Altered expression of synaptotagmin 13 mRNA in adult mouse brain after contextual fear conditioning

Synaptotagmin-13 orchestrates pancreatic endocrine cell egression and islet morphogenesis

During pancreas development endocrine cells leave the ductal epithelium to form the islets of Langerhans, but the morphogenetic mechanisms are incompletely understood. Here, we identify the Ca2+-independent atypical Synaptotagmin-13 (Syt13) as a key regulator of endocrine cell egression and islet formation. We detect specific upregulation of the Syt13 gene and encoded protein in endocrine precursors and the respective lineage during islet formation. The Syt13 protein is localized to the apical membrane of endocrine precursors and to the front domain of egressing endocrine cells, marking a previously unidentified apical-basal to front-rear repolarization during endocrine precursor cell egression. Knockout of Syt13 impairs endocrine cell egression and skews the α-to-β-cell ratio. Mechanistically, Syt13 is a vesicle trafficking protein, transported via the microtubule cytoskeleton, and interacts with phosphatidylinositol phospholipids for polarized localization. By internalizing a subset of plasma membrane proteins at the front domain, including α6β4 integrins, Syt13 modulates cell-matrix adhesion and allows efficient endocrine cell egression. Altogether, these findings uncover an unexpected role for Syt13 as a morphogenetic driver of endocrinogenesis and islet formation.

Synaptotagmin-13 Is a Neuroendocrine Marker in Brain, Intestine and Pancreas

Synaptotagmin-13 (Syt13) is an atypical member of the vesicle trafficking synaptotagmin protein family. The expression pattern and the biological function of this Ca²⁺-independent protein are not well resolved. Here, we have generated a novel Syt13-Venus fusion (Syt13-VF) fluorescence reporter allele to track and isolate tissues and cells expressing Syt13 protein. The reporter allele is regulated by endogenous cis-regulatory elements of Syt13 and the fusion protein follows an identical expression pattern of the endogenous Syt13 protein. The homozygous reporter mice are viable and fertile. We identify the expression of the Syt13-VF reporter in different regions of the brain with high expression in tyrosine hydroxylase (TH)-expressing and oxytocin-producing neuroendocrine cells. Moreover, Syt13-VF is highly restricted to all enteroendocrine cells in the adult intestine that can be traced in live imaging. Finally, Syt13-VF protein is expressed in the pancreatic endocrine lineage, allowing their specific isolation by flow sorting. These findings demonstrate high expression levels of Syt13 in the endocrine lineages in three major organs harboring these secretory cells. Collectively, the Syt13-VF reporter mouse line provides a unique and reliable tool to dissect the spatio-temporal expression pattern of Syt13 and enables isolation of Syt13-expressing cells that will aid in deciphering the molecular functions of this protein in the neuroendocrine system.

The diversity of synaptotagmin isoforms

The synaptotagmin family of molecules is known for regulating calcium-dependent membrane fusion events. Mice and humans express 17 synaptotagmin isoforms, where most studies have focused on isoforms 1, 2, and 7, which are involved in synaptic vesicle exocytosis. Recent work has highlighted how brain function relies on additional isoforms, with roles in postsynaptic receptor endocytosis, vesicle trafficking, membrane repair, synaptic plasticity, and protection against neurodegeneration, for example, in addition to the traditional concept of synaptotagmin-mediated neurotransmitter release - in neurons as well as glia, and at different timepoints. In fact, it is not uncommon for the same isoform to feature several splice isoforms, form homo- and heterodimers, and function in different subcellular locations and cell types. This review aims to highlight the diversity of synaptotagmins, offers a concise summary of key findings on all isoforms, and discusses different ways of grouping these.

Silencing of synaptotagmin 13 inhibits tumor growth through suppressing proliferation and promoting apoptosis of colorectal cancer cells

The treatment of colorectal cancer is currently hampered by the lack of early detection technology. The identification of molecular biomarkers for colorectal cancer is crucial for improving prognosis. Synaptotagmin (SYT) 13 has been reported to be associated with several human tumors, but its role in colorectal cancer remains elusive. In the present study, immunohistochemistry was utilized to detect the expression of SYT13 in colorectal cancer tissues and cells. MTT, colony formation, wound healing and Transwell assays were conducted to evaluate the effect of SYT13 knockdown on the biological behavior of RKO and HCT116 cells. Cell apoptosis and cell cycle profiles were detected by FACS. A mouse xenograft model was constructed to investigate the effect of SYT13 on colorectal cancer in vivo. The results indicated that SYT13 was upregulated in colorectal tumor tissues compared with paracancerous tissues. Silencing of SYT13 inhibited the proliferation, colony formation, migration and invasion ability of RKO and HCT116 cells. Moreover, SYT13 knockdown arrested the cell cycle in the G2 phase, thus inducing cell apoptosis. The in vivo experiments also demonstrated the inhibitory effect of SYT13 on tumor growth. In conclusion, the present study demonstrated that SYT13 may act as a promoter in the development and progression of colorectal cancer and, therefore, may be of value as a target for the development of novel treatment strategies.

Anterior Pituitary Transcriptome Suggests Differences in ACTH Release in Tame and Aggressive Foxes

Domesticated species exhibit a suite of behavioral, endocrinological, and morphological changes referred to as "domestication syndrome." These changes may include a reduction in reactivity of the hypothalamic-pituitary-adrenal (HPA) axis and specifically reduced adrenocorticotropic hormone release from the anterior pituitary. To investigate the biological mechanisms targeted during domestication, we investigated gene expression in the pituitaries of experimentally domesticated foxes (Vulpes vulpes). RNA was sequenced from the anterior pituitary of six foxes selectively bred for tameness ("tame foxes") and six foxes selectively bred for aggression ("aggressive foxes"). Expression, splicing, and network differences identified between the two lines indicated the importance of genes related to regulation of exocytosis, specifically mediated by cAMP, organization of pseudopodia, and cell motility. These findings provide new insights into biological mechanisms that may have been targeted when these lines of foxes were selected for behavior and suggest new directions for research into HPA axis regulation and the biological underpinnings of domestication.

Retinal interneuron survival requires non-cell-autonomous Atrx activity

ATRX is a chromatin remodeling protein that is mutated in several intellectual disability disorders including alpha-thalassemia/mental retardation, X-linked (ATR-X) syndrome. We previously reported the prevalence of ophthalmological defects in ATR-X syndrome patients, and accordingly we find morphological and functional visual abnormalities in a mouse model harboring a mutation occurring in ATR-X patients. The visual system abnormalities observed in these mice parallels the Atrx-null retinal phenotype characterized by interneuron defects and selective loss of amacrine and horizontal cells. The mechanisms that underlie selective neuronal vulnerability and neurodegeneration in the central nervous system upon Atrx mutation or deletion are unknown. To interrogate the cellular specificity of Atrx for its retinal neuroprotective functions, we employed a combination of temporal and lineage-restricted conditional ablation strategies to generate five different conditional knockout mouse models, and subsequently identified a non-cell-autonomous requirement for Atrx in bipolar cells for inhibitory interneuron survival in the retina. Atrx-deficient retinal bipolar cells exhibit functional, structural and molecular alterations consistent with impairments in neuronal activity and connectivity. Gene expression changes in the Atrx-null retina indicate defective synaptic structure and neuronal circuitry, suggest excitotoxic mechanisms of neurodegeneration, and demonstrate that common targets of ATRX in the forebrain and retina may contribute to similar neuropathological processes underlying cognitive impairment and visual dysfunction in ATR-X syndrome.

Elevated adiponectin prevents HIV protease inhibitor toxicity and preserves cerebrovascular homeostasis in mice

HIV protease inhibitors are key components of HIV antiretroviral therapies, which are fundamental in the treatment of HIV infection. However, the protease inhibitors are well-known to induce metabolic dysfunction which can in turn escalate the complications of HIV, including HIV associated neurocognitive disorders. As experimental and epidemiological data support a therapeutic role for adiponectin in both metabolic and neurologic homeostasis, this study was designed to determine if increased adiponectin could prevent the detrimental effects of protease inhibitors in mice. Adult male wild type (WT) and adiponectin-overexpressing (ADTg) mice were thus subjected to a 4-week regimen of lopinavir/ritonavir, followed by comprehensive metabolic, neurobehavioral, and neurochemical analyses. Data show that lopinavir/ritonavir-induced lipodystrophy, hypoadiponectinemia, hyperglycemia, hyperinsulinemia, and hypertriglyceridemia were attenuated in ADTg mice. Furthermore, cognitive function and blood-brain barrier integrity were preserved, while loss of cerebrovascular markers and white matter injury were prevented in ADTg mice. Finally, lopinavir/ritonavir caused significant increases in expression of markers of brain inflammation and decreases in synaptic markers in WT, but not in ADTg mice. Collectively, these data reinforce the pathophysiologic link from metabolic dysfunction to loss of cerebrovascular and cognitive homeostasis; and suggest that preservation and/or replacement of adiponectin could prevent these key aspects of HIV protease inhibitor-induced toxicity in clinical settings.

Synaptic adhesion molecule IgSF11 regulates synaptic transmission and plasticity

Synaptic adhesion molecules regulate synapse development and plasticity through mechanisms that include trans-synaptic adhesion and recruitment of diverse synaptic proteins. We found that the immunoglobulin superfamily member 11 (IgSF11), a homophilic adhesion molecule that preferentially expressed in the brain, is a dual-binding partner of the postsynaptic scaffolding protein PSD-95 and AMPA glutamate receptors (AMPARs). IgSF11 required PSD-95 binding for its excitatory synaptic localization. In addition, IgSF11 stabilized synaptic AMPARs, as determined by IgSF11 knockdown-induced suppression of AMPAR-mediated synaptic transmission and increased surface mobility of AMPARs, measured by high-throughput, single-molecule tracking. IgSF11 deletion in mice led to the suppression of AMPAR-mediated synaptic transmission in the dentate gyrus and long-term potentiation in the CA1 region of the hippocampus. IgSF11 did not regulate the functional characteristics of AMPARs, including desensitization, deactivation or recovery. These results suggest that IgSF11 regulates excitatory synaptic transmission and plasticity through its tripartite interactions with PSD-95 and AMPARs.

Obese-type gut microbiota induce neurobehavioral changes in the absence of obesity

BACKGROUND

The prevalence of mental illness, particularly depression and dementia, is increased by obesity. Here, we test the hypothesis that obesity-associated changes in gut microbiota are intrinsically able to impair neurocognitive behavior in mice.

METHODS

Conventionally housed, nonobese, adult male C57BL/6 mice maintained on a normal chow diet were subjected to a microbiome depletion/transplantation paradigm using microbiota isolated from donors on either a high-fat diet (HFD) or control diet. Following re-colonization, mice were subjected to comprehensive behavioral and biochemical analyses.

RESULTS

The mice given HFD microbiota had significant and selective disruptions in exploratory, cognitive, and stereotypical behavior compared with mice with control diet microbiota in the absence of significant differences in body weight. Sequencing-based phylogenetic analysis confirmed the presence of distinct core microbiota between groups, with alterations in α- and β-diversity, modulation in taxonomic distribution, and statistically significant alterations to metabolically active taxa. HFD microbiota also disrupted markers of intestinal barrier function, increased circulating endotoxin, and increased lymphocyte expression of ionized calcium-binding adapter molecule 1, toll-like receptor 2, and toll-like receptor 4. Finally, evaluation of brain homogenates revealed that HFD-shaped microbiota increased neuroinflammation and disrupted cerebrovascular homeostasis.

CONCLUSIONS

Collectively, these data reinforce the link between gut dysbiosis and neurologic dysfunction and suggest that dietary and/or pharmacologic manipulation of gut microbiota could attenuate the neurologic complications of obesity.

Early growth response 1 (Egr-1) directly regulates GABAA receptor α2, α4, and θ subunits in the hippocampus

The homeostatic regulation of neuronal activity in glutamatergic and GABAergic synapses is critical for neural circuit development and synaptic plasticity. The induced expression of the transcription factor early growth response 1 (Egr-1) in neurons is tightly associated with many forms of neuronal activity, but the underlying target genes in the brain remained to be elucidated. This study uses a quantitative real-time PCR approach, in combination with in vivo chromatin immunoprecipitation, and reveals that GABAA receptor subunit, GABRA2 (α2), GABRA4 (α4), and GABRQ (θ) genes, are transcriptional targets of Egr-1. Transfection of a construct that over-expresses Egr-1 in neuroblastoma (Neuro2A) cells up-regulates the α2, α4, and θ subunits. Given that Egr-1 knockout mice display less GABRA2, GABRA4, and GRBRQ mRNA in the hippocampus, and that Egr-1 directly binds to their promoters and induces mRNA expression, the present findings support a role for Egr-1 as a major regulator for altered GABAA receptor composition in homeostatic plasticity, in a glutamatergic activity-dependent manner. The early growth response 1 (Egr-1) is an inducible transcription factor to mediate rapid gene expression by neuronal activity. However, its underlying molecular target genes and mechanisms are not fully understood. We suggest that GABAA receptor subunits, GABRA2 (α2), GABRA4 (α4), and GABRQ (θ) genes are transcriptional targets of Egr-1. Neuronal activity-dependent up-regulation of Egr-1 might lead to altered subtypes of GABAA receptors for the maintenance of homeostatic excitatory and inhibitory balance for the regulation of synaptic strength.

Designer adiponectin receptor agonist stabilizes metabolic function and prevents brain injury caused by HIV protease inhibitors

HIV protease inhibitors (PI) are fundamental to combination antiretroviral therapy, which has revolutionized HIV clinical care and produced significant reductions in HIV-associated morbidity and mortality. However, PI administration is frequently associated with severe metabolic impairment, including lipodystrophy, dyslipidemia, and insulin resistance; all of which can contribute to cardiovascular and neurologic co-morbidities. Experimental and epidemiological data support a potentially important role for the adipokine adiponectin in both metabolic and neurologic physiology. This study examined if ADP355, a novel, peptide-based adiponectin receptor agonist, could neutralize the detrimental effects of PI treatment in experimental animal models. Adult male C57BL/6 mice were subjected to a clinically relevant, 4-week regimen of lopinavir/ritonavir, with daily injections of ADP355 administered only during the final 2 weeks of PI exposure. Comprehensive metabolic, neurobehavioral, and biochemical analyses revealed that ADP355 administration partially reversed PI-induced loss of subcutaneous adipose tissue, attenuated PI-induced hyperinsulinemia, hypertriglyceridemia, and hypoadiponectinemia, and prevented PI-induced cognitive impairment and brain injury. Collectively, these data reinforce the link between metabolic co-morbidities and cognitive impairment and suggest that pharmacological reactivation of adiponectin pathways could remediate key aspects of PI-induced metabolic syndrome in clinical settings. Furthermore, therapeutic targeting of adiponectin receptors could show utility in reducing the prevalence and/or severity of HIV-associated neurocognitive disorders.

Impaired extinction of learned contextual fear memory in early growth response 1 knockout mice

Inductive expression of early growth response 1 (Egr-1) in neurons is associated with many forms of neuronal activity. However, only a few Egr-1 target genes are known in the brain. The results of this study demonstrate that Egr-1 knockout (KO) mice display impaired contextual extinction learning and normal fear acquisition relative to wild-type (WT) control animals. Genome-wide microarray experiments revealed 368 differentially expressed genes in the hippocampus of Egr-1 WT exposed to different phases of a fear conditioning paradigm compared to gene expression profiles in the hippocampus of KO mice. Some of genes, such as serotonin receptor 2C (Htr2c), neuropeptide B (Npb), neuronal PAS domain protein 4 (Npas4), NPY receptor Y1 (Npy1r), fatty acid binding protein 7 (Fabp7), and neuropeptide Y (Npy) are known to regulate processing of fearful memories, and promoter analyses demonstrated that several of these genes contained Egr-1 binding sites. This study provides a useful list of potential Egr-1 target genes which may be regulated during fear memory processing.