Mitochondrial and lysosomal dysfunction have been implicated in substantia nigra dopaminergic neurodegeneration in Parkinson’s disease (PD), but how these pathways are linked in human neurons remains unclear. Here we studied dopaminergic neurons derived from patients with idiopathic and familial PD. We identified a time-dependent pathological cascade beginning with mitochondrial oxidant stress leading to oxidized dopamine accumulation and ultimately resulting in reduced glucocerebrosidase enzymatic activity, lysosomal dysfunction, and α-synuclein accumulation. This toxic cascade was observed in human, but not in mouse, PD neurons at least in part because of species-specific differences in dopamine metabolism. Increasing dopamine synthesis or α-synuclein amounts in mouse midbrain neurons recapitulated pathological phenotypes observed in human neurons. Thus, dopamine oxidation represents an important link between mitochondrial and lysosomal dysfunction in PD pathogenesis.

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.fluorescent Ca 2+ probe | intracellular organelle tethering | fluorescence resonance energy transfer A lzheimer's disease (AD) is the most common form of dementia in developed countries. The pathogenesis of AD is still largely mysterious, and most basic research in the field is concentrated on rare genetic forms of familial AD (FAD). The majority of FAD cases are caused by point mutations in genes for two homologous proteins, presenilin 1 (PS1) and presenilin 2 (PS2), that are essential components of the γ-secretase complex responsible for the production of the amyloid β peptides (Aβ) (1).Evidence has accumulated suggesting that FAD is linked to an imbalance of cellular Ca 2+ homeostasis (see refs. 2 and 3 for recent reviews). In particular, presenilins appear to play a key role in the control of Ca 2+ concentration within the endoplasmic reticulum (ER), [Ca 2+ ] ER : (i) Several FAD-linked presenilin mutants altered the expression or sensitivity of ER Ca 2+ release channels [ryanodine receptor (RyR) and inositol 1,4,5-trisphosphate receptor (IP 3 R)] in cell lines, neurons, and brain microsomes (see ref. 4 for a recent review); (ii) the sarcoendoplasmic reticulum Ca 2+ ATPase (SERCA) has been proposed as a target of presenilins, although opposite regulatory effects have been reported (5, 6); and (iii) it has been suggested that WT presenilins, but not FAD-linked presenilin mutants, form low-conductance Ca 2+ leak channels in the ER membrane (7,8). This last finding supports the "Ca 2+ overload" hypothesis for FAD, which proposes that the reduced ER Ca 2+ leak c...

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Calcium handling by mitochondria is a key feature in cell life. It is involved in energy production for cell activity, in buffering and shaping cytosolic calcium rises and also in determining cell fate by triggering or preventing apoptosis. Both mitochondria and the mechanisms involved in the control of calcium homeostasis have been extensively studied, but they still provide researchers with long-standing or even new challenges. Technical improvements in the tools employed for the investigation of calcium dynamics have been-and are still-opening new perspectives in this field, and more prominently for mitochondria. In this review we present a state-of-the-art toolkit for calcium measurements, with major emphasis on the advantages of genetically encoded indicators. These indicators can be efficiently and selectively targeted to specific cellular sub-compartments, allowing previously unavailable high-definition calcium dynamic studies. We also summarize the main features of cellular and, in more detail, mitochondrial calcium handling, especially focusing on the latest breakthroughs in the field, such as the recent direct characterization of the calcium microdomains that occur on the mitochondrial surface upon cellular stimulation. Additionally, we provide a major example of the key role played by calcium in patho-physiology by briefly describing the extensively reported-albeit highly controversial-alterations of calcium homeostasis in Alzheimer's disease, casting lights on the possible alterations in mitochondrial calcium handling in this pathology.

Parkinson’s disease (PD) is the second most common neurodegenerative disease in the world. Its causes are poorly understood and there is no proven therapeutic strategy for slowing disease progression. The core motor symptoms of PD are caused by the death of dopaminergic neurons in the substantia nigra pars compacta (SNc). In these neurons, Ca2+entry through plasma membrane Cav1 channels drives a sustained feed-forward stimulation of mitochondrial oxidative phosphorylation. Although this design helps prevent bioenergetic failure when activity needs to be sustained, it leads to basal mitochondrial oxidant stress. Over decades, this basal oxidant stress could compromise mitochondrial function and increase mitophagy, resulting in increased vulnerability to other proteostatic stressors, like elevated alpha synuclein expression. Because this feedforward mechanism is no longer demanded by our lifestyle, it could be dispensed with. Indeed, use of dihydropyridines – negative allosteric modulators of Cav1 Ca2+ channels – comes with little or no effect on brain function but is associated with decreased risk and progression of PD. An ongoing, NIH sponsored, Phase 3 clinical trial in North America is testing the ability of one member of the dihydropyridine class (isradipine) to slow PD progression in early stage patients. The review summarizes the rationale for the trial and outlines some unanswered questions.