Regulator of G protein signaling-4 controls fatty acid and glucose homeostasis

Circulating free fatty acids are a reflection of the balance between lipogenesis and lipolysis that takes place mainly in adipose tissue. We found that mice deficient for regulator of G protein signaling (RGS)-4 have increased circulating catecholamines, and increased free fatty acids. Consequently, RGS4-/- mice have increased concentration of circulating free fatty acids; abnormally accumulate fatty acids in liver, resulting in liver steatosis; and show a higher degree of glucose intolerance and decreased insulin secretion in pancreas. We show in this study that RGS4 controls adipose tissue lipolysis through regulation of the secretion of catecholamines by adrenal glands. RGS4 controls the balance between adipose tissue lipolysis and lipogenesis, secondary to its role in the regulation of catecholamine secretion by adrenal glands. RGS4 therefore could be a good target for the treatment of metabolic diseases.

Hox paralog group 2 genes control the migration of mouse pontine neurons through slit-robo signaling

The pontine neurons (PN) represent a major source of mossy fiber projections to the cerebellum. During mouse hindbrain development, PN migrate tangentially and sequentially along both the anteroposterior (AP) and dorsoventral (DV) axes. Unlike DV migration, which is controlled by the Netrin-1/Dcc attractive pathway, little is known about the molecular mechanisms guiding PN migration along the AP axis. Here, we show that Hoxa2 and Hoxb2 are required both intrinsically and extrinsically to maintain normal AP migration of subsets of PN, by preventing their premature ventral attraction towards the midline. Moreover, the migration defects observed in Hoxa2 and Hoxb2 mutant mice were phenocopied in compound Robo1;Robo2, Slit1;Slit2, and Robo2;Slit2 knockout animals, indicating that these guidance molecules act downstream of Hox genes to control PN migration. Indeed, using chromatin immunoprecipitation assays, we further demonstrated that Robo2 is a direct target of Hoxa2 in vivo and that maintenance of high Robo and Slit expression levels was impaired in Hoxa2 mutant mice. Lastly, the analysis of Phox2b-deficient mice indicated that the facial motor nucleus is a major Slit signaling source required to prevent premature ventral migration of PN. These findings provide novel insights into the molecular control of neuronal migration from transcription factor to regulation of guidance receptor and ligand expression. Specifically, they address the question of how exposure to multiple guidance cues along the AP and DV axes is regulated at the transcriptional level and in turn translated into stereotyped migratory responses during tangential migration of neurons in the developing mammalian brain.

Aquaporins in the brain: from aqueduct to "multi-duct"

The aquaporin channel family was first considered as a family of water channels, however it is now clear that some of these channels are also permeable to small solutes such glycerol, urea and monocarboxylates. In this review, we will consider AQP4 and AQP9 expressed in the rodent brain. AQP4 is present on astrocytic end-feet in contact with brain vessels and could be involved in ionic homeostasis. However, AQP4 may also be involved in cell adhesion. AQP4 expression is highly modified in several brain disorders and it can play a key role in the cerebral edema formation. However, the exact role of AQP4 in edema formation is still debated. Recently, AQP4 has been shown to be also involved in astrocyte migration during glial scar formation. AQP9 is expressed in astrocytes and in catecholaminergic neurons. Two isoforms of AQP9 are expressed in brain cells, the shortest isoform is localized in the inner membrane of mitochondria and the longest in the cell membrane. The level of expression of AQP9 is negatively regulated by high concentrations of insulin. Taken together, these results suggest that AQP9 could be involved in brain energy metabolism. The induction of AQP9 in astrocytes is observed with time after stroke onset suggesting participation in the clearance of excess lactate in the extracellular space. These recent exciting results suggest that AQPs may not only be involved in water homeostasis in the brain but could also participate in other important physiological functions.

Forced expression of Phox2 homeodomain transcription factors induces a branchio-visceromotor axonal phenotype

What causes motor neurons to project into the periphery is not well understood. We here show that forced expression of the homeodomain protein Phox2b, shown previously to be necessary and sufficient for branchio-visceromotor neuron development, and of its paralogue Phox2a imposes a branchiomotor-like axonal phenotype in the spinal cord. Many Phox2-transfected neurons, whose axons would normally stay within the confines of the neural tube, now project into the periphery. Once outside the neural tube, a fraction of the ectopic axons join the spinal accessory nerve, a branchiomotor nerve which, as shown here, does not develop in the absence of Phox2b. Explant studies show that the axons of Phox2-transfected neurons need attractive cues to leave the neural tube and that their outgrowth is promoted by tissues, to which branchio-visceromotor fibers normally grow. Hence, Phox2 expression is a key step in determining the peripheral axonal phenotype and thus the decision to stay within the neural tube or to project out of it.

Precraniate origin of cranial motoneurons

The craniate head is innervated by cranial sensory and motor neurons. Cranial sensory neurons stem from the neurogenic placodes and neural crest and are seen as evolutionary innovations crucial in fulfilling the feeding and respiratory needs of the craniate "new head." In contrast, cranial motoneurons that are located in the hindbrain and motorize the head have an unclear phylogenetic status. Here we show that these motoneurons are in fact homologous to the motoneurons of the sessile postmetamorphic form of ascidians. The motoneurons of adult Ciona intestinalis, located in the cerebral ganglion and innervating muscles associated with the huge "branchial basket," express the transcription factors CiPhox2 and CiTbx20, whose vertebrate orthologues collectively define cranial motoneurons of the branchiovisceral class. Moreover, Ciona's postmetamorphic motoneurons arise from a hindbrain set aside during larval life and defined as such by its position (caudal to the prosensephalic sensory vesicle) and coexpression of CiPhox2 and CiHox1, whose orthologues collectively mark the vertebrate hindbrain. These data unveil that the postmetamorphic ascidian brain, assumed to be a derived feature, in fact corresponds to the vertebrate hindbrain and push back the evolutionary origin of cranial nerves to before the origin of craniates.

Delays in neuronal differentiation in Mash1/Ascl1 mutants

The inactivation of a developmental transcription factor may lead to the complete absence of a specific cell type. More commonly, though, it only partially impairs its generation. The modalities of this partial effect have rarely been documented in any detail. Here, we report a novel function for the bHLH transcription factor Ascl1/Mash1 in the generation of the nucleus of the solitary tract (nTS). In Mash1(-/-) late embryos, the nTS is markedly atrophic. Tracing back the origin of this atrophy, we show that nTS precursors appear in the mutants 1 day later than in the wild type and then accumulate at a slower pace. We also show that the previously reported atrophy of the sympathetic chain in Mash1 mutants is similarly preceded by a delay of 1 to 2 days in the appearance of differentiated ganglionic cells. Finally, we provide evidence that the acceleration imposed by Mash1, regardless of the production of post-mitotic cells, affects differentiation itself, both generic and type-specific.

Primate adult brain cell autotransplantation, a new tool for brain repair?

If successful, autologous brain cell transplantation is an attractive approach to repair lesions and restore function of the central nervous system. We demonstrate that monkey adult brain cells obtained from cortical biopsy and kept in culture for 4 weeks exhibit neural progenitor characteristics. After reimplantation into a lesion area of the donor cerebral cortex, these cells can successfully survive and acquire neuronal characteristics over time. These results open new perspectives in the field of brain repair and may lead to future clinical applications.

Molecular consequences of PHOX2B missense, frameshift and alanine expansion mutations leading to autonomic dysfunction

Heterozygous mutations of the PHOX2B gene account for a broad variety of disorders of the autonomic nervous system, either isolated or combined, including congenital central hypoventilation syndrome (CCHS), tumours of the sympathetic nervous system and Hirschsprung disease. In CCHS, the prevalent mutation is an expansion of a 20-alanine stretch ranging from +5 to +13 alanines, whereas frameshift and missense mutations are found occasionally. To determine the molecular basis of impaired PHOX2B function, we assayed the transactivation and DNA binding properties of wild-type and mutant PHOX2B proteins. Furthermore, we investigated aggregate formation by proteins with polyalanine tract expansions ranging from +5 to +13 alanines using immunofluorescence of transfected cells and gel filtration of in vitro translated proteins. We found that transactivation of the dopamine beta-hydroxylase promoter by PHOX2B proteins with frameshift and missense mutations was abolished or severely curtailed, as was in vitro DNA binding although the proteins localized to the nucleus. The transactivation potential of proteins with polyalanine tract expansions declined with increasing length of the polyalanine stretch, and DNA binding was affected for an expansion of +9 alanines and above. Cytoplasmic aggregation in transfected cells was only observed for the longest expansions, whereas even the short expansion mutants were prone to form multimers in vitro. Such a tendency to protein misfolding could explain loss of transactivation for alanine expansion mutations. However, additional mechanisms such as toxic gain-of-function may play a role in the pathogenic process.

Generation and characterization of Rgs4 mutant mice

RGS proteins are negative regulators of signaling through heterotrimeric G protein-coupled receptors and, as such, are in a position to regulate a plethora of biological phenomena. However, those have just begun to be explored in vivo. Here, we describe a mouse line deficient for Rgs4, a gene normally expressed early on in discrete populations of differentiating neurons and later on at multiple sites of the central nervous system, the cortex in particular, where it is one of the most highly transcribed Rgs genes. Rgs4(lacZ/lacZ) mice had normal neural development and were viable and fertile. Behavioral testing on mutant adults revealed subtle sensorimotor deficits but, so far, supported neither the proposed status of Rgs4 as a schizophrenia susceptibility gene (by showing intact prepulse inhibition in the mutants) nor (unlike another member of the Rgs family, Rgs9) a role of Rgs4 in the acute or chronic response to opioids.

Haploinsufficiency for Phox2b in mice causes dilated pupils and atrophy of the ciliary ganglion: mechanistic insights into human congenital central hypoventilation syndrome

Dilp1 is a semi-dominant mouse mutation that causes dilated pupils when heterozygous and is lethal when homozygous. We report here that it is caused by a point mutation that introduces a stop codon close to the start of the coding sequence of the paired-like homeobox transcription factor Phox2b. Mice carrying a targeted allele of Phox2b also have dilated pupils and the two alleles do not complement. Phox2b is necessary for the development of the autonomic nervous system and when absent one of the consequences is that all parasympathetic ganglia fail to form. Constriction of the pupil is a parasympathetic response mediated by the ciliary ganglion and we find that in Phox2b heterozygous mutants it is highly atrophic. The development of other parasympathetic and sympathetic ganglia appears to be largely unaffected indicating that the ciliary ganglion is exquisitely sensitive to a reduction in dose of this transcription factor. PHOX2B has been implicated in human disease. Mutations, principally leading to polyalanine expansions within the protein, have been found in patients with congenital central hypoventilation syndrome (CCHS), the cardinal feature of which is an inability to breathe unassisted when asleep. Additionally, some CCHS patients have ocular abnormalities, including pupillary defects, although they principally have constricted rather than dilated pupils. The apparent phenotypic differences observed between mice carrying a loss-of-function mutation of Phox2b and CCHS patients indicate that PHOX2B mutations found in CCHS patients, all of which can produce proteins with intact DNA-binding domains, are gain-of-function mutations that alter rather than abolish protein function.

Ascl1/Mash1 is required for the development of central serotonergic neurons

The transcriptional control of the differentiation of central serotonergic (5-HT) neurons in vertebrates has recently come under scrutiny and has been shown to involve the homeobox genes Nkx2-2 and Lmx1b, the Ets-domain gene Pet1 (also known as Fev) and the zinc-finger gene Gata3. The basic helix-loop-helix (bHLH) gene Ascl1 (also known as Mash1) is coexpressed with Nkx2-2 in the neuroepithelial domain of the hindbrain, which gives rise to 5-HT neurons. Here we show in the mouse that Ascl1 is essential for the birth of 5-HT neurons, both as a proneural gene for the production of postmitotic neuronal precursors and as a determinant of the serotonergic phenotype for the parallel activation of Gata3, Lmx1b and Pet1. Thus Ascl1, which is essential for noradrenergic differentiation, is also a determinant of the serotonergic phenotype.

Early acquisition of typical metabolic features upon differentiation of mouse neural stem cells into astrocytes

Specific metabolic features, such as glutamate reuptake, have been associated with normal functions of mature astrocytes. In this study, we examined whether these characteristics are acquired together with classical phenotypic markers of differentiated astrocytes. Differentiation of E14 mouse neurospheres into astrocytes was induced by the addition of fetal bovine serum (FBS). Degree of differentiation was assessed by reverse transcription-polymerase chain reaction (RT-PCR) and immunofluorescence for both GFAP and nestin. Neural stem cells expressed nestin but not GFAP, while differentiated astrocytes were immunopositive for GFAP but displayed low levels of nestin expression. A strong increase in the expression of the glutamate transporter GLAST and the monocarboxylate transporter MCT1 accompanied phenotypic changes. In addition, active glutamate transport appeared in differentiated astrocytes, as well as their capacity to increase aerobic glycolysis in response to glutamate. Leukemia inhibitory factor (LIF) and ciliary neurotrophic factor, but not interleukin-6, triggered the expression of phenotypic and morphological characteristics of astrocytes. In addition, exposure to LIF led to the appearance of metabolic features typically associated with astrocytes. Altogether, our results show that acquisition of some specific metabolic features by astrocytes occurs early in their differentiation process and that LIF represents a candidate signal to induce their expression.

Phox2bcontrols the development of peripheral chemoreceptors and afferent visceral pathways

We report that the afferent relays of visceral (cardiovascular, digestive and respiratory) reflexes, differentiate under the control of the paired-like homeobox gene Phox2b: the neural crest-derived carotid body, a chemosensor organ, degenerates in homozygous mutants, as do the three epibranchial placode-derived visceral sensory ganglia (geniculate, petrosal and nodose), while their central target, the nucleus of the solitary tract,which integrates all visceral information, never forms. These data establish Phox2b as an unusual `circuit-specific' transcription factor devoted to the formation of autonomic reflex pathways. We also show that Phox2b heterozygous mutants have an altered response to hypoxia and hypercapnia at birth and a decreased tyrosine hydroxylase expression in the petrosal chemosensory neurons, thus providing mechanistic insight into congenital central hypoventilation syndrome, which is associated with heterozygous mutations in PHOX2B.

Dynamic expression of RGS4 in the developing nervous system and regulation by the neural type-specific transcription factor Phox2b

Previous studies have shown that members of the family of regulators of G-protein signaling (RGS), including RGS4, have a discrete expression pattern in the adult brain (Gold et al., 1997). Here, we describe for RGS4 a distinct, mostly transient phase of neuronal expression, during embryonic development: transcription of RGS4 occurs in a highly dynamic manner in a small set of peripheral and central neuronal precursors. This expression pattern overlaps extensively with that of the paired-like homeodomain protein Phox2b, a determinant of neuronal identity. In embryos deficient for Phox2b, RGS4 expression is downregulated in the locus coeruleus, sympathetic ganglia, and cranial motor and sensory neurons. Moreover, Phox2b cooperates with the basic helix-loop-helix protein Mash1 to transiently switch on RGS4 after ectopic expression in the chicken spinal cord. Intriguingly, we also identify a heterotrimeric G-protein alpha-subunit, gustducin, as coexpressed with RGS4 in developing facial motor neurons, also under the control of Phox2b. Altogether, these data identify components of the heterotrimeric G-protein signaling pathway as part of the type-specific program of neuronal differentiation.

Aquaporin 1 and aquaporin 4 expression in human brain after subarachnoid hemorrhage and in peritumoral tissue

Aquaporins (AQPs) are a protein family of water channels which facilitate the water flux through the plasmatic membranes. The expression of AQPs has been described in rat brain by several studies. Despite recent reports that have shown an over-expression of AQP1 and 4 in human tumoral cells, little is known about AQP expression in human brain. The purpose of this study was to investigate the expression of AQP1 and AQP4 in human brain after subarachnoid hemorrhage (SAH) and in peritumoral tissue by western blot and immunohistochemistry. The results showed a marked increase of the expression of AQP1 and AQP4. This over-expression occurred on the astrocytic processes and polarization on astrocytic end-feet was lost. No expression was observed on neuronal cells. This study is the first demonstration of the induction of AQP1 and AQP4 on reactive astrocytes in an acute brain injury, such as SAH. These results reinforce the hypothesis that AQPs may be involved in the dynamics of brain edema formation or resolution. Further studies are needed to understand their functional role.

The role of Phox2b in synchronizing pan-neuronal and type-specific aspects of neurogenesis

Within the developing vertebrate nervous system, specific subclasses of neurons are produced in vastly different numbers at defined times and locations. This implies the concomitant activation of a program that controls pan-neuronal differentiation and of a program that specifies neuronal subtype identity, but how these programs are coordinated in time and space is not well understood. Our previous loss- and gain-of-function studies have defined Phox2b as a homeodomain transcription factor that coordinately regulates generic and type-specific neuronal properties. It is necessary and sufficient to impose differentiation towards a branchio- and viscero-motoneuronal phenotype and at the same time promotes generic neuronal differentiation. We have examined the underlying genetic interactions. We show thatPhox2b has a dual action on pan-neuronal differentiation. It upregulates the expression of proneural genes (Ngn2) when expressed alone and upregulates the expression of Mash1 when expressed in combination with Nkx2.2. By a separate pathway, Phox2brepresses expression of the inhibitors of neurogenesis Hes5 andId2. The role of Phox2b in the specification of neuronal subtype identity appears to depend in part on its capacity to act as a patterning gene in the progenitor domain. Phox2b misexpression represses the Pax6 and Olig2 genes, which should inhibit a branchiomotor fate, and induces Nkx6.1 and Nkx6.2, which are expressed in branchiomotor progenitors. We further show that Phox2b behaves like a transcriptional activator in the promotion of both, generic neuronal differentiation and expression of the motoneuronal marker Islet1. These results provide insights into the mechanisms by which a homeodomain transcription factor through interaction with other factors controls both generic and type-specific features of neuronal differentiation.

Phox2 genes - from patterning to connectivity

In the developing brain, many transcription factors are expressed in complex patterns and dynamics, and drive the differentiation of many classes of neurons. How does the spatio-temporal landscape of transcription factor expression map onto the bewildering variety of neuronal types, and, for each of them, the variety of developmental stages they go through? In other words, what is the logic in the transcriptional control of neuronal differentiation? Here, we review what recent work on the two neuronal-type-specific transcription factors Phox2a and Phox2b has contributed to our understanding of this broad question.

Isolation of multipotent neural precursors residing in the cortex of the adult human brain

Multipotent precursors able to generate neurons, astrocytes, and oligodendrocytes have previously been isolated from human brain embryos and recently from neurogenic regions of the adult human brains. The isolation of multipotent neural precursors from adult human should open new perspectives to study adult neurogenesis and for brain repair. The present study describes the in vitro isolation from adult human brains of a progenitor responsive to both epidermal and basic fibroblast growth factors that forms spheres as it proliferates. Single spheres derived from various regions of the brain generate in vitro neurons, astrocytes, and oligodendrocytes. The clonal origin of the spheres was revealed by genomic viral insertion using lentiviral vector. Interestingly, this vector appears to be a potent tool for gene transfer into human neural progeny. Ninety-six percent of the spheres investigated were multipotent. Multipotent precursors were isolated from all brain regions studied, including the temporal and the frontal cortex, the amygdala, the hippocampus, and the ventricular zone. This study is the first evidence that primitive precursors such as multipotent precursors exist in the adult human cortex and can reside far from the ventricles. Neurogenesis derived from adult human progenitors differ to murine neurogenesis by the requirement of laminin for oligodendrocyte generation and by the action of basic-fibroblast growth factor and platelet derived growth factor that prevented the formation of oligodendrocytes and neurons. Moreover, the differentiation of human adult precursors seems to differ from fetal ones: adult precursors do not necessitate the removal of mitogen for differentiation. These results indicate that the study of adult multipotent precursors is a new platform to study adult human neurogenesis, potentially generate neural cells for transplantation, and design protocols for in vivo stimulation.

The Phox2b transcription factor coordinately regulates neuronal cell cycle exit and identity

In the vertebrate neural tube, cell cycle exit of neuronal progenitors is accompanied by the expression of transcription factors that define their generic and sub-type specific properties, but how the regulation of cell cycle withdrawal intersects with that of cell fate determination is poorly understood. Here we show by both loss- and gain-of-function experiments that the neuronal-subtype-specific homeodomain transcription factor Phox2b drives progenitor cells to become post-mitotic. In the absence of Phox2b, post-mitotic neuronal precursors are not generated in proper numbers. Conversely, forced expression of Phox2b in the embryonic chick spinal cord drives ventricular zone progenitors to become post-mitotic neurons and to relocate to the mantle layer. In the neurons thus generated, ectopic expression of Phox2b is sufficient to initiate a programme of motor neuronal differentiation characterised by expression of Islet1 and of the cholinergic transmitter phenotype, in line with our previous results showing that Phox2b is an essential determinant of cranial motor neurons. These results suggest that Phox2b coordinates quantitative and qualitative aspects of neurogenesis, thus ensuring that neurons of the correct phenotype are generated in proper numbers at the appropriate times and locations.

The homeodomain protein CePHOX2/CEH-17 controls antero-posterior axonal growth in C. elegans

An essential aspect of a neuron's identity is the pattern of its axonal projections. In C. elegans, axons extend either longitudinally or circumferentially in response to distinct molecular cues, some of which have been identified. It is currently unclear, however, how the differential capacity to respond to these cues is transcriptionally implemented in distinct neuronal subtypes. Here, we characterise a C. elegans paired-like homeobox gene, CePhox2/ceh-17, expressed in five head neurons, ALA and the 4 SIAs, all of which project axons towards the tail along the lateral and sublateral cords. Abrogation of ceh-17 function, while leaving intact many phenotypic traits of these neurons, disrupts their antero-posterior axonal elongation beyond the mid-body region. Conversely, ectopic expression of ceh-17 in the mechanoreceptors, several of which are known to pioneer their tract, leads to exaggerated longitudinal axonal outgrowth. Thus, ceh-17 is a novel gene involved in fasciculation-independent longitudinal axonal navigation.

Role of the target in the pathfinding of facial visceral motor axons

Axon navigation depends, in part, on guidance cues emanating from the target. We have investigated the possible role of the target in the pathfinding of visceral motor axons to cranial parasympathetic ganglia. Mice homozygous for a tau-LacZ transgene targeted in the Phox2a locus lack the sphenopalatine ganglion, which is the normal target of visceral motor axons of the facial nerve. We found that in these mutants, facial visceral motor axon pathfinding was disrupted, and some axons were misrouted to an alternative parasympathetic ganglion. Moreover, the absence of correct facial visceral motor pathways was concomitant with defects in the pathfinding of rostrally-projecting sympathetic axons.

Control of hindbrain motor neuron differentiation by the homeobox gene Phox2b

Motor neurons are a widely studied model of vertebrate neurogenesis. They can be subdivided in somatic, branchial and visceral motor neurons. Recent studies on the dorsoventral patterning of the rhombencephalon have implicated the homeobox genes Pax6 and Nkx2.2 in the early divergence of the transcriptional programme of hindbrain somatic and visceral motor neuronal differentiation. We provide genetic evidence that the paired-like homeodomain protein Phox2b is required for the formation of all branchial and visceral, but not somatic, motor neurons in the hindbrain. In mice lacking Phox2b, both the generic and subtype-specific programs of motoneuronal differentiation are disrupted at an early stage. Most motor neuron precursors die inside the neuroepithelium while those that emigrate to the mantle layer fail to switch on early postmitotic markers and to downregulate neuroepithelial markers. Thus, the loss of function of Phox2b in hindbrain motor neurons exemplifies a novel control point in the generation of CNS neurons.