Adult hippocampal neurogenesis poststroke: More new granule cells but aberrant morphology and impaired spatial memory

<div><p>Stroke significantly stimulates neurogenesis in the adult dentate gyrus, though the functional role of this postlesional response is mostly unclear. Recent findings suggest that newborn neurons generated in the context of stroke may fail to correctly integrate into pre-existing networks. We hypothesized that increased neurogenesis in the dentate gyrus following stroke is associated with aberrant neurogenesis and impairment of hippocampus-dependent memory. To address these questions we used the middle cerebral artery occlusion model (MCAO) in mice. Animals were housed either under standard conditions or with free access to running wheels. Newborn granule cells were labelled with the thymidine analoque EdU and retroviral vectors. To assess memory performance, we employed a modified version of the Morris water maze (MWM) allowing differentiation between hippocampus dependent and independent learning strategies. Newborn neurons were morphologically analyzed using confocal microscopy and Neurolucida system at 7 weeks. We found that neurogenesis was significantly increased following MCAO. Animals with MCAO needed more time to localize the platform and employed less hippocampus-dependent search strategies in MWM versus controls. Confocal studies revealed an aberrant cell morphology with basal dendrites and an ectopic location (e.g. hilus) of new granule cells born in the ischemic brain. Running increased the number of new neurons but also enhanced aberrant neurogenesis. Running, did not improve the general performance in the MWM but slightly promoted the application of precise spatial search strategies. In conclusion, ischemic insults cause hippocampal-dependent memory deficits which are associated with aberrant neurogenesis in the dentate gyrus indicating ischemia-induced maladaptive plasticity in the hippocampus.</p></div

Stroke animals employ less hippocampus-dependent search strategies.

<p><b>A</b>, Schematic illustration of distinct hippocampus-dependent and -independent search strategies in MWM. <b>B/C,</b> Contribution of single search strategies to group performance in MCAO- and sham operated animals either following standard housing (B) or free access to running wheels (C). Color code as indicated in A.</p

Stroke stimulates neurogenesis in adult dentate gyrus.

<p><b>A,</b> Representative confocal image of dentate gyrus with newly generated neurons (red). B, Higher magnification of new neurons show colabeling of proliferation marker EdU (red) and mature neuronal marker NeuN (blue). <b>C,</b> Quantification of new neurons in animals housed under standard conditions or with free access to running wheels. Bars represent mean ± SEM. Asterisks indicate significant differences (p < 0.05). Scale bars represent 100 μm (A1), 10μm (B1).</p

Stroke induces aberrant neurogenesis.

<p><b>A,</b> Morphology of regular (“non-aberrant”) GFP-positive neurons after retroviral tracing in sham operated control. <b>B1-4,</b> Distinct examples of aberrant neurons following stroke expressing additional basal dendrites toward the hilus (bipolar cells). <b>C1,</b> Regular granule cell with characteristic dendritic tree spanning the granule cell layer. Right panel: higher magnification of one dendrite with spines. <b>C2</b>, Aberrant granule cell extending dendrites towards hilus. Right panel: higher magnification of one dendrite with spines. <b>D1-D2</b>, Quantification of spines in aberrant and regular granule cells (D1, total number of spines. D2, percentage of mushroom spines). Scale bars: 10μm (C1, C2 left) and 5μm (C1, C2 right).</p

Contribution of pooled hippocampus-dependent search strategies.

<p>A/B, Graph depicts percentage of all hippocampus-dependent strategies (direct swim, focal search, directed search) in MCAO- and sham-operated animals either following standard housing (A) or free access to running wheels (B). Graphs represent mean ± SEM. Asterisks indicate significant differences (p < 0.05).</p