Molecular aspects of control in epidermal differentiation

Summary

The mammalian epidermis is organized into layers of structurally different cells—the basal, spinous, granular and cornified layers—which represent steps in the differentiative process that terminates in cornification and desquamation. Investigation of the molecular mechanisms that control this ordered sequence of events provides clues to the etiology of certain epidermal pathologies. DNA synthesis and mitosis are normally restricted to the basal layer. Several substances have been implicated in the mitotic control of epidermal cells, the loss of mitotic activity being the first major step in normal keratinization. Investigations performed in this laboratory indicate that isolated differentiated nuclei can replicate their DNA which they are inhibited from doingin situ. Addition of a high speed supernate from homogenized differentiated cells inhibited this synthetic activityin vitro suggesting the existence of a cytoplasmic inhibitor of DNA synthesis. It is not known whether mitotic inhibition in differentiated epidermal cells is a function of the inhibition of DNA replication.

Contrary to previous assumptions, recent experimental evidence clearly indicates that, unlike DNA synthesis, RNA synthesis occurs in differentiated cells. Correlated with this synthetic activity is the observation that a protein rich in histidine is specifically formed in the granular cells. This protein appears to be a component of the keratohyalin granules which fill the cells of the granular layer. Investigations were conducted in this laboratory to determine whether control of the synthesis of this protein occurs at the level of translation or transcription. Translation,in vitro, of mRNA obtained from isolated populations of each epidermal cell type suggested that control of protein synthesis in the differentiating epidermis is transcriptional, i.e. only in the granular cell is there an mRNA for the histidine-rich protein. Transcription,in vitro, of chormatin isolated from the separated cell populations produced RNA with a ratio of cytidine to uracil consistent with the predicted mRNA for this protein thus providing additional support for the hypothesis that epidermal differentiation is controlled at the level of ‘gene-readout.’