Role of hyaluronan and CD44 in in vitro branching morphogenesis of ureteric bud cells

Mutual interaction between the metanephric mesenchyme (MM) and the ureteric bud (UB) in the developing kidney leads to branching morphogenesis and the formation of the ureteric tree. A UB-derived cell line, stimulated by cond itioned medium derived from an embryonic MM cell line (or, similarly, by 10 % fetal calf serum), forms branching tubules under three-dimensional cultur e conditions (H. Sakurai et al., 1997, Proc. Natl. Acad. Sci. USA 94, 6279 - 6284). The formation of branching tubules in this simple in vitro system for early nephrogenesis is highly sensitive to the matrix environment, a ke y component of which is the glycosaminoglycan hyaluronan (HA). Consistent w ith this, we found that HA in the extracellular environment markedly stimul ated the formation of cellular processes and multicellular cords (early ste ps in branching morphogenesis) and also acted as a tell survival factor. In hibition of HA binding to the cells by addition of blocking antibodies to C D44, the principal cell surface receptor for HA, or degradation of HA by th e addition of Streptomyces hyaluronidase resulted in decreased cell surviva l and diminished morphogenesis, indicating that the HA-CD44 axis plays a ce ntral role in in vitro branching morphogenesis. Analysis of the expression of a large number of genes displayed on a cDNA array revealed that signific ant changes in gene expression in cells undergoing morphogenesis in the pre sence of HA were limited to a small subset of genes regulating apoptosis, p roliferation, and morphogenesis. This included upregulation by HA of its re ceptor, CD44, which was found to largely localize to the tips of branching cellular processes. In the embryonic kidney, HA was found near the developi ng ureteric tree and CD44 was expressed basolaterally in UB-derived structu res. In addition, both UB and MM appear to express HA synthase, suggesting their ability to secrete KA. We propose that HA promotes branching morphoge nesis by creating a positive feedback loop that results in (1) enhanced int eraction of HA-CD44 at branching tips (possibly leading to localization of HA binding morphoregulatory factors at the tips) and (2) an activated trans criptional program favoring cell survival/proliferation and migration/morph ogenesis of cells through matrix by the expression of key morphoregulatory molecules. Furthermore, since HA, hyaluronidase, and CD44 have been functio nally implicated in branching morphogenesis in this model, and since HA, CD 44, and HA synthase are all expressed in an appropriate spatiotemporal fash ion in the developing kidney, we propose that these molecules may, together , constitute a morphoregulatory pathway that plays a key role in sequential cycles of branching morphogenesis in the UB. (C) 2000 Academic Press.