National Parkinson Foundation

National Parkinson Foundation is using its 2017 distribution to fund:

D620N VPS35 knockin mice: a new model of familial Parkinson’s disease

PROJECT TITLE:  D620N VPS35 knockin mice: a new model of familial Parkinson’s disease

Investigators/Authors:  Xi Chen, PhD (mentor: Darren Moore, PhD); Van Andel Research Institute

Objective: The goal of this project is to study the motor function and neuronal perturbations that occur with advancing age in a new model PD mouse, the D620N VPS35 knockin.

Background: Mutations in PARK17, a gene that makes a protein named VPS35, have recently been associated with PD, but until now, it was not clear how they could contribute to the development of the disease. VPS35 plays a central role in a neuron’s protein recycling center as part of the so- called retromer system which helps to shuttle finished proteins to their correct destinations in cells. Most prior research has focused on model systems that lack the gene or overexpress human VPS35. This study aims to test a new mouse model that expresses mutated mouseVPS35 at physiological expression levels.

Methods/Design: We will study motor behavior, dopamine levels, dopaminergic neuronal loss and alterations in neuronal morphology in these mice. Locomotor activity will be monitored using an automated video tracking system. Motor coordination and balance will be assessed by rotarod analysis. Gait analysis will be conducted using a digital Catwalk system. We will measure cadence, stepping pattern, base of support, speed and run duration. We will conduct neurodegenerative, neurochemical and neuropathological assessments at 6, 12, 18 and 24 months of age to correlate these features with the motor assessments and develop a behavioral/pathological profile. We will measure markers of neuropathology, including tau and alpha-synuclein and related pathology. We will also evaluate the impact of the D620N VPS35 mutation on the time-course and pattern of alpha-synuclein propagation, aggregation and dopaminergic neurodegeneration following intra-striatal delivery of mouse alpha-synuclein fibrils.

Relevance to Diagnosis/Treatment of Parkinson’s disease: This study will examine how mutated VPS35 might cooperate with either αSyn or tau to harm dopamine neurons in PD. Understanding these interactions could help develop new drugs that interfere with the actions of either or both proteins to prevent or treat PD.

September 2018 Project Update:

We have so far characterized the VPS35 knockin (KI) mouse model up to 15 months of age. This new mouse model has normal amounts of mouse VPS35, but it is mutated so that the protein does not function properly. We provide evidence that the Parkinson’s disease (PD)-causing D620N mutation is sufficient to reproduce neuropathological hallmarks of PD as indicated by an age-dependent loss of dopamine-producing nerve cells and widespread axonal pathology. Furthermore, D620N KI mice continue to exhibit dysfunctional tau protein, an Alzheimer's disease hallmark, with advancing age. With regard to how the mutation affects alpha-synuclein - a protein in the brain that, when it accumulates abnormally into clumps, is linked to Parkinson's, we find that D620N VPS35 expression does not induce alpha-synuclein neuropathology and has no effect on the alpha-synuclein-induced phenotypes that develop in PD model mice. Therefore, VPS35 KI mice do not have an obvious pathological interaction with alpha-synuclein. D620N VPS35 KI mice are one of the first models of inherited single gene PD that develop the robust and progressive loss of dopamine-producing nerve cells.

September 2019 Project Update:

The cause of Parkinson’s disease (PD) is mostly unknown, but genes, lifestyle and environmental factors are all thought to be involved. In rare instances, an inherited genetic mutation causes PD. In recent years, researchers have identified rare mutations in genes that can directly cause PD or increase the risk of developing the disease. Mutations in the VPS35 gene have recently been associated with inherited PD. However, it is not yet clear how they could contribute to the development of the disease. Most prior research has focused on model systems that lack the gene or overproduce VPS35. We are studying the behavior and neuronal perturbations that occur with advancing age in a highly relevant and new mouse model (D620N VPS35 KI) that expresses normal amounts of mouse VPS35, but it is mutated so that the protein does not function properly. We provide evidence that the PD-causing D620N mutation is sufficient to reproduce neuropathological hallmarks of PD as indicated by an age-dependent loss of dopamine-producing nerve cells and widespread axonal pathology. Furthermore, D620N VPS35 KI mice continue to exhibit dysfunctional microtubule-associated protein tau, an Alzheimer's disease hallmark and a major genetic risk factor for sporadic PD, with advancing age. However, D620N mutation does not lead to overt dendritic spine damages.  Nerve cell dendritic spines are nerve cell projections that act as the bridges connecting one neuron to another via the synapses, and previous research has shown D620N VPS35 expression might result in synaptic defects.
 
Researchers have recently demonstrated that abnormal interaction between proteins implicated could lead to genetic changes that are found in people with PD. Thus we want to understand whether and how mutated VPS35 might cooperate with either α-synuclein (αSyn), a protein in the brain that, when it accumulates abnormally into clumps, is linked to PD, or tau to harm dopamine neurons in PD. Understanding their interactions could help develop new drugs that interfere with the actions of these proteins to prevent or treat PD. With regard to how the mutation affects αSyn, we find that D620N VPS35 expression does not induce αSyn neuropathology and has no effect on the αSyn-induced phenotypes that develop in PD model mice. Therefore, D620N VPS35 KI mice do not have an obvious pathological interaction with αSyn. To further explore the relationship between VPS35 and tau in inducing neurodegeneration in PD, we employed a more rapid and selective mouse model that develops tau pathology and neuronal loss restricted to a focused brain region by unilaterally delivering mutant tau to hippocampus region of D620N VPS35 KI mice at various viral titers. In addition, we are also monitoring the neuron-to-neuron transmission of tau within defined hippocampal neuronal circuits.

In summary, D620N VPS35 KI mice are one of the first models of inherited single gene PD that develop the robust and progressive loss of dopamine-producing nerve cells. We have made good progress in achieving our stated Aims over the past two years. We are continuing to optimize and improve the AAV-tau D620N VPS35 KI mouse models in our ongoing studies to be able to initiate future studies which may have important implications for understanding the development of tau neuropathology in VPS35-linked PD brains.