Differential processes of vascular smooth muscle cell differentiation within elastic and muscular arteries of rats and rabbits: an immunofluorescence study of desmin and vimentin distribution

Tripartite factors leading to molecular divergence between human and murine smooth muscle

A large number of pre-clinical and developmental investigations involve experimental vertebrate animals, of which mice have emerged as a favored organism. Recognition of the differences between humans and mice is essential for assessment of the relevance of animal studies to humans. The primary purpose of this study was to gauge the conservation between human and mouse vascular smooth muscle cell (VSMC) proteins mined from an analysis of the Human Protein Atlas. Two comparison were made: a) immunohistochemistry for 16 proteins in brain, heart, esophagus, bladder, stomach, lung, kidney, and aorta enabled comparison between human and mouse of protein localization in VSMC and non-vascular SMC; and b) multi-species primary protein sequence analysis of an expanded set vascular molecules enabled comparison between VSMC sequences among vertebrate species. In total, three dimensions of diversity were uncovered. First, a significant number of factors show human/mouse differences in cellular expression; these differences occurred in both VSMC and non-vascular SMC in an organ and cell-type dependent fashion. Many markers demonstrated notable cell-to-cell and regional heterogeneity in VSMC of the aorta and non-vascular SMC of the esophagus, bladder, and stomach. Second, species specificity can arise by genetic deletions as exemplified by the human protein adipogenesis regulatory factor (ADIRF), which is not present due to a large sequence gap in mice. Third, we describe significant cross-species protein sequence divergence in selected VSMC proteins which may result in altered orthologue function. In a sample of 346 vascular molecules, 15% demonstrate incomplete vertebrate species gene conservation. Divergence of predicted human/mouse VSMC protein sequences is higher than for endothelial proteins in all species examined. In the future, each of these three cross-species differences could be neutralized using gene manipulation, resulting in improved translational potential of murine experimental models.

An update on clonality: what smooth muscle cell type makes up the atherosclerotic plaque?

Almost 50 years ago, Earl Benditt and his son John described the clonality of the atherosclerotic plaque. This led Benditt to propose that the atherosclerotic lesion was a smooth muscle neoplasm, similar to the leiomyomata seen in the uterus of most women. Although the observation of clonality has been confirmed many times, interest in the idea that atherosclerosis might be a form of neoplasia waned because of the clinical success of treatments for hyperlipemia and because animal models have made great progress in understanding how lipid accumulates in the plaque and may lead to plaque rupture. Four advances have made it important to reconsider Benditt's observations. First, we now know that clonality is a property of normal tissue development. Second, this is even true in the vessel wall, where we now know that formation of clonal patches in that wall is part of the development of smooth muscle cells that make up the tunica media of arteries. Third, we know that the intima, the "soil" for development of the human atherosclerotic lesion, develops before the fatty lesions appear. Fourth, while the cells comprising this intima have been called "smooth muscle cells", we do not have a clear definition of cell type nor do we know if the initial accumulation is clonal. As a result, Benditt's hypothesis needs to be revisited in terms of changes in how we define smooth muscle cells and the quite distinct developmental origins of the cells that comprise the muscular coats of all arterial walls. Finally, since clonality of the lesions is real, the obvious questions are do these human tumors precede the development of atherosclerosis, how do the clones develop, what cell type gives rise to the clones, and in what ways do the clones provide the soil for development and natural history of atherosclerosis?

A systematic review: differentiation of stem cells into functional pericytes

Pericytes are an integral cellular component of vascular structures. Numerous studies have investigated various stem cell types as potential sources of pericytes for application in cell-based therapy. The diverse stem cell types and variable experimental protocols of these studies make it imperative to evaluate the relevant scientific literature on the basis of a unified standard. The purpose of this systematic review is to rigorously evaluate the relevant scientific literature for conclusive evidence that stem cells can differentiate into functional pericytes. An online literature search was conducted up to July 2016. Eligible papers were evaluated on 4 pertinent criteria: 1) appropriate controls, 2) markers to confirm pericyte phenotype, 3) techniques for assessing pericyte functionality, and 4) differentiation efficiency of the protocol. Our search yielded 20 eligible studies (from 2006 to 2016), 12 of which were published in the past 5 yr. Of these 20 articles, only 1 had positive control, and 5 papers evaluated differentiation efficiency. The most commonly used pericyte markers were neuron-glial antigen 2, platelet-derived growth factor receptor-β, and α-smooth muscle actin. Three articles were associated with adipose stem cells, 4 with mesenchymal stem cells, and 7 with pluripotent stem cells, whereas the remaining 6 articles were based on other miscellaneous stem cell types. Stem cells can serve as a potential source of pericytes, but there should be standardized guidelines in future studies for assessing pericyte differentiation.-Xu, J., Gong, T., Heng, B. C., Zhang, C. F. A systematic review: differentiation of stem cells into functional pericytes.

Sympathectomy Causes Aggravated Lesions and Dedifferentiation in Large Rabbit Atherosclerotic Arteries without Involving Nitric Oxide

Previously [Histochem J 1997;29:279-286], we found that sympathectomy induced neointima formation in ear but not cerebral arteries of genetically hyperlipidemic rabbits. To clarify the influence of sympathetic nerves in atherosclerosis, and whether their influence involves vascular NO activity, we studied groups of normocholesterolemic intact (NI) and sympathectomized (NS), and hypercholesterolemic intact (HI) and sympathectomized (HS) rabbits (diet/6-hydroxydopamine for 79 days). Segments of basilar (BA) and femoral (FA) arteries were studied histochemically, to evaluate differentiation (anti-desmin, anti-vimentin, anti-h-caldesmon, and nuclear dye), by confocal microscopy, and by in vitro myography. In BAs, staining of NI and NS groups was similar. In hypercholesterolemic groups, a small neointima developed, more frequently in HS segments where smooth muscle cells (SMCs) positive for all antibodies appeared to be migrating into the neointima. In FAs, SMCs stained for the three antibodies in the NI group, but we observed desmin- and h-caldesmon-negative, vimentin-positive cells in some external medial layers of the NS, HI and HS groups, identical to adventitial fibroblasts. Large neointimas of the HS group contained vimentin-positive and largely desmin- and h-caldesmon-negative cells. Relaxation of BA or FA segments to acetylcholine was not decreased by sympathectomy. Sympathectomy increased the contraction of resting FAs to nitro-L-arginine (p = 0.0379). Thus, sympathectomy aggravates the tendency for FA SMCs to migrate and dedifferentiate, increasing atherosclerotic lesions, without decreasing NO activity, but has only minor effects on BAs.

Differing influence of sympathectomy on smooth muscle cells and fibroblasts in cerebral and peripheral muscular arteries

In the present study, we examined the effect of sympathectomy on the distribution and the relative expression of cytoskeletal proteins used as markers of phenotypic modulation of vascular smooth muscle cells (SMCs) and myofibroblasts (MFBs) in rabbit femoral (FA) and basilar (BA) arteries. Adult rabbits were treated either with repeated 6-hydroxydopamine (6-OHDA) for sympathectomy or with vehicle for control. Cross sections taken from sympathectomized and control arteries 79 days later were immunolabelled for vimentin, desmin, alpha-smooth muscle actin (alpha-SM actin), beta-isoform of actin and h-caldesmon. The distribution of these proteins and the intensity of fluorescent labelled SMCs were examined under a confocal microscope. In the sympathectomized BA, there was no change for desmin, vimentin and h-caldesmon expression, but the expression of both alpha-SM actin and the beta-isoform was significantly higher (+19% and +30%, respectively). In the sympathectomized FA, the expression of the alpha- and beta-isoforms of actin remained unchanged, whereas those of desmin and vimentin were significantly higher (+35% and 17%, respectively) and h-caldesmon expression was lowered by 13%. In contrast to intact FAs, the external layers of sympathectomized FAs revealed migration of fibroblasts from the adventitia and death of SMCs. These results strongly suggest that sympathetic nerves intervene in the cytoskeletal protein remodelling through phenotypic modulation of both SMCs and MFBs during post-natal development, and in pathologies involving similar phenomena, such as atherosclerosis.

Desmin ensheathment ratio as an indicator of vessel stability: evidence in normal development and in retinopathy of prematurity

We developed a measure of pericyte/endothelial interaction, the desmin ensheathment ratio (DER), using the intermediate filament desmin as an indicator of pericyte ensheathment and have examined the DER in normal retinal vascular development and in the kitten retinopathy of prematurity (ROP) model. We also examined the role of mural cells in the pathogenesis of ROP. Postnatal day 1 to 45 kitten retinae were labeled for desmin, alpha-smooth muscle actin (SMA), and isolectin-B4. Newborn kittens exposed to hyperoxia and then returned to room air for 0 to 40 days (dRA) were similarly labeled. The ratio of desmin to lectin labeling on confocal images yielded the DER. Ultrastructural studies showed that mural cells were present on even the most primitive vessels. During normal development, immature vascular beds had DERs of 0.3 to 0.6 whereas mature beds, which predominated by postnatal day 28, had DERs greater than 0.9. Immature pericytes and smooth muscle cells did not prevent hyperoxia-induced vessel regression. During the vasoproliferative stage of ROP, the DERs of intra- and preretinal vessels ranged between 0.2 and 0.5. In the recovery stage, the DER increased in parallel with regression of pathology, reaching 0.9 at 34 dRA. Stabilization of the DER by the fifth postnatal week was temporally coincident with the development of resistance to hyperoxia-induced vessel regression previously reported in the kitten. These observations lead us to suggest that a DER of 0.9 represents a vascular stability threshold and that a low DER observed during ROP raises the possibility that mural cell abnormalities play a key role in the pathogenesis of ROP.

Confocal microscopic evidence of decreased alpha-actin expression within rabbit cerebral artery smooth muscle cells after subarachnoid haemorrhage

Our objective was to determine whether subarachnoid haemorrhage modifies cerebral artery smooth muscle cell phenotype and the contractile protein alpha-actin measured 7 days after haemorrhage. We used a rabbit subarachnoid haemorrhage model and immunofluorescence labelling of alpha-smooth muscle actin, vimentin and desmin. The paired comparison between the haemorrhage and sham rabbits was performed using confocal laser-scanning microscopy. We found in the haemorrhage group significantly less intense alpha-actin immunostaining (p = 0.036) and more intense vimentin immunostaining (p = 0.043) but no significant change in the intensity of desmin staining. Our results indicate an absolute decrease after subarachnoid haemorrhage in the amount of functional alpha-actin and in the light of the literature may suggest a certain degree of dedifferentiation of smooth muscle cells in the cerebral artery wall.

Regional differentiation of desmin, connexin43, and connexin45 expression patterns in rat aortic smooth muscle

The gap-junctional protein, connexin43, is differentially expressed in vascular smooth muscle cells (SMCs) according to phenotype. Previous studies suggest that desmin-negative SMCs are characterized by high levels of connexin43, whereas desmin-positive SMCs (of a more contractile phenotype) typically have low connexin43 levels. In this study, we examine systematically the inverse relationship between connexin43 and desmin in SMCs of defined regions of the rat aortic media and determine whether additional connexin isotypes are expressed and contribute to this relationship. Immunoconfocal microscopy demonstrated that (1) the inverse relationship between connexin43 and desmin expression holds true for the media of sequential aortic zones, with 1 exception, the ascending aorta, and (2) an additional vascular connexin, connexin45, is expressed by aortic SMCs. Examination of connexin43, connexin45, and desmin expression in sequential aortic zones reveals 3 SMC subpopulations. The first, predominating in the aortic arch and thoracic aorta, is desmin negative and contains high connexin43 levels; the second, predominating in the abdominal aorta and iliac artery, is desmin positive and contains low connexin43 levels; and the third, which is restricted to the ascending aorta, is desmin positive and expresses high connexin43 levels. Connexin45 levels are high in the ascending aorta but low in the other aortic segments. In para-aortic veins, a fourth SMC subpopulation appears, one that is desmin positive and contains connexin45 but not connexin43. These results demonstrate that a diversity of connexin expression patterns characterizes distinctive subpopulations of medial SMCs in situ with a potential to contribute to regional differentiation of vascular function.

Contractile and cytoskeletal proteins of smooth muscle cells in rat, rabbit, and human arteries

The aim of this study was to determine whether similar populations of smooth muscle cells, in relation to contractile and cytoskeletal proteins, are present in normal and diseased human coronary arteries and normal and injured rat and rabbit arteries. Rat aortae and rabbit carotid arteries were de-endothelialised and the resulting neointimal thickening examined at set time points 2-24 weeks later. Immunohistochemistry revealed that arteries had three distinct populations of cells in respect to alpha-smooth muscle actin, smooth muscle myosin heavy chain and vimentin (staining intensities '-', '+' or '++' for each protein), but only two populations in respect to desmin ('-' and '+'). The different populations of cells were found in the neointima at all times after injury, in human atherosclerotic plaque and in the media of diseased, injured and uninjured vessels, although in different proportions. It was concluded that arteries of the human, rat and rabbit have cells with a wide spectrum of contractile and cytoskeletal proteins. Expression of the different proteins did not reflect the state of the artery after injury or during the disease process, and was not associated with the expansion of a subset of cells within the artery wall.

Inverse relationship between connexin43 and desmin expression in cultured porcine aortic smooth muscle cells

Our previous work has shown that in vascular tissues the elastic medial regions express high levels of the gap junctional protein, connexin43, but low levels of desmin, while the muscular medial regions express low levels of connexin43 but high levels of desmin. It is uncertain, however, whether this regional difference at the tissue level extends down to the level of the individual cell, or reflects an averaged relationship of groups of cells of different connexin43 and desmin expression. The present study has addressed this question using cultured porcine aortic smooth muscle cells. Immunoconfocal microscopic analysis of single-labeled cells showed that while smooth muscle alpha-actin, calponin and vimentin were positively labeled in the majority of medial smooth muscle cells both in intact porcine aorta and corresponding cultured cells, desmin and connexin43 labeling was highly heterogeneous. In the cultured cells, 0.3-0.5% of cells were found to be desmin-positive, and quantitative analysis after double labeling for desmin and connexin43 revealed that the desmin-positive cells were smaller, and contained significantly lower numbers and smaller sizes of connexin43 gap-junctional spots than did desmin-negative cells. Our findings demonstrate that an inverse expression pattern of connexin43 and desmin holds true at the level of the individual cell. This suggests a close relationship between intrinsic phenotypic control and the regulation of connexin43 expression in the arterial smooth muscle cell.

Differential expression of connexin43 and desmin defines two subpopulations of medial smooth muscle cells in the human internal mammary artery

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 alpha-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.