Caenorhabditis elegans PTR/PTCHD PTR-18 promotes the clearance of extracellular hedgehog-related protein via endocytosis

Spatiotemporal restriction of signaling plays a critical role in animal development and tissue homeostasis. All stem and progenitor cells in newly hatched C. elegans larvae are quiescent and capable of suspending their development until sufficient food is supplied. Here, we show that ptr-18, which encodes the evolutionarily conserved patched-related (PTR)/patched domain-containing (PTCHD) protein, temporally restricts the availability of extracellular hedgehog-related protein to establish the capacity of progenitor cells to maintain quiescence. We found that neural progenitor cells exit from quiescence in ptr-18 mutant larvae even when hatched under starved conditions. This unwanted reactivation depended on the activity of a specific set of hedgehog-related grl genes including grl-7. Unexpectedly, neither PTR-18 nor GRL-7 were expressed in newly hatched wild-type larvae. Instead, at the late embryonic stage, both PTR-18 and GRL-7 proteins were first localized around the apical membrane of hypodermal and neural progenitor cells and subsequently targeted for lysosomal degradation before hatching. Loss of ptr-18 caused a significant delay in GRL-7 clearance, causing this protein to be retained in the extracellular space in newly hatched ptr-18 mutant larvae. Furthermore, the putative transporter activity of PTR-18 was shown to be required for the appropriate function of the protein. These findings not only uncover a previously undescribed role of PTR/PTCHD in the clearance of extracellular hedgehog-related proteins via endocytosis-mediated degradation but also illustrate that failure to temporally restrict intercellular signaling during embryogenesis can subsequently compromise post-embryonic progenitor cell function. Author summary Inside the animal body, many signals are exchanged between tissues and cells to ensure healthy growth and physiological conditions. Too much or less signals have been known to cause various diseases such as cancer and diabetes. When developing animals are not fed enough food, their growth rates generally slow down. We found that when nematodes lacking the PTR-18 protein are hatched out of the egg in the absence of sufficient food, they do not appropriately halt their growth. Analyses at the molecular and cellular levels showed that immediately before hatching, PTR-18 re-uptakes a signaling molecule called GRL-7, which is known to promote growth, from the outside to the inside of cells, resulting in the degradation of GRL-7. As a result, worms lacking PTR-18 still contain excess GRL-7 after hatching, which results in unwanted growth even when ample food is not available. Thus, our findings uncover the role of PTR-18 in terminating a signal on time. Humans also possess a protein called PTCHD1, which is structurally similar to the worm's PTR-18 and has been proposed to cause autistic spectrum disorders and learning disabilities. Our findings may provide a clue to further understand the function of human PTCHD1.

Automated summary

The PTR-18 protein plays a critical role in clearing extracellular hedgehog-related proteins and maintaining quiescence of progenitor cells. Both PTR-18 and GRL-7 proteins are localized around the apical membrane of cells in late embryonic stages and targeted for degradation before hatching.