Differential Expression of Connexin43 and Desmin Defines Two Subpopulations of Medial Smooth Muscle Cells in the Human Internal Mammary Artery

Abstract

Abstract-Upregulation of connexin43-gap junctions is associated with transition of contractile vascular smooth muscle cells (SMCs) to the synthetic state. To determine whether phenotypically distinct subpopulations of medial SMCs differentially express connexin43, we investigated the human distal internal mammary artery, a structurally heterogeneous vessel with features ranging from elastic to elastomuscular to muscular. Immunoconfocal microscopy combined with quantitative analysis and complemented by in situ hybridization showed that SMCs in the elastic medial regions expressed high levels of connexin43 but low levels of desmin, whereas those of muscular medial regions expressed low levels of connexin43 but high levels of desmin. Ultrastructurally, SMCs of both regions were of the contractile phenotype, but the former cells were irregular in shape with relatively prominent synthetic organelles whereas the latter were spindle shaped with fewer synthetic organelles. Vimentin, smooth muscle ␣-actin, calponin, h-caldesmon, and myosin heavy chains (SM1 and SM2) were equally highly expressed by most cells in both subpopulations. The connexin43/desmin expression pattern of SMCs in regions of intimal thickening resembled those of elastic medial regions. These findings refine the view suggested from previous studies that high levels of connexin43 expression are associated with SMCs of a less contractile/more synthetic phenotype. In the internal mammary artery, the 2 subpopulations of SMCs with markedly different connexin43 expression levels both represent a differentiated contractile phenotype, but the subpopulation showing high levels of connexin43-gap junctions is characterized by low levels of desmin and structural features that reflect a more synthetic tendency.

Innate Diversity of Adult Human Arterial Smooth Muscle Cells

“…The finding of 2 SMC types that modestly differ in expression of differentiation markers is noteworthy given a recent description of 2 SMC phenotypes evident in the media of the human internal thoracic artery differing in expression of desmin and connexin43, but not by the stainable presence of 5 other SMC-associated proteins including SM MHC. 13 Although such subtle distinctions in expression of SMC differentiation markers could be caused by variations in the local microenvironment, the present results indicate that they can also reflect intrinsic differences between SMC subtypes.…”

“…[15][16][17] Unfortunately, the extent to which the concepts arising from animal studies can be reliably extrapolated to human arterial behavior is limited. Although there is a small amount of in situ evidence for medial SMC diversity in humans, 11,13 there has been little success at isolating stable and clearly distinguishable subpopulations of SMCs from adult human arteries. The difficulty in cloning human SMCs is well recognized and illustrated by a report of cloning that yielded anchorage-independent cell colonies, implying a loss of physiological growth control as a precursor to cloning.…”

“…In this regard, it is noteworthy that SMC heterogeneity in the media of the internal thoracic artery has recently been documented, based on the distribution of desmin and connexin43. 13 However, it is not known if the observed diversity was because of inherently distinct SMC subsets or because of regional variations in the environmental milieu, nor if the SMC differences observed in situ had functional consequences.…”

Reduced Connexin43 Expression Limits Neointima Formation After Balloon Distension Injury in Hypercholesterolemic Mice

“…14 Because macrophages and other leukocytes account for only a small percentage (Յ1%) of the total cell number in the neointima of balloon-injured rat carotid arteries, 15 the authors proposed that Cx43 communication enabled SMCs to coordinate synthesis of extracellular matrix components, to maintain the functional integrity of the media, and/or to regulate repair and neointimal formation in injured vessels. 16 Enhanced Cx43 expression in vascular SMCs and neointimal macrophages was reported later in restenotic lesions of injured rabbits. 17,18 Indeed, in vitro studies confirmed the presence of Cx43 in stimulated primary macrophages or in macrophage cell lines.…”

Regulation and characteristics of vascular smooth muscle cell phenotypic diversity

“…17 The variations between SMCs within a particular vessel, both with respect to marker gene expression and functional and morphological characteristics, suggest that there may be a genetic basis for SMC diversity. After all, such SMCs share similar embryological origins and experience comparable local conditions.…”