Studies of the role of MAP kinase-activated protein kinase-5 (MK5) in reactive and reparative fibrosis in the murine heart

MK5 is a serine/threonine kinase, originally identified as a p38 Regulated/Activated Protein Kinase (PRAK), activated by p38 MAPK and the atypical MAPKs ERK3 and ERK4. Although MK5 is expressed in the heart, its physiological function is just beginning to be studied. Herein, we studied the effects of chronic pressure overload induced by transverse aortic constriction (TAC) and the effects of myocardial infarction (MI) induced by permanent ligation of the left anterior descending coronary artery (LADL) in heterozygous mice for a functional knockout of MK5 (MK5+/-). We also studied the role of MK5 in cardiac fibroblast function and in extracellular remodeling in healthy heart in vitro using MK5 wild type (MK5+/+), haplodeficient (MK5+/-), deficient (MK5-/-), and siRNA-mediated knockdown (MK5-kd) fibroblasts. At twelve weeks of age, MK5+/- mice were smaller than age-matched wild-type littermates (MK5+/+). Left ventricular end-diastolic diameter and systolic function were reduced in MK5+/- mice. Two weeks post-TAC, heart weight/tibia length ratios (HW/TL) were similarly and significantly increased in both MK5+/+ and MK5+/- hearts. However, eight weeks post-TAC, HW/TL ratios were significantly lower in TAC-MK5+/- mice compared to TAC-MK5+/+ mice. Thus, the progression of hypertrophy in response to chronic pressure overload was attenuated in MK5+/- hearts. Furthermore, two weeks of pressure overload induced increase in collagen type 1 α 1 (COL1A1) mRNA in MK5+/+ mice and this increase was significantly attenuated in MK5+/- mice. As MK5 immunoreactivity was detected in cardiac fibroblasts but not myocytes, these findings suggest that MK5 within cardiac fibroblasts plays an important pro-fibrotic and pro-hypertrophic role in cardiac remodeling during chronic pressure overload. We then examined the effect of reduced MK5 expression on reparative fibrosis following MI. Mortality rates for MK5+/+ and MK5 +/- did not differ significantly over seven days post-MI. In contrast, mortality was higher in MK5+/- mice over twenty-one days. Systolic and diastolic functions were similarly impaired in both MK5+/+ and MK5+/- mice post-MI. Scar area and scar collagen content were reduced in MK5+/- hearts eight days post-MI. Inflammatory cell infiltration was similar in both ligated groups whereas angiogenesis was significantly greater in the peri-infarct zone in LADL-MK5+/- hearts. These results suggest that MK5 may play a role in regulating cardiac fibroblast function. Finally, we examined the effect of reduced MK5 expression on fibroblast function. Motility and proliferation were reduced in MK5-/- fibroblasts compared to MK5+/+ and MK5+/- fibroblasts. The transcriptome for proteins involved in extracellular matrix remodeling (ECM) differed depending on fibroblast genotype. Similarly, collagen 1-α1 and fibronectin secretion was increased in MK5-/- and MK5-kd fibroblasts compared with MK5+/+. In addition, knocking down MK5 decreased myofibroblast contraction. Taken together, these data suggest that MK5 is involved in regulating multiple aspects of cardiac fibroblast function. In conclusion, we have shown a role for MK5 in cardiac remodeling during chronic pressure overload and myocardial infarction. Moreover, we have demonstrated a role for MK5 in cardiac fibroblast function and extracellular matrix remodeling.

Keyphrases: MK5/PRAK, p38 MAPK, Cardiac hypertrophy, Fibroblast, Chronic pressure overload, Reparative fibrosis, Myocardial infarction, Extracellular matrix, p38MAPK