mechanical-vessel-injury-in-zebrafish-embryos

Mechanical Vessel Injury in Zebrafish Embryos

Zebrafish (Danio rerio) embryos have proven to be a powerful model for studying a variety of developmental and disease processes.  External development ...

University of California

Infection of vertebrate hosts with pathogenic Mycobacteria, the agents of tuberculosis, produces granulomas, highly organized structures containing differentiated macrophages and lymphocytes, that sequester the pathogen. Adult zebrafish are naturally susceptible to tuberculosis caused by Mycobacterium marinum. Here, we exploit the optical transparency of zebrafish embryos to image the events of M. marinum infection in vivo. Despite the fact that the embryos do not yet have lymphocytes, infection leads to the formation of macrophage aggregates with pathological hallmarks of granulomas and activation of previously identified granuloma-specific Mycobacterium genes. Thus, Mycobacterium-macrophage interactions can initiate granuloma formation solely in the context of innate immunity. Strikingly, infection can redirect normal embryonic macrophage migration, even recruiting macrophages seemingly committed to their developmentally dictated tissue sites.

Real-time visualization of mycobacterium-macrophage interactions leading to initiation of granuloma formation in zebrafish embryos.

Granulomas are organized host immune structures composed of tightly interposed macrophages and other cells that form in response to a variety of persistent stimuli, both infectious and noninfectious. The tuberculous granuloma is essential for host containment of mycobacterial infection, although it does not always eradicate it. Therefore, it is considered a host-beneficial, if incompletely efficacious, immune response. The Mycobacterium RD1 locus encodes a specialized secretion system that promotes mycobacterial virulence by an unknown mechanism. Using transparent zebrafish embryos to monitor the infection process in real time, we found that RD1-deficient bacteria fail to elicit efficient granuloma formation despite their ability to grow inside of infected macrophages. We showed that macrophages infected with virulent mycobacteria produce an RD1-dependent signal that directs macrophages to aggregate into granulomas. This Mycobacterium-induced macrophage aggregation in turn is tightly linked to intercellular bacterial dissemination and increased bacterial numbers. Thus, mycobacteria co-opt host granulomas for their virulence.

Tuberculous granuloma formation is enhanced by a mycobacterium virulence determinant.

In tuberculosis, infecting mycobacteria are phagocytosed by macrophages, which then migrate into deeper tissue and recruit additional cells to form the granulomas that eventually contain infection. Mycobacteria are exquisitely adapted macrophage pathogens, and observations in the mouse model of tuberculosis have suggested that mycobacterial growth is not inhibited in macrophages until adaptive immunity is induced. Using the optically transparent and genetically tractable zebrafish embryo-Mycobacterium marinum model of tuberculosis, we have directly examined early infection in the presence and absence of macrophages. The absence of macrophages led rapidly to higher bacterial burdens, suggesting that macrophages control infection early and are not an optimal growth niche. However, we show that macrophages play a critical role in tissue dissemination of mycobacteria. We propose that residence within macrophages represents an evolutionary trade-off for pathogenic mycobacteria that slows their early growth but provides a mechanism for tissue dissemination.

Dichotomous role of the macrophage in early Mycobacterium marinum infection of the zebrafish.

One of the strengths of the zebrafish is the ease with which in situ hybridization can be performed to determine spatial and temporal patterns of gene expression in whole embryos. Thus far, colorimetric detection methods are mainly used for these analyses. Here we describe a fluorescent in situ hybridization (FISH) protocol for whole-mount zebrafish embryos using tyramide signal amplification (TSA). An optimal set of reagents was identified that allows for simultaneous localization of gene expression patterns of two genes within the same embryo, permitting identification of colocalized expression within single cells. This protocol can be extended to perform multiplex studies by repetition of the TSA-based detection for each target sequentially with a different fluorescent dye label. To this effect, we demonstrate that this approach can be combined with standard horseradish peroxidase (HRP)-mediated immunocytochemistry procedures in addition to FISH.

Multiplex fluorescent in situ hybridization in zebrafish embryos using tyramide signal amplification.

Tumor necrosis factor (TNF), a key effector in controlling tuberculosis, is thought to exert protection by directing formation of granulomas, organized aggregates of macrophages and other immune cells. Loss of TNF signaling causes progression of tuberculosis in humans, and the increased mortality of Mycobacterium tuberculosis-infected mice is associated with disorganized necrotic granulomas, although the precise roles of TNF signaling preceding this endpoint remain undefined. We monitored transparent Mycobacterium marinum-infected zebrafish live to conduct a stepwise dissection of how TNF signaling operates in mycobacterial pathogenesis. We found that loss of TNF signaling caused increased mortality even when only innate immunity was operant. In the absence of TNF, intracellular bacterial growth and granuloma formation were accelerated and was followed by necrotic death of overladen macrophages and granuloma breakdown. Thus, TNF is not required for tuberculous granuloma formation, but maintains granuloma integrity indirectly by restricting mycobacterial growth within macrophages and preventing their necrosis.

Article	Year
Real-time visualization of mycobacterium-macrophage interactions leading to initiation of granuloma formation in zebrafish embryos. Immunity\| PubMed ID: 12479816	2002
Tuberculous granuloma formation is enhanced by a mycobacterium virulence determinant. PLoS biology\| PubMed ID: 15510227	2004
Dichotomous role of the macrophage in early Mycobacterium marinum infection of the zebrafish. Cell host & microbe\| PubMed ID: 18005715	2007
Multiplex fluorescent in situ hybridization in zebrafish embryos using tyramide signal amplification. Zebrafish\| PubMed ID: 18248170	2005
Tumor necrosis factor signaling mediates resistance to mycobacteria by inhibiting bacterial growth and macrophage death. Immunity\| PubMed ID: 18691913	2008

Hilary Clay

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