Reproducibility of CRISPR-Cas9 methods for generation of conditional mouse alleles: a multi-center evaluation

BACKGROUND

CRISPR-Cas9 gene-editing technology has facilitated the generation of knockout mice, providing an alternative to cumbersome and time-consuming traditional embryonic stem cell-based methods. An earlier study reported up to 16% efficiency in generating conditional knockout (cKO or floxed) alleles by microinjection of 2 single guide RNAs (sgRNA) and 2 single-stranded oligonucleotides as donors (referred herein as "two-donor floxing" method).

RESULTS

We re-evaluate the two-donor method from a consortium of 20 laboratories across the world. The dataset constitutes 56 genetic loci, 17,887 zygotes, and 1718 live-born mice, of which only 15 (0.87%) mice contain cKO alleles. We subject the dataset to statistical analyses and a machine learning algorithm, which reveals that none of the factors analyzed was predictive for the success of this method. We test some of the newer methods that use one-donor DNA on 18 loci for which the two-donor approach failed to produce cKO alleles. We find that the one-donor methods are 10- to 20-fold more efficient than the two-donor approach.

CONCLUSION

We propose that the two-donor method lacks efficiency because it relies on two simultaneous recombination events in cis, an outcome that is dwarfed by pervasive accompanying undesired editing events. The methods that use one-donor DNA are fairly efficient as they rely on only one recombination event, and the probability of correct insertion of the donor cassette without unanticipated mutational events is much higher. Therefore, one-donor methods offer higher efficiencies for the routine generation of cKO animal models.

A Subset of Type I Conventional Dendritic Cells Controls Cutaneous Bacterial Infections through VEGFα-Mediated Recruitment of Neutrophils

Skin conventional dendritic cells (cDCs) exist as two distinct subsets, cDC1s and cDC2s, which maintain the balance of immunity to pathogens and tolerance to self and microbiota. Here, we examined the roles of dermal cDC1s and cDC2s during bacterial infection, notably Propionibacterium acnes (P. acnes). cDC1s, but not cDC2s, regulated the magnitude of the immune response to P. acnes in the murine dermis by controlling neutrophil recruitment to the inflamed site and survival and function therein. Single-cell mRNA sequencing revealed that this regulation relied on secretion of the cytokine vascular endothelial growth factor α (VEGF-α) by a minor subset of activated EpCAM+CD59+Ly-6D+ cDC1s. Neutrophil recruitment by dermal cDC1s was also observed during S. aureus, bacillus Calmette-Guérin (BCG), or E. coli infection, as well as in a model of bacterial insult in human skin. Thus, skin cDC1s are essential regulators of the innate response in cutaneous immunity and have roles beyond classical antigen presentation.

The Generation and Use of Allergen-Specific TCR Transgenic Animals

The generation of allergen-specific TCR transgenic animals allows for the characterization of allergen-specific T-cell responses in vivo and in vitro and is a powerful tool to study adaptive immunity to allergens. Here we describe an approach starting from the isolation of antigen-specific T-cell hybridomas and using PCR, flow cytometric, and co-culture methods to obtain antigen-specific MHC class II-restricted CD4⁺ TCR transgenic mice on the Rag2^-/- background.

ORMDL3 expression levels have no influence on the activity of serine palmitoyltransferase

ORMDL proteins are believed to be negative regulators of serine palmitoyltransferase (SPT), which catalyzes the first and rate limiting step in sphingolipid (SL) de novo synthesis. Several single-nucleotide polymorphisms (SNPs) that are close to the ORMDL3 locus have been reported to increase ORMDL3 expression and to be associated with an elevated risk for early childhood asthma; however, the direct effect of ORMDL3 expression on SPT activity and its link to asthma remains elusive. In this study, we investigated whether ORMDL3 expression is associated with changes in SPT activity and total SL levels. Ormdl3-knockout (Ormdl3^-/-) and transgenic (Ormdl3^Tg/wt) mice were generated to study the effect of ORMDL3 on total SL levels in plasma and tissues. Cellular SPT activity was measured in mouse embryonic fibroblasts from Ormdl3^-/- mice, as well as in HEK293 cells in which ORMDL3 was overexpressed and silenced. Furthermore, we analyzed the association of the reported ORMDL3 asthma SNPs with plasma sphingoid bases in a population-based cohort of 971 individuals. Total C₁₈-long chain bases were not significantly altered in the plasma and tissues of Ormdl3^-/- mice, whereas C₁₈-sphinganine showed a small and significant increase in plasma, lung, and liver tissues. Mouse embryonic fibroblast cells from Ormdl3^-/- mice did not show an altered SPT activity compared with Ormdl3^+/- and Ormdl3^+/+ mice. Overexpression or knockdown of ORMDL3 in HEK293 cells did not alter SPT activity; however, parallel knockdown of all 3 ORMDL isoforms increased enzyme activity significantly. A significant association of the annotated ORMDL3 asthma SNPs with plasma long-chain sphingoid base levels could not be confirmed. ORMDL3 expression levels seem not to be directly associated with changes in SPT activity. ORMDL3 might influence de novo sphingolipid metabolism downstream of SPT.-Zhakupova, A., Debeuf, N., Krols, M., Toussaint, W., Vanhoutte, L., Alecu, I., Kutalik, Z., Vollenweider, P., Ernst, D., von Eckardstein, A., Lambrecht, B. N., Janssens, S., Hornemann, T. ORMDL3 expression levels have no influence on the activity of serine palmitoyltransferase.

Alveolar macrophages develop from fetal monocytes that differentiate into long-lived cells in the first week of life via GM-CSF

Alveolar macrophages differentiate from fetal monocytes in a GM-CSF–dependent fashion and colonize the alveolar space within a few days after birth.

Tissue-resident macrophages can develop from circulating adult monocytes or from primitive yolk sac–derived macrophages. The precise ontogeny of alveolar macrophages (AMFs) is unknown. By performing BrdU labeling and parabiosis experiments in adult mice, we found that circulating monocytes contributed minimally to the steady-state AMF pool. Mature AMFs were undetectable before birth and only fully colonized the alveolar space by 3 d after birth. Before birth, F4/80^hiCD11b^lo primitive macrophages and Ly6C^hiCD11b^hi fetal monocytes sequentially colonized the developing lung around E12.5 and E16.5, respectively. The first signs of AMF differentiation appeared around the saccular stage of lung development (E18.5). Adoptive transfer identified fetal monocytes, and not primitive macrophages, as the main precursors of AMFs. Fetal monocytes transferred to the lung of neonatal mice acquired an AMF phenotype via defined developmental stages over the course of one week, and persisted for at least three months. Early AMF commitment from fetal monocytes was absent in GM-CSF–deficient mice, whereas short-term perinatal intrapulmonary GM-CSF therapy rescued AMF development for weeks, although the resulting AMFs displayed an immature phenotype. This demonstrates that tissue-resident macrophages can also develop from fetal monocytes that adopt a stable phenotype shortly after birth in response to instructive cytokines, and then self-maintain throughout life.

Nuclear and cytoplasmic maturation of in vitro matured human oocytes after temporary nuclear arrest by phosphodiesterase 3-inhibitor

BACKGROUND

The use of hormones for controlled ovarian stimulation results in follicular heterogeneity, with oocytes at diverse stages of nuclear and cytoplasmic development. This study evaluated the impact of temporary nuclear arrest by a specific phosphodiesterase 3-inhibitor (PDE3-I), cilostamide, on nuclear and cytoplasmic maturation of cumulus-free germinal vesicle (GV) human oocytes from controlled ovarian stimulated cycles.

METHODS

GV oocytes (n = 234) were cultured in: (i) medium without the inhibitor (control); (ii) medium supplemented with 1 microM cilostamide and (iii) medium supplemented with 10 microM cilostamide. Oocytes in groups (ii) and (iii) were exposed to cilostamide for 24 h. The PDE3-I was subsequently removed by transfer of oocytes to fresh in vitro maturation (IVM) medium and the reversibility of GV arrest was assessed during IVM culture for maximum 48 h.

RESULTS

Cilostamide (1 and 10 microM) could maintain >80% of the oocytes at the GV stage, without affecting subsequent maturation to metaphase II. Oocytes exposed to 1 microM cilostamide were more likely to have normal bipolar spindles with aligned chromosomes than control oocytes (P < 0.05). When GV chromatin configurations before and after arrest were compared, a significantly higher proportion of oocytes had acquired a nucleolus completely surrounded by a rim of highly condensed chromatin (P < 0.05).

CONCLUSIONS

Temporary nuclear arrest of human GV oocytes with PDE3-I proved to be beneficial for obtaining normal spindle and chromosome configurations after IVM. It resulted also in synchronization within the population of GV oocytes.

Clinical benefit of metaphase I oocytes

BACKGROUND

We studied the benefit of using in vitro matured metaphase I (MI) oocytes for ICSI in patients with a maximum of 6 mature metaphase II (MII) oocytes at retrieval.

METHODS

In 2004, 187 ICSI cycles were selected in which maximum 6 MII oocytes and at least one MI oocyte were retrieved. MI oocytes were put in culture to mature until the moment of ICSI, which was performed between 2 to 11 hours after oocyte retrieval (day 0). In exceptional cases, when the patient did not have any mature oocyte at the scheduled time of ICSI, MI oocytes were left to mature overnight and were injected between 19 to 26 hours after retrieval (day 1). Embryos from MI oocytes were chosen for transfer only when no other good quality embryos from MII oocytes were available. Outcome parameters were time period of in vitro maturation (IVM), IVM and fertilization rates, embryo development, clinical pregnancy rates, implantation rates and total MI oocyte utilization rate.

RESULTS

The overall IVM rate was 43%. IVM oocytes had lower fertilization rates compared to in vivo matured sibling oocytes (52% versus 68%, P < 0.05). The proportion of poor quality embryos was significantly higher in IVM derived oocytes. One pregnancy and live birth was obtained out of 13 transfers of embryos exclusively derived from IVM oocytes. This baby originated from an oocyte that was injected after 22 hrs of IVM.

CONCLUSION

Fertilization of in vitro matured MI oocytes can result in normal embryos and pregnancy, making IVM worthwhile, particularly when few MII oocytes are obtained at retrieval.

Effects of chilling on structural aspects of early preantral mouse follicles

Chilling injury is one of the major limiting factors for achieving optimal cryopreservation of gametes. This study aimed to determine potential chilling-induced damage on several structural aspects of early preantral mouse follicles. Mechanically isolated intact early preantral follicles (type 3b-4) were exposed to 0 degrees C for 1, 5, 10, or 30 min. Control and chilled follicles were analyzed by confocal microscopy after staining for tubulin, F-actin, and chromatin, and by electron microscopy. Chilling for only 1 min was sufficient to cause depolymerization of microtubules in the oocyte and the surrounding granulosa cell layer as evidenced by a substantial decrease in fluorescence intensity after antitubulin labeling. Cooling for longer periods caused alterations in microtubule organization in the follicle-enclosed oocyte. These alterations included the loss of interphase microtubules, concomitant with the formation of perinuclear or cortical microtubule asters and sometimes a complete disappearance of microtubules. The extent of microtubule modification was related to the time of chilling, but was fully reversible after rewarming follicles at 37 degrees C for 1 h. Chilling had only minor effects on the actin-containing elements located predominantly in the oocyte cortex and the transzonal projections. Ultrastructural analysis confirmed that oocyte-somatic cell interactions were present. There was no influence on the chromatin configuration within the follicle-enclosed oocyte. These results indicate that mouse follicles are relatively tolerant to direct chilling injury and, as a consequence, are able to withstand the cooling-warming steps during conventional cryopreservation procedures.

Effect of phosphodiesterase type 3 inhibitor on developmental competence of immature mouse oocytes in vitro

In vitro use of arresters of meiosis could improve cytoplasmic maturation of immature oocytes by controlling the period of prophase I. Phosphodiesterases (PDE) are responsible for the breakdown and concomitant inactivation of the cyclic nucleotides cAMP and cGMP and are implicated in the regulation of oocyte meiotic maturation. Selective inhibitors of phosphodiesterase type 3 (PDE3) prevent meiotic resumption of mammalian oocytes. This study evaluated the impact of meiosis arrest by PDE3 inhibitor, Org 9935, on developmental competence of geminal vesicle (GV)-stage oocytes from small antral follicles. Cumulus-oocyte complexes (COC), retrieved from antral follicles 24 h after eCG exposure and cultured in the presence of PDE3 inhibitor (10 microM) for an additional 24 h, remained arrested in the meiotic prophase. The GV configuration of oocytes before and after the arrest by PDE3 inhibitor was examined. After the period of meiosis arrest, a significantly increased proportion of oocytes had acquired a nucleolus surrounded by a condensed chromatin rim at the GV, which is a morphological correlate of transcriptional repression. Removal of inhibitor resulted in 90.6% +/- 8.3% of oocytes with the first polar body extruded. Fertilization was significantly improved in oocytes that had been arrested compared with oocytes collected 24 h after eCG and undergoing in vitro maturation immediately. Embryonic preimplantation and live offspring rates of arrested oocytes were higher, although not significantly, than those of nonarrested oocytes. These results suggest that a temporal block of meiosis by PDE3 inhibitor promotes developmental competence of mice oocytes retrieved from small antral follicles.