Conditional inactivation of TGF-β type II receptor in smooth muscle cells and epicardium causes lethal aortic and cardiac defects. Transgenic Res. 2010;19:1069-1082. [PMID: 20213136 DOI: 10.1007/s11248-010-9379-4]

Transforming growth factor-β and smooth muscle differentiation

Transforming growth factor (TGF)-β family members are multifunctional cytokines regulating diverse cellular functions such as growth, adhesion, migration, apoptosis, and differentiation. TGF-βs elicit their effects via specific type I and type II serine/threonine kinase receptors and intracellular Smad transcription factors. Knockout mouse models for the different components of the TGF-β signaling pathway have revealed their critical roles in smooth muscle cell (SMC) differentiation. Genetic studies in humans have linked mutations in these signaling components to specific cardiovascular disorders such as aorta aneurysm and congenital heart diseases due to SMC defects. In this review, the current understanding of TGF-β function in SMC differentiation is highlighted, and the role of TGF-β signaling in SMC-related diseases is discussed.

Signaling during epicardium and coronary vessel development

The epicardium, the tissue layer covering the cardiac muscle (myocardium), develops from the proepicardium, a mass of coelomic progenitors located at the venous pole of the embryonic heart. Proepicardium cells attach to and spread over the myocardium to form the primitive epicardial epithelium. The epicardium subsequently undergoes an epithelial-to-mesenchymal transition to give rise to a population of epicardium-derived cells, which in turn invade the heart and progressively differentiate into various cell types, including cells of coronary blood vessels and cardiac interstitial cells. Epicardial cells and epicardium-derived cells signal to the adjacent cardiac muscle in a paracrine fashion, promoting its proliferation and expansion. Recently, high expectations have been raised about the epicardium as a candidate source of cells for the repair of the damaged heart. Because of its developmental importance and therapeutic potential, current research on this topic focuses on the complex signals that control epicardial biology. This review describes the signaling pathways involved in the different stages of epicardial development and discusses the potential of epicardial signals as targets for the development of therapies to repair the diseased heart.

Transforming growth factor-β signaling in myogenic cells regulates vascular morphogenesis, differentiation, and matrix synthesis

OBJECTIVE

Transforming growth factor-β (TGF-β) signaling is required for normal vascular development. We aimed to discover the role of TGF-β signaling in embryonic smooth muscle cells (SMCs).

METHODS AND RESULTS

We bred mice with smooth muscle (SM) 22α-Cre and Tgfbr2(flox) alleles to generate embryos in which the type II TGF-β receptor (TGFBR2; required for TGF-β signaling) was deleted in SMCs. Embryos were harvested between embryonic day (E) 9.5 and E18.5 and examined grossly, microscopically, and by histochemical and RNA analyses. SM22α-Cre(+/0) Tgfbr2(flox/flox) (knockout [KO]) embryos died before E15.5 with defects that included cardiac outflow tract abnormalities, persistence of the right dorsal aorta, and dilation of the distal aorta. Histological analyses suggested normal expression of SMC differentiation markers in KO aortas; however, RNA analyses showed that SMC differentiation markers were increased in KO cardiac outflow vessels but decreased in the descending aorta. KO aortas had only rare mature elastin deposits and contained abnormal aggregates of extracellular matrix proteins. Expression of several matrix proteins was significantly decreased in KO descending aortas but not in cardiac outflow vessels.

CONCLUSIONS

TGF-β signaling in SMCs controls differentiation, matrix synthesis, and vascular morphogenesis. Effects of TGF-β on SMC gene expression appear to differ depending on the location of SMCs in the aorta.

Transforming growth factor beta signaling in adult cardiovascular diseases and repair

The majority of children with congenital heart disease now live into adulthood due to the remarkable surgical and medical advances that have taken place over the past half century. Because of this, adults now represent the largest age group with adult cardiovascular diseases. It includes patients with heart diseases that were not detected or not treated during childhood, those whose defects were surgically corrected but now need revision due to maladaptive responses to the procedure, those with exercise problems and those with age-related degenerative diseases. Because adult cardiovascular diseases in this population are relatively new, they are not well understood. It is therefore necessary to understand the molecular and physiological pathways involved if we are to improve treatments. Since there is a developmental basis to adult cardiovascular disease, transforming growth factor beta (TGFβ) signaling pathways that are essential for proper cardiovascular development may also play critical roles in the homeostatic, repair and stress response processes involved in adult cardiovascular diseases. Consequently, we have chosen to summarize the current information on a subset of TGFβ ligand and receptor genes and related effector genes that, when dysregulated, are known to lead to cardiovascular diseases and adult cardiovascular deficiencies and/or pathologies. A better understanding of the TGFβ signaling network in cardiovascular disease and repair will impact genetic and physiologic investigations of cardiovascular diseases in elderly patients and lead to an improvement in clinical interventions.

Increased atherosclerosis in mice deficient in perilipin1

Background

Perilipin1, a lipid droplet associated protein has an important role in the regulation of lipolysis and lipid storage in adipocytes. Perilipin1 is also expressed in foam cells of atheroma plaques and could therefore play a role in the accumulation of lipids in arterial wall and in the development of atherosclerosis. The aim of the study was to investigate this possible role of perilipin1 in atherogenesis.

Methods

Mice deficient in perilipin1 (Plin1-/-) were crossed with Ldlr-/- mice. Ldlr-/- and Plin1-/- Ldlr-/- mice received an atherogenic diet during 10 or 20 weeks. Blood pressure and plasma lipids concentrations were measured. Aortas were collected at the end of the atherogenic diet periods for quantification of atheroma lesions (en face method), histological and immunohistological studies

Results

Ldlr-/- and Plin1-/- Ldlr-/- mice had comparable blood pressure and plasma lipids levels. Plin1-/- Ldlr-/- mice had a lower body weight and decreased adiposity. The atherosclerotic lesion area in Plin1-/-Ldlr-/- mice was moderately increased after 10 weeks of atherogenic diet (ns) and significantly higher after 20 weeks (p < 0.01). Histology of atheroma plaques was comparable with no sign of increased inflammation in Plin1-/- Ldlr-/- mice.

Conclusion

Perilipin1 ablation in mice results in increased atherosclerosis independently of modifications of risk factors such as raised blood pressure or plasma lipids levels. These data strongly support an atheroprotective role for perilipin1.

Lessons on the pathogenesis of aneurysm from heritable conditions

Aortic aneurysm is common, accounting for 1-2% of all deaths in industrialized countries. Early theories of the causes of human aneurysm mostly focused on inherited or acquired defects in components of the extracellular matrix in the aorta. Although several mutations in the genes encoding extracellular matrix proteins have been recognized, more recent discoveries have shown important perturbations in cytokine signalling cascades and intracellular components of the smooth muscle contractile apparatus. The modelling of single-gene heritable aneurysm disorders in mice has shown unexpected involvement of the transforming growth factor-β cytokine pathway in aortic aneurysm, highlighting the potential for new therapeutic strategies.

TGFβ signaling and congenital heart disease: Insights from mouse studies

Transforming growth factor β (TGFβ) regulates one of the major signaling pathways that control tissue morphogenesis. In vitro experiments using heart explants indicated the importance of this signaling pathway for the generation of cushion mesenchymal cells, which ultimately contribute to the valves and septa of the mature heart. Recent advances in mouse genetics have enabled in vivo investigation into the roles of individual ligands, receptors, and coreceptors of this pathway, including investigation of the tissue specificity of these roles in heart development. This work has revealed that (1) cushion mesenchyme can form in the absence of TGFβ signaling, although mesenchymal cell numbers may be misregulated; (2) TGFβ signaling is essential for correct remodeling of the cushions, particularly those of the outflow tract; (3) TGFβ signaling also has a role in ensuring accurate remodeling of the pharyngeal arch arteries to form the mature aortic arch; and (4) mesenchymal cells derived from the epicardium require TGFβ signaling to promote their differentiation to vascular smooth muscle cells to support the coronary arteries. In addition, a mouse genetics approach has also been used to investigate the disease pathogenesis of Loeys-Dietz syndrome, a familial autosomal dominant human disorder characterized by a dilated aortic root, and associated with mutations in the two TGFβ signaling receptor genes, TGFBR1 and TGFBR2. Further important insights are likely as this exciting work progresses.

Conditional deletion of Dicer in vascular smooth muscle cells leads to the developmental delay and embryonic mortality

Dicer is a RNAase III enzyme that cleaves double stranded RNA and generates small interfering RNA (siRNA) and microRNA (miRNA). The goal of this study is to examine the role of Dicer and miRNAs in vascular smooth muscle cells (VSMCs). We deleted Dicer in VSMCs of mice, which caused a developmental delay that manifested as early as embryonic day E12.5, leading to embryonic death between E14.5 and E15.5 due to extensive hemorrhage in the liver, brain, and skin. Dicer KO embryos showed dilated blood vessels and a disarray of vascular architecture between E14.5 and E15.5. VSMC proliferation was significantly inhibited in Dicer KOs. The expression of VSMC marker genes were significantly downregulated in Dicer cKO embryos. The vascular structure of the yolk sac and embryo in Dicer KOs was lost to an extent that no blood vessels could be identified after E15.5. Expression of most miRNAs examined was compromised in VSMCs of Dicer KO. Our results indicate that Dicer is required for vascular development and regulates vascular remodeling by modulating VSMC proliferation and differentiation.