Lbx1 and Tlx3 are opposing switches in determining GABAergic versus glutamatergic transmitter phenotypes

Connecting the ear to the brain: Molecular mechanisms of auditory circuit assembly

Our sense of hearing depends on precisely organized circuits that allow us to sense, perceive, and respond to complex sounds in our environment, from music and language to simple warning signals. Auditory processing begins in the cochlea of the inner ear, where sounds are detected by sensory hair cells and then transmitted to the central nervous system by spiral ganglion neurons, which faithfully preserve the frequency, intensity, and timing of each stimulus. During the assembly of auditory circuits, spiral ganglion neurons establish precise connections that link hair cells in the cochlea to target neurons in the auditory brainstem, develop specific firing properties, and elaborate unusual synapses both in the periphery and in the CNS. Understanding how spiral ganglion neurons acquire these unique properties is a key goal in auditory neuroscience, as these neurons represent the sole input of auditory information to the brain. In addition, the best currently available treatment for many forms of deafness is the cochlear implant, which compensates for lost hair cell function by directly stimulating the auditory nerve. Historically, studies of the auditory system have lagged behind other sensory systems due to the small size and inaccessibility of the inner ear. With the advent of new molecular genetic tools, this gap is narrowing. Here, we summarize recent insights into the cellular and molecular cues that guide the development of spiral ganglion neurons, from their origin in the proneurosensory domain of the otic vesicle to the formation of specialized synapses that ensure rapid and reliable transmission of sound information from the ear to the brain.

Regulation of cell cycle and DNA repair in post-mitotic GABA neurons in psychotic disorders

Disturbances of cell cycle regulation and DNA repair in post-mitotic neurons have been implicated in degenerative and malignant diseases of the human brain. Recent work is now suggesting that abnormal regulation of these functions in GABA cells of the adult hippocampus may also play a role in two neuropsychiatric disorders. In schizophrenia and bipolar disorder, a network of genes involved in the regulation of GAD₆₇, a marker for the functional differentiation of GABA cells, show pronounced changes in expression and include kainate receptor subunits, TGFβ and Wnt signaling pathways, epigenetic factors and transcription factors. One of these genes, cyclin D2, is involved in the regulation of cell cycle and DNA repair and appears to be a pivotal element in linking GAD₆₇ expression with these functional clusters of genes. Dysfunction of post-mitotic GABAergic neurons in the adult hippocampus of patients with psychotic disorders is associated with changes in the expression of genes that are involved in the maintenance of functional and genomic integrity of GABA cells. The nature of these changes is quite different in schizophrenia and bipolar disorder, suggesting that a common cell phenotype (in this case, decreased GAD₆₇ expression) may involve two fundamentally different molecular endophenotypes and reflect unique susceptibility genes involved in the respective disorders. This article is part of a Special Issue entitled 'Trends in neuropharmacology: in memory of Erminio Costa'.

VGLUT2-dependent glutamate release from nociceptors is required to sense pain and suppress itch

Itch can be suppressed by painful stimuli, but the underlying neural basis is unknown. We generated conditional null mice in which vesicular glutamate transporter type 2 (VGLUT2)-dependent synaptic glutamate release from mainly Nav1.8-expressing nociceptors was abolished. These mice showed deficits in pain behaviors, including mechanical pain, heat pain, capsaicin-evoked pain, inflammatory pain, and neuropathic pain. The pain deficits were accompanied by greatly enhanced itching, as suggested by (1) sensitization of both histamine-dependent and histamine-independent itch pathways and (2) development of spontaneous scratching and skin lesions. Strikingly, intradermal capsaicin injection promotes itch responses in these mutant mice, as opposed to pain responses in control littermates. Consequently, coinjection of capsaicin was no longer able to mask itch evoked by pruritogenic compounds. Our studies suggest that synaptic glutamate release from a group of peripheral nociceptors is required to sense pain and suppress itch. Elimination of VGLUT2 in these nociceptors creates a mouse model of chronic neurogenic itch.

Prrxl1 is required for the generation of a subset of nociceptive glutamatergic superficial spinal dorsal horn neurons

Perception of noxious events relies on activation of complex central neuronal circuits. The spinal cord dorsal horn plays a pivotal role in the process relaying to the brain various types of somatosensory input. These functions are accomplished by distinct sensory neurons specifically organized in different laminae. They differentiate during development in a spatial-temporal order due to the expression of combinatorial sets of homeodomain transcription factors. Here we demonstrate that the differential expression of the homeodomain transcription factors Prrxl1 (DRG11), Tlx3, and Lmx1b defines various subpopulations of spinal cord dorsal horn glutamatergic early born and late born neurons. Accordingly, in the superficial dorsal horn of Prrxl1(-/-) mice, the number of glutamatergic neurons is reduced by 70%, while the number of Golgi-impregnated and noxious-induced Fos immunoreactive neurons is reduced by 85%. These results suggest a crucial role for Prrxl1 in the generation of various subpopulations of nociceptive glutamatergic superficial dorsal horn neurons.

The Gata3 transcription factor is required for the survival of embryonic and adult sympathetic neurons

The transcription factor Gata3 is essential for the development of sympathetic neurons and adrenal chromaffin cells. As Gata3 expression is maintained up to the adult stage, we addressed its function in differentiated sympathoadrenal cells at embryonic and adult stages by conditional Gata3 elimination. Inactivation of Gata3 in embryonic DBH-expressing neurons elicits a strong reduction in neuron numbers due to apoptotic cell death and reduced proliferation. No selective effect on noradrenergic gene expression (TH and DBH) was observed. Interestingly, Gata3 elimination in DBH-expressing neurons of adult animals also results in a virtually complete loss of sympathetic neurons. In the Gata3-deficient population, the expression of anti-apoptotic genes (Bcl-2, Bcl-xL, and NFkappaB) is diminished, whereas the expression of pro-apoptotic genes (Bik, Bok, and Bmf) was increased. The expression of noradrenergic genes (TH and DBH) is not affected. These results demonstrate that Gata3 is continuously required for maintaining survival but not differentiation in the sympathetic neuron lineage up to mature neurons of adult animals.

cJun integrates calcium activity and tlx3 expression to regulate neurotransmitter specification

Neuronal differentiation is accomplished through cascades of intrinsic genetic factors initiated in neuronal progenitors by external gradients of morphogens. Activity has been thought to be important only late in development, but recent evidence suggests that activity also regulates early neuronal differentiation. Activity in post-mitotic neurons before synapse formation can regulate phenotypic specification, including neurotransmitter choice, but the mechanisms are not clear. We identified a mechanism that links endogenous calcium spike activity with an intrinsic genetic pathway to specify neurotransmitter choice in neurons in the dorsal embryonic spinal cord of Xenopus tropicalis. Early activity modulated transcription of the GABAergic/glutamatergic selection gene tlx3 through a variant cAMP response element (CRE) in its promoter. The cJun transcription factor bound to this CRE site, modulated transcription and regulated neurotransmitter phenotype via its transactivation domain. Calcium signaled through cJun N-terminal phosphorylation, which integrated activity-dependent and intrinsic neurotransmitter specification. This mechanism provides a basis for early activity to regulate genetic pathways at critical decision points, switching the phenotype of developing neurons.

Identification of cerebellin2 in chick and its preferential expression by subsets of developing sensory neurons and their targets in the dorsal horn

The cerebellins are a family of four secreted proteins, two of which, Cbln1 and Cbln3, play an important role in the formation and maintenance of parallel fiber-Purkinje cell synapses. We have identified the chicken homologue of Cbln2 and, through the use of in situ hybridization, shown that it is expressed by specific subsets of neurons in the dorsal root ganglia (DRGs) and spinal cord starting shortly after those neurons are generated. In the developing spinal cord, Cbln2 is highly expressed by dI1, dI3, dI5, and dILB dorsal interneurons and to a lesser extent by dI2, dI4, dI6, and dILA dorsal interneurons, but not by ventral (v0-v3) interneurons. After the spinal cord has matured and neurons have migrated to their final destinations, Cbln2 is abundant in the dorsal horn. In the DRGs, Cbln2 is expressed by TrkB+ and TrkC+ sensory neurons, but not by TrkA+ sensory neurons. Interestingly, regions of the spinal cord where TrkB+ and TrkC+ afferents terminate (i.e., laminae II, III, IV, and VI) exhibit the highest levels of Cbln2 expression. Cbln2 is also expressed by preganglionic sympathetic neurons and their targets in the sympathetic chain ganglia. Thus, the results show that Cbln2 is frequently expressed by synaptically connected neuronal populations. This, in turn, raises the possibility that if Cbln2, like Cbln1, plays a role in the formation and maintenance of synapses, it may somehow mediate bi-directional communication between discrete populations of neurons and their appropriate neuronal targets.

Loss of inhibitory interneurons in the dorsal spinal cord and elevated itch in Bhlhb5 mutant mice

Itch is the least well understood of all the somatic senses, and the neural circuits that underlie this sensation are poorly defined. Here we show that the atonal-related transcription factor Bhlhb5 is transiently expressed in the dorsal horn of the developing spinal cord and appears to play a role in the formation and regulation of pruritic (itch) circuits. Mice lacking Bhlhb5 develop self-inflicted skin lesions and show significantly enhanced scratching responses to pruritic agents. Through genetic fate-mapping and conditional ablation, we provide evidence that the pruritic phenotype in Bhlhb5 mutants is due to selective loss of a subset of inhibitory interneurons in the dorsal horn. Our findings suggest that Bhlhb5 is required for the survival of a specific population of inhibitory interneurons that regulate pruritus, and provide evidence that the loss of inhibitory synaptic input results in abnormal itch.

The PAX258 gene subfamily: a comparative perspective

Whole genome duplication events are thought to have substantially contributed to organismal complexity, largely via divergent transcriptional regulation. Members of the vertebrate PAX2, PAX5 and PAX8 gene subfamily derived from an ancient class of paired box genes and arose from such whole genome duplication events. These genes are critical in establishing the midbrain-hindbrain boundary, specifying interneuron populations and for eye, ear and kidney development. Also PAX2 has adopted a unique role in pancreas development, whilst PAX5 is essential for early B-cell differentiation. The contribution of PAX258 genes to their collective role has diverged across paralogues and the animal lineages, resulting in a complex wealth of literature. It is now timely to provide a comprehensive comparative overview of these genes and their ancient and divergent roles. We also discuss their fundamental place within gene regulatory networks and the likely influence of cis-regulatory elements over their differential roles during early animal development.

Analysis of retinal cell development in chick embryo by immunohistochemistry and in ovo electroporation techniques

Background

Retinal cell development has been extensively investigated; however, the current knowledge of dynamic morphological and molecular changes is not yet complete.

Results

This study was aimed at revealing the dynamic morphological and molecular changes in retinal cell development during the embryonic stages using a new method of targeted retinal injection, in ovo electroporation, and immunohistochemistry techniques. A plasmid DNA that expresses the green fluorescent protein (GFP) as a marker was delivered into the sub-retinal space to transfect the chick retinal stem/progenitor cells at embryonic day 3 (E3) or E4 with the aid of pulses of electric current. The transfected retinal tissues were analyzed at various stages during chick development from near the start of neurogenesis at E4 to near the end of neurogenesis at E18. The expression of GFP allowed for clear visualization of cell morphologies and retinal laminar locations for the indication of retinal cell identity. Immunohistochemistry using cell type-specific markers (e.g., Visinin, Xap-1, Lim1+2, Pkcα, NeuN, Pax6, Brn3a, Vimentin, etc.) allowed further confirmation of retinal cell types. The composition of retinal cell types was then determined over time by counting the number of GFP-expressing cells observed with morphological characteristics specific to the various retinal cell types.

Conclusion

The new method of retinal injection and electroporation at E3 - E4 allows the visualization of all retinal cell types, including the late-born neurons, e.g., bipolar cells at a level of single cells, which has been difficult with a conventional method with injection and electroporation at E1.5. Based on data collected from analyses of cell morphology, laminar locations in the retina, immunohistochemistry, and cell counts of GFP-expressing cells, the time-line and dynamic morphological and molecular changes of retinal cell development were determined. These data provide more complete information on retinal cell development, and they can serve as a reference for the investigations in normal retinal development and diseases.

Conservation of gene linkage in dispersed vertebrate NK homeobox clusters

Nk homeobox genes are important regulators of many different developmental processes including muscle, heart, central nervous system and sensory organ development. They are thought to have arisen as part of the ANTP megacluster, which also gave rise to Hox and ParaHox genes, and at least some NK genes remain tightly linked in all animals examined so far. The protostome-deuterostome ancestor probably contained a cluster of nine Nk genes: (Msx)-(Nk4/tinman)-(Nk3/bagpipe)-(Lbx/ladybird)-(Tlx/c15)-(Nk7)-(Nk6/hgtx)-(Nk1/slouch)-(Nk5/Hmx). Of these genes, only NKX2.6-NKX3.1, LBX1-TLX1 and LBX2-TLX2 remain tightly linked in humans. However, it is currently unclear whether this is unique to the human genome as we do not know which of these Nk genes are clustered in other vertebrates. This makes it difficult to assess whether the remaining linkages are due to selective pressures or because chance rearrangements have "missed" certain genes. In this paper, we identify all of the paralogs of these ancestrally clustered NK genes in several distinct vertebrates. We demonstrate that tight linkages of Lbx1-Tlx1, Lbx2-Tlx2 and Nkx3.1-Nkx2.6 have been widely maintained in both the ray-finned and lobe-finned fish lineages. Moreover, the recently duplicated Hmx2-Hmx3 genes are also tightly linked. Finally, we show that Lbx1-Tlx1 and Hmx2-Hmx3 are flanked by highly conserved noncoding elements, suggesting that shared regulatory regions may have resulted in evolutionary pressure to maintain these linkages. Consistent with this, these pairs of genes have overlapping expression domains. In contrast, Lbx2-Tlx2 and Nkx3.1-Nkx2.6, which do not seem to be coexpressed, are also not associated with conserved noncoding sequences, suggesting that an alternative mechanism may be responsible for the continued clustering of these genes.

Multiple transcriptional mechanisms control Ptf1a levels during neural development including autoregulation by the PTF1-J complex

Ptf1a, along with an E protein and Rbpj, forms the transcription factor complex PTF1-J that is essential for proper specification of inhibitory neurons in the spinal cord, retina, and cerebellum. Here we show that two highly conserved noncoding genomic regions, a distal 2.3 kb sequence located 13.4 kb 5' and a 12.4 kb sequence located immediately 3' of the Ptf1a coding region, have distinct activity in controlling Ptf1a expression in all of these domains. The 5' 2.3 kb sequence functions as an autoregulatory element and directs reporter gene expression to all Ptf1a domains in the developing nervous system. The autoregulatory activity of this element was demonstrated by binding of the PTF1-J complex in vitro, Ptf1a localization to this genomic region in vivo, and the in vivo requirement of Ptf1a for the activity of the regulatory element in transgenic mice. In contrast, the 12.4 kb 3' regulatory region does not contain any conserved PTF1 sites, and its expression in transgenic mice is independent of Ptf1a. Thus, regulatory information for initiation of Ptf1a expression in the developing nervous system is located within the 12.4 kb sequence 3' of the Ptf1a gene. Together, these results identify multiple transcriptional mechanisms that control Ptf1a levels, one modulating levels by autoregulation through the PTF1-J complex, and the other a Ptf1a-independent mechanism for initial activation.

Neurog2 is a direct downstream target of the Ptf1a-Rbpj transcription complex in dorsal spinal cord

PTF1-J is a trimeric transcription factor complex essential for generating the correct balance of GABAergic and glutamatergic interneurons in multiple regions of the nervous system, including the dorsal horn of the spinal cord and the cerebellum. Although the components of PTF1-J have been identified as the basic helix-loop-helix (bHLH) factor Ptf1a, its heterodimeric E-protein partner, and Rbpj, no neural targets are known for this transcription factor complex. Here we identify the neuronal differentiation gene Neurog2 (Ngn2, Math4A, neurogenin 2) as a direct target of PTF1-J. A Neurog2 dorsal neural tube enhancer localized 3' of the Neurog2 coding sequence was identified that requires a PTF1-J binding site for dorsal activity in mouse and chick neural tube. Gain and loss of Ptf1a function in vivo demonstrate its role in Neurog2 enhancer activity. Furthermore, chromatin immunoprecipitation from neural tube tissue demonstrates that Ptf1a is bound to the Neurog2 enhancer. Thus, Neurog2 expression is directly regulated by the PTF1-J complex, identifying Neurog2 as the first neural target of Ptf1a and revealing a bHLH transcription factor cascade functioning in the specification of GABAergic neurons in the dorsal spinal cord and cerebellum.

Development of enteric neuron diversity

The mature enteric nervous system (ENS) is composed of many different neuron subtypes and enteric glia, which all arise from the neural crest. How this diversity is generated from neural crest-derived cells is a central question in neurogastroenterology, as defects in these processes are likely to underlie some paediatric motility disorders. Here we review the developmental appearance (the earliest age at which expression of specific markers can be localized) and birthdates (the age at which precursors exit the cell cycle) of different enteric neuron subtypes, and their projections to some targets. We then focus on what is known about the mechanisms underlying the generation of enteric neuron diversity and axon pathfinding. Finally, we review the development of the ENS in humans and the etiologies of a number of paediatric motility disorders.

Manifold functions of the Nail-Patella Syndrome gene Lmx1b in vertebrate development

The LIM (Lin-1, Isl-1 and Mec-3)-homeodomain transcription factor 1 beta (Lmx1b) is widely expressed in vertebrate embryos, and is implicated in the development of diverse structures such as limbs, kidneys, eyes and brains. LMX1B mutations in humans cause an autosomal dominant inherited disease called nail-patella syndrome (NPS), which is characterized by abnormalities of the arms and legs as well as kidney disease and glaucoma. Expression of Lmx1b in the dorsal compartment of growing limb buds is critical for specification of dorsal limb cell fates and consequently dorsoventral patterning of limbs. In addition, Lmx1b is involved in the differentiation of anterior eye structures, formation of the glomerular basement membrane in kidneys and development of the skeleton, especially calvarial bones. In the central nervous system, Lmx1b controls the inductive activity of isthmic organizer, differentiation and maintenance of central serotonergic neurons, as well as the differentiation and migration of spinal dorsal horn neurons. Although details of the genetic programs involved in these developmental events are largely unknown, it is suggested that Lmx1b plays central roles in fate determination or cell differentiation in these tissues. Sustained expression of Lmx1b in the postnatal and mature mouse brain suggests that it also plays important roles in brain maturation and in the regulation of normal brain functions. This review aims to highlight recent insights into the many activities of Lmx1b in vertebrates.

Identification and subclassification of new Atoh1 derived cell populations during mouse spinal cord development

At spinal levels, sensory information pertaining to body positioning (proprioception) is relayed to the cerebellum by the spinocerebellar tracts (SCTs). In the past we revealed the basic helix-loop-helix transcription factor Atoh1 (Math1) to be important for establishing Dorsal Progenitor 1 (DP1) commissural interneurons, which comprise a subset of proprioceptive interneurons. Given there exists multiple subdivisions of the SCT we asked whether Atoh1 may also play a role in specifying other cell types in the spinal cord. Here, we reveal the generation of at least three DP1 derived interneuron populations that reside at spatially restricted positions along the rostral-caudal axis. Each of these cell populations expresses distinct markers and anatomically coincides with the cell bodies of the various subdivisions of the SCT. In addition, we found that as development proceeds (e.g. by E13.5) Atoh1 expression becomes apparent in the dorsal midline in the region of the roof plate (RP). Interestingly, we find that cells derived from Atoh1 expressing RP progenitors express SSEA-1, and in the absence of Atoh1 these progenitors become SOX9 positive. Altogether we reveal the existence of multiple Atoh1 dependent cell types in the spinal cord, and uncover a novel progenitor domain that arises late in development.

Gata2 is a tissue-specific post-mitotic selector gene for midbrain GABAergic neurons

Midbrain GABAergic neurons control several aspects of behavior, but regulation of their development and diversity is poorly understood. Here, we further refine the midbrain regions active in GABAergic neurogenesis and show their correlation with the expression of the transcription factor Gata2. Using tissue-specific inactivation and ectopic expression, we show that Gata2 regulates GABAergic neuron development in the mouse midbrain, but not in rhombomere 1, where it is needed in the serotonergic lineage. Without Gata2, all the precursors in the embryonic midbrain fail to activate GABAergic neuron-specific gene expression and instead switch to a glutamatergic phenotype. Surprisingly, this fate switch is also observed throughout the neonatal midbrain, except for the GABAergic neurons located in the ventral dopaminergic nuclei, suggesting a distinct developmental pathway for these neurons. These studies identify Gata2 as an essential post-mitotic selector gene of the GABAergic neurotransmitter identity and demonstrate developmental heterogeneity of GABAergic neurons in the midbrain.

The bHLH transcription factor Olig3 marks the dorsal neuroepithelium of the hindbrain and is essential for the development of brainstem nuclei

The Olig3 gene encodes a bHLH factor that is expressed in the ventricular zone of the dorsal alar plate of the hindbrain. We found that the Olig3(+) progenitor domain encompassed subdomains that co-expressed Math1, Ngn1, Mash1 and Ptf1a. Olig3(+) cells give rise to neuronal types in the dorsal alar plate that we denote as class A neurons. We used genetic lineage tracing to demonstrate that class A neurons contribute to the nucleus of the solitary tract and to precerebellar nuclei. The fate of class A neurons was not correctly determined in Olig3 mutant mice. As a consequence, the nucleus of the solitary tract did not form, and precerebellar nuclei, such as the inferior olivary nucleus, were absent or small. At the expense of class A neurons, ectopic Lbx1(+) neurons appeared in the alar plate in Olig3 mutant mice. By contrast, electroporation of an Olig3 expression vector in the chick hindbrain suppressed the emergence of Lbx1(+) neurons. Climbing fiber neurons of the inferior olivary nucleus express Foxd3 and require Olig3 as well as Ptf1a for the determination of their fate. We observed that electroporation of Olig3 and Ptf1a expression vectors, but not either alone, induced Foxd3. We therefore propose that Olig3 can cooperate with Ptf1a to determine the fate of climbing fiber neurons of the inferior olivary nucleus.

A differential developmental pattern of spinal interneuron apoptosis during synaptogenesis: insights from genetic analyses of the protocadherin-gamma gene cluster

Although the role of developmental apoptosis in shaping the complement and connectivity of sensory and motoneurons is well documented, the extent to which cell death affects the 13 cardinal classes of spinal interneurons is unclear. Using a series of genetic manipulations in vivo, we demonstrate for the first time a differential pattern of developmental apoptosis in molecularly identified spinal interneuron populations, and implicate the adhesion molecule family encoded by the 22-member protocadherin-gamma (Pcdh-gamma) gene cluster in its control. In constitutive Pcdh-gamma null mouse embryos, many interneuron populations undergo increased apoptosis, but to differing extents: for example, over 80% of En1-positive V1 neurons are lost, whereas only 30% of Chx10-positive V2a neurons are lost and there is no reduction in the number of V1-derived Renshaw cells. We show that this represents an exacerbation of a normal, underlying developmental pattern: the extent of each population's decrease in Pcdh-gamma mutants is precisely commensurate both with the extent of its loss during normal embryogenesis and with the extent of its increase in Bax(-/-) mice, in which apoptosis is genetically blocked. Interneuron apoptosis begins during the first wave of synaptogenesisis in the spinal cord, occurring first among ventral populations (primarily between E14 and E17), and only later among dorsal populations (primarily after P0). Utilizing a new, conditional Pcdh-gamma mutant allele, we show that the gamma-Pcdhs can promote survival non-cell-autonomously: mutant neurons can survive if they are surrounded by normal neurons, and normal neurons can undergo apoptosis if they are surrounded by mutant neurons.

Central respiratory rhythmogenesis is abnormal in lbx1- deficient mice

Lbx1 is a transcription factor that determines neuronal cell fate and identity in the developing medulla and spinal cord. Newborn Lbx1 mutant mice die of respiratory distress during the early postnatal period. Using in vitro brainstem-spinal cord preparations we tested the hypothesis that Lbx1 is necessary for the inception, development and modulation of central respiratory rhythmogenesis. The inception of respiratory rhythmogenesis at embryonic day 15 (E15) was not perturbed in Lbx1 mutant mice. However, the typical age-dependent increase in respiratory frequency observed in wild-type from E15 to P0 was not observed in Lbx1 mutant mice. The slow respiratory rhythms in E18.5 Lbx1 mutant preparations were increased to wild-type frequencies by application of substance P, thyrotropin releasing hormone, serotonin, noradrenaline, or the ampakine drug 1-(1,4-benzodioxan-6-yl-carbonyl) piperidine. Those data suggest that respiratory rhythm generation within the pre-Bötzinger complex (preBötC) is presumably functional in Lbx1 mutant mice with additional neurochemical drive. This was supported by anatomical data showing that the gross structure of the preBötC was normal, although there were major defects in neuronal populations that provide important modulatory drive to the preBötC including the retrotrapezoid nucleus, catecholaminergic brainstem nuclei, nucleus of the solitary tract, and populations of inhibitory neurons in the ventrolateral and dorsomedial medullary nuclei. Finally, we determined that those defects were caused by abnormalities of neuronal specification early in development or subsequent neuronal migration.

Pax2/8 act redundantly to specify glycinergic and GABAergic fates of multiple spinal interneurons

The spinal cord contains several distinct classes of neurons but it is still unclear how many of the functional characteristics of these cells are specified. One of the most crucial functional characteristics of a neuron is its neurotransmitter fate. In this paper, we show that in zebrafish most glycinergic and many GABAergic spinal interneurons express Pax2a, Pax2b and Pax8 and that these transcription factors are redundantly required for the neurotransmitter fates of many of these cells. We also demonstrate that the function of these Pax2/8 transcription factors is very specific: in embryos in which Pax2a, Pax2b and Pax8 are simultaneously knocked-down, many neurons lose their glycinergic and/or GABAergic characteristics, but they do not become glutamatergic or cholinergic and their soma morphologies and axon trajectories are unchanged. In mouse, Pax2 is required for correct specification of GABAergic interneurons in the dorsal horn, but it is not required for the neurotransmitter fates of other Pax2-expressing spinal neurons. Our results suggest that this is probably due to redundancy with Pax8 and that the function of Pax2/8 in specifying GABAergic and glycinergic neuronal fates is much broader than was previously appreciated and is highly conserved between different vertebrates.

Xenopus zinc finger transcription factor IA1 (Insm1) expression marks anteroventral noradrenergic neuron progenitors in Xenopus embryos

The evolutionarily conserved IA1 (Insm1) gene is strongly expressed in the developing nervous system. Here, we show that IA1 is expressed during Xenopus laevis embryogenesis in neural plate primary neurons as well as in a population of uncharacterized anteroventral cells that form in front of the cement gland and that we identified as noradrenergic neurons. We also show that the formation of those anteroventral cells is dependent on BMPs and inhibited by Notch and that it is regulated by the transcription factors Xash1, Phox2, and Hand2. Finally, we provide functional evidence suggesting that IA1 may also play a role in their formation. Together, our results reveal that IA1 constitutes a novel player downstream of Xash1 in the formation of a previously unidentified population of Xenopus noradrenergic primary neurons.

Control of precerebellar neuron development by Olig3 bHLH transcription factor

The rhombic lip (RL) is the neuroepithelium immediately adjacent to the roof plate of the fourth ventricle, and it gives rise to various brainstem and cerebellar cell types. Our study shows that the bHLH (basic helix-loop-helix) transcription factor Olig3 is expressed in the progenitors of RL, and ablation of Olig3 significantly affects the development of RL. In Olig3-/- caudal RL, the expression level of Math1 in the dorsal interneuron 1 (dI1) domain is reduced, and the formation of four mossy-fiber nuclei is compromised; dI2-dI3 neurons are misspecified to dI4 interneurons, and the climbing-fiber neurons (inferior olive nucleus) are completely lost. In addition, the formation of brainstem (nor)adrenergic centers and first-order relay visceral sensory neurons is also dependent on Olig3. Therefore, Olig3 plays an important role in the fate specification and differentiation of caudal RL-derived neurons.

Postnatal ontogeny of the transcription factor Lmx1b in the mouse central nervous system

The expression profile of Lim homeodomain transcription factor Lmx1b in the mouse brain was investigated at different postnatal stages by immunohistochemistry and in situ hybridization. At postnatal day (P) 7, many Lmx1b-expressing neurons were found in the posterior hypothalamic area, supramammillary nucleus, ventral premammillary nucleus, and subthalamic nucleus. In the midbrain, numerous Lmx1b-expressing neurons were present in the substantia nigra pars compacta and ventral tegmental area. In the hindbrain, Lmx1b-expressing neurons were primarily observed in the raphe nuclei, parabrachial nuclei, principal sensory trigeminal nucleus, nucleus of the solitary tract, and laminae I-II of the medullary dorsal horn as well as spinal dorsal horn. Although expression levels diminished as postnatal life progressed, persistent expression throughout the first year of life was observed in many of these regions. In contrast, Lmx1b was present in a few brain regions (e.g., principal sensory trigeminal nucleus) only in early life with expression expiring by P60. Lmx1b was observed in dopaminergic neurons in the midbrain and serotonergic neurons in the hindbrain, as determined by double labeling with specific markers. In addition, we found that Lmx1b-expressing neurons are not GABAergic, and Lmx1b was colocalized with Tlx3 in the parabrachial nuclei, principal sensory trigeminal nucleus, nucleus of the solitary tract. as well as the medullary and spinal dorsal horns, suggesting that Lmx1b-expressing cells in these areas are excitatory neurons. Our data suggest that Lmx1b is involved in the postnatal maturation of certain types of neurons and maintenance of their normal functions in the adult brain.

Regulation of gene expression in the nervous system

The nervous system contains a multitude of cell types which are specified during development by cascades of transcription factors acting combinatorially. Some of these transcription factors are only active during development, whereas others continue to function in the mature nervous system to maintain appropriate gene-expression patterns in differentiated cells. Underpinning the function of the nervous system is its plasticity in response to external stimuli, and many transcription factors are involved in regulating gene expression in response to neuronal activity, allowing us to learn, remember and make complex decisions. Here we review some of the recent findings that have uncovered the molecular mechanisms that underpin the control of gene regulatory networks within the nervous system. We highlight some recent insights into the gene-regulatory circuits in the development and differentiation of cells within the nervous system and discuss some of the mechanisms by which synaptic transmission influences transcription-factor activity in the mature nervous system. Mutations in genes that are important in epigenetic regulation (by influencing DNA methylation and post-translational histone modifications) have long been associated with neuronal disorders in humans such as Rett syndrome, Huntington's disease and some forms of mental retardation, and recent work has focused on unravelling their mechanisms of action. Finally, the discovery of microRNAs has produced a paradigm shift in gene expression, and we provide some examples and discuss the contribution of microRNAs to maintaining dynamic gene regulatory networks in the brain.

A transcriptional network coordinately determines transmitter and peptidergic fate in the dorsal spinal cord

Dorsal horn neurons express many different neuropeptides that modulate sensory perception like the sensation of pain. Inhibitory neurons of the dorsal horn derive from postmitotic neurons that express Pax2, Lbx1 and Lhx1/5, and diversify during maturation. In particular, fractions of maturing inhibitory neurons express various neuropeptides. We demonstrate here that a coordinate molecular mechanism determines inhibitory and peptidergic fate in the developing dorsal horn. A bHLH factor complex that contains Ptf1a acts as upstream regulator and initiates the expression of several downstream transcription factors in the future inhibitory neurons, of which Pax2 is known to determine the neurotransmitter phenotype. We demonstrate here that dynorphin, galanin, NPY, nociceptin and enkephalin expression depends on Ptf1a, indicating that these neuropeptides are expressed in inhibitory neurons. Furthermore, we show that Neurod1/2/6 and Lhx1/5, which act downstream of Ptf1a, control distinct aspects of peptidergic differentiation. In particular, the Neurod1/2/6 factors are essential for dynorphin and galanin expression, whereas the Lhx1/5 factors are essential for NPY expression. We conclude that a transcriptional network operates in maturing dorsal horn neurons that coordinately determines transmitter and peptidergic fate.

Expression and function of Nkx6.3 in vertebrate hindbrain

Homeodomain transcription factors serve important functions in organogenesis and tissue differentiation, particularly with respect to the positional identity of individual cells. The Nkx6 subfamily controls tissue differentiation in the developing central nervous system where they function as transcriptional repressor proteins. Recent work indicates that Nkx6.3 is expressed in hindbrain V2 interneurons that co-express Nkx6.1, suggesting the possibility of functional redundancy. Here, we report that Nkx6.3 expression is specific to Chx10+ V2a interneurons but not to Gata3+ V2b interneurons of the hindbrain, and that Nkx6.3 expression appears to mark cells of the prospective medullary reticular formation. Molecular analysis of Nkx6.3 null embryonic mouse hindbrain did not reveal detectable defects in progenitor markers, motor neuron or V2 interneuron sub-types. Forced expression of Nkx6.3 and Nkx6.1 promote V2 interneuron differentiation in the developing chick hindbrain. These findings indicate Nkx6.3 function is dispensable for CNS development and lead to the proposal that absence of overt defects is due to functional compensation from a related homeodomain transcription factor.

Ptf1a, Lbx1 and Pax2 coordinate glycinergic and peptidergic transmitter phenotypes in dorsal spinal inhibitory neurons

Inhibitory neurons in the dorsal horn synthesize a variety of neurotransmitters, including GABA, glycine and a set of peptides. Here we show that three transcription factors, Ptf1a, Pax2, and Lbx1, which have been reported to promote a GABAergic cell fate, also specify glycinergic and peptidergic transmitter phenotypes. First, Ptf1a appears to be a master regulator, as indicated by a requirement of Ptf1a for the expression of glycinergic marker GlyT2 and a set of peptides, including neuropeptide Y (NPY), nociceptin/orphanin FQ (N/OFQ), somatostatin (SOM), enkephalin (ENK), dynorphin (DYN) and galanin (GAL). Second, Pax2 is a downstream target of Ptf1a and controls subsets of transmitter phenotypes, including the expression of GlyT2, NPY, N/OFQ, DYN, and GAL, but is dispensable for SOM or ENK expression. Third, for Lbx1, due to neuronal cell loss at late stages, our analyses focused on early embryonic stages, and we found that Lbx1 is required for the expression of GlyT2, NPY, N/OFQ and is partially responsible for SOM expression. Our studies therefore suggest a coordinated and hierarchical specification of a variety of neurotransmitters in dorsal spinal inhibitory neurons.

Comparative genomics of Lbx loci reveals conservation of identical Lbx ohnologs in bony vertebrates

Background

Lbx/ladybird genes originated as part of the metazoan cluster of Nk homeobox genes. In all animals investigated so far, both the protostome genes and the vertebrate Lbx1 genes were found to play crucial roles in neural and muscle development. Recently however, additional Lbx genes with divergent expression patterns were discovered in amniotes. Early in the evolution of vertebrates, two rounds of whole genome duplication are thought to have occurred, during which 4 Lbx genes were generated. Which of these genes were maintained in extant vertebrates, and how these genes and their functions evolved, is not known.

Results

Here we searched vertebrate genomes for Lbx genes and discovered novel members of this gene family. We also identified signature genes linked to particular Lbx loci and traced the remnants of 4 Lbx paralogons (two of which retain Lbx genes) in amniotes. In teleosts, that have undergone an additional genome duplication, 8 Lbx paralogons (three of which retain Lbx genes) were found. Phylogenetic analyses of Lbx and Lbx-associated genes show that in extant, bony vertebrates only Lbx1- and Lbx2-type genes are maintained. Of these, some Lbx2 sequences evolved faster and were probably subject to neofunctionalisation, while Lbx1 genes may have retained more features of the ancestral Lbx gene. Genes at Lbx1 and former Lbx4 loci are more closely related, as are genes at Lbx2 and former Lbx3 loci. This suggests that during the second vertebrate genome duplication, Lbx1/4 and Lbx2/3 paralogons were generated from the duplicated Lbx loci created during the first duplication event.

Conclusion

Our study establishes for the first time the evolutionary history of Lbx genes in bony vertebrates, including the order of gene duplication events, gene loss and phylogenetic relationships. Moreover, we identified genetic hallmarks for each of the Lbx paralogons that can be used to trace Lbx genes as other vertebrate genomes become available. Significantly, we show that bony vertebrates only retained copies of Lbx1 and Lbx2 genes, with some Lbx2 genes being highly divergent. Thus, we have established a base on which the evolution of Lbx gene function in vertebrate development can be evaluated.

Regulation of cholinergic phenotype in developing neurons

Specification of neurotransmitter phenotype is critical for neural circuit development and is influenced by intrinsic and extrinsic factors. Recent findings in rat hypothalamus in vitro suggest the role of neurotransmitter glutamate in the regulation of cholinergic phenotype. Here we extended our previous studies on the mechanisms of glutamate-dependent regulation of cholinergic phenotypic properties in hypothalamic neurons. Using immunocytochemistry, electrophysiology, and calcium imaging, we demonstrate that hypothalamic expression of choline acetyltransferase (the cholinergic marker) and responsiveness of neurons to acetylcholine (ACh) receptor agonists increase during chronic administration of an N-methyl-D-aspartate receptor (NMDAR) blocker, MK-801, in developing rats in vivo and genetic and pharmacological inactivation of NMDARs in mouse and rat developing neuronal cultures. In hypothalamic cultures, an inactivation of NMDA receptors also induces ACh-dependent synaptic activity, as do inactivations of PKA, ERK/MAPK, CREB, and NF-kappaB, which are known to be regulated by NMDA receptors. Interestingly, the increase in cholinergic properties in developing neurons that is induced by NMDAR blockade is prevented by the blockade of ACh receptors, suggesting that function of ACh receptor is required for the cholinergic up-regulation. Using dual recording of monosynaptic excitatory postsynaptic currents, we further demonstrate that chronic inactivation of ionotropic glutamate receptors induces the cholinergic phenotype in a subset of glutamatergic neurons. The phenotypic switch is partial as ACh and glutamate are coreleased. The results suggest that developing neurons may not only coexpress multiple transmitter phenotypes, but can also change the phenotypes following changes in signaling in neuronal circuits.

Functional development of the enteric nervous system--from migration to motility

The enteric nervous system (ENS) consists of many different types of enteric neurones forming complex reflex circuits that underlie or regulate many gut functions. Studies of humans with Hirschsprung's disease (distal aganglionosis), and of animal models of Hirschsprung's disease, have led to the identification of many of the genetic, molecular and cellular mechanisms responsible for the colonization of the gut by enteric neurone precursors. However, later events in the ENS development are still poorly understood, including the development of functioning ENS circuits. This article is a personal view of the current state of play in our understanding of the ENS development and of the future of the field.

Tlx1 and Tlx3 coordinate specification of dorsal horn pain-modulatory peptidergic neurons

The dorsal spinal cord synthesizes a variety of neuropeptides that modulate the transmission of nociceptive sensory information. Here, we used genetic fate mapping to show that Tlx3(+) spinal cord neurons and their derivatives represent a heterogeneous population of neurons, marked by partially overlapping expression of a set of neuropeptide genes, including those encoding the anti-opioid peptide cholecystokinin, pronociceptive Substance P (SP), Neurokinin B, and a late wave of somatostatin. Mutations of Tlx3 and Tlx1 result in a loss of expression of these peptide genes. Brn3a, a homeobox transcription factor, the expression of which is partly dependent on Tlx3, is required specifically for the early wave of SP expression. These studies suggest that Tlx1 and Tlx3 operate high in the regulatory hierarchy that coordinates specification of dorsal horn pain-modulatory peptidergic neurons.

Tlx3 exerts context-dependent transcriptional regulation and promotes neuronal differentiation from embryonic stem cells

The T cell leukemia 3 (Tlx3) gene has been implicated in specification of glutamatergic sensory neurons in the spinal cord. In cranial sensory ganglia, Tlx3 is highly expressed in differentiating neurons during early embryogenesis. To study a role of Tlx3 during neural differentiation, mouse embryonic stem (ES) cells were transfected with a Tlx3 expression vector. ES cells stably expressing Tlx3 were grown in the presence or absence of a neural induction medium. In undifferentiated ES cells, there was no significant difference in gene expression in the presence or absence of Tlx3, even after ES cells were cultured for an extensive time period. In contrast, expression levels of Mash1, Ngn1, and NeuroD were significantly higher in Tlx3-expressing cells after neural induction for 4 days compared with those in cells expressing the control vector. At 7 days after neural induction, whereas expression of the proneural genes was down-regulated, VGLUT2, GluR2, and GluR4 were significantly increased in ES cell-derived neurons expressing Tlx3. The sequential and coordinated expression of the proneural and neuronal subtype-specific genes identifies Tlx3 as a selector gene in ES cells undergoing neural differentiation. In addition, the differential effects of Tlx3 overexpression in undifferentiated ES cells compared with ES cell-derived neurons suggest that Tlx3 exerts context-dependent transcriptional signals on its downstream target genes. The context-dependent function of Tlx3 as a selector gene may be used to establish a novel strategy to conditionally generate excitatory glutamatergic neurons from ES cells to cure various types of neurodegenerative disorders.

A nonclassical bHLH Rbpj transcription factor complex is required for specification of GABAergic neurons independent of Notch signaling

Neural networks are balanced by inhibitory and excitatory neuronal activity. The formation of these networks is initially generated through neuronal subtype specification controlled by transcription factors. The basic helix-loop-helix (bHLH) transcription factor Ptf1a is essential for the generation of GABAergic inhibitory neurons in the dorsal spinal cord, cerebellum, and retina. The transcription factor Rbpj is a transducer of the Notch signaling pathway that functions to maintain neural progenitor cells. Here we demonstrate Ptf1a and Rbpj interact in a complex that is required in vivo for specification of the GABAergic neurons, a function that cannot be substituted by the classical form of the bHLH heterodimer with E-protein or Notch signaling through Rbpj. We show that a mutant form of Ptf1a without the ability to bind Rbpj, while retaining its ability to interact with E-protein, is incapable of inducing GABAergic (Pax2)- and suppressing glutamatergic (Tlx3)-expressing cells in the chick and mouse neural tube. Moreover, we use an Rbpj conditional mutation to demonstrate that Rbpj function is essential for GABAergic specification, and that this function is independent of the Notch signaling pathway. Together, these findings demonstrate the requirement for a Ptf1a-Rbpj complex in controlling the balanced formation of inhibitory and excitatory neurons in the developing spinal cord, and point to a novel Notch-independent function for Rbpj in nervous system development.

Update of the pathophysiology of the restless-legs-syndrome

The Restless Legs Syndrome (RLS) is a heterogeneous disease. Symptomatic or secondary forms encompass iron deficiency, uremia, pregnancy, polyneuropathy, and other causes. The so-called idiopathic RLS syndrome preferentially affects patients with a younger onset before the age of 30. Here we summarize pathophysiological results along the anatomical route, beginning at the cortex and followed by the basal ganglia, thalamus, A11 neurones, substantia nigra, brainstem nuclei, and spinal cord. Genetic risk variants for RLS have recently been identified in two genes, one of them the homeobox gene MEIS1, known to be involved in embryonic development and variants in a second locus containing the genes encoding mitogen-activated protein kinase MAP2K5, and the transcription factor LBXCOR1. A third one, the BTBD9 gene with unknown function encodes a BTB(POZ) domain. Accordingly, new concepts on pathophysiology have to bridge conventional knowledge with possible consequences deriving from these findings. Furthermore, this may create a framework to help understand why dopamine, opioid, and some anticonvulsant therapies are effective in RLS patients.

Genome-wide view of cell fate specification: ladybird acts at multiple levels during diversification of muscle and heart precursors

Correct diversification of cell types during development ensures the formation of functional organs. The evolutionarily conserved homeobox genes from ladybird/Lbx family were found to act as cell identity genes in a number of embryonic tissues. A prior genetic analysis showed that during Drosophila muscle and heart development ladybird is required for the specification of a subset of muscular and cardiac precursors. To learn how ladybird genes exert their cell identity functions we performed muscle and heart-targeted genome-wide transcriptional profiling and a chromatin immunoprecipitation (ChIP)-on-chip search for direct Ladybird targets. Our data reveal that ladybird not only contributes to the combinatorial code of transcription factors specifying the identity of muscle and cardiac precursors, but also regulates a large number of genes involved in setting cell shape, adhesion, and motility. Among direct ladybird targets, we identified bric-a-brac 2 gene as a new component of identity code and inflated encoding alphaPS2-integrin playing a pivotal role in cell-cell interactions. Unexpectedly, ladybird also contributes to the regulation of terminal differentiation genes encoding structural muscle proteins or contributing to muscle contractility. Thus, the identity gene-governed diversification of cell types is a multistep process involving the transcriptional control of genes determining both morphological and functional properties of cells.

Analysis of transcription, chromatin dynamics and epigenetic changes in neural genes

The ways in which gene transcription is investigated have undergone radical change since the turn of the millennium. Piece-meal approaches focussed upon model genes have increasingly been complemented by genome-wide approaches that allow interrogation of multiple cohorts of genes or even entire genomes. This sea change has been founded upon the increasing availability of whole genome sequences and the attendant evolution of microarray based discovery platforms. Collectively, these approaches are being used to build a global and dynamic perspective of transcription factor occupancy, co-factor recruitment and epigenetic signature. As yet, few of these approaches have been applied to the study of neuronal gene transcription, but this is set to change. Here, I review these key developments and point to their potential application to the study of transcriptional and epigenetic changes in neurons in health and disease.

Contrasting and brain region-specific roles of neurogenin2 and mash1 in GABAergic neuron differentiation in vitro

We have cultivated highly uniform populations of neural precursor cells, which retain their region-specific identities, from various rat embryonic brain regions. The roles of the proneural basic-helix-loop-helix (bHLH) factors neurogenin2 (Ngn2) and Mash1 in gamma-aminobutyric acid (GABA) neuron differentiation were explored in the region-specific cultures. Consistent with previous in vivo studies, forced expression of Mash1 promoted GABA neuron formation from the precursors derived from the developing forebrains, whereas Ngn2 displayed an inhibitory role in forebrain GABA neuron differentiation. Functional analyses of mutant bHLH proteins indicated that the helix-loop-helix domains of Mash1 and Ngn2, known as the structures for protein-protein interactions, impart the distinct activities. Intriguingly, the regulatory activities of Mash1 and Ngn2 in GABA neuron differentiation from the hindbrain- and spinal cord-derived precursor cells were completely opposite of those observed in the forebrain-derived cultures: increased GABA neuron yield by Ngn2 and decreased yield by Mash1 were shown in the precursors of those posterior brain regions. No clear difference that depended on dorsal-ventral brain regions was observed in the bHLH-mediated activities. Finally, we demonstrated that Otx2, the expression of which is developmentally confined to the regions anterior to the isthmus, is a factor responsible for the anterior-posterior region-dependent opposite effects of the bHLH proteins.

Helt determines GABAergic over glutamatergic neuronal fate by repressing Ngn genes in the developing mesencephalon

The mechanism underlying the determination of neurotransmitter phenotype in the developing mesencephalon, particularly GABAergic versus glutamatergic fate, remains largely unknown. Here, we show in mice that the basic helix-loop-helix transcriptional repressor gene Helt (also known as Megane and Heslike) functions as a selector gene that determines GABAergic over glutamatergic fate in the mesencephalon. Helt was coincidently expressed in all the progenitor domains for mesencephalic GABAergic neurons. In the mesencephalon of Helt-deficient embryos, GABAergic neurons were mostly absent and glutamatergic neurons emerged instead. Conversely, ectopically expressed Helt suppressed glutamatergic formation and induced GABAergic neurogenesis. However, the Helt mutants showed normal progenitor domain formation. In consequence, postmitotic expression of the homeodomain factor Nkx2.2, which was specifically expressed by GABAergic populations in wild-type embryos, was maintained despite the transmitter phenotype conversion from GABAergic to glutamatergic in the Helt mutants, suggesting that Helt is not involved in neuronal identity specification. Furthermore, we identified proneural genes Ngn1 and Ngn2, which were selectively expressed in glutamatergic progenitors in the developing mesencephalon and had the ability to confer the glutamatergic fate, as downstream target genes of Helt. These results suggest that Helt determines GABAergic over glutamatergic fate, at least in part, by repressing Ngn (Neurog) genes and that basic helix-loop-helix transcription factor networks involving Helt and Ngns are commonly used in the mesencephalon for determination of the GABAergic versus glutamatergic transmitter phenotype.

Regulation of the GABA cell phenotype in hippocampus of schizophrenics and bipolars

GABAergic dysfunction is present in the hippocampus in schizophrenia (SZ) and bipolar disorder (BD). The trisynaptic pathway was "deconstructed" into various layers of sectors CA3/2 and CA1 and gene expression profiling performed. Network association analysis was used to uncover genes that may be related to regulation of glutamate decarboxylase 67 (GAD(67)), a marker for this system that has been found by many studies to show decreased expression in SZs and BDs. The most striking change was a down-regulation of GAD(67) in the stratum oriens (SO) of CA2/3 in both groups; CA1 only showed changes in the SO of schizophrenics. The network generated for GAD(67) contained 25 genes involved in the regulation of kainate receptors, TGF-beta and Wnt signaling, as well as transcription factors involved in cell growth and differentiation. In SZs, IL-1beta, (GRIK2/3), TGF-beta2, TGF-betaR1, histone deacetylase 1 (HDAC1), death associated protein (DAXX), and cyclin D2 (CCND2) were all significantly up-regulated, whereas in BDs, PAX5, Runx2, LEF1, TLE1, and CCND2 were significantly down-regulated. In the SO of CA1 of BDs, where GAD67 showed no expression change, TGF-beta and Wnt signaling genes were all up-regulated, but other transcription factors showed no change in expression. In other layers/sectors, BDs showed no expression changes in these GAD(67) network genes. Overall, these results are consistent with the hypothesis that decreased expression of GAD(67) may be associated with an epigenetic mechanism in SZ. In BD, however, a suppression of transcription factors involved in cell differentiation may contribute to GABA dysfunction.

Selective expression of Bhlhb5 in subsets of early-born interneurons and late-born association neurons in the spinal cord

Accumulating evidence has suggested that the basic helix-loop-helix transcription factors play important roles in controlling neuronal fate specification and differentiation in the developing central nervous system. In this study, we report a detailed immunological study on the expression of Bhlhb5 in embryonic mouse spinal cord with a newly developed antibody. At the early stage of neural development, Bhlhb5 is specifically expressed in dI6 dorsal interneurons and in V1 and V2 ventral interneurons. At late stages of development, Bhlhb5 expression is detected in a subset of late-born dorsal association interneurons that migrate into the uppermost layer of the dorsal horn.

Lhx1 and Lhx5 maintain the inhibitory-neurotransmitter status of interneurons in the dorsal spinal cord

Lhx1 and Lhx5 are co-expressed in multiple interneuron cell types in the developing spinal cord. These include early-born dI4 and dI6 inhibitory interneurons, as well as late-born inhibitory dILA neurons (dILA), all of which express the paired-domain transcription factor Pax2. Although it appears that Lhx1 and Lhx5 do not control the initial specification of the neuronal cell types in which they are expressed, we have found a cell-autonomous requirement for either Lhx1 or Lhx5 to maintain the expression of Pax2, Pax5 and Pax8 in dorsal inhibitory neurons at later developmental stages. Lhx1; Lhx5 double-knockout mice exhibit a downregulation of Gad1 and Viaat (Slc32a1) from E13.5 onwards that is closely associated with a decrease in Pax2 expression. Pax2 is a key factor for dorsal GABAergic identity, with the expression of Pax5 and Pax8 being differentially dependent on Pax2 in the dorsal horn. In summary, our findings support a model in which the differentiation of GABAergic interneurons in the dorsal cord depends on Pax2, with Lhx1 and Lhx5 helping to activate and maintain Pax2 expression in these cells. Lhx1 and Lhx5 therefore function together with Pax2, Pax5 and Pax8 to establish a GABAergic inhibitory-neurotransmitter program in dorsal horn interneurons.

SoxD proteins influence multiple stages of oligodendrocyte development and modulate SoxE protein function

The myelin-forming oligodendrocytes are an excellent model to study transcriptional regulation of specification events, lineage progression, and terminal differentiation in the central nervous system. Here, we show that the group D Sox transcription factors Sox5 and Sox6 jointly and cell-autonomously regulate several stages of oligodendrocyte development in the mouse spinal cord. They repress specification and terminal differentiation and influence migration patterns. As a consequence, oligodendrocyte precursors and terminally differentiating oligodendrocytes appear precociously in spinal cords deficient for both Sox proteins. Sox5 and Sox6 have opposite functions than the group E Sox proteins Sox9 and Sox10, which promote oligodendrocyte specification and terminal differentiation. Both genetic as well as molecular evidence suggests that Sox5 and Sox6 directly interfere with the function of group E Sox proteins. Our studies reveal a complex regulatory network between different groups of Sox proteins that is essential for proper progression of oligodendrocyte development.

Electrophysiological and gene expression profiling of neuronal cell types in mammalian neocortex

It is a challenging question to understand how different neuronal types are organized into a complex architecture in the cortex, an architecture which is also adapted in different regions to subserve very different functions. Recent developments in genetic and molecular techniques have opened up the possibility of using gene expression profiling for neuronal cell typing, with the aim of uncovering novel cell types and the underlying mechanisms which generate and maintain neuronal heterogeneity in the cortex. This review introduces some current ideas about neuronal cell types in the cortex and describes recent approaches to expression profiling for defining cortical neuronal cell types.

The development and modulation of nociceptive circuitry

Nociceptive circuitry processes the signals evoked by activating specialized peripheral sensory receptors for pain perception. Recent studies show that the neuronal phenotypes in the dorsal root ganglia and spinal dorsal horn are determined by distinct sets of transcription factors during development. Anatomical analyses with genetic approaches demonstrate that each subset of nociceptive sensory neurons has topographically distinct circuits at both spinal and brain levels. Moreover, the sensitivity of primary afferents can be rapidly regulated not only by phosphorylation of receptors, ion channels and associated regulatory proteins but also by stimulus-induced cell surface expression of G-protein-coupled receptors. In chronic pain states the molecular characteristics of spinal nociceptive circuits are altered, enabling normal peripheral stimuli to induce pain hypersensitivity.

Transcriptional regulation of neuronal phenotype in mammals

Transcription factors (TFs) play pivotal roles in directing the formation of neurons and glia. Here I will review the recent genome-scale analysis of the expression of TFs in the developing mouse nervous system and discuss the logic by which TFs control the establishment of neuronal phenotype. Accumulating evidence suggests that while combinatorial action of TFs is able to define the basic framework of the nervous system, other control mechanisms, such as stochastic and epigenetic regulation of gene expression, also contribute to the generation of nerve cell diversity.