Microtubules are Needed for Dispersal ofα-myosin Heavy Chain mRNA in Rat Neonatal Cardiac Myocytes

In some cell types, microtubules are used for transport of mRNA through the cytoplasm to the translation site. The number of microtubules increases during growth of cardiac myocytes, suggesting a functional role exists. Here, we test the need for microtubules to transportα-myosin heavy chain (α-MyHC) mRNA through the cytoplasm of neonatal cardiac myocytes. Theα-MyHC mRNA concentration was assessed by non-radioactivein situhybridization. The relative mRNA distributions were expressed as slopes (m=OD/μm), since optical density declined linearly from the nucleus to the cell periphery. Spontaneously-contracting myocytes displayed a gradual decrease inα-MyHC mRNA away from the nucleus (m=−1.27±0.12 OD/μm). To test whether microtubules were necessary forα-MyHC mRNA dispersal, contraction was first arrested with the Ca2+-channel blocker verapamil (10μm) for 18 h, which aggregated the mRNA perinuclearly. Contractile activity was then resumed by washing out verapamil and using isoproterenol (10μm) in the presence or absence of a microtubule depolymerizing drug, colchicine (3μm). Within 6 h, theα-MyHC mRNA distribution in myocytes with microtubules returned to normal values (m=−1.11±0.14 OD/μm), while cells lacking microtubules maintained a perinuclear mRNA distribution (m−1.50±0.16 OD/μm;P<0.05 from control). Despite this perinuclear pattern of mRNA distribution, the myocytes still produced new myofibrils. These data indicate that microtubules are necessary for dispersal ofα-MyHC mRNA outward from the nucleus. Furthermore, myofibrillogenesis may occur independently of mRNA localization and microtubule organization.