Rspo1 Is Essential To Normal Mammary Gland Development

The mammary gland, a mammalian specific organ, undergoes cycles of development and differentiation throughout lifespan. The Wnt/β-catenin pathway, found in many developmental processes such as the determination of cell and tissue polarity, stimulation of cell proliferation and differentiation and adult tissue homeostasis, is involved in the biology of the mammary gland.

Recently, its regulation by the R-Spondin(Rspo) genes has been reported. Rspo family genes, which were recently isolated in mouse, Xenopus, monotremes and human, encode secretory proteins containing an N-terminal signal peptide, a cysteine rich region with two furin like domains, a thrombospondin type 1 repeat motif and a C-terminal region comprised of positively charge amino acids. Four members exist in mice and human. In this context, we have studied, the potential implication of these genes in the mice mammary gland biology. Their expression patterns show that they are quantitatively and qualitatively differentially expressed during the different developmental stages of the mammary gland, which suggests that they have different roles. Rspo1 is the most expressed gene and has an expression pattern closely related to Wnt-4 expression pattern. By Immunohistochemistry, RSPO1 protein was found in the epithelial cell with a localization dependant on the mammary development stage. Indeed, a progressive relocalization of the protein from the cytoplasm to the basal membrane and the lame basal is observed during the pregnancy.

This protein is further synthesized by the epithelial cells as it can be observed in the Golgi using electronic microscopy. In order to understand more precisely the potential function of Rspo1, studies were performed using two mice models: one over expressing Rspo1 and one knockout for the gene. Over expression of Rspo1 did not generate any mammary phenotype, while its invalidation induced a defect in the mammary gland development. Indeed, mammary gland from virgin (8-weeks) Rspo1-/- female mice revealed absence of tertiary side branching. In pregnancy, the branching defect persists and is associated to a default of alveolar development (cells that secrete the milk). Rspo1 gene seems to be a critical activator during the normal mammary gland development and differentiation.