receptors.

Serine proteases are considered to be involved in plasticity-related events in the nervous system, but their in vivo targets and the importance of their control by endogenous inhibitors are still not clarified. Here, we demonstrate the crucial role of a potent serine protease inhibitor, protease nexin-1 (PN-1), in the regulation of activity-dependent brain proteolytic activity and the functioning of sensory pathways. Neuronal activity regulates the expression of PN-1, which in turn controls brain proteolytic activity. In PN-1-/- mice, absence of PN-1 leads to increased brain proteolytic activity, which is correlated with an activity-dependent decrease in the NR1 subunit of the NMDA receptor. Correspondingly, reduced NMDA receptor signaling is detected in their barrel cortex. This is coupled to decreased sensory evoked potentials in the barrel cortex and impaired whisker-dependent sensory motor function. Thus, a tight control of serine protease activity is critical for the in vivo function of the NMDA receptors and the proper function of sensory pathways.