Mislocalized Mitochondria Sufficient for Motor Neuron Malaise

Messing with mitochondrial motility, even if the organelles functioned perfectly, was sufficient to cause neurological disease in mice, according to a paper in the August 18 Proceedings of the National Academy of Sciences online. The work, from the laboratory of Janet Shaw at the University of Utah School of Medicine in Salt Lake City, neatly separates movement of mitochondria from two of their main functions—respiration and calcium buffering. Mice lacking a crucial mitochondrial transport factor are healthy at birth, but rapidly develop an apparent upper motor neuron disorder, and die within a month. The animals develop symptoms reminiscent of amyotrophic lateral sclerosis, hereditary spastic paraplegia, and dystonia.

Novel Mouse Model
Mitochondria rely on kinesin and dynein motors to get around, particularly along motor neurons’ lengthy axons. Mitochondrial Rho (Miro) GTPases are among several cellular factors that attach the organelle to these motors, and act as a sort of brake to arrest the cellular powerhouse where they are most needed, for example close to synapses. To examine the relationship between mitochondrial motions and mammalian disease, first author Tammy Nguyen engineered mice with a floxed Miro1 sequence, which she could then delete by introducing the enzyme Cre. First, she made mice that lost Miro1 during the embryo stage. The animals were fine in utero, but upon birth failed to take a single breath.

The authors investigated the nerve circuits required for that first gasp and found the knockouts were missing crucial elements, including motor neurons in a cranial structure called the nucleus ambiguus. They also had a dearth of the spinal cord motor neurons that control the diaphragm muscle, and the phrenic nerve, which runs from the cord to the diaphragm, had fewer branches than it did in wild-type mice (see image below). The authors surmised that their Miro1 knockouts died because of motor neuron defects.

In Miro1 knockout mice (bottom), the phrenic nerve that controls diaphragm muscle contraction lacked the normal branches found in wild-type animals (top).[Image courtesy of Nguyen et al., PNAS.]

To analyze Miro1’s importance in a living model, Nguyen deleted the gene specifically in neurons of the cerebral cortex, hippocampus, and spinal cord. These animals looked just like their control littermates for the first two weeks of life, but then they started clasping their hind paws together, a well-known sign of neurological problems. Over the next few weeks they stopped gaining weight, their spines hunched, their tails stiffened, and they moved spastically. By about 35 days of age, they died.

The authors noted these symptoms resembled upper motor neuron disease, and examined the animals’ spinal cords for markers of neuropathology. They noticed aggregates in the knockout mouse tissues that were reminiscent of Bunina bodies, poorly characterized inclusions commonly observed in the spinal cords of people who died of ALS. However, Bunina bodies are found inside cells, while the aggregates in the Miro1 knockout mice appeared to be extracellular. They might be the remains of dead neurons, the authors suggest.

Do the mice model any particular human disease? At this point, the researchers are uncertain. Because their problems started so early, Shaw initially thought the mice mimicked hereditary spastic paraplegia, a developmental condition. However, she thinks she may be able to extend the animals’ lifespan by placing their food within easier reach, in which case they might model ALS. Some of her colleagues have suggested that the symptoms more closely resemble dystonia. “There is still a lot of characterization to do,” Shaw said. “We need to know more about what is going on in the mouse so we can better relate it to the human condition.”

Serge Przedborski of Columbia University in New York, who was not involved in the study, said it was difficult to determine whether the mice correspond to any human disease. For one, he noted, the animals are only missing Miro1 in certain neurons. He prefers germline models in which all cells share the same genetic defect, such as the mutant superoxide dismutase 1 mice that mimic human ALS. In these mice as well as in people who have the same disease gene, all cells carry the mutation but only motor neurons degenerate.

Przedborski added that spasticity in rodents results from different neurological pathways than it does in people, so the symptoms could be species-specific. Similarly, he said, it is difficult to interpret the Bunina-like bodies because they are not intracellular as in people with ALS.

To understand Miro1’s role better, the authors are planning a new mouse model with Cre controlled by an inducible promoter. With that system, they can remove Miro1 selectively in older animals and potentially create a degenerative condition like ALS. In addition, they are screening people with inherited neurological diseases of unknown genetic origin, looking for Miro1 mutations.

Mitochondrial Stumbles
Regardless of whether they mimic any specific human disease, the mice allowed Nguyen and Shaw to probe how mitochondrial mobility relates to neuron health. Numerous studies have linked defective mitochondria to neurodegeneration (see Jun 2014 news story, news story), but the mitochondria usually exhibit problems both in respiration and localization. However, loss of Miro1 apparently affected only transport, not function.

The mitochondria from the Miro1-negative neurons behaved normally in several assays. In primary cortical neuron cultures, the mitochondrial membrane potential was normal. Nguyen also isolated embryonic fibroblasts from the original knockout mice, lacking Miro1 in every cell, and measured rates of oxygen consumption. Oxygen uptake was also typical in that mitochondria do not need Miro1 for respiration.

Mitochondria also prevent calcium toxicity by buffering the cation, and Miro1 contains several calcium-binding motifs. However, the Miro1-negative fibroblasts managed calcium levels properly.

The only defect Nguyen found was in mitochondrial motion. Though the organelles lacking Miro 1 were able to travel, they exhibited specific defects. In the spinal cord axons of the knockout mice, Nguyen observed fewer than normal mitochondria, indicating improper distribution of the organelles. Using a microscope, she observed that the mitochondria of Miro1-negative cells stalled more often. Specifically, their overall retrograde speed was one-fourth of normal, though their anterograde motion was fine.

“This is the first demonstration that you can cause neurological disease simply by preventing mitochondria from moving to the right place, even if they are functioning,” Shaw concluded. She suggested this model might be useful to test drugs that redistribute the organelle properly. Such medications might see widespread use, Shaw speculated, since many neurological conditions, including Alzheimer’s, appear to involve defective mitochondrial distribution (see Feb 2010 news story, Jul 2009 news story, Apr 2009 news story).

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