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

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 ß-adrenergic activity is markedly and continuously increased in decompensated hypertrophy, and because ß-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 ß-adrenergic overdrive and thus could be reversed by ß-adrenergic blockade. The data in this study, which were designed to answer this question, show that such is the case; that is, ß(1)- (but not ß(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 ß-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 ß-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.

Inhibition of histone deacetylases preserves myocardial performance and prevents cardiac remodeling through stimulation of endogenous angiomyogenesis.

We have previously shown that the inhibition of histone deacetylases (HDACs) protects the heart against acute myocardial ischemia and reperfusion injury. We also demonstrated that HDAC inhibition stimulates myogenesis and angiogenesis in a cultured embryonic stem cell model. We investigate whether in vivo inhibition of HDAC preserves cardiac performance and prevents cardiac remodeling in mouse myocardial infarction (MI) through the stimulation of endogenous regeneration. MI was created by ligation of the left descending artery. Animals were divided into three groups: 1) sham group, animals that underwent thoracotomy without MI; 2) MI, animals that underwent MI; and 3) MI + trichostatin A (TSA), MI animals that received a daily intraperitoneal injection of TSA. In addition, infarcted mice received a daily intraperitoneal injection of TSA (0.1 mg/kg), a selective HDAC inhibitor. 5-Bromo-2-deoxyuridine (50 mg/kg) was delivered every other day to pulse-chase label in vivo endogenous cardiac replication. Eight weeks later, the MI hearts showed a reduction in ventricular contractility. HDAC inhibition increased the improvement of myocardial functional recovery after MI, which was associated with the prevention of myocardial remodeling and reduction of myocardial and serum tumor necrosis factor α. HDAC inhibition enhanced the formation of new myocytes and microvessels, which was consistent with the robust increase in proliferation and cytokinesis in the MI hearts. An increase in angiogenic response was demonstrated in MI hearts receiving TSA treatment. It is noteworthy that TSA treatment significantly inhibited HDAC activity and increased phosphorylation of Akt-1, but decreased active caspase 3. Taken together, our results indicate that HDAC inhibition preserves cardiac performance and mitigates myocardial remodeling through stimulating cardiac endogenous regeneration.

Site-specific microtubule-associated protein 4 dephosphorylation causes microtubule network densification in pressure overload cardiac hypertrophy.

In severe pressure overload-induced cardiac hypertrophy, a dense, stabilized microtubule network forms that interferes with cardiocyte contraction and microtubule-based transport. This is associated with persistent transcriptional up-regulation of cardiac alpha- and beta-tubulin and microtubule-stabilizing microtubule-associated protein 4 (MAP4). There is also extensive microtubule decoration by MAP4, suggesting greater MAP4 affinity for microtubules. Because the major determinant of this affinity is site-specific MAP4 dephosphorylation, we characterized this in hypertrophied myocardium and then assessed the functional significance of each dephosphorylation site found by mimicking it in normal cardiocytes. We first isolated MAP4 from normal and pressure overload-hypertrophied feline myocardium; volume-overloaded myocardium, which has an equal degree and duration of hypertrophy but normal functional and cytoskeletal properties, served as a control for any nonspecific growth-related effects. After cloning cDNA-encoding feline MAP4 and obtaining its deduced amino acid sequence, we characterized by mass spectrometry any site-specific MAP4 dephosphorylation. Solely in pressure overload-hypertrophied myocardium, we identified striking MAP4 dephosphorylation at Ser-472 in the MAP4 N-terminal projection domain and at Ser-924 and Ser-1056 in the assembly-promoting region of the C-terminal microtubule-binding domain. Site-directed mutagenesis of MAP4 cDNA was then used to switch each serine to non-phosphorylatable alanine. Wild-type and mutated cDNAs were used to construct adenoviruses; microtubule network density, stability, and MAP4 decoration were assessed in normal cardiocytes following an equivalent level of MAP4 expression. The Ser-924 --> Ala MAP4 mutant produced a microtubule phenotype indistinguishable from that seen in pressure overload hypertrophy, such that Ser-924 MAP4 dephosphorylation during pressure overload hypertrophy may be central to this cytoskeletal abnormality.

Basis for MAP4 dephosphorylation-related microtubule network densification in pressure overload cardiac hypertrophy.

Increased activity of Ser/Thr protein phosphatases types 1 (PP1) and 2A (PP2A) during maladaptive cardiac hypertrophy contributes to cardiac dysfunction and eventual failure, partly through effects on calcium metabolism. A second maladaptive feature of pressure overload cardiac hypertrophy that instead leads to heart failure by interfering with cardiac contraction and intracellular transport is a dense microtubule network stabilized by decoration with microtubule-associated protein 4 (MAP4). In an earlier study we showed that the major determinant of MAP4-microtubule affinity, and thus microtubule network density and stability, is site-specific MAP4 dephosphorylation at Ser-924 and to a lesser extent at Ser-1056; this was found to be prominent in hypertrophied myocardium. Therefore, in seeking the etiology of this MAP4 dephosphorylation, we looked here at PP2A and PP1, as well as the upstream p21-activated kinase 1, in maladaptive pressure overload cardiac hypertrophy. The activity of each was increased persistently during maladaptive hypertrophy, and overexpression of PP2A or PP1 in normal hearts reproduced both the microtubule network phenotype and the dephosphorylation of MAP4 Ser-924 and Ser-1056 seen in hypertrophy. Given the major microtubule-based abnormalities of contractile and transport function in maladaptive hypertrophy, these findings constitute a second important mechanism for phosphatase-dependent pathology in the hypertrophied and failing heart.

gp-91 mediates histone deacetylase inhibition-induced cardioprotection.

We have recently shown that the inhibition of histone deacetylases (HDAC) protects the heart against ischemia and reperfusion (I/R) injury. The mechanism by which HDAC inhibition induces cardioprotection remains unknown. We sought to investigate whether the genetic disruption of gp-91, a subunit of NADPH-oxidase, would mitigate cardioprotection of HDAC inhibition. Wild-type and gp-91(-)(/-) mice were treated with a potent inhibitor of HDACs, trichostatin A (TSA, 0.1 mg/kg, i.p.). Twenty-four hours later, the perfused hearts were subjected to 30 min of ischemia and 30 min of reperfusion. HDAC inhibition in wild-type mice produced marked improvements in ventricular functional recovery and the reduction of infarct size. TSA-induced cardioprotection was eliminated with genetic deletion of gp91. Notably, Western blot and immunostaining displayed a significant increase in gp-91 in myocardium following HDAC inhibition, which resulted in a mildly subsequent increase in the production of reactive oxygen species (ROS). The pre-treatment of H9c2 cardiomyoblasts with TSA (50 nmol/l) decreased cell necrosis and increased viability in response to simulated ischemia (SI), which was abrogated by the transfection of cells with gp-91 siRNA, but not by scrambled siRNA. Furthermore, treatment of PLB-985 gp91(+/+) cells with TSA increased the resistance to SI, which also diminished with genetic disruption of gp91 in gp91(phox)-deficient PLB-985 cells. TSA treatment inhibited the increased active caspase-3 in H9c2 cardiomyoblasts and PLB-985 gp91(+/+) cells exposed to SI, which were prevented by knockdown of gp-91 by siRNA. These results suggest that a cascade consisting of gp-91 and HDAC inhibition plays an essential role in orchestrating the cardioprotective effect.

A direct test of the hypothesis that increased microtubule network density contributes to contractile dysfunction of the hypertrophied heart.

Contractile dysfunction in pressure overload-hypertrophied myocardium has been attributed in part to the increased density of a stabilized cardiocyte microtubule network. The present study, the first to employ wild-type and mutant tubulin transgenes in a living animal, directly addresses this microtubule hypothesis by defining the contractile mechanics of the normal and hypertrophied left ventricle (LV) and its constituent cardiocytes from transgenic mice having cardiac-restricted replacement of native beta(4)-tubulin with beta(1)-tubulin mutants that had been selected for their effects on microtubule stability and thus microtubule network density. In each case, the replacement of cardiac beta(4)-tubulin with mutant hemagglutinin-tagged beta(1)-tubulin was well tolerated in vivo. When LVs in intact mice and cardiocytes from these same LVs were examined in terms of contractile mechanics, baseline function was reduced in mice with genetically hyperstabilized microtubules, and hypertrophy-related contractile dysfunction was exacerbated. However, in mice with genetically hypostabilized cardiac microtubules, hypertrophy-related contractile dysfunction was ameliorated. Thus, in direct support of the microtubule hypothesis, we show here that cardiocyte microtubule network density, as an isolated variable, is inversely related to contractile function in vivo and in vitro, and microtubule instability rescues most of the contractile dysfunction seen in pressure overload-hypertrophied myocardium.

Inhibition of histone deacetylases triggers pharmacologic preconditioning effects against myocardial ischemic injury.

OBJECTIVES: Recent evidence has demonstrated the importance of histone deacetylases (HDAC) in the control of hypertrophic responses in the heart. However, it remains unknown whether inhibition of HDACs plays a role in myocardial ischemia and reperfusion (I/R) injury. We hypothesize that HDAC inhibition triggers preconditioning-like effects against I/R injury. METHODS AND RESULTS: Isolated mouse hearts were perfused with 3 cycles of 5-minute infusion and 5-minute washout of 50 nM of trichostatin A (TSA), a potent inhibitor of HDACs to mimic early pharmacologic preconditioning. This was followed by 30 min of ischemia and 30 min of reperfusion. In addition, mice were treated with saline or TSA (0.1 mg/kg, i.p.) to investigate delayed pharmacologic preconditioning. Twenty-four hours later, the hearts were subjected to I/R. Ventricular function and infarct size were measured, and HDAC 3, 4 and 5 were assessed by Western blot and immunofluorescence. HDAC and p38 mitogen-activated protein kinase activities were determined. TSA produced marked improvements in post-ischemic ventricular function recovery and a reduction in infarct size in both early and delayed preconditioning. Cardioprotection elicited by TSA was abrogated by SB203580, an inhibitor of p38. HDAC 3, 4 and 5 proteins were detected in mouse myocardium. TSA treatments resulted in a significant inhibition of HDAC activity. HDAC inhibition caused a dramatic increase in phosphorylation of p38 and p38 activity. Notably, HDAC inhibition also resulted in remarkable acetylation of p38 at lysine residues. CONCLUSION: These results suggest that inhibition of HDACs triggers pharmacologic preconditioning to protect the ischemic heart, which involves p38 activation.

Inhibition of beta-adrenergic receptor trafficking in adult cardiocytes by MAP4 decoration of microtubules.

Decreased beta-adrenergic receptor (beta-AR) number occurs both in animal models of cardiac hypertrophy and failure and in patients. beta-AR recycling is an important mechanism for the beta-AR resensitization that maintains a normal complement of cell surface beta-ARs. We have shown that 1) in severe pressure overload cardiac hypertrophy, there is extensive microtubule-associated protein 4 (MAP4) decoration of a dense microtubule network; and 2) MAP4 microtubule decoration inhibits muscarinic acetylcholine receptor recycling in neuroblastoma cells. We asked here whether MAP4 microtubule decoration inhibits beta-AR recycling in adult cardiocytes. [(3)H]CGP-12177 was used as a beta-AR ligand, and feline cardiocytes were isolated and infected with adenovirus containing MAP4 (AdMAP4) or beta-galactosidase (Adbeta-gal) cDNA. MAP4 decorated the microtubules extensively only in AdMAP4 cardiocytes. beta-AR agonist exposure reduced cell surface beta-AR number comparably in AdMAP4 and Adbeta-gal cardiocytes; however, after agonist withdrawal, the cell surface beta-AR number recovered to 78.4 +/- 2.9% of the pretreatment value in Adbeta-gal cardiocytes but only to 56.8 +/- 1.4% in AdMAP4 cardiocytes (P < 0.01). This result was confirmed in cardiocytes isolated from transgenic mice having cardiac-restricted MAP4 overexpression. In functional terms of cAMP generation, beta-AR agonist responsiveness of AdMAP4 cells was 47% less than that of Adbeta-gal cells. We conclude that MAP4 microtubule decoration interferes with beta-AR recycling and that this may be one mechanism for beta-AR downregulation in heart failure.

Inhibition of G protein-coupled receptor trafficking in neuroblastoma cells by MAP 4 decoration of microtubules.

One mechanism for the reappearance of G protein-coupled receptors after agonist activation is microtubule-based transport. In pressure-overload cardiac hypertrophy, there is downregulation of G protein-coupled receptors and the appearance of a densified microtubule network extensively decorated by a microtubule-associated protein, MAP 4. Our hypothesis is that overdecoration of a dense microtubule network with this structural protein, as in hypertrophied myocardium, would impede receptor recovery. We tested this hypothesis by studying muscarinic acetylcholine receptor (mAChR) internalization and recovery after agonist stimulation in neuroblastoma cells. Exposure of cells to carbachol, a muscarinic receptor agonist, decreased membrane receptor binding activity. After carbachol withdrawal, receptor binding recovered toward the initial value. When microtubules were depolymerized before carbachol withdrawal, mAChR recovery was only 44% of that in intact cells. Cells were then infected with an adenovirus containing MAP 4 cDNA. MAP 4 protein decorated the microtubules extensively, and receptor recovery upon carbachol withdrawal was reduced to 54% of control. Thus muscarinic receptor recovery after agonist exposure is microtubule dependent, and MAP 4 decoration of microtubules inhibits receptor recovery.