Kathy A. Steece-Collier, Ph.D. - US grants

Instmctions): Grafting of dopamine (DA) neurons provides benefit in some individuals with Parkinson's disease (PD), however, overall efficacy is less than would be predicted from the degree of DA replacement provided in many individuals. Similarly, while DA grafts in parkinsonian rats can completely reverse amphetamine-induced rotations, more complex motor behaviors often show little to no improvement. Many issues thought to underlie lack of graft success in PD are being investigated. Primary among these is low cell survival following grafting into the aged, parkinsonian brain. However, we hypothesize that there are critical factors not yet considered that contribute to the overall lack of graft success. Specifically, the primary site for afferent input of nigral DA and cortical glutamate neurons are medium spiny neurons (MSNs) within striatum. The numerous dendritic spines found on normal MSNs are critical sites of synaptic integration for DA and glutamate signaling. In advanced PD there is a marked atrophy of dendrites and spines on MSNs (McNeill, 1988; Zaja-Milatovic, 2005; Stephens, 2005). The premise of this project is that these severe morphological alterations will have grave consequences for cell replacement therapies despite the number of cells grafted. Pathological alterations of neuron structure would also be expected to negatively impact traditional dopamine replacement pharmacotherapies. Similar to PD, mice and rats with severe DA depletion also show significant decrease in spine density on MSNs. Importantly, a new mechanism involving dysregulation of intraspine Cavl.3 Ca2+ channels has been found to account for this spine loss. Indeed, absence of Cavl.3 channels in transgenic mice or administration of the Cavl.3 antagonist nimodipine to 6-OHDA lesioned rats can prevent spine loss in the presence of severe striatal DA depletion (Day, 2006). Identification of this mechanism allows testing the hypotheses put forth in this project: 1) degenerative changes in spine density of MSN has a detrimental impact on DA graft efficacy; 2) altered spine morphology plays a role in the development of levodopa-induced and/or DA graft-induced dyskinetic behaviors. The proposed studies will employ the well-established rat model of parkinsonism and dyskinesia. Using light and electron microscopic analyses and multiple behavioral profiles, we will compare therapeutic benefit and/or development of abnormal behaviors between DA-depleted rats with normal spine morphology to those with significant spine atrophy. We will further investigate how the risk factor of advanced age may impact potential dendritic spine regeneration. RELEVANCE (See Instructions): Project 1 will provide novel insight into the role of striatal pathology, specifically loss of dendritic spines on medium spiny output neurons, on dopamine replacement therapy. These studies may allow for improved treatment efficacy for patients with Parkinson's disease.

? DESCRIPTION (provided by applicant): Previous studies, including those in our lab have shown that subcutaneous, slow release pellets containing CaV1.2/1.3 channel antagonists can reduce the expression of levodopa-induced dyskinesias (LID) produced by low dose (6 mg/kg) and high dose levodopa (12.5 mg/kg), however, this effect is partial and lost over time. These data suggest that the CaV1.3 channel is a potential antidyskinetic target, yet whether the limitation in scope and loss of protection over time are related to pharmacological limitation remains unknown. There are several issues that limit validating the involvement of CaV1.3 channel antagonism for any use in Parkinson's disease (PD) with pharmacological agents, which includes: 1) there is no currently available pharmacological agent that can selectively silence CaV1.3 channels without impacting the CaV1.2 channels that are important in cardiovascular function; 2) CaV1.3 channels are incompletely inhibited even by high concentrations of currently available dihydropyridine (DHP) drugs ; and 3) pharmacological blockade traditionally employed results in non-continuous channel blockade, which we propose contributes to the variable or partial protective outcome of all previous clinical and preclinical studies. We posit that the previous studies provide strong and necessary rationale that the CaV1.3 channel is a potential antidyskinetic target and that development of an innovative approach to confirm its possible clinical utility is warranted. To provide unequivocal proof-of- principle evidence, devoid of pharmacological limitations, we propose three Specific Aims (SA) that will allow examination of the impact of continuous, high potency and target-selective, mRNA-level silencing of striatal CaV1.3 channel on LIDs, using the R21 mechanisms to assist in developing and executing these important studies. In SA 1, we will determine the time course of striatal CaV1.3 gene and protein silencing achieved with our recombinant adeno-associated virus (rAAV)-mediated expression of a short hairpin RNA (shRNA) designed against the CaV1.3 mRNA, which will guide the timing of interventions in SA 2 and 3. In SA 2, we will test the hypothesis that constitutive silencing of striatal CaV1.3 channels prior to levodopa exposure will provide potent and enduring amelioration of LIDs. In SA 3, we will test the hypothesis that constitutive silencing of striatal CaV1.3 channels in subjects already expressing LIDs will significantly decrease severity of established LIDs.

Project Summary One often debilitating side-effect of standard pharmacotherapy for Parkinson's disease (PD), levodopa administration, are unwanted involuntary movements known as levodopa-induced dyskinesia (LID). Eliminating LID remains a significant unmet need in PD therapy. There are currently no FDA approved drug treatments for LID, yet up to 90% of individuals with PD develop this side-effect. The L-type calcium channel CaV1.3 is a target of interest for LID prevention. Loss of striatal dopamine (DA) in PD results in dysregulation and overactivity of striatal CaV1.3 channels leading to synaptic pathology, including the loss of dendritic spines on striatal spiny projection neurons that appears to be involved in LID. While initial studies delivering pharmacological CaV1.3 channel antagonists reduced LID dose-dependently, the effects were partial and transient, with potential liability for cardiovascular side-effects due to the lack of specificity of existing drugs for the CaV1.3 channel. To provide unequivocal target validation, free of pharmacological limitations, we developed a rAAV-CaV1.3-shRNA to provide continuous, high potency, target-selective, mRNA-level silencing of striatal CaV1.3 channels. We examined whether genetic silencing of these dysregulated calcium channels could prevent LID induction in previously levodopa naïve parkinsonian rats and/or whether it could reverse these abnormal behaviors in parkinsonian rats already expressing a severe LID phenotype. In our `LID prevention studies' we found that gene level silencing of striatal CaV1.3 channels in severely parkinsonian rats, prior to the introduction of levodopa provides uniform and complete protection against the induction of LID, and that the antidyskinetic benefit is sustained over time even with high doses of daily levodopa. In our `LID reversal studies' we observed that rAAV-mediated CaV1.3 silencing in parkinsonian rats with already established LID could ameliorate these behaviors, with a one-week drug withdrawal 'drug holiday' appearing to be beneficial and/or necessary. Importantly this approach did NOT interfere with motor benefit of levodopa and showed a tendency to enhance motoric response to low dose levodopa. Gene delivery resulting in striatal CaV1.3 silencing provides some of the most profound antidyskinetic benefit reported to date. If these findings can be translated into a clinical application with a similar magnitude, this would provide a much-needed breakthrough in treatment of individuals with PD and would allow the most powerful antiparkinsonian therapy ever identified to work unabated through the duration of the disease. In the current application we propose a series of translational studies in rats and nonhuman primates that will allow us to expand upon these initial proof-of-principle studies and test specific hypotheses of safety and efficacy that will be required for the clinical development of genetic silencing of striatal CaV1.3 channels for LID.

While there are a number of therapeutic options for individuals with Parkinson's disease (PD) these therapies do not work uniformly well in all patients. Indeed, a recent retrospective analysis of the ELLDOPA study reported that early-stage PD subjects receiving equivalent levodopa doses experienced a magnitude of response ranging from a 100% improvement to a 242% worsening as assessed with the United Parkinson's Disease Rating Scale part III (UPDRS-III, motor subscore). This example underscores the incredible heterogeneity in clinical response to standard-of-care anti-parkinsonian therapy, even when disease severity is taken into account. Similar findings have been reported over the past several decades for the experimental regenerative approach of neural grafting. While some PD patients have shown marked and lasting benefit following engraftment of primary dopamine (DA) neurons, many have also shown no or limited benefit. Recent preclinical data in aged parkinsonian rats together with that from two milestone clinical reports provide compelling and sobering data demonstrating that even when robust survival of grafted DA neurons and extensive neurite outgrowth is achieved, obstacle(s) remain that interfere with functional circuit restoration within the aged, parkinsonian brain. As clinical grafting trials are reemerging, it remains uncertain what specific risk factors negatively impact clinical responsiveness to DA terminal remodeling. In attempt to deconstructing the complexity of PD and response to therapy, we recently identified one candidate genetic variant, with prevalence of up to 40% in the human population, which may prove useful in this regard; specifically, a functional single nucleotide polymorphism (SNP) rs6265 in the Bdnf gene for brain-derived neurotrophic factor (BDNF) that results in dysfunctional BDNF release. We have recently observed diminished therapeutic efficacy of oral levodopa in two distinct cohorts of PD patients with this SNP risk allele. In the current application we propose to test the hypothesis that this risk allele also underlies the variability in clinical response to DA neuron grafting in PD patients. Specifically, we hypothesize that BDNF is an unrecognized contributor to the discordant finding of abundant survival of grafted DA neurons and lack of behavioral efficacy reported in a subpopulation of PD patients and in association with normal aging in parkinsonian rats. In this application we propose three Specific Aims to test the overarching hypothesis that impaired BDNF signaling, either through this common SNP and/or advanced age, is a key factor in limiting functional DA terminal remodeling. Toward this end, we have generated a knock-in rat model of the human rs6265 BDNF variant and propose to use this novel tool to characterize its effects and interaction with aging and DA-depletion on the function and synaptic integration of new DA terminals in the parkinsonian striatum using neural grafting as a model system.