Forward suppression in the auditory cortex is caused by the Ca(v)3.1 calcium channel-mediated switch from bursting to tonic firing at thalamocortical projections.

Brief sounds produce a period of suppressed responsiveness in the auditory cortex (ACx). This forward suppression can last for hundreds of milliseconds and might contribute to mechanisms of temporal separation of sounds and stimulus-specific adaptation. However, the mechanisms of forward suppression remain unknown. We used in vivo recordings of sound-evoked responses in the mouse ACx and whole-cell recordings, two-photon calcium imaging in presynaptic terminals, and two-photon glutamate uncaging in dendritic spines performed in brain slices to show that synaptic depression at thalamocortical (TC) projections contributes to forward suppression in the ACx. Paired-pulse synaptic depression at TC projections lasts for hundreds of milliseconds and is attributable to a switch between firing modes in thalamic neurons. Thalamic neurons respond to a brief depolarizing pulse with a burst of action potentials; however, within hundreds of milliseconds, the same pulse repeated again produces only a single action potential. This switch between firing modes depends on Ca(v)3.1 T-type calcium channels enriched in thalamic relay neurons. Pharmacologic inhibition or knockdown of Ca(v)3.1 T-type calcium channels in the auditory thalamus substantially reduces synaptic depression at TC projections and forward suppression in the ACx. These data suggest that Ca(v)3.1-dependent synaptic depression at TC projections contributes to mechanisms of forward suppression in the ACx.

Pancreas-specific deletion of Prox1 affects development and disrupts homeostasis of the exocrine pancreas.

BACKGROUND & AIMS: The exocrine portion of the pancreas functions in digestion and preserves pancreatic homeostasis. Learning how this tissue forms during embryogenesis could improve our understanding of human pancreatic diseases. Expression of the homeobox gene Prox1 in the exocrine pancreas changes throughout development in mice. We investigated the role of Prox1 in development of the exocrine pancreas in mice. METHODS: Mice with pancreas-specific deletion of Prox1 (Prox1(ΔPanc)) were generated and their pancreatic tissues were analyzed using immunohistochemistry, transmission electron microscopy, histologic techniques, quantitative real-time polymerase chain reaction, immunoblotting, and morphometric analysis. RESULTS: Loss of Prox1 from the pancreas led to multiple exocrine alterations, most notably premature acinar cell differentiation, increased ductal cell proliferation, altered duct morphogenesis, and imbalanced expression of claudin proteins. Prox1(ΔPanc) mice also had some minor alterations in islet cells, but beta-cell development was not affected. The exocrine congenital defects of Prox1(ΔPanc) pancreata appeared to initiate a gradual process of deterioration that resulted in extensive loss of acinar cells, lipomatosis, and damage to ductal tissue in adult mice. CONCLUSIONS: Pancreas-specific deletion of Prox1 causes premature differentiation of acinar cells and poor elongation of epithelial branches; these defects indicate that Prox1 controls the expansion of tip progenitors in the early developing pancreas. During later stages of embryogenesis, Prox1 appears to regulate duct cell proliferation and morphogenesis. These findings identify Prox1 as an important regulator of pancreatic exocrine development.

Dynamic distribution of claudin proteins in pancreatic epithelia undergoing morphogenesis or neoplastic transformation.

BACKGROUND: The assembly of distinct proteins into tight junctions results in the formation of a continuous barrier that regulates the paracellular flux of water, ions, and small molecules across epithelia. The claudin protein family encompasses numerous major structural components of tight junctions. These proteins specify the permeability characteristics of tight junctions and consequently, some of the physiological properties of epithelia. Furthermore, defective claudin expression has been found to correlate with some diseases, tumor progression, and defective morphogenesis. Investigating the pattern of claudin expression during embryogenesis or in certain pathological conditions is necessary to begin disclosing the role of these proteins in health and disease. RESULTS: This study analyzed the expression of several claudins during mouse pancreas organogenesis and in pancreatic intraepithelial neoplasias of mouse and human origin. CONCLUSIONS: Our results underscored a distinctive, dynamic distribution of certain claudins in both the developing pancreas and the pancreatic epithelium undergoing neoplastic transformation.

Pdk1 activity controls proliferation, survival, and growth of developing pancreatic cells.

The formation of adequate masses of endocrine and exocrine pancreatic tissues during embryogenesis is essential to ensure proper nutrition and glucose homeostasis at postnatal stages. We generated mice with pancreas-specific ablation of the 3-phosphoinositide-dependent protein kinase 1 (Pdk1) to investigate how signaling downstream of the phosphatidylinositol-3-OH kinase (PI3K) pathway controls pancreas development. Pdk1-conditional knock-out mice were born with conspicuous pancreas hypoplasia, and within a few weeks, they developed severe hyperglycemia. Our detailed characterization of the mutant embryonic pancreas also revealed distinct temporal, cell type-specific requirements of Pdk1 activity in the control of cell proliferation, cell survival, and cell size during pancreas development. These results thus uncover Pdk1 as a novel, crucial regulator of pancreatic growth during embryogenesis. In addition, we provide evidence that Pdk1 activity is required differently in mature pancreatic cell types, since compensatory proliferation and possible mTORC2 activation occurred in exocrine cells but not in beta cells of the Pdk1-deficient postnatal pancreas.

Age-dependent expression pattern in the mammalian brain of a novel, small peptide encoded in the 22q11.2 deletion syndrome region.

22q11.2 deletion syndrome (22q11.2DS) carries increased risk for both physical and psychiatric symptoms, including a high risk for schizophrenia. Understanding the genetic elements within the deletion region therefore has the potential to unlock the mysteries of both diseases. While most of the protein-coding genes in this region have been characterized, novel elements, such as non-coding RNAs and small Open Reading Frames (sORFs) remain unstudied. We have identified a novel, highly-conserved mouse sORF in a region of the mouse genome that is orthologous to a portion of the 22q11.2 deletion. This region was previously associated with age-dependent synaptic plasticity abnormalities. We refer to it as the Plasticity Associated Neural Transcript Short, or Pants. In developing and aging mouse brain, Pants expression is strongest in hippocampus, especially in areas CA3 and CA2, throughout the dorsoventral axis. The Pants peptide is expressed throughout the hippocampus, with an age-dependent increase in stratum lucidum at 16 weeks of age. This expression pattern suggests a potential role for Pants in many hippocampal behaviors, as well as a potential role in the age-dependent neurologic deficits displayed by 22q11.2DS model mice and patients.

Thalamic miR-338-3p mediates auditory thalamocortical disruption and its late onset in models of 22q11.2 microdeletion.

Although 22q11.2 deletion syndrome (22q11DS) is associated with early-life behavioral abnormalities, affected individuals are also at high risk for the development of schizophrenia symptoms, including psychosis, later in life. Auditory thalamocortical (TC) projections recently emerged as a neural circuit that is specifically disrupted in mouse models of 22q11DS (hereafter referred to as 22q11DS mice), in which haploinsufficiency of the microRNA (miRNA)-processing-factor-encoding gene Dgcr8 results in the elevation of the dopamine receptor Drd2 in the auditory thalamus, an abnormal sensitivity of thalamocortical projections to antipsychotics, and an abnormal acoustic-startle response. Here we show that these auditory TC phenotypes have a delayed onset in 22q11DS mice and are associated with an age-dependent reduction of miR-338-3p, a miRNA that targets Drd2 and is enriched in the thalamus of both humans and mice. Replenishing depleted miR-338-3p in mature 22q11DS mice rescued the TC abnormalities, and deletion of Mir338 (which encodes miR-338-3p) or reduction of miR-338-3p expression mimicked the TC and behavioral deficits and eliminated the age dependence of these deficits. Therefore, miR-338-3p depletion is necessary and sufficient to disrupt auditory TC signaling in 22q11DS mice, and it may mediate the pathogenic mechanism of 22q11DS-related psychosis and control its late onset.

Non-coding RNA regulation of synaptic plasticity and memory: implications for aging.

Advancing age is associated with the loss of cognitive ability and vulnerability to debilitating mental diseases. Although much is known about the development of cognitive processes in the brain, the study of the molecular mechanisms governing memory decline with aging is still in its infancy. Recently, it has become apparent that most of the human genome is transcribed into non-coding RNAs (ncRNAs) rather than protein-coding mRNAs. Multiple types of ncRNAs are enriched in the central nervous system, and this large group of molecules may regulate the molecular complexity of the brain, its neurons, and synapses. Here, we review the current knowledge on the role of ncRNAs in synaptic plasticity, learning, and memory in the broader context of the aging brain and associated memory loss. We also discuss future directions to study the role of ncRNAs in the aging process.

Specific disruption of thalamic inputs to the auditory cortex in schizophrenia models.

Auditory hallucinations in schizophrenia are alleviated by antipsychotic agents that inhibit D2 dopamine receptors (Drd2s). The defective neural circuits and mechanisms of their sensitivity to antipsychotics are unknown. We identified a specific disruption of synaptic transmission at thalamocortical glutamatergic projections in the auditory cortex in murine models of schizophrenia-associated 22q11 deletion syndrome (22q11DS). This deficit is caused by an aberrant elevation of Drd2 in the thalamus, which renders 22q11DS thalamocortical projections sensitive to antipsychotics and causes a deficient acoustic startle response similar to that observed in schizophrenic patients. Haploinsufficiency of the microRNA-processing gene Dgcr8 is responsible for the Drd2 elevation and hypersensitivity of auditory thalamocortical projections to antipsychotics. This suggests that Dgcr8-microRNA-Drd2-dependent thalamocortical disruption is a pathogenic event underlying schizophrenia-associated psychosis.

Cleft palate is caused by CNS dysfunction in Gad1 and Viaat knockout mice.

BACKGROUND: Previous studies have shown that disruption of GABA signaling in mice via mutations in the Gad1, Gabrb3 or Viaat genes leads to the development of non-neural developmental defects such as cleft palate. Studies of the Gabrb3 and Gad1 mutant mice have suggested that GABA function could be required either in the central nervous system or in the palate itself for normal palatogenesis. METHODOLOGY/PRINCIPAL FINDINGS: To further examine the role of GABA signaling in palatogenesis we used three independent experimental approaches to test whether Gad1 or Viaat function is required in the fetal CNS for normal palate development. We used oral explant cultures to demonstrate that the Gad1 and Viaat mutant palates were able to undergo palatogenesis in culture, suggesting that there is no defect in the palate tissue itself in these mice. In a second series of experiments we found that the GABA(A) receptor agonist muscimol could rescue the cleft palate phenotype in Gad1 and Viaat mutant embryos. This suggested that normal multimeric GABA(A) receptors in the CNS were necessary for normal palatogenesis. In addition, we showed that CNS-specific inactivation of Gad1 was sufficient to disrupt palate development. CONCLUSIONS/SIGNIFICANCE: Our results are consistent with a role for Gad1 and Viaat in the central nervous system for normal development of the palate. We suggest that the alterations in GABA signaling lead to non-neural defects such as cleft palate as a secondary effect due to alterations in or elimination of fetal movements.

Xenopus Lefty requires proprotein cleavage but not N-linked glycosylation to inhibit nodal signaling.

The Nodal and Nodal-related morphogens are utilized for the specification of distinct cellular identity throughout development by activating discrete target genes in a concentration-dependant manner. Lefty is a principal extracellular antagonist involved in the spatiotemporal regulation of the Nodal morphogen gradient during mesendoderm induction. The Xenopus Lefty proprotein contains a single N-linked glycosylation motif in the mature domain and two potential cleavage sites that would be expected to produce long (Xlefty(L)) and short (Xlefty(S)) isoforms. Here we demonstrate that both isoforms were secreted from Xenopus oocytes, but that Xlefty(L) is the only isoform detected when embryonic tissue was analyzed. In mesoderm induction assays, Xlefty(L) is the functional blocker of Xnr signaling. When secreted from oocytes, vertebrate Lefty molecules were N-linked glycosylated. However, glycan addition was not required to inhibit Xnr signaling and did not influence its movement through the extracellular space. These findings demonstrate that Lefty molecules undergo post-translational modifications and that some of these modifications are required for the Nodal inhibitory function.

Nodal-related gene Xnr5 is amplified in the Xenopus genome.

In Xenopus, six nodal-related genes (Xnrs) have been identified to date. We found numerous tandem duplications of Xnr5 in the Xenopus laevis and Xenopus tropicalis genomes that involve highly conserved copies of coding and regulatory regions. The duplicated versions of Xnr5 were expressed in both the superficial and deep layer of dorsal endoderm and in the deep layer of ventral endoderm, where the initial inducers of mesendoderm formation would be expected to be localized. Overexpression of secreted inhibitors of Xnrs led to a substantially enhanced transcription of the duplicated Xnr5 genes and Xnr6 in embryos. Therefore, Xnr5 and Xnr6 have a novel feedback loop to inhibit transcription of Xnr5 and Xnr6. These results suggest that the initialization of a strong Xnr5 and Xnr6 signal is enabled by the rapid transcription from multiple genes. The novel feedback loop may negatively regulate transcription of Xnr5s and Xnr6 to limit overproduction of these potent inducers, with the Xnr5/Xnr6 signal then activating positive (Xnrs) and negative (Xlefty) loops, which regulate the range of mesodermal tissues produced.