How cells know their place

Analysis of the VMD2 Promoter and Implication of E-box Binding Factors in Its Regulation

The retinal pigment epithelium (RPE) is crucial for the normal development and function of retinal photo-receptors, and mutations in several genes that are preferentially expressed in the RPE have been shown to cause retinal degeneration. We analyzed the 5'-up-stream region of human VMD2, a gene that is preferentially expressed in the RPE and, when mutated, causes Best macular dystrophy. Transgenic mouse studies with VMD2 promoter/lacZ constructs demonstrated that a-253 to +38 bp fragment is sufficient to direct RPE-specific expression in the eye. Transient transfection assays using the D407 human RPE cell line with VMD2 promoter/luciferase reporter constructs identified two positive regulatory regions, -585 to -541 bp for high level expression and -56 to -42 bp for low level expression. Mutation of a canonical E-box located in the -56 to -42 bp region greatly diminished luciferase expression in D407 cells and abolished the bands shifted with bovine RPE nuclear extract in electrophoretic mobility shift assays. Independently a candidate approach was used to select microphthalmia-associated transcription factor (MITF) for testing because it is expressed in the RPE and associated with RPE abnormalities when mutated. MITF-M significantly increased luciferase expression in D407 cells in an E-box-dependent manner. These studies define the VMD2 promoter region sufficient to drive RPE-specific expression in the eye, identify positive regulatory regions in vitro, and suggest that MITF as well as other E-box binding factors may act as positive regulators of VMD2 expression.

RER, an evolutionarily conserved sequence upstream of the rhodopsin gene, has enhancer activity

Previous transgenic mouse experiments localized the mammalian rhodopsin gene promoter to a region just upstream of the mRNA start site, and also suggested the existence of a second more distal regulatory region. A highly conserved 100-base pair (bp) sequence which is homologous to the red and green opsin locus control region is located 1.5-2 kilobases upstream of the rhodopsin gene (depending on the species). In order to test the activity of this 100-bp region, transgenic mice were generated with bovine rhodopsin promoter/lacZ constructs which differed only by the presence or absence of the sequence. Of 11 lines generated, all demonstrated photoreceptor-specific expression of the transgene, but the lines with the putative regulatory region showed significantly higher expression. Additional transgenic lines in which the region was fused to a minimal heterologous promoter did not show transgene expression in the retina. Gel mobility shift and DNase I footprint assays demonstrated that bovine retinal nuclear extracts contain retina-specific as well as ubiquitously expressed factors that interact with the putative regulatory region in a sequence-specific manner. These results indicate that the 100-bp sequence can indeed function in vivo as a rhodopsin enhancer region.