Phenotypic screens for compounds that target the cellular pathologies underlying Parkinson's disease

Parkinson's disease (PD) is a devastating neurodegenerative disease that affects over one million patients in the US. Yet, no disease modifying drugs exist, only those that temporarily alleviate symptoms. Because of its poorly defined and highly complex disease etiology, it is essential to embrace unbiased and innovative approaches for identifying new chemical entities that target the underlying toxicities associated with PD. Traditional target-based drug discovery paradigm can suffer from a bias toward a small number of potential targets. Phenotypic screening of both genetic and pharmacological PD models offers an alternative approach to discover compounds that target the initiating causes and effectors of cellular toxicity. The relative paucity of reported phenotypic screens illustrates the intrinsic difficulty in establishing model systems that are both biologically meaningful and adaptable to high-throughput screening. Parallel advances in PD models and in vivo screening technologies will help create opportunities for identifying new therapeutic leads with unanticipated, breakthrough mechanisms of action.

Introduction

Parkinson's disease (PD) is the second most common neurodegenerative disease, affecting ∼2% of the population over 65 years of age. Patients suffer from progressive loss of muscle control, including tremor, rigidity, bradykinesia, and postural instability. These movement deficits are largely caused by the selective degeneration of dopaminergic (DA) neurons in a brain region called the substantia nigra pars compacta. The histopathological hallmark of PD is the presence of large intracytoplasmic spherical structures (called Lewy Bodies, LBs) that contain the α-synuclein (α-syn) protein [1]. The misfolding and accumulation of a disease-specific protein that causes proteotoxic stress is a common theme among many neurodegenerative diseases, including Alzheimer's and Huntington's diseases. Although most PD arises sporadically, α-syn pathology is almost always detected. Importantly, a causal role for α-syn was strengthened with the identification of rare mutations in the α-syn gene (SNCA) [2]. Extra genomic copies of SNCA also cause familial PD, establishing that simply increasing α-syn dose can cause disease [3]. Intriguingly, aging, which is the most significant risk factor for PD, is accompanied by an increase in α-syn protein levels, possibly sensitizing cells to α-syn misfolding and toxicity [4, 5].

The genetic complexity of PD is exceptionally rich as several other monogenic mutations cause related parkinsonisms. Some mutations, such as those in Parkin, PARK3, LRRK2, PLA2G6, and GBA (Gaucher's locus), accumulate α-syn-positive Lewy Bodies. Others, including mutations in UCHL1, DJ-1, ATP13A2, PARK10, GIGYF2, PARK12, HTRA2, FBX07, PARK16, have less defined α-syn involvement [6]. The diverse functions of these disease-associated genes creates a complexity where different pathological mechanisms, including ubiquitin–proteasome system, oxidative stress, vesicle trafficking, and mitochondrial dysfunction, manifest in related parkinsonisms. Also, several environmental substances, including the mitochondrial toxins MPTP/MPP⁺ and rotenone, as well as manganese, can cause related parkinsonisms.

Model organism studies of ‘PD’ have provided key insights into some basic, underlying toxicities of PD. They do not, however, truly model ‘disease’, which integrates complex interactions between cellular pathology, neuronal networks, multiple systems, and patient heterogeneity. This distinction is crucial in establishing dialogs between basic researchers and clinicians treating PD patients. Herein, we refer to ‘PD models’ without making claims to the actual human disease, but rather to the underlying precipitating toxic events.

Model organisms used to study PD have provided key molecular insights into the underlying cellular pathology. The accumulation of α-syn has been linked to mitochondrial dysfunction, proteasome inhibition, oxidative stress, vesicle trafficking defects, lipid droplet accumulation, calcium dysregulation, α-syn aggregation, and cellular toxicity. These phenotypes are studied in several model systems, including immortalized cell lines, primary neuronal cultures, yeast, fruit flies, nematodes, and rodents [7, 8]. Although no model organism faithfully recapitulates all cellular pathologies, the cross-validation of findings in multiple systems strengthens new connections. Indeed, the combination of diverse cellular pathologies with the aforementioned genetic complexity and environmental links necessitates innovative, unbiased phenotypic screening approaches in simple model organisms.

Despite the prevalence of PD and the substantial efforts in studying disease pathogenesis, no disease modifying agents exist. Current therapies largely manage symptoms through modulating neuronal activity, yet do not significantly modify disease progression [9]. Phenotypic screening provides an opportunity to both study toxic mechanisms and to provide new chemical entities that target the precipitating biology and may themselves modify disease. Herein, we will discuss the promise of phenotypic screens and early successes that can enrich the pipeline for potential therapeutics. We will not discuss the other significant hurdles in drug development, including compound optimization, the difficulties in testing drugs in a slowly progressing disease, or other approaches such as structure-based design.