Autism spectrum disorders (ASD), which affect 1% of the world population, are characterized by impaired social interaction, impaired communication, and restricted behavioural stereotypies. In addition, sleep quality is often impaired in children with ASD which can exacerbate the core symptoms of ASD. For instance, social and communication deficits tend to worsen following a night of poor quality sleep.
Various studies show that 40-80 % of children with ASD have some sleep deficits which can persist into adulthood. These deficits include reduced sleep time, delayed sleep onset, frequent waking after sleep onset, etc. Although these sleep impairments occur in all subtypes of ASD and are one of the biggest difficulties affecting patients with ASD, their etiology is unclear.
Currently, sleep deficits are largely undertreated in this population and pharmaceutical interventions for sleep problems, such as melatonin, do not significantly improve sleep quality in many patients with ASD. It is important to explore the underlying etiology of sleep disorders, including novel molecular targets (e.g. molecules/pathways involved in sleep regulation) that could help us better understand and treat these symptoms.
The R451C missense mutation of neuroligin 3 (Nlgn3R451C), an X-linked gene, was identified in cases of ASD in humans.
ASD are typically diagnosed in humans between the ages 2 to 4, a period characterized by extensive activity-dependent neuronal remodeling. The pathogenesis of ASD has been proposed to depend on 2 mechanisms: abnormal cellular/synaptic growth and alterations in the balance between neuronal excitation/inhibition (E/I).
Neuroligins (Nlgns) are a family of post-synaptic cell adhesion molecules that are important for proper synaptic transmission, plasticity, and neuronal E/I balance. More importantly, the R451C missense mutation of neuroligin 3 (Nlgn3R451C), an X-linked gene, was identified in cases of ASD in humans. Recent work using rodent models demonstrated that neuroligin 3 is important for social interaction, vocal communication, as well as repetitive behavioural stereotypies, which are the key features of ASD. However, the role of neuroligin 3 in sleep regulation had not been examined until recently.
Examining sleep pattern quality
In our study, recently published in Molecular Brain, we used electroencephalography (EEG) and electromyography (EMG) in adult male mice to examine whether this Nlgn3R451C mutation affected sleep pattern and quality. The pattern and quality of sleep are measured using several parameters. Firstly, we measured the proportion of total time spent in each behavioural state (i.e. wakefulness, non-rapid-eye-movement sleep, and rapid-eye-movement sleep) and their distribution across the time-of-day. To measure sleep fragmentation, we analyzed the number and duration of each individual episode for wakefulness, non-rapid-eye-movement (NREM) sleep, and rapid-eye-movement (REM) sleep. The quality of sleep was measured using EEG power spectra.
Our findings showed that Nlgn3R451C mutation does not alter the proportion of total time spent in wakefulness, REM sleep, or contribute to sleep fragmentation. However, the length of NREM sleep was marginally reduced in the mutant mice.
The Nlgn3R451C mutation significantly altered EEG power spectra in our mice.
In addition, the Nlgn3R451C mutation significantly altered EEG power spectra in our mice. For example, Nlgn3R451C mice exhibited lower delta power during NREM sleep, a marker for NREM sleep depth, suggesting impaired NREM sleep quality in Nlgn3R451C mice. Furthermore, Nlgn3R451C mice exhibited elevated beta power during REM sleep.
Beta oscillations are reflective of cortical arousal in sleep in humans, and elevated beta power is reported in some patients with primary insomnia. Hence, this increased beta power in our Nlgn3R451C mice may suggest increased arousal during REM sleep and impaired REM sleep quality. EEG power bands such as theta, alpha, and sigma were also altered in the mutant mice.
Overall, our findings revealed that Nlgn3-mediated mechanisms may be involved in EEG rhythms and sleep quality, proposing this as a new molecular target contributing to sleep deficits in ASD. It is interesting to note that the effects of Nlgn3R451C mutation on sleep are quite different from Nlgn1 (another member of the neuroligin family) knock-out mutation which exhibited decreased high delta, theta, alpha powers during wakefulness while showing a trend of increased delta power during NREM sleep. These differences suggest that the role of neuroligins in sleep regulation is specific to the individual members of the neuroligin family.
It is important for future studies to explore the mechanisms in which neuroligins regulate sleep by examining whether neuroligin mutations alter neuronal E/I ratio (neuronal excitability) in regions that are critical for regulating sleep and arousal (e.g. hypothalamus and thalamus) and how these deficits can be ameliorated in patients with ASD.