Inside the dorsal body wall of a fruit fly larva, two neighbouring sensory neurons face opposite fates as metamorphosis begins: one is destined to die, while its neighbour, ddaC, quietly trims its larval branches and rebuilds an arbor for adult life. A new study in Nature asks how a cell can summon the same executioner proteins that end lives elsewhere and yet remain alive. Its answer points to the proapoptotic Reaper and Grim proteins and to the mitochondria within the cell, a reminder that the machinery of death can also be turned, at lower intensity, toward reshaping a living body.
Fly Sensory Neurons Use Reaper, Grim and Mitochondria to Prune Dendrites Without Dying
The same organelle is needed for the sublethal and the lethal outcome.
Mark
Why does it matter that two neighbouring neurons make opposite choices?
Mimi
Because they sit next to each other in the same animal, so you can compare dying and surviving cells under the same conditions. ddaC prunes and lives, class III neurons die.
Mark
And caspases are active in both?
Mimi
Yes. In ddaC, substrates were cleaved within 20 minutes of puparium formation, but at consistently lower levels than in the dying neighbours.
Luke
Careful with what that shows. Lower levels in ddaC is an observation. The report doesn't establish that the level is what decides survival.
Mimi
Fair. It's a correlation the comparison makes visible, not a demonstrated threshold.
Mark
What about the RHG proteins?
Mimi
Single-cell H99 mutant clones failed to prune, so they're needed inside the cell itself. Reaper and grim seem to cooperate, while hid does not, even though all three are transcribed in ddaC.
Luke
And that reaper-and-grim point comes from mutant combinations, which the authors describe as suggesting it. That's weaker than saying it's proven.
Mark
And the mitochondria?
Mimi
Disrupting their function or transport depleted dendritic mitochondria, blocked caspase activation, and stopped both pruning and the neighbours' apoptosis.
Luke
What's still unknown is how mitochondria enable activation, and how the cell keeps it sublethal. This doesn't say.
Der Puls
- Executioner caspases are switched on inside a neuron that is meant to survive, cleaving their targets within twenty minutes of puparium formation.
- The activity is weaker than in the neighbouring neurons marked for death, yet strong enough to set the pruning process in motion.
- Researchers knock out the Reaper and Grim genes in single cells and watch the branches refuse to sever, locating the trigger inside the pruning neuron itself.
- Disrupting mitochondria starves the dendrites of their organelles, halting caspase activity and stalling pruning while blocking death in the nearby neurons.
- The question of why similar signals lead to survival in one cell and death in the next remains open, with tests in other developmental settings still ahead.
Inside the dorsal body wall of a fruit fly larva, two neighbouring sensory neurons face opposite fates as metamorphosis begins: one is destined to die, while its neighbour, ddaC, quietly trims its larval branches and rebuilds an arbor for adult life. A new study in Nature asks how a cell can summon the same executioner proteins that end lives elsewhere and yet remain alive. Its answer points to the proapoptotic Reaper and Grim proteins and to the mitochondria within the cell, a reminder that the machinery of death can also be turned, at lower intensity, toward reshaping a living body.
In the dorsal body wall of a fruit fly larva, two kinds of sensory neurons lie side by side, and at the start of metamorphosis they diverge. Class III neurons are fated to die by apoptosis, while a class IV neuron called ddaC survives, strips away its larval dendrites, and later grows a new arbor suited to the adult fly. A new study in Nature uses this close neighbourhood to ask how a neuron can run the machinery of death and still live.
The machinery is the caspases, best known as executioners, but during development they also work at sub-lethal levels to guide cell fate, migration and differentiation. How neurons use them without dying has been unclear. Because the dying and pruning neurons sit so close together, the researchers could compare the two directly in an intact animal. Using a sensitive caspase probe, they saw substrates cleaved in ddaC within twenty minutes of puparium formation, well before its dendrites were severed, though at consistently lower levels than in the doomed neighbours.
The team then traced the cause. Removing the proapoptotic RHG proteins inside single ddaC cells stopped pruning, and the mutant combinations showed that reaper and grim, rather than hid, cooperate to sever branches. Mitochondria proved to be the second requirement: when they were disrupted, caspase activation failed and pruning stopped in ddaC, while apoptosis was blocked in the neighbouring neurons. The same organelle thus serves both the sublethal and the lethal outcome.
The authors stop short of explaining why similar signals lead to survival in one cell and death in the next. That comparison sets up the next question, and the researchers plan to test these mechanisms in other developmental contexts where cells use the lethal pathway without dying.
In the dorsal body wall of a fruit fly larva, two kinds of sensory neurons sit side by side, and at the start of metamorphosis they part ways. Class III dendritic arborization neurons die by apoptosis. A class IV neuron called ddaC survives, strips away its larval dendrites, and later regrows an arbor suited to the adult fly. A new study in Nature uses that neighbourhood to ask how a neuron can run the machinery of death and live.
The machinery in question is the caspases. They are best known for executing apoptosis, but during development they are also used sub-lethally, contributing to cell fate specification, migration and terminal differentiation. How neurons switch on executioner caspases without dying has remained unresolved. Because the dying class III neurons and the pruning ddaC neuron lie so close together, the researchers could compare lethal and non-lethal caspase signalling side by side in an intact animal.
To watch the activity, they used SR4VH, a sensitive dual-colour probe for executioner caspases. In ddaC, caspase substrates were cleaved within 20 minutes of puparium formation, well before the proximal dendrites were severed. The levels were consistently lower than in the neighbouring neurons that were headed for death. So the pruning neuron does engage the caspase pathway early, but at a lower intensity than a neuron that is about to die.
The team then asked what drives that activity. They generated single-cell mutant clones lacking the H99 region, which removes the proapoptotic RHG proteins. Pruning failed in those cells, showing that the RHG proteins are required cell-autonomously, meaning inside the pruning neuron itself. The genes reaper, hid and grim are all transcribed in ddaC just as remodelling begins. Yet the mutant combinations pointed to a division of labour: reaper and grim, rather than hid, cooperate to drive branch severing and clearance.
Mitochondria turned out to be the second requirement. When the researchers disrupted mitochondrial function or transport, dendritic mitochondria were depleted and caspase activation did not occur. The consequences ran in both directions. Pruning was blocked in ddaC, and apoptosis was blocked in the neighbouring class III neurons. The same organelle, in other words, is needed for the sublethal and the lethal outcome.
The authors summarise the result in two parts. Reaper and Grim are required cell-autonomously for pruning, and mitochondria are required both for caspase activation and for the severing of branches. The work does not by itself explain why a similar set of signals leads to survival in one cell and death in the next; the lower caspase levels in ddaC are an observation, and the comparison between the two neuron types sets up that question rather than answering it.
The paper credits several outside contributors, including fly lines from the Bloomington Stock Centre and from Tadashi Uemura, Kristin White, Joe Bateman and Alex Whitworth, an antibody against Sox14 from FengWei Yu, and imaging facilities and reagents from Paul Conduit. Nicolas Loncle made the initial observations of caspase activation in sensory neurons with SR4VH, and the work came from the Williams lab. The next step is to test whether these mechanisms operate in other developmental contexts where cells use the lethal pathway without dying.
Bemerkenswerte Zitate
Reaper and Grim are required cell-autonomously for pruning, and mitochondria are required for both caspase activation and branch severing.— Study authors, Nature
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