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Background: Pathological accumulation of aggregated α-synuclein (aSYN) is a common feature of Parkinson's disease (PD). However, the mechanisms by which intracellular aSYN pathology contributes to dysfunction and degeneration of neurons in the brain are still unclear. A potentially relevant target of aSYN is the mitochondrion. To test this hypothesis, genetic and physiological methods were used to monitor mitochondrial function in substantia nigra pars compacta (SNc) dopaminergic and pedunculopontine nucleus (PPN) cholinergic neurons after stereotaxic injection of aSYN pre-formed fibrils (PFFs) into the mouse brain., Methods: aSYN PFFs were stereotaxically injected into the SNc or PPN of mice. Twelve weeks later, mice were studied using a combination of approaches, including immunocytochemical analysis, cell-type specific transcriptomic profiling, electron microscopy, electrophysiology and two-photon-laser-scanning microscopy of genetically encoded sensors for bioenergetic and redox status., Results: In addition to inducing a significant neuronal loss, SNc injection of PFFs induced the formation of intracellular, phosphorylated aSYN aggregates selectively in dopaminergic neurons. In these neurons, PFF-exposure decreased mitochondrial gene expression, reduced the number of mitochondria, increased oxidant stress, and profoundly disrupted mitochondrial adenosine triphosphate production. Consistent with an aSYN-induced bioenergetic deficit, the autonomous spiking of dopaminergic neurons slowed or stopped. PFFs also up-regulated lysosomal gene expression and increased lysosomal abundance, leading to the formation of Lewy-like inclusions. Similar changes were observed in PPN cholinergic neurons following aSYN PFF exposure., Conclusions: Taken together, our findings suggest that disruption of mitochondrial function, and the subsequent bioenergetic deficit, is a proximal step in the cascade of events induced by aSYN pathology leading to dysfunction and degeneration of neurons at-risk in PD., (© 2024. The Author(s).)

How do neurons match generation of adenosine triphosphate by mitochondria to the bioenergetic demands of regenerative activity? Although the subject of speculation, this coupling is still poorly understood, particularly in neurons that are tonically active. To help fill this gap, pacemaking substantia nigra dopaminergic neurons were studied using a combination of optical, electrophysiological, and molecular approaches. In these neurons, spike-activated calcium (Ca 2+ ) entry through Ca v 1 channels triggered Ca 2+ release from the endoplasmic reticulum, which stimulated mitochondrial oxidative phosphorylation through two complementary Ca 2+ -dependent mechanisms: one mediated by the mitochondrial uniporter and another by the malate-aspartate shuttle. Disrupting either mechanism impaired the ability of dopaminergic neurons to sustain spike activity. While this feedforward control helps dopaminergic neurons meet the bioenergetic demands associated with sustained spiking, it is also responsible for their elevated oxidant stress and possibly to their decline with aging and disease.

Loss of functional mitochondrial complex I (MCI) in the dopaminergic neurons of the substantia nigra is a hallmark of Parkinson's disease 1 . Yet, whether this change contributes to Parkinson's disease pathogenesis is unclear 2 . Here we used intersectional genetics to disrupt the function of MCI in mouse dopaminergic neurons. Disruption of MCI induced a Warburg-like shift in metabolism that enabled neuronal survival, but triggered a progressive loss of the dopaminergic phenotype that was first evident in nigrostriatal axons. This axonal deficit was accompanied by motor learning and fine motor deficits, but not by clear levodopa-responsive parkinsonism-which emerged only after the later loss of dopamine release in the substantia nigra. Thus, MCI dysfunction alone is sufficient to cause progressive, human-like parkinsonism in which the loss of nigral dopamine release makes a critical contribution to motor dysfunction, contrary to the current Parkinson's disease paradigm 3,4 ., (© 2021. The Author(s), under exclusive licence to Springer Nature Limited.)

Degeneration of substantia nigra (SN) dopaminergic (DAergic) neurons is responsible for the core motor deficits of Parkinson's disease (PD). These neurons are autonomous pacemakers that have large cytosolic Ca 2+ oscillations that have been linked to basal mitochondrial oxidant stress and turnover. This review explores the origin of Ca 2+ oscillations and their role in the control of mitochondrial respiration, bioenergetics, and mitochondrial oxidant stress.

Parkinson's disease (PD) is a debilitating neurodegenerative disease characterized by a loss of dopaminergic neurons in the substantia nigra (SN). Although mitochondrial dysfunction and dysregulated α-synuclein (aSyn) expression are postulated to play a role in PD pathogenesis, it is still debated why neurons of the SN are targeted while neighboring dopaminergic neurons of the ventral tegmental area (VTA) are spared. Using electrochemical and imaging approaches, we investigated metabolic changes in cultured primary mouse midbrain dopaminergic neurons exposed to a parkinsonian neurotoxin, 1-methyl-4-phenylpyridinium (MPP + ). We demonstrate that the higher level of neurotoxicity in SN than VTA neurons was due to SN neuron-specific toxin-induced increase in cytosolic dopamine (DA) and Ca 2+ , followed by an elevation of mitochondrial Ca 2+ , activation of nitric oxide synthase (NOS), and mitochondrial oxidation. The increase in cytosolic Ca 2+ was not caused by MPP + -induced oxidative stress, but rather depended on the activity of both L-type calcium channels and aSyn expression, suggesting that these two established pathogenic factors in PD act in concert.

Dopaminergic (DA) neurons in the substantia nigra pars compacta (SNc) are richly innervated by GABAergic neurons. The postsynaptic effects of GABA on SNc DA neurons are mediated by a mixture of GABAA and GABAB receptors. Although activation of GABAA receptors inhibits spike generation, the consequences of GABAB receptor activation are less well characterized. To help fill this gap, perforated patch recordings were made from young adult mouse SNc DA neurons. Sustained stimulation of GABAB receptors hyperpolarized SNc DA neurons, as previously described. However, transient stimulation of GABAB receptors by optical uncaging of GABA did not; rather, it reduced the opening of small-conductance, calcium-activated K+ (SK) channels and increased the irregularity of spiking. This modulation was attributable to inhibition of adenylyl cyclase and protein kinase A. Thus, because suppression of SK channel activity increases the probability of burst spiking, transient co-activation of GABAA and GABAB receptors could promote a pause-burst pattern of spiking., Competing Interests: The authors have declared that no competing interests exist.

Mutations in amyloid precursor protein (APP), and presenilin-1 and presenilin-2 (PS1 and PS2) have causally been implicated in Familial Alzheimer's Disease (FAD), but the mechanistic link between the mutations and the early onset of neurodegeneration is still debated. Although no consensus has yet been reached, most data suggest that both FAD-linked PS mutants and endogenous PSs are involved in cellular Ca2+ homeostasis. We here investigated subcellular Ca2+ handling in primary neuronal cultures and acute brain slices from wild type and transgenic mice carrying the FAD-linked PS2-N141I mutation, either alone or in the presence of the APP Swedish mutation. Compared with wild type, both types of transgenic neurons show a similar reduction in endoplasmic reticulum (ER) Ca2+ content and decreased response to metabotropic agonists, albeit increased Ca2+ release induced by caffeine. In both transgenic neurons, we also observed a higher ER-mitochondria juxtaposition that favors increased mitochondrial Ca2+ uptake upon ER Ca2+ release. A model is described that integrates into a unifying hypothesis the contradictory effects on Ca2+ homeostasis of different PS mutations and points to the relevance of these findings in neurodegeneration and aging., (© 2012 The Authors. Aging Cell © 2012 Blackwell Publishing Ltd/Anatomical Society of Great Britain and Ireland.)

An increase in the concentration of cytosolic free Ca(2+) is a key component regulating different cellular processes ranging from egg fertilization, active secretion and movement, to cell differentiation and death. The multitude of phenomena modulated by Ca(2+), however, do not simply rely on increases/decreases in its concentration, but also on specific timing, shape and sub-cellular localization of its signals that, combined together, provide a huge versatility in Ca(2+) signaling. Intracellular organelles and their Ca(2+) handling machineries exert key roles in this complex and precise mechanism, and this review will try to depict a map of Ca(2+) routes inside cells, highlighting the uniqueness of the different Ca(2+) toolkit components and the complexity of the interactions between them.

Presenilin (PS) mutations are the main cause of Familial Alzheimer's Disease (FAD) and have been demonstrated to cause an imbalance of intracellular Ca(2+) homeostasis. Though PS1 and 2 are generally considered to behave similarly in terms of their effects on Ca(2+) handling, we have recently described a novel function, which is unique to PS2, i.e., the modulation of ER-mitochondria juxtaposition. Accordingly, PS2, but not PS1, affects the Ca(2+) cross-talk between these organelles, a key feature in determining cell fate. In particular, PS2 overexpression, and more drastically that of FAD-linked PS2 mutants, strongly increases the interaction between ER and mitochondria, thus facilitating mitochondrial Ca(2+) uptake. The likely mechanisms behind this phenomenon and its potential effects in cell physiology and pathology are discussed.

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.

We have previously shown that familial Alzheimer's disease mutants of presenilin-2 (PS2) and, to a lesser extent, of presenilin-1 (PS1) lower the Ca(2+) concentration of intracellular stores. We here examined the mechanism by which wild-type and mutant PS2 affect store Ca(2+) handling. By using HeLa, SH-SY5Y and MEFs as model cells, and recombinant aequorins as Ca(2+) probes, we show evidence that transient expression of either wild-type or mutant PS2 increases the passive Ca(2+) leakage: both ryanodine- and IP(3)-receptors contribute to Ca(2+) exit out of the ER, whereas the ribosome translocon complex is not involved. In SH-SY5Y cells and MEFs, wild-type and mutant PS2 potently reduce the uptake of Ca(2+) inside the stores, an effect that can be counteracted by over-expression of SERCA-2B. On this line, in wild-type MEFs, lowering the endogenous level of PS2 by RNA interference, increases the Ca(2+)-loading capability of intracellular stores. Furthermore, we show that in PS double knockout MEFs, reduction of Ca(2+) stores is mimicked by the expression of PS2-D366A, a loss-of-function mutant, uncleaved because also devoid of presenilinase activity but not by co-expression of the two catalytic active fragments of PS2. In summary, both physiological and increased levels of wild-type and mutant PS2 reduce the Ca(2+) uptake by intracellular stores. To exert this newly described function, PS2 needs to be in its full-length form, even if it can subsequently be cleaved.