Altered paracrine signaling from the injured knee joint impairs postnatal long bone growth

Look who's TORking: mTOR-mediated integration of cell status and external signals during limb development and endochondral bone growth

The balance of cell proliferation and size is key for the control of organ development and repair. Moreover, this balance has to be coordinated within tissues and between tissues to achieve robustness in the organ's pattern and size. The tetrapod limb has been used to study these topics during development and repair, and several conserved pathways have emerged. Among them, mechanistic target of rapamycin (mTOR) signaling, despite being active in several cell types and developmental stages, is one of the least understood in limb development, perhaps because of its multiple potential roles and interactions with other pathways. In the body of this review, we have collated and integrated what is known about the role of mTOR signaling in three aspects of tetrapod limb development: 1) limb outgrowth; 2) chondrocyte differentiation after mesenchymal condensation and 3) endochondral ossification-driven longitudinal bone growth. We conclude that, given its ability to interact with the most common signaling pathways, its presence in multiple cell types, and its ability to influence cell proliferation, size and differentiation, the mTOR pathway is a critical integrator of external stimuli and internal status, coordinating developmental transitions as complex as those taking place during limb development. This suggests that the study of the signaling pathways and transcription factors involved in limb patterning, morphogenesis and growth could benefit from probing the interaction of these pathways with mTOR components.

CRISPR/Cas9-mediated knock-in strategy at the Rosa26 locus in cattle fetal fibroblasts

The genetic modification of cattle has many agricultural and biomedical applications. However, random integration often leads to the unstable or differentially expression of the exogenous genes, which limit the application and development of transgenic technologies. Finding a safe locus suitable for site-specific insertion and efficient expression of exogenous genes is a good way to overcome these hurdles. In this study, we efficiently integrated three targeted vector into the cattle Rosa26 (cRosa26) by CRISPR/Cas9 technology in which EGFP was driven by CAG, EF1a, PGK and cRosa26 endogenous promoter respectively. The CRISPR/Cas9 knock-in system allows highly efficient gene insertion of different expression units at the cRosa26 locus. We also find that in the four cell lines, EGFP was stable expressed at different times, and the CAG promoter has the highest activity to activate the expression of EGFP, when compared with the cRosa26, EF1a and PGK promoter. Our results proved that cRosa26 was a locus that could integrate different expression units efficiently, and supported the friendly expression of different expression units. Our findings described here will be useful for a variety of studies using cattle.

A New Pipeline to Automatically Segment and Semi-Automatically Measure Bone Length on 3D Models Obtained by Computed Tomography

The characterization of developmental phenotypes often relies on the accurate linear measurement of structures that are small and require laborious preparation. This is tedious and prone to errors, especially when repeated for the multiple replicates that are required for statistical analysis, or when multiple distinct structures have to be analyzed. To address this issue, we have developed a pipeline for characterization of long-bone length using X-ray microtomography (XMT) scans. The pipeline involves semi-automated algorithms for automatic thresholding and fast interactive isolation and 3D-model generation of the main limb bones, using either the open-source ImageJ plugin BoneJ or the commercial Mimics Innovation Suite package. The tests showed the appropriate combination of scanning conditions and analysis parameters yields fast and comparable length results, highly correlated with the measurements obtained via ex vivo skeletal preparations. Moreover, since XMT is not destructive, the samples can be used afterward for histology or other applications. Our new pipelines will help developmental biologists and evolutionary researchers to achieve fast, reproducible and non-destructive length measurement of bone samples from multiple animal species.

A collection of genetic mouse lines and related tools for inducible and reversible intersectional mis-expression

Thanks to many advances in genetic manipulation, mouse models have become very powerful in their ability to interrogate biological processes. In order to precisely target expression of a gene of interest to particular cell types, intersectional genetic approaches using two promoter/enhancers unique to a cell type are ideal. Within these methodologies, variants that add temporal control of gene expression are the most powerful. We describe the development, validation and application of an intersectional approach that involves three transgenes, requiring the intersection of two promoter/enhancers to target gene expression to precise cell types. Furthermore, the approach uses available lines expressing tTA/rTA to control the timing of gene expression based on whether doxycycline is absent or present, respectively. We also show that the approach can be extended to other animal models, using chicken embryos. We generated three mouse lines targeted at the Tigre (Igs7) locus with TRE-loxP-tdTomato-loxP upstream of three genes (p21, DTA and Ctgf), and combined them with Cre and tTA/rtTA lines that target expression to the cerebellum and limbs. Our tools will facilitate unraveling biological questions in multiple fields and organisms.

Size control of the inner ear via hydraulic feedback

Animals make organs of precise size, shape, and symmetry but how developing embryos do this is largely unknown. Here, we combine quantitative imaging, physical theory, and physiological measurement of hydrostatic pressure and fluid transport in zebrafish to study size control of the developing inner ear. We find that fluid accumulation creates hydrostatic pressure in the lumen leading to stress in the epithelium and expansion of the otic vesicle. Pressure, in turn, inhibits fluid transport into the lumen. This negative feedback loop between pressure and transport allows the otic vesicle to change growth rate to control natural or experimentally-induced size variation. Spatiotemporal patterning of contractility modulates pressure-driven strain for regional tissue thinning. Our work connects molecular-driven mechanisms, such as osmotic pressure driven strain and actomyosin tension, to the regulation of tissue morphogenesis via hydraulic feedback to ensure robust control of organ size.

Editorial note: This article has been through an editorial process in which the authors decide how to respond to the issues raised during peer review. The Reviewing Editor's assessment is that all the issues have been addressed (see decision letter).

Systemic signalling and local effectors in developmental stability, body symmetry, and size

Symmetric growth and the origins of fluctuating asymmetry are unresolved phenomena of biology. Small, and sometimes noticeable, deviations from perfect bilateral symmetry reflect the vulnerability of development to perturbations. The degree of asymmetry is related to the magnitude of the perturbations and the ability of an individual to cope with them. As the left and right sides of an individual were presumed to be genetically identical, deviations of symmetry were traditionally attributed to non-genetic effects such as environmental and developmental noise. In this review, we draw attention to other possible sources of variability, especially to somatic mutations and transposons. Mutations are a major source of phenotypic variability and recent genomic data have highlighted somatic mutations as ubiquitous, even in phenotypically normal individuals. We discuss the importance of factors that are responsible for buffering and stabilizing the genome and for maintaining size robustness and quality through elimination of less-fit or damaged cells. However, the important question that arises from these studies is whether this self-correcting capacity and intrinsic organ size controls are sufficient to explain how symmetric structures can reach an identical size and shape. Indeed, recent discoveries in the fruit fly have uncovered a conserved hormone of the insulin/IGF/relaxin family, Dilp8, that is responsible for stabilizing body size and symmetry in the face of growth perturbations. Dilp8 alarm signals periphery growth status to the brain, where it acts on its receptor Lgr3. Loss of Dilp8-Lgr3 signaling renders flies incapable of detecting growth perturbations and thus maintaining a stable size and symmetry. These findings help to understand how size and symmetry of somatic tissues remain undeterred in noisy environments, after injury or illnesses, and in the presence of accumulated somatic mutations.

Cell-nonautonomous local and systemic responses to cell arrest enable long-bone catch-up growth in developing mice

Catch-up growth after insults to growing organs is paramount to achieving robust body proportions. In fly larvae, injury to individual tissues is followed by local and systemic compensatory mechanisms that allow the damaged tissue to regain normal proportions with other tissues. In vertebrates, local catch-up growth has been described after transient reduction of bone growth, but the underlying cellular responses are controversial. We developed an approach to study catch-up growth in foetal mice in which mosaic expression of the cell cycle suppressor p21 is induced in the cartilage cells (chondrocytes) that drive long-bone elongation. By specifically targeting p21 expression to left hindlimb chondrocytes, the right limb serves as an internal control. Unexpectedly, left-right limb symmetry remained normal, revealing deployment of compensatory mechanisms. Above a certain threshold of insult, an orchestrated response was triggered involving local enhancement of bone growth and systemic growth reduction that ensured that body proportions were maintained. The local response entailed hyperproliferation of spared left limb chondrocytes that was associated with reduced chondrocyte density. The systemic effect involved impaired placental function and IGF signalling, revealing bone-placenta communication. Therefore, vertebrates, like invertebrates, can mount coordinated local and systemic responses to developmental insults that ensure that normal body proportions are maintained.