Sleep homeostasis: finding Odysseus among the mnesteres.

IN HOMER'S THE ODYSSEY1 PENELOPE IS ONLY CONVINCED SHE HAS FOUND HER LONG LOST HUSBAND AMONG THE TRIBE OF RANCOROUS SUITORS BY Odysseus' knowledge of the origins of their bed, around which the house was built. Simply seeing him (albeit 20 years later), hearing him and watching him string his “unmanageable for mortals” bow and send a single arrow deftly through 12 ax head sockets was not enough to unveil his identity. Until thoroughly convinced that Odysseus had returned, Penelope maintained her reservation. Sleep research has a task—no less formidable than Penelope's—of sorting through thousands of candidate genes to identify those that truly influence sleep homeostasis and might provide clues about the roles sleep serves. In this month's issue of SLEEP, Mongrain and colleagues2 have used a clever strategy to impressively narrow down the list of candidate sleep genes for sleep homeostasis. Among the (many) confounds in sleep deprivation paradigms is the stress related to the increased handling of sleep-deprived animals, necessary to maintain wakefulness. The stress of sleep deprivation perturbs the hypothalamic pituitary axis, resulting in, among other physiological responses, increased corticosterone.3,4 Corticosterone heads a major stress response with profound effects on nitric oxide synthase activity, glucose utilization, mitochondrial activity, and transcription (sufficient to induce synaptic plasticity and reduce neurotrophic factors).5,6 Identifying a mouse strain with higher cortisol levels yet lower homeostatic drive than two other strains, Mongrain et al., hypothesized that the corticosterone response was not critical for the sleep-deprivation response and set forth to remove corticosterone as a confounder in sleep homeostasis.2 Indeed, mice with adrenalectomy evidenced normal sleep and showed similar homeostatic responses to sleep loss as sham-operated mice. This finding, in turn, enabled the investigators to analyze the microarray gene response to sleep loss without corticosterone's effects on mRNA, thereby narrowing the list from 1500 genes modified by sleep deprivation to 500. Taking advantage of a second circadian time point after spontaneous wakefulness in the mouse when delta power is elevated (6 hours after lights off for the mouse), the investigators were able to further narrow the list to 80 genes modified by extended wakefulness with or without adrenalectomy at both time points.2 Does this significantly narrowed list provide new insight into the role of sleep and homeostatic regulation? Mackiewicz and colleagues recently summarized the collective findings from the past decade of sleep deprivation microarray studies.7 Consistent findings across various experimental paradigms showed that only a few functional groups of genes vary across sleep and wakefulness. Genes upregulated by sleep deprivation include subsets of molecular chaperones and synaptic plasticity genes; while macromolecular synthesis and energy production genes are down-regulated in response to prolonged wakefulness.8–11 In the present study, adrenalectomy removed almost all of the macromolecular synthesis and energy production genes from sleep homeostasis transcriptional responses.2 The list still includes a number of protein chaperone genes that fit nicely with the hypothesis that prolonged sleep deprivation activates chaperone-mediated protein folding (endoplasmic reticulum) and degradation (both ubiquitination and autophagy). The refined list also includes genes involved in neuronal differentiation and growth, including upregulation of many transcription factors. Few genes in this narrowed homeostasis array list, however, are considered to influence neuronal excitability and might, therefore, play a direct role in slow wave sleep or the homeostatic response to sleep loss. Those present in the list include a few ERK/MAPK signaling genes and a few synaptic genes, including Homer1. Homer1 (Homer1a) is the gene that has shown up in all microarray analyses of sleep deprivation, including the present, and for which an increase in its protein in sleep deprivation has been shown.12,13 Is Homer1 our Odysseus? Homer proteins are post-synaptic density and growth cone scaffolding proteins, and Homer1 contributes to intracellular calcium homeostasis through modifying ryanodine and IP3 calcium release in response to receptor activation, including metabotropic glutamate receptors. In addition, Homer1 is critical for axon guidance, where reductions in Homer1 switch axon growth cone responses from attraction to repulsion. In light of these critical and significant roles in normal neuronal function, we excitedly await studies examining local acute modulation (up and down) of Homer1 along various points in the thalamocortical circuit to critically test its role in local and/or whole brain sleep homeostasis. By excluding the corticosterone component of sleep deprivation, have we thrown out Telemachus (Odysseus' son) with the bathwater? In interpreting the microarray findings, it is important to distinguish which questions may be answered effectively with a given experimental paradigm. The adrenalectomy strategy, employed by Mongrain et al.,2 is the proper approach to elucidate clues regarding slow wave sleep responses to sleep deprivation. From this approach, we can conclude that genes for many myelin molecules, membrane and cytoskeleton proteins are suppressed by enforced wakefulness in a corticosterone-dependent fashion. If we wish to answer what processes are disrupted by prolonged sleep restriction, or why do we need to sleep, the adrenals must remain intact, and the approach used by Mackiewicz et al.,13 with several durations of sleep deprivation that provide different levels of homeostatic drive, would be preferable. Let us be as cautious as Penelope in our quest for sleep homeostasis genes. Does a focused proteomics screen (for this mRNA array list) support involvement of these genes in sleep homeostasis? Does Homer1 or any other candidate gene show a dose-dependent response to sleep loss that predicts the homeostatic drive across several strains of mice? Does this gene increase to a larger extent in youth with increased homeostatic drive and does this response deteriorate with aging in parallel with changes in homeostatic drive? Does the candidate gene increase locally in a brain region activated in specific motor or visual challenges across wakefulness and where rebound slow wave sleep is enhanced? Penelope did not find Odysseus by herself; she was helped in her quest by her son, her maid, the gods and Odysseus. We, too, will likely have to put forth a well-planned collaborative effort to find our Odysseus.