Ashok K. Shetty
Professor and Director of Neurosciences, Institute for Regenerative Medicine
Texas, United States
Hippocampus, important for making new memories and maintaining normal mood function [1,2], is one of the brain regions well known for robust compensatory reaction (plasticity) when faced with an injury or loss of neurons. Damage to the hippocampus can occur from multiple causes. These may include head injuries resulting from falls or motor vehicle accidents, loss of blood flow due to clots/plaques in arteries supplying the hippocampus, sudden occurrences of seizures resulting from head trauma, brain infections, brain tumors, stroke, exposure to certain neurotoxins and alcohol/drug withdrawal, prolonged exposure to severe stress, and Alzheimer's disease. Most prominent compensatory changes that occur in the acute phase after injury comprises of increased production of new neurons (neurogenesis) from resident neural stem/progenitor cells and elevated levels of factors (neurotrophic factors) released by surviving neurons and glia to minimize neurodegeneration as well as to promote reorganization of connections of existing neurons [3-5]. Although it is unclear why such changes are so prominent in the hippocampus but mostly a low-key affair in other brain regions, these changes are believed to signify innate mechanisms that restrain the overall dysfunction of the hippocampus.
Nevertheless, early post-injury compensatory changes are insufficient for functional recovery because most injuries to the hippocampus have a tendency to evolve into impairments in functions such as learning, memory and mood, and/or chronic temporal lobe epilepsy characterized by spontaneous seizures [6-8]. These neurological abnormalities in the chronic phase after injury are associated with greatly reduced proliferation of neural stem cells (altered stem cell activity), abnormal migration and integration of newly born neurons (aberrant neurogenesis), and greatly declined concentration of neurotrophic factors [3,4]. Therefore, therapeutic interventions that are competent for averting the evolution of initial hippocampal injury into mood and memory impairments and chronic epilepsy have considerable importance. In particular, there is a need to develop treatment approaches that are efficacious for maintaining a normal extent of new neuron production and appropriate integration of newly added neurons in the injured hippocampus. This need stems from the fact that ongoing neurogenesis in the hippocampus plays a vital role in maintaining normal memory and mood function [1,9,10], and the possibility that altered neurogenesis after injury contributes to an abnormal reorganization of the hippocampal circuitry, memory and mood impairments and chronic epilepsy [11,12].
Neural stem cell grafting therapy is currently considered as one of the most promising approaches for easing hippocampus injury-induced neurological impairments. This notion is supported by several properties of these cells. First, these cells can survive in hypoxic conditions prevailing in the injured brain regions, migrate and integrate into regions exhibiting neuron loss despite an adverse microenvironment . Second, these cells can replace some of the lost interneurons that secrete the inhibitory neurotransmitter gamma-amino butyric acid to regulate the activity of principal neurons and to maintain normal network function. Third, a significant faction of the progeny of neural stem cells can differentiate into astrocytes capable of secreting distinct neurotrophic factors that promote neuroprotection, ease seizures  and improve neurogenesis through stimulation of the proliferation of neural stem cells residing in the hippocampus .
Indeed, several studies in animal models demonstrate the promise of neural stem transplantation approach for easing dysfunction of the hippocampus following injury. For instance, our recent study demonstrated that grafting of neural stem cells expanded from the anterior subventricular zone (a precinct around the lining of forebrain ventricles rich in neural stem/progenitor cells) of postnatal brain into the hippocampus of young adult rats early after injury is highly beneficial for promoting functional recovery . In particular, this study provided clear evidence that grafting of neural stem cells into the hippocampus is a highly efficacious approach for counteracting the hippocampal injury-induced mood and memory dysfunction. Preservation of normal mood and memory function after hippocampal injury was associated with robust survival and pervasive migration of graft-derived cells, differentiation of substantial percentages of graft-derived cells into various subtypes of interneurons secreting the inhibitory neurotransmitter gamma-amino butyric acid, and glial cells such as astrocytes, oligodendrocytes and oligodendrocyte progenitors . Considerable fractions of graft-derived cells also expressed several beneficial neurotrophic factors which include the glial cell line-derived neurotrophic factor, brain-derived neurotrophic factor, fibroblast growth factor, and vascular endothelial growth factor, all of which are neuroprotective and neurogenesis enhancing factors. Indeed, neural stem cell grafting counteracted the injury-induced reductions and abnormalities in the production of new neurons (abnormal neurogenesis). This was accomplished through maintenance of normal level of activity in endogenous neural stem cells residing in the hippocampus and by providing protection to a class of interneurons in the hippocampus that guide newly born neurons generated from stem cells into appropriate areas in the hippocampus . Another recent study in a mouse model of unilateral hippocampal neurodegeneration also reported similar findings . In this study, grafting of neural stem cells overexpressing the insulin-like growth factor-1 into the hippocampus early after injury significantly prevented injury-induced cognitive decline . Additional analyses revealed that early neural stem cell grafting after injury maintained neurogenesis to near normal levels and prevented adverse alterations in astrocytes, a type of non-neuronal cells in the brain that react to injury and contribute to inflammation through proliferation as well as hypertrophy. Moreover, functional benefits of grafting were linked with survival of grafted neural stem cells and their differentiated progeny (neurons and glia).
Studies in several other animal prototypes have also shown beneficial effects of neural stem cell grafts for easing hippocampal injury and inducing functional recovery. Wang et al  examined the effects of grafting of human neural stem cells in an animal model of traumatic brain injury. They found that neural stem cell grafting considerably eased traumatic brain injury induced progressive axonal degeneration. Grafting also blocked the abnormal accumulation of amyloid precursor protein, a protein that forms abnormal aggregates all over the brain in people with Alzheimer's disease and is also believed to play a key role in the evolution of initial traumatic brain injury induced neurodegeneration into a state of cognitive decline. Park and colleagues  examined the effects of grafting of human neural stem cells that are genetically modified to secrete the synthesizing enzyme of acetylcholine (a neurotransmitter vital for making memories) into the hippocampus following an injury. They demonstrated that neural stem cell grafts considerably eased hippocampal injury induced learning and memory deficits and increased the level of acetylcholine.
Likewise, a study in an animal model of Alzheimer's disease has also shown beneficial effects of neural stem cell grafts placed into the hippocampus . This study examined the effects of neural stem cell grafts placed into the hippocampus of aged transgenic mouse (3xTg-AD mouse) exhibiting the properties of Alzheimer's disease. Authors reported that despite the widespread and established pathology typified by amyloid precursor protein plaques and tau-protein mediated neurofibrillary tangles, neural stem cell grafting eased spatial learning and memory deficits seen in Alzheimer's mice. Fascinatingly, grafted neural stem cells did not alter plaques or tangles that were prevalent in the brain but improved cognition through robust enhancement of hippocampal synaptic density, mediated by secretion of brain-derived neurotrophic factor, a protein important for synaptic function, memory and mood. Overall, this study showed that neural stem cell grafting could be efficacious for easing even complex behavioral deficits associated with Alzheimer disease.
Hippocampus injury inflicted by status epilepticus (an occurrence of seizure activity for an extended period) or traumatic brain injury can also lead to chronic epilepsy characterized by spontaneous recurrent seizures. A recent study suggests that early grafting of neural stem cells after status epilepticus can considerably ease spontaneous seizures occurring in the chronic phase after status epilepticus . Status epilepticus was induced through graded intraperitoneal injections of a neurotoxin kainic acid and grafting of neural stem cells was performed seven days after status epilepticus. Analyses of behavioral spontaneous seizures at 3-5 months after status epilepticus revealed greatly reduced frequency and intensity of spontaneous seizures in rats receiving neural stem cells grafts after status epilepticus, in comparison to rats receiving no transplants or dead neural stem cell grafts . Continuous video-electroencephalographic recordings for 120 hours also revealed substantially reduced frequency and intensity of spontaneous seizures in rats receiving neural stem cell grafts. It comprised considerable reductions in the frequencies of all spontaneous seizures as well as stage V seizures (the most severe form of seizures), and the percentage of recorded time spent in seizures. Characterization of recognition memory function using a novel object recognition test demonstrated preserved memory function in rats receiving neural stem cell grafts after status epilepticus. These rats also exhibited greatly reduced depressive-like behavior than the control status epilepticus animals receiving no grafts. Examination of hippocampal tissues revealed an excellent yield of graft-derived cells and differentiation of significant fractions of graft-derived cells into neurons including the inhibitory gamma-amino butyric acid expressing interneurons, and astrocytes secreting neurotrophic factors. Neural stem cell grafting after status epilepticus also preserved host interneurons synthesizing neuropeptide Y, a protein secreted by a subclass of interneurons having anti-seizure and neuroprotective effects. Collectively, this study demonstrated that grafting of neural stem cells into the hippocampus early after status epilepticus is beneficial for restraining the evolution of status epilepticus induced injury into a state of chronic epilepsy and cognitive and mood impairments.
From the results of studies discussed above, it is clear that neural stem cell grafting approach is highly promising for preventing or easing hippocampal injury mediated impairments in memory and mood, and chronic epilepsy development. However, one of the major issues to resolve prior to its clinical application in humans is, how to obtain adequate numbers of human-derived neural stem cells suitable for grafting? Neural stem cells can be obtained from a variety of sources, which include autopsied tissue samples from neural stem cell enriched regions of fetal, postnatal or adult brain. However, these sources may not be sufficient for routine clinical application of neural stem cell grafting for neurological disorders because of issues such as the need to harvest tissues for stem cell expansion within a short period of time after death, and a potential variability between autopsy samples collected from different age groups and at different time intervals after death. Furthermore, expansion of human neural stem cells from autopsied adult brain samples in culture may not match levels observed from rodent brains in terms of obtaining adequate numbers of neural stem cells for grafting.
The above limitations compel investigators to consider additional sources of neural stem cells for clinical use. In this context, neural stem cells obtained from pluripotent human embryonic stem cells isolated from the blastocyst stage of embryo or induced pluripotent stem cells obtained through somatic nuclear transfer or reprogramming of post-mitotic cells such as skin fibroblasts seem ideal because they can easily generate unlimited numbers of neural stem cells for grafting [21-23]. Yet, the potential for forming teratoma (a mixed tumor comprising tissue types generated from ectoderm, endoderm and mesoderm lineage cells), and genetic and epigenetic abnormalities are major barricades for the clinical use of neural stem cells derived from pluripotent stem cells [23,24]. Eradicating the potential for teratoma formation is vital to every clinical transplantation therapy using cells generated from human pluripotent stem cells because the presence of even a single pluripotent stem cell in neural stem cell suspension may pose a threat for generating tumors after grafting. Hence, the cell therapy field using cell types generated from pluripotent stem cells is moving with caution even though preclinical data supporting the safety and functional effectiveness of cells derived from such cells are growing . In particular, the field is looking forward to the development of advanced teratoma elimination techniques in order to avoid tumors from neural stem cells generated from pluripotent stem cells following grafting.
About the Author
Dr. Ashok K. Shetty was born in India. He received his M.S. degree in human anatomy from the Kasturba Medical College, Manipal, India, in 1983. He obtained his Ph.D. degree in neuroscience from the All India Institute of Medical Sciences (AIIMS), New Delhi, India, in 1990. From April 1992 to June 2011, Dr. Shetty worked at the Division of Neurosurgery (Department of Surgery) at the Duke University Medical Center, Durham, North Carolina, USA. He was an Assistant Professor between 1995 and 1999, Associate Professor between 1999 and 2004, and full Professor between 2004 and 2011. Presently, Dr. Shetty is the Director of Neurosciences at the Institute for Regenerative Medicine and Professor of Molecular and Cellular Medicine, Texas A&M Health Science Center College of Medicine at Scott & White, Temple, Texas, USA. He also serves as a Research Career Scientist at the Central Texas Veterans Health Care System, Temple Texas. His major research interests include neural stem cell behavior, neural plasticity and neurogenesis in aging and disease, and stem cell therapy for brain repair in a variety of neurodegenerative disease models. On-going studies in Dr. Shetty’s laboratory at the Institute for Regenerative Medicine are supported by grants from the National Institutes of Health and the Department of Veterans Affairs. From 2004 to 2008, Dr. Shetty served as a Charter Member of the National Institutes of Health Study Section CNNT (Brain Disorders and Clinical Neuroscience ZRG1). He has also served as an ad hoc member of over 20 other National Institutes of Health study sections since 2003. Presently, Dr. Shetty is a charter member of the NIH Study Section, Developmental Brain Disorders. As per the Essential Science Indicators of Thompson Reuters, Dr. Shetty is currently among the top 1% of scientists worldwide in the field of Neuroscience and Behavior, in terms of citations received for his published articles over 10 year period. Dr. Shetty serves as an Editorial Board Member of many prestigious international journals, which include Stem Cells, Aging Cell, Stem Cells International, Stem Cells & Cloning, Current Aging Science, Frontiers in Neurogenesis, and Frontiers in Aging Neuroscience. Dr. Shetty is an Associate Editor of Frontiers in Epilepsy and Co-Editor-in-Chief of the journal, Aging and Disease.
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