Requirement for the homeobox gene Hb9 in the consolidation of motor neuron identity.

The homeobox gene Hb9, like its close relative MNR2, is expressed selectively by motor neurons (MNs) in the developing vertebrate CNS. In embryonic chick spinal cord, the ectopic expression of MNR2 or Hb9 is sufficient to trigger MN differentiation and to repress the differentiation of an adjacent population of V2 interneurons. Here, we provide genetic evidence that Hb9 has an essential role in MN differentiation. In mice lacking Hb9 function, MNs are generated on schedule and in normal numbers but transiently acquire molecular features of V2 interneurons. The aberrant specification of MN identity is associated with defects in the migration of MNs, the emergence of the subtype identities of MNs, and the projection of motor axons. These findings show that HB9 has an essential function in consolidating the identity of postmitotic MNs.     

Active suppression of interneuron programs within developing motor neurons revealed by analysis of homeodomain factor HB9.

Sonic hedgehog (Shh) specifies the identity of both motor neurons (MNs) and interneurons with morphogen-like activity. Here, we present evidence that the homeodomain factor HB9 is critical for distinguishing MN and interneuron identity in the mouse. Presumptive MN progenitors and postmitotic MNs express HB9, whereas interneurons never express this factor. This pattern resembles a composite of the avian homologs MNR2 and HB9. In mice lacking Hb9, the genetic profile of MNs is significantly altered, particularly by upregulation of Chx10, a gene normally restricted to a class of ventral interneurons. This aberrant gene expression is accompanied by topological disorganization of motor columns, loss of the phrenic and abducens nerves, and intercostal nerve pathfinding defects. Thus, MNs actively suppress interneuron genetic programs to establish their identity.     

Chromosomal mapping of ANTP class homeobox genes in amphioxus: piecing together ancestral genomes

Homeobox genes encode DNA-binding proteins, many of which are implicated in the control of embryonic development. Evolutionarily, most homeobox genes fall into two related clades: the ANTP and the PRD classes. Some genes in ANTP class, notably Hox, ParaHox, and NK genes, have an intriguing arrangement into physical clusters. To investigate the evolutionary history of these gene clusters, we examined homeobox gene chromosomal locations in the cephalochordate amphioxus, Branchiostoma floridae. We deduce that 22 amphioxus ANTP class homeobox genes localize in just three chromosomes. One contains the Hox cluster plus AmphiEn, AmphiMnx, and AmphiDll. The ParaHox cluster resides in another chromosome, whereas a third chromosome contains the NK type homeobox genes, including AmphiMsx and AmphiTlx. By comparative analysis we infer that clustering of ANTP class homeobox genes evolved just once, during a series of extensive cis-duplication events of genes early in animal evolution. A trans-duplication event occurred later to yield the Hox and ParaHox gene clusters on different chromosomes. The results obtained have implications for understanding the origin of homeobox gene clustering, the diversification of the ANTP class of homeobox genes, and the evolution of animal genomes.     

Localization of HB9 homeobox gene mRNA and protein during the early stages of chick feather development

We performed in situ hybridization and immunohistochemical analysis of HB9 homeobox gene mRNA and protein, respectively, during chick feather development. HB9 mRNA was highly expressed in epidermal basal cells and dermal cells of the placodes and feather buds, but not in those of the interplacodes and interbud regions. HB9 protein was predominantly expressed in dermal cells of the symmetric short buds and decreased after the asymmetric bud stage when the feather bud had become elongated along the anterior-posterior (A-P) and proximal-distal (P-D) axis. These results suggest that HB9 gene is regulated in a spatiotemporal manner during feather development, and may be involved in early feather bud morphogenesis.     

Localization of HB9 homeodomain protein and characterization of its nuclear localization signal during chick embryonic skin development

We detected HB9 protein during tarsometatarsal scale skin and late feather development. Immunofluorescent analyses with N-terminal 14 amino acids antiserum revealed that HB9 was strongly expressed in epidermal basal cells of the outer scale face in tarsometatarsal scale skin. Specific expression was also detected in dermal cells at the root region of the feather and around the feather follicle. Furthermore, we observed precise distribution of HB9 protein by immunoelectron microscopy. We detected HB9 protein not only in the nucleus, but also in the cytoplasm in tarsometatarsal scale skin. However, in feather skin HB9 protein was found in the nucleus but not in the cytoplasm. Cytoplasmic localization of HB9 protein in tarsometatarsal scale skin was observed especially in the endoplasmic reticulum and the Golgi apparatus. To address the mechanism of nuclear–cytoplasmic translocation, we determined the nuclear localization signal (NLS) sequences by using eukaryotic green fluorescent protein fusion protein in primary keratinocyte culture. Chick HB9 homeoprotein has two types of the NLS sequences in its homeodomain. One of them is a bipartite type as representatively found in Xenopus nucleoplasmin. The other is very similar to hexapeptide NLS sequences identified in pancreatic duodenum homeobox 1 (PDX1). These sequences functioned not only in keratinocytes but also in dermal fibroblasts. They are conserved in Xenopus, mouse, and human HB9 ortholog. These results indicate that HB9 protein might be involved in chick tarsometatarsal scale and feather development and that nuclear import of HB9 protein might be regulated by these NLS sequences in the homeodomain.     

Targeted Depletion of TDP-43 Expression in the Spinal Cord Motor Neurons Leads to the Development of Amyotrophic Lateral Sclerosis-like Phenotypes in Mice

ALS, or amyotrophic lateral sclerosis, is a progressive and fatal motor neuron disease with no effective medicine. Importantly, the majority of the ALS cases are with TDP-43 proteinopathies characterized with TDP-43-positive, ubiquitin-positive inclusions (UBIs) in the cytosol. However, the role of the mismetabolism of TDP-43 in the pathogenesis of ALS with TDP-43 proteinopathies is unclear. Using the conditional mouse gene targeting approach, we show that mice with inactivation of the Tardbp gene in the spinal cord motor neurons (HB9:Cre-Tardbp(lx/-)) exhibit progressive and male-dominant development of ALS-related phenotypes including kyphosis, motor dysfunctions, muscle weakness/atrophy, motor neuron loss, and astrocytosis in the spinal cord. Significantly, ubiquitinated proteins accumulate in the TDP-43-depleted motor neurons of the spinal cords of HB9:Cre-Tardbp(lx/-) mice with the ALS phenotypes. This study not only establishes an important role of TDP-43 in the long term survival and functioning of the mammalian spinal cord motor neurons, but also establishes that loss of TDP-43 function could be one major cause for neurodegeneration in ALS with TDP-43 proteinopathies.     

Expression of the HB9 homeobox gene concomitant with proliferation accompanying epidermal stratification during development of chick embryonic tarsometatarsal skin

A homeobox gene, HB9, has been isolated from the tarsometatarsal skin of 13-day-old chick embryos using a degenerate RT-PCR-based screening method. In situ hybridization analysis revealed that, during development of chick embryonic skin, the HB9 gene was expressed in epidermal basal cells of the placodes, but not in those of interplacodes, and in the dermal cells under the placodes at 9 days before addition of an intermediate layer by proliferation of the basal cells in the placodes. With the onset of epidermal stratification, the direction of the basal cell mitosis changed, with the axis becoming vertical to the epidermal surface. Placodes and interplacodes form outer and inner scales, respectively, after they have elongated distally (Tanaka S, Kato Y (1983b) J Exp Zool 225: 271–283). During scale ridge elongation at 12–15 days, HB9 was strongly expressed in the epidermis of the outer scale face, where the cell proliferation is more active than in the epidermis of the inner scale face; hence, stratification of the outer scale face is more prominent than that of the inner scale face. After 16 days, when mitotic activity in the epidermal basal cells decreases and the thickness of the epidermis is maintained at a constant level, the HB9 expression decreases with the onset of epidermal keratinization. These results suggest that HB9 may be involved in the proliferation of the epidermal basal cells that accompanies epidermal stratification.     

even-skipped determines the dorsal growth of motor axons in Drosophila

Axon pathfinding and target choice are governed by cell type-specific responses to external cues. Here, we show that in the Drosophila embryo, motorneurons with targets in the dorsal muscle field express the homeobox gene even-skipped and that this expression is necessary and sufficient to direct motor axons into the dorsal muscle field. Previously, it was shown that motorneurons projecting to ventral targets express the LIM homeobox gene islet, which is sufficient to direct axons to the ventral muscle field. Thus, even-skipped complements the function of islet, and together these two genes constitute a bimodal switch regulating axonal growth and directing motor axons to ventral or to dorsal regions of the muscle field.     

The development of three identified motor neurons in the larva of an ascidian, Halocynthia roretzi

The generation of distinct classes of motor neurons underlies the development of complex motile behavior in all animals and is well characterized in chordates. Recent molecular studies indicate that the ascidian larval central nervous system (CNS) exhibits anteroposterior regionalization similar to that seen in the vertebrate CNS. To extend the understanding about the diversity of motor neurons in the ascidian larva, we have identified the number, position, and projection of individual motor neurons in Halocynthia roretzi, using a green fluorescent protein under the control of a neuron-specific promoter. Three pairs of motor neurons, each with a distinct shape and innervation pattern, were identified along the anteroposterior axis of the neural tube: the anterior and posterior pairs extend their axons toward dorsal muscle cells, whereas the middle pair project their axons toward ventral muscle. Overexpression of a dominant-negative form of a potassium channel in these cells resulted in paralysis on the injected side, thus these cells must constitute the major population of motor neurons responsible for swimming behavior. Lim class homeobox genes have been known as candidate genes that determine subtypes of motor neurons. Therefore, the expression pattern of Hrlim, which is a Lim class homeobox gene, was examined in the motor neuron precursors. All three motor neurons expressed Hrlim at the tailbud stage, although each down-regulated Hrlim at a different time. Misexpression of Hrlim in the epidermal lineage led to ectopic expression of TuNa2, a putative voltage-gated channel gene normally expressed predominantly in the three pairs of motor neurons. Hrlim may control membrane excitability of motor neurons by regulating ion channel gene expression.