Migrasomes, new vescicles as Hansel and Gretel white pebbles?

Migrasomes, released by migrating cells, belong to the heterogeneous world of extracellular vesicles (EVs). However, they can be distinguished from all other members of EVs by their size, biorigin and protein cargo. As far as we know, they can play important roles in various communication processes, by mediating the release of signals, such as mRNAs, proteins or damaged mitochondria. To extend and better understand the functional roles and importance of migrasomes, it is first essential to well understand the basic molecular mechanisms behind their formation and function. Herein, we endeavor to provide a brief and up-to-date description of migrasome biogenesis, release, characterization, biological properties and functional activities in cell-to-cell communication, and we will discuss and propose putative new functions for these vesicles.

PLK1 inhibition selectively induces apoptosis in ARID1A deficient cells through uncoupling of oxygen consumption from ATP production.

Inhibitors of the mitotic kinase PLK1 yield objective responses in a subset of refractory cancers. However, PLK1 overexpression in cancer does not correlate with drug sensitivity, and the clinical development of PLK1 inhibitors has been hampered by the lack of patient selection marker. Using a high-throughput chemical screen, we discovered that cells deficient for the tumor suppressor ARID1A are highly sensitive to PLK1 inhibition. Interestingly this sensitivity was unrelated to canonical functions of PLK1 in mediating G2/M cell cycle transition. Instead, a whole-genome CRISPR screen revealed PLK1 inhibitor sensitivity in ARID1A deficient cells to be dependent on the mitochondrial translation machinery. We find that ARID1A knock-out (KO) cells have an unusual mitochondrial phenotype with aberrant biogenesis, increased oxygen consumption/expression of oxidative phosphorylation genes, but without increased ATP production. Using expansion microscopy and biochemical fractionation, we see that a subset of PLK1 localizes to the mitochondria in interphase cells. Inhibition of PLK1 in ARID1A KO cells further uncouples oxygen consumption from ATP production, with subsequent membrane depolarization and apoptosis. Knockdown of specific subunits of the mitochondrial ribosome reverses PLK1-inhibitor induced apoptosis in ARID1A deficient cells, confirming specificity of the phenotype. Together, these findings highlight a novel interphase role for PLK1 in maintaining mitochondrial fitness under metabolic stress, and a strategy for therapeutic use of PLK1 inhibitors. To translate these findings, we describe a quantitative microscopy assay for assessment of ARID1A protein loss, which could offer a novel patient selection strategy for the clinical development of PLK1 inhibitors in cancer.

PD-1-induced T cell exhaustion is controlled by a Drp1-dependent mechanism.

Programmed cell death-1 (PD-1) signaling downregulates the T-cell response, promoting an exhausted state in tumor-infiltrating T cells, through mostly unveiled molecular mechanisms. Dynamin-related protein-1 (Drp1)-dependent mitochondrial fission plays a crucial role in sustaining T-cell motility, proliferation, survival, and glycolytic engagement. Interestingly, such processes are exactly those inhibited by PD-1 in tumor-infiltrating T cells. Here, we show that PD-1pos CD8+ T cells infiltrating an MC38 (murine adenocarcinoma)-derived murine tumor mass have a downregulated Drp1 activity and more elongated mitochondria compared with PD-1neg counterparts. Also, PD-1pos lymphocytic elements infiltrating a human colon cancer rarely express active Drp1. Mechanistically, PD-1 signaling directly prevents mitochondrial fragmentation following T-cell stimulation by downregulating Drp1 phosphorylation on Ser616, via regulation of the ERK1/2 and mTOR pathways. In addition, downregulation of Drp1 activity in tumor-infiltrating PD-1pos CD8+ T cells seems to be a mechanism exploited by PD-1 signaling to reduce motility and proliferation of these cells. Overall, our data indicate that the modulation of Drp1 activity in tumor-infiltrating T cells may become a valuable target to ameliorate the anticancer immune response in future immunotherapy approaches.

The long non-coding RNA CDK6-AS1 overexpression impacts on acute myeloid leukemia differentiation and mitochondrial dynamics.

Patients with acute myeloid leukemia (AML) carrying high-risk genetic lesions or high residual disease levels after therapy are particularly exposed to the risk of relapse. Here, we identified the long non-coding RNA CDK6-AS1 able to cluster an AML subgroup with peculiar gene signatures linked to hematopoietic cell differentiation and mitochondrial dynamics. CDK6-AS1 silencing triggered hematopoietic commitment in healthy CD34+ cells, whereas in AML cells the pathological undifferentiated state was rescued. This latter phenomenon derived from RUNX1 transcriptional control, responsible for the stemness of hematopoietic precursors and for the block of differentiation in AML. By CDK6-AS1 silencing in vitro, AML mitochondrial mass decreased with augmented pharmacological sensitivity to mitochondria-targeting drugs. In vivo, the combination of tigecycline and cytarabine reduced leukemia progression in the AML-PDX model with high CDK6-AS1 levels, supporting the concept of a mitochondrial vulnerability. Together, these findings uncover CDK6-AS1 as crucial in myeloid differentiation and mitochondrial mass regulation.

Targeting cancer stem cells in medulloblastoma by inhibiting AMBRA1 dual function in autophagy and STAT3 signalling.

Medulloblastoma (MB) is a childhood malignant brain tumour comprising four main subgroups characterized by different genetic alterations and rate of mortality. Among MB subgroups, patients with enhanced levels of the c-MYC oncogene (MBGroup3) have the poorest prognosis. Here we identify a previously unrecognized role of the pro-autophagy factor AMBRA1 in regulating MB. We demonstrate that AMBRA1 expression depends on c-MYC levels and correlates with Group 3 patient poor prognosis; also, knockdown of AMBRA1 reduces MB stem potential, growth and migration of MBGroup3 stem cells. At a molecular level, AMBRA1 mediates these effects by suppressing SOCS3, an inhibitor of STAT3 activation. Importantly, pharmacological inhibition of autophagy profoundly affects both stem and invasion potential of MBGroup3 stem cells, and a combined anti-autophagy and anti-STAT3 approach impacts the MBGroup3 outcome. Taken together, our data support the c-MYC/AMBRA1/STAT3 axis as a strong oncogenic signalling pathway with significance for both patient stratification strategies and targeted treatments of MBGroup3.

Following the Dynamism of the Mitochondrial Network in T Cells.

The dynamism of mitochondria, considered as complex and motile organelles, is brought about by mitochondria ability to undergo cycles of fission and fusion events, whose fine balance determines their morphology in a specific physiological context. A huge body of evidence makes it possible to associate mitochondrial organization to regulation of an increasing number of key cellular processes, such as biosynthetic pathways, oxidative phosphorylation and ATP production, calcium buffering, mtDNA homeostasis, autophagy, and cell death. Here, we review the recently developed imaging methods for studying mitochondrial dynamics, including live-cell imaging, by using mitochondrial-targeted fluorescent proteins. In more details, we focus our attention on two different protocols in the T cell model, an example of nonadherent cells, which present some particularities and difficulties in the analysis of mitochondrial shape. Also, we discuss some examples of mouse models carrying mitochondria-targeted fluorescent proteins, which allow to investigate the mitochondrial morphology in vivo.

AMBRA1 regulates cyclin D to guard S-phase entry and genomic integrity.

Mammalian development, adult tissue homeostasis and the avoidance of severe diseases including cancer require a properly orchestrated cell cycle, as well as error-free genome maintenance. The key cell-fate decision to replicate the genome is controlled by two major signalling pathways that act in parallel-the MYC pathway and the cyclin D-cyclin-dependent kinase (CDK)-retinoblastoma protein (RB) pathway1,2. Both MYC and the cyclin D-CDK-RB axis are commonly deregulated in cancer, and this is associated with increased genomic instability. The autophagic tumour-suppressor protein AMBRA1 has been linked to the control of cell proliferation, but the underlying molecular mechanisms remain poorly understood. Here we show that AMBRA1 is an upstream master regulator of the transition from G1 to S phase and thereby prevents replication stress. Using a combination of cell and molecular approaches and in vivo models, we reveal that AMBRA1 regulates the abundance of D-type cyclins by mediating their degradation. Furthermore, by controlling the transition from G1 to S phase, AMBRA1 helps to maintain genomic integrity during DNA replication, which counteracts developmental abnormalities and tumour growth. Finally, we identify the CHK1 kinase as a potential therapeutic target in AMBRA1-deficient tumours. These results advance our understanding of the control of replication-phase entry and genomic integrity, and identify the AMBRA1-cyclin D pathway as a crucial cell-cycle-regulatory mechanism that is deeply interconnected with genomic stability in embryonic development and tumorigenesis.

Thioridazine requires calcium influx to induce MLL-AF6-rearranged AML cell death.

In pediatric acute myeloid leukemia (AML), intensive chemotherapy and allogeneic hematopoietic stem cell transplantation are the cornerstones of treatment in high-risk cases, with severe late effects and a still high risk of disease recurrence as the main drawbacks. The identification of targeted, more effective, safer drugs is thus desirable. We performed a high-throughput drug-screening assay of 1280 compounds and identified thioridazine (TDZ), a drug that was highly selective for the t(6;11)(q27;q23) MLL-AF6 (6;11)AML rearrangement, which mediates a dramatically poor (below 20%) survival rate. TDZ induced cell death and irreversible progress toward the loss of leukemia cell clonogenic capacity in vitro. Thus, we explored its mechanism of action and found a profound cytoskeletal remodeling of blast cells that led to Ca2+ influx, triggering apoptosis through mitochondrial depolarization, confirming that this latter phenomenon occurs selectively in t(6;11)AML, for which AF6 does not work as a cytoskeletal regulator, because it is sequestered into the nucleus by the fusion gene. We confirmed TDZ-mediated t(6;11)AML toxicity in vivo and enhanced the drug's safety by developing novel TDZ analogues that exerted the same effect on leukemia reduction, but with lowered neuroleptic effects in vivo. Overall, these results refine the MLL-AF6 AML leukemogenic mechanism and suggest that the benefits of targeting it be corroborated in further clinical trials.

Corrigendum: Recirculation and Residency of T Cells and Tregs: Lessons Learnt in Anacapri.

[This corrects the article DOI: 10.3389/fimmu.2020.00682.].

Correction: JNK1 and ERK1/2 modulate lymphocyte homeostasis via BIM and DRP1 upon AICD induction.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Recirculation and Residency of T Cells and Tregs: Lessons Learnt in Anacapri.

"Location, location, and location": according to this mantra, the place where living beings settle has a key impact on the success of their activities; in turn, the living beings can, in many ways, modify their environment. This idea has now become more and more true for T cells. The ability of T cells to recirculate throughout blood or lymph, or to stably reside in certain tissues, turned out to determine immunity to pathogens, and tumors. If location matters also for human beings, the inspiring environment of Capri Island has contributed to the success of the EFIS-EJI Ruggero Ceppellini Advanced School of Immunology focused on "T cell memory," held in Anacapri from October 12, 2018 to October 15, 2018. In this minireview, we would like to highlight some novel concepts about T cell migration and residency and discuss their implications in relation to recent advances in the field, including the mechanisms regulating compartmentalization and cell cycle entry of T cells during activation, the role of mitochondrial metabolism in T cell movement, and the residency of regulatory T cells.

JNK1 and ERK1/2 modulate lymphocyte homeostasis via BIM and DRP1 upon AICD induction.

The Activation-Induced Cell Death (AICD) is a stimulation-dependent form of apoptosis used by the organism to shutdown T-cell response once the source of inflammation has been eliminated, while allowing the generation of immune memory. AICD is thought to progress through the activation of the extrinsic Fas/FasL pathway of cell death, leading to cytochrome-C release through caspase-8 and Bid activation. We recently described that, early upon AICD induction, mitochondria undergo structural alterations, which are required to promote cytochrome-C release and execute cell death. Here, we found that such alterations do not depend on the Fas/FasL pathway, which is instead only lately activated to amplify the cell death cascade. Instead, such alterations are primarily dependent on the MAPK proteins JNK1 and ERK1/2, which, in turn, regulate the activity of the pro-fission protein Drp1 and the pro-apoptotic factor Bim. The latter regulates cristae disassembly and cooperate with Drp1 to mediate the Mitochondrial Outer Membrane Permeabilization (MOMP), leading to cytochrome-C release. Interestingly, we found that Bim is also downregulated in T-cell Acute Lymphoblastic Leukemia (T-ALL) cells, this alteration favouring their escape from AICD-mediated control.

Targeting Drp1 and mitochondrial fission for therapeutic immune modulation.

Mitochondria are dynamic organelles whose processes of fusion and fission are tightly regulated by specialized proteins, known as mitochondria-shaping proteins. Among them, Drp1 is the main pro-fission protein and its activity is tightly regulated to ensure a strict control over mitochondria shape according to the cell needs. In the recent years, mitochondrial dynamics emerged as a new player in the regulation of fundamental processes during T cell life. Indeed, the morphology of mitochondria directly regulates T cell differentiation, this by affecting the engagment of alternative metabolic routes upon activation. Further, Drp1-dependent mitochondrial fission sustains both T cell clonal expansion and T cell migration and invasivness. By this review, we aim at discussing the most recent findings about the roles played by the Drp1-dependent mitochondrial fission in T cells, and at highlighting how its pharmacological modulation could open the way to future therapeutic approaches to modulate T cell response.

Reversible induction of mitophagy by an optogenetic bimodular system.

Autophagy-mediated degradation of mitochondria (mitophagy) is a key process in cellular quality control. Although mitophagy impairment is involved in several patho-physiological conditions, valuable methods to induce mitophagy with low toxicity in vivo are still lacking. Herein, we describe a new optogenetic tool to stimulate mitophagy, based on light-dependent recruitment of pro-autophagy protein AMBRA1 to mitochondrial surface. Upon illumination, AMBRA1-RFP-sspB is efficiently relocated from the cytosol to mitochondria, where it reversibly mediates mito-aggresome formation and reduction of mitochondrial mass. Finally, as a proof of concept of the biomedical relevance of this method, we induced mitophagy in an in vitro model of neurotoxicity, fully preventing cell death, as well as in human T lymphocytes and in zebrafish in vivo. Given the unique features of this tool, we think it may turn out to be very useful for a wide range of both therapeutic and research applications.

Drp1 Controls Effective T Cell Immune-Surveillance by Regulating T Cell Migration, Proliferation, and cMyc-Dependent Metabolic Reprogramming.

Mitochondria are key players in the regulation of T cell biology by dynamically responding to cell needs, but how these dynamics integrate in T cells is still poorly understood. We show here that the mitochondrial pro-fission protein Drp1 fosters migration and expansion of developing thymocytes both in vitro and in vivo. In addition, we find that Drp1 sustains in vitro clonal expansion and cMyc-dependent metabolic reprogramming upon activation, also regulating effector T cell numbers in vivo. Migration and extravasation defects are also exhibited in Drp1-deficient mature T cells, unveiling its crucial role in controlling both T cell recirculation in secondary lymphoid organs and accumulation at tumor sites. Moreover, the observed Drp1-dependent imbalance toward a memory-like phenotype favors T cell exhaustion in the tumor microenvironment. All of these findings support a crucial role for Drp1 in several processes during T cell development and in anti-tumor immune-surveillance.

AMBRA1 Controls Regulatory T-Cell Differentiation and Homeostasis Upstream of the FOXO3-FOXP3 Axis.

Regulatory T cells (Treg) are necessary to maintain immunological tolerance and are key players in the control of autoimmune disease susceptibility. Expression of the transcription factor FOXP3 is essential for differentiation of Treg cells and indispensable for their suppressive function. However, there is still a lack of knowledge about the mechanisms underlying its regulation. Here, we demonstrate that pro-autophagy protein AMBRA1 is also a key modulator of T cells, regulating the complex network that leads to human Treg differentiation and maintenance. Indeed, through its ability to interact with the phosphatase PP2A, AMBRA1 promotes the stability of the transcriptional activator FOXO3, which, in turn, triggers FOXP3 transcription. Furthermore, we found that AMBRA1 plays a significant role in vivo by regulating Treg cell induction in mouse models of both tumor growth and multiple sclerosis, thus highlighting the role of AMBRA1 in the control of immune homeostasis.

Monitoring the Mitochondrial Dynamics in Mammalian Cells.

Mitochondria exist in a dynamic state inside mammalian cells. They undergo processes of fusion and fission to adjust their shape according to the different cell needs. Different proteins tightly regulate these dynamics: Opa-1 and Mitofusin-1 and Mitofusin-2 are the main profusion proteins, while Drp1 and its different receptors (Mff, Fis1, MiD49, MiD51) regulate mitochondrial fission. The dynamic nature of the mitochondrial network has become evident and detectable, thanks to recent advances in live imaging video microscopy and to the availability of mitochondria-tagged fluorescent proteins. High-resolution confocal reconstruction of mitochondria over time allows researchers to visualize mitochondria shape changes in living cells, under different experimental conditions. Moreover, in recent years, different techniques in living cells have been developed to study the process of mitochondria fusion in more details. Among them are fluorescence recovery after photobleaching (FRAP) of mitochondria-tagged GFP (mtGFP), use of photoactivatable mtGFP, polyethylene glycol (PEG)-based fusion of mtGFP and mtRFP cells, and Renilla luciferase assay (for population studies). In addition, in combination with imaging, the analysis of the expression levels of the different mitochondria-shaping proteins, along with that of their activation status, represents a powerful tool to investigate potential modulations of the mitochondrial network. Here, we review this aspect and then mention a number of techniques, with particular attention to their relative protocols.

AMBRA1-Mediated Mitophagy Counteracts Oxidative Stress and Apoptosis Induced by Neurotoxicity in Human Neuroblastoma SH-SY5Y Cells.

Therapeutic strategies are needed to protect dopaminergic neurons in Parkinson's disease (PD) patients. Oxidative stress caused by dopamine may play an important role in PD pathogenesis. Selective autophagy of mitochondria (mitophagy), mainly regulated by PINK1 and PARKIN, plays an important role in the maintenance of cell homeostasis. Mutations in those genes cause accumulation of damaged mitochondria, leading to nigral degeneration and early-onset PD. AMBRA1ActA is a fusion protein specifically expressed at the mitochondria, and whose expression has been shown to induce a powerful mitophagy in mammalian cells. Most importantly, the pro-autophagy factor AMBRA1 is sufficient to restore mitophagy in fibroblasts of PD patients carrying PINK1 and PARKIN mutations. In this study, we investigated the potential neuroprotective effect of AMBRA1-induced mitophagy against 6-hydroxydopamine (6-OHDA)- and rotenone-induced cell death in human neuroblastoma SH-SY5Y cells. We demonstrated that AMBRA1ActA overexpression was sufficient to induce mitochondrial clearance in SH-SY5Y cells. We found that apoptosis induced by 6-OHDA and rotenone was reversed by AMBRA1-induced mitophagy. Finally, transfection of SH-SY5Y cells with a vector encoding AMBRA1ActA significantly reduced 6-OHDA and rotenone-induced generation of reactive oxygen species (ROS). Altogether, our results indicate that AMBRA1ActA is able to induce mitophagy in SH-SY5Y cells in order to suppress oxidative stress and apoptosis induced by both 6-OHDA and rotenone. These results strongly suggest that AMBRA1 may have promising neuroprotective properties with an important role in limiting ROS-induced dopaminergic cell death, and the utmost potential to prevent PD or other neurodegenerative diseases associated with mitochondrial oxidative stress.

S-nitrosylation drives cell senescence and aging in mammals by controlling mitochondrial dynamics and mitophagy.

S-nitrosylation, a prototypic redox-based posttranslational modification, is frequently dysregulated in disease. S-nitrosoglutathione reductase (GSNOR) regulates protein S-nitrosylation by functioning as a protein denitrosylase. Deficiency of GSNOR results in tumorigenesis and disrupts cellular homeostasis broadly, including metabolic, cardiovascular, and immune function. Here, we demonstrate that GSNOR expression decreases in primary cells undergoing senescence, as well as in mice and humans during their life span. In stark contrast, exceptionally long-lived individuals maintain GSNOR levels. We also show that GSNOR deficiency promotes mitochondrial nitrosative stress, including excessive S-nitrosylation of Drp1 and Parkin, thereby impairing mitochondrial dynamics and mitophagy. Our findings implicate GSNOR in mammalian longevity, suggest a molecular link between protein S-nitrosylation and mitochondria quality control in aging, and provide a redox-based perspective on aging with direct therapeutic implications.