Renitence vacuoles facilitate protection against phagolysosomal damage in activated macrophages

Label-Free Intracellular Multi-Specificity in Yeast Cells by Phase-Contrast Tomographic Flow Cytometry

In-flow phase-contrast tomography provides a 3D refractive index of label-free cells in cytometry systems. Its major limitation, as with any quantitative phase imaging approach, is the lack of specificity compared to fluorescence microscopy, thus restraining its huge potentialities in single-cell analysis and diagnostics. Remarkable results in introducing specificity are obtained through artificial intelligence (AI), but only for adherent cells. However, accessing the 3D fluorescence ground truth and obtaining accurate voxel-level co-registration of image pairs for AI training is not viable for high-throughput cytometry. The recent statistical inference approach is a significant step forward for label-free specificity but remains limited to cells' nuclei. Here, a generalized computational strategy based on a self-consistent statistical inference to achieve intracellular multi-specificity is shown. Various subcellular compartments (i.e., nuclei, cytoplasmic vacuoles, the peri-vacuolar membrane area, cytoplasm, vacuole-nucleus contact site) can be identified and characterized quantitatively at different phases of the cells life cycle by using yeast cells as a biological model. Moreover, for the first time, virtual reality is introduced for handling the information content of multi-specificity in single cells. Full fruition is proofed for exploring and interacting with 3D quantitative biophysical parameters of the identified compartments on demand, thus opening the route to a metaverse for 3D microscopy.

Maintenance and loss of endocytic organelle integrity: mechanisms and implications for antigen cross-presentation

The membranes of endosomes, phagosomes and macropinosomes can become damaged by the physical properties of internalized cargo, by active pathogenic invasion or by cellular processes, including endocytic maturation. Loss of membrane integrity is often deleterious and is, therefore, prevented by mitigation and repair mechanisms. However, it can occasionally be beneficial and actively induced by cells. Here, we summarize the mechanisms by which cells, in particular phagocytes, try to prevent membrane damage and how, when this fails, they repair or destroy damaged endocytic organelles. We also detail how one type of phagocyte, the dendritic cell, can deliberately trigger localized damage to endocytic organelles to allow for major histocompatibility complex class I presentation of exogenous antigens and initiation of CD8⁺ T-cell responses to viruses and tumours. Our review highlights mechanisms for the regulation of endocytic organelle membrane integrity at the intersection of cell biology and immunology that could be co-opted for improving vaccination and intracellular drug delivery.

Epimedokoreanin C, a prenylated flavonoid isolated from Epimedium koreanum, induces non-apoptotic cell death with the characteristics of methuosis in lung cancer cells

Methuosis is a novel type of non-apoptotic cell death characterized by accumulation of cytoplasmic vacuoles. Identification of molecules that induce methuosis may provide alternative therapeutics for cancers that are refractory to apoptosis. Epimedokoreanin C (EKC) is a prenylated flavonoid isolated from a Chinese herb Epimedium koreanum. In this article, we described that EKC reduced cell viability accompanied by extreme vacuolation in human lung cancer cells. The EKC-induced cell death was clarified as non-apoptosis based on the absence of apoptotic changes. The vacuoles stimulated by EKC were supposed to be derived from macropinocytosis based on the engulfment of extracellular fluid tracer, Lucifer Yellow. The vacuoles acquired some characteristics of late endosomes supported that EKC-induced cell death could be described as methuosis. Rac1 and Arf6 were found to be regulated inversely after EKC treatment. Blocking Rac1 activation with the specific Rac1 inhibitor EHT 1864 prevented the accumulation of vacuoles induced by EKC markedly, suggested that the regulation of Rac1 and Arf6 was at least partial mechanism involved in EKC induced methuosis. EKC synergized the effects of doxorubicin and etoposide, demonstrating the effectiveness of using EKC to synergize conventional chemotherapy. Collectively, EKC was demonstrated as a methuosis-like cell death inducer in lung cancer NCI-H292 and A549 cells. It has the potential to be used as an attractive prototype for developing drugs that could kill apoptosis-resistant cancer cells.

Macrophage inflammatory state influences susceptibility to lysosomal damage

Macrophages possess mechanisms for reinforcing the integrity of their endolysosomes against damage. This property, termed inducible renitence, was previously observed in murine macrophages stimulated with LPS, peptidoglycan, IFNγ, or TNFα, which suggested roles for renitence in macrophage resistance to infection by membrane‐damaging pathogens. This study analyzed additional inducers of macrophage differentiation for their ability to increase resistance to lysosomal damage by membrane‐damaging particles. Renitence was evident in macrophages activated with LPS plus IFNγ, PGE₂, or adenosine, and in macrophages stimulated with IFN‐β, but not in macrophages activated with IL‐4 or IL‐10. These responses indicated roles for macrophage subtypes specialized in host defense and suppression of immune responses, but not those involved in wound healing. Consistent with this pattern, renitence could be induced by stimulation with agonists for TLR, which required the signaling adaptors MyD88 and/or TRIF, and by infection with murine norovirus‐1. Renitence induced by LPS was dependent on cytokine secretion by macrophages. However, no single secreted factor could explain all the induced responses. Renitence induced by the TLR3 agonist Poly(I:C) was mediated in part by the type I IFN response, but renitence induced by Pam3CSK4 (TLR2/1), LPS (TLR4), IFNγ, or TNFα was independent of type 1 IFN signaling. Thus, multiple pathways for inducing macrophage resistance to membrane damage exist and depend on the particular microbial stimulus sensed.

Monocytes Expose Factor XIII-A and Stabilize Thrombi against Fibrinolytic Degradation

Factor XIII (FXIII) is a transglutaminase that promotes thrombus stability by cross-linking fibrin. The cellular form, a homodimer of the A subunits, denoted FXIII-A, lacks a classical signal peptide for its release; however, we have shown that it is exposed on activated platelets. Here we addressed whether monocytes expose intracellular FXIII-A in response to stimuli. Using flow cytometry, we demonstrate that FXIII-A antigen and activity are up-regulated on human monocytes in response to stimulation by IL-4 and IL-10. Higher basal levels of the FXIII-A antigen were noted on the membrane of the monocytic cell line THP-1, but activity was significantly enhanced following stimulation with IL-4 and IL-10. In contrast, treatment with lipopolysaccharide did not upregulate exposure of FXIII-A in THP-1 cells. Quantification of the FXIII-A activity revealed a significant increase in THP-1 cells in total cell lysates following stimulation with IL-4 and IL-10. Following fractionation, the largest pool of FXIII-A was membrane associated. Monocytes were actively incorporated into the fibrin mesh of model thrombi. We found that stimulation of monocytes and THP-1 cells with IL-4 and IL-10 stabilized FXIII-depleted thrombi against fibrinolytic degradation, via a transglutaminase-dependent mechanism. Our data suggest that monocyte-derived FXIII-A externalized in response to stimuli participates in thrombus stabilization.

Macropinosomes as units of signal transduction

Macropinosome formation occurs as a localized sequence of biochemical activities and associated morphological changes, which may be considered a form of signal transduction leading to the construction of an organelle. Macropinocytosis may also convey information about the availability of extracellular nutrients to intracellular regulators of metabolism. Consistent with this idea, activation of the metabolic regulator mechanistic target of rapamycin complex-1 (mTORC1) in response to acute stimulation by growth factors and extracellular amino acids requires internalization of amino acids by macropinocytosis. This suggests that macropinocytosis is necessary for mTORC1-dependent growth of metazoan cells, both as a route for delivery of amino acids to sensors associated with lysosomes and as a platform for growth factor-dependent signalling to mTORC1 via phosphatidylinositol 3-kinase (PI3K) and the Akt pathway. Because the biochemical signals required for the construction of macropinosomes are also required for cell growth, and inhibition of macropinocytosis inhibits growth factor signalling to mTORC1, we propose that signalling by growth factor receptors is organized into stochastic, structure-dependent cascades of chemical reactions that both build a macropinosome and stimulate mTORC1. More generally, as discrete units of signal transduction, macropinosomes may be subject to feedback regulation by metabolism and cell dimensions. This article is part of the Theo Murphy meeting issue 'Macropinocytosis'.

Early Interaction of Alternaria infectoria Conidia with Macrophages

Fungi of the genus Alternaria are ubiquitous indoor and outdoor airborne agents, and individuals are daily exposed to their spores. Although its importance in human infections and, particularly in respiratory allergies, there are no studies of how Alternaria spp. spores interact with host cells. Our aim was to study the early interaction of Alternaria infectoria spores with macrophages, the first line of immune defense. RAW 264.7 macrophages were infected with A. infectoria conidia, and the internalization and viability of conidia once inside the macrophages were quantified during the first 6 h of interaction. Live cell imaging was used to study the dynamics of this interaction. TNF-α production was quantified by relative gene expression, and the concentration of other cytokines (IL-1α, IL-1β, IL-6, IL-4, IL-10, IL-17, GM-CSF and INF-γ) and a chemokine, MIP-1α, was quantified by ELISA. Conidia were rapidly internalized by macrophages, with approximately half internalized after 30 min of interaction. During the first 6 h of interaction, macrophages retained the ability to mitotically divide while containing internalized conidia. The classical macrophage-activated morphology was absent in macrophages infected with conidia, and TNF-α and other cytokines and chemokines failed to be produced. Thus, macrophages are able to efficiently phagocyte A. infectoria conidia, but, during the first 6 h, no effective antifungal response is triggered, therefore promoting the residence of these fungal conidia inside the macrophages.