Delays in neuronal differentiation in Mash1/Ascl1 mutants

Ascl1/Mash1 is a novel target of Gli2 during Gli2-induced neurogenesis in P19 EC cells

The Sonic Hedgehog (Shh) signaling pathway is important for neurogenesis in vivo. Gli transcription factors, effector proteins of the Shh signaling pathway, have neurogenic properties in vivo, which are still poorly understood. To study the molecular basis of neurogenic properties of Gli2, we used a well-established embryonic stem cell model, the P19 embryonal carcinoma (EC) cell line, which can be induced to differentiate into neurons in the presence of retinoic acid (RA). We found that, in the absence of RA, overexpression of Gli2 induced P19 EC cells to differentiate into neurons, but not astrocytes during the first ten days of differentiation. To our knowledge, this is the first indication that the expression of Gli factors can convert EC cells into neurons. Furthermore, Gli2 upregulated expression of the neurogenic basic helix-loop-helix (bHLH) factors, such as NeuroD, Neurog1 and Ascl1/Mash1 in P19 EC cells. Using chromatin immunoprecipitation assays, we showed that Gli2 bound to multiple regulatory regions in the Ascl1 gene, including promoter and enhancer regions during Gli2-induced neurogenesis. In addition, Gli2 activated the Ascl1/Mash1 promoter in vitro. Using the expression of a dominant-negative form of Gli2, fused to the Engrailed repression domain, we observed a reduction in gliogenesis and a significant downregulation of the bHLH factors Ascl1/Mash1, Neurog1 and NeuroD, leading to delayed neurogenesis in P19 EC cells, further supporting the hypothesis that Ascl1/Mash1 is a direct target of Gli2. In summary, Gli2 is sufficient to induce neurogenesis in P19 stem cells at least in part by directly upregulating Ascl1/Mash1. Our results provide mechanistic insight into the neurogenic properties of Gli2 in vitro, and offer novel plausible explanations for its in vivo neurogenic properties.

Ongoing roles of Phox2 homeodomain transcription factors during neuronal differentiation

Transcriptional determinants of neuronal identity often stay expressed after their downstream genetic program is launched. Whether this maintenance of expression plays a role is for the most part unknown. Here, we address this question for the paralogous paired-like homeobox genes Phox2a and Phox2b, which specify several classes of visceral neurons at the progenitor stage in the central and peripheral nervous systems. By temporally controlled inactivation of Phox2b, we find that the gene, which is required in ventral neural progenitors of the hindbrain for the production of branchio-visceral motoneuronal precursors, is also required in these post-mitotic precursors to maintain their molecular signature - including downstream transcription factors - and allow their tangential migration and the histogenesis of the corresponding nuclei. Similarly, maintenance of noradrenergic differentiation during embryogenesis requires ongoing expression of Phox2b in sympathetic ganglia, and of Phox2a in the main noradrenergic center, the locus coeruleus. These data illustrate cases where the neuronal differentiation program does not unfold as a transcriptional `cascade' whereby downstream events are irreversibly triggered by an upstream regulator, but instead require continuous transcriptional input from it.

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.

Transcription factor KLF7 regulates differentiation of neuroectodermal and mesodermal cell lineages

Previous gene targeting studies in mice have implicated the nuclear protein Krüppel-like factor 7 (KLF7) in nervous system development while cell culture assays have documented its involvement in cell cycle regulation. By employing short hairpin RNA (shRNA)-mediated gene silencing, here we demonstrate that murine Klf7 gene expression is required for in vitro differentiation of neuroectodermal and mesodermal cells. Specifically, we show a correlation of Klf7 silencing with down-regulation of the neuronal marker microtubule-associated protein 2 (Map2) and the nerve growth factor (NGF) tyrosine kinase receptor A (TrkA) using the PC12 neuronal cell line. Similarly, KLF7 inactivation in Klf7-null mice decreases the expression of the neurogenic marker brain lipid-binding protein/fatty acid-binding protein 7 (BLBP/FABP7) in neural stem cells (NSCs). We also report that Klf7 silencing is detrimental to neuronal and cardiomyocytic differentiation of embryonic stem cells (ESCs), in addition to altering the adipogenic and osteogenic potential of mouse embryonic fibroblasts (MEFs). Finally, our results suggest that genes that are key for self-renewal of undifferentiated ESCs repress Klf7 expression in ESCs. Together with previous findings, these results provide evidence that KLF7 has a broad spectrum of regulatory functions, which reflect the discrete cellular and molecular contexts in which this transcription factor operates.

Distinct requirements for Ascl1 in subpopulations of midbrain GABAergic neurons

Midbrain GABAergic neurons regulate multiple aspects of behavior and play important roles in psychiatric and neurological disease. These neurons constitute several anatomical and functional subpopulations, but their molecular heterogeneity and developmental regulatory mechanisms are poorly understood. Here we have studied the involvement of the proneural gene Ascl1 in the development of the midbrain GABAergic neurons. Analysis of Ascl1 mutant mice demonstrated highly region-specific requirements for Ascl1 for development of different GABAergic neuron subpopulations. Ascl1 is dispensable for the development of the ventral-most midbrain GABAergic neurons associated with dopaminergic nuclei substantia nigra pars reticulata (SNpr) and ventral tegmental area (VTA) GABAergic neurons. In the ventrolateral midbrain, loss of Ascl1 results in markedly delayed neurogenesis in the midbrain domains m3-m5. Within this region, Ascl1 has a unique role in m4, where it also regulates glutamatergic neurogenesis. Our results suggest that the m3-m5 midbrain neuroepithelium gives rise to the GABAergic neuron groups located in the midbrain reticular formation and ventrolateral periaqueductal gray. In contrast to m3-m5, Ascl1 is absolutely required in the dorsal midbrain domains m1-m2, for generation of the GABAergic neurons populating the superior and inferior colliculi as well as dorsal periaqueductal gray. These studies demonstrate different molecular regulatory mechanisms for the distinct midbrain GABAergic neuron subpopulations. Also, our results have implications on understanding the origins of the various midbrain GABAergic neuron groups in the embryonic neuroepithelium.

Development of the autonomic nervous system: a comparative view

In this review we summarize current understanding of the development of autonomic neurons in vertebrates. The mechanisms controlling the development of sympathetic and enteric neurons have been studied in considerable detail in laboratory mammals, chick and zebrafish, and there are also limited data about the development of sympathetic and enteric neurons in amphibians. Little is known about the development of parasympathetic neurons apart from the ciliary ganglion in chicks. Although there are considerable gaps in our knowledge, some of the mechanisms controlling sympathetic and enteric neuron development appear to be conserved between mammals, avians and zebrafish. For example, some of the transcriptional regulators involved in the development of sympathetic neurons are conserved between mammals, avians and zebrafish, and the requirement for Ret signalling in the development of enteric neurons is conserved between mammals (including humans), avians and zebrafish. However, there are also differences between species in the migratory pathways followed by sympathetic and enteric neuron precursors and in the requirements for some signalling pathways.

Studying peripheral sympathetic nervous system development and neuroblastoma in zebrafish

The combined experimental attributes of the zebrafish model system, which accommodates cellular, molecular, and genetic approaches, make it particularly well-suited for determining the mechanisms underlying normal vertebrate development as well as disease states, such as cancer. In this chapter, we describe the advantages of the zebrafish system for identifying genes and their functions that participate in the regulation of the development of the peripheral sympathetic nervous system (PSNS). The zebrafish model is a powerful system for identifying new genes and pathways that regulate PSNS development, which can then be used to genetically dissect PSNS developmental processes, such as tissue size and cell numbers, which in the past haves proved difficult to study by mutational analysis in vivo. We provide a brief review of our current understanding of genetic pathways important in PSNS development, the rationale for developing a zebrafish model, and the current knowledge of zebrafish PSNS development. Finally, we postulate that knowledge of the genes responsible for normal PSNS development in the zebrafish will help in the identification of molecular pathways that are dysfunctional in neuroblastoma, a highly malignant cancer of the PSNS.

Generating diversity: Mechanisms regulating the differentiation of autonomic neuron phenotypes

Sympathetic and parasympathetic postganglionic neurons innervate a wide range of target tissues. The subpopulation of neurons innervating each target tissue can express unique combinations of neurotransmitters, neuropeptides, ion channels and receptors, which together comprise the chemical phenotype of the neurons. The target-specific chemical phenotype shown by autonomic postganglionic neurons arises during development. In this review, we examine the different mechanisms that generate such a diversity of neuronal phenotypes from the pool of apparently homogenous neural crest progenitor cells that form the sympathetic ganglia. There is evidence that the final chemical phenotype of autonomic postganglionic neurons is generated by both signals at the level of the cell body that trigger cell-autonomous programs, as well as signals from the target tissues they innervate.

BMP signaling regulates sympathetic nervous system development through Smad4-dependent and -independent pathways

Induction of the sympathetic nervous system (SNS) from its neural crest (NC) precursors is dependent on BMP signaling from the dorsal aorta. To determine the roles of BMP signaling and the pathways involved in SNS development, we conditionally knocked out components of the BMP pathways. To determine if BMP signaling is a cell-autonomous requirement of SNS development, the Alk3 (BMP receptor IA) was deleted in the NC lineage. The loss of Alk3 does not prevent NC cell migration, but the cells die immediately after reaching the dorsal aorta. The paired homeodomain factor Phox2b, known to be essential for survival of SNS precursors, is downregulated, suggesting that Phox2b is a target of BMP signaling. To determine if Alk3 signals through the canonical BMP pathway, Smad4 was deleted in the NC lineage. Loss of Smad4 does not affect neurogenesis and ganglia formation; however, proliferation and noradrenergic differentiation are reduced. Analysis of transcription factors regulating SNS development shows that the basic helix-loop-helix factor Ascl1 is downregulated by loss of Smad4 and that Ascl1 regulates SNS proliferation but not noradrenergic differentiation. To determine if the BMP-activated Tak1 (Map3k7) pathway plays a role in SNS development, Tak1 was deleted in the NC lineage. We show that Tak1 is not involved in SNS development. Taken together, our results suggest multiple roles for BMP signaling during SNS development. The Smad4-independent pathway acts through the activation of Phox2b to regulate survival of SNS precursors, whereas the Smad4-dependent pathway controls noradrenergic differentiation and regulates proliferation by maintaining Ascl1 expression.

Sonic hedgehog signaling is required for sympathetic nervous system development

Sonic hedgehog (Shh) plays critical roles during nervous system development, yet little is known about its function in the sympathetic nervous system. Using a mouse Shh null line, we examined the roles of Shh during SNS development. Loss of Shh did not prevent formation of the sympathetic trunk, but the ganglia are hypoplastic and misspatterned. Neuronal differentiation was delayed in Shh mutant embryos showing that Shh is required for correct developmental timing in addition to its role in sympathetic nervous system patterning. Immunohistochemical analyses of the ganglia for expression of the pan-neuronal marker beta3-tubulin, the noradrenergic biosynthetic enzyme tyrosine hydroxylase and the glial marker B-FABP showed that Shh is not required for differentiation of sympathetic neurons or glia.

The bHLH transcription factor Hand2 is essential for the maintenance of noradrenergic properties in differentiated sympathetic neurons

The basic helix-loop-helix transcription factor Hand2 is essential for the proliferation and noradrenergic differentiation of sympathetic neuron precursors during development. Here we address the function of Hand2 in postmitotic, differentiated sympathetic neurons. Knockdown of endogenous Hand2 in cultured E12 chick sympathetic neurons by siRNA results in a significant (about 60%) decrease in the expression of the noradrenergic marker genes dopamine-beta-hydroxylase (DBH) and tyrosine hydroxylase (TH). In contrast, expression of the pan-neuronal genes TuJ1, HuC and SCG10 was not affected. To analyze the in vivo role of Hand2 in differentiated sympathetic neurons we used mice harboring a conditional Hand2-null allele and excised the gene by expression of Cre recombinase under control of the DBH promotor. Mouse embryos homozygous for Hand2 gene deletion showed decreased sympathetic neuron number and TH expression was strongly reduced in the residual neuron population. The in vitro Hand2 knockdown also enhances the CNTF-induced expression of the cholinergic marker genes vesicular acetylcholine transporter (VAChT) and choline acetyltransferase (ChAT). Taken together, these findings demonstrate that the Hand2 transcription factor plays a key role in maintaining noradrenergic properties in differentiated neurons.

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.

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.

Insm1 (IA-1) is a crucial component of the transcriptional network that controls differentiation of the sympatho-adrenal lineage

Insm1 (IA-1) encodes a Zn-finger factor that is expressed in the developing nervous system. We demonstrate here that the development of the sympatho-adrenal lineage is severely impaired in Insm1 mutant mice. Differentiation of sympatho-adrenal precursors, as assessed by the expression of neuronal subtype-specific genes such as Th and Dbh, is delayed in a pronounced manner, which is accompanied by a reduced proliferation. Sympathetic neurons eventually overcome the differentiation blockade and mature correctly, but sympathetic ganglia remain small. By contrast, terminal differentiation of adrenal chromaffin cells does not occur. The transcription factors Mash1 (Ascl1), Phox2a, Gata3 and Hand2 (previously dHand) control the differentiation of sympatho-adrenal precursor cells, and their deregulated expression in Insm1 mutant mice demonstrates that Insm1 acts in the transcriptional network that controls differentiation of this lineage. Pronounced similarities between Mash1 and Insm1 phenotypes are apparent, which suggests that Insm1 might mediate aspects of Mash1 function in the subtype-specific differentiation of sympatho-adrenal precursors. Noradrenaline is the major catecholamine produced by developing sympatho-adrenal cells and is required for fetal survival. We demonstrate that the fetal lethality of Insm1 mutant mice is caused by catecholamine deficiency, which highlights the importance of Insm1 in the development of the sympatho-adrenal lineage.

Hand2 determines the noradrenergic phenotype in the mouse sympathetic nervous system

The basic helix-loop-helix (bHLH) transcription factor Hand2 has been shown to play a role in the development of the mammalian sympathetic nervous system (SNS); however, its precise role could not be uncovered because Hand2 is required for early embryonic survival. We therefore generated a conditional Hand2 knockout mouse line by excising Hand2 in Wnt1-Cre-expressing neural crest-derived cells. These mice die at 12.5 dpc with embryos showing severe cardiovascular and facial defects. Crest-derived cells, however, populate sites of SNS development and proliferate normally. Sympathetic precursors differentiate into neurons and express the pan-neuronal markers, beta3-tubulin (Tuj1) and Hu showing that Hand2 is not essential for SNS neuronal differentiation. To determine whether Hand2 regulates noradrenergic differentiation, the levels of the norepinephrine biosynthetic enzymes, tyrosine hydroxylase (TH) and dopamine beta-hydroxylase (DBH) was examined. Both enzymes were dramatically reduced in mutant embryos suggesting that the primary role of Hand2 in the SNS is determination of neuronal phenotype. Loss of Hand2 did not affect the expression of other members of the transcriptional circuit regulating SNS development, including Phox2a/b, Mash1 and Gata2/3; however, Hand2 was required for Hand1 expression. Our data suggest that the major role of Hand2 during SNS development is to permit sympathetic neurons to acquire a catecholaminergic phenotype.

Lbx1 acts as a selector gene in the fate determination of somatosensory and viscerosensory relay neurons in the hindbrain

Distinct types of relay neurons in the hindbrain process somatosensory or viscerosensory information. How neurons choose between these two fates is unclear. We show here that the homeobox gene Lbx1 is essential for imposing a somatosensory fate on relay neurons in the hindbrain. In Lbx1 mutant mice, viscerosensory relay neurons are specified at the expense of somatosensory relay neurons. Thus Lbx1 expression distinguishes between the somatosensory or viscerosensory fate of relay neurons.

The proneural gene Mash1 specifies an early population of telencephalic oligodendrocytes

The bHLH (basic helix-loop-helix) transcription factor Mash1 is best known for its role in the regulation of neurogenesis. However, Mash1 is also expressed in oligodendrocyte precursors and has recently been shown to promote the generation of oligodendrocytes in cell culture, suggesting that it may regulate oligodendrogenesis as well. Here, we show that in the developing ventral forebrain, Mash1 is expressed by a subset of oligodendrocyte precursors (OPCs) as soon as they are generated in the ventricular zone. Using reporter mice, we demonstrate that a subset of OPCs in both the embryonic and postnatal forebrain originate from Mash1-positive progenitors, including a large fraction of adult NG2-positive OPCs. Using Mash1 null mutant mice, we show that Mash1 is required for the generation of an early population of OPCs in the ventral forebrain between embryonic day 11.5 (E11.5) and E13.5, whereas OPCs generated later in embryonic development are not affected. Overexpression of Mash1 in the dorsal telencephalon induces expression of PDGFRalpha (platelet-derived growth factor receptor alpha) but not other OPC markers, suggesting that Mash1 specifies oligodendrogenesis in cooperation with other factors. Analysis of double-mutant mice suggests that Olig2 is one of the factors that cooperate with Mash1 for generation of OPCs. Together, our results show for the first time that Mash1 cooperates in vivo with Olig2 in oligodendrocyte specification, demonstrating an essential role for Mash1 in the generation of a subset of oligodendrocytes and revealing a genetic heterogeneity of oligodendrocyte lineages in the mouse forebrain.