Myeloid-derived Wnts play an indispensible role in macrophage and fibroblast activation and kidney fibrosis

Wnt/β-catenin signaling plays a pivotal role in the pathogenesis of chronic kidney diseases (CKD), which is associated with macrophage activation and polarization. However, the relative contribution of macrophage-derived Wnts in the evolution of CKD is poorly understood. Here we demonstrate a critical role of Wnts secreted by macrophages in regulating renal inflammation and fibrosis after various injuries. In mouse model of kidney fibrosis induced by unilateral ureteral obstruction (UUO), macrophages were activated and polarized to M1 and M2 subtypes, which coincided with the activation of Wnt/β-catenin signaling. In vitro, multiple Wnts were induced in primary cultured bone marrow-derived macrophages (BMDMs) after polarization. Conversely, Wnt proteins also stimulated the activation and polarization of BMDMs to M1 and M2 subtype. Blockade of Wnt secretion from macrophages in mice with myeloid-specific ablation of Wntless (Wls), a cargo receptor that is obligatory for Wnt trafficking and secretion, blunted macrophage infiltration and activation and inhibited the expression of inflammatory cytokines. Inhibition of Wnt secretion by macrophages also abolished β-catenin activation in tubular epithelium, repressed myofibroblast activation and reduced kidney fibrosis after either obstructive or ischemic injury. Furthermore, conditioned medium from Wls-deficient BMDMs exhibited less potency to stimulate fibroblast proliferation and activation, compared to the controls. These results underscore an indispensable role of macrophage-derived Wnts in promoting renal inflammation, fibroblasts activation and kidney fibrosis.

Molecular basis of Wnt biogenesis, secretion, and Wnt7-specific signaling

Wnt proteins are enzymatically lipidated by Porcupine (PORCN) in the ER and bind to Wntless (WLS) for intracellular transport and secretion. Mechanisms governing the transfer of these low-solubility Wnts from the ER to the extracellular space remain unclear. Through structural and functional analyses of Wnt7a, a crucial Wnt involved in central nervous system angiogenesis and blood-brain barrier maintenance, we have elucidated the principles of Wnt biogenesis and Wnt7-specific signaling. The Wnt7a-WLS complex binds to calreticulin (CALR), revealing that CALR functions as a chaperone to facilitate Wnt transfer from PORCN to WLS during Wnt biogenesis. Our structures, functional analyses, and molecular dynamics simulations demonstrate that a phospholipid in the core of Wnt-bound WLS regulates the association and dissociation between Wnt and WLS, suggesting a lipid-mediated Wnt secretion mechanism. Finally, the structure of Wnt7a bound to RECK, a cell-surface Wnt7 co-receptor, reveals how RECKCC4 engages the N-terminal domain of Wnt7a to activate Wnt7-specific signaling.

A stromal lineage maintains crypt structure and villus homeostasis in the intestinal stem cell niche

BACKGROUND

The nutrient-absorbing villi of small intestines are renewed and repaired by intestinal stem cells (ISCs), which reside in a well-organized crypt structure. Genetic studies have shown that Wnt molecules secreted by telocytes, Gli1+ stromal cells, and epithelial cells are required for ISC proliferation and villus homeostasis. Intestinal stromal cells are heterogeneous and single-cell profiling has divided them into telocytes/subepithelial myofibroblasts, myocytes, pericytes, trophocytes, and Pdgfralow stromal cells. Yet, the niche function of these stromal populations remains incompletely understood.

RESULTS

We show here that a Twist2 stromal lineage, which constitutes the Pdgfralow stromal cell and trophocyte subpopulations, maintains the crypt structure to provide an inflammation-restricting niche for regenerating ISCs. Ablating Twist2 lineage cells or deletion of one Wntless allele in these cells disturbs the crypt structure and impairs villus homeostasis. Upon radiation, Wntless haplo-deficiency caused decreased production of anti-microbial peptides and increased inflammation, leading to defective ISC proliferation and crypt regeneration, which were partially rescued by eradication of commensal bacteria. In addition, we show that Wnts secreted by Acta2+ subpopulations also play a role in crypt regeneration but not homeostasis.

CONCLUSIONS

These findings suggest that ISCs may require different niches for villus homeostasis and regeneration and that the Twist2 lineage cells may help to maintain a microbe-restricted environment to allow ISC-mediated crypt regeneration.

Non-canonical Wnt/calcium signaling is protective against podocyte injury and glomerulosclerosis

Activation of canonical Wnt signaling has been implicated in podocyte injury and proteinuria. As Wnts are secreted proteins, whether Wnts derived from podocytes are obligatory for promoting proteinuria remains unknown. To address this, we generated conditional knockout mice where Wntless, a cargo receptor protein required for Wnt secretion, was specifically deleted in glomerular podocytes. Mice with podocyte-specific ablation of Wintless (Podo-Wntless-/-) were phenotypically normal. However, after inducing kidney damage with Adriamycin for six days, Podo-Wntless-/- mice developed more severe podocyte injury and albuminuria than their control littermates. Surprisingly, ablation of Wntless resulted in upregulation of β-catenin, accompanied by reduction of nephrin, podocin, podocalyxin, and Wilms tumor 1 proteins. In chronic injury induced by Adriamycin, increased albuminuria, aggravated podocyte lesions and extracellular matrix deposition were evident in Podo-Wntlessl-/- mice, compared to wild type mice. Mechanistically, specific ablation of Wintless in podocytes caused down-regulation of the nuclear factor of activated T cell 1 (NFAT1) and Nemo-like kinase (NLK), key downstream mediators of non-canonical Wnt/calcium signaling. In vitro, knockdown of either NFAT1 or NLK induced β-catenin activation while overexpression of NLK significantly repressed β-catenin induction and largely preserved nephrin in glomerular podocytes. Thus, our results indicate that podocyte-derived Wnts play an important role in protecting podocytes from injury by repressing β-catenin via activating non-canonical Wnt/calcium signaling.

Colonic healing requires Wnt produced by epithelium as well as Tagln+ and Acta2+ stromal cells

Although Wnt signaling is clearly important for the intestinal epithelial homeostasis, the relevance of various sources of Wnt ligands themselves remains incompletely understood. Blocking the release of Wnt in distinct stromal cell types suggests obligatory functions of several stromal cell sources and yields different observations. The physiological contribution of epithelial Wnt to tissue homeostasis remains unclear. We show here that blocking epithelial Wnts affects colonic Reg4+ epithelial cell differentiation and impairs colonic epithelial regeneration after injury in mice. Single-cell RNA analysis of intestinal stroma showed that the majority of Wnt-producing cells were contained in transgelin (Tagln+) and smooth muscle actin α2 (Acta2+) expressing populations. We genetically attenuated Wnt production from these stromal cells using Tagln-Cre and Acta2-CreER drivers, and found that blockage of Wnt release from either epithelium or Tagln+ and Acta2+ stromal cells impaired colonic epithelial healing after chemical-induced injury. Aggregated blockage of Wnt release from both epithelium and Tagln+ or Acta2+ stromal cells drastically diminished epithelial repair, increasing morbidity and mortality. These results from two uncharacterized stromal populations suggested that colonic recovery from colitis-like injury depends on multiple Wnt-producing sources.

Epithelial Wntless regulates postnatal alveologenesis

Alveologenesis requires the coordinated modulation of the epithelial and mesenchymal compartments to generate mature alveolar saccules for efficient gas exchange. However, the molecular mechanisms underlying the epithelial-mesenchymal interaction during alveologenesis are poorly understood. Here, we report that Wnts produced by epithelial cells are crucial for neonatal alveologenesis. Deletion of the Wnt chaperone protein Wntless homolog (Wls) disrupts alveolar formation, resulting in enlarged saccules in Sftpc-Cre/Nkx2.1-Cre; Wlsloxp/loxp mutants. Although commitment of the alveolar epithelium is unaffected, α-SMA+ mesenchymal cells persist in the alveoli, accompanied by increased collagen deposition, and mutants exhibit exacerbated fibrosis following bleomycin challenge. Notably, α-SMA+ cells include a significant number of endothelial cells resembling endothelial to mesenchymal transition (EndMT), which is also present in Ager-CreER; Wlsloxp/loxp mutants following early postnatal Wls deletion. These findings provide initial evidence that epithelial-derived Wnts are crucial for the differentiation of the surrounding mesenchyme during early postnatal alveologenesis.

Spatial and temporal expression of PORCN is highly dynamic in the developing mouse cochlea

The mammalian organ of Corti is a highly specialized sensory organ of the cochlea with a fine-grained pattern that is essential for auditory function. The sensory epithelium, the organ of Corti consists of a single row of inner hair cells and three rows of outer hair cells that are intercalated by support cells in a mosaic pattern. Previous studies show that the Wnt pathway regulates proliferation, promotes medial compartment formation in the cochlea, differentiation of the mechanosensory hair cells and axon guidance of Type II afferent neurons. WNT ligand expressions are highly dynamic throughout development but are insufficient to explain the roles of the Wnt pathway. We address a potential way for how WNTs specify the medial compartment by characterizing the expression of Porcupine (PORCN), an O-acyltransferase that is required for WNT secretion. We show PORCN expression across embryonic ages (E)12.5 - E14.5, E16.5, and postnatal day (P)1. Our results showed enriched PORCN in the medial domains during early stages of development, indicating that WNTs have a stronger influence on patterning of the medial compartment. PORCN was rapidly downregulated after E14.5, following the onset of sensory cell differentiation; residual expression remained in some hair cells and supporting cells. On E14.5 and E16.5, we also examined the spatial expression of Gsk3β, an inhibitor of canonical Wnt signaling to determine its potential role in radial patterning of the cochlea. Gsk3β was broadly expressed across the radial axis of the epithelium; therefore, unlikely to control WNT-mediated medial specification. In conclusion, the spatial expression of PORCN enriches WNT secretion from the medial domains of the cochlea to influence the specification of cell fates in the medial sensory domain.

The Crucial Role of Wntless in Osteogenesis and Odontogenesis

Wnt signalling pathways have been the focus of intense research activity for decades due to their fundamental role in skeletal and dental development. Wntless, an exclusive chaperone protein for the exocytotis of Wnt ligands, was identified in 2006. In the last decade, the molecular biological studies of Wntless and its genetic studies in human and mice have highlighted the importance of this protein in mineralised tissues, including bone, cartilage and teeth. This article reviews recent developments and discrepancies in the role of Wntless in skeletal and dental development based on mutant phenotypes, as well as the underlying mechanism involved in its molecular and physiological regulation. We conclude that, though some controversial phenotypes exist due to different Cre line resources, Cre recombinase activity and detection time points, Wntless undeniably exerts a context- and stage-dependent regulatory function during the development and homeostasis of both skeletal and dental tissue.

Transforming Growth Factor-β Signaling Regulates Tooth Root Dentinogenesis by Cooperation With Wnt Signaling

Proper differentiation of odontoblasts is crucial for the development of tooth roots. Previous studies have reported the osteogenic/odontogenic potential of pre-odontoblasts during root odontoblast differentiation. However, the underlying molecular pathway that orchestrates these processes remains largely unclear. In this study, ablation of transforming growth factor-β receptor type 2 (Tgfbr2) in root pre-odontoblasts resulted in abnormal formation of root osteodentin, which was associated with ectopic osteogenic differentiation of root odontoblasts. Disrupting TGF-β signaling caused upregulation of Wnt signaling characterized by increased Wnt6, Wnt10a, Tcf-1, and Axin2 expression. Interestingly, inhibiting Wnt signaling by deleting Wntless (wls) in Osteocalcin (Ocn)-Cre; Tgfbr2 ^fl/fl ; Wls ^fl/fl mice or overexpressing the Wnt antagonist Dkk1 in Ocn-Cre; Tgfbr2 ^fl/fl ; ROSA26 ^Dkk1 mice decreased ectopic osteogenic differentiation and arrested odontoblast differentiation. Our results suggest that TGF-β signaling acts with Wnt signaling to regulate root odontogenic differentiation.

Responses of Porcupine and Wntless proteins to oxidative, hypoxic and endoplasmic reticulum stresses

The WNT (Wingless and Int-1) proteins play a role in stem cell development and cell differentiation. Mutations in the WNT proteins lead to the development of various tumours, including gastric tumours. Porcupine (PORCN) is a palmitoyltransferase and Wntless (WLS) is a dedicated WNT transport protein that modify and fold the WNT proteins respectively and are involved in their proper secretion and binding to the frizzled (FZD) receptor and the lipoprotein receptor-related protein 5 or 6 (LRP5/6). We investigated how modifications of PORCN and WLS result in changes in WNT expression and secretion from cells under stress conditions that occur in the tumour microenvironment (hypoxia, oxidative stress, endoplasmic reticulum (ER) stress). In the present study, we found the mRNA expression of both PORCN and WLS were significantly increased with treatments inducing oxidative stress (antimycin A) and proteasome inhibition (MG-132), in human colon cancer (HCT116) and human intestinal epithelial cell-6 (HIEC-6) cells. Treatment with ER stressors thapsigargin, tunicamycin, and dithiolthreitol significantly increased PORCN gene expression, while treatment with thapsigargin and dithiolthreitol increased WLS gene expression. The expression of PORCN and WLS proteins increased with hypoxia and ER stressor treatments in both HCT116 and HIEC-6 cells. All stressors used in this study increased beta-catenin (β-catenin) expression in HCT116 cells. Our results suggest that these stressors alter PORCN, WLS and β-catenin expression and function which may, in turn, alter WNT secretion. Silencing the expression of PORCN and WLS with siRNA expression reduced the expression of WLS and WNT3A in HCT116 cells. The possibility exists that PORCN specifically may be involved in a novel signaling pathway, independent of its palmitoleation of the WNT proteins and its role in their secretion, that is rate-limiting for cancer cell growth and tumorigenesis, within the tumour microenvironment.

Structural Basis of WLS/Evi-Mediated Wnt Transport and Secretion

Wnts are evolutionarily conserved ligands that signal at short range to regulate morphogenesis, cell fate, and stem cell renewal. The first and essential steps in Wnt secretion are their O-palmitoleation and subsequent loading onto the dedicated transporter Wntless/evenness interrupted (WLS/Evi). We report the 3.2 Å resolution cryogenic electron microscopy (cryo-EM) structure of palmitoleated human WNT8A in complex with WLS. This is accompanied by biochemical experiments to probe the physiological implications of the observed association. The WLS membrane domain has close structural homology to G protein-coupled receptors (GPCRs). A Wnt hairpin inserts into a conserved hydrophobic cavity in the GPCR-like domain, and the palmitoleate protrudes between two helices into the bilayer. A conformational switch of highly conserved residues on a separate Wnt hairpin might contribute to its transfer to receiving cells. This work provides molecular-level insights into a central mechanism in animal body plan development and stem cell biology.

Snx3 is important for mammalian neural tube closure via its role in canonical and non-canonical WNT signaling

Disruptions in neural tube (NT) closure result in neural tube defects (NTDs). To understand the molecular processes required for mammalian NT closure, we investigated the role of Snx3, a sorting nexin gene. Snx3^-/- mutant mouse embryos display a fully-penetrant cranial NTD. In vivo, we observed decreased canonical WNT target gene expression in the cranial neural epithelium of the Snx3^-/- embryos and a defect in convergent extension of the neural epithelium. Snx3^-/- cells show decreased WNT secretion, and live cell imaging reveals aberrant recycling of the WNT ligand-binding protein WLS and mis-trafficking to the lysosome for degradation. The importance of SNX3 in WNT signaling regulation is demonstrated by rescue of NT closure in Snx3^-/- embryos with a WNT agonist. The potential for SNX3 to function in human neurulation is revealed by a point mutation identified in an NTD-affected individual that results in functionally impaired SNX3 that does not colocalize with WLS and the degradation of WLS in the lysosome. These data indicate that Snx3 is crucial for NT closure via its role in recycling WLS in order to control levels of WNT signaling.

MicroRNA-31 Regulates Expression of Wntless in Both Drosophila melanogaster and Human Oral Cancer Cells

Recent comparative studies have indicated distinct expression profiles of short, non-coding microRNAs (miRNAs) in various types of cancer, including oral squamous cell carcinoma (OSCC). In this study, we employed a hybrid approach using Drosophila melanogaster as well as OSCC cell lines to validate putative targets of oral cancer-related miRNAs both in vivo and in vitro. Following overexpression of Drosophila miR-31, we found a significant decrease in the size of the imaginal wing discs and downregulation of a subset of putative targets, including wntless (wls), an important regulator of the Wnt signaling pathway. Parallel experiments performed in OSCC cells have also confirmed a similar miR-31-dependent regulation of human WLS that was not initially predicted as targets of human miR-31. Furthermore, we found subsequent downregulation of cyclin D1 and c-MYC, two of the main transcriptional targets of Wnt signaling, suggesting a potential role of miR-31 in regulating the cell cycle and proliferation of OSCC cells. Taken together, our Drosophila-based in vivo system in conjunction with the human in vitro platform will thus provide a novel insight into a mammal-to-Drosophila-to-mammal approach to validate putative targets of human miRNA and to better understand the miRNA-target relationships that play an important role in the pathophysiology of oral cancer.

Frizzled-Dependent Planar Cell Polarity without Secreted Wnt Ligands

Planar cell polarity (PCP) organizes the orientation of cellular protrusions and migratory activity within the tissue plane. PCP establishment involves the subcellular polarization of core PCP components. It has been suggested that Wnt gradients could provide a global cue that coordinates local PCP with tissue axes. Here, we dissect the role of Wnt ligands in the orientation of hairs of Drosophila wings, an established system for the study of PCP. We found that PCP was normal in quintuple mutant wings that rely solely on the membrane-tethered Wingless for Wnt signaling, suggesting that a Wnt gradient is not required. We then used a nanobody-based approach to trap Wntless in the endoplasmic reticulum, and hence prevent all Wnt secretion, specifically during the period of PCP establishment. PCP was still established. We conclude that, even though Wnt ligands could contribute to PCP, they are not essential, and another global cue must exist for tissue-wide polarization.

Wntless regulates lipogenic gene expression in adipocytes and protects against diet-induced metabolic dysfunction

OBJECTIVE

Obesity is a key risk factor for many secondary chronic illnesses, including type 2 diabetes and cardiovascular disease. Canonical Wnt/β-catenin signaling is established as an important endogenous inhibitor of adipogenesis. This pathway is operative in mature adipocytes; however, its roles in this context remain unclear due to complexities of Wnt signaling and differences in experimental models. In this study, we used novel cultured cell and mouse models to investigate functional roles of Wnts secreted from adipocytes.

METHODS

We generated adipocyte-specific Wntless (Wls) knockout mice and cultured cell models to investigate molecular and metabolic consequences of disrupting Wnt secretion from mature adipocytes. To characterize Wls-deficient cultured adipocytes, we evaluated the expression of Wnt target and lipogenic genes and the downstream functional effects on carbohydrate and lipid metabolism. We also investigated the impact of adipocyte-specific Wls deletion on adipose tissues and global glucose metabolism in mice fed normal chow or high-fat diets.

RESULTS

Many aspects of the Wnt signaling apparatus are expressed and operative in mature adipocytes, including the Wnt chaperone Wntless. Deletion of Wntless in cultured adipocytes results in the inhibition of de novo lipogenesis and lipid monounsaturation, likely through repression of Srebf1 (SREBP1c) and Mlxipl (ChREBP) and impaired cleavage of immature SREBP1c into its active form. Adipocyte-specific Wls knockout mice (Wls-/-) have lipogenic gene expression in adipose tissues and isolated adipocytes similar to that of controls when fed a normal chow diet. However, closer investigation reveals that a subset of Wnts and downstream signaling targets are upregulated within stromal-vascular cells of Wls-/- mice, suggesting that adipose tissues defend loss of Wnt secretion from adipocytes. Interestingly, this compensation is lost with long-term high-fat diet challenges. Thus, after six months of a high-fat diet, Wls-/- mice are characterized by decreased adipocyte lipogenic gene expression, reduced visceral adiposity, and improved glucose homeostasis.

CONCLUSIONS

Taken together, these studies demonstrate that adipocyte-derived Wnts regulate de novo lipogenesis and lipid desaturation and coordinate the expression of lipogenic genes in adipose tissues. In addition, we report that Wnt signaling within adipose tissues is defended, such that a loss of Wnt secretion from adipocytes is sensed and compensated for by neighboring stromal-vascular cells. With chronic overnutrition, this compensatory mechanism is lost, revealing that Wls-/- mice are resistant to diet-induced obesity, adipocyte hypertrophy, and metabolic dysfunction.

MON2 Guides Wntless Transport to the Golgi through Recycling Endosomes

Endocytic cargos are transported to recycling endosomes (RE) but how these sorting platforms are generated is not well understood. Here we describe our biochemical and live imaging studies of the conserved MON2-DOPEY complex in RE formation. MON2 mainly co-localized with RE marker RAB4B in peripheral dots and perinuclear region. The peripheral RE approached, interacted with, and separated from sorting nexin 3 (SNX3)-positive early endosomes (EE). Membrane-bound DOPEY2 was recruited to RE dependent upon MON2 expression, and showed binding abilities to kinesin and dynein/dynactin motor proteins. MON2-knockout impaired segregation of RE from EE and led to a decreased tubular recycling endosomal network, whereas RE was accumulated at perinuclear regions in DOPEY2-knockout cells. MON2 depletion also impaired intracellular transferrin receptor recycling, as well as retrograde transport of Wntless during its passage through RE before delivery from EE to the Golgi. Together, these data suggest that the MON2 drives separation of RE from EE and is required for efficient transport of endocytic cargo molecules.Key words: membrane trafficking, MON2, recycling endosomes, Wntless.

Complementary Wnt Sources Regulate Lymphatic Vascular Development via PROX1-Dependent Wnt/β-Catenin Signaling

Wnt/β-catenin signaling is necessary for lymphatic vascular development. Oscillatory shear stress (OSS) enhances Wnt/β-catenin signaling in cultured lymphatic endothelial cells (LECs) to induce expression of the lymphedema-associated transcription factors GATA2 and FOXC2. However, the mechanisms by which OSS regulates Wnt/β-catenin signaling and GATA2 and FOXC2 expression are unknown. We show that OSS activates autocrine Wnt/β-catenin signaling in LECs in vitro. Tissue-specific deletion of Wntless, which is required for the secretion of Wnt ligands, reveals that LECs and vascular smooth muscle cells are complementary sources of Wnt ligands that regulate lymphatic vascular development in vivo. Further, the LEC master transcription factor PROX1 forms a complex with β-catenin and the TCF/LEF transcription factor TCF7L1 to enhance Wnt/β-catenin signaling and promote FOXC2 and GATA2 expression in LECs. Thus, our work defines Wnt sources, reveals that PROX1 directs cell fate by acting as a Wnt signaling component, and dissects the mechanisms of PROX1 and Wnt synergy.

Epithelial Wntless is dispensable for intestinal tumorigenesis in mouse models

Wnt signaling is essential for the maintenance of adult stem cells and its aberrant activation is a stimulator of carcinogenesis. The transmembrane protein, Wntless, is an essential Wnt signaling component through regulating the secretion of Wnt ligands. Here, we generated a mouse model with specific Wntless knockout in intestinal epithelium to study its function in the intestinal epithelium. Wntless knockout exhibits no obvious defects in mice but significantly disrupted proliferation and differentiation of small intestinal organoids. We also discovered that these deficiencies could be partially rescued by Wnt3a supplement but not Wnt9b. To further investigate the role of Wntless in tumorigenesis, APC-deficient spontaneous intestinal tumors and chemical induced colorectal cancer mouse models were employed. To our surprise, intestinal epithelium-specific knockout of Wntless did not cause significant differences in tumor number and size. In summary, our data demonstrated that epithelial Wntless was required for the growth and differentiation of small intestinal organoids but not in live animals, suggesting the other tissues, such as mesenchymal tissue, play critical role for Wnt secretion in both intestinal homeostasis as well as tumorigenesis.

Alternative splicing of the Wnt trafficking protein, Wntless and its effects on protein-protein interactions

Background

Wntless (Wls) is a protein that regulates secretion of Wnt signaling molecules from Wnt-producing cells. Wnt signaling is known to be critical for several developmental and homeostatic processes. However, Wnt-independent functions of Wls are now being elucidated. Primates express an alternative splice variant of Wls (here termed WlsX). WlsX contains an alternatively spliced COOH-terminus, and does not appear to be able to sustain significant levels of WNT secretion because of its inability to undergo retrograde trafficking to the endoplasmic reticulum. The functional significance for this alternatively spliced form of Wls has not yet been elucidated. We previously identified a cohort of Wls interacting proteins using a combination of yeast 2-hybrid and candidate gene approaches.

Results

In the present study, we analyzed the interaction of WlsX with previously identified Wls interactors, and additionally screened for novel protein interactors of WlsX utilizing a membrane yeast two hybrid screen. Three novel Wls interactors, Glycoprotein M6A (GPM6A), Alkylglycerol Monooxygenase (AGMO), and ORAI1 were identified. Each of these novel WlsX interactors, as well as all other Wls interacting proteins identified previously, with the exception of the mu-opioid receptor, were found to interact with both Wls and WlsX splice forms. We show that WlsX can form homodimers, but that WlsX may not interact with Wls.

Conclusions

WlsX has the ability to form homodimers and to interact with most known Wls interacting proteins. Taken together, our results suggest that Wls and WlsX may have overlapping, but distinct functions, including sensitivity to opioid drugs. While studies have focused on the ability of Wls interacting proteins to affect Wnt secretion, future efforts will explore the reciprocal regulation of these proteins by Wls, possibly via Wnt-independent mechanisms.

Wnt/Fgf crosstalk is required for the specification of basal cells in the mouse trachea

Basal progenitor cells are crucial for the establishment and maintenance of the tracheal epithelium. However, it remains unclear how these progenitor cells are specified during foregut development. Here, we found that ablation of the Wnt chaperone protein Gpr177 (also known as Wntless) in mouse tracheal epithelium causes a significant reduction in the number of basal progenitor cells accompanied by cartilage loss in Shh-Cre;Gpr177^loxp/loxp mutants. Consistent with the association between cartilage and basal cell development, Nkx2.1⁺p63⁺ basal cells are co-present with cartilage nodules in Shh-Cre;Ctnnb1^DM/loxp mutants, which maintain partial cell-cell adhesion but not the transcription regulation function of β-catenin. More importantly, deletion of Ctnnb1 in the mesenchyme leads to the loss of basal cells and cartilage, concomitant with reduced transcript levels of Fgf10 in Dermo1-Cre;Ctnnb1^loxp/loxp mutants. Furthermore, deletion of Fgf receptor 2 (Fgfr2) in the epithelium also leads to significantly reduced numbers of basal cells, supporting the importance of Wnt/Fgf crosstalk in early tracheal development.

Wntless, a conserved Wnt-transport protein, is involved in the innate immune response of Macrobrachium rosenbergii

Wnt signaling plays important roles in a variety of developmental and pathological processes. Here we show that Wntless, the main regulator for Wnt secretion, is involved in the innate immune response of the giant freshwater prawn, Macrobrachium rosenbergii. The full-length cDNA of the prawn Wntless (named MrWntless) is 2173 bp in length and contains a 1602-bp open reading frame (ORF), which is conceptually translated into a 533-amino acids sequence. MrWntless protein contains a highly conserved Wnt-binding domain which is required for secretion of Wnt ligands, and exhibits 57-67% identity with known Wntless proteins of other animals. MrWntless was found to be expressed in a variety of prawn tissues including heart, gill, muscle, gut, hepatopancreas and ovary. Moreover, MrWntless expression was significantly increased in the hepatopancreas and gill of the prawns challenged by the bacterial pathogen Aeromonas hydrophila and Vibrio parahaemolyticus. Knockdown of MrWntless by RNA interference in prawns led to dramatically decreased MrWntless expression of approximately 70%. Furthermore, the cumulative mortality rate of the prawn injected with MrWntless dsRNA was greatly increased in response to A. hydrophila challenge compared with the control prawns. Taken together, we provide evidence that prawn Wntless is important for their innate immune response against bacterial pathogens.

Repression of MAP3K1 expression and JNK activity by canonical Wnt signaling

Morphogenesis is a complex and highly coordinated process orchestrated by temporal spatial activity of developmental pathways. How the different pathways interact to guide the developmental program remains an intriguing and open question. MAP3K1-JNK and Wnt are signaling pathways crucial for embryonic eyelid closure, an epithelial morphogenetic event conserved in mammals. Here we used a mouse model of eyelid development and genetic and biochemistry tools to investigate the relationships between the two pathways. We found that Wnt activation repressed MAP3K1 expression. Using Axin-LacZ reporter mice, spatial Wnt activity was detected in the leading edge of the developing eyelid. Conditional knockout of Wntless (Wls) in ocular surface ectoderm blocked eyelid formation, and significantly increased MAP3K1 expression in eyelid cells at the nasal canthus region. Conversely, knockout of Dkk2, encoding a canonical Wnt antagonist, resulted in an increase of Wnt activity in cells at the upper eyelid margin near the nasal canthus. Up-regulation of Wnt signaling in the Dkk2-knockout embryos corresponded to down-regulation of MAP3K1 expression. In vitro data showed that Wnt3a treatment decreased MAP3K1 promoter activity, whereas activation of Wnt by lithium chloride inhibited MAP3K1 expression, and attenuated MAP3K1-mediated JNK activity. Our data identify a unique signal crosstalk between Wnt signaling and the MAP3K1-JNK pathway in epithelial morphogenesis.

Identifying novel members of the Wntless interactome through genetic and candidate gene approaches

Wnt signaling is an important pathway that regulates several aspects of embryogenesis, stem cell maintenance, and neural connectivity. We have recently determined that opioids decrease Wnt secretion, presumably by inhibiting the recycling of the Wnt trafficking protein Wntless (Wls). This effect appears to be mediated by protein-protein interaction between Wls and the mu-opioid receptor (MOR), the primary cellular target of opioid drugs. The goal of this study was to identify novel protein interactors of Wls that are expressed in the brain and may also play a role in reward or addiction. Using genetic and candidate gene approaches, we show that among a variety of protein, Wls interacts with the dopamine transporter (target of cocaine), cannabinoid receptors (target of THC), Adenosine A2A receptor (target of caffeine), and SGIP1 (endocytic regulator of cannabinoid receptors). Our study shows that aside from opioid receptors, Wntless interacts with additional proteins involved in reward and/or addiction. Future studies will determine whether Wntless and WNT signaling play a more universal role in these processes.

A Wntless-SEC12 complex on the ER membrane regulates early Wnt secretory vesicle assembly and mature ligand export

Wntless (Wls) transports Wnt molecules for secretion; however, the cellular mechanism underlying the initial assembly of Wnt secretory vesicles is still not fully defined. Here, we performed proteomic and mutagenic analyses of mammalian Wls, and report a mechanism for formation of early Wnt secretory vesicles on ER membrane. Wls forms a complex with SEC12 (also known as PREB), an ER membrane-localized guanine nucleotide-exchange factor (GEF) activator of the SAR1 (the SAR1A isoform) small GTPase. Compared to palmitoylation-deficient Wnt molecules, binding of mature Wnt to Wls increases Wls-SEC12 interaction and promotes association of Wls with SAR1, the key activator of the COPII machinery. Incorporation of Wls into this exporting ER compartment is affected by Wnt ligand binding and SEC12 binding to Wls, as well as the structural integrity and, potentially, the folding of the cytosolic tail of Wls. In contrast, Wls-SEC12 binding is stable, with the interacting interface biochemically mapped to cytosolic segments of individual proteins. Mutant Wls that fails to communicate with the COPII machinery cannot effectively support Wnt secretion. These data suggest that formation of early Wnt secretory vesicles is carefully regulated to ensure proper export of functional ligands.

Tubule-Derived Wnts Are Required for Fibroblast Activation and Kidney Fibrosis

Cell-cell communication via Wnt ligands is necessary in regulating embryonic development and has been implicated in CKD. Because Wnt ligands are ubiquitously expressed, the exact cellular source of the Wnts involved in CKD remains undefined. To address this issue, we generated two conditional knockout mouse lines in which Wntless (Wls), a dedicated cargo receptor that is obligatory for Wnt secretion, was selectively ablated in tubular epithelial cells or interstitial fibroblasts. Blockade of Wnt secretion by genetic deletion of Wls in renal tubules markedly inhibited myofibroblast activation and reduced renal fibrosis after unilateral ureteral obstruction. This effect associated with decreased activation of β-catenin and downstream gene expression and preserved tubular epithelial integrity. In contrast, fibroblast-specific deletion of Wls exhibited little effect on the severity of renal fibrosis after obstructive or ischemia-reperfusion injury. In vitro, incubation of normal rat kidney fibroblasts with tubule-derived Wnts promoted fibroblast proliferation and activation. Furthermore, compared with kidney specimens from patients without CKD, biopsy specimens from patients with CKD also displayed increased expression of multiple Wnt proteins, predominantly in renal tubular epithelium. These results illustrate that tubule-derived Wnts have an essential role in promoting fibroblast activation and kidney fibrosis via epithelial-mesenchymal communication.

Divergent effects of Porcupine and Wntless on WNT1 trafficking, secretion, and signaling

Loss-of-function studies have identified Porcupine (PORCN) and Wntless (WLS) as essential mediators of Wnt secretion and signaling. Whereas PORCN is thought to palmitoylate Wnt proteins, WLS is believed to transport palmitoylated Wnt proteins to the cell surface. However, little is known about how these two proteins cooperate to regulate Wnt palmitoylation, trafficking, secretion, and signaling. We first investigated possible interactions between PORCN, WLS, and WNT1, by carrying out co-immunoprecipitation studies. These studies demonstrate the existence of a complex containing PORCN and WLS. They further show that PORCN and WLS compete for binding to WNT1. Then, we used gain-of-function studies to investigate the cooperation between PORCN and WLS as well as possible biochemical interactions between PORCN, WLS, and WNT1. Consistent with the proposed roles for PORCN and WLS, we show that overexpression of PORCN promotes palmitoylation of WNT1 while overexpression of WLS does not. Overexpression of PORCN enhances the ability of WLS to promote WNT1 trafficking to the cell surface as well as secretion, but decreases the ability of WLS to activate WNT1 signaling in target cell. These observations suggest that the levels of WNT1 on the cell surface and in the media are not the sole determinants of the activation of Wnt signaling in target cells.

CD133-positive dermal papilla-derived Wnt ligands regulate postnatal hair growth

Active Wnt/β-catenin signaling in the dermal papilla (DP) is required for postnatal hair cycling. In addition, maintenance of the hair-inducing ability of DP cells in vitro requires external addition of Wnt molecules. However, whether DP cells are a critical source of Wnt ligands and induce both autocrine and paracrine signaling cascades to promote adult hair follicle growth and regeneration remains elusive. To address this question, we generated an animal model that allows inducible ablation of Wntless (Wls), a transmembrane Wnt exporter protein, in CD133-positive (CD133+) DP cells. CD133+ cells have been shown to be a specific subpopulation of cells in the DP, which possesses the hair-inducing capability. Here, we show that ablation of Wls expression in CD133+ DP cells results in a shortened period of postnatal hair growth. Mutant hair follicles were unable to enter full anagen (hair growth stage) and progressed toward a rapid regression. Notably, reduced size of the DP and decreased expression of anagen DP marker, versican, were observed in hair follicles when CD133+ DP cells lost Wls expression. Further analysis showed that Wls-deficient CD133+ DP cells led to reduced proliferation and differentiation in matrix keratinocytes and melanocytes that are needed for the generation of the hair follicle structure and a pigmented hair shaft. These findings clearly demonstrate that Wnt ligands produced by CD133+ DP cells play an important role in postnatal hair growth by maintaining the inductivity of DP cells and mediating the signaling cross-talk between the mesenchyme and the epithelial compartment.

Ectodermal Wnt controls nasal pit morphogenesis through modulation of the BMP/FGF/JNK signaling axis

BACKGROUND

Mutations of WNT3, WNT5A, WNT9B, and WNT11 genes are associated with orofacial birth defects, including nonsyndromic cleft lip with cleft palate in humans. However, the source of Wnt ligands and their signaling effects on the orofacial morphogenetic process remain elusive.

RESULTS

Using Foxg1-Cre to impair Wnt secretion through the inactivation of Gpr177/mWls, we investigate the relevant regulation of Wnt production and signaling in nasal-facial development. Ectodermal ablation of Gpr177 leads to severe facial deformities resulting from dramatically reduced cell proliferation and increased cell death due to a combined loss of WNT, FGF and BMP signaling in the developing facial prominence. In the invaginating nasal pit, the Gpr177 disruption also causes a detrimental effect on migration of the olfactory epithelial cells into the mesenchymal region. The blockage of Wnt secretion apparently impairs the olfactory epithelial cells through modulation of JNK signaling.

CONCLUSIONS

Our study thus suggests the head ectoderm, including the facial ectoderm and the neuroectoderm, as the source of canonical as well as noncanonical Wnt ligands during early development of the nasal-facial prominence. Both β-catenin-dependent and -independent signaling pathways are required for proper development of these morphogenetic processes.

Regulation of bone metabolism by Wnt signals

Wnt ligands play a central role in the development and homeostasis of various organs through β-catenin-dependent and -independent signalling. The crucial roles of Wnt/β-catenin signals in bone mass have been established by a large number of studies since the discovery of a causal link between mutations in the low-density lipoprotein receptor-related protein 5 (Lrp5) gene and alternations in human bone mass. The activation of Wnt/β-catenin signalling induces the expression of osterix, a transcription factor, which promotes osteoblast differentiation. Furthermore, this signalling induces the expression of osteoprotegerin, an osteoclast inhibitory factor in osteoblast-lineage cells to prevent bone resorption. Recent studies have also shown that Wnt5a, a typical non-canonical Wnt ligand, enhanced osteoclast formation. In contrast, Wnt16 inhibited osteoclast formation through β-catenin-independent signalling. In this review, we discussed the current understanding of the Wnt signalling molecules involved in bone formation and resorption.

Early avoidance of a heroin-paired taste-cue and subsequent addiction-like behavior in rats

The ability to predict individual vulnerability to substance abuse would allow for a better understanding of the progression of the disease and development of better methods for prevention and/or early intervention. Here we use drug-induced devaluation of a saccharin cue in an effort to predict later addiction-like behavior in a model akin to that used by Deroche-Gamonet et al. (2004) and seek to link such vulnerability to changes in expression of various mu opioid receptor and D2 receptor-interacting proteins in brain. The results show that the greatest heroin-induced suppression of intake of a saccharin cue is associated with the greatest vulnerability to later addiction-like behavior and to differences in the expression of WLS, β-catenin, and NCS-1 in brain compared to rats that exhibited the least suppression of intake of the heroin-paired cue and/or saline controls. Finally, because the self-administration model employed produced no significant differences in drug intake between groups, overall, the resultant changes in protein expression can be more closely linked to individual differences in motivation for drug.

Wntless spatially regulates bone development through β-catenin-dependent and independent mechanisms

BACKGROUND

Canonical and noncanonical Wnt signaling pathways both play pivotal roles in bone development. Wntless/GPR177 is a chaperone protein that is required for secretion of all Wnt ligands. We previously showed that deletion of Wntless within mature osteoblasts severely impaired postnatal bone homeostasis.

RESULTS

In this study, we systemically evaluated how deletion of Wntless in different stages of osteochondral differentiation affected embryonic bone development, by crossing Wntless (Wls)-flox/flox mice with strains expressing cre recombinase behind the following promoters: Osteocalcin, Collagen 2a1, or Dermo1. Ex vivo µCT and whole-mount skeletal staining were performed to examine skeletal mineralization. Histology and immunohistochemistry were used to evaluate cellular differentiation and alterations in Wnt signaling. In this work, we found that Wntless regulated chondrogenesis and osteogenesis through both canonical and noncanonical Wnt signaling.

CONCLUSIONS

These findings provide more insight into the requirements of different Wnt-secretion cell types critical for skeletal development.

Ablation of Wntless in endosteal niches impairs lymphopoiesis rather than HSCs maintenance

Osteoblasts and perivascular stromal cells constitute essential niches for HSC self-renewal and maintenance in the bone marrow. Wnt signaling is important to maintain HSC integrity. However, the paracrine role of Wnt proteins in osteoblasts-supported HSC maintenance and differentiation remains unclear. Here, we investigated hematopoiesis in mice with Wntless (Wls) deficiency in osteoblasts or Nestin-positive mesenchymal progenitor cells, which presumptively block Wnt secretion in osteoblasts. We detected defective B-cell lymphopoiesis and abnormal T-cell infiltration in the bone marrow of Wls mutant mice. Notably, no impact on HSC frequency and repopulation in the bone marrow was observed with the loss of osteoblastic Wls. Our findings revealed a supportive role of Wnts in osteoblasts-regulated B-cell lymphopoiesis. They also suggest a preferential niche role of osteoblastic Wnts for lymphoid cells rather than HSCs, providing new clues for the molecular nature of distinct niches occupied by different hematopoietic cells.

Involvement of epithelial Wntless in the regulation of postnatal hair follicle morphogenesis

The roles of the Wnt cargo receptor Wntless (Wls) during hair follicle (HF) induction and postnatal HF cycling in skin have been elucidated. However, whether Wls regulates postnatal HF morphogenesis remains unclear. In this study, we found that Wls is expressed in developing HF during the morphogenesis stage after birth. By knocking out Wls in mouse skin epithelia with hypomorphic K14-cre, we found that Wls is required for normal HF morphogenesis. Wls-deficient HFs prematurely regressed, which was possibly caused by abnormally activated TGF-β/JNK pathway. Although Wls was reported to be a direct target of the Wnt/β-catenin pathway, we found that epithelial β-catenin was not necessary to maintain Wls expression. Therefore, other signals are involved in regulating Wls transcription in mouse skin.

Rab8a vesicles regulate Wnt ligand delivery and Paneth cell maturation at the intestinal stem cell niche

Communication between stem and niche supporting cells maintains the homeostasis of adult tissues. Wnt signaling is a crucial regulator of the stem cell niche, but the mechanism that governs Wnt ligand delivery in this compartment has not been fully investigated. We identified that Wnt secretion is partly dependent on Rab8a-mediated anterograde transport of Gpr177 (wntless), a Wnt-specific transmembrane transporter. Gpr177 binds to Rab8a, depletion of which compromises Gpr177 traffic, thereby weakening the secretion of multiple Wnts. Analyses of generic Wnt/β-catenin targets in Rab8a knockout mouse intestinal crypts indicate reduced signaling activities; maturation of Paneth cells – a Wnt-dependent cell type – is severely affected. Rab8a knockout crypts show an expansion of Lgr5⁺ and Hopx⁺ cells in vivo. However, in vitro, the knockout enteroids exhibit significantly weakened growth that can be partly restored by exogenous Wnts or Gsk3β inhibitors. Immunogold labeling and surface protein isolation identified decreased plasma membrane localization of Gpr177 in Rab8a knockout Paneth cells and fibroblasts. Upon stimulation by exogenous Wnts, Rab8a-deficient cells show ligand-induced Lrp6 phosphorylation and transcriptional reporter activation. Rab8a thus controls Wnt delivery in producing cells and is crucial for Paneth cell maturation. Our data highlight the profound tissue plasticity that occurs in response to stress induced by depletion of a stem cell niche signal.

Summary: In maturing mouse Paneth cells, Wnt secretion is partly dependent on a Rab8a-mediated anterograde transport of Gpr177. Rab8a is required for Paneth cell maturation.

Control of Wnt5b secretion by Wntless modulates chondrogenic cell proliferation through fine-tuning fgf3 expression

Wnts and Fgfs regulate various tissues development in vertebrates. However, how regional Wnt or Fgf activities are established and how they interact in any given developmental event is elusive. Here, we investigated the Wnt-mediated craniofacial cartilage development in zebrafish and found that fgf3 expression in the pharyngeal pouches is differentially reduced along the anteroposterior axis in wnt5b mutants and wntless (wls) morphants, but its expression is normal in wnt9a and wnt11 morphants. Introducing fgf3 mRNAs rescued the cartilage defects in Wnt5b- and Wls-deficient larvae. In wls morphants, endogenous Wls expression is not detectable but maternally deposited Wls is present in eggs, which might account for the lack of axis defects in wls morphants. Secretion of endogenous Wnt5b but not Wnt11 was affected in the pharyngeal tissue of Wls morphants, indicating that Wls is not involved in every Wnt secretion event. Furthermore, cell proliferation but not apoptosis in the developing jaw was affected in Wnt5b- and Wls-deficient embryos. Therefore, Wnt5b requires Wls for its secretion and regulates the proliferation of chondrogenic cells through fine-tuning the expression of fgf3 during jaw cartilage development.

Wnts produced by Osterix-expressing osteolineage cells regulate their proliferation and differentiation

Wnt signaling is a critical regulator of bone development, but the identity and role of the Wnt-producing cells are still unclear. We addressed these questions through in situ hybridization, lineage tracing, and genetic experiments. First, we surveyed the expression of all 19 Wnt genes and Wnt target gene Axin2 in the neonatal mouse bone by in situ hybridization, and demonstrated--to our knowledge for the first time--that Osterix-expressing cells coexpress Wnt and Axin2. To track the behavior and cell fate of Axin2-expressing osteolineage cells, we performed lineage tracing and showed that they sustain bone formation over the long term. Finally, to examine the role of Wnts produced by Osterix-expressing cells, we inhibited Wnt secretion in vivo, and observed inappropriate differentiation, impaired proliferation, and diminished Wnt signaling response. Therefore, Osterix-expressing cells produce their own Wnts that in turn induce Wnt signaling response, thereby regulating their proliferation and differentiation.

Protein kinase CK2 is required for Wntless internalization and Wnt secretion

Wnt proteins are lipid modified signaling molecules that have essential functions in development and adult tissue homeostasis. Secretion of Wnt is mediated by the transmembrane protein Wntless, which binds Wnt and transports it from the endoplasmic reticulum to the cell surface for release. To maintain efficient Wnt secretion, Wntless is recycled back to the Golgi and the endoplasmic reticulum through endocytosis and retromer dependent endosome to Golgi transport. We have previously identified protein kinase CK2 (CK2) in a genome-wide screen for regulators of Wnt signaling in Caenorhabditis elegans. Here, we show that CK2 function is required in Wnt producing cells for Wnt secretion. This function is evolutionarily conserved, as inhibition of CK2 activity interferes with Wnt5a secretion from mammalian cells. Mechanistically, we show that inhibition of CK2 function results in enhanced plasma membrane localization of Wls in C. elegans and mammalian cells, consistent with the notion that CK2 is involved in the regulation of Wls internalization.

Gpr177-mediated Wnt Signaling is Required for Fungiform Placode Initiation

Fungiform papillae are formed as patterned rows on the surface of the anterior tongue at early organogenesis and contain one taste bud in each papilla to form one of the important sensory organs. Despite the essential role of Wnt/β-catenin signaling in controlling the development of fungiform taste papillae, the universal function of Wnt ligands in the initiation of the fungiform placode has not been completely elucidated. Here, by Shh (Cre) -mediated oral epithelial deletion of Wntless (Gpr177), a regulator essential for intracellular Wnt trafficking, we demonstrate that an overall function of Wnts is required for initiation of the fungiform placode. Multiple Wnts are expressed in the tongue epithelium at E11.5 before initiation of the fungiform placodes. Epithelial Gpr177 loss-of-function, associated with reduction of canonical Wnt signaling in lingual epithelium as exhibited by a loss of TopGal activity and Axin2 expression, results in the failure of fungiform placode initiation, as assessed by diminished expression of several taste placode molecular markers. Moreover, LiCl treatment of Gpr177 epithelial-deficient tongue explants at E11.5, but not at E12.5, restores tongue placode formation, demonstrating that Wnt ligands in the tongue surface prior to but not after fungiform placode initiation are responsible for fungiform papilla initiation. Epithelium-specific expression of an active β-catenin in the Gpr177-deficient tongue leads to fungiform papillae generation, suggesting that an intra-epithelial response to Wnts is required for placode initiation. Together, these results suggest that Gpr177 controls epithelial initiation of the fungiform placode through signaling via epithelial Wnt ligands.

Morphine-induced trafficking of a mu-opioid receptor interacting protein in rat locus coeruleus neurons

Opiate addiction is a devastating health problem, with approximately 2million people currently addicted to heroin or non-medical prescription opiates in the United States alone. In neurons, adaptations in cell signaling cascades develop following opioid actions at the mu opioid receptor (MOR). A novel putative target for intervention involves interacting proteins that may regulate trafficking of MOR. Morphine has been shown to induce a re-distribution of a MOR-interacting protein Wntless (WLS, a transport molecule necessary for secretion of neurotrophic Wnt proteins), from cytoplasmic to membrane compartments in rat striatal neurons. Given its opiate-sensitivity and its well-characterized molecular and cellular adaptations to morphine exposure, we investigated the anatomical distribution of WLS and MOR in the rat locus coeruleus (LC)-norepinephrine (NE) system. Dual immunofluorescence microscopy was used to test the hypothesis that WLS is localized to noradrenergic neurons of the LC and that WLS and MOR co-exist in common LC somatodendritic processes, providing an anatomical substrate for their putative interactions. We also hypothesized that morphine would influence WLS distribution in the LC. Rats received saline, morphine or the opiate agonist [d-Ala2, N-Me-Phe4, Gly-ol5]-enkephalin (DAMGO), and tissue sections through the LC were processed for immunogold-silver detection of WLS and MOR. Statistical analysis showed a significant re-distribution of WLS to the plasma membrane following morphine treatment in addition to an increase in the proximity of gold-silver labels for MOR and WLS. Following DAMGO treatment, MOR and WLS were predominantly localized within the cytoplasmic compartment when compared to morphine and control. In a separate cohort of rats, brains were obtained from saline-treated or heroin self-administering male rats for pulldown co-immunoprecipitation studies. Results showed an increased association of WLS and MOR following heroin exposure. As the LC-NE system is important for cognition as well as decisions underlying substance abuse, adaptations in WLS trafficking and expression may play a role in modulating MOR function in the LC and contribute to the negative sequelae of opiate exposure on executive function.

Wnt signaling regulates homeostasis of the periodontal ligament

BACKGROUND AND OBJECTIVE

In health, the periodontal ligament maintains a constant width throughout an organism's lifetime. The molecular signals responsible for maintaining homeostatic control over the periodontal ligament are unknown. The purpose of this study was to investigate the role of Wnt signaling in this process by removing an essential chaperone protein, Wntless (Wls), from odontoblasts and cementoblasts, and observing the effects of Wnt depletion on cells of the periodontal complex.

MATERIAL AND METHODS

The Wnt responsive status of the periodontal complex was assessed using two strains of Wnt reporter mice: Axin2(LacZ/+) and Lgr5(LacZ/+) . The function of this endogenous Wnt signal was evaluated by conditionally eliminating the Wntless (Wls) gene using an osteocalcin Cre driver. The resulting OCN-Cre;Wls (fl/fl) mice were examined using micro-computed tomography and histology, immunohistochemical analyses for osteopontin, Runx2 and fibromodulin, in-situ hybridization for osterix and alkaline phosphatase activity.

RESULTS

The adult periodontal ligament is Wnt responsive. Elimination of Wnt signaling in the periodontal complex of OCN-Cre;Wls(fl/fl) mice resulted in a wider periodontal ligament space. This pathologically increased periodontal width is caused by a reduction in the expression of osteogenic genes and proteins, which results in thinner alveolar bone. A concomitant increase in fibrous tissue occupying the periodontal space was observed, along with a disruption in the orientation of the periodontal ligament.

CONCLUSION

The periodontal ligament is a Wnt-dependent tissue. Cells in the periodontal complex are Wnt responsive, and eliminating an essential component of the Wnt signaling network leads to a pathological widening of the periodontal ligament space. Osteogenic stimuli are reduced, and a disorganized fibrillary matrix results from the depletion of Wnt signaling. Collectively, these data underscore the importance of Wnt signaling in homeostasis of the periodontal ligament.

Gpr177 regulates pulmonary vasculature development

Establishment of the functional pulmonary vasculature requires intimate interaction between the epithelium and mesenchyme. Previous genetic studies have led to inconsistent conclusions about the contribution of epithelial Wnts to pulmonary vasculature development. This discrepancy is possibly due to the functional redundancy among different Wnts. Here, we use Shh-Cre to conditionally delete Gpr177 (the mouse ortholog of Drosophila Wntless, Wls), a chaperon protein important for the sorting and secretion of Wnt proteins. Deletion of epithelial Gpr177 reduces Wnt signaling activity in both the epithelium and mesenchyme, resulting in severe hemorrhage and abnormal vasculature, accompanied by branching defects and abnormal epithelial differentiation. We then used multiple mouse models to demonstrate that Wnt/β-catenin signaling is not only required for the proliferation and differentiation of mesenchyme, but also is important for the maintenance of smooth muscle cells through the regulation of the transcription factor Kruppel-like factor 2 (Klf2). Together, our studies define a novel mechanism by which epithelial Wnts regulate the normal development and maintenance of pulmonary vasculature. These findings provide insight into the pathobiology of congenital lung diseases, such as alveolar capillary dysplasia (ACD), that have abnormal alveolar development and dysmorphic pulmonary vasculature.

Wntless in Wnt secretion: molecular, cellular and genetic aspects

Throughout the animal kingdom, Wnt-triggered signal transduction pathways play fundamental roles in embryonic development and tissue homeostasis. Wnt proteins are modified as glycolipoproteins and are secreted into the extracellular environment as morphogens. Recent studies on the intracellular trafficking of Wnt proteins demonstrate multiple layers of regulation along its secretory pathway. These findings have propelled a great deal of interest among researchers to further investigate the molecular mechanisms that control the release of Wnts and hence the level of Wnt signaling. This review is dedicated to Wntless, a putative G-protein coupled receptor that transports Wnts intracellularly for secretion. Here, we highlight the conclusions drawn from the most recent cellular, molecular and genetic studies that affirm the role of Wntless in the secretion of Wnt proteins.