It is not known exactly when the improvements

It is not known exactly when the improvements start after MSC transplantation. We found no differences in the striatal neurodegeneration of transplanted and non-transplanted lesioned rats 24h after the transplantation procedure. Thus, a gap might be necessary between cell transplantation and detectable neuroprotective effects. Since MSC-mediated neuroprotection is related to their anti-inflammatory effects and secretion of growth factors, 24h may not be sufficient for MSC to produce substantial amounts of soluble factors and/or be stimulated by the microenvironment to produce further improvement. It is also possible that MSC transplantation protected the partially integrated tissue and neurons that undergo secondary death, but not neurons that die rapidly after QUIN injection. Another potential explanation for the absence of neuroprotection 24h after QUIN lesion might be that transplanted MSC act through interference with a progressive inflammatory process that begins after the initial damage. Our group has recently shown that cell therapy has neuroprotective effects related to reduced microglial activation in hypoxic–ischemic tlr inhibitor damage (Pimentel-Coelho et al., 2010) and reduced astrogliosis after optic nerve crush (Zaverucha-do-Valle et al., 2011). These data corroborate the idea that stress signals in the lesioned tissue can induce MSC to produce a more anti-inflammatory phenotype (Ohtaki et al., 2008). Astrocytes and microglia responses are characteristic in HD models and patients. It was previously shown that in QUIN-lesioned rats, the density of reactive astrogliosis corresponds to the local severity of the neurodegenerative process (Guncova et al., 2011) and in HD patients the microglial activation correlates with severity (Pavese et al., 2006). Thus, the inflammatory processes after MSC transplantation in HD models remain an important issue for further investigation. In HD, the cerebral tissue loss is compensated by the enhancement of the ventricular area, and ventriculomegaly is indicative of a progressive neurodegeneration (Guncova et al., 2011). We found reduced ventriculomegaly 9months after QUIN injection and MSC transplantation. This finding is in accordance with the previous data that late transplantation of MSC leads to reversal of striatal atrophy (Amin et al., 2008), and suggests that the neuroprotective effects of MSC are not transitory. The observed neuroprotective effect of MSC transplantation is also in accordance with previous evidences that bone marrow stem cell transplantation leads to improved motor skills in animal models of HD (Edalatmanesh et al., 2010; Jiang et al., 2011; Lescaudron et al., 2003). We observed a discrete enhancement of FGF-2, but not BDNF striatal expression 7days after lesion in treated rats. FGF-2 is expressed constitutively in the brain (Baird and Walicke, 1989), and can be secreted by cells of the damaged tissue (Reuss and von Bohlen und Halbach, 2003). FGF-2 is neuroprotective in many neurological diseases (Reuss and von Bohlen und Halbach, 2003) and after ischemia it prevents neuronal death in a dose-dependent manner (Nakata et al., 1993). It is known that FGF-2 can reduce cellular death in striatal cultures of the R6/2 transgenic model of HD (K. Jin et al., 2005), and intracerebral injection of this factor leads to functional improvements in R6/2 mice (K. Jin et al., 2005). Our group has previously shown that bone-marrow mononuclear cells can increase neuroprotection, neuroregeneration, and FGF-2 expression in a model of optic nerve crush (Zaverucha-do-Valle et al., 2011). Thus, it is possible that the neuroprotection after QUIN injection is, at least in part, mediated by FGF-2 secreted by transplanted MSC, but it is unlikely that this is the only factor to play a role. FGF-2 activates a range of signal transduction pathways, among which the phosphatidylinositol 3′-kinase/Akt pathway is related to cell survival (K. Jin et al., 2005). We observed a considerable and significant enhancement in SVZ cell proliferation after QUIN injection, in both transplanted and non-transplanted rats. Many investigators have described enhanced proliferation and/or altered SVZ migration in response to brain lesions, including HD (Curtis et al., 2003; Moraes et al., 2009), ischemia (Liu et al., 1998), Alzheimer\'s (K. Jin et al., 2004) and Parkinson\'s (Zhao et al., 2003) diseases (Batista et al., 2006; Goings et al., 2004), giving rise to the possibility of cell replacement from endogenous precursors (Arvidsson et al., 2002). The observed proliferative response in the SVZ of QUIN-lesioned rats was previously correlated with stem cell-factor release from the lesioned striata, which activates the c-kit receptor in neural stem cells and a signaling pathway associated with cellular proliferation and migration (Bantubungi et al., 2008). Although we did not find proliferative modulation associated with MSC transplantation in the SVZ, our analysis was time-limited, and we do not reject the hypothesis that MSC can affect the SVZ microenvironment. One possibility is that the newly generated cells in the SVZ survive longer after MSC transplantation. In accordance with this view, previous findings showed that MSC can enhance neuroblast proliferation and differentiation in the striata of R6/2 mice (Lin et al., 2011). Indeed, we observed Ki-67-positive cells in the striata of both transplanted and non-transplanted QUIN-lesioned rats (data not shown) and it is possible that some of these cells were neuroblasts.