24 Sep 2019

Mitochondrial fragments, jettisoned by stressed cells, create trouble for other cells, according to a study in the September 23 Nature Neuroscience. Researchers led by Daria Mochly-Rosen at Stanford University School of Medicine report that when microglia spat out damaged mitochondria, these cast-offs inflamed astrocytes, which in turn expelled their own mitochondrial fragments. Jetsam from either cell sickened neurons as well, limiting their energy production. Conversely, an inhibitor of mitochondrial fission protected astrocytes and neurons from the effect of externally added mitochondrial fragments, suggesting that mitochondrial fragmentation cascades from cell to cell. Inhibiting fission prevented gliosis and protected neurons in the 5xFAD model of Alzheimer’s pathology as well. Curiously, adding whole, functional mitochondria to neuronal cultures also mitigated the damage from fragmented organelles. “We need to pay attention to mitochondria not only as powerhouses, but also as signaling organelles that communicate between cells,” Mochly-Rosen proposed.

Stressed microglia and astrocytes spit out mitochondrial fragments, which harm cells.

Inhibiting mitochondrial fission protects neurons.

Extracellular whole, functional mitochondria somehow protect neurons.

Derek Narendra at the National Institute of Neurological Disorders and Stroke, Bethesda, Maryland, noted that mitochondria are ancient bacterial invaders of eukaryotic cells, but are tolerated by the body because they are sequestered inside cells. Once released, their proteins and other macromolecules may trigger inflammation. “Mitochondria have long been recognized to be important players in neurodegeneration, but the precise vulnerability has been unclear. This and other recent work suggest that the key may be recognition of these ancient mitochondrial invaders by the immune system,” he wrote to Alzforum (full comment below).

Mitochondrial damage accumulates during aging and Alzheimer’s disease (May 2001 news; Jul 2009 news; Aug 2013 news). Faulty mitochondria mark other neurodegenerative disorders as well, including amyotrophic lateral sclerosis, Parkinson’s, and Huntington’s (Reddy, 2014; Jul 2015 news; Jun 2016 news). Mochly-Rosen and colleagues previously characterized a seven-amino-acid peptide, P110, that prevents aberrant, stress-induced mitochondrial fission while sparing physiological organelle division (Qi et al., 2012). In mouse models of Huntington’s and ALS, P110 improved survival and motor skills, while in 5xFAD mice it improved memory, suggesting broad neuronal protection (Guo et al., 2013; Joshi et al., 2018; Joshi et al., 2018).

Dousing Inflammation. 5xFAD mice develop more astrogliosis (top middle) and microgliosis (bottom middle) than age-matched wild-type (left). An inhibitor of mitochondrial fission (right) protects. [Courtesy of Joshi et al., Nature Neuroscience.]

Exactly how P110 protected neurons in these mouse models was unclear. To find out, first author Amit Joshi analyzed the brains of treated mice. He found a dramatic suppression of microglial and astrocyte activation in all three models (see image at right). To find out if this was a direct effect of P110, Joshi added the inhibitor to microglia expressing a 73-amino-acid polyglutamine expansion (Q73) that causes mitochondria to malfunction and fragment. P110 treatment reduced mitochondrial fission, boosted ATP production, and lowered reactive oxygen species. It also halved production of proinflammatory cytokines, including tumor necrosis factor-α and interleukin-1β. P110 similarly benefitted microglia expressing superoxide dismutase carrying a familial ALS mutation or treated with oligomeric Aβ42.

How might microglia in these models affect other cell types? The authors added media from Q73 microglia to mouse primary astrocyte cultures. In response, the astrocytes pumped out TNF-α and IL-1β. Their mitochondria became dysfunctional and fragmented, and 75 percent more astrocytes died. However, if the Q73 microglia were treated with P110 a day before their media was collected, these effects were largely neutralized and more of the astrocytes survived. Adding P110 directly to astrocytes also protected them from the Q73 microglia-conditioned media.

Likewise, conditioned media from microglia carrying Q73 or SOD1 mutations suppressed cellular respiration and cut short survival of primary neurons. So did media from astrocytes that had been treated with the microglia-conditioned media. Once again, treating either glial culture with P110 first lessened the neuronal damage. P110 added directly to neurons also protected them, suggesting that fission of their mitochondria drove the effects on their health. Taken together, the data implied that fragmentation of mitochondria causes microglia and astrocytes to release factors that can somehow damage mitochondria in other cell types.

The authors wondered if those released “factors” might be mitochondria themselves. Scientists have documented release of intact mitochondria into the extracellular space, and even their uptake by other cells (Falchi et al., 2013; Hayakawa et al., 2016).

Confirming this, the authors found intact functional mitochondria in media from healthy microglial cultures. Media from Q73 microglia cultures contained the same total number of mitochondria as media from the healthy cultures, but only half as many were whole and functional. At the same time, the amount of free-floating mitochondrial proteins in Q73 culture medium rose, suggesting the organelles were leaking contents. Treating Q73 microglial cultures with P110 bumped the number of functional mitochondria almost back to that of control cultures. Microglia carrying ALS mutations or treated with lipopolysaccharide also released damaged mitochondria into the cell medium, and again added P110 ameliorated this. These damaged extracellular mitochondria were a major source of toxicity to other cells, the authors found. Filtering out the organelles protected neurons from conditioned medium to the same extent as did pretreatment of the glial cultures with P110.

Notably, the functional mitochondria in cell media helped neurons. When the authors removed whole mitochondria from control microglial media and then added it to neurons, more cells died than when complete medium was added.

“We were surprised that extracellular mitochondria can affect neighboring cells,” Mochly-Rosen told Alzforum. While the damaged organelles may trigger inflammation by activating microglia or astroglia, their direct effect on neurons remains puzzling, as does the effect of whole mitochondria. Mochly-Rosen is investigating the idea that neurons take up whole or fragmented organelles, with the former bolstering cellular respiration and the latter spreading damage. She is also studying how mitochondria are released from cells in the first place, and exploring a potential role for extracellular mitochondria in other disorders, such as sepsis and cardiac arrest. “It’s possible we’ve found a common mechanism by which cells communicate under pathological stress versus under normal conditions,” she said.

Could reducing aberrant mitochondrial fission be a therapeutic strategy for such conditions? Mochly-Rosen believes so. She noted that P110 enters the brain, shows up in neurons, and appeared safe in a five-month mouse toxicity study. Additional safety studies are ongoing. However, the peptide degrades rapidly, suggesting a more stable molecule might have stronger effects and work better for people. Other work supports the idea that cleaning up damaged mitochondria could help treat Alzheimer’s pathology (Feb 2019 news).—Madolyn Bowman Rogers