Endocytic Trafficking and Human Diseases

粘磷脂构成了一个与瞬时受体电位（TRP）超家族同源的阳离子通道家族。在哺乳动物中，粘脂蛋白家族包括三个成员，分别是粘脂蛋白-1 （MCOLN1）、粘脂蛋白-2 （MCOLN2）和粘脂蛋白-3 (MCOLN3)，它们具有共同的六膜跨越分子结构。在果蝇和秀丽隐杆线虫中也发现了哺乳动物的同源粘液磷脂。MCOLN-1是粘脂蛋白家族中最具特征的成员，因为该蛋白的突变与IV型粘脂病（MLIV）有关，这是一种隐性溶酶体贮积病，其特征是严重的神经和眼科异常。有研究表明，MCOLN1通道的突变会导致后期内体/溶酶体运输的缺陷，从而导致含有磷脂、粘多糖和神经节苷的增大液泡结构的积累。与这一观点一致的是，秀丽隐杆线虫黏脂素-1的同源基因cup-5的功能缺失突变导致了大型内体的形成。溶酶体杂交细胞器，包含溶酶体和内体标记。MCOLN-3也可能在不同的人类病理中发挥作用，因为该基因的突变导致了以色素沉着缺陷和听力丧失为特征的变异-蹒跚慕斯表型。MCOLN3位于毛细胞中，可能与毛细胞成熟和黑素体运输有关。相比之下，MCOLN2的功能和位置仍有待研究。

Mucolipins constitute a family of cation channels with homology to the transient receptor potential (TRP) superfamily. In mammals, mucolipin family includes three members named mucolipin-1 (MCOLN1), mucolipin-2 (MCOLN2), and mucolipin-3 (MCOLN3) that exhibit a common six-membrane-spanning molecular organization. Homologous of mammalian mucolipins have also been described in Drosophila and C. elegans. MCOLN-1 is the best-characterized member of the mucolipin family as mutations in this protein have been linked to mucolipidosis type IV (MLIV), a recessive lysosomal storage disease characterized by severe neurological and ophthalmologic abnormalities. It has been suggested that mutations in MCOLN1 channel causes defects on late-endosomal/lysosomal trafficking resulting in the accumulation of enlarged vacuolar structures that contain phospholipids, mucopolysacharides, and gangliosides. In agreement with this idea, loss-of-function mutations in cup-5, the C. elegans orthologue of mucolipin-1, caused formation of large endosome?lysosome hybrid organelles that contain both late-endosomal and lysosomal markers. MCOLN-3 might also play a role in different human pathologies, as mutations in this gene are responsible for the varitint-waddler mousse phenotype that is characterized by defects on pigmentation and hearing loss. MCOLN3 is located in hair cells and it could be implicated in hair cell maturation and melanosomes trafficking. In contrast, the function and location of MCOLN2 remains to be characterized. One approach to gain information on the function of a protein is to study its trafficking and cellular distribution. For example, we have previously described that, consistent with its role in lysosomal biogenesis, MCOLN1 has specific sorting motifs that mediate interaction with clathrin adaptors and the subsequent delivery of MCOLN1 to lysosomes. Posttranslational modifications may also play an important role in the regulation of protein function. We have established that the C-terminal tail of MCOLN1 is palmitoylated and that this palmitoylation promotes efficient internalization of MCOLN1 from the plasma membrane. We have also found that MCOLN1 is phosphorylated both in vitro and in vivo. We have been able to identify the residues and the kinase responsible for this phosphorylation and are now analyzing the effect of this modification on the activity of MCOLN1 channels in vivo. We have also address the function and cellular distribution of MCOLN2 in HeLa cells. We found that MCOLN2 traffics via the Arf6-dependent pathway and co-localizes with MHCI in both vesicles and long tubular structures. Expression of Arf6Q67L or activation of endogenous Arf6 by transfection with EFA6 or treatment with AlF caused accumulation of MCOLN2 in enlarged vacuoles that also contain MHCI and CD59. Moreover, over-expression of MCOLN2 promoted efficient activation of Arf6. Finally, depletion of endogenous MCOLN2 by expression of a specific shRNA caused missorting of cargo proteins that travel through the Arf6-regulated pathway. Finally, we are analyzing the presence of specific sorting motifs in MCOLN3 and its contribution to the cellular distribution of this protein. We are also addressing the participation of MCOLN3 in the trafficking of melanosomes. Therefore our data reveal that all three members of the mucolipin family may be important regulators of specific intracellular trafficking events.