Chapter 55: neural transplantation

Successes and Hurdles in Stem Cells Application and Production for Brain Transplantation

Brain regenerative strategies through the transplantation of stem cells hold the potential to promote functional rescue of brain lesions caused either by trauma or neurodegenerative diseases. Most of the positive modulations fostered by stem cells are fueled by bystander effects, namely increase of neurotrophic factors levels and reduction of neuroinflammation. Nevertheless, the ultimate goal of cell therapies is to promote cell replacement. Therefore, the ability of stem cells to migrate and differentiate into neurons that later become integrated into the host neuronal network replacing the lost neurons has also been largely explored. However, as most of the preclinical studies demonstrate, there is a small functional integration of graft-derived neurons into host neuronal circuits. Thus, it is mandatory to better study the whole brain cell therapy approach in order to understand what should be better comprehended concerning graft-derived neuronal and glial cells migration and integration before we can expect these therapies to be ready as a viable solution for brain disorder treatment. Therefore, this review discusses the positive mechanisms triggered by cell transplantation into the brain, the limitations of adult brain plasticity that might interfere with the neuroregeneration process, as well as some strategies tested to overcome some of these limitations. It also considers the efforts that have been made by the regulatory authorities to lead to better standardization of preclinical and clinical studies in this field in order to reduce the heterogeneity of the obtained results.

Mechanisms and use of neural transplants for brain repair

Under appropriate conditions, neural tissues transplanted into the adult mammalian brain can survive, integrate, and function so as to influence the behavior of the host, opening the prospect of repairing neuronal damage, and alleviating symptoms associated with neuronal injury or neurodegenerative disease. Alternative mechanisms of action have been postulated: nonspecific effects of surgery; neurotrophic and neuroprotective influences on disease progression and host plasticity; diffuse or locally regulated pharmacological delivery of deficient neurochemicals, neurotransmitters, or neurohormones; restitution of the neuronal and glial environment necessary for proper host neuronal support and processing; promoting local and long-distance host and graft axon growth; formation of reciprocal connections and reconstruction of local circuits within the host brain; and up to full integration and reconstruction of fully functional host neuronal networks. Analysis of neural transplants in a broad range of anatomical systems and disease models, on simple and complex classes of behavioral function and information processing, have indicated that all of these alternative mechanisms are likely to contribute in different circumstances. Thus, there is not a single or typical mode of graft function; rather grafts can and do function in multiple ways, specific to each particular context. Consequently, to develop an effective cell-based therapy, multiple dimensions must be considered: the target disease pathogenesis; the neurodegenerative basis of each type of physiological dysfunction or behavioral symptom; the nature of the repair required to alleviate or remediate the functional impairments of particular clinical relevance; and identification of a suitable cell source or delivery system, along with the site and method of implantation, that can achieve the sought for repair and recovery.

Neural tissue transplantation, repair, and rehabilitation

Transplants of cells and tissues to the central nervous system of adult mammals can, under appropriate conditions, survive, integrate, and function. In particular, the grafted cells can sustain functional recovery in animal models of a range of neurodegenerative conditions including genetic and idiopathic neurodegenerative diseases of adulthood and aging, ischemic stroke, and brain and spinal cord trauma. In a restricted subset of such conditions, cell transplantation has progressed to application in humans in early-stage clinical trials. At the present stage of play, there is clear evidence of clinical efficacy of fetal cell transplants in Parkinson disease (notwithstanding a range of technical difficulties still to be fully resolved), and preliminary claims of promising outcomes in several other severe neurodegenerative conditions, including Huntington disease and stroke. Moreover, the experimental literature is increasingly suggesting that the experience and training of the graft recipient materially affects the functional outcome. For example, environmental enrichment, behavioral activity, and specific training can enhance the recovery process to maximize functional recovery. There are even circumstances where the grafted cells have been demonstrated to restore the neural substrate for new learning. Consequently, it is not sufficient to replace lost cells anatomically; rather, for the grafts to be effective, they need to be integrated functionally into the host circuitry, and the host animal requires training and rehabilitation to maximize function of the reconstructed graft-host circuitry. Such observations require reconsideration of the design of the next generation of clinical trials and subsequent service delivery, to include physiotherapists, cognitive therapists, and rehabilitation experts as core members of the transplant team, along with the neurologists and neurosurgeons that have conventionally led the field.

Transplanting intact donor tissue enhances dopamine cell survival and the predictability of motor improvements in a rat model of Parkinson's disease

Primary cell transplantation is currently the gold standard for cell replacement in Parkinson's disease. However, the number of donors needed to treat a single patient is high, and the functional outcome is sometimes variable. The present work explores the possibility of enhancing the viability and/or functionality of small amounts of ventral mesencephalic (VM) donor tissue by reducing its perturbation during preparation and implantation. Briefly, unilaterally lesioned rats received either: (1) an intact piece of half an embryonic day 13 (E13) rat VM; (2) dissociated cells from half an E13 rat VM; or (3) no transplant. D-amphetamine- induced rotations revealed that animals receiving pieces of VM tissue or dissociated cells showed significant improvement in ipsilateral rotation 4 weeks post transplantation. By 6 weeks post transplantation, animals receiving pieces of VM tissue showed a trend for further improvement, while those receiving dissociated cells remained at their 4 week scores. Postmortem cell counts showed that the number of dopaminergic neurons in dissociated cell transplants was significantly lower than that surviving in transplants of intact tissue. When assessing the correlation between the number of dopamine cells in each transplant, and the improvement in rotation bias in experimental animals, it was shown that transplants of whole pieces of VM tissue offered greater predictability of graft function based on their dopamine cell content. Such results suggest that maintaining the integrity of VM tissue during implantation improves dopamine cell content, and that the dopamine cell content of whole tissue grafts offers a more predictable outcome of graft function in an animal model of Parkinson's disease.

Potentials of endogenous neural stem cells in cortical repair

In the last few decades great thrust has been put in the area of regenerative neurobiology research to combat brain injuries and neurodegenerative diseases. The recent discovery of neurogenic niches in the adult brain has led researchers to study how to mobilize these cells to orchestrate an endogenous repair mechanism. The brain can minimize injury-induced damage by means of an immediate glial response and by initiating repair mechanisms that involve the generation and mobilization of new neurons to the site of injury where they can integrate into the existing circuit. This review highlights the current status of research in this field. Here, we discuss the changes that take place in the neurogenic milieu following injury. We will focus, in particular, on the cellular and molecular controls that lead to increased proliferation in the Sub ventricular Zone (SVZ) as well as neurogenesis. We will also concentrate on how these cellular and molecular mechanisms influence the migration of new cells to the affected area and their differentiation into neuronal/glial lineage that initiate the repair mechanism. Next, we will discuss some of the different factors that limit/retard the repair process and highlight future lines of research that can help to overcome these limitations. A clear understanding of the underlying molecular mechanisms and physiological changes following brain damage and the subsequent endogenous repair should help us develop better strategies to repair damaged brains.