The OK cell line was initiated from the kidney of an adult female North American opossum and was originally intended for use as a source of X chromosomes for studies of X inactivation. The line was soon discovered, however, to display many characteristics of kidney proximal tubule epithelial cells and has since been commonly utilized as a cell culture model for the cell type.

OK cells exhibit a stable nondiploid chromosomal modal number of 23 and display a variety of receptors in culture, including alpha 2 adrenergic, serotonin, parathyroid hormone (PTH), and atrial natriuretic peptide (ANP) receptors. Many studies utilizing OK cells focus upon these receptors, some of which are involved in such notable activities as insulin metabolism and the hormonal regulation of phosphate transport and uptake.

The key unit that comprises the vertebrate kidney is the nephron, which essentially consists of a long tubule and a bundle of capillaries called the glomerulus. Though there is only a single tubule in each nephron, the structure is known by many different names, which refer to specific regions of the tubule. The proximal tubule is the portion of the tubule nearest Bowman’s capsule, which is a sac-like swelling located at the blind end of the tubule that encases the glomerulus. Fluids from the blood contained in the glomerulus are collected in Bowman’s capsule, and the filtrate is then passed on to the proximal tubule followed by the other portions of the nephron, including the loop of Henle and the distal tubule, before emptying into a collecting duct connected to the renal pelvis. Within the proximal tubule, the volume and composition of the filtrate is significantly altered, as some substances, such as salts and water, are reabsorbed, and others, including hydrogen ions and ammonia, are secreted.

Three subcellular localization vectors were utilized to target enhanced fluorescent proteins directly into organelles present in a culture of opossum kidney cells. Use of pDsRed2-Mitochondria, pEGFP-Tubulin, and pECFP-Nucleus plasmid vectors enabled the localization of a red fluorescent protein tag to mitochondria, a green tag to microtubules, and a cyan protein tag to cell nuclei. Images were recorded in grayscale with a 12-bit digital camera coupled to either a Nikon E-600 or Eclipse 80i microscope equipped with bandpass emission fluorescence filter optical blocks. During the processing stage, individual image channels were pseudocolored with RGB values corresponding to each of the fluorophore emission spectral profiles.

Targeting Tubulin in OK Proximal Tubule Epithelial Cells with Immunofluorescence - Immunofluorescence with mouse anti-alpha-tubulin was employed to visualize distribution of microtubules in the log phase monolayer culture of opossum kidney cells presented in this section. The secondary antibody (goat anti-mouse IgG) utilized was conjugated to Marina Blue. DNA in the cell nucleus was counterstained with SYTOX Green.

Localization of Mitochondria, Tubulin, and the Nucleus with Fluorescent Proteins in Opossum Kidney Cells - An adherent culture of opossum kidney epithelial cells (OK line) was transfected with pDsRed2-Mitochondria, pEGFP-Tubulin, and pECFP-Nucleus plasmid subcellular localization vectors, thus localizing a red fluorescent protein tag to the intracellular mitochondrial network, a green tag to microtubules, and a cyan protein tag to cell nuclei.

Distribution of DNA and Microtubules in OK Cell Cultures - The opossum kidney proximal tubule cells illustrated in this section were immunofluorescently labeled with mouse anti-alpha-tubulin primary antibodies followed by goat anti-mouse secondary antibodies conjugated to Marina Blue. Tubulin is the basic component of microtubules. In addition, the specimen was stained with SYTOX Green to target DNA.