Paradoxically, low-level Pb exposure was protective in a severe model of ALS, which correlated with higher levels of VEGF in spinal cord. The present finding was unexpected, since Pb is a well-known environmental neurotoxicant and a risk factor for ALS (Kamel et al. 2005
). We provide evidence that Pb decreases the toxicity of dysfunctional astrocytes occurring in familial ALS by inducing VEGF in SOD1G93A
bearing astrocytes. This response was specific for transgenic astrocytes since VEGF levels in non-transgenic astrocytes were below the limits of detection. The proposed mechanism elicited by Pb might explain recent epidemiological data showing that Pb blood and bone levels positively correlate with longer survival in ALS patients after diagnosis (Kamel et al. 2008
). Hence, Pb exposure may potentially have opposing actions during the course of ALS, initially promoting the degeneration of motor neurons but later abrogating damage and neuroinflammation mediated by dysfunctional glia ().
Figure 6 Proposed mechanisms by which Pb exposure stimulates astrocyte VEGF expression and extends survival in ALS mice. Pb induces VEGF expression specifically in SOD1G93A bearing astrocytes abrogating their neurotoxic phenotype to motor neurons. On the contrary, (more ...)
Pb blood levels after chronic exposure of low-level Pb in drinking water were comparable to those reported in humans following occupational exposition (Roscoe et al. 2002
; Kosnett et al. 2007
). These levels did not produce overt Pb toxicity in mice after 90-100 days of treatment. SOD1G93A
mice exposed to Pb developed as their littermates receiving vehicle and disease onset was not statistically different. But survival was significantly increased, suggesting a disease-modifying effect of Pb. Studies in a mouse model of inherited ALS have suggested that onset is related to motor neuron function, while progression depends upon astrocytes and microglia (Yamanaka et al 2008
, Boillee et al. 2006
). This is consistent with two pathogenic processes responding differently to Pb intoxication.
Protective Pb effects were evidenced by a dramatic reduction of GFAP-immunoreactive astrocytes, indicating a potential downregulation of ALS-linked neuroinflammation (Levine et al. 1999
; Barbeito et al. 2004
). Because ALS progression is regulated by neuroglia (Yamanaka et al. 2008
), progressive Pb accumulation in astrocytes may reduce their deleterious activation. VEGF administration to SOD1G93A
mice also reduce glial reactivity (Zheng et al. 2007
), consistent with the concept of reducing neuroinflammation.
Astrocytes survive accumulating Pb to concentrations that are detrimental to neurons (Tiffany-Castiglioni et al. 1993
). In agreement, both cultured SOD1G93A
and non-transgenic astrocytes are resistant to moderate Pb concentrations that otherwise are toxic for isolated cultured motor neurons. However, preconditioning with Pb caused non-transgenic astrocytes to become neurotoxic to co-cultured motor neurons, suggesting it can also elicit inflammatory-like activation of astrocytes comparable to other stressful stimuli (Cassina et al. 2002
). In contrast, motor neurons co-cultured on SOD1G93A
astrocytes pre-treated with Pb were more likely to survive than on vehicle-treated transgenic astrocytes. These results further suggest that astrocytes bearing the SOD1G93A
mutation are regulated differently from non-transgenic astrocytes.
One distinguishing characteristic of Pb-induced VEGF expression is that it specifically occurs in SOD1G93A
astrocytes. Non-transgenic astrocytes do not express detectable VEGF levels in response to Pb. VEGF is classically induced by activation of hypoxia-inducible factor-1 (HIF-1) mediated by hypoxic stress and hypoxia-induced generation of reactive oxygen species (Xie et al. 2004
; Chandel et al. 2000
). However, VEGF expression is also known to be induced by Pb via a PKC/AP1-dependent and HIF-1-independent signaling pathway in an astrocytic cell line (Hossain et al. 2000
). Decreased VEGF has been implicated in the pathogenesis of ALS (Oosthuyse et al. 2001
; Lambrechts et al. 2003
; Storkebaum et al. 2005
; Wang et al. 2007
). VEGF protects motor neurons in culture and in organotypic cultures (Li et al. 2003
; Tolosa et al. 2008
; Van den Bosch et al. 2004
). Furthermore, VEGF was shown to be released by astrocytes (Van den Bosch et al. 2004
) and exert a potent trophic activity for motor neurons by activating VEGFR-2 (Storkebaum et al. 2005
). Interestingly, our results indicate that Pb increased VEGF expression in the spinal cord of SOD1G93A
mice. Basal VEGF expression was greater in untreated SOD1G93A
mice compared to non-transgenic controls as previously observed (Murakami et al. 2003
). Nevertheless, the protection is appreciated when basal levels are increased suggesting that additional protective mechanisms may be enhancing VEGF release or bioavailability. Additionally, Pb may modulate ALS pathogenesis by affecting other physiological mechanisms. For example, it has been shown that Pb may decrease SOD activity in lead-exposed rats (Babu et al. 2007
), which could have beneficial effects on familiar ALS models. Furthermore, at the cellular level, Pb potently stimulates the antioxidant enzyme hemoxygenase-1 expression in astrocytes (Cabell et al. 2004
), suggesting increased antioxidant defenses and cytoprotective pathways that may also contribute to neuronal protection.
Understanding how Pb stimulates VEGF production in non-neuronal cells by the ALS disease process can reveal a simpler therapeutic route than gene therapy approaches under current development.