Audrey S. Dickey

Mechanism of neuroprotective mitochondrial remodeling by PKA/AKAP1.

The mitochondrial signaling complex PKA/AKAP1 protects neurons against mitochondrial fragmentation and cell death by phosphorylating and inactivating the mitochondrial fission enzyme Drp1.

PKA/AKAP1 and PP2A/Bβ2 Regulate Neuronal Morphogenesis via Drp1 Phosphorylation and Mitochondrial Bioenergetics

Mitochondrial shape is determined by fission and fusion reactions, perturbation of which can contribute to neuronal injury and disease. Mitochondrial fission is catalyzed by dynamin-related protein 1 (Drp1), a large GTPase of the dynamin family that is highly expressed in neurons and regulated by various posttranslational modifications, including phosphorylation. We report here that reversible phosphorylation of Drp1 at a conserved Ser residue by an outer mitochondrial kinase (PKA/AKAP1) and phosphatase (PP2A/Bβ2) impacts dendrite and synapse development in cultured rat hippocampal neurons. PKA/AKAP1-mediated phosphorylation of Drp1 at Ser656 increased mitochondrial length and dendrite occupancy, enhancing dendritic outgrowth but paradoxically decreasing synapse number and density. Opposing PKA/AKAP1, PP2A/Bβ2-mediated dephosphorylation of Drp1 at Ser656 fragmented and depolarized mitochondria and depleted them from dendrites, stunting dendritic outgrowth but augmenting synapse formation. Raising and lowering intracellular calcium reproduced the effects of dephospho-Drp1 and phospho-Drp1on dendrite and synapse development, respectively, while boosting mitochondrial membrane potential with l-carnitine-fostered dendrite at the expense of synapse formation without altering mitochondrial size or distribution. Thus, outer mitochondrial PKA and PP2A regulate neuronal development by inhibiting and promoting mitochondrial division, respectively. We propose that the bioenergetic state of mitochondria, rather than their localization or shape per se, is the key effector of Drp1, altering calcium homeostasis to modulate neuronal morphology and connectivity.

PPAR-δ is repressed in Huntington's disease, is required for normal neuronal function and can be targeted therapeutically.

Huntingtons disease (HD) is a progressive neurodegenerative disorder caused by a CAG trinucleotide repeat expansion in the huntingtin (HTT) gene, which encodes a polyglutamine tract in the HTT protein. We found that peroxisome proliferator-activated receptor delta (PPAR-δ) interacts with HTT and that mutant HTT represses PPAR-δ–mediated transactivation. Increased PPAR-δ transactivation ameliorated mitochondrial dysfunction and improved cell survival of neurons from mouse models of HD. Expression of dominant-negative PPAR-δ in the central nervous system of mice was sufficient to induce motor dysfunction, neurodegeneration, mitochondrial abnormalities and transcriptional alterations that recapitulated HD-like phenotypes. Expression of dominant-negative PPAR-δ specifically in the striatum of medium spiny neurons in mice yielded HD-like motor phenotypes, accompanied by striatal neuron loss. In mouse models of HD, pharmacologic activation of PPAR-δ using the agonist KD3010 improved motor function, reduced neurodegeneration and increased survival. PPAR-δ activation also reduced HTT-induced neurotoxicity in vitro and in medium spiny-like neurons generated from stem cells derived from individuals with HD, indicating that PPAR-δ activation may be beneficial in HD and related disorders.Huntington’s disease (HD) is a progressive neurodegenerative disorder caused by a CAG-polyglutamine repeat expansion in the huntingtin (htt) gene. We found that peroxisome proliferator-activated receptor delta (PPARδ) interacts with htt and that mutant htt represses PPARδ-mediated transactivation. Increased PPARδ transactivation ameliorated mitochondrial dysfunction and improved cell survival of HD neurons. Expression of dominant-negative PPARδ in CNS was sufficient to induce motor dysfunction, neurodegeneration, mitochondrial abnormalities, and transcriptional alterations that recapitulated HD-like phenotypes. Expression of dominant-negative PPARδ specifically in the striatum of medium spiny neurons in mice yielded HD-like motor phenotypes, accompanied by striatal neuron loss. In mouse models of HD, pharmacologic activation of PPAR δ, using the agonist KD3010, improved motor function, reduced neurodegeneration, and increased survival. PPAR δ activation also reduced htt-induced neurotoxicity in vitro and in medium spiny-like neurons generated from human HD stem cells, indicating that PPAR δ activation may be beneficial in individuals with HD and related disorders.

PPARδ activation by bexarotene promotes neuroprotection by restoring bioenergetic and quality control homeostasis

PPARδ activation by bexarotene in a mouse model of Huntington’s disease rescues defective oxidative metabolism and is neuroprotective. Defeating neurotoxicity with a repurposed drug PPARδ is a permissive nuclear receptor that heterodimerizes with the retinoid X receptor (RXR) to activate target genes. Interference with transcription of PPARδ target genes contributes to neurodegeneration in Huntington’s disease (HD). In new work, Dickey et al. evaluated the RXR agonist bexarotene in cellular models of HD and in an HD mouse model. They determined that bexarotene was effective at countering HD neurotoxicity in mouse primary neurons, human HD patient stem cell–derived neurons, and the BAC-HD mouse model. The authors then examined the basis for PPARδ’s neuroprotective effect and found that treatment with RXR/PPARδ agonists enhanced oxidative metabolism, promoted mitochondrial quality control, and boosted protein homeostasis by activating autophagy. Neurons must maintain protein and mitochondrial quality control for optimal function, an energetically expensive process. The peroxisome proliferator–activated receptors (PPARs) are ligand-activated transcription factors that promote mitochondrial biogenesis and oxidative metabolism. We recently determined that transcriptional dysregulation of PPARδ contributes to Huntington’s disease (HD), a progressive neurodegenerative disorder resulting from a CAG-polyglutamine repeat expansion in the huntingtin gene. We documented that the PPARδ agonist KD3010 is an effective therapy for HD in a mouse model. PPARδ forms a heterodimer with the retinoid X receptor (RXR), and RXR agonists are capable of promoting PPARδ activation. One compound with potent RXR agonist activity is the U.S. Food and Drug Administration–approved drug bexarotene. We tested the therapeutic potential of bexarotene in HD and found that bexarotene was neuroprotective in cellular models of HD, including medium spiny-like neurons generated from induced pluripotent stem cells (iPSCs) derived from patients with HD. To evaluate bexarotene as a treatment for HD, we treated the N171-82Q mouse model with the drug and found that bexarotene improved motor function, reduced neurodegeneration, and increased survival. To determine the basis for PPARδ neuroprotection, we evaluated metabolic function and noted markedly impaired oxidative metabolism in HD neurons, which was rescued by bexarotene or KD3010. We examined mitochondrial and protein quality control in cellular models of HD and observed that treatment with a PPARδ agonist promoted cellular quality control. By boosting cellular activities that are dysfunctional in HD, PPARδ activation may have therapeutic applications in HD and potentially other neurodegenerative diseases.

Therapy development in Huntington disease: From current strategies to emerging opportunities

Huntington disease (HD) is a progressive autosomal dominant neurodegenerative disorder in which patients typically present with uncontrolled involuntary movements and subsequent cognitive decline. In 1993, a CAG trinucleotide repeat expansion in the coding region of the huntingtin (HTT) gene was identified as the cause of this disorder. This extended CAG repeat results in production of HTT protein with an expanded polyglutamine tract, leading to pathogenic HTT protein conformers that are resistant to protein turnover, culminating in cellular toxicity and neurodegeneration. Research into the mechanistic basis of HD has highlighted a role for bioenergetics abnormalities stemming from mitochondrial dysfunction, and for synaptic defects, including impaired neurotransmission and excitotoxicity. Interference with transcription regulation may underlie the mitochondrial dysfunction. Current therapies for HD are directed at treating symptoms, as there are no disease‐modifying therapies. Commonly prescribed drugs for involuntary movement control include tetrabenazine, a potent and selective inhibitor of vesicular monoamine transporter 2 that depletes synaptic monoamines, and olanzapine, an atypical neuroleptic that blocks the dopamine D2 receptor. Various drugs are used to treat non‐motor features. The HD therapeutic pipeline is robust, as numerous efforts are underway to identify disease‐modifying treatments, with some small compounds and biological agents moving into clinical trials. Especially encouraging are dosage reduction strategies, including antisense oligonucleotides, and molecules directed at transcription dysregulation. Given the depth and breadth of current HD drug development efforts, there is reason to believe that disease‐modifying therapies for HD will emerge, and this achievement will have profound implications for the entire neurotherapeutics field.

Huntington’s Disease and Other Polyglutamine Repeat Diseases: Molecular Mechanisms and Pathogenic Pathways

Abstract Polyglutamine diseases are inherited, fatal neurodegenerative disorders caused by genomic expansion of exonic cytosine–adenine–guanine (CAG) trinucleotide repeats. These diseases include Huntington’s disease, spinal and bulbar muscular atrophy, dentatorubral–pallidoluysian atrophy, and spinocerebellar ataxia types 1, 2, 3, 6, 7, and 17, each due to CAG expansion in a different gene. Common themes regarding mechanisms of neurodegeneration include protein aggregation, intracellular protein degradation systems, proteolytic processing, mitochondrial dysfunction, nuclear trafficking and subcellular localization, transcriptional dysregulation, and posttranslational modifications. Strategies for developing therapeutics for this class of neurodegenerative diseases include RNA interference, antisense oligonucleotides, zinc finger proteins, and genome editing.

Transcription Modulation of Mitochondrial Function and Related Pathways as a Therapeutic Opportunity in Parkinson’s Disease

As outlined in prior chapters, the etiology of Parkinson’s disease (PD) is complex, involving both genetic and environmental factors, necessitating the development of multiple models to study PD. Mounting evidence has established that α-synuclein plays a central role in PD pathogenesis (Brain Pathology 9:707–720, 1999). In α-synucleinopathies, toxic oligomeric forms of α-synuclein may target intracellular organelles and cellular pathways, including mitochondria, the ubiquitin–proteasome system, and the autophagy–lysosome pathway, leading to neuron dysfunction and cell death. In addition, mitochondrial toxins have been identified in epidemiological studies as contributing to “sporadic” PD, and following the discovery of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced parkinsonism, mitochondrial-based toxin models (paraquat, maneb, rotenone) were developed (Science 219:979–980, 1983; Cell 155:1351–1364, 2013). Mitochondrial dysfunction is thus a common pathological hallmark of PD and may contribute to disease pathogenesis along with altered mitochondrial turnover or reduced mitochondrial biogenesis. Thus, regulators of mitochondrial function are appealing targets for therapeutic strategies to halt or reverse PD neurodegeneration.