The Metabolic Syndrome and Microvascular Complications in a Murine Model of Type 2 Diabetes

To define the components of the metabolic syndrome that contribute to diabetic polyneuropathy (DPN) in type 2 diabetes mellitus (T2DM), we treated the BKS db/db mouse, an established murine model of T2DM and the metabolic syndrome, with the thiazolidinedione class drug pioglitazone. Pioglitazone treatment of BKS db/db mice produced a significant weight gain, restored glycemic control, and normalized measures of serum oxidative stress and triglycerides but had no effect on LDLs or total cholesterol. Moreover, although pioglitazone treatment normalized renal function, it had no effect on measures of large myelinated nerve fibers, specifically sural or sciatic nerve conduction velocities, but significantly improved measures of small unmyelinated nerve fiber architecture and function. Analyses of gene expression arrays of large myelinated sciatic nerves from pioglitazone-treated animals revealed an unanticipated increase in genes related to adipogenesis, adipokine signaling, and lipoprotein signaling, which likely contributed to the blunted therapeutic response. Similar analyses of dorsal root ganglion neurons revealed a salutary effect of pioglitazone on pathways related to defense and cytokine production. These data suggest differential susceptibility of small and large nerve fibers to specific metabolic impairments associated with T2DM and provide the basis for discussion of new treatment paradigms for individuals with T2DM and DPN.     

Desmuslin, an intermediate filament protein that interacts with alpha -dystrobrevin and desmin

Dystrobrevin is a component of the dystrophin-associated protein complex and has been shown to interact directly with dystrophin, alpha1-syntrophin, and the sarcoglycan complex. The precise role of alpha-dystrobrevin in skeletal muscle has not yet been determined. To study alpha-dystrobrevin's function in skeletal muscle, we used the yeast two-hybrid approach to look for interacting proteins. Three overlapping clones were identified that encoded an intermediate filament protein we subsequently named desmuslin (DMN). Sequence analysis revealed that DMN has a short N-terminal domain, a conserved rod domain, and a long C-terminal domain, all common features of type 6 intermediate filament proteins. A positive interaction between DMN and alpha-dystrobrevin was confirmed with an in vitro coimmunoprecipitation assay. By Northern blot analysis, we find that DMN is expressed mainly in heart and skeletal muscle, although there is some expression in brain. Western blotting detected a 160-kDa protein in heart and skeletal muscle. Immunofluorescent microscopy localizes DMN in a stripe-like pattern in longitudinal sections and in a mosaic pattern in cross sections of skeletal muscle. Electron microscopic analysis shows DMN colocalized with desmin at the Z-lines. Subsequent coimmunoprecipitation experiments confirmed an interaction with desmin. Our findings suggest that DMN may serve as a direct linkage between the extracellular matrix and the Z-discs (through plectin) and may play an important role in maintaining muscle cell integrity.     

Radiation heart disease. Analysis of 16 young (aged 15 to 33 years) necropsy patients who received over 3,500 rads to the heart

Certain clinical and necropsy findings are described in 16 young (aged 15 to 33 years) patients who received greater than 3,500 rads to the heart five to 144 months before death. All 16 had some radiation-induced damage to the heart: 15 had thickened pericardia (five of whom had evidence of cardiac tamponade); eight had increased interstitial myocardial fibrosis, particularly in the right ventricle; 12 had fibrous thickening of the mural endocardium and 13 of the valvular endocardium. Except for valvular thickening, the changes were more frequent in the right side of the heart than in the left, presumably because of higher radiation doses to the anterior surface of the heart. In six of the 16 study patients and in one of 10 control subjects, one or more major epicardial coronary arteries were narrowed from 76 to 100 percent in cross-sectional area by atherosclerotic plaque; one patient had a healed myocardial infarct at necropsy and one died suddenly. In 10 patients and in the 10 control subjects, the four major epicardial coronary arteries were examined quantitatively: 6 percent of the 469 five millimeter segments of coronary artery from the patients were narrowed from 76 to 100 percent (controls = 0.2 percent, p = 0.06) and 22 percent were narrowed from 51 to 75 percent (controls = 12 percent). The proximal portion of the arteries in the patients had significantly more narrowing than the distal portions. The arterial plaques in the patients were largely composed of fibrous tissue; the media were frequently replaced by fibrous tissue, and the adventitia were often densely thickened by fibrous tissue. In five patients, there was focal thickening (with or without luminal narrowing) of the intramural coronary arteries. Thus, radiation to the heart may produce a wide spectrum of functional and anatomic changes but particularly damage to the pericardia and the underlying epicardial coronary arteries.     

Insulin-responsive glucose transporter expression in renal microvessels and glomeruli.

The insulin-responsive glucose transporter (GLUT4) is expressed at high levels in fat and skeletal muscle, which account for the majority of insulin-stimulated glucose uptake. However, GLUT4 is also expressed at lower levels in kidney and several other tissues. We have used a variety of protein and mRNA detection techniques to determine the sites of renal GLUT4 expression. Indirect immunofluorescence experiments with two specific anti-peptide antisera detected GLUT4 in the smooth muscle cells of the rat renal microvasculature, in renal glomerulus, and in cultured glomerular mesangial and epithelial cells. PCR amplification of cDNA derived from microdissected renal glomeruli, microvessels and tubules corroborated this distribution of GLUT4, and Northern blotting demonstrated GLUT4 mRNA in cultured glomerular mesangial cells. Both the immunofluorescence and PCR data suggested that GLUT4 is most highly expressed in renal microvessels. Our results show that certain renal cells, such as renal microvascular smooth muscle cells, express the insulin-responsive glucose transporter and therefore may demonstrate altered glucose uptake and metabolism in diabetes mellitus.     

Glucose transporters in diabetic nephropathy

Changes in glucose transporter expression in glomerular cells occur early in diabetes. These changes, especially the GLUT1 increase in mesangial cells, appear to play a pathogenic role in the development of ECM expansion and perhaps other features of diabetic nephropathy. In addition, it appears that at least some diabetic patients may be predisposed to nephropathy because of polymorphisms in their GLUT1 genes. GLUT1 overexpression leads to increased glucose metabolic flux which in turn triggers the polyol pathway and activation of PKC and 1. Activation of these PKC isoforms can lead directly to AP-1 induced increases in fibronectin expression and ECM accumulation. Other, more novel effects of GLUT1 on cellular hypertrophy and injury could also promote changes of diabetic nephropathy. Strategies to prevent GLUT1 overexpression could ameliorate or prevent the progression of diabetic nephropathy.