Systemic hyperosmolality is a severe pathological condition because mammalian cells are not able to tolerate a sustained deviation from osmotic homeostasis. Notable exceptions are renal cells of the kidney inner medulla. These cells utilize mechanisms that allow them to survive, function, and maintain the integrity of their genome during variable and very harsh hyperosmolality. Understanding the molecular basis of such mechanisms provides the knowledge on which to design therapeutic strategies for treating renal diseases and for compensating the pathological effects of systemic hyperosmolality on non-renal cells. We have recently shown that osmotic regulation of DNA repair and chromatin structure represents an important adaptive capacity of renal cells, and that Growth Arrest and DNA Damage inducible genes are induced in renal inner medullary cells exposed to hypertonic stress. Accordingly, our broad objective is to investigate how GADD45 proteins are regulated, in what functional context they are induced, and whether they are necessary for DNA repair and/ or chromatin modulation in renal cells during hypertonic stress. We hypothesize that GADD45 proteins are regulated in various ways in renal inner medullary cells to facilitate cell adaptation to hypertonicity and to protect cells from hypertonicity-induced damage. We will test this hypothesis using the mIMCD3 cell line, primary cultures of rat renal inner medullary cells, and intact animals as models in four aims: Aim 1: By investigating how GADD45 proteins are regulated during hypertonic stress; Aim 2: By analyzing the effect of hypertonicity on genome integrity in the renal inner medulla in intact animals; Aim 3: By testing whether GADD45 proteins are necessary for similar cell functions during hypertonicity as for other types of stress. Aim 4: By studying the proteins that are targeted by GADD45 proteins during hypertonic stress in renal inner medullary cells.