Generation and Validation of Tissue-Specific Knockout Strains for Toxicology Research

Tissue-specific knockout mice are widely used throughout scientific research. A principle method for generating tissue-specific knockout mice is the Cre-loxP system. Here, we give a detailed description of the steps required to generate and validate tissue-specific knockout mice using the Cre-loxP system. The first protocol describes how to use gene targeting in mouse embryonic stem cells to generate mice with conditional alleles. Subsequent protocols describe how to recover Cre transgenic mice from cryopreserved sperm using in vitro fertilization and present a breeding strategy for obtaining tissue-specific knockouts. Finally, methods are provided for validating the knockout mice using PCR of genomic DNA, reverse-transcription PCR and quantitative reverse-transcription PCR of mRNA, and immunoblot analysis of proteins. © 2019 by John Wiley & Sons, Inc.

SLC30A10 transporter in the digestive system regulates brain manganese under basal conditions while brain SLC30A10 protects against neurotoxicity

The essential metal manganese becomes neurotoxic at elevated levels. Yet, the mechanisms by which brain manganese homeostasis is regulated are unclear. Loss-of-function mutations in SLC30A10, a cell surface-localized manganese efflux transporter in the brain and liver, induce familial manganese neurotoxicity. To elucidate the role of SLC30A10 in regulating brain manganese, we compared the phenotypes of whole-body and tissue-specific Slc30a10 knockout mice. Surprisingly, unlike whole-body knockouts, brain manganese levels were unaltered in pan-neuronal/glial Slc30a10 knockouts under basal physiological conditions. Further, although transport into bile is a major route of manganese excretion, manganese levels in the brain, blood, and liver of liver-specific Slc30a10 knockouts were only minimally elevated, suggesting that another organ compensated for loss-of-function in the liver. Additional assays revealed that SLC30A10 was also expressed in the gastrointestinal tract. In differentiated enterocytes, SLC30A10 localized to the apical/luminal domain and transported intracellular manganese to the lumen. Importantly, endoderm-specific knockouts, lacking SLC30A10 in the liver and gastrointestinal tract, had markedly elevated manganese levels in the brain, blood, and liver. Thus, under basal physiological conditions, brain manganese is regulated by activity of SLC30A10 in the liver and gastrointestinal tract, and not the brain or just the liver. Notably, however, brain manganese levels of endoderm-specific knockouts were lower than whole-body knockouts, and only whole-body knockouts exhibited manganese-induced neurobehavioral defects. Moreover, after elevated exposure, pan-neuronal/glial knockouts had higher manganese levels in the basal ganglia and thalamus than controls. Therefore, when manganese levels increase, activity of SLC30A10 in the brain protects against neurotoxicity.

Hypothyroidism induced by loss of the manganese efflux transporter SLC30A10 may be explained by reduced thyroxine production

SLC30A10 and SLC39A14 are manganese efflux and influx transporters, respectively. Loss-of-function mutations in genes encoding either transporter induce hereditary manganese toxicity. Patients have elevated manganese in the blood and brain and develop neurotoxicity. Liver manganese is increased in patients lacking SLC30A10 but not SLC39A14. These organ-specific changes in manganese were recently recapitulated in knockout mice. Surprisingly, Slc30a10 knockouts also had elevated thyroid manganese and developed hypothyroidism. To determine the mechanisms of manganese-induced hypothyroidism and understand how SLC30A10 and SLC39A14 cooperatively mediate manganese detoxification, here we produced Slc39a14 single and Slc30a10/Slc39a14 double knockout mice and compared their phenotypes with that of Slc30a10 single knockouts. Compared with wild-type controls, Slc39a14 single and Slc30a10/Slc39a14 double knockouts had higher manganese levels in the blood and brain but not in the liver. In contrast, Slc30a10 single knockouts had elevated manganese levels in the liver as well as in the blood and brain. Furthermore, SLC30A10 and SLC39A14 localized to the canalicular and basolateral domains of polarized hepatic cells, respectively. Thus, transport activities of both SLC39A14 and SLC30A10 are required for hepatic manganese excretion. Compared with Slc30a10 single knockouts, Slc39a14 single and Slc30a10/Slc39a14 double knockouts had lower thyroid manganese levels and normal thyroid function. Moreover, intrathyroid thyroxine levels of Slc30a10 single knockouts were lower than those of controls. Thus, the hypothyroidism phenotype of Slc30a10 single knockouts is induced by elevated thyroid manganese, which blocks thyroxine production. These findings provide new insights into the mechanisms of manganese detoxification and manganese-induced thyroid dysfunction.

Deficiency in the manganese efflux transporter SLC30A10 induces severe hypothyroidism in mice

Manganese is an essential metal that becomes toxic at elevated levels. Loss-of-function mutations in SLC30A10, a cell-surface-localized manganese efflux transporter, cause a heritable manganese metabolism disorder resulting in elevated manganese levels and parkinsonian-like movement deficits. The underlying disease mechanisms are unclear; therefore, treatment is challenging. To understand the consequences of loss of SLC30A10 function at the organism level, we generated Slc30a10 knock-out mice. During early development, knock-outs were indistinguishable from controls. Surprisingly, however, after weaning and compared with controls, knock-out mice failed to gain weight, were smaller, and died prematurely (by ∼6-8 weeks of age). At 6 weeks, manganese levels in the brain, blood, and liver of the knock-outs were ∼20-60-fold higher than controls. Unexpectedly, histological analyses revealed that the brain and liver of the knock-outs were largely unaffected, but their thyroid exhibited extensive alterations. Because hypothyroidism leads to growth defects and premature death in mice, we assayed for changes in thyroid and pituitary hormones. At 6 weeks and compared with controls, the knock-outs had markedly reduced thyroxine levels (∼50-80%) and profoundly increased thyroid-stimulating hormone levels (∼800-1000-fold), indicating that Slc30a10 knock-out mice develop hypothyroidism. Importantly, a low-manganese diet produced lower tissue manganese levels in the knock-outs and rescued the phenotype, suggesting that manganese toxicity was the underlying cause. Our unanticipated discovery highlights the importance of determining the role of thyroid dysfunction in the onset and progression of manganese-induced disease and identifies Slc30a10 knock-out mice as a new model for studying thyroid biology.

Rfx2 is required for spermatogenesis in the mouse

RFX transcription factors are key regulators of ciliogenesis in vertebrates. In Xenopus and zebrafish embryos, knockdown of Rfx2 causes defects in neural tube closure and in left-right axis patterning. To determine the essential role of the Rfx2 gene in mammalian development, we generated Rfx2-deficient mice using an embryonic stem cell clone containing a lacZ gene trap reporter inserted into the first intron of the Rfx2 gene. We found that the Rfx2 lacZ reporter is expressed in ciliated tissues during mouse development including the node, the floor plate and the dorsal neural tube. However, mice homozygous for the Rfx2 gene trap mutation did not have defects in neural tube closure or in organ situs. The gene trap insertion appears to create a null allele as Rfx2 mRNA was not detected in Rfx2^gt/gt embryos. Although Rfx2-deficient mice do not have an obvious embryonic phenotype, we found that Rfx2^gt/gt males are infertile because of a defect in spermatid maturation at or before the round and elongating spermatid stage. Our results indicate that Rfx2 is not essential for embryonic development in the mouse but is required for spermatogenesis. genesis 53:604-611, 2015. © 2015 Wiley Periodicals, Inc.

Sfrp5 is not essential for axis formation in the mouse

Secreted frizzled related protein (Sfrp) genes encode extracellular factors that can modulate Wnt signaling. During early post-implantation mouse development Sfrp5 is expressed in the anterior visceral endoderm (AVE) and the ventral foregut endoderm. The AVE is important in anterior-posterior axis formation and the ventral foregut endoderm contributes to multiple gut tissues. Here to determine the essential role of Sfrp5 in early mouse development we generated Sfrp5-deficient mice by gene targeting. We report that Sfrp5-deficient mice are viable and fertile. To determine whether the absence of an axis phenotype might be due to genetic redundancy with Dkk1 in the AVE we generated Sfrp5;Dkk1 double mutant mice. AVE development and primitive streak formation appeared normal in Sfrp5(-/-);Dkk1(-/-) embryos. These results indicate that Sfrp5 is not essential for axis formation or foregut morphogenesis in the mouse and also imply that Sfrp5 and Dkk1 together are not essential for AVE development.

Lim 1 is required for nephric duct extension and ureteric bud morphogenesis

The nephric duct plays a central role in orchestrating the development of the mammalian urogenital system. Lim 1 is a homeobox gene required for head and urogenital development in the mouse but most Lim 1-deficient embryos die by embryonic day 10. To determine the role of Lim 1 in the development of the nephric duct, we conditionally removed Lim 1 in the nephric epithelium just after the nephric duct begins to form using a floxed allele of Lim 1 and Pax2-cre transgenic mice. We report that Lim 1 conditional knockout mice have renal hypoplasia and hydronephrosis. Developmental studies revealed that the caudal portion of the nephric duct did not reach the urogenital sinus at embryonic day 10.5, formation of the ureteric bud was delayed, the ureteric bud was smaller and branching of the ureteric bud reduced. We also found that the nephric duct was generally not maintained and extension of the Müllerian duct inhibited. Molecular analysis indicated that Pax2 was expressed normally but the expression of Wnt9b and E-cadherin in the nephric duct was markedly altered. These results suggest that Lim 1 influences nephric duct extension and ureteric bud outgrowth by regulating and or maintaining the differentiation of the nephric epithelium.

Ssdp1 regulates head morphogenesis of mouse embryos by activating the Lim1-Ldb1 complex

The transcriptional activity of LIM-homeodomain (LIM-HD) proteins is regulated by their interactions with various factors that bind to the LIM domain. We show that reduced expression of single-stranded DNA-binding protein 1 (Ssdp1), which encodes a co-factor of LIM domain interacting protein 1 (Ldb1), in the mouse mutant headshrinker (hsk) disrupts anterior head development by partially mimicking Lim1 mutants. Although the anterior visceral endoderm and the anterior definitive endoderm, which together comprise the head organizer, were able to form normally in Ssdp1(hsk/hsk) mutants, development of the prechordal plate was compromised. Head development is partially initiated in Ssdp1(hsk/hsk) mutants, but neuroectoderm tissue anterior to the midbrain-hindbrain boundary is lost, without a concomitant increase in apoptosis. Cell proliferation is globally reduced in Ssdp1(hsk/hsk) mutants, and approximately half also exhibit smaller body size, similar to the phenotype observed in Lim1 and Ldb1 mutants. We also show that Ssdp1 contains an activation domain and is able to enhance transcriptional activation through a Lim1-Ldb1 complex in transfected cells, and that Ssdp1 interacts genetically with Lim1 and Ldb1 in both head development and body growth. These results suggest that Ssdp1 regulates the development of late head organizer tissues and body growth by functioning as an essential activator component of a Lim1 complex through interaction with Ldb1.

The mammalian twisted gastrulation gene functions in foregut and craniofacial development

Extracellular modulators of cell-cell signaling control numerous aspects of organismal development. The Twisted gastrulation (Twsg1) gene product is a small, secreted cysteine-rich protein that has the unusual property of being able to either enhance or inhibit signaling by the bone morphogenetic protein (BMP) subfamily of TGF-beta type factors in a context-dependent manner. In this report, we characterize the early embryonic and skeletal phenotypes associated with loss of Twsg1 function in mice. All Twsg1 mutant mice, irrespective of genetic background, exhibit deletions of neural arches in the cervical vertebrae. In a C57BL/6 background, we also observe pronounced forebrain defects including rostral truncations, holoprosencephaly, cyclopia, as well as alterations in the first branchial arch (BA1) leading to lack of jaw (agnathia). Characterization of marker expression suggests that these defects are attributable to loss of signaling from forebrain-organizing centers including Fgf8 from the anterior neural ridge (ANR) and Shh from the prechordal plate (PrCP). In addition, we find defects in the foregut endoderm and a reduction in Hex expression, which may contribute to both the forebrain and BA1 defects.

Nodal antagonists regulate formation of the anteroposterior axis of the mouse embryo

Patterning of the mouse embryo along the anteroposterior axis during body plan development requires migration of the distal visceral endoderm (DVE) towards the future anterior side by a mechanism that has remained unknown. Here we show that Nodal signalling and the regionalization of its antagonists are required for normal migration of the DVE. Whereas Nodal signalling provides the driving force for DVE migration by stimulating the proliferation of visceral endoderm cells, the antagonists Lefty1 and Cerl determine the direction of migration by asymmetrically inhibiting Nodal activity on the future anterior side.

Requirement of Lim1 for female reproductive tract development

Lim1 encodes a LIM-class homeodomain transcription factor that is essential for head and kidney development. In the developing urogenital system, Lim1 expression has been documented in the Wolffian (mesonephric) duct, the mesonephros, metanephros and fetal gonads. Using, a Lim1 lacZ knock-in allele in mice, we identified a previously unreported urogenital tissue for Lim1 expression, the epithelium of the developing Müllerian duct that gives rise to the oviduct, uterus and upper region of the vagina of the female reproductive tract. Lim1 expression in the Müllerian duct is dynamic, corresponding to its formation and differentiation in females and regression in males. Although female Lim1-null neonates had ovaries they lacked a uterus and oviducts. A novel female mouse chimera assay was developed and revealed that Lim1 is required cell autonomously for Müllerian duct epithelium formation. These studies demonstrate an essential role for Lim1 in female reproductive tract development.

The mouse secreted frizzled-related protein 5 gene is expressed in the anterior visceral endoderm and foregut endoderm during early post-implantation development

The anterior visceral endoderm (AVE) plays an important role in anterior-posterior axis formation in the mouse. The AVE functions in part by expressing secreted factors that antagonize growth factor signaling in the proximal epiblast. Here we report that the Secreted frizzled-related protein 5 (Sfrp5) gene, which encodes a secreted factor that can antagonize Wnt signaling, is expressed in the AVE and foregut endoderm during early mouse development. At embryonic day (E) 5.5, Sfrp5 is expressed in the visceral endoderm at the distal tip region of the embryo and at E6.5 in the AVE opposite the primitive streak. In Lim1 embryos, which lack anterior neural tissue and sometimes form a secondary body axis, Sfrp5-expressing cells fail to move towards the anterior and remain at the distal tip of E6.5 embryos. When compared with Dkk1, which encodes another secreted Wnt antagonist molecule present in the visceral endoderm, Sfrp5 and Dkk1 expression overlap but Sfrp5 is expressed more broadly in the AVE. Between E7.5 and 8, Sfrp5 is expressed in the foregut endoderm underlying the cardiac mesoderm. At E8.5, Sfrp5 is expressed in the ventral foregut endoderm that gives rise to the liver. Additional domains of Sfrp5 expression occur in the dorsal neural tube and in the forebrain anterior to the optic placode. These findings identify a gene encoding a secreted Wnt antagonist that is expressed in the extraembryonic visceral endoderm and anterior definitive endoderm during axis formation and organogenesis in the mouse.

Nodal antagonists in the anterior visceral endoderm prevent the formation of multiple primitive streaks

The anterior visceral endoderm plays a pivotal role in establishing anterior-posterior polarity of the mouse embryo, but the molecular nature of the signals required remains to be determined. Here, we demonstrate that Cerberus-like(-/-);Lefty1(-/-) compound mutants can develop a primitive streak ectopically in the embryo. This defect is not rescued in chimeras containing wild-type embryonic, and Cerberus-like(-/-);Lefty1(-/-) extraembryonic, cells but is rescued in Cerberus-like(-/-); Lefty1(-/-) embryos after removal of one copy of the Nodal gene. Our findings provide support for a model whereby Cerberus-like and Lefty1 in the anterior visceral endoderm restrict primitive streak formation to the posterior end of mouse embryos by antagonizing Nodal signaling. Both antagonists are also required for proper patterning of the primitive streak.

Lim1 activity is required for intermediate mesoderm differentiation in the mouse embryo

During gastrulation and early organogenesis, Lim1 is expressed in the visceral endoderm, the anterior mesendoderm, and the lateral mesoderm that comprises the lateral plate and intermediate mesoderm. A previous study has reported that kidneys and gonads are missing in the Lim1 null mutants (W. Shawlot and R. R. Behringer, 1995, Nature 374, 425-430). Results of the present study show that in the early organogenesis stage mutant embryo, the intermediate mesoderm that contains the urogenital precursor tissues is disorganized and displays diminished expression of PAX2 and the Hoxb6-lacZ transgene. When posterior epiblast cells of the Lim1 null mutant embryo were transplanted to the primitive streak of wild-type host embryos, they were able to colonize the lateral plate and intermediate mesoderm of the host, suggesting that Lim1 activity is not essential for the allocation of epiblast cells to these mesodermal lineages. However, most of the mutant cells that colonized the lateral and intermediate mesoderm of the host embryo did not express the Hoxb6-lacZ transgene, except for some cells that were derived from the distal part of the posterior epiblast. Lim1 activity may therefore be required for the full expression of this transgene that normally marks the differentiation of the lateral plate and intermediate mesoderm.

The cerberus-related gene, Cerr1, is not essential for mouse head formation

The Xenopus cerberus gene encodes a secreted factor expressed in the Spemann organizer that can cause ectopic head formation when its mRNA is injected into Xenopus embryos. In mouse, the cerberus-related gene, Cerr1, is expressed in the anterior mesendoderm that underlies the presumptive anterior neural plate and its expression is downregulated in Lim1 headless embryos. To determine whether Cerr1 is required for head formation we generated a null mutation in Cerr1 by gene targeting in mouse embryonic stem cells. We found that head formation is normal in Cerr1(-/-) embryos and we detected no obvious phenotypic defects in adult Cerr1(-/-) mice. However, in embryonic tissue layer recombination assays, Cerr1(-/-) presomitic/somitic mesoderm, unlike Cerr1-expressing wild-type presomitic/somitic mesoderm, was unable to maintain expression of the anterior neural marker gene Otx2 in ectoderm explants. These findings suggest that establishment of anterior identity in the mouse may involve the action of multiple functionally redundant factors.

Lim1 is required in both primitive streak-derived tissues and visceral endoderm for head formation in the mouse

Lim1 is a homeobox gene expressed in the extraembryonic anterior visceral endoderm and in primitive streak-derived tissues of early mouse embryos. Mice homozygous for a targeted mutation of Lim1 lack head structures anterior to rhombomere 3 in the hindbrain. To determine in which tissues Lim1 is required for head formation and its mode of action, we have generated chimeric mouse embryos and performed tissue layer recombination explant assays. In chimeric embryos in which the visceral endoderm was composed of predominantly wild-type cells, we found that Lim1(−)(/)(−) cells were able to contribute to the anterior mesendoderm of embryonic day 7.5 chimeric embryos but that embryonic day 9.5 chimeric embryos displayed a range of head defects. In addition, early somite stage chimeras generated by injecting Lim1(−)(/)(−) embryonic stem cells into wild-type tetraploid blastocysts lacked forebrain and midbrain neural tissue. Furthermore, in explant recombination assays, anterior mesendoderm from Lim1(−)(/)(−) embryos was unable to maintain the expression of the anterior neural marker gene Otx2 in wild-type ectoderm. In complementary experiments, embryonic day 9.5 chimeric embryos in which the visceral endoderm was composed of predominantly Lim1(−)(/)(−) cells and the embryo proper of largely wild-type cells, also phenocopied the Lim1(−)(/)(−) headless phenotype. These results indicate that Lim1 is required in both primitive streak-derived tissues and visceral endoderm for head formation and that its inactivation in these tissues produces cell non-autonomous defects. We discuss a double assurance model in which Lim1 regulates sequential signaling events required for head formation in the mouse.

HNF3beta and Lim1 interact in the visceral endoderm to regulate primitive streak formation and anterior-posterior polarity in the mouse embryo

Recent embryological and genetic experiments have suggested that the anterior visceral endoderm and the anterior primitive streak of the early mouse gastrula function as head- and trunk-organising centers, respectively. Here, we report that HNF3beta and Lim1 are coexpressed in both organising centers suggesting synergistic roles of these genes in regulating organiser functions and hence axis development in the mouse embryo. To investigate this possibility, we generated compound HNF3beta and Lim1 mutant embryos. An enlarged primitive streak and a lack of axis formation were observed in HNF3beta (−)(/)(−);Lim1(−)(/)(−), but not in single homozygous mutant embryos. Chimera experiments indicate that the primary defect in these double homozygous mutants is due to loss of activity of HNF3beta and Lim1 in the visceral endoderm. Altogether, these data provide evidence that these genes function synergistically to regulate organiser activity of the anterior visceral endoderm. Moreover, HNF3beta (−)(/)(−);Lim1(−)(/)(−) mutant embryos also exhibit defects in mesoderm patterning that are likely due to lack of specification of anterior primitive streak cells.

Sequence and genomic organization of the mouse Lim1 gene

The sequence and genomic organization of the mouse Lim1 gene were determined. The mouse Lim1 gene has five coding exons. The Lim1 transcription initiation start site was determined by 5' RACE. indicating that the first exon encodes the translation initiation codon and a 1360-bp 5' untranslated region. Sequence analysis of the 450-bp upstream of the transcription start site revealed the presence of a CATTAA motif at -32 bp and a CAATT box located in reverse orientation at -68 bp. HNF3 beta and Pbx1 binding sites were also identified. Like most LIM domain encoding genes, the LIM domains of Lim1 are each encoded on separate and adjacent exons. Knowledge of the sequence and structure of the mouse Lim1 gene provides important information for the genetic manipulation of the Lim1 locus.

YAC rescue of downless locus mutations in mice

Mice with mutations at the downless (dl) locus have defects in hair follicle, tooth, sweat gland, preputial gland, Meibomian gland, and tail development. The dl phenotype is analogous to the human genetic disorder termed autosomal hypohidrotic (or anhidrotic) ectodermal dysplasia (HED). On the basis of the identification of two related transgenic insertional mutations in the downless gene, yeast artificial chromosomes (YACs) were identified that map to the critical region of mouse Chromosome (Chr) 10. To determine which of the YACs contain the dl gene, we generated YAC transgenic mice by mouse embryo microinjections. The 200-kb YAC B25.D9 was found to rescue all of the downless defects. In addition, the transgenic YAC rescued the dominant Sleek (Dlslk) allele. Since the sequences within the YAC are entirely deleted in one of the transgenic mutants, our results establish that Sleek encodes a dominant-negative protein whose effects can be reversed by expression of extra copies of the wild-type locus.

Expression of the mouse cerberus-related gene, Cerr1, suggests a role in anterior neural induction and somitogenesis

The Xenopus cerberus gene encodes a secreted factor that is expressed in the anterior endomesoderm of gastrula stage embryos and can induce the formation of ectopic heads when its mRNA is injected into Xenopus embryos [Bouwmeester, T., Kim, S., Lu, B. & De Robertis, E. M. (1996) Nature (London) 382, 595-601]. Here we describe the existence of a cerberus-related gene, Cerr1, in the mouse. Cerr1 encodes a putative secreted protein that is 48% identical to cerberus over a 110-amino acid region. Analysis of a mouse interspecific backcross panel demonstrated that Cerr1 mapped to the central portion of mouse chromosome 4. In early gastrula stage mouse embryos, Cerr1 is expressed in the anterior visceral endoderm and in the anterior definitive endoderm. In somite stage embryos, Cerr1 expression is restricted to the most recently formed somites and in the anterior presomitic mesoderm. Germ layer explant recombination assays demonstrated that Cerr1-expressing somitic-presomitic mesoderm, but not older Cerr1-nonexpressing somitic mesoderm, was able to mimic the anterior neuralizing ability of anterior mesendoderm and maintain Otx2 expression in competent ectoderm. In most Lim1-/- headless embryos, Cerr1 expression in the anterior endoderm was weak or absent. These results suggest that Cerr1 may play a role in anterior neural induction and somite formation during mouse development.

Restricted beta-galactosidase expression of a hygromycin-lacZ gene targeted to the beta-actin locus and embryonic lethality of beta-actin mutant mice

beta-actin is a cytoskeletal protein that is ubiquitously expressed. To exploit the regulation the beta-actin gene, a promoterless hygromycin-lacZ fusion gene with a splice acceptor was introduced into the first intron of the beta-actin locus by homologous recombination in mouse embryonic stem (ES) cells. The targeted ES cells were hygromycin resistant and expressed beta-galactosidase (beta-gal) activity. However, no beta-gal activity was detected in heterozygous embryos. In adult heterozygotes, beta-gal activity was detected only in testes. RT-PCR analysis demonstrated the presence of both beta-actin exon 1-hygromycin- and exon l-exon 2-containing transcripts in homozygous mutant embryos. LacZ-containing transcripts were detected in adult heterozygous tests and, surprisingly, in homozygous mutant embryos. These results demonstrate that the integration of the hygromycin-lacZ gene into the first intron of the beta-actin locus was not productive for the ubiquitous expression of beta-gal activity. Because this integration mimics certain types of gene trap events, it suggests that caution should be used when interpreting beta-gal expression patterns in genetic screens using gene trap strategies. In addition, mice homozygous for the beta-actin mutation developed normally up to embryonic day 8.5 (E8.5) but became growth retarded at E9.5 and subsequently died. The RT-PCR data indicate that this targeted mutation is a hypomorphic allele of beta-actin.

Sequential roles for Otx2 in visceral endoderm and neuroectoderm for forebrain and midbrain induction and specification

The homeobox gene Otx2 is a mouse cognate of the Drosophila orthodenticle gene, which is required for development of the brain, rostral to rhombomere three. We have investigated the mechanisms involved in this neural function and specifically the requirement for Otx2 in the visceral endoderm and the neuroectoderm using chimeric analysis in mice and explant recombination assay. Analyses of chimeric embryos composed of more than 90% of Otx2−/− ES cells identified an essential function for Otx2 in the visceral endoderm for induction of the forebrain and midbrain. The chimeric studies also demonstrated that an anterior neural plate can form without expressing Otx2. However, in the absence of Otx2, expression of important regulatory genes, such as Hesx1/Rpx, Six3, Pax2, Wnt1 and En, fail to be initiated or maintained in the neural plate. Using explant-recombination assay, we could further demonstrate that Otx2 is required in the neuroectodem for expression of En. Altogether, these results demonstrate that Otx2 is first required in the visceral endoderm for the induction, and subsequently in the neuroectoderm for the specification of forebrain and midbrain territories.

Expression of murine Lhx5 suggests a role in specifying the forebrain

A LIM homeobox gene, Lim5, is known to be expressed in the forebrain of Xenopus and zebrafish (Toyama et al. [1995] Dev. Biol. 170:583-593). Results from developmental and comparative studies of its mouse ortholog, Lhx5, indicate that this gene may play important roles in forebrain development. Lhx5 expression is detected in the most anterior portion of the neural tube at the headfold stage, overlapping partially with Otx2 expression domain. After neural tube closure, Lhx5 is expressed as a transverse stripe, covering most of the diencephalic primordium. This expression recedes to restricted areas as Dlx gene expression occurs. By midgestation, both genes, Lhx5 and Dlx5, are expressed in the diencephalon and ventral telencephalon in an alternating complementary pattern. It may be that Dlx inhibits Lhx5, and this may represent a step of early regionalization of the forebrain. Lhx5 is also expressed in midbrain, hindbrain, and spinal cord, overlapping extensively with Lhx1 starting from day E10.5 of gestation. The early, persistent, and dynamic expression of Lhx5 suggests a regulatory function in forebrain formation.

Requirement for Lim1 in head-organizer function

Lim1 is a homeobox gene expressed in the organizer region of mouse embryos. To investigate the role of Lim1 during embryogenesis, a targeted deletion of the Lim1 gene was generated in embryonic stem cells. Embryos homozygous for the null allele lacked anterior head structures but the remaining body axis developed normally. A partial secondary axis developed anteriorly in some mutant embryos. Lim1 is thus an essential regulator of the vertebrate head organizer.

The retinoblastoma protein-binding region of simian virus 40 large T antigen alters cell cycle regulation in lenses of transgenic mice

Regulation of the cell cycle is a critical aspect of cellular proliferation, differentiation, and transformation. In many cell types, the differentiation process is accompanied by a loss of proliferative capability, so that terminally differentiated cells become postmitotic and no longer progress through the cell cycle. In the experiments described here, the ocular lens has been used as a system to examine the role of the retinoblastoma protein (pRb) family in regulation of the cell cycle during differentiation. The ocular lens is an ideal system for such studies, since it is composed of just two cell types: epithelial cells, which are capable of proliferation, and fiber cells, which are postmitotic. In order to inactivate pRb in viable mice, genes encoding either a truncated version of simian virus 40 large T antigen or the E7 protein of human papillomavirus were expressed in a lens-specific fashion in transgenic mice. Lens fiber cells in the transgenic mice were found to incorporate bromodeoxyuridine, implying inappropriate entry into the cell cycle. Surprisingly, the lens fiber cells did not proliferate as tumor cells but instead underwent programmed cell death, resulting in lens ablation and microphthalmia. Analogous lens alterations did not occur in mice expressing a modified version of the truncated T antigen that was mutated in the binding domain for the pRb family. These experimental results indicate that the retinoblastoma protein family plays a crucial role in blocking cell cycle progression and maintaining terminal differentiation in lens fiber cells. Apoptotic cell death ensues when fiber cells are induced to remain in or reenter the cell cycle.

Insertional inactivation of the downless gene in a family of transgenic mice

The mouse downless (dl) gene is a morphogenetic gene that plays a role in dermal-epidermal interaction and regulation of hair follicle induction during fetal development. We report here the identification of a transgenic mouse line with an insertional mutation in the dl gene. The genomic sequences flanking the transgenic insert have been cloned and used as hybridization probes to confirm that the mutant transgenic mice are homozygous for the transgenic insert and that the site of integration lies on mouse chromosome 10. Genomic probes that are close to or within the downless gene are now available and should permit the characterization of a gene that is involved in induction of a specific type of epithelial morphogenesis.