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

Abstract

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.

Meta-Analysis of Caenorhabditis elegans Transcriptomics Implicates Hedgehog-Like Signaling in Host-Microbe Interactions

“…Interestingly, gene per gene comparison of expression data and RNAi-associated pathogen resistance reveals that several of these up-regulated genes (9/16 tested) are necessary for wild-type level resistance to E. faecalis infection, as RNAi against these led to increased worm susceptibility ( Figure 3B ). Among them are HRPs: grl-21 (previously identified as a mediator of environment-host-microbe crosstalk; Lin and Wang, 2017 ), grl-5 , grl-7 , grd-12 , grd-2 , and wrt-4 , and PTRs: ptr-18 (required for HRP clearance; Chiyoda et al, 2021 ) and daf-6 (necessary for sensory organ morphogenesis together with dyf-4 ( Hong et al, 2021 ), and thus possibly for pathogen sensing), as well as another SSD-protein expressing gene: scp-1 . Similarly, RNAi inhibition of the P. aeruginosa -up-regulated ncr-1 and C27B7.7 (DSCAM) led to increased host sensitivity to infection, implicating both genes in the worm immune response to P. aeruginosa gut infections, while RNAi inhibition of ncr-1 (only gene found up-regulated in this context) did not sensitize worms to S. aureus infection.…”

“…HHLS could also impact the gut-brain axis via neurogenic pathways, particularly during development. Indeed, inhibition of HHLS genes can lead to: (1) misshapen or dysfunctional chemosensory organ pockets ( che-14, lit-1, daf-6, mom-4/5 ) ( Michaux et al, 2000 ; Perens and Shaham, 2005 ; Liegeois et al, 2007 ; Oikonomou et al, 2011 ; Oikonomou and Shaham, 2012 ), (2) defects in axonal pathfinding ( wrt-8 and grl-16) ( Riveiro et al, 2017 ), and (3) defects in neuronal progenitor cell cycle schedule and cell fate ( grl-5 , grl-7 , ptr-18 ) ( Kume et al, 2019 ; Chiyoda et al, 2021 ). Thus, as for RNAi effectiveness we initiated genetic knockdown at the late L1 larval stage, some of the effects observed in our study could have resulted from impairment of the late development of the nervous system.…”

“…Although the worm genome expresses a single Gli homolog (TRA-1) primarily involved in sex-determination ( Hodgkin and Brenner, 1977 ; Hodgkin, 1987 ; Ellis, 2020 ), the nematode HHLS pathway lacks the canonical Hedgehog intracellular transduction pathway, and the evolutionary function of the PTR and HRP family expansions remains unclear ( Baker et al, 2021 ). HHLS genes have been involved in C. elegans epithelial function and development ( Hao et al, 2006a ; Liegeois et al, 2006 ; Soloviev et al, 2011 ), cuticular structure and patterning ( Zugasti et al, 2005 ; Hao et al, 2006b , c ; Liegeois et al, 2006 ; Kouns et al, 2011 ; Chiyoda et al, 2021 ; Cohen et al, 2021 ), sensory organ formation ( Michaux et al, 2000 ; Liegeois et al, 2007 ; Oikonomou et al, 2011 ; Wang et al, 2017 ; van der Burght et al, 2020 ), neurogenesis ( Kume et al, 2019 ) and axonal guidance ( Riveiro et al, 2017 ), immunity ( Roberts et al, 2010 ; Lightfoot et al, 2016 ), lipid homeostasis ( Lin and Wang, 2017 ; Del Castillo et al, 2021 ), reproduction ( Kuwabara et al, 2000 ; Templeman et al, 2020 ), and longevity ( Ji et al, 2021 ). In particular, the HHLS pathway was found to mediate the impact of microbial metabolites on host lipid metabolism ( Lin and Wang, 2017 ), supporting a direct role in host-nematode interactions that could in part explain the diversification of HRP ligands and PTR receptors.…”

Patched-Related Is Required for Proper Development of Embryonic Drosophila Nervous System

“…Ptr is relevant to neuronal function in other species. For instance, Ptr-18 (one of the 24 ptr genes found in C. elegans ) is essential for establishing the capacity of neural progenitor cells to maintain quiescence in response to nutritional stresses and provides unique insights into the Ptr role in promoting the clearance of extracellular Hh-related protein by targeting it to lysosomal degradation (Chiyoda et al, 2021 ). Interestingly, PTR-18 is structurally similar to human PTCHD1 (Chiyoda et al, 2021 ), which has been proposed to cause common neurodevelopmental disorders (Noor et al, 2010 ).…”

The Intimate Connection Between Lipids and Hedgehog Signaling

“…Some bilateria, notably Caenorhabditis elegans , have lost the HH pathway but retained PTCH homologs. One of these, PTR-18 clears a secreted protein, GRL-7, distantly related to HH ( Chiyoda et al, 2021 ), suggesting that PTCH can be repurposed to function independently of HH pathway regulation. Another C. elegans PTCH homolog, PTC-3, prevents intracellular cholesterol accumulation ( Cadena del Castillo et al, 2021 ), further supporting the idea that PTCH family members are sterol transporters.…”