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Extension of the Caenorhabditis elegans Pharyngeal M1 neuron axon is regulated by multiple mechanisms.

Refai O, Rohs P, Mains PE, Gaudet J - G3 (Bethesda) (2013)

Bottom Line: We found that extension of the M1 pharyngeal axon, which spans the entire length of the pharynx, occurs in two distinct phases.The first proximal phase does not require genes that function in axon extension (unc-34, unc-51, unc-115, and unc-119), whereas the second distal phase does use these genes and is guided in part by the adjacent g1P gland cell projection. unc-34, unc-51, and unc-115 had incompletely penetrant defects and appeared to act in conjunction with the g1P cell for distal outgrowth.One of these mutations appeared to affect the generation or differentiation of the M1 neuron.

View Article: PubMed Central - PubMed

Affiliation: Department of Biochemistry and Molecular Biology, Alberta Children's Hospital Research Institute, University of Calgary, Calgary, Alberta T2N 4N1, Canada.

ABSTRACT
The guidance of axons to their correct targets is a critical step in development. The C. elegans pharynx presents an attractive system to study neuronal pathfinding in the context of a developing organ. The worm pharynx contains relatively few cells and cell types, but each cell has a known lineage and stereotyped developmental patterns. We found that extension of the M1 pharyngeal axon, which spans the entire length of the pharynx, occurs in two distinct phases. The first proximal phase does not require genes that function in axon extension (unc-34, unc-51, unc-115, and unc-119), whereas the second distal phase does use these genes and is guided in part by the adjacent g1P gland cell projection. unc-34, unc-51, and unc-115 had incompletely penetrant defects and appeared to act in conjunction with the g1P cell for distal outgrowth. Only unc-119 showed fully penetrant defects for the distal phase. Mutations affecting classical neuronal guidance cues (Netrin, Semaphorin, Slit/Robo, Ephrin) or adhesion molecules (cadherin, IgCAM) had, at best, weak effects on the M1 axon. None of the mutations we tested affected the proximal phase of M1 elongation. In a forward genetic screen, we isolated nine mutations in five genes, three of which are novel, showing defects in M1, including axon overextension, truncation, or ectopic branching. One of these mutations appeared to affect the generation or differentiation of the M1 neuron. We conclude that M1 axon extension is a robust process that is not completely dependent on any single guidance mechanism.

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Phenotypes of mutants isolated from the forward genetic screen visualized in L4 or young adults. M1 is marked with glr-2::gfp. Arrowheads indicate cell bodies and arrows denote defects, which are incompletely penetrant in all cases (Figure 3). Other neurons visible in the figure are part of the nonpharyngeal cells in the nerve ring. (A′–E′) Cartoons for each phenotypic class. (A) Wild-type. (B) The unc-51(iv84) shows axon truncation and an ectopic branch. (C) The rpm-1(iv78) results in overextension of M1 terminal branches beyond their normal targets. (D) The mnm-7(iv77) shows ectopic branching from the cell body. (E) The mnm-8(iv82) shows a 95% penetrant M1 missing phenotype, whereas the other neurons that express the glr-2::gfp reporter are still present. Scale bar in (A) = 10 µm.
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fig7: Phenotypes of mutants isolated from the forward genetic screen visualized in L4 or young adults. M1 is marked with glr-2::gfp. Arrowheads indicate cell bodies and arrows denote defects, which are incompletely penetrant in all cases (Figure 3). Other neurons visible in the figure are part of the nonpharyngeal cells in the nerve ring. (A′–E′) Cartoons for each phenotypic class. (A) Wild-type. (B) The unc-51(iv84) shows axon truncation and an ectopic branch. (C) The rpm-1(iv78) results in overextension of M1 terminal branches beyond their normal targets. (D) The mnm-7(iv77) shows ectopic branching from the cell body. (E) The mnm-8(iv82) shows a 95% penetrant M1 missing phenotype, whereas the other neurons that express the glr-2::gfp reporter are still present. Scale bar in (A) = 10 µm.

Mentions: Because the M1 axon defects showed incomplete penetrance in all situations that we examined (with the exception of unc-119), M1 axon extension and guidance may involve novel genes. Thus, we searched for viable mutants with M1 morphology or developmental defects using the integrated glr-2::gfp strain. In a screen of 5000 mutagenized haploid genomes, nine mutations were identified that fell into four phenotypic classes: axon truncation; axon overextension; ectopic branches at the cell body; and M1 absent (Figure 3 and Figure 7). None affected the initial outgrowth of the M1 axon through the isthmus.


Extension of the Caenorhabditis elegans Pharyngeal M1 neuron axon is regulated by multiple mechanisms.

Refai O, Rohs P, Mains PE, Gaudet J - G3 (Bethesda) (2013)

Phenotypes of mutants isolated from the forward genetic screen visualized in L4 or young adults. M1 is marked with glr-2::gfp. Arrowheads indicate cell bodies and arrows denote defects, which are incompletely penetrant in all cases (Figure 3). Other neurons visible in the figure are part of the nonpharyngeal cells in the nerve ring. (A′–E′) Cartoons for each phenotypic class. (A) Wild-type. (B) The unc-51(iv84) shows axon truncation and an ectopic branch. (C) The rpm-1(iv78) results in overextension of M1 terminal branches beyond their normal targets. (D) The mnm-7(iv77) shows ectopic branching from the cell body. (E) The mnm-8(iv82) shows a 95% penetrant M1 missing phenotype, whereas the other neurons that express the glr-2::gfp reporter are still present. Scale bar in (A) = 10 µm.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3815062&req=5

fig7: Phenotypes of mutants isolated from the forward genetic screen visualized in L4 or young adults. M1 is marked with glr-2::gfp. Arrowheads indicate cell bodies and arrows denote defects, which are incompletely penetrant in all cases (Figure 3). Other neurons visible in the figure are part of the nonpharyngeal cells in the nerve ring. (A′–E′) Cartoons for each phenotypic class. (A) Wild-type. (B) The unc-51(iv84) shows axon truncation and an ectopic branch. (C) The rpm-1(iv78) results in overextension of M1 terminal branches beyond their normal targets. (D) The mnm-7(iv77) shows ectopic branching from the cell body. (E) The mnm-8(iv82) shows a 95% penetrant M1 missing phenotype, whereas the other neurons that express the glr-2::gfp reporter are still present. Scale bar in (A) = 10 µm.
Mentions: Because the M1 axon defects showed incomplete penetrance in all situations that we examined (with the exception of unc-119), M1 axon extension and guidance may involve novel genes. Thus, we searched for viable mutants with M1 morphology or developmental defects using the integrated glr-2::gfp strain. In a screen of 5000 mutagenized haploid genomes, nine mutations were identified that fell into four phenotypic classes: axon truncation; axon overextension; ectopic branches at the cell body; and M1 absent (Figure 3 and Figure 7). None affected the initial outgrowth of the M1 axon through the isthmus.

Bottom Line: We found that extension of the M1 pharyngeal axon, which spans the entire length of the pharynx, occurs in two distinct phases.The first proximal phase does not require genes that function in axon extension (unc-34, unc-51, unc-115, and unc-119), whereas the second distal phase does use these genes and is guided in part by the adjacent g1P gland cell projection. unc-34, unc-51, and unc-115 had incompletely penetrant defects and appeared to act in conjunction with the g1P cell for distal outgrowth.One of these mutations appeared to affect the generation or differentiation of the M1 neuron.

View Article: PubMed Central - PubMed

Affiliation: Department of Biochemistry and Molecular Biology, Alberta Children's Hospital Research Institute, University of Calgary, Calgary, Alberta T2N 4N1, Canada.

ABSTRACT
The guidance of axons to their correct targets is a critical step in development. The C. elegans pharynx presents an attractive system to study neuronal pathfinding in the context of a developing organ. The worm pharynx contains relatively few cells and cell types, but each cell has a known lineage and stereotyped developmental patterns. We found that extension of the M1 pharyngeal axon, which spans the entire length of the pharynx, occurs in two distinct phases. The first proximal phase does not require genes that function in axon extension (unc-34, unc-51, unc-115, and unc-119), whereas the second distal phase does use these genes and is guided in part by the adjacent g1P gland cell projection. unc-34, unc-51, and unc-115 had incompletely penetrant defects and appeared to act in conjunction with the g1P cell for distal outgrowth. Only unc-119 showed fully penetrant defects for the distal phase. Mutations affecting classical neuronal guidance cues (Netrin, Semaphorin, Slit/Robo, Ephrin) or adhesion molecules (cadherin, IgCAM) had, at best, weak effects on the M1 axon. None of the mutations we tested affected the proximal phase of M1 elongation. In a forward genetic screen, we isolated nine mutations in five genes, three of which are novel, showing defects in M1, including axon overextension, truncation, or ectopic branching. One of these mutations appeared to affect the generation or differentiation of the M1 neuron. We conclude that M1 axon extension is a robust process that is not completely dependent on any single guidance mechanism.

Show MeSH
Related in: MedlinePlus