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.
Sleepy
mice
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.