Evolution and developmental functions of mesocentin: A novel Caenorhabditis elegans extracellular matrix protein.

摘要

dig-1, a C. elegans gene on LGIII, that when mutated results in a spectrum of loss of adhesion phenotypes, was cloned by microinjection rescue with a candidate gene from the physical map; K07E12.1. RTPCR analysis of dig-1 revealed a novel 13100 aminoacid secreted extracellular matrix molecule with at least 10 alternatively spliced isoforms.; The mesocentin molecule contains both old stable domains and novel modules generated by continuing and numerous recombination, gene conversion and possibly retroposition events. Domain identity in the novel middle repetitive region has apparently been maintained due to strong conversion bias. The evolutionary history of mesocentin is complex with episodes of domain gain in the “repeated” region while the upstream “unique” region has remained quiescent for at least ∼50–100 Myr. Some episodes of gene conversion and duplication are so fresh that DNA sequence identity approaches 100% over long stretches (∼3 kb) even in introns.; The C. elegans mesocentin mRNA sequence was used to uncover the briggsae ortholog in the recently sequenced briggsae genome. Phylogenetic analysis shows that the “unique” region predates the briggsae/elegans speciation while the large “repeated” region expanded independently in both species but to very similar extents implying similar evolutionary pressures are at work in both lineages.; Promotor fusion experiments with dig-1_p:GFP have shown that expression starts in neurulation at the onset of the comma stage (∼300′). Most expression originates in cells of mesodermal lineage hence mesocentin. Expression in the mesoglia, GLRs, is particularly interesting in light of their apparent role in assembling the precise topography of connections of muscle arms to the motorneuron axons in the nerve ring. Muscle expression is also observed but is not unexpected since other ECM molecules such as hemicentin (Vogel and Hedgecock, 2001) are also expressed and exported from the body muscles. Mesocentin may be multifunctional due to its alternative spliced forms and may function as (1) an enthalpic spring to recycle some energy from movement; (2) an instructive molecule in ECM assembly; (3) a source of passive gliding adhesion similar to that suggested for hemicentin and (4) possibly a membrane anchored receptor for ECM molecules or other cell surface ligands.; Finally, experiments with mutants in many known factors affecting epithelia such as cadherins and integrins have demonstrated that none affect the pharynx (Portereiko and Mango, 2000; Leung et al., 1999; Costa et al., 1998). Mutations in dig-1 result in misalignment of the digestive tract including an impressive twisted pharynx phenotype. Thus, mesocentin maybe the first known factor to specifically determine an aspect of pharynx primordium assembly; accurate radial orientation of the anterior cells of the primordium in relation to the posterior ring of arcade cells.