In addition, grafted neuronal elements were closely

associated LDE225 supplier with microglial cells. However, microglia play a dual role and can exert both beneficial or detrimental effects on grafted neurones [78,82]. Resting microglia may have a beneficial role by providing neurotrophic support or sensing the environment to clear cell debris and misfolded proteins [78]. On the other hand, they can migrate to the site of injury and release various pro-inflammatory factors, which can become detrimental when delivered in a chronic and uncontrolled fashion [83–85]. It should be noted that similar observations were made in a transplanted PD patient where solid tissue grafts were also surrounded by an inflammatory response as early as 18 months after surgery [86]. Adequate trophic support is also necessary for graft survival [82,87]. Several studies in PD and HD animal models have repeatedly emphasized that only low number of cells survive transplantation [88–90]. The reason for this is not well understood but it has been hypothesized that deficient trophic support may be implicated. For example, both pretreatment

of cells derived from foetal ventral mesencephalon with basic fibroblast growth factor (bFGF) or glial cell line-derived neurotrophic factor (GDNF) and repeated parenchymal infusion of trophic factors in transplanted 6-hydroxydopamine (6-OHDA)-lesioned Bay 11-7085 rats significantly ameliorate dopaminergic cell survival and promote Silmitasertib mouse fibre outgrowth [91–93]. GDNF pretreatment of embryonic dopaminergic

cells implanted in PD patients showed good survival and led to beneficial effects clinically [94]. However, animal studies in 6-OHDA-lesioned mice have reported that implanted dopaminergic cells pretreated with brain-derived neurotrophic factor (BDNF) show a lower survival rate, albeit leading to improved behavioural recovery [95]. Importantly, treatment with trophic factors favour a TH phenotype in foetal cells in vitro [96,97]. BDNF has been shown to promote survival and to afford protection from excitotoxicity both in vitro [98,99] and in vivo [100]. In normal conditions, BDNF is highly expressed in the cortex, especially in layer V, and retrogradely transported to the striatum [101,102]. The expression of mHtt interferes both with normal BDNF transcription [103] and with the transport of vesicles along the microtubules [82,104]. As mHtt aggregates are found especially in layer V [43] of the cortex, which projects onto the graft p-zones [43,105], grafts may not receive adequate trophic support, making the grafted cells more susceptible to harmful factors derived from the diseased brain (Figure 1). In their report, Keene et al.