Alternative splicing of latrophilin-3 controls synapse formation

The assembly and specification of synapses in the brain is incompletely understood1-3. Latrophilin-3 (encoded by Adgrl3, also known as Lphn3)-a postsynaptic adhesion G-protein-coupled receptor-mediates synapse formation in the hippocampus4 but the mechanisms involved remain unclear. Here we show in mice that LPHN3 organizes synapses through a convergent dual-pathway mechanism: activation of Gαs signalling and recruitment of phase-separated postsynaptic protein scaffolds. We found that cell-type-specific alternative splicing of Lphn3 controls the LPHN3 G-protein-coupling mode, resulting in LPHN3 variants that predominantly signal through Gαs or Gα12/13. CRISPR-mediated manipulation of Lphn3 alternative splicing that shifts LPHN3 from a Gαs- to a Gα12/13-coupled mode impaired synaptic connectivity as severely as the overall deletion of Lphn3, suggesting that Gαs signalling by LPHN3 splice variants mediates synapse formation. Notably, Gαs-coupled, but not Gα12/13-coupled, splice variants of LPHN3 also recruit phase-transitioned postsynaptic protein scaffold condensates, such that these condensates are clustered by binding of presynaptic teneurin and FLRT ligands to LPHN3. Moreover, neuronal activity promotes alternative splicing of the synaptogenic Gαs-coupled variant of LPHN3. Together, these data suggest that activity-dependent alternative splicing of a key synaptic adhesion molecule controls synapse formation by parallel activation of two convergent pathways: Gαs signalling and clustered phase separation of postsynaptic protein scaffolds.

Enhanced Collagen Deposition in the Duodenum of Patients with Hyaline Fibromatosis Syndrome and Protein Losing Enteropathy

Hyaline fibromatosis syndrome (HFS), resulting from ANTXR2 mutations, is an ultra-rare disease that causes intestinal lymphangiectasia and protein-losing enteropathy (PLE). The mechanisms leading to the gastrointestinal phenotype in these patients are not well defined. We present two patients with congenital diarrhea, severe PLE and unique clinical features resulting from deleterious ANTXR2 mutations. Intestinal organoids were generated from one of the patients, along with CRISPR-Cas9 ANTXR2 knockout, and compared with organoids from two healthy controls. The ANTXR2-deficient organoids displayed normal growth and polarity, compared to controls. Using an anthrax-toxin assay we showed that the c.155C>T mutation causes loss-of-function of ANTXR2 protein. An intrinsic defect of monolayer formation in patient-derived or ANTXR2KO organoids was not apparent, suggesting normal epithelial function. However, electron microscopy and second harmonic generation imaging showed abnormal collagen deposition in duodenal samples of these patients. Specifically, collagen VI, which is known to bind ANTXR2, was highly expressed in the duodenum of these patients. In conclusion, despite resistance to anthrax-toxin, epithelial cell function, and specifically monolayer formation, is intact in patients with HFS. Nevertheless, loss of ANTXR2-mediated signaling leads to collagen VI accumulation in the duodenum and abnormal extracellular matrix composition, which likely plays a role in development of PLE.

In vivo imaging reveals an essential role of vasoconstriction in rupture of the ovarian follicle at ovulation

Rupture of the ovarian follicle releases the oocyte at ovulation, a timed event that is critical for fertilization. It is not understood how the protease activity required for rupture is directed with precise timing and localization to the outer surface, or apex, of the follicle. We hypothesized that vasoconstriction at the apex is essential for rupture. The diameter and blood flow of individual vessels and the thickness of the apical follicle wall were examined over time to expected ovulation using intravital multiphoton microscopy. Vasoconstriction of apical vessels occurred within hours preceding follicle rupture in wild-type mice, but vasoconstriction and rupture were absent in Amhr2(cre/+)SmoM2 mice in which follicle vessels lack the normal association with vascular smooth muscle. Vasoconstriction is not simply a response to reduced thickness of the follicle wall; vasoconstriction persisted in wild-type mice when thinning of the follicle wall was prevented by infusion of protease inhibitors into the ovarian bursa. Ovulation was inhibited by preventing the periovulatory rise in the expression of the vasoconstrictor endothelin 2 by follicle cells of wild-type mice. In these mice, infusion of vasoconstrictors (either endothelin 2 or angiotensin 2) into the bursa restored the vasoconstriction of apical vessels and ovulation. Additionally, infusion of endothelin receptor antagonists into the bursa of wild-type mice prevented vasoconstriction and follicle rupture. Processing tissue to allow imaging at increased depth through the follicle and transabdominal ultrasonography in vivo showed that decreased blood flow is restricted to the apex. These results demonstrate that vasoconstriction at the apex of the follicle is essential for ovulation.

Loss of guanylyl cyclase C (GCC) signaling leads to dysfunctional intestinal barrier

Background

Guanylyl Cyclase C (GCC) signaling via uroguanylin (UGN) and guanylin activation is a critical mediator of intestinal fluid homeostasis, intestinal cell proliferation/apoptosis, and tumorigenesis. As a mechanism for some of these effects, we hypothesized that GCC signaling mediates regulation of intestinal barrier function.

Methodology/Principal Findings

Paracellular permeability of intestinal segments was assessed in wild type (WT) and GCC deficient (GCC−/−) mice with and without lipopolysaccharide (LPS) challenge, as well as in UGN deficient (UGN−/−) mice. IFNγ and myosin light chain kinase (MLCK) levels were determined by real time PCR. Expression of tight junction proteins (TJPs), phosphorylation of myosin II regulatory light chain (MLC), and STAT1 activation were examined in intestinal epithelial cells (IECs) and intestinal mucosa. The permeability of Caco-2 and HT-29 IEC monolayers, grown on Transwell filters was determined in the absence and presence of GCC RNA interference (RNAi). We found that intestinal permeability was increased in GCC−/− and UGN−/− mice compared to WT, accompanied by increased IFNγ levels, MLCK and STAT1 activation in IECs. LPS challenge promotes greater IFNγ and STAT1 activation in IECs of GCC−/− mice compared to WT mice. Claudin-2 and JAM-A expression were reduced in GCC deficient intestine; the level of phosphorylated MLC in IECs was significantly increased in GCC−/− and UGN−/− mice compared to WT. GCC knockdown induced MLC phosphorylation, increased permeability in IEC monolayers under basal conditions, and enhanced TNFα and IFNγ-induced monolayer hyperpermeability.

Conclusions/Significance

GCC signaling plays a protective role in the integrity of the intestinal mucosal barrier by regulating MLCK activation and TJ disassembly. GCC signaling activation may therefore represent a novel mechanism in maintaining the small bowel barrier in response to injury.

Incretin receptors for glucagon-like peptide 1 and glucose-dependent insulinotropic polypeptide are essential for the sustained metabolic actions of vildagliptin in mice

OBJECTIVE

Dipeptidyl peptidase-4 (DPP4) inhibitors lower blood glucose in diabetic subjects; however, the mechanism of action through which these agents improve glucose homeostasis remains incompletely understood. Although glucagon-like peptide (GLP)-1 and glucose-dependent insulinotropic polypeptide (GIP) represent important targets for DPP4 activity, whether additional substrates are important for the glucose-lowering actions of DPP4 inhibitors remains uncertain.

RESEARCH DESIGN AND METHODS

We examined the efficacy of continuous vildagliptin administration in wild-type (WT) and dual incretin receptor knockout (DIRKO) mice after 8 weeks of a high-fat diet.

RESULTS

Vildagliptin had no significant effect on food intake, energy expenditure, body composition, body weight gain, or insulin sensitivity in WT or DIRKO mice. However, glycemic excursion after oral glucose challenge was significantly reduced in WT but not in DIRKO mice after vildagliptin treatment. Moreover, vildagliptin increased levels of glucose-stimulated plasma insulin and reduced levels of cholesterol and triglycerides in WT but not in DIRKO mice. Vildagliptin treatment reduced the hepatic expression of genes important for cholesterol synthesis and fatty acid oxidation, including phospho-mevalonate kinase (Mvk), acyl-coenzyme dehydrogenase medium chain (Acadm), mevalonate (diphospho)decarboxylase (Mvd), and Acyl-CoA synthetase (Acsl1), in WT but not in DIRKO mice. However, vildagliptin also reduced levels of hepatic mRNA transcripts for farnesyl di-phosphate transferase (Fdft1), acetyl coenzyme A acyltransferase 1 (Acaa1), and carnitine palmitoyl transferase 1 (Cpt 1) in DIRKO mice. No direct effect of GLP-1 receptor agonists was detected on cholesterol or triglyceride synthesis and secretion in WT hepatocytes.

CONCLUSIONS

These findings illustrate that although GLP-1 and GIP receptors represent the dominant molecular mechanisms for transducing the glucoregulatory actions of DPP4 inhibitors, prolonged DPP4 inhibition modulates the expression of genes important for lipid metabolism independent of incretin receptor action in vivo.

Guanylyl cyclase C suppresses intestinal tumorigenesis by restricting proliferation and maintaining genomic integrity

BACKGROUND AND AIMS

The most commonly lost gene products in colorectal carcinogenesis include guanylin and uroguanylin, endogenous ligands for guanylyl cyclase C (GCC). Beyond intestinal fluid balance, GCC mediates diarrhea induced by bacterial enterotoxins, and an inverse relationship exists between enterotoxigenic Escherichia coli infections producing the exogenous GCC ligand ST and colorectal cancer worldwide. However, the role of GCC in neoplasia remains obscure.

METHODS

Intestinal tumorigenesis was examined in wild-type (Gcc(+/+)) and GCC-deficient (Gcc(-/-)) mice carrying mutations in Apc (Apc(Min/+)) or exposed to the carcinogen azoxymethane. Markers of DNA damage, loss of Apc heterozygosity, and beta-catenin mutations were used to assess genomic integrity. Hyperproliferation was explored using Ki67 and cell cycle markers. Apoptosis was quantified by transferase biotin-dUTP nick end labeling analysis.

RESULTS

In colons of Apc(Min/+) mice, deletion of Gcc increased tumor incidence and multiplicity, reflecting uncoupling of loss of genomic integrity and compensatory apoptosis. Conversely, in the small intestine, elimination of Gcc increased tumorigenesis by enhancing proliferation without altering genomic integrity. Moreover, these distinct but mutually reinforcing mechanisms collaborate in azoxymethane-exposed mice, and deletion of Gcc increased tumor initiation and growth associated with hypermutation and hyperproliferation, respectively, in conjunction with attenuated apoptosis.

CONCLUSIONS

GCC suppresses tumor initiation and growth by maintaining genomic integrity and restricting proliferation. This previously unrecognized role of GCC in inhibiting tumorigenesis, together with the invariant disruption in guanylin and uroguanylin expression early in carcinogenesis, and the uniform over-expression of GCC by tumors, underscores the potential of oral administration of GCC ligands for targeted prevention and therapy of colorectal cancer.

Essential roles of mesenchyme-derived beta-catenin in mouse Müllerian duct morphogenesis

Members of the Wnt family of genes such as Wnt4, Wnt5a, and Wnt7a have been implicated in the formation and morphogenesis of the Müllerian duct into various parts of the female reproductive tract. These WNT ligands elicit their action via either the canonical WNT/beta-catenin or the non-canonical WNT/calcium pathway and could possibly function redundantly in Müllerian duct differentiation. By using the Müllerian duct-specific anti-Müllerian hormone receptor 2 cre (Amhr2-cre) mouse line, we established a conditional knockout model that removed beta-catenin specifically in the mesenchyme of the Müllerian duct. At birth, loss of beta-catenin in the Müllerian duct mesenchyme disrupted the normal coiling of the oviduct in the knockout embryo, resembling the phenotype of the Wnt7a knockout. The overall development of the female reproductive tract was stunted at birth with a decrease in proliferation in the mesenchyme and epithelium. We also discovered that Wnt5a and Wnt7a expression remained normal, excluding the possibility that the phenotypes resulted from a loss of these WNT ligands. We examined the expression of Frizzled (Fzd), the receptors for WNT, and found that Fzd1 is one receptor present in the Müllerian duct mesenchyme and could be the putative receptor for beta-catenin activation in the Müllerian duct. In summary, our findings suggest that mesenchymal beta-catenin is a downstream effector of Wnt7a that mediates the patterning of the oviduct and proper differentiation of the uterus.

Prokineticin receptor 2 (Prokr2) is essential for the regulation of circadian behavior by the suprachiasmatic nuclei

The suprachiasmatic nucleus (SCN), the brain's principal circadian pacemaker, coordinates adaptive daily cycles of behavior and physiology, including the rhythm of sleep and wakefulness. The cellular mechanism sustaining SCN circadian timing is well characterized, but the neurochemical pathways by which SCN neurons coordinate circadian behaviors remain unknown. SCN transplant studies suggest a role for (unidentified) secreted factors, and one potential candidate is the SCN neuropeptide prokineticin 2 (Prok2). Prok2 and its cognate prokineticin receptor 2 (Prokr2/Gpcr73l1) are widely expressed in both the SCN and its neural targets, and Prok2 is light-regulated. Hence, they may contribute to cellular timing within the SCN, entrainment of the clock, and/or they may mediate circadian output. We show that a targeted null mutation of Prokr2 disrupts circadian coordination of the activity cycle and thermoregulation. Specifically, mice lacking Prokr2 lost precision in timing the onset of nocturnal locomotor activity; and under both a light/dark cycle and continuous darkness, there was a pronounced temporal redistribution of activity away from early to late circadian night. Moreover, the coherence of circadian behavior was significantly reduced, and nocturnal body temperature was depressed. Entrainment by light is not, however, dependent on Prokr2, and bioluminescence real-time imaging of organotypical SCN slices showed that the mutant SCN is fully competent as a circadian oscillator. We conclude that Prokr2 is not necessary for SCN cellular timekeeping or entrainment, but it is an essential link for coordination of circadian behavior and physiology by the SCN, especially in defining the onset and maintenance of circadian night.

The Nogo-Nogo receptor pathway limits a spectrum of adult CNS axonal growth

The hypothesis that Nogo-A (Reticulon 4A) and Nogo-66 receptor (NgR1) limit adult CNS axonal growth after injury is supported by both in vitro experiments and in vivo pharmacological studies. However, genetic assessment of the role of Nogo-A in corticospinal tract (CST) axons after spinal cord dorsal hemisection has yielded conflicting results. CST regeneration is detected in homozygous nogo-ab(trap/trap) mice, but not in nogo-ab(atg/atg) mice. CST regeneration is also present after pharmacological NgR blockade, but not in ngr1(-/-) mice. To assess the nogo-ab(atg) and ngr1-null alleles for other axon growth phenotypes, we created unilateral pyramidotomies and monitored the uninjured CST. There is robust pyramidotomy-induced growth of nogo-ab(atg/atg) and ngr1(-/-) CST axons into denervated cervical gray matter. This fiber growth correlates with recovery of fine motor skill in the affected forelimb. Thus nogo-ab and ngr1 play a modulated role in limiting CNS axonal growth across a spectrum of different tracts in various lesion models.

Mutation in the Trapalpha/Ssr1 gene, encoding translocon-associated protein alpha, results in outflow tract morphogenetic defects

Translocon-associated protein complex (TRAP) is thought to be required for efficient protein-specific translocation across the endoplasmic reticulum membrane. We created a mutation in the Trapalpha gene that leads to the synthesis of a truncated TRAPalpha protein fused to ShBle-beta-galactosidase. Analysis of Trapalpha cDNAs reveals that among three different messenger RNAs expressed in the mouse, one of them encodes a slightly larger protein that differs in its C-terminal end. This mRNA, specific for skeletal muscle and heart, is only expressed after birth. Homozygous Trapalpha mutant pups die at birth, likely as a result of severe cardiac defects. Indeed, the septation of the proximal part of the outflow tract is absent, resulting in a double-outlet right ventricle. Studies of protein secretion in transfected embryonic fibroblasts reveal that the TRAP complex does not function properly in homozygous mutant cells and confirm, in vivo, the involvement of TRAP in substrate-specific translocation. Our results provide the first in vivo demonstration that a member of the TRAP complex plays a crucial role in mammalian heart development and suggest that TRAPalpha could be involved in translocation of factors necessary for maturation of endocardial cushions.

Abnormal development of the olfactory bulb and reproductive system in mice lacking prokineticin receptor PKR2

Prokineticins, multifunctional secreted proteins, activate two endogenous G protein-coupled receptors PKR1 and PKR2. From in situ analysis of the mouse brain, we discovered that PKR2 is predominantly expressed in the olfactory bulb (OB). To examine the role of PKR2 in the OB, we created PKR1- and PKR2-gene-disrupted mice (Pkr1(-/-) and Pkr2(-/-), respectively). Phenotypic analysis indicated that not Pkr1(-/-)but Pkr2(-/-)mice exhibited hypoplasia of the OB. This abnormality was observed in the early developmental stages of fetal OB in the Pkr2(-/-) mice. In addition, the Pkr2(-/-) mice showed severe atrophy of the reproductive system, including the testis, ovary, uterus, vagina, and mammary gland. In the Pkr2(-/-) mice, the plasma levels of testosterone and follicle-stimulating hormone were decreased, and the mRNA transcription levels of gonadotropin-releasing hormone in the hypothalamus and luteinizing hormone and follicle-stimulating hormone in the pituitary were also significantly reduced. Immunohistochemical analysis revealed that gonadotropin-releasing hormone neurons were absent in the hypothalamus in the Pkr2(-/-) mice. The phenotype of the Pkr2(-/-) mice showed similarity to the clinical features of Kallmann syndrome, a human disease characterized by association of hypogonadotropic hypogonadism and anosmia. Our current findings demonstrated that physiological activation of PKR2 is essential for normal development of the OB and sexual maturation.

Genetic Targeting of Relaxin and Insl3 Signaling in Mice

We studied ligand-receptor interactions between relaxin (RLN), insulin-like 3 peptide (INSL3), and LGR7 and LGR8 receptors. The phenotypic effects of deficiency for Lgr7 and Lgr8 receptors, transgenic overexpression of Rln1 and Insl3, and different combinations of these mutations in mice were analyzed. It was reported that Rln1-deficient mice exhibit abnormal nipple development, prolonged parturition, age-related pulmonary fibrosis, and abnormalities in the testis and prostate. Mutation of Lgr8 or its cognate ligand Insl3 causes cryptorchidism. Mutant females deficient for the Lgr7 receptor have grossly undeveloped nipples and are unable to feed their progeny. Parturition is prolonged in these females, resulting in a significantly higher number of stillborn pups. Histologic analysis of Lgr7 mutant lung tissues demonstrates increased collagen accumulation and perivenular smooth muscle hypertrophy. However, Lgr7-deficient males do not exhibit abnormalities of male reproductive organs as seen in Rln1 knockout mice. Double-mutant males deficient for Lgr7 and Lgr8 have a normal prostate, suggesting that Lgr8 does not account for differences in Rln1-/- and Lgr7-/- phenotypes. We also produced mice with transgenic overexpression of Rln1 under rat insulin 2 promoter. Rln1 transgenic females exhibited increased nipple size, whereas Rln1 transgenic females deficient in Lgr7 had undeveloped nipples, indicating that Lgr7 is the only receptor for relaxin that mediates this effect. Transgenic overexpression of Rln1 does not affect gonadal descent in females, and transgenic overexpression of Insl3 does not rescue the mutant phenotype of Lgr7-deficient mice, suggesting the non-overlapping functions of two signaling pathways. In summary, our data indicate that the Insl3/Lgr8 and Rln1/Lgr7 pathways are distinct and separate in vivo. Therefore, we propose to rename Lgr8 as Insl3r (Insl3 receptor) and Lgr7 as Rlnr (relaxin receptor).

GREAT/LGR8 is the only receptor for insulin-like 3 peptide

During male development testes descend from their embryonic intraabdominal position into the scrotum. Two genes, encoding the insulin-like 3 peptide (INSL3) and the GREAT/LGR8 G protein-coupled receptor, control the differentiation of gubernaculum, the caudal genitoinguinal ligament critical for testicular descent. It was established that the INSL3 peptide activates GREAT/LGR8 receptor in vitro. Mutations of Insl3 or Great cause cryptorchidism (undescended testes) in mice. Overexpression of the transgenic Insl3 causes male-like gubernaculum differentiation, ovarian descent into lower abdominal position, and reduced fertility in females. To address the question whether Great deletion complements the mutant female phenotype caused by the Insl3 overexpression, we have produced Insl3 transgenic mice deficient for Great. Such females had a wild-type phenotype, demonstrating that Great was the only cognate receptor for Insl3 in vivo. We have established that pancreatic HIT cells, transfected with the INSL3 cDNA, produce functionally active peptide. Analysis of five INSL3 mutant variants detected in cryptorchid patients showed that P49S substitution renders functionally compromised peptide. Therefore, mutations in INSL3 might contribute to the etiology of cryptorchidism. We have also showed that synthetic insulin-like peptides (INSL4 and INSL6) were unable to activate LGR7 or GREAT/LGR8.

Normal reproductive function in InhBP/p120-deficient mice

The inhibins are gonadal transforming growth factor beta superfamily protein hormones that suppress pituitary follicle-stimulating hormone (FSH) synthesis. Recently, betaglycan and inhibin binding protein (InhBP/p120, also known as the product of immunoglobulin superfamily gene 1 [IGSF1]) were identified as candidate inhibin coreceptors, shedding light on the molecular basis of how inhibins may affect target cells. Activins, which are structurally related to the inhibins, act within the pituitary to stimulate FSH production. Betaglycan increases the affinity of inhibins for the activin type IIA (ACVR2) receptor, thereby blocking activin binding and signaling through this receptor. InhBP/p120 may not directly bind inhibins but may interact with the activin type IB receptor, ALK4, and participate in inhibin B's antagonism of activin signaling. To better understand the in vivo functions of InhBP/p120, we characterized the InhBP/p120 mRNAs and gene in mice and generated InhBP/p120 mutant mice by gene targeting in embryonic stem cells. InhBP/p120 mutant male and female mice were viable and fertile. Moreover, they showed no alterations in FSH synthesis or secretion or in ovarian or testicular function. These data contribute to a growing body of evidence indicating that InhBP/p120 does not play an essential role in inhibin biology.

Leukocyte antiadhesive actions of annexin 1: ALXR- and FPR-related anti-inflammatory mechanisms

Recent investigations conducted with human neutrophils have indicated an involvement for the receptor for formylated peptides, termed FPR, and its analog FPRL1 (or ALXR because it is the receptor for the endogenous ligand lipoxin A(4)) in the in vitro inhibitory actions of the glucocorticoid-regulated protein annexin 1 and its peptidomimetics. To translate these findings in in vivo settings, we have used an ischemia/reperfusion (I/R) procedure to promote leukocyte-endothelium interactions in the mouse mesenteric microcirculation. In naive mice, the annexin 1 mimetic peptide Ac2-26 (20 to 100 microg administered intravenously prior to reperfusion) abolished I/R-induced cell adhesion and emigration, but not cell rolling. In FPR-deficient mice, peptide Ac2-26 retained significant inhibitory actions (about 50% of the effects in naive mice), and these were blocked by an FPR antagonist, termed butyloxycarbonyl-Phe-Leu-Phe-Leu-Phe, or Boc2. In vitro, neutrophils taken from these animals could be activated at high concentrations of formyl-Met-Leu-Phe (30 microM; fMLP), and this effect was blocked by cell incubation with peptide Ac2-26 (66 microM) or Boc2 (100 microM). FPR-deficient neutrophils expressed ALXR mRNA and protein. Both ALXR agonists, lipoxin A(4) and peptide Ac2-26, provoked detachment of adherent leukocytes in naive as well as in FPR-deficient mice, whereas the CXC chemokine KC or fMLP were inactive. The present findings demonstrate that endogenous regulatory autocoids such as lipoxin A(4) and annexin 1-derived peptides function to disengage adherent cells during cell-cell interactions.

E. coli heat-stable enterotoxin and guanylyl cyclase C: new functions and unsuspected actions

Some E. coli cause diarrhea by elaborating heat-labile and heat-stable (ST) enterotoxins which stimulate intestinal secretion. E. coli ST's are small peptides which bind to intestinal luminal epithelial cell receptors. The ST receptor, one of a family of receptor-cyclases called guanylyl cyclase C (GC-C), is a membrane spanning protein containing an extracellular binding domain and intracellular protein kinase and catalytic domains. The intestine synthesizes and secretes homologous peptides, guanylin and uroguanylin. The kidney also synthesizes uroguanylin. ST, guanylin or uroguanylin binding to GC-C results in increased cGMP, phosphorylation of the CFTR Cl- channel and secretion. Proguanylin and prouroguanylin circulate in blood and bind to receptors in intestine, kidney, liver, brain etc. In the kidney, they stimulate the excretion of Na+ and K+. Study of GC-C "knock-out" mice reveal that GC-C is important to intestinal salt and water secretion, duodenal bicarbonate secretion, recovery from CCl4-induced liver injury, and to intestinal polyp formation in Min mice lacking GC-C.

The N-formylpeptide receptor (FPR) and a second G(i)-coupled receptor mediate fMet-Leu-Phe-stimulated activation of NADPH oxidase in murine neutrophils

N-Formylypeptides such as fMet-Leu-Phe (fMLF) potently induce superoxide production through NADPH oxidase activation. The receptors that mediate this response have not been defined. Here, we provide definitive proof using a mouse model that formyl peptide receptor (FPR) is a receptor, but not the only receptor, that mediates fMLF-induced oxidase activation. In wild-type (FPR(+/+)) mouse neutrophils, superoxide production is dependent on the concentration of fMLF with an EC(50) of approximately 5 microM and a peak at approximately 50 microM. In contrast, FPR-deficient (FPR(-/-)) mouse neutrophils produced markedly less superoxide with an EC(50) of approximately 50 microM and a peak at approximately 200 microM. Yet, FPR(+/+) and FPR(-/-) neutrophils showed similar oxidase activation kinetics and G(i) protein-dependent pharmacological sensitivities. These results suggested that a second receptor, likely FPR2, mediates superoxide production at high concentrations of fMLF. This less sensitive second pathway may permit continued oxidant generation in response to formyl peptides when FPR is desensitized in high concentrations of the chemotactic gradient.

Genetic analysis of alpha-latrotoxin receptors reveals functional interdependence of CIRL/latrophilin 1 and neurexin 1 alpha

alpha-Latrotoxin triggers massive neurotransmitter release from nerve terminals by binding to at least two distinct presynaptic receptors, neurexin 1 alpha and CIRL1/latrophilin1 (CL1). We have now generated knockout (KO) mice that lack CL1 and analyzed them alone or in combination with neurexin 1 alpha KO mice. Mice lacking only CL1, or both CL1 and neurexin 1 alpha, were viable and fertile. Ca(2+)-independent binding of alpha-latrotoxin to brain membranes was impaired similarly in CL1 single and in CL1/neurexin 1 alpha double KO mice (approximately 75% decrease) but not in neurexin 1 alpha single KO mice. In contrast, Ca(2+)-dependent binding (approximately 2 times above Ca(2+)-independent binding) was altered in both CL1 (approximately 50% decrease) and neurexin 1 alpha single KO mice (approximately 25% decrease) and was decreased further in double KO mice (approximately 75% decrease). Synaptosomes lacking CL1 exhibited the same decrease in alpha-latrotoxin-stimulated glutamate release in the presence and absence of Ca(2+) (approximately 75%). In contrast, synaptosomes lacking neurexin 1 alpha exhibited only a small decrease in alpha-latrotoxin-triggered release in the absence of Ca(2+) (approximately 20%) but a major decrease in the presence of Ca(2+) (approximately 75%). Surprisingly, synaptosomes lacking both CL1 and neurexin 1 alpha displayed a relatively smaller decrease in alpha-latrotoxin-stimulated glutamate release than synaptosomes lacking only CL1 in the absence of Ca(2+) (approximately 50 versus approximately 75%), but the same decrease in the presence of Ca(2+) (approximately 75%). Our data suggest the following two major conclusions. 1) CL1 and neurexin 1 alpha together account for the majority (75%) of alpha-latrotoxin receptors in brain, with the remaining receptor activity possibly due to other CL and neurexin isoforms, and 2) the two receptors act additively in binding alpha-latrotoxin but not in triggering release. Together these data suggest that the two receptors act autonomously in binding of alpha-latrotoxin but cooperatively in transducing the stimulation of neurotransmitter release by alpha-latrotoxin.

The HIV-1 cell entry inhibitor T-20 potently chemoattracts neutrophils by specifically activating the N-formylpeptide receptor

T-20, a synthetic peptide corresponding to the heptad repeat sequence of HIV-1 gp41, blocks HIV-1 entry by targeting gp41, and is currently in clinical trials as an anti-retroviral agent. We recently reported that in vitro T-20 also functions as a phagocyte chemoattractant and a chemotactic agonist at the phagocyte N-formylpeptide receptor (FPR). Here we show that T-20 is also a potent chemotactic agonist in vitro at a related human phagocyte receptor FPRL1R. To test the relative importance of FPR and FPRL1R in primary cells, we identified the corresponding mouse T-20 receptors, mFPR and FPR2, which are both expressed in neutrophils, and compared T-20 action on neutrophils from wild type and mFPR knockout mice. Surprisingly, although T-20 activates mFPR and FPR2 in transfected cells with equal potency and efficacy in both calcium flux and chemotaxis assays, neutrophils from mFPR knockout mice did not respond to T-20. These results provide genetic evidence that FPR is the major phagocyte T-20 receptor in vivo and point to the potential feasibility of studying T-20 effects on immunity in a mouse model. This may help define the cause of local inflammation after T-20 injection that has recently been reported in Phase I clinical trials.

Impaired antibacterial host defense in mice lacking the N-formylpeptide receptor

N-formylpeptides derive from bacterial and mitochondrial proteins, and bind to specific receptors on mammalian phagocytes. Since binding induces chemotaxis and activation of phagocytes in vitro, it has been postulated that N-formylpeptide receptor signaling in vivo may be important in antimicrobial host defense, although direct proof has been lacking. Here we test this hypothesis in mice lacking the high affinity N-formylpeptide receptor (FPR), created by targeted gene disruption. FPR-/- mice developed normally, but had increased susceptibility to challenge with Listeria monocytogenes, as measured by increased mortality compared with wild-type littermates. FPR-/- mice also had increased bacterial load in spleen and liver 2 d after infection, which is before development of a specific cellular immune response, suggesting a defect in innate immunity. Consistent with this, neutrophil chemotaxis in vitro and neutrophil mobilization into peripheral blood in vivo in response to the prototype N-formylpeptide fMLF (formyl-methionyl-leucyl-phenylalanine) were both absent in FPR-/- mice. These results indicate that FPR functions in antibacterial host defense in vivo.

Mice lacking the guanylyl cyclase C receptor are resistant to STa-induced intestinal secretion

Heat-stable enterotoxin (STa) is an important causative agent of diarrheal disease throughout the world. STa is known to bind specifically to receptors in the intestine, provoking intense intestinal secretion. Binding of STa, or of the mammalian endogenous ligands guanylin and uroguanylin, activates the guanylyl cyclase C receptor (GC-C); the resulting elevation of cGMP levels stimulates chloride secretion via CFTR. We have generated knockout mice which completely lack the GC-C receptor. These mice are viable and show no obvious alteration in intestinal fluidity. However, GC-C null mice are refractory to the secretory action of STa, proving that the GC-C receptor is necessary for the diarrheal response induced by STa.