Cytoskeletal role in protection of the failing heart by β-adrenergic blockade

Cheng G, Kasiganesan H, Baicu CF, Wallenborn JG, Kuppuswamy D, Cooper G 4th. Cytoskeletal role in protection of the failing heart by beta-adrenergic blockade. Am J Physiol Heart Circ Physiol 302: H675-H687, 2012. First published November 11, 2011; doi:10.1152/ajpheart.00867.2011.-Formation of a dense microtubule network that impedes cardiac contraction and intracellular transport occurs in severe pressure overload hypertrophy. This process is highly dynamic, since microtubule depolymerization causes striking improvement in contractile function. A molecular etiology for this cytoskeletal alteration has been defined in terms of type 1 and type 2A phosphatase-dependent site-specific dephosphorylation of the predominant myocardial microtubule-associated protein (MAP) 4, which then decorates and stabilizes microtubules. This persistent phosphatase activation is dependent upon ongoing upstream activity of p21-activated kinase-1, or Pak1. Because cardiac beta-adrenergic activity is markedly and continuously increased in decompensated hypertrophy, and because beta-adrenergic activation of cardiac Pak1 and phosphatases has been demonstrated, we asked here whether the highly maladaptive cardiac microtubule phenotype seen in pathological hypertrophy is based on beta-adrenergic overdrive and thus could be reversed by beta-adrenergic blockade. The data in this study, which were designed to answer this question, show that such is the case; that is, beta(1)-(but not beta(2)-) adrenergic input activates this pathway, which consists of Pak1 activation, increased phosphatase activity, MAP4 dephosphorylation, and thus the stabilization of a dense microtubule network. These data were gathered in a feline model of severe right ventricular (RV) pressure overload hypertrophy in response to tight pulmonary artery banding (PAB) in which a stable, twofold increase in RV mass is reached by 2 wk after pressure overloading. After 2 wk of hypertrophy induction, these PAB cats during the following 2 wk either had no further treatment or had beta-adrenergic blockade. The pathological microtubule phenotype and the severe RV cellular contractile dysfunction otherwise seen in this model of RV hypertrophy (PAB No Treatment) was reversed in the treated (PAB beta-Blockade) cats. Thus these data provide both a specific etiology and a specific remedy for the abnormal microtubule network found in some forms of pathological cardiac hypertrophy.