Function And Mechanism Of RanBP3L In BMP Signaling Pathway And Mesenchymal Stem Cell Differentiation

Bone Morphogenetic Proteins(BMPs)are signal molecules belonging to the Transforming Growth Factor beta super family.BMPs have critical roles in regulating embryonic development and maintaining musculoskeletal system homeostasis.Canonical BMP signaling is stringently controlled through reversible phosphorylation and nucleocytoplasmic shuttling of BMP related Smads(Smadl/5/8).We previously identified two protein phosphatases PPM1A and SCP4 are responsible for Smadl/5/8 C-terminal phosphorylation,which terminate BMP signaling transduction.However,how the nuclear export of Smadl/5/8 is regulated remains unclear.In this study,we report the identification and characterization of a new Ran binding protein called RanBP3L,which can specifically recognize and mediate nuclear export of BMP downstream Smadl/5/8 in a Ran-dependent manner.Smadl/5/8 then can be phosphorylated and transduce another round of signaling activation in cytoplasm.In sharp contrast to its ability to regulate BMP signaling,RanBP3L did not interfere with TGF-(3 signaling and mediate Smad2/3 nuclear export.Next we investigated the effects of RanBP3L on BMP-induced transcriptional responses using reporter assay,Real-Time PCR and Western blot analysis.Overexpression of RanBP3L inhibits BMP-induced transcriptional responses,as well as BMP-targeted gene expression.BMPs have critical roles in mesenchymal stem cell differentiation.BMPs can determine the fate of mesenchymal stem cells by stimulating their differentiation into the chondroosteoblastic lineage and meanwhile blocking their differentiation into the myoblastic or adipogenic lineage.We further investigated function of RanBP3L in primary mouse bone marrow-derived mesenchymal stem cell(BMSCs)osteogenesis differentiation model.Ectopic expression of RanBP3L in primary BMSCs can inhibit osteogenic differentiation of BMSCs.While knockdown of RanBP3L accelerates the osteogenic differentiation of BMSCs as well as increases mineral deposition which is the marker of mature osteogenesis.In addition,RanBP3L as an inhibitor of BMP signaling promotes myogenic differentiation of myoblasts.In conclusion,we find a new negative regulator for BMP signaling,which is responsible for nuclear export of BMP-targetd Smadl/5/8.RanBP3L specifically terminates BMP signaling has significant physiology function,which provides evidences and therapeutic application of BMP-related diseases such as skeletal dysfunction and osteoporosis.