Zhong-wei Zhang, PhD - US grants

DESCRIPTION (provided by applicant): Rett syndrome (RTT) is a severe neurological disorder causing progressive loss of motor and language skills in girls. Mutations in the X-linked gene encoding methyl-CpG binding protein 2 (MeCP2) and the loss of MeCP2 function in the brain are the primary cause of RTT. Mecp2-knockout (Mecp2-null) mice and those with truncated MeCP2 recapitulate many features of RTT, and the phenotype in these mice is caused by MeCP2 deficiency in the brain. The lack of change in brain cytoarchitecture in RTT patients and MeCP2 mutant mice suggests that subtle changes in neuronal and synaptic functions are the primary cause. MeCP2 is highly expressed in both excitatory and inhibitory neurons in the brain. While recent studies have demonstrated a key role for MeCP2 in excitatory transmission, little is known about the role of MeCP2 in the development and function of inhibitory synapses. Our preliminary studies demonstrated that GABAergic transmission is significantly altered in the thalamus of Mecp2-null mice. Most interestingly, we found that GABAergic transmission is altered in opposite directions in excitatory and inhibitory neurons. We hypothesize that MeCP2 plays an important role in the development of GABAergic synapses in the thalamus. We will examine this hypothesis using a combination of electrophysiology, anatomy, and immuno- histochemistry in Mecp2-null mice. The two specific aims are to 1) determine changes in GABAergic transmission in the thalamus of Mecp2-null mice during postnatal development;and 2) determine structural basis for GABAergic defects in the thalamus. PUBLIC HEALTH RELEVANCE: This research will advance our understanding of the pathogenesis of Rett syndrome, and may provide opportunity for the development of new and improved treatments.

DESCRIPTION (provided by applicant): The refinement of neuronal connections, which consists of selective elimination and consolidation of synapses, is a key mechanism of neuroplasticity. Disruptions of this process, caused by either environmental or genetic factors or both, are thought to be a substrate for altered information processing and abnormal behaviors associated with common developmental brain disorders including autism and schizophrenia. Mechanisms underlying synaptic refinement during development may also be involved in plasticity in the adult brain associated with learning, memory, and neuronal injuries. The long-term goal of this research is to understand how neurons in immature brains selectively eliminate some synapses while reinforcing others. We use sensory relay synapses in the thalamus of the mouse as a model to investigate the cellular and molecular mechanisms underlying synaptic refinement. We will use a novel slice preparation to quantitatively analyze the number and properties of synapses at the single-cell level in whisker sensory relay neurons in the thalamus of normal and genetically modified mice. We will determine the structural basis of synaptic refinement through quantitative analyses of the synapse using light and electron microscopy. Specific Aim 1 proposes to analyze the effects of sensory deprivation and identify the underlying cellular and molecular mechanisms. We hypothesize that sensory experience plays a critical role in developmental refinement of the synapse. Specific Aim 2 proposes to investigate the roles of NMDA (N-methyl-D-aspartate) receptor subunits NR2A and NR2C at this synapse. We hypothesize that the developmental switch of NMDA receptor composition plays an important role in synaptic refinement. PUBLIC HEALTH RELEVANCE This research will advance our understanding of autism, epilepsy, mood and anxiety disorders, and schizophrenia, and may provide opportunity for the development of new and improved treatments for neural injury.

PROJECT SUMMARY/ABSTRACT Rett syndrome (RTT) is a severe neurodevelopmental disorder causing progressive loss of motor and cognitive functions, impaired social interactions, anxiety, and seizures in young girls. Mutations in the gene encoding MeCP2 (methyl-CpG binding protein 2) are the cause of RTT. It has been hypothesized that defects of neuronal circuit development and function are responsible for neurological symptoms of RTT. The majority of RTT patients have recurrent seizures, suggesting that hyperexcitation of the neocortex is a common feature of RTT. Consistent with clinical data, MeCP2-deficient mice also show cortical hyperexcitation and seizures. Besides epilepsy, hyperexcitation of the neocortex may have major roles in cognitive and behavioral defects of RTT. Reducing cortical hyperexcitation is a potential treatment strategy for RTT. However, mechanisms underlying cortical hyperexcitation in RTT are poorly understood. This application will test the hypothesis that loss of MeCP2 in excitatory neurons in the cortex causes hyperexcitation by downregulating GABAergic transmission in the cortex. In testing this hypothesis, we take advantage of genetic manipulation in the mouse, electrophysiology, and our expertise in physiology and development of cortical neurons. The two specific aims are: 1) to determine the role of MeCP2 in regulating inhibitory neurotransmission in the cortex; and 2) to investigate of the effects of MeCP2 deficiency on cortical excitability and seizures.

DESCRIPTION (provided by applicant): Glutamate receptors that are selective for N-methyl-D-aspartate (NMDA) are a major class of neurotransmitter receptors that are essential for brain function. Abnormal regulation and dysfunction of NMDA receptors have been associated with many brain disorders including epilepsy, autism, stroke, and schizophrenia. NMDA receptors play a critical role in the development of neuronal circuits, but the underlying mechanisms are poorly understood. In this proposal we focus on the role of NMDA receptors in the maturation of neurons and synapses during early life. Specifically, we will address two important questions: 1) What is the role of NMDA receptors in the development of function and morphology of neurons; and 2) What is the role of NMDA receptor alternative splicing in the maturation of excitatory synapses and in the regulation of network excitability. The answers to these questions have broad implications for brain disorders. We use a multidisciplinary approach to address these questions, taking advantage of genetic manipulation, electrophysiology, anatomy, and molecular analyses in the mouse.