Microtubule Stabilization in Pressure Overload Cardiac Hypertrophy

Abstract

Increased microtubule density, for which microtubule stabilization is one potential mechanism, causes contractile dysfunction in cardiac hypertrophy. After microtubule assembly, α-tubulin undergoes two, likely sequential, time-dependent posttranslational changes: reversible carboxy-terminal detyrosination (Tyr-tubulin ↔ Glu-tubulin) and then irreversible deglutamination (Glu-tubulin → Δ2-tubulin), such that Glu- and Δ2-tubulin are markers for long-lived, stable microtubules. Therefore, we generated antibodies for Tyr-, Glu-, and Δ2-tubulin and used them for staining of right and left ventricular cardiocytes from control cats and cats with right ventricular hypertrophy. Tyr- tubulin microtubule staining was equal in right and left ventricular cardiocytes of control cats, but Glu-tubulin and Δ2-tubulin staining were insignificant, i.e., the microtubules were labile. However, Glu- and Δ2-tubulin were conspicuous in microtubules of right ventricular cardiocytes from pressure overloaded cats, i.e., the microtubules were stable. This finding was confirmed in terms of increased microtubule drug and cold stability in the hypertrophied cells. In further studies, we found an increase in a microtubule binding protein, microtubule-associated protein 4, on both mRNA and protein levels in pressure-hypertrophied myocardium. Thus, microtubule stabilization, likely facilitated by binding of a microtubule-associated protein, may be a mechanism for the increased microtubule density characteristic of pressure overload cardiac hypertrophy.

Muscle-specific RING finger-2 (MURF-2) is important for microtubule, intermediate filament and sarcomeric M-line maintenance in striated muscle development

“…Therefore, microtubules appear to function as negative regulators of muscle contraction (Webster, 2000). In agreement with this idea, cardiac hypertrophy and failure correlate with increases in Glu-tubulin and MAP4 levels (Sato et al, 1997;Belmadani et al, 2002;Takahashi et al, 2003). It is an exciting possibility, then, that mis-regulation of MURF-2 (and MURF-3) levels also could contribute to human contractile dysfunction.…”

Sodium Channel Remodeling in Subcellular Microdomains of Murine Failing Cardiomyocytes

“…Several studies have previously shown that Nav1.5 channels are delivered to their final destination on the membrane through the microtubule network 19, 35. Although microtubule network density was increased in TAC cardiomyocytes, the observed increased detyrosinated tubulin (ie, Glu‐tubulin) levels indicate that the microtubule network is less dynamic25 and, thus, potentially less efficient 36, 37, 38. It is, therefore, reasonable to speculate that the increased Glu‐tubulin observed in our experiments is associated with impaired Nav1.5 delivery at any subcellular location.…”

Cytoskeletal networks and the regulation of cardiac contractility: microtubules, hypertrophy, and cardiac dysfunction

“…Instead, active transport is required for this purpose, and microtubules supply this function: the dynein family of motor proteins moves these cargoes along microtubules toward the cell center, and the kinesin family of motor proteins moves these cargoes along microtubules from the nuclei toward the cell periphery. In cardiac hypertrophy, the microtubules not only proliferate but also become heavily decorated by microtubule-associated protein-4, the predominant cardiac microtubule-associated protein (23,28). One consequence of this is that the normal microtubule transport function is sterically inhibited by the presence of microtubule-associated protein-4 on these transport tracks, and as one example that may help explain ␤-adrenergic receptor downregulation in cardiac hypertrophy and failure, we have shown that activated G proteincoupled receptors fail to be recycled properly to the cell membrane (2,3).…”

“…The most definitive solution, however, would be to close the loop and address the microtubule hypothesis directly in a context apart from hemodynamic alterations in vivo or microtubule alterations in vitro. On the basis of some of our other work defining the basic causes of microtubule network densification in cardiac hypertrophy (21,23,28), I very recently tested, via a collaboration with F. Cabral and D. R. Menick, the effects of ␤ 1 -tubulin mutants that had been selected for their effects on microtubule stability and then expressed in the hearts of transgenic mice (4). We found that when intrinsic microtubule stability was increased as an isolated variable, the contractile defects characteristic of pressure-overloaded hypertrophied myocardium were reproduced.…”