It’s Friday night and you are at a concert, wishing you hadn’t woken up at 4:45am to go to spin class. As the night wears on you get more tired and fall asleep on the train ride home. Why do you get tired the longer you stay awake? It’s not your muscles-- they could keep contracting. There are chemical changes to molecules that accumulate the longer we stay awake and they drive this need for sleep. This was shown in a recent paper in Nature by Wang et al. using an interesting mouse mutant.
When mice are sleep deprived, they have an increased need for sleep (just like humans who get more tired the longer they stay awake). Sleep need is measured by putting electrodes on the mouse’s scalp that measure brain waves, which are large synchronized and rhythmic patterns of electrical activity in the brain. When mammals sleep, there are characteristic changes in the brain waves, so we can tell what stage of sleep the animal is in. After sleep deprivation in mice, slow wave activity and the duration of non-REM sleep increase, so this is used to measure sleep need in mice. The researchers who did this study used sleep deprived mice, as well as the Sleepy mutant mouse model (I’m not being cute, this is the actual name of the mutant strain).
The Sleepy mice have a mutation in a gene called Sik3 that encodes for an enzyme. The mutation causes the enzyme to work more efficiently and the mice sleep more, but have an elevated need for sleep (as measured by the brain waves). So these mice are always tired due to one amino acid change in one enzyme – that’s powerful.
Phosphorylated proteins drive sleep need
The researchers compared normal mice with the sleep deprived and Sleepy mice, looking at the chemical changes to the proteins in their brains. The sleep deprived and Sleepy mice had more phosphorylated proteins than the mice who had a normal amount of sleep.
Phosphorylated? That’s a mouth full (here’s how to say it). There is a small molecule called a phosphate, made up of a phosphorous atom surrounded by oxygens. This chemical group is big and charged and will change the shape of the rest of the protein when it is added on. Since phosphorylation changes the shape of proteins, that may also change the way the proteins function.
|Phosphorylation changes the shape of the protein (from Campbell's "Biology")|
The longer the sleep deprived mice stay awake, the more phosphorylated proteins there are. If the mice are allowed to sleep after being deprived, their proteins go back to the unphosphorylated state.
Sleepy mice are always in need of sleep, regardless of how much sleep they get, so their proteins are always phosphorylated. Why do Sleepy mice have so many phosphorylated proteins? Remember that the Sleepy mice have a mutation that makes the Sik3 enzyme more active. Guess what the function of Sik3 is! It is a kinase enzyme, which adds phosphates to proteins. So the poor Sleepy mice accumulate phosphorylation at a higher rate than normal mice, so they will always have an increased need for sleep.
Many of the proteins that are being phosphorylated during the awake state function at the synapse, where neurons communicate with each other. Some neuroscientists believe that memories are encoded while we are awake by changes to synaptic function. These synaptic changes are refined during sleep to consolidate the memories in long-term storage. The authors suggest that the accumulating phosphorylation regulates synapse function and memory formation, though they don’t show evidence for the connection with memory.
In conclusion, next time you are getting tired at that concert, just tell your friends, “My synaptic proteins are too phosphorylated, I need to go home.” They’ll understand.