A conserved transcriptional enhancer that specifies Tyrp1 expression to melanocytes

Abstract

Pigment cells of mammals originate from two different lineages: melanocytes arise from the neural crest, whereas cells of the retinal pigment epithelium (RPE) originate from the optic cup of the developing forebrain. Previous studies have suggested that pigmentation genes are controlled by different regulatory networks in melanocytes and RPE. The promoter of the tyrosinase-related family gene Tyrp1 has been shown to drive detectable transgene expression only to the RPE, even though the gene is also expressed in melanocytes as evident from Tyrp1-mutant mice. This indicates that the regulatory elements responsible for Tyrp1 gene expression in the RPE are not sufficient for expression in melanocytes. We thus searched for a putative melanocyte-specific regulatory sequence and demonstrate that a bacterial artificial chromosome (BAC) containing the Tyrp1 gene and surrounding sequences is able to target transgenic expression to melanocytes and to rescue the Tyrp1b (brown) phenotype. This BAC contains several highly conserved non-coding sequences that might represent novel regulatory elements. We further focused on a sequence located at -15 kb, which we identified as a melanocyte-specific enhancer as shown by cell culture and transgenic mice experiments. In addition, we show that the transcription factor Sox10 can activate this conserved enhancer. The presence of a distal Tyrp1 regulatory element, which specifies melanocyte-specific expression, supports the idea that separate regulatory sequences can mediate differential gene expression in melanocytes and RPE.

RBP-Jκ-dependent Notch signaling enhances retinal pigment epithelial cell proliferation in transgenic mice

“…Notch signaling activity maintains a proliferative state in RPE precursor cells Although melanocytes and RPE cells share distinct regulatory elements controlling the expression of pigment-cell-specific genes (Murisier et al, 2006(Murisier et al, , 2007, both respond similarly to Notch signals. Although Notch activity might increase melanoblast and melanocyte numbers (Schouwey et al, 2010), and is capable to transform melanocytes (Pinnix et al, 2009), sustained Notch signaling activity in Tyrp1::NotchIC/1 RPE results in hyperproliferation and formation of tumors showing pigmented and unpigmented areas.…”

“…We generated transgenic mice expressing the intracellular domain of the Notch1 receptor (NotchIC), which functions as a ligand-activated receptor (Hsieh et al, 1996), under the control of the 1.4 kb Tyrp1 promoter (Figure 2a) directing transgene expression specifically to RPE cells from E10.5 (Jackson et al, 1991;Raymond and Jackson, 1995;Murisier et al, 2006). Even though Notch signals are mainly mediated through the Notch2 receptor in the RPE (Bao and Cepko, 1997), expression of the intracellular domain of Notch1 should be sufficient to induce the activation of RBP-Jk-dependent signaling.…”

“…In addition to neural crest-derived melanocytes found in skin and hair follicles, as well as in the choroid and the iris of the eye, a minor population of pigment cells originates from the optic cup of the developing forebrain and forms the retinal pigment epithelium (RPE), a cell monolayer lying between the photoreceptive neural retina and the choroid (MartinezMorales et al, 2004). Both pigment cells produce melanin, but have adopted distinct regulatory elements controlling the expression of pigment-cell-specific genes, such as tyrosinase, Tyrp1 and Dct, probably because of an independent evolution of these regulatory networks (Murisier et al, 2006(Murisier et al, , 2007. Moreover, severe mutations of Mitf lead to death of melanocytes, whereas RPE cells survive and hyperproliferate (Bumsted and Barnstable, 2000;Nguyen and Arnheiter, 2000;Bharti et al, 2006).…”

Insights into neural crest development and evolution from genomic analysis

“…For example, evolutionarily conserved regions of the gene Mef2c (Agarwal et al 2011), and tyrosinase-related family member Tyrp1 (Murisier et al 2006), can drive specific reporter expression in melanocytes. Interestingly, both enhancers seem to be regulated by the transcription factor Sox10, a key regulator of the melanocytic lineage.…”

Non-Viral Gene Therapy Vectors Carrying Genomic Constructs

“…The ability of genomic constructs to drive tissue-and time-specific expression, unlike cDNA and mini-genes, has made BAC transgenics an additional tool to knockout transgenics for the identification of potential regulatory elements within the locus of a gene. For instance, an enhancer within the locus of the tyrosinase-related family (Tyrp1) gene (Murisier et al, 2006) and a region responsible for tissue-specific expression within the locus of the Neurogenin1 gene (Quiñones et al, 2010) have been discovered in transgenic mice generated with BACs.…”