The HCN1 hyperpolarization-activated cyclic nucleotide regulated cation channel plays an important role in regulating the electrical activity of hippocampal CA1 and neocortical layer V pyramidal neurons. The HCN1 channels are targeted to the very distal apical dendrites of these neurons, where they control the integration of excitatory synaptic inputs. Behavioral studies of HCN1 knockout mice or of rats in which HCN1 is downregulated by siRNA indicate that these channels play important roles in both hippocampal dependent spatial memory and in prefrontal- cortex dependent working memory. HCN1 channel expression has also been shown to be regulated by both physiological patterns of synaptic activity and in response to seizures. Thus, alterations in HCN1 may contribute to the development of epilepsy. Although the role of HCN1 in normal neural function and disease models has been characterized in some detail, relatively little is known about the molecular mechanisms regulating the trafficking of HCN1 to and from the surface membrane or the targeting of HCN1 to the distal apical dendrites. In this application, we propose to address these issues by focusing on a brain-specific protein that we previously found to interact with the HCN channels and regulate their trafficking. This protein, TRIP8b, is associated with HCN1 in the distal dendrites of pyramidal neurons and, in heterologous expression systems, downregulates HCN1 surface membrane expression. In preliminary results we show that different TRIP8b N-terminal splice variants have different effects on HCN1 expression, with one splice variant leading to a marked upregulation of HCN1 expression. Here we propose to examine the pattern of expression of the full range of TRIP8b splice variants and the regulation of this expression in seizure models. We will examine the effects of these isoforms on HCN1 trafficking and targeting to distal dendrites using both heterologous expression systems and in vivo expression using lentiviral vector injection into the CA1 region of mouse hippocampus. The results from this study will have important implications for understanding the molecular basis for how the HCN1 ion channel, a key regulator of neuronal activity that is important for regulating learning and memory, is trafficked to the surface membrane of neurons. As changes in HCN1 channel expression are thought to contribute to the generation of epilepsy, our results may aid in developing novel treatments for this disease.
The results from this study will have important implications for understanding the molecular basis for how the HCN1 ion channel, a key regulator of neuronal activity that is important for regulating learning and memory, is trafficked to the surface membrane of neurons. As changes in HCN1 channel expression are thought to contribute to the generation of epilepsy, our results may aid in developing novel treatments for this disease.
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