Charles Trogdon Anderson - US grants

DESCRIPTION (provided by applicant): Autism spectrum disorder (ASD) affects as many as 1 in 150 children. It is a complex developmental disorder characterized by social deficits, language impairment, and restricted interests (Newschaffer et al., 2007;Levitt and Campbell, 2009). Genetic linkage techniques recently revealed that individuals with ASD are three times more likely to have a mutation in the promoter region of the gene for MET receptor tyrosine kinase (hereafter MET), which results in reduced MET expression (Campbel et al., 2006;Campbel et al., 2007). The MET signaling pathway is important for the normal development of the cerebral cortex (Powell et al., 2001;Powell et al., 2003;Levitt et al., 2004;Gutierrez et al., 2004). Interference with this pathway affects cortical development in a number of ways. Still unknown however, is the extent to which the functional organization of cortical circuits is affected by MET dysfunction. Of particular interest to ASD research is the synaptic organization of the frontal cortex, which is involved in many higher order cognitive aspects of behavior and executive functioning. Evidence suggests long-range under-connectivity between cortical areas in ASD (Horwitz et al., 198;Courchesne and Pierce, 2005;Kana et al., 2007). From this view, ASD is a disorder of cortical circuits. It has been proposed that there is also an over-strengthening of local connectivity in ASD (Courchesne and Pierce, 2005), but this has yet to be directly measured. The availability of a MET- knockout (MET-KO) mouse will allow me to measure changes in synaptic connectivity that result from interference with the MET signaling pathway. I hypothesize that local synaptic connections in frontal cortex are over-strengthened as a result of MET- KO. I will test this hypothesis by measuring the circuit abnormalities of corticostriatal neurons with high throughput circuit mapping techniques (Weiler et al., 2008;Yu et al., 2008;Wood et al., 2009, Anderson 2010). I will focus on corticostriatal neurons because these neurons provide long-range input to other cortical areas (Wilson, 1987;Reiner et al., 2003) and the projection is a key component in loops linking the frontal cortex with the basal ganglia and the thalamus important for the selection and initiation of behavior (Albin et al., 1989). Understanding altered cortical connectivity in MET-KO will be an important step towards characterizing the nature of cortical circuit disorders associated with ASD. It will provide a basis for understanding the mechanisms underlying the changes to the brain in ASD. PUBLIC HEALTH RELEVANCE: As many 1 in 150 children are diagnosed with autism spectrum disorder (ASD), which is a complex developmental disorder characterized by abnormal social interactions, language deficits, and restricted interests. Evidence implicates alterations to the circuits of the frontal cortex in ASD, but to date, no detailed, cell-level resolution of synaptic connectivity in non-syndromic ASD has been obtained. We will characterize the cortical circuits of a mouse model of cortical circuit dysfunction in ASD, which will yield important insight into the mechanism underlying differences in the synaptic organization of the frontal cortex, and help understand the changes that occur in the brains of people with ASD.

DESCRIPTION (provided by applicant): Tinnitus - the perception of phantom sounds -is frequently caused by acoustic trauma. This widespread neurological condition affects approximately 40 million people in the U.S. Recent evidence suggests that the auditory brainstem, and in particular, the dorsal cochlear nucleus (DCN), plays a crucial role in the induction of tinnitus. The DCN displays hyperexcitability in tinnitus, hypothesized to result from endogenous compensatory mechanisms in response to acoustic trauma, termed maladaptive plasticity. Multiple mechanisms have been proposed to account for changes to the DCN in tinnitus, but to date, there has been no consensus as to how this brainstem nucleus transitions into a pathological state during this disorder. The DCN has a well-defined synaptic organization and contains multiple inhibitory and excitatory pathways that shape the response properties of this structure to sound. There are a rich variety of synaptic plasticity mechanisms present in the DCN, so factors that influence synaptic plasticity are potential substrates for the chronic hyperexcitability observed in the DCN during tinnitus. The DCN is unique among the auditory brainstem nuclei because it contains high levels of synaptic zinc - a strong regulator of long-term plasticity. Zinc is released from glutamatergic terminals during synaptic transmission, and because it potently inhibits NMDA receptors, it is poised to have a dramatic effect on synaptic signaling. My preliminary data indicate that mice with behavioral evidence of tinnitus have a dramatic reduction of synaptic zinc released from the DCN. This is a novel neurophysiological correlate of tinnitus. I hypothesize that synaptic zinc is critical for the normal functioning of the DCN and tht the loss of zinc is crucial feature of the pathology of the DCN during tinnitus. My preliminary data suggest that reduced synaptic zinc release leads to reduced inhibitory drive in the DCN. This has the potential to change to the balance of excitation and inhibition in the DCN, and be a contributing factor to the hyperexcitability of this structure in tinnitus. I will use newly developed ratiometric fluorescent zinc sensors and chelators in combination with brain slice electrophysiology to examine the role of synaptic zinc in the DCN in normal hearing and in tinnitus. My results will shed new light onto the mechanisms by which the DCN transitions into a state of pathological hyperactivity and potentially offer new strategies for therapeutic interventions for tinnitus.

Project Summary Many regions of the brain including the cortex, hippocampus, basal ganglia, and limbic structures are highly enriched with synaptic zinc. Synaptic zinc (as Zn2+) is loaded into presynaptic vesicles by zinc transporter 3 (ZnT3), where it is coreleased with glutamate during synaptic transmission. Since synaptic zinc inhibits AMPA and NMDA receptors ? which mediate the majority of excitatory glutamatergic transmission in the brain ? synaptic zinc can modulate excitatory synaptic signaling. ZnT3 KO mice (which lack synaptic zinc) display a range of cognitive and sensory impairments and demonstrate behavioral deficits associated with autism and schizophrenia. Mounting evidence from human populations shows that mutations in certain zinc transporters are linked with major neurological disorders such as schizophrenia. Together, these findings strongly suggest that synaptic zinc signaling is important for neuronal processing. The goals of this project are to understand how synaptic zinc contributes to normal neuronal function and how disruptions in zinc signaling are linked to pathological neuronal conditions. We will take three complimentary experimental approaches to these questions. 1) Using ex vivo brain slice preparations and optogenetic stimulation paradigms, we dissect the roles of synaptic zinc in shaping the dynamics of synaptic transmission at specific synaptic connections in cortical microcircuits. 2) Using in vivo 2-photon calcium imaging, we assess the roles of synaptic zinc in shaping the sensory-evoked responses of specific classes of auditory cortical neurons in awake mice. 3) Using in vitro high-throughput screening assays and rational compound design approaches, we are designing novel tools to modulate the function of specific zinc transport proteins. Together these approaches will allow us to answer fundamental questions concerning the role of synaptic zinc in brain function and provide new mechanistic insights into endogenous mechanisms that shape synaptic and neural processing.