How Spiroplasma Swim

Most motile bacteria use flagella to swim in a liquid environment, but non-flagellated bacteria must achieve directed movement through other means. One such species, Spiroplasma, are tiny helical bacteria that infect plants and insects. Our research elucidated a new strategy for swimming used by these bacteria. By measuring cell kinematics during free swimming we found that propulsion is generated by large kinks which are propagated along the length of the cell bodies. These kinks come in pairs and are generated by processively switching the handedness of the cell helix. We are currently studying the energetics and origins of this fascinating motility apparatus using various techniques in molecular biology in concert with optical trapping and optical microscopy.

Neutrophils are migrating through a microfabricated maze, attracted by the chemoattract and fMLP. Arrows indicate the concentration gradient in the sample.

A fish keratocyte is just touching a microfabricated barrier. It will change direction and move away from the barrier.

Substrate sensing and situational awareness in cells

Many cells in the body can migrate, which is important for processes like development, wound healing and the immune response. The migrating cells are guided by several mechanisms, the best known being chemotaxis. In chemotaxis, cells track the concentration gradient of a chemical that is being released at the destination point, similar to people tracking the smell of freshly baked cookies.

Chemotaxis is often studied on flat microscope slides. However, real cells need to navigate through complex tissue in order to reach their destination, weaving around other cells and the extracellular matrix.

This research focuses on the way cells feel and understand their physical environment. How do cells behave when an object is blocking a straight path to a chemoattractant? Can a chemotactic gradient trap cells in features on a substrate? What logic applies when conflicting cues are felt by the cell? How exactly do cells sense the presence of a structure, and are there any specific signaling pathways involved in this?

These questions are addressed by fabricating small obstacles on microscope slides using lithography with SU-8 negative photoresist.

To study the interplay between substrate features and chemotaxis, neutrophils or similar immune cells are used. For the visualization of the signaling events that may mediate substrate sensing, fish keratocytes are more appropriate, as their flat shape helps imaging them on the microscope.

Neutrophil chemotaxis in gradients of multiple chemoattractants

Neutrophils can sense and migrate up concentration gradients of a wide variety of chemoattractants using G protein-coupled receptors (GPCRs) and associated signaling pathways. When simultaneously confronted with multiple chemoattractant gradients, neutrophils have mechanisms that process these multiple signals to determine the most appropriate direction in which to migrate. For certain combinations of endogenous chemoattractants, this direction will be toward the vector sum of the gradients. When exposed to formylated peptides (fMLP), neutrophils ignore other gradients and prioritize fMLP. We have developed a mathematical model to examine the role that various factors involved in GPCR dynamics play in influencing multiple-signal processing in neutrophil chemotaxis. The model describes receptor-ligand binding, a simple mechanism for signal integration, and resulting migration. Spatial sensing and polarization are modeled implicitly through the distribution of bound receptors on the cellular membrane. Using this model, we are able to quantify the contributions of several parameters to multiple-signal processing and delineate the vector sum and prioritization behaviors.