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Defining smooth muscle cells and smooth muscle injury
William M. Mahoney Jr. and Stephen M. Schwartz
Department of Pathology, University of Washington, Seattle, Washington, USA.
Address correspondence to: Stephen M. Schwartz, University of Washington, Department of Pathology, Box 357335, HSB Rm. 1-416, 1959 NE Pacific Street, Seattle, Washington 98195-7335, USA. Phone: (206) 543-0258; Fax: (206) 543-5657; E-mail: steves@u.washington.edu.
Find articles by Mahoney, W. in: JCI | PubMed | Google Scholar
Department of Pathology, University of Washington, Seattle, Washington, USA.
Address correspondence to: Stephen M. Schwartz, University of Washington, Department of Pathology, Box 357335, HSB Rm. 1-416, 1959 NE Pacific Street, Seattle, Washington 98195-7335, USA. Phone: (206) 543-0258; Fax: (206) 543-5657; E-mail: steves@u.washington.edu.
Find articles by Schwartz, S. in: JCI | PubMed | Google Scholar
Published February 1, 2005 - More info
J Clin Invest. 2005;115(2):221–224. https://doi.org/10.1172/JCI24272.
© 2005 The American Society for Clinical Investigation
Efforts to understand the response of arterial smooth muscle cells (SMCs) to injury have led to confusion, in part because of the as-yet-unconfirmed implication that terms such as dedifferentiation, synthetic phenotype, and phenotypic modulation refer to a specific, common mechanism. This issue of the JCI brings a major new perspective to this subject with a report by Hendrix et al. (1) that builds upon their recent findings (2).
Since the 1970s, most investigators assumed that the loss of properties (i.e., the loss of contractile capacity and the appearance of proteins associated with the extracelluar matrix) observed when SMCs adapted to culture used the same mechanisms required for the response of arterial SMCs to vascular injury, sometimes termed phenotypic modulation (3, 4). However, this theory of a common mechanism underlying the response of SMCs to multiple forms of injury in vivo is largely unsubstantiated. Since the molecular mechanism controlling SMC differentiation and modulation in vivo was poorly understood, Hendrix et al. (1) examined the molecular control of smooth muscle (SM) α-actin expression in response to injury. Their earlier studies showed that expression of SM α-actin is regulated by promoter elements called CArG [CC(A/T)6GG] boxes, which are bound by serum response factor (SRF) either alone or as a macromolecular complex including its specific cofactor, myocardin. Interestingly, cytoskeletal modulation regulates the SRF-myocardin interaction. Myocardin is bound by G-actin in the cytoplasm, and polymerization of actin releases myocardin, allowing it to travel to the nucleus. Once in the nucleus, it acts as a cofactor to enhance the binding of SRF to genes associated with cell replication and the dissociation from genes associated with SMC contractile proteins (5, 6). Therefore, there is a delicate balance between SMCs’ need to respond to various stimuli and the availability of proteins to mediate these processes.
In the study by Hendrix et al. (1), transgenic mice with mutated CArG boxes were created as described in Figure 1. The effects of these mutations were assayed both in vitro and in vivo following injury. Hendrix et al. show that generic CArG binding sites are sufficient for regulation of SM α-actin transcription in vitro. However, SMC-specific, degenerate CArG sequences (Figure 1) are required for appropriate repression of SM α-actin following injury in vivo. Furthermore, the relative levels of myocardin control the interaction between SRF and the degenerate CArG boxes within the SM α-actin promoter. Thus, in vitro dedifferentiation is an imperfect “common” model for SMC response to injury in vivo.
Figure 1Regulation of CArG-specific SMC genes. (A) CArG box elements in the SM α-actin promoter. The SM α-actin promoter contains 2 consensus CArG boxes [CC(A/T)6GG]. However, the wild-type sequences are degenerate relative to the canonical sequence first identified in c-fos (CCATATTAGG). The positions of the specific nucleotide modulations are noted by underline. The other CArG-regulated SMC genes also contain multiple and degenerate CArG box elements. The number of consensus CArG boxes as well as their specific sequences varies from gene to gene. In addition, there are a number of SMC genes that do not contain CArG sequences, which suggests yet another mechanism for controlling gene expression in a gene- and stimulus-specific manner. (B) Molecular control of CArG-specific SMC genes. CArG boxes are bound by SRF. This protein-DNA complex is further regulated through interactions between SRF and specialized cofactors, including myocardin. Ultimately, CArG-specific SMC gene expression is controlled through these macromolecular complexes, which allows the SMC to respond to various stimuli, including injury.
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Can we define SMCs? Is there a single pattern for SMC response to injury?
We already know that, even with no injury, there is great variation in gene expression in various forms of SMCs (Figure 2). For example, SM-memb, a myosin often considered characteristic of all intimal cells, is present in normal intima and in many intimas formed as a result of injury but is absent from the atherosclerotic fibrous cap (9). Similarly, cells making up the fibrous cap have very little mRNA, while cells at the edges of atherosclerotic plaques have abundant mRNA, and these messages code for proteins responsible for matrix synthesis. Finally, microarrays show that the SMCs of the atherosclerotic cap have a phenotype distinct from both nonatherosclerotic intima and the phenotype of medial SMCs (10).
Figure 2The diversity of vascular SM. Within the vasculature, the term smooth muscle cell is used to include any connective tissue cell that forms a coating around the endothelial tubes. These cells may have many different phenotypes, ranging from the typical muscular artery SMC, characterized by a dense filamentous network made of SMC-specific proteins, to cells with much less definitive phenotypes, such as the glomerular mesangial cell and the intralaminar cell of the internal mammary artery, which look more like fibrocytes and lack SMC-specific proteins. Recently, we have begun to realize that SM-like cells may even arise from endothelial cells or circulating precursors. The diversity of the promoter structure described in Figure 1, as well as the presence of non-CArG box promoters in other SMC-restricted genes, may reflect the diverse responses to injury required of the cells making up the vessel wall.
Medial cells of the pulmonary artery fulfill the morphological criteria of SMCs: they are full of contractile apparatus (Figure 2). These cells, however, fail to express classical SMC genes (11). In contrast, in pulmonary hypertension, adventitial fibroblasts have been shown to express the classical SMC genes (12). Similarly, contractile myofibroblasts present in wounds express SM proteins (13). Even endothelial cells are able to delaminate from the monolayer, transdifferentiate, and express at least part of the SMC repertoire of contractile proteins, including SM α-actin (14).
Perhaps the most confusing example of identification of a cell as SM arises in the vasculopathy characteristic of organ transplants. When the abdominal aorta is transplanted across immunological barriers, the donor cells die and are entirely replaced by cells from the host. These precursors of these newly formed SMCs appear to derive from circulating stem cells or they may derive from adventitial fibroblasts (15–18). Recent studies claim that some or all of the cells in the neointima formed by mechanical injury or atherosclerosis also arise outside of the vessel wall (15).
Unfortunately, the answer to the question of identification of a cell as an SMC remains a semantic rather than an objective decision in most cases. It is preferable, in our opinion, to describe a cell’s properties rather than to arbitrarily decide that a cell is or is not an SMC based on one or even a few molecular markers.
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