Self-destructing mitochondria may leave some brain cells vulnerable to ALS

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A newly discovered type of mitochondrial
self-destruction may make some brain cells vulnerable to ALS, also known as Lou
Gehrig’s disease.

In mice genetically engineered to
develop some forms of a degenerative nerve disease similar to amyotrophic
lateral sclerosis, energy-generating organelles called mitochondria
appear to dismantle themselves without help
from usual cell demolition
crews.

This type of power plant
self-destruction was spotted in upper motor neurons, brain nerve cells that
help initiate and control movements, but not in neighboring cells, researchers
report November 7 in Frontiers in
Cellular Neuroscience
. Death of those upper motor neurons is a hallmark of
ALS, and the self-destructing mitochondria may be an early step that sets those
cells up to die later.

Pembe Hande Özdinler, a cellular
neuroscientist at Northwestern University Feinberg School of Medicine in
Chicago, and her colleagues have dubbed the mitochondrial dissolution
“mitoautophagy.” It is a distinct process from mitophagy, the usual way that
cellular structures called autophagosomes and lysosomes remove damaged
mitochondria from the cell, Özdinler says.

Usually, clearing out old or damaged
mitochondria is important for cells to stay healthy. When mitochondria sustain
too much damage, they may trigger the
programmed death of the entire cell, known as apoptosis
(SN: 8/9/18).

Özdinler’s team spotted what she
describes as “awkward” mitochondria in electron microscope images of upper
motor neurons from 15-day-old mice. These unweaned mice are equivalent to human
teenagers, Özdinler says. ALS typically doesn’t strike until people are 40 to
70 years old. But by the time symptoms appear, motor neurons are already
damaged, so Özdinler’s group looked at the young mice to capture the earliest
signs of the disease.

The mice in the study had forms of
ALS-like diseases caused by buildup of one of three abnormal proteins: SOD1,
profilin or TDP-43. Only mice with abnormal TDP-43 or profilin proteins had
mitochondria that dismantled themselves. Mitochondria in rodents with faulty
SOD1 followed the usual removal routes.

Even in the very young TDP-43 mice,
mitochondria in the upper motor neurons looked strange and “not too healthy,” Özdinler
says. “After we systematically analyzed more than 200 cells with thousands of
mitochondria in them, we realized a pattern.”

The researchers propose that
mitochondria progress through several phases of degeneration. First, a mitochondrion
stretches out. “Some of them are extremely long, like we have never seen
before,” Özdinler says. Then, it bends into a U shape. The tips of the U eventually
meet and fuse the organelle into a ringlike structure. Then the inner part of
the ring disintegrates, followed by the outer part of the ring.

“It’s self-eating itself. That’s why we said, ‘This isn’t normal. This we have never seen before,’” Özdinler says. Self-eating mitochondria may somehow make upper motor neurons more vulnerable to ALS later in life. Details of that vulnerability haven’t been worked out yet.

Other researchers who study mitochondria’s
role in health and disease aren’t yet convinced that Özdinler’s team has
discovered a new type of mitochondrial death.

Evandro Fang is a molecular
gerontologist at the University of Oslo who studies how mitochondria are
involved in aging and neurodegenerative diseases. He says the static,
two-dimensional electron microscope images in the study may give a false
impression of what’s going on. Watching what happens to single mitochondria
over time and examining the organelles in 3-D would provide a fuller picture,
he says.

And Özdinler’s group didn’t explain the
molecular mechanism that would cause mitochondria to dissolve themselves, he
says. “We’d better not judge whether it’s right or wrong at this stage, because
it’s too preliminary,” Fang says.

Troubled mitochondria in the liver also
form structures similar to those captured in Özdinler’s microscope images, says
Wen-Xing Ding, a cell biologist at the University of Kansas Medical Center in
Kansas City. Ding has seen sick mitochondria form what he calls mitochondrial
spheroids, reminiscent of the rings Özdinler’s group reports. But neither he
nor Özdinler’s group has quantitative data to show that mitochondrial proteins,
DNA and other components are really cleared from cells, he says.

Mitochondria contain some enzymes that
can break down proteins, but Ding doesn’t think those enzymes could digest the
entire organelle without help from other cellular machinery. Still, something
odd may be going on with mitochondria in some cells, he says. “This is a novel
mitochondrial structure, he says. “Whether this is a novel way to get rid of
mitochondria, I do believe it, but we don’t have clear evidence at the moment.”

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