Neuroprotection  & Regeneration

Research efforts in my laboratory are aimed at understanding how neurons, the fundamental units of the nervous system, can be rescued from injury and toxic insults.  Differentiated neurons are mostly post-mitotic and hence have to be functionally viable throughout the life of the organism. Neurons strive to maintain their functional competence by continually adapting and responding to the ever-changing demands imposed on them. One critical expression of such neuronal plasticity is the ability of the neurons to grow processes and make synaptic contacts after injury to attain functional recovery during regeneration. In contrast, failure of neurons to ward off onslaughts with appropriate adaptive responses leads to pathological conditions involving neurodegeneration. I am interested in studying neuronal responses when exposed to trophic stimuli leading to regeneration and to toxic stimuli resulting in degeneration to understand why and how neurons adapt or fail to do so. I believe that this understanding is crucial for identifying appropriate means of neuroprotection.

Currently I am examining the role of cholinergic components, plant-derived antioxidants and nano materials in neuronal adaptive responses since (a) cholinergic neurotransmitter system is involved in essential physiological functions and behaviors including learning and memory, (b) plant-derived  molecules often have high level of antioxidant property, (c) and nano materials appear to be useful in promoting directional growth of nerve fibers.

Trophic Mechanism

• We are studying the effects of acetylcholinesterase (AChE) on neurite regeneration in dorsal root ganglion (DRG) neurons of rat to examine the potential of using AChE to promote regeneration and restore sensory perception in patients suffering from spinal cord injury (SCI). The majority of the victims of spinal cord injury (SCI) are between the ages of 16 and 45 often requiring costly supportive care and suffer drastic reduction in the quality of life for many decades. Hence finding ways to promote regeneration of the spinal cord (SC) leading to functional recovery has become a critical challenge of utmost urgency to neuroscientists.  Studies from our laboratory and others have shown that AChE which is present throughout the life of the neurons and in their immediate environment promotes neurite growth in invertebrate and vertebrate neurons. We employ cellular and molecular techniques such as cell culture, immunocytochemistry, chromatography, electrophoresis, Western and Northern blots, spectrophotometry, light and fluorescence microscopy, real time imaging, morphometry and microarray analysis to decipher the mechanism of action of AChE  as a neurotrophic factor.

Directed Growth

•  In a collaborative project, we have begun to exploit carbon and magnetic nano tubes to incorporate trophic factors and  use them as guidance cues to promote directed growth of neurites since properly directed growth and synaptic connections are crucial for functional recovery.

Antioxidants

•  In a collaborative project, we are testing “specialized”  metabolites from hairy roots for their efficacy in protecting dopaminergic neurons from oxidative stress.

Degeneration: 

•     Organophosphates (OPs, irreversible inhibitors of AChE) are used as pesticides, agents of chemical warfare and weapons of terrorism because they are lethal when used in concentrations that would inhibit most of the activity of the enzyme AChE. Most research in the past focused on the interactions between OPs and AChE and the resultant lethal effects. However, clinical reports show that chronic exposure to even low levels of OP pesticides has been correlated with sensory impairment among farm workers. Yet little is known about the potential detrimental effects of acute or chronic exposure to low concentrations (non-lethal) of OPs on specific areas of the nervous system or the underlying mechanism(s). Experiments in our laboratory show that exposure to  OP (paraoxon, a metabolite of parathion) results in significant cell death in  a specific sub population of somato-sensory neurons (neurons from DRG) of rat.  My on-going experiments are assessing the time and dose-dependent effects of OPs in isolated neurons in the controlled environment of cell culture employing cellular and molecular techniques.

Cigarette Smoke and Neuron Health:

•  This project is aimed at understanding the effects of cigarette smoke on developing neurons. Tobacco consumption among women is at a national high in Arkansas. The health risks of cigarette smoke among women are of special concern because of its damaging effect on fetal development. Clinical findings reveal that children of smokers exhibit signs of functional impairment of nervous system. Experimental studies on pregnant animals have shown that a well known component of cigarette smoke, nicotine crosses the placenta and results in neurobehavioral disturbances in the newborn. Though these reports provide indirect evidence for the adverse effects of nicotine on developing nervous system, details on the mechanism of action of nicotine and its specific effects on growing neurons are not known. Based on evidence from related research and pilot experiments from our laboratory, our current objective is to determine the effects of chronic nicotine exposure on neurite growth in developing neurons and to investigate the underlying mechanisms.  Neuron culture, morphometry, calcium imaging, receptor binding assays, immunocytochemistry, immuno precipitation and immuno blotting techniques will be used to achieve these specific aims.

Nano toxicity on Neurons

•  Due to their unique physical and chemical properties, nano particles provide several advantages in industrial and biomedical applications. Yet we do not know much about their biocompatibility. We are currently determining the retrograde transport of nano particles from respiratory passages to neurons innervating trachea and lungs. We are planning to study the localization, residence duration and effects of internalized nano particles on neurons in nodose ganglian of rat.

• In a collaborative project we are testing the size and concentration-dependent toxicity of copper nano particle on sensory neurons of rat.

Funding Support: NIH/NINDS,  NIH/NIDA,  NIH/NCRR,  KSCHIRT, Arkansas Bioscience Institute, ASU Faculty Development Award

Selected Publications: 

Srivatsan, M. (2006) An analysis of acetylcholinesterase sequence for predicting mechanisms of its non-catalytic actions. Bioinformation, 1(8): 281-284   

M. Srivatsan, J. Treece and E.E. Shotts (2006) : Nicotine alters nicotinic receptor subunit levels differently in developing mammalian sympathetic neurons. Ann N.Y.Acad. Sci. 1074: 505-513  

Jining Xie, Linfeng Chen, Kiran R Aatre, M Srivatsan and V K Varadan (2006) Somatosensory neurons grown on functionalized carbon nanotube mats. Smart Materials and Structures, 15: N85-88. 

Chad Fite, Malathi Srivatsan (2003)  Neural and circulating cholinesterases of the marine mollusk Aplysia californica.  Chem.  Bull. Vol. 44: 62 -65 

Srivatsan, M. (2001) Effects of organophosphates on survival in dorsal root ganglion neurons of rat in Enzymes, 2001 (proceedings of international symposium on applications of enzymes in  chemical and biological defense). 

Srivatsan, M. (1999) Organophosphates inhibit acetylcholinesterase and  impair neurite growth of cholinergic neurons in Aplysia. Chemico-Biological Interactions, 119-120: 371-378. 

Srivatsan, M. and Peretz, B. (1997) Acetylcholinesterase promotes regeneration of neurites in cultured adult neurons. Neuroscience, 77:921-931. 

Srivatsan, M. and Peretz, B. (1996) Effects of acetylcholinesterase inhibition on behavior is age-dependent in freely moving Aplysia . Behav. Brain Res. 77: 115-124. 

Peretz, B. and Srivatsan, M. (1996) Chronic stimulation increases acetylcholinesterase in old Aplysia Behav. Brain Res. 80:203-210.  

Srivatsan, M. and Peretz, B. (1995) Neurotrophic function of circulating acetylcholinesterase in  Aplysia. pp 449-450, In " Enzymes of the Cholinesterase family" Eds. A.S. Balasubramanyan,  B.P. Doctor, P. Taylor and D.M. Quinn, Plenum Publishing CO., New York, NY.   

Srivatsan, M., Peretz, B., Hallahan, B. and Talwalker, R.  (1992) Acetylcholinesterase and other hemolymph proteins change with age in Aplysia.  J. Comp. Physiol. B. 162: 29-37. 

Peretz B. and Srivatsan, M. (1992) Differences in aging in two neural pathways:  Proposed explanations from the nervous system of Aplysia .  Expr. Geront. 27:83-97. 

Kindy, M.S., Srivatsan, M. and Peretz, B.  (1991) Age-related differential expression of neuropeptide mRNAS in Aplysia. Neuroreport  2:465-468.