We report in this investigation that peripheral stimulation targeting the ischemic cortex helps to recruit more endogenous neuroblasts from the SVZ to migrate to the lesioned barrel cortex, presumably mediated by increased SDF-1/CXCR4 signaling (Imitola et al., 2004
; Sun et al., 2004
; Ohab et al., 2006
; Robin et al., 2006
). This effect combined with the increased VEGF expression and angiogenesis induced by whisker stimulation provide evidence that adequate peripheral activity as a means of physical therapy may benefit the post-ischemic tissue repair in the CNS.
The adult brain harbors two neuroproliferative zones, the subventricular zone (SVZ) and subgranular zone (SGZ), which normally supply neural progenitors to the olfactory bulb and the dentate gyrus of the hippocampus, respectively. Cerebral ischemia alters this normal pattern of adult neurogenesis in two ways: it enhances cell proliferation within the SVZ and SGZ and evokes migration of neuroblasts into areas of damage (Jin et al., 2003
; Thored et al., 2006
; Zhang et al., 2007
). Enhancing this post-stroke neurogenesis and cell migration is expected to promote tissue repair and improve functional outcomes of stroke (Zheng and Chen, 2007
). Peripheral stimuli and physical activity have significant ‘use-dependent’ impacts on the morphological and functional alterations in the adult brain after stroke, the mechanisms of which have been shown to be associated with enhanced angiogenesis and neurogenesis. For example, exercise preconditioning ameliorates inflammatory responses and brain damage in ischemic rats (Ding et al., 2005
). Exposure to an enriched environment after focal cerebral ischemia resulted in enhanced proliferation of neural stem/progenitor cells and neurogenesis in SVZ and a better functional performance in a battery of sensorimotor tasks (Komitova et al., 2005
). Specific rehabilitative training of an impaired forelimb effectively stimulates dentate gyrus neurogenesis and improves spatial learning after focal cortical infarcts (Wurm et al., 2007
). Rehabilitative therapies, therefore, favor the reorganization in the peri-lesion tissue, and contribute to the compensation and/or recovery of the impaired function. As shown in our previous study, whisker-barrel ischemia is an ideal rodent model for testing the use-dependent mechanism, having the advantages of the well-defined whisker-barrel sensorimotor pathway and feasible identification of corresponding morphological and functional changes (Whitaker et al., 2007
). Using the same animal model, the present study showed that although the specific peripheral stimulation did not further increase the number of proliferating cells in the SVZ, it significantly promoted the migration of neuroblasts from the SVZ to the damaged barrel cortex. We also found increased maturation of new neurons that reached the ischemic penumbra region in whisker stimulation group 30 days after stroke. Our results suggest that appropriately increased peripheral activity and afferent signals to the ischemic cortex can recruit more endogenous neural stem cells to migrate to injured region and differentiate into mature neurons, which may imply beneficial effects of specific physical therapy on long-term recovery from ischemic stroke.
CXCR4 is the receptor for the CXC chemokine family member SDF-1 (Matthys et al., 2001
). SDF-1/CXCR4 signaling has been shown to play a major role in directing the migration of neural stem cells to lesioned sites (Imitola et al., 2004
). Neural progenitor cells, derived from adult rat SVZ and cultured in vitro, expressed the CXCR4 receptor and migrated toward a gradient signal of SDF-1 (Sun et al., 2004
). SDF-1 protein expression is clearly upregulated in the periphery of the damaged striatal area within 24 hrs after 2-hr MCA occlusion (Miller et al., 2005
; Thored et al., 2006
). Blocking SDF-1 by a n4eutralizing antibody against CXCR4 significantly attenuated stroke-enhanced cell migration (Ohab et al., 2006
; Robin et al., 2006
). Our study reports the novel observation that peripheral stimulation after stroke enhances the SDF-1/CXCR4 signaling pathway and migration of SVZ-derived neuroblasts toward the damaged cortex. In addition, SDF-1/CXCR4 signaling plays important roles in angiogenesis, which has a close relationship with neuroblast migration and neurogenesis. A recent study demonstrates that the SDF-1/CXCR4 signaling axis induce angiogenesis by increasing expression of VEGF through the activation of PI3K/Akt pathway (Liang et al., 2007
). This is consistent with our observations that SDF-1 expression is detected in vascular endothelial cells and whisker stimulation increases VEGF expression and angiogenesis.
VEGF is an angiogenic factor and exerts biologic functions through two closely related receptor tyrosine kinases VEGFR1 (flt-1) and VEGFR2 (flk-1). Most of the VEGF properties are mediated by its interaction with VEGFR2 (Waltenberger et al., 1994
). VEGF/VEGFR2 has been shown to be essential for endothelial cell proliferation and formation of new microvessels (Breier et al., 1992
). Very recent studies have shown that neuroblasts need the blood vessels to act as a scaffold to migrate to injured regions (Ohab et al., 2006
; Thored et al., 2007
). In addition to its role in inducing angiogenesis, VEGF also stimulates neurogenesis and axonal outgrowth, and promotes the growth and survival of neurons (Sun et al., 2003
; Wang et al., 2007
). VEGF is an attractive guidance cue for the migration of undifferentiated neural progenitors through VEGFR2 signal pathway (Zhang et al., 2003
). We have found that whisker stimulation can increase VEGF expression, angiogenesis and the restoration of local cerebral blood flow (Wei et al., 2005
; Whitaker et al., 2007
). The increase in VEGF gene expression inspected using Western blot was relatively moderate, we assume that multiple gene regulations contribute to the observed neurogenesis and cell migration. It is expected that more intensive whisker stimulation should induce more elevated VEGF expression. We also report for the first time that VEGFR2 was specifically expressed in migrating neuroblasts, which provides the molecular basis for responding to increased VEGF upon peripheral stimulation. Likewise, BDNF regulates neuronal survival, cell migration, and synaptic function (Aguado et al., 2003
; Gorski et al., 2003
), which also have a trend of upregulation by whisker stimulation.
In summary, peripheral whisker activities may optimize the microenvironment of the barrel cortex by upregulating different factors to recruit more endogenous neural stem cells from SVZ, as well as create a more favorable microenvironment for cells survival, differentiation and guided neural network connections, which benefit the tissues repair and function recovery after ischemia. Further studies should be done to explore the molecular and cellular mechanisms involved in the regulation of neurovascular plasticity by peripheral stimulation or physical therapy, which may lead to the development of more specific and efficient therapeutic approaches for stroke patients.