Mitochondria, calcium and cell death: a deadly triad in neurodegeneration

Presenilin 2 modulates endoplasmic reticulum (ER)-mitochondria interactions and Ca2+ cross-talk

Presenilin mutations are the main cause of familial Alzheimer's disease (FAD). Presenilins also play a key role in Ca(2+) homeostasis, and their FAD-linked mutants affect cellular Ca(2+) handling in several ways. We previously have demonstrated that FAD-linked presenilin 2 (PS2) mutants decrease the Ca(2+) content of the endoplasmic reticulum (ER) by inhibiting sarcoendoplasmic reticulum Ca(2+)-ATPase (SERCA) activity and increasing ER Ca(2+) leak. Here we focus on the effect of presenilins on mitochondrial Ca(2+) dynamics. By using genetically encoded Ca(2+) indicators specifically targeted to mitochondria (aequorin- and GFP-based probes) in SH-SY5Y cells and primary neuronal cultures, we show that overexpression or down-regulation of PS2, but not of presenilin 1 (PS1), modulates the Ca(2+) shuttling between ER and mitochondria, with its FAD mutants strongly favoring Ca(2+) transfer between the two organelles. This effect is not caused by a direct PS2 action on mitochondrial Ca(2+)-uptake machinery but rather by an increased physical interaction between ER and mitochondria that augments the frequency of Ca(2+) hot spots generated at the cytoplasmic surface of the outer mitochondrial membrane upon stimulation. This PS2 function adds further complexity to the multifaceted nature of presenilins and to their physiological role within the cell. We also discuss the importance of this additional effect of FAD-linked PS2 mutants for the understanding of FAD pathogenesis.

The mitochondrial dynamics of Alzheimer's disease and Parkinson's disease offer important opportunities for therapeutic intervention

Mitochondrial dynamics play a crucial role in the pathobiology underlying Alzheimer's disease (AD) and Parkinson's disease (PD). Although a complete scientific understanding of these devastating conditions has yet to be realized, alterations in mitochondrial fission and fusion, and in the protein complexes that orchestrate mitochondrial fission and fusion, have been well established in AD- and PD-related neurodegeneration. Whether fission/fusion disruption in the brain is a causal agent in neuronal demise or a product of some other upstream disturbance is still a matter of debate; however, in both AD and PD, the potential for successful therapeutic amelioration of degeneration via mitochondrial protection is high. We here discuss the role of mitochondrial dynamics in AD and PD and assess the need for their therapeutic exploitation.

The IP(3) receptor-mitochondria connection in apoptosis and autophagy

The amount of Ca(2+) taken up in the mitochondrial matrix is a crucial determinant of cell fate; it plays a decisive role in the choice of the cell between life and death. The Ca(2+) ions mainly originate from the inositol 1,4,5-trisphosphate (IP(3))-sensitive Ca(2+) stores of the endoplasmic reticulum (ER). The uptake of these Ca(2+) ions in the mitochondria depends on the functional properties and the subcellular localization of the IP(3) receptor (IP(3)R) in discrete domains near the mitochondria. To allow for an efficient transfer of the Ca(2+) ions from the ER to the mitochondria, structural interactions between IP(3)Rs and mitochondria are needed. This review will focus on the key proteins involved in these interactions, how they are regulated, and what are their physiological roles in apoptosis, necrosis and autophagy. This article is part of a Special Issue entitled: 11th European Symposium on Calcium.

Vulnerability of mesostriatal dopaminergic neurons in Parkinson's disease

The term vulnerability was first associated with the midbrain dopaminergic neurons 85 years ago, before they were identified as monoaminergic neurons, when Foix and Nicolesco (1925) reported the loss of neuromelanin containing neurons in the midbrain of patients with post-encephalitic Parkinson's disease (PD). A few years later, Hassler (1938) showed that degeneration is more intense in the ventral tier of the substantia nigra compacta than in its dorsal tier and the ventral tegmental area (VTA), outlining the concept of differential vulnerability of midbrain dopaminergic (DA-) neurons. Nowadays, we know that other neuronal groups degenerate in PD, but the massive loss of nigral DA-cells is its pathological hallmark, having a pivotal position in the pathophysiology of the disease as it is responsible for the motor symptoms. Data from humans as well as cellular and animal models indicate that DA-cell degeneration is a complex process, probably precipitated by the convergence of different risk factors, mediated by oxidative stress, and involving pathogenic factors arising within the DA-neuron (intrinsic factors), and from its environment and distant interconnected brain regions (extrinsic factors). In light of current data, intrinsic factors seem to be preferentially involved in the first steps of the degenerative process, and extrinsic factors in its progression. A controversial issue is the relative weight of the impairment of common cell functions, such as energy metabolism and proteostasis, and specific dopaminergic functions, such as pacemaking activity and DA handling, in the pathogenesis of DA-cell degeneration. Here we will review the current knowledge about the relevance of these factors at the beginning and during the progression of PD, and in the differential vulnerability of midbrain DA-cells.

Plasma membrane Ca2+-ATPases: Targets of oxidative stress in brain aging and neurodegeneration

The plasma membrane Ca²⁺-ATPase (PMCA) pumps play an important role in the maintenance of precise levels of intracellular Ca²⁺ [Ca²⁺]_i, essential to the functioning of neurons. In this article, we review evidence showing age-related changes of the PMCAs in synaptic plasma membranes (SPMs). PMCA activity and protein levels in SPMs diminish progressively with increasing age. The PMCAs are very sensitive to oxidative stress and undergo functional and structural changes when exposed to oxidants of physiological relevance. The major signatures of oxidative modification in the PMCAs are rapid inactivation, conformational changes, aggregation, internalization from the plasma membrane and proteolytic degradation. PMCA proteolysis appears to be mediated by both calpains and caspases. The predominance of one proteolytic pathway vs the other, the ensuing pattern of PMCA degradation and its consequence on pump activity depends largely on the type of insult, its intensity and duration. Experimental reduction of PMCA expression not only alters the dynamics of cellular Ca²⁺ handling but also has a myriad of downstream consequences on various aspects of cell function, indicating a broad role of these pumps. Age- and oxidation-related down-regulation of the PMCAs may play an important role in compromised neuronal function in the aging brain and its several-fold increased susceptibility to neurodegenerative disorders such as Alzheimer’s disease, Parkinson’s disease, and stroke. Therapeutic approaches that protect the PMCAs and stabilize [Ca²⁺]_i homeostasis may be capable of slowing and/or preventing neuronal degeneration. The PMCAs are therefore emerging as a new class of drug targets for therapeutic interventions in various chronic degenerative disorders.

Calcium flux between the endoplasmic reticulum and mitochondrion contributes to poliovirus-induced apoptosis

We show that poliovirus (PV) infection induces an increase in cytosolic calcium (Ca(2+)) concentration in neuroblastoma IMR5 cells, at least partly through Ca(2+) release from the endoplasmic reticulum lumen via the inositol 1,4,5-triphosphate receptor (IP(3)R) and ryanodine receptor (RyR) channels. This leads to Ca(2+) accumulation in mitochondria through the mitochondrial Ca(2+) uniporter and the voltage-dependent anion channel (VDAC). This increase in mitochondrial Ca(2+) concentration in PV-infected cells leads to mitochondrial dysfunction and apoptosis.

A synergistic dysfunction of mitochondrial fission/fusion dynamics and mitophagy in Alzheimer's disease

Alzheimer's disease (AD), the most common form of dementia in the elderly, can have a late-onset sporadic or an early-onset familial origin. In both cases, the neuropathological hallmarks are the same: senile plaques and neurofibrillary tangles. Despite AD having a proteinopathic nature, there is strong evidence for an organelle dysfunction-related neuropathology, namely dysfunctional mitochondria. In this regard, dysfunctional mitochondria and associated exacerbated generation of reactive oxygen species are among the earliest events in the progression of the disease. Since the maintenance of a healthy mitochondrial pool is essential given the central role of this organelle in several determinant cellular processes, mitochondrial dysfunction in AD would be predicted to have profound pluripotent deleterious consequences. Mechanistically, recent reports suggest that mitochondrial fission/fusion and mitophagy are altered in AD and in in vitro models of disease, and since both processes are reported to be protective, this review will discuss the role of mitochondrial fission/fusion and mitophagy in the pathogenesis of AD.

Dementia, stroke and migraine--some common pathological mechanisms

Dementia, stroke and migraine are very common neurological disorders affecting a large percentage of the population, and leading to a high degree of disability. Often, adequate therapy is not available. Although the symptoms, the progression and the outcome differ in these disorders, to some extent they may share some common pathophysiological mechanisms. The genetic background, an energy deficit, and excitotoxicity, vascular and thrombotic properties can influence all three disorders, resulting in a neuronal dysfunction, increased cellular vulnerability, neurodegeneration and ultimately cell death. All these cellular events occur in dementias and stroke, moreover recent studies suggest that, besides a dysfunction, neuronal damage may be an issue in migraine too. One of the most central events in the multiple mechanisms involved in the pathogenesis of these disorders is a metabolic disturbance of certain brain cells. As mitochondria provide the cells with energy, realization of the importance of these organelles in the aetiopathogenesis of several disorders has emerged in recent years. This review surveys the most important features of the pathogenesis of dementia, stroke and migraine from the aspect of mitochondrial malfunction highlighting some of the considerable connections between these neurological disorders.

A neurological perspective on mitochondrial disease

Disruption of the most fundamental cellular energy process, the mitochondrial respiratory chain, results in a diverse and variable group of multisystem disorders known collectively as mitochondrial disease. The frequent involvement of the brain, nerves, and muscles, often in the same patient, places neurologists at the forefront of the interesting and challenging process of diagnosing and caring for these patients. Mitochondrial diseases are among the most frequently inherited neurological disorders, and can be caused by mutations in mitochondrial or nuclear DNA. Substantial progress has been made over the past decade in understanding the genetic basis of these disorders, with important implications for the general neurologist in terms of the diagnosis, investigation, and multidisciplinary management of these patients.

Mitochondrially targeted anti-cancer agents

Cancer is an ever-increasing problem that is yet to be harnessed. Frequent mutations make this pathology very variable and, consequently, a considerable challenge. Intriguingly, mitochondria have recently emerged as novel targets for cancer therapy. A group of agents with anti-cancer activity that induce apoptosis by way of mitochondrial destabilisation, termed mitocans, have been a recent focus of research. Of these compounds, many are hydrophobic agents that associate with various sub-cellular organelles. Clearly, modification of such structures with mitochondria-targeting moieties, for example tagging them with lipophilic cations, would be expected to enhance their activity. This may be accomplished by the addition of triphenylphosphonium groups that direct such compounds to mitochondria, enhancing their activity. In this paper, we will review agents that possess anti-cancer activity by way of destabilizing mitochondria and their possible targets. We propose that mitochondrial targeting, in particular where the agent associates directly with the target, results in more specific and efficient anti-cancer drugs of potential high clinical relevance.

Alzheimer's disease: diverse aspects of mitochondrial malfunctioning

Alzheimer's disease is a progressive neurodegenerative disorder, either assuming a sporadic, age-associated, late-onset form, or a familial form, with early onset, in a smaller fraction of the cases. Whereas in the familial cases several mutations have been identified in genes encoding proteins related with the pathogenesis of the disease, for the sporadic form several causes have been proposed and are currently under debate. Mitochondrial dysfunction has surfaced as one of the most discussed hypotheses acting as a trigger for the pathogenesis of Alzheimer's disease. Mitochondria assume central functions in the cell, including ATP production, calcium homeostasis, reactive oxygen species generation, and apoptotic signaling. Although their role as the cause of the disease may be controversial, there is no doubt that mitochondrial dysfunction, abnormal mitochondrial dynamics and degradation by mitophagy occur during the disease process, contributing to its onset and progression.

Mitochondria as a target in treatment

Mitochondria are key organelles that perform essential cellular functions and play pivotal roles in cell death and survival signaling. Hence, they represent an attractive target for drugs to treat metabolic, degenerative, and hyperproliferative diseases. Targeting mitochondria with organelle-specific agents or prodrugs has proven to be an effective therapeutic strategy. More specifically, controlling the cellular ROS balance via selective delivery of an antioxidant "payload" into mitochondria is an elegant emerging therapeutic concept. Herein, we review the recent medicinal chemistry and clinical data of these exploratory strategies, which should point the way for future generations of therapeutics.

Basic mechanisms of neurodegeneration: a critical update

Neurodegenerative diseases are characterized by progressive dysfunction of specific populations of neurons, determining clinical presentation. Neuronal loss is associated with extra and intracellular accumulation of misfolded proteins, the hallmarks of many neurodegenerative proteinopathies. Major basic processes include abnormal protein dynamics due to deficiency of the ubiquitin-proteosome-autophagy system, oxidative stress and free radical formation, mitochondrial dysfunction, impaired bioenergetics, dysfunction of neurotrophins, 'neuroinflammatory' processes and (secondary) disruptions of neuronal Golgi apparatus and axonal transport. These interrelated mechanisms lead to programmed cell death is a long run over many years. Neurodegenerative disorders are classified according to known genetic mechanisms or to major components of protein deposits, but recent studies showed both overlap and intraindividual diversities between different phenotypes. Synergistic mechanisms between pathological proteins suggest common pathogenic mechanisms. Animal models and other studies have provided insight into the basic neurodegeneration and cell death programs, offering new ways for future prevention/treatment strategies.

The regulation of OXPHOS by extramitochondrial calcium

Despite extensive research, the regulation of mitochondrial function is still not understood completely. Ample evidence shows that cytosolic Ca2+ has a strategic task in co-ordinating the cellular work load and the regeneration of ATP by mitochondria. Currently, the paradigmatic view is that Cacyt2+ taken up by the Ca2+ uniporter activates the matrix enzymes pyruvate dehydrogenase, alpha-ketoglutarate dehydrogenase and isocitrate dehydrogenase. However, we have recently found that Ca2+ regulates the glutamate-dependent state 3 respiration by the supply of glutamate to mitochondria via aralar, a mitochondrial glutamate/aspartate carrier. Since this activation is not affected by ruthenium red, glutamate transport into mitochondria is controlled exclusively by extramitochondrial Ca2+. Therefore, this discovery shows that besides intramitochondrial also extramitochondrial Ca2+ regulates oxidative phosphorylation. This new mechanism acts as a mitochondrial "gas pedal", supplying the OXPHOS with substrate on demand. These results are in line with recent findings of Satrustegui and Palmieri showing that aralar as part of the malate-aspartate shuttle is involved in the Ca2+-dependent transport of reducing hydrogen equivalents (from NADH) into mitochondria. This review summarises results and evidence as well as hypothetical interpretations of data supporting the view that at the surface of mitochondria different regulatory Ca2+-binding sites exist and can contribute to cellular energy homeostasis. Moreover, on the basis of our own data, we propose that these surface Ca2+-binding sites may act as targets for neurotoxic proteins such as mutated huntingtin and others. The binding of these proteins to Ca2+-binding sites can impair the regulation by Ca2+, causing energetic depression and neurodegeneration.

Mitochondrial dynamics

Mitochondrial dynamics is a key feature for the interaction of mitochondria with other organelles within a cell and also for the maintenance of their own integrity. Four types of mitochondrial dynamics are discussed: Movement within a cell and interactions with the cytoskeleton, fusion and fission events which establish coherence within the chondriome, the dynamic behavior of cristae and their components, and finally, formation and disintegration of mitochondria (mitophagy). Due to these essential functions, disturbed mitochondrial dynamics are inevitably connected to a variety of diseases. Localized ATP gradients, local control of calcium-based messaging, production of reactive oxygen species, and involvement of other metabolic chains, that is, lipid and steroid synthesis, underline that physiology not only results from biochemical reactions but, in addition, resides on the appropriate morphology and topography. These events and their molecular basis have been established recently and are the topic of this review.

Intracellular calcium signalling in Alzheimer's disease

More than two decades ago, dysregulation of the intracellular Ca²⁺ homeostasis was suggested to underlie the development of Alzheimer’s disease (AD). This hypothesis was tested in numerous in vitro studies, which revealed multiple Ca²⁺ signalling pathways able to contribute to AD pathology. It remained, however, unclear whether these pathways are also activated in vivo, in cells involved in signal processing in the living brain. Here we review recent data analysing intracellular Ca²⁺ signalling in vivo in the context of previous in vitro findings. We particularly focus on the processes taking place in the immediate vicinity of amyloid plaques and on their possible role for AD-mediated brain dysfunction.