David G. Simpson

Role of Mechanical Stimulation in the Establishment and Maintenance of Muscle Cell Differentiation

Publisher Summary This chapter discusses the role of mechanical stimulation in the establishment and maintenance of muscle cell differentiation. The striated tissues of the myocardium and axial skeleton are exquisitely sensitive to changes in mechanical load. In the heart, an acute increase in preload can act to augment cardiac output on a beat-to-beat time scale. In skeletal muscle, it serves to accelerate shortening velocity and increase contractile force. During early embryonic life, the developing cardiac myocyte displays a rounded, ovoid shape. Myofibrils are disseminated throughout the sarcoplasm and intercellular junctions are distributed at irregular intervals along the periphery of the cells. With continued development, the myocytes grow severalfold in size and begin to gradually elongate to assume a rod-like shape. The essential role which mechanical forces play in directing cardiac morphogenesis is readily apparent when an intact, developing heart is partially unloaded. Externally derived forces may also serve as an early signal that acts to direct the nucleation of myofibrils in the developing heart. Because the primitive myofibrils of the heart appear to be attached to the sarcolemma through nascent costameres and fascia adherens junctions, they appear to be subject to the extrinsic tensile forces that must accompany these morphogenetic events.

Mechanical regulation of cardiac myofibrillar structure.

The excitation-contraction coupling cycle (ECC) consists of a complex cascade of electrochemical and mechanical events; however, the relative contributions of these different processes in the regulation of cardiac myofibrillar structure are not well understood. There is extensive evidence to suggest that the mechanical aspects of the ECC play a crucial role in controlling the availability of contractile proteins for myofibrillar assembly. To examine if these physical forces might also serve to stabilize the structure of preexisting myofibrils, beating and nonbeating cultures of neonatal cardiac myocytes (NCM) were subjected to a 5% static stretch. Contractile arrest was achieved by treating NCM with 12 microM nifedipine, which resulted in immediate and sustained contractile arrest and initiated the evolution of marked myofibrillar abnormalities within 24 hours. As judged by scanning confocal and transmission electron microscopic examination, an external load appears to partially stabilize myofibrillar structure in nonbeating NCM. These results suggest that the maintenance of myofibrillar structure may be highly dependent upon the mechanical aspects of ECC.

Cardiac Integrins: The Ties That Bind

An elaborate series of morphogenetic events must be precisely coordinated during development to promote the formation of the elaborate three-dimensional structure of the normal heart. In this study we focus on discussing how interconnections between the cardiac myocyte and its surrounding environment regulate cardiac form and function. In vitro experiments from our laboratories provide direct evidence that cardiac cell shape is regulated by a dynamic interaction between constituents of the extracellular matrix (ECM) and by specific members of the integrin family of matrix receptors. Our data indicates that phenotypic information is stored in the tertiary structure and chemical identity of the ECM. This information appears to be actively communicated and transduced by the α1β1 integrin molecule into an intracellular signal that regulates cardiac cell shape and myofibrillar organization. In this study we have assessed the phenotypic consequences of suppressing the expression and accumulation of the α1 integrin molecule in aligned cultures of cardiac myocytes. In related experiments we have examined how the overexpression of α2 and α5 integrin, integrins normally not present or present at very low copy number on the cell surface of neonatal cardiac myocytes, affect cardiac protein metabolism. We also consider how biochemical signals and the mechanical signals mediated by the integrins may converge on common intracellular signaling pathways in the heart. Experiments with the whole embryo culture system indicate that angiotensin II, a peptide that carries information concerning cardiac load, plays a role in controling cardiac looping and the proliferation of myofibrils during development.

Signal Transduction and Myofibrillogenesis in Isolated Neonatal Heart Myocytes In Vitro

A precise sequence of morphological events is associated with the process of myofibrillogenesis in vitro in neonatal cardiac myocytes (Hilenski et al 1991; Sharp et al 1993; Sanger et al 1984). A similar set of sequences probably occurs in vivo; however, the rate of formation is more gradual, thus making investigation of precise events more difficult. In vitro analyses of morphological and biochemical events associated with myofibrillogenesis can provide an understanding of these processes in normal heart development, response to stimulation, and cardiac hypertrophy.