Nuclear transfer and transgenesis in the pig

Genetic diversity, growth and heart function of Auckland Island pigs, a potential source for organ xenotransplantation

One of the prerequisites for successful organ xenotransplantation is a reasonable size match between the porcine organ and the recipient's organ to be replaced. Therefore, the selection of a suitable genetic background of source pigs is important. In this study, we investigated body and organ growth, cardiac function, and genetic diversity of a colony of Auckland Island pigs established at the Center for Innovative Medical Models (CiMM), LMU Munich. Male and female Auckland Island pig kidney cells (selected to be free of porcine endogenous retrovirus C) were imported from New Zealand, and founder animals were established by somatic cell nuclear transfer (SCNT). Morphologically, Auckland Island pigs have smaller body stature compared to many domestic pig breeds, rendering their organ dimensions well-suited for human transplantation. Furthermore, echocardiography assessments of Auckland Island pig hearts indicated normal structure and functioning across various age groups throughout the study. Single nucleotide polymorphism (SNP) analysis revealed higher runs of homozygosity (ROH) in Auckland Island pigs compared to other domestic pig breeds and demonstrated that the entire locus coding the swine leukocyte antigens (SLAs) was homozygous. Based on these findings, Auckland Island pigs represent a promising genetic background for organ xenotransplantation.

How genome editing changed the world of large animal research

The first genetically modified large animals were developed in 1985 by microinjection to increase the growth of agricultural livestock such as pigs. Since then, it has been a difficult trail due to the lack of genetic tools. Although methods and technologies were developed quickly for the main experimental mammal, the mouse, e.g., efficient pronuclear microinjection, gene targeting in embryonic stem cells, and omics data, most of it was-and in part still is-lacking when it comes to livestock. Over the next few decades, progress in genetic engineering of large animals was driven less by research for agriculture but more for biomedical applications, such as the production of pharmaceutical proteins in the milk of sheep, goats, or cows, xeno-organ transplantation, and modeling human diseases. Available technologies determined if a desired animal model could be realized, and efficiencies were generally low. Presented here is a short review of how genome editing tools, specifically CRISPR/Cas, have impacted the large animal field in recent years. Although there will be a focus on genome engineering of pigs for biomedical applications, the general principles and experimental approaches also apply to other livestock species or applications.

Phenotypic features of dystrophin gene knockout pigs harboring a human artificial chromosome containing the entire dystrophin gene

Mammalian artificial chromosomes have enabled the introduction of extremely large amounts of genetic information into animal cells in an autonomously replicating, nonintegrating format. However, the evaluation of human artificial chromosomes (HACs) as novel tools for curing intractable hereditary disorders has been hindered by the limited efficacy of the delivery system. We generated dystrophin gene knockout (DMD-KO) pigs harboring the HAC bearing the entire human DMD via a somatic cell cloning procedure (DYS-HAC-cloned pig). Restored human dystrophin expression was confirmed by immunofluorescence staining in the skeletal muscle of the DYS-HAC-cloned pigs. Viability at the first month postpartum of the DYS-HAC-cloned pigs, including motor function in the hind leg and serum creatinine kinase level, was improved significantly when compared with that in the original DMD-KO pigs. However, decrease in systemic retention of the DYS-HAC vector and limited production of the DMD protein might have caused severe respiratory impairment with general prostration by 3 months postpartum. The results demonstrate that the use of transchromosomic cloned pigs permitted a straightforward estimation of the efficacy of the DYS-HAC carried in affected tissues/organs in a large-animal disease model, providing novel insights into the therapeutic application of exogenous mammalian artificial chromosomes.

Systemic deletion of DMD exon 51 rescues clinically severe Duchenne muscular dystrophy in a pig model lacking DMD exon 52

Duchenne muscular dystrophy (DMD) is a fatal X-linked disease caused by mutations in the DMD gene, leading to complete absence of dystrophin and progressive degeneration of skeletal musculature and myocardium. In DMD patients and in a corresponding pig model with a deletion of DMD exon 52 (DMDΔ52), expression of an internally shortened dystrophin can be achieved by skipping of DMD exon 51 to reframe the transcript. To predict the best possible outcome of this strategy, we generated DMDΔ51-52 pigs, additionally representing a model for Becker muscular dystrophy (BMD). DMDΔ51-52 skeletal muscle and myocardium samples stained positive for dystrophin and did not show the characteristic dystrophic alterations observed in DMDΔ52 pigs. Western blot analysis confirmed the presence of dystrophin in the skeletal muscle and myocardium of DMDΔ51-52 pigs and its absence in DMDΔ52 pigs. The proteome profile of skeletal muscle, which showed a large number of abundance alterations in DMDΔ52 vs. wild-type (WT) samples, was normalized in DMDΔ51-52 samples. Cardiac function at age 3.5 mo was significantly reduced in DMDΔ52 pigs (mean left ventricular ejection fraction 58.8% vs. 70.3% in WT) but completely rescued in DMDΔ51-52 pigs (72.3%), in line with normalization of the myocardial proteome profile. Our findings indicate that ubiquitous deletion of DMD exon 51 in DMDΔ52 pigs largely rescues the rapidly progressing, severe muscular dystrophy and the reduced cardiac function of this model. Long-term follow-up studies of DMDΔ51-52 pigs will show if they develop symptoms of the milder BMD.

Genome Editing of Pig

Pigs have anatomical and physiological characteristics similar to humans; therefore, genetically modified pigs have the potential to become a valuable bioresource in biomedical research. In fact, considering the increasing need for translational research, pigs are useful for studying intractable diseases, organ transplantation, and regenerative medicine as large-scale experimental animals with excellent potential for extrapolation to humans. With the advent of zinc finger nucleases (ZFNs), breakthroughs in genome editing tools such as transcription activator-like effector nucleases (TALENs) and clustered regularly interspaced short palindromic repeats (CRISPR)-CRISPR associated protein 9 (Cas9) have facilitated the efficient generation of genetically modified pigs. Genome editing has been used in pigs for more than 10 years; now, along with knockout pigs, knock-in pigs are also gaining increasing importance. In this chapter, we describe the establishment of gene-modified cells (nuclear donor cells), which are necessary for gene knockout and production of knock-in pigs via somatic cell nuclear transplantation, as well as the production of gene knockout pigs using a simple cytoplasmic injection method.

Genome Editing in Pigs

The generation of genetically engineered (GE) pigs for disease modeling and xenotransplantation has been massively facilitated by the discovery of the CRISPR/Cas9 system. For livestock, genome editing is a powerful tool when used in combination with either somatic cell nuclear transfer (SCNT) or microinjection (MI) into fertilized oocytes. To generate either knockout or knock-in animals using SCNT, genome editing is carried out in vitro. This has the advantage that fully characterized cells are being employed to generate cloned pigs, predetermining their genetic makeups. However, this technique is labor-intensive and, hence, SCNT is better suited for more challenging projects such as the generation of multi-knockout- and knock-in pigs. Alternatively, CRISPR/Cas9 is introduced directly into fertilized zygotes via microinjection to produce knockout pigs more rapidly. Finally, the embryos are each transferred into recipient sows to deliver GE piglets.Both techniques, SCNT and MI, are technically challenging and therefore require skilled expertise, especially when applied for porcine embryos. Here, we present a detailed laboratory protocol for the generation of knockout and knock-in porcine somatic donor cells for SCNT and knockout pigs via microinjection. We describe the state-of-the-art method for isolation, cultivation, and manipulation of porcine somatic cells, which can then be used for SCNT. Moreover, we describe the isolation and maturation of porcine oocytes, their manipulation by microinjection, and the embryo transfer into surrogate sows.

A humanized minipig model for the toxicological testing of therapeutic recombinant antibodies

The safety of most human recombinant proteins can be evaluated in transgenic mice tolerant to specific human proteins. However, owing to insufficient genetic diversity and to fundamental differences in immune mechanisms, small-animal models of human diseases are often unsuitable for immunogenicity testing and for predicting adverse outcomes in human patients. Most human therapeutic antibodies trigger xenogeneic responses in wild-type animals and thus rapid clearance of the drugs, which makes in vivo toxicological testing of human antibodies challenging. Here we report the generation of Göttingen minipigs carrying a mini-repertoire of human genes for the immunoglobulin heavy chains γ1 and γ4 and the immunoglobulin light chain κ. In line with observations in human patients, the genetically modified minipigs tolerated the clinically non-immunogenic IgG1κ-isotype monoclonal antibodies daratumumab and bevacizumab, and elicited antibodies against the checkpoint inhibitor atezolizumab and the engineered interleukin cergutuzumab amunaleukin. The humanized minipigs can facilitate the safety and efficacy testing of therapeutic antibodies.

Early disruption of photoreceptor cell architecture and loss of vision in a humanized pig model of usher syndromes

Usher syndrome (USH) is the most common form of monogenic deaf-blindness. Loss of vision is untreatable and there are no suitable animal models for testing therapeutic strategies of the ocular constituent of USH, so far. By introducing a human mutation into the harmonin-encoding USH1C gene in pigs, we generated the first translational animal model for USH type 1 with characteristic hearing defect, vestibular dysfunction, and visual impairment. Changes in photoreceptor architecture, quantitative motion analysis, and electroretinography were characteristics of the reduced retinal virtue in USH1C pigs. Fibroblasts from USH1C pigs or USH1C patients showed significantly elongated primary cilia, confirming USH as a true and general ciliopathy. Primary cells also proved their capacity for assessing the therapeutic potential of CRISPR/Cas-mediated gene repair or gene therapy in vitro. AAV-based delivery of harmonin into the eye of USH1C pigs indicated therapeutic efficacy in vivo.

Generation of heterozygous PKD1 mutant pigs exhibiting early-onset renal cyst formation

Autosomal dominant polycystic kidney disease (ADPKD) is the most common inherited kidney disease, manifesting as the progressive development of fluid-filled renal cysts. In approximately half of all patients with ADPKD, end-stage renal disease results in decreased renal function. In this study, we used CRISPR-Cas9 and somatic cell cloning to produce pigs with the unique mutation c.152_153insG (PKD1^insG/+). Pathological analysis of founder cloned animals and progeny revealed that PKD1^insG/+ pigs developed many pathological conditions similar to those of patients with heterozygous mutations in PKD1. Pathological similarities included the formation of macroscopic renal cysts at the neonatal stage, number and cystogenic dynamics of the renal cysts formed, interstitial fibrosis of the renal tissue, and presence of a premature asymptomatic stage. Our findings demonstrate that PKD1^insG/+ pigs recapitulate the characteristic symptoms of ADPKD.

Transgenic pigs expressing near infrared fluorescent protein-A novel tool for noninvasive imaging of islet xenotransplants

BACKGROUND

Islet xenotransplantation is a promising concept for beta-cell replacement therapy. Reporter genes for noninvasive monitoring of islet engraftment, graft mass changes, long-term survival, and graft failure support the optimization of transplantation strategies. Near-infrared fluorescent protein (iRFP) is ideal for fluorescence imaging (FI) in tissue, but also for multispectral optoacoustic tomography (MSOT) with an even higher imaging depth. Therefore, we generated reporter pigs ubiquitously expressing iRFP.

METHODS

CAG-iRPF720 transgenic reporter pigs were generated by somatic cell nuclear transfer from FACS-selected stable transfected donor cells. Neonatal pig islets (NPIs) were transplanted into streptozotocin-diabetic immunodeficient NOD-scid IL2Rg^null (NSG) mice. FI and MSOT were performed to visualize different numbers of NPIs and to evaluate associations between signal intensity and glycemia. MSOT was also tested in a large animal model.

RESULTS

CAG-iRFP transgenic NPIs were functionally equivalent with wild-type NPIs. Four weeks after transplantation under the kidney capsule, FI revealed a twofold higher signal for 4000-NPI compared to 1000-NPI grafts. Ten weeks after transplantation, the fluorescence intensity of the 4000-NPI graft was inversely correlated with glycemia. After intramuscular transplantation into diabetic NSG mice, MSOT revealed clear dose-dependent signals for grafts of 750, 1500, and 3000 NPIs. Dose-dependent MSOT signals were also revealed in a pig model, with stronger signals after subcutaneous (depth ∼6 mm) than after submuscular (depth ∼15 mm) placement of the NPIs.

CONCLUSIONS

Islets from CAG-iRFP transgenic pigs are fully functional and accessible to long-term monitoring by state-of-the-art imaging modalities. The novel reporter pigs will support the development and preclinical testing of novel matrices and engraftment strategies for porcine xeno-islets.

A scalable, clinically severe pig model for Duchenne muscular dystrophy

Large animal models for Duchenne muscular dystrophy (DMD) are crucial for evaluation of diagnostic procedures and treatment strategies. Pigs cloned from male cells lacking DMD exon 52 (DMDΔ52) resemble molecular, clinical and pathological hallmarks of DMD, but die before sexual maturity and cannot be propagated by breeding. Therefore, we generated female DMD+/- carriers. A single founder animal had 11 litters with 29 DMDY/-, 34 DMD+/- as well as 36 male and 29 female wild-type offspring. Breeding with F1 and F2 DMD+/- carriers resulted in additional 114 DMDY/- piglets. With intensive neonatal management, the majority survived for 3-4 months, providing statistically relevant cohorts for experimental studies. Pathological investigations and proteome studies of skeletal muscles and myocardium confirmed the resemblance of human disease mechanisms. Importantly, DMDY/- pigs reveal progressive myocardial fibrosis and increased expression of connexin-43, associated with significantly reduced left ventricular ejection fraction already at age 3 months. Furthermore, behavioral tests provided evidence for impaired cognitive ability. Our breeding cohort of DMDΔ52 pigs and standardized tissue repositories provide important resources for studying DMD disease mechanisms and for testing novel treatment strategies.

The Missing Link: Cre Pigs for Cancer Research

The Cre/loxP system is a powerful tool for the generation of animal models with precise spatial and temporal gene expression. It has proven indispensable in the generation of cancer models with tissue specific expression of oncogenes or the inactivation of tumor suppressor genes. Consequently, Cre-transgenic mice have become an essential prerequisite in basic cancer research. While it is unlikely that pigs will ever replace mice in basic research they are already providing powerful complementary resources for translational studies. But, although conditionally targeted onco-pigs have been generated, no Cre-driver lines exist for any of the major human cancers. To model human pancreatic cancer in pigs, Cre-driver lines were generated by CRISPR/Cas9-mediated insertion of codon-improved Cre (iCre) into the porcine PTF1A gene, thus guaranteeing tissue and cell type specific function which was proven using dual fluorescent reporter pigs. The method used can easily be adapted for the generation of other porcine Cre-driver lines, providing a missing tool for modeling human cancers in large animals.

A New Toolbox in Experimental Embryology-Alternative Model Organisms for Studying Preimplantation Development

Preimplantation development is well conserved across mammalian species, but major differences in developmental kinetics, regulation of early lineage differentiation and implantation require studies in different model organisms, especially to better understand human development. Large domestic species, such as cattle and pig, resemble human development in many different aspects, i.e., the timing of zygotic genome activation, mechanisms of early lineage differentiations and the period until blastocyst formation. In this article, we give an overview of different assisted reproductive technologies, which are well established in cattle and pig and make them easily accessible to study early embryonic development. We outline the available technologies to create genetically modified models and to modulate lineage differentiation as well as recent methodological developments in genome sequencing and imaging, which form an immense toolbox for research. Finally, we compare the most recent findings in regulation of the first lineage differentiations across species and show how alternative models enhance our understanding of preimplantation development.

Cas9-expressing chickens and pigs as resources for genome editing in livestock

Genetically modified animals continue to provide important insights into the molecular basis of health and disease. Research has focused mostly on genetically modified mice, although other species like pigs resemble the human physiology more closely. In addition, cross-species comparisons with phylogenetically distant species such as chickens provide powerful insights into fundamental biological and biomedical processes. One of the most versatile genetic methods applicable across species is CRISPR-Cas9. Here, we report the generation of transgenic chickens and pigs that constitutively express Cas9 in all organs. These animals are healthy and fertile. Functionality of Cas9 was confirmed in both species for a number of different target genes, for a variety of cell types and in vivo by targeted gene disruption in lymphocytes and the developing brain, and by precise excision of a 12.7-kb DNA fragment in the heart. The Cas9 transgenic animals will provide a powerful resource for in vivo genome editing for both agricultural and translational biomedical research, and will facilitate reverse genetics as well as cross-species comparisons.

Growth hormone receptor knockout to reduce the size of donor pigs for preclinical xenotransplantation studies

BACKGROUND

Many genetically multi-modified donor lines for xenotransplantation have a background of domestic pigs with rapid body and organ growth. The intrinsic growth potential of porcine xeno-organs may impair their long-term function after orthotopic transplantation in non-human primate models. Since growth hormone is a major stimulator of postnatal growth, we deleted its receptor (GHR-KO) to reduce the size of donor pigs in one step.

METHODS

Heart weight and proteome profile of myocardium were investigated in GHR-KO and control pigs. GHR-KO mutations were introduced using CRISPR/Cas9 in an α1,3-galactosyltransferase (GGTA1)-deficient background expressing the human cluster of differentiation (hCD46) and human thrombomodulin (hTHBD) to generate quadruple-modified (4GM) pigs.

RESULTS

At age 6 months, GHR-KO pigs had a 61% reduced body weight and a 63% reduced heart weight compared with controls. The mean minimal diameter of cardiomyocytes was 28% reduced. A holistic proteome study of myocardium samples from the two groups did not reveal prominent differences. Two 4GM founder sows had low serum insulin-like growth factor 1 (IGF1) levels (24 ± 1 ng/mL) and reached body weights of 70.3 and 73.4 kg at 9 months. Control pigs with IGF1 levels of 228 ± 24 ng/mL reached this weight range three months earlier. The 4GM sows showed normal sexual development and were mated with genetically multi-modified boars. Offspring revealed the expected Mendelian transmission of the genetic modifications and consistent expression of the transgenes.

CONCLUSION

GHR-KO donor pigs can be used at an age beyond the steepest phase of their growth curve, potentially reducing the problem of xeno-organ overgrowth in preclinical studies.

Somatic gene editing ameliorates skeletal and cardiac muscle failure in pig and human models of Duchenne muscular dystrophy

Frameshift mutations in the DMD gene, encoding dystrophin, cause Duchenne muscular dystrophy (DMD), leading to terminal muscle and heart failure in patients. Somatic gene editing by sequence-specific nucleases offers new options for restoring the DMD reading frame, resulting in expression of a shortened but largely functional dystrophin protein. Here, we validated this approach in a pig model of DMD lacking exon 52 of DMD (DMDΔ52), as well as in a corresponding patient-derived induced pluripotent stem cell model. In DMDΔ52 pigs1, intramuscular injection of adeno-associated viral vectors of serotype 9 carrying an intein-split Cas9 (ref. 2) and a pair of guide RNAs targeting sequences flanking exon 51 (AAV9-Cas9-gE51) induced expression of a shortened dystrophin (DMDΔ51-52) and improved skeletal muscle function. Moreover, systemic application of AAV9-Cas9-gE51 led to widespread dystrophin expression in muscle, including diaphragm and heart, prolonging survival and reducing arrhythmogenic vulnerability. Similarly, in induced pluripotent stem cell-derived myoblasts and cardiomyocytes of a patient lacking DMDΔ52, AAV6-Cas9-g51-mediated excision of exon 51 restored dystrophin expression and amelioreate skeletal myotube formation as well as abnormal cardiomyocyte Ca2+ handling and arrhythmogenic susceptibility. The ability of Cas9-mediated exon excision to improve DMD pathology in these translational models paves the way for new treatment approaches in patients with this devastating disease.

Porcine models for studying complications and organ crosstalk in diabetes mellitus

The worldwide prevalence of diabetes mellitus and obesity is rapidly increasing not only in adults but also in children and adolescents. Diabetes is associated with macrovascular complications increasing the risk for cardiovascular disease and stroke, as well as microvascular complications leading to diabetic nephropathy, retinopathy and neuropathy. Animal models are essential for studying disease mechanisms and for developing and testing diagnostic procedures and therapeutic strategies. Rodent models are most widely used but have limitations in translational research. Porcine models have the potential to bridge the gap between basic studies and clinical trials in human patients. This article provides an overview of concepts for the development of porcine models for diabetes and obesity research, with a focus on genetically engineered models. Diabetes-associated ocular, cardiovascular and renal alterations observed in diabetic pig models are summarized and their similarities with complications in diabetic patients are discussed. Systematic multi-organ biobanking of porcine models of diabetes and obesity and molecular profiling of representative tissue samples on different levels, e.g., on the transcriptome, proteome, or metabolome level, is proposed as a strategy for discovering tissue-specific pathomechanisms and their molecular key drivers using systems biology tools. This is exemplified by a recent study providing multi-omics insights into functional changes of the liver in a transgenic pig model for insulin-deficient diabetes mellitus. Collectively, these approaches will provide a better understanding of organ crosstalk in diabetes mellitus and eventually reveal new molecular targets for the prevention, early diagnosis and treatment of diabetes mellitus and its associated complications.

Mild maternal hyperglycemia in INS C93S transgenic pigs causes impaired glucose tolerance and metabolic alterations in neonatal offspring

Alongside the obesity epidemic, the prevalence of maternal diabetes is rising worldwide, and adverse effects on fetal development and metabolic disturbances in the offspring's later life have been described. To clarify whether metabolic programming effects are due to mild maternal hyperglycemia without confounding obesity, we investigated wild-type offspring of INS^C93S transgenic pigs, which are a novel genetically modified large-animal model expressing mutant insulin (INS) C93S in pancreatic β-cells. This mutation results in impaired glucose tolerance, mild fasting hyperglycemia and insulin resistance during late pregnancy. Compared with offspring from wild-type sows, piglets from hyperglycemic mothers showed impaired glucose tolerance and insulin resistance (homeostatic model assessment of insulin resistance: +3-fold in males; +4.4-fold in females) prior to colostrum uptake. Targeted metabolomics in the fasting and insulin-stimulated state revealed distinct alterations in the plasma metabolic profile of piglets from hyperglycemic mothers. They showed increased levels of acylcarnitines, gluconeogenic precursors such as alanine, phospholipids (in particular lyso-phosphatidylcholines) and α-aminoadipic acid, a potential biomarker for type 2 diabetes. These observations indicate that mild gestational hyperglycemia can cause impaired glucose tolerance, insulin resistance and associated metabolic alterations in neonatal offspring of a large-animal model born at a developmental maturation status comparable to human babies.

Editor's choice: Mild maternal hyperglycemia causes impaired glucose tolerance and metabolic alterations in wild-type neonatal offspring of INS^C93S transgenic pigs, a novel large animal model for mutant INS gene-induced diabetes of youth (MIDY).

Early weaning completely eliminates porcine cytomegalovirus from a newly established pig donor facility for xenotransplantation

For clinical xenotransplantation, transplants must be free of porcine cytomegalovirus (PCMV). Piglets become infected primarily in the perinatal period by the mother sow. While individual donor animals can be protected from infection by isolation husbandry, success is not guaranteed and this strategy poses the risk of undetected infections and raises animal welfare questions. Here, we present the establishment of a completely PCMV-negative pig herd for breeding donor animals for xenotransplantation. Eleven pregnant DanAvl Basic hybrid sows were purchased from a designated pathogen-free (DPF), PCMV-positive colony and transferred to a new pig facility at the Centre for Innovative Medical Models (CiMM) 4 weeks prior to farrowing. At the age of 24 hours, piglets were early-weaned and transferred to a commercially available Rescue Deck system dedicated to motherless rearing of piglets. Sows were removed from the facility. The PCMV status of F1-generation animals was determined at regular intervals over a period of 14 months by a sensitive real-time PCR-based detection method testing blood, nasal swabs and cultured peripheral blood mononuclear cells (PBMCs). F1 sows were used as recipients of genetically modified embryos to generate a xenotransplant donor herd. Offspring were tested for PCMV accordingly. All offspring have remained PCMV negative over the whole observation period of 14 months. A completely PCMV-negative pig herd for xenotransplantation has thus been successfully established.

Gene Editing in Primary Cells of Cattle and Pig

Gene Editing by CRISPR/Cas has revolutionized many aspects of biotechnology within a short period of time. This is also true for the genetic manipulation of livestock species, but their specific challenges such as the lack of stem cells, the limited proliferative capacity of primary cells, and the genetic diversity of the pig and cattle populations need consideration when CRISPR/Cas is applied. Here we present guidelines for CRISPRing primary cells in pig and cattle, with a specific focus on testing gRNA in vitro, on generating single cell clones, and on identifying modifications in single cell clones.

Comparative aspects of early lineage specification events in mammalian embryos - insights from reverse genetics studies

Within the mammalian class, formation of the blastocyst is morphologically highly conserved among different species. The molecular and cellular events during preimplantation embryo development have been studied extensively in the mouse as model organism, because multiple genetically defined strains and a plethora of reverse genetics tools are available to dissect specific gene functions and regulatory networks. However, major differences in preimplantation developmental kinetics, implantation, and placentation exist among mammalians, and recent studies in species other than mouse showed, that even regulatory mechanisms of the first lineage differentiation events and maintenance of pluripotency are not always conserved. Here, we focus on the first and the second lineage segregation in mouse and bovine embryos, when the first differentiated cell types emerge. We outline their common features and differences in the regulation of these essential events during embryonic development with a glance at further species. In addition, we show how new reverse genetics strategies aid the study of regulatory circuits in embryos of domestic species, enhancing our overall understanding of mammalian preimplantation development.

Growth hormone receptor-deficient pigs resemble the pathophysiology of human Laron syndrome and reveal altered activation of signaling cascades in the liver

OBJECTIVE

Laron syndrome (LS) is a rare, autosomal recessive disorder in humans caused by loss-of-function mutations of the growth hormone receptor (GHR) gene. To establish a large animal model for LS, pigs with GHR knockout (KO) mutations were generated and characterized.

METHODS

CRISPR/Cas9 technology was applied to mutate exon 3 of the GHR gene in porcine zygotes. Two heterozygous founder sows with a 1-bp or 7-bp insertion in GHR exon 3 were obtained, and their heterozygous F1 offspring were intercrossed to produce GHR-KO, heterozygous GHR mutant, and wild-type pigs. Since the latter two groups were not significantly different in any parameter investigated, they were pooled as the GHR expressing control group. The characterization program included body and organ growth, body composition, endocrine and clinical-chemical parameters, as well as signaling studies in liver tissue.

RESULTS

GHR-KO pigs lacked GHR and had markedly reduced serum insulin-like growth factor 1 (IGF1) levels and reduced IGF-binding protein 3 (IGFBP3) activity but increased IGFBP2 levels. Serum GH concentrations were significantly elevated compared with control pigs. GHR-KO pigs had a normal birth weight. Growth retardation became significant at the age of five weeks. At the age of six months, the body weight of GHR-KO pigs was reduced by 60% compared with controls. Most organ weights of GHR-KO pigs were reduced proportionally to body weight. However, the weights of liver, kidneys, and heart were disproportionately reduced, while the relative brain weight was almost doubled. GHR-KO pigs had a markedly increased percentage of total body fat relative to body weight and displayed transient juvenile hypoglycemia along with decreased serum triglyceride and cholesterol levels. Analysis of insulin receptor related signaling in the liver of adult fasted pigs revealed increased phosphorylation of IRS1 and PI3K. In agreement with the loss of GHR, phosphorylation of STAT5 was significantly reduced. In contrast, phosphorylation of JAK2 was significantly increased, possibly due to the increased serum leptin levels and increased hepatic leptin receptor expression and activation in GHR-KO pigs. In addition, increased mTOR phosphorylation was observed in GHR-KO liver samples, and phosphorylation studies of downstream substrates suggested the activation of mainly mTOR complex 2.

CONCLUSION

GHR-KO pigs resemble the pathophysiology of LS and are an interesting model for mechanistic studies and treatment trials.

Single-cell RNA sequencing reveals developmental heterogeneity of blastomeres during major genome activation in bovine embryos

Embryonic development is initially controlled by maternal RNAs and proteins stored in the oocyte, until gene products gradually generated by the embryo itself take over. Major embryonic genome activation (EGA) in bovine embryos occurs at the eight- to 16-cell stage. Morphological observations, such as size of blastomeres and distribution of microvilli, suggested heterogeneity among individual cells already at this developmental stage. To address cell heterogeneity on the transcriptome level, we performed single-cell RNA sequencing of 161 blastomeres from 14 in vitro produced bovine embryos at Day 2 (n = 6) and Day 3 (n = 8) post fertilization. Complementary DNA libraries were prepared using the Single-Cell RNA-Barcoding and Sequencing protocol and sequenced. Non-supervised clustering of single-cell transcriptome profiles identified six clusters with specific sets of genes. Most embryos were comprised of cells from at least two different clusters. Sorting cells according to their transcriptome profiles resulted in a non-branched pseudo-time line, arguing against major lineage inclination events at this developmental stage. In summary, our study revealed heterogeneity of transcriptome profiles among single cells in bovine Day 2 and Day 3 embryos, suggesting asynchronous blastomere development during the phase of major EGA.

Strong xenoprotective function by single-copy transgenes placed sequentially at a permissive locus

BACKGROUND

Multiple xenoprotective transgenes are best grouped at a single locus to avoid segregation during breeding and simplify production of donor animals.

METHODS

We used transgene stacking to place a human CD55 transgene adjacent to a human heme oxygenase 1 construct at the porcine ROSA26 locus. A transgenic pig was analyzed by PCR, RT-PCR, droplet digital PCR, immunohistochemistry, immunofluorescence, and flow cytometry. Resistance to complement-mediated cell lysis and caspase 3/7 activation were determined in vitro.

RESULTS

The ROSA26 locus was retargeted efficiently, and animals were generated by nuclear transfer. RNA and protein analyses revealed abundant expression in all organs analyzed, including pancreatic beta cells. Transgenic porcine kidney fibroblasts were almost completely protected against complement-mediated lysis and showed reduced caspase 3/7 activation.

CONCLUSION

Step-by-step placement enables highly expressed single-copy xenoprotective transgenes to be grouped at porcine ROSA26.

Modeling lethal X-linked genetic disorders in pigs with ensured fertility

The development of therapies for rare and intractable genetic disorders represents a significant unmet medical need. Disease model pigs characterized by physiological, anatomical, and pathogenetic similarities to humans allow translational studies to be performed, yielding valuable data that can be extrapolated to patients. The establishment of an efficient reproduction system is a key element in the practical application of disease model pigs, which often suffer from reproductive inability due to severe symptoms. Here, we showed that the valuable trait of genetically modified disease model pigs can be maximized by generating unique chimeric boars composed of mutant and normal cells.

Genetically engineered pigs play an indispensable role in the study of rare monogenic diseases. Pigs harboring a gene responsible for a specific disease can be efficiently generated via somatic cell cloning. The generation of somatic cell-cloned pigs from male cells with mutation(s) in an X chromosomal gene is a reliable and straightforward method for reproducing X-linked genetic diseases (XLGDs) in pigs. However, the severe symptoms of XLGDs are often accompanied by impaired growth and reproductive disorders, which hinder the reproduction of these valuable model animals. Here, we generated unique chimeric boars composed of mutant cells harboring a lethal XLGD and normal cells. The chimeric boars exhibited the cured phenotype with fertility while carrying and transmitting the genotype of the XLGD. This unique reproduction system permits routine production of XLGD model pigs through the male-based breeding, thereby opening an avenue for translational research using disease model pigs.

The Munich MIDY Pig Biobank - A unique resource for studying organ crosstalk in diabetes

OBJECTIVE

The prevalence of diabetes mellitus and associated complications is steadily increasing. As a resource for studying systemic consequences of chronic insulin insufficiency and hyperglycemia, we established a comprehensive biobank of long-term diabetic INSC94Y transgenic pigs, a model of mutant INS gene-induced diabetes of youth (MIDY), and of wild-type (WT) littermates.

METHODS

Female MIDY pigs (n = 4) were maintained with suboptimal insulin treatment for 2 years, together with female WT littermates (n = 5). Plasma insulin, C-peptide and glucagon levels were regularly determined using specific immunoassays. In addition, clinical chemical, targeted metabolomics, and lipidomics analyses were performed. At age 2 years, all pigs were euthanized, necropsied, and a broad spectrum of tissues was taken by systematic uniform random sampling procedures. Total beta cell volume was determined by stereological methods. A pilot proteome analysis of pancreas, liver, and kidney cortex was performed by label free proteomics.

RESULTS

MIDY pigs had elevated fasting plasma glucose and fructosamine concentrations, C-peptide levels that decreased with age and were undetectable at 2 years, and an 82% reduced total beta cell volume compared to WT. Plasma glucagon and beta hydroxybutyrate levels of MIDY pigs were chronically elevated, reflecting hallmarks of poorly controlled diabetes in humans. In total, ∼1900 samples of different body fluids (blood, serum, plasma, urine, cerebrospinal fluid, and synovial fluid) as well as ∼17,000 samples from ∼50 different tissues and organs were preserved to facilitate a plethora of morphological and molecular analyses. Principal component analyses of plasma targeted metabolomics and lipidomics data and of proteome profiles from pancreas, liver, and kidney cortex clearly separated MIDY and WT samples.

CONCLUSIONS

The broad spectrum of well-defined biosamples in the Munich MIDY Pig Biobank that will be available to the scientific community provides a unique resource for systematic studies of organ crosstalk in diabetes in a multi-organ, multi-omics dimension.

Improved development of somatic cell cloned bovine embryos by a mammary gland epithelia cells in vitro model

Previous studies have established a bovine mammary gland epithelia cells in vitro model by the adenovirus-mediated telomerase (hTERT-bMGEs). The present study was conducted to confirm whether hTERT-bMGEs were effective target cells to improve the efficiency of transgenic expression and somatic cell nuclear transfer (SCNT). To accomplish this, a mammary-specific vector encoding human lysozyme and green fluorescent protein was used to verify the transgenic efficiency of hTERT-bMGEs, and untreated bovine mammary gland epithelial cells (bMGEs) were used as a control group. The results showed that the hTERT-bMGEs group had much higher transgenic efficiency and protein expression than the bMGEs group. Furthermore, the nontransgenic and transgenic hTERT-bMGEs were used as donor cells to evaluate the efficiency of SCNT. There were no significant differences in rates of cleavage or blastocysts or hatched blastocysts of cloned embryos from nontransgenic hTERT-bMGEs at passage 18 and 28 groups (82.8% vs. 81.9%, 28.6% vs. 24.8%, 58.6% vs. 55.3%, respectively) and the transgenic group (80.8%, 26.5% and 53.4%); however, they were significantly higher than the bMGEs group (71.2%, 12.8% and 14.8%), (p < 0.05). We confirmed that hTERT-bMGEs could serve as effective target cells for improving development of somatic cell cloned cattle embryos.

Genome Editing of Pig

Pigs are important livestock for food and have been used in various biomedical studies, particularly translational research, as experimental animals because of their anatomical and physiological similarity to humans. The recent development of genome editing techniques, such as ZFN, TALEN, and CRISPR/Cas9, has rapidly expanded the use of genome editing tools in a variety of animals, resulting in the relatively easy and efficient generation of gene knock-out pigs. In the past few years, there has been a sustained increase in reports describing the development of genetically modified pigs. This chapter introduces our workflow for establishing the genetically modified cells (nuclear donor cells) necessary to create gene knock-out pigs using somatic cell nuclear transfer and focuses on the actual generation of gene knock-out pigs using a cytoplasmic injection method.

Direct introduction of gene constructs into the pronucleus-like structure of cloned embryos: a new strategy for the generation of genetically modified pigs

Due to a rising demand of porcine models with complex genetic modifications for biomedical research, the approaches for their generation need to be adapted. In this study we describe the direct introduction of a gene construct into the pronucleus (PN)-like structure of cloned embryos as a novel strategy for the generation of genetically modified pigs, termed "nuclear injection". To evaluate the reliability of this new strategy, the developmental ability of embryos in vitro and in vivo as well as the integration and expression efficiency of a transgene carrying green fluorescence protein (GFP) were examined. Eighty percent of the cloned pig embryos (633/787) exhibited a PN-like structure, which met the prerequisite to technically perform the new method. GFP fluorescence was observed in about half of the total blastocysts (21/40, 52.5%), which was comparable to classical zygote PN injection (28/41, 68.3%). In total, 478 cloned embryos injected with the GFP construct were transferred into 4 recipients and from one recipient 4 fetuses (day 68) were collected. In one of the fetuses which showed normal development, the integration of the transgene was confirmed by PCR in different tissues and organs from all three primary germ layers and placenta. The integration pattern of the transgene was mosaic (48 out of 84 single-cell colonies established from a kidney were positive for GFP DNA by PCR). Direct GFP fluorescence was observed macro- and microscopically in the fetus. Our novel strategy could be useful particularly for the generation of pigs with complex genetic modifications.

Efficient production of multi-modified pigs for xenotransplantation by 'combineering', gene stacking and gene editing

Xenotransplantation from pigs could alleviate the shortage of human tissues and organs for transplantation. Means have been identified to overcome hyperacute rejection and acute vascular rejection mechanisms mounted by the recipient. The challenge is to combine multiple genetic modifications to enable normal animal breeding and meet the demand for transplants. We used two methods to colocate xenoprotective transgenes at one locus, sequential targeted transgene placement - ‘gene stacking’, and cointegration of multiple engineered large vectors - ‘combineering’, to generate pigs carrying modifications considered necessary to inhibit short to mid-term xenograft rejection. Pigs were generated by serial nuclear transfer and analysed at intermediate stages. Human complement inhibitors CD46, CD55 and CD59 were abundantly expressed in all tissues examined, human HO1 and human A20 were widely expressed. ZFN or CRISPR/Cas9 mediated homozygous GGTA1 and CMAH knockout abolished α-Gal and Neu5Gc epitopes. Cells from multi-transgenic piglets showed complete protection against human complement-mediated lysis, even before GGTA1 knockout. Blockade of endothelial activation reduced TNFα-induced E-selectin expression, IFNγ-induced MHC class-II upregulation and TNFα/cycloheximide caspase induction. Microbial analysis found no PERV-C, PCMV or 13 other infectious agents. These animals are a major advance towards clinical porcine xenotransplantation and demonstrate that livestock engineering has come of age.

Ubiquitous LEA29Y Expression Blocks T Cell Co-Stimulation but Permits Sexual Reproduction in Genetically Modified Pigs

We have successfully established and characterized a genetically modified pig line with ubiquitous expression of LEA29Y, a human CTLA4-Ig derivate. LEA29Y binds human B7.1/CD80 and B7.2/CD86 with high affinity and is thus a potent inhibitor of T cell co-stimulation via this pathway. We have characterized the expression pattern and the biological function of the transgene as well as its impact on the porcine immune system and have evaluated the potential of these transgenic pigs to propagate via assisted breeding methods. The analysis of LEA29Y expression in serum and multiple organs of CAG-LEA transgenic pigs revealed that these animals produce a biologically active transgenic product at a considerable level. They present with an immune system affected by transgene expression, but can be maintained until sexual maturity and propagated by assisted reproduction techniques. Based on previous experience with pancreatic islets expressing LEA29Y, tissues from CAG-LEA29Y transgenic pigs should be protected against rejection by human T cells. Furthermore, their immune-compromised phenotype makes CAG-LEA29Y transgenic pigs an interesting large animal model for testing human cell therapies and will provide an important tool for further clarifying the LEA29Y mode of action.

Tissue Sampling Guides for Porcine Biomedical Models

This article provides guidelines for organ and tissue sampling adapted to porcine animal models in translational medical research. Detailed protocols for the determination of sampling locations and numbers as well as recommendations on the orientation, size, and trimming direction of samples from ∼50 different porcine organs and tissues are provided in the Supplementary Material. The proposed sampling protocols include the generation of samples suitable for subsequent qualitative and quantitative analyses, including cryohistology, paraffin, and plastic histology; immunohistochemistry;in situhybridization; electron microscopy; and quantitative stereology as well as molecular analyses of DNA, RNA, proteins, metabolites, and electrolytes. With regard to the planned extent of sampling efforts, time, and personnel expenses, and dependent upon the scheduled analyses, different protocols are provided. These protocols are adjusted for (I) routine screenings, as used in general toxicity studies or in analyses of gene expression patterns or histopathological organ alterations, (II) advanced analyses of single organs/tissues, and (III) large-scale sampling procedures to be applied in biobank projects. Providing a robust reference for studies of porcine models, the described protocols will ensure the efficiency of sampling, the systematic recovery of high-quality samples representing the entire organ or tissue as well as the intra-/interstudy comparability and reproducibility of results.

Fetal Sex Modulates Developmental Response to Maternal Malnutrition

The incidence of obesity and metabolic diseases is dramatically high in rapidly developing countries. Causes have been related to intrinsic ethnic features with development of a thrifty genotype for adapting to food scarcity, prenatal programming by undernutrition, and postnatal exposure to obesogenic lifestyle. Observational studies in humans and experimental studies in animal models evidence that the adaptive responses of the offspring may be modulated by their sex. In the contemporary context of world globalization, the new question arising is the existence and extent of sex-related differences in developmental and metabolic traits in case of mixed-race. Hence, in the current study, using a swine model, we compared male and female fetuses that were crossbred from mothers with thrifty genotype and fathers without thrifty genotype. Female conceptuses evidence stronger protective strategies for their adequate growth and postnatal survival. In brief, both male and female fetuses developed a brain-sparing effect but female fetuses were still able to maintain the development of other viscerae than the brain (mainly liver, intestine and kidneys) at the expense of carcass development. Furthermore, these morphometric differences were reinforced by differences in nutrient availability (glucose and cholesterol) favoring female fetuses with severe developmental predicament. These findings set the basis for further studies aiming to increase the knowledge on the interaction between genetic and environmental factors in the determination of adult phenotype

Generation of α1,3-galactosyltransferase and cytidine monophospho-N-acetylneuraminic acid hydroxylase gene double-knockout pigs

Zinc-finger nucleases (ZFNs) and transcription activator-like effector nucleases (TALENs) are new tools for producing gene knockout (KO) animals. The current study reports produced genetically modified pigs, in which two endogenous genes were knocked out. Porcine fibroblast cell lines were derived from homozygous α1,3-galactosyltransferase (GalT) KO pigs. These cells were subjected to an additional KO for the cytidine monophospho-N-acetylneuraminic acid hydroxylase (CMAH) gene. A pair of ZFN-encoding mRNAs targeting exon 8 of the CMAH gene was used to generate the heterozygous CMAH KO cells, from which cloned pigs were produced by somatic cell nuclear transfer (SCNT). One of the cloned pigs obtained was re-cloned after additional KO of the remaining CMAH allele using the same ZFN-encoding mRNAs to generate GalT/CMAH-double homozygous KO pigs. On the other hand, the use of TALEN-encoding mRNAs targeting exon 7 of the CMAH gene resulted in efficient generation of homozygous CMAH KO cells. These cells were used for SCNT to produce cloned pigs homozygous for a double GalT/CMAH KO. These results demonstrate that the combination of TALEN-encoding mRNA, in vitro selection of the nuclear donor cells and SCNT provides a robust method for generating KO pigs.

Viable pigs with a conditionally-activated oncogenic KRAS mutation

Oncogenic mutations of KRAS play a major role in human carcinogenesis. Here we describe viable gene-targeted pigs carrying a latent KRAS (G12D) mutant allele that can be activated by Cre recombination. These have been produced as part of a program to model human cancers in pigs by replicating genetic lesions known to initiate and drive human disease. Cre-activated KRAS (G12D) animals add to a growing set of gene-targeted pigs that includes a Cre-activated oncogenic mutant TP53, a Cre-responsive dual fluorescent reporter and two truncating mutations of APC (adenomatous polyposis coli). These alleles can be combined and activated in various tissues to produce new models for cancer research.