Single cell RT-PCR analysis of ClC-2 mRNA expression in ureteric bud tip

Embryonic epithelia at the tip of the ureteric bud (UB) face the interspace between epithelial and mesenchymal cells and are fundamentally involved in reciprocal signaling during early nephrogenesis. To characterize their membrane conductive proteins, patch-clamp and single cell RT-PCR techniques were applied to embryonic rat UBs [embryonic day 17 (day E17)] microdissected from the outer cortex. Cells at the UB tip had a high whole cell conductance (14 +/- 2 nS/10 pF, n = 8). The main fractional conductance resembled that of Ca-activated Cl channels in nonepithelial cells, with its time-dependent activation at depolarizing and inactivation at hyperpolarizing voltages. A second Cl-selective current fraction, by contrast, activated slowly during strong hyperpolarization, suggestive of a ClC-2-mediated conductance. To determine the origin of this current, cytoplasm was harvested into the patch pipette, RNA was reverse transcribed, and cDNA encoding the glyceraldehyde-3-phosphate dehydrogenase (GAPDH) housekeeper gene or the ClC-2 Cl channel was amplified by polymerase chain reaction (PCR). GAPDH and ClC-2 PCR products were identified in 23 and 8 (out of a total of 57) single cell cDNA samples, respectively. ClC-2 PCR products with two different lengths were obtained, which might be due to two alternatively spliced ClC-2 mRNA isoforms. This first and combined approach by patch-clamp and single cell RT-PCR techniques to embryonic epithelia indicates that 1) cells at the UB tip express a phenotype remarkably different from that of postembryonic collecting duct principal cells and that 2) ClC-2 is likely to have a key role in early nephrogenesis.