Cell calcium oscillations: The origin of their variability

A method for high-content functional imaging of intracellular calcium responses in gelatin-immobilized non-adherent cells

Functional imaging of the intracellular calcium concentration [Ca²⁺]_i using fluorescent indicators is a powerful and frequently applied method for assessing various biological questions in vitro, including ion channel function and intracellular signaling in homeostasis and disease. In functional [Ca²⁺]_i imaging experiments, the fluorescence intensity of single cells is typically recorded during application of a chemical stimulus, i.e. by exchange of modified extracellular media, exposure to drugs and/or ligands. The concomitant mechanical perturbation caused by the perfusion of different solution during experimentation severely hinders calcium imaging in non-adherent cells, including peripheral immune cells, as cells in suspension are dislocated by turbulent flow during chemical stimulation. The quantitative analysis, involving time-courses of intracellular fluorescence signal changes, necessitates cells to remain at the same position throughout the experiment. To prevent dislocation of cells during solution exchange, and to enable imaging as well as analysis of Ca²⁺ responses in immune cells, a gelatin-based method for immobilization of non-adherent cells was developed. Gelatin has been a long-serving material for cell immobilization, e.g. in 3D bio-printing of cells and has thus, also been employed in the context of this study. To demonstrate the applicability of the established method for functional Ca²⁺ imaging in gelatin-immobilized suspension cells, a proof-of-concept study was conducted using human peripheral blood model cell lines (Jurkat/T-lymphocytes and THP-1/monocytes), Ca²⁺ indicators (Fluo-4 and Fura-2) and two different fluorescence microscopy rigs. The data presented that the established methodology is applicable for studying Ca²⁺ signaling by in vitro high-content functional imaging of [Ca²⁺]_i in suspension cells, including but not restricted to human immune cells.

Methods for analysis of calcium/calmodulin signaling in cilia and flagella

The axonemal microtubules of cilia/flagella act as a scaffold for assembly of the protein complexes that ultimately regulate dynein activity to control the size and shape of ciliary bends. Despite our general understanding of the contribution of microtubule sliding to ciliary and flagellar motility, many questions regarding the regulation of dynein remain unanswered. For example, we know that the second messenger calcium plays an important role in modulating dynein activity in response to extracellular cues, but it remains unclear how calcium-binding proteins anchored to the axoneme contribute to this regulation. Recent work has focused on determining the identity and specific functions of these axonemal calcium-binding proteins. Here, we review our current knowledge of calcium-mediated motility and highlight key experiments that have substantially aided our understanding of calcium signaling within the axoneme.