Mitogen-activated protein kinase phosphatase-1 (MKP-1): a critical regulator of innate immune responses

MAP kinase phosphatase-1 deficiency impairs skeletal muscle regeneration and exacerbates muscular dystrophy

In skeletal muscle, the mitogen-activated protein kinase (MAPK) phosphatase-1 (MKP-1) is a critical negative regulator of the MAPKs. Since the MAPKs have been reported to be both positive and negative for myogenesis, the physiological role of MKP-1 in skeletal muscle repair and regeneration has remained unclear. Here, we show that MKP-1 plays an essential role in adult regenerative myogenesis. In a cardiotoxin-induced muscle injury model, lack of MKP-1 impaired muscle regeneration. In mdx mice, MKP-1 deficiency reduced body weight, muscle mass, and muscle fiber cross-sectional area. In addition, MKP-1-deficient muscles exhibit exacerbated myopathy accompanied by increased inflammation. Lack of MKP-1 compromised myoblast proliferation and induced precocious differentiation, phenotypes that were rescued by pharmacological inhibition of p38alpha/beta MAPK. MKP-1 coordinates both myoblast proliferation and differentiation. Mechanistically, MyoD bound to the MKP-1 promoter and activated MKP-1 expression in proliferating myoblasts. Later, during myogenesis, MyoD uncoupled from the MKP-1 promoter leading to the down-regulation of MKP-1 and facilitation of promyogenic p38alpha/beta MAPK signaling. Hence, MKP-1 plays a critical role in muscle stem cells and in the immune response to coordinate muscle repair and regeneration.

Regulation of JNK and p38 MAPK in the immune system: signal integration, propagation and termination

Stress-activated MAP kinases (MAPKs), comprised of JNK and p38, play prominent roles in the innate and adaptive immune systems. Activation of MAPKs is mediated by a three-tiered kinase module comprised of MAPK kinase kinases (MAP3Ks), MAPK kinases (MAP2Ks) and MAPKs through sequential protein phosphorylation. Activated MAPKs, in turn, phosphorylate transcription factors and other targets to regulate gene transcription and immune responses. Recent studies have provided new insight into the upstream and downstream components of the MAPK pathway that facilitate the activation and propagation of MAPK signaling in immune responses. Moreover, MAPK activity is negatively regulated by MAPK phosphatases (MKPs), a group of dual-specificity phosphatases that dephosphorylate and inactivate the MAPKs. Here we discuss the recent advances in our understanding of these regulatory processes in MAPK signaling with a focus on their impacts on immune function.