Matthew Dalva, PhD - US grants

DESCRIPTION (provided by applicant): During development, maps of stimuli in the world are established in the cortex. Understanding the role neuronal activity plays in this process has implications for the effects of abnormal sleep, and of toxins and drugs, during pre- and postnatal development in humans. While understanding the underlying cortical circuitry is fundamental to understanding certain neurological pathologies, including mental retardation. Motivated by both experimental and theoretical studies, a prevailing computational model for how maps are formed has been that afferent activity, either spontaneous or environmentally evoked, drives their development. This model assumes that intracortical connections play a subservient role to feedforward ones, that intrinsically generated spontaneous activity does not significantly influence cortical plasticity, and that intraconnections are more or less fixed during development. However, recent data undermine all of these assumptions: Modeling studies have shown that maps can arise without patterned feedforward connections. Electrophysiological records of spontaneous activity during initial map formation reveal correlations that may arise from intraconnections patterned by molecular cues. And, photo-stimulation experiments suggest that extensive changes occur in intraconnections during subsequent map development. To test the hypothesis that intrinsic spontaneous activity reinforces pre-existing, rudimentary cortical circuits during early map formation, and reinforces changes in cortical maps during subsequent periods of heightened plasticity, this study will: (1) Characterize activity patterns in sleep and wake during initial map formation and subsequent map remodeling in vivo. (2) Map the extent of excitatory and inhibitory intraconnections in vitro. (3) Develop computational models that reproduce the patterns of cortical activity imaged in vivo based on the patterns of lateral connectivity mapped in vitro, and that predict the time-course of map formation and interactions between sleep and wake during map remodeling

DESCRIPTION (provided by applicant): During the past decade stunning advances have been made in imaging, molecular biology and biochemistry that enable the visualization of the behavior of single proteins in vivo. Here, I propose to develop and visualize the temporal and spatial dynamics of intracellular signaling within living neurons. Much as early work in calcium imaging redefined our understanding of the importance of calcium influx by defining its spatial and temporal characteristics, I believe visualizing the spatial and temporal dynamics of intracellular signaling will have similar benefits to our understanding of the nervous system. Initially we have developed indicators that enable visualization of one of the key first steps in many intracellular signaling cascades: tyrosine phosphorylation. During the past several years, we have developed a system that relies on ratiometric imaging of changes in a genetically encoded fluorescent indicator of phosphorylation. We now propose three specific aims to develop these tools into a system for monitoring signaling during neuronal plasticity and development. We propose to: 1) Develop a library of indicators targeted to report activity of specific kinases;2) Develop indicators that localize to specific cellular compartments;3) Develop indicators to report activity of multiple signaling molecules simultaneously. Using our indicators, workers will be able to elucidate the dynamics of signals that underlie synaptic plasticity. Thus, our tools will enable novel insights into essential mechanisms that underlie neuronal plasticity. PUBLIC HEALTH RELEVANCE: Project Narrative Neuronal plasticity underlies many fundamental functions within the brain, while abnormal neuronal plasticity is associated with disease. Excessive plasticity may underlie diseases like epilepsy and addiction, while defects in plasticity could play important roles in epilepsy, neurodegenerative, and autism spectrum disorders. Our research will have broad impacts across all these levels by developing new tools to visualize dynamic neuronal signaling with subcellular resolution.

DESCRIPTION (provided by applicant): Neural progenitor cells originating in the subventricular zone (SVZ) integrate into the neural circuitry that mediates olfactory learning and impacts habit formation. The SVZ produces new neurons throughout development and adulthood that are in part responsible for the life-long elaboration and refinement of the olfactor bulb cytoarchitecture. These new neurons migrate from the SVZ to the olfactory bulb via a pathway called the rostral migratory stream. However, the molecular mechanisms that guide these migrating cells to the olfactory bulb are poorly defined. Building on strong initial findings implicating ephrinB and EphB proteins in control of migration of cells within the rostral migratory steam, we have established novel assay systems that allow us to begin to define the exact role of astrocytes in adult migration. We propose three specific aims: Determine whether expression of ephrin-B2 in the astrocyte sheath constrains migration of adult neural progenitor cells within the rostral migratory steam. Determine whether EphB2 is required for astrocyte dependent-migration of adult neural progenitor cells in the rostral migratory steam. Determine whether astrocytes lining the rostral migratory steam constrain migration by activation of EphB2 in migrating neural progenitor cells. Because previous results have demonstrated progenitor cell proliferation is reduced in depression and EphB and ephrinB expression is altered by changes in neuronal activity our proposed studies will have board impact on the understanding of the basic biology of progenitor cell migration and the cellular-molecular mechanisms controlling pathophysiology.

Project Summary / Abstract: In a rodent model of cervical spinal cord injury (SCI), we propose to examine the contribution of altered EphB2 receptor-NMDA receptor (NMDAR) interaction to both excitatory synaptic neurotransmission in the superficial dorsal horn (DH) and persistent neuropathic pain (NP). The development of NP occurs in a significant portion of individuals affected by SCI, resulting in debilitating and often chronic physical and psychological burdens. Importantly, this pathological pain is particularly refractory to treatment, urgently calling for the identification of mechanistic targets that both robustly regulate pathological pain and avoid the devastating effects of opioid- based interventions. Hyperexcitability of DH circuitry (?central sensitization?) is a major substrate for NP after SCI. Studies have shown that NP is linked to EphB/ephrinB signaling through potentiation of NMDAR function, suggesting that the EphB-NMDAR interaction may be an important target for control of SCI-induced NP. We recently discovered that the EphB2-NMDAR interaction is regulated by a single extracellular amino acid of EphB2 (Y504). We demonstrated in vitro that EphB2-Y504 phosphorylation is required in spinal cord neurons for EphB-NMDAR interaction, NMDAR synaptic localization, and excitatory synapse function. We also found that transduction of DH neurons in vivo with EphB2 that constitutively interacts with the NMDAR results in long- lasting allodynia. We hypothesize that modulating the EphB2-NMDAR interaction in superficial dorsal horn (DH) neurons will impact synaptic localization and function of NMDARs, excitatory synaptic transmission between primary sensory afferents and DH neurons, and NP-related behaviors after cervical contusion SCI. Aim 1. Determine whether interaction with EphB2 drives NMDA receptors to synapses between primary nociceptive afferents and superficial DH neurons following cervical SCI. We will determine whether knocking down EphB2 in both uninjured and cervical contusion SCI mice using DH neuron subtype- specific expression of EphB2-shRNA reduces the localization of NMDAR subunits to excitatory synapses. Aim 2. Determine whether EphB2 regulates excitatory synaptic transmission in DH and NP-related behaviors after cervical SCI. We will determine whether DH neuron subtype-specific knockdown of EphB2 impacts: (2a) synaptic transmission between primary afferents and laminae I-II neurons using whole-cell patch clamp recording in an intact ex vivo preparation; and (2b) initiation and/or persistence of NP-related behaviors. Aim 3. Determine whether EphB2-Y504 phosphorylation regulates EphB2-NMDAR synaptic interaction in the DH and NP-related behaviors after cervical SCI. By expressing wild-type EphB2-Y504 or constitutively-phosphorylated (Y504E) or non-phosphorylatable (Y504F) mutants in a DH neuron subtype- specific manner, we will determine whether modulating EphB2-Y504 phosphorylation impacts: (3a) EphB2- NMDAR interaction, (3a) NMDAR levels at excitatory synapses, (3c) excitatory synaptic transmission between primary sensory afferents and DH neurons, and (3d) NP-related behaviors after cervical contusion SCI.

Project Abstract As much as 20% of the population will suffer from chronic pain lasting for more than 6 months. Chronic pain and its underlying pathophysiology, can result in depression and other debilitating neurological effects and although there are effective treatments for acute pain chronic pain is resistant to most current treatments requiring the development of novel therapeutics that target molecular events underlying these pain states. Neuropathic and persistent post-surgical pain occurs, at least in part, due to long lasting changes in the function of excitatory synaptic transmission in the spinal dorsal horn resulting in enhanced pain signaling (hyperalgesia) and innocuous stimuli evoking pain (allodynia). These synaptic events share many features of neuronal plasticity that has been studied in higher CNS areas. Many of these changes are NMDAR dependent resulting in increased synaptic strength. One mechanism that has emerged underlying these changes in synaptic function is the potentiation of NMDAR function by a direct molecular interaction with the EphB receptor tyrosine kinase. Building on our published work, we will test the hypothesis that an EphB-NMDAR interaction is responsible for the development of a chronic pain state by directing NMDARs to synapses by expressing wild type or mutant EphB2 receptors in vitro and in mice. To test this hypothesis, we will determine the mechanism mediating the EphB-NDMAR interaction, characterize molecules and other tools to disrupt this interaction, and determine whether preventing the EphB-NMDAR interaction will alleviate chronic pain. To address these questions we will undertake three specific aims: 1. Determine the domain on the NMDAR responsible for the EphB-NMDAR interaction. 2. Test the hypothesis that VLK directs phosphorylation of Y504 on EphB2. 3. Determine the functional significance of VLK in pain plasticity. Collectively these aims will create a new knowledge that will provide a deeper understanding of the role of EphB-NMDAR interaction in pain and enable progress toward understanding the basic mechanisms behind chronic pain states.