Activity of putative orexin neurons during cataplexy

Cell-Specific Single Viral Vector CRISPR/Cas9 Editing and Genetically Encoded Tool Delivery in the Central and Peripheral Nervous Systems

CRISPR/Cas9 gene editing represents an exciting avenue to study genes of unknown function and can be combined with genetically encoded tools such as fluorescent proteins, channelrhodopsins, DREADDs, and various biosensors to more deeply probe the function of these genes in different cell types. However, current strategies to also manipulate or visualize edited cells are challenging due to the large size of Cas9 proteins and the limited packaging capacity of adeno-associated viruses (AAVs). To overcome these constraints, we developed an alternative gene editing strategy using a single AAV vector and mouse lines that express Cre-dependent Cas9 to achieve efficient cell-type specific editing across the nervous system. Expressing Cre-dependent Cas9 from a genomic locus affords space to package guide RNAs for gene editing together with Cre-dependent, genetically encoded tools to manipulate, map, or monitor neurons using a single virus. We validated this strategy with three common tools in neuroscience: ChRonos, a channelrhodopsin, for studying synaptic transmission using optogenetics, GCaMP8f for recording Ca2+ transients using photometry, and mCherry for tracing axonal projections. We tested these tools in multiple brain regions and cell types, including GABAergic neurons in the nucleus accumbens, glutamatergic neurons projecting from the ventral pallidum to the lateral habenula, dopaminergic neurons in the ventral tegmental area, and proprioceptive neurons in the periphery. This flexible approach could help identify and test the function of novel genes affecting synaptic transmission, circuit activity, or morphology with a single viral injection.

Cell specific single viral vector CRISPR/Cas9 editing and genetically encoded tool delivery in the central and peripheral nervous systems

Gene manipulation strategies using germline knockout, conditional knockout, and more recently CRISPR/Cas9 are crucial tools for advancing our understanding of the nervous system. However, traditional gene knockout approaches can be costly and time consuming, may lack cell-type specificity, and can induce germline recombination. Viral gene editing presents and an exciting alternative to more rapidly study genes of unknown function; however, current strategies to also manipulate or visualize edited cells are challenging due to the large size of Cas9 proteins and the limited packaging capacity of adeno-associated viruses (AAVs). To overcome these constraints, we have developed an alternative gene editing strategy using a single AAV vector and mouse lines that express Cre-dependent Cas9 to achieve efficient cell-type specific editing across the nervous system. Expressing Cre-dependent Cas9 in specific cell types in transgenic mouse lines affords more space to package guide RNAs for gene editing together with Cre-dependent, genetically encoded tools to manipulate, map, or monitor neurons using a single virus. We validated this strategy with three commonly used tools in neuroscience: ChRonos, a channelrhodopsin, for manipulating synaptic transmission using optogenetics; GCaMP8f for recording Ca2+ transients using fiber photometry, and mCherry for anatomical tracing of axonal projections. We tested these tools in multiple brain regions and cell types, including GABAergic neurons in the nucleus accumbens (NAc), glutamatergic neurons projecting from the ventral pallidum (VP) to the lateral habenula (LHb), dopaminergic neurons in the ventral tegmental area (VTA), and parvalbumin (PV)-positive proprioceptive neurons in the periphery. This flexible approach should be useful to identify novel genes that affect synaptic transmission, circuit activity, or morphology with a single viral injection.

Deficiency of orexin signaling during sleep is involved in abnormal REM sleep architecture in narcolepsy

Narcolepsy is a sleep disorder caused by deficiency of orexin signaling. However, the neural mechanisms by which deficient orexin signaling causes the abnormal rapid eye movement (REM) sleep characteristics of narcolepsy, such as cataplexy and frequent transitions to REM states, are not fully understood. Here, we determined the activity dynamics of orexin neurons during sleep that suppress the abnormal REM sleep architecture of narcolepsy. Orexin neurons were highly active during wakefulness, showed intermittent synchronous activity during non-REM (NREM) sleep, were quiescent prior to the transition from NREM to REM sleep, and a small subpopulation of these cells was active during REM sleep. Orexin neurons that lacked orexin peptides were less active during REM sleep and were mostly silent during cataplexy. Optogenetic inhibition of orexin neurons established that the activity dynamics of these cells during NREM sleep regulate NREM-REM sleep transitions. Inhibition of orexin neurons during REM sleep increased subsequent REM sleep in "orexin intact" mice and subsequent cataplexy in mice lacking orexin peptides, indicating that the activity of a subpopulation of orexin neurons during the preceding REM sleep suppresses subsequent REM sleep and cataplexy. Thus, these results identify how deficient orexin signaling during sleep results in the abnormal REM sleep architecture characteristic of narcolepsy.

Peripheral vs. core body temperature as hypocretin/orexin neurons degenerate: Exercise mitigates increased heat loss

Hypocretins/Orexins (Hcrt/Ox) are hypothalamic neuropeptides implicated in diverse functions, including body temperature regulation through modulation of sympathetic vasoconstrictor tone. In the current study, we measured subcutaneous (Tsc) and core (Tb) body temperature as well as activity in a conditional transgenic mouse strain that allows the inducible ablation of Hcrt/Ox-containing neurons by removal of doxycycline (DOX) from their diet (orexin-DTA mice). Measurements were made during a baseline, when mice were being maintained on food containing DOX, and over 42 days while the mice were fed normal chow which resulted in Hcrt/Ox neuron degeneration. The home cages of the orexin-DTA mice were equipped with running wheels that were either locked or unlocked. In the presence of a locked running wheel, Tsc progressively decreased on days 28 and 42 in the DOX(-) condition, primarily during the dark phase (the major active period for rodents). This nocturnal reduction in Tsc was mitigated when mice had access to unlocked running wheels. In contrast to Tsc, Tb was largely maintained until day 42 in the DOX(-) condition even when the running wheel was locked. Acute changes in both Tsc and Tb were observed preceding, during, and following cataplexy. Our results suggest that ablation of Hcrt/Ox-containing neurons results in elevated heat loss, likely through reduced sympathetic vasoconstrictor tone, and that exercise may have some therapeutic benefit to patients with narcolepsy, a disorder caused by Hcrt/Ox deficiency. Acute changes in body temperature may facilitate prediction of cataplexy onset and lead to interventions to mitigate its occurrence.

The role of orexin neuron activity in sleep/wakefulness regulation

Orexin (also known as hypocretin) is a neuropeptide exclusively synthesized in the neurons of the lateral hypothalamus (LH). Initially orexin was thought to be involved in the regulation of feeding behavior. However, it is now known to also be a critical regulator of sleep/wakefulness, especially the maintenance of wakefulness. Although the somas of orexin neurons are exclusively located in the LH, these neurons send axons throughout the brain and spinal cord. Orexin neurons integrate inputs from various brain regions and project to neurons that are involved in the regulation of sleep/wakefulness. Orexin knockout mice have a fragmentation of sleep/wakefulness and cataplexy-like behavior arrest, which is similar to the sleep disorder narcolepsy. Recent progress with manipulation of neural activity of targeted neurons, using experimental tools such as optogenetics and chemogenetics, has emphasized the role of orexin neuron activity on the regulation of sleep/wakefulness. Recording of orexin neuron activity in vivo using electrophysiological and gene-encoded calcium indicator proteins revealed that these cells have specific activity patterns across sleep/wakefulness state changes. Here, we also discuss not only the role of the orexin peptide, but also the role of other co-transmitters that are synthesized and released from orexin neurons and involved in sleep/wakefulness regulation.

Positive memory increases cataplexy-like behaviors in narcolepsy mice as revealed using conditioned place preference test

BACKGROUND

Cataplexy is a loss of muscle tone that can lead to postural collapse, disturbing the daily life of narcolepsy patients; it is often triggered by positive emotions such as laughter in human patients. Narcolepsy model mice also show cataplexy, and its incidence increases in response to positive emotion-inducing stimuli such as chocolate and female courtship. Although such observation indicates a positive emotion-related nature of cataplexy in narcolepsy mice, they also show cataplexy without any apparent triggering stimulus ~ (spontaneous cataplexy). Therefore, we hypothesized that some spontaneous cataplexy in narcoleptic mice might indicate the remembering of happy moments.

RESULTS

To test our hypothesis, we did a conditioned place preference test on orexin/hypocretin neuron-ablated (ORX-AB) mice, one of the animal models of human narcolepsy, and counted the number of cataplexy-like behaviors. ORX-AB mice successfully remembered the chocolate-associated chamber, and the number of cataplexy-like behaviors significantly increased in the chocolate-associated chamber but not in the control chamber. In addition, ORX-AB mice remembered the aversive odor-associated chamber and avoided entering without affecting the number of cataplexy-like behaviors. Finally, similar activation of the nucleus accumbens, a positive emotion-related nucleus, was observed during both spontaneous and chocolate-induced cataplexy behaviors.

CONCLUSIONS

These results support our hypothesis and will promote the usefulness of a narcolepsy mice model in emotion research and serve as a basis for a better understanding of cataplexy in narcolepsy patients.

Recent advances in neuropeptide-related omics and gene editing: Spotlight on NPY and somatostatin and their roles in growth and food intake of fish

Feeding and growth are two closely related and important physiological processes in living organisms. Studies in mammals have provided us with a series of characterizations of neuropeptides and their receptors as well as their roles in appetite control and growth. The central nervous system, especially the hypothalamus, plays an important role in the regulation of appetite. Based on their role in the regulation of feeding, neuropeptides can be classified as orexigenic peptide and anorexigenic peptide. To date, the regulation mechanism of neuropeptide on feeding and growth has been explored mainly from mammalian models, however, as a lower and diverse vertebrate, little is known in fish regarding the knowledge of regulatory roles of neuropeptides and their receptors. In recent years, the development of omics and gene editing technology has accelerated the speed and depth of research on neuropeptides and their receptors. These powerful techniques and tools allow a more precise and comprehensive perspective to explore the functional mechanisms of neuropeptides. This paper reviews the recent advance of omics and gene editing technologies in neuropeptides and receptors and their progresses in the regulation of feeding and growth of fish. The purpose of this review is to contribute to a comparative understanding of the functional mechanisms of neuropeptides in non-mammalians, especially fish.