Correlation between heart valve interstitial cell stiffness and transvalvular pressure: implications for collagen biosynthesis

Abstract

It has been speculated that heart valve interstitial cells (VICs) maintain valvular tissue homeostasis through regulated extracellular matrix (primarily collagen) biosynthesis. VICs appear to be phenotypically plastic, inasmuch as they transdifferentiate into myofibroblasts during valve development, disease, and remodeling. Under normal physiological conditions, transvalvular pressures (TVPs) on the right and left side of the heart are vastly different. Hence, we hypothesize that higher left-side TVPs impose larger local tissue stress on VICs, which increases their stiffness through cytoskeletal composition, and that this relation affects collagen biosynthesis. To evaluate this hypothesis, isolated ovine VICs from the four heart valves were subjected to micropipette aspiration to assess cellular stiffness, and cytoskeletal composition and collagen biosynthesis were quantified by using the surrogates smooth muscle alpha-actin (SMA) and heat shock protein 47 (HSP47), respectively. VICs from the aortic and mitral valves were significantly stiffer (P < 0.001) than those from the pulmonary and tricuspid valves. Left-side isolated VICs contained significantly more (P < 0.001) SMA and HSP47 than right-side VICs. Mean VIC stiffness correlated well (r = 0.973) with TVP; SMA and HSP47 also correlated well (r = 0.996) with one another. Assays were repeated for VICs in situ, and, as with in vitro results, left-side VIC protein levels were significantly greater (P < 0.05). These findings suggest that VICs respond to local tissue stress by altering cellular stiffness (through SMA content) and collagen biosynthesis. This functional VIC stress-dependent biosynthetic relation may be crucial in maintaining valvular tissue homeostasis and also prove useful in understanding valvular pathologies.

Automated Micropipette Aspiration of Single Cells

“…The determined Young's modulus value for the tested PAVICs is 358.7 ± 192.1 Pa (n=30). This result is in agreement with our previous results (354.8 ± 142.0 Pa) from manual testing [43] and the results reported in [51].…”

“…These cell types were previously tested with manual micropipette aspiration technique and their mechanical properties were reported in literature [32,43,51]. In the validation process, the mechanical properties of these cells were calculated using the previously developed mathematical models.…”

“…It has been shown that the mechanical properties of aortic valves play an important role in the heart's functionality [50,51]. The regulatory valve function is also affected by the valve cells.…”

Active stiffening of mitral valve leaflets in the beating heart

“…They are contractile: epinephrine and angiotensin II induce very slow, tonic contractions (likely actomyosinbased) in cultured and valvular strips (13,38). Their collagen synthesis and smooth muscle ␣-actin content, and hence stiffness, may relate directly to the pressure experienced by the leaflet (28).…”

“…Medical and surgical therapies for this disease have traditionally assumed that the valve leaflets, the key components of the valve, are passive flaps (46). However, muscle fibers (2, 6, 11, 12, 18, 25, 30 -32, 34, 39, 41, 47, 48), blood vessels (6,8,11,24,26,37,39), and nerves (1, 6 -13, 18 -20, 26, 29, 35, 36, 39, 43-45) have been recognized in the MV leaflets of animals and humans since the 19th century, and the physiological function of these entities, along with other contractile elements in the leaflets, such as smooth muscle (8,18,26,47) and valvular interstitial cells (VICs) (4,13,18,23,26,28,38,40), is not understood.…”