Lgr4-deficient mice showed premature differentiation of ureteric bud with reduced expression of Wnt effector Lef1 and Gata3

Deciphering the cellular and molecular landscapes of Wnt/β-catenin signaling in mouse embryonic kidney development

Background

The Wnt/β-catenin signaling pathway is critical in kidney development, yet its specific effects on gene expression in different embryonic kidney cell types are not fully understood.

Methods

Wnt/β-catenin signaling was activated in mouse E12.5 kidneys in vitro using CHIR99021, with RNA sequencing performed afterward, and the results were compared to DMSO controls (dataset GSE131240). Differential gene expression in ureteric buds and cap mesenchyme following pathway activation (datasets GSE20325 and GSE39583) was analyzed. Single-cell RNA-seq data from the Mouse Cell Atlas was used to link differentially expressed genes (DEGs) with kidney cell types. β-catenin ChIP-seq data (GSE39837) identified direct transcriptional targets.

Results

Activation of Wnt/β-catenin signaling led to 917 significant DEGs, including the upregulation of Notum and Apcdd1 and the downregulation of Crym and Six2. These DEGs were involved in kidney development and immune response. Single-cell analysis identified 787 DEGs across nineteen cell subtypes, with Macrophage_Apoe high cells showing the most pronounced enrichment of Wnt/β-catenin-activated genes. Gene expression profiles in ureteric buds and cap mesenchyme differed significantly upon β-catenin manipulation, with cap mesenchyme showing a unique set of DEGs. Analysis of β-catenin ChIP-seq data revealed 221 potential direct targets, including Dpp6 and Fgf12.

Conclusion

This study maps the complex gene expression driven by Wnt/β-catenin signaling in embryonic kidney cell types. The identified DEGs and β-catenin targets elucidate the molecular details of kidney development and the pathway's role in immune processes, providing a foundation for further research into Wnt/β-catenin signaling in kidney development and disease.

Crucial Convolution: Genetic and Molecular Mechanisms of Coiling during Epididymis Formation and Development in Embryogenesis

As embryonic development proceeds, numerous organs need to coil, bend or fold in order to establish their final shape. Generally, this occurs so as to maximise the surface area for absorption or secretory functions (e.g., in the small and large intestines, kidney or epididymis); however, mechanisms of bending and shaping also occur in other structures, notably the midbrain–hindbrain boundary in some teleost fish models such as zebrafish. In this review, we will examine known genetic and molecular factors that operate to pattern complex, coiled structures, with a primary focus on the epididymis as an excellent model organ to examine coiling. We will also discuss genetic mechanisms involving coiling in the seminiferous tubules and intestine to establish the final form and function of these coiled structures in the mature organism.

The Role of LGR4 (GPR48) in Normal and Cancer Processes

Leucine-rich repeats containing G protein-coupled receptor 4 (LGR4) is a receptor that belongs to the superfamily of G protein-coupled receptors that can be activated by R-spondins (RSPOs), Norrin, circLGR4, and the ligand of the receptor activator of nuclear factor kappa-B (RANKL) ligands to regulate signaling pathways in normal and pathological processes. LGR4 is widely expressed in different tissues where it has multiple functions such as tissue development and maintenance. LGR4 mainly acts through the Wnt/β-catenin pathway to regulate proliferation, survival, and differentiation. In cancer, LGR4 participates in tumor progression, invasion, and metastasis. Furthermore, recent evidence reveals that LGR4 is essential for the regulation of the cancer stem cell population by controlling self-renewal and regulating stem cell properties. This review summarizes the function of LGR4 and its ligands in normal and malignant processes.

LGR6 marks nephron progenitor cells

Background

Nephron progenitor cells (NPCs) undergo a stepwise process to generate all mature nephron structures. Mesenchymal to epithelial transition (MET) is considered a multistep process of NPC differentiation to ensure progressive establishment of new nephrons. However, despite this important role, to date, no marker for NPCs undergoing MET in the nephron exists.

Results

Here, we identify LGR6 as a NPC marker, expressed in very early cap mesenchyme, pre‐tubular aggregates, renal vesicles, and in segments of S‐shaped bodies, following the trajectory of MET. By using a lineage tracing approach in embryonic explants in combination with confocal imaging and single‐cell RNA sequencing, we provide evidence for the multiple fates of LGR6+ cells during embryonic nephrogenesis. Moreover, by using long‐term in vivo lineage tracing, we show that postnatal LGR6+ cells are capable of generating the multiple lineages of the nephrons.

Conclusions

Given the profound early mesenchymal expression and MET signature of LGR6⁺ cells, together with the lineage tracing of mesenchymal LGR6⁺ cells, we conclude that LGR6+ cells contribute to all nephrogenic segments by undergoing MET. LGR6+ cells can therefore be considered an early committed NPC population during embryonic and postnatal nephrogenesis with potential regenerative capability.

Lgr6 is expressed in the earliest cap mesenchyme pool, a niche where nephrogenic progenitor cells (NPCs) are found.

Lgr6 marks NPCs undergoing mesenchymal to epithelial transition, following the main process of nephron development.

Using ex vivo and vivo lineage tracing, we show that mesenchymal Lgr6 expressing cells give rise to multiple types of mesenchymal derived nephron segments, including specialized glomerular epithelium, such as podocytes.

Inhibition of GSK3 Represses the Expression of Retinoic Acid Synthetic Enzyme ALDH1A2 via Wnt/β-Catenin Signaling in WiT49 Cells

Organogenesis, including renal development, requires an appropriate retinoic acid concentration, which is established by differential expression of aldehyde dehydrogenase 1 family member A2 (ALDH1A2) and cytochrome P450 family 26 subfamily A/B/C member 1 (CYP26A1/B1/C1). In the fetal kidney, ALDH1A2 expresses in the developing stroma and renal vesicle and its derivatives but does not present in the ureteric bud. It remains unclear what may contribute to this expression pattern. Here we show that the glycogen synthase kinase 3 alpha/beta (GSK3A/B) inhibitor CHIR99021 significantly represses ALDH1A2 expression in WiT49, which is a Wilms’ tumor cell line that exhibits “triphasic” differential potential and is used as a fetal kidney cell model. CHIR99021 fails to suppress ALDH1A2 as β-catenin is inhibited, suggesting that the downregulation of ALDH1A2 by CHIR99021 is through Wnt/β-catenin signaling. Ectopic expression of mouse Wnt1, Wnt3a, Wnt4, and Wnt9b represses ALDH1A2 expression in WiT49 cells. Using immunohistochemistry, we show an inverse correlation of Aldh1a2 expression with β-catenin in rat E18.5 kidney. ChIP demonstrated that β-catenin is recruited to the ALDH1A2 promoter, the conserved intron1G, and another site within intron 1 of ALDH1A2. Using a luciferase assay, we further show that the ALDH1A2 promoter and the intron1G element are involved in the repression of ALDH1A2 expression by CHIR99021. Our work demonstrates that ALDH1A2 expression can be directly repressed by the Wnt/β-catenin signaling in fetal kidney cells, suggesting that Wnt/β-catenin may play a role in maintaining the expression pattern of ALDH1A2 in the fetal kidney, thus controlling the availability and localization of retinoic acid and regulating aspects of kidney development.

The Interplay of MicroRNA-34a, LGR4, EMT-Associated Factors, and MMP2 in Regulating Uveal Melanoma Cells

Purpose

MicroRNA-34a (miR-34a) has been implicated in many biological processes. It is downregulated in uveal melanoma, and introduction of miR-34a inhibits the proliferation and migration of uveal melanoma cells. Leucine-rich repeat-containing G protein-coupled receptor 4 (LGR4) is a novel target of miR-34a identified first in retinal pigment epithelial cells. In this study, we sought to evaluate the interaction of miR-34a and LGR4 in uveal melanoma and its downstream mechanisms.

Methods

The expression of LGR4, epithelial-mesenchymal transition (EMT)-associated factors, and matrix metalloproteinase 2 (MMP2) in uveal melanoma cells was assessed by immunoblotting and immunofluorescence analysis. MicroRNA-34a mimic molecules, LGR4 small interfering RNA (siRNA), or MMP2-specific siRNA were transiently transfected into uveal melanoma cells. In vitro scratch and Transwell assays were used to evaluate the migratory and invasive potential of the resultant uveal melanoma cells.

Results

LGR4 is upregulated in uveal melanoma cells. Introduction of miR-34a significantly decreased the expression level of LGR4. Transfection with miR-34a or knockdown of LGR4 attenuated the aggressiveness of uveal melanoma cells. In addition, there was a decrease in the expression of mesenchymal markers N-cadherin, vimentin, and Snail following miR-34a introduction or knockdown of LGR4. Finally, MMP2 was found to be a downstream effector for miR-34a and LGR4 that regulates the migration and invasion of uveal melanoma cells.

Conclusions

MicroRNA-34a negatively controls LGR4, thereby inhibiting the migration and invasion of uveal melanoma cells. Ultimately, both miR-34a and LGR4 impact the aggressiveness of uveal melanoma with alterations in the markers of the EMT. MMP2 is a downstream effector that influences the metastasis seen with uveal melanoma cells.

Salivary gland stem cells: A review of development, regeneration and cancer

Salivary glands are responsible for maintaining the health of the oral cavity and are routinely damaged by therapeutic radiation for head and neck cancer as well as by autoimmune diseases such as Sjögren's syndrome. Regenerative approaches based on the reactivation of endogenous stem cells or the transplant of exogenous stem cells hold substantial promise in restoring the structure and function of these organs to improve patient quality of life. However, these approaches have been hampered by a lack of knowledge on the identity of salivary stem cell populations and their regulators. In this review we discuss our current knowledge on salivary stem cells and their regulators during organ development, homeostasis and regeneration. As increasing evidence in other systems suggests that progenitor cells may be a source of cancer, we also review whether these same salivary stem cells may also be cancer initiating cells.

Identifying the localization and exploring a functional role for Gprc5c in the kidney

The investigation of orphan GPCRs (GPRs) has the potential to uncover novel insights into whole animal physiology. In this study, our goal was to determine the renal localization of Gprc5c, a receptor that we previously reported to be highly expressed in murine whole kidney, and to examine physiologic parameters in Gprc5c knockout (KO) mice to gain insight into function. Gprc5c localized to the apical membrane of renal proximal tubules (PTs) in mice, rats, and humans. With the comparison of Gprc5c wild-type (WT) and KO mice, we found that Gprc5c KO mice have altered acid-base homeostasis. Specifically, Gprc5c KO mice have lower blood pH and higher urine pH compared with WT mice, with a reduced level of titratable acids in their urine. In an in vitro GPCR internalization assay, we observed that Gprc5c internalization (an index of activation) was triggered by alkaline extracellular pH. Furthermore, with the use of an in vitro BCECF assay, we observed that Gprc5c increases Na⁺/H⁺ exchanger 3 (NHE3) activity at alkaline pH. We also find that the NHE3 activity is reduced in Gprc5c KO mice by 2 photon imaging in seminaphthorhodafluors (SNARF)-4F-loaded kidney sections. NHE3 is a primary contributor to apical transport of H⁺ in the renal PT. Together, these data imply that Gprc5c modulates the renal contribution to systemic pH homeostasis, at least in part, by taking part in the regulation of NHE3.-Rajkumar, P., Cha, B., Yin, J., Arend, L. J., Păunescu, T. G., Hirabayashi, Y., Donowitz, M., Pluznick, J. L. Identifying the localization and exploring a functional role for Gprc5c in the kidney.

Unsung renal receptors: orphan G-protein-coupled receptors play essential roles in renal development and homeostasis

Recent studies have shown that orphan GPCRs of the GPR family are utilized as specialized chemosensors in various tissues to detect metabolites, and in turn to activate downstream pathways which regulate systemic homeostasis. These studies often find that such metabolites are generated by well-known metabolic pathways, implying that known metabolites and chemicals may perform novel functions. In this review, we summarize recent findings highlighting the role of deorphanized GPRs in renal development and function. Understanding the role of these receptors is critical in gaining insights into mechanisms that regulate renal function both in health and in disease.

In Vitro Propagation and Branching Morphogenesis from Single Ureteric Bud Cells

A method to maintain and rebuild ureteric bud (UB)-like structures from UB cells in vitro could provide a useful tool for kidney regeneration. We aimed in our present study to establish a serum-free culture system that enables the expansion of UB progenitor cells, i.e., UB tip cells, and reconstruction of UB-like structures. We found that fibroblast growth factors or retinoic acid (RA) was sufficient for the survival of UB cells in serum-free condition, while the proliferation and maintenance of UB tip cells required glial cell-derived neurotrophic factor together with signaling from either WNT-β-catenin pathway or RA. The activation of WNT-β-catenin signaling in UB cells by endogenous WNT proteins required R-spondins. Together with Rho kinase inhibitor, our culture system facilitated the expansion of UB tip cells to form UB-like structures from dispersed single cells. The UB-like structures thus formed retained the original UB characteristics and integrated into the native embryonic kidneys.

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FGFs and RA signaling sustain UB cell survival in serum-free culture condition

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WNT-β-catenin and RA signaling maintain the expansion of UB tip cells

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WNT proteins in UB cells activate WNT-β-catenin signaling through R-spondins

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Single UB cells form UB-like structures in vitro that integrate into native kidneys

LGR4 Is a Direct Target of MicroRNA-34a and Modulates the Proliferation and Migration of Retinal Pigment Epithelial ARPE-19 Cells

The pathology of proliferative vitreoretinopathy and proliferative diabetic retinopathy is linked to proliferation, migration, and adhesion of the retinal pigment epithelium. MicroRNA-34a (miR-34a) expression modulates changes in proliferation and migration of retinal pigment epithelial cell line ARPE-19. In this study, we determined that miR-34a interacts with LGR4, identified by bioinformatics using TargetScan Human 5.0, to affect these changes. Double luciferase gene reporter assay confirmed miR-34a involvement in mediating control. miR-34a mimic transfection decreased LGR4 expression. Western blot analysis documented corresponding protein expression inhibition. MTS, Ki67 immunostaining, scratch and transwell testing, along with attachment assay showed that miR-34a upregulation inhibited ARPE-19 cell proliferation, migration and attachment partly through downregulation of LGR4 protein expression. Western blot analysis revealed that both miR-34a upregulation and LGR4 downregulation induced declines in E2F1, p-CDC2, CDK2, CDK4 and CDK6 protein expression. Taken together, miR-34a gene expression upregulation inhibits ARPE-19 cell proliferation, migration and adhesion partly by suppressing LGR4 expression. These results substantiate earlier indications that both miR-34a and LGR4 are potential drug targets to prevent fibrosis in a clinical setting.

LGR4 is required for sequential molar development

Tooth development requires proliferation, differentiation, and specific migration of dental epithelial cells, through well-organized signaling interactions with mesenchymal cells. Recently, it has been reported that leucine-rich repeat-containing G protein coupled receptor 4 (LGR4), the receptor of R-spondins, is expressed in many epithelial cells in various organs and tissues and is essential for organ development and stem cell maintenance. Here, we report that LGR4 contributes to the sequential development of molars in mice. LGR4 expression in dental epithelium was detected in SOX2⁺ cells in the posterior end of the second molar (M2) and the early tooth germ of the third molar (M3). In keratinocyte-specific Lgr4-deficient mice (Lgr4^{K5 KO}), the developmental defect became obvious by postnatal day 14 (P14) in M3. Lgr4^{K5 KO} adult mice showed complete absence or the dwarfed form of M3. In M3 development in Lgr4^{K5 KO} mice, at Wnt/β-catenin signal activity was down-regulated in the dental epithelium at P3, as indicated by lymphoid enhancer-binding factor-1 (LEF1) expression. We also confirmed the decrease, in dental epithelium of Lgr4^{K5 KO} mice, of the number of SOX2⁺ cells and the arrest of cell proliferation at P7, and observed abnormal differentiation at P14. Our data demonstrated that LGR4 controls the sequential development of molars by maintaining SOX2⁺ cells in the dental epithelium, which have the ability to form normal molars.

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LGR4 expression was observed in the dental epithelium after birth and moved posteriorly during molar development.

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Keratin5-Cre Tg specific deletion of Lgr4 impaired the development of the third molar.

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LGR4 maintained SOX2 positive and proliferative cells in the dental epithelium of molars.

Novel Regulation of Wnt Signaling at the Proximal Membrane Level

Wnt pathways are crucial for embryonic development and adult tissue homeostasis in all multicellular animals. Our understanding of Wnt signaling networks has grown increasingly complex. Recent studies have revealed many regulatory proteins that function at the proximal membrane level to fine-tune signaling output and enhance signaling specificity. These proteins regulate crucial points in Wnt signaling, including post-translational modification of Wnt proteins, regulation of Wnt receptor degradation, internalization of Wnt receptor complex, and specific ligand-receptor complex formation. Such regulators not only provide us with molecular details of Wnt regulation but also serve as potential targets for therapeutic intervention. In this review we highlight new insights into Wnt regulation at the plasma membrane, especially newly identified feedback regulators.

G Protein-Coupled Receptor Signaling in Stem Cells and Cancer

G protein-coupled receptors (GPCRs) are a large superfamily of cell-surface signaling proteins that bind extracellular ligands and transduce signals into cells via heterotrimeric G proteins. GPCRs are highly tractable drug targets. Aberrant expression of GPCRs and G proteins has been observed in various cancers and their importance in cancer stem cells has begun to be appreciated. We have recently reported essential roles for G protein-coupled receptor 84 (GPR84) and G protein subunit Gαq in the maintenance of cancer stem cells in acute myeloid leukemia. This review will discuss how GPCRs and G proteins regulate stem cells with a focus on cancer stem cells, as well as their implications for the development of novel targeted cancer therapies.

Extra sensory perception: the role of Gpr receptors in the kidney

PURPOSE OF REVIEW

This review will summarize recent literature highlighting the roles of sensory Gpr receptors and their roles in renal function.

RECENT FINDINGS

Chemoreceptors play important roles in renal physiology wherein they modulate renal function in response to ligands from a variety of sources.

SUMMARY

As specialized chemical detectors, chemoreceptors in the kidney monitor the level of a variety of chemical ligands in the body and adjust renal function accordingly. In addition to olfactory receptors and taste receptors, G-protein coupled receptors of the orphan Gpr family are now being found to play a 'sensory' role in renal physiology. Identifying the physiological roles of these receptors and elucidating the cell biology underlying these signaling pathways can give us novel insights into renal function.

Lgr4 is a key regulator of prostate development and prostate stem cell differentiation

Mechanisms modulating prostate cell fate determination remain unexplored. The leucine-rich repeat containing G-protein-coupled receptors (Lgr) have been identified as important stem cell markers in various tissues. Here, we investigated the roles of Lgr4/Gpr48 in prostate stem cells (PSCs) and development. Lgr4 was ubiquitously expressed during early prostate development prior to lineage specification, with adult expression restricted to a few basal cells (principally Lin(-)Sca1(+)CD49f(+)). Lgr4(-/-) mice had compromised branching morphogenesis and delayed epithelial differentiation, leading to decreased prostate size and impaired luminal cell function. In vitro prostate sphere culture revealed that Lgr4(-/-) Lin(-)/Sca1(+)/CD49f(+) cells failed to generate p63(low) cells, indicating a differentiation deficiency. Furthermore, Lgr4 ablation arrested PSC differentiation of in vivo kidney capsule prostate grafts, suggesting that Lgr4 modulates PSC properties independent of hormonal and mesenchymal effects. Analysis of neonatal prostates and prostate spheres revealed a decrease in Wnt, Sonic Hedgehog, and Notch1 expression in Lgr4(-/-) cells. Lgr4 loss blocked differentiation of prostate sphere p63(hi) cells to p63(low). Treatment with exogenous Sonic Hedgehog partially restored the differentiation of p63(hi) cells in Lgr4(-/-) spheres. Taken together, our data revealed the roles of Lgr4 in early prostate development and in stem cell differentiation through regulation of the Wnt, Notch, and Sonic Hedgehog signaling pathways.

The R-spondin/Lgr5/Rnf43 module: regulator of Wnt signal strength

Adult stem cells are controlled by an intricate interplay of potent Wnt agonists, antagonists, and anti-antagonists. This review by de Lau et al. focuses on the complex physical and functional interactions of three recently discovered protein families that control stem cell activity by regulating surface expression of Wnt receptors: Lgr5 and its homologs, the E3 ligases Rnf43 and Znrf3, and the secreted R-spondin ligands.

Lgr5 was originally discovered as a common Wnt target gene in adult intestinal crypts and colon cancer. It was subsequently identified as an exquisite marker of multiple Wnt-driven adult stem cell types. Lgr5 and its homologs, Lgr4 and Lgr6, constitute the receptors for R-spondins, potent Wnt signal enhancers and stem cell growth factors. The Lgr5/R-spondin complex acts by neutralizing Rnf43 and Znrf3, two transmembrane E3 ligases that remove Wnt receptors from the stem cell surface. Rnf43/Znrf3 are themselves encoded by Wnt target genes and constitute a negative Wnt feedback loop. Thus, adult stem cells are controlled by an intricate interplay of potent Wnt agonists, antagonists, and anti-antagonists.

Functional roles of Lgr4 and Lgr5 in embryonic gut, kidney and skin development in mice

Lgr4 and Lgr5 are known markers of adult and embryonic tissue stem cells in various organs. However, whether Lgr4 and Lgr5 are important for embryonic development remains unclear. To study their functions during intestinal crypt, skin and kidney development we now generated mice lacking either Lgr4 (Lgr4KO), Lgr5 (Lgr5KO) or both receptors (Lgr4/5dKO). E16.5 Lgr4KO mice displayed complete loss of Lgr5+/Olfm4+intestinal stem cells, compromised Wnt signaling and impaired proliferation and differentiation of gut epithelium. Similarly, E16.5 Lgr4KO mice showed reduced basal cell proliferation and hair follicle numbers in the developing skin, as well as dilated kidney tubules and ectatic Bowman׳s spaces. Although Lgr4KO and Lgr5KO mice both died perinatally, Lgr5 deletion did not compromise embryonic development of gut, kidney or skin. Concomitant deletion of Lgr4 and Lgr5 did not prevent perinatal lethality, in contrast to a previous report that suggested rescue of Lgr5 KO perinatal lethality by a hypomorphic Lgr4 mutant. While the double deletion did not further promote the phenotypes observed in Lgr4KO intestines, impaired kidney cell proliferation, reduced epidermal thickness, loss of Lgr5+follicular epithelium and impaired hair follicle development were only observed in Lgr4/5dKO mice. This supports complementary functions of both receptors. Our findings clearly establish the importance of Lgr4 and Lgr5 during embryonic gut, skin and kidney development, with a dominant role of Lgr4.

LGR4 expressed in uterine epithelium is necessary for uterine gland development and contributes to decidualization in mice

In previous work we generated mice with a tissue specific ablation of a leucine-rich repeat containing G-protein-coupled receptor 4 (Lgr4) using the Keratin-5 (K5) Cre transgenic mouse strain (Lgr4(K5 KO)). Interestingly, the Lgr4(K5 KO) female mice were subfertile, and their embryos had impaired development. Notably, the contributions of uterine development to the subfertility phenotype were not elucidated in the previous report. In a readdress, the following study explores uterine aberration in Lgr4(K5 KO) female mice. Histological analysis revealed that the uteri of Lgr4(K5 KO) mice displayed altered epithelial differentiation characterized by a reduction in the number of uterine glands. Furthermore, Lgr4 deletion led to the reduced expression of morphoregulatory genes related to the Wnt signaling pathway. Additionally, the uteri of the Lgr4(K5 KO) mice lost the ability to undergo induced decidualization. Quantitative reverse transcription-polymerase chain reaction (qRT-PCR) analysis and administration of recombinant leukemia inhibitory factor (LIF) demonstrated that the impaired decidualization in Lgr4(K5 KO) mice resulted from the decreased secretion of LIF concurrent with a reduction in uterine gland count. Thus, we propose that LGR4 contributes to uterine gland development, which supports decidualization during pregnancy.

Lgr proteins in epithelial stem cell biology

The ultimate success of global efforts to exploit adult stem cells for regenerative medicine will depend heavily on the availability of robust, highly selective stem cell surface markers that facilitate the isolation of stem cells from human tissues. Any subsequent expansion or manipulation of isolated stem cells will also require an intimate knowledge of the mechanisms that regulate these cells, to ensure maintenance of their regenerative capacities and to minimize the risk of introducing undesirable growth traits that could pose health risks for patients. A subclass of leucine-rich repeat-containing G-protein-coupled receptor (Lgr) proteins has recently gained prominence as adult stem cell markers with crucial roles in maintaining stem cell functions. Here, we discuss the major impact that their discovery has had on our understanding of adult stem cell biology in various self-renewing tissues and in accelerating progress towards the development of effective stem cell therapies.

Lgr4 in ocular development and glaucoma

The leucine-rich repeat-containing G protein-coupled receptor 4 (LGR4, also called GPR48) plays a key role in multiple developmental processes, and mice lacking Lgr4 display anterior segment dysgenesis leading to early-onset glaucomatous retinal ganglion cell loss as well as defective eyelid formation. This paper will review Lgr4 signaling and its regulation of the Axenfeld-Rieger syndrome gene Pitx2, a crucial developmental transcription factor. In addition, Wnt signaling plays an important role in eye development, with Norrin functioning to activate the Wnt receptor Frizzled 4 required for proper retinal vascularization. Recent discoveries identifying Lgr4 as a receptor for Norrin highlight the potential for Lgr4 function in retinal vascularization. Finally, several unanswered questions impeding a full understanding of Lgr4 in glaucoma are considered as avenues for further research.

Highly penetrant alterations of a critical region including BDNF in human psychopathology and obesity

CONTEXT Brain-derived neurotrophic factor (BDNF) is suspected of being a causative factor in psychiatric disorders based on case reports or studies involving large structural anomalies. OBJECTIVE To determine the involvement of BDNF in human psychopathology. DESIGN Case-control study. SETTING Microarray-based comparative genomic hybridization data from 7 molecular diagnostic centers including 38 550 affected subjects and 28 705 unaffected subjects. PATIENTS Subjects referred to diagnostic screening centers for microarray-based comparative genomic hybridization for physical or cognitive impairment. MAIN OUTCOME MEASURES Genomic copy number gains and losses. RESULTS We report 5 individuals with psychopathology and genomic deletion of a critical region including BDNF. The defined critical region was never disrupted in control subjects or diagnostic cases without developmental abnormalities. CONCLUSION Hemizygosity of the BDNF region contributes to variable psychiatric phenotypes including anxiety, behavioral, and mood disorders.

The R-spondin protein family

The four vertebrate R-spondin proteins are secreted agonists of the canonical Wnt/β-catenin signaling pathway. These proteins are approximately 35 kDa, and are characterized by two amino-terminal furin-like repeats, which are necessary and sufficient for Wnt signal potentiation, and a thrombospondin domain situated more towards the carboxyl terminus that can bind matrix glycosaminoglycans and/or proteoglycans. Although R-spondins are unable to initiate Wnt signaling, they can potently enhance responses to low-dose Wnt proteins. In humans, rare disruptions of the gene encoding R-spondin1 cause a syndrome of XX sex reversal (phenotypic male), palmoplantar keratosis (a thickening of the palms and soles caused by excess keratin formation) and predisposition to squamous cell carcinoma of the skin. Mutations in the gene encoding R-spondin4 cause anonychia (absence or hypoplasia of nails on fingers and toes). Recently, leucine-rich repeat-containing G-protein-coupled receptor (Lgr)4, Lgr5 and Lgr6, three closely related orphans of the leucine-rich repeat family of G-protein-coupled receptors, have been identified as receptors for R-spondins. Lgr5 and Lgr6 are markers for adult stem cells. Because R-spondins are potent stimulators of adult stem cell proliferation in vivo and in vitro, these findings might guide the therapeutic use of R-spondins in regenerative medicine.

Lgr5(+ve) stem/progenitor cells contribute to nephron formation during kidney development

Multipotent stem cells and their lineage-restricted progeny drive nephron formation within the developing kidney. Here, we document expression of the adult stem cell marker Lgr5 in the developing kidney and assess the stem/progenitor identity of Lgr5(+ve) cells via in vivo lineage tracing. The appearance and localization of Lgr5(+ve) cells coincided with that of the S-shaped body around embryonic day 14. Lgr5 expression remained restricted to cell clusters within developing nephrons in the cortex until postnatal day 7, when expression was permanently silenced. In vivo lineage tracing identified Lgr5 as a marker of a stem/progenitor population within nascent nephrons dedicated to generating the thick ascending limb of Henle's loop and distal convoluted tubule. The Lgr5 surface marker and experimental models described here will be invaluable for deciphering the contribution of early nephron stem cells to developmental defects and for isolating human nephron progenitors as a prerequisite to evaluating their therapeutic potential.

R-Spondin potentiates Wnt/β-catenin signaling through orphan receptors LGR4 and LGR5

The Wnt/β-catenin signaling pathbway controls many important biological processes. R-Spondin (RSPO) proteins are a family of secreted molecules that strongly potentiate Wnt/β-catenin signaling, however, the molecular mechanism of RSPO action is not yet fully understood. We performed an unbiased siRNA screen to identify molecules specifically required for RSPO, but not Wnt, induced β-catenin signaling. From this screen, we identified LGR4, then an orphan G protein-coupled receptor (GPCR), as the cognate receptor of RSPO. Depletion of LGR4 completely abolished RSPO-induced β-catenin signaling. The loss of LGR4 could be compensated by overexpression of LGR5, suggesting that LGR4 and LGR5 are functional homologs. We further demonstrated that RSPO binds to the extracellular domain of LGR4 and LGR5, and that overexpression of LGR4 strongly sensitizes cells to RSPO-activated β-catenin signaling. Supporting the physiological significance of RSPO-LGR4 interaction, Lgr4−/− crypt cultures failed to grow in RSPO-containing intestinal crypt culture medium. No coupling between LGR4 and heterotrimeric G proteins could be detected in RSPO-treated cells, suggesting that LGR4 mediates RSPO signaling through a novel mechanism. Identification of LGR4 and its relative LGR5, an adult stem cell marker, as the receptors of RSPO will facilitate the further characterization of these receptor/ligand pairs in regenerative medicine applications.

Conditional knockout of Lgr4 leads to impaired ductal elongation and branching morphogenesis in mouse mammary glands

We have analyzed the function of LGR4 in the development of various mouse epithelial tissues. Here we first report the retarded invasion of mammary ducts into the fat pad observed in Lgr4(K5 KO) mice at 4 weeks, compared with that of age-matched Lgr4(K5 ctrl). Furthermore, we demonstrate a significant decrease in mammary ductal branching in Lgr4(K5 KO) at several stages (4, 6 and 8 weeks). On the other hand, immunohistochemical analysis of the mammary gland of Lgr4(K5 KO) using anti-αSMA, anti-K18 and anti-laminin antibodies showed structures similar to those of Lgr4(K5 ctrl) mammary glands. In addition, we did not detect significant differences in the expression of ERα, which was suggested to be a downstream molecule of LGR4, and Lgr4(K5 KO) showed no retarded invasion in the response to 17β-estradiol administration. Furthermore, the phosphorylated form of Smad1/5/8 was normally detected in the mammary gland of Lgr4(K5 KO).