Endogenous neural stem cells are able to engage in reparative functions10, 11
. Consistent with this role, we found that NSCs proliferate and engage in repair during the acute phase of EAE, but this capacity is lost during the chronic phase of the disease. The enhanced NSC proliferation seen in this model during acute disease is in contrast with our previous report in the B6 mouse EAE model13
, and in keeping with the prominent forebrain and periventricular pathology in SJL as well as the relapsing nature of the disease. Proliferation of progenitors and enlargement of the SVZ have been reported in lysolecithin-induced demyelination3
and TNF injected EAE animals12
. Other studies in mice with EAE have reported transiently-increased proliferation of PSA-NCAM+ and NG2+ cells in the corpus callosum, which the authors inferred as indirect evidence of enhanced proliferation of stem and precursor cells in the SVZ8, 39, 40
. However, none of these papers has evaluated the behavior of NSCs during the course of relapsing disease, or its relationship to microglia in the niche.
After acute CNS injury, surviving cells produce mediators that enhance stem cell function as reported in stroke models41–45
. Thus, our finding of enhanced NSC proliferation in acute disease is consistent with other models of CNS injury. The interesting finding of decreased proliferative response in the SVZ during the chronic phase of EAE prompted our hypothesis that chronic activation of glial cells induces a transition from a permissive to a non-permissive microenvironment that can affect stem cells. Although we do not discount a role for astrocytes46
, we focused our attention on microglia given our observation of activated microglia during the chronic phase of EAE14
and as reported in MS16
There are conflicting reports on the effect of microglia on NSC function with some reports suggesting a deleterious role21
while others suggest a beneficial role20, 47
. Our data suggest that microglia have both beneficial and deleterious roles at different phases of disease, and that manipulation of microglia activation may impact the repair potential in the CNS. In addition to SVZ progenitors, there are widespread OPC progenitors in the CNS and our data is consistent with the idea that chronic microglia activation may affect OPCs outside of the SVZ impairing oligodendrocyte production.
Future studies are warranted to establish the molecular networks that govern the regenerative vs. degenerative phenotype of microglia during EAE.
Interestingly, we found that modulation of microglia activation even in naïve animals is associated with an increase in the numbers of proliferative SVZ cells (). Our data suggest an inverse relationship between the number of microglia and proliferation in the SVZ, and demonstrate that minocycline can decrease microglia numbers and their activation state in the SVZ, while enhancing proliferation in the SVZ neurogenic niche. These findings suggest that microglia in the SVZ may have a tonic inhibitory role on adult NSC proliferation. A similar effect has been suggested for macrophages/microglia during development48
. In addition chronic microglia activation leads to a non-permissive environment for repair, while specific inactivation of microglia with minocycline ameliorates the loss of reparative potential of endogenous stem cells.
Minocycline has also inhibitory effects on macrophage activation and has been reported to protect animals from EAE26
presumably by inhibiting peripheral immune cell activation. It can also inhibit matrix metalloproteinases and decrease T cell transmigration to the CNS49
. We have used minocycline after the acute phase of the disease to minimize the impact of therapy on peripheral immune activation, although we cannot discount a contribution of macrophage inactivation in our model. Inactivation of microglia may also downregulate expression of MHC class II, which could result in reduced T cell reactivation within the CNS, but we think this is unlikely to contribute to the observed benefits of minocycline here, since the number of T cells in the lesions is minimal during the chronic EAE14
In contrast to a previous report34
we did not find terminal differentiation of neurons in the SVZ. Difference in models and strains may explain this discrepancy. But this raises the question of the fate of progenitors that are generated during acute disease. Our data suggest that the cells remain in a state of altered differentiation and morphology but we cannot rule out the possibility of Dcx+ cell apoptosis since removal of dead cells in the CNS occurs rapidly. Our data show similarity to a recent report in MS tissue that demonstrates a limited expansion of progenitors cells and thickening of the SVZ1
Our findings have implications for therapeutic strategies targeting NSC mobilization from germinal niches by inhibiting microglia activation, but also suggest that caution should be exercised so as not to inhibit beneficial glial activation.