Olfactory map formation in the Drosophila brain: genetic specificity and neuronal variability

Drosophila olfaction: past, present and future

Among the many wonders of nature, the sense of smell of the fly Drosophila melanogaster might seem, at first glance, of esoteric interest. Nevertheless, for over a century, the 'nose' of this insect has been an extraordinary system to explore questions in animal behaviour, ecology and evolution, neuroscience, physiology and molecular genetics. The insights gained are relevant for our understanding of the sensory biology of vertebrates, including humans, and other insect species, encompassing those detrimental to human health. Here, I present an overview of our current knowledge of D. melanogaster olfaction, from molecules to behaviours, with an emphasis on the historical motivations of studies and illustration of how technical innovations have enabled advances. I also highlight some of the pressing and long-term questions.

May the Odds Be Ever in Your Favor: Non-deterministic Mechanisms Diversifying Cell Surface Molecule Expression

How does the information in the genome program the functions of the wide variety of cells in the body? While the development of biological organisms appears to follow an explicit set of genomic instructions to generate the same outcome each time, many biological mechanisms harness molecular noise to produce variable outcomes. Non-deterministic variation is frequently observed in the diversification of cell surface molecules that give cells their functional properties, and is observed across eukaryotic clades, from single-celled protozoans to mammals. This is particularly evident in immune systems, where random recombination produces millions of antibodies from only a few genes; in nervous systems, where stochastic mechanisms vary the sensory receptors and synaptic matching molecules produced by different neurons; and in microbial antigenic variation. These systems employ overlapping molecular strategies including allelic exclusion, gene silencing by constitutive heterochromatin, targeted double-strand breaks, and competition for limiting enhancers. Here, we describe and compare five stochastic molecular mechanisms that produce variety in pathogen coat proteins and in the cell surface receptors of animal immune and neuronal cells, with an emphasis on the utility of non-deterministic variation.

Targeted molecular profiling of rare olfactory sensory neurons identifies fate, wiring, and functional determinants

Determining the molecular properties of neurons is essential to understand their development, function and evolution. Using Targeted DamID (TaDa), we characterize RNA polymerase II occupancy and chromatin accessibility in selected Ionotropic receptor (Ir)-expressing olfactory sensory neurons in Drosophila. Although individual populations represent a minute fraction of cells, TaDa is sufficiently sensitive and specific to identify the expected receptor genes. Unique Ir expression is not consistently associated with differences in chromatin accessibility, but rather to distinct transcription factor profiles. Genes that are heterogeneously expressed across populations are enriched for neurodevelopmental factors, and we identify functions for the POU-domain protein Pdm3 as a genetic switch of Ir neuron fate, and the atypical cadherin Flamingo in segregation of neurons into discrete glomeruli. Together this study reveals the effectiveness of TaDa in profiling rare neural populations, identifies new roles for a transcription factor and a neuronal guidance molecule, and provides valuable datasets for future exploration.

Evolution, developmental expression and function of odorant receptors in insects

Animals rely on their chemosensory system to discriminate among a very large number of attractive or repulsive chemical cues in the environment, which is essential to respond with proper action. The olfactory sensory systems in insects share significant similarities with those of vertebrates, although they also exhibit dramatic differences, such as the molecular nature of the odorant receptors (ORs): insect ORs function as heteromeric ion channels with a common Orco subunit, unlike the G-protein-coupled olfactory receptors found in vertebrates. Remarkable progress has recently been made in understanding the evolution, development and function of insect odorant receptor neurons (ORNs). These studies have uncovered the diversity of olfactory sensory systems among insect species, including in eusocial insects that rely extensively on olfactory sensing of pheromones for social communication. However, further studies, notably functional analyses, are needed to improve our understanding of the origins of the Orco-OR system, the mechanisms of ORN fate determination, and the extraordinary diversity of behavioral responses to chemical cues.

Pioneer interneurons instruct bilaterality in the Drosophila olfactory sensory map

Interhemispheric synaptic connections, a prominent feature in animal nervous systems for the rapid exchange and integration of neuronal information, can appear quite suddenly during brain evolution, raising the question about the underlying developmental mechanism. Here, we show in the Drosophila olfactory system that the induction of a bilateral sensory map, an evolutionary novelty in dipteran flies, is mediated by a unique type of commissural pioneer interneurons (cPINs) via the localized activity of the cell adhesion molecule Neuroglian. Differential Neuroglian signaling in cPINs not only prepatterns the olfactory contralateral tracts but also prevents the targeting of ingrowing sensory axons to their ipsilateral synaptic partners. These results identified a sensitive cellular interaction to switch the sequential assembly of diverse neuron types from a unilateral to a bilateral brain circuit organization.

Chemoreceptor proteins in the Caribbean spiny lobster, Panulirus argus: Expression of Ionotropic Receptors, Gustatory Receptors, and TRP channels in two chemosensory organs and brain

The spiny lobster, Panulirus argus, has two classes of chemosensilla representing “olfaction” and “distributed chemoreception,” as is typical for decapod crustaceans. Olfactory sensilla are found exclusively on antennular lateral flagella and are innervated only by olfactory receptor neurons (ORNs) that project into olfactory lobes organized into glomeruli in the brain. Distributed chemoreceptor sensilla are found on all body surfaces including the antennular lateral flagella (LF) and walking leg dactyls (dactyls), and are innervated by both chemoreceptor neurons (CRNs) and mechanoreceptor neurons that project into somatotopically organized neuropils. Here, we examined expression of three classes of chemosensory genes in transcriptomes of the LF (with ORNs and CRNs), dactyls (with only CRNs), and brain of P. argus: Ionotropic Receptors (IRs), which are related to ionotropic glutamate receptors and found in all protostomes including crustaceans; Gustatory Receptors (GRs), which are ionotropic receptors that are abundantly expressed in insects but more restricted in crustaceans; and Transient Receptor Potential (TRP) channels, a diverse set of sensor-channels that include several chemosensors in diverse animals. We identified 108 IRs, one GR, and 18 homologues representing all seven subfamilies of TRP channels. The number of IRs expressed in the LF is far greater than in dactyls, possibly reflecting the contribution of receptor proteins associated with the ORNs beyond those associated with CRNs. We found co-receptor IRs (IR8a, IR25a, IR76b, IR93a) and conserved IRs (IR21a, IR40a) in addition to the numerous divergent IRs in the LF, dactyl, and brain. Immunocytochemistry showed that IR25a is expressed in ORNs, CRNs, and a specific type of cell located in the brain near the olfactory lobes. While the function of IRs, TRP channels, and the GR was not explored, our results suggest that P. argus has an abundance of diverse putative chemoreceptor proteins that it may use in chemoreception.

Eph Receptor Effector Ephexin Mediates Olfactory Dendrite Targeting in Drosophila

Deciphering the mechanisms of sensory neural map formation is a central aim in neurosciences. Failure to form a correct map frequently leads to defects in sensory processing and perception. The olfactory map develops in subsequent steps initially forming a rough and later a precise map of glomeruli in the antennal lobe (AL), mainly consisting of olfactory receptor neuron (ORN) axons and projection neuron (PN) dendrites. The mechanisms underpinning the later stage of class-specific glomerulus formation are not understood. Recent studies have shown that the important guidance molecule Eph and its ligand ephrin play a role in class-specific PN targeting. Here, we reveal aspects of the mechanism downstream of Eph signaling during olfactory map formation. We show that the Eph-specific RhoGEF Ephexin (Exn) is required to fine tune PN dendrite patterning within specific glomeruli. We provide the first report showing an in vivo neurite guidance defect in an exn mutant. Interestingly, the quality of the phenotypes is different between eph and exn mutants; while loss of Eph leads to strong misprojections of DM3/Or47a neurons along the medial-lateral axis of the antennal lobe (AL), loss of Exn induces ventral ectopic innervation of a neighboring glomerulus. Genetic interaction experiments suggest that differential signaling of the small GTPases Rac1 and Cdc42 mediated by Exn-dependent and -independent Eph signaling fine tunes spatial targeting of PN dendrites within the olfactory map. We propose that their distinct activities on the actin cytoskeleton are required for precise navigation of PN dendrites within the olfactory map. Taken together, our results suggest that the precise connectivity of an individual neuron can depend on different modes of signaling downstream of a single guidance receptor. © 2018 Wiley Periodicals, Inc. Develop Neurobiol 00: 000-000, 2018.

Synaptic circuitry of identified neurons in the antennal lobe of Drosophila melanogaster

In Drosophila melanogaster olfactory sensory neurons (OSNs) establish synapses with projection neurons (PNs) and local interneurons within antennal lobe (AL) glomeruli. Substantial knowledge regarding this circuitry has been obtained by functional studies, whereas ultrastructural evidence of synaptic contacts is scarce. To fill this gap, we studied serial sections of three glomeruli using electron microscopy. Ectopic expression of a membrane-bound peroxidase allowed us to map synaptic sites along PN dendrites. Our data prove for the first time that each of the three major types of AL neurons is both pre- and postsynaptic to the other two types, as previously indicated by functional studies. PN dendrites carry a large proportion of output synapses, with approximately one output per every three input synapses. Detailed reconstructions of PN dendrites showed that these synapses are distributed unevenly, with input and output sites partially segregated along a proximal-distal gradient and the thinnest branches carrying solely input synapses. Moreover, our data indicate synapse clustering, as we found evidence of dendritic tiling of PN dendrites. PN output synapses exhibited T-shaped presynaptic densities, mostly arranged as tetrads. In contrast, output synapses from putative OSNs showed elongated presynaptic densities in which the T-bar platform was supported by several pedestals and contacted as many as 20 postsynaptic profiles. We also discovered synaptic contacts between the putative OSNs. The average synaptic density in the glomerular neuropil was about two synapses/µm(3) . These results are discussed with regard to current models of olfactory glomerular microcircuits across species.

Mechanisms of olfactory receptor neuron specification in Drosophila

Detection of a broad range of chemosensory signals is necessary for the survival of multicellular organisms. Chemical signals are the main facilitators of foraging, escape, and social behaviors. To increase detection coverage, animal sensory systems have evolved to create a large number of neurons with highly specific functions. The olfactory system, much like the nervous system as a whole, is astonishingly diverse. The mouse olfactory system has millions of neurons with over a thousand classes, whereas the more compact Drosophila genome has approximately 80 odorant receptor genes that give rise to 50 neuronal classes and 1300 neurons in the adult.(4) Understanding how neuronal diversity is generated remains one of the central questions in developmental neurobiology. Here, we review the current knowledge on the development of the adult Drosophila olfactory system and the progress that has been made toward answering this central question.

Notch is required in adult Drosophila sensory neurons for morphological and functional plasticity of the olfactory circuit

Olfactory receptor neurons (ORNs) convey odor information to the central brain, but like other sensory neurons were thought to play a passive role in memory formation and storage. Here we show that Notch, part of an evolutionarily conserved intercellular signaling pathway, is required in adult Drosophila ORNs for the structural and functional plasticity of olfactory glomeruli that is induced by chronic odor exposure. Specifically, we show that Notch activity in ORNs is necessary for the odor specific increase in the volume of glomeruli that occurs as a consequence of prolonged odor exposure. Calcium imaging experiments indicate that Notch in ORNs is also required for the chronic odor induced changes in the physiology of ORNs and the ensuing changes in the physiological response of their second order projection neurons (PNs). We further show that Notch in ORNs acts by both canonical cleavage-dependent and non-canonical cleavage-independent pathways. The Notch ligand Delta (Dl) in PNs switches the balance between the pathways. These data define a circuit whereby, in conjunction with odor, N activity in the periphery regulates the activity of neurons in the central brain and Dl in the central brain regulates N activity in the periphery. Our work highlights the importance of experience dependent plasticity at the first olfactory synapse.

Appropriate behavioral responses to changing environmental signals, such as odors, are essential for an organism’s survival. In Drosophila odors are detected by olfactory receptor neurons (ORNs) that synapse with second order projection neurons (PNs) and local interneurons in morphologically identifiable neuropils in the antennal lobe called glomeruli. Chronic odor exposure leads to changes in animal behavior as well as to changes in the activity of neurons in the olfactory circuit and increases in the volume of glomeruli. Here, we establish that Notch, an evolutionarily conserved transmembrane receptor that plays profound and pervasive roles in animal development, is required in adult Drosophila ORNs for functional and morphological plasticity in response to chronic odor exposure. These findings are significant because they point to a role for Notch in regulating activity dependent plasticity. Furthermore, we show that in regulating the odor dependent change in glomerular volume, Notch acts by both non-canonical, cleavage-independent and canonical, cleavage-dependent mechanisms, with the Notch ligand Delta in PNs switching the balance between the pathways. Because both the Notch pathway and the processing of olfactory information are highly conserved between flies and vertebrates these findings are likely to be of general relevance.

Insights into brain development and disease from neurogenetic analyses in Drosophila melanogaster

Groundbreaking work by Obaid Siddiqi has contributed to the powerful genetic toolkit that is now available for studying the nervous system of Drosophila. Studies carried out in this powerful neurogenetic model system during the last decade now provide insight into the molecular mechanisms that operate in neural stem cells during normal brain development and during abnormal brain tumorigenesis. These studies also provide strong support for the notion that conserved molecular genetic programs act in brain development and disease in insects and mammals including humans.

Chitosan/siRNA nanoparticle targeting demonstrates a requirement for single-minded during larval and pupal olfactory system development of the vector mosquito Aedes aegypti

Background

Essentially nothing is known about the genetic regulation of olfactory system development in vector mosquitoes, which use olfactory cues to detect blood meal hosts. Studies in Drosophila melanogaster have identified a regulatory matrix of transcription factors that controls pupal/adult odorant receptor (OR) gene expression in olfactory receptor neurons (ORNs). However, it is unclear if transcription factors that function in the D. melanogaster regulatory matrix are required for OR expression in mosquitoes. Furthermore, the regulation of OR expression during development of the larval olfactory system, which is far less complex than that of pupae/adults, is not well understood in any insect, including D. melanogaster. Here, we examine the regulation of OR expression in the developing larval olfactory system of Aedes aegypti, the dengue vector mosquito.

Results

A. aegypti bears orthologs of eight transcription factors that regulate OR expression in D. melanogaster pupae/adults. These transcription factors are expressed in A. aegypti larval antennal sensory neurons, and consensus binding sites for these transcription factors reside in the 5’ flanking regions of A. aegypti OR genes. Consensus binding sites for Single-minded (Sim) are located adjacent to over half the A. aegypti OR genes, suggesting that this transcription factor functions as a major regulator of mosquito OR expression. To functionally test this hypothesis, chitosan/siRNA nanoparticles were used to target sim during larval olfactory development. These experiments demonstrated that Sim positively regulates expression of a large subset of OR genes, including orco, the obligate co-receptor in the assembly and function of heteromeric OR/Orco complexes. Decreased innervation of the antennal lobe was also noted in sim knockdown larvae. These OR expression and antennal lobe defects correlated with a larval odorant tracking behavioral defect. OR expression and antennal lobe defects were also observed in sim knockdown pupae.

Conclusions

The results of this investigation indicate that Sim has multiple functions during larval and pupal olfactory system development in A. aegypti.

Early olfactory processing in Drosophila: mechanisms and principles

In the olfactory system of Drosophila melanogaster, it is relatively straightforward to target in vivo measurements of neural activity to specific processing channels. This, together with the numerical simplicity of the Drosophila olfactory system, has produced rapid gains in our understanding of Drosophila olfaction. This review summarizes the neurophysiology of the first two layers of this system: the peripheral olfactory receptor neurons and their postsynaptic targets in the antennal lobe. We now understand in some detail the cellular and synaptic mechanisms that shape odor representations in these neurons. Together, these mechanisms imply that interesting neural adaptations to environmental statistics have occurred. These mechanisms also place some fundamental constraints on early sensory processing that pose challenges for higher brain regions. These findings suggest some general principles with broad relevance to early sensory processing in other modalities.

Neural maps in insect versus vertebrate auditory systems

The convergent evolution of hearing in insects and vertebrates raises the question about similarity of the central representation of sound in these distant animal groups. Topographic representations of spectral, spatial and temporal cues have been widely described in mammals, but evidence for such maps is scarce in insects. Recent data on insect sound encoding provides evidence for an early integration of sound parameters to form highly-specific representation that predict behavioral output. In mammals, new studies investigating neural representation of perceptual features in behaving animals allow asking similar questions. A comparative approach may help in understanding principles underlying the formation of perceptual categories and behavioral plasticity.

The zinc finger transcription factor Jing is required for dendrite/axonal targeting in Drosophila antennal lobe development

An important role in olfactory system development is played by transcription factors which act in sensory neurons or in their interneuron targets as cell autonomous regulators of downstream effectors such as cell surface molecules and signalling systems that control neuronal identity and process guidance. Some of these transcriptional regulators have been characterized in detail in the development of the neural elements that innervate the antennal lobe in the olfactory system of Drosophila. Here we identify the zinc finger transcription factor Jing as a cell autonomously acting transcriptional regulator that is required both for dendrite targeting of projection neurons and local interneurons as well as for axonal targeting of olfactory sensory neurons in Drosophila olfactory system development. Immunocytochemical analysis shows that Jing is widely expressed in the neural cells during postembryonic development. MARCM-based clonal analysis of projection neuron and local interneuron lineages reveals a requirement for Jing in dendrite targeting; Jing loss-of-function results in loss of innervation in specific glomeruli, ectopic innervation of inappropriate glomeruli, aberrant profuse dendrite arborisation throughout the antennal lobe, as well as mistargeting to other parts of the CNS. ey-FLP-based MARCM analysis of olfactory sensory neurons reveals an additional requirement for Jing in axonal targeting; mutational inactivation of Jing causes specific mistargeting of some olfactory sensory neuron axons to the DA1 glomerulus, reduction of targeting to other glomeruli, as well as aberrant stalling of axons in the antennal lobe. Taken together, these findings indicate that Jing acts as a key transcriptional control element in wiring of the circuitry in the developing olfactory sensory system in Drosophila.

Using fly genetics to dissect the cytoskeletal machinery of neurons during axonal growth and maintenance

The extension of long slender axons is a key process of neuronal circuit formation, both during brain development and regeneration. For this, growth cones at the tips of axons are guided towards their correct target cells by signals. Growth cone behaviour downstream of these signals is implemented by their actin and microtubule cytoskeleton. In the first part of this Commentary, we discuss the fundamental roles of the cytoskeleton during axon growth. We present the various classes of actin- and microtubule-binding proteins that regulate the cytoskeleton, and highlight the important gaps in our understanding of how these proteins functionally integrate into the complex machinery that implements growth cone behaviour. Deciphering such machinery requires multidisciplinary approaches, including genetics and the use of simple model organisms. In the second part of this Commentary, we discuss how the application of combinatorial genetics in the versatile genetic model organism Drosophila melanogaster has started to contribute to the understanding of actin and microtubule regulation during axon growth. Using the example of dystonin-linked neuron degeneration, we explain how knowledge acquired by studying axonal growth in flies can also deliver new understanding in other aspects of neuron biology, such as axon maintenance in higher animals and humans.

Disruption of Aedes aegypti olfactory system development through chitosan/siRNA nanoparticle targeting of semaphorin-1a

Despite the devastating impact of mosquito-borne illnesses on human health, surprisingly little is known about mosquito developmental biology, including development of the olfactory system, a tissue of vector importance. Analysis of mosquito olfactory developmental genetics has been hindered by a lack of means to target specific genes during the development of this sensory system. In this investigation, chitosan/siRNA nanoparticles were used to target semaphorin-1a (sema1a) during olfactory system development in the dengue and yellow fever vector mosquito Aedes aegypti. Immunohistochemical analyses and anterograde tracing of antennal sensory neurons, which were used to track the progression of olfactory development in this species, revealed antennal lobe defects in sema1a knockdown fourth instar larvae. These findings, which correlated with a larval odorant tracking behavioral phenotype, identified previously unreported roles for Sema1a in the developing insect larval olfactory system. Analysis of sema1a knockdown pupae also revealed a number of olfactory phenotypes, including olfactory receptor neuron targeting and projection neuron defects coincident with a collapse in the structure and shape of the antennal lobe and individual glomeruli. This study, which is to our knowledge the first functional genetic analysis of insect olfactory development outside of D. melanogaster, identified critical roles for Sema1a during Ae. aegypti larval and pupal olfactory development and advocates the use of chitosan/siRNA nanoparticles as an effective means of targeting genes during post-embryonic Ae. aegypti development. Use of siRNA nanoparticle methodology to understand sensory developmental genetics in mosquitoes will provide insight into the evolutionary conservation and divergence of key developmental genes which could be exploited in the development of both common and species-specific means for intervention.

Drosophila Psidin regulates olfactory neuron number and axon targeting through two distinct molecular mechanisms

The formation of neuronal circuits is a key process of development, laying foundations for behavior. The cellular mechanisms regulating circuit development are not fully understood. Here, we reveal Psidin as an intracellular regulator of Drosophila olfactory system formation. We show that Psidin is required in several classes of olfactory receptor neurons (ORNs) for survival and subsequently for axon guidance. During axon guidance, Psidin functions as an actin regulator and antagonist of Tropomyosin. Accordingly, Psidin-deficient primary neurons in culture display growth cones with significantly smaller lamellipodia. This lamellipodial phenotype, as well as the mistargeting defects in vivo, is suppressed by parallel removal of Tropomyosin. In contrast, Psidin functions as the noncatalytic subunit of the N-acetyltransferase complex B (NatB) to maintain the number of ORNs. Psidin physically binds the catalytic NatB subunit CG14222 (dNAA20) and functionally interacts with it in vivo. We define the dNAA20 interaction domain within Psidin and identify a conserved serine as a candidate for phosphorylation-mediated regulation of NatB complex formation. A phosphomimetic mutation of this serine showed severely reduced binding to dNAA20 in vitro. In vivo, it fully rescued the targeting defect but not the reduction in neuron numbers. In addition, we show that a different amino acid point mutation shows exactly the opposite effect by rescuing only the cell number but not the axon targeting defect. Together, our data suggest that Psidin plays two independent developmental roles via the acquisition of separate signaling pathways, both of which contribute to the formation of olfactory circuits.

A biophysical signature of network affiliation and sensory processing in mitral cells

One defining characteristic of the mammalian brain is its neuronal diversity. For a given region, substructure, layer or even cell type, variability in neuronal morphology and connectivity persists. Although it is well known that such cellular properties vary considerably according to neuronal type, the substantial biophysical diversity of neurons of the same morphological class is typically averaged out and ignored. Here we show that the amplitude of hyperpolarization-evoked sag of membrane potential recorded in olfactory bulb mitral cells is an emergent, homotypic property of local networks and sensory information processing. Simultaneous whole-cell recordings from pairs of cells show that the amount of hyperpolarization-evoked sag potential and current (Ih) is stereotypic for mitral cells belonging to the same glomerular circuit. This is corroborated by a mosaic, glomerulus-based pattern of expression of the HCN2 (hyperpolarization-activated cyclic nucleotide-gated channel 2) subunit of the Ih channel. Furthermore, inter-glomerular differences in both membrane potential sag and HCN2 protein are diminished when sensory input to glomeruli is genetically and globally altered so that only one type of odorant receptor is universally expressed. Population diversity in this intrinsic property therefore reflects differential expression between local mitral cell networks processing distinct odour-related information.

Axon degeneration and regeneration: insights from Drosophila models of nerve injury

Axon degeneration is the pivotal pathological event of acute traumatic neural injury as well as many chronic neurodegenerative diseases. It is an active cellular program and yet molecularly distinct from cell death. Much effort is devoted toward understanding the nature of axon degeneration and promoting axon regeneration. However, the fundamental mechanisms of self-destruction of damaged axons remain unclear, and there are still few treatments for traumatic brain injury (TBI) or spinal cord injury (SCI). Genetically approachable model organisms such as Drosophila melanogaster, the fruit fly, have proven exceptionally successful in modeling human neurodegenerative diseases. More recently, this success has been extended into the field of acute axon injury and regeneration. In this review, we discuss recent findings, focusing on how these models hold promise for accelerating mechanistic insight into axon injury and identifying potential therapeutic targets for TBI and SCI.

Teneurins instruct synaptic partner matching in an olfactory map

Neurons are interconnected with extraordinary precision to assemble a functional nervous system. Compared to axon guidance, far less is understood about how individual pre- and post-synaptic partners are matched. To ensure the proper relay of olfactory information in flies, axons of ~50 classes of olfactory receptor neurons (ORNs) form one-to-one connections with dendrites of ~50 classes of projection neurons (PNs). Using genetic screens, we identified two evolutionarily conserved EGF-repeat transmembrane Teneurins, Ten-m and Ten-a, as synaptic partner matching molecules between PN dendrites and ORN axons. Ten-m and Ten-a are highly expressed in select PN-ORN matching pairs. Teneurin loss- and gain-of-function cause specific mismatching of select ORNs and PNs. Finally, Teneurins promote homophilic interactions in vitro, and Ten-m co-expression in non-partner PNs and ORNs promotes their ectopic connections in vivo. We propose that Teneurins instruct matching specificity between synaptic partners through homophilic attraction.

Olfactory maps in the brain

The responses of neural elements in many sensory areas of the brain vary systematically with their physical position, leading to a topographic representation of the outside world. Sensory representation in the olfactory system has been harder to decipher, in part because it is difficult to find appropriate metrics to characterize odor space and to sample this space densely. Recent experiments have shown that the arrangement of glomeruli, the elementary units of processing, is relatively invariant across individuals in a species, yet it is flexible enough to accommodate new sensors that might be added. Evidence supports the existence of coarse spatial domains carved out on a genetic or functional basis, but a systematic organization of odor responses or neural circuits on a local scale is not evident. Experiments and theory that relate the properties of odorant receptors to the detailed wiring diagram of the downstream olfactory circuits and to behaviors they trigger may reveal the design principles that have emerged during evolution.