Suboptimal clinical outcomes are likely to correlate with poor graft survival and function. As suggested by the analysis of post-mortem transplanted brain tissue, various disease-related factors acting in concert may have provided an inhospitable milieu for the grafted tissue, namely (1) an excitotoxic effect exacerbated by the host cortical projections neurones onto the grafted tissue and (2) an impaired uptake of the glutamate excess by astrocytes; (3) poor graft–host interaction; (4) a significant microglial response cuffing check details the grafts; (5) the lack of neurotrophic support; and finally (6) the paucity of blood vessels within the graft (Figure 1). Taken together,

the latter evidence suggests that the negative impact of the pathological environment on graft

survival exceeds any benefit that might be gained from the graft against the disease. Huntington’s disease brains are characterized by abnormal levels of glutamate, especially in the striatum [57] and the impairment of glutamate PD-0332991 cost reuptake mechanisms may play a significant role in striatal neuronal degeneration [58,59]. Synaptic contacts are known to form between glutamatergic axon varicosities and grafted cells, as confirmed both by immunohistochemistry and electron microscopy. Within the graft, these contacts are more abundant onto striatal projection neurones which normally receive cortical glutamatergic innervation [43] (Figure 1). We also observed that the grafts are strikingly more affected by pathological processes than the host striatum, notwithstanding the fact that the grafts are younger and genetically unrelated to the HD patient and that they have been exposed to the disease for only a decade. Instead of a positive influence of grafts on the cortex, the pathology affecting the cortex

appears to induce neuronal degeneration within the grafts [43]. Despite recent evidence supporting the latter hypothesis in animal studies [60], the functional significance of this interaction remains unknown. It is also possible that glutamate is not the sole agent of striatal excitotoxicity [61,62]. For instance, dopamine released by nigral dopaminergic projections might act concomitantly with glutamate to GABA Receptor generate oxidative stress and modulate glutamate release itself [63]. In fact, decortication or 6-hydrohydopamine lesioning of the substantia nigra in R6/2 mice, a model of HD, leads to behavioural improvements and significant increases in longevity. Animals also exhibit lower striatal glutamate concentrations, suggesting overall that the cortical and nigral pathways may act synergistically to induce excitotoxicity [60]. Astrocytes are key players in glutamate uptake and clearance, which takes place mainly via the gap junction [64].