Neuropathology of sporadic Parkinson disease before the appearance of parkinsonism: preclinical Parkinson disease

Abstract

Parkinson disease (PD) is no longer considered a complex motor disorder characterized by parkinsonism but rather a systemic disease with variegated non-motor deficits and neurological symptoms, including impaired olfaction, sleep disorders, gastrointestinal and urinary abnormalities and cardiovascular dysfunction, in addition to other symptoms and signs such as pain, depression and mood disorders. Many of these alterations appear before or in parallel with motor deficits and then worsen with disease progression. Although there is a close relation between motor symptoms and the presence of Lewy bodies (LBs) and neurites filled with abnormal α-synuclein, other neurological alterations are independent of LBs, thereby indicating that different mechanisms probably converge in the degenerative process. This review presents cardinal observations at very early stages of PD and provides personal experience based on the study of a consecutive series of brains with PD-related pathology and without parkinsonism, mainly cases categorized as stages 2-3 of Braak. Alterations in the substantia nigra, striatum and frontal cortex in pPD are here revised in detail. Early modifications in the substantia nigra at pre-motor stages of PD (preclinical PD: pPD) include abnormal small aggregates of α-synuclein which is phosphorylated, nitrated and oxidized, and which exhibits abnormal solubility and truncation. This occurs in association with a plethora of altered molecular events including increased oxidative stress, altered oxidative stress responses, altered balance of L-ferritin and H-ferritin, reduced expression of neuronal globin α and β chains in neurons with α-synuclein deposits, increased expression of endoplasmic reticulum stress markers, increased p62 and ubiquitin immunoreactivity in relation to α-synuclein deposits, and altered distribution of LC3 and other autophagosome/lysosome markers. In spite of the relatively small decrease in the number of dopaminergic neurons in the substantia nigra, which does not reach thresholds causative of parkinsonism, levels of tyrosine hydroxylase and cannabinoid 1 receptor are reduced, whereas levels of adenosine receptor 2A are increased in the caudate in pPD. Moreover, biochemical alterations are also present in the cerebral cortex (at least in the frontal cortex) in pPD including increased oxidative stress and oxidative damage to proteins α-synuclein, β-synuclein, superoxide dismutase 2, aldolase A, enolase 1, and glyceraldehyde dehydrogenase, among others, indicating post-translational modifications of PD-related proteins, and suggesting altered function of pathways involved in glycolysis and energy metabolism in the cerebral cortex in pPD. Current evidence suggests convergence of several altered metabolic pathways leading to chronic neuronal dysfunction, mainly manifested as sub-optimal energy metabolism, altered synaptic function, oxidative and endoplasmic reticulum stress damage and corresponding altered responses, among others. By understanding that these alterations occur at very early stages of PD and that neuronal fatigue and exhaustion may precede, for years, cell death and neuronal loss, we may direct therapeutic strategies towards the prevention and delay of disease progression starting at pre-parkinsonian stages of PD.