Mitochondrial trafficking and morphology in neuronal injury

NP1 regulates neuronal activity-dependent accumulation of BAX in mitochondria and mitochondrial dynamics

In cultured cerebellar granule neurons, low neuronal activity triggers the intrinsic program of apoptosis, which requires protein synthesis-dependent BAX translocation to mitochondria, a process that may underlie neuronal damage in neurodegeneration. However, the mechanisms that link neuronal activity with the induction of the mitochondrial program of apoptosis remain unclear. Neuronal pentraxin 1 (NP1) is a pro-apoptotic protein induced by low neuronal activity that is increased in damaged neurites in Alzheimer's disease-affected brains. Here we report that NP1 facilitates the accumulation of BAX in mitochondria and regulates mitochondrial dynamics during apoptosis in rat and mouse cerebellar granule neurons in culture. Reduction of neuronal activity increases NP1 protein levels in mitochondria and contributes to mitochondrial fragmentation in a Bax-dependent manner. In addition, NP1 is involved in mitochondrial transport in healthy neurons. These results show that NP1 is targeted to mitochondria acting upstream of BAX and uncover a novel function for NP1 in the regulation of mitochondrial dynamics and trafficking during apoptotic neurodegeneration.

Glucose levels regulate the nucleo-mitochondrial distribution of Mig2

Mig2 has been described as a transcriptional factor that in the absence of Mig1 protein is required for glucose repression of the SUC2 gene. Thus, until now, the main role assigned to Mig2 has been the functional redundancy to Mig1. In this study, we report that Mig2 has a double subcellular localization. As expected, in high-glucose conditions it is accumulated in the nucleus but in low-glucose conditions Mig2 has an unexpected mitochondrial localization and role in mitochondrial morphology. We describe that Mig2 physically interacts with the mitochondrial protein Ups1 in a glucose-dependent manner. We also show that Δmig2 mutant cells exhibit a fragmented network of mitochondrial tubules, a phenotype similarly observed in cells lacking Fzo1 and Ups1. Furthermore, Mig2 acts antagonistically with respect to the fission-promoting components, because mitochondrial aggregation induced by DNM1 deletion was rescued in the Δdnm1Δmig2 double mutant. Thus, our studies have revealed an additional role for Mig2 as a novel factor required for the maintenance of fusion-competent mitochondria in Saccharomyces cerevisiae and strongly suggest that Mig2 could be involved in the cross talk between the nucleus and the mitochondria through Ups1 to regulate mitochondrial morphology in a glucose dependent manner.

Molecular mechanisms of neonatal brain injury

Fetal/neonatal brain injury is an important cause of neurological disability. Hypoxia-ischemia and excitotoxicity are considered important insults, and, in spite of their acute nature, brain injury develops over a protracted time period during the primary, secondary, and tertiary phases. The concept that most of the injury develops with a delay after the insult makes it possible to provide effective neuroprotective treatment after the insult. Indeed, hypothermia applied within 6 hours after birth in neonatal encephalopathy reduces neurological disability in clinical trials. In order to develop the next generation of treatment, we need to know more about the pathophysiological mechanism during the secondary and tertiary phases of injury. We review some of the critical molecular events related to mitochondrial dysfunction and apoptosis during the secondary phase and report some recent evidence that intervention may be feasible also days-weeks after the insult.

Proteomic analysis of mitochondria in APOE transgenic mice and in response to an ischemic challenge

Apolipoprotein E (APOE)-ɛ4 is associated with a deleterious outcome after ischemic brain injury, which may involve abnormal regulation of mitochondrial function. We have assessed the mitochondrial proteomic response of APOE-ɛ3 and APOE-ɛ4 transgenic mice to transient global ischemic injury in the hippocampus. A genotype-dependent increase in ApoE levels in mitochondria was observed after ischemia, with APOE-ɛ4 mice showing significantly greater increases than APOE-ɛ3 mice. Quantitative analysis of the mitochondria-enriched fractions was performed using liquid-chromatography mass spectrometry coupled to label-free analysis. Of the 1,067 identified proteins, 274 were mitochondria associated. Mitochondrial protein expression was significantly different between genotypes under basal conditions as well as in response to global ischemia. A total of 12 mitochondrial proteins (including respiratory chain proteins NDUFA11, NDUFS3, NDUF5B, ATP5J, as well as ETFA, CYB5B, ATP6V1A, HSPA1B, OXR1, GLUL, IARS2, and PHYHIPL) were significantly altered with respect to genotype, global ischemia, or their interaction (P<0.01). A compelling interactome, created using proteins found to be significantly modulated by global ischemia (P<0.05), involved proteins that regulate energy production and oxidative stress. Thus, APOE genotype has a differential effect on the mitochondrial protein expression in the absence and presence of an injury, which may underlie the differing genotype susceptibility.

Normal role of the low-molecular-weight neurofilament protein in mitochondrial dynamics and disruption in Charcot-Marie-Tooth disease

Intermediate filaments serve important structural roles, but other cellular functions are increasingly recognized. This study demonstrated normal function of the low-molecular-weight neurofilament protein (NFL) in mitochondrial dynamics and disruption in Charcot-Marie-Tooth disease (CMT) due to mutations in the Nefl gene. In motor neurons of spinal cord cultured from Nefl-knockout mice, mitochondrial length and the rate of fusion were decreased concomitant with increased motility. These parameters were normalized after expression of NFL(wt) on the Nefl(-/-) background, but not by overexpression of the profusion protein, mitofusin 2 (MFN2). The effects of CMT-causing NFL mutants bore similarities to and differences from Nefl knockout. In the early phase of toxicity before disruption of the neurofilament network, NFL(Q333P) and NFL(P8R) integrated into neurofilaments and had effects on mitochondria similar to those with Nefl knockout. The reduction of fusion rate by NFL(Q333P) was partly due to interference with the function of the profusion protein MFN2, which is mutated in CMT2A, functionally linking these forms of CMT. In the later phase of toxicity, mitochondria essentially stopped moving in neurons expressing NFL mutants, probably a consequence of cytoskeletal disruption. Overall, the data point to important functions of neurofilaments in mitochondrial dynamics as well as primary involvement in CMT2E/1F.

Mitochondrial trafficking and the provision of energy and calcium buffering at excitatory synapses

Neuronal postsynaptic currents consume most of the brain's energy supply. Delineating how neurons control the distribution, morphology and function of the energy-producing mitochondria that fuel synaptic communication is therefore important for our understanding of nervous system function and pathology. Here we review recent insights into the molecular mechanisms that control activity-dependent regulation of mitochondrial trafficking, morphology and activity at excitatory synapses. We also consider some implications of this regulation for synaptic function and plasticity and discuss how this may contribute to synaptic dysfunction and signalling in neurological disease, with a focus on Alzheimer's disease.

A pivotal role for PINK1 and autophagy in mitochondrial quality control: implications for Parkinson disease

The PTEN-induced putative kinase 1 (PINK1) is a mitochondrially targeted serine-threonine kinase, which is linked to autosomal recessive familial parkinsonism. Current literature implicates PINK1 as a pivotal regulator of mitochondrial quality control, promoting maintenance of respiring mitochondrial networks through cristae stabilization, phosphorylation of chaperones and possibly regulation of mitochondrial transport or autophagy. Pulse-chase studies indicate that PINK1 is rapidly processed into at least two shorter forms, which are distributed in both mitochondrial and cytosolic compartments. Through indirect regulation of mitochondrial proteases and Drp1, PINK1 may act to facilitate localized repair and fusion in response to minor mitochondrial stress. With severe mitochondrial damage, PINK1 facilitates aggregation and clearance of depolarized mitochondria through interactions with Parkin and possibly Beclin1. This switch in function most probably involves altered processing, post-translational modification and/or localization of PINK1, as overexpression of full-length PINK1 is required for mitochondrial Parkin recruitment. Under conditions of PINK1 deficiency, dysregulation of reactive oxygen species, electron transport chain function and calcium homeostasis trigger altered mitochondrial dynamics, indicating compromise of mitochondrial quality control mechanisms. Nevertheless, Parkin- and Beclin1-regulated mitochondrial autophagy remains effective at recycling PINK1-deficient mitochondria; failure of this final tier of mitochondrial quality control contributes to cell death. Thus, PINK1 plays a pivotal, multifactorial role in mitochondrial homeostasis. As autophagic recycling represents the final tier of mitochondrial quality control, whether PINK1 levels are enhanced or reduced, strategies to promote selective mitophagy and mitochondrial biogenesis may prove effective for multiple forms of Parkinson's disease.