A comparison of the bone and growth phenotype of mdx, mdx:Cmah-/- and mdx:Utrn +/- murine models with the C57BL/10 wild-type mouse

The muscular dystrophy X-linked (mdx) mouse is commonly used as a mouse model of Duchenne muscular dystrophy (DMD). Its phenotype is, however, mild, and other mouse models have been explored. The mdx:Cmah^−/− mouse carries a human-like mutation in the Cmah gene and has a severe muscle phenotype, but its growth and bone development are unknown. In this study, we compared male mdx, mdx:Utrn^+/−, mdx:Cmah^−/− and wild-type (WT) mice at 3, 5 and 7 weeks of age to determine the suitability of the mdx:Cmah^−/− mouse as a model for assessing growth and skeletal development in DMD. The mdx:Cmah^−/− mice were lighter than WT mice at 3 weeks, but heavier at 7 weeks, and showed an increased growth rate at 5 weeks. Cortical bone fraction as assessed by micro-computed tomography was greater in both mdx and mdx:Cmah^−/− mice versus WT mice at 7 weeks. Tissue mineral density was also higher in mdx:Cmah^−/− mice at 3 and 7 weeks. Gene profiling of mdx:Cmah^−/− bone identified increased expression of Igf1, Igf1r and Vegfa. Both the mdx and mdx:Cmah^−/− mice showed an increased proportion of regulated bone marrow adipose tissue (BMAT) but a reduction in constitutive BMAT. The mdx:Cmah^−/− mice show evidence of catch-up growth and more rapid bone development. This pattern does not mimic the typical DMD growth trajectory and therefore the utility of the mdx:Cmah^−/− mouse for studying growth and skeletal development in DMD is limited. Further studies of this model may, however, shed light on the phenomenon of catch-up growth.

This article has an associated First Person interview with the first author of the paper.

Summary: Unlike boys with DMD, the mdx:Cmah^−/− mouse shows increased weight gain and more rapid bone development; therefore, its utility for studying growth and skeletal development in DMD is limited.

Tumour necrosis factor-alpha inhibits adipogenesis via a beta-catenin/TCF4(TCF7L2)-dependent pathway

Tumour necrosis factor-alpha (TNF-alpha), a proinflammatory cytokine, is a potent negative regulator of adipocyte differentiation. However, the mechanism of TNF-alpha-mediated antiadipogenesis remains incompletely understood. In this study, we first confirm that TNF-alpha inhibits adipogenesis of 3T3-L1 preadipocytes by preventing the early induction of the adipogenic transcription factors peroxisome proliferator-activated receptor-gamma (PPARgamma) and CCAAT/enhancer binding protein-alpha (C/EBPalpha). This suppression coincides with enhanced expression of several reported mediators of antiadipogenesis that are also targets of the Wnt/beta-catenin/T-cell factor 4 (TCF4) pathway. Indeed, we found that TNF-alpha enhanced TCF4-dependent transcriptional activity during early antiadipogenesis, and promoted the stabilisation of beta-catenin throughout antiadipogenesis. We analysed the effect of TNF-alpha on adipogenesis in 3T3-L1 cells in which beta-catenin/TCF signalling was impaired, either via stable knockdown of beta-catenin, or by overexpression of dominant-negative TCF4 (dnTCF4). The knockdown of beta-catenin enhanced the adipogenic potential of 3T3-L1 preadipocytes and attenuated TNF-alpha-induced antiadipogenesis. However, beta-catenin knockdown also promoted TNF-alpha-induced apoptosis in these cells. In contrast, overexpression of dnTCF4 prevented TNF-alpha-induced antiadipogenesis but showed no apparent effect on cell survival. Finally, we show that TNF-alpha-induced antiadipogenesis and stabilisation of beta-catenin requires a functional death domain of TNF-alpha receptor 1 (TNFR1). Taken together these data suggest that TNFR1-mediated death domain signals can inhibit adipogenesis via a beta-catenin/TCF4-dependent pathway.

IGF-binding protein-2 protects against the development of obesity and insulin resistance

Proliferation of adipocyte precursors and their differentiation into mature adipocytes contributes to the development of obesity in mammals. IGF-I is a potent mitogen and important stimulus for adipocyte differentiation. The biological actions of IGFs are closely regulated by a family of IGF-binding proteins (IGFBPs), which exert predominantly inhibitory effects. IGFBP-2 is the principal binding protein secreted by differentiating white preadipocytes, suggesting a potential role in the development of obesity. We have generated transgenic mice overexpressing human IGFBP-2 under the control of its native promoter, and we show that overexpression of IGFBP-2 is associated with reduced susceptibility to obesity and improved insulin sensitivity. Whereas wild-type littermates developed glucose intolerance and increased blood pressure with aging, mice overexpressing IGFBP-2 were protected. Furthermore, when fed a high-fat/high-energy diet, IGFBP-2-overexpressing mice were resistant to the development of obesity and insulin resistance. This lean phenotype was associated with decreased leptin levels, increased glucose sensitivity, and lower blood pressure compared with wild-type animals consuming similar amounts of high-fat diet. Our in vitro data suggest a direct effect of IGFBP-2 preventing adipogenesis as indicated by the ability of recombinant IGFBP-2 to impair 3T3-L1 differentiation. These findings suggest an important, novel role for IGFBP-2 in obesity prevention.

The Wnt antagonist Dickkopf-1 and its receptors are coordinately regulated during early human adipogenesis

Secretion of Wnts by adipose cells has an important role in the control of murine adipogenesis. We present the first evidence that a Wnt antagonist, Dickkopf 1 (Dkk1), is secreted by human preadipocytes and promotes adipogenesis. DKK1 mRNA increases six hours after onset of human adipogenesis and this is followed by an increase in Dkk1 protein. With further differentiation, the mRNA and protein levels progressively decline such that they are undetectable in mature adipocytes. The transient induction in DKK1 correlates with downregulation of cytoplasmic and nuclear beta-catenin levels, this being a surrogate marker of canonical Wnt signalling, and Wnt/beta-catenin transcriptional activity. In addition, constitutive expression of Dkk1 in 3T3-L1 preadipocytes promotes their differentiation, further supporting the functional significance of increased Dkk1 levels during human adipogenesis. Concomitant downregulation of the Dkk1 receptors LRP5 and LRP6 is likely to potentiate the ability of Dkk1 to inhibit Wnt signalling and promote differentiation. Notably, Dkk1 is not expressed in primary murine preadipocytes or cell lines. The involvement of Dkk1 in human but not murine adipogenesis indicates that inter-species differences exist in the molecular control of this process. Given the public health importance of disorders of adipose mass, further knowledge of the pathways involved specifically in human adipocyte differentiation might ultimately be of clinical relevance.