1Institute of Neurosciences, The Fourth Military Medical University, Xi’an 710032, Shaanxi, China.
2Department of Neurosurgery, Tangdu Hospital, The Fourth Military Medical University, Xi’an 710032, Shaanxi, China.
3School of Military Preventive Medicine, The Fourth Military Medical University, Xi’an 710032, Shaanxi, China.
4Department of Neurology, Xijing Hospital, The Fourth Military Medical University, Xi’an 710032, Shaanxi, China.
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Lead (Pb2+), a ubiquitous environmental toxicant, may widely affect the function of many organs or systems of human beings, especially the brain. Although lead is believed to transport into the brain through the blood-brain barrier (BBB) and cause direct neuronal injury, growing data have shown that lead exposure could induce brain dysfunction by triggering microglial and astroglial activation, pro-inflammatory cytokine production and inflammatory response, generation of reactive oxygen species and oxidative stress, and finally result in BBB dysfunction and neuronal damage. This review summarizes recent studies regarding microglial and astroglial reaction, neuroinflammation, and neuronal death in the brain following lead insult, suggesting that reactive glial cells may represent a potential target for manipulation of lead-induced neuroinflammatory injury of the brain.
Astroglia, brain, lead toxicity, microglia, neuroinflammation
Lead (Pb2+) is a widely distributed heavy metal and environmental toxicant, and overexposure to lead due to pollution or accident can impair the function of the nervous system, especially learning and memory abilities of developing brains during childhood.[1-4] Epidemiologic data indicate that learning impairment may be caused by moderate lead exposure in young individuals.[2,3] This impairment is largely related to neuronal injury caused by lead toxicity, but the detailed mechanism by which lead exposure induces neuronal injury, neuronal death, and brain dysfunction still remains elusive.[4,5] Several studies have indicated that lead exposure may interfere with calcium signaling, suppress neurogenesis and neuronal differentiation, inhibit formation of long-term potentiation (LTP), influence secretion of neurotransmitters, and even enhance production of amyloid protein.[6-13] Lead can also bind to key metabolic enzymes such as pyruvate kinase, induce reactive oxygen species (ROS), impede the supply of energy to neurons, and cause neuronal apoptosis.[14-17] Moreover, recent studies have shown a crucial involvement of microglial and astroglial cells in neuroinflammatory injury induced by lead exposure.[10,11] Microglia and astrocytes are two major types of glial cells involved in the regulation of the immune response to pathological processes in the brain. Functional activation of microglia and astrocytes and the resulting neuroinflammation are associated with infection, autoimmunity, and pathogenesis of neurodegenerative diseases. In response to lead exposure, microglia and astrocytes can increase the production and release of inflammatory cytokines, enhance ROS generation, impede antioxidant activity, and result in neuronal injury or neuronal loss in the brain or other parts of the central nervous system (CNS).[10,11,19-22]
It is generally regarded that microglial cells are derived from blood monocytes that reset in the CNS during embryonic development and are functionally involved in neuronal maintenance, injury, and repair in a manner similar to peripheral macrophages. Microglial cells are a predominant source of various inflammatory cytokines, that is, interleukin-1 beta (IL-1β), tumor necrosis factor-alpha (TNF-α), and interferon-gamma (IFN-γ), which can then induce a broad spectrum of inflammatory reactions. The activation of microglia and astrocytes in response to internal and external stimuli or insults might further increase the release of cytotoxic substances, pro-inflammatory cytokines, ROS, and excitatory amino acids, thus causing further neuronal injury in the brain.
Obvious morphological change and higher synthesis of cytokines have been observed in activated microglial cells after lead exposure.[10,11,24] For instance, elevated expression of IL-1β and TNF-α is found in the cerebral cortex after lead exposure, as well as increased expression of IL-1β and IL-6 in the hippocampus.[25,26]In vitro experiments have also confirmed the elevation of TNF-α expression after lead exposure. Gene expression analysis has shown that levels of the pro-inflammatory factors IL-6 and TNF-α are significantly perturbed by the lead insult in multiple brain regions.[19,20] These cytokines are co-expressed in glial cells in response to lead crossing the blood-brain barrier (BBB) and might also represent a mechanism for lead toxicity to the immature brain. Conversely, anti-inflammatory factors such as IL-10 and transforming growth factor beta (TGF-β) are decreased in the cortex in response to lead, as detected by real time-polymerase chain reaction.
Lead exposure might destroy the glial support of neuronal cells by increasing ROS and other toxins in microglial cells. The microglial inflammatory response is also associated with the production of ROS and nitric oxide (NO)-dependent reactive nitrogen species (RNS). Nicotinamide adenine dinucleotide phosphate (NADPH) oxidase (NOX), which is ubiquitously expressed in microglia, contributes much to the production of superoxide and the induction of ROS. Furthermore, NOX could be activated in monocytes and microglial cells by IL-1β, TNF-α, IFN-γ, and other pro-inflammatory cytokines. The inducible NO synthase (iNOS) is also prevalent in microglia, and microglial NO generation regulates vascular relaxation and initiates rapidly induced, transiently regulated signaling events. On the other hand, lead also increases NOX, which causes superoxide production and inhibits antioxidant production, and increases the accumulation of ROS in the brain. It is well-known that neurons in the brain are vulnerable to excess ROS and RNS. Oxidative stress could result in the death of newly-born neurons by disrupting signaling processes, dysfunction of ion homeostasis, and protein misfolding.
The signaling pathways involved in lead-induced microglial activation, however, need more investigation. In response to various environmental toxins including lead, microglia could enter the activated state and release ROS. Pattern recognition receptors expressed on microglia might be one common signaling pathway. For example, toll-like receptors act as initiators of the nuclear factor kappa B (NF-κB) pathway when exposed to several toxins, such as lipopolysaccharide (LPS), resulting in the release of pro-inflammatory cytokines. However, it is still not clear how lead could induce microglial activation and trigger inflammatory cytokine production, which remains a critical question to be answered.
The neurovascular unit in the brain comprises of neurons, blood vessels and their adjacent astrocytes.[33,34] The concept of a functional unit is a new one, and emphasizes the interaction between neurons and astrocytes under both normal and pathological physiological conditions. Astrocytes play a critical role in neuron function, including energy support, metabolism, and synapse formation.[35,36] Astrocytes maintain the trans-endothelial electric resistance (TEER) of the BBB. Under pathological conditions, astrocytes might remove toxic substances and balance electrolyte and water levels. It has been found that lead interferes with astrocyte functions such as energy metabolism, immune response, and ROS removal. Furthermore, astrocytes could collaborate with microglia to switch on neuroinflammatory reactions in the brain, and each of these effects can result in BBB dysfunction and injury to neurons.
Lead exposure leads to the insufficient supply of energy from astrocytes to neurons. Astrocytes contain a large number of mitochondria for energy and glutamate metabolism. Neurons in the brain show a preference for lactose and glutamine provided by astrocytes via shuttle routes. Glycogen is exclusively localized in astrocytes in the adult brain and can be metabolized to pyruvate, which is converted to lactate by lactate dehydrogenase mainly in astrocytes and then transported to neurons. When energy is insufficient, astrocytes can also use glycolysis from stored glycogen for the use of neurons.[40,41] Glycogen metabolism in astrocytes is also required for long-termmemory formation in the brain. Lead exposure causes morphological and functional changes in astrocytic mitochondria. Creatine kinase and pyruvate kinase are two key enzymes in astrocytes that are involved in the production of pyruvate and lactate, and lead can bind to their sulfhydryl groups and decrease activity, resulting in an insufficient supply of pyruvate and lactate from astrocytes to neurons. Lead can also act on cytochrome C and adenosine triphosphate synthase to cause dysfunction of the electron transport chain in mitochondria and generation of free radicals.[14,43] Mitochondrial dysfunction in astrocytes affects the survival of motor neurons. Accumulation of free radicals and ROS might enhance the lack of energy and glutamine, eventually causing neuronal apoptosis.
Lead triggers inflammation through a collaboration of astrocytes with microglia. The functional collaboration between astrocytes and microglia might play an important role in neuroinflammation and BBB dysfunction in the brain. Overexpression of inflammatory stimuli in the neurovascular unit may start a response to clear antigenic material, leading to destruction of the BBB as well as neuronal damage. Following lead exposure, astrocytes secrete a number of inflammatory cytokines such as TNF-α, IL-6, and IL-10 into surrounding tissues. These cytokines further mediate the immune response, including activation of microglia and macrophages, and induce other adverse reactions, which might eventually result in the destruction of BBB tight junctions. Matrix metalloproteinases (MMPs) are an important family of proteins composed of a variety of zinc-dependent enzymes that are capable of degrading extracellular matrix proteins such as collagen, gelatin, viscous protein, fibronectin, and proteoglycans. It has been hypothesized that inflammatory cytokines induce production of MMP-2 and MMP-9, two proteinases that degrade the extracellular matrix and basement membrane, in astrocytes, resulting in increased permeability of the BBB. Other studies have shown that low concentrations of pro-inflammatory cytokines (such as TNF-α or IL-1β) or lead did not influence MMP-9 expression when administered separately, but combined administration of lead and cytokines could induce a marked synergistic elevation in MMP-9 expression.
The start of an inflammatory reaction to lead exposure depends on the interaction between the inflammatory responses of astrocytes and microglial cells. Following lead exposure, activation of astrocytes surrounding blood vessels is indicated by increased expression of glial fibrillary acidic protein (GFAP).[49,50] Therefore, the response to lead in astrocytes may affect the BBB. It has been shown that lead in the brain accumulates predominantly in astrocytes, as opposed to neurons.[51,52] Another culture experiment has shown that younger astrocytes accumulate and retain more lead than older astrocytes. To protect neurons against lead, astrocytes serve as a lead pool in the process of neurogenesis. However, because astrocytes are not able to remove lead from their own cytoplasm effectively, the accumulated lead will finally cause progressive damage of astrocytes, the BBB, and nearby neurons.
Liu et al. has proposed that activation of microglia in response to pathological conditions such as trauma, stroke, or neurodegenerative disorders occurs before activation of astrocytes. For instance, the activation of astrocytes occurs subsequently to microglial activation in respect to the cytokine expression sequence in Alzheimer’s disease.[54,55] A study with trimethyltin (TMT) treated rats, a model of neurodegenerative disease, revealed that GFAP significantly increases following microglial activation and that microglial activation requires lower concentrations of TMT than activation of astrocytes. Considering that astrocytes are closer to the peripheral environment anatomically and more easily store toxic substances like lead, it may also be an imperceptible inflammatory signal released from astrocytes such as low amounts of TNF-α, free radicals, or ROS/NO that further initiates activation of microglial cells, leading to an inflammatory response.
Reciprocal activation of microglia and astrocytes mainly depends on inflammatory cytokines or their receptors. Previous studies have shown that cytokines secreted from activated microglia also promote activation of astrocytes. Among those cytokines, IL-1 is a key mediator. IL-1β, mainly from microglia, can be rapidly expressed and may work to increase the secretion other cytokines such as IL-6, mainly from astrocytes, in order to promote inflammation. Moreover, IL-1 might decrease the ability of astrocytes to reabsorb glutamic acid and promote the release of free radicals.[58,59] Experiments have shown that IL-1 receptor antagonists prevent pathological damage to astrocytes, indicating that microglia might indirectly affect the function of astrocytes. In addition, microglial activation also promotes astrocytes to secrete TGF-β1 and IL-10. When the severity of the immune response reaches a certain extent, however, TGF-β initiates a feedback loop to reduce the level of IL-1, inhibiting microglial activation and resulting in suppression of inflammation in the CNS.
Inflammatory cytokines and the inflammatory response are critical in the neurovascular unit and may result in alteration of BBB function. Brain microvascular endothelial cells (BMECs) are considered to be the anatomical and functional basis of the BBB. As they are in direct contact with the circulating blood, BMECs are highly vulnerable to the impact of the blood environment. Studies have revealed that lead toxicity in the BBB or BMECs might influence tight junction proteins. Tight junctions are key functional structures that bond BMECs together. Adhesion proteins are a component of tight junctions, and the zonula occludens (ZO) family plays a key role in connecting transmembrane proteins with actins inside the ECs to complete the structure of tight junctions. In the cultured brain microvessel endothelial cell line RBE4, lead reduces the expression of tight junction proteins and lowers TEER, causing changes in ion permeability at the BBB and brain interstitial fluid ion regulation. As ZO-1 and ZO-2 are intracellular proteins, this suggests that cytoplasmic mechanisms may be associated with this process. Increased permeability of endothelial cells along with a decrease in occludin proteins has been detected following lead exposure. The ZO family also seems to be susceptible to oxidative stress, and tight junctions are destroyed by lead-induced inflammation and ROS, leading to long-term BBB damage. However, other in vitro and in vivo experiments have revealed that claudin-1 mRNA and protein levels are downregulated without significant changes to ZO-1 and atresia proteins.[65,66]
The divalent iron ion channel [divalent metal transporter (DMT)] is a key element for the transport of iron across the BBB. Many experiments have indicated that lead could also pass through DMT in a competitive way and may occupy this transporter when iron is deficient. Lead affects the offset of iron-regulated proteins, which allows it to more easily access endothelial cells.[68,69] When the concentration of iron is elevated, the transport of lead is effectively inhibited. Interestingly, expression of fractalkine (CX3CL1), a mediator of neuron-glial signaling, is also enhanced after exposure to lead, especially in the hippocampus and forebrain. In addition, lead also passes through and interferes with calcium channels, suggesting that lead might be able to cross the BBB in multiple or unknown other ways.
In one model involving exposure to lead, increased β-amyloid (Aβ) levels were found in the choroid plexus. On the choroid epithelial cell surface, a critical transporter known as lipoprotein receptor-related protein-1 (LRP-1) is responsible for transporting Aβ out of the brain. LRP-1 knockout mice show higher levels of amyloid protein following lead exposure. Lead could induce a significant reduction in LRP-1 expression by interfering with the LRP-1 gene promoter. These studies, therefore, suggest that lead neurotoxicity might also be related to memory deficits in the pathogenesis of Alzheimer’s disease.
Lead-induced inflammatory reaction cascades within the neurovascular unit may cause neuronal damage. It has been hypothesized that TNF-α, IL-1β, and IL-6 could cause neuronal apoptosis through glial activation. Possible mechanisms of injury might be ROS production due to the pro-inflammatory cytokine IL-1β or increased glycogen consumption in astrocytes due to TNF-α and IL-1, thereby causing increased levels of toxic substances and affecting the metabolism of the cells. TNF might also be involved in the expression of NO, suggesting another way by which could inactivate LTP. Furthermore, IL-1β acts on endothelial cell tight junction proteins, reducing the amount and location of occludin and increasing the permeability of the BBB. Inflammatory reactions could also change the transport of multiple substances by affecting the role of glutamate receptors. Lead-induced chemokines, mainly secreted from neurons, have been shown to act on microglial receptors and participate in the interactions between neurons and glial cells, resulting in changes in microglial and astrocyte morphology.
Oxidative damage is fatal to brain neurons. In pathological conditions such as hypoxia, traumatic injury, and lead insult, these toxic free radicals might be over-generated and cause secondary injuries to neurons. Compared with neurons, astrocytes have higher levels of antioxidants such as glutathione (GSH), heme-oxygenase 1 and GSH S-transferase. Neurons may maintain their antioxidant capacity by transporting and utilizing these substances, among which the GSH shuttle pathway is likely to be paramount.[79,80] GSH, the most abundant antioxidant in the brain, is mainly generated in astrocytes. Astrocytes store a much higher content of GSH-related enzymes in order to guarantee a supply to neurons. GSH-depleted astrocytes display a reduced ability to protect neurons against oxidative injury.
When lead enters astrocytes, it could directly deplete NADPH. More importantly, it affects glucose 6 phosphate dehydrogenase, a key enzyme of the pentose phosphate pathway, reducing the production of NADPH. Both effects might result in a lack of GSH support from astrocytes to neurons. Lead is able to bind to GSH sulfhydryl groups and disable its ability as a ROS scavenger. Lead exposure results in an accumulation of ROS and a decrease in antioxidants. Increased levels of ROS contribute to higher BBB permeability, inducing oxidative damage to cellular molecules, activation of inflammatory mediators, and the destruction of tight junctions. ROS also inhibit glutamate transporters and cause a secondary glutamate metabolism exception, increasing the role of lead in the destruction of neurons. In addition, studies have shown that lead reduces many antioxidant molecules such as superoxide dismutase and catalase in adult mouse and rat brain.[68,83]
Finally, the phosphorylated cyclic-AMP response element binding (pCREB) is an important transcription factor for long-term memory, and lead could block the cAMP-CREB pathway by reducing pCREB, resulting in a decline in long-term memory.[6,7,13] The effect of lead exposure on CREB protein expression and phosphorylation in the cerebral cortex and hippocampus during postnatal development has been studied. Lead exposure did not affect total CREB levels, but decreased pCREB levels by about 30-38% in both cortex and hippocampus. Disruptions in pCREB expression levels and the binding activity of CREB proteins may decipher intracellular mechanisms of lead neurotoxicity in developing brains.[12,13] In addition, the protein kinase C (PKC)/NF-κB pathway might be involved in lead-induced neuroinflammatory injury to brain neurons, as it represents a key stress response signal to inflammation. The PKC/NF-κB pathway might also play a critical role in cell defense reactions and cell apoptosis. The PKC/NF-κB pathway has been shown to be involved in the regulation of NO and pro-inflammatory cytokine production in the LPS model of inflammation. PKC/NF-κB pathway downstream products such as tumor necrosis factor-related apoptosis inducing ligand, caspase-1, and NOS2 are enhanced in animal models after lead exposure.
In summary, microglial and astroglial responses might be critically involved in neuroinflammation and lead neurotoxicity in the brain. Microglia and astrocytes may have crosstalk or mutual activation by inflammatory cytokines and receptors. Lead (Pb2+) has been shown to contact and interfere with microglia and astrocytes, which may trigger microglial and astrocytic activation, enhance inflammatory cytokine generation and release, increase ROS and oxidative stress, and finally result in BBB dysfunction and neuronal injury [Figure 1]. Further extensive studies are still needed, however, to elucidate the specific signaling pathways for microglia and astrocytes partaking in neuroinflammation in the brain and to find new targets of manipulation for the prevention and treatment of lead neurotoxicity in human beings.
Figure 1. Correlative function of microglial and astroglial cells in the generation of inflammatory cytokines and neuronal injury. The lead (Pb2+) may contact and trigger microglial and astrocytic activation, enhance inflammatory cytokines and response, increase reactive oxygen species and oxidative stress, and finally result in blood‑brain barrier dysfunction, long‑term potentiation inhibition and inflammatory injury of various neurons in the brains
The National Basic Research Program (2012CB525002, 2011CB504103) and National Natural Science Foundation of China (81272346, 31371374).
There are no conflicts of interest.
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