Cellular basis of ray morphology abnormal in C. elegans

Organ morphogenesis is one of the important processes during development. The male tail sensory rays of nematode C. elegans serve as a good model for the study of morphogenesis particularly with a number of Ram (RAy Morphology abnormal) mutants previously identified. In all these mutants, the sensory rays become lumpy instead of being a tapered shape. Based on their molecular natures, different ram genes can be classified into distinct groups, male tail-specific collagen, RAM-1, RAM-2 and RAM-4; ZP protein, RAM-5; and EGF-like domain containing protein, MAB-7 and MAB-29. Even with these molecules characterized, the cellular basis of the sensory ray abnormality is still unknown. In order to define why the rays become lumpy in shape, cellular, molecular biology and genetic techniques were employed to reveal biological events occurring inside swollen rays in different types of ram mutants. ram-2(bx76), ram-2(bx32), ram-5(bx81) and mab-7(e1599) mutants represent each of the subclass of the ram genes. Fluorescent subcellular organelle markers revealed expanded ER, mislocalized nuclei, altered nuclear structure, autophagosomes and lysosomes localized in the swollen rays in ram-2(bx76), ram-2(bx32) and mab-7(e1599) mutant animals, while only expanded ER, mislocalized nuclei were observed in ram-5(bx81) mutant animals. The causal-relationship between the observed subcellular defects and Ram phenotype was tested. Modulation of autophagy level in ram-2(bx76), ram-2(bx32) and mab-7(e1599) mutants showed differential effects on Ram phenotype, suggesting different mechanisms leading to the manifestation of Ram phenotype and distinct roles of autophagy in these mechanisms. In addition, stress response was found to be activated in ram-2(bx32) mutant, but not other ram mutants. In subsequent analysis focusing on ram-2(bx32) mutant, ire-1 signaling pathway was shown to be required for handling the stress. In parallel, a new stress responsive gene, atf-5, was discovered as a potential downstream target of pek-1 signaling pathway. In summary, this study has classified ram genes according to their different biological impacts on cellular behavior. The findings also support the notion that ram-2(bx32) mutants could be an excellent in vivo model for further study of link between ER stress, UPR pathways and other cellular behaviors