Upstream eventBDNF, Reduced
Key Event Relationship Overview
AOPs Referencing Relationship
|AOP Name||Adjacency||Weight of Evidence||Quantitative Understanding|
|Chronic binding of antagonist to N-methyl-D-aspartate receptors (NMDARs) during brain development leads to neurodegeneration with impairment in learning and memory in aging||adjacent||Low|
|Chronic binding of antagonist to N-methyl-D-aspartate receptors (NMDARs) during brain development induces impairment of learning and memory abilities||adjacent||Low|
Life Stage Applicability
Key Event Relationship Description
BDNF influences the apoptosis occurring in developing neurons through two distinct mechanisms (Bernd, 2008). mBDNF can trigger prosurvival signaling after binding to TrkB receptor through inactivation of components of the cell death machinery and also through activation of the transcription factor cAMP-response element binding protein (CREB), which drives expression of the pro-survival gene Bcl-2 (West et al., 2001). On the other hand, proBDNF binds to the p75 neurotrophin receptor (p75NTR) and activates RhoA that regulates actin cytoskeleton polymerization resulting in apoptosis (Lee et al., 2001; Miller and Kaplan, 2001; Murray and Holmes, 2011). It is proved that reduced levels of BDNF can severely interfere with the survival of neurons in different brain regions, leading to cell death (Lee et al., 2001; Miller and Kaplan, 2001; Murray and Holmes, 2011).
Evidence Supporting this KER
BDNF mRNA levels dramatically increase between embryonic days 11 to 13 during rat development, playing important role in neuronal differentiation and survival (reviewed in Murray and Holmes, 2011). The latter has been supported by transgenic experiments where BNDF−/− mice demonstrated a dramatic increase in cell death among developing granule cells leading to impaired development of the layers of the cerebellar cortex (Schwartz et al., 1997). BDNF has also been shown to provide neuroprotection after hypoxic-ischemic brain injury in neonates (P7) but not in older (P21) animals (Cheng et al., 1997; Han and Holtzman, 2000). The neuroprotective role of BDNF has been further supported by the observed correlation between elevated BDNF protein levels and resistance to ischemic damage in hippocampus in vivo (Kokaia et al., 1996) and K+ rich medium-induced apoptosis in vitro (Kubo et al., 1995).
Include consideration of temporal concordance here
Several in vitro and in vivo studies on cortical neurons have demonstrated that the survival of developing neurons is closely related with the activation of the NMDA receptors and subsequent BDNF synthesis/release that fully support the BDNF neurotrophic theory (Ikonomidou et al., 1999; Yoon et al., 2003; Hansen et al., 2004).
Pb2+: Neonatal mice exposed to Pb2+ (350 mg/kg lead twice every 4 h) and sacrificed after 8-24 h show increased apoptotic neurodegeneration above that seen in normal controls. This effect has been recorded only in animals treated with Pb2+ at PND 7, but not at PND 14 (Dribben et al., 2011), confirming the importance of the time of exposure during development in order for Pb2+ to induce apoptosis. Two to four weeks old rats treated for 7 days with 15 mg/kg daily dose of lead acetate show increased apoptosis in hippocampus (Sharifi et al., 2002). In rats (30 PND), it has also been shown that Pb2+ (2, 20 and 200 mg/kg/d) can induce apoptosis (Liu et al., 2010). However, in contrast to the first two in vivo studies, the animals in this experimental approach were old enough to evaluate the most sensitive window of vulnerability of developing neurons to Pb2+ exposure (Liu et al., 2010), confirming that only Pb2+ treatment during synaptogenesis can lead to neuronal cell apoptosis. In vitro evidence of apoptosis induced by Pb2+ also derive from PC12 cells exposed to Pb2+ (0.1, 1, 10 µM) that have shown increased activation of caspase-3 (Xu et al., 2006). Besides PC12 cells (Xu et al., 2006; Sharifi and Mousavi, 2008), lead-induced apoptosis has also been studied in cultured rat cerebellar neurons (Oberto et al., 1996), hippocampal neurons (Niu et al., 2002) and retinal rod cells (He et al., 2000). No significant increase in LDH release was found in neuro-spheres derived from neural stem cells (NSCs) originating from E15 rat cortex (CX), striatum (ST) or ventral mesencephalon (VM) after assessing at 24 h intervals from day 1 through day 7 after addition of lead acetate (0.1–10 μM) (Huang and Schneider, 2004). However, LDH release was increased 1 day after addition of 100 μM lead acetate (1.6-2.1 fold depending on the brain region) and 3 days after addition of 50 μM lead acetate to the culture medium (1.3-1.5 fold depending on the brain region). No significant cell loss was observed in cultures exposed to 0.1–10 μM lead acetate for 7 days after staining with Hoechst 33342. In contrast, significant cell loss was observed 7 days after exposure to 50 μM (35-50% depending on brain region) or 100 μM lead acetate (60-75% depending on brain region) (Huang and Schneider, 2004). In primary rat hippocampal neurons exposed to 1 μM Pb2+ for 5 days during the period of synaptogenesis (DIV7–DIV12), decreased cellular proBDNF protein (40% compared to control) and extracellular levels of mBDNF (25% compared to control) have been recorded (Neal et al., 2010). Significant reductions specifically in dendritic proBDNF protein levels throughout the length of the dendrites have also been described by Stansfield et al. (2012) after exposure to the same concentration of Pb2+ using this in vitro model. In an in vivo study, mice at PND 7 with mean Pb2+ blood levels of 8.10 μg/mL have shown increased apoptosis in the cortex, hippocampus, caudate-putamen, and thalamus compared to controls with F (1,14) = 19.5, 8.40, 4.15, 4.53, respectively (Dribben et al., 2011). These Pb2+ levels in blood (Dribben et al., 2011) were a bit higher than the levels determined in Guilarte et al. 2003 (3.90 μg/dl) that served as the base to calculate in vitro doses in Neal et al. 2010 and Stansfield et al. 2012.
Uncertainties and Inconsistencies
Pb2+: A number of studies demonstrate that deletion of BDNF does not lead to significant apoptotic cell death of neurons in the developing CNS (reviewed in Dekkers et al., 2013). In an in vivo Pb2+ exposure study, where female rats received 1,500 ppm prior, during breeding and lactation shows no changes at mRNA levels of BDNF in different hippocampus section derived from their pups (Guilarte et al., 2003). Regarding Pb2+, the pre- and neonatal exposure of rats to Pb2+ (Pb2+ blood levels below 10 μg/dL) show a decreased number of hippocampus neurons but no morphological or molecular features of severe apoptosis or necrosis have been detected in tested brains (Baranowska-Bosiacka et al., 2013). In contrast to the lack of apoptotic signs, reduced levels of BDNF concentration (pg/mg protein) of BDNF in brain homogenates has been recorded in forebrain cortex (39%) and hippocampus (29%) (Baranowska-Bosiacka et al., 2013). Pregnant rats have been exposed to lead acetate (0.2% in the drinking water) after giving birth until PND 20. At PND 20, blood Pb2+ levels in pups reached at 80 μg/dl. In these animals, the gene expression in different brain regions has been assessed and demonstrated that hippocampus is most sensitive with alterations beginning at PND 12 when caspase 3 mRNA increases after Pb2+ exposure (Chao et al., 2007). However, bcl-x and BDNF mRNA in the hippocampus have been significantly increased after caspase 3 increase, suggesting that the apoptotic signal activates a compensatory response by increasing survival factors like BDNF and that the temporality suggested in this AOP may not be accurate (Chao et al., 2007).
Some of the reported “inconsistencies” may be due to the lack of sufficient details in the reporting since publications vary in what they measure. Some of the referenced studies look at BDNF transcripts, others look at BDNF protein. BDNF processing is highly complex and different mRNA transcripts are known to be implicated in different cellular function.
Several studies addressing apoptosis mainly in the developing cerebral cortex have shown that more mechanism besides neurotrophic factors may be involved. Cytokines, as well as neurotransmitters can potentially activate a number of intracellular proteins that execute cell death (Henderson, 1996; Kroemer et al., 2009), meaning that further branches to this AOP might be added in the future.
Quantitative Understanding of the Linkage
Is it known how much change in the first event is needed to impact the second? Are there known modulators of the response-response relationships? Are there models or extrapolation approaches that help describe those relationships?
No enough data is available to address the questions above.
Known modulating factors
Known Feedforward/Feedback loops influencing this KER
Domain of Applicability
The survival and antiapoptotic role of BDNF has been investigated not only in rodents but also in developing chicken neurons (Hallbook et al., 1995; Frade et al., 1997; Reinprecht et al., 1998). In invertebrates, only recently a protein with possible neurotrophic role has been identified but its influence and function in neuronal cell death of developing neurons has not been investigated yet (Zhu et al., 2008).
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Bernd P. (2008) The role of neurotrophins during early development. Gene Expr. 14: 241-250.
Chao SL, Moss JM, Harry GJ. (2007) Lead-induced Alterations of Apoptosis and Neurotrophic Factor mRNA in the Developing Rat Cortex, Hippocampus, and Cerebellum. J Biochem Mol Toxicol. 21: 265-272.
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