BIOLOGICAL OVERVIEW muscle segment homeobox is homologous to a growing number of divergent homeobox genes in vertebrates that appear to be involved in interactions between epithelial and mesenchymal cells (mesodermal cells that give rise to connective tissue). In vertebrates, msh is first expressed in ectoderm, followed by expression in mesoderm. A similar temporal and spatial pattern of gene activity occurs in flies; a switch from ectodermal to mesodermal expression. This suggests that vertebrate and Drosophila msh play similar developmental roles.

The ectodermal expression pattern is particularly interesting. Expression of msh prefigures the formation of lateral proneural clusters. Early msh expression in ventral stripes is complementary to that of the Hairy stripes, but later forms continuous longitudinal bands of either side of the embryo. Similar to achaete, the expression of msh becomes restricted to delaminating neuroblasts and disappears from ectodermal cells that remain uncommitted. Only one of the four lateral neuroblasts per hemisegment expresses msh (probably identical to NB7-4), whereas two out of four express achaete. In embryos that are mutant for any of the neurogenic genes, all cells from a proneural cluster segregate as neuroblasts and msh remains expressed in one large cluster of neuroblasts per hemisegment. Later expression in seen in proneural clusters that give rise to the second and perhaps third wave (SII and SIII) of segregating neuroblasts (D'Alessio, 1996).

It is suggested that msh functions in a similar fashion to vnd (NK2) in neural patterning, and that these roles are conserved in insects and vertebrates. Interaction between DPP and Short gastrulation (vertebrate homologs BMPs and chordin) restrict msh (vertebrate homologs the Msx genes) expression to the lateral most column of proneural clusters (in vertebrates the lateral-most portions of the neural plate). In a similar fashion vnd is restricted to the medial column of proneural clusters. In vertebrates the vnd homolog Nkx-2 is restricted to the medial region of the neural plate. Both msh and vnd serve a similar conserved function, regulation of expression of the achaete-scute complex to particular neuroblasts. Likewise Msx and Nkx-2 regulate the expression of vertebrate achaete-scute homolog (ash) in the developing neural column (D'Alessio, 1996).

The earliest sign of neural dorsal-ventral patterning is the expression of three homeobox genes in the neuroectoderm -- msh, intermediate neuroblasts defective (ind), and vnd -- which are expressed in dorsal, intermediate, and ventral columns of neuroectoderm, respectively. Previous studies have shown that the Dorsal, Decapentaplegic (Dpp), and EGF receptor (Egfr) signaling pathways regulate embryonic DV patterning, as well as aspects of CNS patterning. This study describes the earliest expression of each DV column gene (vnd, ind, and msh), the regulatory relationships between all three DV column genes, and the role of the Dorsal, Dpp, and Egfr signaling pathways in defining vnd, ind, and msh expression domains. The vnd domain is established by Dorsal and maintained by Egfr, but unlike a previous report vnd is found not to be regulated by Dpp signaling. ind expression requires both Dorsal and Egfr signaling for activation and positioning of its dorsal border, and abnormally high Dpp can repress ind expression. The msh domain is defined by repression: it occurs only where Dpp, Vnd, and Ind activity is low (Von Ohlen, 2000).

Early stage 5 embryos express vnd in a narrow domain similar to its final width; ind and msh are not detected. By the end of stage 5, both vnd and ind are expressed with a one to two cell wide gap; again, this expression is seen in domains similar to their final widths. The gap fills in during development resulting in the precise juxtaposition of the vnd and ind domains. Expression of msh in the trunk is not detected until stage 7. Thus, the timing of gene expression progresses from ventral to dorsal: vnd is detected first, ind appears soon after, and msh is observed last (Von Ohlen, 2000).

msh is expressed in a DV domain that has low Vnd, Ind, and Dpp activity. Overexpression of any of these genes will repress msh expression, and dorsal;dpp embryos that lack all vnd, ind, and dpp expression show ectopic msh expression around the DV axis. Thus, the borders of the msh domain are defined by repression: Vnd and Ind ventrally, and Dpp dorsally. What activates msh expression? msh expression could be activated by 'basal' transcription factors present uniformly in the early embryo. Alternatively, msh expression may be induced by a low level of ubiquitous TGFbeta activity, similar to the observed activation of zebrafish msh homologs. The screw gene encodes a TGFbeta-like protein expressed at low levels throughout the embryo, and although it has no striking CNS phenotype, it would be interesting to see if screw;dpp embryos lose dorsal msh expression, or whether screw;dorsal;dpp embryos lose global msh expression (Von Ohlen, 2000).

It is concluded that the initial diversification of cell fates along the DV axis of the CNS is coordinately established by Dorsal, Dpp, and Egfr signaling pathways. Understanding the mechanisms involved in patterning vnd, ind, and msh expression is important, because DV columnar homeobox gene expression in the neuroectoderm is an early, essential, and evolutionarily conserved step in generating neuronal diversity along the DV axis of the CNS (Von Ohlen, 2000).

GENE STRUCTURE

Bases in 5' UTR - 249 (D'Alessio, 1996)

Bases in 3' UTR - 778

PROTEIN STRUCTURE

Amino Acids - 515 (D'Alessio, 1996).

Structural Domains

The amino acid sequence of the msh homeobox domain is highly homologous to the homeodomains of the Drosophila S59 and empty spiracles genes and the Hox 7 and Hox 8 family of vertebrate homeobox genes. In addition, the 5' end of msh has 52% sequence identity to the 5' end of the empty spiracles gene and encodes several stretches of amino acids rich in serine, alanine, proline, glutamine, and acidic amino acids, indicating potential domains of regulatory activity (Lord, 1995).

A second published sequence for msh differs considerably from the first (D'Alessio, 1996). The homeodomain is highly conserved showing 87% identity to that of C. elegans. Both at their N-termini and C-termini, all homeodomains of this gene family are flanked by extended stretches of highly conserved amino acid sequences. All NK-2 family members have two highly conserved amino acid sequences: (1) the amino terminal TN domain and (2) the NK-2-specific domain lying between the homeobox and the C-terminal sequences. Other portions of the MSH protein do not display any sequence similarities to other members of the MSH family.

The Engrailed homeoprotein is a dominantly acting, so-called 'active' transcriptional repressor, both in cultured cells and in vivo. When retargeted via a homeodomain swap to the endogenous fushi tarazu gene (ftz), Engrailed actively represses ftz, resulting in a ftz mutant phenocopy. Functional regions of Engrailed have been mapped using this in vivo repression assay. In addition to a region containing an active repression domain identified in cell culture assays, there are two evolutionarily conserved regions that contribute to activity. The one that does not flank the HD is particularly crucial to repression activity in vivo. This domain is present not only in all engrailed-class homeoproteins but also in all known members of several other classes, including goosecoid, Nk1, Nk2 (vnd) and muscle segment homeobox. The repressive domain is located in the eh1 region, known as 'region three', found several hundred amino acids N-terminal to the homeodomain. The consensus sequence, arrived at by comparing Engrailed, Msh, Gsc, Nk1 and NK2 proteins from a variety of species, consists of a 23 amino acid homologous motif found in all these proteins. Thus Engrailed's active repression function in vivo is dependent on a highly conserved interaction that was established early in the evolution of the homeobox gene superfamily. Using rescue transgenes it has been shown that the widely conserved in vivo repression domain is required for the normal function of Engrailed in the embryo (Smith, 1996).

date revised:  15 October 2000 

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