Structure, function, and expression of SEL-1, a negative regulator of LIN-12 and GLP-1 in C. elegans

Structural characterization of the Sel1-like repeat protein LceB from Legionella pneumophila

Legionella are freshwater Gram-negative bacteria that in their normal environment infect protozoa. However, this adaptation also allows Legionella to infect human alveolar macrophages and cause pneumonia. Central to Legionella pathogenesis are more than 330 secreted effectors, of which there are nine core effectors that are conserved in all pathogenic species. Despite their importance, the biochemical function of several core effectors remains unclear. To address this, we have taken a structural approach to characterize the core effector of unknown function LceB, or Lpg1356, from Legionella pneumophila. Here, we solve an X-ray crystal structure of LceB using an AlphaFold model for molecular replacement. The experimental structure shows that LceB adopts a Sel1-like repeat (SLR) fold as predicted. However, the crystal structure captured multiple conformations of LceB, all of which differed from the AlphaFold model. A comparison of the predicted model and the experimental models suggests that LceB is highly flexible in solution. Additionally, the molecular analysis of LceB using its close structural homologs reveals sequence and structural motifs of known biochemical function. Specifically, LceB harbors a repeated KAAEQG motif that both stabilizes the SLR fold and is known to participate in protein-protein interactions with eukaryotic host proteins. We also observe that LceB forms several higher-order oligomers in solution. Overall, our results have revealed that LceB has conformational flexibility, self-associates, and contains a molecular surface for binding a target host-cell protein. Additionally, our data provides structural insights into the SLR family of proteins that remain poorly studied.

Sel1l May Contributes to the Determinants of Neuronal Lineage and Neuronal Maturation Regardless of Hrd1 via Atf6-Sel1l Signaling

The endoplasmic reticulum (ER) is the primary site of intracellular quality control involved in the recognition and degradation of unfolded proteins. A variety of stresses, including hypoxia and glucose starvation, can lead to accumulation of unfolded proteins triggering the ER-associated degradation (ERAD) pathway. Suppressor Enhancer Lin12/Notch1 Like (Sel1l) acts as a "gate keeper" in the quality control of de novo synthesized proteins and complexes with the ubiquitin ligase Hrd1 in the ER membrane. We previously demonstrated that ER stress-induced aberrant neural stem cell (NSC) differentiation and inhibited neurite outgrowth. Inhibition of neurite outgrowth was associated with increased Hrd1 expression; however, the contribution of Sel1l remained unclear. To investigate whether ER stress is induced during normal neuronal differentiation, we semi-quantitatively evaluated mRNA expression levels of unfolded protein response (UPR)-related genes in P19 embryonic carcinoma cells undergoing neuronal differentiation in vitro. Stimulation with all-trans retinoic acid (ATRA) for 4 days induced the upregulation of Nestin and several UPR-related genes (Atf6, Xbp1, Chop, Hrd1, and Sel1l), whereas Atf4 and Grp78/Bip were unchanged. Small-interfering RNA (siRNA)-mediated knockdown of Sel1l uncovered that mRNA levels of the neural progenitor marker Math1 (also known as Atoh1) and the neuronal marker Math3 (also known as Atoh3 and NeuroD4) were significantly suppressed at 4 days after ATRA stimulation. Consistent with this result, Sel1l silencing significantly reduced protein levels of immature neuronal marker βIII-tubulin (also known as Tuj-1) at 8 days after induction of neuronal differentiation, whereas synaptogenic factors, such as cell adhesion molecule 1 (CADM1) and SH3 and multiple ankyrin repeat domain protein 3 (Shank3) were accumulated in Sel1l silenced cells. These results indicate that neuronal differentiation triggers ER stress and suggest that Sel1l may facilitate neuronal lineage through the regulation of Math1 and Math3 expression.

The Q system for conditional gene expression is leaky and lacks dynamic range in C. elegans neurons

The Q system allows for conditional gene expression in several organisms, including C. elegans . We aimed to apply this system in C. elegans neurons to obtain temporally-resolved, tissue-specific expression of a fluorescent reporter. We report that, in our hands, there is undesired expression of the reporter in conditions where expression is supposed to be repressed. In addition, in this setup, the signal‑to‑noise ratio of the Q system is unfavorable. We conclude that the Q system is far from optimal in the C. elegans nervous system, and advise cautious use.

Reduction of Derlin activity suppresses Notch-dependent tumours in the C. elegans germ line

Regulating the balance between self-renewal (proliferation) and differentiation is key to the long-term functioning of all stem cell pools. In the Caenorhabditis elegans germline, the primary signal controlling this balance is the conserved Notch signaling pathway. Gain-of-function mutations in the GLP-1/Notch receptor cause increased stem cell self-renewal, resulting in a tumour of proliferating germline stem cells. Notch gain-of-function mutations activate the receptor, even in the presence of little or no ligand, and have been associated with many human diseases, including cancers. We demonstrate that reduction in CUP-2 and DER-2 function, which are Derlin family proteins that function in endoplasmic reticulum-associated degradation (ERAD), suppresses the C. elegans germline over-proliferation phenotype associated with glp-1(gain-of-function) mutations. We further demonstrate that their reduction does not suppress other mutations that cause over-proliferation, suggesting that over-proliferation suppression due to loss of Derlin activity is specific to glp-1/Notch (gain-of-function) mutations. Reduction of CUP-2 Derlin activity reduces the expression of a read-out of GLP-1/Notch signaling, suggesting that the suppression of over-proliferation in Derlin loss-of-function mutants is due to a reduction in the activity of the mutated GLP-1/Notch(GF) receptor. Over-proliferation suppression in cup-2 mutants is only seen when the Unfolded Protein Response (UPR) is functioning properly, suggesting that the suppression, and reduction in GLP-1/Notch signaling levels, observed in Derlin mutants may be the result of activation of the UPR. Chemically inducing ER stress also suppress glp-1(gf) over-proliferation but not other mutations that cause over-proliferation. Therefore, ER stress and activation of the UPR may help correct for increased GLP-1/Notch signaling levels, and associated over-proliferation, in the C. elegans germline.

Notch signaling is a highly conserved signaling pathway that is utilized in many cell fate decisions in many organisms. In the C. elegans germline, Notch signaling is the primary signal that regulates the balance between stem cell proliferation and differentiation. Notch gain-of-function mutations cause the receptor to be active, even when a signal that is normally needed to activate the receptor is absent. In the germline of C. elegans, gain-of-function mutations in GLP-1, a Notch receptor, results in over-proliferation of the stem cells and tumour formation. Here we demonstrate that a reduction or loss of Derlin activity, which is a conserved family of proteins involved in endoplasmic reticulum-associated degradation (ERAD), suppresses over-proliferation due to GLP-1/Notch gain-of-function mutations. Furthermore, we demonstrate that a surveillance mechanism utilized in cells to monitor and react to proteins that are not folded properly (Unfolded Protein Response-UPR) must be functioning well in order for the loss of Derlin activity to supress over-proliferation caused by glp-1/Notch gain-of-function mutations. This suggests that activation of the UPR may be the mechanism at work for suppressing this type of over-proliferation, when Derlin activity is reduced. Therefore, decreasing Derlin activity may be a means of reducing the impact of phenotypes and diseases due to certain Notch gain-of-function mutations.

Genetic modifiers ameliorate endocytic and neuromuscular defects in a model of spinal muscular atrophy

Background

Understanding the genetic modifiers of neurodegenerative diseases can provide insight into the mechanisms underlying these disorders. Here, we examine the relationship between the motor neuron disease spinal muscular atrophy (SMA), which is caused by reduced levels of the survival of motor neuron (SMN) protein, and the actin-bundling protein Plastin 3 (PLS3). Increased PLS3 levels suppress symptoms in a subset of SMA patients and ameliorate defects in SMA disease models, but the functional connection between PLS3 and SMN is poorly understood.

Results

We provide immunohistochemical and biochemical evidence for large protein complexes localized in vertebrate motor neuron processes that contain PLS3, SMN, and members of the hnRNP F/H family of proteins. Using a Caenorhabditis elegans (C. elegans) SMA model, we determine that overexpression of PLS3 or loss of the C. elegans hnRNP F/H ortholog SYM-2 enhances endocytic function and ameliorates neuromuscular defects caused by decreased SMN-1 levels. Furthermore, either increasing PLS3 or decreasing SYM-2 levels suppresses defects in a C. elegans ALS model.

Conclusions

We propose that hnRNP F/H act in the same protein complex as PLS3 and SMN and that the function of this complex is critical for endocytic pathways, suggesting that hnRNP F/H proteins could be potential targets for therapy development.

Quality Control in the Endoplasmic Reticulum: Crosstalk between ERAD and UPR pathways

Endoplasmic reticulum (ER)-associated degradation (ERAD) and the unfolded protein response (UPR) are two key quality-control machineries in the cell. ERAD is responsible for the clearance of misfolded proteins in the ER for cytosolic proteasomal degradation, while UPR is activated in response to the accumulation of misfolded proteins. It has long been thought that ERAD is an integral part of UPR because expression of many ERAD genes is controlled by UPR; however, recent studies have suggested that ERAD has a direct role in controlling the protein turnover and abundance of IRE1α, the most conserved UPR sensor. Here, we review recent advances in our understanding of IRE1α activation and propose that UPR and ERAD engage in an intimate crosstalk to define folding capacity and maintain homeostasis in the ER.

The Expression of Sel1L and Tan-1 in Normal and Neoplastic Cells

We have previously reported on the isolation and chromosomal mapping of a novel human gene (SEL1L), which shows sequence similarity to sel-1, an extragenic suppressor of C. elegans. sel-1 functions as a negative regulator of lin-12 activity, the latter being implicated in the control of diverse cellular differentiation events. In the present study we compare the expression patterns of SEL1L and TAN-1, the human ortholog of lin-12 in normal and neoplastic cells. We found that, whereas both genes are expressed in fetal tissues at similar levels, they are differentially expressed in normal adult and neoplastic cells. In normal adult cells SEL1L is generally present at very low levels; only in the cells of the pancreas does it show maximum expression. By contrast, SEL1L is generally well represented in most neoplastic cells but not in those of pancreatic and gastric carcinomas, where transcription is either downregulated or completely repressed. TAN-1 on the other hand is well represented in almost all normal and neoplastic cells, with very few exceptions. Our observations suggest that SEL1L is presumably implicated in pancreatic and gastric carcinogenesis and that, along with TAN-1, it is very important for normal cell function. Alterations in the expression of SEL1L may be used as a prognostic marker for gastric and pancreatic cancers.

Production of a Monoclonal Antibody Directed against the Recombinant SEL1L Protein

SEL1L, highly similar to the C. elegans sel-1 gene, is a recently cloned human gene whose function is under investigation. SEL1L is differentially expressed in tumors and normal tissues and seems to play a role in tumor growth and aggressiveness. We used the recombinant N-terminus of the SEL1L protein to immunize a Balb/c mouse and produce a monoclonal antibody. A hybridoma secreting an antibody specifically reacting on the SEL1L recombinant fragment was selected. This monoclonal antibody, named MSel1, recognizes the SEL1L protein by Western blotting, immunofluorescence and immunohistochemistry on normal and tumor cells. MSel1 is able to recognize SEL1L even on archival tumor specimens and is therefore particularly appropriate to study SEL1L involvement in tumor progression.

Pancreas lineage allocation and specification are regulated by sphingosine-1-phosphate signalling

During development, progenitor expansion, lineage allocation, and implementation of differentiation programs need to be tightly coordinated so that different cell types are generated in the correct numbers for appropriate tissue size and function. Pancreatic dysfunction results in some of the most debilitating and fatal diseases, including pancreatic cancer and diabetes. Several transcription factors regulating pancreas lineage specification have been identified, and Notch signalling has been implicated in lineage allocation, but it remains unclear how these processes are coordinated. Using a combination of genetic approaches, organotypic cultures of embryonic pancreata, and genomics, we found that sphingosine-1-phosphate (S1p), signalling through the G protein coupled receptor (GPCR) S1pr2, plays a key role in pancreas development linking lineage allocation and specification. S1pr2 signalling promotes progenitor survival as well as acinar and endocrine specification. S1pr2-mediated stabilisation of the yes-associated protein (YAP) is essential for endocrine specification, thus linking a regulator of progenitor growth with specification. YAP stabilisation and endocrine cell specification rely on Gαi subunits, revealing an unexpected specificity of selected GPCR intracellular signalling components. Finally, we found that S1pr2 signalling posttranscriptionally attenuates Notch signalling levels, thus regulating lineage allocation. Both S1pr2-mediated YAP stabilisation and Notch attenuation are necessary for the specification of the endocrine lineage. These findings identify S1p signalling as a novel key pathway coordinating cell survival, lineage allocation, and specification and linking these processes by regulating YAP levels and Notch signalling. Understanding lineage allocation and specification in the pancreas will shed light in the origins of pancreatic diseases and may suggest novel therapeutic approaches.

Genome-wide association study in mice identifies loci affecting liver-related phenotypes including Sel1l influencing serum bile acids

Using publicly available data from inbred mouse strains, we conducted a genome-wide association study to identify loci that accounted for liver-related phenotypes between C57BL/6J and A/J mice fed a Paigen diet. We confirmed genome-wide significant associations for hepatic cholesterol (chromosome 10A2) and serum total bile acid concentration (chromosome 12E) and identified a new locus for liver inflammation (chromosome 7C). Analysis of consomic mice confirmed that chromosome 12 A/J alleles accounted for the variance in serum total bile acid concentrations and had pleiotropic effects on liver mass, serum cholesterol, and serum alanine aminotransferase activity. Using an affected-only haplotype analysis among strains, we refined the chromosome 12E signal to a 1.95 Mb linkage disequilibrium block containing only one gene, sel-1 suppressor of lin-12-like (Sel1l). RNA sequencing and immunoblotting demonstrated that the risk allele locally conferred reduced expression of SEL1L in liver and distantly down-regulated pathways associated with hepatocyte nuclear factor 1 homeobox A (Hnf1a) and hepatocyte nuclear factor 4A (Hnf4a), known modifiers of bile acid transporters and metabolic traits. Consistent with these data, knockdown of SEL1L in HepG2 cells resulted in reduced HNF1A and HNF4A and increased bile acids in culture media; it further captured multiple molecular signatures observed in consomic mouse livers with reduced SEL1L. Finally, dogs harboring a SEL1L mutation and Sel1l(+/-) mice fed a Paigen diet had significantly increased serum total bile acid concentrations, providing independent confirmation linking SEL1L to bile acid metabolism.

CONCLUSION

Genetic analyses of inbred mouse strains identified loci affecting different liver-related traits and implicated Sel1l as a significant determinant of serum bile acid concentration. (Hepatology 2016;63:1943-1956).

Necrotic Cells Actively Attract Phagocytes through the Collaborative Action of Two Distinct PS-Exposure Mechanisms

Necrosis, a kind of cell death closely associated with pathogenesis and genetic programs, is distinct from apoptosis in both morphology and mechanism. Like apoptotic cells, necrotic cells are swiftly removed from animal bodies to prevent harmful inflammatory and autoimmune responses. In the nematode Caenorhabditis elegans, gain-of-function mutations in certain ion channel subunits result in the excitotoxic necrosis of six touch neurons and their subsequent engulfment and degradation inside engulfing cells. How necrotic cells are recognized by engulfing cells is unclear. Phosphatidylserine (PS) is an important apoptotic-cell surface signal that attracts engulfing cells. Here we observed PS exposure on the surface of necrotic touch neurons. In addition, the phagocytic receptor CED-1 clusters around necrotic cells and promotes their engulfment. The extracellular domain of CED-1 associates with PS in vitro. We further identified a necrotic cell-specific function of CED-7, a member of the ATP-binding cassette (ABC) transporter family, in promoting PS exposure. In addition to CED-7, anoctamin homolog-1 (ANOH-1), the C. elegans homolog of the mammalian Ca(2+)-dependent phospholipid scramblase TMEM16F, plays an independent role in promoting PS exposure on necrotic cells. The combined activities from CED-7 and ANOH-1 ensure efficient exposure of PS on necrotic cells to attract their phagocytes. In addition, CED-8, the C. elegans homolog of mammalian Xk-related protein 8 also makes a contribution to necrotic cell-removal at the first larval stage. Our work indicates that cells killed by different mechanisms (necrosis or apoptosis) expose a common "eat me" signal to attract their phagocytic receptor(s); furthermore, unlike what was previously believed, necrotic cells actively present PS on their outer surfaces through at least two distinct molecular mechanisms rather than leaking out PS passively.

Pathogenesis of human mitochondrial diseases is modulated by reduced activity of the ubiquitin/proteasome system

Mitochondria maintain cellular homeostasis by coordinating ATP synthesis with metabolic activity, redox signaling, and apoptosis. Excessive levels of mitochondria-derived reactive oxygen species (ROS) promote mitochondrial dysfunction, triggering numerous metabolic disorders. However, the molecular basis for the harmful effects of excessive ROS formation is largely unknown. Here, we identify a link between mitochondrial stress and ubiquitin-dependent proteolysis, which supports cellular surveillance both in Caenorhabditis elegans and humans. Worms defective in respiration with elevated ROS levels are limited in turnover of a GFP-based substrate protein, demonstrating that mitochondrial stress affects the ubiquitin/proteasome system (UPS). Intriguingly, we observed similar proteolytic defects for disease-causing IVD and COX1 mutations associated with mitochondrial failure in humans. Together, these results identify a conserved link between mitochondrial metabolism and ubiquitin-dependent proteostasis. Reduced UPS activity during pathological conditions might potentiate disease progression and thus provides a valuable target for therapeutic intervention.

Putative tumor suppressor gene SEL1L was downregulated by aberrantly upregulated hsa-mir-155 in human pancreatic ductal adenocarcinoma

Sel-1-like (SEL1L) is a putative tumor suppressor gene that is significantly downregulated in human pancreatic ductal adenocarcinoma (PDA). The mechanism of the downregulation is unclear. Here, we investigated whether aberrantly upregulated microRNAs (miRNAs) repressed the expression of SEL1L. From reported miRNA microarray studies on PDA and predicted miRNA targets, we identified seven aberrantly upregulated miRNAs that potentially target SEL1L. We assessed the expression levels of SEL1L mRNA and the seven miRNAs in human PDA tumors and normal adjacent tissues using real-time quantitative polymerase chain reaction. Then statistical methods were applied to evaluate the association between SEL1L mRNA and the miRNAs. Furthermore, the interaction was explored by functional analysis, including luciferase assay and transient miRNA overexpression. SEL1L mRNA expression levels were found to correlate inversely with the expression of hsa-mir-143, hsa-mir-155, and hsa-mir-223 (P < 0.0001, P < 0.0001, and P = 0.002, respectively). As the number of these overexpressed miRNAs increased, SEL1L mRNA expression progressively decreased (Ptrend  = 0.001). Functional analysis revealed that hsa-mir-155 acted as a suppressor of SEL1L in PDA cell lines. Our study combined statistical analysis with biological approaches to determine the relationships between several miRNAs and the SEL1L gene. The finding that the expression of the putative tumor suppressor SEL1L is repressed by upregulation of hsa-mir-155 helps to elucidate the mechanism for SEL1L downregulation in some human PDA cases. Our results suggest a role for specific miRNAs in the pathogenesis of PDA and indicate that miRNAs have potential as therapeutic targets for PDA.

Direct visualization of specifically modified extracellular glycans in living animals

Modification patterns of the extracellular glycan heparan sulfate coordinate protein function in metazoans, yet in vivo imaging of such non-genetically encoded structures has been impossible. Here we report a transgenic method in Caenorhabditis elegans that allows direct live imaging of specific heparan sulfate modification patterns. This experimental approach reveals a dynamic and cell-specific heparan sulfate landscape and could in principle be adapted to visualize and analyze any extracellular molecule in vivo.

A single-nucleotide polymorphism in tumor suppressor gene SEL1L as a predictive and prognostic marker for pancreatic ductal adenocarcinoma in Caucasians

SEL1L is a putative tumor suppressor gene that is frequently down-regulated in pancreatic ductal adenocarcinoma (PDA). A single-nucleotide polymorphism (SNP) rs12435998 in intron3 of SEL1L has previously been reported to be associated with susceptibility to Alzheimer's disease. We hypothesized that this SNP may influence clinical outcomes of patients with PDA. We analyzed DNA samples from 497 Caucasian patients with pathologically confirmed primary PDA. Of these, 98 had been enrolled in a clinical trial of neoadjuvant chemo-radiotherapy and 77 of the 98 had subsequently undergone pancreaticoduodenectomy (PD). We performed Kaplan-Meier analysis to evaluate the correlation between different SNP genotypes and age at diagnosis, survival time after diagnosis, and survival time after PD. In nonsmokers, we found a significant difference in median age at diagnosis between variant genotypes (AG/GG) carriers and wild-type genotype (AA) carriers (58 vs. 62 yr; log-rank test, P = 0.017). Patients with variant genotypes also showed an increased hazard ratio (HR) of 1.45 [95% confidence interval (CI), 1.07-1.97] relative to wild-type genotype. Among the patients in the clinical trial, the variant genotypes carriers had a median post-PD survival time that was 34.7 months shorter than wild-type genotype carriers (log-rank test, P = 0.019; HR, 1.91; 95% CI, 1.09-3.34). Our results suggest that the rs12435998 SNP in SEL1L gene plays a role in modifying age at diagnosis of PDA in Caucasian nonsmokers. In addition, this SNP may serve as a prognostic marker in PDA patients who undergo the same or similar treatment as the clinical trials.

HPPD: A newly recognized autosomal dominant disorder involving hypertelorism, preauricular sinus, punctal pits, and deafness mapping to chromosome 14q31

We report on a novel autosomal dominant disorder with variable phenotypic expression in a three-generation family; the major features include hypertelorism, preauricular sinus, deafness, and punctal pits with lacrimal-duct obstruction. We ruled out the involvement of EYA1, SIX1, and SIX5 as candidate genes by direct sequencing of their exons and by SNP-based linkage analysis. Subsequent SNP-based whole-genome genotyping and parametric multipoint linkage analysis gave lod scores >1 at 14q31 (LOD = 3.14), 11q25 (LOD = 1.87), and 8p23 (LOD = 1.18). By genotyping additional microsatellite markers at two of these three loci and using an expanded phenotype definition, the LOD at 14q31 increased to 3.34. Direct sequencing of the gene exons within the 14q31 critical interval and a custom aCGH experiment did not show any pathogenic mutation or copy-number changes. Further sequencing of 21 kb of promoter regions showed a novel polymorphism 1,249 bp upstream from the SELIL start codon that segregated with the disease haplotype. Cloning the novel polymorphism into luciferase reporter constructs resulted in a 20% reduction in the expression levels. The identification of this family with a distinctive clinical phenotype and linkage to a novel locus at 14q31 supports the existence of a new syndrome of the branchial cleft.

sel-11 and cdc-42, two negative modulators of LIN-12/Notch activity in C. elegans

Background

LIN-12/Notch signaling is important for cell-cell interactions during development, and mutations resulting in constitutive LIN-12/Notch signaling can cause cancer. Loss of negative regulators of lin-12/Notch activity has the potential for influencing cell fate decisions during development and the genesis or aggressiveness of cancer.

Methodology/Principal Findings

We describe two negative modulators of lin-12 activity in C. elegans. One gene, sel-11, was initially defined as a suppressor of a lin-12 hypomorphic allele; the other gene, cdc-42, is a well-studied Rho GTPase. Here, we show that SEL-11 corresponds to yeast Hrd1p and mammalian Synoviolin. We also show that cdc-42 has the genetic properties consistent with negative regulation of lin-12 activity during vulval precursor cell fate specification.

Conclusions/Significance

Our results underscore the multiplicity of negative regulatory mechanisms that impact on lin-12/Notch activity and suggest novel mechanisms by which constitutive lin-12/Notch activity might be exacerbated in cancer.

UNC-31/CAPS docks and primes dense core vesicles in C. elegans neurons

UNC-31 or its mammalian homologue, Ca(2+)-dependent activator protein for secretion (CAPS), is indispensable for exocytosis of dense core vesicle (DCV) and synaptic vesicle (SV). From N- to the C-terminus, UNC-31 contains putative functional domains, including dynactin 1 binding domain (DBD), C2, PH, (M)UNC-13 homology domain (MHD) and DCV binding domain (DCVBD), the last four we examined in this study. We employed UNC-31 null mutant C. elegans worms to examine whether UNC-31 functions could be rescued by ectopic expression of full length UNC-31 vs each of these four domain-deleted mutants. Full length UNC-31 cDNA rescued the phenotypes of C. elegans null mutants in response to Ca(2+)-elevation in ALA neurons. Surprisingly, MHD deletion also rescued UNC-31 exocytotic function in part because the relatively high Ca(2+) level (pre-flash Ca(2+) was 450 nM) used in the capacitance study could bypass the MHD defect. Nonetheless, the three other domain-truncation cDNAs had almost no rescue on Ca(2+) evoked secretion. Importantly, this genetic null mutant rescue strategy enabled physiological studies at levels of whole organism to single cells, such as locomotion assay, pharmacological study of neurotransmission at neuromuscular junction, in vivo neuropeptide release measurement and analysis of vesicular docking. Our results suggest that each of these UNC-31 domains support distinct sequential molecular actions of UNC-31 in vesicular exocytosis, including steps in vesicle tethering and docking that bridge vesicle with plasma membrane, and subsequently priming vesicle by initiating the formation of soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) core complex.

SEL1L deficiency impairs growth and differentiation of pancreatic epithelial cells

Background

The vertebrate pancreas contains islet, acinar and ductal cells. These cells derive from a transient pool of multipotent pancreatic progenitors during embryonic development. Insight into the genetic determinants regulating pancreatic organogenesis will help the development of cell-based therapies for the treatment of diabetes mellitus. Suppressor enhancer lin12/Notch 1 like (Sel1l) encodes a cytoplasmic protein that is highly expressed in the developing mouse pancreas. However, the morphological and molecular events regulated by Sel1l remain elusive.

Results

We have characterized the pancreatic phenotype of mice carrying a gene trap mutation in Sel1l. We show that Sel1l expression in the developing pancreas coincides with differentiation of the endocrine and exocrine lineages. Mice homozygous for the gene trap mutation die prenatally and display an impaired pancreatic epithelial morphology and cell differentiation. The pancreatic epithelial cells of Sel1l mutant embryos are confined to the progenitor cell state throughout the secondary transition. Pharmacological inhibition of Notch signaling partially rescues the pancreatic phenotype of Sel1l mutant embryos.

Conclusions

Together, these data suggest that Sel1l is essential for the growth and differentiation of endoderm-derived pancreatic epithelial cells during mouse embryonic development.

AMPH-1/Amphiphysin/Bin1 functions with RME-1/Ehd1 in endocytic recycling

RME-1/EHD1 family proteins are key residents of the recycling endosome required for endosome to plasma membrane transport in C. elegans and mammals. Recent studies suggest parallels of the RME-1/EHD proteins to the Dynamin GTPase superfamily of mechanochemical pinchases that promote membrane fission. Here we show that that endogenous C. elegans AMPH-1, the only C. elegans member of Amphiphysin/BIN1 family of BAR-domain proteins, colocalizes with RME-1 on recycling endosomes in vivo, that amph-1 deletion mutants are defective in recycling endosome morphology and function, and that binding of AMPH-1 NPF (D/E) sequences to the RME-1 EH-domain promotes the recycling of transmembrane cargo. We also show a requirement for human BIN1/Amphyphysin 2 in EHD1-regulated endocytic recycling. In vitro we find that the purified recombinant AMPH-1/RME-1 complexes produce short, coated, membrane tubules that are qualitatively distinct from those produced by either protein alone. Our results indicate that AMPH-1 and RME-1 cooperatively regulate endocytic recycling, likely through functions required for the production of cargo carriers exiting the recycling endosome for the cell surface.

Proteome analysis of Plasmodium falciparum extracellular secretory antigens at asexual blood stages reveals a cohort of proteins with possible roles in immune modulation and signaling

The highly co-evolved relationship of parasites and their hosts appears to include modulation of host immune signals, although the molecular mechanisms involved in the host-parasite interplay remain poorly understood. Characterization of these key genes and their cognate proteins related to the host-parasite interplay should lead to a better understanding of this intriguing biological phenomenon. The malaria agent Plasmodium falciparum is predicted to export a cohort of several hundred proteins to remodel the host erythrocyte. However, proteins actively exported by the asexual intracellular parasite beyond the host red blood cell membrane (before merozoite egress) have been poorly investigated so far. Here we used two complementary methodologies, two-dimensional gel electrophoresis/MS and LC-MS/MS, to examine the extracellular secreted antigens at asexual blood stages of P. falciparum. We identified 27 novel antigens exported by P. falciparum in the culture medium of which some showed clustering with highly polymorphic genes on chromosomes, suggesting that they may encode putative antigenic determinants of the parasite. Immunolocalization of four novel secreted proteins confirmed their export beyond the infected red blood cell membrane. Of these, preliminary functional characterization of two novel (Sel1 repeat-containing) parasite proteins, PfSEL1 and PfSEL2 revealed that they down-regulate expression of cell surface Notch signaling molecules in host cells. Also a novel protein kinase (PfEK) and a novel protein phosphatase (PfEP) were found to, respectively, phosphorylate/dephosphorylate parasite-specific proteins in the extracellular culture supernatant. Our study thus sheds new light on malaria parasite extracellular secreted antigens of which some may be essential for parasite development and could constitute promising new drug targets.

Hypoxia induces p53-dependent transactivation and Fas/CD95-dependent apoptosis

p53 triggers apoptosis in response to cellular stress. We analyzed p53-dependent gene and protein expression in response to hypoxia using wild-type p53-carrying or p53 null HCT116 colon carcinoma cells. Hypoxia induced p53 protein levels and p53-dependent apoptosis in these cells. cDNA microarray analysis revealed that only a limited number of genes were regulated by p53 upon hypoxia. Most classical p53 target genes were not upregulated. However, we found that Fas/CD95 was significantly induced in response to hypoxia in a p53-dependent manner, along with several novel p53 target genes including ANXA1, DDIT3/GADD153 (CHOP), SEL1L and SMURF1. Disruption of Fas/CD95 signalling using anti-Fas-blocking antibody or a caspase 8 inhibitor abrogated p53-induced apoptosis in response to hypoxia. We conclude that hypoxia triggers a p53-dependent gene expression pattern distinct from that induced by other stress agents and that Fas/CD95 is a critical regulator of p53-dependent apoptosis upon hypoxia.

SEL1L a multifaceted protein playing a role in tumor progression

Since the cloning in 1997 of SEL1L, the human ortholog of the sel-1 gene of C. elegans, most studies have focused on its role in cancer progression and have provided significant evidences to link its increased expression to a decrease in tumor aggressiveness. SEL1L resides on a "Genome Desert area" on chromosome 14q24.3-31 and is highly conserved in evolution. The function of the SEL1L encoded protein is still very elusive although, several evidences from lower organisms indicate that it plays a major role in protein degradation using the ubiquitin-proteosome system. SEL1L has a very complex structure made up of modules: genomically it consists of 21 exons featuring several alternative transcripts encoding for putative protein isoforms. This structural complexity ensures protein flexibility and specificity, indeed the protein was found in different sub-cellular compartments and may turn on a particular transcript in response to specific stimuli. The overall architecture of SEL1L guarantees an exquisite regulation in the expression of the gene.

Sel1-like repeat proteins in signal transduction

Solenoid proteins, which are distinguished from general globular proteins by their modular architectures, are frequently involved in signal transduction pathways. Proteins from the tetratricopeptide repeat (TPR) and Sel1-like repeat (SLR) families share similar alpha-helical conformations but different consensus sequence lengths and superhelical topologies. Both families are characterized by low sequence similarity levels, rendering the identification of functional homologous difficult. Therefore current knowledge of the molecular and cellular functions of the SLR proteins Sel1, Hrd3, Chs4, Nif1, PodJ, ExoR, AlgK, HcpA, Hsp12, EnhC, LpnE, MotX, and MerG has been reviewed. Although SLR proteins possess different cellular functions they all seem to serve as adaptor proteins for the assembly of macromolecular complexes. Sel1, Hrd3, Hsp12 and LpnE are activated under cellular stress. The eukaryotic Sel1 and Hrd3 proteins are involved in the ER-associated protein degradation, whereas the bacterial LpnE, EnhC, HcpA, ExoR, and AlgK proteins mediate the interactions between bacterial and eukaryotic host cells. LpnE and EnhC are responsible for the entry of L. pneumophila into epithelial cells and macrophages. ExoR from the symbiotic microorganism S. melioti and AlgK from the pathogen P. aeruginosa regulate exopolysaccaride synthesis. Nif1 and Chs4 from yeast are responsible for the regulation of mitosis and septum formation during cell division, respectively, and PodJ guides the cellular differentiation during the cell cycle of the bacterium C. crescentus. Taken together the SLR motif establishes a link between signal transduction pathways from eukaryotes and bacteria. The SLR motif is so far absent from archaea. Therefore the SLR could have developed in the last common ancestor between eukaryotes and bacteria.

Protein profile changes in the human breast cancer cell line MCF-7 in response toSEL1L gene induction

The ectopic expression of the gene SEL1L in the human breast carcinoma cell line MCF-7 resulted in a reduction of the aggressive behaviour of these cells in vitro. In addition, in vivo analysis on a series of primary breast carcinomas revealed an association between the SEL1L protein levels and the patient's overall survival. We aimed to find those proteins, associated with SEL1L, which may be involved in modulating the aggressive or invasive behaviour of breast cancer cells. For this purpose, we used both the proteomic and microarray approaches. Image analysis of two-dimensional electropherograms revealed the presence of 27 qualitative and 35 quantitative variations between the MCF7-SEL1L expressing cells compared to control. Mass spectrometry identified 32 changing proteins mostly involved in cytoskeletal and metabolic activities, stress response and protein folding, selenoprotein synthesis and cellular proliferation. Five of these also showed changes in transcript levels, as assessed by Affymetrix microarray analysis. Interestingly, seven proteins: carbonic anhydrase (CA) II, ovarian/breast septin, S100A16 calcium binding protein, 14-3-3 protein sigma, proteasome subunit beta type 6, Hsp60 and protein disulphide-isomerase A3 merit particular attention since they are known to be involved in cancer, in response to cellular stress and in protein folding.

SEL1L expression in pancreatic adenocarcinoma parallels SMAD4 expression and delays tumor growth in vitro and in vivo

Recent data suggest that SEL1L may play an important role in pancreatic carcinoma, similar to breast cancer, where the expression of SEL1L has been associated with a reduction in both proliferative activity in vitro and clinical tumor aggressiveness. To investigate this possibility, we examined the expression of Sel1L in a series of primary pancreatic carcinomas by immunohistochemistry and characterized the effects of Sel1L overexpression both in vitro and in vivo. In 74 pancreatic cancers analysed, 36% lacked Sel1L expression, although there was no significant correlation between the expression of Sel1L and any clinicopathologic parameter, including survival. However, immunohistochemical reactivity for Sel1L and Dpc4/Smad4 was concordant in 69% of cases (chi(2) test P&<0.004). Overexpression of SEL1L in stably transfected pancreatic cancer cells caused both a decrease in clonogenicity and anchorage-independent growth as well as a significant increase in the levels of activin A and SMAD4. When implanted in nude mice, Suit-2-SEL1L-overexpressing clones displayed a considerably reduced rate of tumor growth. Thus, it can be hypothesized that Sel1L plays an important function in the growth and aggressiveness of pancreatic carcinoma. Moreover, our data provide evidence that SEL1L has an impact on the expression of genes involved in regulation of cellular growth, possibly through the TGF-beta signaling pathway.

Fgf10maintains notch activation, stimulates proliferation, and blocks differentiation of pancreatic epithelial cells

The pancreas is an endodermally derived organ that initially appears as a dorsal and ventral protrusion of the primitive gut epithelium. The pancreatic progenitor cells present in these early pancreatic anlagen proliferate and eventually give rise to all pancreatic cell types. The fibroblast growth factor receptor (FGFR) 2b high-affinity ligand FGF10 has been linked to pancreatic epithelial cell proliferation, and we have shown previously that Notch signalling controls pancreatic cell differentiation by means of lateral inhibition. In the developing pancreas, activated intracellular Notch appears to be required for maintaining cells in the progenitor state, in part by blocking the expression of the pro-endocrine gene neurogenin 3 (ngn3), and hence endocrine cell differentiation. Here, we show that persistent expression of Fgf10 in the embryonic pancreas of transgenic mice also inhibits pancreatic cell differentiation, while stimulating pancreatic epithelial cell proliferation. We provide evidence that one of the effects of the persistent expression of Fgf10 in the developing pancreas is maintained Notch activation, which results in impaired expression of ngn3 within the pancreatic epithelium. Together, our data suggest a role for FGF10/FGFR2b signalling in regulation of pancreatic cell proliferation and differentiation and that FGF10/FGFR2b signalling affects the Notch-mediated lateral inhibition pathway.

Genetic modifiers of the age at diagnosis of diabetes (MODY3) in carriers of hepatocyte nuclear factor-1alpha mutations map to chromosomes 5p15, 9q22, and 14q24

Mutations in hepatocyte nuclear factor (HNF)-1alpha (MODY3) account for the largest proportion of maturity-onset diabetes of the young (MODY) cases in the U.S. This form of diabetes is characterized by impaired insulin secretion in response to glucose, but wide variability exists in the severity of hyperglycemia and in the age at which it becomes clinically manifest. We have previously shown that the age at onset of diabetes in MODY3 families is influenced by familial factors (including modifying genes) and exposure to diabetes in utero. To identify genes influencing the onset of MODY3, we conducted a genome scan in 13 extended MODY families in which diabetes segregates with an HNF-1alpha mutation. Linkage with age at onset of diabetes was assessed by genetic variance component analysis using SOLAR. The locus with the strongest evidence of linkage was on chromosome 14q24 (D14S588; logarithm of odds [LOD] = 2.58, P = 0.0004). This location overlaps with IDDM11 and includes SEL1L, a negative regulator of the Notch pathway that may control islet development. Linkage evidence also supported loci on 5p15 (D5S817; LOD = 2.44, P = 0.0004) and 9q22 (D9S910; LOD = 2.02, P = 0.0018). The latter matches a region linked to 2-h insulin levels in Pima Indians. Less strong linkage evidence was observed at three other regions: chromosomes 3p24 (LOD = 1.44), 7q21 (1.20), and 16q23 (1.51). Our data are consistent with the existence of multiple loci that contribute to the expression of the MODY3 phenotype. Identification of these genes will offer new insights into the pathophysiology of MODY that may, in turn, increase our understanding of the cellular events underlying more common forms of diabetes.

Notch mediates TGF alpha-induced changes in epithelial differentiation during pancreatic tumorigenesis

Notch signaling regulates cell fate decisions in a wide variety of adult and embryonic tissues. Here we show that Notch pathway components and Notch target genes are upregulated in invasive pancreatic cancer, as well as in pancreatic cancer precursors from both mouse and human. In mouse pancreas, ectopic Notch activation results in accumulation of nestin-positive precursor cells and expansion of metaplastic ductal epithelium, previously identified as a precursor lesion for pancreatic cancer. Notch is also activated as a direct consequence of EGF receptor activation in exocrine pancreas and is required for TGF alpha-induced changes in epithelial differentiation. These findings suggest that Notch mediates the tumor-initiating effects of TG alpha by expanding a population of undifferentiated precursor cells.

Cross-species conservation of SEL1L, a human pancreas-specific expressing gene

SEL1L is a recently cloned and organ-specific expressing human gene whose function is still at an embryonic stage but displays several interesting characteristics, among which a remarkable cross-species conservation. During evolution, the gene structural complexity increased, suggesting a diversification of its function; however, several amino acid motifs remain perfectly conserved from the bacteria to the human protein. SEL1L is the human ortholog of the C. elegans gene sel-1; the latter is implicated in the negative regulation of LIN-12/GLP-1/Notch receptor proteins. These receptor proteins play fundamental roles in signal transduction pathways and are key players in cell fate determination during the development of various organs. Studies in model organisms, such as C. elegans, helped to illuminate fundamental mechanisms involved in normal cellular functions and human diseases. This paper describes the conserved nature of SEL1L across a wide range of species suggesting, that the encoded protein most likely exerts a very important biological function; it may belong to a subclass of genes considered to be "essential."

Endocytosis-mediated downregulation of LIN-12/Notch upon Ras activation in Caenorhabditis elegans

The coordination of signals from different pathways is important for cell fate specification during animal development. Here, we define a novel mode of crosstalk between the epidermal growth factor receptor/Ras/mitogen-activated protein kinase cascade and the LIN-12/Notch pathway during Caenorhabditis elegans vulval development. Six vulval precursor cells (VPCs) are initially equivalent but adopt different fates as a result of an inductive signal mediated by the Ras pathway and a lateral signal mediated by the LIN-12/Notch pathway. One consequence of activating Ras is a reduction of LIN-12 protein in P6.p (ref. 2), the VPC believed to be the source of the lateral signal. Here we identify a 'downregulation targeting signal' (DTS) in the LIN-12 intracellular domain, which encompasses a di-leucine-containing endocytic sorting motif. The DTS seems to be required for internalization of LIN-12, and on Ras activation it might mediate altered endocytic routing of LIN-12, leading to downregulation. We also show that if LIN-12 is stabilized in P6.p, lateral signalling is compromised, indicating that LIN-12 downregulation is important in the appropriate specification of cell fates in vivo.

Mutant membrane protein of the budding yeast spindle pole body is targeted to the endoplasmic reticulum degradation pathway

Mutation of either the yeast MPS2 or the NDC1 gene leads to identical spindle pole body (SPB) duplication defects: The newly formed SPB is improperly inserted into the nuclear envelope (NE), preventing the cell from forming a bipolar mitotic spindle. We have previously shown that both MPS2 and NDC1 encode integral membrane proteins localized at the SPB. Here we show that CUE1, previously known to have a role in coupling ubiquitin conjugation to ER degradation, is an unusual dosage suppressor of mutations in MPS2 and NDC1. Cue1p has been shown to recruit the soluble ubiquitin-conjugating enzyme, Ubc7p, to the cytoplasmic face of the ER membrane where it can ubiquitinate its substrates and target them for degradation by the proteasome. Both mps2-1 and ndc1-1 are also suppressed by disruption of UBC7 or its partner, UBC6. The Mps2-1p mutant protein level is markedly reduced compared to wild-type Mps2p, and deletion of CUE1 restores the level of Mps2-1p to nearly wild-type levels. Our data indicate that Mps2p may be targeted for degradation by the ER quality control pathway.

Genetic analysis of endocytosis in Caenorhabditis elegans: coelomocyte uptake defective mutants

The coelomocytes of Caenorhabditis elegans are scavenger cells that continuously and nonspecifically endocytose fluid from the pseudocoelom (body cavity). Green fluorescent protein (GFP) secreted into the pseudocoelom from body wall muscle cells is endocytosed and degraded by coelomocytes. We show that toxin-mediated ablation of coelomocytes results in viable animals that fail to endocytose pseudocoelomic GFP, indicating that endocytosis by coelomocytes is not essential for growth or survival of C. elegans under normal laboratory conditions. We examined known viable endocytosis mutants, and performed RNAi for other known endocytosis genes, for coelomocyte uptake defective (Cup) phenotypes. We also screened for new genes involved in endocytosis by isolating viable mutants with Cup defects; this screen identified 14 different genes, many with multiple alleles. A variety of Cup terminal phenotypes were observed, consistent with defects at various steps in the endocytic pathway. Available molecular information indicates that the Cup mutant screen has identified novel components of the endocytosis machinery that are conserved in mammals but not in Saccharomyces cerevisiae, the only other organism for which large-scale genetic screens for endocytosis mutants have been performed.

Evidence that RME-1, a conserved C. elegans EH-domain protein, functions in endocytic recycling

In genetic screens for new endocytosis genes in Caenorhabditis elegans we identified RME-1, a member of a conserved class of Eps15-homology (EH)-domain proteins. Here we show that RME-1 is associated with the periphery of endocytic organelles, which is consistent with a direct role in endocytic transport. Endocytic defects in rme-1 mutants indicate that the protein is likely to have a function in endocytic recycling. Evidence from studies of mammalian RME-1 also points to a function for RME-1 in recycling, specifically in the exit of membrane proteins from recycling endosomes. These studies show a conserved function in endocytic recycling for the RME-1 family of EH proteins.

Endoplasmic reticulum degradation requires lumen to cytosol signaling. Transmembrane control of Hrd1p by Hrd3p

Endoplasmic reticulum (ER)-associated degradation (ERAD) is required for ubiquitin-mediated destruction of numerous proteins. ERAD occurs by processes on both sides of the ER membrane, including lumenal substrate scanning and cytosolic destruction by the proteasome. The ER resident membrane proteins Hrd1p and Hrd3p play central roles in ERAD. We show that these two proteins directly interact through the Hrd1p transmembrane domain, allowing Hrd1p stability by Hrd3p-dependent control of the Hrd1p RING-H2 domain activity. Rigorous reevaluation of Hrd1p topology demonstrated that the Hrd1p RING-H2 domain is located and functions in the cytosol. An engineered, completely lumenal, truncated version of Hrd3p functioned normally in both ERAD and Hrd1p stabilization, indicating that the lumenal domain of Hrd3p regulates the cytosolic Hrd1p RING-H2 domain by signaling through the Hrd1p transmembrane domain. Additionally, we identified a lumenal region of Hrd3p dispensable for regulation of Hrd1p stability, but absolutely required for normal ERAD. Our studies show that Hrd1p and Hrd3p form a stoichiometric complex with ERAD determinants in both the lumen and the cytosol. The HRD complex engages in lumen to cytosol communication required for regulation of Hrd1p stability and the coordination of ERAD events on both sides of the ER membrane.

Identification of novel loci involved in entry by Legionella pneumophila

Legionella pneumophila is primarily an intracellular pathogen during infection; thus, the mechanisms of entry into host cells are likely to be important for pathogenesis. Several L. pneumophila mutants that display an enhanced-entry (Enh) phenotype were isolated by selecting for bacteria that enter host cells at a higher frequency than wild-type. In the course of characterizing the genetic basis of one of these mutants, C3, a strategy was developed for the isolation of laboratory-media-repressed virulence determinants from L. pneumophila. Screens for dominant mutations using a genomic DNA library from C3 resulted in the isolation of three cosmids that confer an Enh phenotype to wild-type L. pneumophila. Transposon mutagenesis of these cosmids allowed identification of three loci that affect entry. Analysis of the putative proteins encoded by these loci, designated rtxA and enhC, demonstrated similarity to repeats in the structural toxin protein and the secreted Sel-1 protein from Caenorhabditis elegans, respectively. L. pneumophila rtxA and enhC mutants display significantly reduced entry into host cells, compared to wild-type bacteria. The phenotype that the cosmids containing these loci confer is most likely due to elevated expression resulting from their presence on multicopy vectors. The use of increased gene copy number to overexpress genes that are normally repressed under laboratory growth conditions is generally applicable to the isolation of virulence determinants from L. pneumophila and other bacterial pathogens.

Receptor-mediated endocytosis in the Caenorhabditis elegans oocyte

The Caenorhabditis elegans oocyte is a highly amenable system for forward and reverse genetic analysis of receptor-mediated endocytosis. We describe the use of transgenic strains expressing a vitellogenin::green fluorescent protein (YP170::GFP) fusion to monitor yolk endocytosis by the C. elegans oocyte in vivo. This YP170::GFP reporter was used to assay the functions of C. elegans predicted proteins homologous to vertebrate endocytosis factors using RNA-mediated interference. We show that the basic components and pathways of endocytic trafficking are conserved between C. elegans and vertebrates, and that this system can be used to test the endocytic functions of any new gene. We also used the YP170::GFP assay to identify rme (receptor-mediated endocytosis) mutants. We describe a new member of the low-density lipoprotein receptor superfamily, RME-2, identified in our screens for endocytosis defective mutants. We show that RME-2 is the C. elegans yolk receptor.

p24 proteins and quality control of LIN-12 and GLP-1 trafficking in Caenorhabditis elegans

Mutations in the Caenorhabditis elegans sel-9 gene elevate the activity of lin-12 and glp-1, which encode members of the LIN-12/NOTCH family of receptors. Sequence analysis indicates SEL-9 is one of several C. elegans p24 proteins. Allele-specific genetic interactions suggest that reducing sel-9 activity increases the activity of mutations altering the extracellular domains of LIN-12 or GLP-1. Reducing sel-9 activity restores the trafficking to the plasma membrane of a mutant GLP-1 protein that would otherwise accumulate within the cell. Our results suggest a role for SEL-9 and other p24 proteins in the negative regulation of transport of LIN-12 and GLP-1 to the cell surface, and favor a role for p24 proteins in a quality control mechanism for endoplasmic reticulum-Golgi transport.

Cloning and characterization of Sel-1l, a murine homolog of the C. elegans sel-1 gene

The Notch signaling pathway regulates specification and proliferation in a variety of cell lineages in invertebrates and vertebrates. We have cloned a murine homolog of SEL-1, a key negative regulator of the Notch pathway in Caenorhabditis elegans. Murine SEL-1L (mSEL-1L) protein exhibits a high degree of similarity to SEL-1, including a signal peptide and the C-terminal region required for SEL-1 function in C. elegans. This mammalian homolog of sel-1 is widely expressed in adult mouse and human tissues, with particularly high levels in the pancreas. RNA in situ analysis of developing mouse embryos indicates that mSEL-1L is moderately expressed throughout the neural tube and dorsal root ganglia, with particularly high levels in the floor plate of the neural tube beginning at E10.5 and increasing at E11.5. Expression is high at E14.5 and E17.5 in the acini of the pancreas, and moderate in the epithelial cells of the gut villi. We localized the SEL-1L protein to the cytosol, possibly in intracellular vesicles, in a beta-islet-derived tumor cell line (RinM).

LIN-12 protein expression and localization during vulval development in C. elegans

We have used a LIN-12::GFP fusion protein to examine LIN-12 accumulation during cell fate decisions important for vulval development. During the naturally variable anchor cell (AC)/ventral uterine precursor cell (VU) decision of the somatic gonad, a transcription-based feedback mechanism biases two equivalent cells so that one becomes the AC while the other becomes a VU. LIN-12::GFP accumulation reflects lin-12 transcription: LIN-12::GFP is initially present in both cells, but disappears from the presumptive AC and becomes restricted to the presumptive VU. During vulval precursor cell (VPC) fate determination, six equipotential cells uniformly transcribe lin-12 and have invariant fates that are specified by multiple cell-cell interactions. The pattern of LIN-12::GFP accumulation in VPCs and in the VPC lineages is dynamic and does not always reflect lin-12 transcription. In particular, LIN-12::GFP is expressed initially in all six VPCs, but appears to be reduced specifically in P6.p as a consequence of the activation of the Ras pathway by an EGF-like inductive signal from the AC. We propose that downregulation of LIN-12 stability or translation in response to inductive signalling helps impose a bias on lateral signalling and contributes to the invariant pattern of VPC fates.

The LIN-12/Notch signaling pathway and its regulation

Notch, LIN-12, and GLP-1 are receptors that mediate a broad range of cell interactions during Drosophila and nematode development. Signaling by these receptors relies on a conserved pathway with three core components: DSL ligand, LNG receptor, and a CSL effector that links the receptor to its transcriptional response. Although key functional regions have been identified in each class of proteins, the mechanism for signal transduction is not yet understood. Diverse regulatory mechanisms influence signaling by the LIN-12/Notch pathway. Inductive signaling relies on the synthesis of ligand and receptor in distinct but neighboring cells. By contrast, lateral signaling leads to the transformation of equivalent cells that express both ligand and receptor into nonequivalent cells that express either ligand or receptor. This transformation appears to rely on regulatory feedback loops within the LIN-12/Notch pathway. In addition, the pathway can be regulated by intrinsic factors that are asymmetrically segregated during cell division or by extrinsic cues via other signaling pathways. Specificity in the pathway does not appear to reside in the particular ligand or receptor used for a given cell-cell interaction. The existence of multiple ligands and receptors may have evolved from the stringent demands placed upon the regulation of genes encoding them.

sel-10, a negative regulator of lin-12 activity in Caenorhabditis elegans, encodes a member of the CDC4 family of proteins

Mutations that influence lin-12 activity in Caenorhabditis elegans may identify conserved factors that regulate the activity of lin-12/Notch proteins. We describe genetic evidence indicating that sel-10 is a negative regulator of lin-12/Notch-mediated signaling in C. elegans. Sequence analysis shows that SEL-10 is a member of the CDC4 family of proteins and has a potential human ortholog. Coimmunoprecipitation data indicate that C. elegans SEL-10 complexes with LIN-12 and with murine Notch4. We propose that SEL-10 promotes the ubiquitin-mediated turnover of LIN-12/Notch proteins, and discuss potential roles for the regulation of lin-12/Notch activity by sel-10 in cell fate decisions and tumorigenesis.

The ins and outs of notch signaling

The Notch gene encodes a cell surface protein that regulates cell fate choices in vertebrates and invertebrates. Given the wide variety of cell types influenced by Notch, it would seem that the signal relayed through Notch activation is not an instructive one per se. Rather, Notch signaling is thought to influence the cell's ability to respond to instructive signals responsible for specific cell fates. Expression and functional studies of Notch support this idea; however, the possibility of additional functions for Notch cannot be excluded. Much of what we know about the Notch signaling pathway comes from studies with Drosophila Notch and the Caenorhabditis elegans Notch-related genes lin-12 and glp-1. With the isolation of multiple vertebrate Notch genes we are beginning to understand and define Notch signaling in vertebrates as well. A number of excellent reviews have been published summarizing the current status of Notch/LIN-12/GLP-1 signaling in Drosophila and C. elegans, as well as recent findings with the vertebrate counterparts. Here I review the structure of the various Notch proteins and their putative ligands, and discuss possible interactions between Notch, its ligands, and other cellular components that affect Notch signal transduction. A role for Notch signaling during normal development and in disease processes is discussed in an accompanying review by T. Gridley (1997, Mol. Cell. Neurosci. 9: 103-108).