Novel roles of PAK1 in the heart

Abstract

Our work and others' over the past few years have led to the identification of new roles of PAK1 in cardiac physiology, such as the regulation of cardiac ion channel and actomyosin function. More recent studies have revealed that PAK1-deficient mice were vulnerable to cardiac hypertrophy and readily progress to failure under sustained pressure overload and susceptible to ischemia/reperfusion injury. Our further study indicated that the PAK1 activator FTY720 was able to prevent this pressure overload-induced hypertrophy in wild-type mice without compromising their cardiac functions. A cardiac protective effect against ischemia/reperfusion injury by FTY720 was also observed in both rat and mouse models by us and others. Thus, these studies suggest that PAK1 is more important in the heart than previously thought, in particular a therapeutic potential of PAK1 activators. In the future, in-depth investigations are required to further substantiate our hypotheses on mechanisms for PAK1 function in the heart and to explore a therapeutic potential of FTY720 and other PAK1 activators in heart disease conditions.

Figure 1. PAK1 expression in the heart. (A) Western blot detection of PAK1 expression in SAN, left atrial (LA) and ventricular tissues (V). PAK1 expression pattern was examined by immunocytochemical staining in (B) adult rat cardiomyocyte (green, anti-PAK1; red, rhodamine-conjugated phalloidin), in (C) guinea pig senatorial node (SAN) cells (adapted from refs. and 4).

Figure 2. PAK1 regulation of cardiac ion channel activity. (A and B) Representative L-type Ca²⁺ current recordings from cultured SAN cells infected with Ad-LacZ or Ad-PAK1 in the absence and presence of 100 nmol/L ISO for 5 min. Currents were recorded during 200-ms step depolarizations from a holding potential of -50 mV to a range of potentials between -40 and +50 mV. (C and D) Current-voltage relationship of I_CaL in cells infected with Ad-LacZ or Ad-PAK1, in the absence and presence of ISO (adapted from ref. 4).

Figure 3. PAK1 regulation of Ca²⁺ handling. Line-scan images and whole cell-width line plots of fluo-4 fluorescence recorded parallel to the longitudinal axis of the cell from representative field-stimulated adult rat ventricular myocytes expressing LacZ [blue trace, (A)] or CA-PAK1 [red trace, (B)] under control conditions and during steady-state stimulation with 100 nM ISO. (C) Average intracellular Ca²⁺ concentration ([Ca²⁺]_i) transient amplitudes in LacZ (875 ± 153 and 1,889 ± 286 nM without and with ISO, respectively) and CA-PAK1 (845 ± 99 and1,299 ± 138 nM without and with ISO, respectively) cells. (D) Average time constant for [Ca²⁺]_i transient decay (τ_decay) for LacZ (276 ± 8 and 149 ± 6 ms without and with ISO, respectively) and CA-PAK1 (308 ± 10 and 180 ± 10 ms without and with ISO, respectively) cells. Values are means ± SE *, statistically significant difference between LacZ and CA-PAK1. ⁺, statistically significant difference between ISO and +ISO (adapted from ref. 20).

Figure 4. Exacerbated cardiac hypertrophy in PAK1^cko mice after 5 weeks transverse aortic constriction (TAC). (A) Heart weight/Tibia length (HW/TL) ratios of PAK1^f/f and PAK1^cko mice (upper panel). Morphometry demonstrates greater hypertrophy in PAK1^cko-TAC mice (lower panel, scale bar: 2 mm). (B) Measurements of mean cross-sectional areas (upper panel) and hematoxylin and eosin staining of heart cross-sections (lower panel, scale bar: 20 µm) (adapted from ref. 6).

Figure 5. Regulation of cardiac excitation/contraction and hypertrophy by PAK1. In cardiomyocytes, PAK1 regulates activities of ion channels and myofilaments through PP2A activation. Cardiac hypertrophy induced under pathological conditions is suppressed by PAK1, which inactivates the transcription factor NFAT through JNK.