So, we have seen that the more we learn and the more our brain undergoes plastic changes during wake, the more we need to sleep. But all the studies that we have reviewed so far have started from the assumption that the messenger RNAs and the proteins that were differentially expressed between sleep and wake come from the neurons. But, in fact, in the brain, there are many other kinds of cells and more importantly, glial cells, oligodendrocytes and astrocytes. And recent studies are starting to show how sleep and wake can also affect these particular cell types. For instance, there is a new methodology called the translating ribosome affinity purification method, which allows to specifically tag the messenger RNAs that are already attached to ribosomes and therefore are on the way to being translated and becoming proteins in specific cell types only. In this specific line, for instance, only the cells that express the enzyme Cnp, which is a marker for the oligodendrocytes and some of their precursors are targeted. And, therefore, it is possible then to specifically analyze the effects of sleep and wake on oligodendrocytes. And that's what was done in this recent study in which, again, animals, mice in this case, were collected during the day after several hours of either sleep or sleep deprivation, or during the night after several hours of spontaneous wake. And it was found that among all the genes that are expressed in oligodendrocytes up to 1 percent, 300 and so genes were up regulated during sleep and a similar amount during wake. And again, as before, the sleep and wake categories of genes are different. For instance, the wake genes are involved in cell death and specifically apoptosis, in cell differentiation, and in the stress cellular response and instead the sleep genes are involved in cell proliferation, in lipid metabolism, and in myelin formation. And because of these specific group of genes in sleep related to cell proliferation, follow up experiments were then done, specifically using methods such as the BrdU staining to assess the amount of proliferation of the precursors of oligodendrocytes in sleep and wake. And it was found that sleep indeed increases, almost doubles the rate of proliferation of these precursors. So this is just to say that, in fact, certainly, sleep and wake have a profound effect not only on neurons but also on other glial cells. And, of course, many more studies need to be done to clarify those changes. Now, let's move to the second part of the talk where we would summarize the genetic studies, first in humans, showing that genes can have a profound effect on the sleep phenotype. Actually, there is no single sleep phenotype. There are many phenotypes because we can focus on sleep quantity. For instance, the total amount of sleep or the amount of non-REM sleep or quality. For instance, the amount of slow wave activity or we can focus our analysis of the genetic influence on sleep regulation either on the timing of sleep, the circadian regulation, or the homeostatic response. And as you will see, most genes tend to have an effect that is specific for one or few sleep phenotypes. The sleep phenotypes of traits are in general complex traits. So there's not a simple situation in which there is one single gene dictating and totally controlling the trait but there are actually many genes that are epistatic non-addictive interactions among those genes. And, of course, there's a very important interaction between genes and environment. Here is an example of one sleep phenotype, the total sleep duration. This is a pioneer study that asked more than a million Americans about their total amount of sleep. This is the reported amount of sleep where the information was simply obtained through a questionnaire. And you see that both women and men more or less, the majority of them report sleeping seven or eight hours every night. But there are, of course, big variations and there are extremes on both sides of this curve. This table is just meant to give you the following message that many twin studies have already been done assessing many sleep phenotypes. The early studies were mainly using questionnaires. The more recent studies are using objective measures of sleep such as the EEG analysis. The take home message is that most if not all sleep phenotypes are to some extent under genetic control including, for instance, the total amount of sleep or the amount of sleepiness, or in these more recent studies, the expression and the amount of specific EEG frequencies, for instance, the alpha rhythm or again the slow wave activity of non-REM sleep. In fact, the EEG frequencies, the EEG spectrum in general, is extremely hereditable in humans as well as in animals. But this is true not only in sleep but also in wake. A very recent twin study that was not included in the previous table has also assessed the genetic influence on the effect of sleep deprivation. And this was done during a 38 hours of sleep deprivation using the psychomotor vigilant task to assess sustained attention and the impairment in this task. And it was found that a larger percent, 51 percent of the twin variance in this specific measure was due to genetic effects. A very powerful recent approach to try to identify the genes that are controlling or modulating sleep phenotypes is the Genome Wide Association analysis that takes advantage of the presence of marker loci, the chromosomes, and specifically of single nucleotide polymorphism SNPs, which, like the name says, are just difference in the normal population in a single nucleotide in the same position of the two alleles. These studies are extremely powerful statistically, they are unbiased because they don't make any assumption about which genes should be involved, they can now use in very large arrays, target almost a million of these SNPs, and so they are very powerful approach. Unfortunately, they're still quite expensive and the experience so far for the sleep phenotypes has been that many hundreds of subjects are usually required to identify statistically significant loci. And, of course, the studies need replication. Here is a list of human sleep disorders that have some kind of genetic component and they are listed in a decreasing order of genetic influence from fatal familial insomnia, in which you will see there's a very strong genetic effect, down to obstructive sleep apnea syndrome.
