Isolation and developmental expression of Mitf in Xenopus laevis.
Mitf (gene for microphthalmia-associated transcription factor) encodes a transcription factor of the basic/helix-loop-helix/leucine-zipper family and is a key regulator during the development of two different types of melanin-producing cell lineages, namely neural crest-derived melanocytes/melanophores, and the retinal pigment epithelium (RPE) differentiated from the outer layer of the eye cup. Mitf-deficient mice show a lack of melanocytes and small eyes caused by abnormal RPE development. An interesting feature of Mitf is the existence of multiple isoforms with different amino termini and their functions in the development of these melanin-producing pigment cells. In this study, we isolated two Mitf homologues (XlMitfalpha and XlMitfbeta) and their isoforms from Xenopus laevis. Alignment analysis of the amino acid sequences of the N-termini suggests that these isoforms are homologues of mouse Mitf-M (expressed specifically in the melanocyte lineage) and Mitf-A (strongly expressed in the RPE, although this expression is ubiquitous). In Xenopus, XlMitfalpha is strongly expressed in the melanophore lineage (especially in premigratory melanoblasts) and the presumptive RPE and the epiphysis, in which melanin-producing cells differentiate in some vertebrates. Conservation of the Mitf isoforms expected to possess specific functions in the development of melanin-producing cells and of the expressions in such cell types in Xenopus suggest that XlMitf plays a central role in the development of melanin-producing cell lineages, and that, as in mice and humans, most of the signaling molecules or transcription factors implicated genetically in the development of melanin-producing cell lineages affect either Mitf expression or its function (Goding  Genes Dev. 14:1712-1728).
PubMed ID: 15108314
Article link: Dev Dyn.
Genes referenced: crx mitf rpe
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|Figure 2. Expression patterns of XlMitf alpha . A: No hybridization signal was obtained with a sense RNA probe (data not shown for other stages). XlMitf alpha expression is first detected in a population of cells located on the neural tube that are thought to be premigratory melanoblast (B and C, arrowheads) at stage 21/22. B,C: B is a dorsal view of C. D: At stage 23, XlMitf alpha expression is detected in the prospective retinal pigment epithelium (arrow). E-G: At stage 27, XlMitf alpha expression is detected in the otic vesicles (E and F, black arrowheads) and in the epiphysis (E, F, insets in F, G, white arrow). The insets in F show a frontal view of XlMitf alpha and XOtx5b (marker of epiphysis) expression. White arrows show the prospective epiphysis. H: At stage 29/30, XlMitf alpha is strongly expressed in the RPE (black arrow) and epiphysis (white arrow). I: At stage 37/38, XlMitf alpha expression in both melanoblasts and RPE decreases and is almost undetectable. In contrast, XlMitf alpha expression in the lens is detectable at stage 37/38 (black arrow). J: XlDct-expressing cells show melanophores (arrowheads) and RPE (arrow). C-F,H-J show lateral views; A,B,G, dorsal views; insets in F, frontal views. In A-J, left is anterior; F,G, higher magnifications of E. EP, epiphysis; LE, lens; MB, melanoblast; MP, melanophore; OV, otic vesicle; RPE, retinal pigment epithelium.|
|Figure 3. Expression patterns of XlMitf alpha on sections. A-D: Sections of embryos after whole-mount in situ hybridization. A: At stage 23, XlMitf alpha is expressed in cells located on the neural tube (A, black arrowhead). B: At stage 27, XlMitf alpha is expressed in cells located along the medial migration pathway of neural crest cells (yellow arrowhead). XlMitf alpha -positive cells shown by a black arrowhead are thought to have taken either a lateral or a ventral pathway (black arrowhead). C,D: At stage 29/30, XlMitf alpha is expressed in the RPE (black arrows) and epiphysis (white arrowheads in D). E: At stage 37/38, XlMitf alpha expression is detected in the lens (yellow arrows), although that in RPE is decreased. F,G: Serial sections of the trunk of a stage 27 embryo. XlMitf alpha -positive melanoblasts on the neural tube (black arrowhead in F) are also XlDct-positive (black arrowhead in G) in adjacent sections. XlMitf alpha expression is also detected in cells located along the medial pathway (F, yellow arrowheads). The staining in other organ cells not mentioned above, especially in the ectoderm, is background. As for stage 37/38 embryos (E), we performed in situ hybridization on sections to reduce background. BR, brain; EP, epiphysis; LE, lens; NO, notochord; NT, neural tube; RPE, retinal pigment epithelium.|