Exploring the dynamic behavior of leukocytes with zebrafish

Abstract

Cell migration is a complex and intricate network of physical, chemical, and molecular events that ultimately leads to cell motility. This phenomenon is involved in both physiological and pathological processes such as proper immune and inflammatory responses. Dysregulation of cell migration machinery in immune cells can have a tremendous impact on the trajectory of inflammation, infection, and resolution. The small vertebrate, the zebrafish, has a remarkable capacity for genetic and pharmacological manipulation aligned to transparency that enables modulation and visualization of cell migration in vivo noninvasively. Such characteristics revolutionized the field of leukocyte biology, particularly neutrophils. In this review, we will focus on leukocyte migration and highlight findings made in the zebrafish that demonstrate how this small vertebrate system is a unique model to perform in vivo imaging and study mechanisms that regulate the dynamic behavior of immune cells in their native environment under homeostasis or upon challenge.

Keywords: Cell migration; Non-invasive imaging; Zebrafish.

Figure 1:. Amoeboid and mesenchymal migration - Similarities and differences.

Overall, all leukocytes, including neutrophils use an ameboid migration pattern. This type of migration does not rely on proteolytic degradation of the ECM nor strong focal adhesion therefore this type of migration is fast. Establishment of a clear cell polarity is crucial for proper migration with F-actin localizing at the leading edge together with PIP3 gradient. Rho activation and Myosin II generate contractile forces that are responsible for the propulsion of neutrophils. Recently, using LLSM it has been shown that neutrophils display a long Uropod trail that eventually loses while migrating. Interestingly MTOC localizes in the front of the nucleus. Macrophages are known by their morphological plasticity and can also acquire mesenchymal phenotypes with a slow migration speed. This type of migration uses lamellipodia that competes to guide cells through the interstitial tissue. F-actin is localized in lamellipodia, and small myosin filaments attach to focal adhesion allowing cell to interact strongly with ECM. Mesenchymal migration relies heavily on traction. Interestingly, contrary to ameboid cells MTOC is located both at the rear and front of macrophage nucleus.

Figure 2:. Myeloid migration becomes clearer with zebrafish.

In the last decade and a half, the neutrophil community has been engaged in unravelling the mechanism and pathological role of reverse migration. A. Calcium alarm signals in neutrophil clusters locally promote attractant synthesis and are dependent on ATP sensing and contact with necrotic tissue. Clustering neutrophils initiate and propagate calcium alarm signals via Cx43 channels Neutrophil swarms and Cx43 restrict wound colonization by opportunistic bacteria. B. CXCL8 and other chemoattractants regulate neutrophil migration to injuries via CXCR1. CXCR2 on the other hand is needed for forward migration. Additionally, several mediators coordinate neutrophil reverse migration such as PGE2. Some other molecules such as ROS-SFK signal in macrophages and cell-cell contact are also vital for proper neutrophil reverse migration. Cxcl12/CXCR4 signal has been identified as a druggable retention signal for neutrophils at tissue damage that can be blocked using AMD3100.