Neville Vassallo

Cellular prion protein function in copper homeostasis and redox signalling at the synapse

The fundamental physiological function of native cellular prion (PrPC) remains unknown. Herein, the most salient observations as regards prion physiology are critically evaluated. These include: (i) the role of PrPC in copper homeostasis, particularly at the pre‐synaptic membrane; (ii) involvement of PrPC in neuronal calcium disturbances; and (iii) the neuroprotective properties of PrPC in response to copper and oxidative stress. Ultimately, a tentative hypothesis of basic prion function is derived, namely that PrPC acts as a sensor for copper and/or free radical stimuli, thereby triggering intracellular calcium signals that finally translate into modulation of synaptic transmission and maintenance of neuronal integrity.

Inhibition and disaggregation of α-synuclein oligomers by natural polyphenolic compounds.

Alpha‐synuclein binds to Alpha‐synuclein by biophysical (View Interaction 1, 2)

Mitochondrial membrane permeabilisation by amyloid aggregates and protection by polyphenols.

Alzheimers disease and Parkinsons disease are neurodegenerative disorders characterised by the misfolding of proteins into soluble prefibrillar aggregates. These aggregate complexes disrupt mitochondrial function, initiating a pathophysiological cascade leading to synaptic and neuronal degeneration. In order to explore the interaction of amyloid aggregates with mitochondrial membranes, we made use of two in vitro model systems, namely: (i) lipid vesicles with defined membrane compositions that mimic those of mitochondrial membranes, and (ii) respiring mitochondria isolated from neuronal SH-SY5Y cells. External application of soluble prefibrillar forms, but not monomers, of amyloid-beta (Aβ42 peptide), wild-type α-synuclein (α-syn), mutant α-syn (A30P and A53T) and tau-441 proteins induced a robust permeabilisation of mitochondrial-like vesicles, and triggered cytochrome c release (CCR) from isolated mitochondrial organelles. Importantly, the effect on mitochondria was shown to be dependent upon cardiolipin, an anionic phospholipid unique to mitochondria and a well-known key player in mitochondrial apoptosis. Pharmacological modulators of mitochondrial ion channels failed to inhibit CCR. Thus, we propose a generic mechanism of thrilling mitochondria in which soluble amyloid aggregates have the intrinsic capacity to permeabilise mitochondrial membranes, without the need of any other protein. Finally, six small-molecule compounds and black tea extract were tested for their ability to inhibit permeation of mitochondrial membranes by Aβ42, α-syn and tau aggregate complexes. We found that black tea extract and rosmarinic acid were the most potent mito-protectants, and may thus represent important drug leads to alleviate mitochondrial dysfunction in neurodegenerative diseases.

Amyloid precursor protein intracellular domain modulates cellular calcium homeostasis and ATP content

Consecutive cleavages of amyloid precursor protein (APP) generate APP intracellular domain (AICD). Its cellular function is still unclear. In this study, we investigated the functional role of AICD in cellular Ca2+ homeostasis. We could confirm previous observations that endoplasmic reticulum Ca2+ stores contain less calcium in cells with reduced APP γ‐secretase cleavage products, increased AICD degradation, reduced AICD expression or in cells lacking APP. In addition, we observed an enhanced resting cytosolic calcium concentration under conditions where AICD is decreased or missing. In view of the reciprocal effects of Ca2+ on mitochondria and of mitochondria on Ca2+ homeostasis, we analysed further the cellular ATP content and the mitochondrial membrane potential. We observed a reduced ATP content and a mitochondrial hyperpolarisation in cells with reduced amounts of AICD. Blockade of mitochondrial oxidative phosphorylation chain in control cells lead to similar alterations as in cells lacking AICD. On the other hand, substrates of Complex II rescued the alteration in Ca2+ homeostasis in cells lacking AICD. Based on these observations, our findings indicate that alterations observed in endoplasmic reticulum Ca2+ storage in cells with reduced amounts of AICD are reciprocally linked to mitochondrial bioenergetic function.

The centrality of mitochondria in the pathogenesis and treatment of Parkinson's disease.

Parkinsons disease (PD) is an incurable neurodegenerative disorder leading to progressive motor impairment and for which there is no cure. From the first postmortem account describing a lack of mitochondrial complex I in the substantia nigra of PD sufferers, the direct association between mitochondrial dysfunction and death of dopaminergic neurons has ever since been consistently corroborated. In this review, we outline common pathways shared by both sporadic and familial PD that remarkably and consistently converge at the level of mitochondrial integrity. Furthermore, such knowledge has incontrovertibly established mitochondria as a valid therapeutic target in neurodegeneration. We discuss several mitochondria‐directed therapies that promote the preservation, rescue, or restoration of dopaminergic neurons and which have been identified in the laboratory and in preclinical studies. Some of these have progressed to clinical trials, albeit the identification of an unequivocal disease‐modifying neurotherapeutic is still elusive. The challenge is therefore to improve further, not least by more research on the molecular mechanisms and pathophysiological consequences of mitochondrial dysfunction in PD.

AMPA-receptor-mediated excitatory synaptic transmission is enhanced by iron-induced α-synuclein oligomers

J. Neurochem. (2011) 117, 868–878.

Mutant presenilin 1 alters synaptic transmission in cultured hippocampal neurons.

Mutations in presenilins are the major cause of familial Alzheimer disease, but the precise pathogenic mechanism by which presenilin (PS) mutations cause synaptic dysfunction leading to memory loss and neurodegeneration remains unclear. Using autaptic hippocampal cultures from transgenic mice expressing human PS1 with the A246E mutation, we demonstrate that mutant PS1 significantly depressed the amplitude of evoked α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid and N-methyl-d-aspartate receptor-mediated synaptic currents. Analysis of the spontaneous miniature synaptic activity revealed a lower frequency of miniature currents but normal miniature amplitude. Both alterations could be rescued by the application of a γ-secretase blocker. On the other hand, the application of synthetic soluble Aβ42 in wild-type neurons induced the PS1 mutant phenotype on synaptic strength. Together, these findings strongly suggest that the expression of mutant PS1 in cultured neurons depresses synaptic transmission by causing a physical reduction in the number of synapses. This hypothesis is consistent with morphometic and semiquantitative immunohistochemical analysis, revealing a decrease in synaptophysin-positive puncta in PS1 mutant hippocampal neurons.

Mediterranean diet and dementia of the Alzheimer type.

Dementia of the Alzheimer type is the most common form of dementia affecting mostly the elderly population. It is a progressive and fatal neurodegenerative disorder with characteristic neuropathology and clinical symptomology. In the coming years, the number of individuals with Alzheimers disease (AD) will increase as the elderly population worldwide is expected to grow significantly thus putting an added strain on national health care systems as well as caregivers who will inevitably carry most of the care burden. Thus it has been suggested that early intervention strategies which delay or halt the disease progression will have a strong impact on clinical outcomes. Changes in lifestyle habits such as diet modification or supplementation have been indicated as probable protective factors for a number of chronic conditions including AD. Particular attention has recently been devoted to the Mediterranean diet which is rich in the antioxidants Vitamins C and E, polyunsaturated fatty acids and polyphenolic compounds. Several in vitro, animal and population-based studies reported a positive effect between adherence to a Mediterranean diet and AD prevention, although contrasting views remain. This review will focus on the latest developments and findings in the ongoing research investigating the relationship between Mediterranean diet and its major constituents in AD onset and progression.

Interaction of α-synuclein with biomembranes in Parkinson's disease —role of cardiolipin

One of the key molecular events underlying the pathogenesis of Parkinsons disease (PD) is the aberrant misfolding and aggregation of the α-synuclein (αS) protein into higher-order oligomers that play a key role in neuronal dysfunction and degeneration. A wealth of experimental data supports the hypothesis that the neurotoxicity of αS oligomers is intrinsically linked with their ability to interact with, and disrupt, biological membranes; especially those membranes having negatively-charged surfaces and/or lipid packing defects. Consequences of αS-lipid interaction include increased membrane tension, permeation by pore formation, membrane lysis and/or leakage due to the extraction of lipids from the bilayer. Moreover, we assert that the interaction of αS with a liquid-disordering phospholipid uniquely enriched in mitochondrial membranes, namely cardiolipin (1,3-diphosphatidyl-sn-glycerol, CL), helps target the αS oligomeric complexes intracellularly to mitochondria. Binding mediated by CL may thus represent an important pathomechanism by which cytosolic αS could physically associate with mitochondrial membranes and disrupt their integrity. Impaired mitochondrial function culminates in a cellular bioenergetic crisis and apoptotic death. To conclude, we advocate the accelerated discovery of new drugs targeting this pathway in order to restore mitochondrial function in PD.

Cellular prion protein modulates the intracellular calcium response to hydrogen peroxide

The physiological function of the cellular prion protein (PrPC) is still under intense investigation. It has been suggested that PrPC has a protective role in neuronal cells, particularly against environmental stress caused by reactive oxygen species (ROS). Here we analysed the acute effect of a major ROS, hydrogen peroxide (H2O2), on intracellular calcium homeostasis in cultured cerebellar granule cells and immortalized hippocampal neuronal cells. Both neuronal cell culture models showed that the rise in intracellular calcium following application of H2O2 was strongly dependent on the presence of PrPC. Moreover, the N‐terminal octapeptide repeats of PrPC were required for this effect, because neuronal cells expressing a PrPC lacking the N‐terminus resembled the PrPC‐deficient phenotype. Neurones deficient of fyn kinase, or pharmacological inhibition of fyn, also abrogated the calcium response to H2O2 treatment, indicating that fyn activation is a critical step within the PrPC signalling cascade. Finally, we identified a possible role of this PrPC signalling pathway in the neuroprotective response of PrPC to oxidative stress. In conclusion, we put forward the hypothesis that PrPC functions as a sensor for H2O2, thereby activating a protective signalling cascade involving fyn kinase that leads to calcium release from intracellular stores.