Mastermind-like transcriptional co-activator-mediated Notch signaling is indispensable for maintaining conjunctival epithelial identity.

Conjunctival goblet cells primarily synthesize mucins to lubricate the ocular surface, which is essential for normal vision. Notch signaling has been known to associate with goblet cell differentiation in intestinal and respiratory tracts, but its function in ocular surface has yet to be fully characterized. Herein, we demonstrate that conditional inhibition of canonical Notch signaling by expressing dominant negative mastermind-like 1 (dnMaml1) in ocular surface epithelia resulted in complete suppression of goblet cell differentiation during and subsequent to development. When compared with the ocular surface of wild-type mice (OS(Wt)), expression of dnMaml1 at the ocular surface (OS(dnMaml1)) caused conjunctival epithelial hyperplasia, aberrant desquamation, failure of Mucin 5ac (Muc5ac) synthesis, subconjunctival inflammation and epidermal metaplasia in cornea. In addition, conditional deletion of Notch1 from the ocular surface epithelia partially recapitulated OS(dnMaml1) phenotypes. We have demonstrated that N1-ICD (Notch1 intracellular domain) transactivated the mouse Krüppel-like factor 4 (Klf) promoter and that Klf4 directly bound to and significantly potentiated the Muc5ac promoter. By contrast, OS(dnMaml1) dampened Klf4 and Klf5 expression, and diminished Muc5ac synthesis. Collectively, these findings indicated that Maml-mediated Notch signaling plays a pivotal role in the initiation and maintenance of goblet cell differentiation for normal ocular surface morphogenesis and homeostasis through regulation of Klf4 and Klf5.

Lack of the mesodermal homeodomain protein MEOX1 disrupts sclerotome polarity and leads to a remodeling of the cranio-cervical joints of the axial skeleton.

Meox1 and Meox2 are two related homeodomain transcription factor genes that together are essential for the development of all somite compartments. Here we show that mice homozygous for Meox1 mutations alone have abnormalities that are restricted to the sclerotome and its derivatives. A prominent and consistent phenotype of these mutations is a remodeling of the cranio-cervical joints whose major feature is the assimilation of the atlas into the basioccipital bone so that the skull rests on the axis. These abnormalities can be traced back to changes in the relative rates of cell proliferation in the rostral and caudal sclerotome compartments, and they are associated with alterations in the expression of at least three transcription factor genes, Tbx18, Uncx, and Bapx1. As previously observed for Bapx1, MEOX1 protein occupies evolutionarily conserved promoter regions of Tbx18 and Uncx, suggesting that Meox1 regulates these genes at least in part directly. Hence, Meox1 is part of a regulatory circuit that serves an essential, non-redundant function in the maintenance of rostro-caudal sclerotome polarity and axial skeleton formation.

An opsin 5-dopamine pathway mediates light-dependent vascular development in the eye.

During mouse postnatal eye development, the embryonic hyaloid vascular network regresses from the vitreous as an adaption for high-acuity vision. This process occurs with precisely controlled timing. Here, we show that opsin 5 (OPN5; also known as neuropsin)-dependent retinal light responses regulate vascular development in the postnatal eye. In Opn5-null mice, hyaloid vessels regress precociously. We demonstrate that 380-nm light stimulation via OPN5 and VGAT (the vesicular GABA/glycine transporter) in retinal ganglion cells enhances the activity of inner retinal DAT (also known as SLC6A3; a dopamine reuptake transporter) and thus suppresses vitreal dopamine. In turn, dopamine acts directly on hyaloid vascular endothelial cells to suppress the activity of vascular endothelial growth factor receptor 2 (VEGFR2) and promote hyaloid vessel regression. With OPN5 loss of function, the vitreous dopamine level is elevated and results in premature hyaloid regression. These investigations identify violet light as a developmental timing cue that, via an OPN5-dopamine pathway, regulates optic axis clearance in preparation for visual function.

The other pigment cell: specification and development of the pigmented epithelium of the vertebrate eye.

Vertebrate retinal pigment epithelium (RPE) cells are derived from the multipotent optic neuroepithelium, develop in close proximity to the retina, and are indispensible for eye organogenesis and vision. Recent advances in our understanding of RPE development provide evidence for how critical signaling factors operating in dorso-ventral and distal-proximal gradients interact with key transcription factors to specify three distinct domains in the budding optic neuroepithelium: the distal future retina, the proximal future optic stalk/optic nerve, and the dorsal future RPE. Concomitantly with domain specification, the eye primordium progresses from a vesicle to a cup, RPE pigmentation extends towards the ventral side, and the future ciliary body and iris form from the margin zone between RPE and retina. While much has been learned about the molecular networks controlling RPE cell specification, key questions concerning the cell proliferative parameters in RPE and the subsequent morphogenetic events still need to be addressed in greater detail.

Chx10 repression of Mitf is required for the maintenance of mammalian neuroretinal identity.

During vertebrate eye development, the cells of the optic vesicle (OV) become either neuroretinal progenitors expressing the transcription factor Chx10, or retinal pigment epithelium (RPE) progenitors expressing the transcription factor Mitf. Chx10 mutations lead to microphthalmia and impaired neuroretinal proliferation. Mitf mutants have a dorsal RPE-to-neuroretinal phenotypic transformation, indicating that Mitf is a determinant of RPE identity. We report here that Mitf is expressed ectopically in the Chx10(or-J/or-J) neuroretina (NR), demonstrating that Chx10 normally represses the neuroretinal expression of Mitf. The ectopic expression of Mitf in the Chx10(or-J/or-J) NR deflects it towards an RPE-like identity; this phenotype results not from a failure of neuroretinal specification, but from a partial loss of neuroretinal maintenance. Using Chx10 and Mitf transgenic and mutant mice, we have identified an antagonistic interaction between Chx10 and Mitf in regulating retinal cell identity. FGF (fibroblast growth factor) exposure in a developing OV has also been shown to repress Mitf expression. We demonstrate that the repression of Mitf by FGF is Chx10 dependent, indicating that FGF, Chx10 and Mitf are components of a pathway that determines and maintains the identity of the NR.