Singular expression of olfactory receptor genes

Patterns of early embryogenesis and growth in the olfactory system of chick (Gallus gallus domesticus) based on iodine-enhanced micro-computed tomography

BACKGROUND

The vertebrate olfactory system entails a complex set of neural/support structures that bridge morphogenetic regions. The developmental mechanisms coordinating this bridge remain unclear, even for model organisms such as chick, Gallus gallus. Here, we combine previous growth data on the chick olfactory apparatus with new samples targeting its early embryogenesis. The purpose is to illuminate how early developmental dynamics integrate with scaling relationships to produce adult form and, potentially, evolutionary patterns. Olfactory structures, including epithelium, turbinate, nerve, and olfactory bulb, are considered in the context of neighboring nasal and brain structures.

RESULTS

Axonal outgrowth from the olfactory epithelium, which eventually connects receptor neurons with the brain, begins earlier than previously established. This dynamic marks the beginning of a complex pattern of early differential growth wherein the olfactory bulbs scale with positive allometry relative to both brain volume and turbinate area, which in turn scale isometrically with one another.

CONCLUSIONS

The mechanisms driving observed patterns of organogenesis and growth remain unclear awaiting experimental evidence. We discuss competing hypotheses, including the possibility that broad-based isometry of olfactory components reflects constraints imposed by high levels of functional/structural integration. Such integration would include the frontonasal prominence having a strong influence on telencephalic patterning.

Molecular, cellular, and developmental organization of the mouse vomeronasal organ at single cell resolution

We have generated single cell transcriptomic atlases of vomeronasal organs (VNO) from juvenile and adult mice. Combined with spatial molecular imaging, we uncover a distinct, previously unidentified class of cells that express the vomeronasal receptors (VRs) and a population of canonical olfactory sensory neurons in the VNO. High-resolution trajectory and cluster analyses reveal the lineage relationship, spatial distribution of cell types, and a putative cascade of molecular events that specify the V1r, V2r, and OR lineages from a common stem cell population. The expression of vomeronasal and olfactory receptors follow power law distributions, but there is high variability in average expression levels between individual receptor and cell types. Substantial co-expression is found between receptors across clades, from different classes, and between olfactory and VRs, with nearly half from pairs located on the same chromosome. Interestingly, the expression of V2r, but not V1r, genes is associated with various transcription factors, suggesting distinct mechanisms of receptor choice associated with the two cell types. We identify association between transcription factors, surface axon guidance molecules, and individual VRs, thereby uncovering a molecular code that guides the specification of the vomeronasal circuitry. Our study provides a wealth of data on the development and organization of the accessory olfactory system at both cellular and molecular levels to enable a deeper understanding of vomeronasal system function.

Bioinformatics Analysis and Experimental Validation of Differential Genes and Pathways in Bone Nonunions

Non-union fractures pose a significant clinical challenge, often leading to prolonged pain and disability. Understanding the molecular mechanisms underlying non-union fractures is crucial for developing effective therapeutic interventions. This study integrates bioinformatics analysis and experimental validation to unravel key genes and pathways associated with non-union fractures. We identified differentially expressed genes (DEGs) between non-union and fracture healing tissues using bioinformatics techniques. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analyses were employed to elucidate the biological processes and pathways involved. Common DEGs were identified, and a protein-protein interaction (PPI) network was constructed. Fibronectin-1 (FN1), Thrombospondin-1 (THBS1), and Biglycan (BGN) were pinpointed as critical target genes for non-union fracture treatment. Experimental validation involved alkaline phosphatase (ALP) and Alizarin Red staining to confirm osteogenic differentiation. Our analysis revealed significant alterations in pathways related to cell behavior, tissue regeneration, wound healing, infection, and immune responses in non-union fracture tissues. FN1, THBS1, and BGN were identified as key genes, with their upregulation indicating potential disruptions in the bone remodeling process. Experimental validation confirmed the induction of osteogenic differentiation. The study provides comprehensive insights into the molecular mechanisms of non-union fractures, emphasizing the pivotal roles of FN1, THBS1, and BGN in extracellular matrix dynamics and bone regeneration. The findings highlight potential therapeutic targets and pathways for further investigation. Future research should explore interactions between these genes, validate results using in vivo fracture models, and develop tailored treatment strategies for non-union fractures, promising significant advances in clinical management.

Molecular, Cellular, and Developmental Organization of the Mouse Vomeronasal organ at Single Cell Resolution

We have generated single cell transcriptomic atlases of vomeronasal organs (VNO) from juvenile and adult mice. Combined with spatial molecular imaging, we uncover a distinct, previously unidentified class of cells that express the vomeronasal receptors and a population of canonical olfactory sensory neurons in the VNO. High resolution trajectory and cluster analyses reveal the lineage relationship, spatial distribution of cell types, and a putative cascade of molecular events that specify the V1r, V2r, and OR lineages from a common stem cell population. The expression of vomeronasal and olfactory receptors follow power law distributions, but there is high variability in average expression levels between individual receptor and cell types. Substantial co-expression is found between receptors across clades, from different classes, and between olfactory and vomeronasal receptors, with nearly half from pairs located on the same chromosome. Interestingly, the expression of V2r, but not V1r, genes is associated with various transcription factors, suggesting distinct mechanisms of receptor choice associated with the two cell types. We identify association between transcription factors, surface axon guidance molecules, and individual VRs, thereby uncovering a molecular code that guides the specification of the vomeronasal circuitry. Our study provides a wealth of data on the development and organization of the accessory olfactory system at both cellular and molecular levels to enable a deeper understanding of vomeronasal system function.

Understanding the molecular mechanisms of odorant binding and activation of the human OR52 family

Structural and mechanistic studies on human odorant receptors (ORs), key in olfactory signaling, are challenging because of their low surface expression in heterologous cells. The recent structure of OR51E2 bound to propionate provided molecular insight into odorant recognition, but the lack of an inactive OR structure limited understanding of the activation mechanism of ORs upon odorant binding. Here, we determined the cryo-electron microscopy structures of consensus OR52 (OR52cs), a representative of the OR52 family, in the ligand-free (apo) and octanoate-bound states. The apo structure of OR52cs reveals a large opening between transmembrane helices (TMs) 5 and 6. A comparison between the apo and active structures of OR52cs demonstrates the inward and outward movements of the extracellular and intracellular segments of TM6, respectively. These results, combined with molecular dynamics simulations and signaling assays, shed light on the molecular mechanisms of odorant binding and activation of the OR52 family.

Identity, lineage and fates of a temporally distinct progenitor population in the embryonic olfactory epithelium

We defined a temporally and transcriptionally divergent precursor cohort in the medial olfactory epithelium (OE) shortly after it differentiates as a distinct tissue at mid-gestation in the mouse. This temporally distinct population of Ascl1+ cells in the dorsomedial OE is segregated from Meis1+/Pax7+ progenitors in the lateral OE, and does not appear to be generated by Pax7+ lateral OE precursors. The medial Ascl1+ precursors do not yield a substantial number of early-generated ORNs. Instead, they first generate additional proliferative precursors as well as a distinct population of frontonasal mesenchymal cells associated with the migratory mass that surrounds the nascent olfactory nerve. Parallel to these in vivo distinctions, isolated medial versus lateral OE precursors in vitro retain distinct proliferative capacities and modes of division that reflect their in vivo identities. At later fetal stages, these early dorsomedial Ascl1+ precursors cells generate spatially restricted subsets of ORNs as well as other non-neuronal cell classes. Accordingly, the initial compliment of ORNs and other OE cell types is derived from at least two distinct early precursor populations: lateral Meis1/Pax7+ precursors that generate primarily early ORNs, and a temporally, spatially, and transcriptionally distinct subset of medial Ascl1+ precursors that initially generate additional OE progenitors and apparent migratory mass cells before yielding a subset of ORNs and likely supporting cell classes.

A coordinated transcriptional switching network mediates antigenic variation of human malaria parasites

Malaria parasites avoid immune clearance through their ability to systematically alter antigens exposed on the surface of infected red blood cells. This is accomplished by tightly regulated transcriptional control of individual members of a large, multicopy gene family called var and is the key to both the virulence and chronic nature of malaria infections. Expression of var genes is mutually exclusive and controlled epigenetically, however how large populations of parasites coordinate var gene switching to avoid premature exposure of the antigenic repertoire is unknown. Here, we provide evidence for a transcriptional network anchored by a universally conserved gene called var2csa that coordinates the switching process. We describe a structured switching bias that shifts overtime and could shape the pattern of var expression over the course of a lengthy infection. Our results provide an explanation for a previously mysterious aspect of malaria infections and shed light on how parasites possessing a relatively small repertoire of variant antigen-encoding genes can coordinate switching events to limit antigen exposure, thereby maintaining chronic infections.

Placing human gene families into their evolutionary context

Following the draft sequence of the first human genome over 20 years ago, we have achieved unprecedented insights into the rules governing its evolution, often with direct translational relevance to specific diseases. However, staggering sequence complexity has also challenged the development of a more comprehensive understanding of human genome biology. In this context, interspecific genomic studies between humans and other animals have played a critical role in our efforts to decode human gene families. In this review, we focus on how the rapid surge of genome sequencing of both model and non-model organisms now provides a broader comparative framework poised to empower novel discoveries. We begin with a general overview of how comparative approaches are essential for understanding gene family evolution in the human genome, followed by a discussion of analyses of gene expression. We show how homology can provide insights into the genes and gene families associated with immune response, cancer biology, vision, chemosensation, and metabolism, by revealing similarity in processes among distant species. We then explain methodological tools that provide critical advances and show the limitations of common approaches. We conclude with a discussion of how these investigations position us to gain fundamental insights into the evolution of gene families among living organisms in general. We hope that our review catalyzes additional excitement and research on the emerging field of comparative genomics, while aiding the placement of the human genome into its existentially evolutionary context.

Ecological constraints on highly evolvable olfactory receptor genes and morphology in neotropical bats

Although evolvability of genes and traits may promote specialization during species diversification, how ecology subsequently restricts such variation remains unclear. Chemosensation requires animals to decipher a complex chemical background to locate fitness-related resources, and thus the underlying genomic architecture and morphology must cope with constant exposure to a changing odorant landscape; detecting adaptation amidst extensive chemosensory diversity is an open challenge. In phyllostomid bats, an ecologically diverse clade that evolved plant visiting from a presumed insectivorous ancestor, the evolution of novel food detection mechanisms is suggested to be a key innovation, as plant-visiting species rely strongly on olfaction, supplementarily using echolocation. If this is true, exceptional variation in underlying olfactory genes and phenotypes may have preceded dietary diversification. We compared olfactory receptor (OR) genes sequenced from olfactory epithelium transcriptomes and olfactory epithelium surface area of bats with differing diets. Surprisingly, although OR evolution rates were quite variable and generally high, they are largely independent of diet. Olfactory epithelial surface area, however, is relatively larger in plant-visiting bats and there is an inverse relationship between OR evolution rates and surface area. Relatively larger surface areas suggest greater reliance on olfactory detection and stronger constraint on maintaining an already diverse OR repertoire. Instead of the typical case in which specialization and elaboration are coupled with rapid diversification of associated genes, here the relevant genes are already evolving so quickly that increased reliance on smell has led to stabilizing selection, presumably to maintain the ability to consistently discriminate among specific odorants-a potential ecological constraint on sensory evolution.

Asynchronous Replication Timing: A Mechanism for Monoallelic Choice During Development

Developmental programming is carried out by a sequence of molecular choices that epigenetically mark the genome to generate the stable cell types which make up the total organism. A number of important processes, such as genomic imprinting, selection of immune or olfactory receptors, and X-chromosome inactivation in females are dependent on the ability to stably choose one single allele in each cell. In this perspective, we propose that asynchronous replication timing (ASRT) serves as the basis for a sophisticated universal mechanism for mediating and maintaining these decisions.

Diversity in olfactory receptor repertoires is associated with dietary specialization in a genus of frugivorous bat

Mammalian olfactory receptor genes (ORs) are a diverse family of genes encoding proteins that directly interact with environmental chemical cues. ORs evolve via gene duplication in a birth-death fashion, neofunctionalizing and pseudogenizing over time. Olfaction is a primary sense used for food detection in plant-visiting bats, but the relationship between dietary specialization and OR repertoire diversity is unclear. Within neotropical Leaf-nosed bats (Phyllostomidae), many lineages are plant specialists, and some have a distinct OR repertoire compared to insectivorous species. Yet, whether specialization on particular plant genera is associated with the evolution of specialized, less diverse OR repertoires has never been tested. Using targeted sequence capture, we sequenced the OR repertoires of three sympatric species of short-tailed fruit bats (Carollia), which vary in their degree of specialization on the fruits of Piper plants. We characterized orthologous vs duplicated receptors among Carollia species, and explored the diversity and redundancy of the receptor gene repertoire. At the species level, the most dedicated Piper specialist, Carollia castanea, had lower OR diversity compared to the two generalists (C. sowelli and C. perspicillata), but we discovered a few unique sets of ORs within C. castanea with high redundancy of similar gene duplicates. These unique receptors potentially enable C. castanea to detect Piper fruit odorants better than its two congeners. Carollia perspicillata, the species with the most generalist diet, had a higher diversity of intact receptors, suggesting the ability to detect a wider range of odorant molecules. Variation among ORs may be a factor in the coexistence of these sympatric species, facilitating the exploitation of different plant resources. Our study sheds light on how gene duplication and changes in OR diversity may play a role in dietary adaptations and underlie ecological interactions between bats and plants.

Olfactory receptor gene evolution is unusually rapid across Tetrapoda and outpaces chemosensory phenotypic change

Chemosensation is the most ubiquitous sense in animals, enacted by the products of complex gene families that detect environmental chemical cues and larger-scale sensory structures that process these cues. While there is a general conception that olfactory receptor (OR) genes evolve rapidly, the universality of this phenomenon across vertebrates, and its magnitude, are unclear. The supposed correlation between molecular rates of chemosensory evolution and phenotypic diversity of chemosensory systems is largely untested. We combine comparative genomics and sensory morphology to test whether OR genes and olfactory phenotypic traits evolve at faster rates than other genes or traits. Using published genomes, we identified ORs in 21 tetrapods, including amphibians, reptiles, birds, and mammals and compared their rates of evolution to those of orthologous non-OR protein-coding genes. We found that, for all clades investigated, most OR genes evolve nearly an order of magnitude faster than other protein-coding genes, with many OR genes showing signatures of diversifying selection across nearly all taxa in this study. This rapid rate of evolution suggests that chemoreceptor genes are in "evolutionary overdrive," perhaps evolving in response to the ever-changing chemical space of the environment. To obtain complementary morphological data, we stained whole fixed specimens with iodine, µCT-scanned the specimens, and digitally segmented chemosensory and nonchemosensory brain regions. We then estimated phenotypic variation within traits and among tetrapods. While we found considerable variation in chemosensory structures, they were no more diverse than nonchemosensory regions. We suggest chemoreceptor genes evolve quickly in reflection of an ever-changing chemical space, whereas chemosensory phenotypes and processing regions are more conserved because they use a standardized or constrained architecture to receive and process a range of chemical cues.

Uncovering the mouse olfactory long non-coding transcriptome with a novel machine-learning model

Very little is known about long non-coding RNAs (lncRNAs) in the mammalian olfactory sensory epithelia. Deciphering the non-coding transcriptome in olfaction is relevant because these RNAs have been shown to play a role in chromatin modification and nuclear architecture reorganization, processes that accompany olfactory differentiation and olfactory receptor gene choice, one of the most poorly understood gene regulatory processes in mammals. In this study, we used a combination of in silico and ex vivo approaches to uncover a comprehensive catalogue of olfactory lncRNAs and to investigate their expression in the mouse olfactory organs. Initially, we used a novel machine-learning lncRNA classifier to discover hundreds of annotated and unannotated lncRNAs, some of which were predicted to be preferentially expressed in the main olfactory epithelium and the vomeronasal organ, the most important olfactory structures in the mouse. Moreover, we used whole-tissue and single-cell RNA sequencing data to discover lncRNAs expressed in mature sensory neurons of the main epithelium. Candidate lncRNAs were further validated by in situ hybridization and RT-PCR, leading to the identification of lncRNAs found throughout the olfactory epithelia, as well as others exquisitely expressed in subsets of mature olfactory neurons or progenitor cells.

Monoallelic, antisense and total RNA transcription in an in vitro neural differentiation system based on F1 hybrid mice

We developed an in vitro system to differentiate embryonic stem cells (ESCs) derived from reciprocally crossed F1 hybrid mice into neurons, and used it to investigate poly(A)+ and total RNA transcription at different stages of cell differentiation. By comparing expression profiles of transcripts assembled from 20 RNA sequencing datasets [2 alleles×(2 cell lines×4 time-points+2 mouse brains)], the relative influence of strain, cell and parent specificities to overall expression could be assessed. Divergent expression profiles of ESCs converged tightly at neural progenitor stage. Patterns of temporal variation of monoallelically expressed transcripts and antisense transcripts were quantified. Comparison of sense and antisense transcript pairs within the poly(A)+ sample, within the total RNA sample, and across poly(A)+ and total RNA samples revealed distinct rates of pairs showing anti-correlated expression variation. Unique patterns of sharing of poly(A)+ and poly(A)- transcription were identified in distinct RNA species. Regulation and functionality of monoallelic expression, antisense transcripts and poly(A)- transcription remain elusive. We demonstrated the effectiveness of our approach to capture these transcriptional activities, and provided new resources to elucidate the mammalian developmental transcriptome.

Monoallelic expression in melanoma

BACKGROUND

Monoallelic expression (MAE) is a frequent genomic phenomenon in normal tissues, however its role in cancer is yet to be fully understood. MAE is defined as the expression of a gene that is restricted to one allele in the presence of a diploid heterozygous genome. Constitutive MAE occurs for imprinted genes, odorant receptors and random X inactivation. Several studies in normal tissues have showed MAE in approximately 5-20% of the cases. However, little information exists on the MAE rate in cancer. In this study we assessed the presence and rate of MAE in melanoma. The genetic basis of melanoma has been studied in depth over the past decades, leading to the identification of mutations/genetic alterations responsible for melanoma development.

METHODS

To examine the role of MAE in melanoma we used 15 melanoma cell lines and compared their RNA-seq data with genotyping data obtained by the parental TIL (tumor infiltrating lymphocytes). Genotyping was performed using the Illumina HumanOmni1 beadchip. The RNA-seq library preparation and sequencing was performed using the Illumina TruSeq Stranded Total RNA Human Kit and subsequently sequenced using a HiSeq 2500 according to manufacturer's guidelines. By comparing genotyping data with RNA-seq data, we identified SNPs in which DNA genotypes were heterozygous and corresponding RNA genotypes were homozygous. All homozygous DNA genotypes were removed prior to the analysis. To confirm the validity to detect MAE, we examined heterozygous DNA genotypes from X chromosome of female samples as well as for imprinted and olfactory receptor genes and confirmed MAE.

RESULTS

MAE was detected in all 15 cell lines although to a different rate. When looking at the B-allele frequencies we found a preferential pattern of complete monoallelic expression rather then differential monoallelic expression across the 15 melanoma cell lines. As some samples showed high differences in the homozygous and heterozygous call rate, we looked at the single chromosomes and showed that MAE may be explained by underlying large copy number imbalances in some instances. Interestingly these regions included genes known to play a role in melanoma initiation and progression. Nevertheless, some chromosome regions showed MAE without CN imbalances suggesting that additional mechanisms (including epigenetic silencing) may explain MAE in melanoma.

CONCLUSION

The biological implications of MAE are yet to be realized. Nevertheless, our findings suggest that MAE is a common phenomenon in melanoma cell lines. Further analyses are currently being undertaken to evaluate whether MAE is gene/pathway specific and to understand whether MAE can be employed by cancers to achieve a more aggressive phenotype.

Tailoring Formulations for Intranasal Nose-to-Brain Delivery: A Review on Architecture, Physico-Chemical Characteristics and Mucociliary Clearance of the Nasal Olfactory Mucosa

The blood-brain barrier and the blood-cerebrospinal fluid barrier are major obstacles in central nervous system (CNS) drug delivery, since they block most molecules from entering the brain. Alternative drug delivery routes like intraparenchymal or intrathecal are invasive methods with a remaining risk of infections. In contrast, nose-to-brain delivery is a minimally invasive drug administration pathway, which bypasses the blood-brain barrier as the drug is directed from the nasal cavity to the brain. In particular, the skull base located at the roof of the nasal cavity is in close vicinity to the CNS. This area is covered with olfactory mucosa. To design and tailor suitable formulations for nose-to-brain drug delivery, the architecture, structure and physico-chemical characteristics of the mucosa are important criteria. Hence, here we review the state-of-the-art knowledge about the characteristics of the nasal and, in particular, the olfactory mucosa needed for a rational design of intranasal formulations and dosage forms. Also, the information is suitable for the development of systemic or local intranasal drug delivery as well as for intranasal vaccinations.

Evolutionary ecology of chemosensation and its role in sensory drive

All behaviors of an organism are rooted in sensory processing of signals from its environment, and natural selection shapes sensory adaptations to ensure successful detection of cues that maximize fitness. Sensory drive, or divergent selection for efficient signal transmission among heterogeneous environments, has been a useful hypothesis for describing sensory adaptations, but its current scope has primarily focused on visual and acoustic sensory modalities. Chemosensation, the most widespread sensory modality in animals that includes the senses of smell and taste, is characterized by rapid evolution and has been linked to sensory adaptations to new environments in numerous lineages. Yet, olfaction and gustation have been largely underappreciated in light of the sensory drive hypothesis. Here, we examine why chemosensory systems have been overlooked and discuss the potential of chemosensation to shed new insight on the sensory drive hypothesis and vice versa. We provide suggestions for developing a framework to better incorporate studies of chemosensory adaptation that have the potential to shape a more complete, coherent, and holistic interpretation of the sensory drive.

Single olfactory receptors set odor detection thresholds

In many species, survival depends on olfaction, yet the mechanisms that underlie olfactory sensitivity are not well understood. Here we examine how a conserved subset of olfactory receptors, the trace amine-associated receptors (TAARs), determine odor detection thresholds of mice to amines. We find that deleting all TAARs, or even single TAARs, results in significant odor detection deficits. This finding is not limited to TAARs, as the deletion of a canonical odorant receptor reduced behavioral sensitivity to its preferred ligand. Remarkably, behavioral threshold is set solely by the most sensitive receptor, with no contribution from other highly sensitive receptors. In addition, increasing the number of sensory neurons (and glomeruli) expressing a threshold-determining TAAR does not improve detection, indicating that sensitivity is not limited by the typical complement of sensory neurons. Our findings demonstrate that olfactory thresholds are set by the single highest affinity receptor and suggest that TAARs are evolutionarily conserved because they determine the sensitivity to a class of biologically relevant chemicals.

Odorous chemicals broadly activate subsets of olfactory receptors in the nose, but how individual receptors contribute to behavioral sensitivity is not clear. Here, the authors demonstrate that detection thresholds in mice are set solely by the highest affinity receptor for a given odorant.

Sema7A/PlxnCl signaling triggers activity-dependent olfactory synapse formation

In mammals, neural circuits are formed based on a genetic program and further refined by neuronal activity during the neonatal period. We report that in the mouse olfactory system, the glomerular map is not merely refined but newly connected to second-order neurons by odorant-receptor-derived neuronal activity. Here, we analyzed a pair of molecules, Sema7A, expressed in olfactory sensory neurons (OSNs) in an activity-dependent manner, and PlxnC1, localized to dendrites of mitral/tufted (M/T) cells in the first week after birth. In Sema7A or PlxnC1 knockout (KO) mice, initiation of synapse formation and dendrite selection of M/T cells were perturbed. Reconstitution and rescue experiments demonstrated that Sema7A-PlxnC1 interaction is essential to form the post-synaptic assembly. Pharmacological blocking experiments indicated that synaptic transmission triggers primary dendrite selection by synaptic competition. We conclude that Sema7A signaling is key to inducing activity-dependent post-synapse events and dendrite selection in M/T-cells during the neonatal period.

Access to the odor world: olfactory receptors and their role for signal transduction in insects

The sense of smell enables insects to recognize and discriminate a broad range of volatile chemicals in their environment originating from prey, host plants and conspecifics. These olfactory cues are received by olfactory sensory neurons (OSNs) that relay information about food sources, oviposition sites and mates to the brain and thus elicit distinct odor-evoked behaviors. Research over the last decades has greatly advanced our knowledge concerning the molecular basis underlying the reception of odorous compounds and the mechanisms of signal transduction in OSNs. The emerging picture clearly indicates that OSNs of insects recognize odorants and pheromones by means of ligand-binding membrane proteins encoded by large and diverse families of receptor genes. In contrast, the mechanisms of the chemo-electrical transduction process are not fully understood; the present status suggests a contribution of ionotropic as well as metabotropic mechanisms. In this review, we will summarize current knowledge on the peripheral mechanisms of odor sensing in insects focusing on olfactory receptors and their specific role in the recognition and transduction of odorant and pheromone signals by OSNs.

Perspective: Is Random Monoallelic Expression a Contributor to Phenotypic Variability of Autosomal Dominant Disorders?

Several factors have been proposed as contributors to interfamilial and intrafamilial phenotypic variability in autosomal dominant disorders, including allelic variation, modifier genes, environmental factors and complex genetic and environmental interactions. However, regardless of the similarity of genetic background and environmental factors, asymmetric limb or trunk anomalies in a single individual and variable manifestation between monozygotic twins have been observed, indicating other mechanisms possibly involved in expressivity of autosomal dominant diseases. One such example is Holt-Oram syndrome (HOS), which is characterized by congenital cardiac defects and forelimb anomalies, mainly attributed to mutations in the TBX5 gene. We hypothesize that monoallelic expression of the TBX5 gene occurs during embryo development, and, in the context of a mutation, random monoallelic expression (RME) can create discrepant functions in a proportion of cells and thus contribute to variable phenotypes. A hybrid mouse model was used to investigate the occurrence of RME with the Tbx5 gene, and single-cell reverse transcription PCR and restriction digestion were performed for limb bud cells from developing embryos (E11.5) of the hybrid mice. RME of Tbx5 was observed in approximately two-thirds of limb bud cells. These results indicate that RME of the Tbx5 gene occurs frequently during embryo development, resulting in a mosaic expression signature (monoallelic, biallelic, or null) that may provide a potential explanation for the widespread phenotypic variability in HOS. This model will further provide novel insights into the variability of autosomal dominant traits and a better understanding of the complex expressivity of disease conditions.

Programming asynchronous replication in stem cells

Many regions of the genome replicate asynchronously and are expressed monoallelically. It is thought that asynchronous replication may be involved in choosing one allele over the other, but little is known about how these patterns are established during development. We show that, unlike somatic cells, which replicate in a clonal manner, embryonic and adult stem cells are programmed to undergo switching, such that daughter cells with an early-replicating paternal allele are derived from mother cells that have a late-replicating paternal allele. Furthermore, using ground-state embryonic stem (ES) cells, we demonstrate that in the initial transition to asynchronous replication, it is always the paternal allele that is chosen to replicate early, suggesting that primary allelic choice is directed by preset gametic DNA markers. Taken together, these studies help define a basic general strategy for establishing allelic discrimination and generating allelic diversity throughout the organism.

Variation in olfactory neuron repertoires is genetically controlled and environmentally modulated

The mouse olfactory sensory neuron (OSN) repertoire is composed of 10 million cells and each expresses one olfactory receptor (OR) gene from a pool of over 1000. Thus, the nose is sub-stratified into more than a thousand OSN subtypes. Here, we employ and validate an RNA-sequencing-based method to quantify the abundance of all OSN subtypes in parallel, and investigate the genetic and environmental factors that contribute to neuronal diversity. We find that the OSN subtype distribution is stereotyped in genetically identical mice, but varies extensively between different strains. Further, we identify cis-acting genetic variation as the greatest component influencing OSN composition and demonstrate independence from OR function. However, we show that olfactory stimulation with particular odorants results in modulation of dozens of OSN subtypes in a subtle but reproducible, specific and time-dependent manner. Together, these mechanisms generate a highly individualized olfactory sensory system by promoting neuronal diversity.

DOI:http://dx.doi.org/10.7554/eLife.21476.001

Smells are simply chemicals in the air that are recognized by nerves in our nose. Each nerve has a receptor that can identify a limited number of chemicals, and the nerve then relays this information to the brain. Animals have hundreds to thousands of different types of these nerves meaning that they can detect a wide array of smells.

Smell receptors are proteins, and the genes that encode these proteins can be very different in two unrelated people. This could partly explain, for example, why some people find certain odors intense and unpleasant while others do not. However, having different genes for smell receptors does not by itself completely explain why some people are more sensitive than others to particular smells. The amounts of each nerve type in the nose might also differ between people and have an effect, but to date it has not been possible to accurately count them all.

Ibarra-Soria et al. have now devised a new method to essentially count the number of each nerve type in the noses of mice from different breeds. The method makes use of a technique called RNA-sequencing, which can reveal which genes are active at any one time, and thus show how many nerves are producing each type of smell receptor. Ibarra-Soria et al. learned that different breeds of mice had remarkably different compositions of nerves in their noses. Further analysis revealed that this was due to changes to the DNA code near to the genes that encode the smell receptor.

Next, Ibarra-Soria et al. sought to find out how the amount of each nerve type is controlled by giving mice water with different smells for weeks and looking how this affected their noses. These experiments revealed that a small number of the nerve types became more or less common after exposure to a smell. The altered nerves were directly involved in recognizing the smells, proving that the very act of smelling can change the make-up of nerves in a mouse’s nose.

These results confirm that the diversity in the nose of each individual is not only dictated by the types of receptors found in there, but also by the number of each nerve type. The next challenge is to understand better how these differences change the way people perceive smells.

DOI:http://dx.doi.org/10.7554/eLife.21476.002

Lamin B1 is required for mature neuron-specific gene expression during olfactory sensory neuron differentiation

B-type lamins are major constituents of the nuclear lamina in all metazoan cells, yet have specific roles in the development of certain cell types. Although they are speculated to regulate gene expression in developmental contexts, a direct link between B-type lamins and developmental gene expression in an in vivo system is currently lacking. Here, we identify lamin B1 as a key regulator of gene expression required for the formation of functional olfactory sensory neurons. By using targeted knockout in olfactory epithelial stem cells in adult mice, we show that lamin B1 deficient neurons exhibit attenuated response to odour stimulation. This deficit can be explained by decreased expression of genes involved in mature neuron function, along with increased expression of genes atypical of the olfactory lineage. These results support that the broadly expressed lamin B1 regulates expression of a subset of genes involved in the differentiation of a specific cell type.

Lysine-specific demethylase-1 (LSD1) depletion disrupts monogenic and monoallelic odorant receptor (OR) expression in an olfactory neuronal cell line

Function of the mammalian olfactory system depends on specialized olfactory sensory neurons (OSNs) that each express only one allele ("monoallelic") of one odorant receptor (OR) gene ("monogenic"). The lysine-specific demethylase-1 (LSD1) protein removes activating H3K4 or silencing H3K9 methylation marks in a variety of developmental contexts, and is thought to be important for proper OR regulation. Most of the focus in the field has been on a potential "activating" function for LSD1; e.g., in the demethylation of H3K9 associated with the expressed OR allele. Here we show that depletion of LSD1 in an immortalized olfactory-placode-derived cell line (OP6) results in multigenic and multiallelic OR transcription per cell, while not seemingly disrupting the ability of these cells to activate new OR genes during clonal expansion. These results are consistent with LSD1 having a role in silencing additional OR alleles, as opposed to being required for the activation of OR alleles, within the OP6 cellular context.

Mechanisms controlling diversification of olfactory sensory neuron classes

Animals survive in harsh and fluctuating environments using sensory neurons to detect and respond to changes in their surroundings. Olfactory sensory neurons are essential for detecting food, identifying danger, and sensing pheromones. The ability to sense a large repertoire of different types of odors is crucial to distinguish between different situations, and is achieved through neuronal diversity within the olfactory system. Here, we review the developmental mechanisms used to establish diversity of olfactory sensory neurons in various model organisms, including Caenorhabditis elegans, Drosophila, and vertebrate models. Understanding and comparing how different olfactory neurons develop within the nervous system of different animals can provide insight into how the olfactory system is shaped in humans.

Lhx2 Determines Odorant Receptor Expression Frequency in Mature Olfactory Sensory Neurons

A developmental program of epigenetic repression prepares each mammalian olfactory sensory neuron (OSN) to strongly express one allele from just one of hundreds of odorant receptor (OR) genes, but what completes this process of OR gene choice by driving the expression of this allele is incompletely understood. Conditional deletion experiments in mice demonstrate that Lhx2 is necessary for normal expression frequencies of nearly all ORs and all trace amine-associated receptors, irrespective of whether the deletion of Lhx2 is initiated in immature or mature OSNs. Given previous evidence that Lhx2 binds OR gene control elements, these findings indicate that Lhx2 is directly involved in driving OR expression. The data also support the conclusion that OR expression is necessary to allow immature OSNs to complete differentiation and become mature. In contrast to the robust effects of conditional deletion of Lhx2, the loss of Emx2 has much smaller effects and more often causes increased expression frequencies. Lhx2:Emx2 double mutants show opposing effects on Olfr15 expression that reveal independent effects of these two transcription factors. While Lhx2 is necessary for OR expression that supports OR gene choice, Emx2 can act differently; perhaps by helping to control the availability of OR genes for expression.

Genomic organization and evolution of olfactory receptors and trace amine-associated receptors in channel catfish, Ictalurus punctatus

BACKGROUND

Channel catfish (Ictalurus punctatus) live in turbid waters with limited visibility to chase prey within a certain distance. This can be compensated through detecting specific water-soluble substances by the olfactory receptors (ORs) and trace amine associated receptors (TAARs) expressed on the olfactory epithelium.

METHODS

We identified the OR and TAAR repertoires in channel catfish, and characterized the genomic organizations of these two gene families by data mining available genomic resources.

RESULTS

A total of 47 putative OR genes and 36 putative TAAR genes were identified in the channel catfish genome, including 27 functional OR genes and 28 functional TAAR genes. Phylogenetic and orthogroup analyses were conducted to illustrate the evolutionary dynamics of the vertebrate ORs and TAARs. Collinear analysis revealed the presence of two conserved orthologous blocks that contain OR genes between the catfish genome and zebrafish genome. The complete loss of a conserved motif in fish OR family H may contribute to the divergence of family H from other families. The dN/dS analysis indicated that the highest degree of selection pressure was imposed on TAAR subfamily 14 among all fish ORs and TAARs.

CONCLUSIONS

The present study provides understanding of the evolutionary dynamics of the two gene families (OR and TAAR) associated with olfaction in channel catfish.

GENERAL SIGNIFICANCE

This is the first systematic study of ORs and TAARs in catfish, which could provide valuable genomic resources for further investigation of olfactory mechanisms in teleost fish.

Revisiting the Vomeronasal System From an Integrated Perspective

"Olfactory subsystems" is a relatively new terminology to refer to the different regions of the nasal cavity featuring olfactory sensory neurons. In mice, the olfactory chemical cues are detected in four well delimited areas: the main olfactory epithelium, the septal organ, Grüneberg's ganglion, and the sensory epithelium of the vomeronasal organ. Nevertheless, such distribution is by no means exhibited by all mammals. In microsmatic mammals -humans included- the only existing olfactory subsystem is the main olfactory epithelium. This raises the question of whether the lack of certain olfactory structures in those species implies that they are unable to identify certain olfactory signals, or on the contrary, their main olfactory epithelium assumes such role. It would be interesting to determine, in the context of biomedical research, if the sense of smell in humans is fully or partially endowed with the wide range of functions assigned to the vomeronasal system in mice. If it is not, presumptive implications of the lack of such functions should be addressed in human health and well-being. Anat Rec, 299:1488-1491, 2016. © 2016 Wiley Periodicals, Inc.

The nuclear envelope and gene organization in parasitic protozoa: Specializations associated with disease

The parasitic protozoa Trypanosoma brucei and Plasmodium falciparum are lethal human parasites that have developed elegant strategies of immune evasion by antigenic variation. Despite the vast evolutionary distance between the two taxa, both parasites employ strict monoallelic expression of their membrane proteins, variant surface glycoproteins in Trypanosomes and the var, rif and stevor genes in Plasmodium, in order to evade their host's immune system. Additionally, both telomeric location and epigenetic controls are prominent features of these membrane proteins. As such, telomeres, chromatin structure and nuclear organization all contribute to control of gene expression and immune evasion. Here, we discuss the importance of epigenetics and sub-nuclear context for the survival of these disease-causing parasites.

Histone and DNA Modifications as Regulators of Neuronal Development and Function

DNA and histone modifications, together with constraints imposed by nuclear architecture, contribute to the transcriptional regulatory landscape of the nervous system. Here, we provide select examples showing how these regulatory layers, often referred to as epigenetic, contribute to neuronal differentiation and function. We describe the interplay between DNA methylation and Polycomb-mediated repression during neuronal differentiation, the role of DNA methylation and long-range enhancer-promoter interactions in Protocadherin promoter choice, and the contribution of heterochromatic silencing and nuclear organization in singular olfactory receptor expression. Finally, we explain how the activity-dependent expression of a histone variant determines the longevity of olfactory sensory neurons.

Random monoallelic expression of autosomal genes: stochastic transcription and allele-level regulation

Random monoallelic expression (RME) of genes represents a striking example of how stochastic molecular processes can result in cellular heterogeneity. Recent transcriptome-wide studies have revealed both mitotically stable and cell-to-cell dynamic forms of autosomal RME, with the latter presumably resulting from burst-like stochastic transcription. Here, we discuss the distinguishing features of these two forms of RME and revisit literature on their nature, pervasiveness and regulation. Finally, we explore how RME may contribute to phenotypic variation, including the incomplete penetrance and variable expressivity often seen in genetic disease.

Allelome.PRO, a pipeline to define allele-specific genomic features from high-throughput sequencing data

Detecting allelic biases from high-throughput sequencing data requires an approach that maximises sensitivity while minimizing false positives. Here, we present Allelome.PRO, an automated user-friendly bioinformatics pipeline, which uses high-throughput sequencing data from reciprocal crosses of two genetically distinct mouse strains to detect allele-specific expression and chromatin modifications. Allelome.PRO extends approaches used in previous studies that exclusively analyzed imprinted expression to give a complete picture of the ‘allelome’ by automatically categorising the allelic expression of all genes in a given cell type into imprinted, strain-biased, biallelic or non-informative. Allelome.PRO offers increased sensitivity to analyze lowly expressed transcripts, together with a robust false discovery rate empirically calculated from variation in the sequencing data. We used RNA-seq data from mouse embryonic fibroblasts from F1 reciprocal crosses to determine a biologically relevant allelic ratio cutoff, and define for the first time an entire allelome. Furthermore, we show that Allelome.PRO detects differential enrichment of H3K4me3 over promoters from ChIP-seq data validating the RNA-seq results. This approach can be easily extended to analyze histone marks of active enhancers, or transcription factor binding sites and therefore provides a powerful tool to identify candidate cis regulatory elements genome wide.

A Unique Virulence Gene Occupies a Principal Position in Immune Evasion by the Malaria Parasite Plasmodium falciparum

Mutually exclusive gene expression, whereby only one member of a multi-gene family is selected for activation, is used by the malaria parasite Plasmodium falciparum to escape the human immune system and perpetuate long-term, chronic infections. A family of genes called var encodes the chief antigenic and virulence determinant of P. falciparum malaria. var genes are transcribed in a mutually exclusive manner, with switching between active genes resulting in antigenic variation. While recent work has shed considerable light on the epigenetic basis for var gene activation and silencing, how switching is controlled remains a mystery. In particular, switching seems not to be random, but instead appears to be coordinated to result in timely activation of individual genes leading to sequential waves of antigenically distinct parasite populations. The molecular basis for this apparent coordination is unknown. Here we show that var2csa, an unusual and highly conserved var gene, occupies a unique position within the var gene switching hierarchy. Induction of switching through the destabilization of var specific chromatin using both genetic and chemical methods repeatedly led to the rapid and exclusive activation of var2csa. Additional experiments demonstrated that these represent "true" switching events and not simply de-silencing of the var2csa promoter, and that activation is limited to the unique locus on chromosome 12. Combined with translational repression of var2csa transcripts, frequent "default" switching to this locus and detection of var2csa untranslated transcripts in non-pregnant individuals, these data suggest that var2csa could play a central role in coordinating switching, fulfilling a prediction made by mathematical models derived from population switching patterns. These studies provide the first insights into the mechanisms by which var gene switching is coordinated as well as an example of how a pharmacological agent can disrupt antigenic variation in Plasmodium falciparum.

Developmental regulation of neural map formation in the mouse olfactory system

In the mouse olfactory system, various odorants are detected by approximately 1000 different odorant receptors (ORs) expressed in the olfactory sensory neurons (OSNs). It is well established that each OSN expresses only one functional OR gene in a monoallelic manner. Furthermore, OSN axons expressing the same OR converge to a set of glomeruli in the olfactory bulb (OB). During embryonic development, a coarse map is formed by the combination of two genetically programmed processes. One is OR-independent axonal projection along the dorsal-ventral (D-V) axis, and the other is OR-dependent projection along the anterior-posterior (A-P) axis. D-V projection is regulated by the anatomical location of OSNs within the olfactory epithelium (OE), whereas A-P projection is instructed by expressed OR molecules using cyclic adenosine monophosphate (cAMP) signals. After birth, the map is further refined in an activity-dependent manner by its conversion from a continuous to a discrete map through segregation of glomerular structures. Here, we summarize recent progress from our laboratory in understanding neural map formation in the mouse olfactory system.

Class I odorant receptors, TAS1R and TAS2R taste receptors, are markers for subpopulations of circulating leukocytes

Our cellular immune system has to cope constantly with foodborne substances that enter the bloodstream postprandially. Here, they may activate leukocytes via specific but yet mostly unknown receptors. Ectopic RNA expression out of gene families of chemosensory receptors, i.e., the ∼400 ORs, ∼25 TAS2R bitter-taste receptors, and the TAS1R umami- and sweet-taste receptor dimers by which we typically detect foodborne substances, has been reported in a variety of peripheral tissues unrelated to olfaction or taste. In the present study, we have now discovered, by gene-specific RT-PCR experiments, the mRNA expression of most of the Class I ORs (TAS1R) and TAS2R in 5 different types of blood leukocytes. Surprisingly, we did not detect Class II OR mRNA. By RT-qPCR, we show the mRNA expression of human chemosensory receptors and their cow orthologs in PMN, thus suggesting an evolutionary concept. By immunocytochemistry, we demonstrate that some olfactory and taste receptors are expressed, on average, in 40-60% of PMN and T or B cells and largely coexpress in the same subpopulation of PMN. The mRNA expression and the size of subpopulations expressing certain chemosensory receptors varied largely among individual blood samples, suggesting a regulated expression of olfactory and taste receptors in these cells. Moreover, we show mRNA expression of their downstream signaling molecules and demonstrate that PTX abolishes saccharin- or 2-PEA-induced PMN chemotactic migration, indicating a role for Gi-type proteins. In summary, our data suggest "chemosensory"-type subpopulations of circulating leukocytes.

A Host-Pathogen Interaction Reduced to First Principles: Antigenic Variation in T. brucei

Antigenic variation is a common microbial survival strategy, powered by diversity in expressed surface antigens across the pathogen population over the course of infection. Even so, among pathogens, African trypanosomes have the most comprehensive system of antigenic variation described. African trypanosomes (Trypanosoma brucei spp.) are unicellular parasites native to sub-Saharan Africa, and the causative agents of sleeping sickness in humans and of n'agana in livestock. They cycle between two habitats: a specific species of fly (Glossina spp. or, colloquially, the tsetse) and the bloodstream of their mammalian hosts, by assuming a succession of proliferative and quiescent developmental forms, which vary widely in cell architecture and function. Key to each of the developmental forms that arise during these transitions is the composition of the surface coat that covers the plasma membrane. The trypanosome surface coat is extremely dense, covered by millions of repeats of developmentally specified proteins: procyclin gene products cover the organism while it resides in the tsetse and metacyclic gene products cover it while in the fly salivary glands, ready to make the transition to the mammalian bloodstream. But by far the most interesting coat is the Variant Surface Glycoprotein (VSG) coat that covers the organism in its infectious form (during which it must survive free living in the mammalian bloodstream). This coat is highly antigenic and elicits robust VSG-specific antibodies that mediate efficient opsonization and complement mediated lysis of the parasites carrying the coat against which the response was made. Meanwhile, a small proportion of the parasite population switches coats, which stimulates a new antibody response to the prevalent (new) VSG species and this process repeats until immune system failure. The disease is fatal unless treated, and treatment at the later stages is extremely toxic. Because the organism is free living in the blood, the VSG:antibody surface represents the interface between pathogen and host, and defines the interaction of the parasite with the immune response. This interaction (cycles of VSG switching, antibody generation, and parasite deletion) results in stereotypical peaks and troughs of parasitemia that were first recognized more than 100 years ago. Essentially, the mechanism of antigenic variation in T. brucei results from a need, at the population level, to maintain an extensive repertoire, to evade the antibody response. In this chapter, we will examine what is currently known about the VSG repertoire, its depth, and the mechanisms that diversify it both at the molecular (DNA) and at the phenotypic (surface displayed) level, as well as how it could interact with antibodies raised specifically against it in the host.

Mammalian olfactory receptors: molecular mechanisms of odorant detection, 3D-modeling, and structure-activity relationships

This chapter describes the main characteristics of olfactory receptor (OR) genes of vertebrates, including generation of this large multigenic family and pseudogenization. OR genes are compared in relation to evolution and among species. OR gene structure and selection of a given gene for expression in an olfactory sensory neuron (OSN) are tackled. The specificities of OR proteins, their expression, and their function are presented. The expression of OR proteins in locations other than the nasal cavity is regulated by different mechanisms, and ORs display various additional functions. A conventional olfactory signal transduction cascade is observed in OSNs, but individual ORs can also mediate different signaling pathways, through the involvement of other molecular partners and depending on the odorant ligand encountered. ORs are engaged in constitutive dimers. Ligand binding induces conformational changes in the ORs that regulate their level of activity depending on odorant dose. When present, odorant binding proteins induce an allosteric modulation of OR activity. Since no 3D structure of an OR has been yet resolved, modeling has to be performed using the closest G-protein-coupled receptor 3D structures available, to facilitate virtual ligand screening using the models. The study of odorant binding modes and affinities may infer best-bet OR ligands, to be subsequently checked experimentally. The relationship between spatial and steric features of odorants and their activity in terms of perceived odor quality are also fields of research that development of computing tools may enhance.

Epigenetic regulation of monoallelic rearrangement (allelic exclusion) of antigen receptor genes

While most genes in the mammalian genome are transcribed from both parental chromosomes in cells where they are expressed, approximately 10% of genes are expressed monoallelically, so that any given cell will express either the paternal or maternal allele, but not both. The antigen receptor genes in B and T cells are well-studied examples of a gene family, which is expressed in a monoallelic manner, in a process coined "allelic exclusion." During lymphocyte development, only one allele of each antigen receptor undergoes V(D)J rearrangement at a time, and once productive rearrangement is sensed, rearrangement of the second allele is prevented. In this mini review, we discuss the epigenetic processes, including asynchronous replication, nuclear localization, chromatin condensation, histone modifications, and DNA methylation, which appear to regulate the primary rearrangement of a single allele, while blocking the rearrangement of the second allele.

Single sea urchin phagocytes express messages of a single sequence from the diverse Sp185/333 gene family in response to bacterial challenge

Immune systems in animals rely on fast and efficient responses to a wide variety of pathogens. The Sp185/333 gene family in the purple sea urchin, Strongylocentrotus purpuratus, consists of an estimated 50 (±10) members per genome that share a basic gene structure but show high sequence diversity, primarily due to the mosaic appearance of short blocks of sequence called elements. The genes show significantly elevated expression in three subpopulations of phagocytes responding to marine bacteria. The encoded Sp185/333 proteins are highly diverse and have central effector functions in the immune system. In this study we report the Sp185/333 gene expression in single sea urchin phagocytes. Sea urchins challenged with heat-killed marine bacteria resulted in a typical increase in coelomocyte concentration within 24 h, which included an increased proportion of phagocytes expressing Sp185/333 proteins. Phagocyte fractions enriched from coelomocytes were used in limiting dilutions to obtain samples of single cells that were evaluated for Sp185/333 gene expression by nested RT-PCR. Amplicon sequences showed identical or nearly identical Sp185/333 amplicon sequences in single phagocytes with matches to six known Sp185/333 element patterns, including both common and rare element patterns. This suggested that single phagocytes show restricted expression from the Sp185/333 gene family and infers a diverse, flexible, and efficient response to pathogens. This type of expression pattern from a family of immune response genes in single cells has not been identified previously in other invertebrates.

Random monoallelic expression: regulating gene expression one allele at a time

Monoallelic gene expression is a remarkable process in which transcription occurs from only one of two homologous alleles in a diploid cell. Interestingly, between 0.5% and 15% of autosomal genes exhibit random monoallelic gene expression, in which different cells express only one allele independently of the underlying genomic sequence, in a cell type-specific manner. Recently, genome-wide studies have increased our understanding of the cell type-specific incidence of random monoallelic gene expression, and how the imbalance in allelic expression is distinguished within the cell and potentially maintained across cell generations. Monoallelic gene expression is likely generated through stochastic independent regulation of the two alleles upon differentiation, and has varied implications for the cell and organism, in particular with respect to disease.

Sensational placodes: neurogenesis in the otic and olfactory systems

For both the intricate morphogenetic layout of the sensory cells in the ear and the elegantly radial arrangement of the sensory neurons in the nose, numerous signaling molecules and genetic determinants are required in concert to generate these specialized neuronal populations that help connect us to our environment. In this review, we outline many of the proteins and pathways that play essential roles in the differentiation of otic and olfactory neurons and their integration into their non-neuronal support structures. In both cases, well-known signaling pathways together with region-specific factors transform thickened ectodermal placodes into complex sense organs containing numerous, diverse neuronal subtypes. Olfactory and otic placodes, in combination with migratory neural crest stem cells, generate highly specialized subtypes of neuronal cells that sense sound, position and movement in space, odors and pheromones throughout our lives.