Current Gene Therapy - Volume 11, Issue 2, 2011

The better the understanding of cellular and molecular events involved in nervous system degeneration and regeneration became during the last decades, the more likely have gene therapy approaches become to improve nervous system regeneration and to find their position in clinical settings. Gene therapy based strategies for neuroregeneration mainly aim to provide target specific neurotrophic support to enhance viability of diseased or trauma-affected neurons [1, 2]. In addition to support of neuronal survival, axonal regeneration across long distances and re-establishment of functional circuits are needed for successful repair of peripheral and central nervous system deficits. Therefore not only the introduction of genes directly into the neuronal or glial target cells with the aim to support their survival and functionality but also the transplantation of genetically modified supportive cells have reached the level of clinical research [3, 4]. With this special issue we want to give insight in gene therapeutic approaches targeting the injured peripheral nervous system, the injured dorsal root entry zone and spinal cord as well as the inner retina. Four comprehensive reviews discuss current developments in biotechnology as well as the most promising strategies to include gene therapy into combinatory therapeutic treatments to restore peripheral as well as central nervous system functions. Furthermore, advantages as well as limitations, especially regarding the clinical application of gene therapy of the lesioned nervous system, are reviewed. Because use of viral vectors has been shown to be the most efficient way to introduce the expression of potentially therapeutic gene products into the nervous system, a special focus has been put on viral vector systems like adeno-associated and lentiviral vectors. The first contribution by Mason et al. discusses efforts to develop gene therapy as an adjunct strategy to promote peripheral nerve regeneration following neurosurgical repair which alone often cannot avoid a considerable degree of functional impairment. Viral-based gene transfer strategies are reviewed and analyzed regarding their potential to enforce the regeneration outcome and their clinical applicability. While Mason et al. mainly describe the introduction of neurotrophic support for neuroregeneration, the second review by Lavdas et al. gives insights on the promising genetic manipulation of cell adhesion molecules in the peripheral as well as central nervous system. Cell adhesion molecules are crucial players in axonal pathfinding and formation of functional circuits which makes them along with neurotrophic factors the most promising candidates to help long distance axonal regeneration and appropriate target reinnervation. Bo et al. provide up-to-date information on the in vivo, ex vivo and combinatorial gene delivery of a wide range of therapeutic molecules to overcome the non-permissive properties that are inhibiting axonal regeneration after spinal root and spinal cord injury. Thus the third review clearly demonstrates, how experimental gene transfer could pave way for the development of new therapeutic strategies. The mammalian visual system, in particular the eye and the optic nerve, are widely used for experimental studies to investigate the cellular and molecular mechanisms of central nervous system injury and repair. As a matter of course the current state of viral-based gene transfer techniques to target inner retinal neurons are provided by the fourth review from Hellstrom and Harvey. This review therefore covers another important and evolving field of gene therapy approaches in neuroregeneration that has so far received less attention than gene delivery to the outer retina for the treatment of deficits in photoreceptor and retinal epithelium function [5]. This special issue was set up to provide an overview on cutting-edge developments in gene transfer technology that will in the future hopefully enable successful regeneration within the different parts of the nervous system presented. It was a great pleasure to work with the authors and I highly appreciate the dedication and expert knowledge they contributed to this special issue.

Peripheral nerve injury in humans often leads to incomplete functional recovery. In this review we discuss the potential for gene therapy to be used as a strategy alongside surgical repair techniques for the study of peripheral nerve regeneration in rodent models and with a view to its eventual use for the promotion of successful regeneration in the clinic. Gene therapy vectors based on herpes simplex virus, adenovirus, lentivirus and adeno-associated virus have been developed to deliver genes to the neurons of the peripheral nervous system, i.e. primary sensory neurons in the dorsal root ganglia and primary motor neurons. Adenoviral and lentiviral vectors have also been used to transduce Schwann cells and fibroblasts in the injured nerve. We present an overview of these vectors, their application so far in the peripheral nervous system, their potential as vectors for enhancing peripheral nerve repair, and the successful interventions that have been demonstrated in animal models. We also discuss some of the limitations of current vectors and how they may be overcome. While the technology for gene delivery is approaching a state of readiness for clinical translation, the current range of therapeutic genes for the repair of the traumatically injured peripheral nerve is mostly limited to neurotrophic factors delivered to neurons, Schwann cells or possibly the target organs. Finally, therefore, we consider what type of therapeutic transgene may be desirable to enhance nerve regeneration in the future.

The inability of the central nervous system (CNS) to efficiently repair damages results in severe functional impairment after trauma or neurodegenerative / demyelinating diseases. Regeneration failure is attributed to inhibitory molecules creating a nonpermissive environment for axonal regrowth, and dictates the necessity for the development of novel therapeutic strategies. An emerging approach for improving regeneration is the use of gene therapy to manipulate cell adhesion molecule expression in experimental animal models of degeneration. Alternatively, cell transplantation to replace lost neurons and the grafting of myelinating cells to repair demyelinating lesions are promising approaches for treating CNS injuries and demyelination. Schwann cells (SCs), oligodendrocyte progenitors, olfactory ensheathing cells and embryonic and neural stem cells have been shown to form myelin after transplantation into the demyelinated CNS. The repair capacity of the peripheral nervous system (PNS) is much higher, but there is still a limit to the amount of nerve loss that can be bridged after injury, and longer nerve gaps call for the use of conduits populated with living cells. In both cases, the interaction of grafted cells with the host environment is of paramount importance for the incorporation and functional integration of these cells and the manipulation of cell adhesion molecules is an attractive approach towards achieving this goal. In this review we summarize data from the recent literature regarding the manipulation of cell adhesion molecule expression towards CNS and PNS repair and discuss the prospects for future therapeutic applications.

Recent understanding in pathophysiological mechanisms of spinal cord and spinal root injuries has facilitated the development of new strategies to promote neural repair. Gene therapy approaches have been viewed as the ideal means to achieve long-term local delivery of therapeutic molecules in the central nervous system (CNS). Ex vivo gene delivery offers the additional advantage of providing cellular support for regenerating axons. In this review, we summarize the studies on viral vector-mediated gene delivery to spinal cord in animal models, both in vivo and ex vivo. Most of the studies reported so far are aimed at delivery of various growth factors, such as neurotrophins and neuropoietic cytokines. Other molecules tested include those that interfere with intracellular processes to prevent cell death, or increase intrinsic regenerating state of injured neurons, or modify the CNS environment to make it permissive for axon growth. Several different combinatorial strategies involving gene delivery are also discussed as it has been recognized that successful neural repair may require the synergistic actions of multiple therapeutic managements.

Recent clinical trials have shown that the use of replication deficient viral vectors to genetically modify cells in the retina can be of therapeutic benefit in the treatment of certain inherited degenerative conditions that compromise photoreceptor, and hence visual, function. This review is focussed primarily on the use of recombinant adeno-associated viral (rAAV) vectors to target neurons in inner retina, specifically retinal ganglion cells (RGCs). Genetic modification of RGCs may be of value in various ophthalmic conditions in which there is documented loss of RGCs or damage to their centrally projecting axons. Such conditions include glaucoma, optic neuritis, vascular disruption or trauma, and neurological degenerative conditions such as Alzheimer's disease. Furthermore, because the retina and optic nerve (ON) form part of the CNS, the visual system is a useful experimental model in which to study the molecular and cellular mechanisms that underlie degenerative as well as regenerative responses of adult CNS neurons after injury. Gene therapy studies from a number of laboratories are first reviewed, involving not only rAAV-based treatments but also application of lentiviral and adenoviral vectors. Recent work from our own laboratory is then summarized, in which intravitreal injection of rAAV2 serotype vectors is used to introduce growth promoting genes into injured RGCs. rAAV encoding a secretable form of ciliary neurotrophic factor (CNTF) has proved to be particularly effective in promoting RGC survival and axon regeneration after optic nerve crush or after transection followed by a peripheral nerve autograft. In the latter situation we have found that RGCs and their regenerated axons are maintained for at least 15 months after the initial injury. We have also combined rAAV gene therapy with pharmacotherapy to determine if cAMP elevation and additional intravitreal injections of growth factors can act synergistically with vector-based delivery of growth-promoting genes.

Intravenous enzyme replacement therapy has been developed as a viable treatment for most of the somatic pathologies associated with the mucopolysaccharide storage disorders. However, approximately two thirds of individuals affected by a mucopolysaccharide storage disorder also display neurological disease, in these instances intravenous enzyme replacement therapy is not viable as the blood-brain barrier severely limits enzyme distribution from the peripheral circulation into the central nervous system. Accordingly, much research is now focussed on developing therapies that specifically address neurological disease, or somatic and neurological disease in combination. Therapies designed to address the underlying cause of central nervous system pathology, that is the lysosomal storage itself, can be broadly divided into two groups, those that continue the rationale of enzyme replacement, and those that address the supply side of the storage equation; that is the production of storage material. Enzyme replacement can be further divided by technology (principally direct enzyme replacement, gene replacement and cell transplantation). Here we review the current state of the art for these strategies and suggest possible future directions for research in this field. In particular, we suggest that any one approach in itself is unlikely to be as efficacious as a carefully considered combination therapy, be it a combination of some sort of enzyme replacement with substrate deprivation, or a combination of two different replacement technologies or strategies.

Classical non-viral methods of gene transfer, such as chemical transfection, have met with limited success of instillation of genetic material into non-proliferating cells in vitro. Among the different kinds of viral vectors, Lentiviral vectors (LVs) have emerged as robust and versatile tool for ex vivo and in vivo gene delivery into multiple cell types including non-dividing cells such as neurons. The capacity of LVs to maintain stable, long-term transgene expression and the substantial flexibility in the design of the expression cassettes account for their increasing use in various pre-clinical and clinical applications. Additionally, LVs have been hugely successful in reprogramming induced pluripotent stem cells (iPSCs). Recent development using LVs in conjunction with a Cre-Lox based reversible system has opened up many new possibilities towards therapeutic application of iPSC technology in various clinical settings. Moreover, improvements in term of biosafety and efficacy, achieved either by modifying the vector design or by involving integration-deficient LVs (IDLVs), have important implications for adoption of LV as the vector of choice for clinical trials. Several human gene therapy clinical trials evaluating the use of LVs for treatment of human diseases such as Parkinson's disease, β- thalassemia, X-linked adrenoleukodystrophy (ALD), and AIDS are currently ongoing. This review will describe the state of the art achieved by LV technology, its impact on biomedical research, and implications to human clinical trials as therapeutic gene delivery vehicle for a wide range of infectious and genetic diseases.