Transplantation of neural stem cells in a rat model of stroke: assessment of short-term graft survival and acute host immunological response

In vivo monitoring of cellular transplants by magnetic resonance imaging and positron emission tomography

Cellular loss is a common pathological observation in many disease conditions. Recent evidence that these cells can be replaced has generated huge excitement over possible clinical applications. The use of stem or progenitor cells, which can differentiate into site-appropriate phenotypes required to "repair" the damaged tissue, has already demonstrated potential in animal models, but many aspects of this novel treatment strategy require further elucidation. Most importantly, the monitoring of the safety of cellular transplants in patients remains a challenge. Traditional histological methods do not address the dynamic nature of transplant-induced recovery and highlight the necessity of in vivo imaging to probe the survival, migration and functional consequences of transplanted cells. This paper reviews how non-invasive imaging technology can be used to serially assess intact living organisms in order to visualise and monitor cellular transplants.

Intracerebral xenografts of mouse bone marrow cells in adult rats facilitate restoration of cerebral blood flow and blood–brain barrier

We examined in the present study alterations in cerebral blood flow (CBF) and blood-brain barrier (BBB) permeability following intrastriatal transplantation of mouse bone marrow stromal cells (BMSCs) or saline infusion in adult Sprague-Dawley rats. Laser Doppler revealed that transplanted animals exhibited near normal cerebral blood flow (CBF, 150 perfusion units) at a much earlier period post-transplantation (day 4) compared to animals that received saline infusion (day 12) (p's<0.05). Similarly, Evans Blue assay demonstrated that transplanted animals exhibited near complete BBB reconstitution at day 5 post-transplantation, whereas animals that received saline infusion continued to display a compromised BBB up to 11 days post-transplantation. Transplanted animals displayed a cell dose-dependent CBF and BBB restoration. Enzyme-linked immunosorbent assay (ELISA) of transplanted BMSCs revealed elevated levels of transforming growth factor-beta superfamily of neurotrophic factors. Moreover, despite the absence of immunosuppression in this cross-species transplantation, at least in the acute phase (12 days post-transplantation), surviving xenografts were detected during periods of restored CBF and BBB permeability. These observations suggest that restoration of CBF and BBB permeability accompanies the reported functional outcomes associated with intracerebral transplantation of BMSCs.

Wide therapeutic time window for nimesulide neuroprotection in a model of transient focal cerebral ischemia in the rat

Results from several studies indicate that cyclooxygenase-2 (COX-2) is involved in ischemic brain injury. The purpose of this study was to evaluate the neuroprotective effects of the selective COX-2 inhibitor nimesulide on cerebral infarction and neurological deficits in a standardized model of transient focal cerebral ischemia in rats. Three doses of nimesulide (3, 6 and 12 mg/kg; i.p.) or vehicle were administered immediately after stroke and additional doses were given at 6, 12, 24, 36 and 48 h after ischemia. In other set of experiments, the effect of nimesulide was studied in a situation in which its first administration was delayed for 3-24 h after ischemia. Total, cortical and subcortical infarct volumes and functional outcome (assessed by neurological deficit score and rotarod performance) were determined 3 days after ischemia. The effect of nimesulide on prostaglandin E(2) (PGE(2)) levels in the injured brain was also investigated. Nimesulide dose-dependently reduced infarct volume and improved functional recovery when compared to vehicle. Of interest is the finding that neuroprotection conferred by nimesulide (reduction of infarct size and neurological deficits and improvement of rotarod performance) was also observed when treatment was delayed until 24 h after ischemia. Further, administration of nimesulide in a delayed treatment paradigm completely abolished PGE(2) accumulation in the postischemic brain, suggesting that COX-2 inhibition is a promising therapeutic strategy for cerebral ischemia to target the late-occurring inflammatory events which amplify initial damage.

Mapping transplanted stem cell migration after a stroke: a serial, in vivo magnetic resonance imaging study

Preferential migration of stem cells toward the site of a lesion is a highly desirable property of stem cells that allows flexibility in the site of graft implantation in the damaged brain. In rats with unilateral stroke damage, neural stem cells transplanted into the contralateral hemisphere migrate across to the lesioned hemisphere and populate the area around the ischaemic infarct. To date, the migration of neural stem cells in the damaged brain has been mainly inferred from snapshot histological images. In this study, we demonstrate that by pre-labelling neural stem cells with the bimodal contrast agent Gadolinium-RhodamIne Dextran [GRID, detectable by both magnetic resonance imaging (MRI) and fluorescent microscopy], the transhemispheric migration of transplanted neural stem cells contralateral to a stroke lesion can be followed in vivo by serial MRI and corroborated by subsequent histological analyses. Our results indicate that neural stem cells migrated from the injection tract mainly along the corpus callosum within 7 days of transplantation and extensively re-populated the peri-lesion area by 14 days following implantation. In contrast, neural stem cells transplanted into sham controls did not show any substantial migration outside of the injection tract, suggesting that the transcallosal migration observed in the stroke-lesioned animals is due to neural stem cells being attracted by the lesion site. In vivo tracking of the migration of neural stem cells responding to damage will greatly enhance our understanding of optimal transplantation strategies as well as how neural stem cells promote functional and anatomical recovery in neurological disorders.

Regulatory genes controlling cell fate choice in embryonic and adult neural stem cells

Neural stem cells are the most immature progenitor cells in the nervous system and are defined by their ability to self-renew by symmetric division as well as to give rise to more mature progenitors of all neural lineages by asymmetric division (multipotentiality). The interest in neural stem cells has been growing in the past few years following the demonstration of their presence also in the adult nervous system of several mammals, including humans. This observation implies that the brain, once thought to be entirely post-mitotic, must have at least a limited capacity for self-renewal. This raises the possibility that the adult nervous system may still have the necessary plasticity to undergo repair of inborn defects and acquired injuries, if ways can be found to exploit the potential of neural stem cells (either endogenous or derived from other sources) to replace damaged or defective cells. A full understanding of the molecular mechanisms regulating generation and maintenance of neural stem cells, their choice between different differentiation programmes and their migration properties is essential if these cells are to be used for therapeutic applications. Here, we summarize what is currently known of the genes and the signalling pathways involved in these mechanisms.

Retinal transplantation: progress and problems in clinical application

There is currently no real treatment for blinding disorders that stem from the degeneration of cells in the retina and affect at least 50 million individuals worldwide. The excitement that accompanied the first studies showing the potential of retinal cell transplantation to alleviate the progress of blindness in such diseases as retinitis pigmentosa and age-related macular degeneration has lost some of its momentum, as attempts to apply research to the clinic have failed so far to provide effective treatments. What these studies have shown, however, is not that the approach is flawed but rather that the steps that need to be taken to achieve a viable, clinical treatment are many. This review summarizes the course of retinal transplant studies and points to obstacles that still need to be overcome to improve graft survival and efficacy and to develop a protocol that is effective in a clinical setting. Emphasis is given particularly to the consequences of introducing transplants to sites that have been considered immunologically privileged and to the role of the major histocompatibility complex classes I and II molecules in graft survival and rejection.

Gene therapy in autoimmune, demyelinating disease of the central nervous system

Multiple sclerosis (MS) is an immune-mediated disease of the central nervous system (CNS), where suspected autoimmune attack causes nerve demyelination and progressive neurodegeneration and should benefit from both anti-inflammatory and neuroprotective strategies. Although neuroprotection strategies are relatively unexplored in MS, systemic delivery of anti-inflammatory agents to people with MS has so far been relatively disappointing. This is most probably because of the limited capacity of these molecules to enter the target tissue, because of exclusion by the blood-brain barrier. The complex natural history of MS also means that any therapeutic agents will have to be administered long-term. Gene therapy offers the possibility of site-directed, long-term expression, and is currently being preclinically investigated in experimental autoimmune encephalomyelitis (EAE), an animal model of MS. While some immune effects may be targeted in the periphery using DNA vaccination, strategies both viral and nonviral are being developed to target agents into the CNS either via direct delivery or using the trafficking properties of cell-carrier systems. Targeting of leucocyte activation, cytokines and nerve growth factors have shown some promising benefit in animal EAE systems, the challenge will be their application in clinical use.

In vitro expression of major histocompatibility class I and class II antigens by conditionally immortalized murine neural stem cells

The expression of major histocompatibility complex (MHC) antigens on the surface of cells is intimately linked to in vivo graft survival. It has been previously shown that the conditionally immortalized temperature-sensitive Maudsley hippocampal clone 36 (MHP36) neural stem cells show good long-term graft survival and do not elicit an acute immunological response following transplantation. Here we report that MHP36 cells express both MHC class I and class II antigens when grown in culture under proliferative conditions (33 degrees C), whereas cells with a differentiated morphology in the non-proliferative (37-39 degrees C) condition express low to undetectable levels of either MHC molecules. However, morphologically undifferentiated cells persisting under non-proliferating conditions continued to express both MHC antigens. The downregulation of MHC antigens upon differentiation following cell transplantation could therefore contribute to the graft survival of MHP36 cells.

Potential use of stem cells in neuroreplacement therapies for neurodegenerative diseases

The use of stem cells for neuroreplacement therapy is no longer science fiction--it is science fact. We have succeeded in the development of neural and mesenchymal stem cell transplantation to produce neural cells in the brain. We have also seen improvement in cognitive function following stem cell transplantation in a memory-impaired aged animal model. These results promise a bright future for stem cell therapies in neurodegenerative diseases. Before we begin to think about clinical applications beyond the present preclinical studies, we have to consider the pathophysiological environment of individual diseases and weigh the factors that affect stem cell biology. Here, I not only review potential therapeutic applications of stem cell strategies in neurodegenerative diseases, but also discuss stem cell biology regarding factors that are altered under disease conditions.