The p38 MAPK (mitogen-activated protein kinase) signalling pathway allows cells to interpret a wide range of external signals and respond appropriately by generating a plethora of different biological effects. The diversity and specificity in cellular outcomes is achieved with an apparently simple linear architecture of the pathway, consisting of a core of three protein kinases acting sequentially. In the present review, we dissect the molecular mechanisms underlying p38 MAPK functions, with special emphasis on the activation and regulation of the core kinases, the interplay with other signalling pathways and the nature of p38 MAPK substrates as a source of functional diversity. Finally, we discuss how genetic mouse models are facilitating the identification of physiological functions for p38 MAPKs, which may impinge on their eventual use as therapeutic targets.

p38alpha is a stress-activated protein kinase that negatively regulates malignant transformation induced by oncogenic H-Ras, although the mechanisms involved are not fully understood. Here, we show that p38alpha is not a general inhibitor of oncogenic signaling, but that it specifically modulates transformation induced by oncogenes that produce reactive oxygen species (ROS). This inhibitory effect is due to the ROS-induced activation of p38alpha early in the process of transformation, which induces apoptosis and prevents the accumulation of ROS and their carcinogenic effects. Accordingly, highly tumorigenic cancer cell lines have developed a mechanism to uncouple p38alpha activation from ROS production. Our results indicate that oxidative stress sensing plays a key role in the inhibition of tumor initiation by p38alpha.

In this report, we investigate the role of the RNA-binding protein HuR during skeletal myogenesis. At the onset of myogenesis in differentiating C2C12 myocytes and in vivo in regenerating mouse muscle, HuR cytoplasmic abundance increased dramatically, returning to a predominantly nuclear presence upon completion of myogenesis. mRNAs encoding key regulators of myogenesis-specific transcription (myogenin and MyoD) and cell cycle withdrawal (p21), bearing AU-rich regions, were found to be targets of HuR in a differentiationdependent manner. Accordingly, mRNA half-lives were highest during differentiation, declining when differentiation was completed. Importantly, HuR-overexpressing C2C12 cells displayed increased target mRNA expression and half-life and underwent precocious differentiation. Our findings underscore a critical function for HuR during skeletal myogenesis linked to HuR's coordinate regulation of muscle differentiation genes.

Activation of p38 mitogen-activated protein kinase (MAPK) plays an important role in the G 2 /M cell cycle arrest induced by DNA damage, but little is known about the role of this signaling pathway in the G 1 /S transition. Upregulation of the cyclin-dependent kinase inhibitor p21Cip1 is thought to make a major contribution to the G 1 /S cell cycle arrest induced by ␥ radiation. We show here that inhibition of p38 MAPK impairs p21Cip1 accumulation and, as a result, the ability of cells to arrest in G 1 in response to ␥ radiation. We found that p38 MAPK induces p21Cip1 mRNA stabilization, without affecting its transcription or the stability of the protein. In particular, p38 MAPK phosphorylates the mRNA binding protein HuR on Thr118, which results in cytoplasmic accumulation of HuR and its enhanced binding to the p21 Cip1 mRNA. Our findings help to understand the emerging role of p38 MAPK in the cellular responses to DNA damage and reveal the existence of p53-independent networks that cooperate in modulating p21Cip1 levels at the G 1 /S checkpoint.

Cohesin is a protein complex originally identified for its role in sister chromatid cohesion, although increasing evidence portrays it also as a major organizer of interphase chromatin. Vertebrate cohesin consists of Smc1, Smc3, Rad21/Scc1 and either stromal antigen 1 (SA1) or SA2. To explore the functional specificity of these two versions of cohesin and their relevance for embryonic development and cancer, we generated a mouse model deficient for SA1. Complete ablation of SA1 results in embryonic lethality, while heterozygous animals have shorter lifespan and earlier onset of tumourigenesis. SA1-null mouse embryonic fibroblasts show decreased proliferation and increased aneuploidy as a result of chromosome segregation defects. These defects are not caused by impaired centromeric cohesion, which depends on cohesin-SA2. Instead, they arise from defective telomere replication, which requires cohesion mediated specifically by cohesin-SA1. We propose a novel mechanism for aneuploidy generation that involves impaired telomere replication upon loss of cohesin-SA1, with clear implications in tumourigenesis.

Shugoshin-2 (SGOL2) is one of the two mammalian orthologs of the Shugoshin/Mei-S322 family of proteins that regulate sister chromatid cohesion by protecting the integrity of the multiprotein cohesin complexes. This protective system is essential for faithful chromosome segregation during mitosis and meiosis, which is the physical basis of Mendelian inheritance. Regardless of its evolutionary conservation from yeast to mammals, little is known about the in vivo relevance and specific role that SGOL2 plays in mammals. Here we show that disruption of the gene encoding mouse SGOL2 does not cause any alteration in sister chromatid cohesion in embryonic cultured fibroblasts and adult somatic tissues. Moreover, mutant mice develop normally and survive to adulthood without any apparent alteration. However, both male and female Sgol2-deficient mice are infertile. We demonstrate that SGOL2 is necessary for protecting centromeric cohesion during mammalian meiosis I. In vivo, the loss of SGOL2 promotes a premature release of the meiosis-specific REC8 cohesin complexes from anaphase I centromeres. This molecular alteration is manifested cytologically by the complete loss of centromere cohesion at metaphase II leading to single chromatids and physiologically with the formation of aneuploid gametes that give rise to infertility.[Keywords: Cohesion; chromosome segregation; Shugoshin-2; mouse; mitosis; meiosis] Supplemental material is available at http://www.genesdev.org.

Vertebrates have two cohesin complexes that consist of Smc1, Smc3, Rad21/Scc1 and either SA1 or SA2, but their functional specificity is unclear. Mouse embryos lacking SA1 show developmental delay and die before birth.Comparison of the genome-wide distribution of cohesin in wild-type and SA1-null cells reveals that SA1 is largely responsible for cohesin accumulation at promoters and at sites bound by the insulator protein CTCF. As a consequence, ablation of SA1 alters transcription of genes involved in biological processes related to Cornelia de Lange syndrome (CdLS), a genetic disorder linked to dysfunction of cohesin. We show that the presence of cohesin-SA1 at the promoter of myc and of protocadherin genes positively regulates their expression, a task that cannot be assumed by cohesin-SA2. Lack of SA1 also alters cohesin-binding pattern along some gene clusters and leads to dysregulation of genes within. We hypothesize that impaired cohesin-SA1 function in gene expression underlies the molecular aetiology of CdLS.

Two variant cohesin complexes containing SMC1, SMC3, RAD21 and either STAG/SA1 or SA2 are present in all cell types. We report here their genomic distribution and their specific contributions to genome organization in human cells. While both variants are found at CTCF sites, a fraction of cohesin-SA2 localizes to enhancers lacking CTCF, is linked to tissue-specific transcription and cannot be replaced by cohesin-SA1 when SA2 is absent, a condition observed in several tumours. Downregulation of either variant has different consequences for gene expression and genome architecture. Our results suggest that cohesin-SA1 preferentially contributes to the stabilization of TAD boundaries together with CTCF, while cohesin-SA2 promotes cell type-specific contacts between enhancers and promoters independently of CTCF. Loss of SA2 rewires local chromatin contacts and alters gene expression. These findings provide insights on how cohesin mediates chromosome folding and establish a novel framework to address the consequences of cohesin mutations in cancer.