The potential of neural grafts to replace adult brain tissue in a structurally and
functionally relevant way is an essential prerequisite for reconstructive approaches.
While numerous studies have focused on the controlled derivation of neuronal subtypes
from human ES cells, the ability of these in vitro generated neurons to innervate
the host CNS has remained largely unexplored.
To evaluate the potential of human ES cell-derived neurons to differentiate and grow
projections in the adult brain, micrografts were stereotactically targeted at the
primary motor cortex or the hippocampus of rodents. Most donor neurons acquired an
inhibitory (GAD+) phenotype, excitatory (vGlut2+) neurons were, in both locations
present to a lesser extent. Six months after transplantation, no markers indicative
of immature cells or ongoing proliferation were detectable. Donor neurons with complex
morphologies were detected up until at least one year after transplantation without
any evidence of tumor formation.
Remarkably, engrafted human neurons frequently showed projection patterns comparable
to neighboring endogenous cells. Donor cells transplanted into the dentate gyrus preferentially
projected via the mossy fiber pathway to the ipsi-lateral CA3 region, where xenogeneic
synapses were identified. In addition, donor axons entered the fimbria, crossed to
the contra-lateral hemisphere and established contact with the contra-lateral hippocampus.
Human neurons engrafted in the motorcortex projected into the corpus callosum from
where they re-entered the cortex on the contra-lateral side, thereby mimicking the
projection pattern of commissural neurons. Subsets of human axons also entered the
capsula interna as well as the basal ganglia.
The ability of human ES cell-derived neurons to establish a region specific long-range
axonal projection profile in the adult brain suggests that these cells are responsive
to endogenous guidance cues and may thus be particularly suitable for circuit reconstruction.
Supported by the DFG and the Hertie Foundation.
