Phosphorylation of tau and α-synuclein in synaptic-enriched fractions of the frontal cortex in Alzheimer’s disease, and in Parkinson’s disease and related α-synucleinopathies

Mediators of tau phosphorylation in the pathogenesis of Alzheimer's disease

The need for disease-modifying drugs for Alzheimer's disease has become increasingly important owing to escalating disease prevalence and the associated socio-economic burden. Until recently, reducing brain amyloid accumulation has been the main therapeutic focus; however, increasing evidence suggests that targeting abnormal tau phosphorylation could be beneficial. Tau is phosphorylated by several protein kinases and this is balanced by dephosphorylation by protein phosphatases. Phosphorylation at specific sites can influence the physiological functions of tau, including its role in binding to and stabilizing the neuronal cytoskeleton. aberrant phosphorylation of tau could render it susceptible to potentially pathogenic alterations, including conformational changes, proteolytic cleavage and aggregation. While strategies that reduce tau phosphorylation in transgenic models of disease have been promising, our understanding of the mechanisms through which tau becomes abnormally phosphorylated in disease is lacking.

Simultaneous characterization of glyco- and phosphoproteomes of mouse brain membrane proteome with electrostatic repulsion hydrophilic interaction chromatography

Characterization of glyco- and phosphoproteins as well as their modification sites poses many challenges, the greatest being loss of their signals during mass spectrometric detection due to substoichiometric amounts and the ion suppression effect caused by peptides of high abundance. We report here an optimized protocol using electrostatic repulsion hydrophilic interaction chromatography for the simultaneous enrichment of glyco- and phosphopeptides from mouse brain membrane protein digest. With this protocol, we successfully identified 544 unique glycoproteins and 922 glycosylation sites, which were significantly higher than those from the commonly used hydrazide chemistry method (192 glycoproteins and 345 glycosylation sites). Moreover, a total of 383 phosphoproteins and 915 phosphorylation sites were recovered from the sample, suggesting that this protocol has the potential to enrich both glycopeptides and phosphopeptides simultaneously. Of the total 995 glycosylation sites identified from both methods, 96% were considered new as they were either annotated as putative or not documented in the newly released Swiss-Prot database. Thus, this study could be of significant value in complementing the current glycoprotein database and provides a unique opportunity to study the complex interaction of two different post-translational modifications in health and disease without being affected by interexperimental variations.

A tangled web - tau and sporadic Parkinson's disease

Parkinson's disease (PD) represents a major challenge for health care systems around the world: it is the most common degenerative movement disorder of old age, affecting over 100,000 people in the UK alone (Schrag et al., 2000). Despite the remarkable success of treatments directed at potentiating or replacing dopamine within the brain, which can relieve symptoms for over a decade, PD remains an incurable and invariably fatal disorder. As such, efforts to understand the processes that lead to cell death in the brains of patients with PD are a priority for neurodegenerative researchers. A great deal of progress has been made in this regard by taking advantage of advances in genetics, initially by the identification of genes responsible for rare Mendelian forms of PD (outlined in Table 1), and more recently by applying genome wide association studies (GWAS) to the sporadic form of the disease (Hardy et al., 2009). Several such GWAS have now been carried out, with a meta-analysis currently under way. Using over 6000 cases and 10,000 controls, two of these studies have identified variation at a number of loci as being associated with an increased risk of disease (Satake et al., 2009; Simon-Sanchez et al., 2009). Three genes stand out as candidates from these studies - the SNCA gene, coding for α-synuclein, the LRRK2 gene, coding for leucine rich repeat kinase 2, and MAPT, coding for the microtubule-associated protein tau. Mutations at all three of these loci have been associated with Mendelian forms of disease presenting with the clinical syndrome of Parkinsonism, however only SNCA and LRRK2 have been previously associated with pathologically defined PD (Hardy et al., 2009). Point mutations in α-synuclein, along with gene multiplication events, result in autosomal dominant PD, often with a significant dementia component. In addition to this, α-synuclein is the principle component of the main pathological hallmark of idiopathic PD, the Lewy body, making it an unsurprising hit in the GWAS (Spillantini et al., 1997). Mutations in LRRK2 are the most common genetic cause of PD, and so again made this gene a likely candidate as a susceptibility locus for the sporadic form of disease (Kumari and Tan, 2009). More surprising, perhaps, was the identification of tau as a susceptibility factor for Parkinson's. In this review we will outline the role of tau in neurodegeneration and in different forms of Parkinsonism, and speculate as to what the functional basis of the association between MAPT and PD might be.

Formation and development of Lewy pathology: a critical update

Filamentous protein inclusions in neurons (Lewy bodies, LB) and dystrophic neurites containing pathologic alpha-synuclein (alpha Syn) are the morphologic hallmarks of sporadic Parkinson disease (PD) and dementia with Lewy bodies (DLB), but are also found in aged subjects and in a variety of neurogenerative disorders. They occur in the central, peripheral, and autonomic nervous system as an essential or coincident feature. Their formation runs through several phases from initial dust-like particles cross-linked with alpha Syn to aggregation of ubiquitinated dense filaments, formation of LBs, finally degradation and death of the afflicted neurons. Pathologic accumulation of alpha Syn/LBs proposed by Braak et al. (Neurobiol Aging 24:197-211, 2003), following a predictable sequence of lesions in six stages with ascending progression from medullary and olfactory nuclei to the cortex, has been considered to be linked to clinical dysfunctions. The consensus pathologic guidelines of DLB (Neurology 65:1863-1872, 2005), by semiquantitative scoring to alpha Syn pathology (LB density and distribution) in specific brain regions, distinguish three phenotypes (brainstem, transitional/limbic, and diffuse neocortical), and also consider concomitant Alzheimer-related pathology. alpha Syn pathology in the amygdala is often associated with Alzheimer disease. Although some retrospective clinico-pathologic studies have largely confirmed the Braak LB staging system, it shows neither correlation to the clinical severity and duration of parkinsonism nor to nigral alpha Syn burden and cell loss which significantly correlates with resulting striatal loss of dopamine, dopamine transporter and tyrosine hydroxylase, duration and severity of motor dysfunction. Between 6.3 and 43% of clinically manifested PD cases did not follow this pattern, and in 7-8.3% of those with alpha Syn-positive inclusions in midbrain and cortex the medullary nuclei were spared. On the other hand, 30-55% of elderly subjects with widespread Lewy pathology revealed no neuropsychiatric symptoms or were not classifiable. Therefore, detection and staging of Lewy pathology without assessment of neuronal loss in specific areas may not have clinical impact and its predictive validity is questionable. For demented patients, modified criteria for categorization of Lewy pathology were proposed. If robust correlations between clinical course and Lewy/alpha Syn pathology are to be confirmed by future studies, the currently used morphologic staging/classification systems should be revised accordingly.

Alpha-synuclein immunopositive aggregates in the myenteric plexus of the aging Fischer 344 rat

Dystrophic axons and terminals are common in the myenteric plexus and smooth muscle of the gastrointestinal (GI) tract of aged rats. In young adult rats, alpha-synuclein in its normal state is abundant throughout the myenteric plexus, making this protein-which is prone to fibrillization-a candidate marker for axonopathies in the aged rat. To determine if aggregation of alpha-synuclein is involved in the formation of age-related enteric neuropathies, we sampled the stomach, small intestine and large intestine of adult, middle-aged, and aged virgin male Fischer 344 rats stained for alpha-synuclein in both its normal and pathological states. Alpha-synuclein-positive dystrophic axons and terminals were present throughout the GI tract of middle-aged and aged rats, with immunohistochemical double labeling demonstrating co-localization within nitric oxide synthase-, calretinin-, calbindin-, or tyrosine hydroxylase-positive markedly swollen neurites. However, other dystrophic neurites positive for each of these four markers were not co-reactive for alpha-synuclein. Similarly, a subpopulation of alpha-synuclein inclusions contained deposits immunostained with an anti-tau phospho-specific Ser(262) antibody, but not all of these hyperphosphorylated tau-positive aggregates were co-localized with alpha-synuclein. The presence of heteroplastic and potentially degenerating neural elements and protein aggregates both positive and negative for alpha-synuclein suggests a complex chronological relationship between the onset of degenerative changes and the accumulation of misfolded proteins. Additionally, proteins other than alpha-synuclein appear to be involved in age-related axonopathies. Finally, this study establishes the utility of the aging Fischer 344 rat for the study of synucleopathies and tauopathies in the GI tract.

Synaptic degeneration in Alzheimer's disease

Synaptic loss is the major neurobiological substrate of cognitive dysfunction in Alzheimer's disease (AD). Synaptic failure is an early event in the pathogenesis that is clearly detectable already in patients with mild cognitive impairment (MCI), a prodromal state of AD. It progresses during the course of AD and in most early stages involves mechanisms of compensation before reaching a stage of decompensated function. This dynamic process from an initially reversible functionally responsive stage of down-regulation of synaptic function to stages irreversibly associated with degeneration might be related to a disturbance of structural brain self-organization and involves morpho-regulatory molecules such as the amyloid precursor protein. Further, recent evidence suggests a role for diffusible oligomers of amyloid beta in synaptic dysfunction. To form synaptic connections and to continuously re-shape them in a process of ongoing structural adaptation, neurons must permanently withdraw from the cell cycle. Previously, we formulated the hypothesis that differentiated neurons after having withdrawn from the cell cycle are able to use molecular mechanisms primarily developed to control proliferation alternatively to control synaptic plasticity. The existence of these alternative effector pathways within neurons might put them at risk of erroneously converting signals derived from plastic synaptic changes into the program of cell cycle activation, which subsequently leads to cell death. The molecular mechanisms involved in cell cycle activation might, thus, link aberrant synaptic changes to cell death.

Therapy for Alzheimer's Disease: How Effective are Current Treatments?

Available symptomatic therapies for the treatment of Alzheimer's disease (AD) have been based on known neurotransmitter dysfunctions associated with the illness. The second-generation cholinesterase inhibitors and the N-methyl D-aspartate receptor antagonist memantine have been widely prescribed and studied. Meta-analyses of these therapies were reviewed, focusing on effectiveness and tolerability. Although many of the meta-analyses demonstrate statistically significant improvements, some question if these benefits are sufficient to justify their current widespread and protracted use. This has spurred the development of new disease-modifying therapies that aim to have a greater impact on this debilitating illness.

Alpha-Synuclein contributes to GSK-3beta-catalyzed Tau phosphorylation in Parkinson's disease models

We have shown in the parkinsonism-inducing neurotoxin MPP(+)/MPTP model that alpha-Synuclein (alpha-Syn), a presynaptic protein causal in Parkinson's disease (PD), contributes to hyperphosphorylation of Tau (p-Tau), a protein normally linked to tauopathies, such as Alzheimer's disease (AD). Here, we investigated the kinase involved and show that the Tau-specific kinase, glycogen synthase kinase 3beta (GSK-3beta), is robustly activated in various MPP(+)/MPTP models of Parkinsonism (SH-SY5Y cotransfected cells, mesencephalic neurons, transgenic mice overexpressing alpha-Syn, and postmortem striatum of PD patients). The activation of GSK-3beta was absolutely dependent on the presence of alpha-Syn, as indexed by the absence of p-GSK-3beta in cells lacking alpha-Syn and in alpha-Syn KO mice. MPP(+) treatment induced translocation and accumulation of p-GSK-3beta in nuclei of SH-SY5Y cells and mesencephalic neurons. Through coimmunoprecipitation (co-IP), we found that alpha-Syn, pSer396/404-Tau, and p-GSK-3beta exist as a heterotrimeric complex in SH-SY5Y cells. GSK-3beta inhibitors (lithium and TDZD-8) protected against MPP(+)-induced events in SH-SY5Y cells, preventing cell death and p-GSK-3beta formation, by reversing increases in alpha-Syn accumulation and p-Tau formation. These data unveil a previously unappreciated role of alpha-Syn in the induction of p-GSK-3beta, and demonstrate the importance of this kinase in the genesis and maintenance of neurodegenerative changes associated with PD.

Early-onset familial lewy body dementia with extensive tauopathy: a clinical, genetic, and neuropathological study

We describe a Spanish family in which 3 of 4 siblings had dementia with Lewy bodies, 2 of them starting at age 26 years and the other at 29 years. The father has recently been diagnosed with Lewy body disease, with onset at 77 years. Neuropathological examination of the brain of the index patient disclosed unusual features characterized by diffuse Lewy body disease and generalized neurofibrillary tangle pathology but with no amyloid deposits in any region. Moreover, Lewy body pathology colocalized with neurofibrillary tangles in most affected neurons. Mutation screening that included all coding exons of presenilin 1 (PSEN1), presenilin 2 (PSEN2), alpha-synuclein (SNCA), beta-synuclein (SNCB), microtubule-associated protein tau (MAPT), leucine-rich repeat kinase 2 (LRRK2), glucocerebrosidase (GBA), and exons 16 and 17 of the amyloid precursor protein (APP) genes did not identify any mutation. Genome-wide single nucleotide polymorphism was performed in 4 family members and ruled out any pathogenic duplication or deletion in the entire genome. In summary, we report a unique family with pathologically confirmed early-onset dementia with Lewy bodies with widespread tau and alpha-synuclein deposition. The absence of mutations in genes known to cause Lewy body disease suggests that a novel locus or loci are implicated in this neurodegenerative disease.

Neonatal exposure to PFOS and PFOA in mice results in changes in proteins which are important for neuronal growth and synaptogenesis in the developing brain

Perfluorooctane sulfonate (PFOS) and perfluorooctanoic acid (PFOA) belong to the family of perfluorinated compounds. They are used in industrial and consumer applications, e.g., clothing fabrics, carpets, and food packaging. PFOS and PFOA are present in the environment and are found in dust and human milk, which implies that newborns and toddlers can be directly exposed to these agents during brain development. Recently, we reported that PFOS and PFOA can cause neurobehavioral defects and changes in the cholinergic system, in the adult animal, when given directly to neonatal mice, and thereby showing similarities with other investigated persistent organic pollutants, such as dichloro-diphenyl-trichloroethan, polychlorinated biphenyls, and polybrominated diphenyl ethers (PBDEs). In recent studies, we have also seen that highly brominated PBDEs can affect the levels of proteins that are important for neuronal growth and synaptogenesis in the neonatal mouse brain. The present study shows that a single oral dose of either 21 micromol PFOS or PFOA/kg body weight (11.3 or 8.70 mg), given directly to the neonatal mice on postnatal day 10, significantly increased the levels of CaMKII, GAP-43, and synaptophysin in the hippocampus of the neonatal mouse. Both compounds significantly increased the levels of synaptophysin and tau in cerebral cortex, and PFOA also increased the levels of tau in hippocampus. These proteins are important for normal brain development, and altered levels of these proteins during a critical period of the brain growth spurts could be one of the mechanisms behind earlier reported behavioral defects.