Our senses are remarkable in detecting stimuli from the external world, extracting features from the signals and translating the information into meaningful perception in the brain. Sensory perceptions lead to logic, measured behavioral responses at times, but other times we experience impulsive urges. The complex network of neurons in our brain underlies our senses and behaviors, but we know little about how the brain works. The goal of the research in my laboratory is to identify the neural circuitry that detects, parses, and integrates specific sensory information and to elucidate the molecular mechanisms that specify the circuitry. We employ a combination of methodologies that include molecular genetics, optical imaging, electrophysiology, behavioral assays, and systems biology to tackle a range of problems. Our current focuses are in the mammalian main olfactory and vomeronasal systems.
In terrestrial vertebrates, the sense of smell provides important information about the environment and enables the animals to detect food, avoid predators, and find mates. Olfaction is also a sense that conveys strong emotional values. Odors are detected by the sensory neurons in the olfactory epithelia in the nose, which pass the information to the main olfactory bulb (MOB) for further processing. The mouse olfactory system expresses approximately 1300 odorant receptor (OR) genes. Neurons expressing the same OR gene converge onto two specific olfactory glomeruli in the bulb. Each glomerulus in the MOB corresponding to a single type of OR genes. The layout of the olfactory bulb is highly stereotypic and poses interesting questions as to not only how such a specific sensory map develops, but also what particular function it serves.
Molecular mechanism involved in the development of the main olfactory system
We have used genetic methods to manipulate the development of the olfactory map. By selectively dampening neural excitability in the olfactory epithelial neurons during early development, olfactory axons in the transgenic mice show a developmental delay in comparison to axons from wild-type mice. Moreover, the silenced olfactory neurons fail to target to the proper glomeruli in the olfactory bulb such that neurons expressing the same receptor genes innervate multiple glomeruli rather than the two fixed ones seen in the wild-type. This offers the opportunity to investigate the molecular mechanism involved in the development of olfactory circuitry. We are taking several approaches, including deep sequencing and bioinformatic analyses to identify the molecules that are specifically regulated by neural activity.
Plasticity of the olfactory circuitry
The olfactory circuit is plastic. Olfactory sensory neurons in the epithelia, as well as the local neurons in the olfactory bulb, are continuously regenerated and replaced throughout the life of the animal. The connectivity among the neurons in the olfactory bulb undergoes continuous modification during development and in adulthood. However, this plasticity declines with aging and the onset of some neurological diseases such as Alzheimer’s and Parkinson’s. We have used mouse genetics to study the plasticity during early development. Currently, we are developing novel transgenic strategies to both manipulate and examine plastic changes in specific neurons.
Each of the approximately 2000 olfactory glomeruli in the bulb is tuned to different odors. Collectively, they form a code set that encodes odor information. The information is further processed by the mitral cells in the main olfactory bulb (MOB) and transmitted to different cortical areas. We have generated transgenic mice that express the calcium sensor G-CaMP2 and imaged odor-evoked responses from live animals. We perform systems level examination of glomerular response to large dimensional odor stimuli. These analyses reveal the organization principle of glomeruli. Using automated odor-discrimination paradigms developed in the lab, we can predict and test odor-discrimination based on the activation patterns in the olfactory bulb. We are currently investigating intensity-invariant coding of odors and the segmentation of odor mixtures.
The olfactory bulb is a network of interconnected neurons, composing not only the incoming olfactory sensory axon and the projecting mitral cell, but also many types of interneurons. Each glomerulus forms a computational unit that processes odor information. By using multi-electrode recording from mitral cells in live animals and comparing their odor response to that of the glomeruli, we are investigating the transformation of odor codes occurring in the bulb. Furthermore, we are adopting a transgenic approach using molecular tools such as channelrhodopsin and halorhodopsin to investigate the network interactions in the bulb.
Pheromones, a set of chemical signals emitted by the animals for intra-species chemo-communication, convey information about the sexual, social, and reproductive status of other individuals. Pheromones trigger a restricted repertoire of innate and stereotyped behaviors such as mating rituals, territorial aggression, and neuroendocrine responses. Pheromones are detected by their receptor neurons in the vomeronasal organ (VNO), which project axons to the accessory olfactory bulb (AOB). The vomeronasal circuit is largely genetically determined. There is an intrinsic link between sensory input and behavioral output in this circuit, making them an attractive and tractable system to study sensory processing and developmental changes in the circuitry.
Information coding in the vomeronasal organ
The VNO expresses about 300 G-protein coupled, seven-transmembrane domain receptor proteins, the vomeronasal receptors. The receptors respond specifically to individual pheromones conveying distinct social cues. We have generated transgenic mice expressing the genetically encoded calcium sensor, G-CaMP2, to visualize the response to pheromone responses in VNO slices. Through systems level analyses of VNO responses to pheromones from different individual mice, we have identified the specific neuronal populations encoding information about gender, strain, and hormonal statuses of individual animals. By combining optical imaging and single-cell degenerate RT-PCR, we can identify specific receptors expressed by these neurons.
The neural circuit that processes pheromone information
Individual VNO neuron expresses only one specific type of vomeronasal receptor. Neurons expressing different receptors have distinct but stereotyped projection patterns. Pheromone information is initially encoded in the differential activation of VNO sensory neurons and it will be transformed into a topographic representation in the AOB to elicit innate behaviors. By identifying the receptors expressed by specific neurons, we can genetically trace the projection of their axons to elucidate the circuit logic of pheromone information processing in the brain.
Identification of specific pheromones
By combining optical imaging, HPLC, LC/MS, proteomics, siingle-cell genomics, and behavioral assays, we are identifying the specific chemicals that convey information about gender, strain, and hormonal status of individual mice. The identifications, together with a number of behavioral paradigms that probe innate responses, allow us to further understand the neural circuitry involved in pheromone-mediated behaviors.
Signaling mechanism of pheromone detection
We are investigating the molecular mechanisms of pheromone activation. Previous experiments have identified Trp2, a cationic channel exclusively expressed in the VNO, as the main ion channel mediating pheromone-triggered responses. By combining patch clamp recording, fluorescent imaging, and mouse genetics, we have identified additional ion channels mediating pheromones responses.