SLEEP L2: Franks, Wisden. 2023. Cell Res - LKB1/SIK3

Question 1

intro

Answer 1

• in mammals, sleep homeostat is inferred from changes in delta power in the EEG, w/increased delta power indicating deeper NREM sleep, observed after sleep deprivation

Question 2

Sleepy mice

Answer 2

• exhibit increased NREM sleep with higher delta power
• sleepy gene encodes constitutively active SIK3 (salt-inducible-kinase 3), a Ser-thr protein kinase expressed widely in brain and peripheral tissues
• SIK kinases are activated by LKB1 (liver kinase b1) and inhibited by PKA (protein kinase A)
• upon activation, SIK3 P HDACs (histone deacetylases) like HDAC4/5 preventing their nuclear translocation –> this mechanism regualtes gene expression via interactions with TFs such as CREB

Question 3

elucidating how SIK3 regulates sleep quantity and quality

2 studies expanded on this: Kim et al 2022; Zhou et al 2022

Answer 3

• unbiased genetic screen identified Sleepy2 gene, mutations of which also increase NREM sleep duration & higher delta power
• Sleepy2 gene = mutation in HDAC4 gene = reduced HDAC4 protein levels
• using mouse crosses for conditional genetic manipulation & AAV transgene delivery, both studies investigated components of the LKB1-SIK3-HDAC4/5 pathway in various brain regions
• eg. LKB1 KO in brain decreased NREM sleep & delta power; conditional KO of HDAC4 & 5 in adult brains increased NREM sleep & delta power

Question 4

proposed model of HDAC4 & 5

Answer 4

• HDAC4 & 5, in conjunction with TF CREB, inhibit the transcription of sleep-associated genes in glutamatergic neurons in the neocortex
• as wakefulness continues, increased activation of SIK3 by LKB1 P, leads to P of HDAC4/5, causing them to be sequestered in the cytoplasm –> allows for activation of sleep-promoting genes
• LKB1-SIK3-HDAC4/5 pathway extends NREM sleep time by altering gene expression in neurons in posterior hypothalamus, and increases NREM delta power by activation in neocortex (as observed in Sleepy mice)
• interestingly, mice w/gain-of-function mutations in HDAC4 (forcing it to remain in nucleus), still exhibit sleep homeostasis –> suggesting pathway’s dispensability in regulating sleep need

Question 5

connection between LKB1-SIK3-HDAC4/5 pathway and circadian system

Answer 5

• remains unclear, but studies have indicated that SIK3 depletion leads to circadian disruptions and upregulation of the circadian clock protein PER2, hypothesised to be P and destabilised by SIK3

Question 6

delta power assumptions

Answer 6

• assumptions that increased delta power indicates increased sleep may not always hold true, as delta power can be influenced by various factors independent of sleep need
• LKB1 pathway through changes in gene expression, could impact NREM delta power by altering neural synchronicity, which may not necessarily correlate w/sleep itself
• manipulations of neural circuitry can also affect sleep duration w/o directly altering sleep drive
• eg. changes in GABA-A receptor activity in the reticular thalamic nucleus can enhance delta power, while selective elimination of GABA neurons in the midbrain VTA can reduce NREM sleep time
• illustrates that different manipulations can lead to similar sleep-related outcomes, potentially creating a misleading impression of causality

Question 7

research on worms…

Answer 7

• has shown that SIK3 kinase homologue promotes both lipid mobilization and a sleep-like state before moulting, suggesting a connection between energy metabolism and sleep regulation
• in mammals, this pathway could similarly integrate metabolic processes and sleep, potentially explaining why sleep deprivation leads to negative effects on well-being