Microtubule Involvement in the Adaptation to Altered Mechanical Load in Developing Chick Myocardium

Abstract

Abstract-Mechanical load regulates ventricular growth, function, and structure from the earliest stages of cardiac morphogenesis through senescence. Dramatic changes in cardiac form and function have been defined for developing cardiovascular systems, and changes in mechanical loading conditions can produce structural malformations such as left heart hypoplasia. To date, relatively little is known regarding the interactions between changes in mechanical load, morphogenesis, and the material properties of the embryonic heart. We tested the hypothesis that passive material properties in the embryonic heart change in response to altered mechanical load and that microtubules play an important role in this adaptive response. We measured biaxial passive stress-strain relations in left ventricular (LV) myocardial strips in chick embryos at Hamburger-Hamilton stage 27 following left atrial ligation (LAL) at stage 21 to reduce LV volume load and create left heart hypoplasia. Following LAL, myocardial stresses at given strains and circumferential stiffness increased versus control strips. Western blot analysis of LAL embryos showed an increase in both total and polymerized ␤-tubulin and confocal microscopy confirmed an increase in microtubule density in the LV compact layer versus control. Following colchicine treatment, LV stresses and stiffness normalized in LAL specimens and microtubule density following colchicine was similar in LAL to control. In contrast, Taxol treatment increased myocardial stresses and stiffness in control strips to levels beyond LAL specimens. Thus, the material properties of the developing myocardium are regulated by mechanical load and microtubules play a role in this adaptive response during cardiac morphogenesis.

Mechanical modulation of cardiac microtubules

“…In contrast, the hypertrophic response in these cells was found not to be dependent upon an intact microtubule cytoskeleton by Sadoshima et al [61]. In contrast to findings in adult hearts (see below), Schroder et al [64] observed that unloading the embryonic ventricle in chicks increased microtubule density, the reason for this different response in embryonic hearts is unknown. Hypoosmotic swelling has been shown to disrupt microtubules in neonatal cells, though neither taxol nor colchicine affected volume regulation [41].…”

Normal Shear Stress and Vascular Smooth Muscle Cells Modulate Migration of Endothelial Cells Through Histone Deacetylase 6 Activation and Tubulin Acetylation

“…4,12,33 Dynamic microtubules are crucial for modulating cell motility, 5 and the adaptation of cell to vary of mechanical level. 27 Microtubule destabilization significantly enhanced aspects of their motility. 5 Acetylation plays a role in the maintenance of stable populations of microtubules.…”

Systolic and Diastolic Ventricular Function Assessed by Pressure-Volume Loops in the Stage 21 Venous Clipped Chick Embryo

“…After clipping, ventricular contractility development may lag behind as a consequence of impaired cardiac development. Mechanical load regulates ventricular growth (30,31). Venous obstruction temporarily reduces mechanical load on the myocardium and thus probably slows down the ventricular growth and differentiation process (3).…”

“…Changes in diastolic function may be a critical regulatory pathway during cardiac morphogenesis (34). Recently Schroder et al (31) found an increase in microtubule density and ␤-tubulin protein after reduced mechanical load in the stage 27 embryonic heart after left atrial ligation at stage 21. They observed increased myocardial stiffness in response to reduced mechanical load.…”