Oligodendrocytes Engineered With Migratory Proteins as Effective Graft Source for Cell Transplantation in Multiple Sclerosis

Abstract

Multiple sclerosis (MS) is characterized by widespread immunomodulatory demyelination of the central nervous system (CNS), resulting in nerve cell dysfunction. Accordingly, treatment strategies have been centered on immunodulation and remyelination, with the former primarily focused on reducing the pathology rather than enhancing myelin repair, which the latter targets. While conceding to the emerging view of heterogeneity in the pathology of MS, which precludes variations in degree of immune response (i.e., inflammation) and demyelination, the concept of enhancing myelin repair is appealing since it is likely to provide both disease-reducing and disease-inhibiting therapeutic approaches to MS. In this regard, we and several others have proposed that cell replacement therapy is an effective strategy to repair the myelin in MS. Here we hypothesize that transplantation of mouse bone marrow-derived oligodendrocytes (BMDOs) and BMDOs transfected with ephrin proteins (BMDO + ephrin), which are known to enhance cell and axonal migratory capacity, may produce therapeutic benefits in animal models of MS.

INTRODUCTION

Multiple sclerosis (MS) is a severe central nervous system (CNS) autoimmune disease characterized by widespread immunomodulatory demyelination of the CNS resulting in nerve cell dysfunction (31,51). It is a chronic, progressive disorder that afflicts about 400,000 people in the US and about 2.5 million adults worldwide (51). Recent studies have estimated that per-patient direct medical costs of MS amount to approximately $20,000 to $30,000 per year (29). In addition to such treatment costs, the expense of personal health services and earnings loss (indirect cost), as well as pain and suffering (intangible cost), has been estimated at an average of $17,581–$21,231 and $15,315 per patient per year, respectively (11,29). Furthermore, because MS is a disabling neurologic disease of young adults, productivity loss is also a major concern. Thus, MS is very costly to the individual, health care system, and society.

Treatment strategies for MS have been centered on immunomodulation and remyelination, with the former primarily focused on reducing the pathology rather than enhancing myelin repair, which the latter targets. While conceding to the emerging view of heterogeneity in the pathology of MS (34,41), which precludes variations in degree of immune response (i.e., inflammation) and demyelination, the concept of enhancing myelin repair is appealing since it is likely to provide both disease-reducing and disease-inhibiting therapeutic approaches to MS. In this regard, we and several others have proposed stem cell transplantation in the promotion of myelin replacement around demyelinated axons in MS (17,20,23). In the last decade, clinical studies were conducted to evaluate the effects of stem cell treatments (e.g., hematopoietic and mesenchymal stem cells) on MS patients. These studies have reported possible remyelination or stabilization and improvement of MS patients treated with stem cells (24,26). Thus, although substantial clinical and biological issues remain to be resolved, remyelination treatment by cell-based therapy represents a realistic approach in the generation of promising treatments for MS.

Depletion of oligodendrocytes, cells that engage with and remyelinate the demyelinated axons, is a major hallmark of MS (3–5,36,52,55,56). Thus, replacement of oligodendrocytes in MS lesions is an ideal approach to enhance remyelination, resulting in improved long-term outcomes in MS patients. In this article, we discuss the relevance of oligodendrocyte transplantation for MS and hypothesize the therapeutic potential of bone marrow-derived oligodendrocytes (BMDOs) and BMDO transfected with ephrin (BMDO + ephrin). The use of ephrin proteins is assumed to improve cell and axonal migratory capacity of oligodendrocytes, thereby enhancing the capacity of the transplants to remyelinate multifocal lesions in MS.

WHY CELL TRANSPLANTATION FOR MS?

Immunotherapeutic treatments [e.g., interferons (IFNs)] for MS exert beneficial effects, especially when initiated early in the course of illness. However, the outcomes remain partial, with progression of the disease observed over time (13). Such limited efficacy of immunotherapies can be ascribed, in part, to the heterogeneity of MS pathology (41), suggesting that treatment strategies need to be catered to the individual patient. Indeed, IFN-β demonstrated benefits in secondary progressive MS (10) but showed no efficacy on the primary outcome measure or on most of the secondary outcome measures in primary progressive MS (13,33). Alternatively, immunotherapies may be considered as disease-reducing rather than disease-inhibiting treatment, in that the hallmark MS pathology of demyelination is not addressed by such therapy.

In order to provide a disease-inhibiting therapy for MS, treating demyelination should be a primary target. In recent years, demyelinating MS lesions have been characterized by neuronal or axonal injury, which may account for most or all of the permanent clinical deficits (30,44,50). Neuronal injury in MS is characterized by reductions in the neuronal marker, N-acetyl aspartate, and cerebral and spinal cord atrophy as detected by magnetic resonance imaging techniques (14,15,21), whereas axonal injury is revealed by the presence of axonal spheroids or terminal ovoids, reductions in silver staining or neurofilament immunostaining, and accumulations of amyloid precursor protein in axons (35,50). Although there is concern about the variable degree of demyelination in MS (53), previous studies reported that axonal injury persists in all types of MS lesions (2), suggesting that demyelination accompanies both acute and chronic MS. Furthermore, while “injury” may be permanent (i.e., axonal transection) or transient (i.e., perturbed axonal transport), the progressive nature of MS suggests that deteriorating axonal abnormalities are more likely to translate into a more permanent axonal damage over the course of the disease.

Demyelination in MS may actually stimulate the CNS into a self-regenerative mode in that host remyelination can occur during early demyelination process. Although the adult CNS has long been considered nonregenerative, accumulating evidence over the last two decades suggests plasticity of the adult brain. Damage to the CNS induces spontaneous remyelination via glial cells from neighboring spared tissue (41). We, and several others, have demonstrated that this glial response may in fact promote neuroprotective and regenerative effects (9,38,49), although equally compelling evidence argues for glial response as exacerbating the degenerative process (22,45,46). Whether glial response is pro- or antiregenerative, CNS remyelination, especially in chronic MS and generally in late stages of most neurodegenerative disorders, eventually fails because few spared oligodendrocytes remain [(27,47) but see Murray et al. (39)]. Thus, there is a need to introduce new cells capable of remyelination in the injured CNS.

Years of basic laboratory research have provided direct evidence that neural transplantation promotes functional protection and repair in many models of CNS disorders (1,7,8,23,43). This surgical procedure has been employed in the clinic for Parkinson’s, Huntington’s, and stroke patients (6,42,48). However, logistical and ethical concerns impede fetal neural transplantation therapy. To this end, alternatives to fetal cells as donor cell grafts have been examined. Indeed, autologous Schwann cells, mesenchymal or autologous hematopoietic stem cells (HSCs) have been transplanted in MS patients (12,18,47). A recent small-scale study reported improvement in CNS structure, function, and physiology after systemic infusion of autologous bone marrow-derived mesenchymal stem cells in secondary progressive MS patients (12). A number of clinical studies also reported beneficial effects on disease progression of MS patients transplanted with HSCs (26). Although further large-scale, longitudinal studies need to be performed before stem cells could be considered as a treatment option for progressive MS, the above-mentioned studies provide “proof of concept” for stem cell-based therapy in MS.

WHY BONE MARROW-DERIVED OLIGODENDROCYTES (BMDOs)?

The rationale for choosing BMDOs as the transplant source is twofold in that bone marrow transplantation has been performed in MS patients and animal models, and oligodendrocyte depletion is a major hallmark of MS (3–5,36,52,55,56). While bone marrow is a good source of stem cells, a cell source that can be committed to a lineage differentiation primarily altered in MS (i.e., oligodendrocytes) is deemed more efficacious (20,24,32).

Of note, although HSC transplantation showed promise for MS, initial clinical studies on HSC transplantation produced mixed results. Accordingly, neurologic improvements were seen in some MS patients, while disease progressed in others after treatment with HSCs (18,26). The heterogenous composition of autologous HSC grafts used in the clinical trials, derived from either bone marrow or peripheral blood, might have produced deleterious side effects. Indeed, the transplants were seen “purged with lymphocytes,” and mortality risks increased in some patients (18). In contrast, transplantation of putatively “pure” oligodendrocytes has been demonstrated to promote remyelination in experimental animals (4,16,19,54), accompanied by improved neurological function (25). However, migration of grafted cells along the injured host demyelinated cells, coupled with the inhibitory nature of normal host white matter (40), greatly limits the capacity of the transplants to remyelinate the characteristic multifocal MS lesions. Multiple intracerebral transplantation, to encompass the widespread demyelination in MS, is not feasible as this will expose the patient to surgical trauma. Intraventricular transplantation may be a more plausible route since most MS lesions are periventricular (32). Nonetheless, migration of the grafted cells from the ventricles toward the lesions may still present some limitations. If a method can be identified to promote long-distance migration of oligodendrocytes toward demyelinated MS lesions, then more extensive myelin repair and functional outcome can be achieved.

OLIGODENDROCYTES TRANSFECTED WITH EPHRIN: EFFECTIVE GRAFT SOURCE FOR CELL TRANSPLANTATION IN MS?

The variable outcomes observed in the recent clinical transplantation in MS may be due, in part, to heterogenous cell populations, which comprise the current autologous graft material. Thus, increasing the homogeneity of donor cells, specifically by culturing and harvesting pure oligodendrocytes, may enhance the functional effects of the grafts in MS. Accordingly, we hypothesize that transplantation of BMDOs will provide better functional outcomes than crude bone marrow cells. We further hypothesize that a critical factor that impedes the success of bone marrow transplantation in MS is the limited capacity of the grafted cells to repair widespread demyelination, especially in the terminal stage of the disease. In order to promote remyelination in the chronic MS phase, the grafted cells should have the ability to migrate to the diseased regions. In this regard, the capacity of BMDOs to exhibit cell and axonal migration can be enhanced via lentiviral vector gene therapy. Ephrin and their receptors (Eph) are contact-mediated molecules that aid in the migration of neuronal precursors during development (28,37). Eph–ephrin interactions between oligodendrocyte precursor cells (OPCs) and axons have been shown to modulate the migration of OPCs (37). In this respect, we hypothesize that BMDOs loaded with ephrin (BMDO + ephrin) may increase graft migration of oligodendrocytes. Eventually, we suggest that pure oligodendrocytes transfected with ephrin are an effective graft source for cell transplantation in MS. Future studies will determine whether these transplants promote recovery of motor function in animal models of MS [e.g., animals subjected to acute and chronic experimental autoimmune encephalomyelitis (EAE)]. Moreover, we will characterize survival and maturation of grafted oligodendrocytes, host cell remyelination, and graft migration (in the case of BMDO + ephrin) in MS animals.

CONCLUDING REMARKS

The limited efficacy of immunomodulatory treatments (e.g., IFNs) for MS means that more rational, if not better, approaches in the treatment of MS have been sought, such as enhancement of neuroregeneration (e.g., remyelination) via stem cell transplantation. Oligodendrocytes are a good stem cell source because lineage differentiation of these cells is primarily altered in MS (20,32). Moreover, we hypothesize that transplanting BMDOs loaded with ephrin (BMDO + ephrin) may increase the migratory capacity of these grafts and eventually promote remyelination, behavioral recovery, and improvement of motor functions in MS animals. Of course, several issues remain to be addressed prior to establishing the optimal efficacy of BMDO + ephrin transplantation in MS animal models. For instance, timing of transplantation after EAE induction is critical for successful functional outcomes. Therefore, early transplantation postimmunization should be performed, especially for chronic EAE. Furthermore, aside from looking into remyelination in white matter tracts of the cerebral hemispheres after BMDO + ephrin transplantation in animal models of MS, we also need to characterize the effects of these transplants in the spinal cord. It would also be worthwhile to explore other proteins aside from ephrin (e.g., syndecans) that could enhance migration of transplanted oligodendrocytes.