Stratifin is redundant in cell cycle regulation but essential for the translational control during skin development

Three phenotypically similar mouse mutations that cause severe defects in skin terminal differentiation have been identified in stratifin ( Sfn, also known as 14-3-3sigma), I-Kappa Kinase alpha (IKKalpha) and Interleukin regulatory factor 6 (Irf6). IKKalpha and Irf6 have been proposed to regulate skin differentiation at the level of transcription. Sfn has been studied intensively as a negative cell cycle regulator during DNA damage response; however, its role in skin development has not been fully characterized.;Our detailed phenotypic analysis along with previous studies demonstrated that IKKalpha and Irf6 deficient skin shared more severe defects in terminal differentiation when compared to SfnEr/Er skin. IKKalpha and Irf6 deficient skin lack both granular layer and stratum corneum. Suprabasal cells in IKKalpha and Irf6 deficient skin express spinous cell markers and are proliferative indicating that IKKalpha and Irf6 are important for the maturation of the spinous cells from mitotically active intermediate cells. Conversely, SfnEr/Er epidermis has partially formed granular layer and SfnEr/Er suprabasal cells are mitotically inactive indicating Sfn is essential for the maturation of the granular cells.;To further define the relationship of these genes, global gene expression analysis was performed to correlate gene expression with phenotype. To this end, we discovered that consistent with the phenotypes observed, SfnEr/Er skin had a distinct gene expression profile whereas IKKalpha deficient and Irf6 deficient skin shared more similar expression profiles. Notably, there were a sub-set of differentiation-specific genes in SfnEr/Er skin whose mRNA transcript levels did not correlate with their respective protein expression. Therefore, we hypothesized that Sfn regulates these targets post-transcriptionally.;Unexpectedly Gene ontology analysis of these datasets revealed that cell cycle regulation is uniquely induced in SfnEr/Er skin. In contrast to the proposed function of Sfn in mediating cell cycle arrest in tumor cells, SfnEr/Er keratinocytes maintained normal proliferation in vivo by inducing alternative cell cycle regulators and repressing DNA synthesis.;Cell cycle profile analysis revealed a cytokenisis defect in SfnEr/Er keratinocytes indicating an impaired translation switch from cap-dependant to cap-independent translation in Sfn Er/Er keratinocytes. We proposed that Sfn acts at the level of translation to control keratinocyte differentiation by mediating the translation switch from cap-dependant to cap-independent translation. To directly test this hypothesis and identify additional targets of Sfn, microarray analysis was performed on polysome-bound RNA isolated from SfnEr/Er and WT skin. Polysome-bound RNA microarray analysis confirmed that loss of Sfn specifically inhibited the translation of a subset of differentiation-specific genes. We also identified 45 genes specifically repressed by the loss of Sfn.;Our results provided the first direct evidence that translation control is a vital part of the terminal differentiation program. Transcription factors including IKKalpha and Irf6 were induced upon terminal differentiation and orchestrated the transcription of the differentiation specific-genes. Sfn switched the translation from cap-dependent to cap-independent to ensure the efficient and coordinated translation of the differentiation-specific genes. Translation control served as the last regulatory step to propel the completion of the cornification process after keratinocytes lost their nuclei.