Eric Levine - US grants

DESCRIPTION (from applicant's abstract): Neurotrophins play critical roles in the development of the nervous system by promoting neuronal survival and differentiation. These factors have also been implicated in the pathophysiologic mechanisms underlying Alzheimer's and other neurodegenerative diseases. At the cellular level, neurotrophins play a dynamic role in modulating synaptic transmission in the hippocampus, a brain structure of particular interest because of its proposed role in learning and memory processes as well as its selective vulnerability to injury. This research project uses electrophysiological approaches to characterize the molecular targets and signaling cascades underlying neurotrophin-induced synaptic plasticity. Recent evidence has demonstrated specific biochemical effects of neurotrophins on glutamate receptors, mediated by activation of trkB tyrosine kinase receptors in postsynaptic membranes. The proposed studies will examine the physiological context of these novel neurotrophin effects. The proposed studies will examine the physiological context of these novel neurotrophin effects, focusing specifically on functional modulation of glutamate receptor activity. Initial experiments use neuronal cultures and patch-clamp recordings to characterize the regulation of channel kinetics and elucidate underlying protein kinase signaling cascades. Later studies will explore neurotrophin modulation within the more complex cellular environment of hippocampal slices, investigating the roles of neurotrophins in synaptic plasticity and neuronal-glial interactions. These studies will contribute to a broader understanding of neurotrophin action in the brain, with potential therapeutic applications in neurogenerative disease.

DESCRIPTION (provided by applicant): The cerebral cortex is involved in a huge diversity of function, ranging from sensory processing and motor coordination to perception, generation of language, aM other higher-order cognitive abilities. These varied processes rely on the function of pyramidal cells, which are responsible for connections between cortical areas as well as connections to subcortical structures. Pyramidal cell activity, in turn, is tightly controlled by distinct classes of GABAergic inhibitory interneurons, which innervate functionally segregated domains on pyramidal cells to regulate action potential timing, the efficacy of excitatory inputs, and synchronous activity. These interneurons fire at high rates in vivo and provide potent inhibition to pyramidal cells, thus regulation of this inhibitory tone is essential for proper cortical function. Recent anatomical and physiological data indicate that the cannabinoid system plays an important role in modulating GABAergic interneurons in the neocortex. The type 1 cannabinoid (CB 1) receptor is one of the most highly expressed G-protein coupled receptors in the forebrain, and mediates the effects of exogenous cannabinoids on cognitive, sensory, and motor processes. Endogenous cannabinoid ligands are synthesized and released from pyramidal neurons with a high degree of spatial and temporal specificity, and act at least in part by binding to receptors on the presynaptic terminals of interneurons to regulate GABA release. The specificity of the endogenous system suggests that the disruptive effects of exogenous cannabinoids on cognitive processes may result from the non-selective global activation of this system. The long-term objective of this research is to understand the physiological significance of endogenous cannabinoid signaling in the regulation of neocortical function. The specific goals of the proposed studies are to: 1) determine the impact of endogenous cannabinoids on cortical synaptic inhibition, 2) test the hypothesis that cannabinoids selectively modulate a particular functional class of inhibitory afferents to pyramidal cells, and 3) investigate the consequences of cannabinoid signaling for pyramidal cell activity.

DESCRIPTION (provided by applicant): The goal of this project is to explore previously-unknown interactions between brain-derived neurotrophic factor (BDNF) and the endocannabinoid system in the cerebral cortex. Both BDNF and endocannabinoids are highly expressed throughout the sensory, motor, and association cortices, and there is a striking overlap of expression of trkB neurotrophin receptors and type 1 cannabinoid (CB1) receptors across cortical layers, with highest levels of expression in cortical layers 2/3 and 5. Disruption f either of these neuromodulatory systems has been implicated in several neurologic and psychiatric diseases, including anxiety, depression, schizophrenia, and seizure disorders, and both systems are currently major targets for the development of novel therapeutics. We found that the effects of BDNF at inhibitory cortical synapses are mediated by the BDNF-induced mobilization of endocannabinoids acting at presynaptic CB1 receptors. The proposed studies will explore the signaling mechanisms underlying BDNF-evoked synthesis and release of endocannabinoids at inhibitory synapses using electrophysiological, molecular biological, and pharmacological approaches. The proposed studies will also extend these findings to examine BDNF-cannabinoid interactions at excitatory synapses, and explore the functional relevance of these interactions in regulating activity-dependent synaptic plasticity. Knowledge gained from these studies will provide insights into the regulation and interdependence of BDNF and endocannabinoid signaling. New mechanistic insights regarding the interaction between these neuromodulators could provide the basis for novel therapeutic approaches to neurologic and psychiatric disease.

DESCRIPTION (provided by applicant): Individuals with a deletion of chromosome 15q11-q13 suffer from Angelman syndrome (AS), a neurogenetic developmental disorder characterized by intellectual disability, motor ataxia, absent speech, and seizures. The specific gene that is responsible for AS encodes the ubiquitin protein ligase UBE3A. In AS, deficits in synaptic signaling and plasticity appear to play a critical role in the disease phenotype, but the exact functional role of UBE3A and its relevant downstream targets are unknown. In order to develop appropriate treatments for AS, it is necessary to understand the pathophysiological changes caused by UBE3A deletion. Until recently it has not been possible to examine the functional properties of brain neurons in affected individuals. The discovery of genomic reprogramming of human somatic cells into induced pluripotent stem cell (iPSC) lines provides a novel way to model human diseases with complex genetics. We have recently succeeded in reprogramming dermal fibroblasts from AS patients, as well as age-matched control subjects, into iPSCs, and then differentiated these cells into functional neurons that maintain the imprinting phenotype of UBE3A expression seen in AS patients. We are now poised to take advantage of these novel patient-derived cell lines to test specific hypotheses about the underlying physiological defects in AS. The first aim uses electrophysiological and immunocytochemical approaches to explore the intrinsic functional properties of iPSC-derived neurons and activity-dependent plasticity of synaptic connections. Rescue experiments will focus on the roles of the immediate early gene ARC and the calcium/calmodulin-dependent protein kinase CaMKII in AS-associated deficits in synaptic signaling. The second aim will explore changes in synapse number and dendritic spine density in AS-derived neurons. Rescue experiments will target the role of ephexin-5, a substrate of UBE3A that plays a role in regulating excitatory synapse number during development. Overall, this approach may prove useful for identifying novel targets for drug discovery and for screening potential therapeutics aimed at ameliorating and/or curing the seizures, movement disorders, and language and cognitive impairments in Angelman syndrome. PUBLIC HEALTH RELEVANCE: Angelman syndrome is a neurogenetic developmental disorder characterized by intellectual disability, motor ataxia, absent speech, and seizures. The proposed research uses a novel human stem cell culture model to investigate the cellular and molecular basis of these deficits. The long term objective is to identify novel therapeutic targets for treating Angelman syndrome and other autism-related disorders.

Duplications of chromosome 15q11-q13 are one of the most common chromosomal anomalies associated with autism. In addition to the social, speech/language, and repetitive behavior deficits associated with autism, individuals with duplications of chromosome 15q (Dup15q) also suffer from features that are frequently co-morbid with idiopathic autism--developmental delay, motor skills delay, and seizures. Dup15q syndrome is a fully- penetrant disorder caused by the presence of one or two extra copies of a region spanning ~15-20 genes, but the specific gene(s) responsible are not clear. Based on human genetic studies and mouse models, we hypothesize that increased expression of UBE3A plus at least one other gene leads to the phenotypic manifestations of Dup15q syndrome. To test this hypothesis, we will use CRISPR and LoxP technologies to genetically correct human induced pluripotent stem cells (iPSCs) derived from individuals with Dup15q syndrome. We will then perform detailed electrophysiological characterization of neurons generated from these isogenic Dup15q/control iPSC pairs to identify cellular phenotypes associated with Dup15q syndrome. Preliminary data suggests that Dup15q iPSC-derived neurons are hyperexcitable and have deficits in synaptic plasticity and homeostatic synaptic scaling. Finally, we will use CRISPR inhibition and antisense oligonucleotide technologies to reduce expression of genes in the duplicated region individually or in combination to determine their contribution to the cellular pathophysiology in human Dup15q neurons. Successful completion of these experiments will identify cellular phenotypes underlying Dup15q syndrome as well as the genes contributing to them. The information garnered here will create cellular resources for drug discovery, inform construction of better mouse models for Dup15q syndrome, and identify neuronal pathophysiology that may contribute to syndromic and idiopathic forms of autism.

PROJECT SUMMARY The goal of this project is to explore the functional relevance of interactions between brain-derived neurotrophic factor (BDNF) and endogenous cannabinoids (eCB) in regulating activity-dependent synaptic plasticity in the neocortex and hippocampus. Although there is growing evidence for crosstalk between BDNF and eCBs, little is known regarding potential synaptic interactions. We have previously characterized the synaptic effects of eCBs and BDNF in layer 2/3 and layer 5 of somatosensory cortex as well as the CA1 area of hippocampus, and we have recently shown that the presynaptic effects of BDNF at cortical and hippocampal inhibitory synapses are mediated by the BDNF-induced release of eCBs from postsynaptic pyramidal cells. We have also found that BDNF causes release of eCBs at excitatory synapses, and this eCB signaling mitigates the direct facilitatory effects of BDNF at these synapses. We are now poised to explore the functional relevance of these interactions in regulating activity-dependent synaptic plasticity. In particular, we will examine the interactions between endogenous BDNF-induced eCB release and activity-dependent eCB release in regulating the magnitude and direction of plasticity at excitatory and inhibitory synapses. These studies will combine electrophysiology and calcium imaging with pharmacological and genetic approaches to manipulate these signaling systems. We will also examine these signaling interactions using mice engineered to express common human single-nucleotide polymorphisms (SNPs) that affect either endogenous BDNF or anandamide levels. Importantly, we will carry out parallel studies using cultured human induced pluripotent stem cell (iPSC)-derived neurons generated from individuals who carry these same SNPs.