Fig. 6.

Regulatory network of melanin deposition in chicken breast muscle. A Breast muscle image from three chicken breeds. (B1–3) Relative expression of TFAP2A, TFAP2B, MITF and PDZK1 gene mRNA from skin (B1), breast muscle (B2) and leg muscle (B3) at 1 day of age of three chicken breeds. C Western-blot analysis for protein expression levels of TFAP2A and TFAP2B. For full length blot information see additional file 5. D Melanin synthesized within melanosomes of melanocytes by a series of reactions that are catalyzed by specific melanogenic enzymes and transcriptional factors in muscle of chickens. Production of these enzymes is driven by the MITF transcription factor whose activity is regulated by a number of signaling pathways including MAPK (red), cAMP/ PKA (orange) and WNT (green). These signaling pathways are activated upstream by receptors such as KIT, MC5R and FZD5. TFAP2A and TFAP2B could regulate the KIT gene expression, which is the important gene affecting upstream of MAPK signaling pathways in melanin deposition. Similarly, the MC5R gene could regulate MITF gene through CREB3L3, and the GPCG gene could regulate MITF gene through PDZK1. As the target gene of MITF, TYR, TYRP1 and Plem17 directly participate in the tyrosine metabolism signaling pathway with the DCT, IL4I1, HPD, ADH1C and ADH6 genes. MLANA, GPNMB and PMEL could affect the melanosome development. Furthermore, we proposed that lipid molecule 16:0 LYSO PE could be converted into LPA under EPNN2, and then LPA could activate upstream receptors GPCR to regulate the PDZK1 gene expression involved in tyrosine metabolism and the melanogenesis signaling pathways