Rosa26/Safe-Harbor Reporter Knock-in Model
Materials Required
Principle
The Rosa26 locus is widely used as a genomic \"safe harbor\" site because it supports stable and ubiquitous transgene expression without overt disruption of endogenous organismal development, enabling consistent reporter or functional gene expression across tissues in multiple mammalian species[1][3][4]. Reporter knock-in models at Rosa26 are typically generated by inserting a transgene cassette (e.g., fluorescent proteins or Cre-dependent reporters) into the locus using either CRISPR/Cas9-mediated homology-directed repair in zygotes or homologous recombination in embryonic stem cells, resulting in germline-transmissible alleles[1][2][3]. After correct integration, reporter expression is driven by inserted regulatory elements (commonly ubiquitous promoters such as CAG or endogenous Rosa26 regulatory context), allowing visualization of gene expression patterns or lineage tracing through fluorescence or recombinase-dependent activation[1][3]. These models function as readouts of genome editing efficiency and provide a stable platform for in vivo tracking of cellular populations or gene regulation dynamics[3][4].
MCE has not independently verified the accuracy of these methods. They are for reference only.
Experimental Materials
• SgRNA targeting Rosa26 intronic regions: guides Cas9 to the safe-harbor locus for precise genome cleavage[1][2].
• Donor DNA plasmid (reporter cassette such as EGFP/tdTomato or Cre-dependent constructs): provides the template for targeted insertion[1][3].
• Homology arms (short and long arms flanking insertion cassette): facilitate precise recombination at Rosa26 locus[1][2].
• Embryo culture media and microinjection buffers: maintain viability of zygotes during manipulation[1][2].
• Fluorescent reporter detection systems (e.g., EGFP/tdTomato fluorescence readouts): used to confirm successful knock-in and tissue-wide expression[3][4].
• PCR genotyping reagents: used to validate correct integration at 5’ and 3’ junctions[1][2].
• Pronuclear microinjection system: delivers CRISPR components and donor DNA into fertilized zygotes[1][2].
• Embryo culture incubator: supports development of injected embryos to blastocyst or transfer stage[1].
• PCR and gel electrophoresis systems: used for screening founder animals or modified cells[1][2].
• Fluorescence microscope: detects reporter expression in embryos or tissues[3].
• Southern blot apparatus (when used): confirms correct genomic integration and copy number[1]
Experimental Procedure
Preparation Steps
• Superovulation is induced in donor females to obtain fertilized embryos for microinjection, followed by collection of zygotes at the pronuclear stage suitable for genome editing[1][2].• Donor vectors are constructed with reporter genes flanked by Rosa26 homology arms, typically optimized for efficient homology-directed repair[1].
• CRISPR reagents (Cas9 mRNA or protein and sgRNA) are prepared under embryo-compatible conditions for direct microinjection or electroporation into zygotes[1][2].
• Embryos are cultured briefly in vitro before transfer to pseudopregnant surrogate females[1].
Operation Steps
• CRISPR/Cas9 components and donor DNA are co-injected into fertilized zygotes at the pronuclear stage to induce a targeted double-strand break at the Rosa26 locus and stimulate homology-directed repair-mediated insertion of the reporter cassette[1][2].• For mouse models, large inserts (approximately 8-11 kb) have been successfully integrated using this approach, demonstrating feasibility for complex reporter constructs[1].
• Injected embryos are cultured and transferred into surrogate females for gestation and birth of founder animals[1][2]. Founder screening is performed using PCR amplification of 5’ and 3’ junctions to confirm precise integration at the Rosa26 locus, followed by fluorescence assessment of reporter expression in live tissues or embryos[1][3].
• In some systems, additional validation such as Southern blot analysis is used to confirm single-copy insertion and correct genomic architecture[1].
Data Acquisition and Analysis
• Successful knock-in is indicated by ubiquitous or tissue-wide reporter expression consistent with Rosa26-driven transcriptional activity, often observable across multiple organs in F1 animals[3][4].• PCR-positive founders are further analyzed for germline transmission by breeding, and offspring are assessed for stable reporter expression across tissues[2][3].
• Experimental controls include wild-type animals (negative fluorescence control) and known Rosa26 reporter lines (positive control for expression consistency)[3].
• Quantitative assessment may include fluorescence imaging intensity comparison and genotyping confirmation across biological replicates[2][3].
Troubleshooting
Problem: Few or no correctly targeted founders obtained after microinjection.
Possible Cause
Inefficient homology-directed repair or suboptimal donor design.Literature-supported Solution
Optimization of homology arm design and use of Cas9 protein or mRNA with sgRNA in zygotes has been shown to improve knock-in efficiency, including recovery of up to a substantial fraction of correctly targeted embryos under optimized conditions[1].Problem 2: Mosaic reporter expression in founder animals
Problem: Founders show patchy or inconsistent reporter fluorescence.
Possible Cause
CRISPR editing occurring after first cell division leading to mosaicism.Literature-supported Solution
Delivery of CRISPR components into pronuclear-stage zygotes prior to the first mitotic division reduces mosaicism and improves uniform integration of Rosa26 reporter alleles[2].Problem 3: Lack of detectable reporter expression despite correct genotyping
Problem: PCR-confirmed knock-in animals fail to show fluorescence.
Possible Cause
Disruption of promoter activity or incorrect cassette orientation.Literature-supported Solution
Use of validated ubiquitous promoter-driven cassettes (e.g., CAG-driven reporters) and confirmed Rosa26-compatible constructs is critical for ensuring robust expression after integration[1][3].References:
- [1]. Chu VT, Weber T, Graf R, Sommermann T, Petsch K, Sack U, et al. Efficient generation of Rosa26 knock-in mice using CRISPR/Cas9 in C57BL/6 zygotes. BMC Biotechnology. 2016;16:23.
- [2]. Abe T, et al. Efficient CRISPR/Cas9-mediated knockin of reporter genes in rats at ROSA26 by pronuclear microinjection. Development, Growth Differentiation. 2025;67:215-225.
- [3]. Wu Y, et al. Characterization of knockin mice at the Rosa26, Tac1 and Plekhg1 loci generated by homologous recombination in oocytes. PLoS ONE. 2018;13:e0193129. [Content Brief]
- [4]. Kobayashi T, et al. Identification of Rat Rosa26 Locus Enables Generation of Knock-In Rat Lines Ubiquitously Expressing tdTomato. Stem Cells and Development. 2012;21(16):2981-2986. [Content Brief]
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