Involvement of α4 integrins in maintenance of cardiac sympathetic axons

Sympathetic neurons extend and maintain axons that innervate the myocardium, and proper innervation is important for cardiac function. However, the molecular basis for axon outgrowth and maintenance is not well understood. We have shown previously that the integrin α4β1 is expressed on developing axons, and the α4 function is important for the development of innervation in vivo [Wingerd, K.L., Goodman, N.L., Tresser, J.W., Smail, M.M., Leu, S.T., Rohan, S.J., Pring, J.L., Jackson, D.Y., and Clegg, D.O., 2002. Alpha 4 integrins and vascular cell adhesion molecule-1 play a role in sympathetic innervation of the heart. J. Neurosci. 22,10772-10780]. Here we examine the function of α4β1 integrins in the maintenance of cardiac sympathetic innervation in vitro and in vivo, and investigate integrin expression and function after myocardial infarction and in hypertensive rats. On substrates of vascular cell adhesion molecule-1 (VCAM-1), α4β1 was required for both initial outgrowth and maintenance of neurites in vitro. On fibronectin substrates, initial outgrowth requires only α4 integrins, but maintenance requires both α4 integrins and RGD-dependent integrins. In vivo, in adult Long Evans rats, inhibition of α4 integrins resulted in decreased maintenance of sympathetic fibers innervating the apex of the heart. However, α4 integrins were not detected on most sympathetic axons that sprout after myocardial infarction, and α4 function was not required for sprouting. Spontaneously hypertensive rats (SHR) have increased numbers of cardiac sympathetic fibers compared to the parental Wistar strain, but many of these lack α4 expression, and α4 function is not required for maintenance of these fibers in the heart. These results suggest that developing sympathetic axons and sprouting sympathetic axons use different mechanisms of outgrowth, and that maintenance of cardiac sympathetic innervation involves α4 integrins in some rat strains.