The POU domain factor Skin-1a represses the keratin 14 promoter independent of DNA binding. A possible role for interactions between Skn-1a and CREB-binding protein/p300.

The genes encoding keratin 5 and 14 are highly expressed in the basal cell layer keratinocytes of the epidermis, but both genes are silenced when keratinocytes move into the suprabasal compartment. The POU homeodomain factors Skn-1a and Tst-1, which are expressed in epidermis, may play a role in the suprabasal repression of the keratin 5 and 14 genes because keratin 14 mRNA expression persists in suprabasal cells in Skn-1/Tst-1 double knockout mice. In transfection experiments, both Skn-1a and Tst-1 repress the keratin 14 promoter, with the POU domain being sufficient for repression. The region of the keratin 14 gene sufficient and required for repression by Skn-1a is a 100-base pair sequence lacking POU-binding sites adjacent to the transcription start site. DNA-binding defective mutants of Skn-1a and Tst-1 are as effective at mediating repression as the wild type proteins, suggesting that protein-protein interactions rather than direct DNA binding are important for repression. We also show that CREB-binding protein (CBP)/p300 co-activators are strong activators of keratin 14 gene expression, acting through sequences close to the keratin 14 promoter. Further, CBP interacts directly with the POU domain of Skn-1a, and increasing concentrations of CBP can overcome Skn-1a-mediated repression, suggesting that POU domain factors may repress keratin 14 gene expression by interfering with the activity of co-activators such as CBP/p300.

Expression of LMO-4 in the central nervous system of the embryonic and adult mouse.

LMO (LIM-only) proteins constitute a family of coregulators thought to act as adapters in transcriptionally active complexes. In this manuscript, we describe in situ hybridization studies to elucidate the expression of the recently identified LMO-4 in embryonic and adult mouse brain. We found that LMO-4 expression was very widespread at early (E10.5-11.5) embryonic stages with more intense hybridization signal in certain regions. The expression became more regionalized over mid (E14.5-15.5) and late (E17.5-P0) embryonic stages. Throughout development the ventricular zone, consisting of less differentiated cells, showed less intense signal than the subventricular and marginal zones. Further, expression of LMO-4 in and surrounding myelinating axons, representing Schwann cells, was seen at late embryonic stages. In the adult brain, a restricted number of regions displayed significant LMO-4 expression. Interestingly, these were the same regions that showed most consistent and intense hybridization signal during embryonic development. These regions included the cerebral cortex (layer 2-6), the hippocampus (e.g., the CA3 layer), the dentate gyrus, the basal ganglia, the lateral amygdala, olfactory structures, the paraventricular, dorsomedial and ventromedial hypothalamic nuclei, zona incerta, intralaminar and reticular nuclei of the ventral thalamus, medullary motor structures and the cerebellum. In summary, the regionalized expression of LMO-4 during development suggest a role for this gene in differentiation and specification of specific brain regions involved in various functions, such as autonomic, motor and neuroendocrine regulation.

Mouse deformed epidermal autoregulatory factor 1 recruits a LIM domain factor, LMO-4, and CLIM coregulators.

Nuclear LIM domains interact with a family of coregulators referred to as Clim/Ldb/Nli. Although one family member, Clim-2/Ldb-1/Nli, is highly expressed in epidermal keratinocytes, no nuclear LIM domain factor is known to be expressed in epidermis. Therefore, we used the conserved LIM-interaction domain of Clim coregulators to screen for LIM domain factors in adult and embryonic mouse skin expression libraries and isolated a factor that is highly homologous to the previously described LIM-only proteins LMO-1, -2, and -3. This factor, referred to as LMO-4, is expressed in overlapping manner with Clim-2 in epidermis and in several other regions, including epithelial cells of the gastrointestinal, respiratory and genitourinary tracts, developing cartilage, pituitary gland, and discrete regions of the central and peripheral nervous system. Like LMO-2, LMO-4 interacts strongly with Clim factors via its LIM domain. Because LMO/Clim complexes are thought to regulate gene expression by associating with DNA-binding proteins, we used LMO-4 as a bait to screen for such DNA-binding proteins in epidermis and isolated the mouse homologue of Drosophila Deformed epidermal autoregulatory factor 1 (DEAF-1), a DNA-binding protein that interacts with regulatory sequences first described in the Deformed epidermal autoregulatory element. The interaction between LMO-4 and mouse DEAF-1 maps to a proline-rich C-terminal domain of mouse DEAF-1, distinct from the helix-loop-helix and GATA domains previously shown to interact with LMOs, thus defining an additional LIM-interacting domain.