Molecular pathophysiology of mucolipidosis type IV: pH dysregulation of the mucolipin-1 cation channel

Cation channel activity of mucolipin-1: the effect of calcium

Mucolipidosis type IV (MLIV) is a rare, neurogenetic disorder characterized by developmental abnormalities of the brain, and impaired neurological, ophthalmological, and gastric function. Considered a lysosomal disease, MLIV is characterized by the accumulation of large vacuoles in various cell types. Recent evidence indicates that MLIV is caused by mutations in MCOLN1, the gene that encodes mucolipin-1 (ML1), a 65-kDa protein showing sequence homology and topological similarities with polycystin-2 and other transient receptor potential (TRP) channels. In this report, our observations on the channel properties of ML1, and molecular pathophysiology of MLIV are reviewed and expanded. Our studies have shown that ML1 is a multiple sub-conductance, non-selective cation channel. MLIV-causing mutations result in functional differences in the channel protein. In particular, the V446L and DeltaF408 mutations retain channel function but have interesting functional differences with regards to pH dependence and Ca(2+) transport. While the wild-type protein is inhibited by Ca(2+) transport, mutant ML1 is not. Atomic force microscopy imaging of ML1 channels shows that changes in pH modify the aggregation and size of the ML1 channels, which has an impact on vesicular fusogenesis. The new evidence provides support for a novel role of ML1 cation channels in vesicular acidification and normal endosomal function.

TRP channels: a TR(I)P through a world of multifunctional cation channels

The "transient receptor potential" (TRP) family of ion channels comprises more than 50 cation-permeable channels expressed from yeast to man. On the basis of structural homology, the TRP family can be subdivided in to seven main subfamilies: the TRPC ('Canonical') group, the TRPV ('Vanilloid') group, the TRPM ('Melastatin') group, the TRPP ('Polycystin'), the TRPML ('Mucolipin'), the TRPA ('Ankyrin') and the TRPN ('NOMP') family. The cloning and characterization of members of this cation channel family has exploded during recent years, leading to a plethora of data concerning TRPs in a variety of cell types, tissues and species. This paper briefly reviews the TRP superfamily and the basic properties of its many members as a reader's guide in this Special Issue. Hopefully, a better understanding of TRP channel physiology will provide important insight into the relationship between TRP channel dysfunction and human diseases.

Cellular and molecular function of mucolipins (TRPML) and polycystin 2 (TRPP2)

Mucolipins (transient receptor potential mucolipin, TRPML) and polycystin-2 proteins (transient receptor potential polycystin, TRPP) constitute two small families of cation channels with motif and sequence similarities to the transient receptor potential (TRP) class of non-selective cation channels. Genetic defects in TRPML1 and TRPML3 in humans and in animal models cause the accumulation of large vacuoles, leading to a variety of cellular phenotypes including neurological and neurosensory deficiencies. TRPML1 is a Ca(2+)-, K(+)-, and Na(+)-permeable cation channel sensitive to pH changes, and regulates a critical step in the maturation of late endosomes to lysosomes. Mutations of TRPP2 in humans result in autosomal dominant polycystic kidney disease. Molecular studies have demonstrated that TRPP2 and TRPP3 proteins function as Ca(2+)-regulated, non-selective cation channels. During embryogenesis TRPP2 is active in node monocilia and plays a role in the establishment of left-right asymmetry. Recent results have indicated that TRPP2 interacts with polycystin-1 and that their interaction is important for their function as mechanosensitive channels at the primary cilium of renal epithelial cells. The interaction of polycystin family members appears to be conserved and is critical for fertilization and mating behavior. An emerging concept from the studies of the polycystin family is that they function as cation-influx based devices for sensing extracellular signals on ciliated structures. Here we review the function of TRPML1 and TRPP2 as representative members of these families, focusing on the genetics, physiology, and biochemistry.

CUPpling calcium to lysosomal biogenesis

Transport from late endosomes to lysosomes results in the formation of an endosome-lysosome hybrid organelle from which late endosomes and lysosomes must be re-formed. Recent studies indicate that the transient receptor potential (TRP)-related channel mucolipin-1 (the loss of which causes mucolipidosis type IV) and its Caenorhabditis elegans orthologue CUP-5 might control the process of lysosome re-formation by regulating calcium flux.

Polycystin-2—an intracellular or plasma membrane channel?

The intracellular location of polycystin-2 is a hotly debated topic in the field of polycystic kidney disease. Two not necessarily mutually exclusive hypotheses state that polycystin-2 is located in the endoplasmic reticulum or in the plasma membrane, respectively. Although a variety of techniques have been employed to prove one or the other location, no definite consensus has been reached yet. It is generally acknowledged, however, that the COOH-terminus of polycystin-2 contains a retention signal for the endoplasmic reticulum. Another facet has been added to the discussion due to the fact that many genes mutated in patients with cystic kidney diseases, among them PKD2, encode proteins which have been detected in primary cilia. Since there is no evidence that the endoplasmic reticulum extends into the primary cilium, polycystin-2 has to reach the plasma membrane at least in this case. An unbiased approach towards elucidating the physiological location of polycystin-2 would involve the characterization of its intracellular trafficking. Using the COOH-terminus of polycystin-2 in a two-hybrid screen, my group has identified a novel coiled-coil protein which we call PIGEA-14 (polycystin-2 interactor, Golgi- and endoplasmic reticulum-associated protein with a molecular weight of 14 kDa). PIGEA-14 also interacts with GM130, a protein associated with the Golgi matrix, and may therefore represent one important component of the trafficking machinery for polycystin-2.

The TRP superfamily of cation channels

The transient receptor potential (TRP) protein superfamily consists of a diverse group of cation channels that bear structural similarities to Drosophila TRP. TRP channels play important roles in nonexcitable cells; however, an emerging theme is that many TRP-related proteins are expressed predominantly in the nervous system and function in sensory physiology. The TRP superfamily is divided into seven subfamilies, the first of which is composed of the "classical" TRPs" (TRPC subfamily). Some TRPCs may be store-operated channels, whereas others appear to be activated by production of diacylglycerol or regulated through an exocytotic mechanism. Many members of a second subfamily (TRPV) function in sensory physiology and respond to heat, changes in osmolarity, odorants, and mechanical stimuli. Two members of the TRPM family function in sensory perception and three TRPM proteins are chanzymes, which contain C-terminal enzyme domains. The fourth and fifth subfamilies, TRPN and TRPA, include proteins with many ankyrin repeats. TRPN proteins function in mechanotransduction, whereas TRPA1 is activated by noxious cold and is also required for the auditory response. In addition to these five closely related TRP subfamilies, which comprise the Group 1 TRPs, members of the two Group 2 TRP subfamilies, TRPP and TRPML, are distantly related to the group 1 TRPs. Mutations in the founding members of these latter subfamilies are responsible for human diseases. Each of the TRP subfamilies are represented by members in worms and flies, providing the potential for using genetic approaches to characterize the normal functions and activation mechanisms of these channels.

Mucolipin 1: endocytosis and cation channel--a review

Mucolipidosis type IV (MLIV) is a neurodegenerative, recessive, lysosomal storage disorder characterized by psychomotor retardation and visual impairment due to various ophthalmologic abnormalities. MLIV is found in relatively high frequency in the Ashkenazi Jewish population. The disease is caused by mutations in the gene MCOLN1, which encodes the protein mucolipin 1 (MLN1), a member of the mucolipins family. MLN1 is a non-specific cation channel, and its putative structure attributes it to the TRP superfamily; thus, the gene is also referred as TRPML1. Over 16 MLIV-causing mutations, including two founder mutations in the Ashkenazi population, have been identified hitherto. Atypical increased lysosomal storage in MLIV is present in the cells of all patients. This accumulation is caused by an abnormal endocytosis process of the membrane components to late endosomes to the lysosomes, resulting in an apparent block in the traffic process in pre-lysosomal vacuoles with intraluminal pH of >5.0. MLN1 was localized in cultured cells to late endosomes and lysosomes. The exact function of this cation channel in the late stages of lysosomal maintenance is currently under study.

PIGEA-14, a Novel Coiled-coil Protein Affecting the Intracellular Distribution of Polycystin-2

Employing a yeast two-hybrid screen with the COOH terminus of polycystin-2, one of the proteins mutated in patients with polycystic kidney disease, we were able to isolate a novel protein that we call PIGEA-14 (polycystin-2 interactor, Golgi- and endoplasmic reticulum-associated protein with a molecular mass of 14 kDa). Molecular modeling only predicts a coiled-coil motif, but no other functional domains, in PIGEA-14. In a subsequent two-hybrid screen using PIGEA-14 as a bait, we found GM130, a component of the cis-compartment of the Golgi apparatus. Co-expression of the PIGEA-14 and PKD2 cDNAs in LLC-PK(1) and HeLa cells resulted in a redistribution of PIGEA-14 and polycystin-2 to the trans-Golgi network, which suggests that PIGEA-14 plays an important role in regulating the intracellular location of polycystin-2 and possibly other intracellular proteins. Our results also indicate that the intracellular trafficking of polycystin-2 is regulated both at the level of the endo-plasmic reticulum and that of the trans-Golgi network.

Overexpression of wild-type and mutant mucolipin proteins in mammalian cells: effects on the late endocytic compartment organization

Mucolipin-1 is a 65-kDa membrane protein encoded by the MCOLN1 gene, which is mutated in patients with mucolipidosis type IV (MLIV), a rare neurodegenerative lysosomal storage disorder. We studied the subcellular localization of wild-type and three different mutant forms (T232P, F408del and F465L) of mucolipin by expressing Myc-tagged proteins in HeLa cells. The overexpressed wild-type mucolipin colocalizes to late endocytic structures and induces an aberrant distribution of these compartments. F408del and F465L MLIV mutant proteins show a distribution similar to the wild-type protein, whereas T232P is retained in the endoplasmic reticulum. Among the mutants, only F408del induces a redistribution of the late endocytic compartment. These findings suggest that the overexpression of the mucolipin cation channel influences the dynamic equilibrium of late endocytic compartments.