Generation and usage of aequorin lentiviral vectors for Ca²⁺ measurement in sub-cellular compartments of hard-to-transfect cells

Targeted aequorin-based Ca²⁺ probes represent an unprecedented tool for the reliable measurement of Ca²⁺ concentration and dynamics in different sub-cellular compartments. The main advantages of aequorin are its proteinaceous nature, which allows attachment of a signal peptide for targeting aequorin to virtually any sub-cellular compartment; its low Ca²⁺-binding capacity; the wide range of Ca²⁺ concentrations that can be measured, ranging from sub-micromolar to millimolar; its robust performance in aggressive environments, e.g., the strong acidic pH of the lysosomal lumen. Lentiviral vectors represent a popular tool to transduce post-mitotic or hard-to-transfect cells both in vitro and in vivo. Furthermore, it has great potential for gene therapy. Last generation lentiviral vectors represent a perfect compromise for combining large insert size, ease of production and handling, and high degree of biosafety. Here, we describe strategies for cloning aequorin probes - targeted to the cytosol, sub-plasma membrane cytosolic domains, the mitochondrial matrix, and the endoplasmic reticulum lumen - into lentiviral vectors. We describe methods for the production of lentiviral particles, and provide examples of measuring Ca²⁺ dynamics by such aequorin-encoding lentiviral vectors in sub-cellular compartments of hard-to-transfect cells, including immortalized striatal neurons, primary cerebellar granule neurons and endothelial progenitor cells, which provide suitable in vitro models for the study of different human diseases.