Scientists have found new evidence in lab-grown mouse brain
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cells, called astrocytes, that one root of Alzheimer’s disease may be a
simple imbalance in acid-alkaline or pH chemistry inside
endosomes, the nutrient and chemical cargo shuttles in cells.
Astrocytes work to clear so-called amyloid beta proteins from the
spaces between neurons, but decades of evidence has shown that if the
clearing process goes awry, amyloid proteins pile up around neurons,
leading to the characteristic amyloid plaques and nerve cell
degeneration that are the hallmarks of memory-destroying Alzheimer’s
disease.
The new study, described online in Proceedings of the National Academy of Sciences,
also reports that the scientists gave drugs called histone deacetylase
(HDAC) inhibitors to pH-imbalanced mice cells engineered with a common
Alzheimer’s gene variant.
The experiment successfully reversed the pH
problem and improved the capacity for amyloid beta clearance.
HDAC inhibitors are approved by the U.S. Food and Drug Administration
for use in people with certain types of blood cancers, but not in
people with Alzheimer’s. They cautioned that most HDAC inhibitors cannot
cross the blood-brain barrier, a significant challenge to the direct
use of the drugs for brain disorders.
The scientists say they are
planning additional experiments to see if HDAC inhibitors have a similar
effect in lab-grown astrocytes from Alzheimer’s patients, and that
there is the potential to design HDAC inhibitors that can cross the
barrier.
However, the scientists caution that even before those experiments
can happen, far more research is needed to verify and explain the
precise relationship between amyloid proteins and Alzheimer’s disease,
which affects an estimated 50 million people worldwide. To date, there
is no cure and no drugs that can predictably or demonstrably prevent or
reverse Alzheimer’s disease symptoms.
“By the time Alzheimer’s disease is diagnosed, most of the
neurological damage is done, and it’s likely too late to reverse the
disease’s progression,”
says Rajini Rao, Ph.D., professor of physiology
at the Johns Hopkins University School of Medicine.
“That’s why we need
to focus on the earliest pathological symptoms or markers of Alzheimer’s
disease, and we know that the biology and chemistry of endosomes is an
important factor long before cognitive decline sets in.”
Nearly 20 years ago, scientists at Johns Hopkins and New York
University discovered that endosomes, circular compartments that ferry
cargo within cells, are larger and far more abundant in brain cells of
people destined to develop Alzheimer’s disease.
This hinted at an
underlying problem with endosomes that could lead to an accumulation of
amyloid protein in spaces around neurons, says Rao.
To shuttle their cargo from place to place, endosomes use chaperones
— proteins that bind to specific cargo and bring them back and forth
from the cell’s surface. Whether and how well this binding occurs
depends on the proper pH level inside the endosome, a delicate balance
of acidity and alkalinity, or acid and base, that makes endosomes float
to the surface and slip back down into the cell.
Embedded in the endosome membrane are proteins that shuttle charged
hydrogen atoms, known as protons, in and out of endosomes. The amount of
protons inside the endosome determines its pH.
When fluids in the endosome become too acidic, the cargo is trapped
within the endosome deep inside the cell. When the endosome contents are
more alkaline, the cargo lingers at the cell’s surface for too long.
To help determine whether such pH imbalances occur in Alzheimer’s
disease, Johns Hopkins graduate student Hari Prasad scoured scientific
studies of Alzheimer’s disease looking for genes that were dialed down
in diseased brains compared with normal ones. Comparing a dataset of 15
brains of Alzheimer’s disease patients with 12 normal ones, he found
that 10 of the 100 most frequently down-regulated genes were related to
the proton flow in the cell.
In another set of brain tissue samples from 96 people with
Alzheimer’s disease and 82 without it, gene expression of the proton
shuttle in endosomes, known as NHE6, was approximately 50 percent lower
in people with Alzheimer’s disease compared with those with normal
brains. In cells grown from people with Alzheimer’s disease and in mouse
astrocytes engineered to carry a human Alzheimer’s disease gene
variant, the amount of NHE6 was about half the amount found in normal
cells.
To measure the pH balance within endosomes without breaking open the
astrocyte, Prasad and Rao used pH sensitive probes that are absorbed by
endosomes and emit light based on pH levels.
They found that mouse cell
lines containing the Alzheimer’s disease gene variant had more acidic
endosomes (average of 5.37 pH) than cell lines without the gene variant
(average of 6.21 pH).
“Without properly functioning NHE6, endosomes become too acidic and
linger inside astrocytes, avoiding their duties to clear amyloid beta
proteins,”
says Rao.
While it’s likely that changes in NHE6 happen over time in people who
develop sporadic Alzheimer’s disease, people who have inherited
mutations in NHE6 develop what’s known as Christianson syndrome in
infancy and have rapid brain degeneration.
Prasad and Rao also found that a protein called LRP1, which picks up
amyloid beta proteins outside the astrocyte and delivers them to
endosomes, was half as abundant on the surface of lab grown mouse
astrocytes engineered with a human gene variant called APOE4, commonly
linked to Alzheimer’s disease.
Looking for ways to restore the function of NHE6, Prasad searched
databases of yeast studies to find that HDAC inhibitors tend to increase
expression of the NHE6 gene in yeast. This gene is very similar across
species, including flies, mice and humans.
Prasad and Rao tested nine types of HDAC inhibitors on cell cultures
of mouse astrocytes engineered with the APOE4 gene variant.
Broad-spectrum HDAC inhibitors increased NHE6 expression to levels
associated with mouse astrocytes that did not have the Alzheimer’s gene
variant.
They also found that HDAC inhibitors corrected the pH imbalance
inside endosomes and restored LRP1 to the astrocyte surface, resulting
in efficient clearance of amyloid beta protein.
Story source:
Johns Hopkins Medicine
