Normal behavioral functions rely on precise and complex neural circuits linking large ensembles of neurons. In development, axons and dendrites extend in a highly directed manner and elaborate intricate branches and arbors to establish organized patterns of synaptic connections. A genetic program plays a profound role in the formation of these networks by specifying neuronal cell types, positioning neurons, guiding axons and dendrites and forming appropriate functional synapses. Neural activity shapes circuit development and coordinate with the genetic program.
After the nervous system matures, certain areas of nervous system remain highly plastic in adulthood, whereas others are much less malleable. The regenerative ability of the adult central nervous system is generally highly limited. Abnormal circuit development and degenerative disorders lead to behavioral deficits.
1. Molecular and Cellular Mechanisms of Axon Guidance
Guidance cues for axon wiring
Axonal connections are highly organized and precisely patterned. Much of the organization is achieved by the action of a large number of guidance cues, which control the direction of axon pathfinding and target selection in development. We are studying the role of essential molecular determinants in central nervous system axon wiring, particularly extracellular morphogens and classic axon guidance molecules by using spinal cord commissural neurons, dorsal root ganglion neurons, retinal ganglion cells, corticospinal tracts, and serotonergic and dopaminergic neurons Lyuksyutova et al. . Liu et al. Fenstermaker et al Salinas and Zou Zou and Lyuksyutova
Signaling and cell biological mechanisms
The growth cone is a specialized structure localized at the tip of a growing axon responsible for sensing and responding to the guidance cues present in its micro-enviroment. Guidance molecules are detected by receptors on the surface of growth cones. Once the cues bind to their receptors, signals are then transduced across the plasma membrane and interpreted in the growth cones. We found that signaling pathways that specify the apical-basal and planar cell polarity of epithelial cells mediate Wnt signaling in axon guidance Wolf et al. Fenstermaker et al. Shafer et al. We recently found that apical-basal polarity signaling component, aPKC, promotes the endocytosis of a planar cell polarity component, Frizzled3, and may be part of an amplification mechanism for asymmetric signalling among growth cone filopodia Onishi et al.
Axons face multiple guidance molecules and need to integrate signaling activities to make correct guidance decisions. For axons that travel a long distance, they often have complex trajectories that are made of segments punctuated by intermediate targets before they reach their final target area. Growth cones commonly stay at the intermediate targets for certain period of time, change their responsiveness and become sensitive to new guidance cues when they leave the intermediate targets. We are studying how growth cones integrate signaling pathways activated by different guidance cues and how their sensitivity to guidance cues change while growing across intermediate targets. We found a classic morphogen, Sonic Hedgehog can switch on responsiveness of commissural axons to Semaphorin at the midline Parra and Zou
The cytoskeleton and the growth cone membrane undergo significant reorganization, regulated by signals activated by guidance molecules. But little is known about how these changes cause growth cone turning. We are using the Wnt family guidance molecules as a model system to study the fundamental cellular machinery responsible for growth cone turning and to understand how this machinery responds to guidance cues. For this, we are using live imaging techniques combined with biochemistry and molecular biology approaches.
2. Wiring for Function
The connections in the nervous system are highly specific and organized. Part of the organization is set up during pathfinding where axon-axon interactions self sort each other and directional cues lead them to proper target area. Once axons arrive at the target area, they need to establish various patterns of synaptic connections by seeking out correct post-synaptic partners.
One type of synaptic connection pattern is topographic connections, which convey smooth and continuous positional information between two connected areas. Our studies suggest that diffusible guidance cues, such as Wnts, also play a role in specifying topographic position by controlling axon target selection. We found that Wnt3 and EphrinB1 are two counterbalancing mapping forces in retinotectal projections along the dorsal-ventral (medial-lateral) axis. Schmitt et al We are currently testing whether this "two-molecular-gradient model" is a general mechanism for topographic mapping and how two opposing mapping activities lay out topographic connections. Another common wiring strategy is laminar-specific targeting. For example, different retinal ganglion cell axons find their synaptic targets in different recipient layers in the optic tectum (in chick) and superior colliculus (in mammals). Layer-specific targeting contributes significantly to the specific patterns of synaptic connections and creates cellular and subcellular precisions, which are essential for encoding behavior. We are investigating the mechanisms of this level of brain wiring to understand how functional neural circuits emerge from the molecular cues and their potential interactions with neural activity in this process. We are also studying how specificity of synaptic connections are achieved in the hippocampus for memory formation and how abnormal connectivity contributes to neurological and psychiatric disorders.
3. Axon regeneration and neural circuit repair
In the adult mammalian central nervous system axons generally do not naturally regenerate. We are interested in understanding the mechanisms regulating axon regrowth and regeneration and how these mechanisms could be used to repair the injured central nervous system.
Traumatic injury in the central nervous system leads to changes of gene expression in the injured areas as well as in neurons whose connections are altered. Some of these induced genes are important regulators of developmental processes, such as axon guidance. However, the roles of these reinduced developmental genes in injury response are unclear. We are studying the role of reinduced Wnt signaling system in injury response, including regulating axon regeneration. Liu et al. The reinduced Wnt inhibitory system also limits sensory axon regeneration even after conditioning lesion, suggesting that Wnt-Ryk signaling is a general barrier for axon regeneration in the central nervous system. Hollis et al. The reinduced Wnt signaling system likely influences glial and inflammatory responses that play significant roles. We are dissecting the role of Wnt signaling in astrogliosis, myelination and microglia activation. Our final goal is to develop therapeutic approaches based on more complete knowledge about CNS injury to promote regeneration and functional recovery.
4. Mechanisms of Neurodegenerative Disorders
Axon loss in degenerative disorders or traumatic injury leads to permanent changes/loss of circuits and function, making it challenging to improve function in affected patients. Because in the mammalian central nervous system axons generally do not naturally regenerate, another approach to try to prevent, or treat neurodegenerative disorders would be by protecting the existing neurons and axons, and impeding their degeneration. We are studying the mechanisms that control axon stability and protection, as well as the ones that lead to axon degeneration in several neurodegenerative disease models, such as amyotrophic lateral sclerosis (ALS) and traumatic injury. We recently found that the expression of Wnt signaling components that are critical for axon guidance in development is drastically changed in a mouse model of ALS. Tury et al. Our aim is to understand the neurobiological mechanisms that regulate the stability of axons with the aim to develop new therapies for axon protection.