We are investigating molecular mechanisms that underpin the ability of animals to survive and thrive in a wide range of environmental salinities. To study how such mechanisms have evolved we use different models of aquatic vertebrates (fish) and invertebrates. Ttilapia are extremely euryhaline fish and well suited for deciphering mechanisms of physiological salinity acclimation. Daphnia are microcrustaceans with very short generation time and two different modes of reproduction. They inhabit a wide range of ecological habitats and are well suited for studying genetic mechanisms of salinity adaptation. Besides these main models we also work with other species of fish and invertebrates to gain a better understanding of evolutionary mechanisms, trends, and principles that are associated with the physiological trait euryhalinity.

Sharks are primitive fishes (elasmobranchs) that are exposed to abiotic environmental stresses. Although most sharks do not inhabit freshwater, bullsharks are capable of living in both seawater and fresh water. Other shark species have evolved capacities for extreme hypoxia tolerance. We are interested in how the stress response network operates in these sharks and how it compares to the stress response network in tilapia and sturgeon to gain insight into the evolutionary diversification of the environmental stress response network in primitive vertebrates.

The overall goal of our studies on euryhaline fishes, in particular with regard to the osmoregulatory function of gills, is to identify and functionally understand key components of the stress response network to develop predictive models and more robust and predictive biomarkers of environmental stress exposure in fishes. By comparing such networks in several evolutionarily distinct lineages of fishes with those of mammals we aim to get insight into how stress response networks of vertebrates have evolved and how they can be most effectively manipulated for the benefit of animal and human health. Practical implications of our research also include improvement of aquaculture practices (tilapia, sturgeon), conservation efforts aimed at maintaining ecosystem health and protection of endangered species (sturgeon, sharks), and invasive species control (tilapia).

Cells of the renal papilla (inner medulla) of mammals are unique and different from most other mammalian cell types because they tolerate very large osmotic fluctuations and very high osmolality. In line with their high osmotic stress tolerance they also have high tolerance towards other types of stress, including nephrotoxin exposure and ischemia / reperfusion (hypoxic and oxidative stress).

Renal cells have the same genome as other cells in an organism and the high stress tolerance of renal papillary cells is a result of their expressed proteome. Therefore, we study the proteome of renal papillary cells because it is enriched for proteins representing nodes in the stress response network of mammals.

We have discovered that hyperosmotic stress (hypertonicity) causes DNA damage, in particular DNA double-strand breaks. This discovery led to a series of follow-up studies showing that the intracellular signaling networks activated during hyperosmotic stress resemble those activated during other types of stress. Therefore stressor-specificity in effector mechanisms is likely a result of variable activation of specific sets of sensors, the combination of which controls the flow of information through the stress response network rather then utilization of separate networks. Our finding that DNA damage in the form of double-strand breaks occurs during hypertonic stress in renal papillary cells raises three general questions. First, how does hypertonicity cause DNA damage? Second, how are central nodes in the stress response network regulated in response to hypertonic stress-induced DNA damage? Third, how do cells know when hypertonicity and DNA damage exceed their tolerance limit and repair capacity to initiate apoptosis? In addressing these important questions we investigate the regulation and function of central nodes in the stress response network of renal papillary cells, including ATM, TSC-22D2, and p38 MAP kinase. We are particularly interested in how these proteins control chromatin compactness, cell cycle checkpoints, and cell survival or death pathways during hypertonic and nephrotoxin stress. We also study the mechanism that allows renal papillary cells to quantify osmotic stress to activate the appropriate compensatory mechanisms.

Cells of the renal papilla (inner medulla) of mammals are unique and different from most other mammalian cell types because they tolerate very large fluctuations and very high osmolality. In line with their high osmotic stress tolerance they also have high tolerance towards other types of stress, including nephrotoxin exposure and ischemia / reperfusion (hypoxic and oxidative stress).

Renal cells have the same genome as other cells in an organism and the high stress tolerance of renal papillary cells is a result of their expressed proteome. Therefore, we study the proteome of renal papillary cells because it is enriched for proteins representing nodes in the stress response network of mammals (Valkova and Kültz, 2006).

We have discovered that hyperosmotic stress (hypertonicity) causes DNA damage, in particular DNA double-strand breaks (Kültz and Chakravarty, 2001). This discovery led to a series of follow-up studies showing that the intracellular signaling networks activated during hyperosmotic stress resemble those activated during other types of stress. Therefore stressor-specificity in effector mechanisms is likely a result of altered flow of information through the stress response network rather then utilization of separate networks.

Four central nodes in the mammalian stress response network are ataxia telangiectasia mutated (ATM) kinase, a TGF-beta-stimulated clone 22 (TSC-22) domain protein, p38 mitogen-activated protein kinase, and the phospho-protein adapter protein 14-3-3. These proteins orchestrate cell cycle checkpoints, apoptosis, and DNA repair processes during stress.

The overall goal of our studies on mammalian renal papillary cells is to know a sufficient number of elements and how they are regulated during stress to be able to reliably model information flow through the stress response network in response to specific types and specific degrees of environmental stress. We focus our efforts on hyperosmotic stress because hyperosmolality in the renal inner medulla is a prerequisite for the urinary concentrating mechanism.

These studies will further our understanding of how the stress response network of mammalian cells operates. Such understanding is crucial for developing cures of diseases and disorders that arise because of malfunction in the stress response network, including cancer, ataxia telengiectasia, and polycystic kidney disease.

Two marine sponge species of the genus Tetilla are excellent models for studying the evolution of the osmotic stress response network. The two species (Tetilla mutabilis and Tetilla leptoderma) are very closely related but occur in completely different habitats. T. mutabilis inhabits the mid- to low intertidal zone of mudflats in the Gulf of California and the Pacific coast of Mexico and southern California. Individuals of this species are emersed during ebb-tides and exposed to extreme environmental stress under these conditions, including UV, heat, hyperosmolality (on hot days), hypo-osmolality (on rainy days), and oxidative stress. In contrast, T. leptoderma inhabits a very homeostatic environment along the Antarctic shore from 4 to >2000 m depth. Thus, it has evolved for a very long time under conditions that are characterized by the absence of osmotic stress and its response to such stress is much more limited compared to T. mutabilis.

We compare proteome changes that occur in these two congeneric sponge species as a result of hyperosmolality in the absence and presence of other environmental stresses such as heat and oxidative stress. Furthermore, we study the regulation of identified stress proteins by posttranslational modification and interaction with other proteins. The overall goal of these studies is to know the critical proteins and mechanisms within the osmotic stress response network that have evolved to confer high environmental (in particular osmotic) stress tolerance to primitive metazoa.

Sturgeons are primitive fishes that are often exposed to environmental stress in their native habitat. Two species of sturgeons (green and white sturgeon) occur in the San Francisco Bay-Delta water system. In this diverse ecosystem they are exposed to salinity, temperature, and toxicant stresses resulting from urbanization, mining, water diversion, agriculture, industrial pollution, and global climate change. Because of their taxonomic position near the root of the vertebrate phylogenetic tree sturgeons are excellent models for studying the evolution of the stress response network in vertebrates. Green and white sturgeons are economically important for aquaculture (caviar) and sport fishing. They are ecologically important as key species at the top of the trophic chain in aquatic ecosystems. Many sturgeon populations are dramatically declining and several species, including green sturgeon, are listed as threatened or endangered and, therefore, are important from a conservation biology perspective.