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Leucine-rich repeats and immunoglobulin-like domains (LRIG) are transmembrane proteins shown to promote bone morphogenetic protein (BMP) signaling in Caenorhabditis elegans, Drosophila melanogaster, and mammals. BMPs comprise a subfamily of the transforming growth factor beta (TGFβ) superfamily, or TGFβ family, of ligands. In mammals, LRIG1 and LRIG3 promote BMP4 signaling. BMP6 signaling, but not BMP9 signaling, is also regulated by LRIG proteins, although the specific contributions of LRIG1, LRIG2, and LRIG3 have not been investigated, nor is it known whether other mammalian TGFβ family members are regulated by LRIG proteins. To address these questions, we took advantage of Lrig-null mouse embryonic fibroblasts (MEFs) with doxycycline-inducible LRIG1, LRIG2, and LRIG3 alleles, which were stimulated with ligands representing all the major TGFβ family subgroups. By analyzing the signal mediators pSmad1/5 and pSmad3, as well as the induction of Id1 expression, we showed that LRIG1 promoted BMP2, BMP4, and BMP6 signaling and suppressed GDF7 signaling; LRIG2 promoted BMP2 and BMP4 signaling; and LRIG3 promoted BMP2, BMP4, BMP6, and GDF7 signaling. BMP9 and BMP10 signaling was not regulated by individual LRIG proteins, however, it was enhanced in Lrig-null cells. LRIG proteins did not regulate TGFβ1-induced pSmad1/5 signaling, or GDF11- or TGFβ1-induced pSmad3 signaling. Taken together, our results show that some, but not all, TGFβ family ligands are regulated by LRIG proteins and that the three LRIG proteins display differential regulatory effects. LRIG proteins thereby provide regulatory means for the cell to further diversify the signaling outcomes generated by a limited number of TGFβ family ligands and receptors.

Netrin-1 is a secreted protein that is well known for its involvement in axonal guidance during embryonic development and as an enhancer of cancer cell metastasis. Despite extensive efforts, the molecular mechanisms behind many of the physiological functions of netrin-1 have remained elusive. Here, we show that netrin-1 functions as a suppressor of bone morphogenetic protein (BMP) signaling in various cellular systems, including a mutually inhibitory interaction with the BMP-promoting function of leucine-rich repeats and immunoglobulin-like domains (LRIG) proteins. The BMP inhibitory function of netrin-1 in mouse embryonic fibroblasts was dependent on the netrin receptor neogenin, with the expression level regulated by both netrin-1 and LRIG proteins. Our results reveal a previously unrecognized function of netrin-1 that may help to explain several of the developmental, physiological, and cancer-promoting functions of netrins at the signal transduction level.

Leucine-rich repeats and immunoglobulin-like domains (LRIG) proteins have been implicated as regulators of growth factor signaling; however, the possible redundancy among mammalian LRIG1, LRIG2, and LRIG3 has hindered detailed elucidation of their physiological functions. Here, we show that Lrig-null mouse embryonic fibroblasts (MEFs) are deficient in adipogenesis and bone morphogenetic protein (BMP) signaling. In contrast, transforming growth factor-beta (TGF-beta) and receptor tyrosine kinase (RTK) signaling appeared unaltered in Lrig-null cells. The BMP signaling defect was rescued by ectopic expression of LRIG1 or LRIG3 but not by expression of LRIG2. Caenorhabditis elegans with mutant LRIG/sma-10 variants also exhibited a lipid storage defect. Human LRIG1 variants were strongly associated with increased body mass index (BMI) yet protected against type 2 diabetes; these effects were likely mediated by altered adipocyte morphology. These results demonstrate that LRIG proteins function as evolutionarily conserved regulators of lipid metabolism and BMP signaling and have implications for human disease. Herdenberg et al. show that adipogenesis and BMP signaling are altered in mouse cells deficient in LRIG (Leucine-rich repeats and immunoglobulin-like domains) proteins. They find that mutant LRIG/sma-10 variant worms exhibit lipid storage defects and that human LRIG1 variants are associated with higher body mass index, yet protect against type 2 diabetes. This study suggests an evolutionarily conserved role of LRIG proteins for lipid metabolism and BMP signaling.