Syntaxin-17-Dependent Mitochondrial Dynamics is Essential for Protection Against Oxidative-Stress-Induced Apoptosis

Syntaxin17 Restores Lysosomal Function and Inhibits Pyroptosis Caused by Acinetobacter baumannii

Acinetobacter baumannii (A. baumannii) causes autophagy flux disorder by degrading STX17, resulting in a serious inflammatory response. It remains unclear whether STX17 can alter the inflammatory response process by controlling autolysosome function. This study aimed to explore the role of STX17 in the regulation of pyroptosis induced by A. baumannii. Our findings indicate that overexpression of STX17 enhances autophagosome degradation, increases LAMP1 expression, reduces Cathepsin B release, and improves lysosomal function. Conversely, knockdown of STX17 suppresses autophagosome degradation, reduces LAMP1 expression, augments Cathepsin B release, and accelerates lysosomal dysfunction. In instances of A. baumannii infection, overexpression of STX17 was found to improve lysosomal function and reduce the expression of mature of GSDMD and IL-1β, along with the release of LDH, thus inhibiting pyroptosis caused by A. baumannii. Conversely, knockdown of STX17 led to increased lysosomal dysfunction and further enhanced the expression of mature of GSDMD and IL-1β, and increased the release of LDH, exacerbating pyroptosis induced by A. baumannii. These findings suggest that STX17 regulates pyroptosis induced by A. baumannii by modulating lysosomal function.

Chloroquine synergizes doxorubicin efficacy in cervical cancer cells through flux impairment and down regulation of proteins involved in the fusion of autophagosomes to lysosomes

Drug repurposing holds abundant opportunity in the development of novel anticancer drugs. Chloroquine (CQ), a FDA approved anti-malarial drug, is demonstrated to enhance anticancer efficacy of standard anticancer drugs including doxorubicin (DOX) in several types of cancer cells. Here, we aimed to exploit the chemosensitizing effects of CQ against DOX in human cervical cancer (HeLa) cells that remains to be investigated yet. We show that a combination of DOX (40 nM) and CQ (40 μM) resulted in a synergistic cytotoxicity (combination index; CI < 1) in HeLa cells compared to the DOX or CQ alone. Synergistic effect of the combination (DOX + CQ) was associated with the impaired autophagic flux and enhanced apoptosis. Following treatment with the combination (DOX + CQ), the level of p62/SQSTM and LC-3II proteins was increased, while a decrease was noted in the expression of LAMP-2, Syntaxin17, Rab 5, and Rab 7 proteins that play critical roles in the fusion of autophagosomes to lysosomes. Autophagy inhibition by combination (DOX + CQ) enhanced the apoptotic cell death synergistically by increasing the cleavage of procaspase-3 and PARP1. Further, a prior incubation of HeLa cells with Z-VAD-FMK (a pan-caspase inhibitor) for 4 h, suppressed the combination (DOX + CQ)-induced cell death. Our data suggest that a combination of DOX + CQ had a better anti-cancer efficacy in HeLa cells than either of the drugs alone. Thus, CQ, as a repurposed drug, may hold the potential to synergize anticancer effects of DOX in cervical cancer cells.

New insights regarding SNARE proteins in autophagosome-lysosome fusion

Macroautophagy/autophagy refers to the engulfment of cellular contents selected for lysosomal degradation. The final step in autophagy is the fusion of autophagosome with the lysosome, which is mediated by SNARE proteins. Of the SNAREs, autophagosome-localized Q-SNAREs, such as STX17 and SNAP29, and lysosome-localized R-SNAREs, such as VAMP8 or VAMP7, have been reported to be involved. Recent studies also reveal participation of the R-SNARE, YKT6, in autophagosome-lysosome fusion. These SNAREs, with the help of other regulatory factors, act coordinately to spatiotemporally control the fusion process. Besides regulating autophagosome-lysosome fusion, some SNAREs, such as STX17, also function in other autophagic processes, including autophagosome formation and mitophagy. A better understanding of the functions of SNAREs will shed light on the molecular mechanisms of autophagosome-lysosome fusion as well as on the mechanisms by which autophagy is globally regulated.Abbreviations: ATG: autophagy related; DNM1L: dynamin 1 like; ER: endoplasmic reticulum; GABARAP: GABA type A receptor-associated protein; GABARAPL1: GABA type A receptor associated protein like 1; IRGM: immunity related GTPase M; LAMP2: lysosomal associated membrane protein 2; MAP1LC3B/LC3: microtubule associated protein 1 light chain 3 beta; MTOR: mechanistic target of rapamycin kinase; PIK3R4: phosphoinositide-3-kinase regulatory subunit 4; PLEKHM1: pleckstrin homology and RUN domain containing M1; PRKN: PRKN RBR E3 ubiquitin protein ligase; RAB2A: RAB2A, member RAS oncogene family; RAB33B: RAB33B, member RAS oncogene family; RAB7A: RAB7A, member RAS oncogene family; RB1CC1: RB1 inducible coiled-coil 1; RTN3: reticulon 3; RUBCNL: rubicon like autophagy enhancer; SNARE: soluble N-ethylmaleimide-sensitive factor attachment protein receptor; SNAP29: synaptosomal associated protein 29; STX17: syntaxin 17; ULK1: unc-51 like autophagy activating kinase 1; VAMP7: vesicle associated membrane protein 7; VAMP8: vesicle associated membrane protein 8; YKT6: YKT6 v-SNARE homolog.

Mitochondrial Fission Protein 1: Emerging Roles in Organellar Form and Function in Health and Disease

Mitochondrial fission protein 1 (Fis1) was identified in yeast as being essential for mitochondrial division or fission and subsequently determined to mediate human mitochondrial and peroxisomal fission. Yet, its exact functions in humans, especially in regard to mitochondrial fission, remains an enigma as genetic deletion of Fis1 elongates mitochondria in some cell types, but not others. Fis1 has also been identified as an important component of apoptotic and mitophagic pathways suggesting the protein may have multiple, essential roles. This review presents current perspectives on the emerging functions of Fis1 and their implications in human health and diseases, with an emphasis on Fis1's role in both endocrine and neurological disorders.

When Friendship Turns Sour: Effective Communication Between Mitochondria and Intracellular Organelles in Parkinson's Disease

Parkinson's disease (PD) is a complex neurodegenerative disease with pathological hallmarks including progressive neuronal loss from the substantia nigra pars compacta and α-synuclein intraneuronal inclusions, known as Lewy bodies. Although the etiology of PD remains elusive, mitochondrial damage has been established to take center stage in the pathogenesis of PD. Mitochondria are critical to cellular energy production, metabolism, homeostasis, and stress responses; the association with PD emphasizes the importance of maintenance of mitochondrial network integrity. To accomplish the pleiotropic functions, mitochondria are dynamic not only within their own network but also in orchestrated coordination with other organelles in the cellular community. Through physical contact sites, signal transduction, and vesicle transport, mitochondria and intracellular organelles achieve the goals of calcium homeostasis, redox homeostasis, protein homeostasis, autophagy, and apoptosis. Herein, we review the finely tuned interactions between mitochondria and surrounding intracellular organelles, with focus on the nucleus, endoplasmic reticulum, Golgi apparatus, peroxisomes, and lysosomes. Participants that may contribute to the pathogenic mechanisms of PD will be highlighted in this review.

The YAP/SERCA2a signaling pathway protects cardiomyocytes against reperfusion-induced apoptosis

Mitochondria and the endoplasmic reticulum (ER) are known to promote cardiac ischemia/reperfusion (I/R) injury. Overexpression of yes-associated protein (YAP) and/or sarcoplasmic reticulum calcium ATPase 2a (SERCA2a) has been shown to protect cardiomyocytes against I/R-induced injury. Here, we show that activation of the YAP/SERCA2a pathway attenuated mitochondrial damage and ER stress (ERS) to maintain cardiomyocyte viability in the setting of I/R injury. Our results demonstrate that I/R treatment reduced the transcription and expression of YAP and SERCA2a, along with a decline in cardiomyocyte viability. The overexpression of YAP promoted SERCA2a transcription, whereas SERCA2a upregulation did not affect the YAP transcription, suggesting that YAP functions upstream of SERCA2a. Activation of the YAP/SERCA2a pathway suppressed mitochondrial damage by sustaining the mitochondrial redox balance and restoring mitochondrial bioenergetics. Additionally, its activation repressed ERS, reduced calcium overload, and eventually blocked caspase activation. The knockdown of SERCA2a suppressed the protective effects of YAP overexpression on mitochondrial damage and ERS. Overall, our findings reveal that the YAP/SERCA2a pathway attenuates the mitochondrial damage and ERS in response to cardiac I/R injury by regulating the mitochondria–ER communication.

Role of mitochondrial quality control in the pathogenesis of nonalcoholic fatty liver disease

Nutrient oversupply and mitochondrial dysfunction play central roles in nonalcoholic fatty liver disease (NAFLD). The mitochondria are the major sites of β-oxidation, a catabolic process by which fatty acids are broken down. The mitochondrial quality control (MQC) system includes mitochondrial fission, fusion, mitophagy and mitochondrial redox regulation, and is essential for the maintenance of the functionality and structural integrity of the mitochondria. Excessive and uncontrolled production of reactive oxygen species (ROS) in the mitochondria damages mitochondrial components, including membranes, proteins and mitochondrial DNA (mtDNA), and triggers the mitochondrial pathway of apoptosis. The functionality of some damaged mitochondria can be restored by fusion with normally functioning mitochondria, but when severely damaged, mitochondria are segregated from the remaining functional mitochondrial network through fission and are eventually degraded via mitochondrial autophagy, also called as mitophagy. In this review, we describe the functions and mechanisms of mitochondrial fission, fusion, oxidative stress and mitophagy in the development and progression of NAFLD.