In the current study, we have demonstrated that a widely and clinically used HMG-CoA reductase inhibitor (statin), atorvastatin, when administered to mice starting 1 day after MCAO, evokes significant improvement in functional neurologic recovery. This observation is consistent with our previous studies using this compound in rats (Chen et al, 2003b
). In addition, we have also shown that atorvastatin promotes: (1) neurogenesis and neuronal plasticity as well as increases the expression of VEGF, VEGFR2 and BDNF in the ischemic border after stroke in mice; (2) endothelial cell proliferation and VEGFR2 and BDNF expression; (3) neural cell migration; (4) BDNF expression, which mediates neural cell migration in SVZ explants. Collectively, these in vivo
and in vitro
data strongly support a role for atorvastatin in promoting brain plasticity and recovery from stroke.
Atorvastatin increases VEGF, VEGFR2 expression as well as promotes endothelial cell proliferation in the ischemic border. Vascular endothelial growth factor is an angiogenic factor. Vascular endothelial growth factor exerts biologic functions via two closely related receptor tyrosine kinases VEGFR1 (flt-1) and VEGFR2 (flk-1). Most of the VEGF properties, such as mitogenicity, chemotaxis and induction of morphologic change, are mediated by its interaction with VEGFR2 (Waltenberger et al, 1994
). Vascular endothelial growth factor/VEGFR2 has been shown to be essential for endothelial cell proliferation and differentiation (Breier et al, 1992
). Binding of VEGF to VEGFR2 leads to the receptor phosphorylation and subsequent activation of PI3K/Akt and other downstream signaling proteins (Gliki et al, 2002
; Nakashio et al, 2002
). Statin-induced capillary-like tube formation is inhibited by a neutralizing antibody against VEGFR2 and inhibition of PI3K (Chen et al, 2003b
). We propose that administration of atorvastatin promotes VEGF and VEGFR2 expression in the ischemic border, which may facilitate induction of endothelial cell proliferation and angiogenesis.
The 10 mg/kg dose of atorvastatin used in this study is consistent with the neuroprotective dose of atorvastatin used in pretreatment of mice after stroke (Gertz et al, 2003
; Laufs et al, 2000
). We and others have shown that in vitro
and in vivo
, atorvastatin has a U-shaped dose–response curve on inducing angiogenesis (Chen et al, 2003b
; Urbich et al, 2002
; Weis et al, 2002
). However, a 2.5 mg/kg dose of atorvastatin was shown to decrease angiogenesis in brain tumor and in a model of inflammation (Weis et al, 2002
). This apparent discrepancy may be attributed to our use of the stroke model versus tumor model.
In addition to its role in inducing angiogenesis, VEGF also stimulates neurogenesis and axonal outgrowth, and improves the survival of mouse superior cervical, dorsal root ganglion neurons, and mesencephalic neurons (Hess et al, 2002
; Sondell et al, 1999
). Vascular endothelial growth factor is mitogenic for astrocytes and promotes growth/survival of neurons (Silverman et al, 1999
). Our data have shown that atorvastatin promotes angiogenesis, neuronal plasticity as well as increases VEGF/VEGFR2 expression. We propose that the atorvastatin-induced increase of VEGF/VEGFR2 may not only cause angiogenesis but also provide a supportive microenvironment, which can enhance the neuronal and synaptic plasticity.
Neurogenesis and synaptic reorganization are important for functional improvement after stroke (Gomez-Fernandez, 2000
; Hallett, 2001
; Shingo et al, 2001
). Our data show that atorvastatin treatment after stroke induces the expression of BDNF and synaptic proteins, and neuronal migration in the ischemic border, and some neuronal migrating cells are localized to blood vessels. These data are consistent with other reported studies showing that increasing synaptic activity elicits compensatory angiogenesis (Black et al, 1989
). An index of axonal sprouting, GAP43, appears to be colocalized with angiogenesis after MCAO (Kawamata et al, 1997
). Neurogenesis occurs in close proximity to blood vessels, where VEGF expression is high and angiogenesis is ongoing (Palmer et al, 2000
). The newly activated and expanded vasculature substantially increases the production and release of BDNF, whose induction is both spatially and temporally associated with recruitment of new neurons (Leventhal et al, 1999
). Cerebral endothelial cells are a potential source for bioactive BDNF (Bayas et al, 2002
). Vascular endothelial cells may contribute to circulating BDNF (Nakahashi et al, 2000
). Atorvastatin and BDNF induce neuronal migration in the SVZ explant culture. Inhibition of BDNF decreases neuronal migration in the SVZ explant. These data suggest that neuronal plasticity is closely linked with angiogenesis and BDNF may facilitate atorvastatin-induced neuronal plasticity after stroke.
In summary, treatment of stroke at 24 hours after MCAO in mice with a widely used atorvastatin reduces neurologic deficits associated with stroke. Statin-mediated functional benefit might be derived from the upregulation of trophic factors such as VEGF/VEGFR2, BDNF and induction of angiogenesis, neurogenesis, and synaptic plasticity.