ErbB4 in parvalbumin-positive interneurons is critical for neuregulin 1 regulation of long-term potentiation

Neuregulin 1 (NRG1) is a trophic factor that acts by stimulating ErbB receptor tyrosine kinases and has been implicated in neural development and synaptic plasticity. In this study, we investigated mechanisms of its suppression of long-term potentiation (LTP) in the hippocampus. We found that NRG1 did not alter glutamatergic transmission at SC-CA1 synapses but increased the GABA(A) receptor-mediated synaptic currents in CA1 pyramidal cells via a presynaptic mechanism. Inhibition of GABA(A) receptors blocked the suppressing effect of NRG1 on LTP and prevented ecto-ErbB4 from enhancing LTP, implicating a role of GABAergic transmission. To test this hypothesis further, we generated parvalbumin (PV)-Cre;ErbB4(-/-) mice in which ErbB4, an NRG1 receptor in the brain, is ablated specifically in PV-positive interneurons. NRG1 was no longer able to increase inhibitory postsynaptic currents and to suppress LTP in PV-Cre;ErbB4(-/-) hippocampus. Accordingly, contextual fear conditioning, a hippocampus-dependent test, was impaired in PV-Cre;ErbB4(-/-) mice. In contrast, ablation of ErbB4 in pyramidal neurons had no effect on NRG1 regulation of hippocampal LTP or contextual fear conditioning. These results demonstrate a critical role of ErbB4 in PV-positive interneurons but not in pyramidal neurons in synaptic plasticity and support a working model that NRG1 suppresses LTP by enhancing GABA release. Considering that NRG1 and ErbB4 are susceptibility genes of schizophrenia, these observations contribute to a better understanding of how abnormal NRG1/ErbB4 signaling may be involved in the pathogenesis of schizophrenia.     

Decoding the rules of recruitment of excitatory interneurons in the adult zebrafish locomotor network

Significance

Spinal neural networks generate locomotion. An adjustment of the locomotion speed entails a precise order of recruitment of excitatory interneurons (e.g., V2a interneurons) within these networks. We show, using the adult zebrafish spinal cord, that the recruitment order of V2a interneurons is not topographic and does not conform to input resistance. The incremental recruitment of these interneurons is determined by scaling the excitatory drive with input resistance. We also show that locomotor networks are composed of multiple microcircuits recruited in a continuum. Thus, we provide insights into the recruitment mechanisms of spinal microcircuits that ensure optimal execution of locomotor movements.     

Embryonic gonadotropin-releasing hormone signaling is necessary for maturation of the male reproductive axis

Gonadotropin-releasing hormone (GnRH) signaling regulates reproductive physiology in mammals. GnRH is released by a subset of hypothalamic neurons and binds to GnRH receptor (GnRHR) on gonadotropes in the anterior pituitary gland to control production and secretion of gonadotropins that in turn regulate the activity of the gonads. Central control of reproduction is well understood in adult animals, but GnRH signaling has also been implicated in the development of the reproductive axis. To investigate the role of GnRH signaling during development, we selectively ablated GnRHR-expressing cells in mice. This genetic strategy permitted us to identify an essential stage in male reproductive axis development, which depends on embryonic GnRH signaling. Our experiments revealed a striking dichotomy in the gonadotrope population of the fetal anterior pituitary gland. We show that luteinizing hormone-expressing gonadotropes, but not follicle-stimulating hormone-expressing gonadotropes, express the GnRHR at embryonic day 16.75. Furthermore, we demonstrate that an embryonic increase in luteinizing hormone secretion is needed to promote development of follicle-stimulating hormone-expressing gonadotropes, which might be mediated by paracrine interactions within the pituitary. Moreover, migration of GnRH neurons into the hypothalamus appeared normal with appropriate axonal connections to the median eminence, providing genetic evidence against autocrine regulation of GnRH neurons. Surprisingly, genetic ablation of GnRHR expressing cells significantly increased the number of GnRH neurons in the anterior hypothalamus, suggesting an unexpected role of GnRH signaling in establishing the size of the GnRH neuronal population. Our experiments define a functional role of embryonic GnRH signaling.     

A critical developmental role for tgfbr2 in myogenic cell lineages is revealed in mice expressing SM22-Cre, not SMMHC-Cre

Smooth muscle cell (SMC)-specific deletion of transforming growth factor beta (TGF-beta) signaling would help elucidate the mechanisms through which TGF-beta signaling contributes to vascular development and disease. We attempted to generate mice with SMC-specific deletion of TGF-beta signaling by mating mice with a conditional ("floxed") allele for the type II TGF-beta receptor (tgfbr2flox) to mice with SMC-targeted expression of Cre recombinase. We bred male mice transgenic for smooth muscle myosin heavy chain (SMMHC)-Cre with females carrying tgfbr2flox. Surprisingly, SMMHC-Cre mice recombined tgfbr2flox at low levels in SMC and at high levels in the testis. Recombination of tgfbr2flox in testis correlated with high-level expression of SMMHC-Cre in testis and germline transmission of tgfbr2null. In contrast, mice expressing Cre from a SM22alpha promoter (SM22-Cre) efficiently recombined tgfbr2flox in vascular and visceral SMC and the heart, but not in testis. Use of the R26R reporter allele confirmed that Cre-mediated recombination in vascular SMC was inefficient for SMMHC-Cre mice and highly efficient for SM22-Cre mice. Breedings that introduced the SM22-Cre allele into tgfbr2flox/flox zygotes in order to generate adult mice that are hemizygous for SM22-Cre and homozygous for tgfbr2flox- and would have conversion of tgfbr2flox/flox to tgfbr2null/null in SMC-produced no live SM22-Cre : tgfbr2flox/flox pups (P<0.001). We conclude: (1) "SMC-targeted" Cre lines vary significantly in specificity and efficiency of Cre expression; (2) TGF-beta signaling in the subset of cells that express SM22alpha is required for normal development; (3) generation of adult mice with absent TGF-beta signaling in SMC remains a challenge.     

Developmental synaptic regulator,TWEAK/Fn14 signaling,is a determinant of synaptic function in models of stroke and neurodegeneration

Identifying molecular mediators of neural circuit development and/or function that contribute to circuit dysfunction when aberrantly reengaged in neurological disorders is of high importance. The role of the TWEAK/Fn14 pathway, which was recently reported to be a microglial/neuronal axis mediating synaptic refinement in experience-dependent visual development, has not been explored in synaptic function within the mature central nervous system. By combining electrophysiological and phosphoproteomic approaches, we show that TWEAK acutely dampens basal synaptic transmission and plasticity through neuronal Fn14 and impacts the phosphorylation state of pre- and postsynaptic proteins in adult mouse hippocampal slices. Importantly, this is relevant in two models featuring synaptic deficits. Blocking TWEAK/Fn14 signaling augments synaptic function in hippocampal slices from amyloid-beta–overexpressing mice. After stroke, genetic or pharmacological inhibition of TWEAK/Fn14 signaling augments basal synaptic transmission and normalizes plasticity. Our data support a glial/neuronal axis that critically modifies synaptic physiology and pathophysiology in different contexts in the mature brain and may be a therapeutic target for improving neurophysiological outcomes.

Neural circuit patterning, refinement, and plasticity are enabled by the dynamic strengthening, weakening, and pruning of chemical synapses in response to circuit activity. However, synapse loss and reduced plasticity are early hallmarks of chronic neurological disorders such as autism, schizophrenia and Alzheimer’s disease (AD) (1–3). It is therefore hypothesized that the underlying molecular mechanisms of pruning, although normally balanced in health, are dysregulated in disease. Particularly interesting is the notion that the mechanisms responsible for the reduction in functional synapses in disease reflect the aberrant reactivation of pathways important for synapse elimination in development. For example, in an AD model, synapse elimination was shown to be mediated by the complement pathway in the hippocampus (HC), reflecting aberrant reactivation of complement-dependent synapse elimination that occurs in the dorsal lateral geniculate nucleus (dLGN) of the thalamus during visual development (4). In such a paradigm, the reactivation of developmental mechanisms enables pathways that can act universally across different ages, circuits, and brain regions. Thus, the mechanisms underlying normal circuit development and their potential reactivation as key contributors to neurological diseases are areas of deep interest.In addition to chronic neurological disorders, circuitry changes also occur in acute ischemic stroke, the second leading cause of death worldwide and a cause of debilitating long-term disability. Interruptions in blood flow that deprive neurons of oxygen and nutrients result in significant cell death, followed by deficits in neurophysiological activity that are associated with poor motor recovery (5). Remarkably, the adult brain can undergo some degree of spontaneous poststroke recovery, apparently by engaging neuroplasticity mechanisms including remapping, synaptogenesis, and synaptic strengthening (5, 6). Despite these adaptations, over half of ischemic stroke patients fail to recover completely and continue to experience persistent long-term disability (7). The underlying signaling pathways that regulate synaptic physiology after stroke are an active topic of investigation.TNF-like weak inducer of apoptosis (TWEAK) protein, originally discovered as a cytokine produced by macrophages (8), signals through its injury-inducible transmembrane receptor, FGF-inducible molecule-14 (Fn14) (9). Consequently, the function of TWEAK/Fn14 signaling was elucidated as a driver of tissue remodeling in contexts of injury and disease in a variety of organ systems (10). Recently, findings have suggested a role for the TWEAK/Fn14 pathway in the central nervous system (CNS). Namely, several compelling observations indicate that TWEAK signaling through Fn14 might be a key molecular modulator of synaptic function in contexts of neurological challenge. TWEAK and Fn14 are up-regulated in the CNS in AD (11, 12, 13 and SI Appendix, Fig. S6A) and after ischemic stroke in humans and mice (14–16). Importantly, TWEAK/Fn14 signaling was also recently shown to be a pathway necessary for synapse maturation during experience-dependent visual development. Light-induced up-regulation of Fn14 in thalamocortical excitatory neurons and corresponding up-regulation of TWEAK in microglia mediate the elimination of weak synapses and strengthening of remaining synapses in the dLGN (17, 18). Indeed, the communication between neurons and supporting microglia has emerged as a key mechanism regulating neuronal circuitry, with microglia deploying their ramified processes to continuously survey and refine synapses in response to neural activity. Interestingly, TWEAK expression has also been shown to be microglia-enriched in the mouse cortex (19), suggesting that it may play a role in multiple brain regions. Thus, like the complement pathway, the TWEAK/Fn14 pathway could be an important regulator of synapse biology in visual development which is re-engaged and acts generally in different ages and brain regions to contribute to pathology.The involvement of TWEAK/Fn14 signaling in synapse physiology or pathophysiology outside of the developing visual system is unknown. We considered it to be a strong candidate modifier of synaptic function in adults given that Fn14 is up-regulated and required for synaptic refinement in experience-dependent visual development, and TWEAK and Fn14 are up-regulated in contexts of neurological injury/disease, suggesting that the TWEAK/Fn14 system is tuned to periods of substantial change in neuronal activity levels or environment (e.g., eye opening, ischemic stroke). We employed HC slices to test the hypothesis that the TWEAK/Fn14 pathway regulates synaptic function in adult mice and in different disease contexts and delineate its mechanism of action. Herein, we reveal that TWEAK, through neuronal Fn14, mediates acute dampening of basal synaptic transmission and synaptic plasticity in hippocampal slices from mature mice. Furthermore, we demonstrate that TWEAK/Fn14 signaling broadly impacts the phosphorylation state of critical synaptic proteins, suggesting a general role in synapse modulation. Finally, we show that pathway deficiency or pharmacological inhibition of TWEAK/Fn14 signaling augments synaptic transmission and plasticity in amyloid-beta (Aβ)–overexpressing mice and post ischemic stroke animals, two model systems featuring synaptic functional deficits. Thus, our results support that TWEAK/Fn14 constitutes a synaptic regulatory pathway with therapeutic potential for CNS disorders in the adult brain.     

IL-21 signaling is critical for the development of type I diabetes in the NOD mouse

IL-21 is a pleiotropic type I cytokine that shares the common cytokine receptor γ chain and plays important roles for normal Ig production, terminal B cell differentiation to plasma cells, and Th17 differentiation. IL-21 is elevated in several autoimmune diseases, and blocking its action has attenuated disease in MRL/lpr mice and in collagen-induced arthritis. The diabetes-associated Idd3 locus is at the Il2/Il21 locus, and elevated IL-21 was observed in the nonobese diabetic (NOD) mouse and suggested to contribute to diabetes by augmenting T cell homeostatic proliferation. To determine the role of IL-21 in diabetes, Il21r-knockout (KO) mice were backcrossed to NOD mice. These mice were devoid of lymphocytic infiltration into the pancreas, and only 1 of 20 animals had an elevated glucose compared with 60% of NOD mice on a wild-type (WT) background. Although TCR and Treg-related responses were normal, these mice had reduced Th17 cells and significantly higher levels of mRNAs encoding members of the Reg (regenerating) gene family whose transgenic expression protects against diabetes. Our studies establish a critical role for IL-21 in the development of type I diabetes in the NOD mouse, with obvious potential implications for type I diabetes in humans.