Shape driven confluent rigidity transition in curved biological tissues

Collective cell motions underlie structure formation during embryonic development. Tissues exhibit emergent multicellular characteristics such as jamming, rigidity transitions, and glassy dynamics, but there remain questions about how those tissue scale dynamics derive from local cell level properties. Specifically, there has been little consideration of the interplay between local tissue geometry and cellular properties influencing larger scale tissue behaviours. Here we consider a simple two dimensional computational vertex model for confluent tissue monolayers, which exhibits a rigidity phase transition controlled by the shape index (ratio of perimeter to square root area) of cells, on a spherical surface. We show that the critical point for the rigidity transition is a function of curvature such that more highly curved systems are more likely to be in a less rigid, more fluid, phase. A phase diagram we generate for the curvature and shape index constitutes a testable prediction from the model. The curvature dependence is interesting because it suggests a natural explanation for more dynamic tissue remodelling and facile growth in regions of higher surface curvature, without invoking the need for biochemical or other physical differences. This has potential ramifications for our understanding of morphogenesis of budding and branching structures

Transgenic force sensors and software to measure force transmission across the mammalian nuclear envelope in vivo

Nuclear mechanotransduction is a growing field with exciting implications for the regulation of gene expression and cellular function. Mechanical signals may be transduced to the nuclear interior biochemically or physically through connections between the cell surface and chromatin. To define mechanical stresses upon the nucleus in physiological settings, we generated transgenic mouse strains that harbour FRET-based tension sensors or control constructs in the outer and inner aspects of the nuclear envelope. We knocked-in a published esprin-2G sensor to measure tensions across the LINC complex and generated a new sensor that links the inner nuclear membrane to chromatin. To mitigate challenges inherent to fluorescence lifetime analysis in vivo, we developed software (FLIMvivo) that markedly improves the fitting of fluorescence decay curves. In the mouse embryo, the sensors responded to cytoskeletal relaxation and stretch applied by micro-aspiration. They reported organ-specific differences and a spatiotemporal tension gradient along the proximodistal axis of the limb bud, raising the possibility that mechanical mechanisms coregulate pattern formation. These mouse strains and software are potentially valuable tools for testing and refining mechanotransduction hypotheses in vivo

Dysregulation of Indian hedgehog - parathyroid hormone related protein signalling in cartilaginous neoplasia

grantor: University of TorontoEnchondroma is a benign cartilage-forming tumour of bone that might be caused by dysregulation of growth plate signals. A paracrine feedback loop couples Indian hedgehog (IHH), which stimulates growth plate chondrocyte proliferation, to Parathyroid hormone related protein (PTHrP), which regulates chondrocyte differentiation. Here I show that these two signalling pathways were uncoupled and IHH was dysinhibited in explant cultures of human enchondromas and their malignant counterparts, chondrosarcomas. A heterozygous variant 'Type I PTH'/'PTHrP receptor' ('PTHR1') was discovered in the germline of one patient and as a somatic change in another with enchondromatosis. In this condition, multiple enchondromas may lead to deformity and chondrosarcoma. The variant PTHR1 suppressed Cyclic adenosine monophosphate baseline level in a dominant fashion and abolished Inositol triphosphate accumulation 'in vitro'. Transgenic mice expressing the variant receptor under the regulatory elements of 'Type II' collagen developed enchondromatosis. I show that the transcription factor Gli2 was a positive regulator of Ihh function in the murine growth plate, while Gli3 was a negative regulator. Overexpression of 'Gli2' was found in human enchondromas, and was sufficient to cause enchondromatosis in transgenic mice. A lack of ' Gli3' accelerated another condition of benign chondrocyte neoplasia, synovial chondromatosis. Patched 1 (PTCH1), a Hedghehog receptor, formed a complex with PTHR1, and this complex was required for effective accumulation of PTHR1 second messengers. The variant PTHR1 did not associate with PTCH1, and constitutively activated Hedgehog signalling in a manner that was likely dependent upon suppression of Protein kinase A. Dysregulation of growth plate signals causes certain benign cartilage tumours. Agents that block Hedgehog signalling in particular, might be useful in preventing the deleterious sequelae of enchondromas.Ph.D

Magnetic Micromanipulation for In Vivo Measurement of Stiffness Heterogeneity and Anisotropy in the Mouse Mandibular Arch

The mechanical properties of tissues are pivotal for morphogenesis and disease progression. Recent approaches have enabled measurements of the spatial distributions of viscoelastic properties among embryonic and pathological model systems and facilitated the generation of important hypotheses such as durotaxis and tissue-scale phase transition. There likely are many unexpected aspects of embryo biomechanics we have yet to discover which will change our views of mechanisms that govern development and disease. One area in the blind spot of even the most recent approaches to measuring tissue stiffness is the potentially anisotropic nature of that parameter. Here, we report a magnetic micromanipulation device that generates a uniform magnetic field gradient within a large workspace and permits measurement of the variation of tissue stiffness along three orthogonal axes. By applying the device to the organ-stage mouse embryo, we identify spatially heterogenous and directionally anisotropic stiffness within the mandibular arch. Those properties correspond to the domain of expression and the angular distribution of fibronectin and have potential implications for mechanisms that orient collective cell movements and shape tissues during development. Assessment of anisotropic properties extends the repertoire of current methods and will enable the generation and testing of hypotheses

Live imaging YAP signalling in mouse embryo development

YAP protein is a critical regulator of mammalian embryonic development. By generating a near-infrared fusion YAP reporter mouse line, we have achieved high-resolution live imaging of YAP localization during mouse embryonic development. We have validated the reporter by demonstrating its predicted responses to blocking LATS kinase activity or blocking cell polarity. By time lapse imaging preimplantation embryos, we revealed a mitotic reset behaviour of YAP nuclear localization. We also demonstrated deep tissue live imaging in post-implantation embryos and revealed an intriguing nuclear YAP pattern in migrating cells. The YAP fusion reporter mice and imaging methods will open new opportunities for understanding dynamic YAP signalling in vivo in many different situations

Bone Health in Osteosarcoma at Presentation and Its Impact on Cancer Treatment: A Case Series of 3 Pediatric Patients

Osteosarcoma is the most common pediatric malignant bone tumor. Concomitant osteoporosis has typically been attributed to oncologic therapy. The present case series is aimed to describe 3 patients who presented with osteoporosis or osteopenia before, or early in, their oncology treatment. In our patients, bone health and its complications had significant impacts including pain, reduced mobility, prolonged admission, and delays in recovery. Our patients experienced improvement with resection of their primary tumor and with bisphosphonate infusion. Future studies are required to determine the prevalence osteoporosis at presentation of osteosarcoma and the role of bisphosphonates