16 Mar 2012

Lysosome malfunction caused by an organelle's inability to clear lipids or other waste from the cell plays a central role in lysosomal storage diseases (LSDs). Though those diseases have disparate causes, could a similar mechanism be to blame? A particular lysosome calcium channel, which signals the organelle to process its contents, malfunctions in these disorders, suggests a March 13 Nature Communications paper from Haoxing Xu, University of Michigan, Ann Arbor, and colleagues. The channel releases too little calcium in cellular models of certain LSDs or in the presence of certain lipids, but boosting that channel's activity improves lysosomal trafficking, the researchers found. The results could point to new treatments for LSDs, write the authors.

"This is quite a novel finding," said Jochen Walter, University of Bonn, Germany, who was not involved in the study. It could extend not only to LSDs that are fatal very early in life, but to more prolonged disorders such as Alzheimer’s disease, he told ARF. "It shows that stimulation of lysosomal activity and vesicular transport by calcium activation could be beneficial for diseased cells."

Lysosomes are intracellular acidic sacs filled with enzymes that hydrolyze lipids and proteins, and cells recycle many unwanted components through these sacs. If lysosomes or select enzymes are not working properly, components can build up inside, resulting in an LSD. For example, in Niemann-Pick's disease, lysosomes lack—or have insufficient—enzymes to break down sphingomyelin, a type of lipid present in membranes, including the myelin sheath, and the fat builds up in cells until the cell dies.

One ion channel in particular—the Transient Receptor Potential Channel Mucolipin-1 (TRPML1)—has been linked to LSDs. The Trpml1 gene encodes an iron and calcium ion channel expressed in late endosomes and lysosomes. A mutation in the human gene causes mucolipidosis type 4 (ML4). ML4 symptoms include swollen lysosomes, impaired membrane trafficking, and neurodegeneration, and they bear a striking resemblance to those of Niemann-Pick's (NP) disease type C (NPC). Xu and researchers wondered if the same channel could be at the heart of NPC or other LSDs.

To test this idea, first author Dongbiao Shen and colleagues developed a genetically encoded fluorescent reporter to detect calcium release specifically from lysosomes. They transfected that reporter into Chinese hamster ovary (CHO cells) that were null for the NPC1 protein, which encodes a protein that transports low-density lipoproteins to lysosomal compartments. Without the protein, the cells accumulate excess lipids in late endosomes and show signs of NPC. The researchers stimulated TRPML1 calcium release by applying an agonist, mucolipin synthetic agonist 1 (ML-SA1), that they had identified in small molecule screens. NPC CHO cells released 75 percent less calcium from TRPML1 channels compared to wild-type CHO cells. A similar reduction compared to wild-type cells was also apparent in fibroblasts from human patients with NPC and NP disease type A (NPA), and cells that had been pharmacologically induced to exhibit NPC-like symptoms by way of a small-molecule drug called U18666A.

Using a patch-clamp device, the researchers were able to directly measure calcium currents from single agonist-stimulated channels in the lysosome. This is a more detailed, though more invasive, measurement that involved isolating the lysosome. About 70 percent less current flowed through TRPML1 channels from NPC human fibroblasts than from wild-type fibroblasts. Since a comparison of mRNAs suggested that the channel was properly expressed in diseased cells, the TRPML1 channel itself seemed to be impaired.

If TRPML1 was present and active in these cells, then what explains the channel’s poor conductance? Xu and colleagues wondered if the accumulation of sphingomyelin that occurs in NPC could be at fault. They found that bathing normal human embryonic kidney cells in sphingomyelin—but not cholesterol, glycosphingolipids, or sphingomyelin breakdown products—drastically reduced agonist-evoked calcium release from TRPML1 channels. However, if the channel was mutated to stay open and locked, sphingomyelin had no impact. If the sphingomyelin-degrading enzyme SMase was added, current even exceeded that induced by the agonist. These findings pointed a finger at sphingomyelin as the channel inhibitor, said Xu, though he is not yet sure how the lipid closes or blocks the channel.

To find out if TRPML1 channel malfunction was a cause or consequence of Niemann-Pick’s disease, the group applied the ML-SA1 agonist to macrophages from NPC1-null mice to enhance TRPML1 calcium release. Macrophages are a phagocytic cell type affected by NPC. In these cells, the agonist restored previously blocked vesicle transport from late endosomes to the Golgi apparatus, though not to control levels.

"We were quite excited to find that a similar mechanism might apply to both Niemann-Pick's disease and mucolipidosis type 4 disease," said Xu. The channel is perhaps involved in some cases of the more common neurodegenerative disorders such as AD and PD, he added, in which lysosome function, protein or lipid accumulation, and autophagy are also affected (see ARF related news story and ARF news story). A human genetic connection to AD or PD has not been reported thus far. A study published last year could provide one connection. Walter and his lab reported that excess sphingolipids lead to more activity for γ-secretase (which cleaves the Aβ precursor protein, APP), higher levels of Aβ, and a stalled final stage of autophagy in human cells (see ARF related news story). "The effects overall are quite similar—when you add sphingomyelin you affect both APP processing and this channel activity," said Walter. "It could well be that deficiency or partial inhibition of this channel would also affect APP processing," though that has not yet been tested, he added.

Previous work suggested that it is not calcium signaling per se, but calcium stores that are defective in Niemann-Pick's disease (Lloyd-Evans et al., 2008). The two studies together indicate that calcium is important, said Xu. He and his colleagues are still working out calcium's exact role in lysosome function, but for now they borrow the concept of its role in neurotransmitter release, where the ion triggers vesicle fusion with the presynaptic membrane. In the case of the lysosome, the calcium may be released in response to a full organelle to recruit protein machinery that stimulates budding of the lysosomal membrane and gets rid of the waste, he said.

Since the ML-SA1 agonist may be a good drug candidate for stimulating the system, Xu next plans to test its safety and effectiveness in animal models of LSDs. He is also interested in looking at the channel's function in cellular models of AD and PD.—Gwyneth Dickey Zakaib

Is Calcium Channel Major Player in Lysosome Diseases?

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