Inhibition of the master regulator of Listeria monocytogenes virulence enables bacterial clearance from spacious replication vacuoles in infected macrophages

A hallmark of Listeria (L.) monocytogenes pathogenesis is bacterial escape from maturing entry vacuoles, which is required for rapid bacterial replication in the host cell cytoplasm and cell-to-cell spread. The bacterial transcriptional activator PrfA controls expression of key virulence factors that enable exploitation of this intracellular niche. The transcriptional activity of PrfA within infected host cells is controlled by allosteric coactivation. Inhibitory occupation of the coactivator site has been shown to impair PrfA functions, but consequences of PrfA inhibition for L. monocytogenes infection and pathogenesis are unknown. Here we report the crystal structure of PrfA with a small molecule inhibitor occupying the coactivator site at 2.0 Å resolution. Using molecular imaging and infection studies in macrophages, we demonstrate that PrfA inhibition prevents the vacuolar escape of L. monocytogenes and enables extensive bacterial replication inside spacious vacuoles. In contrast to previously described spacious Listeria-containing vacuoles, which have been implicated in supporting chronic infection, PrfA inhibition facilitated progressive clearance of intracellular L. monocytogenes from spacious vacuoles through lysosomal degradation. Thus, inhibitory occupation of the PrfA coactivator site facilitates formation of a transient intravacuolar L. monocytogenes replication niche that licenses macrophages to effectively eliminate intracellular bacteria. Our findings encourage further exploration of PrfA as a potential target for antimicrobials and highlight that intra-vacuolar residence of L. monocytogenes in macrophages is not inevitably tied to bacterial persistence.

Effects of High Magnetic Fields on the Diffusion of Biologically Active Molecules

The diffusion of biologically active molecules is a ubiquitous process, controlling many mechanisms and the characteristic time scales for pivotal processes in living cells. Here, we show how a high static magnetic field (MF) affects the diffusion of paramagnetic and diamagnetic species including oxygen, hemoglobin, and drugs. We derive and solve the equation describing diffusion of such biologically active molecules in the presence of an MF as well as reveal the underlying mechanism of the MF’s effect on diffusion. We found that a high MF accelerates diffusion of diamagnetic species while slowing the diffusion of paramagnetic molecules in cell cytoplasm. When applied to oxygen and hemoglobin diffusion in red blood cells, our results suggest that an MF may significantly alter the gas exchange in an erythrocyte and cause swelling. Our prediction that the diffusion rate and characteristic time can be controlled by an MF opens new avenues for experimental studies foreseeing numerous biomedical applications.

Neutrophil transit time and localization within the megakaryocyte define morphologically distinct forms of emperipolesis

Neutrophils transit through megakaryocytes in a process termed emperipolesis, but it is unknown whether this interaction is a single type of cell-in-cell interaction or a set of distinct processes. Using a murine in vitro model, we characterized emperipolesis by live-cell spinning disk microscopy and electron microscopy. Approximately half of neutrophils exited the megakaryocyte rapidly, typically in 10 minutes or less, displaying ameboid morphology as they passed through the host cell (fast emperipolesis). The remaining neutrophils assumed a sessile morphology, most remaining within the megakaryocyte for at least 60 minutes (slow emperipolesis). These neutrophils typically localized near the megakaryocyte nucleus. By ultrastructural assessment, all internalized neutrophils remained morphologically intact. Most neutrophils resided within emperisomes, but some could be visualized exiting the emperisome to enter the cell cytoplasm. Neutrophils in the cytoplasm assumed close contact with the platelet-forming demarcation membrane system or the perinuclear endoplasmic reticulum. These findings reveal that megakaryocyte emperipolesis reflects at least two distinct processes differing in transit time and morphology, fast and slow emperipolesis, suggesting divergent physiologic functions.

Membrane fission during bacterial spore development requires DNA-driven cellular inflation

Bacteria require membrane fission for cell division and endospore formation. FisB catalyzes membrane fission during sporulation, but the molecular basis is unclear as it cannot remodel membranes by itself. Sporulation initiates with an asymmetric division that generates a large mother cell and a smaller forespore that contains only 1/4 of its complete genome. As the mother cell membranes engulf the forespore, a DNA translocase pumps the rest of the chromosome into the small forespore compartment, inflating it due to increased turgor. When the engulfing membranes undergo fission, the forespore is released into the mother cell cytoplasm. Here we show that forespore inflation and FisB accumulation are both required for efficient membrane fission. We suggest that high membrane tension in the engulfment membrane caused by forespore inflation drives FisB-catalyzed membrane fission. Collectively our data indicate that DNA-translocation has a previously unappreciated second function in energizing FisB-mediated membrane fission under energy-limited conditions.

Asymmetric binomial statistics explains organelle partitioning variance in cancer cell proliferation

Asymmetric inheritance of organelles and compounds between daughter cells is considered a hallmark for differentiation and rejuvenation in stem-like and cancer cells, as much as a mechanism for enhancing resistance in bacteria populations. In non-differentiating homogeneous cancer cells, asymmetric division is still poorly investigated. Here, we present a method based on the binomial partitioning process that allows the measurement of asymmetric organelle partitioning with multiple live cell markers without genetically mutating the cells. We demonstrate our method by measuring simultaneously the partitioning of three cellular elements, i.e., cytoplasm, membrane, and mitochondria in human Jurkat T-cells. We found that although cell cytoplasm is partitioned symmetrically, mitochondria and membrane lipids are asymmetrically partitioned between daughter cells. Moreover, we observe that mitochondria and membrane lipids present a stable positive correlation with cytoplasm, incompatibly with a binomial partition mechanism produced by two independent partitioning processes. Our experimental apparatus, combined with our theoretical framework, could be generalized to different cell kinds, providing a tool for understanding partitioning-driven biological processes.

Bombyx mori Nucleopolyhedrovirus p26 Is Associated with Viral Late Stage Replication

Bombyx mori nucleopolyhedrovirus (BmNPV) p26 is conserved among all Lepidoptera baculoviruses that have been completely sequenced thus far, and some baculoviruses even have two copies of p26, which suggested that p26 may play an important role in the virus infection cycle. This study aimed to characterize BmNPV p26. We found that BmNPV p26 transcripts were detectable as early as 3 h post-infection (hpi), and the transcript levels rapidly increased starting from 12 hpi. Western blot analysis using an anti-p26 polyclonal antibody demonstrated that the corresponding protein was also detectable from 6 hpi in BmNPV-infected cell lysates. Immunofluorescence analysis demonstrated that p26 was mainly dispersed in the infected cell cytoplasm, whereas the over-expressed fusion protein EGFP-p26 also accumulated in the nucleus. These results indicated that p26 is an early BmNPV gene and has functions both in the cytoplasm and the nucleus. RNAi-based knockdown of p26 could produce infectious virus and normal-appearing virions but decreased budded virus (BV) production in BmNPV-infected cells at 72 hpi. Moreover, the results of further quantitative PCR (Q-PCR) analysis indicated that the gp64 and p74 transcripts levels decreased significantly. These results indicated that BmNPV p26 may be associated with BmNPV replication during the late infection stage.

FisB relies on homo-oligomerization and lipid binding to catalyze membrane fission in bacteria

Little is known about mechanisms of membrane fission in bacteria despite their requirement for cytokinesis. The only known dedicated membrane fission machinery in bacteria, fission protein B (FisB), is expressed during sporulation in Bacillus subtilis and is required to release the developing spore into the mother cell cytoplasm. Here, we characterized the requirements for FisB-mediated membrane fission. FisB forms mobile clusters of approximately 12 molecules that give way to an immobile cluster at the engulfment pole containing approximately 40 proteins at the time of membrane fission. Analysis of FisB mutants revealed that binding to acidic lipids and homo-oligomerization are both critical for targeting FisB to the engulfment pole and membrane fission. Experiments using artificial membranes and filamentous cells suggest that FisB does not have an intrinsic ability to sense or induce membrane curvature but can bridge membranes. Finally, modeling suggests that homo-oligomerization and trans-interactions with membranes are sufficient to explain FisB accumulation at the membrane neck that connects the engulfment membrane to the rest of the mother cell membrane during late stages of engulfment. Together, our results show that FisB is a robust and unusual membrane fission protein that relies on homo-oligomerization, lipid binding, and the unique membrane topology generated during engulfment for localization and membrane scission, but surprisingly, not on lipid microdomains, negative-curvature lipids, or curvature sensing.

Self-Organization of Cellular Units

The purpose of this review is to explore self-organizing mechanisms that pattern microtubules (MTs) and spatially organize animal cell cytoplasm, inspired by recent experiments in frog egg extract. We start by reviewing conceptual distinctions between self-organizing and templating mechanisms for subcellular organization. We then discuss self-organizing mechanisms that generate radial MT arrays and cell centers in the absence of centrosomes. These include autocatalytic MT nucleation, transport of minus ends, and nucleation from organelles such as melanosomes and Golgi vesicles that are also dynein cargoes. We then discuss mechanisms that partition the cytoplasm in syncytia, in which multiple nuclei share a common cytoplasm, starting with cytokinesis, when all metazoan cells are transiently syncytial. The cytoplasm of frog eggs is partitioned prior to cytokinesis by two self-organizing modules, protein regulator of cytokinesis 1 (PRC1)-kinesin family member 4A (KIF4A) and chromosome passenger complex (CPC)-KIF20A. Similar modules may partition longer-lasting syncytia, such as early Drosophila embryos. We end by discussing shared mechanisms and principles for the MT-based self-organization of cellular units. Expected final online publication date for the Annual Review of Cell and Developmental Biology, Volume 37 is October 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

α-Helices in the Type III Secretion Effectors: A Prevalent Feature with Versatile Roles

Type III Secretion Systems (T3SSs) are multicomponent nanomachines located at the cell envelope of Gram-negative bacteria. Their main function is to transport bacterial proteins either extracellularly or directly into the eukaryotic host cell cytoplasm. Type III Secretion effectors (T3SEs), latest to be secreted T3S substrates, are destined to act at the eukaryotic host cell cytoplasm and occasionally at the nucleus, hijacking cellular processes through mimicking eukaryotic proteins. A broad range of functions is attributed to T3SEs, ranging from the manipulation of the host cell’s metabolism for the benefit of the bacterium to bypassing the host’s defense mechanisms. To perform this broad range of manipulations, T3SEs have evolved numerous novel folds that are compatible with some basic requirements: they should be able to easily unfold, pass through the narrow T3SS channel, and refold to an active form when on the other side. In this review, the various folds of T3SEs are presented with the emphasis placed on the functional and structural importance of α-helices and helical domains.

Neutrophil transit time and localization within the megakaryocyte define morphologically distinct forms of emperipolesis

In emperipolesis, neutrophils transit through megakaryocytes, but it is unknown whether this interaction represents a single type of cell-in-cell interaction or a set of distinct processes. Using an in vitro model of murine emperipolesis, we characterized neutrophils entering megakaryocytes using live-cell spinning disk microscopy and electron microscopy. Approximately half of neutrophils exited the megakaryocyte rapidly, typically in 10 minutes or less, displaying ameboid morphology as they passed through the host cell (fast emperipolesis). The remaining neutrophils assumed a sessile morphology, most remaining within the megakaryocyte for at least 60 minutes (slow emperipolesis). These neutrophils typically localized near the megakaryocyte nucleus. By ultrastructural assessment, all internalized neutrophils remained morphologically intact. Most neutrophils resided within emperisomes, but some could be visualized exiting the emperisome into the cell cytoplasm. Neutrophils in the cytoplasm assumed close contact with the platelet-forming demarcation membrane system or with the perinuclear endoplasmic reticulum, as confirmed by immunofluorescence microscopy. Together, these findings reveal that megakaryocyte emperipolesis reflects at least two processes, fast and slow emperipolesis, each with its own characteristic transit time, morphology, and intracellular localization, suggesting distinct functions.Key PointsNeutrophil passage through megakaryocytes, termed emperipolesis, diverges into fast and slow forms that differ in transit time, morphology, and intracellular localizationDuring emperipolesis, neutrophils can reside in vacuoles (emperisomes) or escape into the cell cytoplasm to assume positions near the megakaryocyte’s demarcation membrane system, endoplasmic reticulum, or nucleus.