Reporter alleles that inform on differences in Cre recombinase expression

A most formidable arsenal: genetic technologies for building a better mouse

The mouse is one of the most widely used model organisms for genetic study. The tools available to alter the mouse genome have developed over the preceding decades from forward screens to gene targeting in stem cells to the recent influx of CRISPR approaches. In this review, we first consider the history of mice in genetic study, the development of classic approaches to genome modification, and how such approaches have been used and improved in recent years. We then turn to the recent surge of nuclease-mediated techniques and how they are changing the field of mouse genetics. Finally, we survey common classes of alleles used in mice and discuss how they might be engineered using different methods.

Refinement of inducible gene deletion in embryos of pregnant mice

CreERT2‐mediated gene recombination is widely applied in developmental biology research. Activation of CreERT2 is typically achieved by injection of tamoxifen in an oily vehicle into the peritoneal cavity of mid‐gestation pregnant mice. This can be technically challenging and adversely impacts welfare. Here we characterize three refinements to this technique: Pipette feeding (not gavage) of tamoxifen, ex vivo CreERT2 activation in whole embryo culture and injection of cell‐permeable TAT‐Cre into Cre‐negative cultured embryos. We demonstrate that pipette feeding of tamoxifen solution to the mother on various days of gestation reliably activates embryonic CreERT2, illustrated here using β‐Actin^CreERT2, Sox2^CreERT2, T^CreERT2, and Nkx1.2^CreERT2. Pipette feeding of tamoxifen induces dose‐dependent recombination of Rosa26^mTmG reporters when administered at E8.5. Activation of two neuromesodermal progenitor‐targeting Cre drivers, T^CreERT2, and Nkx1.2^CreERT2, produces comparable neuroepithelial lineage tracing. Dose‐dependent CreERT2 activation can also be achieved by brief exposure to 4OH‐tamoxifen in whole embryo culture, allowing temporal control of gene deletion and eliminating the need to treat pregnant mice. Rosa26^mTmG reporter recombination can also be achieved regionally by injecting TAT‐Cre into embryonic tissues at the start of culture. This allows greater spatial control over Cre activation than can typically be achieved with endogenous CreERT2, for example by injecting TAT‐Cre on one side of the midline. We hope that our description and application of these techniques will stimulate refinement of experimental methods involving CreERT2 activation for gene deletion and lineage tracing studies. Improved temporal (ex vivo treatment) and spatial (TAT‐Cre injection) control of recombination will also allow previously intractable questions to be addressed.

Heterogeneous Cellular Contributions to Elastic Laminae Formation in Arterial Wall Development

RATIONALE

Elastin is an important ECM (extracellular matrix) protein in large and small arteries. Vascular smooth muscle cells (SMCs) produce the layered elastic laminae found in elastic arteries but synthesize little elastin in muscular arteries. However, muscular arteries have a well-defined internal elastic lamina (IEL) that separates endothelial cells (ECs) from SMCs. The extent to which ECs contribute elastin to the IEL is unknown.

OBJECTIVE

To use targeted elastin (Eln) deletion in mice to explore the relative contributions of SMCs and ECs to elastic laminae formation in different arteries.

METHODS AND RESULTS

We used SMC- and EC-specific Cre recombinase transgenes with a novel floxed Eln allele to focus gene inactivation in mice. Inactivation of Eln in SMCs using Sm22aCre resulted in depletion of elastic laminae in the arterial wall with the exception of the IEL and SMC clusters in the outer media near the adventitia. Inactivation of elastin in ECs using Tie2Cre or Cdh5Cre resulted in normal medial elastin and a typical IEL in elastic arteries. In contrast, the IEL was absent or severely disrupted in muscular arteries. Interruptions in the IEL resulted in neointimal formation in the ascending aorta but not in muscular arteries.

CONCLUSIONS

Combined with lineage-specific fate mapping systems, our knockout results document an unexpected heterogeneity in vascular cells that produce the elastic laminae. SMCs and ECs can independently form an IEL in most elastic arteries, whereas ECs are the major source of elastin for the IEL in muscular and resistance arteries. Neointimal formation at IEL disruptions in the ascending aorta confirms that the IEL is a critical physical barrier between SMCs and ECs in the large elastic arteries. Our studies provide new information about how SMCs and ECs contribute elastin to the arterial wall and how local elastic laminae defects may contribute to cardiovascular disease.

A cell fitness selection model for neuronal survival during development

Developmental cell death plays an important role in the construction of functional neural circuits. In vertebrates, the canonical view proposes a selection of the surviving neurons through stochastic competition for target-derived neurotrophic signals, implying an equal potential for neurons to compete. Here we show an alternative cell fitness selection of neurons that is defined by a specific neuronal heterogeneity code. Proprioceptive sensory neurons that will undergo cell death and those that will survive exhibit different molecular signatures that are regulated by retinoic acid and transcription factors, and are independent of the target and neurotrophins. These molecular features are genetically encoded, representing two distinct subgroups of neurons with contrasted functional maturation states and survival outcome. Thus, in this model, a heterogeneous code of intrinsic cell fitness in neighboring neurons provides differential competitive advantage resulting in the selection of cells with higher capacity to survive and functionally integrate into neural networks.

Cellular and Molecular Analysis of Dendritic Morphogenesis in a Retinal Cell Type That Senses Color Contrast and Ventral Motion

As neuronal dendrites develop, they acquire cell-type-specific features including characteristic size, shape, arborization, location and synaptic patterns. These features, in turn, are major determinants of type-specific neuronal function. Because neuronal diversity complicates the task of relating developmental programs to adult structure and function, we analyzed dendritic morphogenesis in a single retinal ganglion cell (RGC) type in mouse called J-RGC. We documented the emergence of five dendritic features that underlie J-RGC physiology: (1) dendritic field size, which approximate receptive field size; (2) dendritic complexity, which affects how J-RGCs sample space; (3) asymmetry, which contributes to direction-selectivity; (4) restricted lamination within the inner plexiform layer (IPL), which renders J-RGCs responsive to light decrements; and (5) distribution of synaptic inputs, which generate a color-opponent receptive field. We found dendritic growth in J-RGCs is accompanied by a refinement in dendritic self-crossing. Asymmetry arises by a combination of selective pruning and elaboration, whereas laminar restriction results from biased outgrowth toward the outermost IPL. Interestingly, asymmetry develops in a protracted dorsoventral wave, whereas lamination does so in a rapid centrifugal wave. As arbors mature, they acquire excitatory and inhibitory synapses, with the latter forming first and being concentrated in proximal dendrites. Thus, distinct mechanisms operate in different spatiotemporal dimensions of J-RGC dendritic patterning to generate the substrate for specific patterns of synaptogenesis. Finally, we asked whether the defining molecular signature of J-RGCs, the adhesion molecule JAM-B, regulates morphogenesis, and showed that it promotes dendro-dendritic interactions. Our results reveal multiple mechanisms that shape a dendritic arbor.SIGNIFICANCE STATEMENT Visual perception begins in the retina, where distinct types of retinal ganglion cells (RGCs) are tuned to specific visual features such as direction of motion. The features to which each RGC type responds are determined largely by the number and type of synaptic inputs it receives, and these, in turn, are greatly influenced by the size, shape, arborization pattern, and location of its dendrites. We analyzed dendritic morphogenesis in a functionally characterized RGC type, the J-RGC, demonstrating distinct mechanisms that operate in different dimensions to generate the dendritic scaffold and synaptic patterns for feature detection. Our work elucidates cellular and molecular mechanisms that shape dendritic arbors and synaptic distribution, enabling J-RGC connectivity and thus, function.

Cre Recombinase and Other Tyrosine Recombinases

Tyrosine-type site-specific recombinases (T-SSRs) have opened new avenues for the predictable modification of genomes as they enable precise genome editing in heterologous hosts. These enzymes are ubiquitous in eubacteria, prevalent in archaea and temperate phages, present in certain yeast strains, but barely found in higher eukaryotes. As tools they find increasing use for the generation and systematic modification of genomes in a plethora of organisms. If applied in host organisms, they enable precise DNA cleavage and ligation without the gain or loss of nucleotides. Criteria directing the choice of the most appropriate T-SSR system for genetic engineering include that, whenever possible, the recombinase should act independent of cofactors and that the target sequences should be long enough to be unique in a given genome. This review is focused on recent advancements in our mechanistic understanding of simple T-SSRs and their application in developmental and synthetic biology, as well as in biomedical research.

Analysis of GzmbCre as a Model System for Gene Deletion in the Natural Killer Cell Lineage

The analysis of gene function in mature and activated natural killer cells has been hampered by the lack of model systems for Cre-mediated recombination in these cells. Here we have investigated the utility of Gzmb^Cre for recombination of loxp sequences in these cells predicated on the observation that Gzmb mRNA is highly expressed in mature and activated natural killer cells. Using two different reporter strains we determined that gene function could be investigated in mature natural killer cells after Gzmb^Cre mediated recombination in vitro in conditions that lead to natural killer cell activation such as in the cytokine combination of interleukin 2 and interleukin 12. We demonstrated the utility of this model by creating Gzmb^Cre;Rosa26^IKKbca mice in which Cre-mediated recombination resulted in expression of constitutively active IKKβ, which results in activation of the NFκB transcription factor. In vivo and in vitro activation of IKKβ in natural killer cells revealed that constitutive activation of this pathway leads to natural killer cell hyper-activation and altered morphology. As a caveat to the use of Gzmb^Cre we found that this transgene can lead to recombination in all hematopoietic cells the extent of which varies with the particular loxp flanked allele under investigation. We conclude that Gzmb^Cre can be used under some conditions to investigate gene function in mature and activated natural killer cells.