Affiliations: Molecular Neurogenetics Unit, Center for Human Genetic Research, Massachusetts General Hospital, Boston, MA, USA
Note:  Correspondence to: Michael A. Myre, Ph.D., Center for Human Genetic Research, Massachusetts General Hospital, Simches Research Building, 185 Cambridge Street, Boston, MA 02114, USA. Tel.: +1 617 643 5536; E-mail: [email protected]
Abstract: Background: The CAG triplet repeat expansion mutation in the HTT locus, which results in neurodegeneration in Huntington's disease, elongates a polyglutamine tract in huntingtin, a HEAT/HEAT-like protein that has been highly structurally conserved through evolution. In several organisms, huntingtin is necessary for proper cell-cell adhesion and normal development. Objective: Dictyostelium discoideum huntingtin null (htt−) cells display a variety of developmental abnormalities and completely fail to acquire EDTA-resistant homotypic cell adhesion during starvation in suspension culture. Methods: Here, we have assessed the hypothesis that htt may be a genetic interactor of csaA, a major regulator of EDTA-resistant homotypic cell adhesion in D. discoideum. Immunoblot analysis demonstrated that csaA protein expression is dysregulated in htt− cells. Results: Unexpectedly, csaA overexpression, previously shown to rescue csaA− cell adhesion, failed to rescue the htt− adhesion defect. Thus, while htt was required for proper expression of the csaA protein, csaA overexpression was not sufficient to confer EDTA-resistant adhesion in the context of the htt− genetic background in contrast to parental cells. This implies a novel role for htt in conferring csaA-dependent, EDTA-resistant cell adhesion that warrants further investigation. Calcium supplementation restored both endogenous csaA protein levels and EDTA-resistant adhesion in htt− cells. Conclusions: Our data suggests the existence of an additional mechanism that overcomes the EDTA-resistant adhesion defect of htt− cells in the early development of D. discoideum.
Keywords: Cell adhesion, contact site A, csaA, csA, gp80, htt, hd, huntingtin, Dictyostelium discoideum, calcium, development