Assay for detection of transient intracellular Ca2+

A method of producing stable calcium sensor cell clones comprising:

• i) generating a nucleic acid sequence comprising a first nucleic acid encoding a chimeric protein and a second nucleic acid encoding either:
• a) a protein comprising amino acids 1271-1311 of rat NR2B and capable of binding to the T-site of Ca2+/calmodulin dependent protein kinase II (CaMKII);
• or
• b) a fusion protein comprising amino acids 1271-1311 of rat NR2B and capable of binding to the T-site of CaMKII, conjugated to a mitochondrial localizing sequence;
• wherein the chimeric protein comprises α-CaMKII, or a mutant thereof maintaining a functioning T-site, and a fluorescent protein, and wherein the nucleic acid sequence further comprises an internal ribosome entry site (IRES) sequence between the first nucleic acid and the second nucleic acid;
• ii) generating an expression vector with a unique antibiotic selection marker expression cassette for the nucleic acid sequence mentioned in item (i) by subcloning the nucleic acid sequence mentioned in item (i) into the expression vector;
• iii) transfecting cells with the expression vector generated as mentioned in item (ii);
• iv) subjecting the cells to antibiotic selection pressure using the antibiotic referred in item (ii);
• v) sorting all the cells expressing the fluorescent protein mentioned in item (i) by fluorescence activated cell sorter (FACS) technology and subjecting the sorted cells again to antibiotic selection pressure as mentioned in item (iv);
• vi) repeating cycles comprising the procedures mentioned in item (iv) followed by item (v) until almost all cells are positive for the fluorescent protein;
• and
• vii) subjecting the cells obtained in item (vi) that stably express the fluorescent protein to clonal selection by seeding the cells at low density and separating colonies formed from single cells by colony picking method and by expanding the colonies.