Functional characterization of an orexin neuropeptide in amphioxus reveals an ancient origin of orexin/orexin receptor system in chordate

The Role and Mechanisms of the Hypocretin System in Zebrafish (Danio rerio)

Sleep is the most important physiological function of all animals studied to date. Sleep disorders include narcolepsy, which is characterized by excessive daytime sleepiness, disruption of night sleep, and muscle weakness-cataplexy. Narcolepsy is known to be caused by the degeneration of orexin-synthesizing neurons (hypocretin (HCRT) neurons or orexin neurons) in the hypothalamus. In mammals, HCRT neurons primarily regulate the sleep/wake cycle, nutrition, reward seeking, and addiction development. The hypocretin system of the brain is involved in a number of neurological disorders. The distinctive pathologies associated with the disruption of HCRT neurons are narcolepsy and cataplexy, which are caused by the loss of hypocretin neurons that produce HCRT. In Danio, the hypocretin system is also involved in the regulation of sleep and wakefulness. It is represented by a single hcrt gene that encodes the peptides HCRT1 and HCRT2, as well as one HCRT receptor (HCRTR), which is structurally closest to the mammalian HCRTR2. The overexpression of the hcrt gene in Danio rerio larvae causes wakefulness, whereas the physical destruction of HCRT cells or a pharmacological blockade of the type 2 hypocretin receptor leads to fragmentation of sleep in fish larvae, which is also observed in patients with narcolepsy. These data confirm the evolutionary conservatism of the hypocretin system. Thus, Danio rerio is an ideal model for studying the functions of HCRT neural networks and their functions.

Neuropeptides and receptors in the cephalochordate: A crucial model for understanding the origin and evolution of vertebrate neuropeptide systems

Genomes and transcriptomes from diverse organisms are providing a wealth of data to explore the evolution and origin of neuropeptides and their receptors in metazoans. While most neuropeptide-receptor systems have been extensively studied in vertebrates, there is still a considerable lack of understanding regarding their functions in invertebrates, an extraordinarily diverse group that account for the majority of animal species on Earth. Cephalochordates, commonly known as amphioxus or lancelets, serve as the evolutionary proxy of the chordate ancestor. Their key evolutionary position, bridging the invertebrate to vertebrate transition, has been explored to uncover the origin, evolution, and function of vertebrate neuropeptide systems. Amphioxus genomes exhibit a high degree of sequence and structural conservation with vertebrates, and sequence and functional homologues of several vertebrate neuropeptide families are present in cephalochordates. This review aims to provide a comprehensively overview of the recent findings on neuropeptides and their receptors in cephalochordates, highlighting their significance as a model for understanding the complex evolution of neuropeptide signaling in vertebrates.

Identification, Expression and Evolutional Analysis of Two cyp19-like Genes in Amphioxus

The mechanism of sex determination and differentiation in animals remains a central focus of reproductive and developmental biology research, and the regulation of sex differentiation in amphioxus remains poorly understood. Cytochrome P450 Family 19 Subfamily A member 1 (CYP19A1) is a crucial sex differentiation gene that catalyzes the conversion of androgens into estrogens. In this study, we identified two aromatase-like genes in amphioxus: cyp19-like1 and cyp19-like2. The cyp19-like1 is more primitive and may represent the ancestral form of cyp19 in zebrafish and other vertebrates, while the cyp19-like2 is likely the result of gene duplication within amphioxus. To gain further insights into the expression level of these two aromatase-like, we examined their expression in different tissues and during different stages of gonad development. While the expression level of the two genes differs in tissues, both are highly expressed in the gonad primordium and are primarily localized to microsomal membrane systems. However, as development proceeds, their expression level decreases significantly. This study enhances our understanding of sex differentiation mechanisms in amphioxus and provides valuable insights into the formation and evolution of sex determination mechanisms in vertebrates.

Characterization of a putative orexin receptor in Ciona intestinalis sheds light on the evolution of the orexin/hypocretin system in chordates

Tunicates are evolutionary model organisms bridging the gap between vertebrates and invertebrates. A genomic sequence in Ciona intestinalis (CiOX) shows high similarity to vertebrate orexin receptors and protostome allatotropin receptors (ATR). Here, molecular phylogeny suggested that CiOX is divergent from ATRs and human orexin receptors (hOX1/2). However, CiOX appears closer to hOX1/2 than to ATR both in terms of sequence percent identity and in its modelled binding cavity, as suggested by molecular modelling. CiOX was heterologously expressed in a recombinant HEK293 cell system. Human orexins weakly but concentration-dependently activated its Gq signalling (Ca2+ elevation), and the responses were inhibited by the non-selective orexin receptor antagonists TCS 1102 and almorexant, but only weakly by the OX1-selective antagonist SB-334867. Furthermore, the 5-/6-carboxytetramethylrhodamine (TAMRA)-labelled human orexin-A was able to bind to CiOX. Database mining was used to predict a potential endogenous C. intestinalis orexin peptide (Ci-orexin-A). Ci-orexin-A was able to displace TAMRA-orexin-A, but not to induce any calcium response at the CiOX. Consequently, we suggested that the orexin signalling system is conserved in Ciona intestinalis, although the relevant peptide-receptor interaction was not fully elucidated.

Hypocretin/Orexin Receptor Pharmacology and Sleep Phases

The hypocretins/orexins are two excitatory neuropeptides, alternately called HCRT1 or orexin-A and HCRT2 or orexin-B, that are the endogenous ligands for two G-protein-coupled receptors, HCRTR1/OX1R and HCRTR2/OX2R. Shortly after the discovery of this system, degeneration of hypocretin/orexin-producing neurons was implicated in the etiology of the sleep disorder narcolepsy. The involvement of this system in a disorder characterized by the loss of control over arousal state boundaries also suggested its role as a critical component of endogenous sleep-wake regulatory circuitry. The broad projections of the hypocretin/orexin-producing neurons, along with differential expression of the two receptors in the projection fields of these neurons, suggest distinct roles for these receptors. While HCRTR1/OX1R is associated with regulation of motivation, reward, and autonomic functions, HCRTR2/OX2R is strongly linked to sleep-wake control. The association of hypocretin/orexin with these physiological processes has led to intense interest in the therapeutic potential of compounds targeting these receptors. Agonists and antagonists for the hypocretin/orexin receptors have shown potential for the treatment of disorders of excessive daytime somnolence and nocturnal hyperarousal, respectively, with the first antagonists approved by the US Food and Drug Administration (FDA) in 2014 and 2019 for the treatment of insomnia. These and related compounds have also been useful tools to advance hypocretin/orexin neurobiology.