Meisosomes, folded membrane microdomains between the apical extracellular matrix and epidermis

PIKI-1, a class II phosphatidylinositol 3-kinase, functions in endocytic trafficking

Membrane trafficking, including endocytosis and exocytosis, is a complex process that is coordinated by trafficking-associated proteins, cargo molecules, the cytoskeleton, and membrane lipid composition. The NIMA-related kinases NEKL-2 (human NEK8/9) and NEKL-3 (human NEK6/7) are conserved regulators of membrane trafficking in Caenorhabditis elegans and are required for successful molting. Through a genetic approach, we isolated reduction-of-function mutations in piki-1 that suppress nekl-associated molting defects. piki-1 encodes the sole predicted C. elegans Class II phosphatidylinositol 3-kinase (PI3Ks), an understudied class of lipid modifiers that contribute to the production of phosphatidylinositol 3-phosphate (PI(3)P) and phosphatidylinositol 3,4-bisphosphate (PI(3,4)P2). Using a set of genetically encoded lipid sensors, we found that PIKI-1 was responsible for the production of PI(3,4)P2 in the C. elegans epidermis but played only a minor role in the control of PI(3)P levels. Consistent with this, both PI(3,4)P2 and PIKI-1 colocalized to early endosomes, and reduction of PIKI-1 function strongly affected early endosomal morphology and protein composition. Additionally, reduced PIKI-1 function led to excess tubulation of endosomal compartments associated with recycling or the degradation of cellular debris. In contrast to previous studies using mammalian cell culture, PIKI-1 was largely dispensable for clathrin-mediated endocytosis in the context of the worm epidermis, which is a polarized epithelium. Notably, reduction of PIKI-1 function strongly mitigated defects in early endosomes associated with the depletion of NEKL-2. We propose that reduction of PIKI-1 function may suppress nekl molting defects by partially restoring endocytic trafficking within specific compartments, including the early endosome. We also show that inhibition of the PI(3,4)P2-binding protein HIPR-1 (HIP1/HIPR1) suppresses nekl molting defects, suggesting that reduced PI(3,4)P2 levels alter endosomal protein recruitment in a manner that antagonizes NEKL-2 function.

A knock-in translational reporter for NLP-29 reveals AMP secretion to the apical extracellular matrices following epidermal damage in Caenorhabditis elegans

Antimicrobial peptides (AMPs) are small proteins produced and secreted as part of the innate immune response to infection and wounding. They target pathogens and can also function as signalling molecules, for example, promoting sleep in response to injury in C. elegans . A transcriptional reporter transgene for nlp-29 has been pivotal in studying AMP gene expression and regulation, but to understand AMPs antimicrobial and signalling roles, protein expression and trafficking needs to be monitored. We have now created a knock-in translational reporter allele for nlp-29 , with NLP-29 fused to mKate2, that enables visualisation of this secreted AMP. Using the NLP-29::mKate2 reporter, we demonstrate that NLP-29 is secreted into the cuticle upon genetic or physical cuticle damage. NLP-29::mKate2 will therefore be a valuable tool for visualising the secretion of this peptide in C. elegans and thus to dissect the different roles of this key AMP.

A defining member of the new cysteine-cradle family is an aECM protein signalling skin damage in C. elegans

Apical extracellular matrices (aECMs) act as crucial barriers, and communicate with the epidermis to trigger protective responses following injury or infection. In Caenorhabditis elegans, the skin aECM, the cuticle, is produced by the epidermis and is decorated with periodic circumferential furrows. We previously showed that mutants lacking cuticle furrows exhibit persistent immune activation (PIA), providing a valuable model to study the link between cuticle damage and immune response. In a genetic suppressor screen, we identified spia-1 as a key gene downstream of furrow collagens and upstream of immune signalling. spia-1 expression oscillates during larval development, peaking between each moult together with patterning cuticular components. It encodes a secreted protein that localises to furrows. SPIA-1 shares a novel cysteine-cradle domain with other aECM proteins. SPIA-1 mediates immune activation in response to furrow loss and is proposed to act as an extracellular signal activator of cuticle damage. This research provides a molecular insight into intricate interplay between cuticle integrity and epidermal immune activation in C. elegans.

Genome-wide analysis of Smad and Schnurri transcription factors in C. elegans demonstrates widespread interaction and a function in collagen secretion

Smads and their transcription factor partners mediate the transcriptional responses of target cells to secreted ligands of the transforming growth factor-β (TGF-β) family, including those of the conserved bone morphogenetic protein (BMP) family, yet only a small number of direct target genes have been well characterized. In C. elegans, the BMP2/4 ortholog DBL-1 regulates multiple biological functions, including body size, via a canonical receptor-Smad signaling cascade. Here, we identify functional binding sites for SMA-3/Smad and its transcriptional partner SMA-9/Schnurri based on ChIP-seq peaks (identified by modEncode) and expression differences of nearby genes identified from RNA-seq analysis of corresponding mutants. We found that SMA-3 and SMA-9 have both overlapping and unique target genes. At a genome-wide scale, SMA-3/Smad acts as a transcriptional activator, whereas SMA-9/Schnurri direct targets include both activated and repressed genes. Mutations in sma-9 partially suppress the small body size phenotype of sma-3, suggesting some level of antagonism between these factors and challenging the prevailing model for Schnurri function. Functional analysis of target genes revealed a novel role in body size for genes involved in one-carbon metabolism and in the endoplasmic reticulum (ER) secretory pathway, including the disulfide reductase dpy-11. Our findings indicate that Smads and SMA-9/Schnurri have previously unappreciated complex genetic and genomic regulatory interactions that in turn regulate the secretion of extracellular components like collagen into the cuticle to mediate body size regulation.

Genome-wide analysis of Smad and Schnurri transcription factors in C. elegans demonstrates widespread interaction and a function in collagen secretion

Smads and their transcription factor partners mediate the transcriptional responses of target cells to secreted ligands of the Transforming Growth Factor-β (TGF-β) family, including those of the conserved bone morphogenetic protein (BMP) family, yet only a small number of direct target genes have been well characterized. In C. elegans, the BMP2/4 ortholog DBL-1 regulates multiple biological functions, including body size, via a canonical receptor-Smad signaling cascade. Here, we identify functional binding sites for SMA-3/Smad and its transcriptional partner SMA-9/Schnurri based on ChIP-seq peaks (identified by modEncode) and expression differences of nearby genes identified from RNA-seq analysis of corresponding mutants. We found that SMA-3 and SMA-9 have both overlapping and unique target genes. At a genome-wide scale, SMA-3/Smad acts as a transcriptional activator, whereas SMA-9/Schnurri direct targets include both activated and repressed genes. Mutations in sma-9 partially suppress the small body size phenotype of sma-3, suggesting some level of antagonism between these factors and challenging the prevailing model for Schnurri function. Functional analysis of target genes revealed a novel role in body size for genes involved in one-carbon metabolism and in the endoplasmic reticulum (ER) secretory pathway, including the disulfide reductase dpy-11. Our findings indicate that Smads and SMA-9/Schnurri have previously unappreciated complex genetic and genomic regulatory interactions that in turn regulate the secretion of extracellular components like collagen into the cuticle to mediate body size regulation.

A defining member of the new cysteine-cradle family is an aECM protein signalling skin damage in C. elegans

Apical extracellular matrices (aECMs) act as crucial barriers, and communicate with the epidermis to trigger protective responses following injury or infection. In Caenorhabditis elegans, the skin aECM, the cuticle, is produced by the epidermis and is decorated with periodic circumferential furrows. We previously showed that mutants lacking cuticle furrows exhibit persistent immune activation (PIA). In a genetic suppressor screen, we identified spia-1 as a key gene downstream of furrow collagens and upstream of immune signalling. spia-1 expression oscillates during larval development, peaking between each moult together with patterning cuticular components. It encodes a secreted protein that localises to furrows. SPIA-1 shares a novel cysteine-cradle domain with other aECM proteins. SPIA-1 mediates immune activation in response to furrow loss and is proposed to act as a sensor of cuticle damage. This research provides a molecular insight into intricate interplay between cuticle integrity and epidermal immune activation in C. elegans.

C. elegans epicuticlins define specific compartments in the apical extracellular matrix and function in wound repair

The apical extracellular matrix (aECM) of external epithelia often contains lipid-rich outer layers that contribute to permeability barrier function. The external aECM of nematodes is known as the cuticle and contains an external lipid-rich layer - the epicuticle. Epicuticlins are a family of tandem repeat cuticle proteins of unknown function. Here, we analyze the localization and function of the three C. elegans epicuticlins (EPIC proteins). EPIC-1 and EPIC-2 localize to the surface of the cuticle near the outer lipid layer, as well as to interfacial cuticles and adult-specific struts. EPIC-3 is expressed in dauer larvae and localizes to interfacial aECM in the buccal cavity. Skin wounding in the adult induces epic-3 expression, and EPIC proteins localize to wound sites. Null mutants lacking EPIC proteins are viable with reduced permeability barrier function and normal epicuticle lipid mobility. Loss of function in EPIC genes modifies the skin blistering phenotypes of Bli mutants and reduces survival after skin wounding. Our results suggest EPIC proteins define specific cortical compartments of the aECM and promote wound repair.

Structural and physiological functions of Caenorhabditis elegans epidermis

Research on the skin is continuously evolving, and it is imperative to select a streamlined and efficient research model. Caenorhabditis elegans is a free-leaving nematode whose epidermis serves as the primary barrier epithelium, composed of a collagen matrix. Differentiation of the epidermis begins in the middle of embryonic development, including polarization of the cytoskeleton and formation of cell junctions. Cuticle secretion is one of the main developmental and physiological features of the epidermis. Mutations in the collagen genes of individual worms lead to cuticle defects, thereby changing the shape of the animals. The complete genome sequence of C. elegans indicates that more than 170 different collagen genes may be related to this structure. Collagen is a structural protein that plays an important role in the development of extracellular matrix. Different collagen genes are expressed at different stages of matrix synthesis, which may help form specific interactions between different collagens. The differentiated epidermis also plays a key role in the transmission of hormonal signals, fat storage, and ion homeostasis and is closely related to the development and function of the nervous system. The epidermis also provides passive and active defenses against pathogens that penetrate the skin and can repair wounds. In addition, age-dependent epidermal degeneration is a prominent feature of aging and may affect aging and lifespan. This review we highlight recent findings of the structure and related physiological functions of the cuticle of C. elegans. In contrast to previous studies, we offer novel insights into the utilization of C. elegans as valuable models for skin-related investigations. It also encourages the use of C. elegans as a skin model, and its high-throughput screening properties facilitate the acceleration of fundamental research in skin-related diseases.

The Caenorhabditis elegans cuticle and precuticle: a model for studying dynamic apical extracellular matrices in vivo

Apical extracellular matrices (aECMs) coat the exposed surfaces of animal bodies to shape tissues, influence social interactions, and protect against pathogens and other environmental challenges. In the nematode Caenorhabditis elegans, collagenous cuticle and zona pellucida protein-rich precuticle aECMs alternately coat external epithelia across the molt cycle and play many important roles in the worm's development, behavior, and physiology. Both these types of aECMs contain many matrix proteins related to those in vertebrates, as well as some that are nematode-specific. Extensive differences observed among tissues and life stages demonstrate that aECMs are a major feature of epithelial cell identity. In addition to forming discrete layers, some cuticle components assemble into complex substructures such as ridges, furrows, and nanoscale pillars. The epidermis and cuticle are mechanically linked, allowing the epidermis to sense cuticle damage and induce protective innate immune and stress responses. The C. elegans model, with its optical transparency, facilitates the study of aECM cell biology and structure/function relationships and all the myriad ways by which aECM can influence an organism.

Increased expression of metabolism and lysosome-associated genes in a C. elegans dpy-7 cuticle furrow mutant

The collagen-based epidermal 'cuticle' of Caenorhabditis elegans functions as an extracellular sensor for damage that regulates genes promoting osmotic balance, innate immunity, and detoxification. Prior studies demonstrate that SKN-1 , an ortholog of the mammalian Nrf transcription factors, activates core detoxification genes downstream from cuticle damage. Prior RNAseq data suggested that expression of five genes with functions in redox balance, ATP homeostasis, and lysosome function ( gst-15 , gst-24 , cyts-1 , argk-1 , and mfsd-8.4 ) were increased in a cuticle collagen mutant; this study employed RT-qPCR to verify this observation and to test the role of SKN-1 . Activation of all five genes was verified in dpy-7 mutants, but none were reduced by skn-1 (RNAi) suggesting parallel or distinct regulatory mechanisms.

Sur7 mediates a novel pathway for PI4,5P2 regulation in C. albicans that promotes stress resistance and cell wall morphogenesis

The human fungal pathogen Candida albicans can cause lethal systemic infections due to its ability to resist stress from the host and to undergo invasive hyphal growth. Previous studies showed that plasma membrane MCC/eisosome domains were important for virulence by promoting the ability of Sur7 to mediate normal cell wall morphogenesis and stress resistance. The sur7Δ mutant displayed abnormal clusters of PI4,5P2, suggesting that misregulation of this lipid underlies the sur7Δ phenotype. To test this, we increased PI4,5P2 levels by deleting combinations of the three PI4,5P2 5' phosphatase genes (INP51, INP52, and INP54) and found that some combinations, such as inp51Δ inp52Δ, gave phenotypes similar the sur7Δ mutant. In contrast, deleting one copy of MSS4, the gene that encodes the 5' kinase needed to create PI4,5P2, reduced the abnormal PI4,5P2 clusters and also decreased the abnormal cell wall and stress sensitive phenotypes of the sur7Δ mutant. Additional studies support a model that the abnormal PI4,5P2 patches recruit septin proteins, which in turn promote aberrant cell wall growth. These results identify Sur7 as a novel regulator of PI4,5P2 and highlight the critical role of PI4,5P2 in the regulation of C. albicans virulence properties.

ADARs regulate cuticle collagen expression and promote survival to pathogen infection

BACKGROUND

In all organisms, the innate immune system defends against pathogens through basal expression of molecules that provide critical barriers to invasion and inducible expression of effectors that combat infection. The adenosine deaminase that act on RNA (ADAR) family of RNA-binding proteins has been reported to influence innate immunity in metazoans. However, studies on the susceptibility of ADAR mutant animals to infection are largely lacking.

RESULTS

Here, by analyzing adr-1 and adr-2 null mutants in well-established slow-killing assays, we find that both Caenorhabditis elegans ADARs are important for organismal survival to gram-negative and gram-positive bacteria, all of which are pathogenic to humans. Furthermore, our high-throughput sequencing and genetic analysis reveal that ADR-1 and ADR-2 function in the same pathway to regulate collagen expression. Consistent with this finding, our scanning electron microscopy studies indicate adr-1;adr-2 mutant animals also have altered cuticle morphology prior to pathogen exposure.

CONCLUSIONS

Our data uncover a critical role of the C. elegans ADAR family of RNA-binding proteins in promoting cuticular collagen expression, which represents a new post-transcriptional regulatory node that influences the extracellular matrix. In addition, we provide the first evidence that ADAR mutant animals have altered susceptibility to infection with several opportunistic human pathogens, suggesting a broader role of ADARs in altering physical barriers to infection to influence innate immunity.

Nanoscale patterning of collagens in C. elegans apical extracellular matrix

Apical extracellular matrices (aECMs) are complex extracellular compartments that form important interfaces between animals and their environment. In the adult C. elegans cuticle, layers are connected by regularly spaced columnar structures known as struts. Defects in struts result in swelling of the fluid-filled medial cuticle layer ('blistering', Bli). Here we show that three cuticle collagens BLI-1, BLI-2, and BLI-6, play key roles in struts. BLI-1 and BLI-2 are essential for strut formation whereas activating mutations in BLI-6 disrupt strut formation. BLI-1, BLI-2, and BLI-6 precisely colocalize to arrays of puncta in the adult cuticle, corresponding to struts, initially deposited in diffuse stripes adjacent to cuticle furrows. They eventually exhibit tube-like morphology, with the basal ends of BLI-containing struts contact regularly spaced holes in the cuticle. Genetic interaction studies indicate that BLI strut patterning involves interactions with other cuticle components. Our results reveal strut formation as a tractable example of precise aECM patterning at the nanoscale.

Signaling circuits and the apical extracellular matrix in aging: connections identified in the nematode Caenorhabditis elegans

Numerous conserved signaling pathways play critical roles in aging, including insulin/IGF-1, TGF-β, and Wnt pathways. Some of these pathways also play prominent roles in the formation and maintenance of the extracellular matrix. The nematode Caenorhabditis elegans has been an enduringly productive system for the identification of conserved mechanisms of biological aging. Recent studies in C. elegans highlight the regulatory circuits between conserved signaling pathways and the extracellular matrix, revealing a bidirectional relationship between these factors and providing a platform to address how regulation of and by the extracellular matrix can impact lifespan and organismal health during aging. These discoveries provide new opportunities for clinical advances and novel therapeutic strategies.

Body stiffness is a mechanical property that facilitates contact-mediated mate recognition in Caenorhabditis elegans

Physical contact is prevalent in the animal kingdom to recognize suitable mates by decoding information about sex, species, and maturity. Although chemical cues for mate recognition have been extensively studied, the role of mechanical cues remains elusive. Here, we show that C. elegans males recognize conspecific and reproductive mates through short-range cues, and that the attractiveness of potential mates depends on the sex and developmental stages of the hypodermis. We find that a particular group of cuticular collagens is required for mate attractiveness. These collagens maintain body stiffness to sustain mate attractiveness but do not affect the surface properties that evoke the initial step of mate recognition, suggesting that males utilize multiple sensory mechanisms to recognize suitable mates. Manipulations of body stiffness via physical interventions, chemical treatments, and 3D-printed bionic worms indicate that body stiffness is a mechanical property for mate recognition and increases mating efficiency. Our study thus extends the repertoire of sensory cues of mate recognition in C. elegans and provides a paradigm to study the important roles of mechanosensory cues in social behaviors.

C. elegans Hemidesmosomes Sense Collagen Damage to Trigger Innate Immune Response in the Epidermis

The collagens are an enormous family of extracellular matrix proteins that play dominant roles in cell adhesion, migration and tissue remodeling under many physiological and pathological conditions. However, their function mechanisms in regulating innate immunity remain largely undiscovered. Here we use C. elegans epidermis as the model to address this question. The C. elegans epidermis is covered with a collagen-rich cuticle exoskeleton and can produce antimicrobial peptides (AMPs) against invading pathogens or physical injury. Through an RNAi screen against collagen-encoding genes, we found that except the previously reported six DPY collagens and the BLI-1 collagen, the majority of collagens tested appear unable to trigger epidermal immune defense when damaged. Further investigation suggests that the six DPY collagens form a specific substructure, which regulates the interaction between BLI-1 and the hemidesmosome receptor MUP-4. The separation of BLI-1 with MUP-4 caused by collagen damage leads to the detachment of the STAT transcription factor-like protein STA-2 from hemidesmosomes and the induction of AMPs. Our findings uncover the mechanism how collagens are organized into a damage sensor and how the epidermis senses collagen damage to mount an immune defense.