Drosophila phosphatidylinositol-4 kinase fwd promotes mitochondrial fission and can suppress Pink1/parkin phenotypes

Mitochondria-Plasma Membrane Contact Sites: Emerging Regulators of Mitochondrial Form and Function

Sites of close apposition between organelles, known as membrane contact sites (MCSs), are critical regulators of organelle function. Mitochondria form elaborate reticular networks that perform essential metabolic and signaling functions. Many mitochondrial functions are regulated by MCSs formed between mitochondria and other organelles. In this review, we aim to bring attention to an understudied, but physiologically important, MCS between mitochondria and the plasma membrane (PM). We first describe the molecular mechanism of mitochondria-PM tethering in budding yeast and discuss its role in regulating multiple biological processes, including mitochondrial dynamics and lipid metabolism. Next, we discuss the evidence for mitochondria-PM tethering in higher eukaryotes, with a specific emphasis on mitochondria-PM contacts in retinal cells, and speculate on their functions. Finally, we discuss unanswered questions to guide future research into the function of mitochondria-PM contact sites.

Mitochondrial Complex I and ROS control synapse function through opposing pre- and postsynaptic mechanisms

Neurons require high amounts energy, and mitochondria help to fulfill this requirement. Dysfunctional mitochondria trigger problems in various neuronal tasks. Using the Drosophila neuromuscular junction (NMJ) as a model synapse, we previously reported that Mitochondrial Complex I (MCI) subunits were required for maintaining NMJ function and growth. Here we report tissue-specific adaptations at the NMJ when MCI is depleted. In Drosophila motor neurons, MCI depletion causes profound cytological defects and increased mitochondrial reactive oxygen species (ROS). But instead of diminishing synapse function, neuronal ROS triggers a homeostatic signaling process that maintains normal NMJ excitation. We identify molecules mediating this compensatory response. MCI depletion in muscles also enhances local ROS. But high levels of muscle ROS cause destructive responses: synapse degeneration, mitochondrial fragmentation, and impaired neurotransmission. In humans, mutations affecting MCI subunits cause severe neurological and neuromuscular diseases. The tissue-level effects that we describe in the Drosophila system are potentially relevant to forms of mitochondrial pathogenesis.

SARS-CoV-2 membrane protein induces neurodegeneration via affecting Golgi-mitochondria interaction

BACKGROUND

Neurological complications are a significant concern of Coronavirus Disease 2019 (COVID-19). However, the pathogenic mechanism of neurological symptoms associated with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection is poorly understood.

METHODS

We used Drosophila as a model to systematically analyze SARS-CoV-2 genes encoding structural and accessory proteins and identified the membrane protein (M) that disrupted mitochondrial functions in vivo. The M protein was stereotaxically injected to further assess its effects in the brains of wild-type (WT) and 5 × FAD mice. Omics technologies, including RNA sequencing and interactome analysis, were performed to explore the mechanisms of the effects of M protein both in vitro and in vivo.

RESULTS

Systematic analysis of SARS-CoV-2 structural and accessory proteins in Drosophila identified that the M protein induces mitochondrial fragmentation and dysfunction, leading to reduced ATP production, ROS overproduction, and eventually cell death in the indirect flight muscles. In WT mice, M caused hippocampal atrophy, neural apoptosis, glial activation, and mitochondrial damage. These changes were further aggravated in 5 × FAD mice. M was localized to the Golgi apparatus and genetically interacted with four wheel drive (FWD, a Drosophila homolog of mammalian PI4KIIIβ) to regulate Golgi functions in flies. Fwd RNAi, but not PI4KIIIα RNAi, reversed the M-induced Golgi abnormality, mitochondrial fragmentation, and ATP reduction. Inhibition of PI4KIIIβ activity suppressed the M-induced neuronal cell death. Therefore, M induced mitochondrial fragmentation and apoptosis likely through disruption of Golgi-derived PI(4)P-containing vesicles.

CONCLUSIONS

M disturbs the distribution and function of Golgi, leading to mitochondrial abnormality and eventually neurodegeneration via a PI4KIIIβ-mediated mechanism. This study reveals a potential mechanism for COVID-19 neurological symptoms and opens a new avenue for development of therapeutic strategies targeting SARS-CoV-2 M or mitochondria.

Apolipoprotein-L Functions in Membrane Remodeling

The mammalian Apolipoprotein-L families (APOLs) contain several isoforms of membrane-interacting proteins, some of which are involved in the control of membrane dynamics (traffic, fission and fusion). Specifically, human APOL1 and APOL3 appear to control membrane remodeling linked to pathogen infection. Through its association with Non-Muscular Myosin-2A (NM2A), APOL1 controls Golgi-derived trafficking of vesicles carrying the lipid scramblase Autophagy-9A (ATG9A). These vesicles deliver APOL3 together with phosphatidylinositol-4-kinase-B (PI4KB) and activated Stimulator of Interferon Genes (STING) to mitochondrion-endoplasmic reticulum (ER) contact sites (MERCSs) for the induction and completion of mitophagy and apoptosis. Through direct interactions with PI4KB and PI4KB activity controllers (Neuronal Calcium Sensor-1, or NCS1, Calneuron-1, or CALN1, and ADP-Ribosylation Factor-1, or ARF1), APOL3 controls PI(4)P synthesis. PI(4)P is required for different processes linked to infection-induced inflammation: (i) STING activation at the Golgi and subsequent lysosomal degradation for inflammation termination; (ii) mitochondrion fission at MERCSs for induction of mitophagy and apoptosis; and (iii) phagolysosome formation for antigen processing. In addition, APOL3 governs mitophagosome fusion with endolysosomes for mitophagy completion, and the APOL3-like murine APOL7C is involved in phagosome permeabilization linked to antigen cross-presentation in dendritic cells. Similarly, APOL3 can induce the fusion of intracellular bacterial membranes, and a role in membrane fusion can also be proposed for endothelial APOLd1 and adipocyte mAPOL6, which promote angiogenesis and adipogenesis, respectively, under inflammatory conditions. Thus, different APOL isoforms play distinct roles in membrane remodeling associated with inflammation.

Imbalance of mitochondrial quality control regulated by STING and PINK1 affects cyfluthrin-induced neuroinflammation

Nervous system diseases are a global health problem, and with the increase in the elderly population around the world, their incidence will also increase. Harmful substances in the environment are closely related to the occurrence of nervous system diseases. China is a large agricultural country, and thus the insecticide cyfluthrin has been widely used. Cyfluthrin is neurotoxic, but the mechanism of this injury is not clear. Inflammation is an important mechanism for the occurrence of nervous system diseases. Mitochondria are the main regulators of the inflammatory response, and various cellular responses, including autophagy, directly affect the regulation of inflammatory processes. Mitochondrial damage is related to mitochondrial quality control (MQC) and PTEN-induced kinase 1 (PINK1). As an anti-inflammatory factor, stimulator of interferon genes (STING) participates in the regulation of inflammation. However, the relationship between STING and mitochondria in the process of cyfluthrin-induced nerve injury is unclear. This study established in vivo and in vitro models of cyfluthrin exposure to explore the role of MQC and to clarify the mechanism of action of STING and PINK1. Our results showed that cyfluthrin can increase the reactive oxygen species (ROS) level, resulting in mitochondrial damage and inflammation. In this process, an imbalance in MQC leads to the aggravation of mitochondrial damage, and high STING expression drives the occurrence of inflammation. We established a differential expression model of STING and PINK1 to further determine the underlying mechanism and found that the interaction between STING and PINK1 regulates MQC to affect the levels of mitochondrial damage and inflammation. When STING and PINK1 expression are downregulated, mitochondrial damage and STING-induced inflammation are significantly alleviated. In summary, a synergistic effect between STING and PINK1 on cyfluthrin-induced neuroinflammation may exist, which leads to an imbalance in MQC by inhibiting mitochondrial biogenesis and division/fusion, and PINK1 can reduce STING-driven inflammation.

Differential functions of phosphatidylinositol 4-kinases in neurotransmission and synaptic development

Phosphoinositides, such as PI(4,5)P2, are known to function as structural components of membranes, signalling molecules, markers of membrane identity, mediators of protein recruitment and regulators of neurotransmission and synaptic development. Phosphatidylinositol 4-kinases (PI4Ks) synthesize PI4P, which are precursors for PI(4,5)P2, but may also have independent functions. The roles of PI4Ks in neurotransmission and synaptic development have not been studied in detail. Previous studies on PI4KII and PI4KIIIβ at the Drosophila larval neuromuscular junction have suggested that PI4KII and PI4KIIIβ enzymes may serve redundant roles, where single PI4K mutants yielded mild or no synaptic phenotypes. However, the precise synaptic functions (neurotransmission and synaptic growth) of these PI4Ks have not been thoroughly studied. Here, we used PI4KII and PI4KIIIβ null mutants and presynaptic-specific knockdowns of these PI4Ks to investigate their roles in neurotransmission and synaptic growth. We found that PI4KII and PI4KIIIβ appear to have non-overlapping functions. Specifically, glial PI4KII functions to restrain synaptic growth, whereas presynaptic PI4KIIIβ promotes synaptic growth. Furthermore, loss of PI4KIIIβ or presynaptic PI4KII impairs neurotransmission. The data presented in this study uncover new roles for PI4K enzymes in neurotransmission and synaptic growth.

Mitochondria-ER-PM contacts regulate mitochondrial division and PI(4)P distribution

The mitochondria-ER-cortex anchor (MECA) forms a tripartite membrane contact site between mitochondria, the endoplasmic reticulum (ER), and the plasma membrane (PM). The core component of MECA, Num1, interacts with the PM and mitochondria via two distinct lipid-binding domains; however, the molecular mechanism by which Num1 interacts with the ER is unclear. Here, we demonstrate that Num1 contains a FFAT motif in its C-terminus that interacts with the integral ER membrane protein Scs2. While dispensable for Num1's functions in mitochondrial tethering and dynein anchoring, the FFAT motif is required for Num1's role in promoting mitochondrial division. Unexpectedly, we also reveal a novel function of MECA in regulating the distribution of phosphatidylinositol-4-phosphate (PI(4)P). Breaking Num1 association with any of the three membranes it tethers results in an accumulation of PI(4)P on the PM, likely via disrupting Sac1-mediated PI(4)P turnover. This work establishes MECA as an important regulatory hub that spatially organizes mitochondria, ER, and PM to coordinate crucial cellular functions.

The Janus-faced functions of Apolipoproteins L in membrane dynamics

The functions of human Apolipoproteins L (APOLs) are poorly understood, but involve diverse activities like lysis of bloodstream trypanosomes and intracellular bacteria, modulation of viral infection and induction of apoptosis, autophagy, and chronic kidney disease. Based on recent work, I propose that the basic function of APOLs is the control of membrane dynamics, at least in the Golgi and mitochondrion. Together with neuronal calcium sensor-1 (NCS1) and calneuron-1 (CALN1), APOL3 controls the activity of phosphatidylinositol-4-kinase-IIIB (PI4KB), involved in both Golgi and mitochondrion membrane fission. Whereas secreted APOL1 induces African trypanosome lysis through membrane permeabilization of the parasite mitochondrion, intracellular APOL1 conditions non-muscular myosin-2A (NM2A)-mediated transfer of PI4KB and APOL3 from the Golgi to the mitochondrion under conditions interfering with PI4KB-APOL3 interaction, such as APOL1 C-terminal variant expression or virus-induced inflammatory signalling. APOL3 controls mitophagy through complementary interactions with the membrane fission factor PI4KB and the membrane fusion factor vesicle-associated membrane protein-8 (VAMP8). In mice, the basic APOL1 and APOL3 activities could be exerted by mAPOL9 and mAPOL8, respectively. Perspectives regarding the mechanism and treatment of APOL1-related kidney disease are discussed, as well as speculations on additional APOLs functions, such as APOL6 involvement in adipocyte membrane dynamics through interaction with myosin-10 (MYH10).

Partial loss of MCU mitigates pathology in vivo across a diverse range of neurodegenerative disease models

Mitochondrial calcium (Ca2+) uptake augments metabolic processes and buffers cytosolic Ca2+ levels; however, excessive mitochondrial Ca2+ can cause cell death. Disrupted mitochondrial function and Ca2+ homeostasis are linked to numerous neurodegenerative diseases (NDs), but the impact of mitochondrial Ca2+ disruption is not well understood. Here, we show that Drosophila models of multiple NDs (Parkinson's, Huntington's, Alzheimer's, and frontotemporal dementia) reveal a consistent increase in neuronal mitochondrial Ca2+ levels, as well as reduced mitochondrial Ca2+ buffering capacity, associated with increased mitochondria-endoplasmic reticulum contact sites (MERCs). Importantly, loss of the mitochondrial Ca2+ uptake channel MCU or overexpression of the efflux channel NCLX robustly suppresses key pathological phenotypes across these ND models. Thus, mitochondrial Ca2+ imbalance is a common feature of diverse NDs in vivo and is an important contributor to the disease pathogenesis. The broad beneficial effects from partial loss of MCU across these models presents a common, druggable target for therapeutic intervention.

Mitochondrial CISD1/Cisd accumulation blocks mitophagy and genetic or pharmacological inhibition rescues neurodegenerative phenotypes in Pink1/parkin models

BACKGROUND

Mitochondrial dysfunction and toxic protein aggregates have been shown to be key features in the pathogenesis of neurodegenerative diseases, such as Parkinson's disease (PD). Functional analysis of genes linked to PD have revealed that the E3 ligase Parkin and the mitochondrial kinase PINK1 are important factors for mitochondrial quality control. PINK1 phosphorylates and activates Parkin, which in turn ubiquitinates mitochondrial proteins priming them and the mitochondrion itself for degradation. However, it is unclear whether dysregulated mitochondrial degradation or the toxic build-up of certain Parkin ubiquitin substrates is the driving pathophysiological mechanism leading to PD. The iron-sulphur cluster containing proteins CISD1 and CISD2 have been identified as major targets of Parkin in various proteomic studies.

METHODS

We employed in vivo Drosophila and human cell culture models to study the role of CISD proteins in cell and tissue viability as well as aged-related neurodegeneration, specifically analysing aspects of mitophagy and autophagy using orthogonal assays.

RESULTS

We show that the Drosophila homolog Cisd accumulates in Pink1 and parkin mutant flies, as well as during ageing. We observed that build-up of Cisd is particularly toxic in neurons, resulting in mitochondrial defects and Ser65-phospho-Ubiquitin accumulation. Age-related increase of Cisd blocks mitophagy and impairs autophagy flux. Importantly, reduction of Cisd levels upregulates mitophagy in vitro and in vivo, and ameliorates pathological phenotypes in locomotion, lifespan and neurodegeneration in Pink1/parkin mutant flies. In addition, we show that pharmacological inhibition of CISD1/2 by rosiglitazone and NL-1 induces mitophagy in human cells and ameliorates the defective phenotypes of Pink1/parkin mutants.

CONCLUSION

Altogether, our studies indicate that Cisd accumulation during ageing and in Pink1/parkin mutants is a key driver of pathology by blocking mitophagy, and genetically and pharmacologically inhibiting CISD proteins may offer a potential target for therapeutic intervention.

A new role for phosphoinositides in regulating mitochondrial dynamics

Phosphoinositides are a minor group of membrane-associated phospholipids that are transiently generated on the cytoplasmic leaflet of many organelle membranes and the plasma membrane. There are seven functionally distinct phosphoinositides, each derived via the reversible phosphorylation of phosphatidylinositol in various combinations on the inositol ring. Their generation and termination is tightly regulated by phosphatidylinositol-kinases and -phosphatases. These enzymes can function together in an integrated and coordinated manner, whereby the phosphoinositide product of one enzyme may subsequently serve as a substrate for another to generate a different phosphoinositide species. This regulatory mechanism not only enables the transient generation of phosphoinositides on membranes, but also more complex sequential or bidirectional conversion pathways, and phosphoinositides can also be transferred between organelles via membrane contacts. It is this capacity to fine-tune phosphoinositide signals that makes them ideal regulators of membrane organization and dynamics, through their recruitment of signalling, membrane altering and lipid transfer proteins. Research spanning several decades has provided extensive evidence that phosphoinositides are major gatekeepers of membrane organization, with roles in endocytosis, exocytosis, autophagy, lysosome dynamics, vesicular transport and secretion, cilia, inter-organelle membrane contact, endosome maturation and nuclear function. By contrast, there has been remarkably little known about the role of phosphoinositides at mitochondria - an enigmatic and major knowledge gap, with challenges in reliably detecting phosphoinositides at this site. Here we review recent significant breakthroughs in understanding the role of phosphoinositides in regulating mitochondrial dynamics and metabolic function.

Apolipoproteins L1 and L3 control mitochondrial membrane dynamics

Apolipoproteins L1 and L3 (APOLs) are associated at the Golgi with the membrane fission factors phosphatidylinositol 4-kinase-IIIB (PI4KB) and non-muscular myosin 2A. Either APOL1 C-terminal truncation (APOL1Δ) or APOL3 deletion (APOL3-KO [knockout]) reduces PI4KB activity and triggers actomyosin reorganization. We report that APOL3, but not APOL1, controls PI4KB activity through interaction with PI4KB and neuronal calcium sensor-1 or calneuron-1. Both APOLs are present in Golgi-derived autophagy-related protein 9A vesicles, which are involved in PI4KB trafficking. Like APOL3-KO, APOL1Δ induces PI4KB dissociation from APOL3, linked to reduction of mitophagy flux and production of mitochondrial reactive oxygen species. APOL1 and APOL3, respectively, can interact with the mitophagy receptor prohibitin-2 and the mitophagosome membrane fusion factor vesicle-associated membrane protein-8 (VAMP8). While APOL1 conditions PI4KB and APOL3 involvement in mitochondrion fission and mitophagy, APOL3-VAMP8 interaction promotes fusion between mitophagosomal and endolysosomal membranes. We propose that APOL3 controls mitochondrial membrane dynamics through interactions with the fission factor PI4KB and the fusion factor VAMP8.

Drosophila as a model to study autophagy in neurodegenerative diseases and digestive tract

Autophagy regulates cellular homeostasis by degrading and recycling cytosolic components and damaged organelles. Disruption of autophagic flux has been shown to induce or facilitate neurodegeneration and accumulation of autophagic vesicles is overt in neurodegenerative diseases. The fruit fly Drosophila has been used as a model system to identify new factors that regulate physiology and disease. Here we provide a historical perspective of how the fly models have offered mechanistic evidence to understand the role of autophagy in neurodegenerative diseases including Alzheimer's disease, Parkinson's disease, Charcot-Marie-Tooth neuropathy, and polyglutamine disorders. Autophagy also plays a pivotal role in maintaining tissue homeostasis and protecting organism health. The gastrointestinal tract regulates organism health by modulating food intake, energy balance, and immunity. Growing evidence is strengthening the link between autophagy and digestive tract health in recent years. Here, we also discuss how the fly models have advanced the understanding of digestive physiology regulated by autophagy.

Deconstructing the molecular genetics behind the PINK1/Parkin axis in Parkinson’s disease using Drosophila melanogaster as a model organism

Background

Parkinson’s disease (PD) is a multifactorial neurodegenerative disorder marked by the death of nigrostriatal dopaminergic neurons in response to the compounding effects of oxidative stress, mitochondrial dysfunction and protein aggregation. Transgenic Drosophila models have been used extensively to decipher the underlying genetic interactions that exacerbate neural health in PD. Autosomal recessive forms of the disease have been linked to mutations in the serine/threonine kinase PINK1(PTEN-Induced Putative Kinase 1) and E3 ligase Parkin, which function in an axis that is conserved in flies. This review aims to probe the current understanding of PD pathogenesis via the PINK1/Parkin axis while underscoring the importance of several molecular and pharmacologic rescues brought to light through studies in Drosophila.

Main body

Mutations in PINK1 and Parkin have been shown to affect the axonal transport of mitochondria within dopaminergic neurons and perturb the balance between mitochondrial fusion/fission resulting in abnormal mitochondrial morphology. As per studies in flies, ectopic expression of Fwd kinase and Atg-1 to promote fission and mitophagy while suppressing fusion via MUL1 E3 ligase may aid to halt mitochondrial aggregation and prolong the survival of dopaminergic neurons. Furthermore, upregulation of Hsp70/Hsp90 chaperone systems (Trap1, CHIP) to target misfolded mitochondrial respiratory complexes may help to preserve their bioenergetic capacity. Accumulation of reactive oxygen species as a consequence of respiratory complex dysfunction or antioxidant enzyme deficiency further escalates neural death by inducing apoptosis, lipid peroxidation and DNA damage. Fly studies have reported the induction of canonical Wnt signalling to enhance the activity of transcriptional co-activators (PGC1α, FOXO) which induce the expression of antioxidant enzymes. Enhancing the clearance of free radicals via uncoupling proteins (UCP4) has also been reported to ameliorate oxidative stress-induced cell death in PINK1/Parkin mutants.

Conclusion

While these novel mechanisms require validation through mammalian studies, they offer several explanations for the factors propagating dopaminergic death as well as promising insights into the therapeutic importance of transgenic fly models in PD.

Disruption of Mitochondrial Homeostasis: The Role of PINK1 in Parkinson's Disease

The progressive reduction of the dopaminergic neurons of the substantia nigra is the fundamental process underlying Parkinson’s disease (PD), while the mechanism of susceptibility of this specific neuronal population is largely unclear. Disturbances in mitochondrial function have been recognized as one of the main pathways in sporadic PD since the finding of respiratory chain impairment in animal models of PD. Studies on genetic forms of PD have provided new insight on the role of mitochondrial bioenergetics, homeostasis, and autophagy. PINK1 (PTEN-induced putative kinase 1) gene mutations, although rare, are the second most common cause of recessively inherited early-onset PD, after Parkin gene mutations. Our knowledge of PINK1 and Parkin function has increased dramatically in the last years, with the discovery that a process called mitophagy, which plays a key role in the maintenance of mitochondrial health, is mediated by the PINK1/Parkin pathway. In vitro and in vivo models have been developed, supporting the role of PINK1 in synaptic transmission, particularly affecting dopaminergic neurons. It is of paramount importance to further define the role of PINK1 in mitophagy and mitochondrial homeostasis in PD pathogenesis in order to delineate novel therapeutic targets.

Mitochondrial fission, integrity and completion of mitophagy require separable functions of Vps13D in Drosophila neurons

A healthy population of mitochondria, maintained by proper fission, fusion, and degradation, is critical for the long-term survival and function of neurons. Here, our discovery of mitophagy intermediates in fission-impaired Drosophila neurons brings new perspective into the relationship between mitochondrial fission and mitophagy. Neurons lacking either the ataxia disease gene Vps13D or the dynamin related protein Drp1 contain enlarged mitochondria that are engaged with autophagy machinery and also lack matrix components. Reporter assays combined with genetic studies imply that mitophagy both initiates and is completed in Drp1 impaired neurons, but fails to complete in Vps13D impaired neurons, which accumulate compromised mitochondria within stalled mito-phagophores. Our findings imply that in fission-defective neurons, mitophagy becomes induced, and that the lipid channel containing protein Vps13D has separable functions in mitochondrial fission and phagophore elongation.

Neuronal autophagy and mitophagy in Parkinson's disease

Autophagy is the process by which cells can selectively or non-selectively remove damaged proteins and organelles. As the cell's main means of sequestering damaged mitochondria for removal, mitophagy is central to cellular function and survival. Research on autophagy and mitochondrial quality control has increased exponentially in relation to the pathogenesis of numerous disease conditions, from cancer and immune diseases to chronic neurodegenerative diseases like Parkinson's disease (PD). Understanding how components of the autophagic/mitophagic machinery are affected during disease, as well as the contextual relationship of autophagy with determining neuronal health and function, is essential to the goal of designing therapies for human disease. In this review, we will summarize key signaling molecules that consign damaged mitochondria for autophagic degradation, describe the relationship of genes linked to PD to autophagy/mitophagy dysfunction, and discuss additional roles of both mitochondrial and cytosolic pools of PTEN-induced kinase 1 (PINK1) in mitochondrial homeostasis, dendritic morphogenesis and inflammation.

Nicotine Causes Mitochondrial Dynamics Imbalance and Apoptosis Through ROS Mediated Mitophagy Impairment in Cardiomyocytes

Nicotine contained in traditional cigarettes, hookahs, and e-cigarettes is an important risk factor for cardiovascular disease. Our previous study showed that macroautophagic flux impairment occurred under nicotine stimulation. However, whether nicotine influences mitochondrial dynamics in neonatal rat ventricular myocytes (NRVMs) is unclear. The purpose of this study was to explore the effects and potential mechanism of nicotine on mitophagy, mitochondrial dynamics, apoptosis, and the relationship between these processes in NRVMs. Our results showed that nicotine exposure increased mitochondria-derived superoxide production, decreased mitochondrial membrane potential, and impaired PINK1/Parkin-mediated mitophagic flux in NRVMs. Interestingly, nicotine significantly promoted dynamin-related protein 1 (Drp1)-mediated mitochondrial fission and suppressed mitofusin (MFN)-mediated fusion, which was also observed in the bafilomycin A1-treated group. These results suggest that mitophagic flux impairment may contribute to Drp-1-mediated mitochondrial fission. Finally, nicotine caused excessive mitochondrial fission and contributed to apoptosis, which could be alleviated by mdivi-1, an inhibitor of Drp1. In addition to CTSB, as we previously reported, the enzyme activity of cathepsin L (CTSL) was also decreased in lysosomes after stimulation with nicotine, which may be the main cause of the hindered mitophagic flux induced by nicotine in NRVMs. Pretreatment with Torin 1, which is an inhibitor of mTOR, activated CTSL and ameliorated nicotine-induced mTOR activation and mitophagy impairment, decreased mitochondria-derived superoxide production, and blunted mitochondrial fission and apoptosis. Pretreatment with the ROS scavenger N-acetyl-cysteine (NAC) or inhibitors of p38 and JNK, which could also alleviate mitophagy impairment, exhibited similar effects as Torin1 on mitochondria. Taken together, our study demonstrated that nicotine treatment may lead to an increase in Drp1-mediated mitochondrial fission by blocking mitophagic flux by weakening the enzyme activity of CTSL and activating the ROS/p38/JNK signaling pathway. Excessive mitochondrial fission induced by nicotine ultimately leads to apoptosis. Torin1 restored the decreased CTSL enzyme activity by removing excessive ROS and alleviated the effects of nicotine on mitophagic flux, mitochondrial dynamics, and apoptosis. These results may provide new evidence on the relationship between mitophagic flux and mitochondrial dynamics and new perspectives on nicotine’s effects on mitochondrial dynamics in cardiomyocytes.

Deregulation of phosphatidylinositol-4-phosphate in the development of amyotrophic lateral sclerosis 8

Amyotrophic lateral sclerosis 8 (ALS8) is one of a heterogeneous group of progressive neurodegenerative disorders characterized by the death of motor neurons. ALS8 is caused by mutations in VAPB, a protein that acts at multiple membrane contact sites between the endoplasmic reticulum (ER) and almost all other organelles and thus affects functions at diverse cellular locations. One prominent function mediated by VAPB at these sites is lipid exchange, and a recurrent phenotype observed in all models investigating knockout or knockdown of VAPs is a significant increase in the levels of phosphatidylinositol-4-phosphate (PI4P). Here we consider the relevance of this PI4P deregulation in the development of ALS8 that might represent a potential target for therapeutic intervention.

The Complex Dance of Organelles during Mitochondrial Division

Mitochondria are dynamic organelles that undergo cycles of fission and fusion events depending on cellular requirements. During mitochondrial division, the GTPase dynamin-related protein-1 is recruited to endoplasmic reticulum (ER)-induced mitochondrial constriction sites where it drives fission. However, the events required to complete scission of mitochondrial membranes are not well understood. Here, we emphasize the recently described roles for Golgi-derived phosphatidylinositol 4-phosphate (PI4P)-containing vesicles in the last steps of mitochondrial division. We then propose how trans-Golgi network vesicles at mitochondria-ER contact sites and PI4P generation could mechanistically execute mitochondrial division, by recruiting PI4P effectors and/or the actin nucleation machinery. Finally, we speculate on mechanisms to explain why such a complex dance of different organelles is required to facilitate the remodelling of mitochondrial membranes.