Space: A Final Frontier for Vacuolar Pathogens

Abstract

There is a fundamental gap in our understanding of how a eukaryotic cell apportions the limited space within its cell membrane. Upon infection, a cell competes with intracellular pathogens for control of this same precious resource. The struggle between pathogen and host provides us with an opportunity to uncover the mechanisms regulating subcellular space by understanding how pathogens modulate vesicular traffic and membrane fusion events to create a specialized compartment for replication. By comparing several important intracellular pathogens, we review the molecular mechanisms and trafficking pathways that drive two space allocation strategies, the formation of tight and spacious pathogen-containing vacuoles. Additionally, we discuss the potential advantages of each pathogenic lifestyle, the broader implications these lifestyles might have for cellular biology and outline exciting opportunities for future investigation.

Keywords: Brucella; Chlamydia; Coxiella; Legionella; Mycobacterium; intracellular pathogens; membrane fusion; vesicular trafficking.

Figure 1:. Tight and spacious represent two broad classes of PVs.

A) For intracellular bacteria that reside in tight vacuoles, the vacuolar membrane divides with each bacterium, resulting in a single cell within each PV Inset is a transmission electron micrograph of a Brucella abortus-infected J774.A1 macrophage (6). Copyright 2006, American Society for Microbiology. B) In contrast, spacious vacuoles expand as the bacteria divide, so that all individuals are housed within a single, large vacuole. Inset is a transmission electron micrograph of a Coxiella burnetii-infected L929 cell (7). Copyright 1993, American Society for Microbiology.

Figure 2:. Fusogenic properties of tight versus spacious vacuoles.

A) Spacious vacuoles allow membrane fusion to occur with each other (homotypic fusion), as well as host-derived vesicles (heterotypic fusion), fueling their growth. B) Tight vacuoles inhibit or avoid fusion with different host subcellular compartments and with each other, and as a result maintain their small size as they replicate.

Figure 3:. Tight and spacious vacuoles require differing amounts of membrane and space.

In each graph, the total surface area (A) or total volume (B) is shown for increasing numbers of bacteria of the same size. Surface area (SA) and volume (V) were calculated assuming spherical bacteria and vacuoles. Total surface area for tight vacuoles was calculated using the equation SA_T = (4πr²)n, where SA_T equals total surface area of all tight vacuoles, r is bacterial radius (1 μm) and n represents the number of bacteria. Total volume for tight vacuoles was calculated using the equation V_T = πr³)n, where V_T is the total volume of tight vacuoles, r is bacterial radius (1 μm) and n is the number of bacteria. For spacious vacuoles, we assume that bacteria are packed in an irregular configuration with a density limit that does not exceed 63.4% (93). Therefore, the volume of spacious vacuole containing n spheres (V_S) is calculated as V_S = V_T/0.634. The surface area of the corresponding spacious vacuole (SA_V) is calculated by first determining the radius of a sphere of volume V_S, which is r = [(V_S)(3/4π)]^1/3. The surface area SA_V can then be calculated as follows: SA_V = (4πr²) = (4π)[(V_S)(3/4π)]^2/3.