Introduction: Stem cell transplantation has emerged as a promising new experimental treatment for stroke; understanding its mechanism of action will facilitate the translation of stem cell therapy to the clinic. Previous work suggests that stem cells function by enhancing endogenous brain repair processes including structural brain plasticity. We hypothesize that stem cells will enhance synaptic structural remodeling in the post-ischemic brain. To test this we use array tomography, a new high-resolution proteomic imaging method, to determine how the number and subtype of synapses are affected by transplantation of human neural progenitor cells (hNPCs).
Methods: Vehicle or hNPCs were transplanted into the ischemic cortex of Nude rats 7 days after distal middle cerebral artery occlusion. Behavior was evaluated weekly. For array tomography, small tissue sections were removed from the peri-infarct cortex at 1 week and 4 weeks post-transplantation and ribbons of serial ultrathin sections were obtained. Ribbons were stained with antibodies for synaptic proteins, images taken in cortical layer 2/3 and layer 5, and the resultant staining pattern analyzed to quantify different synapse sub-types.
Results: Transplantation of hNPCs significantly improved behavioral recovery after stroke compared to vehicle-treated rats (4 weeks post-transplantation; p<0.01).
hNPCs had no detectable effect on the density of GABAergic synapses in either layer 5 or 2/3 at 1week post-transplantation. Conversely, hNPC-treated rats had a higher density of VGluT1-containing glutamatergic synapses (0.223 vs 0.185 synapses/µm3, p<0.05, n=6) in layer 5 at 4 weeks post-transplantation, compared to vehicle-treated rats. This synaptic increase was cortical layer-specific, observed in layer 5 but not in layer 2/3.
Conclusions: Stem cells enhance synaptic remodeling after stroke and this is coincident with stem cell-induced functional recovery. These results provide new information about the post-stroke reorganization of synaptic connectivity after stem cell therapy.
Patient Care: Understanding the mechanisms of cortical plasticity can give some information about indentifying therapeutic targets for stroke patients.
Learning Objectives: By the conclusion of this session, participants should be able to understand the importance of synaptic structural changes to circuits during the functional recovery after stem cell transplantation for stroke.