Biomedical subjects

W F Blakemore

Publications and source records attributed to W F Blakemore.

At least 19 recordsLinked to original sources

PubMed2006

Age increases axon loss associated with primary demyelination in cuprizone-induced demyelination in C57BL/6 mice.

Axon loss is recognised as a significant contributor to the progression of the disability associated with multiple sclerosis. Although evidence of axon damage is found in areas of chronic demyelination it is more frequently seen in association with acute demyelination. This study compares the incidence of axon degeneration associated with the areas undergoing demyelination in young adult (8-10 weeks) and aged (6-7 months) C57BL/6 mice in cuprizone intoxication; a widely used model of demyelination. The incidence of axon transection, as indicated by the presence of SMI 32 positive axonal spheroids, and evidence of axon loss in the medial corpus callosum, were significantly greater in aged mice, as was the magnitude of the macrophage and astrocyte response to demyelination. Aged C57BL/6 mice are thus more prone to axon degeneration in association with demyelination than young adult mice. A retrospective study indicated that the incidence of axon degeneration was much higher in C57BL/6 mice than in the Swiss albino mice used in the early cuprizone intoxication studies which were fed much higher doses of cuprizone. These results indicate both a genetic and age susceptibility to demyelination-associated axon transection.

Aging↗

PubMed2006

Haplotype matching is not an essential requirement to achieve remyelination of demyelinating CNS lesions.

Transplantation of oligodendrocyte precursor cells (OPCs) results in efficient remyelination in animal models of demyelination. However, the experiments so far undertaken have not addressed the need for tissue-type matching to achieve graft-mediated remyelination. Examination of MHC expression (main determinant of allograft rejection) by OPCs showed nondetectable levels under standard culture conditions and upregulation of MHC Class I expression only upon exposure to interferon gamma. We therefore hypothesized that MHC matching of OPC grafts may not be crucial to achieve transplant-mediated remyelination. Transplant experiments performed using a nonself repairing toxin-induced demyelination model showed that, similarly to allogeneic neurons, survival of allogeneic oligodendrocyte lineage cells is influenced by donor-host haplotype combination and graft composition. Transplantation of allogeneic mixed glial cell cultures resulted in remyelination failure by 1 month postengraftment due to a rejection response targeting both myelinating oligodendrocytes and OPCs, suggesting that inflammation-induced upregulation of OPC MHC I expression results in susceptibility to cytotoxic T cell attack. In contrast, remyelination persisted for at least 2 months following transplantation of OPC-enriched cultures whose overall MHC expression level was significantly decreased. While OPC-enriched preparations elicited delayed type hypersensitivity responses in hosts sensitized to alloantigens, allografting of such preparations into a central nervous system demyelinating lesion did not result in recipient priming. We conclude that while allografted oligodendrocyte lineage cells become targets of a graft rejection response once this response has been initiated, transplantation of OPC-enriched preparations can evade priming against alloantigens and graft rejection. This finding indicates that close tissue matching may not be an essential requirement for successful transplant-mediated remyelination.

Animals↗

PubMed2005

Glial grafting for demyelinating disease.

Remyelination of demyelinated central nervous system (CNS) axons is considered as a potential treatment for multiple sclerosis, and it has been achieved in experimental models of demyelination by transplantation of pro-myelinating cells. However, the experiments undertaken have not addressed the need for tissue-type matching in order to achieve graft-mediated remyelination since they were performed in conditions in which the chance for graft rejection was minimized. This article focuses on the factors determining survival of allogeneic oligodendrocyte lineage cells and their contribution to the remyelination of demyelinating CNS lesions. The immune status of the CNS as well as the suitability of different models of demyelination for graft rejection studies are discussed, and ways of enhancing allogeneic oligodendrocyte-mediated remyelination are presented. Finally, the effects of glial graft rejection on host remyelination are described, highlighting the potential benefits of the acute CNS inflammatory response for myelin repair.

Central Nervous System↗

PubMed2005

Inflammation stimulates remyelination in areas of chronic demyelination.

A major challenge in multiple sclerosis research is to understand the cause or causes of remyelination failure and to devise ways of ameliorating its consequences. This requires appropriate experimental models. Although there are many models of acute demyelination, at present there are few suitable models of chronic demyelination. The taiep rat is a myelin mutant that shows progressive myelin loss and, by 1 year of age, its CNS tissue has many features of chronic areas of demyelination in multiple sclerosis: chronically demyelinated axons present in an astrocytic environment in the absence of acute inflammation. Using the taiep rat and a combination of X-irradiation and cell transplantation, it has been possible to address a number of questions concerning remyelination failure in chronic multiple sclerosis lesions, such as whether chronically demyelinated axons have undergone changes that render them refractory to remyelination and why remyelination is absent when oligodendrocyte progenitor cells (OPCs) are present. Our experiments show that (i) transplanted OPCs will not populate OPC-containing areas of chronic demyelination; (ii) myelination competent OPCs can repopulate OPC-depleted chronically demyelinated astrocytosed tissue, but this repopulation does not result in remyelination--closely resembling the situation found in some multiple sclerosis plaques; and (iii) the induction of acute inflammation in this non-remyelinating situation results in remyelination. Thus, we can conclude that axonal changes induced by chronic demyelination are unlikely to contribute to remyelination failure in multiple sclerosis. Rather, remyelination fails either because OPCs fail to repopulate areas of demyelination or because if OPCs are present they are unable to generate remyelinating oligodendrocytes owing to the presence of inhibitory factors and/or a lack of the stimuli required to activate these cells to generate remyelinating oligodendrocytes. This non-remyelinating situation can be transformed to a remyelinating one by the induction of acute inflammation.

Acute Disease↗

PubMed2005

Repopulation of oligodendrocyte progenitor cell depleted tissue in a model of chronic demyelination.

Some, but not all chronically demyelinated MS lesions are depleted of oligodendrocyte progenitor cells (OPCs) suggesting that OPCs are destroyed during the process of demyelination and some factor impedes OPC repopulation of the depleted tissue. The chronically demyelinated axons in MS lie in an astrocytic environment and it has been proposed that this might impede entry of OPCs into such regions. By depleting a short length of spinal cord of its OPCs using 40 Gy of X-irradiation in both normal rats and rats with progressive myelin loss accompanied by an astrocytosis (taiep rats), we investigated whether such changes affect the ability of OPCs to repopulate OPC-depleted tissue. In both taiep and normal rats, the rate of repopulation decreases with age, but no difference was detected in the rate at which OPCs repopulated normally myelinated and chronically demyelinated and astrocytosed tissue. This indicates that, if the astrocytic environment of the taiep CNS is comparable to that found in MS lesions, then the presence of chronically demyelinated axons and astrocytosis in chronic MS lesions does not represent a barrier to repopulation of the tissue by OPCs. However, similar to the situation in the normal adult rodent CNS, the rate of repopulation by endogenous OPCs in aged taiep rats is very slow, approximately 0.2 mm per week.

Age Factors↗

PubMed2005

The case for a central nervous system (CNS) origin for the Schwann cells that remyelinate CNS axons following concurrent loss of oligodendrocytes and astrocytes.

In certain experimental and naturally occurring pathological situations in the central nervous system (CNS), demyelinated axons are remyelinated by Schwann cells. It has always been assumed that these Schwann cells are derived from Schwann cells associated with peripheral nerves. However, it has become apparent that CNS precursors can give rise to Schwann cells in vitro and following transplantation into astrocyte-free areas of demyelination in vivo. This paper compares the behaviour of remyelinating Schwann cells following transplantation of peripheral nerve derived Schwann cells over, and into, astrocyte-depleted areas of demyelination to that which follows transplantation of CNS cells and that seen in normally remyelinating ethidium bromide induced demyelinating lesions. It concludes that while the examination of normally remyelinating lesions can not resolve the origin of the remyelinating Schwann cells, the results from transplantation studies provide strong evidence that the Schwann cells that remyelinate CNS axons are most likely generated from CNS precursors. In addition these studies also indicate that the precursors that give rise to these Schwann cells are the same cells that give rise to remyelinating oligodendrocytes.

Animals↗

PubMed2005

Repopulation of oligodendrocyte progenitor cell-depleted tissue in a model of chronic demyelination.

Some, but not all, chronically demyelinated multiple sclerosis (MS) lesions are depleted of oligodendrocyte progenitor cells (OPCs) suggesting that OPCs are destroyed during the process of demyelination and some factor impedes OPC repopulation of the depleted tissue. The chronically demyelinated axons in MS lie in an astrocytic environment and it has been proposed that this might impede entry of OPCs into such regions. By depleting a short length of spinal cord of its OPCs using 40 Gy of X-irradiation in both normal rats and rats with progressive myelin loss accompanied by an astrocytosis (taiep rats), we investigated whether such changes affect the ability of OPCs to repopulate OPC-depleted tissue. In both taiep and normal rats, the rate of repopulation decreases with age, but no difference was detected in the rate at which OPCs repopulated normally myelinated and chronically demyelinated and astrocytosed tissue. This indicates that, if the astrocytic environment of the taiep central nervous system (CNS) is comparable to that found in MS lesions, then the presence of chronically demyelinated axons and astrocytosis in chronic MS lesions does not represent a barrier to repopulation of the tissue by OPCs. However, similar to the situation in the normal adult rodent CNS, the rate of repopulation by endogenous OPCs in aged taiep rats is very slow, approximately 0.2 mm per week.

Age Factors↗

PubMed2004

The number of cells expressing the myelin-supporting oligodendrocyte marker PLP-exon 3b remains unchanged in Wallerian degeneration.

Following spinal cord trauma there is controversy as to whether myelin-supporting oligodendrocytes at a distance from areas of spinal cord damage undergo apoptosis. To examine the response of oligodendrocytes to axon degeneration, we counted the number of oligodendrocytes and oligodendrocyte precursors in the dorsal funiculi during the course of Wallerian degeneration. Axons were disrupted at T13 and the number of labelled cells in the dorsal funiculi counted at T12, 4 days and 2, 4, and 8 weeks after injury using riboprobes to exon-3b of the PLP gene whose expression is considered to restricted to myelin-supporting oligodendrocytes, PDGFRalpha which is regarded as a marker of oligodendrocyte precursors, and MOG a marker previously used to identify myelin-supporting oligodendrocytes. We found that the number of PLP-exon-3b labelled cells remained constant during the course of Wallerian degeneration while the number of cells labelled with the riboprobes to PDGFRalpha and MOG increased. Significantly the number of MOG-positive cells was increased at times when the number of PDGFRalpha labelled cells was highest. The number of PDGFRalpha labelled cells decreased with time while the number PLP-exon-3b labelled cells remained constant. It is therefore possible that the apoptotic oligodendrocytes identified in previous studies could represent degenerating oligodendrocyte precursors or their progeny rather than degenerating myelin-supporting oligodendrocytes.

Animals↗

PubMed2004

The remyelinating potential and in vitro differentiation of MOG-expressing oligodendrocyte precursors isolated from the adult rat CNS.

There is a long-standing controversy as to whether oligodendrocytes may be capable of cell division and thus contribute to remyelination. We recently published evidence that a subpopulation of myelin oligodendrocyte glycoprotein (MOG)-expressing cells in the adult rat spinal cord co-expressed molecules previously considered to be restricted to oligodendrocyte progenitors [G. Li et al. (2002) Brain Pathol., 12, 463-471]. To further investigate the properties of MOG-expressing cells, anti-MOG-immunosorted cells were grown in culture and transplanted into acute demyelinating lesions. The immunosorting protocol yielded a cell preparation in which over 98% of the viable cells showed anti-MOG- and O1-immunoreactivity; 12-15% of the anti-MOG-immunosorted cells co-expressed platelet-derived growth factor alpha receptor (PDGFRalpha) or the A2B5-epitope. When cultured in serum-free medium containing EGF and FGF-2, 15-18% of the anti-MOG-immunosorted cells lost anti-MOG- and O1-immunoreactivity and underwent cell division. On removal of these growth factors, cells differentiated into oligodendrocytes, or astrocytes and Schwann cells when the differentiation medium contained BMPs. Transplantation of anti-MOG-immunosorted cells into areas of acute demyelination immediately after isolation resulted in the generation of remyelinating oligodendrocytes and Schwann cells. Our studies indicate that the adult rat CNS contains a significant number of oligodendrocyte precursors that express MOG and galactocerebroside, molecules previously considered restricted to mature oligodendrocytes. This may explain why myelin-bearing oligodendrocytes were considered capable of generating remyelinating cells. Our study also provides evidence that the adult oligodendrocyte progenitor can be considered as a source of the Schwann cells that remyelinate demyelinated CNS axons following concurrent destruction of oligodendrocytes and astrocytes.

Animals↗

PubMed2003

Dysfunctional oligodendrocyte progenitor cell (OPC) populations may inhibit repopulation of OPC depleted tissue.

We have attempted to extend a previously described rat model of focal oligodendrocyte progenitor cell (OPC) depletion, using 40 Gy X-irradiation (Chari and Blakemore [2002] Glia 37:307-313), to the adult mouse spinal cord, to examine the ability of OPCs present in adjacent normal areas to colonise areas of progenitor depletion. In contrast to rat, OPCs in the mouse spinal cord appeared to be a comparatively radiation-resistant population, as 30-35% of OPCs survived in X-irradiated tissue (whereas <1% of OPCs survive in X-irradiated rat spinal cord). The numbers of surviving OPCs remained constant with time indicating that this population was incapable of regenerating itself in response to OPC loss. Additionally, these OPCs did not contribute to remyelination of axons when demyelinating lesions were placed in X-irradiated tissue, suggesting that the surviving cells are functionally impaired. Importantly, the length of the OPC-depleted area did not diminish with time, as would be expected if progressive repopulation of OPC-depleted areas by OPCs from normal areas was occurring. Our findings therefore raise the possibility that the presence of a residual dysfunctional OPC population may inhibit colonisation of such areas by normal OPCs.

Animals↗

PubMed2003

Neuropathological assessment of artemether-treated severe malaria.

In animals, high doses of intramuscular artemether and artemotil have been shown to cause an unusual pattern of selective damage to certain brainstem nuclei, especially those implicated in hearing and balance. We aimed to investigate whether a similar pattern arises in human adults. We examined the brainstems of adults who died after treatment with high dose artemether or quinine for severe falciparum malaria for evidence of a pattern of selective neuronal damage. Neuropathological findings were similar in recipients of quinine (n=15) and artemether (n=6; total artemether doses received 4-44 mg/kg). No evidence was recorded for artemether-induced neurotoxic effects.

Adult↗

PubMed2003

Decline in rate of colonization of oligodendrocyte progenitor cell (OPC)-depleted tissue by adult OPCs with age.

Rates of remyelination decline with age and this has been attributed to slower recruitment of oligodendrocyte progenitor cells (OPCs) into areas of demyelination and slower differentiation of OPCs into remyelinating oligodendrocytes. When considering causes for reduced recruitment rates, intrinsic causes (alterations in biological properties of OPCs) need to be separated from extrinsic causes (age-related differences in the lesion environment). Using 40 Gy of X-irradiation to deplete tissue of its endogenous OPC-population, we examined the effects of age on the rate at which adult rat OPCs colonize OPC-depleted tissue. We found a significant reduction in the rate of colonization between 2 and 10 months of age (0.6 mm/week versus 0.38 mm/week). To determine if this represented an intrinsic property of OPCs or was due to changes in the environment that the cells were recolonizing, OPCs from 10-month-old animals were transplanted into 2-month-old hosts and OPCs from 2-month-old animals were transplanted into 10-month-old hosts. These experiments showed that the transplanted OPCs retained their age-related rate of colonization, indicating that the decline in colonizing rates of OPCs with age reflects an intrinsic property of OPCs. This age-related decline in the ability of OPCs to repopulate OPC-depleted tissue has implications for understanding remyelination failure in multiple sclerosis (MS) and developing therapies for remyelination failure.

Aging↗

PubMed2002

Schwann cell remyelination is not replaced by oligodendrocyte remyelination following ethidium bromide induced demyelination.

Demyelinated CNS axons can be remyelinated by Schwann cells. A recent study concluded that with time Schwann cell remyelination is replaced by oligodendrocyte remyelination [9]. To examine this, the extent of Schwann cell and oligodendrocyte remyelination at 4, 6.5 and 24 weeks was determined for ethidium bromide lesions made in the spinal cords of rats. Although the extent of oligoden-drocyte remyelination increased with time there was no significant change in the amount of Schwann cell remyelination. This indicates that Schwann cell remyelination is stable and is not replaced by oligodendrocyte remyelination.

Animals↗

PubMed2002

Modelling large areas of demyelination in the rat reveals the potential and possible limitations of transplanted glial cells for remyelination in the CNS.

Transplantation of myelin-forming glial cells may provide a means of achieving remyelination in situations in which endogenous remyelination fails. For this type of cell therapy to be successful, cells will have to migrate long distances in normal tissue and within areas of demyelination. In this study, 40 Gy of X-irradiation was used to deplete tissue of endogenous oligodendrocyte progenitors (OPCs). By transplanting neonatal OPCs into OPC-depleted tissue, we were able to examine the speed with which neonatal OPCs repopulate OPC-depleted tissue. Using antibodies to NG-2 proteoglycan and in situ hybridisation to detect platelet-derived growth factor alpha-receptor Ralpha (PDGFRalpha) mRNA to visualise OPCs, we were able to show that neonatal OPCs repopulate OPC-depleted normal tissue 3-5 times more rapidly than endogenous OPCs. Transplanted neonatal OPCs restore OPC densities to near-normal values and when demyelinating lesions were made in tissue into which transplanted OPCs had been incorporated 1 month previously, we were able to show that the transplanted cells retain a robust ability to remyelinate axons after their integration into host tissue. In order to model the situation that would exist in a large OPC-depleted area of demyelination such as may occur in humans; we depleted tissue of its endogenous OPC population and placed focal demyelinating lesions at a distance (< or =1 cm) from a source of neonatal OPCs. In this situation, cells would have to repopulate depleted tissue in order to reach the area of demyelination. As the repopulation process would take time, this model allowed us to examine the consequences of delaying the interaction between OPCs and demyelinated axons on remyelination. Using this approach, we have obtained data that suggest that delaying the time of the interaction between OPCs and demyelinated axons restricts the expression of the remyelinating potential of transplanted OPCs.

Animals↗

PubMed2002

New insights into remyelination failure in multiple sclerosis: implications for glial cell transplantation.

This review considers aspects of remyelination that require further clarification if successful strategies are to be devised to enhance remyelination in multiple sclerosis (MS). We speculate, based on our understanding of the rate with which oligodendrocyte progenitor cells (OPCs) repopulate OPC-depleted tissue in adult rats, that OPC depletion during the demyelination process could explain why remyelination fails in MS. We show that loss of OPCs in the context of large areas of demyelination would have serious consequences for remyelination as the rates of colonization of tissue by adult OPCs would lead to a situation where the cellular events associated with demyelination become uncoupled from the interaction of OPCs with demyelinated axons. Experimental studies indicate that transplanted neonatal OPCs would be able to repopulate large areas of demyelination with much greater efficiency than endogenous OPCs. This suggests that cell transplantation will have considerable potential to achieve remyelination in situations where the endogenous repair process is failing due to concurrent death of oligodendroytes and OPCs. However, we suggest that for this approach to be effective, it will be critical that the environment is permissive for remyelination.

Animals↗

PubMed2001

Depletion of endogenous oligodendrocyte progenitors rather than increased availability of survival factors is a likely explanation for enhanced survival of transplanted oligodendrocyte progenitors in X-irradiated compared to normal CNS.

Oligodendrocyte progenitors (OPs) survive and migrate following transplantation into adult rat central nervous system (CNS) exposed to high levels of X-irradiation but fail to do so if they are transplanted into normal adult rat CNS. In the context of developing OP transplantation as a potential therapy for repairing demyelinating diseases it is clearly of some importance to understand what changes have occurred in X-irradiated CNS that permit OP survival. This study addressed two alternative hypotheses. Firstly, X-irradiation causes an increase in the availability of OP survival factors, allowing the CNS to support a greater number of progenitors. Secondly, X-irradiation depletes the endogenous OP population thereby providing vacant niches that can be occupied by transplanted OPs. In situ hybridization was used to examine whether X-irradiation causes an increase in mRNA expression of five known OP survival factors, CNTF, IGF-I, PDGF-A, NT-3 and GGF-2. The levels of expression of these factors at 4 and 10 days following exposure of the adult rat spinal cord to X-irradiation remain the same as the expression levels in normal tissue. Using intravenous injection of horseradish peroxidase, no evidence was found of X-irradiation-induced change in blood-brain barrier permeability that might have exposed X-irradiated tissue to serum-derived survival factors. However, in support of the second hypothesis, a profound X-irradiation-induced decrease in the number of OPs was noted. These data suggest that the increased survival of transplanted OPs in X-irradiated CNS is not a result of the increases in the availability of the OP survival factors examined in this study but rather the depletion of endogenous OPs creating 'space' for transplanted OPs to integrate into the host tissue.

Animals↗

PubMed2001

Remyelination of demyelinated CNS axons by transplanted human schwann cells: the deleterious effect of contaminating fibroblasts.

Areas of demyelination can be remyelinated by transplanting myelin-forming cells. Schwann cells are the naturally remyelinating cells of the peripheral nervous system and have a number of features that may make them attractive for cell implantation therapies in multiple sclerosis, in which spontaneous but limited Schwann cell remyelination has been well documented. Schwann cells can be expanded in vitro, potentially affording the opportunity of autologous transplantation; and they might also be spared the demyelinating process in multiple sclerosis. Although rat, cat, and monkey Schwann cells have been transplanted into rodent demyelinating lesions, the behavior of transplanted human Schwann cells has not been evaluated. In this study we examined the consequences of injecting human Schwann cells into areas of acute demyelination in the spinal cords of adult rats. We found that transplants containing significant fibroblast contamination resulted in deposition of large amounts of collagen and extensive axonal degeneration. However, Schwann cell preparations that had been purified by positive immunoselection using antibodies to human low-affinity nerve growth factor receptor containing less than 10% fibroblasts were associated with remyelination. This result indicates that fibroblast contamination of human Schwann cells represents a greater problem than would have been appreciated from previous studies.

Animals↗

PubMed2000

Transplanted glial cells migrate over a greater distance and remyelinate demyelinated lesions more rapidly than endogenous remyelinating cells.

Glial cell transplantation offers a means of remyelinating areas of demyelination in situations where endogenous remyelination fails. How effective such a strategy would be if undertaken in human demyelinating disease is not yet clear since it is very difficult to create large areas of demyelination in adult rodents that would mimic the situation found in a human disease such as multiple sclerosis. When CNS tissue is subjected to 40 Grays of X-irradiation, remyelination is suppressed in the X-irradiated area unless cells migrate into, or are introduced into the X-irradiated area. In the present experiments, by appropriate positioning of lead shielding we have created a "starting gate" from which oligodendrocyte progenitors must depart in order to colonise areas of demyelination. When the starting gate is located at one end of the area of demyelination, endogenous cells fail to colonise throughout an area of demyelination over the ensuing month. In contrast, when transplanted oligodendrocyte precursors are faced with the same situation, the whole area of demyelination is remyelinated over the same period. To determine how far transplanted cells can migrate to areas of demyelination and also to study how quickly the cells can colonise areas of demyelination we injected cells at some distance from areas of demyelination made in X-irradiated tissue. In these experiments, we found that transplanted cells could repopulate up to 9 mm in 2 months compared to 4 mm recorded for endogenous cells (Franklin et al. [1997] J. Neurosci. Res. 50:337-344). These experiments demonstrate that transplanted cells have a far greater ability to colonise areas of demyelination than endogenous cells.

Animals↗