Abstract
LGR4 is an R-spondin receptor with strong positive effect on Wnt signaling. It plays a critical role in development as its ablation in the mouse led to total embryonic/neonatal lethality with profound defects in multiple organs. Haplotype insufficiency of LGR4 in human was associated with several diseases, including increased risk of squamous cell carcinoma of the skin, reduced birth weights, electrolyte imbalance, and decreased levels of testosterone, which are similar to the phenotypes of LGR4-hypomorphic mice. Tissue distribution of LGR4 was extensively analyzed in the mouse using gene-trap reporter enzyme alleles. However, its expression pattern in human tissues remained largely unknown. We have developed LGR4-specific monoclonal antibodies and used them to examine the expression of LGR4 in selected adult human and mouse tissues by immunohistochemical analysis. Intense LGR4-like immunoreactivity was observed in the epidermis and hair follicle of the skin, pancreatic islet cells, and epithelial cells in both the male and female reproductive organs. Of particular interest is that LGR4 is highly expressed in germ cells and pancreatic islet cells, which have important implications given the role of R-spondin-LGR4 signaling in the survival of adult stem cells. In addition, the majority of colon tumors showed elevated levels of LGR4 receptor. Overall, the expression pattern of LGR4 in human tissues mapped by this IHC analysis is similar to that in the mouse as revealed from gene trap alleles. Importantly, the pattern lends strong support to the important role of LGR4 in the development and maintenance of skin, kidney, reproductive systems, and other organs.