Quantification of Live Bacterial Sensing for Chemotaxis and Phagocytosis and of Macropinocytosis

Paraburkholderia Symbionts Display Variable Infection Patterns That Are Not Predictive of Amoeba Host Outcomes

Symbiotic interactions exist within a parasitism to mutualism continuum that is influenced, among others, by genes and context. Dynamics of intracellular invasion, replication, and prevalence may underscore both host survivability and symbiont stability. More infectious symbionts might exert higher corresponding costs to hosts, which could ultimately disadvantage both partners. Here, we quantify infection patterns of diverse Paraburkholderia symbiont genotypes in their amoeba host Dictyostelium discoideum and probe the relationship between these patterns and host outcomes. We exposed D. discoideum to thirteen strains of Paraburkholderia each belonging to one of the three symbiont species found to naturally infect D. discoideum: Paraburkholderia agricolaris, Paraburkholderia hayleyella, and Paraburkholderia bonniea. We quantified the infection prevalence and intracellular density of fluorescently labeled symbionts along with the final host population size using flow cytometry and confocal microscopy. We find that infection phenotypes vary across symbiont strains. Symbionts belonging to the same species generally display similar infection patterns but are interestingly distinct when it comes to host outcomes. This results in final infection loads that do not strongly correlate to final host outcomes, suggesting other genetic factors that are not a direct cause or consequence of symbiont abundance impact host fitness.

DPF is a cell-density sensing factor, with cell-autonomous and non-autonomous functions during Dictyostelium growth and development

Background

Cellular functions can be regulated by cell-cell interactions that are influenced by extra-cellular, density-dependent signaling factors. Dictyostelium grow as individual cells in nutrient-rich sources, but, as nutrients become depleted, they initiate a multi-cell developmental program that is dependent upon a cell-density threshold. We hypothesized that novel secreted proteins may serve as density-sensing factors to promote multi-cell developmental fate decisions at a specific cell-density threshold, and use Dictyostelium in the identification of such a factor.

Results

We show that multi-cell developmental aggregation in Dictyostelium is lost upon minimal (2-fold) reduction in local cell density. Remarkably, developmental aggregation response at non-permissive cell densities is rescued by addition of conditioned media from high-density, developmentally competent cells. Using rescued aggregation of low-density cells as an assay, we purified a single, 150-kDa extra-cellular protein with density aggregation activity. MS/MS peptide sequence analysis identified the gene sequence, and cells that overexpress the full-length protein accumulate higher levels of a development promoting factor (DPF) activity than parental cells, allowing cells to aggregate at lower cell densities; cells deficient for this DPF gene lack density-dependent developmental aggregation activity and require higher cell density for cell aggregation compared to WT. Density aggregation activity co-purifies with tagged versions of DPF and tag-affinity-purified DPF possesses density aggregation activity. In mixed development with WT, cells that overexpress DPF preferentially localize at centers for multi-cell aggregation and define cell-fate choice during cytodifferentiation. Finally, we show that DPF is synthesized as a larger precursor, single-pass transmembrane protein, with the p150 fragment released by proteolytic cleavage and ectodomain shedding. The TM/cytoplasmic domain of DPF possesses cell-autonomous activity for cell-substratum adhesion and for cellular growth.

Conclusions

We have purified a novel secreted protein, DPF, that acts as a density-sensing factor for development and functions to define local collective thresholds for Dictyostelium development and to facilitate cell-cell communication and multi-cell formation. Regions of high DPF expression are enriched at centers for cell-cell signal-response, multi-cell formation, and cell-fate determination. Additionally, DPF has separate cell-autonomous functions for regulation of cellular adhesion and growth.

Live Monitoring and Analysis of Fungal Growth, Viability, and Mycelial Morphology Using the IncuCyte NeuroTrack Processing Module

Pathogenic fungi remain a major cause of infectious complications in immunocompromised patients. Microscopic techniques are crucial for our understanding of fungal biology, host-pathogen interaction, and the pleiotropic effects of antifungal drugs on fungal cell growth and morphogenesis. Taking advantage of the morphological similarities of neuronal cell networks and mycelial growth patterns, we employed the IncuCyte time-lapse microscopy system and its NeuroTrack image analysis software package to study growth and branching of a variety of pathogenic yeasts and molds. Using optimized image processing definitions, we validated IncuCyte NeuroTrack analysis as a reliable and efficient tool for translational applications such as antifungal efficacy evaluation and coculture with host immune effector cells. Hence, the IncuCyte system and its NeuroTrack module provide an appealing platform for efficient in vitro studies of antifungal compounds and immunotherapeutic strategies in medical mycology.

Efficient live-imaging methods are pivotal to understand fungal morphogenesis, especially as it relates to interactions with host immune cells and mechanisms of antifungal drugs. Due to the notable similarities in growth patterns of neuronal cells and mycelial networks, we sought to repurpose the NeuroTrack (NT) processing module of the IncuCyte time-lapse microscopy system as a tool to quantify mycelial growth and branching of pathogenic fungi. We showed the robustness of NT analysis to study Candida albicans and five different molds and confirmed established characteristics of mycelial growth kinetics. We also documented high intra- and interassay reproducibility of the NT module for a spectrum of spore inocula and culture periods. Using GFP-expressing Aspergillus fumigatus and Rhizopus arrhizus, the feasibility of fluorescence-based NT analysis was validated. In addition, we performed proof-of-concept experiments of NT analysis for several translational applications such as studying the morphogenesis of a filamentation-defective C. albicans mutant, the effects of different classes of antifungals (polyenes, azoles, and echinocandins), and coculture with host immune cells. High accuracy was found, even at high immune cell-to-fungus ratios or in the presence of fungal debris. For antifungal efficacy studies, addition of a cytotoxicity dye further refined IncuCyte-based analysis, facilitating real-time determination of fungistatic and fungicidal activity in a single assay. Complementing conventional MIC-based assays, NT analysis is an appealing method to study fungal morphogenesis and viability in the context of antifungal compound screening and evaluation of novel immune therapeutics.