Current Drug Targets - Volume 6, Issue 1, 2005

Gene and Cell therapies have paved the way to a new era in the treatment of human diseases. Knowledge about the potentiality and limits of such therapeutical tools is of great interest. The use of antisense oligonucleotides and more recently, of small interfering RNAs (siRNAs) as selective inhibitors of gene expression, offers a rational approach to the prevention and treatment of some gene-mediated disorders. In this gene therapy approach, oligonucleotides or siRNAs block the expression of specific target genes involved in the development of the pathological processes. The use of antisense molecules to modify gene expression has been found to be variable in its efficacy and reliability, raising objections about their use as therapeutic agents. However, several antisense candidate molecules are undergoing separate clinical trials. It is still too early to tell whether the entire class of antisense drugs will prove to be clinically effective. It is, however, quite surprising that all efforts devoted to clinical trials of dozens of antisense compounds have so far produced only a commercial drug, targeted against a side effect of HIV infection and hence, with a limited market value. Nevertheless, the antisense oligonucleotides could be one of the few strategies that could be used to treat some neurodegenerative diseases. In this issue, some contributions discuss the promises and concerns linked to the development of a new generation of antisense molecules for treatment of neural diseases. Concerns about antisense therapeutics have induced researchers to focus on other gene therapy tools. Gene expression downregulation by siRNAs (the so called RNA interference) is one of the most exciting discoveries of the past decade in functional genomics. Some authors have reported that the potency, effectiveness, duration of action, and sequence specificity of siRNAs are greater than those obtained with antisense molecules. For this reason, RNA interference is rapidly becoming an important method for analyzing gene functions in eukaryotes, and promises the development of therapeutic gene silencing. This topic is also discussed in this issue. Stem cell therapy seeks to reverse the ravages of damaged tissues by injecting living stem cells from animal organs, embryos or fetuses into patients. Traditional cell therapy is founded on the belief that, when healthy cells are injected into patients, cells will automatically find their way to damaged tissues and stimulate the body's own healing process. For example, there is evidence that liver stem cells injected into the human body naturally migrate to the host liver and stimulate regeneration. Stem cell therapy is promoted as an alternative therapy for several pathologies, such as cancer, atherosclerosis, and several neurodegenerative diseases. Unfortunately, there are a number of potential side effects regarding which, the individuals considering this therapy should be made aware. Indeed, cell therapy may be dangerous and some cases in medical literature reported of patient deaths directly linked to the therapy. Patients may contract bacterial and viral infections carried by the donor cells, and have experienced life-threatening and even fatal allergic reactions. Donor cells may seriously compromise the immune system. Thus, despite extensive research, still there are problems with stem cell therapy, since in many cases, deep and exhaustive studies to find out the exact biology of stem cells are omitted, and there are increasing pressures to start with insufficiently controlled clinical trials. It is very important to address all these issues, and therefore, some contributions are focused on these key topics and give an in-depth contribution to the knowledge of the state of art in cell therapy, with particular emphasis on the treatment of neural diseases.

Advances in cell biology have encouraged the hope that stem cell-based therapy can be used to heal central nervous system (CNS) diseases. Here, we will review the potential application of neural cell transplantation for the treatment of multiple sclerosis (MS) and other demyelinating disorders, mention some problematic issues that still face this therapeutic approach, and describe novel noninvasive methods for in vivo tracking of transplanted cells.

The complexity of the central nervous system (CNS) exposes it to a number of different diseases, often caused by only small variations in gene sequence or expression level. Antisense oligonucleotides and RNA interference-mediated therapies hold great promise for the treatment of CNS diseases in which neurodegeneration is linked to overproduction of endogenous protein or to synthesis of aberrant proteins coded by dominant mutant alleles. Nevertheless, difficulties related to the crossing of the blood-brain barrier, expression vectors, molecule design and to the choosing of the correct target, should be effectively solved. This review summarizes some of the most recent findings concerning the administration of potential nucleic acid-based therapeutic drugs, as well as the most promising studies performed both in vitro and in animal models of disease. Finally, some current clinical trials involving antisense oligonucleotides or silencing RNA for therapy of neurological disorders are illustrated. Results of current studies and clinical trials are exciting, and further results will be certainly reached with increasing knowledge of blood-brain barrier transporters, of genes involved in neurological disease and in new vectors for efficient delivery to brain.

Bone marrow derived mesenchymal stem cells (MSC) are adult stem cells that reside within the bone marrow compartment. In the traditional developmental model, adult stem cells are able to differentiate only to the tissue in which they reside. Recent data have challenged the committed fate of the adult stem cells, presenting evidence for their multilineage differentiation potential. In addition, potential therapeutic benefits of MSC administration have been the main concern of much research, including clinical trials. These studies promote adult stem cell therapy by shedding some light on the therapeutic potential of MSC and their mechanism of action. Many doubts have found their way into MSC research. They question MSC potency and beneficial contribution. However, these obstacles should not arrest but set a challenge to MSC researchers to examine their achievements under a magnifying glass. Therapeutic benefits of MSC exogenous delivery do not run counter to its possible participation in endogenous repair. Several reports imply MSC involvement in physiological repair but no explicit data support this hypothesis. This review tries to put MSC research into perspective. Possible therapeutic applications of MSC therapy for damaged tissue replacement, tissue engineering and the underlying repair mechanisms will be discussed. In addition, reported data about MSC possible involvement in physiological multiple tissue repair, their homing to injury and site-specific differentiation will be presented.

Huntington's disease (HD) is an incurable, adult-onset, dominantly inherited neurodegenerative disease, caused by a CAG expansion in the 5' coding region of the gene HD [encoding huntingtin (htt), which is ubiquitously expressed in all tissues]. The disease progresses inexorably with devastating clinical effects on motor, cognitive and psychological functions; death occurring approximately 18 years from the time of onset. These clinical symptoms primarily relate to the progressive death of medium-spiny GABA-ergic neurons of the striatum and in the deep layers of the cortex; during the later stages of the disease, the degeneration extends to a variety of brain regions, including the hypothalamus and hippocampus. The mechanism by which mutant htt leads to neuronal cell death and the question of why striatal neurons are targeted both remain to be further investigated. Certainly htt is required for cell survival and impairment of wild-type htt function can be involved in neurodegeneration, but considerable evidence also shows that trinucleotide repeat expansion into glutamine (polyQ domain) endows the protein with a newly acquired toxic activity. The increasing availability of HD animal models have allowed not only to investigate the function of htt, but also to screen and test potential therapeutic drugs in the promising area of neurotherapeutics. So, thorough analysis of these molecular and biochemical events, assessing the validity of candidate mechanisms, provides a means to identify effective therapeutic strategies for cellular repair. Here, the rationale and efficacy of different therapies are compared and alternative therapies are reviewed including intrastriatal transplantation of human fetal striatal tissue to support the cell replacement strategy in HD. Since functional restoration through neuronal replacement probably could be combined with neuroprotective strategies for optimum clinical benefit, in vivo and ex vivo gene therapy for delivery of neuroprotective growth factor molecules are also considered.

There is growing evidence to suggest that reservoirs of stem cells may reside in several types of adult tissue. These cells may retain the potential to transdifferentiate from one phenotype to another, presenting exciting possibilities for cellular therapies. Recent discoveries in the area of neural differentiation are particularly exciting given the limited capacity of neural tissue for intrinsic repair and regeneration. Adult adipose tissue is a rich source of mesenchymal stem cells, providing an abundant and accessible source of adult stem cells. These cells have been termed adipose derived stem cells (ASC). The characterization of these ASCs has defined a population similar to marrow-derived and skeletal muscle-derived stem cells. The success seen in differentiating ASC into various mesenchymal lineages has generated interest in using ASC for neuronal differentiation. Initial in vitro studies characterized the morphology and protein expression of ASC after exposure to neural induction agents. Additional in vitro data suggests the possibility that ASCs are capable of neuronal activity. Progress in the in vitro characterization of ASCs has led to in vivo modeling to determine the survival, migration, and engraftment of transplanted ASCs. While work to define the mechanisms behind the transdifferentiation of ASCs continues, their application to neurological diseases and injuries should also progress. The subject of this review is the capacity of adipose derived stem cells (ASC) for neural transdifferentiation and their application to the treatment of various neurologic disorders.

The mechanical force incurred by spinal cord injury results in degenerative neural tissue damage beyond the site of initial injury. By nature, the central nervous system (CNS) does not regenerate itself. Cell therapy, in particular, stem cell implantation has become a possible solution for spinal cord injury. Embryonic stem cells and fetal stem cells are the forefathers of the field of stem cell therapy. Isolation and preparation of specific populations of adult stem cells have evolved to the point of stable, long-term culturing with the capability to differentiate into neural phenotypes from all three of the neural lineages: neurons, astrocytes, and oligodendrocytes. Thus, adult stem cells will transcend ethical concerns, technical difficulties, and probably immunorejection. A variety of adult stem cells have been implanted in a rat model of spinal cord injury, ranging from olfactory ensheathing cells, cultured spinal cord stem cells, bone marrow derived stem cells, dermis derived stem cells, and a few others. Although no definite decisions on which adult stem cells are most effective for this CNS injury, their ability to incorporate into the spinal cord, differentiate, and to improve locomotor recovery hold promise for a cure.

Neuropathic pain is defined as pain initiated or caused by a primary lesion or dysfunction in the nervous system. It is a devastating and difficult to manage consequence of peripheral nerve injury and has a variety of clinical symptoms. Neuropathic pain is a major health problem. It has been estimated that 70% of patients with advanced cancer and inflammatory pathologies are afflicted by chronic pain. About 95% of patients with spinal cord injuries have neuropathic pain problems. Chronic pain is debilitating and cause of depression and decreasing quality of life. Pharmacological treatment for the symptoms of painful neuropathy is difficult, because there has been limited understanding of the underlying causes and systemic levels that an effective dose can have on multiple side effects. The use of molecular methods, such as gene therapy, stem cell therapy and viral vector for delivery of biologic antinociceptive molecules, has led to a better understanding of the underlying mechanisms of the induction of intractable neuropathic pain.

Cell therapy will probably become a major therapeutic strategy for neuronal disorders in the coming years. Nevertheless, due to poor survival of grafted cells and limited differentiation and integration in the host tissue, certain ameliorations must be envisaged. To address these difficulties, several strategies have been developed and among them, two methods seem particularly promising : in situ controlled drug delivery and implantation of cells adhered on biomaterial-based scaffolds. Indeed, the ability of drugs, such as growth factors, to regulate neuronal survival and/or plasticity infers the use of these molecules to treat neurodegeneration associated with human diseases. Moreover, the synthesis of cell scaffolds which mimic the extra-cellular matrix can help guide morphogenesis and tissue repair. Furthermore, cells can be cultivated on these matrices that may eventually make graft therapy a more practical approach for the treatment of neurological diseases. Nevertheless, for those two encouraging approaches multiple parameters have to be considered, such as the drug targeting strategy, but also the physical and morphological characteristics of the scaffold and the type of cells to be conveyed. This review thus focuses on those two promising strategies and also on their possible association to improve stem cell therapy of neurodegenerative disorders. Indeed, tissue replacement by grafting cells within or adhered onto drug delivering biomaterial-based devices, has recently been reported and seems to be very promising.

Neural stem cells (NSCs) are present not only in the developing nervous systems, but also in the adult human central nervous system (CNS). It is long thought that the subventricular zone of the lateral ventricles and the dentate gyrus of the hippocampus are the main sources of human adult NSCs, which are considered to be a reservoir of new neural cells. Recently adult NSCs with potential neural capacity have been isolated from white matter and inferior prefrontal subcortex in the human brain. Rapid advances in the stem cell biology have raised appealing possibilities of replacing damaged or lost neural cells by transplantation of in vitro -expanded stem cells and/or their neuronal progeny. However, sources of stem cells, large scale expansion, control of the differentiations, and tracking in vivo represent formidable challenges. In this paper we review the characteristics of the adult human NSCs, their potentiality in terms of proliferation and differentiation capabilities, as well as their large scale expansion for clinical needs. This review focuses on the major advances in brain stem cell-based therapy from the clinical perspective, and summarizes our work in clinical phase I-II trials with autologuous transplantation of adult NSCs for patients with open brain trauma. It also describes multiple approaches to monitor adult human NSCs labeled superparamagnetic nanoparticles after transplantation and explores the intriguing possibility of stem cell transplantation.

Neural stem/progenitor cells capable of generating new neurons and glia, reside in specific areas of the adult mammalian central nervous system (CNS), including the ependymal region of the spinal cord and the subventricular zone (SVZ), hippocampus, and dentate gyrus of the brain. Much is known about the neurogenic regions in the CNS, and their response to various stimuli including injury, neurotrophins (NFs), morphogens, and environmental factors like learning, stress, and aging. This work has shaped our current views about the CNS's potential to recover lost tissue and function post-traumatically and the therapies to support the intrinsic regenerative capacity of the brain or spinal cord. Recently, intensive research has explored the potential of harvesting, culturing, and transplanting neural stem/progenitors as a therapeutic intervention for spinal cord injury (SCI) and traumatic brain injury (TBI). Another strategy has focused on maximizing the potential of this endogenous population of cells by stimulating their recruitment, proliferation, migration, and differentiation in vivo following traumatic lesions to the CNS. The promise of such experimental treatments has prompted tissue and biomaterial engineers to implant synthetic three-dimensional biodegradable scaffolds seeded with neural stem/progenitors into CNS lesions. Although there is no definitive answer about the ideal cell type for transplantation, strong evidence supports the use of region specific neural stem/progenitors. The technical and logistic considerations for transplanting neural stem/progenitors are extensive and crucial to optimizing and maintaining cell survival both before and after transplantation, as well as for tracking the fate of transplanted cells. These issues have been systematically addressed in many animal models, that has improved our understanding and approach to clinical therapeutic paradigms.