Overlapping roles of two Hox genes and the exd ortholog ceh-20 in diversification of the C. elegans postembryonic mesoderm

Members of the Hox family of homeoproteins and their cofactors play a central role in pattern formation of all germ layers. During postembryonic development of C. elegans, non-gonadal mesoderm arises from a single mesoblast cell M. Starting in the first larval stage, M divides to produce 14 striated muscles, 16 non-striated muscles, and two non-muscle cells (coelomocytes). We investigated the role of the C. elegans Hox cluster and of the exd ortholog ceh-20 in patterning of the postembryonic mesoderm. By examining the M lineage and its differentiation products in different Hox mutant combinations, we found an essential but overlapping role for two of the Hox cluster genes, lin-39 and mab-5, in diversification of the postembryonic mesoderm. This role of the two Hox gene products required the CEH-20 cofactor. One target of these two Hox genes is the C. elegans twist ortholog hlh-8. Using both in vitro and in vivo assays, we demonstrated that twist is a direct target of Hox activation. We present evidence from mutant phenotypes that twist is not the only target for Hox genes in the M lineage: in particular we show that lin-39 mab-5 double mutants exhibit a more severe M lineage defect than the hlh-8 null mutant.     

Formation of the egg-laying system in Pristionchus pacificus requires complex interactions between gonadal, mesodermal and epidermal tissues and does not rely on single cell inductions

The invariant cell lineage of nematodes allows the formation of organ systems, like the egg-laying system, to be studied at a single cell level. The Caenorhabditis elegans egg-laying system is made up of the vulva, the mesodermal gonad and muscles and several neurons. The gonad plays a central role in patterning the underlying ectoderm to form the vulva and guiding the migration of the sex myoblasts to their final position. In Pristionchus pacificus, the egg-laying system is homologous to C. elegans, but comparative studies revealed several differences at the cellular and molecular levels during vulval formation. For example, the mesoblast M participates in lateral inhibition, a process that influences the fate of two vulval precursor cells. Here, we describe the M lineage in Pristionchus and show that both the dorsal and ventral M sublineages are involved in lateral inhibition. Mutations in the homeotic gene Ppa-mab-5 cause severe misspecification of the M lineage, resembling more the C. elegans Twist than the mab-5 phenotype. Ectopic differentiation of P8.p in Ppa-mab-5 results from at least two separate interactions between M and P8.p. Thus, interactions among the Pristionchus egg-laying system are complex, involving multiple cells of different tissues occurring over a distance.     

An antagonistic role for the C. elegans Schnurri homolog SMA-9 in modulating TGFbeta signaling during mesodermal patterning

In C. elegans, the Sma/Mab TGFbeta signaling pathway regulates body size and male tail patterning. SMA-9, the C. elegans homolog of Schnurri, has been shown to function as a downstream component to mediate the Sma/Mab TGFbeta signaling pathway in these processes. We have discovered a new role for SMA-9 in dorsoventral patterning of the C. elegans post-embryonic mesoderm, the M lineage. In addition to a small body size, sma-9 mutant animals exhibit a dorsal-to-ventral fate transformation within the M lineage. This M lineage defect of sma-9 mutants is unique in that animals carrying mutations in all other known components of the TGFbeta pathway exhibit no M lineage defects. Surprisingly, mutations in the core components of the Sma/Mab TGFbeta signaling pathway suppressed the M lineage defects of sma-9 mutants without suppressing their body size defects. We show that this suppression specifically happens within the M lineage. Our studies have uncovered an unexpected role of SMA-9 in antagonizing the TGFbeta signaling pathway during mesodermal patterning, suggesting a novel mode of function for the SMA-9/Schnurri family of proteins.     

The Caenorhabditis elegans presenilin sel-12 is required for mesodermal patterning and muscle function

Mutations in presenilin genes impair Notch signalling and, in humans, have been implicated in the development of familial Alzheimer's disease. We show here that a reduction of the activity of the Caenorhabditis elegans presenilin sel-12 results in a late defect during sex muscle development. The morphological abnormalities and functional deficits in the sex muscles contribute to the egg-laying defects seen in sel-12 hermaphrodites and to the severely reduced mating efficiency of sel-12 males. Both defects can be rescued by expressing sel-12 from the hlh-8 promoter that is active during the development of the sex muscle-specific M lineage, but not by expressing sel-12 from late muscle-specific promoters. Both weak and strong sel-12 mutations cause defects in the sex muscles that resemble the defects we found in lin-12 hypomorphic alleles, suggesting a previously uncharacterised LIN-12 signalling event late in postembryonic mesoderm development. Together with a previous study indicating a role of lin-12 and sel-12 during the specification of the pi cell lineage required for proper vulva-uterine connection, our data suggest that the failure of sel-12 animals to lay eggs properly is caused by defects in at least two independent signalling events in different tissues during development.     

A Twist in fate: evolutionary comparison of Twist structure and function

The general requirement to induce mesoderm and allocate cells into different mesodermal tissues such as body muscle or heart is common in many animal embryos. Since the discovery of the twist gene, there has been great progress toward unraveling the molecular mechanisms that control mesoderm specification and differentiation. Twist was first identified in Drosophila as a gene crucial for proper gastrulation and mesoderm formation. In the fly embryo, Twist continues to play additional roles, allocating mesodermal cells into the body wall muscle fate and patterning a subset of these muscles. Twist is also required for proper differentiation of the adult musculature. Twist homologues have been identified in a great variety of organisms, which span the phylogenetic tree. These organisms include other invertebrates such as jellyfish, nematode, leech and lancelet as well as vertebrates such as frog, chick, fish, mouse and human. The Twist family shares both homology in structure across the basic helix-loop-helix domain and in expression during mesoderm and muscle development in most species. Here we review the current state of knowledge of the Twist family and consider how Twist functions during development. Moreover, we highlight experimental evidence that shows common themes that Twist employs during specification and patterning of the mesoderm among evolutionarily distant organisms. Conserved principles and the molecular mechanisms underlying them are discussed.     

The Caenorhabditis elegans genes egl-27 and egr-1 are similar to MTA1, a member of a chromatin regulatory complex, and are redundantly required for embryonic patterning

We show here that two functionally redundant Caenorhabditis elegans genes, egl-27 and egr-1, have a fundamental role in embryonic patterning. When both are inactivated, cells in essentially all regions of the embryo fail to be properly organised. Tissue determination and differentiation are unaffected and many zygotic patterning genes are expressed normally, including HOX genes. However, hlh-8, a target of the HOX gene mab-5, is not expressed. egl-27 and egr-1 are members of a gene family that includes MTA1, a human gene with elevated expression in metastatic carcinomas. MTA1 is a component of a protein complex with histone deacetylase and nucleosome remodelling activities. We propose that EGL-27 and EGR-1 function as part of a chromatin regulatory complex required for the function of regional patterning genes.     

The myogenic potency of HLH-1 reveals wide-spread developmental plasticity in early C. elegans embryos

In vertebrates, striated muscle development depends on both the expression of members of the myogenic regulatory factor family (MRFs) and on extrinsic cellular cues, including Wnt signaling. The 81 embryonically born body wall muscle cells in C. elegans are comparable to the striated muscle of vertebrates. These muscle cells all express the gene hlh-1, encoding HLH-1 (CeMyoD) which is the only MRF-related factor in the nematode. However, genetic studies have shown that body wall muscle development occurs in the absence of HLH-1 activity, making the role of this factor in nematode myogenesis unclear. By ectopically expressing hlh-1 in early blastomeres of the C. elegans embryo, we show that CeMyoD is a bona fide MRF that can convert almost all cells to a muscle-like fate, regardless of their lineage of origin. The window during which ectopic HLH-1 can function is surprisingly broad, spanning the first 3 hours of development when cell lineages are normally established and non-muscle cell fate markers begin to be expressed. We have begun to explore the maternal factors controlling zygotic hlh-1 expression. We find that the Caudal-related homeobox factor PAL-1 can activate hlh-1 in blastomeres that either lack POP-1/TCF or that have down-regulated POP-1/TCF in response to Wnt/MAP kinase signaling. The potent myogenic activity of HLH-1 highlights the remarkable developmental plasticity of early C. elegans blastomeres and reveals the evolutionary conservation of MyoD function.