Date of Award

5-2009

Document Type

On-Campus Dissertation

Degree Name

Doctor of Philosophy (PhD)

Program

Anatomy and Neurobiology

Research Advisor

Angela R. Cantrell, Ph.D.

Committee

William E. Armstrong, Ph.D. Robert C. Foehring, Ph.D. Anton J. Reiner, Ph.D. Steven J. Tavalin, Ph.D.

Abstract

Huntington’s disease (HD) is an autosomal dominant degenerative disease that is caused by an expansion mutation in the huntingtin protein that lengthens a naturally occurring trinucleotide CAG repeat in exon 1 resulting in striatal degeneration. The mechanism by which striatal neurons undergo selective degeneration is not fully understood, although studies have suggested several different intrinsic and extrinsic mechanisms involving early neuronal dysfunction in the cortex and striatum. One way in which cortical neuron function may be compromised is through alterations in the functional properties of the voltage gated ion channels underlying the action potential and synaptic transmission. This study investigates the functional properties of high voltage activated (HVA) Ca2+ channels in cortical neurons from HD transgenic mice and wild type littermates. Due to the importance of this class of channels in neuronal function and signaling, changes in their functional properties would be expected to have a profound influence and may be important to understanding disease progression and pathology.

Experiments were conducted using the transgenic R6/2 HD mouse model, which, has a CAG expansion of approximately 144 repeats and displays progressive motor and behavioral dysfunction similar to those found in HD patients. The patch clamp method was used to record whole-cell HVA Ca2+ currents in acutely dissociated cortical neurons from symptomatic R6/2 and wild type mice. Electrophysiological results indicated there was an increase in the whole-cell HVA Ca2+ current in R6/2 pyramidal neurons compared to wild type neurons that was not the result of a shift in the voltage dependence of activation or from changes in the properties of inactivation. Pharmacological experiments indicated that the increase involved primarily the N-type and other non-L-type Ca2+ currents. Together these results suggest that alterations in the physiological properties of HVA Ca2+ currents in symptomatic R6/2 cortical neurons may play a role in cortical dysfunction in HD.

Similar experiments were conducted on early symptomatic R6/2 and wild type dissociated cortical neurons. In these experiments, we found that whole-cell Ca2+ current was significantly smaller in early symptomatic R6/2 neurons compared to wild type neurons. Surprisingly, pharmacological experiments indicated that L-, N-, P-, and Q-type Ca2+ currents were decreased in this age range. These findings suggest there are alterations in HVA Ca2+ currents early in the disease before the onset of overt physical symptoms. However, the pattern of dysfunction appears to change with disease progression.

During the course of these experiments, a spontaneous mutation in the R6/2 colony occurred resulting in a new mouse named the R6/2X. The R6/2X mouse has a CAG repeat length of over 400, a longer life span, and does not develop intranuclear inclusions (NIIs). Even though the R6/2X mice live longer, they eventually develop the same motor and behavioral symptoms as the R6/2 mice. We were curious to see if neurons from these mice displayed similar alterations in Ca2+ channel function. Experiments on symptomatic R6/2X cortical neurons revealed a significantly larger Ca2+ current density compared to wild type neurons. However, the increase involved non-L-type Ca2+ current in R6/2X cortical neurons. In experiments involving early symptomatic R6/2X and wild type cortical neurons, no alterations were found in the physiological properties of HVA Ca2+ currents. These findings suggest that the development of NIIs may not be necessary in the development of alterations in HVA Ca2+ current in cortical neurons.

In an Hdh null mutant mouse model, we found a significantly greater whole-cell HVA Ca2+ current in cortical neurons of Hdh null mice compared to controls that involved increased non-L-type currents. These findings suggest that alterations in HVA Ca2+ current and the resulting cortical neuronal dysfunction in the Hdh null and R6/2 mice may be the consequence of the loss of normal huntingtin function.

Finally, in whole-cell current clamp experiments in symptomatic R6/2 and wild type littermate brain slices we found significant differences in the resting membrane potential and the single action potential amplitude, ½ width, and dV/dt of depolarization in R6/2 cortical neurons. Examination of the medium and slow afterhyperpolarizations (AHPs) revealed no significant difference between R6/2 and wild type cortical neurons. The slope of the F-I graph revealed no significant difference in the shape of the steady-state F-I curves in R6/2 and wild type cortical neurons. However, there were differences in the tau and percentage of adaptation at certain current injections between R6/2 and wild type cortical neurons. Although we did not find alterations in the characteristics of repetitive firing associated with HVA Ca2+ channels this finding does not rule out the impact of the increased Ca2+ current on other Ca2+-dependent mechanisms. The changes in the action potential characteristics we found in symptomatic R6/2 layer V cortical neurons indicate possible alterations in Na+ and K+ channel function.

We conclude that the expansion mutation of the huntingtin protein in HD mouse models is capable of altering the function of HVA Ca2+ channels and other voltage-gated ion channels in cortical pyramidal neurons. The alteration of the function of Ca2+ channels in cortical pyramidal neurons may result in changes in neuron Ca2+ homeostasis, gene expression, synaptic integration, and neurotransmitter release. All of which may be important to the underlying disease mechanism or progression.

DOI

10.21007/etd.cghs.2009.0051

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