Stress and odorant receptor feedback during a critical period after hatching regulates olfactory sensory neuron differentiation in Drosophila

New dimensions in the molecular genetics of insect chemoreception

Chemoreception is the foundation of olfaction and taste, which in insects underlie the detection of humans to whom they spread disease and crops that they ravage. Recent advances have provided clear and in some cases surprising new insights into the molecular genetics of chemoreception. We describe mechanisms that govern the choice of a single Odorant receptor gene by an olfactory receptor neuron in Drosophila. We highlight genetic and epigenetic mechanisms by which chemoreceptor expression can be modulated. Exitrons, RNA editing, and pseudo-pseudogenes in chemosensory systems are described. We summarize key insights from the recent structural determinations of odorant and taste receptors. Finally, new molecular components of chemosensory systems, including long noncoding RNAs, are described.

Insect olfactory neurons: receptors, development, and function

Insects represent the most diverse group of animals in the world. While the olfactory systems of different species share general principles of organization, they also exhibit a wide range of structural and functional diversity. Scientists have gained tremendous insight into olfactory neural development and function, notably in Drosophila, but also in other insect species (see reviews by Benton, 2022; Robertson, 2019; Yan et al., 2020). In the last few years, new evidence has steadily mounted, for example, the stoichiometry of odorant receptor and co-receptor (OR-Orco) complex. This review aims to highlight the recent progress on four aspects: (1) the structure and function of the OR-Orco complex, (2) chemosensory gene co-expression, (3) diverse neural developmental processes, and (4) the role of genes and neurons in olfactory development and olfactory-mediated behavior.

Astrocyte regulation of critical period plasticity across neural circuits

Critical periods are brief windows of heightened neural circuit plasticity that allow circuits to permanently reset their structure and function to facilitate robust organismal behavior. Understanding the cellular and molecular mechanisms that instruct critical period timing is of broad clinical interest, as altered developmental plasticity is linked to multiple neurodevelopmental disorders. While intrinsic, neuronal mechanisms shape both neural circuit remodeling and critical period timing, recent data indicate that signaling from astrocytes and surrounding glia can both promote and limit critical period plasticity. In this short review, we discuss recent breakthroughs in our understanding of astrocytes in critical period plasticity and highlight pioneering work in Drosophila.

Odorant receptor co-receptors affect expression of tuning receptors in Drosophila

Insects detect odorants using two large families of heteromeric receptors, the Odorant Receptors (ORs) and Ionotropic Receptors (IRs). Most OR and IR genes encode odorant-binding "tuning" subunits, whereas four (Orco, Ir8a, Ir25a, and Ir76b) encode co-receptor subunits required for receptor function. Olfactory neurons are thought to degenerate in the absence of Orco in ants and bees, and limited data suggest this may happen to some olfactory neurons in Drosophila fruit flies as well. Here, we thoroughly examined the role of co-receptors on olfactory neuron survival in Drosophila. Leveraging knowledge that olfactory neuron classes are defined by the expression of different tuning receptors, we used tuning receptor expression in antennal transcriptomes as a proxy for the survival of distinct olfactory neuron classes. Consistent with olfactory neuron degeneration, expression of many OR-family tuning receptors is decreased in Orco mutants relative to controls, and transcript loss is progressive with age. The effects of Orco are highly receptor-dependent, with expression of some receptor transcripts nearly eliminated and others unaffected. Surprisingly, further studies revealed that olfactory neuron classes with reduced tuning receptor expression generally survive in Orco mutant flies. Furthermore, there is little apoptosis or neuronal loss in the antenna of these flies. We went on to investigate the effects of IR family co-receptor mutants using similar approaches and found that expression of IR tuning receptors is decreased in the absence of Ir8a and Ir25a, but not Ir76b. As in Orco mutants, Ir8a-dependent olfactory neurons mostly endure despite near-absent expression of associated tuning receptors. Finally, we used differential expression analysis to identify other antennal genes whose expression is changed in IR and OR co-receptor mutants. Taken together, our data indicate that odorant co-receptors are necessary for maintaining expression of many tuning receptors at the mRNA level. Further, most Drosophila olfactory neurons persist in OR and IR co-receptor mutants, suggesting that the impact of co-receptors on neuronal survival may vary across insect species.

Experience-dependent tuning of the olfactory system

Insects rely on their sense of smell to navigate complex environments and make decisions regarding food and reproduction. However, in natural settings, the odors that convey this information may come mixed with environmental odors that can obscure their perception. Therefore, recognizing the presence of informative odors involves generalization and discrimination processes, which can be facilitated when there is a high contrast between stimuli, or the internal representation of the odors of interest outcompetes that of concurrent ones. The first two layers of the olfactory system, which involve the detection of odorants by olfactory receptor neurons and their encoding by the first postsynaptic partners in the antennal lobe, are critical for achieving such optimal representation. In this review, we summarize evidence indicating that experience-dependent changes adjust these two levels of the olfactory system. These changes are discussed in the context of the advantages they provide for detection of informative odors.

Neofunctionalization of ciliary BBS proteins to nuclear roles is likely a frequent innovation across eukaryotes

The eukaryotic BBSome is a transport complex within cilia and assembled by chaperonin-like BBS proteins. Recent work indicates nuclear functions for BBS proteins in mammals, but it is unclear how common these are in extant proteins or when they evolved. We screened for BBS orthologues across a diverse set of eukaryotes, consolidated nuclear association via signal sequence predictions and permutation analysis, and validated nuclear localization in mammalian cells via fractionation and immunocytochemistry. BBS proteins are-with exceptions-conserved as a set in ciliated species. Predictions highlight five most likely nuclear proteins and suggest that nuclear roles evolved independently of nuclear access during mitosis. Nuclear localization was confirmed in human cells. These findings suggest that nuclear BBS functions are potentially not restricted to mammals, but may be a common frequently co-opted eukaryotic feature. Understanding the functional spectrum of BBS proteins will help elucidating their role in gene regulation, development, and disease.

Hedgehog-mediated gut-taste neuron axis controls sweet perception in Drosophila

Dietary composition affects food preference in animals. High sugar intake suppresses sweet sensation from insects to humans, but the molecular basis of this suppression is largely unknown. Here, we reveal that sugar intake in Drosophila induces the gut to express and secrete Hedgehog (Hh) into the circulation. We show that the midgut secreted Hh localize to taste sensilla and suppresses sweet sensation, perception, and preference. We further find that the midgut Hh inhibits Hh signalling in the sweet taste neurons. Our electrophysiology studies demonstrate that the midgut Hh signal also suppresses bitter taste and some odour responses, affecting overall food perception and preference. We further show that the level of sugar intake during a critical window early in life, sets the adult gut Hh expression and sugar perception. Our results together reveal a bottom-up feedback mechanism involving a "gut-taste neuron axis" that regulates food sensation and preference.

Early-life exposure to tobacco smoke alters airway signaling pathways and later mortality in D. melanogaster

Early life environmental influences such as exposure to cigarette smoke (CS) can disturb molecular processes of lung development and thereby increase the risk for later development of chronic respiratory diseases. Among the latter, asthma and chronic obstructive pulmonary disease (COPD) are the most common. The airway epithelium plays a key role in their disease pathophysiology but how CS exposure in early life influences airway developmental pathways and epithelial stress responses or survival is poorly understood. Using Drosophila melanogaster larvae as a model for early life, we demonstrate that CS enters the entire larval airway system, where it activates cyp18a1 which is homologues to human CYP1A1 to metabolize CS-derived polycyclic aromatic hydrocarbons and further induces heat shock protein 70. RNASeq studies of isolated airways showed that CS dysregulates pathways involved in oxidative stress response, innate immune response, xenobiotic and glutathione metabolic processes as well as developmental processes (BMP, FGF signaling) in both sexes, while other pathways were exclusive to females or males. Glutathione S-transferase genes were further validated by qPCR showing upregulation of gstD4, gstD5 and gstD8 in respiratory tracts of females, while gstD8 was downregulated and gstD5 unchanged in males. ROS levels were increased in airways after CS. Exposure to CS further resulted in higher larval mortality, lower larval-pupal transition, and hatching rates in males only as compared to air-exposed controls. Taken together, early life CS induces airway epithelial stress responses and dysregulates pathways involved in the fly's branching morphogenesis as well as in mammalian lung development. CS further affected fitness and development in a highly sex-specific manner.

Copy number changes in co-expressed odorant receptor genes enable selection for sensory differences in drosophilid species

Despite numerous examples of chemoreceptor gene family expansions and contractions, how these relate to modifications in the sensory neuron populations in which they are expressed remains unclear. Drosophila melanogaster's odorant receptor (Or) family is ideal for addressing this question because most Ors are expressed in distinct olfactory sensory neuron (OSN) types. Between-species changes in Or copy number may therefore indicate increases or reductions in the number of OSN populations. Here we investigated the Or67a subfamily, which exhibits copy number variation in D. melanogaster and its closest relatives: D. simulans, D. sechellia and D. mauritiana. These species' common ancestor had three Or67a paralogues that had already diverged adaptively. Following speciation, two Or67a paralogues were lost independently in D. melanogaster and D. sechellia, with ongoing positive selection shaping the intact genes. Unexpectedly, the functionally diverged Or67a paralogues in D. simulans are co-expressed in a single neuron population, which projects to a glomerulus homologous to that innervated by Or67a neurons in D. melanogaster. Thus, while sensory pathway neuroanatomy is conserved, independent selection on co-expressed receptors has contributed to species-specific peripheral coding. This work reveals a type of adaptive change largely overlooked for olfactory evolution, raising the possibility that similar processes influence other cases of insect Or co-expression.

Olfactory Receptor Gene Regulation in Insects: Multiple Mechanisms for Singular Expression

The singular expression of insect olfactory receptors in specific populations of olfactory sensory neurons is fundamental to the encoding of odors in patterns of neuronal activity in the brain. How a receptor gene is selected, from among a large repertoire in the genome, to be expressed in a particular neuron is an outstanding question. Focusing on Drosophila melanogaster, where most investigations have been performed, but incorporating recent insights from other insect species, we review the multilevel regulatory mechanisms of olfactory receptor expression. We discuss how cis-regulatory elements, trans-acting factors, chromatin modifications, and feedback pathways collaborate to activate and maintain expression of the chosen receptor (and to suppress others), highlighting similarities and differences with the mechanisms underlying singular receptor expression in mammals. We also consider the plasticity of receptor regulation in response to environmental cues and internal state during the lifetime of an individual, as well as the evolution of novel expression patterns over longer timescales. Finally, we describe the mechanisms and potential significance of examples of receptor co-expression.