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Macroautophagy decreases with age, and this change is considered a hallmark of the aging process. It remains unknown whether mitophagy, the essential selective autophagic degradation of mitochondria, also decreases with age. In our analysis of mitophagy in multiple organs in the mito-QC reporter mouse, mitophagy is either increased or unchanged in old versus young mice. Transcriptomic analysis shows marked upregulation of the type I interferon response in the retina of old mice, which correlates with increased levels of cytosolic mtDNA and activation of the cGAS/STING pathway. Crucially, these same alterations are replicated in primary human fibroblasts from elderly donors. In old mice, pharmacological induction of mitophagy with urolithin A attenuates cGAS/STING activation and ameliorates deterioration of neurological function. These findings point to mitophagy induction as a strategy to decrease age-associated inflammation and increase healthspan. Dysregulated autophagy and mitochondrial function are two well-described hallmarks of aging. Here, the authors describe an unexpected age-associated upregulation of mitophagy in response to neuroinflammation triggered by leaked mtDNA.

Background Age-related macular degeneration (AMD) is the leading cause of blindness in elderly people in the developed world, and the number of people affected is expected to almost double by 2040. The retina presents one of the highest metabolic demands in our bodies that is partially or fully fulfilled by mitochondria in the neuroretina and retinal pigment epithelium (RPE), respectively. Together with its post-mitotic status and constant photooxidative damage from incoming light, the retina requires a tightly-regulated housekeeping system that involves autophagy. The natural polyphenol Urolithin A (UA) has shown neuroprotective benefits in several models of aging and age-associated disorders, mostly attributed to its ability to induce mitophagy and mitochondrial biogenesis. Sodium iodate (SI) administration recapitulates the late stages of AMD, including geographic atrophy and photoreceptor cell death. Methods A combination of in vitro, ex vivo and in vivo models were used to test the neuroprotective potential of UA in the SI model. Functional assays (OCT, ERGs), cellular analysis (flow cytometry, qPCR) and fine confocal microscopy (immunohistochemistry, tandem selective autophagy reporters) helped address this question. Results UA alleviated neurodegeneration and preserved visual function in SI-treated mice. Simultaneously, we observed severe proteostasis defects upon SI damage induction, including autophagosome accumulation, that were resolved in animals that received UA. Treatment with UA restored autophagic flux and triggered PINK1/Parkin-dependent mitophagy, as previously reported in the literature. Autophagy blockage caused by SI was caused by severe lysosomal membrane permeabilization. While UA did not induce lysosomal biogenesis, it did restore upcycling of permeabilized lysosomes through lysophagy. Knockdown of the lysophagy adaptor SQSTM1/p62 abrogated viability rescue by UA in SI-treated cells, exacerbated lysosomal defects and inhibited lysophagy. Conclusions Collectively, these data highlight a novel putative application of UA in the treatment of AMD whereby it bypasses lysosomal defects by promoting p62-dependent lysophagy to sustain proteostasis. Graphical Abstract

The vimentin network displays remarkable plasticity to support basic cellular functions and reorganizes during cell division. Here, we show that in several cell types vimentin filaments redistribute to the cell cortex during mitosis, forming a robust framework interwoven with cortical actin and affecting its organization. Importantly, the intrinsically disordered tail domain of vimentin is essential for this redistribution, which allows normal mitotic progression. A tailless vimentin mutant forms curly bundles, which remain entangled with dividing chromosomes leading to mitotic catastrophes or asymmetric partitions. Serial deletions of vimentin tail domain gradually impair cortical association and mitosis progression. Disruption of f-actin, but not of microtubules, causes vimentin bundling near the chromosomes. Pathophysiological stimuli, including HIV-protease and lipoxidation, induce similar alterations. Interestingly, full filament formation is dispensable for cortical association, which also occurs in vimentin particles. These results unveil implications of vimentin dynamics in cell division through its interplay with the actin cortex. The intermediate filament vimentin reorganizes during mitosis, but its molecular regulation and impact on the cell during cell division is unclear. Here, the authors show that vimentin filaments redistribute to the cell cortex during mitosis intertwining with and affecting actin organization.

The SARS-CoV-2 Spike protein mediates docking of the virus onto cells prior to viral invasion. Several cellular receptors facilitate SARS-CoV-2 Spike docking at the cell surface, of which ACE2 plays a key role in many cell types. The intermediate filament protein vimentin has been reported to be present at the surface of certain cells and act as a co-receptor for several viruses; furthermore, its potential involvement in interactions with Spike proteins has been proposed. Nevertheless, the potential colocalization of vimentin with Spike and its receptors on the cell surface has not been explored. Here we have assessed the binding of Spike protein constructs to several cell types. Incubation of cells with tagged Spike S or Spike S1 subunit led to discrete dotted patterns at the cell surface, which consistently colocalized with endogenous ACE2, but sparsely with a lipid raft marker. Vimentin immunoreactivity mostly appeared as spots or patches unevenly distributed at the surface of diverse cell types. Of note, vimentin could also be detected in extracellular particles and in the cytoplasm underlying areas of compromised plasma membrane. Interestingly, although overall colocalization of vimentin-positive spots with ACE2 or Spike was moderate, a selective enrichment of the three proteins was detected at elongated structures, positive for acetylated tubulin and ARL13B. These structures, consistent with primary cilia, concentrated Spike binding at the top of the cells. Our results suggest that a vimentin-Spike interaction could occur at selective locations of the cell surface, including ciliated structures, which can act as platforms for SARS-CoV-2 docking.

The type III intermediate filament proteins vimentin and GFAP are modulated by oxidants and electrophiles, mainly through perturbation of their single cysteine residues. Desmin, the type III intermediate filament protein specific to muscle cells, is critical for muscle homeostasis, playing a key role in sarcomere organization and mitochondrial function. Here, we have studied the impact of oxidants and cysteine-reactive agents on desmin behavior. Our results show that several reactive species and drugs induce covalent modifications of desmin in vitro, of which its single cysteine residue, C333, is an important target. Moreover, stimuli eliciting oxidative stress or lipoxidation, including H2O2, 15-deoxy-prostaglandin J2, and CoCl2-elicited chemical hypoxia, provoke desmin disorganization in H9c2 rat cardiomyoblasts transfected with wild-type desmin, which is partially attenuated in cells expressing a C333S mutant. Notably, in cells lacking other cytoplasmic intermediate filaments, network formation by desmin C333S appears less efficient than that of desmin wt, especially when these proteins are expressed as fluorescent fusion constructs. Nevertheless, in these cells, the desmin C333S organization is also protected from disruption by oxidants. Taken together, our results indicate that desmin is a target for oxidative and electrophilic stress, which elicit desmin remodeling conditioned by the presence of its single cysteine residue.

Protein lipoxidation is a non-enzymatic post-translational modification that consists of the covalent addition of reactive lipid species to proteins. This occurs under basal conditions but increases in situations associated with oxidative stress. Protein targets for lipoxidation include metabolic and signalling enzymes, cytoskeletal proteins, and transcription factors, among others. There is strong evidence for the involvement of protein lipoxidation in disease, including atherosclerosis, neurodegeneration, and cancer. Nevertheless, the involvement of lipoxidation in cellular regulatory mechanisms is less understood. Here we review basic aspects of protein lipoxidation and discuss several features that could support its role in cell signalling, including its selectivity, reversibility, and possibilities for regulation at the levels of the generation and/or detoxification of reactive lipids. Moreover, given the great structural variety of electrophilic lipid species, protein lipoxidation can contribute to the generation of multiple structurally and functionally diverse protein species. Finally, the nature of the lipoxidised proteins and residues provides a frameshift for a complex interplay with other post-translational modifications, including redox and redox-regulated modifications, such as oxidative modifications and phosphorylation, thus strengthening the importance of detailed knowledge of this process.

Mitochondrial quality control is finely tuned by mitophagy, the selective degradation of mitochondria through autophagy, and mitochondrial biogenesis. Removal of damaged mitochondria is essential to preserve cellular bioenergetics and prevent detrimental events such as sustained mitoROS production, pro-apoptotic cytochrome c release or mtDNA leakage. The array of tools available to study mitophagy is very limited but in constant development. Almost a decade ago, we developed a method to assess mitophagy flux using MitoTracker Deep Red in combination with lysosomal inhibitors. Now, using the novel tandem-fluorescence reporter mito -QC (mCherry-GFP-FIS1 101−152 ) that allows to differentiate between healthy mitochondria (mCherry + GFP + ) and mitolysosomes (mCherry + GFP − ), we have developed a robust and quantitative method to assess mitophagy by flow cytometry. This approach has been validated in ARPE-19 cells using PINK1/Parkin-dependent (CCCP) and PINK1/Parkin-independent (DFP) positive controls and complementary techniques. Furthermore, we show that the mito -QC reporter can be multiplexed, especially if using spectral flow cytometry, to simultaneously study other cellular parameters such as viability or ROS production. Using this technique, we evaluated and characterized two prospective mitophagy inducers and further dissected their mechanism of action. Finally, using mito -QC reporter mice, we developed a protocol to measure mitophagy levels in the retina ex vivo . This novel methodology will propel mitophagy research forward and accelerate the discovery of novel mitophagy modulators.

Intermediate filaments are critical regulators of cell responses and organizers of cellular structures. GFAP (glial fibrillary acidic protein) is an intermediate filament protein mainly expressed in astrocytes. GFAP mutations are associated with Alexander disease (AxD), a type of leukodystrophy that causes degeneration of astrocytes and ultimately neurodegeneration. AxD astrocytes display protein aggregation, proteostasis defects and altered organelle homeostasis. We previously showed that expression of GFAP AxD mutants in astrocytes provoked mitochondrial alterations and oxidative stress. Here we have used an astrocytoma cell model to explore the impact of GFAP AxD mutants on the lysosomal degradation pathway. Expression of GFAP AxD mutants in this model elicits marked alterations in lysosomal distribution. Cells expressing the GFAP R239C mutant display defective lysosomal activity and intraluminal acidification. Lysosomes are primary sites of oxidative damage. Moreover, expression of GFAP R239C increases susceptibility of lysosomes to oxidative stress, resulting in a greater loss of lysosomal mass and compromised membrane integrity, as revealed by increased intraluminal recruitment of galectins, with respect to cells expressing GFAP wt. Notably, lysosomes in GFAP R239C expressing cells are also more vulnerable to chemically-induced rupture. Interestingly, lysosomes of cells expressing GFAP wt are able to rapidly recover after removal of the damaging agent. In sharp contrast, recovery of acidic vesicles is severely impaired in cells expressing GFAP R239C, suggesting a defect in lysosomal repair. Taken together, our results show that expression of the GFAP AxD mutant is sufficient to deeply perturb lysosomal distribution, function and repair. These alterations could contribute to proteostasis defects and cellular toxicity in AxD.Competing Interest StatementThe authors have declared no competing interest.

The vimentin network displays remarkable plasticity to support basic cellular functions. Here, we show that in several cell types vimentin filaments redistribute to the cell periphery during mitosis, forming a robust scaffold interwoven with cortical actin and affecting the mitotic cortex properties. Importantly, the intrinsically disordered tail domain of vimentin is essential for this redistribution, which allows normal mitotic progression. A tailless vimentin mutant forms curly bundles, which remain entangled with dividing chromosomes leading to mitotic catastrophes or asymmetric partitions. Serial deletions of the tail domain induce increasing impairments of cortical association and mitosis progression. Disruption of actin, but not of microtubules, mimics the impact of tail deletion. Pathophysiological stimuli, including HIV-protease and lipoxidation, induce similar alterations. Interestingly, filament integrity is dispensable for cortical association, which also occurs in vimentin particles. These results unveil novel implications of vimentin dynamics in cell division by means of its interplay with the mitotic cortex. Footnotes * Main text, figures, supplemental figures. Supplemental videos have been added.