Fabrice Richard

A dual role for planar cell polarity genes in ciliated cells

Significance Ependymal cilia are required for circulation of the cerebrospinal fluid and neurogenesis. To function properly, ependymal cilia must coordinate their beats in individual cells and across the tissue. Planar cell polarity (PCP) orients cilia in a given cell, thereby enabling their concerted beating. Here, we describe previously unidentified functions for PCP in cilia organization at the cell and tissue levels. We show that PCP is important for the correct positioning of the primary cilium in radial progenitors and motile cilia in ependymal cells and provide evidence that cilia positioning is important for function. We also describe cytoskeletal changes during ependymal differentiation and shed light on mechanisms by which polarity is acquired by radial progenitors and passed on to ependymal cells. In the nervous system, cilia dysfunction perturbs the circulation of the cerebrospinal fluid, thus affecting neurogenesis and brain homeostasis. A role for planar cell polarity (PCP) signaling in the orientation of cilia (rotational polarity) and ciliogenesis is established. However, whether and how PCP regulates cilia positioning in the apical domain (translational polarity) in radial progenitors and ependymal cells remain unclear. By analysis of a large panel of mutant mice, we show that two PCP signals are operating in ciliated cells. The first signal, controlled by cadherin, EGF-like, laminin G-like, seven-pass, G-type receptor (Celsr) 2, Celsr3, Frizzled3 (Fzd3) and Van Gogh like2 (Vangl2) organizes multicilia in individual cells (single-cell polarity), whereas the second signal, governed by Celsr1, Fzd3, and Vangl2, coordinates polarity between cells in both radial progenitors and ependymal cells (tissue polarity). Loss of either of these signals is associated with specific defects in the cytoskeleton. Our data reveal unreported functions of PCP and provide an integrated view of planar polarization of the brain ciliated cells.

Hook2 is involved in the morphogenesis of the primary cilium.

Hook2 partitions between the Golgi apparatus and the centrosome, and its depletion hinders ciliogenesis after mother centriole maturation without Golgi breakdown. Hook2 interacts with PCM1 and Rab8a, and Hook2-depleted cells can be forced to grow primary cilia by overexpressing GFP::Rab8a, indicating that Rab8a acts downstream of Hook2 and PCM1.

Viscoelastic modeling and quantitative experimental characterization of normal and osteoarthritic human articular cartilage using indentation.

The viscoelastic behavior of articular cartilage changes with progression of osteoarthritis. The objective of this study is to quantify this progression and to propose a viscoelastic model of articular cartilage taking into account the degree of osteoarthritis that which be easily used in predictive numerical simulations of the hip joint behavior. To quantify the effects of osteoarthritis (OA) on the viscoelastic behavior of human articular cartilage, samples were obtained from the hip arthroplasty due to femoral neck fracture (normal cartilage) or advanced coxarthrosis (OA cartilage). Experimental data were obtained from instrumented indentation tests on unfrozen femoral cartilage collected and studied in the day following the prosthetic hip surgery pose. By using an inverse method coupled with a numerical modeling (FEM) of all experimental data of the indentation tests, the viscoelastic properties of the two states were quantified. Mean values of viscoelastic parameters were significantly lower for OA cartilage than normal (instantaneous and relaxed tension moduli, viscosity coefficient). Based on the results and in the thermodynamic framework, a constitutive viscoelastic model taking into account the degree of osteoarthritis as an internal variable of damage is proposed. The isotropic phenomenological viscoelastic model including degradation provides an accurate prediction of the mechanical response of the normal human cartilage and OA cartilage with advanced coxarthrosis but should be further validated for intermediate degrees of osteoarthritis.

Apico-basal elongation requires a drebrin-E–EB3 complex in columnar human epithelial cells

Although columnar epithelial cells are known to acquire an elongated shape, the mechanisms involved in this morphological feature have not yet been completely elucidated. Using columnar human intestinal Caco2 cells, it was established here that the levels of drebrin E, an actin-binding protein, increase in the terminal web both in vitro and in vivo during the formation of the apical domain. Drebrin E depletion was found to impair cell compaction and elongation processes in the monolayer without affecting cell polarity or the formation of tight junctions. Decreasing the drebrin E levels disrupted the normal subapical F-actin–myosin-IIB–βII-spectrin network and the apical accumulation of EB3, a microtubule-plus-end-binding protein. Decreasing the EB3 levels resulted in a similar elongation phenotype to that resulting from depletion of drebrin E, without affecting cell compaction processes or the pattern of distribution of F-actin–myosin-IIB. In addition, EB3, myosin IIB and βII spectrin were found to form a drebrin-E-dependent complex. Taken together, these data suggest that this complex connects the F-actin and microtubule networks apically during epithelial cell morphogenesis, while drebrin E also contributes to stabilizing the actin-based terminal web.

Inner-membrane proteins PMI/TMEM11 regulate mitochondrial morphogenesis independently of the DRP1/MFN fission/fusion pathways

Mitochondria are highly dynamic organelles that can change in number and morphology during cell cycle, development or in response to extracellular stimuli. These morphological dynamics are controlled by a tight balance between two antagonistic pathways that promote fusion and fission. Genetic approaches have identified a cohort of conserved proteins that form the core of mitochondrial remodelling machineries. Mitofusins (MFNs) and OPA1 proteins are dynamin‐related GTPases that are required for outer‐ and inner‐mitochondrial membrane fusion respectively whereas dynamin‐related protein 1 (DRP1) is the master regulator of mitochondrial fission. We demonstrate here that the Drosophila PMI gene and its human orthologue TMEM11 encode mitochondrial inner‐membrane proteins that regulate mitochondrial morphogenesis. PMI‐mutant cells contain a highly condensed mitochondrial network, suggesting that PMI has either a pro‐fission or an anti‐fusion function. Surprisingly, however, epistatic experiments indicate that PMI shapes the mitochondria through a mechanism that is independent of drp1 and mfn. This shows that mitochondrial networks can be shaped in higher eukaryotes by at least two separate pathways: one PMI‐dependent and one DRP1/MFN‐dependent.

The safety-factor calibration of laminates for long-term applications : Behavior model and reliability method

Abstract This paper presents a method for determining safety factors which must be taken into account when designing laminates for long-term applications. The method requires two steps: the first is a behavioral model able to predict time-dependent phenomena, and the second is a reliability method. The meso/macro approach is used in the behavioral model. This paper presents a damaged elasto-viscoplastic model able to predict the behavior of a layer made of a polymer reinforced with unidirectional fibers. This model requires a non-linear laminate theory which provides the response of laminate structures. The method for calibrating safety factors is based on the first-order reliability method. A description of the behavioral model in this method is presented. Finally, as a working example, the design of filament-wound pipe for fluid transportation is presented. A functional criterion is introduced to prevent leakage. The effect of material parameters is analyzed.

Identification of geometrical parameters of femoral heads for hip joint mechanical model

The femoral head is a protrusion that fits together with the acetabulum of the pelvis. This joint called hip joint is an important joint in the body. It transmits the body weight to the lower limb and allows humans to move. In most textbooks, there is an excellent congruence between the articular surfaces, thus describing the femoral head and the acetabulum as perfect spheres. Some studies (Hammond and Charnley 1967; Blowers et al. 1972) confirm that the shape of the femoral head is a sphere. However, Cathcart (1972), Oonishi et al. (1976) and Menschik (1997) describe shapes that deviate from the pure spherical. The results of these studies are not consistent and sparse. Works on the geometric shapes of osteoarthritis heads are even rarer. Understanding the shape of the femoral head would allow to better understand the onset of certain diseases like osteoarthritis or efficiently analyse the pressure distribution in the joint and to improve resurfacing prosthesis. The objective of this study was to identify the shape and geometrical parameters associated with arthrosic and non-pathological femoral heads via the use of a 3D scanning (digitisation) and of an identification by an inverse method.

Identification of material parameters of cartilage for hip joint mechanical model

Cartilage is an essential element of joint. It allows the joint surfaces to slide against one another and to distribute the pressure between them. Without cartilage, many movements would be impossible or at least painful. Understanding the mechanical behaviour of cartilage and associated parameters would allow, through appropriate modelling, to better understand the onset of certain diseases such as osteoarthritis or efficiently analyse the pressure distribution in the joint. Many studies have been conducted on the mechanical properties of cartilage. It was shown that cartilage has a viscoelastic behaviour (Juras et al. 2009; Villars et al. in press). Moreover, some studies (Kurrat and Oberländer 1978) have reported a heterogeneity of femoral head cartilage thickness, cartilage being more or less thick depending on the considered area. It would be interesting to study the heterogeneity of the viscoelastic properties of cartilage.