A Proposed Role for Glyphosate in Shared Pathway Disruptions in
Autism, Alzheimer’s Disease and Cancer
Perhaps surprisingly, autism and Alzheimer’s disease overlap
considerably in symptoms, genetics, and mechanisms, suggesting a similar
underlying pathology [105]. PIN1 is normally expressed at very high
levels in neurons, but it is inhibited in neurons in Alzheimer’s disease
via multiple mechanisms, including downregulation, oxidation,
phosphorylation and sequestration [106]. Just as autism rates over
time are highly correlated with glyphosate usage on core crops,
age-adjusted deaths due to Alzheimer’s disease over time had a
correlation coefficient R of 0.917 with glyphosate usage on corn and soy
crops in the United States, with a p-value of 2.2E-7 for the probability
of a chance occurrence [57].
Also surprisingly, autism and cancer share common features involving
mitochondrial dysfunction and dysregulated metabolic pathways, including
p53, AKT, mTOR, WNT, NOTCH, and MAPK [107]. There is considerable
overlap in the risk genes linked to both conditions [108].
Dysregulated ERK/MAPK signaling can lead to mitochondrial dysfunction
upon toxic exposures in both neurons and tumor cells [109]. PIN1
overexpression is a well-known feature of tumor cells, where it
facilitates mitosis, proliferation, and metastasis [110].
By contrast, we argue here that PIN1 is suppressed in neurons in
association with autism, as is the case for many neurodegenerative
diseases, especially Alzheimer’s [111,112]. A neuron is a fully
differentiated cell type that is intricately connected to large networks
of other neurons. They do not have the option of dividing in the face of
stressors. As a consequence, upon toxic exposures, their regulatory
pathways have an opposite effect on PIN1 compared to cancer cells, and
this drives them toward apoptotic signalling rather than inducing
cellular proliferation following severe DNA damage. Oxidative stress can
cause severe DNA damage [113]. Much of what is known about PIN1’s
function in cells was discovered through studies on cancer cells. In
future sections we will reference this literature to help explain the
many roles of PIN1 in biology.
Overexpressed mTOR Signaling Pathway: Autism is associated with
increased dendritic spine density and reduced developmental spine
pruning in neurons. These defects are correlated with overexpressed mTOR
signaling and impaired autophagy. This potential for autophagy
impairment would result in serious interference in the associated cell
death mechanisms that can consequently lead to development of disease
[114]. Designer mice have been created with constitutively expressed
mTOR activity due to a defect in gene expression of proteins that
inhibit mTOR. These mice exhibit autistic behaviors and have the same
characteristic spine pruning defects in pyramidal neurons in the
temporal lobe, along with impaired autophagy [115]. Microglia play
an important role in synaptic pruning, as mice with deficient autophagy
in microglia are impaired in synaptic pruning and exhibit behaviors
characteristic of autism [116].
Wnt/β-catenin Signaling Pathway: Several of the genes
associated with autism converge in the regulation of the Wnt/β-catenin
signaling pathway. Both gain and loss of function in this pathway
contribute to abnormalities in embryonic brain development associated
with autism [117]. Cultivated neurons exposed to glyphosate were
impaired in differentiation and growth, eliciting shorter and unbranched
axons, and developing less complex dendritic arbors compared to controls
[118]. These features are characteristic of impaired neuronal
maturation in autism [119]. In the glyphosate study, it was found
that both the expression of Wnt5a and the activity of the
serine/threonine kinase CaMKII were decreased [118].
Autophosphorylation of CaMKII increases its activity and prolongs the
duration of its active state. A missense mutation in
CaMKII that results in impaired
autophosphorylation and more rapid turnover is associated with autism.
When this mutation was introduced in mice, they displayed autistic-like
behaviors [120]. Rat pups exposed to glyphosate in utero showed
inhibition of the Wnt5a-CaMKII signaling pathway, associated with
defects in motor activity and cognitive function [121].
The corpus callosum is the largest white matter tract in the brain, and
it connects the two cerebral hemispheres together. Wnt5a-evoked CaMKII
signaling instructs specific growth and guidance behaviors in the corpus
callosum, controlling its development [122]. Intriguingly, the
corpus callosum is thin and underdeveloped in PIN1 knockout mice
[123]. Many neurodevelopmental disorders have been linked to
malformation of the corpus callosum, including autism, ADHD, and
schizophrenia [124]. BTBR mice, a well-established model for mouse
autism, exhibit complete agenesis of the corpus callosum [125]. A
large percentage of humans suffering from agenesis of the corpus
callosum display many traits associated with autism, including deficits
in communication and social skills and repetitive behaviors [126].
The DAPK1 regulatory function, which inhibits PIN1 activity, is central
to CaMKII autophosphorylation of threonine at position 286 (Thr286),
which is required for synaptic long-term potentiation (LTP) and
depression (LTD) [127,128]. Both LTP and LTD, which oppose each
other, are involved in synaptic plasticity. The autophosphorylation of
CaMKII
results in the binding of the molecule to the N-methyl-D-aspartate
(NMDA) receptor (NMDAR) subunit GluN2B and accumulation of CaMKII during
LTP. DAPK1, through activating calcineurin (CaN), blocks CaMKII from
binding to GluN2B by competitive inhibition. This creates a fine
balance, regulated by Ca2+/CaM influx, which determines whether CaMKII
will become attached to GluN2B (establishing LTP), or not (establishing
LTD), a balance that is deregulated in autism. Specifically, DAPK1
overexpression, which we have argued can happen due to glyphosate’s
inhibition of melatonin synthesis, inhibits the accumulation of CaMKII
in the synapse following LTP stimuli [128].
Wnt/β-catenin signaling increases both mRNA expression and protein
synthesis of the cell adhesion molecule neuroligin-3, which in turn is
essential for the maturation of synapses. Genetic mutations in
neuroligin-3 are associated with autism [129]. β-catenin is a major
substrate for PIN1 in neural progenitor cells. The developing brain of
PIN1 knockout mice shows reduced expression of β-catenin during
differentiation, leading to significantly fewer upper layer neurons in
the motor cortex [130].
The Integrin β1/FAK/SRC Signaling Pathway: Tetraspanin 7
(TSPAN7) is a master regulator of morphological changes that take place
during cell differentiation through cytoskeleton remodeling [131].
Focal adhesion kinase (FAK) is a tyrosine kinase that regulates cellular
adhesion, motility, proliferation, and survival, and it interacts with
TSPAN7 through the Integrin β1/FAK/SRC signaling pathway. FAK plays an
important role in neural migration, dendritic morphology, axonal
branching, and synapse formation, all of which are dysregulated in
autism [132]. Activation and autophosphorylation of FAK promote
neurite formation in neurons [133]. TSPAN7 knockout rats exhibit
autism-like behaviors, and this has been linked to impairments in this
pathway, which is critical for neurite outgrowth [134].
One of the major pathogenic features of autism is reduced cell
migration. Lymphoblasts share features with neurons, and it has been
discovered that growing them in culture can serve a useful role to
identify systemic pathologies in pathways linked to specific diseases.
The protein expression level of FAK is significantly decreased in
autistic lymphoblasts, and this was associated with increased adhesion
properties and decreased migration, attributed to impaired FAK function
in the lymphoblasts [132]. PIN1 isomerizes two prolines in FAK
adjacent to Ser-910 and Ser-571, and this leads to dephosphorylation of
FAK Tyr-397 [110]. PIN1 isomerization is required to recruit the
phosphatase to FAK. Dephosphorylation of FAK at Tyr397 inhibits FAK
kinase activity, promoting the disassembly of focal adhesion and
enhancing tumor cell metastasis [135]. Experimentally, FAK Tyr-397
dephosphorylation causes cells to round up and lose their attachment to
focal adhesions, promoting cellular migration [136]. Thus, low
expression of PIN1 would explain the pathology observed in autism,
because it leads to suppression of FAK Tyr-397 dephosphorylation.
Autistic neurons have reduced migratory capabilities due to increased
adhesion complexes that lead to migratory defects, a process in which
PIN1 inactivation and DAPK1 over-activation are major participators.
This defect interferes with the maturation process of neurons in the
brain during neurodevelopment.