Biomedical subjects
J Hodgkin
Publications and source records attributed to J Hodgkin.
The caenorhabditis elegans fate-determining gene mab-9 encodes a T-box protein required to pattern the posterior hindgut.
Caenorhabditis elegans mab-9 mutants are defective in hindgut and male tail development because of cell fate transformations in two posterior blast cells, B and F. We have cloned mab-9 and show that it encodes a member of the T-box family of transcriptional regulators. MAB-9 localizes to the nucleus of B and F and their descendents during development, suggesting that it acts cell autonomously in the posterior hindgut to direct cell fate. T-box genes related to brachyury have also been implicated in hindgut patterning, and our results support models for an evolutionarily ancient role for these genes in hindgut formation.
MRT-2 checkpoint protein is required for germline immortality and telomere replication in C. elegans.
The germ line is an immortal cell lineage that is passed indefinitely from one generation to the next. To identify the genes that are required for germline immortality, we isolated Caenorhabditis elegans mutants with mortal germ lines--worms that can reproduce for several healthy generations but eventually become sterile. One of these mortal germline (mrt) mutants, mrt-2, exhibits progressive telomere shortening and accumulates end-to-end chromosome fusions in later generations, indicating that the MRT-2 protein is required for telomere replication. In addition, the germ line of mrt-2 is hypersensitive to X-rays and to transposon activity. Therefore, mrt-2 has defects in responding both to damaged DNA and to normal double-strand breaks present at telomeres. mrt-2 encodes a homologue of a checkpoint gene that is required to sense DNA damage in yeast. These results indicate that telomeres may be identified as a type of DNA damage and then repaired by the telomere-replication enzyme telomerase.
The gon-1 gene is required for gonadal morphogenesis in Caenorhabditis elegans.
In wild-type Caenorhabditis elegans, the gonad is a complex epithelial tube that consists of long arms composed predominantly of germline tissue as well as somatic structures specialized for particular reproductive functions. In gon-1 mutants, the adult gonad is severely disorganized with essentially no arm extension and no recognizable somatic structure. The developmental defects in gon-1 mutants are limited to the gonad; other cells, tissues, and organs appear to develop normally. Previous work defined the regulatory "leader" cells as crucial for extension of the gonadal arms (J. E. Kimble and J. G. White, 1981, Dev. Biol. 81, 208-219). In gon-1 mutants, the leader cells are specified correctly, but they fail to migrate and gonadal arms are not generated. In addition, gon-1 is required for morphogenesis of the gonadal somatic structures. This second role appears to be independent of that required for leader migration. Parallel studies have shown that gon-1 encodes a secreted metalloprotease (R. Blelloch and J. Kimble, 1999, Nature 399, 586-590). We discuss how a metalloprotease may control two aspects of gonadal morphogenesis.
ETR-1, a homologue of a protein linked to myotonic dystrophy, is essential for muscle development in Caenorhabditis elegans.
Post-transcriptional gene processing by RNA-binding proteins (RBPs) has crucial roles during development [1] [2]. Here, we report the identification of ETR-1 (ELAV-type RNA-binding protein), a muscle-specific RBP in the nematode Caenorhabditis elegans. ETR-1 is related to the family of RBPs defined by the protein ELAV, which is essential for neurogenesis in the fruit fly Drosophila; members of the family possess two consecutive RNA recognition motifs (RRMs) separated from a third, carboxy-terminal RRM by a tether region of variable length [3] [4] [5] [6]. Its closest homologue, CUG-binding protein (CUG-bp), is a human RBP that has been implicated in the disease myotonic dystrophy and binds CUG repeats in the 3' untranslated region (UTR) of the mRNA for myotonic dystrophy protein kinase (DMPK) [7] [8]. Inactivation of etr-1 by RNA-mediated interference resulted in embryonic lethality. Embryos failed to elongate and became paralysed, a phenotype characteristic of C. elegans Pat mutants, which are defective in muscle formation and function [9]. The data indicate that etr-1 is essential for muscle development in C. elegans, perhaps by playing a role in post-transcriptional processing of some muscle component, and thus suggesting a possible conservation of gene function with human CUG-bp.
Sex, cell death, and the genome of C. elegans.
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Stu-7/air-2 is a C. elegans aurora homologue essential for chromosome segregation during embryonic and post-embryonic development.
We have isolated a new sterile uncoordinated C. elegans mutant, stu-7, which is defective in post-embryonic cell divisions in a regionally-specific fashion. The anterior of the worm is relatively unaffected whereas the mid-body and/or posterior are markedly thin, often resulting in worms having a central 'waist'. We have cloned stu-7 and found that it encodes a member of the recently expanding aurora sub-family of serine/threonine kinases. Elimination of maternal as well as zygotic stu-7 expression reveals that stu-7 is essential for mitosis from the first embryonic cell cycle onwards and is required for chromosome segregation though not for centrosome separation or for setting up a bipolar spindle. Multicopy expression of stu-7 also causes mitotic defects, suggesting that the level of this protein must be tightly controlled in order to maintain genetic stability during development.
Genetic and molecular analysis of fox-1, a numerator element involved in Caenorhabditis elegans primary sex determination.
fox-1 was previously identified as a candidate numerator element based on its overexpression phenotype. FOX-1 is an RRM-type RNA-binding protein, which can bind RNAs in vitro. Western analysis detects FOX-1 throughout development. fox-1::lacZ comes on ubiquitously early during embryogenesis. Postembryonically, fox-1::lacZ is expressed sex specifically in a subset of cells in the head and tail. We describe a Tc1-derived deletion allele [fox-1(Delta)] that removes the RRM domain. fox-1(Delta) confers no phenotype in XXs, but can rescue XO-specific lethality and feminization caused by duplications of the left end of the X. fox-1(Delta) synergizes with putative numerators, resulting in abnormal XX development. Genetic analysis indicated that fox-1(Delta) leads to a slight increase in xol-1 activity, while fox-1(gf) leads to partial loss of xol-1 activity, and xol-1 is epistatic to fox-1. RNase protection experiments revealed increased levels of the 2.2-kb xol-1 message in fox-1(Delta) animals, and reduced levels in fox-1(gf) animals. Additionally, fox-1(Delta) impairs male mating efficiency, which, we propose, represents another function of fox-1, independent of xol-1 and its role in sex determination.
Evidence for evolutionary conservation of sex-determining genes.
Most metazoans occur as two sexes. Surprisingly, molecular analyses have hitherto indicated that sex-determining mechanisms differ completely between phyla. Here we present evidence to the contrary. We have isolated the male sexual regulatory gene mab-3 from the nematode Caenorhabditis elegans and found that it is related to the Drosophila melanogaster sexual regulatory gene doublesex (dsx)2. Both genes encode proteins with a DNA-binding motif that we have named the 'DM domain'. Both genes control sex-specific neuroblast differentiation and yolk protein gene transcription; dsx controls other sexually dimorphic features as well. The form of DSX that is found in males can direct male-specific neuroblast differentiation in C. elegans. This structural and functional similarity between phyla suggests a common evolutionary origin of at least some aspects of sexual regulation. We have identified a human gene, DMT1, that encodes a protein with a DM domain and find that DMT1 is expressed only in testis. DMT1 maps to the distal short arm of chromosome 9, a location implicated in human XY sex reversal. Proteins with DM domains may therefore also regulate sexual development in mammals.
Changing styles in C. elegans genetics.
The past 30 years have taken the nematode Caenorhabditis elegans from obscurity, as a nondescript member of a large but unglamorous invertebrate phylum, to a position as one of the major model organisms. This year, it will acquire a particular celeberity as the owner of the first animal genome to be sequenced in its entirety. In this review we consider the ways in which genetical investigations of this species have begun to change and what some of the consequences of the completion of the sequence are likely to be.
Seven types of pleiotropy.
Pleiotropy, a situation in which a single gene influences multiple phenotypic traits, can arise in a variety of ways. This paper discusses possible underlying mechanisms and proposes a classification of the various phenomena involved.
Natural variation and copulatory plug formation in Caenorhabditis elegans.
Most of the available natural isolates of the nematode Caenorhabditis elegans have been examined and compared with the standard laboratory wild type (Bristol N2). Molecular markers, in particular transposon restriction fragment length polymorphisms, were used to assign these isolates to 22 different races, for which brood size and spontaneous male frequency were determined. Several distinctive traits were observed in some of these races. One example is mab-23, in a race from Vancouver, which leads to severe distortion of male genitalia and prevents male mating. Another is gro-1, segregating in a Californian race, which is associated with slow growth, heat resistance and longevity. Many races differ from N2 in carrying a dominant allele at the plg-1 locus, causing copulatory plug formation by males. Properties and possible advantages of the plugging trait have been investigated. The dominant plg-1 allele does not lead to increased male mating efficiency, but males from a Stanford race (CB4855), in which the plugging trait was first observed, are much more virile than N2 males. Crosses between N2 and CB4855 indicate that the higher virility is due to multiple factors. Size differences between N2 and CB4855 are associated with factors mapping to LGV and LGX.
The tra-3 sex determination gene of Caenorhabditis elegans encodes a member of the calpain regulatory protease family.
The Caenorhabditis elegans sex determination gene tra-3 is required for the correct sexual development of the soma and germ line in hermaphrodites, while being fully dispensable in males. Genetic analysis of tra-3 has suggested that its product may act as a potentiator of another sex determination gene, tra-2. Molecular analysis reported here reveals that the predicted tra-3 gene product is a member of the calpain family of calcium-regulated cytosolic proteases, though it lacks the calcium binding regulatory domain. Calpains are regulatory processing proteases, exhibiting marked substrate specificity, and mutations in the p94 isoform underlie the human hereditary condition limb-girdle muscular dystrophy type 2A. The molecular identity of TRA-3 is consistent with previous genetic analysis which suggested that tra-3 plays a very selective modulatory role and is required in very small amounts. Based on these observations and new genetic data, we suggest a refinement of the position of tra-3 within the sex determination cascade and discuss possible mechanisms of action for the TRA-3 protein.
Evolution of sex determination in caenorhabditis: unusually high divergence of tra-1 and its functional consequences.
The tra-1 gene is a terminal regulator of somatic sex in Caenorhabditis elegans: high tra-1 activity elicits female development, low tra-1 activity elicits male development. To investigate the function and evolution of tra-1, we examined the tra-1 gene from the closely related nematode C. briggsae. Ce-tra-1 and Cb-tra-1 are unusually divergent. Each gene generates two transcripts, but only one of these is present in both species. This common transcript encodes TRA-1A, which shows only 44% amino acid identity between the species, a figure much lower than that for previously compared genes. A Cb-tra-1 transgene rescues many tissues of tra-1(null) mutants of C. elegans but not the somatic gonad or germ line. This transgene also causes nongonadal feminization of XO animals, indicating incorrect sexual regulation. Alignment of Ce-TRA-1A and Cb-TRA-1A defines several conserved regions likely to be important for tra-1 function. The phenotypic differences between Ce-tra-1(null) mutants rescued by Cb-tra-1 transgenes and wild-type C. elegans indicate significant divergence of regulatory regions. These molecular and functional studies suggest that evolution of sex determination in nematodes is rapid and genetically complex.
The nematode Caenorhabditis elegans and its genome.
Over the past two decades, the small soil nematode Caenorhabditis elegans has become established as a major model system for the study of a great variety of problems in biology and medicine. One of its most significant advantages is its simplicity, both in anatomy and in genomic organization. The entire haploid genetic content amounts to 100 million base pairs of DNA, about 1/30 the size of the human value. As a result, C. elegans has also provided a pilot system for the construction of physical maps of larger animal and plant genomes, and subsequently for the complete sequencing of those genomes. By mid-1995, approximately one-fifth of the complete DNA sequence of this animal had been determined. Caenorhabditis elegans provides a test bed not only for the development and application of mapping and sequencing technologies, but also for the interpretation and use of complete sequence information. This article reviews the progress so far toward a realizable goal--the total description of the genome of a simple animal.
Genome maps. VI. Caenorhabditis elegans. Wall chart.
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Dominant feminizing mutations implicate protein-protein interactions as the main mode of regulation of the nematode sex-determining gene tra-1.
The tra-1 gene is the terminal global selector of somatic sex in Caenorhabditis elegans: High tra-1 activity elicits female somatic development while low tra-1 activity elicits male development. Previous genetic studies defined a cascade of negatively interacting genes that regulates tra-1 activity in response to the primary sex-determining signal. Here, we investigate the last step in this regulatory cascade, by studying rare gain-of-function (gf) mutations of tra-1 that direct female somatic development irrespective of the upstream sex-determining signal. These mutations appear to abolish negative regulation of tra-1 in male tissues. We identify the lesions associated with 29 of these mutations and find that all affect a short stretch of amino acid residues present in both protein products of the tra-1 gene. Twenty-six alleles are associated with single nonconservative amino acid substitutions. Two alleles affect tra-1 RNA splicing and generate messages that omit part or all of the exon encoding this short stretch. These results suggest that sexual regulation of tra-1 is achieved post-translationally, by an inhibitory protein-protein interaction. The amino acid stretch altered by the tra-1(gf) mutations may define a site of interaction for negative regulators of tra-1. The stretch includes a potential phosphorylation site for glycogen synthase kinase 3 and may be conserved in the human gene GLI3, a homolog of tra-1 identified previously.