Erythroid-specific inactivation of Slc12a6/Kcc3 by EpoR promoter-driven Cre expression reduces K-Cl cotransport activity in mouse erythrocytes.

Investigation of erythrocytes from spontaneous or engineered germ-line mutant mice has been instrumental in characterizing the physiological functions of components of the red cell cytoskeleton and membrane. However, the red blood cell expresses some proteins whose germline loss-of-function is embryonic-lethal, perinatal-lethal, or confers reduced post-weaning viability. Promoter regions of erythroid-specific genes have been used to engineer erythroid-specific expression of Cre recombinase. Through breeding with mice carrying appropriately spaced insertions of loxP sequences, generation of erythroid-specific knockouts has been carried out for signaling enzymes, transcription factors, peptide hormones, and single transmembrane span signaling receptors. We report here the use of Cre recombinase expression driven by the erythropoietin receptor (EpoR) promoter to generate EpoR-Cre;Kcc3f/f mice, designed to express erythroid-specific knockout of the KCC3 K-Cl cotransporter encoded by Kcc3/Slc12A6. We confirm KCC3 as the predominant K-Cl cotransporter of adult mouse red cells in mice with better viability than previously exhibited by Kcc3-/- germline knockouts. We demonstrate roughly proportionate preservation of K-Cl stimulation by hypotonicity, staurosporine, and urea in the context of reduced, but not abrogated, K-Cl function in EpoR-Cre;Kcc3f/f mice. We also report functional evidence suggesting incomplete recombinase-mediated excision of the Kcc3 gene in adult erythroid tissues.

An efficient inducible RPE-Selective cre transgenic mouse line.

Cre-mediated modulation of gene function in the murine retinal pigment epithelium (RPE) has been widely used, but current postnatal RPE-selective Cre driver lines suffer from limited recombination efficiency and/or ectopic or mosaic expression. We sought to generate a transgenic mouse line with consistently efficient RPE-selective Cre activity that could be temporally regulated. We used ϕC31 integrase to insert a DNA construct encoding a human BEST1 promoter fragment driving a Cre recombinase estrogen receptor fusion (BEST1-CreERT2) at the Rosa26 locus of C57BL/6J mice. Rosa26BEST1-CreERT2 mice were bred with a tdTomato reporter line and to mice with a Cre-conditional allele of Tfam. 4-hydroxytamoxifen or vehicle was delivered by four consecutive daily intraperitoneal injections. TdTomato was robustly expressed in the RPE of mice of both sexes for inductions beginning at P14 (males 90.7 ± 4.5%, females 84.7 ± 3.2%) and at 7 weeks (males 84.3 ± 7.0%, females 82 ± 3.6%). <0.6% of Muller glia also expressed tdTomato, but no tdTomato fluorescence was observed in other ocular cells or in multiple non-ocular tissues, with the exception of sparse foci in the testis. No evidence of retinal toxicity was observed in mice homozygous for the transgene induced beginning at P14 and assessed at 7-10 months. RPE-selective ablation of Tfam beginning at P14 led to reduced retinal thickness at 8 months of age and diminished retinal electrical responses at 12 months, as expected. These findings demonstrate that we have generated a mouse line with consistently efficient, tamoxifen-mediated postnatal induction of Cre recombination in the RPE and a small fraction of Muller glia. This line should be useful for temporally regulated modulation of gene function in the murine RPE.

Exclusive labeling of direct and indirect pathway neurons in the mouse neostriatum by an adeno-associated virus vector with Cre/lox system.

We developed an adeno-associated virus (AAV) vector-based technique to label mouse neostriatal neurons comprising direct and indirect pathways with different fluorescent proteins and analyze their axonal projections. The AAV vector expresses GFP or RFP in the presence or absence of Cre recombinase and should be useful for labeling two cell populations exclusively dependent on its expression. Here, we describe the AAV vector design, stereotaxic injection of the AAV vector, and a highly sensitive immunoperoxidase method for axon visualization. For complete details on the use and execution of this protocol, please refer to Okamoto et al. (2020).

A new mouse model to study restoration of interleukin-6 (IL-6) expression in a Cre-dependent manner: microglial IL-6 regulation of experimental autoimmune encephalomyelitis.

BACKGROUND: Interleukin-6 (IL-6) is a pleiotropic cytokine that controls numerous physiological processes both in basal and neuroinflammatory conditions, including the inflammatory response to experimental autoimmune encephalomyelitis (EAE). IL-6 is produced by multiple peripheral and central cells, and until now, the putative roles of IL-6 from different cell types have been evaluated through conditional cell-specific IL-6 knockout mice. Nevertheless, these mice probably undergo compensatory responses of IL-6 from other cells, which makes it difficult to assess the role of each source of IL-6. METHODS: To give some insight into this problem, we have produced a novel mouse model: a conditional reversible IL-6 KO mouse (IL6-DIO-KO). By using double-inverted, open-reading-frame (DIO) technology, we created a mouse line with the loss of Il6 expression in all cells that can be restored by the action of Cre recombinase. Since microglia are one of the most important sources and targets of IL-6 into the central nervous system, we have recovered microglial Il6 expression in IL6-DIO-KO mice through breeding to Cx3cr1-CreER mice and subsequent injection of tamoxifen (TAM) when mice were 10-16 weeks old. Then, they were immunized with myelin oligodendrocyte glycoprotein 35-55 peptide (MOG35-55) 7 weeks after TAM treatment to induce EAE. Clinical symptoms and demyelination, CD3 infiltration, and gliosis in the spinal cord were evaluated. RESULTS: IL6-DIO-KO mice were resistant to EAE, validating the new model. Restoration of microglial Il6 was sufficient to develop a mild version of EAE-related clinical symptoms and neuropathology. CONCLUSIONS: IL6-DIO-KO mouse is an excellent model to understand in detail the role of specific cellular sources of IL-6 within a recovery-of-function paradigm in EAE.

Optimization of tamoxifen-induced Cre activity and its effect on immune cell populations.

Tamoxifen (TAM) inducible Cre recombinase system is an essential tool to study gene function when early ablation or overexpression can cause developmental defects or embryonic lethality. However, there remains a lack of consensus on the optimal route and dosage of TAM administration in vivo. Here, we assessed dosage and delivery of TAM for activation of Cre in immune cell subsets assessed longitudinally and spatially using transgenic mice with ubiquitously expressed Cre/ER and the Cre-inducible fluorescent reporter YFP. After comparing two TAM delivery methods (intraperitoneal versus oral gavage) and different doses, we found that 3 mg of TAM administered orally for five consecutive days provides maximal reporter induction with minimal adverse effects in vivo. Serum levels of TAM peaked 1 week after initiating treatment then slowly decreased, regardless of dosing and delivery methods. TAM concentration in specific tissues (liver, spleen, lymph nodes, and thymus) was also dependent on delivery method and dose. Cre induction was highest in myeloid cells and B cells and substantially lower in T cells, and double-positive thymocytes had a notably higher response to TAM. In addition to establishing optimal dose and administration of TAM, our study reveals a disparate activity of Cre in different cell immune populations when using Cre/ER models.

Characterization of kindled VGAT-Cre mice as a new animal model of temporal lobe epilepsy.

OBJECTIVE: Development of novel therapies for temporal lobe epilepsy is hindered by a lack of models suitable for drug screening. While testing the hypothesis that "inhibiting inhibitory neurons" was sufficient to induce seizures, it was discovered that a mild electrical kindling protocol of VGAT-Cre mice led to spontaneous motor and electrographic seizures. This study characterizes these seizures and investigates the mechanism. METHODS: Mice were implanted with electroencephalographic (EEG) headsets that included a stimulating electrode in the hippocampus before being electrically kindled. Seizures were evaluated by review of EEG recordings and behavior. γ-Aminobutyric acidergic (GABAergic) neurotransmission was evaluated by quantitative polymerase chain reaction, immunocytochemistry, Western blot, and electrophysiology. RESULTS: Electrical kindling of VGAT-Cre mice induces spontaneous recurring seizures after a short latency (6 days). Seizures occur 1-2 times per day in both male and female mice, with only minimal neuronal death. These mice express Cre recombinase under the control of the vesicular GABA transporter (VGAT), a gene that is specifically expressed in GABAergic inhibitory neurons. The insertion of Cre disrupts the expression of VGAT mRNA and protein, and impairs GABAergic synaptic transmission in the hippocampus. SIGNIFICANCE: Kindled VGAT-Cre mice can be used to study the mechanisms involved in epileptogenesis and may be useful for screening novel therapeutics.

Nonspecific Expression in Limited Excitatory Cell Populations in Interneuron-Targeting Cre-driver Lines Can Have Large Functional Effects.

Transgenic Cre-recombinase expressing mouse lines are widely used to express fluorescent proteins and opto-/chemogenetic actuators, making them a cornerstone of modern neuroscience. The investigation of interneurons in particular has benefitted from the ability to genetically target specific cell types. However, the specificity of some Cre driver lines has been called into question. Here, we show that nonspecific expression in a subset of hippocampal neurons can have substantial nonspecific functional effects in a somatostatin-Cre (SST-Cre) mouse line. Nonspecific targeting of CA3 pyramidal cells caused large optogenetically evoked excitatory currents in remote brain regions. Similar, but less severe patterns of nonspecific expression were observed in a widely used SST-IRES-Cre line, when crossed with a reporter mouse line. Viral transduction on the other hand yielded more specific expression but still resulted in nonspecific expression in a minority of pyramidal layer cells. These results suggest that a careful analysis of specificity is mandatory before the use of Cre driver lines for opto- or chemogenetic manipulation approaches.

Phenotypic Diversity Caused by Differential Expression of SFTPC-Cre-Transgenic Alleles.

Type II alveolar epithelial cells (AEC2s) play an essential role in the function and maintenance of the pulmonary epithelium. Several transgenic mice have been developed to study the function of these cells in vivo by using the human SFTPC promoter to drive expression of Cre recombinase. The precise activity of each of these transgenic alleles has not been studied, and previous reports suggest that their activity can depend on breeding strategies. We bred mice with a conditional allele of the essential telomere capping protein TRF2 with two different SFTPC-Cre-transgenic strains and observed opposite phenotypes (100% lethality vs. 100% viability). We characterized the Cre recombinase activity in these two transgenic lines and found that the contrasting phenotypes were driven by difference in embryonic expression of the two transgenes, likely due to position effects or differences in the transgenic constructs. We also tested if SFTPC-Cre activity was dependent on maternal or paternal inheritance. When paternally inherited, both SFTPC-Cre alleles produced offspring with constitutive reporter activity independent of the inheritance of the Cre allele, suggesting that Cre recombinase was expressed in the male germline before meiosis. Immunohistochemical analysis of the testis showed reporter activity during spermatogenesis. Analysis of single-cell RNA sequencing data from murine and human testis demonstrated SFTPC expression uniquely during human spermatogenesis, suggesting that use of the human promoter in these constructs is responsible for male germline activity. Our data highlight the importance of careful analysis of transgenic allele activity and identify an SFTPC-Cre allele that is useful for panepithelial targeting in the mouse.

Addition of an N-Terminal Poly-Glutamate Fusion Tag Improves Solubility and Production of Recombinant TAT-Cre Recombinase in Escherichia coli.

Cre recombinase is widely used to manipulate DNA sequences for both in vitro and in vivo research. Attachment of a trans-activator of transcription (TAT) sequence to Cre allows TATCre to penetrate the cell membrane, and the addition of a nuclear localization signal (NLS) helps the enzyme to translocate into the nucleus. Since the yield of recombinant TAT-Cre is limited by formation of inclusion bodies, we hypothesized that the positively charged arginine-rich TAT sequence causes the inclusion body formation, whereas its neutralization by the addition of a negatively charged sequence improves solubility of the protein. To prove this, we neutralized the positively charged TAT sequence by proximally attaching a negatively charged poly-glutamate (E12) sequence. We found that the E12 tag improved the solubility and yield of E12-TAT-NLS-Cre (E12-TAT-Cre) compared with those of TAT-NLS-Cre (TATCre) when expressed in E. coli. Furthermore, the growth of cells expressing E12-TAT-Cre was increased compared with that of the cells expressing TAT-Cre. Efficacy of the purified TATCre was confirmed by a recombination test on a floxed plasmid in a cell-free system and 293 FT cells. Taken together, our results suggest that attachment of the E12 sequence to TAT-Cre improves its solubility during expression in E. coli (possibly by neutralizing the ionic-charge effects of the TAT sequence) and consequently increases the yield. This method can be applied to the production of transducible proteins for research and therapeutic purposes.

Expression and purification of codon-optimized cre recombinase in E. coli.

The presence of antibiotic resistance genes in genetically modified bacteria raises a regulatory concern in the production of therapeutic proteins and additionally reduces the number of plasmids available for propagation in a cell. Cre recombinase from bacteriophage P1, involved in Cre/loxP mechanism is one of the widely used systems for selectable marker gene removal. We have overexpressed codon-optimized cre gene in pColdIV and pET28a(+) vector systems and purified His6-Cre recombinase by immobilized metal affinity chromatography. N-terminal His6 tagged Cre recombinase obtained was approximately 26 fold purified and promoted the site-specific recombination of two loxP sites of linearized pLox2+ vector allowing the excision of a re-circularized plasmid and a short stretch of DNA containing the recombined loxP site. The results of the expression using two vectors, purification and activity assessment of His6 tagged Cre recombinase is presented here.

A novel conditional ZsGreen-expressing transgenic reporter rat strain for validating Cre recombinase expression.

The Cre/loxP recombination system has revolutionized the ability to genetically manipulate animal genomes in order to conditionally control gene expression. With recent advances in genome editing, barriers to manipulating the rat genome have been overcome and it is now possible to generate new rat strains (Cre drivers) in which Cre recombinase expression is carefully controlled temporally and/or spatially. However, the ability to evaluate and characterize these Cre driver strains is limited by the availability of reliable reporter rat strains. Here, we describe the generation and characterization of a new transgenic rat strain in which conditional expression of the ZsGreen fluorescent protein gene requires the presence of exogenous Cre recombinase. Breeding Cre-expressing rat strains to this stable ZsGreen reporter strain provides an ideal method for validating new rat Cre driver lines and will greatly accelerate the characterization pipeline.

Knock-In Rat Lines with Cre Recombinase at the Dopamine D1 and Adenosine 2a Receptor Loci.

Genetically modified mice have become standard tools in neuroscience research. Our understanding of the basal ganglia in particular has been greatly assisted by BAC mutants with selective transgene expression in striatal neurons forming the direct or indirect pathways. However, for more sophisticated behavioral tasks and larger intracranial implants, rat models are preferred. Furthermore, BAC lines can show variable expression patterns depending upon genomic insertion site. We therefore used CRISPR/Cas9 to generate two novel knock-in rat lines specifically encoding Cre recombinase immediately after the dopamine D1 receptor (Drd1a) or adenosine 2a receptor (Adora2a) loci. Here, we validate these lines using in situ hybridization and viral vector mediated transfection to demonstrate selective, functional Cre expression in the striatal direct and indirect pathways, respectively. We used whole-genome sequencing to confirm the lack of off-target effects and established that both rat lines have normal locomotor activity and learning in simple instrumental and Pavlovian tasks. We expect these new D1-Cre and A2a-Cre rat lines will be widely used to study both normal brain functions and neurological and psychiatric pathophysiology.

Achieving tight control of a photoactivatable Cre recombinase gene switch: new design strategies and functional characterization in mammalian cells and rodent.

A common mechanism for inducibly controlling protein function relies on reconstitution of split protein fragments using chemical or light-induced dimerization domains. A protein is split into fragments that are inactive on their own, but can be reconstituted after dimerization. As many split proteins retain affinity for their complementary half, maintaining low activity in the absence of an inducer remains a challenge. Here, we systematically explore methods to achieve tight regulation of inducible proteins that are effective despite variation in protein expression level. We characterize a previously developed split Cre recombinase (PA-Cre2.0) that is reconstituted upon light-induced CRY2-CIB1 dimerization, in cultured cells and in vivo in rodent brain. In culture, PA-Cre2.0 shows low background and high induced activity over a wide range of expression levels, while in vivo the system also shows low background and sensitive response to brief light inputs. The consistent activity stems from fragment compartmentalization that shifts localization toward the cytosol. Extending this work, we exploit nuclear compartmentalization to generate light-and-chemical regulated versions of Cre recombinase. This work demonstrates in vivo functionality of PA-Cre2.0, describes new approaches to achieve tight inducible control of Cre DNA recombinase, and provides general guidelines for further engineering and application of split protein fragments.

Diverse spinal commissural neuron populations revealed by fate mapping and molecular profiling using a novel Robo3Cre mouse.

The two sides of the nervous system coordinate and integrate information via commissural neurons, which project axons across the midline. Commissural neurons in the spinal cord are a highly heterogeneous population of cells with respect to their birthplace, final cell body position, axonal trajectory, and neurotransmitter phenotype. Although commissural axon guidance during development has been studied in great detail, neither the developmental origins nor the mature phenotypes of commissural neurons have been characterized comprehensively, largely due to lack of selective genetic access to these neurons. Here, we generated mice expressing Cre recombinase from the Robo3 locus specifically in commissural neurons. We used Robo3 Cre mice to characterize the transcriptome and various origins of developing commissural neurons, revealing new details about their extensive heterogeneity in molecular makeup and developmental lineage. Further, we followed the fate of commissural neurons into adulthood, thereby elucidating their settling positions and molecular diversity and providing evidence for possible functions in various spinal cord circuits. Our studies establish an important genetic entry point for further analyses of commissural neuron development, connectivity, and function.

Comparison of the Opn-CreER and Ck19-CreER Drivers in Bile Ducts of Normal and Injured Mouse Livers.

Inducible cyclization recombinase (Cre) transgenic mouse strains are powerful tools for cell lineage tracing and tissue-specific knockout experiments. However, low efficiency or leaky expression can be important pitfalls. Here, we compared the efficiency and specificity of two commonly used cholangiocyte-specific Cre drivers, the Opn-iCreERT2 and Ck19-CreERT drivers, using a tdTomato reporter strain. We found that Opn-iCreERT2 triggered recombination of the tdTomato reporter in 99.9% of all cholangiocytes while Ck19-CreERT only had 32% recombination efficiency after tamoxifen injection. In the absence of tamoxifen, recombination was also induced in 2% of cholangiocytes for the Opn-iCreERT2 driver and in 13% for the Ck19-CreERT driver. For both drivers, Cre recombination was highly specific for cholangiocytes since recombination was rare in other liver cell types. Toxic liver injury ectopically activated Opn-iCreERT2 but not Ck19-CreERT expression in hepatocytes. However, ectopic recombination in hepatocytes could be avoided by applying a three-day long wash-out period between tamoxifen treatment and toxin injection. Therefore, the Opn-iCreERT2 driver is best suited for the generation of mutant bile ducts, while the Ck19-CreERT driver has near absolute specificity for bile duct cells and is therefore favorable for lineage tracing experiments.

Selectable marker recycling in the nonconventional yeast Xanthophyllomyces dendrorhous by transient expression of Cre on a genetically unstable vector.

Selectable marker recycling is a basic technique in bioengineering. However, this technique is usually unavailable in non-model microorganisms. In this study, we proposed a simple and efficient method for selectable marker recycling in the astaxanthin-synthesizing yeast Xanthophyllomyces dendrorhous. This method was based on a Cre-loxP system, in which the transient expression of the Cre recombinase was controlled by a genetically unstable vector independent of episomal plasmids and inducible promoters. The selectable markers in single-gene locus and multigene loci were removed along with the loss of the Cre vector with a ratio of 100% and 29%, respectively. The significance of the method was highlighted by the finding that stable autotrophic mutants were not readily obtained in X. dendrorhous. Comparative studies in X. dendrorhous and the non-homologous end joining dominant yeast Yarrowia lipolytica suggested that the method could be universally used in homologous recombination dominant yeasts.

A novel family of tyrosine integrases encoded by the temperate pleolipovirus SNJ2.

Genomes of halophilic archaea typically contain multiple loci of integrated mobile genetic elements (MGEs). Despite the abundance of these elements, however, mechanisms underlying their site-specific integration and excision have not been investigated. Here, we identified and characterized a novel recombination system encoded by the temperate pleolipovirus SNJ2, which infects haloarchaeon Natrinema sp. J7-1. SNJ2 genome is inserted into the tRNAMet gene and flanked by 14 bp direct repeats corresponding to attachment core sites. We showed that SNJ2 encodes an integrase (IntSNJ2) that excises the proviral genome from its host cell chromosome, but requires two small accessory proteins, Orf2 and Orf3, for integration. These proteins were co-transcribed with IntSNJ2 to form an operon. Homology searches showed that IntSNJ2-type integrases are widespread in haloarchaeal genomes and are associated with various integrated MGEs. Importantly, we confirmed that SNJ2-like recombination systems are encoded by haloarchaea from three different genera and are critical for integration and excision. Finally, phylogenetic analysis suggested that IntSNJ2-type recombinases belong to a novel family of archaeal integrases distinct from previously characterized recombinases, including those from the archaeal SSV- and pNOB8-type families.

Recoding of synonymous genes to expand evolutionary landscapes requires control of secondary structure affecting translation.

Synthetic DNA design needs to harness the many information layers embedded in a DNA string. We previously developed the Evolutionary Landscape Painter (ELP), an algorithm that exploits the degeneracy of the code to increase protein evolvability. Here, we have used ELP to recode the integron integrase gene (intI1) in two alternative alleles. Although synonymous, both alleles yielded less IntI1 protein and were less active in recombination assays than intI1. We spliced the three alleles and mapped the activity decrease to the beginning of alternative sequences. Mfold predicted the presence of more stable secondary structures in the alternative genes. Using synonymous mutations, we decreased their stability and recovered full activity. Following a design-build-test approach, we have now updated ELP to consider such structures and provide streamlined alternative sequences. Our results support the possibility of modulating gene activity through the ad hoc design of 5' secondary structures in synthetic genes.

Rapid characterization of the CHO platform cell line and identification of pseudo attP sites for PhiC31 integrase.

The Chinese Hamster Ovary (CHO) cell lines, applicable to post-translational modifications, are preferred systems for biopharmaceutical protein production. In this study, by using the Jump-In™ TI™ technology which employs PhiC31 and R4 bacteriophage recombinases, a platform CHO-K1 cell line containing a R4-attP site was generated. Here, a combination of Quantitative Fluorescent-Polymerase Chain Reaction (QF-PCR) and semi-random, two-step PCR (ST-PCR), was performed to feature the platform cell clones. Our results show that QF-PCR and ST-PCR, can be utilized for efficient and accelerated cell line characterization. By applying these approaches, we were able to accurately identify the copy number of integrated R4-attP sites and the genomic position of recombination of many clones. Three novel PhiC31 pseudo-attP sites were identified on chromosomes 1, 3 and 6, and their genomic features were analyzed. The characterized platform cell lines are stable, and because of single-copy, site-specific R4 recognition attP site, the cell lines could be retargeted for recombinant protein production and drug discovery applications.

Genetic tracing of hepatocytes in liver homeostasis, injury, and regeneration.

The liver possesses a remarkable capacity to regenerate after damage. There is a heated debate on the origin of new hepatocytes after injuries in adult liver. Hepatic stem/progenitor cells have been proposed to produce functional hepatocytes after injury. Recent studies have argued against this model and suggested that pre-existing hepatocytes, rather than stem cells, contribute new hepatocytes. This hepatocyte-to-hepatocyte model is mainly based on labeling of hepatocytes with Cre-recombinase delivered by the adeno-associated virus. However, the impact of virus infection on cell fate determination, consistency of infection efficiency, and duration of Cre-virus in hepatocytes remain confounding factors that interfere with the data interpretation. Here, we generated a new genetic tool Alb-DreER to label almost all hepatocytes (>99.5%) and track their contribution to different cell lineages in the liver. By "pulse-and-chase" strategy, we found that pre-existing hepatocytes labeled by Alb-DreER contribute to almost all hepatocytes during normal homeostasis and after liver injury. Virtually all hepatocytes in the injured liver are descendants of pre-existing hepatocytes through self-expansion. We concluded that stem cell differentiation is unlikely to be responsible for the generation of a substantial number of new hepatocytes in adult liver. Our study also provides a new mouse tool for more precise in vivo genetic study of hepatocytes in the field.