Abstract 363: Upregulation Of PP2A Activity Induces Microtubule Network Densification in Pressure Overload-Induced Hypertrophy
Type 1 protein phosphatase (PP1) and protein phosphatase 2A (PP2A) account for >90% of total cardiac phosphatase activity. Activated PP1 and PP2A can cause cardiac dysfunction through effects on calcium metabolism. We have shown that activated PP1 can also causes cardiac dysfunction through microtubule (MT) proliferation, since PP1-induced microtubule-associated protein 4 (MAP4) dephosphorylation causes MAP4 to bind and stabilize MTs. However, PP2A may both increase PP1 activity and in addition is the more important phosphatase for regulating MAP4 dephosphorylation and thus MT stability. Therefore, we hypothesized that PP2A may be causally linked to depressed cardiac function via microtubule network densification. PP2A activity and protein were measured in feline right ventricular (RV) pressure overload. PP2A activity increased within 48 hours of cardiac pressure overloading and remained upregulated for as long as we looked (at least 10 weeks); PP2A protein level was unchanged. Transgenic mice overexpressing the catalytic subunit α of PP2A in the cardiocytes (PP2ATgM) (JBC, Vol. 279, 40827–34, 2004) were then used to determine directly the role of PP2A on MT proliferation in vivo. The polymerized fraction of cardiac αβ-tubulin heterodimers was two-fold higher than in wild-type (WT) controls, and a dense, heavily MAP4-decorated cardiocyte microtubule network was present. To determine the effects of active PP2A on cardiac MT stability, colchicine (0.5 mg/kg, i.p.) was given to the mice. At 4 hours, the polymerized αβ-tubulin heterodimer fraction was significantly increased in PP2A overexpressing hearts. Thus, increased activity of PP2A increases MAP4 microtubule affinity, which in turn generates a dense network of stabilized microtubules. Our findings suggest that PP2A may represent a novel therapeutic target to modulate altered MAP4 phosphorylation and thus microtubule network stability and density in heart failure.