Wei Li, Ph.D. - US grants

PROJECT SUMMARY/ABSTRACT Rett syndrome (RTT), an X-linked autism spectrum disorder, is a devastating childhood disability and has a tremendous impact on individuals (1:10,000 births), their families and society. The majority of RTT cases are caused by loss-of-function mutations in the gene encoding methyl-CpG-binding protein 2 (MeCP2). RTT girls are born without any obvious problems but abruptly develop a host of neurological deficits, including irregular breathing, loss of purposeful hand movement and speech, seizures, and intellectual disability. Among these symptoms, motor dysfunction and deficits in attention-related behavioral shifting are the most profound, bearing resemblance to several brain disorders of basal ganglia origin. Disease-modifying therapies that are designed for the treatment of RTT require a clear understanding of the underlying pathophysiology at the molecular, cellular, and network levels. Dysfunction of the striatum, where most inputs enter the basal ganglia, may play a significant role in RTT pathogenesis. The striatum receives glutamatergic excitatory projections from the thalamus, which integrate and modulate cortical inputs for proper striatal output. Our preliminary data demonstrate altered function and plasticity of thalamo-striatal synapses in Mecp2 knockout (KO) mice, which may contribute to dysfunction of cortico-striatal synaptic. Our hypothesis is that altered thalamo-striatal synaptic function in Mecp2 KO mice disintegrates the striatal role in integrating cortical inputs. We propose two Specific Aims: (1) characterize thalamic neurons and their synaptic projections to the dorsal striatum of Mecp2 KO mice; (2) test whether the impact of thalamic inputs on cortico-striatal system in dorsal striatum is impaired in Mecp2 KO mice. We anticipate that these experiments will yield novel information regarding the consequences of MeCP2 loss on thalamo-striatal synaptic transmission and plasticity, as well as on its integration and control of cortico-striatal connections. These findings will uncover fundamental brain mechanisms involved in RTT neuropathology, and aid to develop and test novel therapeutic approaches. Our studies will also have deep implications for the understanding and treatment of other neurological disorders with common symptoms and neural substrates, including Huntington and Parkinson diseases, as well as Tourette and Angelman syndromes.

ABSTRACT Astrocytes are a key element of brain connectivity because they regulate neuronal communication at synapses. Bergmann glial cells (BGCs), a major type of astrocytes in the cerebellum, are intimately associated with excitatory glutamatergic synapses between Purkinje cell (PC) spines and parallel fibers or climbing fibers, playing a significant role in synaptic stability and long-term plasticity. BGCs express a high density of Ca2+- permeable GluA2-lacking AMPA receptors (CP-AMPARs), which are required for the maintenance of the BGC processes that ensheath PC spine synapses. Genetic deletion of CP-AMPARs results in impaired associative motor learning. Our preliminary data indicate that CP-AMPARs in BGCs are subject to dopaminergic (DAergic) modulation. Pharmacological stimulation of D1 receptors enhances CP-AMPARs-mediated currents in BGCs, and phosphorylates the GluA1 subunit at Ser845. Using transgenic mice with fluorescently-tagged D1 receptors and anterograde axonal tracing, we determined that D1 receptors are exclusively expressed in BGCs, which receive DAergic afferents from the ventral tegmental area and substantia nigra pars compacta in the midbrain. Intriguingly, D1 receptor activation also increases PC firing and locomotor activity. Based on these preliminary findings, we hypothesize that activation of D1 receptors induces insertion of CP- AMPARs in BGCs, which in turn modulates PC synaptic function. To test this hypothesis, we will use a combination of experimental approaches, including whole-cell recordings with optogenetic stimulation, immunocytochemistry, molecular biology, and pharmacology. We propose two Specific Aims: (1) characterize membrane insertion of GluA1 in Bergmann glial cells induced by DA; and (2) define the role of DAergic modulation of Bergmann glial cells in PC activity. We expect to generate conceptually novel knowledge regarding DA modulation of cerebellar circuitry function by defining the contribution of CP-AMPARs in BGCs to synaptic function in PCs. These new findings will not only deepen our understanding of DAergic function in the healthy brain, but also provide additional avenues for the development of disease-modifying therapies for motor-related neurological disorders.