Mitochondrial trafficking to synapses in cultured primary cortical neurons

Intracellular zinc release, 12-lipoxygenase activation and MAPK dependent neuronal and oligodendroglial death

Zinc translocation from presynaptic nerve terminals to postsynaptic neurons has generally been considered the critical step leading to the accumulation of intracellular free zinc and subsequent neuronal injury. Recent evidence, however, strongly suggests that the liberation of zinc from intracellular stores upon oxidative and nitrative stimulation contributes significantly to the toxicity of this metal not only to neurons, but also to oligodendrocytes. The exact cell death signaling pathways triggered by zinc are beginning to be deciphered. In this review, we describe how the activation of 12-lipoxygenase and mitogen-activated protein kinase (MAPK) contribute to the toxicity of liberated zinc to neurons and oligodendrocytes.

A constitutive, transient receptor potential-like Ca2+ influx pathway in presynaptic nerve endings independent of voltage-gated Ca2+ channels and Na+/Ca2+ exchange

Calcium levels in the presynaptic nerve terminal are altered by several pathways, including voltage-gated Ca(2+) channels, the Na(+)/Ca(2+) exchanger, Ca(2+)-ATPase, and the mitochondria. The influx pathway for homeostatic control of [Ca(2+)](i) in the nerve terminal has been unclear. One approach to detecting the pathway that maintains internal Ca(2+) is to test for activation of Ca(2+) influx following Ca(2+) depletion. Here, we demonstrate that a constitutive influx pathway for Ca(2+) exists in presynaptic terminals to maintain internal Ca(2+) independent of voltage-gated Ca(2+) channels and Na(+)/Ca(2+) exchange, as measured in intact isolated nerve endings from mouse cortex and in intact varicosities in a neuronal cell line using fluorescence spectroscopy and confocal imaging. The Mg(2+) and lanthanide sensitivity of the influx pathway, in addition to its pharmacological and short hairpin RNA sensitivity, and the results of immunostaining for transient receptor potential (TRP) channels indicate the involvement of TRPC channels, possibly TRPC5 and TRPC1. This constitutive Ca(2+) influx pathway likely serves to maintain synaptic function under widely varying levels of synaptic activity.

Mitochondrial Dynamics and Peripheral Neuropathy

Peripheral neuropathy is perhaps the archetypal disease of axonal degeneration, characteristically involving degeneration of the longest axons in the body. Evidence from both inherited and acquired forms of peripheral neuropathy strongly supports that the primary pathology is in the axons themselves and points to disruption of axonal transport as an important disease mechanism. Recent studies in human genetics have further identified abnormalities in mitochondrial dynamics—the fusion, fission, and movement of mitochondria— as a player in the pathogenesis of inherited peripheral neuropathy. This review provides an update on the mechanisms of mitochondrial trafficking in axons and the emerging relationship between the disruption of mitochondrial dynamics and axonal degeneration. Evidence suggests mitochondria are a “critical cargo” whose transport is necessary for proper axonal and synaptic function. Importantly, understanding the regulation of mitochondrial movement and the consequences of decreased axonal mitochondrial function may define new paths for therapeutic agents in peripheral neuropathy and other neurodegenerative diseases. NEUROSCIENTIST 14(1):12—18, 2008. DOI: 10.1177/1073858407307354

Local influence of mitochondrial calcium transport in retinal amacrine cells

Ca2+-dependent synaptic transmission from retinal amacrine cells is thought to be initiated locally at dendritic processes. Hence, understanding the spatial and temporal impact of Ca2+ transport is fundamental to understanding how amacrine cells operate. Here, we provide the first examination of the local effects of mitochondrial Ca2+ transport in neuronal processes. By combining mitochondrial localization with measurements of cytosolic Ca2+, the local impacts of mitochondrial Ca2+ transport for two types of Ca2+ signals were investigated. Disruption of mitochondrial Ca2+ uptake with carbonyl cyanide 4-(trifluoromethoxy) phenylhydrazone (FCCP) produces cytosolic Ca2+ elevations. The amplitudes of these elevations decline with distance from mitochondria suggesting that they are related to mitochondrial Ca2+ transport. The time course of the FCCP-dependent Ca2+ elevations depend on the availability of ER Ca2+ and we provide evidence that Ca2+ is released primarily via nearby ryanodine receptors. These results indicate that interactions between the ER and mitochondria influence cytosolic Ca2+ in amacrine cell processes and cell bodies. We also demonstrate that the durations of glutamate-dependent Ca2+ elevations are dependent on their proximity to mitochondria in amacrine cell processes. Consistent with this observation, disruption of mitochondrial Ca2+ transport alters the duration of glutamate-dependent Ca2+ elevations near mitochondria but not at sites more than 10 microm away. These results indicate that mitochondria influence local Ca2+-dependent signaling in amacrine cell processes.

Proteomic analysis of activity-dependent synaptic plasticity in hippocampal neurons

Following long-term treatment with bicuculline and tetrodotoxin (TTX) aimed at modifying synaptic activity in cultured neurons, we used a proteomic approach to identify the associated changes in protein expression. The neurons were left untreated, or treated with bicuculline or TTX, and fractionated by means of differential detergent extraction, after which the proteins in each fraction were separated by means of two-dimensional (2D) gel electrophoresis, and 57 proteins of interest were identified by mass spectrometry. The proteins that showed altered expression and/or post-translational modifications include proteins or enzymes involved in regulating cell and protein metabolism, the cytoskeleton, or mitochondrial activity. These results suggest that extensive alterations in neuronal protein expression take place as a result of increased or decreased synaptic activity.

Intracellular zinc release, 12-lipoxygenase activation and MAPK dependent neuronal and oligodendroglial death

Zinc translocation from presynaptic nerve terminals to postsynaptic neurons has generally been considered the critical step leading to the accumulation of intracellular free zinc and subsequent neuronal injury. Recent evidence, however, strongly suggests that the liberation of zinc from intracellular stores upon oxidative and nitrative stimulation contributes significantly to the toxicity of this metal not only to neurons, but also to oligodendrocytes. The exact cell death signaling pathways triggered by zinc are beginning to be deciphered. In this review, we describe how the activation of 12-lipoxygenase and mitogen-activated protein kinase (MAPK) contribute to the toxicity of liberated zinc to neurons and oligodendrocytes.

Abeta oligomers induce neuronal oxidative stress through an N-methyl-D-aspartate receptor-dependent mechanism that is blocked by the Alzheimer drug memantine

Oxidative stress is a major aspect of Alzheimer disease (AD) pathology. We have investigated the relationship between oxidative stress and neuronal binding of Abeta oligomers (also known as ADDLs). ADDLs are known to accumulate in brain tissue of AD patients and are considered centrally related to pathogenesis. Using hippocampal neuronal cultures, we found that ADDLs stimulated excessive formation of reactive oxygen species (ROS) through a mechanism requiring N-methyl-d-aspartate receptor (NMDA-R) activation. ADDL binding to neurons was reduced and ROS formation was completely blocked by an antibody to the extracellular domain of the NR1 subunit of NMDA-Rs. In harmony with a steric inhibition of ADDL binding by NR1 antibodies, ADDLs that were bound to detergent-extracted synaptosomal membranes co-immunoprecipitated with NMDA-R subunits. The NR1 antibody did not affect ROS formation induced by NMDA, showing that NMDA-Rs themselves remained functional. Memantine, an open channel NMDA-R antagonist prescribed as a memory-preserving drug for AD patients, completely protected against ADDL-induced ROS formation, as did other NMDA-R antagonists. Memantine and the anti-NR1 antibody also attenuated a rapid ADDL-induced increase in intraneuronal calcium, which was essential for stimulated ROS formation. These results show that ADDLs bind to or in close proximity to NMDA-Rs, triggering neuronal damage through NMDA-R-dependent calcium flux. This response provides a pathologically specific mechanism for the therapeutic action of memantine, indicates a role for ROS dysregulation in ADDL-induced cognitive impairment, and supports the unifying hypothesis that ADDLs play a central role in AD pathogenesis.

ADP regulates movements of mitochondria in neurons

Mitochondria often reside in subcellular regions with high metabolic demands. We examined the mechanisms that can govern the relocation of mitochondria to these sites in respiratory neurons. Mitochondria were visualized using tetramethylrhodamineethylester, and their movements were analyzed by applying single-particle tracking. Intracellular ATP ([ATP](i)) was assessed by imaging the luminescence of luciferase, the fluorescence of the ATP analog TNP-ATP, and by monitoring the activity of K(ATP) channels. Directed movements of mitochondria were accompanied by transient increases in TNP-ATP fluorescence. Application of glutamate and hypoxia reversibly decreased [ATP](i) levels and inhibited the directed transport. Injections of ATP did not rescue the motility of mitochondria after its inhibition by hypoxia. Introduction of ADP suppressed mitochondrial movements and occluded the effects of subsequent hypoxia. Mitochondria decreased their velocity in the proximity of synapses that correlated with local [ATP](i) depletions. Using a model of motor-assisted transport and Monte Carlo simulations, we showed that mitochondrial traffic is more sensitive to increases in [ADP](i) than to [ATP](i) depletions. We propose that consumption of synaptic ATP can produce local increases in [ADP](i) and facilitate the targeting of mitochondria to synapses.

Simultaneous age-related depolarization of mitochondrial membrane potential and increased mitochondrial reactive oxygen species production correlate with age-related glutamate excitotoxicity in rat hippocampal neurons

Mitochondria are implicated in glutamate excitotoxicity by causing bioenergetic collapse, loss of Ca(2+) homeostasis, and generation of reactive oxygen species (ROS), all of which become increasingly important clinically with age. Little is known about how aging affects the relative importance of mitochondrial membrane potential (DeltaPsi(m)) and ROS production. To determine aging affects on DeltaPsi(m) and ROS production in individual somal and axonal/dendritic mitochondria, we compared ROS production while simultaneously monitoring DeltaPsi(m) before and after glutamate treatment of live neurons from embryonic (day 18), middle-aged (9-12 months), and old (24 months) rats. At rest, old neuronal mitochondria 1) showed a higher rate of ROS production that was particularly strong in axonal/dendritic mitochondria relative to that in middle-age neurons, 2) were more depolarized in comparison with neurons of other ages, and 3) showed no differences in ROS or DeltaPsi(m) as a function of distance from the nucleus. All DeltaPsi(m) grouped into three classes of high (less than -120 mV), medium (-85 to -120 mV), and low (greater than -85 mV) polarization that shifted toward the lower classes with age at rest. Glutamate exposure dramatically depolarized the DeltaPsi(m) in parallel with greatly increased ROS production, with a surprising absence of an effect of age or distance from the nucleus on these mitochondrial parameters. These data suggest that old neurons are more susceptible to glutamate excitotoxicity because of an insidious depolarization of DeltaPsi(m) and rate of ROS generation at rest that lead to catastrophic failure of phosphorylative and reductive energy supplies under stress.

Neuron-specific conditional expression of a mitochondrially targeted fluorescent protein in mice

Mitochondrial dysfunction contributes to the pathophysiology of both acute and chronic neurodegenerative disorders. Quantification of mitochondrial bioenergetic properties generally requires the use of isolated brain mitochondria. However, the involvement of neuronal mitochondrial dysfunction in these disorders is limited by the lack of markers, and therefore isolation procedures, that distinguish neuronal compared with astrocyte mitochondria. To address this and other issues concerning neuronal mitochondria in the CNS, transgenic mice were generated that express a fluorescent protein targeted specifically to neurons. A neuron-specific promoter, CaMKIIalpha (calcium/calmodulin-dependent kinase IIalpha) driven tTA (tetracycline transactivator) mice were crossed with TRE (tetracycline responsive element) driven mitochondrial targeted enhanced yellow fluorescent protein (eYFP) mice. Expression of eYFP in the bigenic mouse brain was observed only in neuronal mitochondria of striatum, forebrain, and hippocampus and was enhanced by the removal of the tetracycline analog doxycycline (Dox) in the diet. The respiratory control ratio of synaptic and nonsynaptic mitochondria isolated from eYFP-expressing mice was the same as control mice, suggesting that neuronal mitochondria expressing eYFP maintain normal bioenergetic functions. More importantly, the development of Dox-inducible, neuron targeted mito/eYFP transgenic mice offer a unique in vivo model for delineating the participation of neuronal mitochondria in neuronal survival and death.

Mitochondrial trafficking and morphology in healthy and injured neurons

Mitochondria are the primary generators of ATP and are important regulators of intracellular calcium homeostasis. These organelles are dynamically transported along lengthy neuronal processes, presumably for appropriate distribution to cellular regions of high metabolic demand and elevated intracellular calcium, such as synapses. The removal of damaged mitochondria that produce harmful reactive oxygen species and promote apoptosis is also thought to be mediated by transport of mitochondria to autophagosomes. Mitochondrial trafficking is therefore important for maintaining neuronal and mitochondrial health while cessation of movement may lead to neuronal and mitochondrial dysfunction. Mitochondrial morphology is also dynamic and is remodeled during neuronal injury and disease. Recent studies reveal different manifestations and mechanisms of impaired mitochondrial movement and altered morphology in injured neurons. These are likely to cause varied courses toward neuronal degeneration and death. The goal of this review is to provide an appreciation of the full range of mitochondrial function, morphology and trafficking, and the critical role these parameters play in neuronal physiology and pathophysiology.

Acute impairment of mitochondrial trafficking by beta-amyloid peptides in hippocampal neurons

Defects in axonal transport are often associated with a wide variety of neurological diseases including Alzheimer's disease (AD). Beta-amyloid (Abeta) is a major component of neuritic plaques associated with pathological conditions of AD brains. Here, we report that a brief exposure of cultured hippocampal neurons to Abeta molecules resulted in rapid and severe impairment of mitochondrial transport without inducing apparent cell death and significant morphological changes. Such acute inhibition of mitochondrial transport was not associated with a disruption of mitochondria potential nor involved aberrant cytoskeletal changes. Abeta also did not elicit significant Ca2+ signaling to affect mitochondrial trafficking. However, stimulation of protein kinase A (PKA) by forskolin, cAMP analogs, or neuropeptides effectively alleviated the impairment. We also show that Abeta inhibited mitochondrial transport by acting through glycogen synthase kinase 3beta (GSK3beta). Given that mitochondria are crucial organelles for many cellular functions and survival, our findings thus identify an important acute action of Abeta molecules on nerve cells that could potentially contribute to various abnormalities of neuronal functions under AD conditions. Manipulation of GSK3beta and PKA activities may represent a key approach for preventing and alleviating Abeta cytotoxicity and AD pathological conditions.