Metabolic activation of molecular oxygen frequently gives rise to reactive oxygen species (ROS), which may be toxic to biological systems. Cells have evolved antioxidant protection mechanisms, including enzymes and low molecular weight molecules. Cellular antioxidative capacity may be overwhelmed by a flux of ROS, resulting in oxidative stress to cells and tissues. Oxidative stress was shown to be involved in most human pathologies, including cardiovascular diseases, cancer and neurodegenerative diseases. However, ROS also play a central role in various essential cellular processes such as apoptosis, which is the physiological suicide pathway. Our laboratory is involved in basic and applied-oriented research on the deleterious and beneficial effects of ROS in human pathologies and environmental stresses encountered by lichens and bacteria.
- ROS, apoptosis and neurological disorders. Several lines of evidence suggest that in neurodegenerative diseases neurons die by ROS-mediated apoptotic mechanisms. In our cell culture models for Parkinson’s disease we have shown that apoptosis, induced by the endogenous neurotransmitter dopamine, plays a major role in ROS-induced nigral degeneration and that thiol antioxidants inhibited it (1,3,8). We are currently studying new compounds that were shown for the first time in our laboratory to inhibit cell death induced by dopamine, and other oxidants and neurotoxins (unpublished). Our studies with transgenic mice showed that the gene products of bcl-2 and bax, that are involved in regulation of apoptosis, affected the antioxidant status of neurons, and consequently their resistance to oxidative stress imposed by neurotoxins (4,5,9). In our Alzheimer’s disease (AD)-related research we are collaborating with Prof. Mark Smith (Institute of Pathology, Case Western Reserve University, Cleveland, Ohio, USA). We have shown that upstream caspases are clearly found in association with the intraneuronal pathology in human brains, while downstream caspases are present only at control levels. This suggests that initiation of apoptosis in AD does not proceed to caspase-dependent cell death. We proposed to term this phenomenon of apoptotic avoidance “abortive apoptosis” or “abortosis” (13,16,17,18). In an animal model we studied the effect of close head injury on apolipoprotein E deficient mice. We have shown that the antioxidative metabolism of these animals is altered both prior to and following head injury. It is proposed that antioxidative mechanisms may play a role in mediating the neuronal maintenance and repair derangements of the apolipoprotein E-deficient mice (7).
- Liver cirrhosis. In collaboration with Prof. Rafael Bruck from the Wolfson Medical Center and Sackler School of Medicine we are studying animal models for the involvement of ROS in liver cirrhosis and various potential antioxidant drugs. We have found that TAA-induced liver cirrhosis is associated with oxidative damage and was ameliorated by the hydroxyl radical scavengers, dimethylsulfoxide (DMSO) and dimethylthiourea (DMTU) (12), as well as the antioxidants pyrrolidine dithiocarbamate (14) and melatonin (21).
- Premature babies. In collaboration with Prof. Cathy Hammerman and Prof. Michael Kaplan from Shaare Zedek Medical Center and the Hebrew University-Hadassah Medical School, Jerusalem, we are studying the involvement of ROS in treatment of premature babies and animal models of their pathologies. The intestinal mucosa is hypersensitive to ischemia/reperfusion injuries (IR) that may be associated with necrotizing enterocolitis (NEC), an inflammation causing injury to the bowel that affects mainly premature babies. In a mouse model of NEC - intestine IR - we have found that pentoxyfilline (6) and bilirubin (15) ameliorate the oxidative damage.
- Lichens. Lichens are symbiotic organisms comprised of fungi and algae that are able to survive environmental adversities such as desiccation and nutrient deprivation. We are studying the involvement of ROS in response of lichens to stressful changes in environmental conditions. Rehydration of the lichen Ramalina lacera caused a burst of production of ROS that was accompanied by membrane damage and alterations in cellular antioxidants (19). Transplantation of this lichen from clean to polluted areas resulted in accumulation of polluting metal ions, membrane damage, lipid peroxidation and alterations in antioxidants (20). We are developing the methodology to use lichens for an early warning-biomonitoring system for air pollution.
- Bacteria. In freshwater and marine habitats bacteria are challenged by various stresses, including nutrient deprivation and oxidative stress, and have evolved survival protection mechanisms Viable but nonculturable cells (VBNC) are bacterial cells that are alive by criteria of integrity and metabolic activities but lost their ability to divide. Our surveys in Lake Kinneret (in collaboration with the Kinneret Limnological Laboratory) as well as "in situ" experiments have shown that oxygen and light enhance conversion of bacteria to VBNC (10,11). We propose that VBNC is a prokaryotic manifestation, and possibly a phylogenetic precursor of eukaryotic apoptosis, and therefore we propose to designate it "proapoptosis" (2). In multicellular eukaryotic organisms apoptosis is responsible for elimination of cells as a necessary condition for differentiation and development, and consequently for survival of the organism. Prokaryotic apoptosis is a stress-adaptation mechanism for preservation of the species by what may be defined as "population-differentiation”. At the genetic level, formation of VBNC may be controlled by chromosomal genes, as well as plasmid-carried genes, such as the toxin/antitoxin (TA) systems. We have shown that TA systems are widely distributed in bacteria isolated from lake Kinneret, and that, unlike other suggestions, one such toxin, MazF, promotes conversion of E. coli to VBNC (unpublished).