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M Chalfie

Publications and source records attributed to M Chalfie.

At least 55 records · Page 3Linked to original sources
PubMed1989

Genetic control of differentiation of the Caenorhabditis elegans touch receptor neurons.

The genetic control of neuronal differentiation has been studied by examining mutations that affect the development and function of the six touch receptor neurons of the nematode Caenorhabditis elegans. By screening for touch-insensitive mutants, it has been possible to identify 18 genes (represented by 417 mutations) that are required at various stages in the developmental program for touch cell differentiation. Two of the genes are needed for the generation of precursors in the touch cell lineages; without the precursors, touch cells are not made. A third gene, mec-3, specifies the differentiation of the touch cells, probably by acting as a transcription factor. The remaining 15 genes are likely targets of mec-3 action; mutants defective in these genes have nonfunctioning, yet differentiated, touch cells. Some of these latter genes are needed for the formation of cell-specific components of the touch cells, such as a set of microtubules that are only found in these cells. The study of the touch genes should help us understand how touch cell fate is determined, how microtubule form is specified, and, perhaps, how mechanical stimuli are transduced.

Animals↗
PubMed1989

Genetic and molecular analysis of a Caenorhabditis elegans beta-tubulin that conveys benzimidazole sensitivity.

Benzimidazole anti-microtubule drugs, such as benomyl, induce paralysis and slow the growth of the nematode Caenorhabditis elegans. We have identified 28 mutations in C. elegans that confer resistance to benzimidazoles. All resistant mutations map to a single locus, ben-1. Virtually all these mutations are genetically dominant. Molecular cloning and DNA sequence analysis established that ben-1 encodes a beta-tubulin. Some resistant mutants are completely deleted for the ben-1 gene. Since the deletion strains appear to be fully resistant to the drugs, the ben-1 product appears to be the only benzimidazole-sensitive beta-tubulin in C. elegans. Furthermore, since animals lacking ben-1 are viable and coordinated, the ben-1 beta-tubulin appears to be nonessential for growth and movement. The ben-1 function is likely to be redundant in the nematode genome.

Alleles↗
PubMed1989

The mec-3 gene of Caenorhabditis elegans requires its own product for maintained expression and is expressed in three neuronal cell types.

The homeo-box-containing gene mec-3 of the nematode Caenorhabditis elegans, is expressed in several sensory neurons, as assayed by expression of a mec-3-lacZ fusion. These cells are the touch receptors, which mediate the response to gentle touch, and the FLP and PVD neurons. PVD mediates a response to harsh mechanical stimuli, and FLP has an ultrastructure suggestive of a mechanoreceptor, but its function is unknown. mec-3 is necessary for the differentiation of the touch receptors, because in mec-3 mutants, the touch receptors do not function and have none of their distinguishing features. mec-3 is also needed for PVD function: The PVD neurons no longer mediate a response to harsh mechanical stimuli in the mutants. The expression of the mec-3-lacZ fusion, and presumably mec-3 itself, is altered by mutations in several genes originally identified by their effects on touch cell development. unc-86, another homeo-box-containing gene, is necessary for all mec-3-lacZ expression, but also affects several other lineages and cells in which mec-3 is not expressed. mec-3 activity appears to be required for maintained expression of the mec-3-lacZ fusion in all cells in which it is expressed. In a mec-17 mutant, mec-3-lacZ expression is not maintained in the touch receptors, but is not affected in the FLP and PVD neurons. These findings suggest that combinatorial mechanisms of gene regulation control both the expression of mec-3 itself and its action in promoting the terminal differentiation of various cell types.

Animals↗
PubMed1989

mec-7 is a beta-tubulin gene required for the production of 15-protofilament microtubules in Caenorhabditis elegans.

In the nematode Caenorhabditis elegans, microtubules with 15 protofilaments are a specialized feature of six touch-receptor neurons; microtubules found in other C. elegans neurons have 11 protofilaments. Mutations in the gene mec-7 result in touch-insensitive animals whose touch cells lack the 15-protofilament microtubules. We have characterized 54 mutations in the mec-7 gene. The absence of mec-7 activity results selectively in the recessive loss of touch sensitivity. Partial loss-of-function alleles result in a partial loss of touch sensitivity. Dominant mutations, which are isolated at an unusually high proportion, may encode abnormal products. We have cloned the mec-7 gene; it encodes a beta-tubulin which is 90-93% identical to vertebrate beta-tubulin. Our results are consistent with the hypothesis that tubulin heterogeneity contributes to the formation of structurally and functionally distinct sets of microtubules.

Amino Acid Sequence↗
PubMed1988

mec-3, a homeobox-containing gene that specifies differentiation of the touch receptor neurons in C. elegans.

The mec-3 gene is essential for proper differentiation of the set of six touch receptor neurons in C. elegans. In mutants lacking mec-3 activity, the touch receptors express none of their unique differentiated features and appear to be transformed into other types of neurons. We cloned the mec-3 gene by transposon tagging and showed that a mec-3 mutant can be rescued by germ line transformation using a 5.6 kb genomic DNA fragment. In a strain in which transforming mec-3 DNA is present in about 50 copies per haploid genome, additional cells express a mec-3-dependent phenotype. The putative coding sequence of mec-3 contains a homeobox, suggesting that the mec-3 protein specifies the expression of touch cell differentiation by binding to DNA and regulating transcription of genes that encode the differentiated features of these cells.

Alleles↗
PubMed1988

Cell-cell interactions in the guidance of late-developing neurons in Caenorhabditis elegans.

The initial outgrowth of developing neuronal processes can be affected by a number of extrinsic interactions. Cell-cell interactions are also important in a later stage of neuronal outgrowth when processes grow into the region of their targets. The correct positioning of the process of a postembryonic sensory neuron, the touch cell AVM of the nematode Caenorhabditis elegans, at its synaptic targets requires the presence of a pair of embryonic interneurons, the BDU cells. These cells receive synapses from AVM but do not participate in the touch reflex circuit. Therefore, the AVM-BDU synapses may be required to stabilize the association between these cells and assist in the guidance of the AVM processes through a mature neuropil.

Animals↗
PubMed1986

Mutations affecting microtubule structure in Caenorhabditis elegans.

Three types of microtubules are seen in the neuronal processes of the nematode Caenorhabditis elegans. Single cytoplasmic microtubules of most neurones have 11 protofilaments whereas those of six touch receptor cells have 15 protofilaments. The axonemes of sensory cilia have nine outer doublets with a variable number (up to seven) of singlet microtubules. Mutations in 11 genes affect the appearance of these microtubules.

Animals↗
PubMed1985

The neural circuit for touch sensitivity in Caenorhabditis elegans.

The neural pathways for touch-induced movement in Caenorhabditis elegans contain six touch receptors, five pairs of interneurons, and 69 motor neurons. The synaptic relationships among these cells have been deduced from reconstructions from serial section electron micrographs, and the roles of the cells were assessed by examining the behavior of animals after selective killing of precursors of the cells by laser microsurgery. This analysis revealed that there are two pathways for touch-mediated movement for anterior touch (through the AVD and AVB interneurons) and a single pathway for posterior touch (via the PVC interneurons). The anterior touch circuitry changes in two ways as the animal matures. First, there is the formation of a neural network of touch cells as the three anterior touch cells become coupled by gap junctions. Second, there is the addition of the AVB pathway to the pre-existing AVD pathway. The touch cells also synapse onto many cells that are probably not involved in the generation of movement. Such synapses suggest that stimulation of these receptors may modify a number of behaviors.

Animals↗
PubMed1983

Induction of neuronal branching in Caenorhabditis elegans.

The two postembryonic touch receptor neurons in the nematode Caenorhabditis elegans arise from essentially identical cell lineages and have the same ultrastructural features. The cells are found in different positions in the animal, however, and differ in neuronal branching, connectivity, and function. These structural and functional differences are not seen when cells are placed in similar positions by mutation or laser-induced damage. Thus, some, but probably not all, of the differentiated properties of these cells are a consequence of their cellular environment.

Animals↗
PubMed1982

Serotonin and octopamine in the nematode Caenorhabditis elegans.

The biogenic amines serotonin and octopamine are present in the nematode Caenorhabditis elegans. Serotonin, detected histochemically in whole mounts, is localized in two pharyngeal neurons that appear to be neurosecretory. Octopamine, identified radioenzymatically in crude extracts, probably is also localized in a few neurons. Exogenous serotonin and octopamine elicit specific and opposite behavioral responses in Caenorhabditis elegans, suggesting that these compounds function physiologically as antagonists.

Age Factors↗
PubMed1982

Structural and functional diversity in the neuronal microtubules of Caenorhabditis elegans.

Tannic acid fixation reveals differences in the number of protofilaments between microtubules (MTs) in the nematode Caenorhabditis elegans. Most cells have MTs with 11 protofilaments but the six touch receptor neurons (the microtubule cells) have MTs with 15 protofilaments. No 13-protofilament (13-p) MT has been seen. The modified cilia of sensory neurons also possess unusual structures. The cilia contain nine outer doublets with A subfibers of 13 protofilaments and B subfibers of 11 protofilaments and a variable number of inner singlet MTs containing 11 protofilaments. The 15-p MTs but not the 11-p MTs are eliminated by colchicine-treatment or by mutation of the gene mec-7. Concomitantly, touch sensitivity is also lost. However, whereas colchicine treatment leads to the loss of all MTs from the microtubule cells, mutations in mec-7 result in the partial replacement of the 15-p MTs with 11-p MTs. Benzimidazoles (benomyl and nocodazole) have more general effects on C. elegans (slow growth, severe uncoordination, and loss of processes from the ventral cord) but do not affect the 15-p MTs. Benomyl will, however, disrupt the replacement 11-p MTs found in the microtubule cells of mec-7 mutants. The 11-p and 15-p MTs also respond differently to temperature and fixation conditions. It is likely that either type of MT will suffice for the proper outgrowth of the microtubule cell process, but only the 15-p MT can function in the specialized role of sensory transduction of the microtubule cells.

Animals↗
PubMed1981

Mutations that lead to reiterations in the cell lineages of C. elegans.

Cells in the nematode Caenorhabditis elegans arise from invariant cell lineages. Mutations in two genes, unc-86 and lin-4, alter multiple and mutually exclusive sets of these lineages. In these mutants, particular cells repeat division patterns normally associated with their parental or grandparental progenitors. The effects of unc-86 are highly specific, altering in equivalent ways the lineages of three post-embryonic neuroblasts that in the wild-type undergo similar division patterns. The effects of lin-4 are more varied, resulting in a number of types of lineage reiterations as well as in supernumerary molts and the continued synthesis of larval-specific cuticle. The reiteration of a given cell division or pattern of cell divisions leads to the repeated generation of cells indistinguishable (by both light and electron microscopy) from those produced after the same division or pattern of cell divisions in the wild-type. This correlation between lineage history and cell fate suggests that in C. elegans a particular sequence of cell divisions may be necessary for the generation of a particular cell type. Reiterative lineages, often referred to as stem cell lineages, may be basic to the development of nematodes and other organisms. We suggest that the wild-type unc-86 and lin-4 genes act to modify latent reiterative cell lineages, which are revealed when the activity of one of these genes is eliminated.

Animals↗
PubMed1979

Organization of neuronal microtubules in the nematode Caenorhabditis elegans.

We have studied the organization of microtubules in neurons of the nematode Caenorhabditis elegans. Six neurons, which we call the microtubule cells, contain bundles of darkly staining microtubules which can be followed easily in serial-section electron micrographs. Reconstruction of individual microtubules in these cells indicate that most, if not all, microtubules are short compared with the length of the cell process. Average microtubule length varies characteristically with cell type. The arrangement of microtubules gives an overall polarity to each bundle: the distal ends of the microtubles are on the outside of the bundle, whereas the proximal ends are preferentially inside. The distal and proximal ends each have a characteristic appearance indicating that these microtubules may have a polarity of their own. Short microtubules in processes of other neurons in C. elegans have also been observed.

Animals↗