The presents findings demonstrate the first evidence that AD pathology and exercise training contribute to the mRNA level of angiostatin in AD brain. Also, our current study provides evidence that exercise training ameliorates cognitive dysfunction in AD, possibly through VEGF signaling. Specifically, our results demonstrate that 1) the impaired cognitive function is ameliorated by exercise training in AD rat; 2) exercise training increased the reduced mRNA expression level of VEGF signaling including HIF1α, VEGF, and VEGFR2 in hippocampus from AD rat, 3) mRNA expression level of angiostatin was elevated in hippocampus from AD rat, and exercise training abrogated exacerbated expression.
Memory and cognitive deficits are the most primary clinical manifestation in patients with AD, are implicated with the deposition of Aβ plaques in the hippocampal area and CAA in the brain (Rosa & Fahnestock, 2014). Previous studies have shown that spatial learning impairment and memory deficits were developed after Aβ microinjection in the hippocampus of rodent models (Sharma, Verma, Kapoor, Saini, & Nehru, 2016; Wu et al., 2017). Our data aligned with the previous finding that the escape latency and traveled distance were significantly increased in AD group compared to CON group in the acquisition phase (Fig. 2a & b), as well as a significant decrease in the percentage of time spent in the target quadrant in AD group compared to CON group in probe phase (Fig. 2c). Our findings suggest that Aβ injection directly impairs cognitive function in the rat model. The beneficial effect of exercise training on cognitive decline in patients with AD (Gomes-Osman et al., 2017; Morris et al., 2017) and animal models (Azimi et al., 2018; Kim et al., 2014; X.-Q. Wang & Wang, 2016) are well demonstrated. Exercise training elevates LRP-1 expression in the AD hippocampus, in turn, increase Aβ clearance, finally improving cognitive function and preventing the progression of AD pathology (Khodadadi et al., 2018). Our results also revealed that exercise training ameliorated spatial learning and memory impairment in AD-EX group compared to AD group (Fig. 2a, b, & c). Collectively, current findings provide evidence that exercise training has protective effects on cognitive dysfunction, possibly through the prevention of Aβ pathology.
VEGF is one of the main angiogenesis factors regulating neurogenesis and is also involved in AD pathogenesis as well as cognitive impairment in AD (Guo et al., 2019; Provias & Jeynes, 2014; P. Wang et al., 2011). Previous studies reported that the mRNA and protein expression of HIF-1 (Liu et al., 2008; Schubert, Soucek, & Blouw, 2009), VEGF, VEGFR2 are downregulated in the hippocampus and cortex of the AD brain (Guo et al., 2019; Provias & Jeynes, 2014; P. Wang et al., 2011). These findings align with our results, in part, the mRNA levels of HIF-1, VEFG, and VEGFR2 were significantly reduced in the hippocampus of AD group compared to CON groups (Fig. 3a, b & c). Moreover, VEGF treatment ameliorates cognitive deficits in AD by reducing Aβ accumulation in AD brain (Cao et al., 2004; Guo et al., 2019; P. Wang et al., 2011) and cerebral vessels with reduction of Aβ-induced vascular regression and apoptosis (Religa et al., 2013). The expression of VEGF is decreased with an elevation of Aβ, while VEGF treatment improves the cognitive impairment, concurrently decreased the BACE1 (β-site APP cleaving enzyme) and increased ADAM10 (α-secretase cleaving APP) expression in the TG2576 mice brain (Guo et al., 2019). Furthermore, Aβ acts as an anti-angiogenic factor inhibiting the migration and permeability of VEGF in endothelial cells, in turn, blocks VEGF signaling by direct interaction with VEGFR2 in Aβ-treated HUVECs and HBMECs (Patel et al., 2010). Elevated Aβ plaque formation in brain tissue and CAA might impair Aβ clearance process possibly through a decrease of VEGF signaling, and it exacerbates AD pathology and cognitive decline in the AD. These previous findings support our data, in part, reduced mRNA level of VEGF signaling induced by Aβ injection might accelerate cognitive deficits in AD group (Fig. 3a-c &Fig. 3). Collectively, current data suggest that the reduced the mRNA expression of VEGF signaling might induce cognitive impairment, possibly through exacerbating amyloidogenic pathway in AD brain.
Several clinical investigations report that exercise training delay or prevent progression of AD pathogenesis and cognitive impairment (Kim et al., 2014; Morris et al., 2017) via a decrease in Aβ accumulation, which was accompanied by increased mRNA levels of HIF-1, VEGF and VEGFR2 in the hippocampus of rats (Ding et al., 2006; Dornbos III et al., 2013; Lou et al., 2008), as well as patient with AD (Pedrinolla et al., 2020). Aforementioned, VEGF signaling improves cognitive function by modulating amyloidogenic pathway (Bürger et al., 2009; Guo et al., 2019; P. Wang et al., 2011). VEGF administration increased ADAM10 and reduced BACE1 expressions in the brain of Tg2576 mice, which led to the improvement in cognitive function (Guo et al., 2019). Furthermore, Wang et al. also reported that VEGF treatment in PDGF-hAPP transgenic mice protects memory impairment and decreased Aβ deposition in the brain (P. Wang et al., 2011). These findings suggested that exercise-induced improvement of VEGF signaling might increase in both central and peripheral Aβ clearance in the AD brain, eventually ameliorating learning and memory deficits. Our findings also showed that exercise training elevates the mRNA levels of HIF-1, VEGF, and VEGFR2 in the hippocampus of AD-EX group compared to AD groups (Fig. 3a, b & c) with improved cognitive function (Fig. 2a, b & c).These findings collectively imply that elevated VEGF signaling by exercise training might ameliorate cognitive decline, possibly through converting amyloidogenic to a non-amyloidogenic pathway in AD mice brain.
Angiostatin is identified as an endogenous angiogenic inhibitor that inhibiting both angiogenesis and vascular permeability (Eriksson et al., 2003). It has been indicated that angiostatin attaches to integrins and inhibits p42/p44 MAP kinase pathway that playing a role in regulating VEGF and controlling its downstream pathways (Sima, Zhang, Shao, Fant, & Ma, 2004). Chung et al. have reported that angiostatin production is inversely associated with VEGF expression, but its expression is positively correlated with MMP-2 & 9 expressions in human diabetic arterial vasculature (Chung et al., 2006). This study results explained that elevated angiostatin could be attributed to the pathogenesis of impaired angiogenesis in diabetes melitus. Furthermore, we firstly show that the mRNA level of angiostatin was elevated in AD hippocampus with reduced VEGF mRNA level (Fig. 3d) while 10 weeks of aerobic training reduced the mRNA level of angiostatin in cardiac and hindlimb muscles from rats with myocardiac infartion (Ranjbar et al., 2016). Our study demonstrates that exercise training abrogated mRNA level of angiostatin in the AD hippocampus (Fig. 3d) and it suggests that reduced mRNA level of angiostatin by exercise training might increase VEGF signaling, in turn, finally improve cognitive function in AD.
In conclusion, the current study highlights the importance of exercise training as an effective approach to reduce/prevent the deterioration of cognitive deficits in AD, possibly through enhancing VEGF signaling and reducing angiostatin. Further investigation is required to elucidate the direct causality between VEGF signaling and angiostatin on cognitive impairment in AD. These findings will provide the missing clues for developing a therapeutic strategy to protect and/or prevent cognitive decline in AD.