Purdue researcher finds genetic link to calcium regulation in cells

November 08, 1999

WEST LAFAYETTE, Ind. -- A Purdue University researcher has discovered a gene in fruit flies that may play a key role in regulating the flow of calcium into cells.

Mutations in the gene, called inaF, also appear to suppress the effects of degeneration caused by the overactivity of some calcium channels. The findings may lead to answers to long-standing questions about how these calcium channels work.

The study, published in the Nov. 9 issue of the Proceedings of the National Academy of Sciences, may have long-term implications for treating some degenerative eye and brain diseases, including Alzheimer's, that have been linked to a buildup of calcium in the cell.

In the study, biology Professor William L. Pak and six co-researchers isolated several new strains of genetically mutant Drosophila, or fruit flies, to study how calcium levels are maintained within nerve cells.

The study was conducted on new strains containing mutations in a gene called trp or another gene called inaF.

The trp gene encodes a protein making up the channels in the cell membrane that allow calcium ions to flow into the cell. These channels, called TRP channels, play a critical role in the process by which light signals are converted to nerve signals in the eyes of fruit flies, though the mechanism by which they carry out this process is unknown.

Similar channels, also called TRP channels, have been identified in animals and humans.

The inaF gene, which was identified and named by Pak's group, encodes a protein that appears to play a critical role in the TRP channel's function.

In a group of flies containing mutations in the trp gene, the Pak group identified a new mutation that produced flies with traits different from previously identified trp mutants and caused the flies' photoreceptors to undergo rapid and massive degeneration.

Their study suggests that this degeneration was caused by overactive TRP channels, which allowed calcium ions to accumulate in the cells, thus killing them.

In a second group of flies, Pak disrupted the inaF gene so that it could not produce the INAF protein. In addition to not producing the INAF protein, the flies in this group showed a significant reduction in the amount of TRP protein produced and nearly shut down the activities of the TRP channel, Pak says.

Though part of the shutdown of the TRP channels may be attributed to a reduction in the amount of the TRP protein available, Pak's group found additional evidence that mutations in the inaF gene also have a direct effect on TRP channel activity. In flies containing mutations in both the inaF gene and the trp gene, the study found that the degenerative effects displayed by fruit flies with a trp mutation are suppressed.

"The suppression is rather dramatic," Pak says. "Flies normally have two copies of the trp gene. Even if only one of these contains the new mutation, the photoreceptors will degenerate completely within two to three days.

"If inaF mutations are introduced into the same flies, no degeneration is observed for more than two weeks."

Pak says the findings suggest that the INAF protein may play a regulatory role in the function of the TRP channel.

"We still need to show that this is indeed the case," Pak says. "However, if it turns out to be true, INAF would be the first example of a regulator protein for the TRP-related calcium channels."

Because the TRP protein is related to a number of proteins found in humans, the findings may have long-term implications for finding new ways to suppress or control some degenerative diseases.

"Several different forms of TRP have been identified in humans, and some of these are heavily expressed in the brain," Pak says. "Since mutations in the Drosophila trp gene can cause massive degeneration of affected tissues, it is not difficult to imagine that similar mutations in the human genes that encode for these TRP proteins may also cause degeneration of affected tissues."

Because TRP proteins appear to be heavily expressed in the brain, the brain could be particularly vulnerable to such degenerative disorders, he says. "In fact, there are reports that disruptions in the cells' calcium levels may be a step leading to brain degeneration in diseases such as Alzheimer's Disease."

The researchers have yet to determine if INAF proteins play a role in the functioning of human cells, Pak says.

"If this were the case, we not only would have a new mechanism of brain degeneration but also a potential means of suppressing or controlling the degeneration," he says.
-end-
Writer: Susan Gaidos, (765) 494-2081; susan_gaidos@uns.purdue.edu

Related Web sites:
William Pak's home page:

ABSTRACT

INAF, a protein required for transient receptor potential Ca2+ channel function
Chenjian Li, Chaoxian Geng, Hung-Tat Leung, Young Seok Hong, Lydia L. R. Strong, Stephen Schneuwly, and William L. Pak

The trp gene of Drosophila encodes a subunit of a class of Ca2+ -selective light-activated channels that carry the bulk of the phototransduction current. Transient receptor potential (TRP) homologs have been identified throughout animal phylogeny. In vertebrates, TRP-related channels have been suggested to mediate "store-operated Ca2+ entry," which is important in Ca2+ homeostasis in a wide variety of cell types. However, the mechanisms of activation and regulation of the TRP channel are not known. Here, we report on the Drosophila inaF gene, which encodes a highly eye-enriched protein, INAF, that appears to be required for TRP channel function. A null mutation in this gene significantly reduces the amount of the TRP protein and, in addition, specifically affects the TRP channel function so as to nearly shut down its activity. The inaF mutation also dramatically suppresses the severe degeneration caused by a constitutively active mutation in the trp gene. Although the reduction in the amount of the TRP protein may contribute to these phenotypes, several lines of evidence support the view that inaF mutations also more directly affect the TRP channel function, suggesting that the INAF protein may have a regulatory role in the channel function.

Purdue University

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