DMPK expression and localization
Efforts to initially understand the relationship between DM pathology and extended trinucleotide repeats in the DMPK gene include analysis of DMPK mRNA levels in patient tissues. Most of these studies have found that DM1 mutations cause a decrease in total DMPK mRNA. Based on mRNA data, the heart of DM subjects and skeletal muscle with low DMPK protein content in the heart were found. In skeletal muscle of DM1 patients, a decrease in DMPK concentration to about 50% was found, and the decrease in protein appeared to be independent of the CTG repeat length. These studies indicate that inadequate haplotype function is a potential mechanism for disease expression, DMPK expression of mutant alleles is greatly reduced, while normal expression of unaffected alleles is greatly reduced. Regarding the expression pattern of DMPK tissue, in situ hybridization analysis showed that DMPK mRNA was expressed in a series of adult mouse tissues, including skeletal muscle, heart, smooth muscle, bone, testis, pituitary, brain, eye, skin, thymus, lung, intestinal epithelium, Cartilage and liver. No DMPK mRNA was detected in the ovary, pancreas or kidney. In developing mouse embryos, DMPK mRNA is detected primarily in the skeletal structure of the lungs and intestines, all major muscles of the heart muscle, muscle, and smooth muscle.
Structure and activity of DMPK
The phylogenetic tree containing DMPK also includes ROCK (also known as Rho kinase), which was originally described as mediating the formation of myosin light chain (MLC) through the action of stress fibers and focal plaques induced by RhoA. Cell division control protein 42 (Cdc42) binding kinase (MRCK) and grapefruit kinase are related to DMPK. Typically, these kinases consist of an amino-terminal kinase domain followed by a potential coiled-coil-forming region and another functional motif at the carboxy terminus.
DMPK targets
DMPK preferably has an arginine residue (serine or threonine) upstream of the phosphate receptor site, followed by a hydrophobic residue (leucine or valine), and then another peptide base of arginine Thing. This consensus motif -RxxS/TL/VR- is similar to PKC, which preferably has upstream and downstream basic residues, and similarities to CaMK II and phosphorylase kinase, preferably downstream hydrophobic residues. Therefore, the DMPK phosphorylation motif appears to overlap, but differs from the previously described kinase classes. Screening a library of about 35 synthetic peptides showed that the optimal DMPK substrate sequence should consist of 3 to 4 arginine (or lysine) residues at different positions on the N-terminus of the phosphate receptor. Both studies have identified the smallest DMPK substrate consensus motif with a positively charged amino acid at the -2 position of the Ser/Thr phosphate receptor and a hydrophobic residue at the +1 position.
DMPK functions
1. Skeletal muscle integrity
DMPK is mainly expressed in skeletal muscle and myocardium. In skeletal muscle, the DMPK gene contains a low-level promoter that works with enhancer elements in the first intron with a conserved MyoD-responsive E-box. In L6E9 and C2C12 muscle cells, the expression of DMPK is upregulated through typical myogenic pathways (ie, phosphatidylinositol 3 kinase, nuclear factor B, nitric oxide synthase, and p38 mitogen-activated protein kinase), thereby supporting Functional significance of DMPK.
2. Myosin phosphatase target subunit
DMPK phosphorylates the C-terminal domain of myosin phosphatase target subunit (MYPT1) in vitro, in a similar way to Rho-kinase, and this phosphorylation inhibits the phosphatase activity of MYPT1. A consequence of myosin phosphatase inhibition is to increase the level of myosin light chain phosphorylation, which leads to Ca2+ sensitization in smooth muscle and cytoskeletal rearrangements (increase in stress fibers and focal adhesions) in non-muscle cells. Further studies on MYPT1 phosphorylation status and activity in DMPK knockout and DMPK overexpressing mice may help us to understand the role of DMPK in cytoskeletal reorganization.
3. Cell metabolism
DMPK is highly expressed in muscle, which is a key target tissue for insulin-dependent regulation of glucose metabolism [68]. We have recently shown that dmpk−/− mice exhibit decreased insulin sensitivity in cardiac and skeletal muscles but normal insulin signaling in adipose tissue and liver, in which DMPK is not detected [69]. Dmpk−/− mice show impaired insulin-induced glucose uptake and GLUT4 translocation in muscle. Metabolic alteration in dmpk−/− mice is reflected in glucose intolerance and increased circulating insulin and lipid levels in fed mice. As observed in DM1 patients, dmpk−/− mice show higher concentrations of plasma insulin than wild-type mice in glucose tolerance tests.
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