A Mouse Model to Test Novel Therapeutics for Parkinson's Disease: an Update on the Thy1-aSyn ("line 61") Mice

Abstract

Development of neuroprotective therapeutics for Parkinson's disease (PD) is facing a lack of translation from pre-clinical to clinical trials. One strategy for improvement is to increase predictive validity of pre-clinical studies by using extensively characterized animal models with a comprehensive set of validated pharmacodynamic readouts. Mice over-expressing full-length, human, wild-type alpha-synuclein under the Thy-1 promoter (Thy1-aSyn line 61) reproduce key features of sporadic PD, such as progressive loss of striatal dopamine, alpha-synuclein pathology, deficits in motor and non-motor functions, and elevation of inflammatory markers. Extensive work with this model by multiple laboratories over the past decade further increased confidence in its robustness and validity, especially for analyzing pathomechanisms of alpha-synuclein pathology and down-stream pathways, and for pre-clinical drug testing. Interestingly, while postnatal transgene expression is widespread in central and peripheral neurons, the extent and progression of down-stream pathology differs between brain regions, thereby replicating the characteristic selective vulnerability of neurodegenerative diseases. In-depth characterization of these readouts in conjunction with behavioral deficits has led to more informative endpoints for pre-clinical trials. Each drug tested in Thy1-aSyn line 61 enhances knowledge on how molecular targets, pathology, and functional behavioral readouts are interconnected, thereby further optimizing the platform towards predictive validity for clinical trials. Here, we present the current state of the art using Thy1-aSyn line 61 for drug target discovery, validation, and pre-clinical testing.

Keywords: Alpha-synuclein; Neuroprotection; Parkinson’s disease; Progressive; Therapy.

Fig. 1

Exemplified progression of neurodegenerative processes and therapeutic interventions. For a specific neuron/neuronal population, there are early processes that predispose to neurodegeneration, which induce several downstream secondary processes that with time progress independently from the primary cause (e.g., GBA mutation to lysosomal dysfunction to proteinopathy to mitochondrial damage). Together with additional processes that initiate by a toxic event (pesticides, virus [17]), they create a burden that ultimately cumulates in cell death (apoptosis). Alpha-synuclein (aSyn) pathology may spread to other cells and induce degenerative processes. Cell debris from dying neurons initiate microgliosis and release of cytotoxic cytokines. At time of diagnosis, the majority of dopamine neurons is degenerated and the remaining neurons degenerate in a few years [18]. Neuroprotective therapy would have to substantially halt several neurodegenerative processes and probably reduce microgliosis for overt benefits on dopamine related endpoints. Solutions to enhance neuroprotective benefits on motor symptoms could be (i) starting prior to overt dopamine neuron loss, if early diagnosis becomes more reliable, or (ii) target several degenerative processes and the spreading to less affected neurons, which would require drug combinations. Alternatively, adapt clinical readouts away from classical motor symptoms to more sensitive pharmacodynamics biomarkers, if they become available, including non-motor symptoms projected to develop in stratified patient groups (e.g., cognitive decline in GBA mutation carriers). To address and foresee ongoing efforts and successes to overcome these challenges, pre-clinical studies need to cover dopamine related endpoints in combination with a comprehensive set of pharmacodynamic readouts to be informative for current and future clinical trial designs

Fig. 2

Disease model validity and drug effects. Construct validity describes how closely the make-up of the model replicates etiology and pathophysiology of the disease (e.g., SNCA triplication — overexpression). Face validity describes how the model replicates the down-stream pathology (dopamine loss) and the symptoms (motor, non-motor). Predictive validity describes whether the model can correctly predict drug effects in patients (e.g., established for toxin models for L-dopa symptomatic effects). Neuroprotective/disease-modifying drugs target the disease progression early at the construct, e.g., inhibition of aggregation, while symptomatic drugs target at face validity (dopamine replacement). A single drug may target different pathways/brain regions at early pathophysiology, thereby impacting different symptoms and pathologies (e.g., limbic system versus nigrostriatal system), and may even have an additional symptomatic effect (on- or off-target). Disease-modifying drugs that in addition have symptomatic effects on motor deficits have a higher probability to cross the threshold of sufficient clinical efficacy. It would be useful to detect such multiple effects on readout at the pre-clinical stage. In clinical trials, changes in the brain cannot be directly measured or observed, therefore specific clinical trial designs are necessary to distinguish slowing of disease progression from symptomatic effects. Well-characterized models with defined connections between construct and face validity can help to identify the drug target underlying effects on symptoms and pathology

Fig. 3

Thy1-aSyn line 61 summary of pathological and behavioral phenotypes developed with age. DA, dopamine (red is extracellular dopamine measured via microdialysis, blue is overall tissue dopamine); PK/PD, pharmacokinetics/pharmacodynamics with exposure–response relationship; POP, proof of principle how a drug with a specific mode of action alters pharmacodynamic responses. Early and progressive pathologies linked to robust functional behavioral readouts allow testing at young ages. Testing closer to age at dopamine loss (14 months) allows measuring delay of striatal neurodegeneration, or pharmacodynamic response under the condition of dopamine depletion. Ctx, cortex; HC, hippocampus; MAP-2, microtubule-associated protein 2; NE, norepinephrine; p-aSyn, phosphorylated alpha-synuclein; SN, substantia nigra; Str, striatum; TH, tyrosine hydroxylase; TNFa, tumor necrosis factor alpha