Subcellular localization of Mitf in monocytic cells

Mitf over-expression leads to microphthalmia and coloboma in Mitf-cre mice

The Mitf transcription factor is a critical regulator of the melanocyte lineage and eye development. Mitf activity in different cell types is controlled in part by ten alternative promoters and their resulting isoforms. A useful tool for melanocyte-based research, Mitf-cre was designed to express Cre from the Mitf-M promoter, which is melanocyte specific. However, Mitf-cre mice are also microphthalmic, perhaps because of insertional mutagenesis or disrupted gene expression. Here, we investigated these possibilities and described the eye phenotype. Targeted locus amplification indicated that the transgene integrated on chromosome 2, in between Spred1 and Meis2. The BAC transgene used to make Mitf-cre was larger than expected, carrying three upstream alternative promoters, Mitf-H, Mitf-D, and Mitf-B, which could express their isoforms intact off the transgene. RT-qPCR using eye tissue demonstrated a 5-fold increase in Mitf transcripts containing exon 1B1b, which is shared by Mitf-H, Mitf-D, and Mitf-B, while Spred1 and Meis2 did not differ in their expression. These findings clarify and support the usage of Mitf-cre in conditional mutagenesis in melanocytes. The specific over-expression of these isoforms, which are preferentially expressed in the RPE, presents a unique resource for those interested in eye development and coloboma.

TGF-β2 upregulates tyrosinase activity via Opsin3 in human skin melanocytes in vitro

Opsin3 (OPN3) is a potential key regulator of human melanocyte melanogenesis. How OPN3-mediated regulation of melanocyte melanogenesis is triggered is largely unclear. Transforming growth factor-β (TGF-β) can inhibit the growth of human melanocytes and reduce melanin synthesis in melanocytes. However, whether TGF-β2 can modulate pigmentation in normal human primary melanocytes via OPN3 is entirely unknown. Here, we constructed a co-culture model with human epidermal melanocytes and keratinocytes. Higher OPN3, tyrosinase (TYR), tyrosinase-related protein (TRP)-1, and TRP-2 expression and higher tyrosinase activity were detected in co-cultured cells than in mono-cultured cells. Moreover, elevated levels of TGF-β2 were detected in the culture supernatant of melanocytes co-cultured with keratinocytes. OPN3 inhibition in melanocytes decreased TYR, TRP-1, and TRP-2 expression and downregulated tyrosinase activity. Our findings indicate that TGF-β2 upregulates TYR, TRP-1 and TRP-2 expression in human melanocytes via OPN3 and downstream calcium-dependent G protein-coupled signalling pathways to induce melanogenesis. Interestingly, treatment with the TGF-β2 receptor inhibitor LY2109761 (10 μM) did not inhibit TGF-β2-induced melanocyte melanogenesis via OPN3. Collectively, our data suggest that TGF-β2 upregulates tyrosinase activity via OPN3 through a TGF-β2R-independent and calcium-dependent G protein-coupled signalling pathway.

Subcellular localization and stability of MITF are modulated by the bHLH-Zip domain

Microphthalmia-associated transcription factor (MITF) is a member of the basic helix-loop-helix leucine zipper (bHLH-Zip) family and functions as the master regulator of the melanocytic lineage. MITF-M is the predominant isoform expressed in melanocytes and melanoma cells, and, unlike other MITF isoforms, it is constitutively nuclear. Mutational analysis revealed three karyophilic signals in the bHLH-Zip domain of MITF-M, spanning residues 197-206, 214-217, and 255-265. Structural characterization of the MITF protein showed that basic residues within these signals are exposed for interactions in the absence of DNA. Moreover, our data indicate that neither DNA binding nor dimerization of MITF-M are required for its nuclear localization. Finally, dimerization-deficient MITF-M mutants exhibited a significantly reduced stability in melanoma cells when compared to the wild-type protein. Taken together, we have shown that, in addition to its well-established role in DNA binding and dimer formation, the bHLH-Zip domain of MITF modulates the transcription factor's subcellular localization and stability.

Regulation of cell proliferation in the retinal pigment epithelium: Differential regulation of the death-associated protein like-1 DAPL1 by alternative MITF splice forms

Vertebrate eye development and homoeostasis critically depend on the regulation of proliferation of cells forming the retinal pigment epithelium (RPE). Previous results indicated that the death-associated protein like-1 DAPL1 cell autonomously suppresses RPE proliferation in vivo and in vitro. Here, we show in human RPE cell lines that the pigment cell transcription factor MITF regulates RPE cell proliferation by upregulating DAPL1 expression. DAPL1 regulation by MITF is, however, mediated predominantly by (-) MITF, one of two alternative splice isoforms of MITF that lacks six residues located upstream of the DNA-binding basic domain. Furthermore, we find that the regulation of DAPL1 by MITF is indirect in that (-) MITF stimulates the transcription of Musashi homolog-2 (MSI2), which negatively regulates the processing of the anti-DAPL1 microRNA miR-7. Our results provide molecular insights into the regulation of RPE cell proliferation and quiescence and may help us understand the mechanisms of normal RPE maintenance and of eye diseases associated with either RPE hyperproliferation or the lack of regenerative proliferation.

KITD816V Induces SRC-Mediated Tyrosine Phosphorylation of MITF and Altered Transcription Program in Melanoma

The oncogenic D816V mutation of the KIT receptor is well characterized in systemic mastocytosis and acute myeloid leukemia. Although KIT^D816V has been found in melanoma, its function and involvement in this malignancy is not understood. Here we show that KIT^D816V induces tyrosine phosphorylation of MITF through a triple protein complex formation between KIT, MITF, and SRC family kinases. In turn, phosphorylated MITF activates target genes that are involved in melanoma proliferation, cell-cycle progression, suppression of senescence, survival, and invasion. By blocking the triple protein complex formation, thus preventing MITF phosphorylation, the cells became hypersensitive to SRC inhibitors. We have therefore delineated a mechanism behind the oncogenic effects of KIT^D816V in melanoma and provided a rationale for the heightened SRC inhibitor sensitivity in KIT^D816V transformed cells.Implications: This study demonstrates that an oncogenic tyrosine kinase mutant, KIT^D816V, can alter the transcriptional program of the transcription factor MITF in melanoma Mol Cancer Res; 15(9); 1265-74. ©2017 AACR.

Efficient delivery and functional expression of transfected modified mRNA in human embryonic stem cell-derived retinal pigmented epithelial cells

Gene- and cell-based therapies are promising strategies for the treatment of degenerative retinal diseases such as age-related macular degeneration, Stargardt disease, and retinitis pigmentosa. Cellular engineering before transplantation may allow the delivery of cellular factors that can promote functional improvements, such as increased engraftment or survival of transplanted cells. A current challenge in traditional DNA-based vector transfection is to find a delivery system that is both safe and efficient, but using mRNA as an alternative to DNA can circumvent these major roadblocks. In this study, we show that both unmodified and modified mRNA can be delivered to retinal pigmented epithelial (RPE) cells with a high efficiency compared with conventional plasmid delivery systems. On the other hand, administration of unmodified mRNA induced a strong innate immune response that was almost absent when using modified mRNA. Importantly, transfection of mRNA encoding a key regulator of RPE gene expression, microphthalmia-associated transcription factor (MITF), confirmed the functionality of the delivered mRNA. Immunostaining showed that transfection with either type of mRNA led to the expression of roughly equal levels of MITF, primarily localized in the nucleus. Despite these findings, quantitative RT-PCR analyses showed that the activation of the expression of MITF target genes was higher following transfection with modified mRNA compared with unmodified mRNA. Our findings, therefore, show that modified mRNA transfection can be applied to human embryonic stem cell-derived RPE cells and that the method is safe, efficient, and functional.

Molecular analysis of the amphioxus frontal eye unravels the evolutionary origin of the retina and pigment cells of the vertebrate eye

The origin of vertebrate eyes is still enigmatic. The "frontal eye" of amphioxus, our most primitive chordate relative, has long been recognized as a candidate precursor to the vertebrate eyes. However, the amphioxus frontal eye is composed of simple ciliated cells, unlike vertebrate rods and cones, which display more elaborate, surface-extended cilia. So far, the only evidence that the frontal eye indeed might be sensitive to light has been the presence of a ciliated putative sensory cell in the close vicinity of dark pigment cells. We set out to characterize the cell types of the amphioxus frontal eye molecularly, to test their possible relatedness to the cell types of vertebrate eyes. We show that the cells of the frontal eye specifically coexpress a combination of transcription factors and opsins typical of the vertebrate eye photoreceptors and an inhibitory Gi-type alpha subunit of the G protein, indicating an off-responding phototransductory cascade. Furthermore, the pigmented cells match the retinal pigmented epithelium in melanin content and regulatory signature. Finally, we reveal axonal projections of the frontal eye that resemble the basic photosensory-motor circuit of the vertebrate forebrain. These results support homology of the amphioxus frontal eye and the vertebrate eyes and yield insights into their evolutionary origin.

Histochemistry and cell biology: the annual review 2010

This review summarizes recent advances in histochemistry and cell biology which complement and extend our knowledge regarding various aspects of protein functions, cell and tissue biology, employing appropriate in vivo model systems in conjunction with established and novel approaches. In this context several non-expected results and discoveries were obtained which paved the way of research into new directions. Once the reader embarks on reading this review, it quickly becomes quite obvious that the studies contribute not only to a better understanding of fundamental biological processes but also provide use-oriented aspects that can be derived therefrom.