Artwork: Adapted by Geoff Hyde, from an original photo by Étienne-Jules Marey.

Can you remember running down a set of stairs really fast without thinking - and then wondering in amazement, “How did I do that?”. Well in a way “you” didn’t. Just as your breathing and heartbeat do not require your conscious input, the control of other rhythmic activities such as walking and running is largely automatic. This can free up your often already-overloaded conscious mind to think about more important things, for example where to run to! In any animal, the automatic component of each rhythmic motion is enabled by a specialised part of the central nervous system that functions as a “Central Pattern Generator” (CPG) for that particular motion. CPGs are a fascinating and important area of animal biology, and last year Gayatri Venkiteswaran and Gaiti Hasan of NCBS published a paper in PNAS that demonstrated a previously unrecognised feature of the nerves that directly influence the CPG that controls flight in the fruitfly, Drosophila. And, right now, the movie that illustrates the most significant findings of their study is a finalist in the competition for Drosophila Image Award of 2010, an honour awarded each year by the Genetics Society of America.

CPGs come in two basic types and the rhythmic nature of flight in Drosophila is probably controlled by the type that consists of a minimal circuit of just two neurons. Oscillations result from a tussle between the neurons, each taking turns to switch off its adversary. In the other type of CPG - sometimes referred to as “endogenous” -  the oscillations occur within an individual neuron, and reflect intrinsic fluctuations of its internal electrical activity. Our understanding of each type of CPG is expanding rapidly every year, and often in quite dramatic ways. For example, breathing in vertebrates has long been thought to be under the control of a single endogenous CPG, but a recent paper on rat respiration suggests that there is is one endogenous CPG for inspiration and a separate one for expiration. Even in sleep, breathing in and out may be two independent activities, co-ordinated somehow by another higher centre.

The study of Gayatri and Gaiti in PNAS also suggests something quite dramatically different about the cellular physiology of the neurons that make up, or are closely linked to, the Drosophila flight CPG. All neurons have a mechanism to generate a surge in internal calcium levels, most typically by allowing calcium from outside the cell to temporarily flood in through channels in the cell’s outer membrane. The history of calcium surges in a neuron can determine what type of role a developing neuron will eventually perform or, at maturity, how strongly it interconnects with its neighbours. Up until very recently it was thought that neurons had only one way to trigger the outer membrane channels that let in the extracellular calcium (by increasing the voltage across the membrane) but in another study, previous to the current one, Gayatri, Gaiti and colleagues found evidence suggesting that neurons of the flight CPG might also employ another trigger mechanism, one that is used by ordinary cells.

In non-neuronal cells, surges in internal calcium often involve an intriguing two-stage process. In stage 1, calcium in the cytoplasm is raised by the release of calcium from an internal reservoir (the endoplasmic reticulum). In stage 2, if the reservoir is depleted, a protein that is normally bound to it (STIM) senses the depletion and then migrates to the cell’s outer membrane to switch on calcium channels there, further boosting the initial surge. Of equal importance, the incoming calcium can now replenish the reservoir. In their previous study, Gayatri and her colleagues blocked stage 1 (using fruit flies with a mutant version of a protein critical for this step - InsP3R) and found that this compromised the flies’ ability to carry out various types of rhythmic movement, including flight and walking. A stage-1-blocked, flight-compromised fly can be seen in the accompanying video (fly on the left).

This result raised the question of whether movement-regulating neurons also employ the stage 2 mechanism. In the current study, Gayatri and Gaiti found evidence that, for neurons associated with the flight CPG, this was indeed the case. They focussed on a pair of stage 2 proteins: STIM and orai, the latter being the main component of the calcium channel that STIM switches on. In otherwise normal flies, if they depressed the activity of either of these two proteins (using mutants), then flies could not fly properly, a similar compromise to that seen when stage 1 was blocked in the previous study. Astonishing further evidence of stage 2 involvement was seen when orai levels were raised in flies suffering from a stage 1 block. Flies with high orai levels regained much of their ability to respond to the cue used to elicit wing beating (an air puff, see fly at right in video).

The studies indicate that CPG-related neurons, unlike other neurons, can draw in calcium from outside the cell via two different channels - one type triggered by increased transmembrane voltage, the other triggered by a protein sensitive to the calcium levels of an internal reservoir. CPG-related neurons may need to manage their calcium levels even more carefully than other neurons because the activities they control must often be sustained for prolonged periods, sometimes an entire life-time.

The study also suggests possible medical therapies for people suffering from certain types of neuronal degeneration. In particular, the gait-disrupting disease spinocerebellar ataxia 15 is caused by a deleterious mutation in the same protein that was the target of the Gayatri’s stage 1 block. If those affected by this mutation can have their orai levels increased, they might experience a recovery analogous to that seen in Drosophila.

Gayatri Venkiteswaran