Virus-assisted mapping of neural inputs to a feeding center in the hypothalamus

Fluorescent Protein Approaches in Alpha Herpesvirus Research

In the nearly two decades since the popularization of green fluorescent protein (GFP), fluorescent protein-based methodologies have revolutionized molecular and cell biology, allowing us to literally see biological processes as never before. Naturally, this revolution has extended to virology in general, and to the study of alpha herpesviruses in particular. In this review, we provide a compendium of reported fluorescent protein fusions to herpes simplex virus 1 (HSV-1) and pseudorabies virus (PRV) structural proteins, discuss the underappreciated challenges of fluorescent protein-based approaches in the context of a replicating virus, and describe general strategies and best practices for creating new fluorescent fusions. We compare fluorescent protein methods to alternative approaches, and review two instructive examples of the caveats associated with fluorescent protein fusions, including describing several improved fluorescent capsid fusions in PRV. Finally, we present our future perspectives on the types of powerful experiments these tools now offer.

A perisinusoidal niche for extramedullary haematopoiesis in the spleen

Haematopoietic stresses mobilize haematopoietic stem cells (HSCs) from the bone marrow to the spleen and induce extramedullary haematopoiesis (EMH). However, the cellular nature of the EMH niche is unknown. Here we assessed the sources of the key niche factors, SCF (also known as KITL) and CXCL12, in the mouse spleen after EMH induction by myeloablation, blood loss, or pregnancy. In each case, Scf was expressed by endothelial cells and Tcf21(+) stromal cells, primarily around sinusoids in the red pulp, while Cxcl12 was expressed by a subset of Tcf21(+) stromal cells. EMH induction markedly expanded the Scf-expressing endothelial cells and stromal cells by inducing proliferation. Most splenic HSCs were adjacent to Tcf21(+) stromal cells in red pulp. Conditional deletion of Scf from spleen endothelial cells, or of Scf or Cxcl12 from Tcf21+ stromal cells, severely reduced spleen EMH and reduced blood cell counts without affecting bone marrow haematopoiesis. Endothelial cells and Tcf21(+) stromal cells thus create a perisinusoidal EMH niche in the spleen, which is necessary for the physiological response to diverse haematopoietic stresses.

Presynaptic Inputs to Any CNS Projection Neuron Identified by Dual Recombinant Virus Infection

Although neuroanatomical tracing studies have defined the origin and targets of major projection neurons (PN) of the central nervous system (CNS), there is much less information about the circuits that influence these neurons. Recently, genetic approaches that use Cre recombinase-dependent viral vectors have greatly facilitated such circuit analysis, but these tracing approaches are limited by the availability of Cre-expressing mouse lines and the difficulty in restricting Cre expression to discrete regions of the CNS. Here, we illustrate an alternative approach to drive Cre expression specifically in defined subsets of CNS projection neurons, so as to map both direct and indirect presynaptic inputs to these cells. The method involves a combination of Cre-dependent transneuronal viral tracers that can be used in the adult and that does not require genetically modified mice. To trigger Cre-expression we inject a Cre-expressing adenovirus that is retrogradely transported to the projection neurons of interest. The region containing the retrogradely labeled projection neurons is next injected with Cre-dependent pseudorabies or rabies vectors, which results in labeling of poly- and monosynaptic neuronal inputs, respectively. In proof-of-concept experiments, we used this novel tracing system to study the circuits that engage projection neurons of the superficial dorsal horn of the spinal cord and trigeminal nucleus caudalis, neurons of the parabrachial nucleus of the dorsolateral pons that project to the amygdala and cortically-projecting neurons of the lateral geniculate nucleus. Importantly, because this dual viral tracing method does not require genetically derived Cre-expressing mouse lines, inputs to almost any projection system can be studied and the analysis can be performed in larger animals, such as the rat.

Central nervous system control of gastrointestinal motility and secretion and modulation of gastrointestinal functions

Although the gastrointestinal (GI) tract possesses intrinsic neural plexuses that allow a significant degree of autonomy over GI functions, the central nervous system (CNS) provides extrinsic neural inputs that regulate, modulate, and control these functions. While the intestines are capable of functioning in the absence of extrinsic inputs, the stomach and esophagus are much more dependent upon extrinsic neural inputs, particularly from parasympathetic and sympathetic pathways. The sympathetic nervous system exerts a predominantly inhibitory effect upon GI muscle and provides a tonic inhibitory influence over mucosal secretion while, at the same time, regulates GI blood flow via neurally mediated vasoconstriction. The parasympathetic nervous system, in contrast, exerts both excitatory and inhibitory control over gastric and intestinal tone and motility. Although GI functions are controlled by the autonomic nervous system and occur, by and large, independently of conscious perception, it is clear that the higher CNS centers influence homeostatic control as well as cognitive and behavioral functions. This review will describe the basic neural circuitry of extrinsic inputs to the GI tract as well as the major CNS nuclei that innervate and modulate the activity of these pathways. The role of CNS-centered reflexes in the regulation of GI functions will be discussed as will modulation of these reflexes under both physiological and pathophysiological conditions. Finally, future directions within the field will be discussed in terms of important questions that remain to be resolved and advances in technology that may help provide these answers.

Neuroanatomy goes viral!

The nervous system is complex not simply because of the enormous number of neurons it contains but by virtue of the specificity with which they are connected. Unraveling this specificity is the task of neuroanatomy. In this endeavor, neuroanatomists have traditionally exploited an impressive array of tools ranging from the Golgi method to electron microscopy. An ideal method for studying anatomy would label neurons that are interconnected, and, in addition, allow expression of foreign genes in these neurons. Fortuitously, nature has already partially developed such a method in the form of neurotropic viruses, which have evolved to deliver their genetic material between synaptically connected neurons while largely eluding glia and the immune system. While these characteristics make some of these viruses a threat to human health, simple modifications allow them to be used in controlled experimental settings, thus enabling neuroanatomists to trace multi-synaptic connections within and across brain regions. Wild-type neurotropic viruses, such as rabies and alpha-herpes virus, have already contributed greatly to our understanding of brain connectivity, and modern molecular techniques have enabled the construction of recombinant forms of these and other viruses. These newly engineered reagents are particularly useful, as they can target genetically defined populations of neurons, spread only one synapse to either inputs or outputs, and carry instructions by which the targeted neurons can be made to express exogenous proteins, such as calcium sensors or light-sensitive ion channels, that can be used to study neuronal function. In this review, we address these uniquely powerful features of the viruses already in the neuroanatomist's toolbox, as well as the aspects of their biology that currently limit their utility. Based on the latter, we consider strategies for improving viral tracing methods by reducing toxicity, improving control of transsynaptic spread, and extending the range of species that can be studied.

CXCL12-Producing Vascular Endothelial Niches Control Acute T Cell Leukemia Maintenance

The role of the microenvironment in T cell acute lymphoblastic leukemia (T-ALL), or any acute leukemia, is poorly understood. Here we demonstrate that T-ALL cells are in direct, stable contact with CXCL12-producing bone marrow stroma. Cxcl12 deletion from vascular endothelial, but not perivascular, cells impeded tumor growth, suggesting a vascular niche for T-ALL. Moreover, genetic targeting of Cxcr4 in murine T-ALL after disease onset led to rapid, sustained disease remission, and CXCR4 antagonism suppressed human T-ALL in primary xenografts. Loss of CXCR4 targeted key T-ALL regulators, including the MYC pathway, and decreased leukemia initiating cell activity in vivo. Our data identify a T-ALL niche and suggest targeting CXCL12/CXCR4 signaling as a powerful therapeutic approach for T-ALL.

The neurobiological drive for overeating implicated in Prader-Willi syndrome

Prader-Willi syndrome (PWS) is a genetic imprinting disorder characterized mainly by hyperphagia and early childhood obesity. Previous fMRI studies examined the activation of eating-related neural circuits in PWS patients with or without exposures to food cues and found an excessive eating motivation and a reduced inhibitory control of cognitive processing of food. However, the effective connectivity between various brain areas or neural circuitry critically implicated in both the biological and behavioral control of overeating in PWS is largely unexplored. The current study combined resting-state fMRI and Granger causality analysis (GCA) techniques to investigate interactive causal influences among key neural pathways underlying overeating in PWS. We first defined the regions of interest (ROIs) that demonstrated significant alterations of the baseline brain activity levels in children with PWS (n = 21) as compared to that of their normal siblings controls (n = 18), and then carried out GCA to characterize the region-to-region interactions among these ROIs. Our data revealed significantly enhanced causal influences from the amygdala to the hypothalamus and from both the medial prefrontal cortex and anterior cingulate cortex to the amygdala in patients with PWS (P < 0.001). These alterations offer new explanations for hypothalamic regulation of homeostatic energy intake and impairment in inhibitory control circuit. The deficits in these dual aspects may jointly contribute to the extreme hyperphagia in PWS. This study provides both a new methodological and a neurobiological perspective to aid in a better understanding of neural mechanisms underlying obesity in the general public. This article is part of a Special Issue entitled 1618.

Distinct brainstem and forebrain circuits receiving tracheal sensory neuron inputs revealed using a novel conditional anterograde transsynaptic viral tracing system

Sensory nerves innervating the mucosa of the airways monitor the local environment for the presence of irritant stimuli and, when activated, provide input to the nucleus of the solitary tract (Sol) and paratrigeminal nucleus (Pa5) in the medulla to drive a variety of protective behaviors. Accompanying these behaviors are perceivable sensations that, particularly for stimuli in the proximal end of the airways, can be discrete and localizable. Airway sensations likely reflect the ascending airway sensory circuitry relayed via the Sol and Pa5, which terminates broadly throughout the CNS. However, the relative contribution of the Sol and Pa5 to these ascending pathways is not known. In the present study, we developed and characterized a novel conditional anterograde transneuronal viral tracing system based on the H129 strain of herpes simplex virus 1 and used this system in rats along with conventional neuroanatomical tracing with cholera toxin B to identify subcircuits in the brainstem and forebrain that are in receipt of relayed airway sensory inputs via the Sol and Pa5. We show that both the Pa5 and proximal airways disproportionately receive afferent terminals arising from the jugular (rather than nodose) vagal ganglia and the output of the Pa5 is predominately directed toward the ventrobasal thalamus. We propose the existence of a somatosensory-like pathway from the proximal airways involving jugular ganglia afferents, the Pa5, and the somatosensory thalamus and suggest that this pathway forms the anatomical framework for sensations arising from the proximal airway mucosa.

Vesicular stomatitis virus enables gene transfer and transsynaptic tracing in a wide range of organisms

Current limitations in technology have prevented an extensive analysis of the connections among neurons, particularly within nonmammalian organisms. We developed a transsynaptic viral tracer originally for use in mice, and then tested its utility in a broader range of organisms. By engineering the vesicular stomatitis virus (VSV) to encode a fluorophore and either the rabies virus glycoprotein (RABV‐G) or its own glycoprotein (VSV‐G), we created viruses that can transsynaptically label neuronal circuits in either the retrograde or anterograde direction, respectively. The vectors were investigated for their utility as polysynaptic tracers of chicken and zebrafish visual pathways. They showed patterns of connectivity consistent with previously characterized visual system connections, and revealed several potentially novel connections. Further, these vectors were shown to infect neurons in several other vertebrates, including Old and New World monkeys, seahorses, axolotls, and Xenopus. They were also shown to infect two invertebrates, Drosophila melanogaster, and the box jellyfish, Tripedalia cystophora, a species previously intractable for gene transfer, although no clear evidence of transsynaptic spread was observed in these species. These vectors provide a starting point for transsynaptic tracing in most vertebrates, and are also excellent candidates for gene transfer in organisms that have been refractory to other methods. J. Comp. Neurol. 523:1639–1663, 2015. © 2015 Wiley Periodicals, Inc.

Dentate gyrus-CA3 glutamate release/NMDA transmission mediates behavioral despair and antidepressant-like responses to leptin

Compelling evidence supports the important role of the glutamatergic system in the pathophysiology of major depression and also as a target for rapid-acting antidepressants. However, the functional role of glutamate release/transmission in behavioral processes related to depression and antidepressant efficacy remains to be elucidated. In this study, glutamate release and behavioral responses to tail suspension, a procedure commonly used for inducing behavioral despair, were simultaneously monitored in real time. The onset of tail suspension stress evoked a rapid increase in glutamate release in hippocampal field CA3, which declined gradually after its offset. Blockade of N-methyl-D-aspartic acid (NMDA) receptors by intra-CA3 infusion of MK-801, a non-competitive NMDA receptor antagonist, reversed behavioral despair. A subpopulation of granule neurons that innervated the CA3 region expressed leptin receptors and these cells were not activated by stress. Leptin treatment dampened tail suspension-evoked glutamate release in CA3. On the other hand, intra-CA3 infusion of NMDA blocked the antidepressant-like effect of leptin in reversing behavioral despair in both the tail suspension and forced swim tests, which involved activation of Akt signaling in DG. Taken together, these results suggest that the DG-CA3 glutamatergic pathway is critical for mediating behavioral despair and antidepressant-like responses to leptin.

Hematopoietic stem and progenitor cells regulate the regeneration of their niche by secreting Angiopoietin-1

Hematopoietic stem cells (HSCs) are maintained by a perivascular niche in bone marrow but it is unclear whether the niche is reciprocally regulated by HSCs. Here, we systematically assessed the expression and function of Angiopoietin-1 (Angpt1) in bone marrow. Angpt1 was not expressed by osteoblasts. Angpt1 was most highly expressed by HSCs, and at lower levels by c-kit⁺ hematopoietic progenitors, megakaryocytes, and Leptin Receptor⁺ (LepR⁺) stromal cells. Global conditional deletion of Angpt1, or deletion from osteoblasts, LepR⁺ cells, Nes-cre-expressing cells, megakaryocytes, endothelial cells or hematopoietic cells in normal mice did not affect hematopoiesis, HSC maintenance, or HSC quiescence. Deletion of Angpt1 from hematopoietic cells and LepR⁺ cells had little effect on vasculature or HSC frequency under steady-state conditions but accelerated vascular and hematopoietic recovery after irradiation while increasing vascular leakiness. Hematopoietic stem/progenitor cells and LepR⁺ stromal cells regulate niche regeneration by secreting Angpt1, reducing vascular leakiness but slowing niche recovery.

DOI:http://dx.doi.org/10.7554/eLife.05521.001

In adults, blood cells develop from a set of stem cells that are found in bone marrow. There are also specialized blood vessels and cells called ‘stromal cells’ within the bone marrow that provide these stem cells with oxygen, nutrients, and other molecules. This local environment, or ‘niche’, plays an important role in regulating the maintenance of these stem cells. But it has not been known whether stem cells can reciprocally regulate their niches.

Unfortunately, radiation used to treat cancer obliterates the stem cells and their niche; both must recover after such a treatment before the patient can produce blood cells normally again. A protein called Angpt1 is thought to play a role in this post-treatment recovery. Angpt1 is known to regulate blood vessels in the bone marrow, and one influential study had previously suggested that bone cells produce Angpt1, which promotes and regulates the maintenance of the stem cells within the niche. However, this previous study did not directly test this. Thus, it was not clear whether Angpt1 promotes the regeneration of the stem cells themselves or if it regulates the rebuilding of the niche.

Now, Zhou, Ding and Morrison have genetically engineered mice to make a ‘reporter’ molecule—which glows green when viewed under a microscope—wherever and whenever the gene for Angpt1 is active. These experiments showed where the protein is produced, and unexpectedly revealed that the bone cells do not make Angpt1. Instead, it is the stem cells and the stromal cells in the niche that made the protein. Further experiments showed that deleting the gene for Angpt1 from mice, or just from their bone cells, did not affect blood cell production; nor did it affect the maintenance or regulation of the stem cells.

Next, Zhou, Ding and Morrison looked at whether Angpt1 might be involved in rebuilding the niche after being exposed to radiation. Some of these irradiated mice had been genetically engineered to lack Angpt1; and, in these mice, blood stem cells and blood cell production recovered more quickly than in mice with Angpt1. The blood vessels in the niche also grew back more quickly in the irradiated mice that lacked Angpt1. However, these regenerated blood vessels were leaky. This suggests that blood stem cells produce Angpt1 to slow the recovery of the niche and reduce leakage from the blood vessels. Thus, blood stem cells can regulate the regeneration of the niches that maintain them.

DOI:http://dx.doi.org/10.7554/eLife.05521.002

Challenges in retinal circuit regeneration: linking neuronal connectivity to circuit function

Tremendous progress has been made in retinal regeneration, as exemplified by successful transplantation of retinal pigment epithelia and photoreceptor cells in the adult retina, as well as by generation of retinal tissue from embryonic stem cells and induced pluripotent cells. However, it remains unknown how new photoreceptors integrate within retinal circuits and contribute to vision restoration. There is a large gap in our understanding, at both the cellular and behavioral levels, of the functional roles of new neurons in the adult retina. This gap largely arises from the lack of appropriate methods for analyzing the organization and function of new neurons at the circuit level. To bridge this gap and understand the functional roles of new neurons in living animals, it will be necessary to identify newly formed connections, correlate them with function, manipulate their activity, and assess the behavioral outcome of these manipulations. Recombinant viral vectors are powerful tools not only for controlling gene expression and reprogramming cells, but also for tracing cell fates and neuronal connectivity, monitoring biological functions, and manipulating the physiological state of a specific cell population. These virus-based approaches, combined with electrophysiology and optical imaging, will provide circuit-level insight into neural regeneration and facilitate new strategies for achieving vision restoration in the adult retina. Herein, we discuss challenges and future directions in retinal regeneration research.

Protein tyrosine phosphatase-1B contributes to LPS-induced leptin resistance in male rats

Leptin resistance is induced by the feedback inhibitors tyrosine phosphatase-1B (PTP1B) and decreased Src homology 2 domain-containing tyrosine phosphatase-2 (SHP-2) signaling. To investigate the participation of PTP1B and SHP-2 in LPS-induced leptin resistance, we injected repeated (6-LPS) intraperitoneal LPS doses (100 μg/kg ip) for comparison with a single (1-LPS) treatment and evaluated the expression of SHP-2, PTP1B, p-ERK1/2, and p-STAT3 in the hypothalamus of male Wistar rats. The single LPS treatment increased the expression of p-STAT3 and PTP1B but not SHP-2. The repeated LPS treatment reduced SHP-2, increased PTP1B, and did not change p-STAT3. We observed that the PTP1B expression induced by the endotoxin was highly colocalized with leptin receptor cells in the hypothalamus of LepRb-IRES-Cre-tdTomato reporter mice. The single, but not the repeated, LPS treatment decreased the food intake and body weight. Leptin had no stimulatory effect on the hypophagia, body weight loss, or pSTAT3 expression in 6-LPS rats, indicating leptin unresponsiveness. Notably, the PTP1B inhibitor (3.0 nmol/rat in 5 μl icv) restored the LPS-induced hypophagia in 6-LPS rats and restored the ability of leptin to reduce food intake and body weight as well as to phosphorylate STAT3 in the arcuate, paraventricular, and ventromedial nuclei of the hypothalamus. The present data suggest that an increased PTP1B expression in the hypothalamus underlies the development of leptin resistance during repeated exposure to LPS. Our findings contribute to understanding the mechanisms involved in leptin resistance during low-grade inflammation as seen in obesity.

Conditional genetic transsynaptic tracing in the embryonic mouse brain

Anatomical path tracing is of pivotal importance to decipher the relationship between brain and behavior. Unraveling the formation of neural circuits during embryonic maturation of the brain however is technically challenging because most transsynaptic tracing methods developed to date depend on stereotaxic tracer injection. To overcome this problem, we developed a binary genetic strategy for conditional genetic transsynaptic tracing in the mouse brain. Towards this end we generated two complementary knock-in mouse strains to selectively express the bidirectional transsynaptic tracer barley lectin (BL) and the retrograde transsynaptic tracer Tetanus Toxin fragment C from the ROSA26 locus after Cre-mediated recombination. Cell-specific tracer production in these mice is genetically encoded and does not depend on mechanical tracer injection. Therefore our experimental approach is suitable to study neural circuit formation in the embryonic murine brain. Furthermore, because tracer transfer across synapses depends on synaptic activity, these mouse strains can be used to analyze the communication between genetically defined neuronal populations during brain development at a single cell resolution. Here we provide a detailed protocol for transsynaptic tracing in mouse embryos using the novel recombinant ROSA26 alleles. We have utilized this experimental technique in order to delineate the neural circuitry underlying maturation of the reproductive axis in the developing female mouse brain.

Long-term Cre-mediated retrograde tagging of neurons using a novel recombinant pseudorabies virus

Brain regions contain diverse populations of neurons that project to different long-range targets. The study of these subpopulations in circuit function and behavior requires a toolkit to characterize and manipulate their activity in vivo. We have developed a novel set of reagents based on Pseudorabies Virus (PRV) for efficient and long-term genetic tagging of neurons based on their projection targets. By deleting IE180, the master transcriptional regulator in the PRV genome, we have produced a mutant virus capable of infection and transgene expression in neurons but unable to replicate in or spread from those neurons. IE180-null mutants showed no cytotoxicity, and infected neurons exhibited normal physiological function more than 45 days after infection, indicating the utility of these engineered viruses for chronic experiments. To enable rapid and convenient construction of novel IE180-null recombinants, we engineered a bacterial artificial chromosome (BAC) shuttle-vector system for moving new constructs into the PRV IE180-null genome. Using this system we generated an IE180-null recombinant virus expressing the site-specific recombinase Cre. This Cre-expressing virus (PRV-hSyn-Cre) efficiently and robustly infects neurons in vivo and activates transgene expression from Cre-dependent vectors in local and retrograde projecting populations of neurons in the mouse. We also generated an assortment of recombinant viruses expressing fluorescent proteins (mCherry, EGFP, ECFP). These viruses exhibit long-term labeling of neurons in vitro but transient labeling in vivo. Together these novel IE180-null PRV reagents expand the toolkit for targeted gene expression in the brain, facilitating functional dissection of neuronal circuits in vivo.

A fourth generation of neuroanatomical tracing techniques: exploiting the offspring of genetic engineering

The first three generations of neuroanatomical tract-tracing methods include, respectively, techniques exploiting degeneration, retrograde cellular transport and anterograde cellular transport. This paper reviews the most recent development in third-generation tracing, i.e., neurochemical fingerprinting based on BDA tracing, and continues with an emerging tracing technique called here 'selective fluorescent protein expression' that in our view belongs to an entirely new 'fourth-generation' class. Tracing techniques in this class lean on gene expression technology designed to 'label' projections exclusively originating from neurons expressing a very specific molecular phenotype. Genetically engineered mice that express cre-recombinase in a neurochemically specific neuronal population receive into a brain locus of interest an injection of an adeno-associated virus (AAV) carrying a double-floxed promoter-eYFP DNA sequence. After transfection this sequence is expressed only in neurons metabolizing recombinase protein. These particular neurons promptly start manufacturing the fluorescent protein which then accumulates and labels to full detail all the neuronal processes, including fibers and terminal arborizations. All other neurons remain optically 'dark'. The AAV is not replicated by the neurons, prohibiting intracerebral spread of 'infection'. The essence is that the fiber projections of discrete subpopulations of neurochemically specific neurons can be traced in full detail. One condition is that the transgenic mouse strain is recombinase-perfect. We illustrate selective fluorescent protein expression in parvalbumin-cre (PV-cre) mice and choline acetyltransferase-cre (ChAT-cre) mice. In addition we compare this novel tracing technique with observations in brains of native PV mice and ChAT-GFP mice. We include a note on tracing techniques using viruses.

The neuroanatomical function of leptin in the hypothalamus

The anorexigenic hormone leptin plays an important role in the control of food intake and feeding-related behavior, for an important part through its action in the hypothalamus. The adipose-derived hormone modulates a complex network of several intercommunicating orexigenic and anorexigenic neuropeptides in the hypothalamus to reduce food intake and increase energy expenditure. In this review we present an updated overview of the functional role of leptin in respect to feeding and feeding-related behavior per distinct hypothalamic nuclei. In addition to the arcuate nucleus, which is a major leptin sensitive hub, leptin-responsive neurons in other hypothalamic nuclei, including the, dorsomedial-, ventromedial- and paraventricular nucleus and the lateral hypothalamic area, are direct targets of leptin. However, leptin also modulates hypothalamic neurons in an indirect manner, such as via the melanocortin system. The dissection of the complexity of leptin's action on the networks involved in energy balance is subject of recent and future studies. A full understanding of the role of hypothalamic leptin in the regulation of energy balance requires cell-specific manipulation using of conditional deletion and expression of leptin receptors. In addition, optogenetic and pharmacogenetic tools in combination with other pharmacological (such as the recent discovery of a leptin receptor antagonist) and neuronal tracing techniques to map the circuit, will be helpful to understand the role of leptin receptor expressing neurons. Better understanding of these circuits and the involvement of leptin could provide potential sites for therapeutic interventions in obesity and metabolic diseases characterized by dysregulation of energy balance.

Transneuronal circuit analysis with pseudorabies viruses

Our ability to understand the function of the nervous system is dependent upon defining the connections of its constituent neurons. Development of methods to define connections within neural networks has always been a growth industry in the neurosciences. Transneuronal spread of neurotropic viruses currently represents the best means of defining synaptic connections within neural networks. The method exploits the ability of viruses to invade neurons, replicate, and spread through the intimate synaptic connections that enable communication among neurons. Since the method was first introduced in the 1970s, it has benefited from an increased understanding of the virus life cycle, the function of viral genome, and the ability to manipulate the viral genome in support of directional spread of virus and the expression of transgenes. In this unit, we review these advances in viral tracing technology and the way in which they may be applied for functional dissection of neural networks.

Leptin-receptor-expressing mesenchymal stromal cells represent the main source of bone formed by adult bone marrow

Studies of the identity and physiological function of mesenchymal stromal cells (MSCs) have been hampered by a lack of markers that permit both prospective identification and fate mapping in vivo. We found that Leptin Receptor (LepR) is a marker that highly enriches bone marrow MSCs. Approximately 0.3% of bone marrow cells were LepR(+), 10% of which were CFU-Fs, accounting for 94% of bone marrow CFU-Fs. LepR(+) cells formed bone, cartilage, and adipocytes in culture and upon transplantation in vivo. LepR(+) cells were Scf-GFP(+), Cxcl12-DsRed(high), and Nestin-GFP(low), markers which also highly enriched CFU-Fs, but negative for Nestin-CreER and NG2-CreER, markers which were unlikely to be found in CFU-Fs. Fate-mapping showed that LepR(+) cells arose postnatally and gave rise to most bone and adipocytes formed in adult bone marrow, including bone regenerated after irradiation or fracture. LepR(+) cells were quiescent, but they proliferated after injury. Therefore, LepR(+) cells are the major source of bone and adipocytes in adult bone marrow.

The neuroinvasive profiles of H129 (herpes simplex virus type 1) recombinants with putative anterograde-only transneuronal spread properties

The use of viruses as transneuronal tracers has become an increasingly powerful technique for defining the synaptic organization of neural networks. Although a number of recombinant alpha herpesviruses are known to spread selectively in the retrograde direction through neural circuits only one strain, the H129 strain of herpes simplex virus type 1, is reported to selectively spread in the anterograde direction. However, it is unclear from the literature whether there is an absolute block or an attenuation of retrograde spread of H129. Here, we demonstrate efficient anterograde spread, and temporally delayed retrograde spread, of H129 and three novel recombinants. In vitro studies revealed no differences in anterograde and retrograde spread of parental H129 and its recombinants through superior cervical ganglion neurons. In vivo injections of rat striatum revealed a clear bias of anterograde spread, although evidence of deficient retrograde transport was also present. Evidence of temporally delayed retrograde transneuronal spread of H129 in the retina was observed following injection of the lateral geniculate nucleus. The data also demonstrated that three novel recombinants efficiently express unique fluorescent reporters and have the capacity to infect the same neurons in dual infection paradigms. From these experiments we conclude that H129 and its recombinants not only efficiently infect neurons through anterograde transneuronal passage, but also are capable of temporally delayed retrograde transneuronal spread. In addition, the capacity to produce dual infection of projection targets following anterograde transneuronal passage provides an important addition to viral transneuronal tracing technology.

SLAM family markers resolve functionally distinct subpopulations of hematopoietic stem cells and multipotent progenitors

Hematopoietic stem cells (HSCs) and multipotent hematopoietic progenitors (MPPs) are routinely isolated using various markers but remain heterogeneous. Here we show that four SLAM family markers, CD150, CD48, CD229, and CD244, can distinguish HSCs and MPPs from restricted progenitors and subdivide them into a hierarchy of functionally distinct subpopulations with stepwise changes in cell-cycle status, self-renewal, and reconstituting potential. CD229 expression largely distinguished lymphoid-biased HSCs from rarely dividing myeloid-biased HSCs, enabling prospective enrichment of these HSC subsets. Differences in CD229 and CD244 expression resolved CD150(-)CD48(-/low)Lineage(-/low)Sca-1(+)c-Kit(+) cells into a hierarchy of highly purified MPPs that retained erythroid and platelet potential but exhibited progressive changes in mitotic activity and reconstituting potential. Use of these markers, and reconstitution assays, showed that conditional deletion of Scf from endothelial cells and perivascular stromal cells eliminated the vast majority of bone marrow HSCs, including nearly all CD229(-/low) HSCs, demonstrating that quiescent HSCs are maintained by a perivascular niche.

Controlling feeding behavior by chemical or gene-directed targeting in the brain: what's so spatial about our methods?

Intracranial chemical injection (ICI) methods have been used to identify the locations in the brain where feeding behavior can be controlled acutely. Scientists conducting ICI studies often document their injection site locations, thereby leaving kernels of valuable location data for others to use to further characterize feeding control circuits. Unfortunately, this rich dataset has not yet been formally contextualized with other published neuroanatomical data. In particular, axonal tracing studies have delineated several neural circuits originating in the same areas where ICI injection feeding-control sites have been documented, but it remains unclear whether these circuits participate in feeding control. Comparing injection sites with other types of location data would require careful anatomical registration between the datasets. Here, a conceptual framework is presented for how such anatomical registration efforts can be performed. For example, by using a simple atlas alignment tool, a hypothalamic locus sensitive to the orexigenic effects of neuropeptide Y (NPY) can be aligned accurately with the locations of neurons labeled by anterograde tracers or those known to express NPY receptors or feeding-related peptides. This approach can also be applied to those intracranial "gene-directed" injection (IGI) methods (e.g., site-specific recombinase methods, RNA expression or interference, optogenetics, and pharmacosynthetics) that involve viral injections to targeted neuronal populations. Spatial alignment efforts can be accelerated if location data from ICI/IGI methods are mapped to stereotaxic brain atlases to allow powerful neuroinformatics tools to overlay different types of data in the same reference space. Atlas-based mapping will be critical for community-based sharing of location data for feeding control circuits, and will accelerate our understanding of structure-function relationships in the brain for mammalian models of obesity and metabolic disorders.

Projections of nucleus accumbens adenosine A2A receptor neurons in the mouse brain and their implications in mediating sleep-wake regulation

Adenosine A_2A receptors (A_2ARs) in the nucleus accumbens (Acb) have been demonstrated to play an important role in the arousal effect of adenosine receptor antagonist caffeine, and may be involved in physiological sleep. To better understand the functions of these receptors in sleep, projections of A_2AR neurons were mapped utilizing adeno-associated virus (AAV) encoding humanized Renilla green fluorescent protein (hrGFP) as a tracer for long axonal pathways. The Cre-dependent AAV was injected into the core (AcbC) and shell (AcbSh) of the Acb in A_2AR-Cre mice. Immunohistochemistry was then used to visualize hrGFP, highlighting the perikarya of the A_2AR neurons in the injection sites, and their axons in projection regions. The data revealed that A_2AR neurons exhibit medium-sized and either round or elliptic perikarya with their processes within the Acb. Moreover, the projections from the Acb distributed to nuclei in the forebrain, diencephalon, and brainstem. In the forebrain, A_2AR neurons from all Acb sub-regions jointly projected to the ventral pallidum, the nucleus of the diagonal band, and the substantia innominata. Heavy projections from the AcbC and the ventral AcbSh, and weaker projections from the medial AcbSh, were observed in the lateral hypothalamus and lateral preoptic area. In the brainstem, the Acb projections were found in the ventral tegmental area, while AcbC and ventral AcbSh also projected to the median raphe nucleus, the dorsal raphe nucleus, and the ventrolateral periaqueductal gray. The results supply a solid base for understanding the roles of the A_2AR and A_2AR neurons in the Acb, especially in the regulation of sleep.

A critical view of the use of genetic tools to unveil neural circuits: the case of leptin action in reproduction

The remarkable development and refinement of the Cre-loxP system coupled with the nonstop production of new mouse models and virus vectors have impelled the growth of various fields of investigation. In this article, I will discuss the data collected using these genetic tools in our area of interest, giving specific emphasis to the identification of the neuronal populations that relay leptin action in reproductive physiology. A series of mouse models that allow manipulation of the leptin receptor gene have been generated. Of those, I will discuss the use of two models of leptin receptor gene reexpression (LepR(neo/neo) and LepR(loxTB/loxTB)) and one model of leptin signaling blockade (LepR(flox/flox)). I will also highlight the differences of using stereotaxic delivery of virus vectors expressing DNA-recombinases (Flp and Cre) and mouse models expressing Cre-recombinase. Our findings indicate that leptin action in the ventral premammillary nucleus is sufficient, but not required, for leptin action in reproduction and that leptin action in Kiss1 neurons arises after pubertal maturation; therefore, direct leptin signaling in Kiss1 neurons is neither required nor sufficient for the permissive action of leptin in pubertal development. It also became evident that the full action of leptin in the reproductive neuroendocrine axis requires the engagement of an integrated circuitry, yet to be fully unveiled.

Defining functional gene-circuit interfaces in the mouse nervous system

Complexity in the nervous system is established by developmental genetic programs, maintained by differential genetic profiles and sculpted by experiential and environmental influence over gene expression. Determining how specific genes define neuronal phenotypes, shape circuit connectivity and regulate circuit function is essential for understanding how the brain processes information, directs behavior and adapts to changing environments. Mouse genetics has contributed greatly to current percepts of gene-circuit interfaces in behavior, but considerable work remains. Large-scale initiatives to map gene expression and connectivity in the brain, together with advanced techniques in molecular genetics, now allow detailed exploration of the genetic basis of nervous system function at the level of specific circuit connections. In this review, we highlight several key advances for defining the function of specific genes within a neural network.

Neither signal transducer and activator of transcription 3 (STAT3) or STAT5 signaling pathways are required for leptin's effects on fertility in mice

The hormone leptin is critical for the regulation of energy balance and fertility. The long-form leptin receptor (LepR) regulates multiple intracellular signaling cascades, including the classic Janus kinase-signal transducer and activator of transcription (STAT) pathways. Previous studies have shown that deletion of STAT3 or the closely related STAT5 from the brain results in an obese phenotype, but their roles in fertility regulation are not clear. This study tested whether STAT3 and STAT5 pathways of leptin signaling are required for fertility, and whether absence of one pathway might be compensated for by the other in a redundant manner. A Cre-loxP approach was used to generate 3 models of male and female transgenic mice with LepR-specific deletion of STAT3, STAT5, or both STAT3 and STAT5. Body weight, puberty onset, estrous cyclicity, and fertility were measured in all knockout (KO) mice and their control littermates. Knocking out STAT3 or both STAT3 and 5 from LepR expressing cells, but not STAT5 alone, led to significant increase in body weight. All STAT3 and STAT5 single KO mice exhibited normal puberty onset and subsequent fertility compared to their control littermates. Surprisingly, all STAT3 and STAT5 double KO mice also exhibited normal puberty onset, estrous cyclicity, and fertility, although they had severely disrupted body weight regulation. These results suggest that, although STAT3 signaling is crucial for body weight regulation, neither STAT3 nor STAT5 is required for the regulation of fertility by leptin. It remains to be determined what other signaling molecules mediate this effect of leptin, and whether they interact in a redundant manner.

Connections of the superior colliculus to shoulder muscles of the rat: a dual tracing study

Previous investigations indicate that the superior colliculus (SC) is involved in the initiation and execution of forelimb movements. In the present study we investigated the tectofugal, in particular the tecto-reticulo-spinal projections to the shoulder and arm muscles in the rat. We simultaneously retrogradely labeled the premotor neurons in the brainstem by injection of the pseudorabies virus PrV Bartha 614 into the m. rhomboideus minor and m. acromiodeltoideus, and anterogradely visualized the tectofugal projections by intracollicular injection of the tracer FITC dextrane. Our results demonstrate that the connection of the SC to the skeletal muscles of the forelimb is at least trisynaptic. This was confirmed by long survival times after virus injections into the muscles (98-101 h) after which numerous neurons in the deep layers of the SC were labeled. Transsynaptically retrogradely labeled brainstem neurons connected disynaptically to the injected muscles with adjacent tectal terminals were predominantly located in the gigantocellular nuclear complex of the reticular formation. In addition, putative relay neurons were found in the caudal part of the pontine reticular nucleus. Both tectal projections to the nucleus gigantocellularis and the pontine reticular nucleus were bilateral but ipsilaterally biased. We suggest this projection to be involved in more global functions in motivated behavior like general arousal allowing fast voluntary motor activity.

Hypothalamic survival circuits: blueprints for purposive behaviors

Neural processes that direct an animal's actions toward environmental goals are critical elements for understanding behavior. The hypothalamus is closely associated with motivated behaviors required for survival and reproduction. Intense feeding, drinking, aggressive, and sexual behaviors can be produced by a simple neuronal stimulus applied to discrete hypothalamic regions. What can these "evoked behaviors" teach us about the neural processes that determine behavioral intent and intensity? Small populations of neurons sufficient to evoke a complex motivated behavior may be used as entry points to identify circuits that energize and direct behavior to specific goals. Here, I review recent applications of molecular genetic, optogenetic, and pharmacogenetic approaches that overcome previous limitations for analyzing anatomically complex hypothalamic circuits and their interactions with the rest of the brain. These new tools have the potential to bridge the gaps between neurobiological and psychological thinking about the mechanisms of complex motivated behavior.

Haematopoietic stem cells and early lymphoid progenitors occupy distinct bone marrow niches

Although haematopoietic stem cells (HSCs) are commonly assumed to reside within a specialized microenvironment, or niche, most published experimental manipulations of the HSC niche have affected the function of diverse restricted progenitors. This raises the fundamental question of whether HSCs and restricted progenitors reside within distinct, specialized niches or whether they share a common niche. Here we assess the physiological sources of the chemokine CXCL12 for HSC and restricted progenitor maintenance. Cxcl12(DsRed) knock-in mice (DsRed-Express2 recombined into the Cxcl12 locus) showed that Cxcl12 was primarily expressed by perivascular stromal cells and, at lower levels, by endothelial cells, osteoblasts and some haematopoietic cells. Conditional deletion of Cxcl12 from haematopoietic cells or nestin-cre-expressing cells had little or no effect on HSCs or restricted progenitors. Deletion of Cxcl12 from endothelial cells depleted HSCs but not myeloerythroid or lymphoid progenitors. Deletion of Cxcl12 from perivascular stromal cells depleted HSCs and certain restricted progenitors and mobilized these cells into circulation. Deletion of Cxcl12 from osteoblasts depleted certain early lymphoid progenitors but not HSCs or myeloerythroid progenitors, and did not mobilize these cells into circulation. Different stem and progenitor cells thus reside in distinct cellular niches in bone marrow: HSCs occupy a perivascular niche and early lymphoid progenitors occupy an endosteal niche.

Systemic and local infection routes govern different cellular dissemination pathways during gammaherpesvirus infection in vivo

Human gammaherpesviruses cause morbidity and mortality associated with infection and transformation of lymphoid and endothelial cells. Knowledge of cell types involved in virus dissemination from primary virus entry to virus latency is fundamental for the understanding of gammaherpesvirus pathogenesis. However, the inability to directly trace cell types with respect to virus dissemination pathways has prevented definitive conclusions regarding the relative contribution of individual cell types. Here, we describe that the route of infection affects gammaherpesvirus dissemination pathways. We constructed a recombinant murine gammaherpesvirus 68 (MHV-68) variant harboring a cassette which switches fluorescent markers in a Cre-dependent manner. Since the recombinant virus which was constructed on the wild-type background was attenuated, in this study we used an M1-deleted version, which infected mice with normal kinetics. Infection of Cre-transgenic mice with this convertible virus was used to estimate the quantitative contribution of defined cell types to virus productivity and dissemination during the acute phase of MHV-68 infection. In systemic infection, we found splenic vascular endothelial cells (EC) among the first and main cells to produce virus. After local infection, the contribution of EC to splenic virus production did not represent such early kinetics. However, at later time points, B cell-derived viruses dominated splenic productivity independently of systemic or local infection. Systemic versus local infection also governed the cell types involved in loading peritoneal exudate cells, leading to latency in F4/80- and CD11b-positive target cells. Systemic infection supported EC-driven dissemination, whereas local infection supported B cell-driven dissemination.

Rapid glutamate release in the mediobasal hypothalamus accompanies feeding and is exaggerated by an obesogenic food

The mediobasal hypothalamus (MBH) plays a central role in the regulation of food intake and energy balance. Although the excitatory neurotransmitter glutamate is implicated in energy balance regulation by the MBH, the hypothesis that feeding elicits local glutamate release remains untested. To test this hypothesis, we employed a glutamate biosensor that measures glutamate concentrations at 1-s intervals in conscious, freely behaving rats. Results indicate that feeding is associated with an increase of MBH glutamate concentration that occurs within 1-2 s of oral contact with a food pellet, and the glutamate response to a palatable high-fat pellet is greatly exaggerated relative to chow. In contrast, glutamate responses were not observed during water ingestion or other observed behaviors. These findings indicate that feeding is associated with rapid release of glutamate in the MBH, that this release is exaggerated with an obesogenic food, and that this response is likely stimulated by orosensory factors.

Model systems for studying kisspeptin signalling: mice and cells

Kisspeptins are a family of overlapping neuropeptides, encoded by the Kiss1 gene, that are required for activation and maintenance of the mammalian reproductive axis. Kisspeptins act within the hypothalamus to stimulate release of gonadotrophic releasing hormone and activation of the pituitary-gonadal axis. Robust model systems are required to dissect the regulatory mechanisms that control Kiss1 neuronal activity and to examine the molecular consequences of kisspeptin signalling. While studies in normal animals have been important in this, transgenic mice with targeted mutations affecting the kisspeptin signalling pathway have played a significant role in extending our understanding of kisspeptin physiology. Knock-out mice recapitulate the reproductive defects associated with mutations in humans and provide an experimentally tractable model system to interrogate regulatory feedback mechanisms. In addition, transgenic mice with cell-specific expression of modulator proteins such as the CRE recombinase or fluorescent reporter proteins such as GFP allow more sophisticated analyses such as cell or gene ablation or electrophysiological profiling. At a less complex level, immortalized cell lines have been useful for studying the role of kisspeptin in cell migration and metastasis and examining the intracellular signalling events associated with kisspeptin signalling.

Expression of mutant huntingtin in leptin receptor-expressing neurons does not control the metabolic and psychiatric phenotype of the BACHD mouse

Metabolic and psychiatric disturbances occur early on in the clinical manifestation of Huntington’s disease (HD), a neurodegenerative disorder caused by an expanded CAG repeat in the huntingtin (HTT) gene. Hypothalamus has emerged as an important site of pathology and alterations in this area and its neuroendocrine circuits may play a role in causing early non-motor symptoms and signs in HD. Leptin is a hormone that controls energy homeostasis by signaling through leptin receptors in the hypothalamus. Disturbed leptin action is implicated in both obesity and depression and altered circulating levels of leptin have been reported in both clinical HD and rodent models of the disease. Pathological leptin signaling may therefore be involved in causing the metabolic and psychiatric disturbances of HD. Here we tested the hypothesis that expression of mutant HTT in leptin receptor carrying neurons plays a role in the development of the non-motor phenotype in the BACHD mouse model. Our results show that inactivation of mutant HTT in leptin receptor-expressing neurons in the BACHD mouse using cross-breeding based on a cre-loxP system did not have an effect on the metabolic phenotype or anxiety-like behavior. The data suggest that mutant HTT disrupts critical hypothalamic pathways by other mechanisms than interfering with intracellular leptin signaling.

Whole-brain mapping of direct inputs to midbrain dopamine neurons

Recent studies indicate that dopamine neurons in the ventral tegmental area (VTA) and substantia nigra pars compacta (SNc) convey distinct signals. To explore this difference, we comprehensively identified each area's monosynaptic inputs using the rabies virus. We show that dopamine neurons in both areas integrate inputs from a more diverse collection of areas than previously thought, including autonomic, motor, and somatosensory areas. SNc and VTA dopamine neurons receive contrasting excitatory inputs: the former from the somatosensory/motor cortex and subthalamic nucleus, which may explain their short-latency responses to salient events; and the latter from the lateral hypothalamus, which may explain their involvement in value coding. We demonstrate that neurons in the striatum that project directly to dopamine neurons form patches in both the dorsal and ventral striatum, whereas those projecting to GABAergic neurons are distributed in the matrix compartment. Neuron-type-specific connectivity lays a foundation for studying how dopamine neurons compute outputs.

Forebrain GABAergic neuron precursors integrate into adult spinal cord and reduce injury-induced neuropathic pain

Neuropathic pain is a chronic debilitating disease characterized by mechanical allodynia and spontaneous pain. Because symptoms are often unresponsive to conventional methods of pain treatment, new therapeutic approaches are essential. Here, we describe a strategy that not only ameliorates symptoms of neuropathic pain but is also potentially disease modifying. We show that transplantation of immature telencephalic GABAergic interneurons from the mouse medial ganglionic eminence (MGE) into the adult mouse spinal cord completely reverses the mechanical hypersensitivity produced by peripheral nerve injury. Underlying this improvement is a remarkable integration of the MGE transplants into the host spinal cord circuitry, in which the transplanted cells make functional connections with both primary afferent and spinal cord neurons. By contrast, MGE transplants were not effective against inflammatory pain. Our findings suggest that MGE-derived GABAergic interneurons overcome the spinal cord hyperexcitability that is a hallmark of nerve injury-induced neuropathic pain.

Anterograde neuronal circuit tracing using a genetically modified herpes simplex virus expressing EGFP

Insights into the anatomical organization of complex neural circuits provide important information about function, and thus tools that facilitate neuroanatomical studies have proved invaluable in neuroscience. Advances in molecular cloning have allowed the production of novel recombinant neuroinvasive viruses for use in transynaptic neural tracing studies. However, the vast majority of these viruses have motility in the retrograde direction only, therefore limiting their use to studies of synaptic input circuitry. Here we describe the construction of an EGFP reporting herpes simplex virus, strain H129, which preferentially moves along synaptically connected neurons in the anterograde direction. In vitro and in vivo characterization studies confirm that the HSV-1 H129-EGFP retains comparable replication and neuroinvasiveness as the wildtype H129 virus. As a proof of principle we confirm anterograde movement of the H129-EGFP along polysynaptic pathways by inoculating the upper airways and tracking time-dependent EGFP expression in previously described ascending sensory pathways. These data confirm a genomic locus for recombining HSV-1 H129 such that normal viral function and replication is maintained. Novel viral recombinants such as HSV-1 H129-EGFP will be useful tools for delineating the central organization of peripheral sensory pathways as well as the synaptic outputs from central neuronal populations.

Radio-wave heating of iron oxide nanoparticles can regulate plasma glucose in mice

Medical applications of nanotechnology typically focus on drug delivery and biosensors. Here, we combine nanotechnology and bioengineering to demonstrate that nanoparticles can be used to remotely regulate protein production in vivo. We decorated a modified temperature-sensitive channel, TRPV1, with antibody-coated iron oxide nanoparticles that are heated in a low-frequency magnetic field. When local temperature rises, TRPV1 gates calcium to stimulate synthesis and release of bioengineered insulin driven by a Ca(2+)-sensitive promoter. Studying tumor xenografts expressing the bioengineered insulin gene, we show that exposure to radio waves stimulates insulin release from the tumors and lowers blood glucose in mice. We further show that cells can be engineered to synthesize genetically encoded ferritin nanoparticles and inducibly release insulin. These approaches provide a platform for using nanotechnology to activate cells.

Loss of neuronal potassium/chloride cotransporter 3 (KCC3) is responsible for the degenerative phenotype in a conditional mouse model of hereditary motor and sensory neuropathy associated with agenesis of the corpus callosum

Disruption of the potassium/chloride cotransporter 3 (KCC3), encoded by the SLC12A6 gene, causes hereditary motor and sensory neuropathy associated with agenesis of the corpus callosum (HMSN/ACC), a neurodevelopmental and neurodegenerative disorder affecting both the peripheral nervous system and CNS. However, the precise role of KCC3 in the maintenance of ion homeostasis in the nervous system and the pathogenic mechanisms leading to HMSN/ACC remain unclear. We established two Slc12a6 Cre/LoxP transgenic mouse lines expressing C-terminal truncated KCC3 in either a neuron-specific or ubiquitous fashion. Our results suggest that neuronal KCC3 expression is crucial for axon volume control. We also demonstrate that the neuropathic features of HMSN/ACC are predominantly due to a neuronal KCC3 deficit, while the auditory impairment is due to loss of non-neuronal KCC3 expression. Furthermore, we demonstrate that KCC3 plays an essential role in inflammatory pain pathways. Finally, we observed hypoplasia of the corpus callosum in both mouse mutants and a marked decrease in axonal tracts serving the auditory cortex in only the general deletion mutant. Together, these results establish KCC3 as an important player in both central and peripheral nervous system maintenance.

Neuronal projections from V1 to V2 in amblyopia

The mechanism of amblyopia in children with congenital cataract is not understood fully, but studies in macaques have shown that geniculate synapses are lost in striate cortex (V1). To search for other projection abnormalities in amblyopia, the pathway from V1 to V2 was examined using a triple-label technique in three animals raised with monocular suture. [(3)H]proline was injected into one eye to label the ocular dominance columns. Cholera toxin B subunit conjugated to gold (CTB-Au) was injected into V2 to label V1 projection neurons. Alternate sections were processed for cytochrome oxidase (CO) and CTB-Au, or dipped for autoradiography. Eight fields of CTB-Au-labeled cells in V1 opposite injection sites were plotted in layers 2/3 or 4B. After thin stripe injection, labeled cells were concentrated in CO patches. Despite column shrinkage, cells in deprived and normal columns were equal in size and density in both layers 2/3 and 4B. After pale or thick stripe injection, labeled cells were concentrated in interpatches. Only 23% of projection neurons originated from deprived columns. This reduction exceeded the degree of column shrinkage, a result explained by the fact that column shrinkage causes disproportionate loss of interpatch territory. These data indicate that early monocular form deprivation does not alter the segregation of patch and interpatch pathways to V2 stripes or cause selective loss or atrophy of V1 projection neurons. The effect of shrinkage of geniculocortical afferents in layer 4C following visual deprivation is not amplified further by attenuation of the amblyopic eye's projections from V1 to V2.

Dendritically targeted Bdnf mRNA is essential for energy balance and response to leptin

Mutations in the Bdnf gene, which produces transcripts with either short or long 3' untranslated regions (3' UTRs), cause human obesity; however, the precise role of brain-derived neurotrophic factor (BDNF) in the regulation of energy balance is unknown. Here we show the relationship between Bdnf mRNA with a long 3' UTR (long 3' UTR Bdnf mRNA), leptin, neuronal activation and body weight. We found that long 3' UTR Bdnf mRNA was enriched in the dendrites of hypothalamic neurons and that insulin and leptin could stimulate its translation in dendrites. Furthermore, mice harboring a truncated long Bdnf 3' UTR developed severe hyperphagic obesity, which was completely reversed by viral expression of long 3' UTR Bdnf mRNA in the hypothalamus. In these mice, the ability of leptin to activate hypothalamic neurons and inhibit food intake was compromised despite normal activation of leptin receptors. These results reveal a novel mechanism linking leptin action to BDNF expression during hypothalamic-mediated regulation of body weight, while also implicating dendritic protein synthesis in this process.

Functional implications of limited leptin receptor and ghrelin receptor coexpression in the brain

The hormones leptin and ghrelin act in apposition to one another in the regulation of body weight homeostasis. Interestingly, both leptin receptor expression and ghrelin receptor expression have been observed within many of the same nuclei of the central nervous system (CNS), suggesting that these hormones may act on a common population of neurons to produce changes in food intake and energy expenditure. In the present study we explored the extent of this putative direct leptin and ghrelin interaction in the CNS and addressed the question of whether a loss of ghrelin signaling would affect sensitivity to leptin. Using histological mapping of leptin receptor and ghrelin receptor expression, we found that cells containing both leptin receptors and ghrelin receptors are mainly located in the medial part of the hypothalamic arcuate nucleus. In contrast, coexpression was much less extensive elsewhere in the brain. To assess the functional consequences of this observed receptor distribution, we explored the effect of ghrelin receptor deletion on leptin sensitivity. In particular, the responses of ad libitum-fed, diet-induced obese and fasted mice to the anorectic actions of leptin were examined. Surprisingly, we found that deletion of the ghrelin receptor did not affect the sensitivity to exogenously administrated leptin. Thus, we conclude that ghrelin and leptin act largely on distinct neuronal populations and that ghrelin receptor deficiency does not affect sensitivity to the anorexigenic and body weight-lowering actions of leptin.

Endothelial and perivascular cells maintain haematopoietic stem cells

Multiple cell types have been proposed to create niches for haematopoietic stem cells (HSCs). However, the expression patterns of HSC maintenance factors have not been systematically studied and no such factor has been conditionally deleted from any candidate niche cell. Thus, the cellular sources of these factors are undetermined. Stem Cell Factor (SCF) is a key niche component that maintains HSCs. Using Scf^gfp knock-in mice we found Scf was primarily expressed by perivascular cells throughout bone marrow. HSC frequency and function were not affected when Scf was conditionally deleted from haematopoietic cells, osteoblasts, Nestin-Cre, or Nestin-CreER-expressing cells. However, HSCs were depleted from bone marrow when Scf was deleted from endothelial cells or Leptin receptor (Lepr)-expressing perivascular stromal cells. Most HSCs were lost when Scf was deleted from both endothelial and Lepr-expressing perivascular cells. HSCs reside in a perivascular niche in which multiple cell types express factors that promote HSC maintenance.

New technologies for imaging synaptic partners

Understanding the brain will require unraveling its synaptic circuitry, but methods that can reliably identify connected neurons are often excruciatingly slow. Although light microscopy can provide much higher throughput, synapses are smaller than the diffraction limit and cannot readily be assigned to particular presynaptic and postsynaptic cells without specialized labeling methods. Here we review the ongoing development of techniques that allow direct imaging of neural networks by specifically marking connected cells or their synapses.

A Cre-dependent, anterograde transsynaptic viral tracer for mapping output pathways of genetically marked neurons

Neurotropic viruses that conditionally infect or replicate in molecularly defined neuronal subpopulations, and then spread transsynaptically, are powerful tools for mapping neural pathways. Genetically targetable retrograde transsynaptic tracer viruses are available to map the inputs to specific neuronal subpopulations, but an analogous tool for mapping synaptic outputs is not yet available. Here we describe a Cre recombinase-dependent, anterograde transneuronal tracer, based on the H129 strain of herpes simplex virus (HSV). Application of this virus to transgenic or knockin mice expressing Cre in peripheral neurons of the olfactory epithelium or the retina reveals widespread, polysynaptic labeling of higher-order neurons in the olfactory and visual systems, respectively. Polysynaptic pathways were also labeled from cerebellar Purkinje cells. In each system, the pattern of labeling was consistent with classical circuit-tracing studies, restricted to neurons, and anterograde specific. These data provide proof-of-principle for a conditional, nondiluting anterograde transsynaptic tracer for mapping synaptic outputs from genetically marked neuronal subpopulations.

The acute effects of leptin require PI3K signaling in the hypothalamic ventral premammillary nucleus

Evidence suggests that the role played by the adipocyte-derived hormone leptin in female reproductive physiology is mediated in part by neurons located within the ventral premammillary nucleus (PMV). Leptin activates PMV neurons; however, the intracellular signaling pathway and channel(s) involved remain undefined. Notably, leptin's excitatory and inhibitory effects within hypothalamic and brainstem nuclei share the intracellular signaling cascade phosphoinositide 3 kinase (PI3K). Therefore, we assessed whether PI3K signaling is required for the acute effect of leptin to alter cellular activity of PMV neurons that express leptin receptors (LepR PMV neurons). Leptin caused a rapid depolarization in the majority of LepR PMV neurons in patch-clamp recordings of hypothalamic slices, while a subset of LepR PMV neurons were hyperpolarized in response to leptin. Data were obtained from both male and female mice and results demonstrate that the acute effect of leptin on LepR PMV neurons was identical for both sexes. Pharmacological inhibition of PI3K prevented the acute leptin-induced change in neuronal activity of LepR PMV neurons, indicating a PI3K-dependent mechanism of leptin action. Similarly, mice with genetically disrupted PI3K signaling in LepR PMV neurons failed to alter cellular activity in response to leptin. Moreover, the leptin-induced depolarization was dependent on a putative TRPC channel. In contrast, the leptin-induced-hyperpolarization required the activation of a putative Katp channel. Collectively, these results suggest that PI3K signaling in LepR PMV neurons is essential for leptin-induced alteration in cellular activity, and these data may suggest a cellular correlate in which leptin contributes to the initiation of reproductive development.