Renal Cellular and Molecular Biology Section
Maurice B Burg, MD, Principal Investigator
DNA damage exists in mouse inner medullas in vivo under the normal condition of high NaCl and urea. Read more.
Our focus has been on the mechanisms by which cells in the renal medulla withstand the high concentrations of salt (NaCl) and urea that occur there when the kidney produces concentrated urine and that are much higher than elsewhere in the body. We initially identified some organic osmolytes (sorbitol, glycine betaine, glycerophosphocholine (GPC) and myo-inositol) that renal medullary cells accumulate during antidiuresis. Then, we established tissue culture models to elucidate the protective function of the osmolytes and the mechanisms by which they are accumulated (reviewed in Garcia-Perez, et al, 1991 ).
We discovered the biochemical mechanisms by which the organic osmolytes are accumulated. High NaCl increases synthesis of sorbitol from glucose by increasing the amount of the enzyme, aldose reductase, and increases transport of glycine betaine and myo-inositol into the cells by increasing the number of transporters. The transcription of the aldose reductase and transporter genes is osmotically regulated. We identified an osmotic response element (ORE) in the aldose reductase gene and similar elements have been identified in the transporter genes. Currently, we are studying a transcription factor, TonEBP, whose binding to the ORE and whose transactivating activity are increased by hypertonicity, signaled by protein kinases, including PKA (Ferraris, et al, 2002).
Our ongoing work also concerns the genomic stress that can occur at extremes of high NaCl or urea and result in cell cycle arrest and apoptosis. Recent findings are that high NaCl causes DNA damage and impairs DNA repair, not only in cell culture, but also in vivo (Dmitrieva, et al, 2003). Moreover, the damage is less if the cells are not proliferating rapidly (Zhang, et al, 2002) and if the changes in NaCl and urea concentration are slow (Cai, et al, 2002, 2004), as occurs in the renal inner medulla in vivo.
DNA damage exists in mouse inner medullas in vivo under the normal condition of high NaCl and urea.
Above: Single cell gel electrophoresis (comet) assay of DNA damage in cells from inner medulla and cortex. Damaged DNA appears in the "tails" of the "comets."
Below: Terminal deoxynucleotidyltransferase-mediated dUTP nick end labeling assay of DNA breaks performed on mouse kidney sections. DNA damage (dark staining) is widespread in the inner medulla but not in cortex. The DNA damage is repaired within 2 hours after medullary osmolality is decreased by furosemide (not shown).
For details see: Cells adapted to high NaCl have many DNA breaks and impaired DNA repair both in cell culture and in vivo. Natalia I. Dmitrieva, Qi Cai, and Maurice B. Burg, PNAS 101: 2317-2322, 2004.