Mice express S and M opsins that form visual pigments for the detection of light and visual signaling in cones. Here, we show that S opsin transcription is higher than that of M opsin, which supports ultraviolet (UV) sensitivity greater than midwavelength sensitivity. Surprisingly, most cones coexpress both S and M opsins in a common cone cell type throughout the retina. All cones express M opsin, but the levels are graded from dorsal to ventral. The levels of S opsin are relatively constant. However, in the far dorsal retina, S opsin is repressed stochastically, such that some cones express M opsin only. These observations indicate that two different mechanisms control M and S opsin expression. We suggest that a common cone type is patterned across the retinal surface to produce phenotypic cone subtypes.

Wound healing is primarily controlled by the proliferation and migration of keratinocytes and fibroblasts as well as the complex interactions between these two cell types. To investigate the interactions between keratinocytes and fibroblasts and the effects of direct cell-to-cell contact on the proliferation and migration of keratinocytes, keratinocytes and fibroblasts were stained with different fluorescence dyes and co-cultured with or without transwells. During the early stage (first 5 days) of the culture, the keratinocytes in contact with fibroblasts proliferated significantly faster than those not in contact with fibroblasts, but in the late stage (11th to 15th day), keratinocyte growth slowed down in all cultures unless EGF was added. In addition, keratinocyte migration was enhanced in co-cultures with fibroblasts in direct contact, but not in the transwells. Furthermore, the effects of the fibroblasts on keratinocyte migration and growth at early culture stage correlated with heparin-binding EGF-like growth factor (HB-EGF), IL-1α and TGF-β1 levels in the cultures where the cells were grown in direct contact. These effects were inhibited by anti-HB-EGF, anti-IL-1α and anti-TGF-β1 antibodies and anti-HB-EGF showed the greatest inhibition. Co-culture of keratinocytes and IL-1α and TGF-β1 siRNA-transfected fibroblasts exhibited a significant reduction in HB-EGF production and keratinocyte proliferation. These results suggest that contact with fibroblasts stimulates the migration and proliferation of keratinocytes during wound healing, and that HB-EGF plays a central role in this process and can be up-regulated by IL-1α and TGF-β1, which also regulate keratinocyte proliferation differently during the early and late stage.

Cone photoreceptors in the murine retina are patterned by dorsal repression and ventral activation of S opsin. TR␤2, the nuclear thyroid hormone receptor ␤ isoform 2, regulates dorsal repression. To determine the molecular mechanism by which TR␤2 acts, we compared the spatiotemporal expression of TR␤2 and S opsin from embryonic day (E) 13 through adulthood in C57BL/6 retinae. TR␤2 and S opsin are expressed in cone photoreceptors only. Both are transcribed by E13, and their levels increase with cone genesis. TR␤2 is expressed uniformly, but transiently, across the retina. mRNA levels are maximal by E17 at completion of cone genesis and again minimal before P5. S opsin is also transcribed by E13, but only in ventral cones. Repression in dorsal cones is established by E17, consistent with the occurrence of patterning during cone cell genesis. The uniform expression of TR␤2 suggests that repression of S opsin requires other dorsalspecific factors in addition to TR␤2. The mechanism by which TR␤2 functions was probed in transgenic animals with TR␤2 ablated, TR␤2 that is DNA binding defective, and TR␤2 that is ligand binding defective. These studies show that TR␤2 is necessary for dorsal repression, but not ventral activation of S opsin. TR␤2 must bind DNA and the ligand T3 (thyroid hormone) to repress S opsin. Once repression is established, T3 no longer regulates dorsal S opsin repression in adult animals. The transient, embryonic action of TR␤2 is consistent with a role (direct and/or indirect) in chromatin remodeling that leads to permanent gene silencing in terminally differentiated, dorsal cone photoreceptors.

No abstract

Arrestin, which plays a role in the termination of the visual transduction cascade, is one of several photoreceptor proteins whose mRNA levels are increased by light. Retinoic acid, a by-product of photoreceptor signaling and a potent modulator of hormonal transcription control, is one candidate for regulating the arrestin mRNA levels. Here we show that retinoic acid, injected intraperitoneally into dark-adapted mice, increases the arrestin mRNA levels and mimics the effect of light. Injection of 1 mumol of retinoic acid produces a maximal increase in arrestin mRNA levels. The mRNA level reaches a maximum 3 h after injection and slowly declines thereafter. The observations suggest that retinoic acid may mediate the increase in arrestin mRNA produced by light.