DreEX0044597 @ danRer10

MAP2K7 recognizes its target JNK by a novel mechanism involving a partially cooperative interaction of three low affinity D-sites in the N-terminal domain of MAP2K7, in the alternative exon. Mutations of the conserved residues within any one of the three docking sites (D1, D2, and D3) disrupted the ability of the N-terminal domain of MAP2K7beta to bind JNK1 by about 50-70%. Moreover, mutation of any two of the three D-sites reduced binding by about 80-90%, and mutation of all three reduced binding by 95%.

In response to T-cell activation, the Jun kinase (JNK) kinase MAP kinase kinase 7 (MKK7, MAP2K7) is alternatively spliced to favor an isoform that lacks exon 2. This isoform restores a JNK-docking site within MAP2K7 that is disrupted in the larger isoform. Consistently, skipping of MAP2K7 exon 2 enhances JNK pathway activity, as indicated by c-Jun phosphorylation and up-regulation of TNF-alpha. Moreover, this splicing event is itself dependent on JNK signaling. Thus, MAP2K7 alternative splicing represents a positive feedback loop through which JNK promotes its own signaling. Repression of MAP2K7 exon 2 is dependent on the presence of flanking sequences and the JNK-induced expression of the RNA-binding protein CELF2, which binds to these regulatory elements. Finally, _25% of T-cell receptor-mediated alternative splicing events are dependent on JNK signaling. These JNK-dependent events are also significantly enriched for responsiveness to CELF2.

Down-regulation of MBNL1 predicted poor overall survival in breast, lung, and stomach adenocarcinomas and increased relapse and distant metastasis in triple-negative breast cancer. A discrete set of alternative splicing events (ASEs) are shared between MBNL1-low cancers and embryonic stem cells including a MAP2K7 delta-exon2 splice variant that leads to increased stem/progenitor-like properties via JNK activation. Accordingly, JNK inhibition is capable of reversing MAP2K7 delta-exon2-driven tumor dedifferentiation in MBNL1-low cancer cells. To directly examine the role of MAP2K7 delta-exon2 splice isoform in cellular dedifferentiation and cancer stem/progenitor-like properties, the authors used an antisense morpholino (AMO) directed against the 5_ splice site of MAP2K7 exon2 to force MAP2K7 exon2 skipping in HFE-145 cells. This produced robust exon2 skipping and activation of JNK signaling as previously reported. Most importantly it drove up-regulation of CSC and pluripotency markers (CD133, NANOG, and SOX2) and increased trans-well migration and invasion, thereby recapitulating stemness phenotype observed upon loss of MBNL1 expression. Surprisingly, the MAP2K7 AMO also led to up-regulation of MAP2K7 and a modest (_30%) down-regulation of MBNL1 proteins, suggesting feedback signaling.