Cell Sources for Tissue Engineering Strategies to Treat Calcific Valve Disease

Abstract

Cardiovascular calcification is an independent risk factor and an established predictor of adverse cardiovascular events. Despite concomitant factors leading to atherosclerosis and heart valve disease (VHD), the latter has been identified as an independent pathological entity. Calcific aortic valve stenosis is the most common form of VDH resulting of either congenital malformations or senile "degeneration." About 2% of the population over 65 years is affected by aortic valve stenosis which represents a major cause of morbidity and mortality in the elderly. A multifactorial, complex and active heterotopic bone-like formation process, including extracellular matrix remodeling, osteogenesis and angiogenesis, drives heart valve "degeneration" and calcification, finally causing left ventricle outflow obstruction. Surgical heart valve replacement is the current therapeutic option for those patients diagnosed with severe VHD representing more than 20% of all cardiac surgeries nowadays. Tissue Engineering of Heart Valves (TEHV) is emerging as a valuable alternative for definitive treatment of VHD and promises to overcome either the chronic oral anticoagulation or the time-dependent deterioration and reintervention of current mechanical or biological prosthesis, respectively. Among the plethora of approaches and stablished techniques for TEHV, utilization of different cell sources may confer of additional properties, desirable and not, which need to be considered before moving from the bench to the bedside. This review aims to provide a critical appraisal of current knowledge about calcific VHD and to discuss the pros and cons of the main cell sources tested in studies addressing in vitro TEHV.

Keywords: calcification; heterotopic bone formation; in vitro; tissue engineering heart valves; valve heart disease.

Figure 1

Differential pathology and clinical impact of valvular vs. vascular calcification flowchart. Cardiovascular calcification is an active and degenerative bone-like process affecting the cardiovascular tissues. Both vessels and valves show an athero-inflammatory background and, despite the commonalities and overlap of several risk factors (such as aging, hyperlipidaemia or kidney disease), both atherosclerosis and calcific VHD are two independent pathologic entities. The biological progression of the disease, tissue characteristics and clinical impact stand those differences. The result is the independent plaque rupture primary outcome found in the progression of VHD. An increased stiffness or sclerosis induces an increased aortic pulse wave, triggering hypertension, and a reduction in coronary perfusion. Besides, the pressure overload caused by a sclerotic pre-stadium and observed in the progression of the VHD leads to LV structural and hemodynamic changes. Symptomatology onset and calcification burden are poor prognosis predictors associated with multiple adverse cardiovascular complications, such as left ventricular hypertrophy (LVH), aortic valve stenosis, congestive heart failure (HF), ascending aorta aneurysm, myocardial infarction (MI), and peripheral vascular disease (PVD).

Figure 2

Pathophysiology of valve calcification. VICs are myofibroblast-like cells endowed with high plasticity, allowing them to participate in reparative, regenerative, and pathological processes. Underlying athero-inflammatory disease, aging as well as other clinical conditions such as chronic kidney disease or hyperlipidaemia, can trigger the acquisition of synthetic, proliferative and migratory phenotypes by VICs. These conditions are often associated with a reduction of defense mechanism against calcification. Two major features are recognized during valve calcification: (i) ECM remodeling, including the synthesis of a collagen-enriched matrix and its mineralization, and (ii) osteo/chondroblast differentiation of VICs. Osteogenic processes are associated with the abundant synthesis of collagen type-I and other ECM remodeling processes causing stiffness changes capable of perpetuate and extend the osteogenesis in the valve. A prestadium of sclerosis in well-functional valves finally leading to stenosis and obstruction in an active heterotopic bone-like formation process has been described. Observations in human samples also suggest the possibility of dystrophic mineralisation in advanced end-stages of calcific VHD. Pyrophosphate, osteopontin, osteoprotegerin, gamma-carboxylated matrix Gla protein and circulating fetuin-A are listed as physiologic inhibitors of vascular calcification; while hyperphosphatemia and hypercalcemia, expression of bone morphogenetic proteins or alkaline phosphatase and activation of osteo/chondroblast regulators (Runx2, Osterix, Wnt) are counted among the pro-calcifying elements orchestrating osteoblast-like transdifferentiation. PPi, pyrophosphate; cMGP, gamma-carboxylated matrix Gla protein; BMP-2, bone morphogenetic protein 2; ECM, extracellular matrix; CKD, chronic kidney disease; CHD, congestive heart disease; VIC, valve interstitial cells; VEC, valve endothelial cells; GAGs, glycosaminoglycans; Ca₁₀(PO₄)₆(OH)₂, hydroxyapatite; Pi, inorganic phosphate; ALP, alkaline phosphatase; Bglap, bone Gla protein or osteocalcin.

Figure 3

Main TEHV approaches comparison. Two main TEHV strategies are being explored; in vitro and in situ. In vitro TEHV consist of the combination of, ideally, autologous cells (e.g., MSCs, fibroblasts, EPCs, HSC, PCs or others) that may, or may not, be previously differentiated into the target cell. Those cells are seeded on a scaffold or graft preparation and statically or dynamically matured prior orthotopic implantation. On the other hand, in situ TEHV is based on the use of an acellular scaffold/graft capable to recruit endogenous cells which will remodel and integrate. Both in vitro and in situ meet at the scaffold/graft preparation step. Scaffold/grafts can be based on decellularized xeno/homografts or synthetic totally/partially biodegradable materials (hydrogels, porous, or fibrous). Both approaches can be enriched with biomolecules or drugs stimulating desired cell recruitment and phenotypes or inhibiting deleterious events such as calcification, angiogenesis, etc of the TEHV. After implantation, the TEHV must be able to regenerate and integrate into the recipient host and be native-like functional. MSCs, mesenchymal stromal cells; EPCs, endothelial progenitor cells; HSC, hematopoietic stem cells; PCs, pericytes.