Worm Breeder's Gazette 13(2): 67 (February 1, 1994)
These abstracts should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author.
UNC-6 is a laminin-related extracellular matrix protein which guides cell and pioneer axon migration. Several experiments are now in progress to define the spatial and temporal expression patterns of UNC-6 .Preliminary results have been obtained with epitope-tagged UNC-6 .A DNA fragment encoding three tandem copies of a foreign viral epitope (HA tag) was incorporated in-frame into the coding sequence of the cloned unc-6 gene. The site of incorporation was at the N-terminus immediately following the signal peptide. This site was chosen because a comparison of laminin domain Vl sequences and the UNC-6 sequence revealed this region is highly variable in terms of amino acid composition and length. This suggested the region might not be functionally important and was likely to be exposed outside the globular portion of the protein. The plasmid was microinjected into null mutants ( ev400 )and rescued progeny were selected. An integrated line has been examined immunochemically using antibody to the HA tag.
The earliest staining ( ~270 min) is of the left and right P cell groups. These epidermal (hypodermal) cells form the midventral surface and are ventral cord blast cells. The early pattern is transitory and throughout the rest of embryogenesis individual neurons and axons become visible. Also, from ~400-440 min six cells (not yet identified) are visible in the head. These cells may be important during development of sensory processes and/or the nerve ring. In larva the ventral nerve chord and pairs of postembryonic neurons (VA and VB) are easily identified.
One of the visible neurons appears to be the PVQ (L/R) cells. These are early pioneers of the posterior ventral nerve chord. It is possible the other neurons staining in embryos are also pioneering axons. We suggest the early expression of UNC-6 by the ventral subset of epidermal cells provides a necessary spatial cue for the first pioneering axons. The pioneering axons and/or those immediately following (which do not form commissures), extend together along the entire ventral side of the animal and express UNC-6 as a cue for later migrations. In larva, the postembryonic neurons maintains expression in the ventral nerve chord .
Although the epitope-tagged construct does rescues unc-6 null animals, it is not known how the distribution of UNC-6 may be altered. For example, the additional amino acids may modify secretion of the protein. In fact, strains expressing an UNC-6 promotor:lacZ construct show a similar pattern (Andrew Fire, Carnegie Institution of Washington). We are currently examining the expression pattern using antibodies.