Published online
doi:10.1083/jcb.1764iti5
The Journal of Cell Biology, Vol. 176, No. 4, 372a-
The Rockefeller University Press, 0021-9525 $30.00
© LeBrasseur
Pinched off membrane
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Prominin-1 (red) and a midbody marker (green) are found on midbodies (arrowheads) and in lumenal particles (inset).
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Cells slough off extra membrane by pinching it off into the extracellular space, say Dubreuil et al. on page 483. The discarded membrane can be released by either cilia or midbodies, with potentially distinct outcomes.
Membrane particles can be seen floating in the extracellular space of the developing neural tube. These particles are distinct from signaling exosomes, leaving their origin in question. The authors now show that one origin of these particles is the midbody—the membrane structure that transiently connects two daughter cells and contains the contractile ring.
The group identified the midbody as the source by examining EMs of neural tissue serial sections. They showed that particles emerge from midbodies of dividing neuroepithelial cells. The midbody particles are released at a developmental stage when these cells switch from symmetric, proliferative divisions to the asymmetric, differentiation-inducing divisions that produce neurons.
Differentiation might be a consequence of particle release, as the discarded membrane is enriched in a stem cell marker called Prominin-1. If this protein confers a stem cell phenotype, its irreversible expulsion might drive differentiation. The authors are now testing whether cancer cells have lost the ability to discard the Prominin-1 particles, thus preserving the stem cell-like state.
At the onset of neurogenesis, neuroepithelial cells released Prominin-1 particles that are smaller than midbodies and appear to originate instead from cilia. Their release might help to shrink the membrane of the cilium, which disappears during mitosis.
The authors speculate that free-floating Prominin-1 bodies might also relay signaling information. Perhaps, for instance, the particles notify neighbors that a division has taken place as part of a size control mechanism.
The components that make up this new membrane disposal pathway are still unknown, but they should be widely expressed; preliminary results indicate that epidermal and mesenchymal tissues also release midbody particles. 
Nicole LeBrasseur
lebrasn{at}rockefeller.edu
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Midbody and primary cilium of neural progenitors release extracellular membrane particles enriched in the stem cell marker prominin-1
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