Ptf1a determines GABAergic over glutamatergic neuronal cell fate in the spinal cord dorsal horn

Prdm13 forms a feedback loop with Ptf1a and is required for glycinergic amacrine cell genesis in the Xenopus Retina

Background

Amacrine interneurons that modulate synaptic plasticity between bipolar and ganglion cells constitute the most diverse cell type in the retina. Most are inhibitory neurons using either GABA or glycine as neurotransmitters. Although several transcription factors involved in amacrine cell fate determination have been identified, mechanisms underlying amacrine cell subtype specification remain to be further understood. The Prdm13 histone methyltransferase encoding gene is a target of the transcription factor Ptf1a, an essential regulator of inhibitory neuron cell fate in the retina. Here, we have deepened our knowledge on its interaction with Ptf1a and investigated its role in amacrine cell subtype determination in the developing Xenopus retina.

Methods

We performed prdm13 gain and loss of function in Xenopus and assessed the impact on retinal cell fate determination using RT-qPCR, in situ hybridization and immunohistochemistry.

Results

We found that prdm13 in the amphibian Xenopus is expressed in few retinal progenitors and in about 40% of mature amacrine cells, predominantly in glycinergic ones. Clonal analysis in the retina reveals that prdm13 overexpression favours amacrine cell fate determination, with a bias towards glycinergic cells. Conversely, knockdown of prdm13 specifically inhibits glycinergic amacrine cell genesis. We also showed that, as in the neural tube, prdm13 is subjected to a negative autoregulation in the retina. Our data suggest that this is likely due to its ability to repress the expression of its inducer, ptf1a.

Conclusions

Our results demonstrate that Prdm13, downstream of Ptf1a, acts as an important regulator of glycinergic amacrine subtype specification in the Xenopus retina. We also reveal that Prdm13 regulates ptf1a expression through a negative feedback loop.

Distinct Activities of Tfap2A and Tfap2B in the Specification of GABAergic Interneurons in the Developing Cerebellum

GABAergic inhibitory neurons in the cerebellum are subdivided into Purkinje cells and distinct subtypes of interneurons from the same pool of progenitors, but the determinants of this diversification process are not well defined. To explore the transcriptional regulation of the development of cerebellar inhibitory neurons, we examined the role of Tfap2A and Tfap2B in the specification of GABAergic neuronal subtypes in mice. We show that Tfap2A and Tfap2B are expressed in inhibitory precursors during embryonic development and that their expression persists into adulthood. The onset of their expression follows Ptf1a and Olig2, key determinants of GABAergic neuronal fate in the cerebellum; and, their expression precedes Pax2, an interneuron-specific factor. Tfap2A is expressed by all GABAergic neurons, whereas Tfap2B is selectively expressed by interneurons. Genetic manipulation via in utero electroporation (IUE) reveals that Tfap2B is necessary for interneuron specification and is capable of suppressing the generation of excitatory cells. Tfap2A, but not Tfap2B, is capable of inducing the generation of interneurons when misexpressed in the ventricular neuroepithelium. Together, our results demonstrate that the differential expression of Tfap2A and Tfap2B defines subtypes of GABAergic neurons and plays specific, but complementary roles in the specification of interneurons in the developing cerebellum.

Repression by PRDM13 is critical for generating precision in neuronal identity

The mechanisms that activate some genes while silencing others are critical to ensure precision in lineage specification as multipotent progenitors become restricted in cell fate. During neurodevelopment, these mechanisms are required to generate the diversity of neuronal subtypes found in the nervous system. Here we report interactions between basic helix-loop-helix (bHLH) transcriptional activators and the transcriptional repressor PRDM13 that are critical for specifying dorsal spinal cord neurons. PRDM13 inhibits gene expression programs for excitatory neuronal lineages in the dorsal neural tube. Strikingly, PRDM13 also ensures a battery of ventral neural tube specification genes such as Olig1, Olig2 and Prdm12 are excluded dorsally. PRDM13 does this via recruitment to chromatin by multiple neural bHLH factors to restrict gene expression in specific neuronal lineages. Together these findings highlight the function of PRDM13 in repressing the activity of bHLH transcriptional activators that together are required to achieve precise neuronal specification during mouse development.

The Retina of Asian and African Elephants: Comparison of Newborn and Adult

Elephants are precocial mammals that are relatively mature as newborns and mobile shortly after birth. To determine whether the retina of newborn elephants is capable of supporting the mobility of elephant calves, we compared the retinal structures of 2 newborn elephants (1 African and 1 Asian) and 2 adult animals of both species by immunohistochemical and morphometric methods. For the first time, we present here a comprehensive qualitative and quantitative characterization of the cellular composition of the newborn and the adult retinas of 2 elephant species. We found that the retina of elephants is relatively mature at birth. All retinal layers were well discernible, and various retinal cell types were detected in the newborns, including Müller glial cells (expressing glutamine synthetase and cellular retinal binding protein; CRALBP), cone photoreceptors (expressing S-opsin or M/L-opsin), protein kinase Cα-expressing bipolar cells, tyrosine hydroxylase-, choline acetyltransferase (ChAT)-, calbindin-, and calretinin-expressing amacrine cells, and calbindin-expressing horizontal cells. The retina of newborn elephants contains discrete horizontal cells which coexpress ChAT, calbindin, and calretinin. While the overall structure of the retina is very similar between newborn and adult elephants, various parameters change after birth. The postnatal thickening of the retinal ganglion cell axons and the increase in ganglion cell soma size are explained by the increase in body size after birth, and the decreases in the densities of neuronal and glial cells are explained by the postnatal expansion of the retinal surface area. The expression of glutamine synthetase and CRALBP in the Müller cells of newborn elephants suggests that the cells are already capable of supporting the activities of photoreceptors and neurons. As a peculiarity, the elephant retina contains both normally located and displaced giant ganglion cells, with single cells reaching a diameter of more than 50 µm in adults and therefore being almost in the range of giant retinal ganglion cells found in aquatic mammals. Some of these ganglion cells are displaced into the inner nuclear layer, a unique feature of terrestrial mammals. For the first time, we describe here the occurrence of many bistratified rod bipolar cells in the elephant retina. These bistratified bipolar cells may improve nocturnal contrast perception in elephants given their arrhythmic lifestyle.

Cellular and Molecular Underpinnings of Neuronal Assembly in the Central Auditory System during Mouse Development

During development, the organization of the auditory system into distinct functional subcircuits depends on the spatially and temporally ordered sequence of neuronal specification, differentiation, migration and connectivity. Regional patterning along the antero-posterior axis and neuronal subtype specification along the dorso-ventral axis intersect to determine proper neuronal fate and assembly of rhombomere-specific auditory subcircuits. By taking advantage of the increasing number of transgenic mouse lines, recent studies have expanded the knowledge of developmental mechanisms involved in the formation and refinement of the auditory system. Here, we summarize several findings dealing with the molecular and cellular mechanisms that underlie the assembly of central auditory subcircuits during mouse development, focusing primarily on the rhombomeric and dorso-ventral origin of auditory nuclei and their associated molecular genetic pathways.

Genetic silencing of olivocerebellar synapses causes dystonia-like behaviour in mice

Theories of cerebellar function place the inferior olive to cerebellum connection at the centre of motor behaviour. One possible implication of this is that disruption of olivocerebellar signalling could play a major role in initiating motor disease. To test this, we devised a mouse genetics approach to silence glutamatergic signalling only at olivocerebellar synapses. The resulting mice had a severe neurological condition that mimicked the early-onset twisting, stiff limbs and tremor that is observed in dystonia, a debilitating movement disease. By blocking olivocerebellar excitatory neurotransmission, we eliminated Purkinje cell complex spikes and induced aberrant cerebellar nuclear activity. Pharmacologically inhibiting the erratic output of the cerebellar nuclei in the mutant mice improved movement. Furthermore, deep brain stimulation directed to the interposed cerebellar nuclei reduced dystonia-like postures in these mice. Collectively, our data uncover a neural mechanism by which olivocerebellar dysfunction promotes motor disease phenotypes and identify the cerebellar nuclei as a therapeutic target for surgical intervention.

Dystonia is thought to be driven by impairments in cerebellar signalling. The authors use a mouse genetic approach to silence excitatory transmission in the inferior olive to cerebellum pathway, resulting in dystonia-like signs in the animals which can be alleviated using DBS stimulation of the pathway.

The dorsal spinal cord and hindbrain: From developmental mechanisms to functional circuits

Neurons of the dorsal hindbrain and spinal cord are central in receiving, processing and relaying sensory perception and participate in the coordination of sensory-motor output. Numerous cellular and molecular mechanisms that underlie neuronal development in both regions of the nervous system are shared. We discuss here the mechanisms that generate neuronal diversity in the dorsal spinal cord and hindbrain, and emphasize similarities in patterning and neuronal specification. Insight into the developmental mechanisms has provided tools that can help to assign functions to small subpopulations of neurons. Hence, novel information on how mechanosensory or pain sensation is encoded under normal and neuropathic conditions has already emerged. Such studies show that the complex neuronal circuits that control perception of somatosensory and viscerosensory stimuli are becoming amenable to investigations.

Homeotic Transformations of Neuronal Cell Identities

Homeosis is classically defined as the transformation of one body part into something that resembles another body part. We propose here to broaden the concept of homeosis to the many neuronal cell identity transformations that have been uncovered over the past few years upon removal of specific regulatory factors in organisms from Caenorhabditis elegans to Drosophila, zebrafish, and mice. The concept of homeosis provides a framework for the evolution of cell type diversity in the brain.

Lmx1b is required for the glutamatergic fates of a subset of spinal cord neurons

Background

Alterations in neurotransmitter phenotypes of specific neurons can cause imbalances in excitation and inhibition in the central nervous system (CNS), leading to diseases. Therefore, the correct specification and maintenance of neurotransmitter phenotypes is vital. As with other neuronal properties, neurotransmitter phenotypes are often specified and maintained by particular transcription factors. However, the specific molecular mechanisms and transcription factors that regulate neurotransmitter phenotypes remain largely unknown.

Methods

In this paper we use single mutant, double mutant and transgenic zebrafish embryos to elucidate the functions of Lmx1ba and Lmx1bb in the regulation of spinal cord interneuron neurotransmitter phenotypes.

Results

We demonstrate that lmx1ba and lmx1bb are both expressed in zebrafish spinal cord and that lmx1bb is expressed by both V0v cells and dI5 cells. Our functional analyses demonstrate that these transcription factors are not required for neurotransmitter fate specification at early stages of development, but that in embryos with at least two lmx1ba and/or lmx1bb mutant alleles there is a reduced number of excitatory (glutamatergic) spinal interneurons at later stages of development. In contrast, there is no change in the numbers of V0v or dI5 cells. These data suggest that lmx1b-expressing spinal neurons still form normally, but at least a subset of them lose, or do not form, their normal excitatory fates. As the reduction in glutamatergic cells is only seen at later stages of development, Lmx1b is probably required either for the maintenance of glutamatergic fates or to specify glutamatergic phenotypes of a subset of later forming neurons. Using double labeling experiments, we also show that at least some of the cells that lose their normal glutamatergic phenotype are V0v cells. Finally, we also establish that Evx1 and Evx2, two transcription factors that are required for V0v cells to acquire their excitatory neurotransmitter phenotype, are also required for lmx1ba and lmx1bb expression in these cells, suggesting that Lmx1ba and Lmx1bb act downstream of Evx1 and Evx2 in V0v cells.

Conclusions

Lmx1ba and Lmx1bb function at least partially redundantly in the spinal cord and three functional lmx1b alleles are required in zebrafish for correct numbers of excitatory spinal interneurons at later developmental stages. Taken together, our data significantly enhance our understanding of how spinal cord neurotransmitter fates are regulated.

Loss of Ptf1a Leads to a Widespread Cell-Fate Misspecification in the Brainstem, Affecting the Development of Somatosensory and Viscerosensory Nuclei

The brainstem contains diverse neuronal populations that regulate a wide range of processes vital to the organism. Proper cell-fate specification decisions are critical to achieve neuronal diversity in the CNS, but the mechanisms regulating cell-fate specification in the developing brainstem are poorly understood. Previously, it has been shown that basic helix-loop-helix transcription factor Ptf1a is required for the differentiation and survival of neurons of the inferior olivary and cochlear brainstem nuclei, which contribute to motor coordination and sound processing, respectively. In this study, we show that the loss of Ptf1a compromises the development of the nucleus of the solitary tract, which processes viscerosensory information, and the spinal and principal trigeminal nuclei, which integrate somatosensory information of the face. Combining genetic fate-mapping, birth-dating, and gene expression studies, we found that at least a subset of brainstem abnormalities in Ptf1a(-/-) mice are mediated by a dramatic cell-fate misspecification in rhombomeres 2-7, which results in the production of supernumerary viscerosensory and somatosensory neurons of the Lmx1b lineage at the expense of Pax2(+) GABAergic viscerosensory and somatosensory neurons, and inferior olivary neurons. Our data identify Ptf1a as a major regulator of cell-fate specification decisions in the developing brainstem, and as a previously unrecognized developmental regulator of both viscerosensory and somatosensory brainstem nuclei.

SIGNIFICANCE STATEMENT

Cell-fate specification decisions are critical for normal CNS development. Although extensively studied in the cerebellum and spinal cord, the mechanisms mediating cell-fate decisions in the brainstem, which regulates a wide range of processes vital to the organism, remain largely unknown. Here we identified mouse Ptf1a as a novel regulator of cell-fate decisions during both early and late brainstem neurogenesis, which are critical for proper development of several major classes of brainstem cells, including neurons of the somatosensory and viscerosensory nuclei. Since loss-of-function PTF1A mutations were described in human patients, we suggest Ptf1a-dependent cell-fate misspecification as a novel mechanism of human brainstem pathology.

Horizontal Cells, the Odd Ones Out in the Retina, Give Insights into Development and Disease

Thorough investigation of a neuronal population can help reveal key aspects regarding the nervous system and its development. The retinal horizontal cells have several extraordinary features making them particularly interesting for addressing questions regarding fate assignment and subtype specification. In this review we discuss and summarize data concerning the formation and diversity of horizontal cells, how morphology is correlated to molecular markers, and how fate assignment separates the horizontal lineage from the lineages of other retinal cell types. We discuss the novel and unique features of the final cell cycle of horizontal cell progenitors and how they may relate to retinoblastoma carcinogenesis.

Regulating the dorsal neural tube expression of Ptf1a through a distal 3' enhancer

Generating the correct balance of inhibitory and excitatory neurons in a neural network is essential for normal functioning of a nervous system. The neural network in the dorsal spinal cord functions in somatosensation where it modulates and relays sensory information from the periphery. PTF1A is a key transcriptional regulator present in a specific subset of neural progenitor cells in the dorsal spinal cord, cerebellum and retina that functions to specify an inhibitory neuronal fate while suppressing excitatory neuronal fates. Thus, the regulation of Ptf1a expression is critical for determining mechanisms controlling neuronal diversity in these regions of the nervous system. Here we identify a sequence conserved, tissue-specific enhancer located 10.8kb 3' of the Ptf1a coding region that is sufficient to direct expression to dorsal neural tube progenitors that give rise to neurons in the dorsal spinal cord in chick and mouse. DNA binding motifs for Paired homeodomain (Pd-HD) and zinc finger (ZF) transcription factors are required for enhancer activity. Mutations in these sequences implicate the Pd-HD motif for activator function and the ZF motif for repressor function. Although no repressor transcription factor was identified, both PAX6 and SOX3 can increase enhancer activity in reporter assays. Thus, Ptf1a is regulated by active and repressive inputs integrated through multiple sequence elements within a highly conserved sequence downstream of the Ptf1a gene.

Zinc finger transcription factor Casz1 expression is regulated by homeodomain transcription factor Prrxl1 in embryonic spinal dorsal horn late-born excitatory interneurons

The transcription factor Casz1 is required for proper assembly of vertebrate vasculature and heart morphogenesis as well as for temporal control of Drosophila neuroblasts and mouse retina progenitors in the generation of different cell types. Although Casz1 function in the mammalian nervous system remains largely unexplored, Casz1 is expressed in several regions of this system. Here we provide a detailed spatiotemporal characterization of Casz1 expression along mouse dorsal root ganglion (DRG) and dorsal spinal cord development by immunochemistry. In the DRG, Casz1 is broadly expressed in sensory neurons since they are born until perinatal age. In the dorsal spinal cord, Casz1 displays a more dynamic pattern being first expressed in dorsal interneuron 1 (dI1) progenitors and their derived neurons and then in a large subset of embryonic dorsal late-born excitatory (dILB) neurons that narrows gradually to become restricted perinatally to the inner portion. Strikingly, expression analyses using Prrxl1-knockout mice revealed that Prrxl1, a key transcription factor in the differentiation of dILB neurons, is a positive regulator of Casz1 expression in the embryonic dorsal spinal cord but not in the DRG. By performing chromatin immunoprecipitation in the dorsal spinal cord, we identified two Prrxl1-bound regions within Casz1 introns, suggesting that Prrxl1 directly regulates Casz1 transcription. Our work reveals that Casz1 lies downstream of Prrxl1 in the differentiation pathway of a large subset of dILB neurons and provides a framework for further studies of Casz1 in assembly of the DRG-spinal circuit.

Spinal Inhibitory Interneuron Diversity Delineates Variant Motor Microcircuits

Animals generate movement by engaging spinal circuits that direct precise sequences of muscle contraction, but the identity and organizational logic of local interneurons that lie at the core of these circuits remain unresolved. Here, we show that V1 interneurons, a major inhibitory population that controls motor output, fractionate into highly diverse subsets on the basis of the expression of 19 transcription factors. Transcriptionally defined V1 subsets exhibit distinct physiological signatures and highly structured spatial distributions with mediolateral and dorsoventral positional biases. These positional distinctions constrain patterns of input from sensory and motor neurons and, as such, suggest that interneuron position is a determinant of microcircuit organization. Moreover, V1 diversity indicates that different inhibitory microcircuits exist for motor pools controlling hip, ankle, and foot muscles, revealing a variable circuit architecture for interneurons that control limb movement.

Evx1 and Evx2 specify excitatory neurotransmitter fates and suppress inhibitory fates through a Pax2-independent mechanism

BACKGROUND

For neurons to function correctly in neuronal circuitry they must utilize appropriate neurotransmitters. However, even though neurotransmitter specificity is one of the most important and defining properties of a neuron we still do not fully understand how neurotransmitter fates are specified during development. Most neuronal properties are determined by the transcription factors that neurons express as they start to differentiate. While we know a few transcription factors that specify the neurotransmitter fates of particular neurons, there are still many spinal neurons for which the transcription factors specifying this critical phenotype are unknown. Strikingly, all of the transcription factors that have been identified so far as specifying inhibitory fates in the spinal cord act through Pax2. Even Tlx1 and Tlx3, which specify the excitatory fates of dI3 and dI5 spinal neurons work at least in part by down-regulating Pax2.

METHODS

In this paper we use single and double mutant zebrafish embryos to identify the spinal cord functions of Evx1 and Evx2.

RESULTS

We demonstrate that Evx1 and Evx2 are expressed by spinal cord V0v cells and we show that these cells develop into excitatory (glutamatergic) Commissural Ascending (CoSA) interneurons. In the absence of both Evx1 and Evx2, V0v cells still form and develop a CoSA morphology. However, they lose their excitatory fate and instead express markers of a glycinergic fate. Interestingly, they do not express Pax2, suggesting that they are acquiring their inhibitory fate through a novel Pax2-independent mechanism.

CONCLUSIONS

Evx1 and Evx2 are required, partially redundantly, for spinal cord V0v cells to become excitatory (glutamatergic) interneurons. These results significantly increase our understanding of the mechanisms of neuronal specification and the genetic networks involved in these processes.

An emerging role for prdm family genes in dorsoventral patterning of the vertebrate nervous system

The embryonic vertebrate neural tube is divided along its dorsoventral (DV) axis into eleven molecularly discrete progenitor domains. Each of these domains gives rise to distinct neuronal cell types; the ventral-most six domains contribute to motor circuits, while the five dorsal domains contribute to sensory circuits. Following the initial neurogenesis step, these domains also generate glial cell types—either astrocytes or oligodendrocytes. This DV pattern is initiated by two morphogens—Sonic Hedgehog released from notochord and floor plate and Bone Morphogenetic Protein produced in the roof plate—that act in concentration gradients to induce expression of genes along the DV axis. Subsequently, these DV-restricted genes cooperate to define progenitor domains and to control neuronal cell fate specification and differentiation in each domain. Many genes involved in this process have been identified, but significant gaps remain in our understanding of the underlying genetic program. Here we review recent work identifying members of the Prdm gene family as novel regulators of DV patterning in the neural tube. Many Prdm proteins regulate transcription by controlling histone modifications (either via intrinsic histone methyltransferase activity, or by recruiting histone modifying enzymes). Prdm genes are expressed in spatially restricted domains along the DV axis of the neural tube and play important roles in the specification of progenitor domains, as well as in the subsequent differentiation of motor neurons and various types of interneurons. Strikingly, Prdm proteins appear to function by binding to, and modulating the activity of, other transcription factors (particularly bHLH proteins). The identity of key transcription factors in DV patterning of the neural tube has been elucidated previously (e.g. the nkx, bHLH and pax families), but it now appears that an additional family is also required and that it acts in a potentially novel manner.

Genetically identified spinal interneurons integrating tactile afferents for motor control

Our movements are shaped by our perception of the world as communicated by our senses. Perception of sensory information has been largely attributed to cortical activity. However, a prior level of sensory processing occurs in the spinal cord. Indeed, sensory inputs directly project to many spinal circuits, some of which communicate with motor circuits within the spinal cord. Therefore, the processing of sensory information for the purpose of ensuring proper movements is distributed between spinal and supraspinal circuits. The mechanisms underlying the integration of sensory information for motor control at the level of the spinal cord have yet to be fully described. Recent research has led to the characterization of spinal neuron populations that share common molecular identities. Identification of molecular markers that define specific populations of spinal neurons is a prerequisite to the application of genetic techniques devised to both delineate the function of these spinal neurons and their connectivity. This strategy has been used in the study of spinal neurons that receive tactile inputs from sensory neurons innervating the skin. As a result, the circuits that include these spinal neurons have been revealed to play important roles in specific aspects of motor function. We describe these genetically identified spinal neurons that integrate tactile information and the contribution of these studies to our understanding of how tactile information shapes motor output. Furthermore, we describe future opportunities that these circuits present for shedding light on the neural mechanisms of tactile processing.

Polyamine biosynthesis is critical for growth and differentiation of the pancreas

The pancreas, in most studied vertebrates, is a compound organ with both exocrine and endocrine functions. The exocrine compartment makes and secretes digestive enzymes, while the endocrine compartment, organized into islets of Langerhans, produces hormones that regulate blood glucose. High concentrations of polyamines, which are aliphatic amines, are reported in exocrine and endocrine cells, with insulin-producing β cells showing the highest concentrations. We utilized zebrafish as a model organism, together with pharmacological inhibition or genetic manipulation, to determine how polyamine biosynthesis functions in pancreatic organogenesis. We identified that inhibition of polyamine biosynthesis reduces exocrine pancreas and β cell mass, and that these reductions are at the level of differentiation. Moreover, we demonstrate that inhibition of ornithine decarboxylase (ODC), the rate-limiting enzyme in polyamine biosynthesis, phenocopies inhibition or knockdown of the enzyme deoxyhypusine synthase (DHS). These data identify that the pancreatic requirement for polyamine biosynthesis is largely mediated through a requirement for spermidine for the downstream posttranslational modification of eIF5A by its enzymatic activator DHS, which in turn impacts mRNA translation. Altogether, we have uncovered a role for polyamine biosynthesis in pancreatic organogenesis and identified that it may be possible to exploit polyamine biosynthesis to manipulate pancreatic cell differentiation.

Sensory and spinal inhibitory dorsal midline crossing is independent of Robo3

Commissural neurons project across the midline at all levels of the central nervous system (CNS), providing bilateral communication critical for the coordination of motor activity and sensory perception. Midline crossing at the spinal ventral midline has been extensively studied and has revealed that multiple developmental lineages contribute to this commissural neuron population. Ventral midline crossing occurs in a manner dependent on Robo3 regulation of Robo/Slit signaling and the ventral commissure is absent in the spinal cord and hindbrain of Robo3 mutants. Midline crossing in the spinal cord is not limited to the ventral midline, however. While prior anatomical studies provide evidence that commissural axons also cross the midline dorsally, little is known of the genetic and molecular properties of dorsally-crossing neurons or of the mechanisms that regulate dorsal midline crossing. In this study, we describe a commissural neuron population that crosses the spinal dorsal midline during the last quarter of embryogenesis in discrete fiber bundles present throughout the rostrocaudal extent of the spinal cord. Using immunohistochemistry, neurotracing, and mouse genetics, we show that this commissural neuron population includes spinal inhibitory neurons and sensory nociceptors. While the floor plate and roof plate are dispensable for dorsal midline crossing, we show that this population depends on Robo/Slit signaling yet crosses the dorsal midline in a Robo3-independent manner. The dorsally-crossing commissural neuron population we describe suggests a substrate circuitry for pain processing in the dorsal spinal cord.

From induction to conduction: how intrinsic transcriptional priming of extrinsic neuronal connectivity shapes neuronal identity

Every behaviour of an organism relies on an intricate and vastly diverse network of neurons whose identity and connectivity must be specified with extreme precision during development. Intrinsically, specification of neuronal identity depends heavily on the expression of powerful transcription factors that direct numerous features of neuronal identity, including especially properties of neuronal connectivity, such as dendritic morphology, axonal targeting or synaptic specificity, ultimately priming the neuron for incorporation into emerging circuitry. As the neuron's early connectivity is established, extrinsic signals from its pre- and postsynaptic partners feedback on the neuron to further refine its unique characteristics. As a result, disruption of one component of the circuitry during development can have vital consequences for the proper identity specification of its synaptic partners. Recent studies have begun to harness the power of various transcription factors that control neuronal cell fate, including those that specify a neuron's subtype-specific identity, seeking insight for future therapeutic strategies that aim to reconstitute damaged circuitry through neuronal reprogramming.

Misexpression of ptf1a in cortical pyramidal cells in vivo promotes an inhibitory peptidergic identity

The intracellular transcriptional milieu wields considerable influence over the induction of neuronal identity. The transcription factor Ptf1a has been proposed to act as an identity "switch" between developmentally related precursors in the spinal cord (Glasgow et al., 2005; Huang et al., 2008), retina (Fujitani et al., 2006; Dullin et al., 2007; Nakhai et al., 2007; Lelièvre et al., 2011), and cerebellum (Hoshino et al., 2005; Pascual et al., 2007; Yamada et al., 2014), where it promotes an inhibitory over an excitatory neuronal identity. In this study, we investigate the potency of Ptf1a to cell autonomously confer a specific neuronal identity outside of its endogenous environment, using mouse in utero electroporation and a conditional genetic strategy to misexpress Ptf1a exclusively in developing cortical pyramidal cells. Transcriptome profiling of Ptf1a-misexpressing cells using RNA-seq reveals that Ptf1a significantly alters pyramidal cell gene expression, upregulating numerous Ptf1a-dependent inhibitory interneuron markers and ultimately generating a gene expression profile that resembles the transcriptomes of both Ptf1a-expressing spinal interneurons and endogenous cortical interneurons. Using RNA-seq and in situ hybridization analyses, we also show that Ptf1a induces expression of the peptidergic neurotransmitter nociceptin, while minimally affecting the expression of genes linked to other neurotransmitter systems. Moreover, Ptf1a alters neuronal morphology, inducing the radial redistribution and branching of neurites in cortical pyramidal cells. Thus Ptf1a is sufficient, even in a dramatically different neuronal precursor, to cell autonomously promote characteristics of an inhibitory peptidergic identity, providing the first example of a single transcription factor that can direct an inhibitory peptidergic fate.

Molecular and cellular development of spinal cord locomotor circuitry

The spinal cord of vertebrate animals is comprised of intrinsic circuits that are capable of sensing the environment and generating complex motor behaviors. There are two major perspectives for understanding the biology of this complicated structure. The first approaches the spinal cord from the point of view of function and is based on classic and ongoing research in electrophysiology, adult behavior, and spinal cord injury. The second view considers the spinal cord from a developmental perspective and is founded mostly on gene expression and gain-of-function and loss-of-function genetic experiments. Together these studies have uncovered functional classes of neurons and their lineage relationships. In this review, we summarize our knowledge of developmental classes, with an eye toward understanding the functional roles of each group.

Tfap2a and 2b act downstream of Ptf1a to promote amacrine cell differentiation during retinogenesis

Retinogenesis is a precisely controlled developmental process during which different types of neurons and glial cells are generated under the influence of intrinsic and extrinsic factors. Three transcription factors, Foxn4, RORβ1 and their downstream effector Ptf1a, have been shown to be indispensable intrinsic regulators for the differentiation of amacrine and horizontal cells. At present, however, it is unclear how Ptf1a specifies these two cell fates from competent retinal precursors. Here, through combined bioinformatic, molecular and genetic approaches in mouse retinas, we identify the Tfap2a and Tfap2b transcription factors as two major downstream effectors of Ptf1a. RNA-seq and immunolabeling analyses show that the expression of Tfap2a and 2b transcripts and proteins is dramatically downregulated in the Ptf1a null mutant retina. Their overexpression is capable of promoting the differentiation of glycinergic and GABAergic amacrine cells at the expense of photoreceptors much as misexpressed Ptf1a is, whereas their simultaneous knockdown has the opposite effect. Given the demonstrated requirement for Tfap2a and 2b in horizontal cell differentiation, our study thus defines a Foxn4/RORβ1-Ptf1a-Tfap2a/2b transcriptional regulatory cascade that underlies the competence, specification and differentiation of amacrine and horizontal cells during retinal development.

Control of axon guidance and neurotransmitter phenotype of dB1 hindbrain interneurons by Lim-HD code

Hindbrain dorsal interneurons (HDIs) are implicated in receiving, processing, integrating, and transmitting sensory inputs from the periphery and spinal cord, including the vestibular, auditory, and proprioceptive systems. During development, multiple molecularly defined HDI types are set in columns along the dorsoventral axis, before migrating along well-defined trajectories to generate various brainstem nuclei. Major brainstem functions rely on the precise assembly of different interneuron groups and higher brain domains into common circuitries. Yet, knowledge regarding interneuron axonal patterns, synaptic targets, and the transcriptional control that govern their connectivity is sparse. The dB1 class of HDIs is formed in a district dorsomedial position along the hindbrain and gives rise to the inferior olive nuclei, dorsal cochlear nuclei, and vestibular nuclei. dB1 interneurons express various transcription factors (TFs): the pancreatic transcription factor 1a (Ptf1a), the homeobox TF-Lbx1 and the Lim-homeodomain (Lim-HD), and TF Lhx1 and Lhx5. To decipher the axonal and synaptic connectivity of dB1 cells, we have used advanced enhancer tools combined with conditional expression systems and the PiggyBac-mediated DNA transposition system in avian embryos. Multiple ipsilateral and contralateral axonal projections were identified ascending toward higher brain centers, where they formed synapses in the Purkinje cerebellar layer as well as at discrete midbrain auditory and vestibular centers. Decoding the mechanisms that instruct dB1 circuit formation revealed a fundamental role for Lim-HD proteins in regulating their axonal patterns, synaptic targets, and neurotransmitter choice. Together, this study provides new insights into the assembly and heterogeneity of HDIs connectivity and its establishment through the central action of Lim-HD governed programs.

Sox9 is critical for suppression of neurogenesis but not initiation of gliogenesis in the cerebellum

Background

The high mobility group (HMG) family transcription factor Sox9 is critical for induction and maintenance of neural stem cell pool in the central nervous system (CNS). In the spinal cord and retina, Sox9 is also the master regulator that defines glial fate choice by mediating the neurogenic-to-gliogenic fate switch. On the other hand, the genetic repertoire governing the maintenance and fate decision of neural progenitor pool in the cerebellum has remained elusive.

Results

By employing the Cre/loxP strategy, we specifically inactivated Sox9 in the mouse cerebellum. Unexpectedly, the self-renewal capacity and multipotency of neural progenitors at the cerebellar ventricular zone (VZ) were not perturbed upon Sox9 ablation. Instead, the mutants exhibited an increased number of VZ-derived neurons including Purkinje cells and GABAergic interneurons. Simultaneously, we observed continuous neurogenesis from Sox9-null VZ at late gestation, when normally neurogenesis ceases to occur and gives way for gliogenesis. Surprisingly, glial cell specification was not affected upon Sox9 ablation.

Conclusion

Our findings suggest Sox9 may mediate the neurogenic-to-gliogenic fate switch in mouse cerebellum by modulating the termination of neurogenesis, and therefore indicate a functional discrepancy of Sox9 between the development of cerebellum and other major neural tissues.

Expression of gastrin-releasing peptide by excitatory interneurons in the mouse superficial dorsal horn

Background

Gastrin-releasing peptide (GRP) and its receptor have been shown to play an important role in the sensation of itch. However, although GRP immunoreactivity has been detected in the spinal dorsal horn, there is debate about whether this originates from primary afferents or local excitatory interneurons. We therefore examined the relation of GRP immunoreactivity to that seen with antibodies that label primary afferent or excitatory interneuron terminals. We tested the specificity of the GRP antibody by preincubating with peptides with which it could potentially cross-react. We also examined tissue from a mouse line in which enhanced green fluorescent protein (EGFP) is expressed under control of the GRP promoter.

Results

GRP immunoreactivity was seen in both primary afferent and non-primary glutamatergic axon terminals in the superficial dorsal horn. However, immunostaining was blocked by pre-incubation of the antibody with substance P, which is present at high levels in many nociceptive primary afferents. EGFP⁺ cells in the GRP-EGFP mouse did not express Pax2, and their axons contained the vesicular glutamate transporter 2 (VGLUT2), indicating that they are excitatory interneurons. In most cases, their axons were also GRP-immunoreactive. Multiple-labelling immunocytochemical studies indicated that these cells did not express either of the preprotachykinin peptides, and that they generally lacked protein kinase Cγ, which is expressed by a subset of the excitatory interneurons in this region.

Conclusions

These results show that GRP is expressed by a distinct population of excitatory interneurons in laminae I-II that are likely to be involved in the itch pathway. They also suggest that the GRP immunoreactivity seen in primary afferents in previous studies may have resulted from cross-reaction of the GRP antibody with substance P or the closely related peptide neurokinin A.

The E3 ubiquitin ligase thyroid hormone receptor-interacting protein 12 targets pancreas transcription factor 1a for proteasomal degradation

Pancreas transcription factor 1a (PTF1a) plays a crucial role in the early development of the pancreas and in the maintenance of the acinar cell phenotype. Several transcriptional mechanisms regulating expression of PTF1a have been identified. However, regulation of PTF1a protein stability and degradation is still unexplored. Here, we report that inhibition of proteasome leads to elevated levels of PTF1a and to the existence of polyubiquitinated forms of PTF1a. We used the Sos recruitment system, an alternative two-hybrid system method to detect protein-protein interactions in the cytoplasm and to map the interactome of PTF1a. We identified TRIP12 (thyroid hormone receptor-interacting protein 12), an E3 ubiquitin-protein ligase as a new partner of PTF1a. We confirmed PTF1a/TRIP12 interaction in acinar cell lines and in co-transfected HEK-293T cells. The protein stability of PTF1a is significantly increased upon decreased expression of TRIP12. It is reduced upon overexpression of TRIP12 but not a catalytically inactive TRIP12-C1959A mutant. We identified a region of TRIP12 required for interaction and identified lysine 312 of PTF1a as essential for proteasomal degradation. We also demonstrate that TRIP12 down-regulates PTF1a transcriptional and antiproliferative activities. Our data suggest that an increase in TRIP12 expression can play a part in PTF1a down-regulation and indicate that PTF1a/TRIP12 functional interaction may regulate pancreatic epithelial cell homeostasis.

A transcription factor network specifying inhibitory versus excitatory neurons in the dorsal spinal cord

The proper balance of excitatory and inhibitory neurons is crucial for normal processing of somatosensory information in the dorsal spinal cord. Two neural basic helix-loop-helix transcription factors (TFs), Ascl1 and Ptf1a, have contrasting functions in specifying these neurons. To understand how Ascl1 and Ptf1a function in this process, we identified their direct transcriptional targets genome-wide in the embryonic mouse neural tube using ChIP-Seq and RNA-Seq. We show that Ascl1 and Ptf1a directly regulate distinct homeodomain TFs that specify excitatory or inhibitory neuronal fates. In addition, Ascl1 directly regulates genes with roles in several steps of the neurogenic program, including Notch signaling, neuronal differentiation, axon guidance and synapse formation. By contrast, Ptf1a directly regulates genes encoding components of the neurotransmitter machinery in inhibitory neurons, and other later aspects of neural development distinct from those regulated by Ascl1. Moreover, Ptf1a represses the excitatory neuronal fate by directly repressing several targets of Ascl1. Ascl1 and Ptf1a bind sequences primarily enriched for a specific E-Box motif (CAGCTG) and for secondary motifs used by Sox, Rfx, Pou and homeodomain factors. Ptf1a also binds sequences uniquely enriched in the CAGATG E-box and in the binding motif for its co-factor Rbpj, providing two factors that influence the specificity of Ptf1a binding. The direct transcriptional targets identified for Ascl1 and Ptf1a provide a molecular understanding of how these DNA-binding proteins function in neuronal development, particularly as key regulators of homeodomain TFs required for neuronal subtype specification.

Transformation of the cerebellum into more ventral brainstem fates causes cerebellar agenesis in the absence of Ptf1a function

Model organism studies have demonstrated that cell fate specification decisions play an important role in normal brain development. Their role in human neurodevelopmental disorders, however, is poorly understood, with very few examples described. The cerebellum is an excellent system to study mechanisms of cell fate specification. Although signals from the isthmic organizer are known to specify cerebellar territory along the anterior-posterior axis of the neural tube, the mechanisms establishing the cerebellar anlage along the dorsal-ventral axis are unknown. Here we show that the gene encoding pancreatic transcription factor PTF1A, which is inactivated in human patients with cerebellar agenesis, is required to segregate the cerebellum from more ventral extracerebellar fates. Using genetic fate mapping in mice, we show that in the absence of Ptf1a, cells originating in the cerebellar ventricular zone initiate a more ventral brainstem expression program, including LIM homeobox transcription factor 1 beta and T-cell leukemia homeobox 3. Misspecified cells exit the cerebellar anlage and contribute to the adjacent brainstem or die, leading to cerebellar agenesis in Ptf1a mutants. Our data identify Ptf1a as the first gene involved in the segregation of the cerebellum from the more ventral brainstem. Further, we propose that cerebellar agenesis represents a new, dorsal-to-ventral, cell fate misspecification phenotype in humans.

Mechanisms regulating GABAergic neuron development

Neurons using gamma-aminobutyric acid (GABA) as their neurotransmitter are the main inhibitory neurons in the mature central nervous system (CNS) and show great variation in their form and function. GABAergic neurons are produced in all of the main domains of the CNS, where they develop from discrete regions of the neuroepithelium. Here, we review the gene expression and regulatory mechanisms controlling the main steps of GABAergic neuron development: early patterning of the proliferative neuroepithelium, production of postmitotic neural precursors, establishment of their identity and migration. By comparing the molecular regulation of these events across CNS, we broadly identify three regions utilizing distinct molecular toolkits for GABAergic fate determination: telencephalon-anterior diencephalon (DLX2 type), posterior diencephalon-midbrain (GATA2 type) and hindbrain-spinal cord (PTF1A and TAL1 types). Similarities and differences in the molecular regulatory mechanisms reveal the core determinants of a GABAergic neuron as well as provide insights into generation of the vast diversity of these neurons.

Neuronal Ig/Caspr recognition promotes the formation of axoaxonic synapses in mouse spinal cord

Inhibitory microcircuits are wired with a precision that underlies their complex regulatory roles in neural information processing. In the spinal cord, one specialized class of GABAergic interneurons (GABApre) mediates presynaptic inhibitory control of sensory-motor synapses. The synaptic targeting of these GABAergic neurons exhibits an absolute dependence on proprioceptive sensory terminals, yet the molecular underpinnings of this specialized axoaxonic organization remain unclear. Here, we show that sensory expression of an NB2 (Contactin5)/Caspr4 coreceptor complex, together with spinal interneuron expression of NrCAM/CHL1, directs the high-density accumulation of GABAergic boutons on sensory terminals. Moreover, genetic elimination of NB2 results in a disproportionate stripping of inhibitory boutons from high-density GABApre-sensory synapses, suggesting that the preterminal axons of GABApre neurons compete for access to individual sensory terminals. Our findings define a recognition complex that contributes to the assembly and organization of a specialized GABAergic microcircuit.

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Sensory Ig/Caspr4 complex directs inhibitory synapse formation in mouse spinal cord

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Eliminating NB2 results in a reduced number of GABApre-sensory synapses

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Quantitative modeling suggests competition for formation of axoaxonic synapses

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Role for a contactin/Caspr complex in central synapse formation

Normal and abnormal coding of somatosensory stimuli causing pain

Noxious stimuli usually cause pain and pain usually arises from noxious stimuli, but exceptions to these apparent truisms are the basis for clinically important problems and provide valuable insight into the neural code for pain. In this Review, we discuss how painful sensations arise. We argue that, although primary somatosensory afferents are tuned to specific stimulus features, natural stimuli often activate more than one type of afferent. Manipulating coactivation patterns can alter perception in ways that argue against each type of afferent acting independently (as expected for strictly labeled lines), suggesting instead that signals conveyed by different types of afferents interact. Deciphering the central circuits that mediate those interactions is critical for explaining the generation and modulation of neural signals that ultimately elicit pain. The advent of genetic and optical dissection techniques promise to dramatically accelerate progress toward this goal, which will facilitate the rational design of future pain therapeutics.

The Prdm13 histone methyltransferase encoding gene is a Ptf1a-Rbpj downstream target that suppresses glutamatergic and promotes GABAergic neuronal fate in the dorsal neural tube

The basic helix-loop-helix (bHLH) transcriptional activator Ptf1a determines inhibitory GABAergic over excitatory glutamatergic neuronal cell fate in progenitors of the vertebrate dorsal spinal cord, cerebellum and retina. In an in situ hybridization expression survey of PR domain containing genes encoding putative chromatin-remodeling zinc finger transcription factors in Xenopus embryos, we identified Prdm13 as a histone methyltransferase belonging to the Ptf1a synexpression group. Gain and loss of Ptf1a function analyses in both frog and mice indicates that Prdm13 is positively regulated by Ptf1a and likely constitutes a direct transcriptional target. We also showed that this regulation requires the formation of the Ptf1a-Rbp-j complex. Prdm13 knockdown in Xenopus embryos and in Ptf1a overexpressing ectodermal explants lead to an upregulation of Tlx3/Hox11L2, which specifies a glutamatergic lineage and a reduction of the GABAergic neuronal marker Pax2. It also leads to an upregulation of Prdm13 transcription, suggesting an autonegative regulation. Conversely, in animal caps, Prdm13 blocks the ability of the bHLH factor Neurog2 to activate Tlx3. Additional gain of function experiments in the chick neural tube confirm that Prdm13 suppresses Tlx3(+)/glutamatergic and induces Pax2(+)/GABAergic neuronal fate. Thus, Prdm13 is a novel crucial component of the Ptf1a regulatory pathway that, by modulating the transcriptional activity of bHLH factors such as Neurog2, controls the balance between GABAergic and glutamatergic neuronal fate in the dorsal and caudal part of the vertebrate neural tube.

Ontogeny of excitatory spinal neurons processing distinct somatic sensory modalities

Spatial and temporal cues govern the genesis of a diverse array of neurons located in the dorsal spinal cord, including dI1-dI6, dIL(A), and dIL(B) subtypes, but their physiological functions are poorly understood. Here we generated a new line of conditional knock-out (CKO) mice, in which the homeobox gene Tlx3 was removed in dI5 and dIL(B) cells. In these CKO mice, development of a subset of excitatory neurons located in laminae I and II was impaired, including itch-related GRPR-expressing neurons, PKCγ-expressing neurons, and neurons expressing three neuropeptide genes: somatostatin, preprotachykinin 1, and the gastrin-releasing peptide. These CKO mice displayed marked deficits in generating nocifensive motor behaviors evoked by a range of pain-related or itch-related stimuli. The mutants also failed to exhibit escape response evoked by dynamic mechanical stimuli but retained the ability to sense innocuous cooling and/or warm. Thus, our studies provide new insight into the ontogeny of spinal neurons processing distinct sensory modalities.

Several cis-regulatory elements control mRNA stability, translation efficiency, and expression pattern of Prrxl1 (paired related homeobox protein-like 1)

The homeodomain transcription factor Prrxl1/DRG11 has emerged as a crucial molecule in the establishment of the pain circuitry, in particular spinal cord targeting of dorsal root ganglia (DRG) axons and differentiation of nociceptive glutamatergic spinal cord neurons. Despite Prrxl1 importance in the establishment of the DRG-spinal nociceptive circuit, the molecular mechanisms that regulate its expression along development remain largely unknown. Here, we show that Prrxl1 transcription is regulated by three alternative promoters (named P1, P2, and P3), which control the expression of three distinct Prrxl1 5'-UTR variants, named 5'-UTR-A, 5'-UTR-B, and 5'-UTR-C. These 5'-UTR sequences confer distinct mRNA stability and translation efficiency to the Prrxl1 transcript. The most conserved promoter (P3) contains a TATA-box and displays in vivo enhancer activity in a pattern that overlaps with the zebrafish Prrxl1 homologue, drgx. Regulatory modules present in this sequence were identified and characterized, including a binding site for Phox2b. Concomitantly, we demonstrate that zebrafish Phox2b is required for the expression of drgx in the facial, glossopharyngeal, and vagal cranial ganglia.

Hox genes and region-specific sensorimotor circuit formation in the hindbrain and spinal cord

Homeobox (Hox) genes were originally discovered in the fruit fly Drosophila, where they function through a conserved homeodomain as transcriptional regulators to control embryonic morphogenesis. In vertebrates, 39 Hox genes have been identified and like their Drosophila counterparts they are organized within chromosomal clusters. Hox genes interact with various cofactors, such as the TALE homeodomain proteins, in recognition of consensus sequences within regulatory elements of their target genes. In vertebrates, Hox genes display spatially restricted patterns of expression within the developing hindbrain and spinal cord, and are considered crucial determinants of segmental identity and cell specification along the anterioposterior and dorsoventral axes of the embryo. Here, we review their later roles in the assembly of neuronal circuitry, in stereotypic neuronal migration, axon pathfinding, and topographic connectivity. Importantly, we will put some emphasis on how their early-segmented expression patterns can influence the formation of complex vital hindbrain and spinal cord circuitries.

A transcription factor code defines nine sensory interneuron subtypes in the mechanosensory area of the spinal cord

Interneurons in the dorsal spinal cord process and relay innocuous and nociceptive somatosensory information from cutaneous receptors that sense touch, temperature and pain. These neurons display a well-defined organization with respect to their afferent innervation. Nociceptive afferents innervate lamina I and II, while cutaneous mechanosensory afferents primarily innervate sensory interneurons that are located in lamina III-IV. In this study, we outline a combinatorial transcription factor code that defines nine different inhibitory and excitatory interneuron populations in laminae III-IV of the postnatal cord. This transcription factor code reveals a high degree of molecular diversity in the neurons that make up laminae III-IV, and it lays the foundation for systematically analyzing and manipulating these different neuronal populations to assess their function. In addition, we find that many of the transcription factors that are expressed in the dorsal spinal cord at early postnatal times continue to be expressed in the adult, raising questions about their function in mature neurons and opening the door to their genetic manipulation in adult animals.

Detailed expression analysis of regulatory genes in the early developing human neural tube

Studies in model organisms constitute the basis of our understanding of the principal molecular mechanisms of cell fate determination in the developing central nervous system. Considering the emergent applications in stem cell-based regenerative medicine, it is important to demonstrate conservation of subtype specific gene expression programs in human as compared to model vertebrates. We have examined the expression patterns of key regulatory genes in neural progenitor cells and their neuronal and glial descendants in the developing human spinal cord, hindbrain, and midbrain, and compared these with developing mouse and chicken embryos. As anticipated, gene expression patterns are highly conserved between these vertebrate species, but there are also features that appear unique to human development. In particular, we find that neither tyrosine hydroxylase nor Nurr1 are specific markers for mesencephalic dopamine neurons, as these genes also are expressed in other neuronal subtypes in the human ventral midbrain and in human embryonic stem cell cultures directed to differentiate towards a ventral mesencephalic identity. Moreover, somatic motor neurons in the ventral spinal cord appear to be produced by two molecularly distinct ventral progenitor populations in the human, raising the possibility that the acquisition of unique ventral progenitor identities may have contributed to the emergence of neural subtypes in higher vertebrates.

Corticospinal tract insult alters GABAergic circuitry in the mammalian spinal cord

During perinatal development, corticospinal tract (CST) projections into the spinal cord help refine spinal circuitry. Although the normal developmental processes that are controlled by the arrival of corticospinal input are becoming clear, little is known about how perinatal cortical damage impacts specific aspects of spinal circuit development, particularly the inhibitory microcircuitry that regulates spinal reflex circuits. In this study, we sought to determine how ischemic cortical damage impacts the synaptic attributes of a well-characterized population of inhibitory, GABAergic interneurons, called GABApre neurons, which modulates the efficiency of proprioceptive sensory terminals in the sensorimotor reflex circuit. We found that putative GABApre interneurons receive CST input and, using an established mouse model of perinatal stroke, that cortical ischemic injury results in a reduction of CST density within the intermediate region of the spinal cord, where these interneurons reside. Importantly, CST alterations were restricted to the side contralateral to the injury. Within the synaptic terminals of the GABApre interneurons, we observed a dramatic upregulation of the 65-isoform of the GABA synthetic enzyme glutamic acid decarboxylase (GAD65). In accordance with the CST density reduction, GAD65 was elevated on the side of the spinal cord contralateral to cortical injury. This effect was not seen for other GABApre synaptic markers or in animals that received sham surgery. Our data reveal a novel effect of perinatal stroke that involves severe deficits in the architecture of a descending spinal pathway, which in turn appear to promote molecular alterations in a specific spinal GABAergic circuit.

The homeodomain factor Gbx1 is required for locomotion and cell specification in the dorsal spinal cord

Dorsal horn neurons in the spinal cord integrate and relay sensory information to higher brain centers. These neurons are organized in specific laminae and different transcription factors are involved in their specification. The murine homeodomain Gbx1 protein is expressed in the mantle zone of the spinal cord at E12.5-13.5, correlating with the appearance of a discernable dorsal horn around E14 and eventually defining a narrow layer in the dorsal horn around perinatal stages. At postnatal stages, Gbx1 identifies a specific subpopulation of GABAergic neurons in the dorsal spinal cord. We have generated a loss of function mutation for Gbx1 and analyzed its consequences during spinal cord development. Gbx1^−/− mice are viable and can reproduce as homozygous null mutants. However, the adult mutant mice display an altered gait during forward movement that specifically affects the hindlimbs. This abnormal gait was evaluated by a series of behavioral tests, indicating that locomotion is impaired, but not muscle strength or motor coordination. Molecular analysis showed that the development of the dorsal horn is not profoundly affected in Gbx1^−/− mutant mice. However, analysis of terminal neuronal differentiation revealed that the proportion of GABAergic inhibitory interneurons in the superficial dorsal horn is diminished. Our study unveiled a role for Gbx1 in specifying a subset of GABAergic neurons in the dorsal horn of the spinal cord involved in the control of posterior limb movement.

Distinct enhancers of ptf1a mediate specification and expansion of ventral pancreas in zebrafish

Development of the pancreas and cerebellum require Pancreas-specific transcription factor-1a (Ptf1a), which encodes a subunit of the transcription factor complex PTF1. Ptf1a is required in succession for specification of the pancreas, proper allocation of pancreatic progenitors to endocrine and exocrine fates, and the production of digestive enzymes from the exocrine acini. In several neuronal structures, including the cerebellum, hindbrain, retina and spinal cord, Ptf1a is transiently expressed and promotes inhibitory neuron fates at the expense of excitatory fates. Transcription of Ptf1a in mouse is maintained in part by PTF1 acting on an upstream autoregulatory enhancer. However, the transcription factors and enhancers that initially activate Ptf1a expression in the pancreas and in certain structures of the nervous system have not yet been identified. Here we describe a zebrafish autoregulatory element, conserved among teleosts, with activity similar to that described in mouse. In addition, we performed a comprehensive survey of all non-coding sequences in a 67kb interval encompassing zebrafish ptf1a, and identified several neuronal enhancers, and an enhancer active in the ventral pancreas prior to activation of the autoregulatory enhancer. To test the requirement for autoregulatory control during pancreatic development, we restored ptf1a function through BAC transgenesis in ptf1a morphants, either with an intact BAC or one lacking the autoregulatory enhancer. We find that ptf1a autoregulation is required for development of the exocrine pancreas and full rescue of the ptf1a morphant phenotype. Similarly, we demonstrate that a ptf1a locus lacking the early enhancer region is also capable of rescue, but only supports formation of a hypoplastic exocrine pancreas. Through our dissection of the complex regulatory control of ptf1a, we identified separate cis-regulatory elements that underlie different aspects of its expression and function, and further demonstrated the requirement of maintained ptf1a expression for normal pancreatic morphogenesis. We also identified a novel enhancer that mediates initiation of ptf1a expression in the pancreas, through which the signals that specify the ventral pancreas are expected to exert their action.

Prdm13 mediates the balance of inhibitory and excitatory neurons in somatosensory circuits

Generating a balanced network of inhibitory and excitatory neurons during development requires precise transcriptional control. In the dorsal spinal cord, Ptf1a, a basic helix-loop-helix (bHLH) transcription activator, maintains this delicate balance by inducing homeodomain (HD) transcription factors such as Pax2 to specify the inhibitory lineage while suppressing HD factors such as Tlx1/3 that specify the excitatory lineage. We uncover the mechanism by which Ptf1a represses excitatory cell fate in the inhibitory lineage. We identify Prdm13 as a direct target of Ptf1a and reveal that Prdm13 actively represses excitatory cell fate by binding to regulatory sequences near the Tlx1 and Tlx3 genes to silence their expression. Prdm13 acts through multiple mechanisms, including interactions with the bHLH factor Ascl1, to repress Ascl1 activation of Tlx3. Thus, Prdm13 is a key component of a highly coordinated transcriptional network that determines the balance of inhibitory versus excitatory neurons in the dorsal spinal cord.

Genome-wide expression analysis of Ptf1a- and Ascl1-deficient mice reveals new markers for distinct dorsal horn interneuron populations contributing to nociceptive reflex plasticity

Inhibitory interneurons of the spinal dorsal horn play critical roles in the processing of noxious and innocuous sensory information. They form a family of morphologically and functionally diverse neurons that likely fall into distinct subtypes. Traditional classifications rely mainly on differences in dendritic tree morphology and firing patterns. Although useful, these markers are not comprehensive and cannot be used to drive specific genetic manipulations targeted at defined subsets of neurons. Here, we have used genome-wide expression profiling of spinal dorsal horns of wild-type mice and of two strains of transcription factor-deficient mice (Ptf1a(-/-) and Ascl1/Mash1(-/-) mice) to identify new genetic markers for specific subsets of dorsal horn inhibitory interneurons. Ptf1a(-/-) mice lack all inhibitory interneurons in the dorsal horn, whereas only the late-born inhibitory interneurons are missing in Ascl1(-/-) mice. We found 30 genes that were significantly downregulated in the dorsal horn of Ptf1a(-/-) mice. Twenty-one of those also showed reduced expression in Ascl1(-/-) mice. In situ hybridization analyses of all 30 genes identified four genes with primarily non-overlapping expression patterns in the dorsal horn. Three genes, pDyn coding the neuropeptide dynorphin, Kcnip2 encoding a potassium channel associated protein, and the nuclear receptor encoding gene Rorb, were expressed in Ascl1-dependent subpopulations of the superficial dorsal horn. The fourth gene, Tfap2b, encoding a transcription factor, is expressed mainly in a Ascl1-independent subpopulation of the deep dorsal horn. Functional experiments in isolated spinal cords showed that the Ascl1-dependent inhibitory interneurons are key players of nociceptive reflex plasticity.

Program specificity for Ptf1a in pancreas versus neural tube development correlates with distinct collaborating cofactors and chromatin accessibility

The lineage-specific basic helix-loop-helix transcription factor Ptf1a is a critical driver for development of both the pancreas and nervous system. How one transcription factor controls diverse programs of gene expression is a fundamental question in developmental biology. To uncover molecular strategies for the program-specific functions of Ptf1a, we identified bound genomic regions in vivo during development of both tissues. Most regions bound by Ptf1a are specific to each tissue, lie near genes needed for proper formation of each tissue, and coincide with regions of open chromatin. The specificity of Ptf1a binding is encoded in the DNA surrounding the Ptf1a-bound sites, because these regions are sufficient to direct tissue-restricted reporter expression in transgenic mice. Fox and Sox factors were identified as potential lineage-specific modifiers of Ptf1a binding, since binding motifs for these factors are enriched in Ptf1a-bound regions in pancreas and neural tube, respectively. Of the Fox factors expressed during pancreatic development, Foxa2 plays a major role. Indeed, Ptf1a and Foxa2 colocalize in embryonic pancreatic chromatin and can act synergistically in cell transfection assays. Together, these findings indicate that lineage-specific chromatin landscapes likely constrain the DNA binding of Ptf1a, and they identify Fox and Sox gene families as part of this process.

Transcription factors define the neuroanatomical organization of the medullary reticular formation

The medullary reticular formation contains large populations of inadequately described, excitatory interneurons that have been implicated in multiple homeostatic behaviors including breathing, viserosensory processing, vascular tone, and pain. Many hindbrain nuclei show a highly stereotyped pattern of localization across vertebrates suggesting a strong underlying genetic organization. Whether this is true for neurons within the reticular regions of hindbrain is unknown. Hindbrain neurons are derived from distinct developmental progenitor domains each of which expresses distinct patterns of transcription factors (TFs). These neuronal populations have distinct characteristics such as transmitter identity, migration, and connectivity suggesting developmentally expressed TFs might identify unique subpopulations of neurons within the reticular formation. A fate-mapping strategy using perinatal expression of reporter genes within Atoh1, Dbx1, Lmx1b, and Ptf1a transgenic mice coupled with immunohistochemistry (IHC) and in situ hybridization (ISH) were used to address the developmental organization of a large subset of reticular formation glutamatergic neurons. All hindbrain lineages have relatively large populations that extend the entire length of the hindbrain. Importantly, the location of neurons within each lineage was highly constrained. Lmx1b- and Dbx1- derived populations were both present in partially overlapping stripes within the reticular formation extending from dorsal to ventral brain. Within each lineage, distinct patterns of gene expression and organization were localized to specific hindbrain regions. Rostro-caudally sub-populations differ sequentially corresponding to proposed pseudo-rhombomereic boundaries. Dorsal-ventrally, sub-populations correspond to specific migratory positions. Together these data suggests the reticular formation is organized by a highly stereotyped developmental logic.