Abnormal Mitochondrial Distribution Research Articles

Geniposide Alleviates Amyloid-Induced Synaptic Injury by Protecting Axonal Mitochondrial Trafficking

Synaptic and mitochondrial pathologies are early events in the progression of Alzheimer's disease (AD). Normal axonal mitochondrial function and transport play crucial roles in maintaining synaptic function by producing high levels of adenosine triphosphate and buffering calcium. However, there can be abnormal axonal mitochondrial trafficking, distribution, and fragmentation, which are strongly correlated with amyloid-β (Aβ)-induced synaptic loss and dysfunction. The present study examined the neuroprotective effect of geniposide, a compound extracted from gardenia fruit in Aβ-treated neurons and an AD mouse model. Geniposide alleviated Aβ-induced axonal mitochondrial abnormalities by increasing axonal mitochondrial density and length and improving mitochondrial motility and trafficking in cultured hippocampal neurons, consequently ameliorating synaptic damage by reversing synaptic loss, addressing spine density and morphology abnormalities, and ameliorating the decreases in synapse-related proteins in neurons and APPswe/PS1dE9 mice. These findings provide new insights into the effects of geniposide administration on neuronal and synaptic functions under conditions of Aβ enrichment.

RAD51 maintains chromosome integrity and mitochondrial distribution during porcine oocyte maturation in vitro.

DNA repair protein RAD51 homolog 1 (RAD51) plays a central role in homologous recombination (HR) repair of DNA breaks. HR depends on the formation of a RAD51 recombinase filament that facilitates strand invasion. However, the role of RAD51 during porcine oocyte maturation is unknown. The objective of this study was to investigate the expression and function of RAD51 during porcine oocyte maturation in vitro. RAD51 was mainly localized to the nucleus at the germinal vesicle (GV) stage, and was widely distributed in the cytoplasm between the GV breakdown (GVBD) and metaphase II stage. DNA damage induced by etoposide was accompanied by the formation of RAD51 foci that were colocalized with γH2AX. Inhibition of RAD51 increased DNA damage and induced metaphase I arrest along with spindle defects, chromosomal misalignment, and abnormal spindle assembly checkpoint (SAC) activity. Inhibition of RAD51 also increased ROS levels and led to an abnormal mitochondrial distribution. Our results indicate that RAD51 plays a critical role in maintaining chromosome integrity and mitochondrial activity during porcine oocyte maturation.

Mutations in TFAM, encoding mitochondrial transcription factor A, cause neonatal liver failure associated with mtDNA depletion.

In humans, mitochondrial DNA (mtDNA) depletion syndromes are a group of genetically and clinically heterogeneous autosomal recessive disorders that arise as a consequence of defects in mtDNA replication or nucleotide synthesis. Clinical manifestations are variable and include myopathic, encephalomyopathic, neurogastrointestinal or hepatocerebral phenotypes. Through clinical exome sequencing, we identified a homozygous missense variant (c.533C>T; p.Pro178Leu) in mitochondrial transcription factor A (TFAM) segregating in a consanguineous kindred of Colombian-Basque descent in which two siblings presented with IUGR, elevated transaminases, conjugated hyperbilirubinemia and hypoglycemia with progression to liver failure and death in early infancy. Results of the liver biopsy in the proband revealed cirrhosis, micro- and macrovesicular steatosis, cholestasis and mitochondrial pleomorphism. Electron microscopy of muscle revealed abnormal mitochondrial morphology and distribution while enzyme histochemistry was underwhelming. Electron transport chain testing in muscle showed increased citrate synthase activity suggesting mitochondrial proliferation, while respiratory chain activities were at the lower end of normal. mtDNA content was reduced in liver and muscle (11% and 21% of normal controls respectively). While Tfam mRNA expression was upregulated in primary fibroblasts, Tfam protein level was significantly reduced. Furthermore, functional investigations of the mitochondria revealed reduced basal respiration and spare respiratory capacity, decreased mtDNA copy number and markedly reduced nucleoids. TFAM is essential for transcription, replication and packaging of mtDNA into nucleoids. Tfam knockout mice display embryonic lethality secondary to severe mtDNA depletion. In this report, for the first time, we associate a homozygous variant in TFAM with a novel mtDNA depletion syndrome.

Mitochondrial abnormalities in the myofibrillar myopathies.

Myofibrillar myopathies are a genetically diverse group of skeletal muscle disorders, with distinctive muscle histopathology. Causative mutations have been identified in the genes MYOT, LDB3, DES, CRYAB, FLNC, BAG3, DNAJB6, FHL1, PLEC and TTN, which encode proteins which either reside in the Z-disc or associate with the Z-disc. Mitochondrial abnormalities have been described in muscle from patients with a myofibrillar myopathy. We reviewed the literature to determine the extent of mitochondrial dysfunction in each of the myofibrillar myopathy subtypes. Abnormal mitochondrial distribution is a frequent finding in each of the subtypes, but a high frequency of COX-negative or ragged red fibres, a characteristic finding in some of the conventional mitochondrial myopathies, is a rare finding. Few invitro studies of mitochondrial function have been performed in affected patients.

Distribution pattern of cytoplasmic organelles, spindle integrity, oxidative stress, octamer-binding transcription factor 4 (Oct4) expression and developmental potential of oocytes following multiple superovulation.

The aim of the present study was to determine the effects of repeated superovulation on oocyte quality and embryo developmental potential. Female Swiss albino mice were injected with 5IU pregnant mare's serum gonadotropin followed 48h by 10IU human chorionic gonadotropin. Mice were superovulated up to four times with a gap of 7 days between each superovulation cycle. Ovarian weight increased significantly with an increasing number of superovulation cycles. Although the first stimulation cycle resulted in a threefold increase in the number of oocytes, the number of oocytes decreased gradually after subsequent stimulations. Increased cytoplasmic fragmentation, abnormal mitochondrial distribution, aggregation of Golgi apparatus, spindle damage, increased intracellular oxidative stress and a decrease in expression of octamer-binding transcription factor 4 (Oct4) expression were observed in these oocytes. Further, embryos derived from mice subjected to multiple stimulation cycles exhibited a low blastocyst rate, decreased hatching rate and increased apoptosis in blastocysts. In conclusion, the present study demonstrates that repeated superovulation adversely affects mouse oocyte quality by altering the distribution of cytoplasmic organelles, increasing oxidative stress and decreasing Oct4 expression, resulting in poor developmental potential of the embryos.

Adverse effects of obesity and/or high-fat diet on oocyte quality and metabolism are not reversible with resumption of regular diet in mice.

Obesity adversely affects reproduction and results in oocyte defects in both mice and humans. In the present study we used a mouse model to examine whether the adverse effects of an obesogenic diet on oocyte metabolism and morphology can be reversed by return to a control diet. The intervention group consisted of C57BL6/J mice placed on a high-fat diet (HFD; 35.8% fat and 20.2% protein by nutritional content) for 6 weeks and then switched to an isocaloric control diet (CD; 13% fat and 25% protein) for 8 weeks (HFD/CD mice). The control group consisted of age-matched C57BL6/J mice maintained on CD for 14 weeks (CD/CD mice). Although metabolic parameters (weight, glucose tolerance and cholesterol levels) of HFD/CD mice returned to normal after this 'diet reversal' period, several oocyte defects were not reversible. These HFD/CD oocytes demonstrated significantly higher percentages of abnormal meiotic spindles, lower mitochondrial membrane potential and lower ATP and citrate levels, and higher percentages of abnormal lipid accumulation and mitochondrial distribution compared with CD/CD mice. These results suggest that the negative effects of an obesogenic diet on oocyte quality are not reversible, despite reversal of metabolic parameters. These data may provide better insight when counselling obese women regarding reproductive options and success.

Miro1 is Dispensable for Calcium-Mediated Inhibition of Mitochondrial Movement

Miro1 and 2 are (Rhot1 and 2) are two highly similar GTPases that are bound to the surface of mitochondria and possess EF-hand calcium binding motifs. Several groups have reported that Miro is involved in mitochondrial motility and inheritance, and particularly its calcium regulation, but the roles of the two isoforms have not been established. Genetic deletion of Miro1 in mouse is lethal at birth (Nguyen et al., 2014) and fibroblasts (MEFs) derived from Miro1 KO embryos show abnormal mitochondrial distribution, but the calcium-dependent inhibition of motility is unaffected and the respiratory and calcium buffering capacities are normal. Neuron-specific knockout of Miro1 leads to progressive deficits of upper motor neuron function, however mitochondria in processes of cortical neurons from Miro1 KO and wild-type embryos showed comparable calcium-sensitive motility inhibition. While no significant increase in Miro2 protein was observed in Miro1 KO MEFs, these data raise two possibilities: Miro1 and 2 are interchangeable with regard to calcium regulation of mitochondrial motility or Miro2 is the key player in this regard. To finally resolve this question, we are in the process of generating Miro2 KO and Miro1/2 KO cell lines.

Skeletal muscle proteomic signature and metabolic impairment in pulmonary hypertension.

Exercise limitation comes from a close interaction between cardiovascular and skeletal muscle impairments. To better understand the implication of possible peripheral oxidative metabolism dysfunction, we studied the proteomic signature of skeletal muscle in pulmonary arterial hypertension (PAH). Eight idiopathic PAH patients and eight matched healthy sedentary subjects were evaluated for exercise capacity, skeletal muscle proteomic profile, metabolism, and mitochondrial function. Skeletal muscle proteins were extracted, and fractioned peptides were tagged using an iTRAQ protocol. Proteomic analyses have documented a total of 9 downregulated proteins in PAH skeletal muscles and 10 upregulated proteins compared to healthy subjects. Most of the downregulated proteins were related to mitochondrial structure and function. Focusing on skeletal muscle metabolism and mitochondrial health, PAH patients presented a decreased expression of oxidative enzymes (pyruvate dehydrogenase, p < 0.01) and an increased expression of glycolytic enzymes (lactate dehydrogenase activity, p < 0.05). These findings were supported by abnormal mitochondrial morphology on electronic microscopy, lower citrate synthase activity (p < 0.01) and lower expression of the transcription factor A of the mitochondria (p < 0.05), confirming a more glycolytic metabolism in PAH skeletal muscles. We provide evidences that impaired mitochondrial and metabolic functions found in the lungs and the right ventricle are also present in skeletal muscles of patients. • Proteomic and metabolic analysis show abnormal oxidative metabolism in PAH skeletal muscle. • EM of PAH patients reveals abnormal mitochondrial structure and distribution. • Abnormal mitochondrial health and function contribute to exercise impairments of PAH. • PAH may be considered a vascular affliction of heart and lungs with major impact on peripheral muscles.

Oxidative stress-mediated activation of extracellular signal-regulated kinase contributes to mild cognitive impairment-related mitochondrial dysfunction

Mild cognitive impairment (MCI) occurs during the predementia stage of Alzheimer disease (AD) and is characterized by a decline in cognitive abilities that frequently represents a transition between normal cognition and AD dementia. Its pathogenesis is not well understood. Here, we demonstrate the direct consequences and potential mechanisms of oxidative stress and mitochondrial dynamic and functional defects in MCI-derived mitochondria. Using a cytoplasmic hybrid (cybrid) cell model in which mitochondria from MCI or age-matched non-MCI subjects were incorporated into a human neuronal cell line depleted of endogenous mitochondrial DNA, we evaluated the mitochondrial dynamics and functions, as well as the role of oxidative stress in the resultant cybrid lines. We demonstrated that increased expression levels of mitofusin 2 (Mfn2) are markedly induced by oxidative stress in MCI-derived mitochondria along with aberrant mitochondrial functions. Inhibition of oxidative stress rescues MCI-impaired mitochondrial fusion/fission balance as shown by the suppression of Mfn2 expression, attenuation of abnormal mitochondrial morphology and distribution, and improvement in mitochondrial function. Furthermore, blockade of MCI-related stress-mediated activation of extracellular signal-regulated kinase (ERK) signaling not only attenuates aberrant mitochondrial morphology and function but also restores mitochondrial fission and fusion balance, in particular inhibition of overexpressed Mfn2. Our results provide new insights into the role of the oxidative stress–ERK–Mfn2 signal axis in MCI-related mitochondrial abnormalities, indicating that the MCI phase may be targetable for the development of new therapeutic approaches that improve mitochondrial function in age-related neurodegeneration.

Effects of postmortem interval on mouse ovary oocyte survival and maturation.

To study the time- and temperature-dependent survival of ovarian oocytes collected from postmortem carcass, ICR mice were killed and placed for different periods (0, 1, 2, 4, 6, 8 and 10 h) at different temperatures (25°C, 4°C and 37°C). After preservation, oocyte morphology, germinal vesicle (GV) oocyte number, oocyte meiotic maturation percentage, mitochondrial distribution and intracellular glutathione (GSH) level were evaluated. The results showed no surviving oocytes could be collected by 2h, 6h, and 12 h after carcass preservation at 37°C, 25°C and 4°C, respectively. The number of collected GV oocytes in the ovary deceased as the preservation time lasted at the same temperature. Meanwhile at the same point in time, the ratio of germinal vesicle breakdown (GVBD) and the first polar body emission (PBE) gradually reduced as preservation temperature increased. In addition, the percentage of abnormal mitochondrial distribution in the preserved oocytes was obviously higher than that in the control oocytes, while GSH level was not altered in collected oocytes. Unexpectedly, neither chromosome arrangement nor spindle organization was affected as long as the oocytes from preserved carcasses could complete maturation. These data are helpful for proper use of ovary oocytes from postmortem carcass of valuable individuals.

Silencing of the Charcot–Marie–Tooth disease-associated gene GDAP1 induces abnormal mitochondrial distribution and affects Ca2 + homeostasis by reducing store-operated Ca2 + entry

GDAP1 is an outer mitochondrial membrane protein that acts as a regulator of mitochondrial dynamics. Mutations of the GDAP1 gene cause Charcot–Marie–Tooth (CMT) neuropathy. We show that GDAP1 interacts with the vesicle-organelle trafficking proteins RAB6B and caytaxin, which suggests that GDAP1 may participate in the mitochondrial movement within the cell. GDAP1 silencing in the SH-SY5Y cell line induces abnormal distribution of the mitochondrial network, reduces the contact between mitochondria and endoplasmic reticulum (ER) and alters the mobilization of mitochondria towards plasma membrane upon depletion of ER-Ca2+ stores. GDAP1 silencing does not affect mitochondrial Ca2+ uptake, ER-Ca2+, or Ca2+ flow from ER to mitochondria, but reduces Ca2+ inflow through store-operated Ca2+ entry (SOCE) following mobilization of ER-Ca2+ and SOCE-driven Ca2+ entry in mitochondria. Our studies suggest that the pathophysiology of GDAP1-related CMT neuropathies may be associated with abnormal distribution and movement of mitochondria throughout cytoskeleton towards the ER and subplasmalemmal microdomains, resulting in a decrease in SOCE activity and impaired SOCE-driven Ca2+ uptake in mitochondria.

Significant changes in mitochondrial distribution in different pain models of mice

Mitochondria play an important role in pathophysiology of inflammatory and neuropathic pain but the mechanism is unclear. So far no comprehensive study exists that evaluates the changes of mitochondrial dynamics following the pain. In this study, we detected the mitochondrial distribution and subcellular morphology by using intrathecal injection of mitochondrial marker, Mitotracker Red® CM-H2XRox (Mito-Red) and confocal microscopic analysis in models of formalin-induced acute inflammatory pain, Complete Freund's Adjuvant (CFA)-induced persistent pain and spared nerve injury (SNI)-induced neuropathic pain. The results demonstrated that subcutaneous formalin injection did not affect the number of Mito-Red cells within the spinal dorsal horn at both acute and tonic phases, but significantly increased the number of cluster type mitochondria in superficial spinal dorsal horn (laminas I–II) at tonic phase. Differently, the number of Mito-Red cells significantly increased in superficial and deep spinal dorsal horn (laminas III–V) following persistent CFA and SNI neuropathic pain. Moreover, both CFA and SNI remarkably increased the number of cluster type mitochondria and decreased the number of granule type mitochondria, in both superficial and deep spinal dorsal horn. So we concluded that abnormal mitochondrial distribution contributes to neuropathic and some forms of inflammatory pain.

Effects of Hoechst 33342 staining and ultraviolet irradiation on mitochondrial distribution and DNA copy number in porcine oocytes and preimplantation embryos

Hoechst 33342 (H342), in combination with ultraviolet (UV) irradiation, is frequently used to aid or confirm the enucleation of porcine oocytes in somatic cell nuclear transfer programs. The exposure of oocytes to H342 and UV irradiation has a deleterious effect on the development of in vitro-fertilized porcine oocytes, with increasing exposure to UV irradiation (up to 30 sec) having more drastic effects. It has been hypothesized that this decrease in embryonic development could be due to damage to the mitochondrial DNA (mtDNA). To investigate this hypothesis, we analyzed the mitochondrial distribution and DNA copy number of in vitro-matured porcine oocytes exposed to H342/UV and the subsequent embryonic development compared with the mitochondrial distribution and DNA copy number of in vivo-derived oocytes and embryos. Using quantitative, real-time polymerase chain reaction (qPCR) protocols to analyze mtDNA and confocal laser scanning microscopy with MitoTracker Deep Red to determine mitochondrial distribution, we demonstrated that the simultaneous exposure of in vitro-matured porcine oocytes to H342 staining and UV irradiation is associated with reduced oocyte developmental competence and abnormal mitochondrial distribution in the resulting cleaved embryos. In addition, 2- to 4-cell embryos derived from oocytes exposed to H342/UV showed a significant decrease in mtDNA copy number. These results should be considered when H342/UV procedure is used during nuclear transfer in recipient porcine oocytes.

Comparative Cytotoxicity Study of Water-Soluble Carbon Nanoparticles on Plant Cells

This study provides a parallel cytotoxicity study of water-soluble single-walled carbon nanotubes and fullerene C70(C(COOH)2)4-8 on Nicotiana tabacum BY-2 cells. After BY-2 cells were treated with single-walled carbon nanotubes or C70(C(COOH)2)4-8, the cells adopted a shrunken morphology and exhibited lower proliferation rate. Fluorescence imaging of the treated cells revealed increased reactive oxygen species generation, abnormal mitochondrial distribution, decreased mitochondrial activity, and impaired actin cytoskeleton arrangement. However, no obvious cells death was induced by either single-walled carbon nanotubes or C70(C(COOH)2)4-8. Compared with single-walled carbon nanotubes, C70(C(COOH)2)4-8 exerted more serious adverse effects on BY-2 cells.

Microtubule Affinity-regulating Kinase 2 (MARK2) Turns on Phosphatase and Tensin Homolog (PTEN)-induced Kinase 1 (PINK1) at Thr-313, a Mutation Site in Parkinson Disease: EFFECTS ON MITOCHONDRIAL TRANSPORT

The kinase MARK2/Par-1 plays key roles in several cell processes, including neurodegeneration such as Alzheimer disease by phosphorylating tau and detaching it from microtubules. In search of interaction partners of MARK2, we identified phosphatase and tensin homolog (PTEN)-induced kinase 1 (PINK1), which is important for the survival of neurons and whose mutations are linked to familial Parkinson disease (PD). MARK2 phosphorylated and activated the cleaved form of PINK1 (ΔN-PINK1; amino acids 156-581). Thr-313 was the primary phosphorylation site, a residue mutated to a non-phosphorylatable form (T313M) in a frequent variant of PD. Mutation of Thr-313 to Met or Glu in PINK1 showed toxic effects with abnormal mitochondrial distribution in neurons. MARK2 and PINK1 were found to colocalize with mitochondria and regulate their transport. ΔN-PINK1 promoted anterograde transport and increased the fraction of stationary mitochondria, whereas full-length PINK1 promoted retrograde transport. In both cases, MARK2 enhanced the effects. The results identify MARK2 as an upstream regulator of PINK1 and ΔN-PINK1 and provide insights into the regulation of mitochondrial trafficking in neurons and neurodegeneration in PD.

Abnormal Mitochondria Organization and Oxidative Activity in the Palate Muscles of Long-Term Snorers with Obstructive Sleep Apnea

Background: Histopathological alterations and a reduced number of capillaries have been observed in the palate muscles of snorers with obstructive sleep apnea syndrome (OSAS). These changes may create a substrate for decreased microcirculation, impaired aerobic metabolism and muscle dysfunction and contribute to upper airway obstruction during sleep. Objectives: The aim was to analyze mitochondria distribution and oxidative enzyme activity in relation to capillary supply in the palate muscles of patients with a history of long-term snoring and OSAS. Methods: Palatopharyngeus (PP) and uvula (UV) muscle samples were obtained from 8 patients undergoing uvulopalatopharyngoplasty due to habitual snoring and OSAS. The muscles were analyzed with enzyme- and immunohistochemistry and morphometry. Results: Abnormalities in the internal organization of mitochondria and oxidative activity were observed in 39 ± 15% of the fibers in the PP and 4 ± 3% in the UV, but not in control samples. The majority of these fibers had a lobulated contour and trabecular internal organization of mitochondria. The number of capillaries around abnormal fibers (PP 0.9 ± 0.3, UV 0.4 ± 0.1) was lower than in fibers of a normal appearance in both patients (PP 1.4 ± 0.6, UV 1.2 ± 0.3) and references (PP 2.7 ± 0.7, UV 1.9 ± 0.9) (p < 0.05). Conclusions: Abnormal mitochondrial distribution, a low capillary supply and signs of impaired oxidative activity suggest that muscle dysfunction of the palate muscles in long-term snorers may contribute to the upper airway obstruction during sleep. The cause of these abnormalities remains unclear, but local muscle and nerve trauma due to vibration and stretch is a possible etiology.

Severe Neurologic Impairment in Mice with Targeted Disruption of the Electrogenic Sodium Bicarbonate Cotransporter NBCe2 (Slc4a5 Gene)

The choroid plexus lining the four ventricles in the brain is where the majority of cerebrospinal fluid (CSF) is produced. The secretory function of the choroid plexus is mediated by specific transport systems that allow the directional flux of nutrients and ions into the CSF and the removal of toxins. Normal CSF dynamics and chemistry ensure that the environment for neural function is optimal. Here, we report that targeted disruption of the Slc4a5 gene encoding the electrogenic sodium bicarbonate cotransporter NBCe2 results in significant remodeling of choroid plexus epithelial cells, including abnormal mitochondrial distribution, cytoskeletal protein expression, and ion transporter polarity. These changes are accompanied by very significant abnormalities in intracerebral ventricle volume, intracranial pressure, and CSF electrolyte levels. The Slc4a5(-/-) mice are significantly more resistant to induction of seizure behavior than wild-type controls. In the retina of Slc4a5(-/-) mice, loss of photoreceptors, ganglion cells, and retinal detachment results in visual impairment assessed by abnormal electroretinogram waveforms. Our findings are the first demonstration of the fundamental importance of NBCe2 in the biology of the nervous system.

The role of hepatitis B virus X protein is related to its differential intracellular localization

Chronic hepatitis B virus (HBV) infection has been strongly associated with hepatocellular carcinoma. HBV encodes an oncogenic hepatitis B virus X protein (HBx), which is a multifunctional regulator that modulates signal transduction, transcription, cell cycle progress, protein degradation, apoptosis, and genetic stability through direct and indirect interaction with host factors. The subcellular localization of HBx is primarily cytoplasmic, with a small fraction in the nucleus. In addition, high levels of HBx expression lead to an abnormal mitochondrial distribution. The dynamic distribution of HBx could be important to the multiple functions of HBx at different stages of the HBV life cycle. This short review presents an overview of the differential roles of HBx as a function of its intracellular localization.

Maternal Diabetes Causes Mitochondrial Dysfunction and Meiotic Defects in Murine Oocytes

The adverse effects of maternal diabetes on embryo development and pregnancy outcomes have recently been shown to occur as early as the one-cell zygote stage. The hypothesis of this study was that maternally inherited mitochondria in oocytes from diabetic mice are abnormal and thus responsible in part for this latency of developmental compromise. In ovulated oocytes from diabetic mice, transmission electron microscopy revealed an alteration in mitochondrial ultrastructure, and the quantitative analysis of mitochondrial DNA copy number demonstrated an increase. The levels of ATP and tricarboxylic acid cycle metabolites in diabetic oocytes were markedly reduced compared with controls, suggesting a mitochondrial metabolic dysfunction. Abnormal distribution of mitochondria within maturing oocytes also was seen in diabetic mice. Furthermore, oocytes from diabetic mice displayed a higher frequency of spindle defects and chromosome misalignment in meiosis, resulting in increased aneuploidy rates in ovulated oocytes. Collectively, our results suggest that maternal diabetes results in oocyte defects that are transmitted to the fetus by two routes: first, meiotic spindle and chromatin defects result in nondisjunction leading to embryonic aneuploidy; second, structural and functional abnormalities of oocyte mitochondria, through maternal transmission, provide the embryo with a dysfunctional complement of mitochondria that may be propagated during embryogenesis.

Impaired Balance of Mitochondrial Fission and Fusion in Alzheimer's Disease

Mitochondrial dysfunction is a prominent feature of Alzheimer's disease (AD) neurons. In this study, we explored the involvement of an abnormal mitochondrial dynamics by investigating the changes in the expression of mitochondrial fission and fusion proteins in AD brain and the potential cause and consequence of these changes in neuronal cells. We found that mitochondria were redistributed away from axons in the pyramidal neurons of AD brain. Immunoblot analysis revealed that levels of DLP1 (also referred to as Drp1), OPA1, Mfn1, and Mfn2 were significantly reduced whereas levels of Fis1 were significantly increased in AD. Despite their differential effects on mitochondrial morphology, manipulations of these mitochondrial fission and fusion proteins in neuronal cells to mimic their expressional changes in AD caused a similar abnormal mitochondrial distribution pattern, such that mitochondrial density was reduced in the cell periphery of M17 cells or neuronal process of primary neurons and correlated with reduced spine density in the neurite. Interestingly, oligomeric amyloid-beta-derived diffusible ligands (ADDLs) caused mitochondrial fragmentation and reduced mitochondrial density in neuronal processes. More importantly, ADDL-induced synaptic change (i.e., loss of dendritic spine and postsynaptic density protein 95 puncta) correlated with abnormal mitochondrial distribution. DLP1 overexpression, likely through repopulation of neuronal processes with mitochondria, prevented ADDL-induced synaptic loss, suggesting that abnormal mitochondrial dynamics plays an important role in ADDL-induced synaptic abnormalities. Based on these findings, we suggest that an altered balance in mitochondrial fission and fusion is likely an important mechanism leading to mitochondrial and neuronal dysfunction in AD brain.