PD is a multisystemic disease of the nervous system
James Parkinson described the disease in 1817 with the term shaking palsy. In 1920, Frederick Lewy described the typical neuronal inclusions, later named Lewy bodies. Konstatin Tretiakoff (in 1919) and Rolf Hassler (in 1938) established that the substantia nigra was the main cerebral area affected. In 1950, Arvid Carlsson identified dopamine as the major biochemical defect in the substantia nigra and causative of the dopaminergic denervation of the basal ganglia. The first trials with levo-dopa were carried out in 1967–1968.
The discovery of dopamine defects and the therapeutic effects of dopa or agonists have been central to our understanding and treatment of major motor symptoms (parkinsonian symptoms) in PD. Pioneering and subsequent studies support the importance of molecular and biochemical studies of the brain in human neurodegenerative diseases.
However, PD is not simply a motor disorder. Rather, olfactory dysfunction, dysautonomia, sleep fragmentation, rapid eye movement behaviour disorder, mood and anxiety disorders, and depression are common in PD; these alterations may precede parkinsonian symptoms and they usually increase in intensity with the progression of the disease. Changes in personality and moderate or mild cognitive debilitation are found in PD. Neuropsychiatric alterations and cognitive decline may occur at early stages of parkinsonism, suggesting that they are an integral part of PD from the beginning of the disease in some patients. Characteristically, symptoms are often subtle at the beginning and difficult to detect without neuropsychological tests, although they become aggravated with the progression of the disease. Deficits mainly affect executive function, including working memory and visuospatial capacity.
A correlation between non-motor alterations and neuropathological substrates has been established using several instrumental classifications of LB pathology that help to explain the progression from the medulla oblongata (and olfactory bulb) to the midbrain, diencephalic nuclei and neocortex . Stage 1 is characterized by LBs and neurites in the dorsal IX/X motor nuclei and/or intermediate reticular zone; there is also myentheric plexus involvement. Stage 2 affects the medulla oblongata and pontine tegmentum and covers the pathology of stage 1 plus lesions in the caudal raphe nuclei, gigantocellular reticular nucleus and ceruleus–subceruleus complex; the olfactory bulb is also involved. Stage 3 refers to the pathology of stage 2 plus midbrain lesions, particularly in the pars compacta of the substantia nigra. Stage 4 includes basal prosencephalon and mesocortex pathology (i.e. cortical involvement confined to the transentorhinal region and allocortex, and CA2 plexus) in addition to lesions in the midbrain, pons and medulla oblongata. Stage 5 extends to sensory association areas of the neocortex and prefrontal neocortex. In addition, stage 6 includes lesions in first-order sensory association areas of the neocortex and pre-motor areas; occasionally, there are also mild changes in primary sensory areas and the primary motor field.
The involvement of distinct areas and regions of the central, peripheral and autonomous nervous system indicates that PD is a multisystemic disease of the nervous system. Although several atypical cases (approximately 30%) do not follow a clear gradient of LB pathology from the medulla oblongata to the neocortex, Braak classification has been useful for proposing an instrumental approach that suggests spread or progression of the disease in most cases.
An important point is that motor symptoms and certain non-motor symptoms correlate with LB pathology at specific regions of the peripheral and central nervous system, whereas others do not. As a paradigm of this assertion, cognitive impairment and dementia in PD barely correlate with LB pathology in the cerebral cortex , thus inferring that other factors, in addition to altered α-synuclein, probably play key roles in their pathogenesis.
On the other hand, cases with LB pathology in the brain stem without parkinsonism are considered as incidental PD or pre-motor PD [17–19]. Recent studies have shown biochemical alterations in various regions of the nervous system even at these very early stages of the disease.
Taken together, these observations suggest that biochemical rather than morphological studies represent an appropriate approach for increasing our understanding of the molecular pathology of non-motor symptoms in PD.
Below, we report information obtained from the biochemical and molecular studies of human autopsy samples stored in brain banks focused on the cerebral cortex in PD.
Neurochemical alterations related to cognitive impairment in PD
Studies in recent years have changed our understanding of the many mechanisms involved in the deterioration of cognition in PD. On the one hand, neuroimaging studies have shown altered dopaminergic, serotoninergic, cholinergic and noradrenergic innervation of the cerebral cortex, thus providing robust information related to impaired cortical inputs representing major deficits in cortical function. On the other hand, evidence is available showing intrinsic abnormalities in the cerebral cortex in PD even at pre-motor stages of the disease when the brains are analyzed at post-mortem using appropriate methods .
Intrinsic cortical alterations are summarized below.
Mitochondria and energy machinery failure
Mitochondria and energy machinery failure are well-known abnormalities in the substantia nigra in PD, and the mutation of several genes encoding proteins related to the mitochondria is causative of familial PD. Recent biochemical studies have revealed mitochondrial abnormalities in the cerebral cortex as well, including decreased brain cortex and mitochondrial O2 uptake and reduced complex I activity in PD. This is accompanied by a higher mitochondrial nitric oxide synthase activity, cytochrome content, expression of superoxide dismutase (SOD)2 and oxidative damage in PD compared to age-matched controls .
Increased oxidative damage has been detected in the frontal cortex in PD, in addition to that reported several years ago in the substantia nigra. Redox proteomics has been useful for identifying protein targets of oxidative damage in aged control and diseased brains . Several proteins are targets of oxidative damage in the frontal cortex even at very early stages of PD-related pathology, including α-synuclein, β-synuclein and SOD2 [23,24]. Other relevant proteins are also oxidatively damaged in PD: UCHL1, SOD1 and DJ-1 [25–27]. In addition, increased oxidative damage to aldolase A, enolase 1 and glyceraldehyde-3 phosphate dehydrogenase, which are all involved in glycolysis and energy metabolism, is found in the frontal cortex in pre-motor stages of PD and in established parkinsonian PD .
Altered mRNA expression
Altered mRNA expression has been reported in the posterior cingulate cortex in PD compared to age-matched controls, and this increases in PD cases with dementia . An interesting point is the observation of down-regulation of numerous genes involved in mRNA splicing, thereby implicating alterations in mRNA processing in the pathogenesis of dementia in PD . mRNA expression in the cerebral cortex differs in PD compared to Alzheimer’s disease, thus indicating disease-specific modifications in gene expression .
Abnormal microRNA down-regulation
Abnormal microRNA miR34b and miR34c down-regulation has been described in the amygdala, substantia nigra, frontal cortex and cerebellum in PD. Down-regulation of miR-34b or miR-34c in differentiated SH-SY5Y dopaminergic neuronal cells results in a moderate decrease in cell viability that is accompanied by altered mitochondrial function and dynamics, oxidative stress, and a decrease in total cellular ATP content. This is accompanied by decreased DJ1 and parkin expression. These findings support the notion that deregulation of miR-34b/c in PD triggers downstream transcriptome alterations underlying mitochondrial dysfunction and down-regulation of DJ1 and parkin in PD .
Altered protein expression in neocortex
Proteomics using bidimensional gels in post-mortem brain samples is still subject to validation by other methods . However, precise results have been obtained using subcellular fractionation, such as after the isolation of midbrain and cortical LBs. Research on protein expression in the neocortex in PD is limited [33,34]. In one of these studies, mortalin, a mitochondrial protein also named mitochondrial heat shock protein-70, was found to be decreased with progression of the disease in the cerebral cortex in PD , corroborating earlier observations of mortalin alteration in the substantia nigra in PD, and supporting the involvement of mortalin in the progression of PD . This is an important point because mortalin maintains mitochondrial homeostasis, antagonizes oxidative stress damage, interacts with several damaged proteins and cooperates with PD-linked proteins as parkin in mitochondrial function .
Interestingly, glutathione S-transferase pi, which is involved in the regulation of oxidative stress, is also dysregulated in the cerebral cortex with progression of the disease in PD .
Lipid composition is altered in total membranes and in lipid rafts
Abnormal lipid composition occurs in the frontal cortex at very early stages of PD-related pathology, with significantly increased expression levels of the highly peroxidizable docosahexanoic acid and an increased peroxidability index . Altered lipid composition is particularly marked in lipid rafts in which dramatic reductions are seen in n-3 and n-6 long chain polyunsaturated fatty acid content, mainly docosahexaenoic acid and arachidonic acid, as well as increased medium- and long-chain saturated fatty acids compared to control brains, thus leading to increased membrane viscosity and most likely increased oxidative stress .
Impaired cortical metabolism
Impaired cortical metabolism has also been supported using other methods. Cerebral glucose metabolism is reduced in the cerebral cortex in PD patients suffering from cognitive impairment [38–41]. Longitudinal studies have shown that idiopatic PD is accompanied by decreased metabolism in selected cortical areas and that the progression of dementia in the same series of cases was associated with mixed subcortical and cortical deficits .
Abnormal proteins at the synapses
Abnormal proteins at the synapses may account for the altered cortical function observed in PD. Tau phosphorylation and α-synuclein phosphorylation are increased in synaptic-enriched fractions of frontal cortex homogenates in PD in the absence of LBs in the same tissue samples . This indicates that there are early α-synuclein alterations at the synapse even in cases with no cognitive impairment . Recent observations have further demonstrated the presence of small abnormal aggregates of α-synuclein at the synapses [44,45]. It is worth emphasizing that altered α-synuclein may result in altered protein–protein interactions, leading to altered synaptic function. Thus, abnormal interactions have been reported between α-synuclein and Rab3a, a protein involved in synaptic vesicle trafficking; Rab5, a protein involved in dopamine endocytosis; and Rab8, a protein engaged in transport .