Expression pattern of NOGO and NgR genes during human development

An unexpected role of neurite outgrowth inhibitor A as regulator of tooth enamel formation

Neurite outgrowth inhibitor A (Nogo-A) is a major player in neural development and regeneration and the target of clinical trials aiming at promoting the regeneration of the central nervous system upon traumatic and ischemic injury. In this work, we investigated the functions of Nogo-A during tooth development to determine its role in dental physiology and pathology. Using immunohistochemistry and in situ hybridization techniques, we showed that Nogo-A is highly expressed in the developing mouse teeth and, most specifically, in the ameloblasts that are responsible for the formation of enamel. Using both Nogo-A knockout and K14-Cre;Nogo-A fl/fl transgenic mice, we showed that Nogo-A deletion in the dental epithelium leads to the formation of defective enamel. This phenotype is associated with overexpression of a set of specific genes involved in ameloblast differentiation and enamel matrix production, such as amelogenin, ameloblastin and enamelin. By characterising the interactome of Nogo-A in the dental epithelium of wild-type and mutant animals, we found that Nogo-A directly interacts with molecules important for regulating gene expression, and its deletion disturbs their cellular localisation. Furthermore, we demonstrated that inhibition of the intracellular, but not cell-surface, Nogo-A is responsible for gene expression modulation in ameloblasts. Taken together, these results reveal an unexpected function for Nogo-A in tooth enamel formation by regulating gene expression and cytodifferentiation events.

Comparative characterization of CD271+ and CD271- subpopulations of CD34+ human adipose-derived stromal cells

Adipose-derived stromal/stem cells (ASCs) are promising candidates for cell-based therapies. However, the lack of markers able to unequivocally identify these cells, the differential expression of cell surface molecules among stromal progenitors from different tissues and cellular alterations caused by culture are phenomena that need to be comprehensively addressed in order to improve ASC purification and consequently refine our knowledge about their function and therapeutic efficiency. In this study, we investigated the potential of CD271, a marker used for purification of bone marrow-derived mesenchymal stem cells, on enriching ASCs from CD34⁺ stromal cells of human adipose tissue. Putative ASC populations were sorted based on CD271 expression (CD45^- CD31^- CD34⁺ CD271⁺ and CD45^- CD31^- CD34⁺ CD271^- cells) and compared regarding their clonogenic efficiency, proliferation, immunophenotypic profile, and multilineage potential. To shed light on their native identity, we also interrogated the expression of key perivascular cell markers in freshly isolated cells. CD271^- cells displayed twofold higher clonogenic efficiency than CD271⁺ cells. Upon culture, the progeny of both populations displayed similar immunophenotypic profile and in vitro adipogenic and chondrogenic potentials, while CD271⁺ cells produced more calcified extracellular matrix. Interestingly, uncultured freshly isolated CD271⁺ cells displayed higher expression of pericyte-associated markers than CD271^- cells and localized in the inner region of the perivascular wall. Our results demonstrate that cells with in vitro ASC traits can be obtained from both CD271⁺ and CD271^- stromal populations of human adipose tissue. In addition, gene expression profiling and in situ localization analyses indicate that the CD271⁺ population displays a pericytic phenotype.

Non-canonical actions of Nogo-A and its receptors

Nogo-A is a myelin associated protein and one of the most potent neurite growth inhibitors in the central nervous system. Interference with Nogo-A signaling has thus been investigated as therapeutic target to promote functional recovery in CNS injuries. Still, the finding that Nogo-A presents a fairly ubiquitous expression in many types of neurons in different brain regions, in the eye and even in the inner ear suggests for further functions besides the neurite growth repression. Indeed, a growing number of studies identified a variety of functions including regulation of neuronal stem cells, modulation of microglial activity, inhibition of angiogenesis and interference with memory formation. Aim of the present commentary is to draw attention on these less well-known and sometimes controversial roles of Nogo-A. Furthermore, we are addressing the role of Nogo-A in neuropathological conditions such as ischemic stroke, schizophrenia and neurodegenerative diseases.

NogoR1 and PirB signaling stimulates neural stem cell survival and proliferation

Neural stem cells (NSCs) and neural progenitors (NPs) in the mammalian neocortex give rise to the main cell types of the nervous system. The biological behavior of these NSCs and NPs is regulated by extracellular niche derived autocrine-paracrine signaling factors on a developmental timeline. Our previous reports [Plos One 2010;5:e15341; J Neurochem 2011;117:565-578] have shown that chondroitin sulfate proteoglycan and ApolipoproteinE are autocrine-paracrine survival factors for NSCs. NogoA, a myelin related protein, is expressed in the cortical ventricular zones where NSCs reside. However, the functional role of Nogo signaling proteins in NSC behavior is not completely understood. In this study, we show that NogoA receptors, NogoR1 and PirB, are expressed in the ventricular zone where NSCs reside between E10.5 and 14.5 but not at E15.5. Nogo ligands stimulate NSC survival and proliferation in a dosage-dependent manner in vitro. NogoR1 and PirB are low and high affinity Nogo receptors, respectively and are responsible for the effects of Nogo ligands on NSC behavior. Inhibition of autocrine-paracrine Nogo signaling blocks NSC survival and proliferation. In NSCs, NogoR1 functions through Rho whereas PirB uses Shp1/2 signaling pathways to control NSC behavior. Taken together, this work suggests that Nogo signaling is an important pathway for survival of NSCs.

The P75 neurotrophin receptor regulates proliferation of the human MG63 osteoblast cell line

The 75 kDa transmembrane protein, p75(NTR), is a marker of mesenchymal stem cells (MSCs). Isolated MSCs are capable of differentiating into osteoblasts, but the molecular function of p75(NTR) in MSCs and osteoblasts is poorly understood. The aim of this study was to examine the function of p75(NTR) in the human MG63 osteoblast cell line compared to the murine MC3T3E-1 pre-osteoblast cell line. MG63 cells and MC3T3-E1 cells expressing exogenous p75(NTR) protein (denoted as p75-MG63 and p75GFP-E1, respectively) were generated to compare osteogenic differentiation and cell proliferation abilities. Overexpression of p75(NTR) induced alkaline phosphatase activity and the mRNA expression of osteoblast-related genes such as osterix and bone sialoprotein in both p75-MG63 and p75GFP-E1. Interestingly, exogenous p75(NTR) stimulated cell proliferation and cell cycle progression in p75GFP-E1, but not in p75-MG63. To elucidate any different effects of p75(NTR) expression on osteogenic differentiation and cell proliferation, we examined the mRNA expression of tropomyosin receptor kinase (trk) genes (trkA, trkB, trkC) and Nogo receptor (NgR), which are binding partners of p75(NTR). Although trkA, trkB, and trkC were detected in both p75-MG63 and p75GFP-E1, only NgR was detected in p75-MG63. We then used the K252a inhibitor of the trks to identify the signaling pathway for osteogenic differentiation and cell proliferation. Inhibition of trks by K252a suppressed p75(NTR)-mediated osteogenic differentiation of p75GFP-E1, whereas deletion of the GDI domain in P75(NTR) from the p75-MG63 produced enhanced cell proliferation compared to p75-MG63. These results suggest that p75(NTR) signaling associated with trk receptors promotes both cell proliferation and osteoblast differentiation, but that p75(NTR)-mediated proliferation may be suppressed by signaling from the p75(NTR)/NgR complex.

Down-regulation of Nogo-A by collagen scaffolds impregnated with bone marrow stromal cell treatment after traumatic brain injury promotes axonal regeneration in rats

Nogo-A is a major form of growth inhibitory molecule (growth-IM) which inhibits axonal regeneration and neurite regrowth after neural injury. Bone marrow stromal cells (MSCs) have been shown to inhibit Nogo-A expression in vitro and in cerebral ischemic animal models. The present study was designed to investigate the effects of treatment with human MSCs (hMSCs) impregnated into collagen scaffolds on the expression of Nogo-A and axonal plasticity after traumatic brain injury (TBI). Adult male Wistar rats were injured with controlled cortical impact and treated either with saline, hMSCs-alone or hMSCs impregnated into collagen scaffolds (scaffold+hMSC) transplanted into the lesion cavity 7 days after TBI. Rats were sacrificed 14 days after TBI and brain tissues were harvested for immunohistochemical studies, Western blot analysis, laser capture microdissections and qRT-PCR to evaluate axonal density and Nogo-A protein and gene expressions. Our data showed that treatment of TBI with scaffold+hMSC significantly decreased TBI-induced Nogo-A protein expression and increased axonal density compared to saline and hMSC-alone treatments. In addition, scaffold+hMSC transplantation decreased Nogo-A transcription in oligodendrocytes after TBI. Scaffold+hMSC treatment was superior to hMSC-alone treatment in suppressing Nogo-A expression and enhancing axonal regeneration after TBI. Our data suggest that transplanting hMSCs with scaffolds down-regulates Nogo-A transcription and protein expression which may partially contribute to the enhanced axonal regeneration after TBI.

Inhibition of retinal ganglion cell axonal outgrowth through the Amino-Nogo-A signaling pathway

Nogo-A is a myelin-derived inhibitor playing a pivotal role in the prevention of axonal regeneration. A functional domain of Nogo-A, Amino-Nogo, exerts an inhibitory effect on axonal regeneration, although the mechanism is unclear. The present study investigated the role of the Amino-Nogo-integrin signaling pathway in primary retinal ganglion cells (RGCs) with respect to axonal outgrowth, which is required for axonal regeneration. Immunohistochemistry showed that integrin αv, integrin α5 and FAK were widely expressed in the visual system. Thy-1 and GAP-43 immunofluorescence showed that axonal outgrowth of RGCs was promoted by Nogo-A siRNA and a peptide antagonist of the Nogo-66 functional domain of Nogo-A (Nep1-40), and inhibited by a recombinant rat Nogo-A-Fc chimeric protein (Δ20). Western blotting revealed increased integrin αv and p-FAK expression in Nogo-A siRNA group, decreased integrin αv expression in Δ20 group and decreased p-FAK expression in Nep1-40 group. Integrin α5 expression was not changed in any group. RhoA G-LISA showed that RhoA activation was inhibited by Nogo-A siRNA and Δ20, but increased by Nep1-40 treatment. These results suggest that Amino-Nogo inhibits RGC axonal outgrowth primarily through the integrin αv signaling pathway.

The p75 neurotrophin receptor regulates MC3T3-E1 osteoblastic differentiation

While the role of p75(NTR) signaling in the regulation of nerve-related cell growth and survival has been well documented, its actions in osteoblasts are poorly understood. In this study, we examined the effects of p75(NTR) on osteoblast proliferation and differentiation using the MC3T3-E1 pre-osteoblast cell line. Proliferation and osteogenic differentiation were significantly enhanced in p75(NTR)-overexpressing MC3T3-E1 cells (p75GFP-E1). In addition, expression of osteoblast-specific osteocalcin (OCN), bone sialoprotein (BSP), and osterix mRNA, ALP activity, and mineralization capacity were dramatically enhanced in p75GFP-E1 cells, compared to wild MC3T3-E1 cells (GFP-E1). To determine the binding partner of p75(NTR) in p75GFP-E1 cells during osteogenic differentiation, we examined the expression of trkA, trkB, and trkC that are known binding partners of p75(NTR), as well as NgR. Pharmacological inhibition of trk tyrosine kinase with the K252a inhibitor resulted in marked reduction in the level of ALPase under osteogenic conditions. The deletion of the GDI binding domain in the p75(NTR)-GFP construct had no effect on mineralization. Taken together, our studies demonstrated that p75(NTR) signaling through the trk tyrosine kinase pathway affects osteoblast functions by targeting osteoblast proliferation and differentiation.

Expression of nogo-a is decreased with increasing gestational age in the human fetal brain

Nogo is a member of the reticulon family. Our understanding of the physiological functions of the Nogo-A protein has grown over the last few years, and this molecule is now recognized as one of the most important axonal regrowth inhibitors present in central nervous system (CNS) myelin. Nogo-A plays other important roles in nervous system development, epilepsy, vascular physiology, muscle pathology, stroke, inflammation, and CNS tumors. Since the exact role of Nogo-A protein in human brain development is still poorly understood, we studied its cellular and regional distribution by immunohistochemistry in the frontal lobe of 30 human fetal brains. Nogo-A was expressed in the following cortical zones: ependyma, ventricular zone, subventricular zone, intermediate zone, subplate, cortical plate, and marginal zone. The number of positive cells decreased significantly with increasing gestational age in the subplate and marginal zone. Using different antibodies, changes in isoform expression and dimerization states could be shown between various cortical zones. The results demonstrate a significant change in the expression of Nogo-A during the development of the human brain. The effects of its time- and region-specific regulation have to be further studied in detail.

CD271/p75(NTR) inhibits the differentiation of mesenchymal stem cells into osteogenic, adipogenic, chondrogenic, and myogenic lineages

We describe a novel role for CD271 in the differentiation of mesenchymal stem cells (MSCs), including deciduous dental pulp stem cells (DDPSCs) and murine multipotent MSCs (C3H10T1/2 cells). The CD271(+) subpopulation of deciduous dental pulp cells (CD271(+)/DDPSCs) and the forced expression of CD271 in C3H10T1/2 (10T271) were analyzed by fluorescence-activated cell sorting. CD271 expression was detected in DDPSCs that expressed both CD44 and CD90, simultaneously, and the clonogenic capacity of the CD271(+)/DDPSCs was higher than that of the CD271(-)/DDPSCs that expressed both CD44 and CD90. Further, the differentiation of CD271(+)/DDPSCs into osteoblasts and adipocytes was inhibited although CD271(-)/DDPSCs were capable of differentiating into osteoblasts and adipocytes. CD271 was overexpressed in C3H10T1/2 cells, which have the potential to differentiate into osteoblasts, adipocytes, chondrocytes, and myocytes. CD271 inhibited the differentiation of C3H10T1/2 cells into any of these lineages. These results indicate a role for CD271 in inhibiting the differentiation of MSCs.

Nogo-a regulates neural precursor migration in the embryonic mouse cortex

Although Nogo-A has been intensively studied for its inhibitory effect on axonal regeneration in the adult central nervous system, little is known about its function during brain development. In the embryonic mouse cortex, Nogo-A is expressed by radial precursor/glial cells and by tangentially migrating as well as postmigratory neurons. We studied radially migrating neuroblasts in wild-type and Nogo-A knockout (KO) mouse embryos. In vitro analysis showed that Nogo-A and its receptor components NgR, Lingo-1, TROY, and p75 are expressed in cells emigrating from embryonic forebrain–derived neurospheres. Live imaging revealed an increased cell motility when Nogo-A was knocked out or blocked with antibodies. Antibodies blocking NgR or Lingo-1 showed the same motility-enhancing effect supporting a direct role of surface Nogo-A on migration. Bromodeoxyuridine (BrdU) labeling of embryonic day (E)15.5 embryos demonstrated that Nogo-A influences the radial migration of neuronal precursors. At E17.5, the normal transient accumulation of radially migrating precursors within the subventricular zone was not detectable in the Nogo-A KO mouse cortex. At E19, migration to the upper cortical layers was disturbed. These findings suggest that Nogo-A and its receptor complex play a role in the interplay of adhesive and repulsive cell interactions in radial migration during cortical development.

Patterns of Nogo-A, NgR, and RhoA expression in the brain tissues of rats with focal cerebral infarction

Nogo-A and its Nogo receptor (NgR) have been shown to inhibit plasticity after central nervous system lesions. Therefore, we hypothesized that Nogo-A and its receptor NgR will be upregulated and will activate RhoA, and thus, they play a role in the damage in the infarction developed. To test this hypothesis, a focal cerebral infarction model was created by coagulation of the right middle cerebral artery (MCA) and ipsilateral common carotid artery (CCA), as well as the simultaneous transient occlusion of the contralateral CCA for 30 min in 60 adult Sprague-Dawley rats. The rat brains were treated at 6 h, 12 h, 24 h, 48 h, 96 h, and 7 d after cerebral infarction. Sham controls were collected to determine histopathologic damage and Nogo-A, NgR, and RhoA expression using hematoxylin-eosin, immunohistochemical staining, Western blot analysis, and fluorimeter-based quantitive reverse transcriptase-polymerase chain reaction. The results indicate that cerebral infarction produced damage and edema on nerve cells in the infarction area, becoming most prominent at 24h after modeling. Meanwhile, a marked increase of Nogo-A, NgR, and RhoA expression was found at 6h in model groups compared with the sham controls, which peaked at 24 h after the operation. Immunohistochemical staining and Western blot analysis also showed upregulated Nogo-A located in the myelin sheath of the infarction area, NgR expressed on the surface of neurons and their processes, and RhoA expressed inside the cytoplasm of neurons in infarction brain. In conclusion, the upregulation of Nogo-A, NgR, and RhoA in the infarction area may be an important feature of cerebral infarction and may play a role in the pathologic progression of this lesion.

Spatio-temporal expression pattern of receptors for myelin-associated inhibitors in the developing rat olfactory system

The myelin-associated inhibitory proteins (Nogo-A, MAG and OMgp) that prevent axon regeneration in adult CNS, mediate their effects via a receptor referred as NgR1. Beside their inhibitory role in the adult CNS, Nogo-A and NgR1 might also be functionally involved in the developing nervous system. At the present time, no detailed study is available regarding either the onset of NgR1 expression during development or its spatio-temporal pattern of expression relative to the presence of Nogo-A. Two homologs of NgR1, NgR2 and NgR3, have been recently identified, but their function in the nervous system is still unknown in adult as well as during development. We have examined the spatio-temporal expression pattern of both NgR1, NgR2 and NgR3 mRNAs and corresponding proteins in the developing rat olfactory system using in situ hybridization and immunohistochemistry. From E15-E16 onwards, NgR1 mRNA was expressed by differentiating neurons in both the olfactory epithelium and the olfactory bulb. At all developmental stages, including adult animals, NgR1 protein was preferentially targeted to olfactory axons emerging from the olfactory epithelium. Using double-immunostainings in the postnatal olfactory mucosa, we confirm the neuronal localization of NgR1 and its preferential distribution along the olfactory axons. The NgR2 and NgR3 transcripts and their proteins display similar expression profiles in the olfactory system. Together, our data suggest that, in non-pathological conditions, NgR1 and its homologs may play a role in axon outgrowth in the rat olfactory system and may be relevant for the confinement of neural projections within the developing olfactory bulb.

SLC25A12 expression is associated with neurite outgrowth and is upregulated in the prefrontal cortex of autistic subjects

Autism is a neurodevelopmental disorder with a strong genetic component, probably involving several genes. Genome screens have provided evidence of linkage to chromosome 2q31-q33, which includes the SLC25A12 gene. Association between autism and single-nucleotide polymorphisms in SLC25A12 has been reported in various studies. SLC25A12 encodes the mitochondrial aspartate/glutamate carrier functionally important in neurons with high-metabolic activity. Neuropathological findings and functional abnormalities in autism have been reported for Brodmann's area (BA) 46 and the cerebellum. We found that SLC25A12 was expressed more strongly in the post-mortem brain tissues of autistic subjects than in those of controls, in the BA46 prefrontal cortex but not in cerebellar granule cells. SLC25A12 expression was not modified in brain subregions of bipolar and schizophrenic patients. SLC25A12 was expressed in developing human neuronal tissues, including neocortical regions containing excitatory neurons and neocortical progenitors and the ganglionic eminences that generate neocortical inhibitory interneurons. At mid-gestation, when gyri and sulci start to develop, SLC25A12 molecular gradients were identified in the lateral prefrontal and ventral temporal cortex. These fetal structures generate regions with abnormal activity in autism, including the dorsolateral prefrontal cortex (BA46), the pars opercularis of the inferior frontal cortex and the fusiform gyrus. SLC25A12 overexpression or silencing in mouse embryonic cortical neurons also modified dendrite length and the mobility of dendritic mitochondria. Our findings suggest that SLC25A12 overexpression may be involved in the pathophysiology of autism, modifying neuronal networks in specific subregions, such as the dorsolateral prefrontal cortex and fusiform gyrus, at both pre- and postnatal stages.

Nogo receptor antagonizes p75NTR-dependent motor neuron death

The Nogo-66 receptor (NgR) plays a critical role in restricting axon regeneration in the central nervous system. This inhibitory action is in part mediated by a neuronal receptor complex containing p75NTR, a multifunctional receptor also well known to trigger cell death upon binding to neurotrophins such as NGF. In the present study, we show that Pep4 and NEP1-40, which are two peptides derived from the Nogo-66 sequence that modulate NgR-mediated neurite outgrowth inhibition, prevent NGF-stimulated p75NTR-dependent death of cultured embryonic motor neurons. They also confer protection on spinal cord motor neurons after neonatal sciatic nerve axotomy. These findings demonstrate an as-yet-unknown function of NgR in maintaining neuronal survival that may be relevant for motor neuron development and degeneration.

NOGO-A induction and localization during chick brain development indicate a role disparate from neurite outgrowth inhibition

Background

Nogo-A, a myelin-associated protein, inhibits neurite outgrowth and abates regeneration in the adult vertebrate central nervous system (CNS) and may play a role in maintaining neural pathways once established. However, the presence of Nogo-A during early CNS development is counterintuitive and hints at an additional role for Nogo-A beyond neurite inhibition.

Results

We isolated chicken NOGO-A and determined its sequence. A multiple alignment of the amino acid sequence across divergent species, identified five previously undescribed, Nogo-A specific conserved regions that may be relevant for development. NOGO gene transcripts (NOGO-A, NOGO-B and NOGO-C) were differentially expressed in the CNS during development and a second NOGO-A splice variant was identified. We further localized NOGO-A expression during key phases of CNS development by in situ hybridization. CNS-associated NOGO-A was induced coincident with neural plate formation and up-regulated by FGF in the transformation of non-neural ectoderm into neural precursors. NOGO-A expression was diffuse in the neuroectoderm during the early proliferative phase of development, and migration, but localized to large projection neurons of the optic tectum and tectal-associated nuclei during architectural differentiation, lamination and network establishment.

Conclusion

These data suggest Nogo-A plays a functional role in the determination of neural identity and/or differentiation and also appears to play a later role in the networking of large projection neurons during neurite formation and synaptogenesis. These data indicate that Nogo-A is a multifunctional protein with additional roles during CNS development that are disparate from its later role of neurite outgrowth inhibition in the adult CNS.

Reticulons as markers of neurological diseases: focus on amyotrophic lateral sclerosis

Reticulons (RTNs) are a family of proteins that are primarily associated with the endoplasmic reticulum. In mammals, four genes have been identified and referred as to rtn1, 2, 3 and the neurite outgrowth inhibitor rtn4/nogo. These genes generate multiple isoforms that contain a common C-terminal reticulon homology domain of 150-200 amino-acid residues. The N-terminal regions of RTNs are highly variable, and result from alternative splicing or differential promoter usage. Although widely distributed, the functions of RTNs are still poorly understood. Much interest has been focused on rtn4/nogo because of its activity as a potent inhibitor of axonal growth and repair. In the present study, we update recent knowledge on mammalian RTNs paying special attention to the involvement of these proteins as markers of neurological diseases. We also present recent data concerning RTN expression in amyotrophic lateral sclerosis, a fatal degenerative disorder characterized by loss of upper and lower motor neurons, and muscle atrophy. The rearrangement of RTN expression is regulated not only in suffering skeletal muscle but also preceding the onset of symptoms, and may relate to the disease process.

Nogo-A expression in the human hippocampus in normal aging and in Alzheimer disease

Myelin-associated proteins are involved in the formation and stabilization of myelin sheaths. In addition, they prevent axon regeneration and plasticity in the adult brain. Recent evidence suggests that the expression of certain myelin-associated proteins (e.g. Nogo-A) can be regulated by synaptic activity or by over-expression after neural lesions in brain syndromes such as temporal lobe epilepsy. However, no studies on Alzheimer disease (AD) have been reported in which cell loss and significant synaptic reorganization occurs. In the present study, we analyze in detail the expression of Nogo-A in the hippocampal formation in normal human aging and in AD. Our results indicate that Nogo-A is expressed by oligodendrocytes and neurons in the aged hippocampal formation. In addition, both granule cells and mossy fiber connections are also labeled in the old-aged hippocampi. Interestingly, Nogo-A is over-expressed by hippocampal neurons in AD and is associated with beta-amyloid deposits in senile plaques. Taken together, our results reinforce the hypothesis that Reticulon proteins such as Nogo-A participate in the neuronal responses stemming from hippocampal formation during senescence, and particularly in AD. These findings also indicate that Reticulon proteins could be considered as new putative drug targets in therapies of neurodegenerative disorders.

Ret deficiency in mice impairs the development of A5 and A6 neurons and the functional maturation of the respiratory rhythm

Although a normal respiratory rhythm is vital at birth, little is known about the genetic factors controlling the prenatal maturation of the respiratory network in mammals. In Phox2a mutant mice, which do not express A6 neurons, we previously hypothesized that the release of endogenous norepinephrine by A6 neurons is required for a normal respiratory rhythm to occur at birth. Here we investigated the role of the Ret gene, which encodes a transmembrane tyrosine kinase receptor, in the maturation of norepinephrine and respiratory systems. As Ret-null mutants (Ret-/-) did not survive after birth, our experiments were performed in wild-type (wt) and Ret-/- fetuses exteriorized from pregnant heterozygous mice at gestational day 18. First, in wt fetuses, quantitative in situ hybridization revealed high levels of Ret transcripts in the pontine A5 and A6 areas. Second, in Ret-/- fetuses, high-pressure liquid chromatography showed significantly reduced norepinephrine contents in the pons but not the medulla. Third, tyrosine hydroxylase immunocytochemistry revealed a significantly reduced number of pontine A5 and A6 neurons but not medullary norepinephrine neurons in Ret-/- fetuses. Finally, electrophysiological and pharmacological experiments performed on brainstem 'en bloc' preparations demonstrated impaired resting respiratory activity and abnormal responses to central hypoxia and norepinephrine application in Ret-/- fetuses. To conclude, our results show that Ret gene contributes to the prenatal maturation of A6 and A5 neurons and respiratory system. They support the hypothesis that the normal maturation of the respiratory network requires afferent activity corresponding to the A6 excitatory and A5 inhibitory input balance.

Inhibition of Nogo: a key strategy to increase regeneration, plasticity and functional recovery of the lesioned central nervous system

In the adult central nervous system (CNS) myelin and oligodendrocytes, Nogo-A exerts a growth inhibitory function leading to restricted axonal regeneration. After development of different anti-Nogo-A antibodies and other Nogo-A blocking reagents their application has recently been studied in various in vivo animal models of spinal cord injury and stroke. These studies show that intracerebral application of Nogo-A-inactivating reagents leads to enhanced regeneration and compensatory sprouting, structural reorganization or plasticity, and functional recovery as seen in different behavioural analyses.