Our main interest is to understand how neural circuits are used to process sensory information and integrate it with the internal state of the organism to produce the appropriate behavioral responses to the changing environment. In our studies, we combine anatomy, functional imaging and behavioral analysis. Our lab uses mice and fruit flies as model organisms, but we collaborate with other labs on using other models. We have developed a new method for transsynaptic labeling of neural circuits in fruit flies termed trans-Tango. We have implemented trans-Tango to study a variety of circuits in fruit flies, including the circuits involved in olfactory and gustatory processing. Our studies in mice focus on the olfactory system. We are currently developing the mouse version of trans-Tango with the objective of following the projections of olfactory circuits to higher brain.
Neural circuit analysis with trans-Tango
We have developed a new method for transsynaptic labeling of neural circuits termed trans-Tango. At the core of trans-Tango is a synthetic signaling pathway that is introduced into all neurons in the animal. This pathway converts receptor activation at the cell-surface into reporter expression through site-specific proteolysis. Specific labeling is achieved by presenting a tethered ligand at the synapses of genetically defined neurons, thereby activating the pathway in their postsynaptic partners and providing genetic access to these neurons. We first established trans-Tango in fruit flies (Talay and Richman et al., Neuron, 2017), validating it in the olfactory system and then implementing it in the gustatory system to reveal second-order projections within gustatory circuits.
We are now expanding trans-Tango beyond circuit tracing. Because it gives genetic access to second-order neurons in a circuit, trans-Tango has the potential to drive expression of any genetic tool available for studying neuronal function. Toward this end, we are developing optimized configurations of trans-Tango for calcium imaging, optogenetic and thermogenetic manipulation, intersectional labeling and more. These versions of trans-Tango are likely to be important for dissecting circuits in higher brain, where recurrent connectivity and divergent projections are common. Our vision is to establish a modular toolkit around trans-Tango to facilitate powerful and sophisticated strategies for neural circuit analysis.
One current area of research in the lab is the neural circuitry of gustatory processing. Flies respond to sweet and bitter tastants with different stereotyped behaviors: sweet substances, often calorie rich, are appetitive and elicit acceptance, while bitter compounds are usually harmful, and they elicit rejection and avoidance. The linkage between stimulus quality and behavioral response suggests that sweet and bitter tastants are represented differently in the brain. It is currently unknown whether flies process taste information via labeled line like mammals, or whether a distributed representation for taste stimuli is operative, as is the case in moths. In addition, while flies have gustatory neurons in various parts of their body, it is unclear how the somatotopic dimension of gustatory information in represented in the central brain. We hope to shed light on open questions such as these using trans-Tango, functional imaging, circuit manipulation and behavior. In addition, along with our extensive network of collaborators, we are implementing trans-Tango in the study of a variety of circuits and behaviors.
Furthermore, we are developing versions of trans-Tango for studying neural circuits in other model organisms. Our main effort in the lab is to establish mouse trans-Tango. In collaborations with colleagues in other institutions we are establishing versions of trans-Tango in zebrafish, chicken and C. elegans.
Finally, we are also collaborating on establishing configurations of trans-Tango for studying metastatic cancer, interactions in the immune system and inter-cellular interactions during development.
The mouse olfactory system
The second part of our research focuses on the olfactory system in mice. We study how the various olfactory sub-systems mediate innate and learned behavioral responses to odor stimuli. In addition, we are interested in uncovering the mechanisms ensuring that each olfactory sensory neuron expresses a single type of odorant receptor and in characterizing the role of the chosen receptor in determining the projection of the sensory neuron that expresses it.
In mice, a subset of olfactory sensory neurons expresses trace amine associated receptors (TAARs) that respond to volatile amines. Most of these ligands are believed to elicit aversion in mice. We showed that like ORs, TAARs are monoallelically expressed and localized both in cilia, the site of odor detection, and in axons, where they may participate in guidance. TAAR-expressing neurons (TRNs) project to discrete glomeruli predominantly confined within a dorsal band in the olfactory bulb. Remarkably, Taar expression involves different regulatory logic than OR expression. Together, these observations led us to the conclusion that the TRNs constitute a distinct olfactory subsystem (Johnson et al., Proc. Natl. Acad. Sci. USA, 2012). Our overall hypothesis is that TRNs integrate into hard-wired circuits that enable them to extract specific environmental cues and drive robust innate behavioral responses. We generated several knockin and knockout mouse lines and use them in a multipronged approach spanning molecular, neuroanatomical, neurophysiological and behavioral levels of interrogation to test this hypothesis. We test whether TAAR-expressing neurons are molecularly distinct and whether activation of these neurons optogenetically is sufficient to elicit innate behaviors. Finally, we intend to employ trans-Tango to trace the projections from the TAAR glomeruli and examine whether they are stereotyped and target central brain areas known to control aversive behavioral responses.
Neurons in the basal layer of the vomeronasal organ in mice are different than most other sensory neurons in the nose in that they do not obey the one receptor per neuron rule. Each neuron in the basal layer expresses several receptors from the Vmn2r family of G protein coupled receptors. The Vmn2r family consists of four classes designated A, B, C and D. Members of classes A, B and D are more closely related to one another than to the seven members of class C (Vmn2r1-7). Each basal VSN expresses one Vmn2rC and one Vmn2rABD. We are using a multi-tiered strategy encompassing mouse genetics, molecular and biochemical studies, neuroanatomical examination, and behavioral analysis to examine the roles of the interactions between the different classes of Vmn2rs.
We have a long-standing interest in the development, wiring and plasticity of the olfactory circuits, and the roles played by the odorant receptors (ORs) in these processes. Each olfactory sensory neuron (OSN) in the nose expresses one odorant receptor (OR) out of ~1,300 genes. All the OSNs that express a given OR project to the same neuropil structures called glomeruli in the olfactory bulb, forming a topographic map. OSNs continue to be born and to integrate into the olfactory circuits throughout the life of the animal, posing a challenge of maintaining proper wiring. In one study (Tsai and Barnea, Science, 2014), we generated transgenic mice for ectopic expression of a specific OR in an inducible manner. We perturbed the formation of the olfactory sensory map by inducing expression of the ectopic OR at different time points in the life of the animal. We observed non-cell-autonomous effects of the ectopic OR expression on axons expressing this OR from the endogenous genomic locus. Our studies uncovered a critical period for establishing the topographic map of glomeruli in the olfactory bulb. During this critical period, particular glomeruli are marked as targets for OSNs expressing specific ORs, thus providing a potential mechanism for input stability in the olfactory circuitry.
