Access Type

Open Access Dissertation

Date of Award

January 2012

Degree Type

Dissertation

Degree Name

Ph.D.

Department

Biological Sciences

First Advisor

Karen A. Beningo

Abstract

Cellular migration is a vital process central to many physiological events including development, immune surveillance and wound healing. However, migration and invasion are not unique to normal physiology, they are also key determinants in the progression of disease states such as cancer. Given the significance of migration it is important that we understand how the process is regulated intracellularly and the various stimuli that can promote it. Even though the role of biochemical factors in mediating migration has been studied extensively, the role of biophysical factors in modulating migration and invasion is less appreciation. The biochemical and biophysical components of cell and tissue microenvironments influence cellular behavior. This is true for both normal and disease conditions. For example, the role of substrate stiffness and extracellular matrix (ECM) composition in cell proliferation, spreading, preferential migration and even stem cell differentiation has been observed. However, a number of questions remain unanswered, such as the ability of cells to sense locally applied mechanical stimuli and how this mechanosensing is regulated. Would the regulation be different if cancer cells were to sense the applied stimulus? Studies have shown that as cells migrate they produce contractile forces called traction forces that are generated by the cellular cytoskeleton and transmitted onto the substrate. Yet the signaling mechanism that promotes this force production or how is it regulated is not well characterized. In attempt at address these questions, we have identified the importance of locally applied mechanical stimuli in cancer cell invasion and we have also identified a major link in the traction force production pathway. Our study on the influence of local mechanical stimuli on cancer cell invasion suggests that the stimuli produced as a result of ECM remodeling by and migration of non-cancerous cells present in the tumor microenvironment could enhance tumor cell invasion. This enhanced invasion is dependent on actin and cofilin, and the ECM protein, fibronectin.

In gaining understanding of the mechanisms and interplay between traction force and mechanosensing we have focused on the Calpain protease. We previously identified that the calpain small subunit, calpain 4 (Capn4), influences force production independent of the proteolytic activity of the catalytic subunits calpain 1 and 2, yet their mechanosensing mechanism overlaps. To further explore the relationship, we asked how Capn4 could regulate force production. We have found that Capn4 indirectly mediates tyrosine phosphorylation of a lectin binding protein, galectin-3. Upon phosphorylation, galectin-3 is secreted into the ECM from where it is able to modulate traction force and associated events involving focal adhesion maturation and adhesion strength. It however, does not influence mechanosensing. Together these results further emphasize the point that cell migration and invasion is significantly influenced by the biochemical and biophysical components and properties on the microenvironment. Further studies will elucidate these pathways and provide greater insight for bioengineering and medical advances.

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