MICROTUBULES ARE NEEDED FOR DISPERSAL OF ALPHA-MYOSIN HEAVY-CHAIN MESSENGER-RNA IN RAT NEONATAL CARDIAC MYOCYTES

In some cell types, microtubules are used for transport of mRNA throug h the cytoplasm to the translation site. The number of microtubules in creases during growth of cardiac myocytes, suggesting a functional rol e exists. Here, we test the need for microtubules to transport a-myosi n heavy chain (alpha-MyHC) mRNA through the cytoplasm of neonatal card iac myocytes. The alpha-MyHC mRNA concentration was assessed by non-ra dioactive in situ hybridization, The relative mRNA distributions were expressed as slopes (m = OD/mu m), since optical density declined line arly from the nucleus to the cell periphery. Spontaneously-contracting myocytes displayed a gradual decrease in alpha-MyHC mRNA away from th e nucleus (m = -1.27 +/- 0.12 OD/mu m). To test whether microtubules w ere necessary for a-MyHC mRNA dispersal, contraction was first arreste d with the Ca2+-channel blocker verapamil (10 mu M) for 18 h. which ag gregated the mRNA perinuclearly. Contractile activity was then resumed by washing out verapamil and using isoproterenol (10 mu M) in the pre sence or absence of a microtubule depolymerizing drug, colchicine (3 m u M). Within 6 h, the alpha-MyHC mRNA distribution in myocytes with mi crotubules returned to normal values (m = -1.11 +/- 0.14 OD/mu m), whi le cells lacking microtubules maintained a perinuclear mRNA distributi on (m-1.50 +/- 0.16 OD/mu m: P<0.05 from control). Despite this perinu clear pattern of mRNA distribution, the myocytes still produced new my ofibrils. These data indicate that microtubules are necessary for disp ersal of alpha-MyHC mRNA outward from the nucleus, Furthermore, myofib rillogenesis may occur independently of mRNA localization and microtub ule organization. (C) 1998 Academic Press