4)
Temporality of aluminum accumulation
occurring before AD:
This criterion requires that the causative agent
occurs prior to the outcome. Therefore
chronic aluminum exposure must precede AD if chronic aluminum intake is the
environmental cause of AD.
For the last 125 years we
have lived in the “aluminum age” during which there has been a steady increase
in our exposure to aluminum. We ingest food, pharmaceuticals, and drink water
containing aluminum, we apply aluminum containing products to our skin, we are
vaccinated with aluminum containing vaccines, and we inhale air containing
aluminum112. This results in
a slow accumulation of aluminum in our brains from the fetal stage to old age28,49,113,114. Therefore humans living in an industrialized
society accumulate aluminum in certain regions of their brains many years
before the onset of AD. There are three
sub-cellular changes in brain physiology that occur prior to overt AD in humans
and all three lead to AD:
·
Progressive aluminum accumulation in neurons
·
Hyperphosphorylation of tau due to aluminum inhibition of an enzyme
·
Oxidative stress due to aluminum
Increasing aluminum exposure and accumulation is in lock-step with the
increasing frequency of AD. AD was
described as a rare disease in The Lancet fifteen years after Alzheimer’s 1911
paper115. The reported number of AD cases rose from one in 1907 to
more than 90 by 1935116.
Subsequently the age-adjusted death rate for AD in the U.S. rose from
0.4 per 100,000 in 1979 to 25 per 100,000 in 2010117,118. In the 25
year span from 1980 to 2004 the annual U.S. death rate from AD in those over 65
rose from 1,037 to 65,313 per year117.
It is estimated that in North America the mean aluminum intake is 24mg
of aluminum per day, equivalent to more than 8.76 grams per year118. The demand for aluminum products has
increased requiring more and more aluminum be extracted and refined from
bauxite deposits. The current annual
global demand for aluminum is 11 kg per person119. This means approximately 0.08% of the
aluminum produced each year is ingested.
Demand for aluminum has increased 30-fold since 1950 and is estimated to
increase by 3-fold current levels by 2050119. Using these data on aluminum demand, it is estimated
that human exposure to aluminum has and will continue to increase at a rate of 90
fold over the 100 year period from 1950 to 205039. This means we only ingested 0.29 grams of
aluminum per year in 1950 and by 2050 we will be ingesting more than 26 grams
per year.
So temporality exists as aluminum accumulates in our bodies prior to the
onset of AD. In addition the rate of
ingestion and accumulation of aluminum is increasing and this accounts for the
rising prevalence of AD.
5)
Biological Gradient with
Dose-response Effects of Aluminum and AD:
In 1996 McLachlan, et al. observed a dose-response in the amount of
aluminum in drinking water with the risk of AD in humans75. Each subject’s residential and drinking water
history for the 10-year period prior to death were taken into account. The drinking water subjects were exposed to
varied from less than 100mcg/L to 175mcg/L.
A single pathologist performed a histopathological examination of all
614 brains included in this study. The brains were assigned to AD or control
groups based upon clinical history and the presence or absence of plaques and
NFTs. The results in the following table demonstrate a dose-response
relationship between aluminum in drinking water and AD.
6)
Biological Plausibility
of Aluminum Neurotoxicity Causing AD:
It is known that aluminum facilitates the formation of Aβ plaques and NFTs in the brain that are two hallmarks of AD16,17,24,25.
Aluminum causes oxidative stress that kills mitochondria and ultimately kills neurons53. This results in mitochondrial disease and increased
atrophy of some brain regions both of which are clinical symptoms of AD.
Aluminum also disrupts memory storage that is a behavioral symptom of AD44.
Some metal ions, such as
aluminum, act as physiological stressors in the brain by stimulating brain
cells to produce oxidizing chemicals (a.k.a. ROS)121,122. This ROS
can damage and kill mitochondria and neurons creating inflammation in the
brain. Aluminum tops the list of metal ion inducers of ROS in human brain’s
glial cells58.
It has been observed from microscopic evidence that aluminum causes
lesions in the brain’s perforant pathway that result in short term memory loss44. Aluminum also acts as individual ions to
block the neurochemistry of long and short term memory storage123.
This mechanism of action explains why very small amounts of aluminum in the
brain (i.e. on the order of several parts per million or micrograms per gram of
brain on a dry weight basis) can have a very large impact on memory storage.
Calmodulin is a
calcium-binding messenger protein required for memory formation and
storage. Aluminum ions modify its
structure thereby inhibiting its function123. This prevents calmodulin from regulating
calcium levels in neurons and also prevents the activation of four key enzymes
that control memory formation and storage in neurons.
The neurochemical explanation
of how memories are stored in neural networks is still evolving. However
considerable detail has already been discovered. The ground-work was laid by Donald Hebb in
1949124 when he described a theory of neuronal learning as:
“Neurons that fire together - wire together and neurons that are
out of sync - do not link”.
The neurochemical mechanism
that supports Hebbian Theory involves the synchronized firing of several
different types of neuroreceptors at a synapse between two neurons. When this occurs in synchrony it leads first
to stronger or potentiated neuronal connection between the two neurons. This connection is then made even stronger by
several types of neuroreceptors moving their location in order to increase
their density at the synapse. The theory
that describes this two- step process of strengthening neuro-circuits is called
spike-timing dependent plasticity (STDP)125. The successful result of this process is called
long term potentiation (LTP). STDP and
LTP are theorized to be the way memories are stored. A lack of synchrony in the process leads to
no potentiation and is called long term depression (LTD) or lost memories. Aluminum ions inhibit calmodulin from activating
four key enzymes involved in LTP16,101,123,126-129. Thereby aluminum
ions encourage LTD and cause memory loss (see Neurochemistry of Memory
Impairment by Aluminum for details on role of these four enzymes in memory
storage).
The biological plausibility
of aluminum causing AD is well established by those studies that have connected
aluminum’s neurotoxicity with the hallmarks and symptoms of AD.
7)
Coherence of what we know about how aluminum neurotoxicity causes
AD:
Aluminum taken in by ingestion alone is estimated to be 24mg a day of
which approximately 0.2% is absorbed into our blood118,130,131. We know
that aluminum accumulates more in some areas of the human brain such as memory
processing regions86. This accumulation
likely results in chronic aluminum neurotoxicity and the hallmarks and symptoms
of AD. The cells in these regions have
very high energy needs. The high rate of
energy utilization increases the demand for iron. Transferrin is the molecule that carries iron
to these cells. Therefore these cells
have a high density of transferrin receptors on their membrane in order to
facilitate iron uptake. Aluminum and
iron ions are almost equivalent in size and can have the same ionic charge. This allows aluminum to be carried by
transferrin into these cells in higher than normal amounts even though the
cells have no need for aluminum.
Some metal ions act as
physiological stressors in the brain by stimulating brain cells to produce oxidizing
chemicals (a.k.a. ROS)121,122. The metal ions stimulate inducible
nitric oxide synthase (iNOS) in microglial and astroglial cells of the brain to
produce nitric oxide (NO) that reacts to produce ROS122. This ROS
can damage and kill neurons creating inflammation in the brain. The following table shows how much ROS is
produced from a cell culture of human glial cells exposed to 50nM aqueous
solutions of various common metal ions58. Aluminum tops the list of metal ion inducers
of ROS in human brain’s glial cells.
The brain damage caused
by aluminum inducing ROS could partially account for the neuronal death that
underlies brain atrophy. This atrophy is seen in those areas the brain that are
aluminum “hot spots” and it parallels aluminum accumulation in those areas of our
brains as we age86,94.
Neurofibrillary tangle (NFT) formation
in the brain is a hallmark of AD.
Aluminum has been shown to participate in NFT formation in both
pre-tangle and tangle-bearing cells132. Aluminum
inhibits the activity of enzyme PP2A that clips off excess phosphoryl groups on
a structural protein of the brain called tau22,133. Aluminum also
inhibits the expression of a gene involved in making PP2A100.
Aluminum creates a lack of active PP2A that results in tau being coated with
more than the normal number of phosphoryl groups. This accounts for low PP2A
activity and paired helical filaments (PHFt) found in the brains of AD patients133. In AD brains aluminum
secondarily aggregates the PHFt into
granules that fuse and grow into cytoplasmic pools of PHFt and aluminum that give rise
to NFT filaments132. Aluminum and PHFt give rise to NFTs in brain cells, including large pyramidal and
stellate cells, particularly in the brains of those with AD132. Pyramidal cells are found in many regions of
the brain, including the hippocampus, entorhinal, and prefrontal cortex.
Aβ plaque formation in the brain is another hallmark of AD. Aβ plaques form from Aβ peptides that are
cleaved from large Aβ precursor proteins (APP).
This process is called amyloidogenic cleavage and alteration of this
process is a key feature of AD134.
Beneficial non-amyloidogenic cleavage of APP leads to a secreted product
that is important for promoting neurite growth and maintaining brain
tissue. Protein phosphorylation
stimulates the beneficial non-amyloidogenic pathway. Both protein kinase C (PKC) activity that
increases phosphorylation and protein phosphatase 2A (PP2A) activity that
decreases phosphorylation are involved in the control of how much of each competing
pathway is used for APP cleavage135. Activation of PKC decreases
production of Aβ peptides by 50-80% and increases the beneficial non-amyloidogenic
cleavage by 30-50%135. Nanomolar concentrations of aluminum reduce
PKC activity by 90%136.
Therefore inhibition of PKC activity by aluminum directs APP to the
amyloidogenic pathway resulting in more Aβ peptide135. This situation is partially modulated by
aluminum’s inhibition of PP2A135.
Microtubules are important
neuronal structural features that are required for strength, rigidity, and
transportation of cell constituents between the nucleus of the cell and the
synapses. Human pyramidal cells that contain
NFTs and/or high levels of aluminum accumulation are microtubule-depleted44.
Aluminum-induced microtubule depletion
is possibly more fundamental to AD neuropathology than AB oligomers, AB plaques,
or NFTs74. This is because microtubule depletion is more damaging to
neuronal connectivity and function than these hallmarks of AD neuropathology
that may represent protective cell responses to aluminum 132,137.
Aluminum-induced microtubule depleted cells have axonal and dendritic dieback
that is consistent with AD being associated with neuronal disconnection. In addition aluminum-induced microtubule
depletion leads to synapse breakdown and depletion44,138. This
explains why humans with AD have impaired axonal transport139,140.
Neuronal death is marked in
the brain by ghost NFTs that can be the result of aluminum accumulation in the
neuron prior to death. NFTs inside the
pyramidal cells tend to displace the cell nucleus to the periphery resulting in
denucleation. The denucleated cell is unable to renew cellular membranes and
eventually the cell membrane ruptures74. This results in an extracellular ghost NFTs
that act as tombstones of former neurons and a hallmark of AD.