Caspases indirectly regulate cleavage of the mitochondrial fusion GTPase OPA1 in neurons undergoing apoptosis

Cell death in development, maintenance, and diseases of the nervous system

Cell death, be it of neurons or glial cells, marks the development of the nervous system. Albeit relatively less so than in tissues such as the gut, cell death is also a feature of nervous system homeostasis-especially in context of adult neurogenesis. Finally, cell death is commonplace in acute brain injuries, chronic neurodegenerative diseases, and in some central nervous system tumors such as glioblastoma. Recent studies are enumerating the various molecular modalities involved in the execution of cells. Intimately linked with cell death are mechanisms of disposal that remove the dead cell and bring about a tissue-level response. Heretofore, the association between these methods of dying and physiological or pathological responses has remained nebulous. It is envisioned that careful cartography of death and disposal may reveal novel understandings of disease states and chart new therapeutic strategies in the near future.

LRRK2 at the Crossroad of Aging and Parkinson's Disease

Parkinson's disease (PD) is a heterogeneous neurodegenerative disease characterized by the progressive loss of dopaminergic neurons in the substantia nigra pars compacta and the widespread occurrence of proteinaceous inclusions known as Lewy bodies and Lewy neurites. The etiology of PD is still far from clear, but aging has been considered as the highest risk factor influencing the clinical presentations and the progression of PD. Accumulating evidence suggests that aging and PD induce common changes in multiple cellular functions, including redox imbalance, mitochondria dysfunction, and impaired proteostasis. Age-dependent deteriorations in cellular dysfunction may predispose individuals to PD, and cellular damages caused by genetic and/or environmental risk factors of PD may be exaggerated by aging. Mutations in the LRRK2 gene cause late-onset, autosomal dominant PD and comprise the most common genetic causes of both familial and sporadic PD. LRRK2-linked PD patients show clinical and pathological features indistinguishable from idiopathic PD patients. Here, we review cellular dysfunctions shared by aging and PD-associated LRRK2 mutations and discuss how the interplay between the two might play a role in PD pathologies.

CoenzymeQ10-Induced Activation of AMPK-YAP-OPA1 Pathway Alleviates Atherosclerosis by Improving Mitochondrial Function, Inhibiting Oxidative Stress and Promoting Energy Metabolism

Atherosclerosis (AS) is an excessive chronic inflammatory hyperplasia caused by the damage of vascular endothelial cell morphology and function. Changes in mitochondrial internal conformation and increase of reactive oxygen species (ROS) can lead to energy metabolism disorders in mitochondria, which further affects the occurrence of atherosclerosis by impairing vascular endothelial function. Coenzyme Q10 (CoQ10) is one of the components of mitochondrial respiratory chain, which has the functions of electron transfer, reducing oxidative stress damage, improving mitochondrial function and promoting energy metabolism. The main purpose of this study is to investigate the protective effects of CoQ10 against AS by improving mitochondrial energy metabolism. Both in high fat diet (HFD) fed APOE ^-/- mice and in ox-LDL-treated HAECs, CoQ10 significantly decreased the levels of TG, TC and LDL-C and increased the levels of HDL-C, thus playing a role in regulating lipid homeostasis. Meanwhile, CoQ10 decreased the levels of LDH and MDA and increased the levels of SOD and GSH, thus playing a role in regulating oxidation level. CoQ10 also inhibited the over-release of ROS and increased ATP content to improve mitochondrial function. CoQ10 also decreased the levels of related inflammatory factors (ICAM-1, VCAM-1, IL-6, TNF-α and NLRP3). In order to study the mechanism of the experiment, AMPK and YAP were silenced in vitro. The further study suggested AMPK small interfering RNA (siRNA) and YAP small interfering RNA (siRNA) affected the expression of OPA1, a crucial protein regulating the balance of mitochondrial fusion and division and decreased the therapeutic effects of CoQ10. These results indicated that CoQ10 improved mitochondrial function, inhibited ROS production, promoted energy metabolism and attenuated AS by activating AMPK-YAP-OPA1 pathway. This study provides a possible new mechanism for CoQ10 in the treatment of AS and may bring a new hope for the prevention and treatment of AS in the future.

Regulation of Mammalian Mitochondrial Dynamics: Opportunities and Challenges

Mitochondria are highly dynamic organelles and important for a variety of cellular functions. They constantly undergo fission and fusion events, referred to as mitochondrial dynamics, which affects the shape, size, and number of mitochondria in the cell, as well as mitochondrial subcellular transport, mitochondrial quality control (mitophagy), and programmed cell death (apoptosis). Dysfunctional mitochondrial dynamics is associated with various human diseases. Mitochondrial dynamics is mediated by a set of mitochondria-shaping proteins in both yeast and mammals. In this review, we describe recent insights into the potential molecular mechanisms underlying mitochondrial fusion and fission, particularly highlighting the coordinating roles of different mitochondria-shaping proteins in the processes, as well as the roles of the endoplasmic reticulum (ER), the actin cytoskeleton and membrane phospholipids in the regulation of mitochondrial dynamics. We particularly focus on emerging roles for the mammalian mitochondrial proteins Fis1, Mff, and MIEFs (MIEF1 and MIEF2) in regulating the recruitment of the cytosolic Drp1 to the surface of mitochondria and how these proteins, especially Fis1, mediate crosstalk between the mitochondrial fission and fusion machineries. In summary, this review provides novel insights into the molecular mechanisms of mammalian mitochondrial dynamics and the involvement of these mechanisms in apoptosis and autophagy.

Dynamin-like protein 1 cleavage by calpain in Alzheimer's disease

Abnormal mitochondrial dynamics contributes to mitochondrial dysfunction in Alzheimer's disease (AD), yet the underlying mechanism remains elusive. In the current study, we reported that DLP1, the key mitochondrial fission GTPase, is a substrate of calpain which produced specific N-terminal DLP1 cleavage fragments. In addition, various AD-related insults such as exposure to glutamate, soluble amyloid-β oligomers, or reagents inducing tau hyperphosphorylation (i.e., okadaic acid) led to calpain-dependent cleavage of DLP1 in primary cortical neurons. DLP1 cleavage fragments were found in cortical neurons of CRND8 APP transgenic mice which can be inhibited by calpeptin, a potent small molecule inhibitor of calpain. Importantly, these N-terminal DLP1 fragments were also present in the human brains, and the levels of both full-length and N-terminal fragments of DLP1 and the full-length and calpain-specific cleavage product of spectrin were significantly reduced in AD brains along with significantly increased calpain. These results suggest that calpain-dependent cleavage is at least one of the posttranscriptional mechanisms that contribute to the dysregulation of mitochondrial dynamics in AD.

Genetic analysis in a cohort of patients with hereditary optic neuropathies in Southwest of China

We report the results of molecular screening in 121 patients with suspected hereditary optic neuropathies. The 34 primary and 9 secondary LHON mutations were screened in all the patients. In the familial cases, OPA1 was also tested when negative finding for the mtDNA mutations screening. Molecular defects were identified in 35 patients (28.9% of screened patients). Among these, 33 patients (94.3%) had an mtDNA mutation, including m.11778G > A (69.7%), m.14484 T > C, m.3460G > A, m.3635G > A, m.14502 T > C and three secondary mutations m.3316G > A, m.3394 T > C, m.3497C > T. Two novel OPA1 mutations, c.1301 T > G (p.Leu434Arg) and c.985-1G > A (IVS9-1G > A), were also detected in families with the evidence of father-to-son transmission. In conclusion, we reported the results of the molecular screening of 121 patients with hereditary optic neuropathies from southwest of China. Our results highlight the importance of investigating LHON-causing mtDNA mutations and OPA1 mutations in cases of suspected hereditary optic neuropathy.

Neuronal Cell Death

Neuronal cell death occurs extensively during development and pathology, where it is especially important because of the limited capacity of adult neurons to proliferate or be replaced. The concept of cell death used to be simple as there were just two or three types, so we just had to work out which type was involved in our particular pathology and then block it. However, we now know that there are at least a dozen ways for neurons to die, that blocking a particular mechanism of cell death may not prevent the cell from dying, and that non-neuronal cells also contribute to neuronal death. We review here the mechanisms of neuronal death by intrinsic and extrinsic apoptosis, oncosis, necroptosis, parthanatos, ferroptosis, sarmoptosis, autophagic cell death, autosis, autolysis, paraptosis, pyroptosis, phagoptosis, and mitochondrial permeability transition. We next explore the mechanisms of neuronal death during development, and those induced by axotomy, aberrant cell-cycle reentry, glutamate (excitoxicity and oxytosis), loss of connected neurons, aggregated proteins and the unfolded protein response, oxidants, inflammation, and microglia. We then reassess which forms of cell death occur in stroke and Alzheimer's disease, two of the most important pathologies involving neuronal cell death. We also discuss why it has been so difficult to pinpoint the type of neuronal death involved, if and why the mechanism of neuronal death matters, the molecular overlap and interplay between death subroutines, and the therapeutic implications of these multiple overlapping forms of neuronal death.

Control of Mitochondrial Dynamics by Fas-induced Caspase-8 Activation in Hippocampal Neurons

Cells undergo apoptosis mainly via two pathways-the mitochondrial pathway and the cytosolic pathway. It has been well documented that activation of the mitochondrial pathway promotes mitochondrial fragmentation and inhibition of mitochondrial fragmentation partly represses cell death. However, the mitochondrial events following activation of the cytosolic pathway are less understood. In this study, we treated Fas-activating antibody and found mitochondrial fragmentation without cell death in hippocampal primary neurons and HT-22 cell lines. Fas antibody treatment, in fact, promoted rapid activation of caspase-8, while executioner caspase-3 activation was not observed. Furthermore, blockage of caspase-8 efficiently prevented Fas antibody-induced mitochondrial fragmentation. These results suggest that the cytosolic pathway induced by death receptor activation promotes caspase-8-dependent mitochondrial fission.

Redox Homeostasis and Mitochondrial Dynamics

Within living cells, mitochondria are considered relevant sources of reactive oxygen species (ROS) and are exposed to reactive nitrogen species (RNS). During the last decade, accumulating evidence suggests that mitochondrial (dys)function, ROS/RNS levels, and aberrations in mitochondrial morphology are interconnected, albeit in a cell- and context-dependent manner. Here it is hypothesized that ROS and RNS are involved in the short-term regulation of mitochondrial morphology and function via non-transcriptional pathways. We review the evidence for such a mechanism and propose that it allows homeostatic control of mitochondrial function and morphology by redox signaling.

Deregulation of mitochondria-shaping proteins Opa-1 and Drp-1 in manganese-induced apoptosis

Mitochondria are dynamic organelles that undergo fusion and fission processes. These events are regulated by mitochondria-shaping proteins. Changes in the expression and/or localization of these proteins lead to a mitochondrial dynamics impairment and may promote apoptosis. Increasing evidence correlates the mitochondrial dynamics disruption with the occurrence of neurodegenerative diseases. Therefore, we focused on this topic in Manganese (Mn)-induced Parkinsonism, a disorder associated with Mn accumulation preferentially in the basal ganglia where mitochondria from astrocytes represent an early target. Using MitoTracker Red staining we observed increased mitochondrial network fission in Mn-exposed rat astrocytoma C6 cells. Moreover, Mn induced a marked decrease in fusion protein Opa-1 levels as well as a dramatic increase in the expression of fission protein Drp-1. Additionally, Mn provoked a significant release of high MW Opa-1 isoforms from the mitochondria to the cytosol as well as an increased Drp-1 translocation to the mitochondria. Both Mdivi-1, a pharmacological Drp-1 inhibitor, and rat Drp-1 siRNA reduced the number of apoptotic nuclei, preserved the mitochondrial network integrity and prevented cell death. CsA, an MPTP opening inhibitor, prevented mitochondrial Δψm disruption, Opa-1 processing and Drp-1 translocation to the mitochondria therefore protecting Mn-exposed cells from mitochondrial disruption and apoptosis. The histological analysis and Hoechst 33258 staining of brain sections of Mn-injected rats in the striatum showed a decrease in cellular mass paralleled with an increase in the occurrence of apoptotic nuclei. Opa-1 and Drp-1 expression levels were also changed by Mn-treatment. Our results demonstrate for the first time that abnormal mitochondrial dynamics is implicated in both in vitro and in vivo Mn toxicity. In addition we show that the imbalance in fusion/fission equilibrium might be involved in Mn-induced apoptosis. This knowledge may provide new therapeutic tools for the treatment of Manganism and other neurodegenerative diseases.

Cannabidiol normalizes caspase 3, synaptophysin, and mitochondrial fission protein DNM1L expression levels in rats with brain iron overload: implications for neuroprotection

We have recently shown that chronic treatment with cannabidiol (CBD) was able to recover memory deficits induced by brain iron loading in a dose-dependent manner in rats. Brain iron accumulation is implicated in the pathogenesis of neurodegenerative diseases, including Parkinson's and Alzheimer's, and has been related to cognitive deficits in animals and human subjects. Deficits in synaptic energy supply have been linked to neurodegenerative diseases, evidencing the key role played by mitochondria in maintaining viable neural cells and functional circuits. It has also been shown that brains of patients suffering from neurodegenerative diseases have increased expression of apoptosisrelated proteins and specific DNA fragmentation. Here, we have analyzed the expression level of brain proteins involved with mitochondrial fusion and fission mechanisms (DNM1L and OPA1), the main integral transmembrane protein of synaptic vesicles (synaptophysin), and caspase 3, an apoptosis-related protein, to gain a better understanding of the potential of CBD in restoring the damage caused by iron loading in rats. We found that CBD rescued iron-induced effects, bringing hippocampal DNM1L, caspase 3, and synaptophysin levels back to values comparable to the control group. Our results suggest that iron affects mitochondrial dynamics, possibly trigging synaptic loss and apoptotic cell death and indicate that CBD should be considered as a potential molecule with memory-rescuing and neuroprotective properties to be used in the treatment of cognitive deficits observed in neurodegenerative disorders.

N-terminal cleavage of the mitochondrial fusion GTPase OPA1 occurs via a caspase-independent mechanism in cerebellar granule neurons exposed to oxidative or nitrosative stress

Neuronal cell death via apoptosis or necrosis underlies several devastating neurodegenerative diseases associated with aging. Mitochondrial dysfunction resulting from oxidative or nitrosative stress often acts as an initiating stimulus for intrinsic apoptosis or necrosis. These events frequently occur in conjunction with imbalances in the mitochondrial fission and fusion equilibrium, although the cause and effect relationships remain elusive. Here, we demonstrate in primary rat cerebellar granule neurons (CGNs) that oxidative or nitrosative stress induces an N-terminal cleavage of optic atrophy-1 (OPA1), a dynamin-like GTPase that regulates mitochondrial fusion and maintenance of cristae architecture. This cleavage event is indistinguishable from the N-terminal cleavage of OPA1 observed in CGNs undergoing caspase-mediated apoptosis (Loucks et al., 2009) and results in removal of a key lysine residue (K301) within the GTPase domain. OPA1 cleavage in CGNs occurs coincident with extensive mitochondrial fragmentation, disruption of the microtubule network, and cell death. In contrast to OPA1 cleavage induced in CGNs by removing depolarizing extracellular potassium (5K apoptotic conditions), oxidative or nitrosative stress-induced OPA1 cleavage caused by complex I inhibition or nitric oxide, respectively, is caspase-independent. N-terminal cleavage of OPA1 is also observed in vivo in aged rat and mouse midbrain and hippocampal tissues. We conclude that N-terminal cleavage and subsequent inactivation of OPA1 may be a contributing factor in the neuronal cell death processes underlying neurodegenerative diseases, particularly those associated with aging. Furthermore, these data suggest that OPA1 cleavage is a likely convergence point for mitochondrial dysfunction and imbalances in mitochondrial fission and fusion induced by oxidative or nitrosative stress.

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.

Neuroprotective effects of anthocyanins on apoptosis induced by mitochondrial oxidative stress

OBJECTIVES

Mitochondrial oxidative stress (MOS) is a major factor in the underlying pathology of many neurodegenerative diseases. Here, we investigated the neuroprotective effects of a unique class of nutraceutical antioxidants, anthocyanins, against MOS-induced death of cultured cerebellar granule neurons (CGNs). Callistephin and kuromanin are anthocyanins derived from strawberries and black rice, respectively, whose neuroprotective properties have yet to be examined in detail.

METHODS

Glutathione (GSH)-sensitive MOS and intrinsic apoptosis were induced in CGNs by incubation with the Bcl-2 inhibitor, HA14-1. The effects of anthocyanin co-incubation on CGN survival were assessed.

RESULTS

The anthocyanins demonstrated significant protection from MOS-induced apoptosis which was equivalent to that provided by the green tea polyphenol, epigallocatechin 3-gallate; however, neither anthocyanin was as effective as GSH at rescuing CGNs. Inhibition of Bcl-2 caused a significant reduction of mitochondrial GSH which was prevented by the anthocyanins. Furthermore, the anthocyanins inhibited iron-induced lipid peroxidation in rat brain homogenates and prevented cardiolipin oxidation induced by MOS in CGNs. MOS-induced mitochondrial fragmentation and proteolytic cleavage of the optic atrophy 1 (OPA1) fusion GTPase were also attenuated by the anthocyanins. Finally, the anthocyanins significantly enhanced GSH peroxidase activity in a cell-free assay.

DISCUSSION

These data show that anthocyanins suppress MOS-induced apoptosis by preserving mitochondrial GSH and inhibiting cardiolipin oxidation and mitochondrial fragmentation. These nutraceutical antioxidants warrant further study as potential therapeutic agents for neurodegenerative diseases caused by MOS.

Automatic morphological subtyping reveals new roles of caspases in mitochondrial dynamics

Morphological dynamics of mitochondria is associated with key cellular processes related to aging and neuronal degenerative diseases, but the lack of standard quantification of mitochondrial morphology impedes systematic investigation. This paper presents an automated system for the quantification and classification of mitochondrial morphology. We discovered six morphological subtypes of mitochondria for objective quantification of mitochondrial morphology. These six subtypes are small globules, swollen globules, straight tubules, twisted tubules, branched tubules and loops. The subtyping was derived by applying consensus clustering to a huge collection of more than 200 thousand mitochondrial images extracted from 1422 micrographs of Chinese hamster ovary (CHO) cells treated with different drugs, and was validated by evidence of functional similarity reported in the literature. Quantitative statistics of subtype compositions in cells is useful for correlating drug response and mitochondrial dynamics. Combining the quantitative results with our biochemical studies about the effects of squamocin on CHO cells reveals new roles of Caspases in the regulatory mechanisms of mitochondrial dynamics. This system is not only of value to the mitochondrial field, but also applicable to the investigation of other subcellular organelle morphology.

Mitochondria are “cellular power plants” that synthesize adenosine triphosphate (ATP) from degradation of nutrients, providing chemical energy for cellular activities. In addition, mitochondria are involved in a range of other cellular processes, such as signaling, cell differentiation, cell death, cell cycle and cell growth. Dysfunctional mitochondrial dynamics have been linked to several neurodegenerative diseases, and may play a role in the aging process. Previous studies on the correlation between mitochondrial morphological changes and pathological processes involve mostly manual or semi-automated classification and quantification of morphological features, which introduces biases and inconsistency, and are labor intensive. In this work we have developed an automated quantification system for mitochondrial morphology, which is able to extract and distinguish six representative morphological subtypes within cells. Using this system, we have analyzed 1422 cells and extracted more than 200 thousand individual mitochondrion, and calculated morphological statistics for each cell. From the numerical results we were able to derive new biological conclusions about mitochondrial morphological dynamics. With this new system, investigations of mitochondrial morphology can be scaled up and objectively quantified, allowing standardization of morphological distinctions and replicability between experiments. This system will facilitate future research on the relation between subcellular morphology and various physiological processes.

Calpain plays a central role in 1-methyl-4-phenylpyridinium (MPP+)-induced neurotoxicity in cerebellar granule neurons

1-Methyl-4-phenylpyridinium (MPP(+))-induced neurotoxicity has previously been attributed to either caspase-dependent apoptosis or caspase-independent cell death. In the current study, we found that MPP(+) induces a unique, non-apoptotic nuclear morphology coupled with a caspase-independent but calpain-dependent mechanism of cell death in primary cultures of rat cerebellar granule neurons (CGNs). Using a terminal deoxynucleotidyl transferase dUTP nick end-labeling (TUNEL) assay in CGNs exposed to MPP(+), we observed that these neurons are essentially devoid of caspase-dependent DNA fragments indicative of apoptosis. Moreover, proteolysis of a well recognized caspase-3 substrate, poly (ADP ribose) polymerase (PARP), was not observed in CGNs exposed to MPP(+). In contrast, calpain-dependent proteolysis of fodrin and pro-caspases-9 and -3 occurred in this model coupled with inhibition of caspase-3/-7 activities. Notably, several key members of the Bcl-2 protein family appear to be prominent calpain targets in MPP(+)-treated CGNs. Bid and Bax were proteolyzed to truncated forms thought to have greater pro-death activity at mitochondria. Moreover, the pro-survival Bcl-2 protein was degraded to a form predicted to be inactive at mitochondria. Cyclin E was also cleaved by calpain to an active low MW fragment capable of facilitating cell cycle re-entry. Finally, MPP(+)-induced neurotoxicity in CGNs was significantly attenuated by a cocktail of calpain and caspase inhibitors in combination with the antioxidant glutathione. Collectively, these results demonstrate that caspases do not play a central role in CGN toxicity induced by exposure to MPP(+), whereas calpain cleavage of key protein targets, coupled with oxidative stress, plays a critical role in MPP(+)-induced neurotoxicity. Our findings underscore the complexity of MPP(+)-induced neurotoxicity and suggest that calpain may play a fundamental role in causing neuronal death downstream of mitochondrial oxidative stress and dysfunction.