Basis for MAP4 Dephosphorylation-related Microtubule Network Densification in Pressure Overload Cardiac Hypertrophy

Abstract

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. Pathological cardiac hypertrophy may be accompanied by increased density and MAP4 2 decoration of the cardiomyocyte microtubule network (1, 2), which causes defects in cellular contractile (3, 4) and transport (5, 6) function. We recently have described, in pathological but not physiological cardiac hypertrophy, site-specific dephosphorylation of three MAP4 serine residues (7); one site is in the MAP4 projection domain, and two are in the microtubule-binding domain. Of these, the striking dephosphorylation at feline MAP4 Ser-924 corresponding to human MAP4 Ser-914 within the first of the four KXGS repeats of the MAP4 microtubule-binding domain was especially interesting, because adenoviral expression of a dephosphomimetic Ser-924 3 Ala feline MAP4 mutant in normal cardiomyocytes phenocopied the features of microtubule network densification, stabilization, and MAP4 overdecoration seen in pathological cardiac hypertrophy. Conversely, adenoviral expression of a phosphomimetic Ser-924 3 Asp feline MAP4 mutant in normal cells caused microtubule depolymerization.This first MAP4 microtubule-binding domain KXGS motif repeat is closely homologous to the corresponding first repeat in the neuronal MAP Tau, with feline MAP4 Ser-924 corresponding to human full-length Tau Ser-262, and it is known that phosphorylation of both MAP4 and Tau at the respective serine residues virtually abolishes MAP binding to microtubules, with consequent microtubule network destabilization (8, 9). Indeed, hyperphosphorylation of Tau Ser-262, leading to aggregation of the detached Tau into...

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“…We therefore asked whether a directionally opposite phosphatase dysregulation from that seen in Alzheimer's disease could be responsible for the site-specific MAP4 dephosphorylation that appears to be responsible for microtubule stabilization in pathological cardiac hypertrophy. Our recent data indicate that such is the case, since increased activity of the predominant myocardial protein phosphatases, both that of PP1 and especially that of PP2A, is a striking and persistent feature of severe pressure overload hypertrophy (7). Finally, in asking what is responsible for persistent activation of PP2A in this setting, the observation Fig.…”

“…that activity of an upstream multifunctional enzyme, Pak1, is increased in early cardiac hypertrophy (33) provided another important insight, since in our hands Pak1 activity is persistently increased in severe pressure overload hypertrophy and overexpression of Pak1 in normal cardiomyocytes reproduces the hypertrophic microtubule phenotype (7).…”

“…The assays were performed in a 50-ml buffer of 50 mM Tris·HCl, 0.1 mM Na 2EDTA, 5 mM DTT, 5 mM caffeine, and 0.01% Brij 35 (pH 7.0) at 30°C. The PP1 activity was measured with 32 P-labeled myelin basic protein as a substrate in the presence of either 4 nM okadaic acid to block PP2A activity or 1 mM okadaic acid to block both PP1 and PP2A activity (7,57). Thus PP1 activity was calculated from okadaic acid-sensitive protein phosphatase activity by deducting phosphatase activity with 1 mM okadaic acid from activity with 4 nM okadaic acid.…”

“…Furthermore, this phosphatase activation is apparently driven by ongoing activation of the upstream stress-related kinase p21-activated kinase-1, or Pak1. Again, genetic induction of increased activity of upstream Pak1 or downstream PP2A or PP1 in normal cardiomyocytes and/or myocardium reproduced each of the major features of the pathological cardiac microtubule network (7).…”

AMPK attenuates microtubule proliferation in cardiac hypertrophy

“…However, when phosphorylated at Ser-924 of the KXGS motif (equivalent to Ser-914 in mice) within the MT binding domain, this interaction is inhibited and MT stability is reduced (4). Cheng et al (3) showed that increased phospha- tase activity during pressure overload results in dephosphorylation of MAP4 Ser924 , thereby promoting MT assembly and stabilization. We find that activation of AMPK increases MAP4 phosphorylation at the analogous site in rat cardiomyocytes.…”