Suma Rao, MD
Suma Rao, MD -- Award winner -- Third Year Fellow
Division of Neonatology / Sidhartha Tan, MD, advisor
"Neuronal nitric oxide synthase and ongoing rabbit fetal brain injury following in vivo antenatal hypoxia-ischemia."
Perinatal hypoxic-ischemic (HI) brain injury is a significant risk factor for neonatal morbidity and even mortality. HI injury can result in long term neurologic consequences such as cerebral palsy, cognitive delays and learning disabilities in both preterm and term infants. Neuronal injury can continue to occur several hours and days after an HI event. There is a lack of substantial data looking at molecular mechanisms involved in ongoing or subacute phase of injury. My research was focused on studying the involvement of nitric oxide (NO) in ongoing injury to rabbit fetal neuronal cells several days after suffering HI injury in utero. Though advances in neonatal medicine have led to improved survival of preterm neonates, the incidence of cerebral palsy has not been substantially reduced. Despite the improved management strategies for babies with HI injury, there is still a paucity of specific postnatal interventions that offer neuroprotection after hypoxia-ischemia has already occurred in utero. By studying the molecular mechanisms involved in injury, we can develop specific postnatal interventions to prevent or limit the extent of brain injury.
Our laboratory has established a rabbit model of global hypoxia-ischemia in utero that results in a cerebral palsy phenotype. This provides a clinically relevant animal model to study the various mechanisms of injury and potential neuroprotectants. Most studies to date have looked at involvement of NO in the acute phase after HI injury. In order to study the ongoing involvement of NO, I prepared brain slices from rabbit fetuses that had experienced hypoxia-ischemia in utero. The slices were cultured for 3 to 6 days, and then subjected to flow cytometry analysis. To our knowledge, this is the first model of brain slices that are obtained after HI event in utero, cultured and and then analyzed by flow cytometry in the subacute phase. We analyzed mitochondrial function in the brain slices after being cultured with a novel designer drug called JI8, a highly selective neuronal nitric oxide synthase (nNOS) inhibitor. nNOS is the predominant source of NO in the brain. We found that JI8, the selective nNOS inhibitor, offers significant neuroprotection especially in the parietal cortex and hippocampus for as long as six days after the HI event. This could have tremendous clinical implications and potential therapeutic benefits. Also, inhibition of iNOS (inducible nitric oxide synthase) does not offer any neuroprotection. In summary, our innovative brain slice model offers the opportunity to analyze the various cellular mechanisms involved in ongoing HI injury to brain in the subacute phase. It also provides a quick and easy model to study the effects of several potential neuroprotectants.
Our lab has also shown that nNOS inhibition with JI8 before HI event can ameliorate the motor deficits in rabbit fetuses. Unfortunately, clinically it is not possible to identify all the fetuses that are likely to suffer HI antenatally. My studies have shown that nNOS inhibition with JI8 after in utero HI injury can still offer neuroprotection for several days after the event. So far, we have not found any toxic effects of the drug in the concentrations used, but further studies are under way to evaluate toxicity in detail. This drug needs to be studied further so that it can be used clinically. Another area that is being studied is the use of RNAi (RNA interference) to suppress the expression of nNOS and therefore NO, to limit the extent of brain injury.
Dr. Sidhartha Tan, MD, is my mentor. His lab has done extensive research on hypoxic ischemic brain injury. He has established the global model of HI in rabbit fetuses that results in the cerebral palsy phenotype – the first clinically relevant animal model of cerebral palsy. The lab has done research on mechanisms of brain injury, timing, MRI predictors of pattern of injury, potential methods of neuroprotection, etc.
My research could not have been possible without the tremendous help from my mentor and my laboratory colleagues. We truly work as a family. I have presented my research at national and international meetings such as Pediatric Academic Societies in Toronto, Society for Neuroscience in San Diego; and Mead Johnson Nutritionals conference on perinatal research in Park City, Utah. This research opportunity has enabled me to make a contribution, albeit small, to the scientific world.