Deamidation enables pathogenic SMAD6 variants to activate the BMP signaling pathway

The BMP signaling pathway plays a crucial role in regulating early embryonic development and tissue homeostasis.SMAD6 encodes a negative regulator of BMP,and rare variants of SMAD6 are recurrently found in individuals with birth defects.However,we observed that a subset of rare pathogenic variants of SMAD6 consistently exhibited positive regulatory effects instead of the initial negative effects on the BMP signaling pathway.We sought to determine whether these SMAD6 variants have common pathogenic mechanisms.Here,we showed that pathogenic SMAD6 variants accompanying this functional reversal exhibit similar increases in deamidation.Mechanistically,increased deamidation of SMAD6 variants promotes the accumulation of the BMP receptor BMPR1A and the formation of new complexes,both of which lead to BMP signaling pathway activation.Specifically,two residues,N262 and N404,in SMAD6 were identified as the crucial sites of deamidation,which was catalyzed primarily by glutamine-fructose-6-phosphate transaminase 2(GFPT2).Additionally,treatment of cells harboring SMAD6 variants with a deamidase inhibitor restored the inhibitory effect of SMAD6 on the BMP signaling pathway.Conversely,when wild-type SMAD6 was manually simulated to mimic the deamidated state,the reversed function of activating BMP signaling was reproduced.Taken together,these findings show that deamidation of SMAD6 plays a crucial role in the functional reversal of BMP signaling activity,which can be induced by a subset of various SMAD6 variants.Our study reveals a common pathogenic mechanism shared by these variants and provides a potential strategy for preventing birth defects through deamidation regulation,which might prevent the off-target effects of gene editing.

Calycosin-7-O-β-D-glucopyranoside stimulates osteoblast differentiation through regulating the BMP/WNT signaling pathways

The iso fl avone calycosin-7-O-β-D-glucopyranoside(CG) is a principal constituent of Astragalus membranaceus(AR) and has been reported to inhibit osteoclast development in vitro and bone loss in vivo. The aim of this study was to investigate the osteogenic effects of CG and its underlying mechanism in ST2 cells. The results show that exposure of cells to CG in osteogenic differentiation medium increases ALP activity, osteocalcin(Ocal) m RNA expression and the osteoblastic mineralization process. Mechanistically, CG treatment increased the expression of bone morphogenetic protein 2(BMP-2), p-Smad 1/5/8, β-catenin and Runx2, all of which are regulators of the BMP- or wingless-type MMTV integration site family(WNT)/β-catenin-signaling pathways. Moreover, the osteogenic effects of CG were inhibited by Noggin and DKK-1 which are classical inhibitors of the BMP and WNT/β-catenin-signaling pathways, respectively. Taken together, the results indicate that CG promotes the osteoblastic differentiation of ST2 cells through regulating the BMP/WNT signaling pathways. On this basis, CG may be a useful lead compound for improving the treatment of bone-decreasing diseases and enhancing bone regeneration.

The role of Smad6 in immunity of the pearl oyster Pinctada fucata martensii

Inhibitory Smads(I-Smads),which belong to the Smad family and inhibit bone morphogenic protein 2(BMP2)signaling by a variety of mechanisms,can suppress innate immunity responses in vertebrates.However,there are no reports for the role of Smad6 in immunity in mollusks.In this study,we showed that Smad6 of the pearl oyster Pinctada fucata martensii was located in the Smad6 cluster of the phylogenetic tree;mRNA expression of Smad6 and Smad3 was up-regulated after lipopolysaccharide and polyinosinic:polycytidylic challenge;and transcript levels of Smad6 and Smad3 showed opposite patterns during wound healing.Under salinity stress,water inflow and outflow in the gills appear to be regulated by BMP2-Smads signals,and BMP2-Smads signaling may be closely related to the immune response.Our results indicate that Smad6 is involved in immunity,that it plays a positive role in the response to immune challenge and an inhibitory role during wound healing,and that Smad6 and Smad3 may work against each other.