Programming neural Hoxd10: in vivo evidence that early node-associated signals predominate over paraxial mesoderm signals at posterior spinal levels

Motor neurons with limb-innervating character in the cervical spinal cord are sculpted by apoptosis based on the Hox code in chick embryo

In the developing chick embryo, a certain population of motor neurons (MNs) in the non-limb-innervating cervical spinal cord undergoes apoptosis between embryonic days 4 and 5. However, the characteristics of these apoptotic MNs remain undefined. Here, by examining the spatiotemporal profiles of apoptosis and MN subtype marker expression in normal or apoptosis-inhibited chick embryos, we found that this apoptotic population is distinguishable by Foxp1 expression. When apoptosis was inhibited, the Foxp1⁺ MNs survived and showed characteristics of lateral motor column (LMC) neurons, which are of a limb-innervating subtype, suggesting that cervical Foxp1⁺ MNs are the rostral continuation of the LMC. Knockdown and misexpression of Foxp1 did not affect apoptosis progression, but revealed the role of Foxp1 in conferring LMC identity on the cervical MNs. Furthermore, ectopic expression of Hox genes that are normally expressed in the brachial region prevented apoptosis, and directed Foxp1⁺ MNs to LMC neurons at the cervical level. These results indicate that apoptosis in the cervical spinal cord plays a role in sculpting Foxp1⁺ MNs committed to LMC neurons, depending on the Hox expression pattern.

Transgenic studies on homeobox genes in nervous system development: spina bifida in Isl1 transgenic mice

To develop in vivo assays for homeobox gene function in neural development, we generated transgenic mice in which the expression of a homeobox gene is altered only within the nervous system, in neurons or neuronal precursor cells. Transgenic expression of Hoxc8 did not result in gross abnormalities, while a Hoxd4 transgene caused death shortly after birth. In neural progenitor cells, the motorneuron-specific homeodomain transcription factor Isl1 induced early developmental defects, including absence of anterior neural structures, profound defects in the neuroepithelium and defective neural tube closure. A fraction of Isl1 transgenic mice exhibited spina bifida. Isl1 transgene expression was also associated with decreased proliferation and increased Pbx1 expression in the ventral neural tube. Our results suggest a function for some homeobox genes in development of the nervous system, and that cell-type- and region-specific transgenic models will be useful to identify the cellular and molecular targets of homeobox transcription factors in nervous system development.

Localised axial progenitor cell populations in the avian tail bud are not committed to a posterior Hox identity

The outgrowth of the vertebrate tail is thought to involve the proliferation of regionalised stem/progenitor cell populations formed during gastrulation. To follow these populations over extended periods, we used cells from GFP-positive transgenic chick embryos as a source for donor tissue in grafting experiments. We determined that resident progenitor cell populations are localised in the chicken tail bud. One population, which is located in the chordoneural hinge (CNH), contributes descendants to the paraxial mesoderm, notochord and neural tube, and is serially transplantable between embryos. A second population of mesodermal progenitor cells is located in a separate dorsoposterior region of the tail bud, and a corresponding population is present in the mouse tail bud. Using heterotopic transplantations, we show that the fate of CNH cells depends on their environment within the tail bud. Furthermore, we show that the anteroposterior identity of tail bud progenitor cells can be reset by heterochronic transplantation to the node region of gastrula-stage chicken embryos.

Hox gene expression patterns in Lethenteron japonicum embryos--insights into the evolution of the vertebrate Hox code

The Hox code of jawed vertebrates is characterized by the colinear and rostrocaudally nested expression of Hox genes in pharyngeal arches, hindbrain, somites, and limb/fin buds. To gain insights into the evolutionary path leading to the gnathostome Hox code, we have systematically analyzed the expression pattern of the Hox gene complement in an agnathan species, Lethenteron japonicum (Lj). We have isolated 15 LjHox genes and assigned them to paralogue groups (PG) 1-11, based on their deduced amino acid sequences. LjHox expression during development displayed gnathostome-like spatial patterns with respect to the PG numbers. Specifically, lamprey PG1-3 showed homologous expression patterns in the rostral hindbrain and pharyngeal arches to their gnathostome counterparts. Moreover, PG9-11 genes were expressed specifically in the tailbud, implying its posteriorizing activity as those in gnathostomes. We conclude that these gnathostome-like colinear spatial patterns of LjHox gene expression can be regarded as one of the features already established in the common ancestor of living vertebrates. In contrast, we did not find evidence for temporal colinearity in the onset of LjHox expression. The genomic and developmental characteristics of Hox genes from different chordate species are also compared, focusing on evolution of the complex body plan of vertebrates.

Expression patterns of Hox10 paralogous genes during lumbar spinal cord development

We have examined the expression of three paralogous Hox genes from E11.5 through E15.5 in the mouse spinal cord. These ages coincide with major phases of spinal cord neurogenesis, neuronal differentiation, cell migration, gliogenesis, and motor neuron cell death. The three genes, Hoxa10, Hoxc10, and Hoxd10, are all expressed in the lumbar spinal cord and have distinct expression patterns. Mutations in these three genes are known to affect motor neuron patterning. All three genes show lower levels of expression at the rostral limits of their domains, with selective regions of higher expression more caudally. Hoxa10 and Hoxd10 expression appears confined to postmitotic cell populations in the intermediate and ventral gray, while Hoxc10 is also expressed in proliferating cells in the dorsal ventricular zone. Hoxc10 and Hoxd10 expression is clearly excluded from the lateral motor columns at rostral lumbar levels but is present in this region more caudally. Double labeling demonstrates that Hoxc10 expression is correlated with ventrolateral LIM gene expression in the caudal part of the lumbar spinal cord.

An early broad competence of motoneurons to express ER81 is later sculpted by the periphery

The ETS transcription factor ER81 is expressed in sensory neurons and motoneurons that innervate the adductor and femorotibialis muscles in chick hindlimb and is essential for the development of monosynaptic connections between these two populations of neurons. Neurons need a signal(s) from limb bud mesoderm to initiate ER81 expression. It is not known whether the mature expression pattern arises because adductor and femorotibialis motoneurons are uniquely competent to respond to peripheral signals and express ER81, or whether all motoneurons are competent to express ER81, but normally only adductor and femorotibialis motoneurons are exposed to the requisite activating signal. To investigate these possibilities, we examined ER81 expression in motoneurons that encountered limb tissue surgically mismatched with their target identity at stages after motor pool identities are established. We found that ER81 expression was not invariably linked to motor pool identity or target innervation and was more malleable in later-born femorotibialis motoneurons than in earlier-born adductor motoneurons. We also found that ER81 expression is regulated differently in sensory neurons and motoneurons. Most striking was the observation that motoneurons caudal to the normal adductor and femorotibialis pools could express ER81 when exposed to the appropriate peripheral signals, although this competence did not extend through the entire lumbosacral (LS) region. Thus, it appears that a prepattern of competence to express ER81 is established in early LS motoneurons, most likely in concert with their target identity, and that the expression domains of motoneurons are subsequently refined by peripheral signals at later stages.

All-trans retinoic acid-responsive genes identified in the human SH-SY5Y neuroblastoma cell line and their regulated expression in the nervous system of early embryos

The vitamin A metabolite, all-trans retinoic acid (atRA), is required for embryonic development. atRA binds to the nuclear retinoic acid receptors and regulates the transcription of specific target genes. In order to identify atRA-induced genes that play a role in neural development, a subtractive library was created from SH-SY5Y neuroblastoma cells, a human cell line that exhibits changes in cell adhesion and neurite outgrowth after exposure to the vitamin A acid. We report here the identification of 14 genes that are rapidly induced by atRA (retinoic acid induced in neuroblastoma or RAINB), eight of which were previously not known to be atRA responsive (BTBD11, calmin, cyclin M2, ephrin B2, HOXD10, NEDD9, RAINB6 and tenascin R). mRNA regulation by atRA was confirmed in SH-SY5Y cells by Northern blotting, and gene regulation was studied in additional human cell lines using the quantitative polymerase chain reaction. The majority of the atRA-responsive clones revealed in this screen are highly expressed in the nervous system of developing rat embryos. Further, the expression of several of these genes is perturbed in developing rat embryos exposed to excess atRA or conversely, deprived of sufficient retinoid during early development. We propose that a subset of these genes lie downstream of atRA and its receptors in the regulation of neurite outgrowth and cell adhesion in both neural and non-neural tissues within the developing embryo.

Ectopic expression of Hoxd10 in thoracic spinal segments induces motoneurons with a lumbosacral molecular profile and axon projections to the limb

Hox genes encode anterior-posterior identity during central nervous system development. Few studies have examined Hox gene function at lumbosacral (LS) levels of the spinal cord, where there is extensive information on normal development. Hoxd10 is expressed at high levels in the embryonic LS cord but not the thoracic cord. To test the hypothesis that restricted expression of Hoxd10 contributes to the attainment of an LS identity, and specifically an LS motoneuron identity, Hoxd10 was ectopically expressed in thoracic segments in chick embryos by means of in ovo electroporation. Regional motoneuron identity was assessed after the normal period of motoneuron differentiation. Subsets of motoneurons in transfected thoracic segments developed a molecular profile normally shown by LS motoneurons, including Lim 1 and RALDH2 expression. In addition, motoneurons in posterior thoracic segments showed novel axon projections to two muscles in the anterodorsal limb, the sartorius and anterior iliotibialis muscles. At thoracic levels, we also found a decrease in motoneuron numbers and a reduction in gonad size. These last findings suggest that early and high levels of Hox expression impeded motoneuron development and neural-mesodermal interactions. Despite these adverse effects, our data indicate that Hoxd10 expression is sufficient to induce LS motoneuron identity and axon trajectories characteristic of motoneurons in the LS region.

Opposing FGF and retinoid pathways: a signalling switch that controls differentiation and patterning onset in the extending vertebrate body axis

Construction of the trunk/caudal region of the vertebrate embryo involves a set of distinct molecules and processes whose relationships are just coming into focus. In addition to the subdivision of the embryo into head and trunk domains, this "caudalisation" process requires the establishment and maintenance of a stem zone. This sequentially generates caudal tissues over a long period which then undergo differentiation and patterning in the extending body axis. Here we review recent studies that show that changes in the signalling properties of the paraxial mesoderm act as a switch that controls onset of differentiation and pattern in the spinal cord. These findings identify distinct roles for different caudalising factors; in particular, Fibroblast Growth Factor (FGF) inhibits differentiation in the caudal stem zone, while Retinoic acid (RA) provided rostrally by somitic mesoderm is required for neuronal differentiation and establishment of ventral neural pattern. Furthermore, the mutual opposition of FGF and RA pathways controls not only neural differentiation but also mesoderm segmentation and might also underlie the progressive assignment of rostrocaudal identity by regulating Hox gene availability and activation.