Specialized Molecular Pathways Drive the Formation of Light-Scattering Assemblies in Leucophores

Pigmentation plays a vital role in the survival of organisms, supporting functions such as camouflage, communication, and mate attraction. In vertebrates, these functions are mediated by specialized pigment cells known as chromatophores of which, uric acid crystal-forming leucophores remain the least understood, with little known about their molecular mechanisms. A key question in pigment cell biology is whether different crystal chemistries require distinct molecular pathways, or whether similar cellular processes drive the formation of diverse crystals. This study was designed to unravel the uncharacterized process of uric acid crystallization in leucophores and compare them to guanine crystal formation in iridophores and pterin formation in xanthophores. The results of our transcriptomic, ultrastructural, and metabolomic analyses, demonstrate that leucophores share molecular pathways with iridophores, particularly those connected to organelle organization and purine metabolism, but express discrete genes involved in uric acid biosynthesis and storage. Additionally, leucophores share intracellular trafficking and pterin biosynthesis genes with xanthophores, suggesting universally conserved processes. Ultrastructural studies reveal star-like fibrous structures in leucosomes, which likely serve as scaffolds for unique one-dimensional uric acid assemblies that radiate from the core and act as efficient light scatterers. These findings provide new insights into leucophore cell biology and the specialized mechanisms driving molecular crystalline assembly, and reveal that while some cellular processes are conserved, the specific chemistry of each crystal type drives the evolution of distinct molecular pathways. Overall design: Cells were isolated from adult HdrR Medaka fish skin and scales and sorted via Fluorescence-Activated Cell Sorting (FACS), pigment cells enrichment was done according to the present of both GFP and mCherry signals.

色素沉着（pigmentation）对生物体的生存至关重要，可介导伪装、通讯及求偶等核心生理功能。在脊椎动物中，上述功能由一类特化的色素细胞（chromatophores）介导，其中以形成尿酸晶体的白色素细胞（leucophores）研究最为匮乏，其分子机制迄今仍鲜为人知。
色素细胞生物学领域的核心问题之一在于：不同晶体的化学组成是否需要截然不同的分子通路，亦或是相似的细胞过程可驱动多种晶体的形成。本研究旨在阐明白色素细胞中尿酸结晶的未表征过程，并将其与虹彩细胞（iridophores）的鸟嘌呤（guanine）晶体形成、黄色素细胞（xanthophores）的蝶啶（pterin）形成过程进行对比。
我们的转录组学（transcriptomic）、超微结构（ultrastructural）及代谢组学（metabolomic）分析结果显示，白色素细胞与虹彩细胞共享部分分子通路，尤其是与细胞器组织及嘌呤代谢相关的通路，但同时也表达参与尿酸生物合成与储存的特异性基因。此外，白色素细胞还与黄色素细胞共享细胞内运输及蝶啶生物合成相关基因，这提示存在普遍保守的细胞过程。
超微结构研究发现，白色体（leucosomes）内存在星状纤维结构，该结构或可作为独特一维尿酸组装体的支架——此类组装体从核心向外辐射，可作为高效的光散射结构。本研究结果为白色素细胞生物学及介导分子晶体组装的特化机制提供了新视角，并揭示：尽管部分细胞过程具有保守性，但不同晶体类型的特异性化学组成推动了独特分子通路的演化。
整体实验设计：从成年HdrR青鳉（Medaka）的皮肤及鳞片中分离细胞，通过荧光激活细胞分选（Fluorescence-Activated Cell Sorting, FACS）进行分选，依据GFP与mCherry双信号的共表达实现色素细胞的富集。