Quis custodiet ipsos custodes?

Who will guard the guards themselves?

- Juvenal in Satires (Satire VI, lines 347-8)

The rise and fall of calcium levels within cells is an integral part of information flow in living systems. Cells rely on calcium as a key agent in several signalling pathways essential to our lives - muscle contraction, immune reactions, nerve function, light sensing and many other such processes. Calcium ions, however, cannot cross membranes directly, and must do so through specific protein channels that serve as 'gateways'. One class of these gateways are named 'Orai', after the mythological Greek Orai, also known as Horae or the gatekeepers of heaven. But what guards the guards, in this case the gatekeepers themselves?

Latest research from the National Centre for Biological Sciences (NCBS), Bangalore, gives us new insights into how Orai proteins are regulated. Researcher Bipan Kumar Deb from Gaiti Hasan's group has discovered that the protein Septin 7 guards Orai function by acting as a 'molecular brake' to Orai activation. Genetic experiments performed by Trayambak Pathak, also an author in the work, were followed up with molecular and cellular investigations by Deb, leading to this discovery, which has been published in the journal Nature Communications.

Previous work from this group had established a critical role for calcium signalling and Orai in development of the flight circuit in Drosophila fly brains. During this investigation, Hasan's group showed the importance of Store Operated Calcium Entry or SOCE for dopamine biosynthesis in these neurons. SOCE is a process by which calcium ions slowly enter nerve cells to refill stores that have been depleted by cellular activity. Using this system as a platform, the team began further investigations into the ways by which calcium entry into cells is regulated.

These studies led to the discovery of Septin 7 as a negative regulator of Orai in fly neurons. This means that if Septin 7 levels are decreased in a cell, calcium entry via Orai goes up, leading to higher calcium concentrations in the cell. This calcium entry was found to operate through a mechanism independent of depletion of endoplasmic reticulum calcium stores. Further, the increased calcium entry due to lowering Septin 7 levels compensated for experimentally reduced SOCE in the Drosophila flight circuit. Importantly, this compensation did not occur when levels of both Septin 7 and Orai are lowered simultaneously. This underscores that Septin 7 acts primarily through Orai regulation. On the other hand, increasing the levels of Septin 7 by overexpression of the gene reduced SOCE in cells, adding further support to its role as a negative regulator of Orai function. In all, these experiments have discovered a role for Septin 7 as a key regulator of a membrane calcium channel that modulates calcium homeostasis in cells.

Septins are a class of cytoskeletal proteins that are important for various functions such as cellular movement and cell division. "Although several Septins are known to affect calcium signalling via Orai, all of these are positive regulators of Orai. This is one of the most interesting aspects of this work - Septin 7 is the first Septin shown to be a negative regulator of Orai and calcium signalling," says Gaiti Hasan.

The negative nature of Orai regulation by Septin 7 could be a therapeutically important one. "Most drugs work by inhibiting the function of a protein, and inhibiting most proteins causes the processes they are involved in to be reduced. In this case, inhibiting Septin 7 can actually raise intracellular calcium levels," says Bipan Kumar Deb, the lead author of the paper.

"In the context of neural function, we know that under some conditions, reduced calcium signalling can lead to neurodegeneration. Rare genetic disorders such as spinocerebellar ataxias are thought to be caused by dysregulation of calcium signalling. Future therapies for certain classes of such disorders could focus on Septin 7 as a therapeutic target," adds Gaiti Hasan.

The next challenge the team intends to address involves studying the role of Septin 7 in Orai regulation in mammalian cells. Future work also involves studying how Septin 7 functions in cells other than neuronal cells, specifically, in cells of the immune system. "The Orai gene was first discovered in patients with Severe Combined Immuno-Deficency (SCID) syndrome, and subsequently an essential role for Orai in immune cell function was uncovered," says Deb. "Therefore, it is likely that Septin 7 may also play a role in calcium signalling mechanisms of the immune system, and we are already planning experiments to pursue this line of investigation," he adds with enthusiasm.

The work described here was published in a paper titled 'Store-independent modulation of Ca²⁺ entry through Orai by Septin 7' in May 2016 in the journal Nature Communications. The paper can be accessed at: http://www.nature.com/ncomms/2016/160526/ncomms11751/full/ncomms11751.html