Pdx-1 is required for activation in vivo from a duodenum-specific enhancer

Patterning the gastrointestinal epithelium to confer regional-specific functions

The gastrointestinal (GI) tract, in simplest terms, can be described as an epithelial-lined muscular tube extending along the cephalocaudal axis from the oral cavity to the anus. Although the general architecture of the GI tract organs is conserved from end to end, the presence of different epithelial tissue structures and unique epithelial cell types within each organ enables each to perform the distinct digestive functions required for efficient nutrient assimilation. Spatiotemporal regulation of signaling pathways and downstream transcription factors controls GI epithelial morphogenesis during development to confer essential regional-specific epithelial structures and functions. Here, we discuss the fundamental functions of each GI tract organ and summarize the diversity of epithelial structures present along the cephalocaudal axis of the GI tract. Next, we discuss findings, primarily from genetic mouse models, that have defined the roles of key transcription factors during epithelial morphogenesis, including p63, SOX2, SOX15, GATA4, GATA6, HNF4A, and HNF4G. Additionally, we examine how the Hedgehog, WNT, and BMP signaling pathways contribute to defining unique epithelial features along the cephalocaudal axis of the GI tract. Lastly, we examine the molecular mechanisms controlling regionalized cytodifferentiation of organ-specific epithelial cell types within the GI tract, concentrating on the stomach and small intestine. The delineation of GI epithelial patterning mechanisms in mice has provided fundamental knowledge to guide the development and refinement of three-dimensional GI organotypic culture models such as those derived from directed differentiation of human pluripotent stem cells and those derived directly from human tissue samples. Continued examination of these pathways will undoubtedly provide vital insights into the mechanisms of GI development and disease and may afford new avenues for innovative tissue engineering and personalized medicine approaches to treating GI diseases.

In vitro patterning of pluripotent stem cell-derived intestine recapitulates in vivo human development

The intestine plays a central role in digestion, nutrient absorption and metabolism, with individual regions of the intestine having distinct functional roles. Many examples of region-specific gene expression in the adult intestine are known, but how intestinal regional identity is established during development is a largely unresolved issue. Here, we have identified several genes that are expressed in a region-specific manner in the developing human intestine. Using human embryonic stem cell-derived intestinal organoids, we demonstrate that the duration of exposure to active FGF and WNT signaling controls regional identity. Short-term exposure to FGF4 and CHIR99021 (a GSK3β inhibitor that stabilizes β-catenin) resulted in organoids with gene expression patterns similar to developing human duodenum, whereas longer exposure resulted in organoids similar to ileum. When region-specific organoids were transplanted into immunocompromised mice, duodenum-like organoids and ileum-like organoids retained their regional identity, demonstrating that regional identity of organoids is stable after initial patterning occurs. This work provides insights into the mechanisms that control regional specification of the developing human intestine and provides new tools for basic and translational research.

Summary: Human embryonic stem cell-derived intestinal organoids can be patterned into duodenum-like or ileum-like tissue, recapitulating in vivo human development.

PDX1 regulation of FABP1 and novel target genes in human intestinal epithelial Caco-2 cells

The transcription factor pancreatic and duodenal homeobox 1 (PDX1) plays an essential role in pancreatic development and in maintaining proper islet function via target gene regulation. Few intestinal PDX1 targets, however, have been described. We sought to define novel PDX1-regulated intestinal genes. Caco-2 human intestinal epithelial cells were engineered to overexpress PDX1 and gene expression profiles relative to control cells were assessed. Expression of 80 genes significantly increased while that of 49 genes significantly decreased more than 4-fold following PDX1 overexpression in differentiated Caco-2 cells. Analysis of the differentially regulated genes with known functional annotations revealed genes encoding transcription factors, growth factors, kinases, digestive glycosidases, nutrient transporters, nutrient binding proteins, and structural components. The gene for fatty acid binding protein 1, liver, FABP1, is repressed by PDX1 in Caco-2 cells. PDX1 overexpression in Caco-2 cells also results in repression of promoter activity driven by the 0.6kb FABP1 promoter. PDX1 regulation of promoter activity is consistent with the decrease in FABP1 RNA abundance resulting from PDX1 overexpression and identifies FABP1 as a candidate PDX1 target. PDX1 repression of FABP1, LCT, and SI suggests a role for PDX1 in patterning anterior intestinal development.

Direct transcriptional regulation of Gata4 during early endoderm specification is controlled by FoxA2 binding to an intronic enhancer

The embryonic endoderm is a multipotent progenitor cell population that gives rise to the epithelia of the digestive and respiratory tracts, the liver and the pancreas. Among the transcription factors that have been shown to be important for endoderm development and gut morphogenesis is GATA4. Despite the important role of GATA4 in endoderm development, its transcriptional regulation is not well understood. In this study, we identified an intronic enhancer from the mouse Gata4 gene that directs expression to the definitive endoderm in the early embryo. The activity of this enhancer is initially broad in all endodermal progenitors, as demonstrated by fate mapping analysis using the Cre/loxP system, but becomes restricted to the dorsal foregut and midgut, and associated organs such as dorsal pancreas and stomach. The function of the intronic Gata4 enhancer is dependent upon a conserved Forkhead transcription factor-binding site, which is bound by recombinant FoxA2 in vitro. These studies identify Gata4 as a direct transcriptional target of FoxA2 in the hierarchy of the transcriptional regulatory network that controls the development of the definitive endoderm.

An endoderm-specific transcriptional enhancer from the mouse Gata4 gene requires GATA and homeodomain protein-binding sites for function in vivo

Several transcription factors function in the specification and differentiation of the endoderm, including the zinc finger transcription factor GATA4. Despite its essential role in endoderm development, the transcriptional control of the Gata4 gene in the developing endoderm and its derivatives remains incompletely understood. Here, we identify a distal enhancer from the Gata4 gene, which directs expression exclusively to the visceral and definitive endoderm of transgenic mouse embryos. The activity of this enhancer is initially broad within the definitive endoderm but later restricts to developing endoderm-derived tissues, including pancreas, glandular stomach, and duodenum. The activity of this enhancer in vivo is dependent on evolutionarily-conserved HOX- and GATA-binding sites, which are bound by PDX-1 and GATA4, respectively. These studies establish Gata4 as a direct transcriptional target of homeodomain and GATA transcription factors in the endoderm and support a model in which GATA4 functions in the transcriptional network for pancreas formation.

Pdx1 inactivation restricted to the intestinal epithelium in mice alters duodenal gene expression in enterocytes and enteroendocrine cells

Null mutant mice lacking the transcription factor pancreatic and duodenal homeobox 1 (Pdx1) are apancreatic and survive only a few days after birth. The role of Pdx1 in regulating intestinal gene expression has therefore yet to be determined in viable mice with normal pancreatic development. We hypothesized that conditional inactivation of Pdx1 restricted to the intestinal epithelium would alter intestinal gene expression and cell differentiation. Pdx1(flox/flox);VilCre mice with intestine-specific Pdx1 inactivation were generated by crossing a transgenic mouse strain expressing Cre recombinase, driven by a mouse villin 1 gene promoter fragment, with a mutant mouse strain homozygous for loxP site-flanked Pdx1. Pdx1 protein is undetectable in all epithelial cells in the intestinal epithelium of Pdx1(flox/flox);VilCre mice. Goblet cell number and mRNA abundance for mucin 3 and mucin 13 genes in the proximal small intestine are comparable between Pdx1(flox/flox);VilCre and control mice. Similarly, Paneth cell number and expression of Paneth cell-related genes Defa1, Defcr-rs1, and Mmp7 in the proximal small intestine remain statistically unchanged by Pdx1 inactivation. Although the number of enteroendocrine cells expressing chromogranin A/B, gastric inhibitory polypeptide (Gip), or somatostatin (Sst) is unaffected in the Pdx1(flox/flox);VilCre mice, mRNA abundance for Gip and Sst is significantly reduced in the proximal small intestine. Conditional Pdx1 inactivation attenuates intestinal alkaline phosphatase (IAP) activity in the duodenal epithelium, consistent with an average 91% decrease in expression of the mouse enterocyte IAP gene, alkaline phosphatase 3 (a novel Pdx1 target candidate), in the proximal small intestine following Pdx1 inactivation. We conclude that Pdx1 is necessary for patterning appropriate gene expression in enterocytes and enteroendocrine cells of the proximal small intestine.

GATA-4 upregulates glucose-dependent insulinotropic polypeptide expression in cells of pancreatic and intestinal lineage

A thorough examination of glucose-dependent insulinotropic polypeptide (GIP) expression has been hampered by difficulty in isolating widely dispersed, GIP-producing enteroendocrine K-cells. To elucidate the molecular mechanisms governing the regulation of GIP expression, 14 intestinal and pancreatic cell lines were assessed for their suitability for studies examining GIP expression. Both STC-1 cells and the pancreatic cell line betaTC-3 were found to express GIP mRNA and secrete biologically active GIP. However, levels of GIP mRNA and bioactive peptide and the activity of transfected GIP reporter constructs were significantly lower in betaTC-3 than STC-1 cells. When betaTC-3 cells were analyzed for transcription factors known to be important for GIP expression, PDX-1 and ISL-1, but not GATA-4, were detected. Double staining for GIP-1 and GATA-4 in mouse duodenum demonstrated GATA-4 expression in intestinal K-cells. Exogenous expression of GATA-4 in betaTC-3 cells led to marked increases in both GIP transcription and secretion. Lastly suppression of GATA-4 via RNA interference, in GTC-1 cells, a subpopulation of STC-1 cells with high endogenous GIP expression resulted in a marked an attenuation of GIP promoter activity. Our data support the hypothesis that GATA-4 may function to augment or enhance GIP expression rather than act as an initiator of GIP transcription.

Temporal regulation of enhancer function in intestinal epithelium: a role for Onecut factors

An intestine-specific gene regulatory region was previously identified near the second exon of the human adenosine deaminase (ADA) gene. In mammalian intestine, ADA is expressed at high levels only along the villi of the duodenal epithelium, principally if not exclusively in enterocytes. Within the ADA intestinal regulatory region, a potent duodenum-specific enhancer was identified that controls this pattern of expression. This enhancer has been shown to rely on PDX-1, GATA factors, and Cdx factors for its function. Upstream of the enhancer, a separate temporal regulatory region was identified that has no independent enhancer capability but controls the timing of enhancer activation. DNase I footprinting and electrophoretic mobility shift assays were used to begin to characterize temporal region function at the molecular level. In this manner, binding sites for the Onecut (OC) family of factors, YY1, and NFI family members were identified. Identification of the OC site was especially interesting, because almost nothing is known about the function of OC factors in intestine. In transgenic mice, mutation of the OC site to ablate binding resulted in a delay of 2-3 weeks in enhancer activation in the developing postnatal intestine, a result very similar to that observed previously when the entire temporal region was deleted. In mammals, the OC family is comprised of OC-1/HNF-6, OC-2, and OC-3. An examination of intestinal expression patterns showed that all three OC factors are expressed at detectable levels in adult mouse duodenum, with OC-2 predominant. In postnatal day 2 mice only OC-2 and OC-3 were detected in intestine, with OC-2 again predominant.

Lactase gene promoter fragments mediate differential spatial and temporal expression patterns in transgenic mice

Lactase gene expression is spatiotemporally regulated during mammalian gut development. We hypothesize that distinct DNA control regions specify appropriate spatial and temporal patterning of lactase gene expression. In order to define regions of the lactase promoter involved in mediating intestine-specific and spatiotemporal restricted expression, transgenic mice harboring 100 bp, 1.3- and 2.0- kb fragments of the 5' flanking region of the rat lactase gene cloned upstream of a luciferase reporter were characterized. The 100-bp lactase promoter-reporter transgenic mouse line expressed maximal luciferase activity in the intestine with a posterior shift in spatial restriction and ectopic expression in the stomach and lung. The temporal pattern of expression mediated by the 1.3-kb promoter?reporter transgene increases with postnatal maturation in contrast with the postnatal decline mediated by the 2.0-kb promoter-reporter transgene and the endogenous lactase gene. The differential transgene expression patterns mediated by the lactase promoter fragments suggests that intestine-specific spatial and temporal control elements reside in distinct regions of the DNA sequences upstream of the lactase gene transcription start-site.

Decreased sucrase and lactase activity in iron deficiency is accompanied by reduced gene expression and upregulation of the transcriptional repressor PDX-1

Disaccharidases are important digestive enzymes whose activities can be reduced by iron deficiency. We hypothesise that this is due to reduced gene expression, either by impairment to enterocyte differentiation or by iron-sensitive mechanisms that regulate mRNA levels in enterocytes. Iron-deficient Wistar rats were generated by dietary means. The enzyme activities and kinetics of sucrase and lactase were tested as well as the activity of intestinal alkaline phosphatase (IAP)-II because it is unrelated to carbohydrate digestion. mRNA levels of beta-actin, sucrase, lactase, and the associated transcription factors pancreatic duodenal homeobox (PDX)-1, caudal-related homeobox (CDX)-2, GATA-binding protein (GATA)-4, and hepatocyte nuclear factor (HNF)-1 were measured by real-time PCR. Spatial patterns of protein and gene expression were assessed by immunofluorescence and in situ hybridization, respectively. It was found that iron-deficient rats had significantly lower sucrase (19.5% lower) and lactase (56.8% lower) but not IAP-II activity than control rats. Kinetic properties of both enzymes remained unchanged from controls, suggesting a decrease in the quantity of enzyme present. Sucrase and lactase mRNA levels were reduced by 44.5% and 67.9%, respectively, by iron deficiency, suggesting that enzyme activity is controlled primarily by gene expression. Iron deficiency did not affect the pattern of protein and gene expression along the crypt to villus axis. Expression of PDX-1, a repressor of sucrase and lactase promoters, was 4.5-fold higher in iron deficiency, whereas CDX-2, GATA-4, and HNF-1 levels were not significantly different. These data suggest that decreases in sucrase and lactase activities result from a reduction in gene expression, following from increased levels of the transcriptional repressor PDX-1.

Cdx binding determines the timing of enhancer activation in postnatal duodenum

In mammalian intestine, adenosine deaminase (ADA) is expressed at high levels only along the villi of the duodenal epithelium. A duodenum-specific enhancer identified in the second intron of the human ADA gene controls this pattern of expression. This enhancer faithfully recapitulates this expression pattern in transgenic mice, when included in CAT reporter gene constructions. Multiple binding sites for PDX-1 and GATA factors were previously identified within the approximately 300-bp region that encompasses the enhancer. Mutation analyses demonstrated that binding of PDX-1 and of GATA-4 was absolutely essential for enhancer function. In the present study, we have identified additional enhancer binding sites for Cdx factors, for YY1, and for NFI family members. Detailed EMSA studies were used to confirm binding at these sites. This brings the number of confirmed binding sites within the enhancer to thirteen, with five different factors or family of factors contributing to the putative enhanceosome complex. Mutation analysis was utilized to examine the specific roles of the newly identified sites. Two sites were identified that bound both Cdx1 and Cdx2. Mutations were identified in these two sites that completely and specifically eliminated Cdx binding. In transgenic mice, these enhancer mutations dramatically changed the developmental timing of enhancer activation (delaying it by 2-3 weeks) without affecting other aspects of enhancer function. In the chromatin context of certain transgenic insertion sites, mutation of the two YY1 sites to specifically ablate binding caused a delay in enhancer activation similar to that observed with the Cdx mutations. No overt changes were observed from mutation of the NFI site.

Transcriptional regulation of the lactase-phlorizin hydrolase promoter by PDX-1

Lactase-phlorizin hydrolase gene expression is spatially restricted along the anterior-posterior gut axis. Lactase gene transcription is maximal in the distal duodenum and jejunum in adult mammals and is barely detectable in the proximal duodenum. By contrast, pancreatic duodenal homeobox-1 (PDX-1) protein is expressed maximally in the proximal duodenum. This study aimed to determine the role of PDX-1 in regulating lactase gene promoter activity in intestinal epithelial cells. Caco-2 cells were cotransfected with lactase promoter-reporter constructs in the presence of a PDX-1 expression vector and assayed for luciferase activity. PDX-1 cotransfection results in repression of lactase promoter activity. Sequence analysis of the lactase promoter revealed a putative PDX-1 DNA binding site in the proximal 100-bp lactase gene promoter. EMSAs demonstrated that PDX-1 can interact with the lactase promoter binding site but not with a site in which the core PDX-1 binding sequence TAAT is mutated. Site-directed mutagenesis of the PDX-1 core binding site in the lactase promoter-reporter construct suggests that PDX-1 can function independently of DNA binding to its consensus binding site. Stable overexpression of PDX-1 results in repression of the endogenous human lactase gene in differentiated Caco-2 cells. Given the contrasting spatial expression pattern, PDX-1 may function to specify the anterior boundary of lactase expression in the small intestine and is thus a candidate regulator of anterior spatial restriction in the gut.

Iron supplementation during infancy—effects on expression of iron transporters, iron absorption, and iron utilization in rat pups

BACKGROUND

Studies conducted in human infants suggest developmental changes in the regulation of iron absorption; however, little is known about the molecular mechanisms regulating iron absorption during infancy. Two intestinal iron transporters, divalent metal transporter 1 (DMT1) and ferroportin 1 (FPN1), were recently identified.

OBJECTIVE

The objective was to investigate at a molecular level the regulation of iron absorption during infancy in a rat pup model. We examined the developmental expression of DMT1 and FPN1 and the effects of iron supplementation on their expression and on iron absorption and utilization during infancy.

DESIGN

Rat pups were given daily oral doses of 0, 30, or 150 microg Fe from day 2 to day 20 after birth. On days 10 and 20 after birth, (59)Fe absorption, tissue minerals, and intestinal DMT1, FPN1, and ferritin expression were examined. To assess developmental expression, DMT1 and FPN1 were examined in control rats from days 1 to 50 after birth.

RESULTS

Intestinal DMT1 and FPN1 were significantly affected by age; expression increased dramatically by day 40. On day 10, no significant effect of iron supplementation on DMT1 and FPN1 gene expression or on iron absorption was observed. By day 20, DMT1 and FPN1 expression and iron absorption had decreased significantly with iron supplementation.

CONCLUSIONS

During early infancy, rat pups are unable to down-regulate intestinal iron transporters or iron absorption in response to iron supplementation, whereas down-regulation occurs during late infancy. The current findings provide evidence of the developmental regulation of iron absorption, which emphasizes the need for caution when giving iron supplements to infants at an early age.

Pancreatic-duodenal homeobox 1 regulates expression of liver receptor homolog 1 during pancreas development

Liver receptor homolog 1 (LRH-1) and pancreatic-duodenal homeobox 1 (PDX-1) are coexpressed in the pancreas during mouse embryonic development. Analysis of the regulatory region of the human LRH-1 gene demonstrated the presence of three functional binding sites for PDX-1. Electrophoretic mobility shift assays and chromatin immunoprecipitation analysis showed that PDX-1 bound to the LRH-1 promoter, both in cultured cells in vitro and during pancreatic development in vivo. Retroviral expression of PDX-1 in pancreatic cells induced the transcription of LRH-1, whereas reduced PDX-1 levels by RNA interference attenuated its expression. Consistent with direct regulation of LRH-1 expression by PDX-1, PDX-1(-/-) mice expressed smaller amounts of LRH-1 mRNA in the embryonic pancreas. Taken together, our data indicate that PDX-1 controls LRH-1 expression and identify LRH-1 as a novel downstream target in the PDX-1 regulatory cascade governing pancreatic development, differentiation, and function.

High-level activation by a duodenum-specific enhancer requires functional GATA binding sites

The purine metabolic gene adenosine deaminase (ADA) is expressed at high levels in a well-defined spatiotemporal pattern in the villous epithelium of proximal small intestine. A duodenum-specific enhancer module responsible for this expression pattern has been identified in the second intron of the human ADA gene. It has previously been shown that binding of the factor PDX-1 is essential for function of this enhancer. The studies presented here examine the proposed roles of GATA factors in the enhancer. Site-directed mutagenesis of the enhancer's GATA binding sites crippled enhancer function in 10 lines of transgenic mice, with 9 of the lines demonstrating <1% of normal activity. Detailed studies along the longitudinal axis of mouse small intestine indicate that GATA-4 and GATA-5 mRNA levels display a reciprocal pattern, with low levels of GATA-6 throughout. Interestingly, gel shift studies with duodenal nuclear extracts showed binding only by GATA-4.

Intron 1 is required for cell type-specific, but not injury-responsive, peripherin gene expression

The "primitive" neurons of the peripheral nervous system (PNS) have the remarkable ability to regenerate new fibers. This regenerative process requires a sequence of gene activation and repression that is poorly understood. One gene that is almost exclusively expressed in neurons of the PNS and is activated after nerve injury is the peripherin intermediate filament gene, but little is known about the genomic elements that control either its restricted expression or its response to nerve injury in adult mice. Previous studies suggested that both 5' flanking sequence and intragenic regions were required for cell type-specific and injury-specific expression. To determine which intragenic regions were critical, mice were generated that expressed peripherin transgenes lacking different introns. Analyses of these mice revealed that deletion of introns 2-8 had no effect on either the cell type-specific or injury-specific expression of the peripherin gene; however, the remaining intron, intron 1, differentially bound Sp1 transcription-related proteins/protein complexes in extracts from peripherin-expressing and nonexpressing tissues. Furthermore, a transgene that lacked intron 1 was not expressed in many neurons that contain endogenous peripherin but was activated after injury. Thus, accurate cell type-specific peripherin gene expression in the PNS depends on elements within intron 1, but other sequences, most likely in the 5'flanking region, are required for activating the peripherin gene in response to nerve injury.

Cis elements of the villin gene control expression in restricted domains of the vertical (crypt) and horizontal (duodenum, cecum) axes of the intestine

Villin, an actin bundling protein found in the apical brush border of absorptive tissues, is one of the first structural genes to be transcriptionally activated in the embryonic intestinal endoderm. In the adult, villin is broadly expressed in every cell of the intestinal epithelium on both the vertical axis (crypt to villus tip) and the horizontal axis (duodenum through colon) of the intestine. Here, we document that a 12.4-kilobase region of the mouse villin gene drives high level expression of two different reporter genes (LacZ and Cre recombinase) within the entire intestinal epithelium of transgenic mice. Deletion of a portion of this transgene results in reduction of beta-galactosidase activity in restricted domains of the small intestine (duodenum) and large intestine (cecum). In addition, expression is reduced in the crypt compartment throughout the intestine. Thus, the global expression pattern of villin in the intestine is apparently the consequence of an amalgam of distinct and individual domain-specific control processes. That is, expression of villin in the duodenum and cecum requires different regulatory sequences than the rest of the intestine, and the expression of villin in crypts is regulated by different circuitry than expression of villin on villus tips.

Bile acids regulate the ontogenic expression of ileal bile acid binding protein in the rat via the farnesoid X receptor

BACKGROUND & AIMS

In the rat, an increase in ileal bile acid binding protein (IBABP) expression occurs during the third postnatal week. In vitro studies suggest that bile acids (BAs) increase IBABP transcription by activating the BA receptor, farnesoid X receptor (FXR). Thus, we investigated the role of BAs on the ontogenic expression of IBABP and whether FXR may mediate these effects.

METHODS

Suckling rats were gavage-fed taurocholate for 3 days or were allowed to develop normally. Ileums were collected for Northern and Western blot analyses. Electrophoretic mobility shift assays for functional FXR were performed using nuclear extracts from ileums of both adult and developing rats.

RESULTS

Taurocholate gavage significantly elevated IBABP messenger RNA and protein levels in suckling animals. Gelshift assays using adult ileal nuclear extracts incubated with a radiolabeled consensus inverted repeat-1 oligonucleotide (response element for FXR) revealed a high-molecular weight DNA/protein complex. Cold competition and supershift assays showed that this complex is sequence specific and confirmed that FXR is a component of the complex. Gelshift assays with nuclear extracts from rat ileum at different ages revealed absence of the DNA/protein complex in the second postnatal week when there is lack of IBABP expression and presence of these complexes at later ages when there is normally high expression. Western blot analyses showed FXR and its heterodimer partner, retinoid X receptor alpha, protein levels are low in the ileum during the suckling period and increase during the third postnatal week.

CONCLUSIONS

BAs play a role in the normal developmental expression of IBABP through FXR activation, and decreased functional FXR in ileal nuclei during the suckling period may account, in part, for the lack of IBABP expression at this time.

New advances in the molecular and cellular biology of the small intestine

Research into the structure and function of the small intestinal mucosa is becoming increasingly focused on the molecular and cellular biology of this fascinating tissue. There is a growing understanding of the factors determining the expression of specific genes at different stages of development and differentiation in the multiple cell types, and several important transcription factors have emerged. Recent publications have included studies of the effects of commensal bacteria on gene expression and the molecules producing apoptosis. Mechanisms of the intestinal adaptation to injury or surgery involve a number of hormones; current research has shown a major role for glucagon-like peptide 2.