hibris,an Ig domain protein involved in execution of muscle differentiation. R.D. Artero , M. Baylies. Molecular Biology Program, Mem. Sloan-Kettering Cancer Center, 1275 York Avenue, New York, NY 10021.

   Terminal muscle differentiation involves the acquisition of a given size, shape, innervation and attachments to the epidermis. Using a molecular approach we have identified several new genes involved in this identity process and here we report the characterization of one, hibris (hbs).In situhybridization and antibody immunostainings show that hbsis expressed in the developing mesoderm at several points. First, hbsis expressed in patches similar to bagpipein the visceral mesoderm anlagen. Slightly later, at stages 11 and 12, it is found in the somatic mesoderm and in the progenitors of the heart. Finally, hbsis expressed in the epidermis at the muscle attachment sites. Overexpression experiments indicate that hbsfunctions downstream of Notch in the somatic mesoderm. Insight to hbsfunction has been gained using both loss and gain of function approaches. The smallest deficiency that removes hbsreveals a dramatic muscle phenotype, including: (1) muscle losses, (2) muscles adhering to one another, (3) improper muscle attachments (including muscle to muscle attachments), (4) failure in proper gut constriction and (5) failure in CNS condensation and patterning. RNA interference confirms hbsfunction during myogenesis. In summary, hbsis necessary at least for the formation of some muscles (the ones absent in the RNA injected embryos) and for the final differentiation process of others. Our gain of function experiments support our loss of function data. We have used the Gal4/UAS system to overexpress hbsin places where it is normally expressed and/or ectopically. Overexpression of a full length hbsprotein in embryonic mesoderm partially blocks myoblast fusion. We also detect improper muscle attachments to the tendon cells when hbsexpression is maintained in the muscles. A model of hbsfunction will be presented at the meeting.