Retrofusion of intralumenal MVB membranes parallels viral infection and coexists with exosome release

The endosomal system constitutes a highly dynamic vesicle network used to relay materials and signals between the cell and its environment.¹ Once internalized, endosomes gradually mature into late acidic compartments and acquire a multivesicular body (MVB) organization through invagination of the limiting membrane (LM) to form intraluminal vesicles (ILVs).² Cargoes sequestered into ILVs can either be delivered to lysosomes for degradation or secreted following fusion of the MVB with the plasma membrane.³ It has been speculated that commitment to ILVs is not a terminal event, and that a return pathway exists, allowing “back-fusion” or “retrofusion” of intraluminal membranes to the LM.⁴ The existence of retrofusion as a way to support membrane equilibrium within the MVB has been widely speculated in various cell biological contexts, including exosome uptake⁵ and major histocompatibility complex class II (MHC class II) antigen presentation.6, 7, 8, 9 Given the small physical scale, retrofusion of ILVs cannot be measured with conventional techniques. To circumvent this, we designed a chemically tunable cell-based system to monitor retrofusion in real time. Using this system, we demonstrate that retrofusion occurs as part of the natural MVB lifestyle, with attributes parallel to those of viral infection. Furthermore, we find that retrofusion and exocytosis coexist in an equilibrium, implying that ILVs inert to retrofusion comprise a significant fraction of exosomes destined for secretion. MVBs thus contain three types of ILVs: those committed to lysosomal degradation, those retrofusing ILVs, and those subject to secretion in the form of exosomes.

Video abstract

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MVBs are complex organelles with intraluminal vesicles bound by the limiting membrane

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Intraluminal membranes are in a dynamic equilibrium with the limiting membrane

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Retrofusion of internal vesicles is controlled by processes used for viral fusion

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Exosomes arise from internal MVB vesicles not participating in retrofusion

Endosomal microdomains: Formation and function

It is widely recognized that after endocytosis, internalized cargo is delivered to endosomes that act as sorting stations. The limiting membrane of endosomes contain specialized subregions, or microdomains, that represent distinct functions of the endosome, including regions competing for cargo capture leading to degradation or recycling. Great progress has been made in defining the endosomal protein coats that sort cargo in these domains, including Retromer that recycles transmembrane cargo, and ESCRT (endosomal sorting complex required for transport) that degrades transmembrane cargo. In this review, we discuss recent work that is beginning to unravel how such coat complexes contribute to the creation and maintenance of endosomal microdomains. We highlight data that indicates that adjacent microdomains do not act independently but rather interact to cross-regulate. We posit that these interactions provide an agile means for the cell to adjust sorting in response to extracellular signals and intracellular metabolic cues.

ALIX Regulates Tumor-Mediated Immunosuppression by Controlling EGFR Activity and PD-L1 Presentation

The immunosuppressive transmembrane protein PD-L1 was shown to traffic via the multivesicular body (MVB) and to be released on exosomes. A high-content siRNA screen identified the endosomal sorting complexes required for transport (ESCRT)-associated protein ALIX as a regulator of both EGFR activity and PD-L1 surface presentation in basal-like breast cancer (BLBC) cells. ALIX depletion results in prolonged and enhanced stimulation-induced EGFR activity as well as defective PD-L1 trafficking through the MVB, reduced exosomal secretion, and its redistribution to the cell surface. Increased surface PD-L1 expression confers an EGFR-dependent immunosuppressive phenotype on ALIX-depleted cells. An inverse association between ALIX and PD-L1 expression was observed in human breast cancer tissues, while an immunocompetent mouse model of breast cancer revealed that ALIX-deficient tumors are larger and show an increased immunosuppressive environment. Our data suggest that ALIX modulates immunosuppression through regulation of PD-L1 and EGFR and may, therefore, present a diagnostic and therapeutic target for BLBC.

Hierarchical expression of genes controlled by the Bacillus subtilis global regulatory protein CodY

Global regulators that bind strategic metabolites allow bacteria to adapt rapidly to dynamic environments by coordinating the expression of many genes. We report an approach for determining gene regulation hierarchy using the regulon of the Bacillus subtilis global regulatory protein CodY as proof of principle. In theory, this approach can be used to measure the dynamics of any bacterial transcriptional regulatory network that is affected by interaction with a ligand. In B. subtilis, CodY controls dozens of genes, but the threshold activities of CodY required to regulate each gene are unknown. We hypothesized that targets of CodY are differentially regulated based on varying affinity for the protein's many binding sites. We used RNA sequencing to determine the transcription profiles of B. subtilis strains expressing mutant CodY proteins with different levels of residual activity. In parallel, we quantified intracellular metabolites connected to central metabolism. Strains producing CodY variants F71Y, R61K, and R61H retained varying degrees of partial activity relative to the WT protein, leading to gene-specific, differential alterations in transcript abundance for the 223 identified members of the CodY regulon. Using liquid chromatography coupled to MS, we detected significant increases in branched-chain amino acids and intermediates of arginine, proline, and glutamate metabolism, as well as decreases in pyruvate and glycerate as CodY activity decreased. We conclude that a spectrum of CodY activities leads to programmed regulation of gene expression and an apparent rerouting of carbon and nitrogen metabolism, suggesting that during changes in nutrient availability, CodY prioritizes the expression of specific pathways.

Use of the dominant T wave to enhance reliability of T-wave offset identification

T-wave offset (Toff) identification may be jeopardized by the presence of a significant inter-method (IMV) and inter-lead (ILV) Toff variability. Thus, the aim of the present study was to investigate if the dominant T wave (DTW) may be used to enhance Toff-identification reliability. DTWs and 15-lead ECG T waves of 46 control healthy subjects (CHS) and 103 acute myocardial infarction patients (AMIP) were analyzed for Toff identification using Zhang et al.'s (M1) and Daskalov and Christov's (M2) methods. Results indicate that IMV is significantly reduced when identifying Toff from the DTW rather than from single ECG leads in both populations (CHS: 5ms vs. 5-15ms; AMIP: 10ms vs. 10-20ms). Moreover, when analyzing ILV, Toff was found to be equivalent (correlation=0.71-0.98; P<10(-14)) to the median Toff among leads, but required only one identification instead of 15. Thus, the DTW can be used to enhance Toff-identification reliability.

Brugia pahangi: immunization with early L3 ES alters parasite migration, and reduces microfilaremia and lymphatic lesion formation in gerbils (Meriones unguiculatus)

Previous studies have shown that intradermally (ID) injected Brugia pahangi L3 s migrate through various tissues and into the lymphatics of gerbils in a distinct pattern. Excretory/secretory products (ES) produced at the time of invasion of B. pahangi are likely to be important in this early migration phase of the parasite life cycle in their rodent host. Hence, early L3 ES was collected from 24h in vitro cultures of B. pahangi L3 larvae and used in immunization experiments to investigate the effect of immunity to early L3 ES on worm migration, survival and development of B. pahangi. Immunization of gerbils with ES in RIBI adjuvant produced antibodies to numerous ES proteins eliciting a strong humoral response to ES and indirect fluorescent antibody (IFA) assay using anti-ES serum recognized the ES proteins on the surface of B. pahangi L3 larvae. Following ES immunization, gerbils were challenged either ID or intraperitoneally (IP) with 100 L3 s of B. pahangi and euthanized at 3 or 106 days post inoculation (DPI). Immunization with early ES slowed the migration of ID inoculated L3 at 3 DPI and significantly altered the locations of adult worms at 106 DPI. Immunization did not induce protection in any treatment group. However, immunized animals had significantly fewer microfilariae per female worm suggesting the antigens in ES are important in microfilariae development or survival in the host. The number of lymphatic granulomas was also significantly reduced in ES immunized animals. It is important to note that microfilariae serve as a nidus in these granulomas. Our results shows immunization with early Brugia malayi L3 ES alters the worm migration, affects circulating microfilarial numbers and reduces lymphatic granulomas associated with B. pahangi infection in gerbils.