Cell Lineage Tracing: Colocalization of Cell Lineage Markers with a Fluorescent Reporter

Dynamic interactions between cartilaginous and tendinous/ligamentous primordia during musculoskeletal integration

Proper connections between cartilaginous and muscular primordia through tendinous/ligamentous primordia are essential for musculoskeletal integration. Herein, we report a novel double-reporter mouse model for investigating this process via fluorescently visualising scleraxis (Scx) and SRY-box containing gene 9 (Sox9) expression. We generated ScxTomato transgenic mice and crossed them with Sox9EGFP knock-in mice to obtain ScxTomato;Sox9EGFP mice. Deep imaging of optically cleared double-reporter embryos at E13.5 and E16.5 revealed previously unknown differences in the dynamic interactions between cartilaginous and tendinous/ligamentous primordia in control and Scx-deficient mice. Tendon/ligament maturation was evaluated through simultaneous detection of fluorescence and visualisation of collagen fibre formation using second harmonic generation imaging. Lack of deltoid tuberosity in Scx-deficient mice caused misdirected muscle attachment with morphological changes. Loss of Scx also dysregulated progenitor cell fate determination in the chondrotendinous junction, resulting in the formation of a rounded enthesis rather than the protruding enthesis observed in the control. Hence, our double-reporter mouse system, in combination with loss- or gain-of-function approaches, is a unique and powerful tool that could be used to gain a comprehensive understanding of musculoskeletal integration.

Current Practice in Bicistronic IRES Reporter Use: A Systematic Review

Bicistronic reporter assays have been instrumental for transgene expression, understanding of internal ribosomal entry site (IRES) translation, and identification of novel cap-independent translational elements (CITE). We observed a large methodological variability in the use of bicistronic reporter assays and data presentation or normalization procedures. Therefore, we systematically searched the literature for bicistronic IRES reporter studies and analyzed methodological details, data visualization, and normalization procedures. Two hundred fifty-seven publications were identified using our search strategy (published 1994-2020). Experimental studies on eukaryotic adherent cell systems and the cell-free translation assay were included for further analysis. We evaluated the following methodological details for 176 full text articles: the bicistronic reporter design, the cell line or type, transfection methods, and time point of analyses post-transfection. For the cell-free translation assay, we focused on methods of in vitro transcription, type of translation lysate, and incubation times and assay temperature. Data can be presented in multiple ways: raw data from individual cistrons, a ratio of the two, or fold changes thereof. In addition, many different control experiments have been suggested when studying IRES-mediated translation. In addition, many different normalization and control experiments have been suggested when studying IRES-mediated translation. Therefore, we also categorized and summarized their use. Our unbiased analyses provide a representative overview of bicistronic IRES reporter use. We identified parameters that were reported inconsistently or incompletely, which could hamper data reproduction and interpretation. On the basis of our analyses, we encourage adhering to a number of practices that should improve transparency of bicistronic reporter data presentation and improve methodological descriptions to facilitate data replication.