Delineating the Role of MITF Isoforms in Pigmentation and Tissue Homeostasis
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ABSTRACT:
ORGANISM(S): Mus musculus
PROVIDER: GSE138538 | GEO | 2019/12/31
REPOSITORIES: GEO
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Project description:Damage to the gene regulatory network governing terminal differentiation of melanocytes leads to pigmentation phenotypes and increases the risk for melanoma. Microphthalmia-associated transcription factor (MITF) directly activates expression of melanocyte differentiation effectors, and levels of MITF have been proposed to govern the melanoma phenotype. Mutations in the gene encoding Transcription Factor Activator Protein 2 alpha (TFAP2A) cause reduced pigmentation in model organisms and premature hair graying in humans, and TFAP2A expression tends to be lower in advanced melanoma tumors than in benign nevi. However, the transcriptional targets of TFAP2A in melanocytes, and the epistatic relationship of TFAP2A and MITF, have been unclear. Using microarray-based analysis of zebrafish tfap2a mutant embryos, we generated a profile of genes whose expression is Tfap2a-dependent. We conducted anti-TFAP2A chromatin immunoprecipitation followed by next-generation sequencing (ChIP-seq) in immortalized mouse melanocytes and human primary melanocytes, and discovered that TFAP2A peaks are present near the promoters of Tfap2a-dependent genes expressed in melanocytes, and also at the majority of enhancers active in melanocytes. Comparison of TFAP2A ChIP-seq data to published MITF ChIP-seq data showed that the set of genes with promoters bound by both MITF and TFAP2A is enriched for the gene ontology term “pigment cell differentiation.” Deletion analysis of one such co-bound promoter, for Transient Receptor Potential Melastatin-like 1(TRPM1), confirmed that its expression depends on the presence of MITF binding sites as previously shown, but also depends on the presence of TFAP2A binding sites. Finally, we find that mitfa and tfap2a interact genetically in zebrafish. Collectively, these results show that TFAP2A, operating in parallel with MITF, directly regulates effectors of terminal differentiation in melanocytes and melanoma.
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Project description:The iris is a fine structure that controls the amount of light that enters the eye. Iris diseases that result in visual disability and blindness include iritis, primary angle closure glaucoma, and pigment dispersion syndrome. A detailed investigation of the proteome of the normal human iris may provide a foundation for new investigations into the iris biology and pathophysiology. We conducted an in-depth proteomic analysis of five normal irides. Proteins were fractionated using SDS-PAGE. After in-gel digestion, peptides were analyzed using LC-MS/MS on an Orbitrap Elite mass spectrometer. We identified 3,000 non-redundant proteins in the human iris, including 26 unambiguous protein isoforms. The proteins identified in the iris included numerous proteins involved in smooth muscle motility, melanosome development, extracellular matrix, and selenoproteins involved in redox signaling. The MS proteome database of the human iris may serve as a valuable resource for future investigations of the eye in health and disease.
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Project description:Skin pigmentation is paused following sun exposure, however the mechanism behind this pausing is unknown. Here we found that the UVB-induced DNA repair system, led by the ATM protein kinase, represses MITF transcriptional activity of pigmentation genes while placing MITF in DNA repair mode, thus directly inhibiting pigment production. Phosphoproteomics analysis revealed ATM to be the most significantly enriched pathway among all UVB-induced DNA repair systems. ATM inhibition in mouse or human skin, either genetically or chemically, induces pigmentation. Upon UVB, MITF transcriptional activation is blocked due to ATM dependent phosphorylation of MITF on S414, which modifies MITF activity and interactome towards DNA repair including binding to TRIM28 and RBBP4. Accordingly, MITF genome-occupancy is enriched in sites of high DNA damage that are likely repaired. This suggests that ATM harnesses the pigmentation key activator, for the necessary rapid, efficient DNA repair, thus optimizing the chances of the cell to survive.
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Project description:The close integration of the MAPK, PI3K and WNT signaling pathways underpins much of development and is deregulated in cancer. In principle, combinatorial post-translational modification of key lineage–specific transcription factors would be an effective means to integrate critical signaling events. Understanding how this might be achieved is central to deciphering the impact of microenvironmental cues in development and disease. The microphthalmia associated transcription factor, MITF, plays a crucial role in the development of melanocytes, the retinal pigment epithelium, osteoclasts and mast cells, and acts as a lineage survival oncogene in melanoma. MITF coordinates survival, differentiation, cell cycle progression, cell migration, metabolism and lysosome biogenesis. Yet how the activity of this key transcription factor is controlled remains poorly understood. Here we show that GSK3b, downstream from both the PI3K and Wnt pathways, and BRAF/MAPK signaling converge to control MITF nuclear export. Phosphorylation of the melanocyte MITF-M isoform in response to BRAF/MAPK signaling primes for phosphorylation by GSK3b, a kinase inhibited by both PI3K and Wnt signaling. Dual phosphorylation, but not monophosphorylation, then promotes MITF nuclear export by activating a previously unrecognized hydrophobic export signal. Nonmelanocyte MITF isoforms exhibit poor regulation by MAPK signaling, but instead their export is controlled by mTOR. We uncover here an unanticipated mode of MITF regulation that integrates the output of key developmental and cancer-associated signaling pathways to gate MITF flux through the import-export cycle. The results have significant implications for our understanding of melanoma progression and stem cell renewal
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Project description:Mutations in the gene encoding transcription factor TFAP2A result in pigmentation anomalies in model organisms and premature hair graying in humans. However, the pleiotropic functions of TFAP2A and its redundantly-acting paralogs have made the precise contribution of TFAP2-type activity to melanocyte differentiation unclear. Defining this contribution may help to explain why TFAP2A expression is reduced in advanced-stage melanoma compared to benign nevi. To identify genes with TFAP2A-dependent expression in melanocytes, we profile zebrafish tissue and mouse melanocytes deficient in Tfap2a, and find that expression of a small subset of genes underlying pigmentation phenotypes is TFAP2A-dependent, including Dct, Mc1r, Mlph, and Pmel. We then conduct TFAP2A ChIP-seq in mouse and human melanocytes and find that a much larger subset of pigmentation genes is associated with active regulatory elements bound by TFAP2A. These elements are also frequently bound by MITF, which is considered the “master regulator” of melanocyte development. For example, the promoter of TRPM1 is bound by both TFAP2A and MITF, and we show that the activity of a minimal TRPM1 promoter is lost upon deletion of the TFAP2A binding sites. However, the expression of Trpm1 is not TFAP2A-dependent, implying that additional TFAP2 paralogs function redundantly to drive melanocyte differentiation, which is consistent with previous results from zebrafish. Paralogs Tfap2a and Tfap2b are both expressed in mouse melanocytes, and we show that mouse embryos with Wnt1-Cre-mediated deletion of Tfap2a and Tfap2b in the neural crest almost completely lack melanocytes but retain neural crest-derived sensory ganglia. These results suggest that TFAP2 paralogs, like MITF, are also necessary for induction of the melanocyte lineage. Finally, we observe a genetic interaction between tfap2a and mitfa in zebrafish, but find that artificially elevating expression of tfap2a does not increase levels of melanin in mitfa hypomorphic or loss-of-function mutants. Collectively, these results show that TFAP2 paralogs, operating alongside lineage-specific transcription factors such as MITF, directly regulate effectors of terminal differentiation in melanocytes. In addition, they suggest that TFAP2A activity, like MITF activity, has the potential to modulate the phenotype of melanoma cells.
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Project description:In developing melanocytes and in melanoma cells, multiple paralogs of the Activating-enhancer-binding Protein 2 family of transcription factors (TFAP2) contribute to expression of genes encoding pigmentation regulators, but their interaction with Microphthalmia transcription factor (MITF), a master regulator of these cells, is unclear. Supporting the model that Tfap2 facilitates MITF's ability to activate expression of pigmentation genes, single-cell seq analysis of zebrafish embryos revealed that pigmentation genes are only expressed in the subset of mitfa-expressing cells that also express Tfap2 paralogs. To test this model in SK-MEL-28 melanoma cells we deleted the two TFAP2 paralogs with highest expression, TFAP2A and TFAP2C, creating TFAP2 knockout (TFAP2-KO) cells. We then assessed gene expression, chromatin accessibility, binding of TFAP2A and of MITF, and the chromatin marks H3K27Ac and H3K27Me3 which are characteristic of active enhancers and silenced chromatin, respectively. Integrated analyses of these datasets indicate TFAP2 paralogs directly activate enhancers near genes enriched for roles in pigmentation and proliferation, and directly repress enhancers near genes enriched for roles in cell adhesion. Consistently, compared to WT cells, TFAP2-KO cells proliferate less and adhere to one another more. TFAP2 paralogs and MITF co-operatively activate a subset of enhancers, with the latter necessary for MITF binding and chromatin accessibility. By contrast, TFAP2 paralogs and MITF do not appear to co-operatively inhibit enhancers. These studies reveal a mechanism by which TFAP2 profoundly influences the set of genes activated by MITF, and thereby the phenotype of pigment cells and melanoma cells.
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