Clock imageAah, the cursed jet lag! How I loathe thee! Source: Hank Conner www.flickr.com

In Around the World in 80 days, the protagonists faced a lifetime’s worth of calamities – failing suspension bridges, raids, abductions, hurricanes–but they were spared from one giant headache–jet lag.

Jet lag is no doubt an unfortunately side effect of man’s dream of flight. While the occasional sleep disturbance may seem little more than a nuisance, repeated jet lag (or continuous shift work) is an insidious killer. Epidemiological studies repeatedly show increased risk of cancer, cardiovascular disease, sleep disorders and metabolic derangement in those forced to frequently reset their internal clocks.

Given the contemporary nature of the malaise, perhaps it’s not so surprising that jet lag has stubbornly eluded a cure. In a peculiar turn of events, a recent study points to a water-retention hormone as a crucial candidate in the battle against jet lag. Mice lacking the vasopressin hormone receptors boasted a more flexible–and seemingly fully functional–internal clock, allowing them to rapidly phase-shift and adjust to new light-dark cycles. These became the perfect globetrotters.

How in the world can deletion of vasopressin receptors lead to jet lag-resistant mice?

Move to the (circadian) rhythm
Outside the realm of conscious perception, our physiological processes oscillate rhythmically in a daily manner, driving cycles of stereotypic behaviors that adapt us to external time cues. Circadian rhythms control many aspects of our physical well-being, allowing the smooth running of sleep-wake cycles, thermo- and hormone- regulation and feeding. Being “on time” internally makes us tick.

Most, if not all cells can generate internal time in a cell-autonomous manner, with a period of roughly 24hrs. In broad strokes, these biological clocks work by expressing “clock” genes in a series of overlapping loops, in which their protein products directly inhibit the activity of the genes in a negative feedback manner. Degradation of these repressor protein products allows clock gene translation to start anew, thus beginning a new circadian cycle. Incredibly, the loops interlock in such a way that “boosts” the oscillation, forming a sustained and high-amplitude signal. (Fun fact: one of the core genes is named CLOCK, or Circadian Locomotor Output Cycles Kaput! Who says scientists don’t have a sense of humor?)

However, individual cellular clocks do not run amok–their activities are tightly coordinated by the “master clock”, a group of roughly 10,000 neurons situated in the suprachiasmatic nucleus (SCN), a part of the hypothalamus at the base of the brain. Unlike peripheral biological clocks, SCN neurons directly receive information from the eyes, making them the only part of our clock machinery capable of establishing rhythmic periodicity based on external light cues. What’s more, SCN neurons work in synchrony, “linking up” their individual molecular loops to establish rhythm at the NETWORK level. By amplifying individual pacemakers and stabilizing wonky components through this coupling action, SCN neurons are capable of generating a much more precise, sustained and robust circadian clock than any other tissue.

However, network inertia is both a blessing and a curse. A stable master clock wards off perturbations that may else wise seriously tamper with our physiology, but it also takes its sweet time adjusting to altered light-dark cycles. While SCN neurons CAN sense a sudden shift in external light cues, its intrinsic stability acts as a buffer to slow the re-tuning of the clock. This temporary misalignment between internal timekeeping and external cues causes us to feel exhausted, disoriented and generally crappy, which persist until our clocks “catch up”.

Vasopressin: too much of a good thing?
Arginine vasopressin is better known as a hormone that regulates fluid balance by controlling how much water is excreted in urine. However, it is also released by SCN neurons as a neurotransmitter to facilitate communication between adjacent cells. While obviously important, the hormone’s effect on circadian rhythms is largely a mystery. This is precisely what the authors set out to study.

The researchers put two groups of mice–one “normal”, one lacking vasopressin receptors–into specialized cages and subjected them to 12hr light/dark cycles while recording their locomotor activity with infrared sensors. Two weeks later, the researchers advanced the light-dark cycle by 8hrs (so that 8pm turns into 12pm, for example) and watched how the mice reacted.

Normal mice, being nocturnal, generally perk up and start moving around once the lights go out. The day after the time-shift, they delayed their activities until deep into the night. Every subsequent day, these mice shifted their “waking” schedule slightly forward, so that after 8-10 days, their behavioral rhythm finally aligned with the beginning of the night. In stark contrast, mutant mice took less than 4 days to completely readjust their behavior to the new schedule.

Rapid adaptation also happened on molecular level: after initial disruption, circadian expression of clock genes sprung back into rhythm in the SCN, liver and kidneys, taking half the time than normally required. As these mice were born without vasopressin receptors, to rule out developmental defects, researchers infused a drug into the SCN of normal mice to temporarily block both receptors. Once again, the mice rapidly adapted, though at a slightly slower rate than the mutants.

This isn’t the first time scientists have tinkered genes to make jet-lag resistant mice, but those previous mice had a little problem–their internal clocks could not withstand the mutations, eventually winding down to a complete stop. While they SEEMED to rapidly realign their behavior–that is, increase their movement “at night”–it was simply a reflection of their inherent fear of light, with nothing to do with circadian rhythms.

Here’s where these vasopressin receptor-lacking mice stand out. In constant darkness, they showed prominent rhythmic activity of clock gene expression, body temperature and behavior, which is indicative of a ticking timekeeper. On the circuit level, they displayed robust synchrony in hundreds of cellular oscillations across the SCN, as visualized beautifully by a bioluminescent reporter. These results–on both circuitry and behavioral levels–strongly suggest a functional internal clock.

Adaptive instability: a win for the slackers?
So here we have a peculiar timekeeper: one that seems fully functional, which requires stability, yet one capable of large leaps in time, which requires flexibility. How could this be?

To get at the mechanism, researchers used a drug to inhibit protein synthesis, which effectively cuts off all clock gene-protein feedback loops and halts the clocks. After removal of the blocker, normal SCN neurons rapidly reestablished and resynchronized their molecular loops, so that they once again fell back into their original rhythm. Mutant SCN neurons, on the other hand, could not manage to reassemble into network-level oscillations, even though individually they could restart their own cellular clocks.

Thus, it seems that in these mutants, a “synchronization” message fails to broadcast due to the lack of vasopressin receptors–hence once disrupted SCN neurons have a hard time falling back in sync. This isn’t necessary a bad thing. In a steady state, looser communication allows SCN neurons to maintain normal circadian cycles with the extra perk of rapid readjustment. In the face of extreme perturbations however, they loose their synchrony and internal timekeeping fails.

Have scientists truly found a way to “hack” circadian rhythm? I’m hesitant to say yes. Growing up without vasopressin receptors almost certainly altered how SCN neurons communicate with each other in the mutants’ brains. Although temporary blockade of the receptors with drugs also produced rapid clock readjustment, the effect was much less dramatic, strongly suggesting that other factors are at play in a normal brain. Indeed, several previous studies have identified molecular “brakes” that limit SCN’s ability to re-adjust to a new time zone. Targeting vasopressin is also problematic: blocking the receptor peripherally disrupts blood pressure, salt levels and leaves you running to the bathroom every five minutes.

While still ways from conquering jet lag, this study nevertheless beautifully illustrates how neuropeptides can produce network-level oscillations with a stark behavioral outcome. It also introduces a startling master clock capable of both good time keeping and sensitivity to light, something once thought to be mutually exclusive. Whether we should try to reproduce this genetic quirk in humans though is something worth pondering: after 4 billion years of evolution, do we really want to tinker with our circadian clocks?

References:

Y Yamaguchi et al.(2013) Mice genetically deficient in vasopressin V1a and V1b receptors are resistant to jet lag. Science 342 (6154): 85-90. doi: 10.1126/science.1238599

MH Hastings (2013) A looser clock to cure jet lag. Science. 342(6154): 52-53. doi: 10.1126/science.1245474