Abbreviations
Introduction
Cardiovascular disease (CVD) contributes to roughly 10,000 deaths per year in Ireland, 50% of which are directly related to ‘ischemic heart diseases’ according to a Health Service Executive report on CVD, conducted in 2009 (Department of Health and Children, 2010). This subgroup of CVD, more commonly referred to as coronary heart disease includes angina and myocardial infarction, both of which are characterised by the restriction of blood flow and thus oxygen to the myocardium via narrowing of blood vessels, (). Ischemic strokes, which make up over 85% of all incidents of stroke, may occur if the carotid vessels to the brain become narrowed, if an atherosclerotic thrombosis forms in the cerebral arteries themselves or by an embolus, which forms in peripheral arteries and travels to the brain via the bloodstream.
The narrowing of coronary and cerebral vessels can be caused by various mechanisms with primary research focusing on atherosclerosis, a disease led by lipid deposition in the arteries. Atherogenesis begins with damage to the endothelial lining of blood vessels , due to laminar flow being disrupted as discussed by Jeng-Jiann Chiu and Shu Chien in their 2011 paper on the vascular endothelium (). Endothelial dysfunction impacts many endocrine roles of the monolayer outlined in Table 1, but in relation to atherosclerosis development the endothelium damage can allow the intima be infiltrated by low density lipoproteins (LDL) to form the early stage pathogenesis ‘fatty streak’. The NOX family of NADPH membrane-bound enzyme complexes and disruption of the nitric oxide synthase system promote the progression of the atherosclerotic through formation of reactive oxygen species, e.g superoxide O2- which, with other free radicals originating from exogenous sources can oxidise the LDL residing in the intima and blood. Oxidized LDL is cytotoxic and triggers an inflammatory response – the release of classical CD14++CD16- monocyte recruitment chemokines, TNF-alpha and Interleukin 1 into the bloodstream and surrounding tissues. These cytokines also upregulate the protein VCAM-1 on the surface of the endothelial cells, the site at which a receptor/ligand bond forms with Integrin a4beta1 displayed on the monocytes surface. Frostegard et al, in a 1989 experiment showed that oxidized LDL induced monocytes to show a macrophage phenotype in the presence of M-colony stimulating factor (M-CSF). Macrophages phagocytose oxidized LDL via scavenger receptors CD36 and SRA, forming foam cells. The foam cells cause a positive feedback network recruiting more immune cells to the lesion by secreting pro-inflammatory cytokines. As foam cells increase in numbers the plaque progresses, initially causing the media to expand allowing the lumen to say the same width, with blood flow not yet being impeded. Once the blood vessel can accommodate no more expansion the plaque grows into the lumen. Smooth muscle cells (SMC) the dominant cell group in the media migrate from this area to the intima to form a protective layer surrounding the plaque, called a fibrous cap. Clinical events associated with CVD can result if the plaque ruptures to form a clot, while stable blockages may stay asymptomatic. Stable blockages have a much thicker layer of SMC’s and macrophages covering the plaque.
This pathway of atherosclerosis was seen as the major mechanism for artery remodelling and clinical events but it did not answer the question of why seemingly healthy individuals, with low cholesterol levels suffered from cardiac incidents. Atherosclerotic coronary disease is mainly found in older populations, whereas intima media thickening (IMT), as shown in Figure due to non-atherosclerotic coronary disease has been identified in 33% juvenile sudden-cardiac deaths cases during post-mortem. While a 2008 study showed that this non-atherosclerotic form of IMT is consistent with normal processes of aging, it may be accelerated by certain risk factors such as hypertension, injury caused by surgical interventions and hyperglycaemia.
Both pathways cause a decrease in lumen diameter with intimal media thickening playing a role in each, but IMT is of tremendous importance in non-lipid driven arteriosclerosis. Questions arose over the source of the cells which led to the thickened layer of media and intima. This debate of where in the body these cells originated from has spanned many decades, with various proposals being presented as the prime model for non-atherosclerotic vessel remodelling and IMT. Newer evidence showing that the body’s own progenitor cells having a major participatory role in the pathogenesis of atherosclerosis and arthrosclerosis being published in recent years. Currently the understanding is that a relatively newly discovered group of stem cells called ‘Resident Vascular Stem Cells’ exist within the blood vessel and differentiate to smooth muscle like cells.
The timeline of arthrosclerosis research is complex and contains many different viewpoints as newer ideas formed and mechanisms being discovered challenged our previous knowledge, forcing new theories to be hypothesised.
In this review the mechanisms of stem cell differentiation and migration, the impact stem cells may have on disease will be discussed and how traditional therapeutics for CVD may be over taken by stem cell therapies as our understanding of the activation of progenitor cells increases.
Sources of Intima and Medial Thickening Cells; Past Theories
Removing certain lifestyle risk factors will reduce clinical incidences of cardiovascular events, but understanding the mechanisms behind the why, where, and how cells in the blood vessel form neointimal tissue will further develop our understanding and ultimately our therapies for cardiovascular disease.
The original dogma surrounding the origin of cells that contribute to the neointimal area was based on the fact that they were identified as smooth muscle cells (SMC). Vascular smooth muscle cells (VSMC) express specific markers on their surface, such as alpha-actin 2 , myosin heavy chain-11 (My-11), SM22a , smoothelin, h-caldesmon, and calponin, any of which can be tracked when being expressed using immunohisto-chemical techniques and lineage tracing. CRE
All smooth muscle cells express alpha-actin 2 (α-SMA) including the cells within the neointima were positive, leading scientists to believe that these new cells derived from mature smooth muscle. This theory can from cells retained the ability to revert to a more plastic, proliferative phenotype similar to the state they existed in during vasculogenesis. At this stage, My-11+ marked cells differentiated cells should be green
When the SMC dedifferentiated from unipotent cells to a multipotent phenotype the GFP marker was no longer visible as the more specific markers for differentiated SMC’s e.g. My-11 were no longer expressed, as shown in Figure when compared to a control blood vessel.
Inducible marking
Reporter switing – positive cells dedifferentiated green
Red negative for my-11
Results dhouwed that my-11 positive cells dedifferentiate
Brainbow – my-11 inducible cre
Track where the colours go. One population of SMC made the new intima layer. Distinct layer
Atherosclerotic legions marked SMC can dedifferentiate and transdifferentiate to macrophages CD68 labelled yellow
This prevailing theory was challenged with the evidence supporting that progenitor cells in the blood and tissues could differentiate to SMC’s.
Firstly it was believed that a cell type in bone marrow could differentiate to vascular smooth muscle cells after Shimizu et al. published experimental
results in Nature in the year 2001. The bone marrow contains various cell groups including haematopoietic stem cells, endothelial progenitor cells, mesenchymal stem cells.
Scientists noted that transplanted organs and vessels from donor-host transplants (allograft) suffered with major complications from IMT and wanted to investigate the source of these cells. Previous experimental data had been inconclusive on the origin of the cells, but it was assumed that donor tissue contributed, Shimizu with emerging information on bone marrow cells hypothesised that host progenitor cells were involved. It was predicted that haematopoietic stem cells and endothelial stem cells would differentiate to SM-like cells. With the use of transgenic B6 ROSA26 beta- galactosidase bone-marrow mice and an aortic transplant from wild-type mice the results showed positive correlation between smooth muscle cell markers and X-gal reacting with the beta- galactosidase in the neointima cells around the transplant, thus proving their hypothesis. Further transplant experiments were conducted and published in Nature Medicine over the next few months with results which seemed to confirm the idea of heptatonic stem cells (c-Kit+/Sca-1+/lin-), tagged with green florescent protein (GFP) as shown in Figure involvement in arterial remodelling (Sata et al., 2002).
Though other scientists were producing data that was consistent with the original hypothesis, Shimizu noted in his report that the “lack of uniform X-gal staining” in areas of the vessel which were staining positive for alpha-actin suggested the possibility of further sources of SM progenitor cells. Hu et al challenged this opinion when it was reported in 2002 that repeated testing demonstrated no beta-galactsidase activity in SM22-a positive cells in the neointima (Hu et al., 2002).
Currently, it is generally accepted that bone marrow stem cells, while can play a role in formation of new endothelial cells, (Jackson et al., 2001) do not play a major part of intima/media thickening in arteriosclerosis. Bone marrow stem cells located around allografts within vessels is a trigged inflammatory response to injury of the vessel wall with migration of immune cells to the area and not the main source of progenitor SMC’s.
Discovery of Stem Cells in Vascular Walls
Stem cells (SC) can be defined as being either from embryonic or adult cell lineage. Human embryonic stem cells were first derived from in vitro fertilization (IVF) created embryos in 1998 during the blastocyst stage of development (Thomson et al., 1998). They exhibit pluripotency, the ability to differentiate into any cell type, whereas adult stem cells have undergone specialisation to become multipotent or monopotent cells, the ability to differentiate into one or many cell types. Adult stem cells are necessary for tissue repair and regeneration while we age and can be divided into the major categories of hematopoietic stem cells, endothelial stem cells, and mesenchymal stem cells first discovered to be generated in the bone marrow. Other stem cell populations have been subsequently found in the intestinal epithelium, neural cells, liver, skeletal muscle and various other tissues (Ratajczak et al,. 2007).
Studies in 2003 had identified cells within the tunica media that had the capability to differentiate into multiple lineages after culturing the cells in specific induction media. The major finding of this study indicated that the cells were positive for CD29+ and CD44+ proteins on their surface, shown in Figure.
In addition to these mesenchymal stem cell like markers they indicated the possibility of becoming bone, cartilage and muscle cells, they did not possess the capacity to differentiate into adipose progenitor cells separating themselves from classical MSC’s found in bone marrow.
Prof. Tang of Berkley University in 2013, published results in Nature Communications relating to the discovery of a niche population of MSC-like multipotent vascular stem cells (MVSC) residing within vessel walls, specifically the media layer via immunostaining techniques. The research group used lineage tracing of the myosin-11 marker to show that the newly discovered stem cell in the vessel did not arise from terminally differentiated SMC in the vessel. The cells which expressed both mesenchymal SC markers and neural markers were shown to contribute to vascular intima/media thickening after injury of the vessel wall instead of the popular SMC dedifferentiation and bone marrow progenitor cell theories. These new MVSC’s identified expressed the stem cell markers; Sox17, Sox10, S100β and neural filament-medium polypeptide (NFMP), the neural stem cell marker.
The discovery of resident vascular stem cells led scientists to believe that the SMC’s involved in IMT may have differentiated from these progenitor cells located in the vessel walls.
Role of Resident Vascular Stem Cells in Vascular Disease
Treatments of atherosclerotic plaques includes angioplasty and the fitting of a vascular stent. Following these procedures there is a chance of life threatening restenosis occurring due to differentiation of stem cells to smooth muscle cells.
Potential Therapeutic Uses of Vascular Stem Cells
Conclusion
References
https://irishheart.ie/wp-content/uploads/2016/12/changing_cardiovascular_health.pdf
1. Thomson JA, Itskovitz-Eldor J, Shapiro SS, et al. Embryonic stem cell linesderived from human blastocysts. Science. 1998;282:1145–1147.