TIS7 induces transcriptional cascade of methylosome components required for muscle differentiation

The negative adipogenesis regulator Dlk1 is transcriptionally regulated by Ifrd1 (TIS7) and translationally by its orthologue Ifrd2 (SKMc15)

Delta-like homolog 1 (Dlk1), an inhibitor of adipogenesis, controls the cell fate of adipocyte progenitors. Experimental data presented here identify two independent regulatory mechanisms, transcriptional and translational, by which Ifrd1 (TIS7) and its orthologue Ifrd2 (SKMc15) regulate Dlk1 levels. Mice deficient in both Ifrd1 and Ifrd2 (dKO) had severely reduced adipose tissue and were resistant to high-fat diet-induced obesity. Wnt signaling, a negative regulator of adipocyte differentiation, was significantly upregulated in dKO mice. Elevated levels of the Wnt/β-catenin target protein Dlk1 inhibited the expression of adipogenesis regulators Pparg and Cebpa, and fatty acid transporter Cd36. Although both Ifrd1 and Ifrd2 contributed to this phenotype, they utilized two different mechanisms. Ifrd1 acted by controlling Wnt signaling and thereby transcriptional regulation of Dlk1. On the other hand, distinctive experimental evidence showed that Ifrd2 acts as a general translational inhibitor significantly affecting Dlk1 protein levels. Novel mechanisms of Dlk1 regulation in adipocyte differentiation involving Ifrd1 and Ifrd2 are based on experimental data presented here.

Binding of the protein ICln to α-integrin contributes to the activation of IClswell current

ICl_swell is the chloride current induced by cell swelling, and plays a fundamental role in several biological processes, including the regulatory volume decrease (RVD). ICln is a highly conserved, ubiquitously expressed and multifunctional protein involved in the activation of ICl_swell. In platelets, ICln binds to the intracellular domain of the integrin αIIb chain, however, whether the ICln/integrin interaction plays a role in RVD is not known. Here we show that a direct molecular interaction between ICln and the integrin α-chain is not restricted to platelets and involves highly conserved amino acid motifs. Integrin α recruits ICln to the plasma membrane, thereby facilitating the activation of ICl_swell during hypotonicity. Perturbation of the ICln/integrin interaction prevents the transposition of ICln towards the cell surface and, in parallel, impedes the activation of ICl_swell. We suggest that the ICln/integrin interaction interface may represent a new molecular target enabling specific ICl_swell suppression in pathological conditions when this current is deregulated or plays a detrimental role.

Histone phosphorylation by TRPM6's cleaved kinase attenuates adjacent arginine methylation to regulate gene expression

TRPM6 and TRPM7 are members of the melastatin-related transient receptor potential (TRPM) subfamily of ion channels. Deletion of either gene in mice is embryonically lethal. TRPM6/7 are the only known examples of single polypeptides containing both an ion channel pore and a serine/threonine kinase (chanzyme). Here we show that the C-terminal kinase domain of TRPM6 is cleaved from the channel domain in a cell type-specific fashion and is active. Cleavage requires that the channel conductance is functional. The cleaved kinase translocates to the nucleus, where it is strictly localized and phosphorylates specific histone serine and threonine (S/T) residues. TRPM6-cleaved kinases (M6CKs) bind subunits of the protein arginine methyltransferase 5 (PRMT5) molecular complex that make important epigenetic modifications by methylating histone arginine residues. Histone phosphorylation by M6CK results in a dramatic decrease in methylation of arginines adjacent to M6CK-phosphorylated amino acids. Knockout of TRPM6 or inactivation of its kinase results in global changes in histone S/T phosphorylation and changes the transcription of hundreds of genes. We hypothesize that M6CK associates with the PRMT5 molecular complex in the nucleus, directing M6CK to a specific genomic location and providing site-specific histone phosphorylation. M6CK histone phosphorylation, in turn, regulates transcription by attenuating the effect of local arginine methylation.