Cross-talk between stem cells and the dysfunctional brain is facilitated by manipulating the niche: evidence from an...

In the injured brain, the behavior of neural stem/progenitor cells (NSCs) is regulated by multiple converging factors encountered in the niche, which is composed of several neural and non-neural cell types. Signals emanating from the host influence the migration, survival, distribution, and fate of transplanted NSCs, which in turn can create host microenvironments that favor a return to homeostasis. We tested the hypothesis that overexpression of key facilitatory molecules that define the injury niche might enhance this bidirectional stem cell-host interaction to therapeutic advantage. As proof of concept, we investigated whether conditioning the niche with the neural cell adhesion molecule L1 might enhance recovery in a prototypical neurodegenerative milieu-the MPTP-induced model of Parkinson's disease in aged mice-where cross-talk between NSCs and imperiled host dopaminergic neurons is known to be pivotal in rescuing the function and connectivity of the latter. In lesioned mice (and in unlesioned controls), we overexpressed L1 in the NSCs to be transplanted into the ventral mesencephalon. Several pairwise experimental combinations were tested based on variations of engrafting L1 overexpressing versus nonoverexpressing NSCs into wild-type (WT) versus L1-overexpressing transgenic mice (specifically L1 transcribed from the GFAP promoter and, hence, overexpressed in host astrocytes). Enrichment for L1-particularly when expressed simultaneously in both donor NSCs and host brain-led to rapid and extensive distribution of exogenous NSCs, which in turn rescued (with an efficacy greater than in nonengineered controls) dysfunctional host dopaminergic nigral neurons, even when grafting was delayed by a month. L1 overexpression by NSCs also enhanced their own differentiation into tyrosine hydroxylase-expressing neurons in both WT and transgenic hosts. Graft-host interactions were thus favored by progressively increasing levels of L1. More broadly, this study supports the view that manipulating components of the niche (such as an adhesion molecule) that facilitate cross-talk between stem cells and the dysfunctional brain may offer new strategies for more efficacious neurotransplantation, particularly when treatment is delayed as in chronic lesions or advanced stages of a neurodegenerative disease.