Ionizing radiation enhances the engraftment of transplanted in vitro-derived multipotent astrocytic stem cells

Combined Ionizing Radiation Exposure by Gamma Rays and Carbon-12 Nuclei Increases Neurotrophic Factor Content and Prevents Age-Associated Decreases in the Volume of the Sensorimotor Cortex in Rats

In orbital and ground-based experiments, it has been demonstrated that ionizing radiation (IR) can stimulate the locomotor and exploratory activity of rodents, but the underlying mechanism of this phenomenon remains undisclosed. Here, we studied the effect of combined IR (0.4 Gy γ-rays and 0.14 Gy carbon-12 nuclei) on the locomotor and exploratory activity of rats, and assessed the sensorimotor cortex volume by magnetic resonance imaging-based morphometry at 1 week and 7 months post-irradiation. The sensorimotor cortex tissues were processed to determine whether the behavioral and morphologic effects were associated with changes in neurotrophin content. The irradiated rats were characterized by increased locomotor and exploratory activity, as well as novelty-seeking behavior, at 3 days post-irradiation. At the same time, only unirradiated rats experienced a significant decrease in the sensorimotor cortex volume at 7 months. While there were no significant differences at 1 week, at 7 months, the irradiated rats were characterized by higher neurotrophin-3 and neurotrophin-4 content in the sensorimotor cortex. Thus, IR prevents the age-associated decrease in the sensorimotor cortex volume, which is associated with neurotrophic and neurogenic changes. Meanwhile, IR-induced increases in locomotor activity may be the cause of the observed changes.

Stem Cell Therapies for the Resolution of Radiation Injury to the Brain

Purpose of review

To encapsulate past and current research efforts focused on stem cell transplantation strategies to resolve radiation-induced cognitive dysfunction.

Recent Findings

Transplantation of human stem cells in the irradiated brain was first shown to resolve radiation-induced cognitive dysfunction in a landmark paper by Acharya et al., appearing in PNAS in 2009. Since that time, work from the same laboratory as well as other groups have reported on the beneficial (as well as detrimental) effects of stem cell grafting after cranial radiation exposure. Improved learning and memory found many months after engraftment has since been associated with a preservation of host neuronal morphology, a suppression of neuroinflammation, improved myelination and increased cerebral blood flow. Interestingly, many (if not all) of these beneficial effects can be demonstrated by substituting stem cells with microvesicles derived from human stem cells during transplantation, thereby eliminating many of the more long-standing concerns related to immunorejection and teratoma formation.

Summary

Stem cell and microvesicle transplantation into the irradiated brain of rodents has uncovered some unexpected benefits that hold promise for ameliorating many of adverse neurocognitive complications associated with major cancer treatments. Properly developed, such approaches may provide much needed clinical recourse to millions of cancer survivors suffering from the unintended side effects of their cancer therapies.

A nonhuman primate model of human radiation-induced venocclusive liver disease and hepatocyte injury

BACKGROUND

Human liver has an unusual sensitivity to radiation that limits its use in cancer therapy or in preconditioning for hepatocyte transplantation. Because the characteristic veno-occlusive lesions of radiation-induced liver disease do not occur in rodents, there has been no experimental model to investigate the limits of safe radiation therapy or explore the pathogenesis of hepatic veno-occlusive disease.

METHODS AND MATERIALS

We performed a dose-escalation study in a primate, the cynomolgus monkey, using hypofractionated stereotactic body radiotherapy in 13 animals.

RESULTS

At doses ≥40 Gy, animals developed a systemic syndrome resembling human radiation-induced liver disease, consisting of decreased albumin, elevated alkaline phosphatase, loss of appetite, ascites, and normal bilirubin. Higher radiation doses were lethal, causing severe disease that required euthanasia approximately 10 weeks after radiation. Even at lower doses in which radiation-induced liver disease was mild or nonexistent, latent and significant injury to hepatocytes was demonstrated by asialoglycoprotein-mediated functional imaging. These monkeys developed hepatic failure with encephalopathy when they received parenteral nutrition containing high concentrations of glucose. Histologically, livers showed central obstruction via an unusual intimal swelling that progressed to central fibrosis.

CONCLUSIONS

The cynomolgus monkey, as the first animal model of human veno-occlusive radiation-induced liver disease, provides a resource for characterizing the early changes and pathogenesis of venocclusion, for establishing nonlethal therapeutic dosages, and for examining experimental therapies to minimize radiation injury.

Growth hormone responsive neural precursor cells reside within the adult mammalian brain

The detection of growth hormone (GH) and its receptor in germinal regions of the mammalian brain prompted our investigation of GH and its role in the regulation of endogenous neural precursor cell activity. Here we report that the addition of exogenous GH significantly increased the expansion rate in long-term neurosphere cultures derived from wild-type mice, while neurospheres derived from GH null mice exhibited a reduced expansion rate. We also detected a doubling in the frequency of large (i.e. stem cell-derived) colonies for up to 120 days following a 7-day intracerebroventricular infusion of GH suggesting the activation of endogenous stem cells. Moreover, gamma irradiation induced the ablation of normally quiescent stem cells in GH-infused mice, resulting in a decline in olfactory bulb neurogenesis. These results suggest that GH activates populations of resident stem and progenitor cells, and therefore may represent a novel therapeutic target for age-related neurodegeneration and associated cognitive decline.

Exercise increases neural stem cell number in a growth hormone-dependent manner, augmenting the regenerative response in aged mice

The exercise-induced enhancement of learning and memory, and its ability to slow age-related cognitive decline in humans led us to investigate whether running stimulates periventricular (PVR) neural stem cells (NSCs) in aging mice, thereby augmenting the regenerative capacity of the brain. To establish a benchmark of normal aging on endogenous NSCs, we harvested the PVR from serial vibratome sections through the lateral ventricles of juvenile (6-8 weeks), 6-, 12-, 18-, and 24-month-old mice, culturing the cells in the neural colony-forming cell assay. A significant decline in NSC frequency was apparent by 6 months ( approximately 40%), ultimately resulting in a approximately 90% reduction by 24 months. Concurrent with this decline was a progressive loss in regenerative capacity, as reflected by an incomplete repopulation of neurosphere-forming cells following gamma cell irradiation-induced depletion of the PVR. However, voluntary exercise (i.e., 21 days of running) significantly increased NSC frequency in mice > or = 18 months of age, augmenting the regeneration of irradiation-ablated periventricular cells and restoring NSC numbers to youthful levels. Importantly, and consistent with the demonstrated ability of growth hormone (GH) to increase NSC proliferation, and the elevated secretion of GH during exercise, exercise failed to stimulate NSCs in GH receptor-null mice. These findings now provide a novel basis for understanding the ability of exercise to delay the onset and rate of decline in neurodegenerative conditions not typically associated with the hippocampus and suggest that the GH-dependent activation of endogenous NSCs may be effective in reversing or preventing age-related neurodegeneration in humans.

Neural progenitors of the postnatal and adult mouse forebrain retain the ability to self-replicate, form neurospheres, and undergo multipotent differentiation in vivo

Somatic stem cells are reservoirs to replace lost cells or damaged tissue. Cells with neural stem cell (NSC) characteristics can be isolated from the postnatal mammalian brain into adulthood and expanded as neurospheres. We addressed the ability of these in vitro expanded putative NSCs to retain progenitor characteristics in vivo, in analogy to hematopoietic stem cells. When transplanted in utero, both postnatal and adult neural progenitors colonize host brains and contribute neurons and glia. In stark contrast to what has been reported when transplanted in postnatal hosts, epidermal growth factor-expanded cells also remain self-replicating and multipotent in vivo over many months and can be serially transplanted into multiple hosts. Surprisingly, embryonically transplanted NSCs remain in the neurogenic regions in adult hosts, where they express progenitor cell markers and continue to proliferate even after 6 months without tumor formation. These data indicate that spherogenic cells of the postnatal and adult mammalian brain retain their potential in vitro and in vivo throughout the life of the organism and beyond transplantation, which has important implications for cell replacement strategies.

Transplantation of neural stem/progenitor cells into developing and adult CNS

Neural transplantation has been a long-standing goal for the treatment of neurological injury and disease. The recent discovery of persistent pools of neural stem cells within the adult mammalian brain has re-ignited interest in transplant therapeutics. Since neural stem cells are self-renewing, it may be possible to culture and expand neural stem cells and their progenitor cell progeny to sufficient numbers for use in autologous, self-repair strategies. Such approaches will require optimized cultivation protocols, as well as extensive testing of candidate donor cells to assess their capacity for engraftment, survival, and integration. In this chapter, we describe the transplantation of neural stem/progenitor cells-cultivated as either neurospheres or neurogenic astrocyte monolayers-into the persistently neurogenic olfactory bulb system of the adult mouse forebrain, and into the cerebellum of neonatal mutant mice.

The neurosphere assay, a method under scrutiny

OBJECTIVES

The aim of this review is to provide an overview of the fundamental features of the neurosphere assay (NSA), which was initially described in 1992, and has since been used not only to detect the presence of stem cells in embryonic and adult mammalian neural tissues, but also to study their characteristics in vitro. Implicit in this review is a detailed examination of the limitations of the NSA, and how this assay is most accurately and appropriately used. Finally we will point out criteria that should be challenged to design alternative ways to overcome the limits of this assay.

METHODS

NSA is used to isolate putative neural stem cells (NSCs) from the central nervous system (CNS) and to demonstrate the critical stem cell attributes of proliferation, extensive self-renewal and the ability to give rise to a large number of differentiated and functional progeny. Nevertheless, the capability of neural progenitor cells to form neurospheres precludes its utilisation to accurately quantify bona fide stem cell frequency based simply on neurosphere numbers. New culture conditions are needed to be able to distinguish the activity of progenitor cells from stem cells.

CONCLUSION

A commonly used, and arguably misused, methodology, the NSA has provided a wealth of information on precursor activity of cells derived from the embryonic through to the aged CNS. Importantly, the NSA has contributed to the demise of the 'no new neurogenesis' dogma, and the beginning of a new era of CNS regenerative medicine. Nevertheless, the interpretations arising from the utilisation of the NSA need to take into consideration its limits, so as not to be used beyond its specificity and sensitivity.

Quiescent adult neural stem cells are exceptionally sensitive to cosmic radiation

Generation of new neurons in the adult brain, a process that is likely to be essential for learning, memory, and mood regulation, is impaired by radiation. Therefore, radiation exposure might have not only such previously expected consequences as increased probability of developing cancer, but might also impair cognitive function and emotional stability. Radiation exposure is encountered in settings ranging from cancer therapy to space travel; evaluating the neurogenic risks of radiation requires identifying the at-risk populations of stem and progenitor cells in the adult brain. Here we have used a novel reporter mouse line to find that early neural progenitors are selectively affected by conditions simulating the space radiation environment. This is reflected both in a decrease in the number of these progenitors in the neurogenic regions and in an increase in the number of dying cells in these regions. Unexpectedly, we found that quiescent neural stem cells, rather than their rapidly dividing progeny, are most sensitive to radiation. Since these stem cells are responsible for adult neurogenesis, their death would have a profound impact on the production of new neurons in the irradiated adult brain. Our finding raises an important concern about cognitive and emotional risks associated with radiation exposure.

Survival of transplanted neural progenitor cells enhanced by brain irradiation

OBJECT

Authors of previous studies have reported that adult transplanted neural progenitor cells (NPCs) are suitable for brain cell replacement or gene delivery. In this study, the authors evaluated survival and integration of adult rat-derived NPCs after transplantation and explored the potential impact on transplant survival of various mechanical and biological factors of clinical importance.

METHODS

Adult female Fischer 344 rats were used both as a source and recipient of transplanted NPCs. Both 9L and RG2 rat glioma cells were used to generate in vivo brain tumor models. On the 5th day after tumor implantation, NPCs expressing green fluorescent protein (GFP) were administered either intravenously (3.5 x 10(7) cells) or by stereotactic injection (1 x 10(4)-1 x 10(6) cells) into normal or tumor-bearing brain. The authors evaluated the effect of delivery method (sharp compared with blunt needles, normal compared with zero-volume needles, phosphate-buffered saline compared with medium as vehicle), delivery sites (intravenous compared with intratumoral compared with intraparenchymal), and pretreatment with an immunosuppressive agent (cyclosporin) or brain irradiation (20-40 Gy) on survival and integration of transplanted NPCs.

RESULTS

Very few cells survived when less than 10(5) cells were transplanted. When 10(5) cells or more were transplanted, only previously administered brain irradiation significantly affected survival and integration of NPCs. Although GFP-containing NPCs could be readily detected 1 day after injection, few cells survived 4 days to 1 week unless preceded by whole-brain radiation (20 or 40 Gy in a single fraction), which increased the number of GFP-containing NPCs within the tissue more than fivefold.

CONCLUSIONS

The authors' findings indicate that most NPCs, including those from a syngeneic autologous source, do not survive at the site of implantation, but that brain irradiation can facilitate subsequent survival in both normal and tumor-bearing brain. An understanding of the mechanisms of this effect could lead to improved survival and clinical utility of transplanted NPCs.

Astrocytic stem cells in the adult brain

The adult mammalian brain harbors a population of neural stem cells (NSCs) that are responsible for persistent neurogenesis in the olfactory system and hippocampus and may also play a role in tumorigenesis. Here, the authors review the evidence that NSCs within the adult brain are a special type of astrocyte. In addition, the authors examine the phylogenetic and ontogenetic relations between this astrocyte stem cell and related members of the astrocyte family. Finally, the authors compare and contrast the functional characteristics of NSCs and hematopoietic stem cells and review the potential oncogenic transformation of astrocyte NSCs that may underlie brain tumorigenesis as seen in glioblastoma and other primary brain tumors.

Neurogenic astrocytes transplanted into the adult mouse lateral ventricle contribute to olfactory neurogenesis, and reveal a novel intrinsic subependymal neuron

Spatially and temporally restricted populations of neurogenic astrocytes can generate multipotent neurospheres in vitro. To examine the ability of neurogenic astrocytes to respond to in vivo differentiation cues within a germinal matrix, we provided cultured neonatal cerebellar astrocytes access to the subependymal zone (SEZ) by grafting them directly into the lateral ventricle of adult mice. Here we report three events that follow such transplants. 1) Donor cells attach to periventricular structures, and form "neoplastic-like" spheres that penetrate the ventricular wall. These attached spheres can persist for months, as they give rise to "clones" of cells that infiltrate forebrain parenchyma. 2) Many donor cells enter the rostral migratory stream and migrate into the olfactory bulb where a small percentage differentiates as olfactory interneurons. 3) Finally, within the SEZ, some donor cells formed cell clusters that appear to interact with the SEZ neuronal precursor chains, and some donor cells differentiate into distinctive neurons with extensive, beady projections precisely confined between the ependymal layer and the striatum. Further analysis of normal SEZ anatomy reveals indigenous neurons with identical morphologies--some of which are contacted by 5-HT+ fibers--that we propose represent a heretofore uncharacterized, intrinsic SEZ neuron of unknown function. These results suggest that cultured astrocytes derived from non-SEZ brain regions can respond in different ways to in vivo cues provided by the adult lateral ventricle and SEZ by differentiating into neurons that eventually inhabit both the olfactory bulb and SEZ proper.

In Vitro-Derived “Neural Stem Cells” Function as Neural Progenitors Without the Capacity for Self-Renewal

Hematopoietic stem cells have been defined by their ability to self-renew and successfully reconstitute hematopoiesis throughout the life of a transplant recipient. Neural stem cells (NSCs) are believed to exist in the regenerating regions of the brain in adult mice: the subependymal zone (SEZ) of the lateral ventricles (LVs) and the hippocampal dentate gyrus. Cells from the SEZ can be cultured to generate neurospheres or multipotent astrocytic stem cells (MASCs), both of which demonstrate the stem cell qualities of multipotency and self-renewal in vitro. Whether neurospheres and MASCs possess the true stem cell quality of functional self-renewal in vivo is unknown. The definitive tests for this unique capability are long-term engraftment and serial transplantation. Both neurospheres and MASCs transplanted into the LVs of C57BL/6 mice resulted in short-term engraftment into the recipient brain, with donor-derived migratory neuroblasts visible in the rostral migratory stream and olfactory bulb after transplantation. To test in vivo expansion/self-renewal of the transplanted cells, we attempted to reisolate donor-derived neurospheres and MASCs. Even when rigorous drug selection was used to select for rare events, no donor-derived neurospheres or MASCs could be reisolated. Furthermore, donor-derived migratory neuroblasts were not observed in the rostral migratory stream (RMS) for more than 1 month after transplantation, indicating a transient rather than long-term engraftment. Therefore, in vitro-derived neurospheres and MASCs do not function as NSCs with long-term, self-renewal capabilities in vivo but instead represent short-term neural progenitor cells as defined by an in vivo functional assay.