Lead and Mercury Flashcards
what is the most efficient route of absorbing lead
inhalation exposure to leas is a more efficient route of absorption than ingestion
Lead in the lungs
Lead particles that are < 1 μm can penetrate to alveoli and absorbed via phagocytosis
Rates of absorption and retention of lead in adults and children
- Adults absorb 5-15% of ingested lead and retain less than 5% of what is absorbed
- Children absorb 42% of ingested lead with 32% retention
How is lead absorption in the intestines mediated
- Absorption from the intestine is mediate by passive and facilitated diffusion
- Not clear exactly which transporters are involved
Lead Transporters
- Divalent Metal Transporter1 (DMT1)
- Calcium binding protein (calbindin)
- can bind to both calcium and lead
How does lead pass the BBB
- Once in the bloodstream, lead most likely crosses the BBB through passive diffusion
- Passive diffusion most occurrent
- can also transfer across by cation transporters (DMT-1)
Lead and Endothelial Cells
- Lead can accumulate in endothelial cells of the BBB and epithelial cells of the blood-CSF barrier causing them to become leaky
What are Claudins
Claudins are transmembrane proteins that form the backbone of the tight junctions at the BBB
How does lead interfere with the endothelial cell function
- Accumulation in the BBB epithelial cells
- Lead exposure can decrease the mRNA and protein levels of Claudin-1
- Claudin-1 is the tight junction backbone
- Therefore, Lead effects the attachment function of the Claudin-1 and interferes with the tight junction’s original form
Lead, DMT1, and the Brain. A story of betrayal
- DMT1 is highly expressed in the brain endothelial cells of the BBB
- DMT is also expressed in the striatum, cortex, hippocampus, and cerebellum
- DMT helps direct lead within specific areas of the brain
This is called Preferential Accumulation
- evidence of this is that lead preferentially accumulated in the hippocampus
Brain Efflux mediated by ATP-dependent calcium pumps
- Lead can substitute for calcium ions and cross the BBB
- substitution of calcium and using calcium’s form to get past the BBB
Explain the Brain’s lead to calcium ratio
Higher among of lead in the blood = higher levels of calcium deposits in the brain
How does Brain Calcification occur
- Related to increased calcium in the blood or inflammation or damage to the brain
What is Brain Calcification correlated with
- Raised blood lead levels
- Associated with dementia, loss of visual acuity, psychotic episodes
- linked with many diseases and deteriorations
Where can Brain Calcification patterns be found
Calcification patterns found in the subcortical area, basal ganglia, vermis, cerebellum, thalamus, pons
Explain Iron’s Modifying Effect on Lead Exposure
- Iron-deficiency organisms retain 5x more lead then normal iron levelled organisms
- Iron is the sole user of the transporter
- when deficient, lead has multiple transporters available to it. Increasing rates of absorption
DMT1
- absorption of lead regulated through DMT1
- DMT1 is regulated at the mRNA level by Iron
- increased DMT1 due to Iron deficiency
- results in increased absorption of lead
Explain the process and results of the Lead in Rats Study
Process
- Gestation and Lactation
- mother’s exposed to high levels of lead
- Pups then exposed from the mother - Direct Pup lead exposure
- After the pup has been weaned from their mother
Results:
- lead reduces cell proliferation in the hippocampus dentate gyrus
- lead reduces cell survival in the dorsal dentate gyrus
- lead reduces BrdU labelling cells in the dentate gyrus
Cellular Effects of Lead: DCX-labelled fibres
- Lead reduces the density and length of DCX-labelled fibres in the outer molecular layer (OML) of the dorsal dentate gyrus (DG)
- Lead alters cell morphology of DCX-positive neurons in the dorsal DG (causing irregular orientations of apical dendrites (bushy- spiral-like))
Cellular Effects of Lead: Mossy Fibres in the CA3
- Mossy fibres in the CA3 region of the hippocampus
- lead exposure reduces mossy fibres in the stratum oriens
- lead exposure alters granule cell neurogenesis and morphology in the hippocampus
- effects alter neuronal circuity in the hippocampus with detrimental effects on synaptic plasticity and learning
Why is lead to toxic
- Main reason lead is so toxic is due to its ability to substitute for diverse polyvalent cations, including calcium, zinc, and magnesium at their binding sites
- These binding sites have a higher affinity for lead
Mechanisms of Lead Toxicity
- lead has substitution properties for cations at their binding sites
- These cations (calcium, zinc, magnesium) have diverse functions within the body such as;
- Many proteins as structural components
- Signalling networks are based on the association and dissociation of these cations from the proteins which they bind
- Catalytic roles
what is one of the most important targets of lead in the NS
One of the most important targets of lead in the nervous system are voltage-gated calcium channels
- E.x., N-methyl-D-aspartic acid (NMDA) receptor
Role of Voltage-gated calcium channels (VGCCs)
- Allow the flow of many mono- and polyvalent cations (e.x., sodium, potassium, calcium)
Characteristics of VGCC
- Wide permeability range due to the ionic-channel pore diameter and length
- Channel can harbour in its interior more than one ion simultaneously
Mechanisms of Lead Toxicity: Role of Calcium
- EEE locus: four glutamate residues form the selectivity filter, a high-affinity cation-binding site
- The electrostatic interactions among the carboxylic groups of glutamate and the permeating ion determine the affinity of the selectivity filter for the ion
- higher the cation affinity = faster the cation’s entrance to the VGCC channel pore and the slower its exit at the cytoplasmic end - The higher affinity of lead for the EEE locus on VGCCs c
Characteristics of the EEEE
- interactions within the EEEE locus are competitive
- Usually the cation with the highest affinity that binds to the selectivity filter and displaces other ions from the VGCCs channel pore
Characteristics of VGCC
- Selective
- Their pore-forming region has greater affinity for calcium
- A lower affinity for other cations, such as sodium or potassium, which are more abundant
- However, some heavy metals, such as lead, have a higher affinity for the EEEE than calcium
Lead, EEE and VGCC, a story of affinity
The higher affinity of lead for the EEEE locus on VGCCs causes
- Lead to displace calcium form the locus
- Lead to flow slowly flow through the pore acting as a channel blocker
Peak amplitude of NMDA-induced currents inhibition
- The peak amplitude of NMDA-induced currents are preferentially inhibited by lead
- Onset of the effect of lead is rapid (seconds)
- Offset is slow (> 30 minutes)
NMDA-induced currents in immature hippocampal neurons
- NMDA-induced current in immature hippocampal neurons is particularly sensitive to the inhibitory effects of lead
- Peak amplitude reduced in all cultured hippocampal neurons
- Amplitude reduction was greatest in the younger neurons (age 1-10 days) verse older neurons (21-30 days)
What happens when VGCCs are blocked
- Blocking VGCCs can disrupt intracellular calcium dynamics
- Which can affect synaptic development and plasticity resulting in disease states
Explain the expression of Brain-derived neurotrophic factor (BDNF)
- Brain-derived neurotrophic factor (BDNF) expression is a key calcium dependent pathway
- Exposure to lead can reduce BDNF transcripts and protein levels
- BDNF supplementation can fully mitigate the effects of lead exposure on presynaptic function and protein expression
Explain BDNF dependency
- BDNF neurotransmission is also dependent on calcium pathways
- Reductions in extracellular levels of BDNF may also be due to the disruption of the transport of BDNF vesicles to dendritic sites
- Protein released as neurons
NMDS and BDNF a story of release
- NMDA receptor release of BDNF may play a great role in dendritic BDNF release rather than axonic BDNF release
- Due to the fact that majority of NMDA receptors are postsynaptically located
Explain the calcium dependent pathway of Protein Kinase C
- Protein kinase C (PKC) is another calcium dependent pathway
- Calcium is a natural physiological activator of PKC
- Lead can substitute for calcium and can result in the activation of PKC
- Workers occupationally exposed to lead had higher levels of PKC activity in bone and blood samples
- High PKC activity was inversely related to performance on neuro-behavioural tests
What happens when PKC (calcium pathway) is impaired
Impaired PKC function can compromise second messenger systems within the cells
May lead to altered gene expression and protein synthesis
What is Long-Term Potentiation in terms of Lead
Exposure to lead in vitro and in vivo can inhibit LTP
- The exact target is unclear
What is NMDS receptor role in LTP
NMDA receptor mediated influx of calcium is necessary for the initiation of LTP
- Blocking of voltage-gated calcium channels may be part responsible
Explain the relationship between Lead and Nitric Oxide synthesis
Persistent increase in presynaptic transmitter release may be evoked by retrograde factor, nitric oxide (NO)
- Low levels of lead inhibits nitric oxide synthase (NOS) activity in the rat hippocampus, cortex, and cerebellum
What are the Mechanisms of Lead Toxicity
- Long-Term Potentiation (LTP)
2. Oxidative Stress
What is Oxidative Stress in terms of Lead Toxicity
- Lead can accumulate in mitochondria
- Mitochondria regulate intracellular calcium concentrations
- Increased entry of calcium into mitochondria can increase the production of reactive oxygen species (ROS)
- Can result in oxidative stress, inflammation, apoptosis (Cell death)
What are the forms of Mercury
- elements
- mercury vapour
- inorganic salts
- organic compounds
What is the most toxic form of Mercury
organic Hg compounds (especially methylmercury MeHg)
Explain what happens to Mercury in the environment
Elemental forms of Hg can become methylated in the environment
What is Methylmercury in terms of toxicity and how does is it expressed in the food chain
- The most toxicologically important forms of mercury
- MeHg enters the aquatic food chain with plankton, then herbivorous fish, carnivorous fish, sea mammals and then people
Toxicity: How is MeHg absorbed and distributed
- Well absorbed from the gastrointestinal tract
- 95% of MeHg from fish is absorbed
- Distributed to all tissues in about 30 hours
- 10% of absorbed MeHg is distributed to the brain
Toxicity: Explain the mechanisms of MeHg bonds
- MeHg is bound to thiol-containing molecules, such as cysteine
- When bound to cysteine it is structurally similar to L-methionine
- Mimics L-methionine to cross the BBB and placenta via neutral amino acid carriers (LAT system)
- Can also cross into neurons and astrocytes via same mechanism
Toxicity: How is MeHg distributed and excreted
- MeHg is slowly metabolized to inorganic Hg by microflora in the intestines
- 90% of MeHg is eliminated via feces and less than 10% in urine
- Half-life of MeHs is approximately 45-70 days
- MeHg accumulates in hair
What are the Acute clinical symptoms of MeHg poisoning
- tremors
- convulsions
- Irritability
- lethargy
- confusion
- decreased reflexes
- nerve conduction
- hearing
What are the Chronic clinical symptoms of MeHg poisoning
- Tremors
- ataxia
- unsteady gait
- Insomnia
- memory loss
- shyness
- depression
- anorexia
- headache
- dysarthria
- parenthesis
- death
What amount of Mercury causes Chronic mercury poisoning?
A daily intake of more than 0.3 mg will produce chronic mercury poisoning in the average 70 kg adult
Minamata Disease
- mercury poisoning in Japanese inhabitants around bays
- waste containing mercury chloride was released into the bays and became concentrated in the fish
- Resulted in the births of infants suffering from degenerative neurological disorders, blindness, deafness
what are two reasons that developing organisms are more vulnerable to mercury intoxication
- They are undergoing a period of rapid development
2. They have not fully developed the capacity to breakdown and excrete MeHg
Toxicity: MeHg and the placenta
- MeHg readily crosses the placenta to the fetus
- MeHg crosses more readily that all other forms of Hg
- Concentrations of MeHg in the fetal brain can be 5 to 7 times higher than maternal blood
Toxicity: MeHg and the Cerebral Edema
- Cerebral edema is common during acute poisoning
- Neuropathological observations indicate prominent changes to the cerebral cortex and cerebellum
- Necrosis of gray matter and cerebral atrophy
- Lysis of cell membranes
- Gliosis
Toxicity: what are the preferential accumulation sites of MeHg
Astrocytes
Toxicity: MeHg and Neurons
- neurons are highly susceptible to MeHg-induced toxicity
- Neuronal cells from different brain structures have been reported as potential targets for the toxic effects of MeHg
- Hippocampus, cerebral cortex, and cerebellum
MeHg toxic effects on the Cerebellum
- cerebellum is a brain region highly sensitive to MeHg exposure
- Reductions in cerebellar white matter volume and cerebellar width
- Abnormal migration of cerebellar granulae and Purkinje cells
- Large numbers of gemistocytic astrocytes
MeHg degeneration of the Cerebellum
- Degeneration and loss of Purkinje and granule cells
- Reduced size of cerebellar lamella
- Disarrangement and irregular thickness of external granular layer (EGL)
- Granule cells and Purkinje neurons display nuclear pyknosis (apoptosis or necrosis)
What are Bergmann Glial Cells
Bergmann glial cell fibers provide the scaffolding for granule cell migration
MeHg and Bergamann cells in Rats
- Developing rats exposed in utero and via lactation to MeHg display abnormal thickening and twisting of the Bergmann fibers
- Suggests that irregular development of these fibres may directly interfere with granule cell migration
- Results in heterotopic localization of these cells in the cerebellar cortex
What doe In-Vitro application or MeHg in microglial/astrocytes cultures lead to
- Reduced secretion of pro-inflammatory cytokine interleukin-6
- Decreased glutamate uptake by astrocytes
Excitotoxicity (presence of MeHg) - Cell death
- Increases inflammation, - - cell stress, and cell death
In vivo (animal models) MeHg intoxication resulted in astrocytosis (or the abnormal increase in astrocytes)
- Corresponds to elevated levels of apoptosis in the cerebellum
- Suggests astrocytes are up-regulated following MeHg-induced neuronal cell death
MeHg and Astrocytes
- Astrocytes also swell in response to neuronal damage
- Results in reduced extracellular volume
- Leads to increased neuronal excitability
- MeHg exposure prevents the natural decrease in astrocytic swelling that occurs over time
- Results in the elevated release of endogenous glutamate and aspartate which can lead to neuronal death
Behavioural outcomes related to cerebellar dame following MeHg exposure
- impairments in balance
- motor coordination
- fine motor movement
- locomotor activity
*tested in mice on the roterod pole test following acute exposure to low dose MeHg
What is the Cerebellum responsible for?
- Relation to motor coordination and balance
- Learning, memory, conditioning
Why are their variations in the vulnerability to MeHg intoxication across ages
- The cerebellum begins to develop in utero and continues throughout the postnatal period
- age may be directly related to the developmental processes that are occurring during the different developmental time periods
Three stem mechanism of MeHg Toxicity: Oxidative Stress
- Interaction with glutathione (GSH)
- Interference with normal maturation of the antioxidant GSH system during weanling period
- Disrupts the mitochondrial electron transport chain
First stage of MeHg Oxidative Stress Mechanism: Interaction with glutathione (GSH)
- Hg atom directly interacts with the thiol group of GSH
- results in the formation of an excitable GS-MeHg complex which reduces GSH levels
- Contributes to oxidative stress as lower GSH levels increases brain become more susceptible to reactive oxygen species)
Overview of GSH
- GSH is a compound in the antioxidant system
- GSH contains a thiol group
In Vitro GSH levels based on MeHg exposure
- In vitro, decreased GSH levels were reported following MeHg exposure in
- Neuronal and glial primary cultures
- Mitochondria from the mouse brain
In Vivo GSH levels based on MeHg exposure
- In vivo, decrease GSH levels in the cerebellum after exposure to MeHg
- Weaning animals more susceptible than adults
Second stage of MeHg Oxidative Stress Mechanism: Interferes with normal maturation of the antioxidant GSH system during weanling period
- Early Postnatal period MeHg exposure
- Interferes with the physiological increase in glutathione reductase (GR_ and glutathione Peroxidase (GPx) in the CNS
- GR and GPx are central enzymes involved with the detoxification of peroxides and the reduction of oxidized glutathione (GSSG)
- Renders brain more susceptible to ROS
Third stage of MeHg Oxidative Stress Mechanism:
Disrupts the mitochondrial electron transport chain
- Leads to increased formation of ROS
- ROS species include hydrogen peroxide and superoxide anion
MeHg neurotoxicity in Glutamate dyshomeostasis
- MeHg disrupting homeostatic levels of Glutamate in the brain
- It is important to maintain Glutamate balance as too much Glutamate in the synaptic cleft causes it to act as a toxin
- ## This toxin causes neuronal damage and cell death
The role of Glutamate in our bodies
- Glutamate is the major excitatory neurotransmitter in the mammalian CNS
Plays a role in:
- behaviour
- development
- learning
- memory
- response to injury
Glutamate Equilibrium Maintenance
Equilibrium of extracellular levels of glutamate is maintained primarily by glutamate transporters on astrocytes
What are the two methods of MeHg toxicity on Glutamate
- Increase glutamate release
2. Inhibition of glutamate uptake
First Method of MeHg toxicity on Glutamate: Increasing release
- MeHg increased the spontaneous, above average release of glutamate in cerebellum and neuronal cells extracellular space
- Results in decreased vesicular uptake of glutamate
Second Method of MeHg toxicity on Glutamate: Inhibition of Glutamate uptake
- MeHg readily inhibits glutamate uptake into cultured astrocytes, neurons, and synaptic vesicles
- Related to inhibitory effects on astrocyte glutamate transporters
- Increased levels of hydrogen peroxide mediate the inhibitory effects
What happens when there is increased Glutamate in the synapse
Increased glutamate in the synapse can leads to the over-activation of NMDA glutamate receptors resulting in increased calcium influx into neurons
- The influx of calcium can active important pathways involved in cell death
- Chain of events > increased production of cell species > cell death - The calcium can be taken up by mitochondria where it can stimulate the generation of ROS
