Epiphyseal primary cilia axoneme are preferentially orientated, independent of centriolar position and irrespective of ambulatory limb loading.

Abstract

How cells and their organelles are positioned in three-dimensional, organ level, anatomical context, is rarely investigated. Here we focus on cells, centrioles and primary cilia in the growing limb. Through the ciliums mechanobiological role in skeletal development, we explored the mechanobiology of morphogenesis. A transgenic mouse model (Centrin 2-GFP.ARL13B-mCherry), combined with an image analysis pipeline, can map cellular size, positions and orientations, centriole position and ciliary axoneme orientation, all with respect to the anatomy of the epiphysis or growth plate. The line was crossed with an ift88fl/flCreERT2 line to enable ciliary ift88 deletion. We used limb immobilization, to test for a role of mechanical forces associated with ambulatory loading, in the organization of these elements and transcriptomics to understand the role of forces in regulating growth plate morphogenic programs. The pipeline can accurately quantify expected patterns of cell orientation and size through zones of the growth plate. Analysis across thousands of cells, through regions and zones of multiple murine growth plates, reveals cilia prevalence is increased in the periphery, highest in the resting zone in the outer limb, harboring stem cells. Cilia length is greatest in the hypertrophic cells about to die or transdifferentiate, as part of the formation of bone from cartilage by endochondral ossification. The inducible and cartilage-specific, deletion of ciliary gene ift88, alters cell orientation and sizes and reduces ciliation in the areas where endochondral ossification is most disrupted, the periphery and expanded hypertrophic zones, linking changes in structure to function. Most strikingly, centriole position, including that of the basal body, from which the ciliary axoneme is extended, is not preferentially organised. In contrast, cilia axoneme orientation is preferentially organised. Axonemes are directed posterior or anterior, 45 degrees to the axis of the limb, irrespective of their position, which is defined by basal body position. Immobilization of the limb for 2 weeks markedly alters the transcriptomic profile of the growth plate, with changes to size and orientation of cells and alterations in matrix and cytoskeletal profiles. Within altered genes, primary cilia genes themselves are regulated, including those indicative of altered cilia signaling such as hedgehog signaling. However, despite the role of cilia in mechanobiology of the growing limb, and ciliary signature within changes to loading of the limb, cilia orientation is unaltered by the removal of ambulatory associated forces. Patterns of ciliation in control and IFT88cKO mice help reconcile the previously observed anisotropic effects of cilia perturbation, focusing study on stem cell-resting chondrocytes and hypertrophy, when considering the mechanobiological role of cilia in limb development. Endochondral ossification is apparently highly sensitive to ambulatory loading at transcriptomic level, including effects on ciliary genes and signaling. A highly organized orientation of these putative antennae is governed by centriole position-independent mechanisms and is independent to changes to ambulatory loading, indicating a cell intrinsic mechanism. The resilient position of axonemes in the limb, points to mechano-regulatory mechanisms for how cilia integrate biophysical signals. We propose that predominant ventral or dorsal orientation at 45 degrees to horizonal plane but never parallel to cranial-chordal or medial-lateral axis, ensures multiple signal integration and avoids single signal blindness.