Out FOXing Parkinson Disease: Where Development Meets Neurodegeneration

Abstract

Parkinson disease (PD) is the most common neurodegenerative movement disorder in humans, but its cause in the majority of patients is unknown. Fewer than 10% of cases appear to be inherited, and only rarely are the known Mendelian mutations identified in patients with PD. In addition to genetic factors, environmental toxins—especially those affecting mitochondrial function—have also been implicated [1,2], demonstrating a clear role for gene–environment interactions in most cases of sporadic PD. Patients afflicted with PD experience slow, selective, often asymmetric loss of dopaminergic (DA) neurons projecting from the substantia nigra (SN) to the striatum, but not of DA neurons located elsewhere in the brain (e.g., ventral tegmental area and retina). Once more than 80%–90% of DA neurons in the SN are lost, patients suffer the characteristic symptoms of tremor, slow movements (bradykinesia), rigidity, and postural instability. To study PD, researchers have used two basic types of animal models: (1) acute lesioning of DA neurons/tracts, either by surgery or toxic insult, and (2) genetic models that rely on expression of one of the rare, dominant, and highly penetrant mutations that cause PD or knockout of a similar, recessively inherited allele (e.g., α-synuclein, Parkin, and leucine-rich repeat kinase-2 [LRRK2]). Although these models have led to huge advances in our understanding of PD—implicating mitochondrial dysfunction, oxidative damage, and protein handling systems in pathogenesis—they typically either fail to recapitulate the gradual nature of this degenerative disease or lack the defining pathology (e.g., Parkin and α-synuclein transgenic mice fail to show progressive nigrostriatal degeneration, although rotenone-treated mice do show Lewy bodies (intracytoplasmic inclusions that are composed primarily of α-synuclein and ubiqutin)) [3,4]. Because no single animal model is likely to be sufficient for the study of any particular human neurodegenerative disease, new models are needed to accelerate our ability to devise and test improved disease-altering therapies. It is in this regard that a new paper in PLoS Biology by Kittappa et al. [5] warrants special attention. They demonstrate that knockout of a transcription factor FoxA2, which is critical for DA neuron specification and survival in mice causes a late-onset, asymmetric degenerative condition affecting motor systems in a manner very similar to PD. This model is entirely distinct in its basic mechanism from the aforementioned models, because it involves partial loss of a transcription factor that is critical for DA neuron cell fate. One of the most striking features of the majority of adult-onset neurodegenerative diseases is their regional or cell-type specificity in humans. Yet, many of the known causative mutations reside in proteins that are widely expressed throughout the central nervous system, such as synuclein, LRK2, and Parkin in PD, beta amyloid and presenilin-1 (PS1) in Alzheimer disease, and tau and progranulin in frontotemporal dementia. The current study in mice, and previous work in humans and other model organisms, highlight the potential role that neurodevelopmental factors may play in the cell-type–specific or regional neuronal vulnerability seen in neurodegenerative disease [6,7] As this work by Kittappa and colleagues demonstrates, understanding early developmental processes such as cell-type specification and survival may have major implications for aging-related diseases.