Genetic and physical mapping of the GLUR5 glutamate receptor gene on human chromosome 21

Glutamate receptors (GluRs) mediate excitatory neurotransmission and may have important roles in central nervous system disorders. To characterize the human GLUR5 gene, which is located on human chromosome 21q22.1, we isolated cDNAs, genomic phage lambda clones, and yeast artificial chromosomes (YACs) and developed sequence tagged sites (STSs) and simple sequence length polymorphisms (SSLPs) for GLUR5. Genetic mapping with a tetranucleotide AGAT repeat named GLUR5/AGAT (six alleles observed, 70% heterozygosity) placed GLUR5 5 cM telomeric to APP (D21S210) and 3 cM centromeric to SOD1 (D21S223). The human GLUR5 gene is located near the familial amyotrophic lateral sclerosis (FALS) locus; linkage analysis of GLUR5 SSLPs in FALS pedigrees yielded negative lod scores, consistent with the recent association of the FALS locus with the SOD1 gene. Physical mapping of GLUR5 using a YAC contig suggested that the GLUR5 gene spans approximately 400-500kb, and is within 280kb of D21S213. The large size of the GLUR5 gene raises questions regarding its functional significance. Our GLUR5 YAC contig includes clones found in the Genethon chromosome 21 YAC contig, and reference to the larger contig indicates the orientation centromere--D21S213-GLUR5 5' end-GLUR5/AGAT--GLUR5 3' end--SOD1. The development of GLUR5/AGAT should permit rapid determination of the status of the GLUR5 gene in individuals with partial trisomy or monosomy of chromosome 21. Such studies may provide insights concerning the possible role of GLUR5 in Down syndrome.

The Caenorhabditis elegans gene lin-26 is required to specify the fates of hypodermal cells and encodes a presumptive zinc-finger transcription factor

The mutation lin-26(n156) prevents vulva formation in C. elegans by transforming the vulval precursor cells into neurons or neuroblasts. We have isolated and characterized three new lin-26 alleles, which result in embryonic lethality. These mutations cause a few other hypodermal cells to express a neural fate and most hypodermal cells to degenerate. lin-26 encodes a presumptive zinc-finger transcription factor. Our data indicate that lin-26 is required for cells to acquire the hypodermal fate.

The ins and outs of programmed cell death during C. elegans development

During the development of the C. elegans hermaphrodite, 131 of the 1090 cells generated undergo programmed cell death. Genetic studies have identified mutations in 14 genes that specifically affect this process. These genes define a genetic pathway for programmed cell death in C. elegans. Two genes, ced-3 and ced-4, are required for cells to undergo programmed cell death, while a third gene, ced-9, protects cells that should live from undergoing programmed cell death. The proteins encoded by ced-3 and ced-9 show significant similarity to proteins that affect programmed cell death in vertebrates, suggesting that the molecular cell death pathway in which ced-3, ced-4, and ced-9 act has been conserved between nematodes and vertebrates.

The Caenorhabditis elegans locus lin-15, a negative regulator of a tyrosine kinase signaling pathway, encodes two different proteins

The Caenorhabditis elegans locus lin-15 negatively regulates an intercellular signaling process that induces formation of the hermaphrodite vulva. The lin-15 locus controls two separate genetic activities. Mutants that lack both activities have multiple, ectopic pseudo-vulvae resulting from the overproduction of vulval cells, whereas mutants defective in only one lin-15 activity appear wild-type. lin-15 acts non-cell-autonomously to prevent the activation of a receptor tyrosine kinase/ras signaling pathway. We report here the molecular characterization of the lin-15 locus. The two lin-15 activities are encoded by contiguous genomic regions and by two distinct, non-overlapping transcripts that may be processed from a single mRNA precursor by trans-splicing. Based on the DNA sequence, the 719- and 1,440-amino acid lin-15 proteins are not similar to each other or to known proteins. lin-15 multivulva mutants, which are defective in both lin-15 activities, contain deletions and insertions that affect the lin-15 genomic region.

Programmed cell death in Caenorhabditis elegans

Programmed cell death in the nematode Caenorhabditis elegans requires the activities of the genes ced-3 and ced-4 and is antagonized by the activity of the gene ced-9. Cloning of these C. elegans genes has shown that two of them encode proteins with similarity to vertebrate cell death genes and has revealed that nematodes and mammals share a common pathway for programmed cell death.

Activation of C. elegans cell death protein CED-9 by an amino-acid substitution in a domain conserved in Bcl-2

The Caenorhabditis elegans gene ced-9 and the human proto-oncogene bcl-2, both of which protect cells from programmed cell death, are members of the same gene family. ced-9 and bcl-2 were discovered because of the effects of dominant gain-of-function mutations. Such bcl-2 mutations, which are commonly found in follicular lymphoma, are translocations that result in over-expression of a normal Bcl-2 protein in B cells. Here we report that, by contrast, the ced-9(n1950) gain-of-function mutation affects the open reading frame of ced-9 and results in a glycine-to-glutamate substitution in a region highly conserved among all ced-9/bcl-2 family members. We conclude that this glycine has an important function in ced-9 regulation, and we suggest that alteration of this glycine in other members of the ced-9/bcl-2 family might lead to oncogenic activation. We also present genetic evidence suggesting that the CED-9 protein might exist in two distinct forms that have opposite effects on cell death.

Genetic linkage analysis of familial amyotrophic lateral sclerosis using human chromosome 21 microsatellite DNA markers

Amyotrophic lateral sclerosis (ALS: Lou Gehrig's Disease) is a lethal neurodegenerative disease of upper and lower motorneurons in the brain and spinal cord. We previously reported linkage of a gene for familial ALS (FALS) to human chromosome 21 using 4 restriction fragment length polymorphism DNA markers [Siddique et al.: N Engl J Med 324:1381-1384, 1991] and identified disease-associated mutations in the superoxide dismutase (SOD)-1 gene in some ALS families [Rosen et al.: Nature 362:59-62, 1993]. We report here the genetic linkage data that led us to examine the SOD-1 gene for mutations. We also report a new microsatellite DNA marker for D21S63, derived from the cosmid PW517 [VanKeuren et al.: Am J Hum Genet 38:793-804, 1986]. Ten microsatellite DNA markers, including the new marker D21S63, were used to reinvestigate linkage of FALS to chromosome 21. Genetic linkage analysis performed with 13 ALS families for these 10 DNA markers confirmed the presence of a FALS gene on chromosome 21. The highest total 2-point LOD score for all families was 4.33, obtained at a distance of 10 cM from the marker D21S223. For 5 ALS families linked to chromosome 21, a peak 2-point LOD score of 5.94 was obtained at the DNA marker D21S223. A multipoint score of 6.50 was obtained with the markers D21S213, D21S223, D21S167, and FALS for 5 chromosome 21-linked ALS families. The haplotypes of these families for the 10 DNA markers revealed recombination events that further refined the location of the FALS gene to a segment of approximately 5 megabases (Mb) between D21S213 and D21S219.(ABSTRACT TRUNCATED AT 250 WORDS)

The Caenorhabditis elegans gene lin-44 controls the polarity of asymmetric cell divisions

The generation and orientation of cellular and organismic polarity are fundamental aspects of development. Mutations in the gene lin-44 of the nematode Caenorhabditis elegans reverse both the relative positions of specific sister cells and the apparent polarities of these cells. Thus, lin-44 mutants appear to generate polar cells but to misorient these cells along the body axis of the animal. We postulate that lin-44 acts to specify the orientation of polar cells.

C. elegans cell survival gene ced-9 encodes a functional homolog of the mammalian proto-oncogene bcl-2

The activity of the C. elegans gene ced-9 is required to protect cells that normally survive from undergoing programmed cell death. Here we describe the cloning and molecular characterization of this gene. ced-9 is an element of a polycistronic locus that also contains the gene cyt-1, which encodes a protein similar to cytochrome b560 of complex II of the mitochondrial respiratory chain. ced-9 encodes a 280 amino acid protein showing sequence and structural similarities to the mammalian proto-oncogene bcl-2. Overexpression of bcl-2 can mimic the protective effect of ced-9 on C. elegans cell death and can prevent the ectopic cell deaths that occur in ced-9 loss-of-function mutants. These results suggest that ced-9 and bcl-2 are homologs and that the molecular mechanism of programmed cell death has been conserved from nematodes to mammals.

A MAP kinase homolog, mpk-1, is involved in ras-mediated induction of vulval cell fates in Caenorhabditis elegans

During development of the Caenorhabditis elegans hermaphrodite, the gonadal anchor cell induces nearby Pn.p cells to adopt vulval fates. The response to this signal is mediated by a receptor tyrosine kinase signal transduction pathway that has been remarkably well conserved during metazoan evolution. Because mitogen-activated protein (MAP) kinases are activated by receptor tyrosine kinase pathways in vertebrate cells, we hypothesized that C. elegans MAP kinase homologs may play a role in vulval induction. Two C. elegans MAP kinase genes, mpk-1 and mpk-2 (mpk, MAP kinase), were cloned using degenerate oligonucleotide primers and PCR amplification; in parallel, genes involved in vulval induction were identified by screening for mutations that suppress the vulval defects caused by an activated let-60 ras gene. One such suppressor mutation is an allele of mpk-1. We used a new type of mosaic analysis to show that mpk-1 acts cell autonomously in the Pn.p cells. Our results show that mpk-1 plays an important functional role as an activator in ras-mediated cell signaling in vivo.

The C. elegans cell death gene ced-3 encodes a protein similar to mammalian interleukin-1 beta-converting enzyme

We have cloned the C. elegans cell death gene ced-3. A ced-3 transcript is most abundant during embryogenesis, the stage during which most programmed cell deaths occur. The predicted CED-3 protein shows similarity to human and murine interleukin-1 beta-converting enzyme and to the product of the mouse nedd-2 gene, which is expressed in the embryonic brain. The sequences of 12 ced-3 mutations as well as the sequences of ced-3 genes from two related nematode species identify sites of potential functional importance. We propose that the CED-3 protein acts as a cysteine protease in the initiation of programmed cell death in C. elegans and that cysteine proteases also function in programmed cell death in mammals.

Migrations of the Caenorhabditis elegans HSNs are regulated by egl-43, a gene encoding two zinc finger proteins

During embryonic development, the two Caenorhabditis elegans HSN motor neurons migrate from their birthplace in the tail to positions near the middle of the embryo. Here, we demonstrate that of all cells that undergo long-range migrations, only the HSNs are affected in animals that lack function of the egl-43 gene. We also show that egl-43 function is required for normal development of phasmid neurons, which are sensory neurons located in the tail. The egl-43 gene encodes two proteins containing zinc finger motifs that are similar to the zinc fingers of the murine Evi-1 proto-oncoprotein. Our genetic and molecular results suggest that egl-43 encodes two transcription factors and acts to control HSN migration and phasmid neuron development, presumably by regulating other genes that function directly in these processes.

Three new classes of mutations in the Caenorhabditis elegans muscle gene sup-9

We are studying five interacting genes involved in the regulation or coordination of muscle contraction in Caenorhabditis elegans. A distinctive "rubber-band" muscle-defective phenotype was previously shown to result from rare altered-function mutations in either of two of these genes, unc-93 and sup-10. null mutations in sup-9, sup-10, sup-18 or unc-93 act as essentially recessive suppressors of these rubber-band mutations. In this work, we identify three new classes of sup-9 alleles: altered-function rubber-band, partial loss-of-function and dominant-suppressor. The existence of rubber-band mutations in sup-9, sup-10 and unc-93 and the suppression of these mutations by null mutations in any of these three genes suggest that these proteins are required at the same step in muscle contraction. Moreover, allele-specific interactions shown by the partial loss-of-function mutations indicate that the products of these interacting genes may physically contact each other in a multiple subunit protein complex. Finally, the phenotypes of double rubber-band mutant combinations suggest that the rubber-band mutations affect a neurogenic rather than a myogenic input in excitation-contraction coupling in muscle.

Odorant-selective genes and neurons mediate olfaction in C. elegans

Olfaction is a versatile and sensitive mechanism for detecting volatile odorants. We show that the nematode C. elegans detects many volatile chemicals, which can be attractants, repellents, or attractants at low concentrations and repellents at high concentrations. Through laser ablation, we have identified chemosensory neurons that detect volatile odorants. Chemotaxis to volatile odorants requires different sensory neurons from chemotaxis to water-soluble attractants, indicating that C. elegans might have senses that correspond to smell and taste, respectively. Single neurons have complex sensory properties, since six distinguishable volatile odorants are sensed by only two types of sensory neurons. Chemotaxis to subsets of volatile odorants is disrupted by mutations in the odr genes, which might be involved in odorant sensation or signal transduction.

The GABAergic nervous system of Caenorhabditis elegans

gamma-Aminobutyric acid (GABA) is the most abundant inhibitory neurotransmitter in vertebrates and invertebrates. GABA receptors are the target of anxiolytic, antiepileptic and antispasmodic drugs, as well as of commonly used insecticides. How does a specific neurotransmitter such as GABA control animal behaviour? To answer this question, we identified all neurons that react with antisera raised against the neurotransmitter GABA in the nervous system of the nematode Caenorhabditis elegans. We determined the in vivo functions of 25 of the 26 GABAergic neurons by killing these cells with a laser microbeam in living animals and by characterizing a mutant defective in GABA expression. On the basis of the ultrastructurally defined connectivity of the C. elegans nervous system, we deduced how these GABAergic neurons act to control the body and enteric muscles necessary for different behaviours. Our findings provide evidence that GABA functions as an excitatory as well as an inhibitory neurotransmitter.

Genes required for GABA function in Caenorhabditis elegans

gamma-Aminobutyric acid (GABA) neurotransmission is widespread in vertebrate and invertebrate nervous systems. Here we use a genetic approach to identify molecules specific to GABA function. On the basis of the known in vivo roles of GABAergic neurons in controlling behaviour of the nematode Caenorhabditis elegans, we identified mutants defective in GABA-mediated behaviours. Five genes are necessary either for GABAergic neuronal differentiation or for pre- or postsynaptic GABAergic function. The gene unc-30 is required for the differentiation of a specific type of GABAergic neuron, the type-D inhibitory motor neuron. The gene unc-25 is necessary for GABA expression and probably encodes the GABA biosynthetic enzyme glutamic acid decarboxylase. The genes unc-46 and unc-47 seem to be required for normal GABA release. Finally, the gene unc-49 is apparently necessary postsynaptically for the inhibitory effect of GABA on the body muscles and might encode a protein needed for the function of a GABAA-like receptor. Some of these genes are likely to encode previously unidentified proteins required for GABA function.

Control of cell fates in the central body region of C. elegans by the homeobox gene lin-39

Cells in the mid-body region of the nematode C. elegans develop differently from their anterior or posterior homologs. The gene lin-39 is required for mid-body region-specific development. In lin-39 mutants, mid-body cells express fates characteristic of more anterior or posterior homologs, and the migration of a neuroblast through the mid-body is defective. lin-39 acts cell autonomously in these mid-body cells and in the migrating neuroblast. lin-39 encodes a protein with an Antennapedia class homeodomain, most similar to those of the Drosophila homeotic genes Deformed and Sex combs reduced, and is located in a homeotic gene cluster with two other regional homeotic genes, mab-5 and egl-5. lin-39 and mab-5 function combinatorially in 2 ectodermal cells and have redundant functions in gonad development.

The Caenorhabditis elegans unc-31 gene affects multiple nervous system-controlled functions

We have devised a method for selecting Caenorhabditis elegans mutants that execute feeding motions in the absence of food. One mutation isolated in this way is an allele of the gene unc-31, first discovered by S. Brenner in 1974, because of its effects on locomotion. We find that strong unc-31 mutations cause defects in four functions controlled by the nervous system. Mutant worms are lethargic, feed constitutively, are defective in egg-laying and produce dauer larvae that fail to recover. We discuss two extreme models to explain this pleiotropy: either unc-31 affects one or a few neurons that coordinately control several different functions, or it affects many neurons that independently control different functions.

A dual mechanosensory and chemosensory neuron in Caenorhabditis elegans

After light touch to its nose, the nematode Caenorhabditis elegans halts forward locomotion and initiates backing. Here we show that three classes of neurons (ASH, FLP, and OLQ) sense touch to the nose and hence are required for this avoidance response. ASH, FLP, and OLQ have sensory endings that contain axonemal cilia. Mutant animals that have defective ciliated sensory endings as well as laser-operated animals that lack ASH, FLP, and OLQ fail to respond to touch to the nose. Together with the previous work of others, these results demonstrate that C. elegans has at least five morphologically distinct classes of mechanosensory neurons. Interestingly, the ASH neuron also acts as a chemosensory neuron; it mediates the avoidance of noxious chemicals. Since ASH possesses both chemosensory and mechanosensory modalities, this neuron might be functionally analogous to vertebrate nociceptors, which mediate the sensation of pain.

Cell interactions control the direction of outgrowth, branching and fasciculation of the HSN axons of Caenorhabditis elegans

The two serotonergic HSN motor neurons of the nematode Caenorhabditis elegans innervate the vulval muscles and stimulate egg laying by hermaphrodites. By analyzing mutant and laser-operated animals, we find that both epithelial cells of the developing vulva and axons of the ventral nerve cord are required for HSN axonal guidance. Vulval precursor cells help guide the growth cone of the emerging HSN axon to the ventral nerve cord. Vulval cells also cause the two HSN axons to join the ventral nerve cord in two separate fascicles and to defasciculate from the ventral nerve cord and branch at the vulva. The axons of either the PVP or PVQ neurons are also necessary for the HSN axons to run in two separate fascicles within the ventral nerve cord. Our observations indicate that the outgrowth of the HSN axon is controlled in multiple ways by both neuronal and nonneuronal cells.

The Caenorhabditis elegans cell death gene ced-4 encodes a novel protein and is expressed during the period of extensive programmed cell death

Mutations in the gene ced-4 block almost all of the programmed cell deaths that normally occur during Caenorhabditis elegans development. We have cloned the ced-4 gene using a ced-4 mutation caused by the insertion of the transposon Tc4. When microinjected into a ced-4 animal, a 4.4 kb DNA fragment derived from the wild-type strain and corresponding to the region of the Tc4 insertion in the mutant ced-4(n1416) rescues the Ced-4 mutant phenotype. The ced-4 gene encodes a 2.2 kb RNA transcript. This mRNA is expressed primarily during embryogenesis, when most programmed cell deaths occur. The Ced-4 protein, as deduced from cDNA and genomic DNA clones, is 549 amino acids in length. Two regions of the putative Ced-4 protein product show some similarity to known calcium-binding domains.