February 9, 2011

9:10 AM

Mitochondrial and bioenergetic dysfunction in Huntington's disease: Overview

Timothy Greenamyre, MD, PhD – University of Pittsburgh Medical Center

Huntington's disease (HD) is caused by an expansion of CAG triplet repeats in the gene encoding huntingtin protein; however, the mechanism by which the altered protein exerts its pleiotropic and deleterious effects remains elusive. Several lines of evidence support the notion that abnormal energy metabolism and mitochondrial dysfunction may be responsible for much of the observed HD pathophysiology. In this talk, I will present an overview of mitochondrial and bioenergetic abnormalities including altered calcium handling, changes in mitochondrial structure and dynamics, and transcriptional dysregulation, which are all thought to contribute to neurodegeneration and neuronal loss in in vitro and in vivo HD models. Furthermore, I will provide current evidence in support of metabolic disturbances in HD patients, and an overview of recent HD clinical trials using the bioenergetic agents. Understanding huntingtin-induced perturbations to energy homeostasis and free radical production should provide greater validation for this mechanism and a more defined set of targets for therapeutic intervention.

9:50 AM

Evaluating regional cerebral dysfunction and bioenergetics by magnetic resonance spectroscopic imaging

Hoby Hetherington, MSc, PhD – Yale University, School of Medicine

Magnetic Resonance Spectroscopic Imaging (MRSI) is a valuable tool in evaluating neuronal injury and dysfunction in a variety of pathologies including epilepsy, multiple sclerosis and Alzheimer’s disease. Measurements of N-acetyl aspartate (NAA), by ¹H MRSI have demonstrated substantial reductions in these patient groups. Through studies involving multiple research groups it is clear that: 1) NAA is primarily localized to neurons and synthesized in neuronal mitochondria; 2) reductions in NAA are not due solely to neuronal loss, but do reflect measures of neuronal injury, which occur prior to neuronal loss in animals models of epilepsy and are reversible MS and epilepsy; 3) NAA reductions do reflect true neuronal dysfunction and are correlated with reductions in cognitive performance in epilepsy, MS and AD; 4) in healthy control subjects NAA levels are linearly correlated with ADP suggesting that NAA levels are a response to energetic demands and 5) in mitochondrial preparations NAA and ATP synthesis rates are linearly correlated and sensitive to mitochondrial poisons. Taken together this supports the role of NAA as an early marker for neuronal bioenergetic defects. Although NAA provides a highly sensitive surrogate marker for the presence of impaired neuronal function and bioenergetics, it is an indirect measure of cerebral bioenergetics and has been hypothesized to be sensitive to other physiologic perturbations including osmotic perturbations. Direct measurements of cerebral bioenergetics can be obtained using ³¹P MRSI. Although its lower intrinsic sensitivity degrades the achievable spatial resolution (6cc v 0.5cc for 1H), measurements of phosphocreatine, ATP and inorganic phosphate have been used to demonstrate the presence of energetic impairment in epilepsy and its reversibility with effective medical interventions. These regional measures have been correlated with changes on a cellular level including glial proliferation, and a variety of measures of hyper excitability in resected epileptogenic tissue. Application of these methods to HD in animal models and patients is relatively straight forwards, with the caveat that spectral quality from regions about the basal ganglia are often degraded in the human brain due to susceptibility artifacts arising from air-tissue interfaces. Fortunately, a recent advance in shim hardware design has overcome this limitation.

11:00 AM

What can be learned from another neurodegenerative disease: Mitochondrial dynamics impairment in Parkinson’s disease

Sarah B. Berman, MD, PhD – University of Pittsburgh

Mitochondrial dysfunction is increasingly linked to both Huntington’s disease (HD) and Parkinson’s disease (PD). There are many similarities in links to impaired mitochondrial function in HD and PD, including evidence for a role of oxidative stress, bioenergetic dysfunction, alterations in mitochondrial biogenesis regulation, fission, fusion, and trafficking, all important components of mitochondrial homeostasis. I will present an overview of mitochondrial impairment in PD, including involvement of oxidative stress, changes in mitochondrial dynamics and quality control (fission, fusion, trafficking, biogenesis and degradation through mitophagy) and mitochondrial DNA damage. I will discuss our ongoing research into the role of mitochondrial homeostasis, dynamics and bioenergetics in Parkinson’s disease and how this might be relevant in furthering understanding of neuropathology and neuroprotection in HD.

11:40 AM

Optimization of brain-penetrant isozyme selective PDHK inhibitors as potential therapeutic agents for HD

Leticia Toledo-Sherman, PhD – CHDI

Huntington’s disease (HD) is associated with impaired energy metabolism. The activity of the pyruvate dehydrogenase complex (PDHC), a key component of the energetic-metabolic machinery of cells, has been found to be reduced in R6/2 mice at 12 weeks of age and in the caudate of human HD postmortem brains, with the most severe deficits correlating with disease progression. The PDHC is composed of three key enzymes, PDH (E1₂E1₂), E2 and E3, and an auxiliary binding protein, E3bp. This complex catalyses the conversion of pyruvate to acetyl coenzyme A, an essential molecule in many biological processes, including, fatty acid metabolism, cholesterol biosynthesis, histone acetylation, and the synthesis of the neurotransmitter acetylcholine. The PDHC is tightly regulated by two enzymes, pyruvate dehydrogenase kinase (PDHK), which down-regulates its activity via phosphorylation at any of three serine residues on the PDH component of the enzyme complex, and pyruvate dehydrogenase phosphatase, which dephosphorylates those residues resulting in activation of PDHC. Thus, inhibition of PDHK, and consequently up-regulation and restoration of PDHC activity, could be a promising therapeutic intervention in HD.

Evidence in support of this therapeutic strategy has been demonstrated with DCA, an inhibitor of PDHKs that stimulates the activity of the PDHC; treatment with DCA had beneficial effects in the R6/2 mice, showing significant increase in survival, improved motor function, delayed loss of body weight, and attenuation in the development of striatal neuron atrophy. However, given DCA’s poor drug profile, it is not a feasible molecule to prosecute as a therapeutic agent for HD. Fortuitously, there are safe PDHK inhibitors known, such as Nov3r and AZD7545, but these are not optimal CNS agents. Neither AZD7545 nor Nov3r are selective against PDHK1 and only Nov3r penetrates the brain. Thus, is desirable to obtain compounds which are not only brain penetrant, but also selective for PDHK2/3, the predominant neuronal isoforms of PDHK, and thus have a good safety-efficacy window. Taking these latter agents as benchmarks, CHDI in collaboration with AMRI has initiated a lead optimization program to deliver brain penetrant selective inhibitors of PDHK2/3 for proof-of-concept studies in HD. The team has developed, validated and recently implemented biochemical assays for PDHK1-4 isoforms, as well as a functional cell-based assay. To attain blood brain barrier permeability, ADME profiles and Pharmacokinetic analysis of the benchmark compounds has been used to identify the liabilities of these compounds and the physical properties that need to be optimized. Similarly, to attain selectivity, a thorough structural analysis of the crystal structures of all PDHK isosymes has been performed and molecular modeling methods are being applied to design novel compounds. Even though, this program is in early stages the team has already obtained novel compounds with inhibitory activities similar or better than the benchmark compounds that also display promising selectivity profiles. Progress on this program will be presented at the meeting.

4:00 PM

Molecular approaches to drug receptors: Relevance to HD

Solomon H. Snyder, MD – Johns Hopkins Medical School

Except for enzyme inhibitors, molecular strategies for drug development, especially in the neurosciences, were quite limited till the 1970s. Test tube monitoring of neurotransmitter uptake permitted advances, such as discriminating norepinephrine and serotonin uptake. Reversible ligand binding, exemplified in opiate receptors and then receptors for most neurotransmitters, led to an explication of many psychotropic drugs’ therapeutic and adverse actions. Similar approaches elucidated second messengers such as IP3 receptors. For HD, a major challenge has been to understand the selective striatal damage, despite the ubiquitous localizations of huntingtin. Binding of the small G protein Rhes, which is selectively enriched in the striatum, to mutant huntingtin (mHtt) mediates its sumoylation. Sumoylation disaggregates mHtt leading to augmented neurotoxicity. The Rhes-mHtt interaction may explain the striatal selectivity of HD neuropathology (Subramaniam et al Science 324: 1327, 2009). One might screen for drugs that selectively block Rhes-mHtt binding and thus could delay HD symptom onset. As Rhes is striatal selective, the risk of adverse effects for such drugs would be minimized. Rhes, like the G protein Rheb, activates mTOR and is the principal stimulus to striatal protein translation. Sequestration of Rhes by mHtt might diminish its access to mTOR, accounting for striatal shrinkage in HD.