It's long been known that people living with HIV experience a loss of white
matter in their brains. As opposed to "gray matter," which is composed of
the cell bodies of neurons, white matter is made up of a fatty substance
called myelin that coats neurons, offering protection and helping them
transmit signals quickly and efficiently. A reduction in white matter is
associated with motor and cognitive impairment.
Earlier work by a team from the University of Pennsylvania and Children's
Hospital of Philadelphia (CHOP) found that antiretroviral therapy (ART)--the
lifesaving suite of drugs that many people with HIV use daily--can reduce
white matter, but it wasn't clear how the virus itself contributed to this
loss.
In a new study using both human and rodent cells, the team has hammered out
a detailed mechanism, revealing how HIV prevents the myelin-making brain
cells called oligodendrocytes from maturing, thus putting a wrench in white
matter production. When the researchers applied a compound blocking this
process, the cells were once again able to mature.
The work is published in the journal Glia.
"Even when people with HIV have their disease well-controlled by
antiretrovirals, they still have the virus present in their bodies, so this
study came out of our interest in understanding how HIV infection itself
affects white matter," says Kelly Jordan-Sciutto, a professor in Penn's
School of Dental Medicine and co-senior author on the study. "By
understanding those mechanisms, we can take the next step to protect people
with HIV infection from these impacts."
"When people think about the brain, they think of neurons, but they often
don't think about white matter, as important as it is," says Judith
Grinspan, a research scientist at CHOP and the study's other co-senior
author. "But it's clear that myelination is playing key roles in various
stages of life: in infancy, in adolescence, and likely during learning in
adulthood too. The more we find out about this biology, the more we can do
to prevent white matter loss and the harms that can cause."
Jordan-Sciutto and Grinspan have been collaborating for several years to
elucidate how ART and HIV affect the brain, and specifically
oligodendrocytes, a focus of Grinspan's research. Their previous work on
antiretrovirals had shown that commonly used drugs disrupted the function of
oligodendrocytes, reducing myelin formation.
In the current study, they aimed to isolate the effect of HIV on this
process. Led by Lindsay Roth, who recently earned her doctoral degree within
the Biomedical Graduate Studies group at Penn and completed a postdoctoral
fellowship working with Jordan-Sciutto and Grinspan, the investigation began
by looking at human macrophages, one of the major cell types that HIV
infects.
Scientists had hypothesized that HIV's impact on the brain arose indirectly
through the activity of these immune cells since the virus doesn't infect
neurons or oligodendrocytes. To learn more about how this might affect white
matter specifically, the researchers took the fluid in which macrophages
infected with HIV were growing and applied it to rat oligodendrocyte
precursor cells, which mature into oligodendrocytes. While this treatment
didn't kill the precursor cells, it did block them from maturing into
oligodendrocytes. Myelin production was subsequently also reduced.
"Immune cells that are infected with the virus secrete harmful substances,
which normally target invading organisms, but can can also kill nearby
cells, such as neurons, or stop them from differentiating," Grinspan says.
"So the next step was to figure out what was being secreted to cause this
effect on the oligodendrocytes."
The researchers had a clue to go on: Glutamate, a neurotransmitter, is known
to have neurotoxic effects when it reaches high levels. "If you have too
much glutamate, you're in big trouble," says Grinspan. Sure enough, when the
researchers applied a compound that blunts glutamate levels to HIV-infected
macrophages before the transfer of the growth medium to oligodendrocyte
precursors, the cells were able to mature into oligodendrocytes. The result
suggests that glutamate secreted by the infected macrophages was the culprit
behind the precursor cells getting "stuck" in their immature form.
There was another mechanism, however, that the researchers suspected might
be involved: the integrated stress response. This response integrates
signals from four different signaling pathways, resulting in changes in gene
expression that serve to protect the cell from stress or to prompt the cell
to die, if the stress is overwhelming. Earlier findings from
Jordan-Sciutto's lab had found the integrated stress response was activated
in other types of brain cells in patients who had cognitive impairment
associated with HIV infection, so the team looked for its involvement in
oligodendrocytes as well.
Indeed, they found evidence that the integrated stress response was
activated in cultures of oligodendrocyte precursor cells.
Taking this information with what they had found out about glutamate,
"Lindsay was able to tie these two things together," Jordan-Sciutto says.
She demonstrated that HIV-infected macrophages secreted glutamate, which
activated the integrated stress response by turning on a pathway governed by
an enzyme called PERK. "If you blocked glutamate, you prevented the
activation of the integrated stress response," Jordan-Sciutto says.
To take these findings further, and potentially test out new drug targets to
address HIV-related cognitive impairments, the team hopes to use a
well-characterized rat model of HIV infection.
"HIV is a human disease, so it's a hard one to model," says Grinspan. "We
want to find out if this model recapitulates human disease more accurately
than others we've used in the past."
By tracking white matter in this animal model and comparing it to imaging
studies done on patients with HIV, they hope to get at a better
understanding of what factors shape white matter loss. They're particularly
interested in looking at a cohort of adolescents being treated at CHOP, as
teens are a group in whom HIV infection rates are climbing.
Ultimately, the researchers want to discern the effects of the virus from
the drugs used to treat it in order to better evaluate the risks of each.
"When we put people on ART, especially kids or adolescents, it's important
to understand the implications of doing that," says Jordan-Sciutto.
"Antiretrovirals may prevent the establishment of a viral reservoir in the
central nervous system, which would be wonderful, but we also know that the
drugs can cause harm, particularly to white matter.
"And then of course we can't forget the 37 million HIV-infected individuals
who live outside the United States and may not have access to
antiretrovrials like the patients here," she says. "We want to know how we
can help them too."
Reference:
Roth LM, Akay-Espinoza C, Grinspan JB, Jordan-Sciutto KL. HIV-induced
neuroinflammation inhibits oligodendrocyte maturation via
glutamate-dependent activation of the PERK arm of the integrated stress
response. Glia. 2021. doi:
10.1002/glia.24033