Our group uses a combination of biophysical and bioimaging techniques to study cell migration. Cell migration is a critical process in every living organism. For example, when wounds heal, skin cells migrate to close the wound. During cancer metastasis, cancer cells migrate and spread to a distant part of the body to initiate the formation of a new tumor. We develop novel assays to measure the biophysical properties of cell migration in three-dimensional microenvironments, and elucidate the signaling and gene regulatory networks that govern these properties, especially the role of the cytoskeleton. From these studies, we hope to identify novel therapeutic targets to perturb cell migration, which can eventually be translated into drugs to promote wound healing or stop cancer cell invasion.
Recently, we have looked at the migration of neutrophils. Neutrophils are white blood cells that play an important role in the host defence against pathogenic microbial agents. Under resting conditions, approximately 98% of the mature neutrophils in the body are stored within the bone marrow, with approximately 2% of the total mature neutrophils distributed in the blood stream and tissues. Upon exposure to inflammatory stimuli, neutrophils are rapidly mobilized from the bone marrow into the circulation, and are recruited to the site of inflammation where they carry out their functions and promote tissue remodelling. The distribution of neutrophils between bone marrow and blood compartment is mediated by egression and retention signals provided by chemokines CXCL2 and CXCL12, respectively. In bone marrow, CXCL12 constitutively expressed by stroma cells promotes neutrophil accumulation in the bone marrow through interaction with CXCR4. Moreover, the CXCR4-CXCL12 signalling axis also activates several signalling processes such as cell migration and proliferation.
Defects in neutrophil mobilization from the bone marrow into circulation leads to recurrent infections, as evinced by patients with the Warts, Hypogammaglobulinemia, Infections, and Myelokathexis (WHIM) syndrome. WHIM syndrome is an inherited primary immunodeficiency disease that subject the affected individual to recurrent bacterial infections due to low counts of most leukocytes, including neutrophils, circulating in the blood. WHIM patients expressing truncated CXCR4 display active receptor signalling that decays over a longer time as compared to WT leukocytes upon ligand stimulation. It has been shown that CXCL12-activated WHIM leukocytes show sustained activation of downstream signalling pathways such as pERK with greater signalling amplitude. Mutant leukocytes also exhibit higher sensitivity to CXCL12 as a larger percentage of cells migrated towards CXCL12 in transwell assays compared to WT cells.
To understand neutrophil migration in WHIM patients, we have characterized the mechanochemical properties of wild type (WT) and WHIM neutrophils (Figure 1) and show that neutrophils isolated from WHIM mouse model are primed for cell migration. These mutant cells exhibit higher traction stresses, faster migration speed, and form more lamellipodia on stimulation with CXCL12 as compared to WT cells. Filopodia and lamellipodia are typically formed as a cell migrate. Specifically, filopodia protrusions are associated with extracellular matrix sensing to identify targets for adhesion which may mature into larger adhesion structures upon lamellipodia advancement. Lamellipodia, on the other hand, are associated with efficient cell migration. Hence, increased frequency of lamellipodia-type protrusions in WHIM neutrophils may result in increase in cell migration speed. In agreement, we found that cells forming the lamellipodia-type of protrusions tend to migrate at a faster speed than cells forming the filopodia-type protrusions. WHIM neutrophils are also characterized by larger initial increase in cell migration speed upon ligand stimulation, which decreases more slowly over time as compared to WT neutrophils, suggesting both increased CXCR4 sensitivity to CXCL12 and a longer CXCR4 internalization adaptation time.
To understand how increased CXCR4 internalization adaptation time and CXCR4 sensitivity to CXCL12 enhances WHIM neutrophil accumulation in the bone marrow, we have proposed a mechanochemical model of the CXCR4-CXCL12 interaction, incorporating CXCR4 activation by CXCL12 and CXCR4 receptor internalization through β-arrestin-mediated endocytosis (Figure 2). The model explained how reducing the receptor internalization rate, as in the case in WHIM neutrophils, can result in longer CXCR4 internalization adaptation time and increased CXCR4 sensitivity to CXCL12. Through the model, we also explained that both longer CXCR4 internalization adaptation times and increased CXCR4 sensitivity collectively contribute to the increased accumulation of WHIM neutrophils in the bone marrow. We have also proposed, through traction stress measurements, that WHIM neutrophils are more adherent to the substratum which may contribute to the increased accumulation of WHIM neutrophils in the bone marrow. Lastly, differences in the type of cell protrusions produced by WT and WHIM neutrophils may suggest differences in the signalling pathways involved in the activation of the ras-related superfamily of small GTPase (Rac and Cdc42). Although more work is necessary to elucidate the differences in cell-substrate adhesions and small GTPase activation due to CXCR4-CXCL12 signalling in WT and WHIM neutrophils, our work has offered a mechanistic framework to understand WT and WHIM neutrophil migration and accumulation in the bone marrow.
Chiam Keng Hwee is a theorist working at the interface of physics and biology, collaborating very closely with experimental groups in developing theories and models for a variety of problems in mechanobiology and biological physics, systems biology, and biological fluid mechanics. He received his Ph.D. in physics from the California Institute of Technology in 2003 and his B.S.E. in physics from the University of Michigan in 1997.
Keng Hwee's group uses a combination of biophysical and bioinformatics tools to study cell migration. Cell migration is a critical process in every living organism, central to, for example, the morphogenesis of embryos, formation of tissues and organs, wound repair, as well as in less welcoming scenarios such as cancer metastasis. Some of their current projects include the study of amoeboid modes of cancer cell invasion, collective modes of epithelial cell sheet migration, and swimming and swarming modes of bacterial cell motility.
He hopes that their approach will enable them to identify potential targets to perturb cell migration, which can eventually be translated into drugs to stop cancer cell invasion, promote wound and skin healing, or stop the aggregation of bacterial cells into biofilms.
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