Ion channel mediated mechanotransduction in immune cells

Abstract

The immune system performs critical functions to defend against invading pathogens and maintain tissue homeostasis. Immune cells reside within or are recruited to a host of mechanically active tissues throughout the body and, as a result, are exposed to varying types and degrees of mechanical stimuli. Despite their abundance in such tissues, the role of mechanical stimuli in influencing immune cell function and the molecular mechanisms responsible for mechanics-mediated changes are still poorly understood. The recent emergence of mechanically-gated ion channels, particularly Piezo1, has provided an exciting avenue of research within the fields of mechanobiology and immunology. Numerous studies have identified roles for mechanically-gated ion channels in mechanotransduction within various different cell types, with a few recent studies in immune cells. These initial studies provide strong evidence that mechanically-gated ion channels play pivotal roles in regulating the immune system. In this review, we discuss characteristics of ion channel mediated force transduction, review the current techniques used to quantify and visualize ion channel activity in response to mechanical stimuli, and finally we provide an overview of recent studies examining the role of mechanically-gated ion channels in modulating immune cell function.

Keywords: Piezo1; T cell; adaptive immunity; innate immunity; macrophage; mechanotransduction.

Figure 1:. Common mechanical stimuli experienced by innate and adaptive immune cells.

Schematic illustrating the different mechanical stimuli experienced by immune cells as they extravasate through blood vessels and adhere to mechanically active tissues or foreign implantable materials.

Figure 2:. Techniques to activate mechanically-gated ion channels.

A variety of techniques have been developed to study mechanically-gated ion channels, as depicted above. These often aim to mimic the vast mechanical stimuli cells are subject to in situ. The channel activation mechanisms shown here can be used in conjunction with methods that record channel activity, such as patch clamp or imaging. They can also be combined with molecular techniques to measure the downstream effects of channel gating.

Figure 3:. Patch clamp configurations.

In the cell-attached configuration, the pipette is sealed to a small patch of the cell membrane and the activity of one or several ion channels within the membrane patch are recorded. If the pipette is pulled away, the membrane patch detaches from the rest of the cell resulting in the inside-out configuration. Here the intracellular side of the ion channel is in contact with the bath solution. If suction is applied whilst in the cell-attached configuration then whole-cell can be achieved. Here the combined activity of all ion channels within the cell are recorded and the intracellular solution is replaced by the pipette solution. If the pipette is pulled away, small regions of membrane detach on either side of the pipette and can re-join to form the outside-out configuration. Here the extracellular side of the ion channel is in contact with the bath solution.

Figure 4:. Piezo1-mediated regulation of innate immune cell function and disease.

Summary of recent studies that identify roles for myeloid cell specific Piezo1 in regulating lung fibrosis [95], tumor growth [100], aortic valve stenosis [104], iron metabolism [106], and the foreign body response [83]. Mechanical stimuli identified as key regulators of channel activity are indicated in parentheses. Data shown indicates that myeloid cell specific Piezo1 deletion (Piezo1^ΔLysM) reduced bacteria-induced lung fibrosis, tumor growth, and fibrous encapsulation of stiff implanted materials as is indicated by a reduction in the Ashcroft clinical score, tumor weight, and fibrous capsule thickness, respectively. Moreover, siRNA mediated Piezo1 knockdown or pharmacological inhibition of mechanically-gated ion channels revealed a reduction in monocyte cell adhesion, whereas myeloid cell specific heterozygous (Het-GOF^const.) of homozygous (Hom-GOF^const.) Piezo1 GOF mutation resulted in enhanced iron deposition in the liver. Arrows indicate presence of iron in hepatocytes.

Figure 5:. Effects of Piezo1 deletion in CD4⁺ T cell subset functions.

CD4⁺ T cells were activated either by peptide recognition on antigen presenting cell (APC) or by TCR-crosslinking antibodies immobilized on beads or a flat-substrate (left). T cell subsets generated by TCR activation in association with specific cytokine cocktails as indicated. Panel on the right shows the effects of Piezo1 deletion in various subsets (see text for details).