. Extracellular amyloid formation and associated pathology in neural grafts. Nat Neurosci. 2003 Apr;6(4):370-7. PubMed.

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  1. Alzheimer’s Disease-like Molecular Pathology in Neural Grafts

    Meyer-Luehmann and colleagues published an interesting paper describing the extracellular formation of amyloid plaques and pathology in a neural graft model. The authors transplanted cell suspensions of embryonic cortical and hippocampal tissue from AβPP23-transgenic mice into three-month-old wild-type and transgenic hosts. Such transgenic grafts into wild-type animals showed no amyloid deposits up to 20 months after transplantation. The grafted neurons showed no reduction in hAβPP expression, Aβ levels, or evidence of a humoral response to the grafted tissue. However, when transgenic or wild-type tissue was transplanted into transgenic hosts, amyloid deposition was observed as soon as three months after transplantation. Reactive gliosis was observed at the host-graft interface. These Aβ deposits were surrounded by activated astrocytes and microglia, dystrophic synaptic boutons, and abnormally stained acetylcholine-positive fibers. Some of these dystrophic neurons also showed evidence of abnormal tau phosphorylation. The presence of Aβ deposits in grafts derived from wild-type vs. the absence of Aβ in transgenic grafts into wild-type mice lead the authors to suggest that there was extracellular diffusion of soluble and insoluble Aβ into and out of the grafted tissue, respectively, in these two graft conditions. These results suggest that human Aβ‚ together with other unknown factors that influence its diffusion, clearance and deposition, may be central to the pathogenesis of Alzheimer’s disease (AD).

    We would like to draw attention to Sykovà et al., 1999 (2) and Martins et al., 2001 (3), studies that also describe a fetal neural transplantation model. Transplants of fetal cerebral cortex onto the midbrain region of neonatal rat hosts results in extensive and chronic reactive gliosis within the grafted tissue. This gliosis is evident as early as one month after transplantation, and is associated with altered AβPP metabolism, a decrease in presenilin-1 levels, and an increase in apolipoprotein E (ApoE) protein levels. Furthermore, altered extracellular matrix deposition and a change in the diffusion characteristics of the extracellular space are observed. Conversely, when the same embryonic tissue is transplanted homotopically into the cortex of neonatal rats, minimal gliosis is observed and no consequent change in AβPP metabolism occurs.

    The graft environment mirrored what is observed in the AD brain. However, rodent Aβ is less toxic and less fibrillogenic than human Aβ (4), so we extended our chronic gliosis model to Tg 2576 transgenic mice that express human Aβ (5). This transgenic tissue was grafted into wild-type hosts and also into ApoE knockout hosts. We again observed chronic gliosis in a region-specific manner, which was associated with altered AβPP metabolism and a significant increase in levels of the C-99 fragment, the precursor of Aβ. Consistent with Meyer-Luehmann et al., after 10 months we found no frank Aβ deposition in transgenic grafts into wild-type hosts. However, our data do suggest that chronic reactive gliosis may alter the metabolism of AβPP to favor Aβ production. As such, the phenomena of astrocytic and microglial reactivity found in AD brains may be a primary pathological trigger, rather than a secondary response that facilitates plaque formation.

    Meyer-Luehmann and colleagues also investigated the possibility that activated glia in wild-type grafts may be responsible for Aβ deposits. They performed a stab injury to the hippocampal region of six-month-old transgenic mice and examined the material three months later. No Aβ deposits were observed, despite some microgliosis, suggesting that a single episode of surgical trauma per se was an inadequate insult to trigger Aβ deposition. However, this is consistent with a stab injury being associated with an acute effect on gliosis and inflammation (6,7), unlike the sustained and chronic reactivity observed in our transplantation model.

    It is apparent from these studies that Aβ deposition is a region-specific event. In our neonatal transplant model, altered Aβ metabolism and reactive gliosis is only observed in cortical grafts to the midbrain. Meyer-Luehmann et al. observed Aβ deposition in areas known to exhibit the most severe neuropathology, such as the hippocampus and thalamus in the AβPP23 mouse. Aβ deposits were not seen in grafts placed on the striatum, a region of relatively low AD neuropathology.

    Such transplantation studies provide a useful tool to study the mechanisms of reactive gliosis and the pathogenesis of AD. Further studies are clearly warranted into the transport of all forms of Aβ between different brain regions, Aβ clearance mechanisms, and the role of the blood-brain barrier in this process. In this regard, molecular chaperones such as the major AD genetic risk factor, ApoE, may play key roles in isoform-specific binding and clearance of Aβ (8,9). This process also depends on the amount of ApoE produced (10), which is most pertinent to the region-specific effects described above.

    The extracellular milieu may also be integral to the pathogenesis of AD. Chronic gliosis is associated with altered production of extracellular matrix components, and this, coupled with hypertrophied astrocytic processes, may affect the diffusion of neurotransmitters, and other substances such as Aβ. It is becoming clear that reactive gliosis is a complex event, as evidenced by the fine balance between pro- and antiinflammatory signals in normal brain. One can easily envisage a situation where an accumulation of insults over time may disturb this balance, resulting in the cascade of neurodegenerative events observed in AD. Transplantation studies should facilitate investigation into the process of reactive gliosis, the effect of molecular chaperones and the extracellular space in Aβ clearance, and the toxic effects of Aβ on both neurons and glial cells. Thus, transplantation studies should provide valuable insights into the molecular mechanisms of AD pathogenesis and serve as an excellent model for the evaluation of potential therapeutic agents.

    References:
    1. Meyer-Luehmann, M et al. Extracellular amyloid formation and associated pathology in neural grafts. Nat. Neurosci. 2003 Apr;6:370-377. Abstract

    2. Sykovà E, Roitbak T, Mazel T, Simonova Z, Harvey AR. Astrocytes, oligodendroglia, extracellular space volume and geometry in rat fetal brain grafts. Neuroscience 1999;91(2):219-234. Abstract

    3. Martins RN et al. Altered expression of apolipoprotein E, amyloid precursor protein and presenilin-1 is associated with chronic reactive gliosis in rat cortical tissue. Neuroscience 2001;106(3):555-567. Abstract

    4. Dyrks Y, Dyrks E, Masters CL. and Beyreuther, K. Amyloidogenicity of rodent and human beta A4 sequences. FEBS Lett. 1993;324:231-236. Abstract

    5. Bates KA, Fonte J, Robertson T, Martins RN and Harvey AR. Chronic gliosis triggers Alzheimer disease-like processing of amyloid precursor protein. Neuroscience. 2002;113(4):785-796. Abstract

    6. Amat JA, Ishiguro H, Nakamura K and Norton WT. Phenotypic diversity and kinetics of proliferating microglia and astrocytes following cortical stab wounds. Glia. 1996 Apr;16(4):368-82. Abstract

    7. Roitbak T and Sykovà E. Diffusion barriers evoked in rat cortex by reactive astrogliosis. Glia. 1999 Oct;28(1):40-48. Abstract

    8. Yang DS, Smith JD, Zhou Z, Gandy SE and Martins RN. Characterization of the binding of amyloid-beta peptide to cell culture-derived native apolipoprotein E2, E3, and E4 isoforms and to isoforms from human plasma. J Neurochem. 1997 Feb;68(2):721-5. Abstract

    9. Yang DS et al. Apolipoprotein E promotes the binding and uptake of beta-amyloid into Chinese hamster ovary cells in an isoform-specific manner. Neuroscience. 1999;90(4):1217-1226. Abstract

    10. Laws SM, Hone E, Gandy S and Martins RN. Expanding the association between the ApoE gene and the risk of Alzheimer's disease: possible roles for ApoE promoter polymorphisms and alterations in ApoE transcription. J Neurochem. 2003 Mar;84(6):1215-1236. Abstract

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