Molecular mechanisms of Platynereis larval settlement

Some marine zooplankton are only temporary residents of the plankton, representing the larval stages of marine invertebrates such as corals, worms, snails, clams and crabs. After a period in the open water, these larvae must settle to the ocean floor where they will live out their adult lives. It is important that larvae are able to find the best place in which to settle, as this will affect their future success and survival.

 

Many marine larvae rely on a chemosensory organ in the head, known as the apical organ, to detect chemical cues from their environment that are indicative of favourable settlement sites. How environmental cues are interpreted by the apical organ to generate behavioural responses during settlement is not yet understood.

 

Studying Platynereis larvae, we have identified a neuropeptide, myoinhibitory peptide (MIP), as an apical organ signaling molecule that triggers larval settlement. In lab culture, 2 day Platynereis larvae actively swim in a water column. Exposure of the larvae to synthetic MIP however, rapidly induces sinking of the larvae followed by exploratory crawling on the bottom of the container. We have shown that MIP is expressed in chemosensory-neurosecretory cells of the apical organ and signals to its receptor, an ortholog of the Drosophila sex peptide receptor, which is expressed in neighbouring neurosecretory cells. The MIP-receptor cells likely then initiate further downstream endocrine signaling events that regulate the complex behaviors associated with settlement. The signaling pathways downstream of MIP and its receptor are a subject that we are currently investigating further.

 

Interestingly, MIPs are present in many protostomes (also called allatostatin-B) and are part of the Wamide family of neuropeptides, which are also involved in regulating settlement and metamorphosis in larvae of Cnidarians such as corals and sea anemones. This indicates that the initiation of life cycle transitions may have been an ancestral function of MIPs.

MIP peptide treatment triggers Platynereis larval settlement

MIP treatment triggers Platynereis larval settlement in 2 day larvae. The video shows a timelapse recording of untreated larvae (left) and larvae in the presence of 5 µM MIP. In the right side larval tracks are marked in white, and the current larval position in red.

Chemosensory neurons in the larval apical organ

Chemosensory cells in the 2 day Platynereis larvae. These cells can be stained by dye-filling with Mitotracker. Some of these cells express the neuropeptide MIP. The image is color coded for depth. Scale bar 20 µM.
SEM image of a Platynereis trochophore larva, apical view.
Close-up view of two apical organ MIP-expressing chemosensory-neurosecretory neurons in a 29 h old larva, labeled with Mitotracker. The image is color coded for depth.

Expression of MIP and its receptor in the apical organ

The neuropeptide MIP (red) is expressed in chemosensory-neurosecretory cells in the apical organ of 2 day Platynereis. Immunostaining against acetylated tubulin (white) highlights the cila and axonal scaffold of the larva.
MIP (red) and the MIP receptor (cyan) is expressed in adjacent cells, indicating a paracrine signaling mechanism. Immunostaining against acetylated tubulin (white) highlights the cila and axonal scaffold of the larva.

Serial TEM reconstruction of a pair of MIP-expressing chemosensory-neurosecretory neurons

Transmission electron microscopy reconstruction of a pair of MIP-expressing chemosensory-neurosecretory cells. The cells have long branching microvilli and are packed with many dense core vesicles, indicating that they have both chemosensory and neurosecretory character.  

References

Conzelmann M, Williams EA, Tunaru S, Randel N, Shahidi R, Asadulina A, Berger J, Offermanns S, Jékely G. Conserved MIP receptor-ligand pair regulates Platynereis larval settlement. Opens external link in new windowProc Natl Acad Sci U S A. 2013 May 14;110(20):8224-9. Epub 2013 Apr 8.