Jun Is Required in Isl1-Expressing Progenitor Cells for Cardiovascular Development

Jun is a highly conserved member of the multimeric activator protein 1 transcription factor complex and plays an important role in human cancer where it is known to be critical for proliferation, cell cycle regulation, differentiation, and cell death. All of these biological functions are also crucial for embryonic development. Although all Jun null mouse embryos die at mid-gestation with persistent truncus arteriosus, a severe cardiac outflow tract defect also seen in human congenital heart disease, the developmental mechanisms are poorly understood. Here we show that murine Jun is expressed in a restricted pattern in several cell populations important for cardiovascular development, including the second heart field, pharyngeal endoderm, outflow tract and atrioventricular endocardial cushions and post-migratory neural crest derivatives. Several genes, including Isl1, molecularly mark the second heart field. Isl1 lineages include myocardium, smooth muscle, neural crest, endocardium, and endothelium. We demonstrate that conditional knockout mouse embryos lacking Jun in Isl1-expressing progenitors display ventricular septal defects, double outlet right ventricle, semilunar valve hyperplasia and aortic arch artery patterning defects. In contrast, we show that conditional deletion of Jun in Tie2-expressing endothelial and endocardial precursors does not result in aortic arch artery patterning defects or embryonic death, but does result in ventricular septal defects and a low incidence of semilunar valve defects, atrioventricular valve defects and double outlet right ventricle. Our results demonstrate that Jun is required in Isl1-expressing progenitors and, to a lesser extent, in endothelial cells and endothelial-derived endocardium for cardiovascular development but is dispensable in both cell types for embryonic survival. These data provide a cellular framework for understanding the role of Jun in the pathogenesis of congenital heart disease.


Introduction
Jun (a.k.a. c-Jun) is a highly conserved member of the multimeric activator protein 1 (AP-1) transcription factor complex [1]. The AP-1 protein complexes are a heterogeneous group of transcriptionally active dimers and include members of the Jun family (Jun, Jund, Junb), Fos family (c-Fos, Fosb, Fosl1, Fosl2) and other transcription factor families such as ATF and Maf [2]. Several Jun mutant mice have been generated to study AP-1 function. While Jun heterozygous mice are normal [3], all Jun null embryos die between E12.5 and E14.5 with persistent truncus arteriosus (PTA) [3,4,5]. PTA is a severe developmental cardiac abnormality seen in many patients as an isolated finding or as part of a syndrome such as DiGeorge/22q11 deletion syndrome. Jun proteins can form homo-or heterodimers to differentially regulate transcription [1]. Examination of the promiscuity of these dimer protein-protein interactions has revealed that as part of a DNAbinding complex, Jun is critical for multiple biological processes including cell proliferation, apoptosis, cell cycle progression and differentiation [6,7,8,9]. Although these cellular phenomena are critical for mammalian development and for diseases such as cancer, data regarding the role of Jun during embryogenesis is limited.
The cardiac outflow tract (OFT) incorporates the lineages of multiple cardiac progenitors and its development is dependent upon the complex interaction of several cell types. Neural crest (NC) cells migrate from the dorsal neural tube to the developing aorticopulmonary septation complex to mediate septation of the truncus arteriosus into the main pulmonary artery and aorta [10]. These NC cells contribute to the OFT endocardial cushion mesenchyme which is also comprised of endothelial-derived endocardial cells [11]. Second heart field (SHF) progenitors contribute to the OFT myocardium and smooth muscle [12,13] while endothelial progenitors give rise to the mature endothelial cells and semilunar valves of the OFT [14,15]. Defects seen in Jun null embryos are striking and may be mediated by Jun function in one or more of these cell populations involved in OFT development. Here we show that murine Jun is expressed in a restricted pattern in several cell populations important for cardiovascular development, including the SHF, pharyngeal endoderm, OFT endocardial cushions, atrioventricular (AV) endocardial cushions and post-migratory NC derivatives. Using tissue-specific conditional deletion studies in mice, we demonstrate that Jun is required in Isl1-expressing progenitors and, to a lesser extent, in endothelial cells and endothelial-derived endocardium for cardiovascular development but is dispensable in both cell types for embryonic survival.

Jun is Detected during Mid-gestation in Restricted Cell Populations
Several cell populations are important for normal OFT development and septation. The 100% incidence of PTA in Jun null embryos indicates that Jun is clearly required in one or more of these cell populations. An overview of Jun's spatial and temporal expression pattern during embryonic development in the mouse is lacking in the literature, particularly prior to E14.5. In limited expression analyses by in situ hybridization and Northern blot, it has been reported that Jun mRNA is expressed in the developing heart, cartilage, gut, central nervous system, lung, kidney, adrenal gland and placenta of the developing mouse [16,17,18,19,20]. To determine the specific cell populations in which Jun might be functioning to regulate cardiac morphogenesis, we examined the expression of Jun by in situ hybridization and immunohistochemistry at several stages of embryonic development between E8.5 and E15.5. Our Jun expression analysis revealed expression in multiple tissues important for heart development and aortic arch artery remodeling. At E8.5, Jun was expressed in the pharyngeal endoderm, dorsal aortae, common atrial chamber, endocardial cushions and in regions populated by SHF mesoderm (Fig. 1A). The anterior SHF expression was stronger than the posterior SHF (Fig. 1A). The expression of Jun in the SHF was also evident at E9.5 by whole mount in situ hybridization (Fig. 1B, C). This is consistent with our previous observation of Jun expression in SHFderived OFT myocardium [21]. At E9.5, Jun was expressed in the otic vesicle, telencephalon, somites, and aortic arch arteries (Fig. 1B, C). The expression in the telencephalon, somites and pharyngeal arches is consistent with publically available in situ hybridization data at E11 (http://goo.gl/DoJro) [22]. At E10.5, Jun was highly expressed in the OFT endocardial cushions, AV endocardial cushions and cranial nerve IX (Fig. 1D). The high levels of Jun expression in the OFT endocardial cushions persists until E11.5 (Fig. 1E), where expression in pericardium (Fig. 1E) and dorsal root ganglia (data not shown) was also evident. At E15.5, Jun was broadly expressed in the myocardium and both the semilunar and AV valves (Fig. S1).

Conditional Deletion of Jun in Isl1-expressing Progenitors Results in Severe Cardiovascular Malformations
Jun is expressed in the SHF (Fig. 1) and SHF-derived myocardium (Fig. S1) [21]. The SHF comprises a specialized subset of cardiac progenitor cells derived from early splanchnic mesoderm [13]. These progenitors are marked by the expression of genes such as Isl1 [13,23,24] and play a critical role in the development of the right ventricle and OFT. To determine if Jun is expressed in Isl1-positive cells, we examined the expression of Jun and Isl1 at E8.5 using immunohistochemistry. Our analysis using confocal microscopy revealed co-localization of Jun and Isl1 in the anterior SHF mesoderm ( Fig. 2A, C-F) and pharyngeal endoderm ( Fig. 2A, B).
Isl1 Cre/+ ; Jun flox/flox embryos were present at the expected Mendelian ratios (E14.5-P0, n = 23, 100% of predicted; Table 1) and thus survived longer than Jun null embryos [3,4,5]. This suggests that Jun function in Isl1-expressing cells is not responsible for the embryonic mortality seen in Jun null embryos and supports the hypothesis of Hilberg and Eferl et al. that the embryonic death in Jun nulls is attributable to impaired hepatogenesis [3,5] rather to the cardiac defects. We examined E14.5 to P0 Isl1 Cre/+ ; Jun flox/ flox and control embryos for cardiac defects. Upon careful examination, we found that 32% of Isl1 Cre/+ ; Jun flox/flox conditional mutant embryos (n = 22) showed aortic arch artery remodeling defects (Fig. 4B, B9; Table 2). These defects included interrupted aortic arch (IAA) type B, hypoplasia of the B segment of the aortic arch and aberrant retro-esophageal right subclavian artery. OFT defects were observed in 88% of conditional mutant embryos (n = 8) and included ventricular septal defect (VSD), double outlet right ventricle, and semilunar valve hyperplasia (Fig. 4F, K-N; Table 2). Although there is some evidence that Isl1 progenitors may contribute to the mature AV valves [35], we observed very few GFP-expressing cells derived from Isl1expressing progenitors cells in the developing AV cushions (Fig. 3A, F). Consistent with this observation, we did not observe any AV valve defects in Isl1 Cre/+ ; Jun flox/flox embryos (Fig. 4G, H; Table 2).
Because Isl1 Cre/+ is a loss-of-function allele [25], we looked for evidence of a genetic interaction between Isl1 and Jun. Neither Isl1 +/2 nor Isl1 Cre/+ mice have discernible cardiac defects [23,25,36], thus heterozygosity of Isl1 from the Cre knock-in could not account for the phenotype in Isl1 Cre/+ ; Jun flox/flox mutants. We observed a single Isl1 Cre/+ ; Jun flox/+ embryo (1 of 22) with IAA type B. No other OFT or aortic arch abnormalities were noted among Isl1 Cre/+ ; Jun flox/+ embryos. While this suggests a subtle genetic interaction, it does not account for the significantly higher incidence of defects observed in Isl1 Cre/+ ; Jun flox/flox embryos.
Given Jun's role in proliferation and apoptosis [6,7], we investigated whether an alteration in one or both of these cellular processes may contribute to the OFT defects observed in Jun mutant mice. Using phospho-histone H3 (pHH3) and TUNEL as markers of cell proliferation and apoptosis, we could not detect any statistically significant differences in the percent of pHH3-or TUNEL-positive cells in serial sections of the OFT of E10.5 Isl1 Cre/+ ; Jun flox/flox , Isl1 Cre/+ ; Jun flox/2 and control littermates ( Fig. 5A-E). Although the differences were not statistically significant, there was a trend toward a decrease in proliferating cells and an increase in apoptosis in the OFT of mutant embryos. These findings suggest that the observed OFT defects may not be solely the result of altered proliferation or apoptosis, but raises the possibility that in combination, these alterations could contribute to the phenotype.
Endothelial progenitors, contributing to the developing OFT endocardial cushion mesenchyme, and cardiac NC cells play an important role in the development of the OFT [10,11,14,15]. We used NFATc1 as an endocardial marker [14,37] and Tfap2a (a.k.a. AP-2a) as a marker of migrating NC cells (and ectoderm) [36,38] to determine if conditional deletion of Jun in Isl1expressing progenitors affected the developing endocardium or NC cells. We could not detect any significant differences in NFATc1 or Tfap2a expression in E10.5 Isl1 Cre/+ ; Jun flox/2 embryos compared with control littermates (Fig. 5F-I). These findings suggest that the defects observed in Isl1 conditional Jun mutant embryos are not attributable to a failure of endocardial formation or cardiac NC migration.

Jun is Required in the Endocardium and Endothelial Progenitors for Valve and Ventricular Septum Formation but not for Aortic Arch Artery Remodeling
Although Isl1-expressing progenitors populate the SHF mesoderm, Isl1 lineages also include other cell populations such as the endocardium and scattered endothelial cells in the aortic arch arteries [13,23,24,26,39]. Endothelial cells, giving rise to the endocardium, undergo an epithelial-to-mesenchymal transforma-tion to contribute to the OFT cushion mesenchymal tissue during OFT septation [11]. The endocardial cushions subsequently give rise to the valves of the mature heart [14,15]. Our finding that Jun was strongly expressed in the OFT endocardial cushion mesenchyme ( Fig. 1D, E) together with the prominent endocardial cushions noted in Jun null embryos [5] suggests that Jun may be required in the endothelial progenitors giving rise to endocardial cushion mesenchyme. Hence, we performed fate-mapping studies of endothelial cells in Jun mutant embryos. Endothelial cells and endothelial-derived endocardium were marked with ß-galactosidase or GFP expression by crossing transgenic Tie2-Cre mice with R26R or mT/mG Cre reporter mice [34,40,41]. Tie2-Cre mice have been extensively characterized and shown to drive expression specifically in the embryonic endothelium and endocardium as early as E7.5 [41]. In global Jun nulls at E10.5 (Fig. 6D, E), and in endothelial-specific conditional mutants at E15.5 (Fig. 6F), we observed the contribution of Tie2-expressing endothelial progenitors to the endothelial cushion mesenchyme and endothelial lining of the truncus arteriosus and aortic sac and to the semilunar valves in a pattern similar to control embryos ( Fig. 6A-C). Although these fate mapping experiments suggest that Jun is not required in Tie2-expressing endothelial progenitors, it remains possible that the loss of Jun in the endocardium results in a functional defect resulting in cardiac defects or aortic arch artery remodeling defects.
To determine if Jun was required for OFT development and aortic arch artery remodeling, we performed a conditional deletion of Jun in endothelium and endothelial-derived endocardium by crossing Jun flox/flox mice with transgenic Tie2-Cre mice. To ensure that Tie2-Cre was efficiently deleting Jun in endothelial progenitors we performed co-immunostaining for Jun, the endothelial marker, CD31 (Pecam1) and the smooth muscle cell marker, a-smooth We analyzed embryonic and neonatal Tie2-Cre; Jun flox/flox mutants for a recapitulation of the cardiac phenotype observed in the Jun null and Isl1 conditional Jun mutant embryos. We quantified the incidence of embryonic death, aortic arch remodeling defects, and cardiac defects and found that, similar to the Isl1 Cre/+ ; Jun flox/flox embryos, the endothelial-specific knockouts of Jun survive until late gestation or even until birth. Tie2-Cre; Jun flox/flox embryos were present at the expected Mendelian ratios (E15.5-P0; Table 1). We examined E15.5 to P0 Tie2-Cre; Jun flox/flox and control embryos for cardiac defects. Upon careful examination, we did not observe any Tie2-Cre; Jun flox/flox conditional mutant embryos (n = 10) with aortic arch artery remodeling defects ( Table 2). 43% of conditional mutant embryos had a perimembranous VSD and 14% had double outlet right ventricle, mitral or pulmonary valve hyperplasia (Fig. 8A, C; Table 2). We also noted thinning of the compact myocardium of the right ventricle (Fig. 8A) in 43% (n = 3/7) of Tie2-Cre; Jun flox/flox embryos. This phenotype is similar to that previously described in the global Jun null embryos [5]. To determine if these defects were due to a cell non-autonomous effect on NC cell migration, we performed Tfap2a immunostaining of Tie2-Cre conditional Jun mutant and control embryos. We observed a similar pattern of Tfap2a expression in the developing OFT and pharyngeal arches in E10.5 Tie2-Cre; Jun flox/2 embryos compared with controls ( Fig. 6G-J) suggesting that defects observed in Tie2 conditional Jun mutant embryos are not attributable to a failure of cardiac NC cell migration. Thus, Jun is not required in endothelial cells and endothelial-derived endocardial cushions for aortic arch artery remodeling or embryonic survival, but is required for the formation of the ventricular septum, compact myocardium and to a lesser extent for formation of the OFT and valves.

Discussion
Congenital heart disease (CHD) is the most commonly occurring major birth defect in humans with an incidence of 6 per 1000 live births [42]. OFT malformations, including PTA, constitute the largest class of life-threatening CHD. It has been more than a decade since it was recognized that the global loss of Jun in mice uniformly results in PTA [5]. Information regarding Jun expression in cardiac progenitors during critical stages of heart development has been poorly described and thus it has remained unclear in what cell populations Jun may regulate transcription. We show that Jun is expressed in a restricted pattern in several cell populations including the SHF, pharyngeal endoderm, OFT and AV endocardial cushions and post-migratory NC derivatives such as cranial nerve IX and the dorsal root ganglia.
Although Jun null embryos die with PTA, we did not observe PTA in conditional Jun mutants. There are at least two explanations for this finding. There may be functional redundancy among Jun proteins and/or Jun may be required in other cardiac progenitors not expressing Isl1. There is evidence to suggest some degree of functionally redundancy among Jun proteins, such as Jun, Junb and Jund, during heart development. Jund 2/2 mice have no cardiac defects [43] and Junb 2/2 null embryos die at E8.5-E10 due to multiple defects in extra-embryonic tissues [44]. Wagner et al. have generated mice in which either Junb or Jund is knocked-in to the Jun locus to test whether Junb or Jund, under the control of the endogenous Jun regulatory elements, can rescue the Jun null phenotype [20,45]. While both of these knock-in mice rescue the mortality at E13 seen in Jun 2/2 embryos [20,45], the rescue of the cardiac defects is not as straightforward. Jun Junb/Junb embryos survive to E18.5 in Mendelian ratios but continue to have PTA (similar to Jun 2/2 embryos [20]) and VSDs. The same authors then tested whether overexpression of Junb, with Junb transgenic mice under the control of human ubiquitin C promoter [46] were able to rescue the Jun null phenotype. Junb-Tg; Jun 2/2 embryos are born with no cardiac defects demonstrating redundancy that is dependent on gene dosage [20]. Jun Jund/Jund embryos survive to E18.5 in Mendelian ratios but also display PTA similar to Jun 2/2 embryos [45], suggesting that Jund is not functionally redundant for Jun during cardiac development. The rescue of embryonic lethality but not heart defects indicates that different developmental processes have different sensitivities to Jun dosage. It is unclear whether Jund overexpression can rescue the cardiac defects see in Jun null embryos. Future studies to test whether decreasing the dosage of Jund or Junb will uncover functional redundancy during heart development may include the analysis of heart defects in Isl1 Cre/+ ; Jun flox/flox ; Junb +/2 or Isl1 Cre/ + ; Jun flox/flox ; Jund +/2 mutants.
Jun transcriptional activity is dependent on Jun N-terminal kinase (JNK) phosphorylation [47,48]. This JNK-dependent Jun phosphorylation is dispensable for embryonic development, including for cardiogenesis. Behrens et al. demonstrated this by generating mice in which the Jun locus was mutated to prevent JNK phosphorylation (JunAA) [49]. JunAA homozygous mice, unlike Jun 2/2 mice, are born in Mendelian ratios and are healthy and fertile, without heart defects, as adults. The notion that JNKdependent phosphorylation is not essential for heart development is further supported by the observation that Jnk mutant mice survive without heart defects [50].
An alternative explanation for the PTA seen in Jun null embryos but not in the conditional mutants is that Jun may have roles in multiple cell populations involved in regulating OFT septation. The proximal portion of the OFT cushions (conus) becomes the subpulmonary infundibulum and failure of this structure to form properly results in VSDs. The distal OFT cushions (truncus) gives rise to the semilunar valves and intrapericardial portions of the aorta and pulmonary artery. More distally, the dorsal wall of the aortic sac becomes the aorticopulmonary septum. In humans, defects in this structure result in an aorticopulmonary window [51,52]. A failure of septation in both the distal OFT and the aortic sac-derived portion of the great arteries results in PTA which usually, but not always [53], is associated with VSDs. The PTA in Jun null embryos [5] together with our observation of VSDs in the absence of both PTA and a common truncal valve, supports a model in which ''conal'' septation, regulated by Jun in Isl1-progenitors, is mechanistically separate from both ''truncal'' and aorticopulmonary septation. Jun may play a role in a non-Isl1expressing domain to regulate ''truncal'' and aorticopulmonary septation.
Heart development is dependent upon the complex interaction and contribution of several cell types. There are multiple lines of evidence supporting the notion that within the early mesoderm, there is a common cardiovascular progenitor that gives rise to myocardial, smooth muscle and endothelial lineages [54]. Within the splanchnic mesoderm, the SHF, a group of cardiovascular precursors destined to give rise to the right ventricle and OFT, is molecularly marked by the expression of Tbx1, a Mef2c regulatory module, and by Isl1 [13,23,55,56,57]. Data from both murine models and from humans suggest that ISL1 plays a role in heart development and possibly human CHD. There is a report of a diabetic patient who harbors an ISL1 mutation [58], but there have been none described in patients with CHD. Despite this, there is evidence to suggest that common ISL1 single nucleotide polymorphisms (SNPs) are associated with human CHD [59]. It remains to be determined if these SNPs are causative or if they are linked to an associated causative locus. The current study provides data to support a possible role for JUN in ISL1 progenitors and thus adds to the likelihood that this association may be causative. Multipotent Isl1-positive progenitors, when isolated and cultured from either mouse or human embryonic hearts, are capable of differentiating into each of these three lineages [24,60]. This has also been demonstrated using a potentially overlapping population of Nkx2.5-positive progenitor cells [61]. This ex vivo data is consistent with in vivo fate-mapping studies, using an Isl1 inducible Cre, showing the contribution of Isl1 derivatives to the same three lineages [28]. Recent evidence suggests that some Isl1-expressing  Table 1. Summary of genotypes for Jun conditional mutants. progenitor cells may also be of a NC lineage (discussed below) [62].
Our analysis of mutants lacking Jun in the endothelial lineage reveals that Jun is required for heart development in some, but not all, Tie2-derived endothelial cells and endothelial-derived endo-   cardium. The thinning of the compact myocardium of the right ventricle observed in the Tie2 conditional Jun mutants suggests a model in which there is signaling from the endocardium to regulate the differentiation of primitive myocardial epithelium into compact myocardium. Pathways involved in reciprocal paracrine signaling between the endocardium and myocardium include Neuregulin-1, EphrinB2, Notch, Neurofibromin 1, VEGF, Angiopoietin-1, and Fgf [63,64,65,66,67]. The role of endocardial Jun in one of these pathways or in a parallel pathway remains to be determined. It is intriguing that there was no compact zone thinning of the left ventricle suggesting that endocardial Jun is not required for the differentiation of all primitive myocardial epithelium. Our findings of VSDs in both Isl1 and Tie2 conditional Jun mutants also support this paracrine model although we cannot conclude from our data whether or not the critical subset of Tie2derived cells is descended from Isl1-positive progenitors. Currently available techniques do not allow us to conditionally delete Jun in Tie2/Isl1-positive progenitors in vivo while leaving Jun unaltered in Tie2-positive Isl1-negative progenitors. Thus, it remains to be tested if Jun is dispensable in the Isl1-negative endothelial lineage. Alternatively, the VSDs we observed may be a common phenotype resulting from independent requirement for Jun in both Isl1-negative endothelial/endocardial cells and Tie2-negative Isl1 progenitors. Our finding of aortic arch artery patterning defects in Isl1specific Jun deleted embryos raises the possibility of a cell autonomous effect of Jun in an Isl1-derived smooth muscle lineage or alternatively a cell non-autonomous effect on another cell population such as NC. There are other examples of genetic alterations in the SHF affecting a tissue-tissue interaction with NC cells. We have shown that deletion of Notch in the SHF using Isl1 Cre or Mef2c-AHF-Cre results in severe NC-related cardiac defects including PTA and IAA [36], defects identical to those seen in our Isl1 Cre/+ ; Jun flox/flox embryos and in Jun null embryos [5]. Further tissue-restricted deletion studies are required to determine the relative requirement for Jun specifically in the smooth muscle or myocardial lineages.
Our expression analysis indicates that Jun is expressed in postmigratory NC derivatives. NC cells are a specialized subset of neuroepithelial cells in the dorsal neural tube that migrate ventrally and contribute to a diverse array of tissues. Raivich et al. have previously reported that Jun is expressed in neuroepithelial cells [68] yet deletion of Jun in neuroepithelial cells with Nestin-Cre did not result in congenital heart defects [30,69]. It is unclear if Nestin-Cre is expressed in a cardiac NC subset of neuroepithelial cells. Cardiac NC cells, originating between the mid-otic placode and the third somite, invade the pharyngeal arches and encompass the aortic arch arteries around E10. By E10.5 they populate the cardiac OFT as two columns of cells subsequently forming a portion of the mesenchymal tissue in the OFT endocardial cushions. Notably, these cushions are abnormal in Jun null embryos [5]. Ultimately, descendants of this subset of NC cells contribute to the aorticopulmonary septum, dividing the truncus arteriosus into the aorta and pulmonary artery [10,70,71]. Although there is evidence that the complete loss of Jun does not globally affect the fate of NC cells, by E12.5 there are fewer connexin43-labeled cardiac NC cells populating the right ventricular OFT of Jun null embryos [5]. In contrast, our findings suggest that the defects observed in Isl1 and Tie2 conditional Jun mutant embryos are not attributable to a cell non-autonomous defect of cardiac NC migration. In the global Jun null embryo, it is unknown whether the fewer connexin43-labeled cells is due to a secondary cell non-autonomous mechanism affecting NC as the result of a loss of Jun function in another tissue such as SHF or pharyngeal endoderm. This is further complicated by the recent observation that some Isl1-expressing progenitor cells may be of a NC lineage [62]. This novel and surprising data raises the possibility that the defects observed in Isl1-conditional Jun mutants may reflect a cell autonomous defect of NC. The effect of conditional deletion of Jun in Isl1-expressing progenitors on NC cell differentiation was not determined in the current study but is an important area for future investigation. Tissue-restricted deletion studies using Wnt1-Cre or Pax3 Cre/+ mice [72,73] are required to determine the requirement for Jun specifically in NC derivatives.
Jun, as part of the AP-1 transcription factor complex, is a positive regulator of cell proliferation [7,8,32] and positively regulates cell cycle progression through p19, p53, cyclin D1 and cyclin A pathways [6,8,29,74,75,76,77]. It regulates the differentiation of varied cell populations such as hematopoietic cells and keratinocytes [6,9] and has also been shown to regulate apoptosis in such cells as fibroblasts, hepatocytes and neurons [6,8,78,79]. Although these pleiotropic cellular functions are likely to be important for all progenitors, our observation that Isl1and Tie2specific Jun knockouts survive embryogenesis in Mendelian ratios indicates that Jun is dispensable in these progenitors for overall embryonic survival. Though Jun is not required for formation or maintenance of the blood vessels (at least after the time of Tie2-Cre-mediated recombination), it is not completely dispensable in endothelial precursors as they display a low incidence of cardiac defects. Out data do not exclude the possibility of subtle defects in endothelial cell function in these Jun mutant embryos.
The molecular mechanisms by which Jun functions during heart development, in Isl1-expressing cells and other cells remains to be determined. In the embryonic human heart, ISL1 is strongly expressed in highly proliferative SHF-derived cell populations in the arterial and venous poles [80]. We observed a statistically insignificant trend toward a decrease in proliferating cells and an increase in apoptosis in the OFT of mutant embryos. These individual differences were modest when compared with controls, but in combination these two alterations may contribute to defects in OFT septation or complete formation of the ventricular septum. There are several transcriptional networks and signaling pathways known to regulate proliferation in the SHF. These include Nkx2.5, Wnt/ß-catenin, Notch, Fgf, Shh, Isl1, and Tbx1 signaling pathways (reviewed in [13]). TBX1 is a molecular marker of the SHF and accruing evidence points to a causative role for this transcription factor in the pathogenesis of DiGeorge syndrome (DGS). It has been proposed that the loss of TBX1 in patients with DGS results in defective proliferation in the SHF [81,82], although little mechanistic data have yet been published to support this notion. Cardiac defects observed in patients with DGS and in Tbx1 mutant mice are strikingly similar to those seen in Jun mutant mouse embryos [83,84,85] and to mice in which NC has been disrupted [10]. Despite the similarity with NC mutants and reports of a disruption in the distribution of NC-derived cells in Tbx1 nulls, Tbx1 is not expressed in NC [86]. One hypothesis is that Jun and Tbx1 could be acting in concert to regulate the proliferation of SHF progenitors and subsequently affect interactions with other tissues such as NC. Future studies are required to determine if Jun may function in a Tbx1-dependent pathway and to further elucidate cell autonomous and cell non-autonomous mechanisms of Jun function during heart development.

Ethics Statement
Animal studies were conducted in accordance with the National Institutes of Health National Research Council Guide for the Care and Use of Laboratory Animals and were approved by the Children's Hospital of Philadelphia Research Institute Institutional Animal Care and Use Committee (No. 2008-6-840; Department of Health and Human Services Animal Welfare Assurance #A3442-01).

X-Gal Staining
Whole mount embryos were stained for ß-galactosidase activity using previously described methods [87].

Confocal Microscopy
Confocal images were acquired with the Zeiss LSM 510/NLO META confocal microscope using 20x, 0.8 NA Plan-Apochromat air immersion and 63x, 1.4 NA Plan-Apochromat oil immersion objectives.

Immunohistochemistry
Immunofluorescence (IF) and horseradish peroxidase (HRP) immunostaining were performed as described previously [88] using rabbit monoclonal anti-Jun 60A8

Whole Mount in situ Hybridization
Whole mount in situ hybridization was performed as described [89] with modifications. Digoxigenin-UTP-labeled RNA probes were generated from plasmid template containing mouse Jun (IMAGE clone 3493248; GenBank accession number BC002081) by XmaI restriction digestion and transcription with T7 RNA polymerase. Embryos were fixed in 4% PFA overnight, then dehydrated in a graded series of methanol in PBST. The embryos were serially rehydrated to PBST. Embryos were permeabilized in RIPA buffer thrice for 5 minutes at room temperature. Embryos were refixed in 4% PFA/0.2% glutaraldehyde in PBS for 20 minutes, washed and hybridized overnight at 70uC in hybridization solution containing 1 mg probe/mL. Embryos were washed and hybridized with alkaline phosphatase conjugated anti-digoxigenin antibody (Roche) overnight at 4uC, washed for 24 hours and developed in BM purple (Roche).