Evidence-Based Network Modelling to Simulate Nucleus Pulposus Multicellular Activity in Different Nutritional and Pro-Inflammatory Environments.

Initiation of intervertebral disc degeneration is thought to be biologically driven. This reflects a process, where biochemical and mechanical stimuli affect cell activity (CA) that compromise the tissue strength over time. Experimental research enhanced our understanding about the effect of such stimuli on different CA, such as protein synthesis or mRNA expression. However, it is still unclear how cells respond to their native environment that consists of a "cocktail" of different stimuli that might locally vary. This work presents an interdisciplinary approach of experimental and in silico research to approximate Nucleus Pulposus CA within multifactorial biochemical environments. Thereby, the biochemical key stimuli glucose, pH, and the proinflammatory cytokines TNF-α and IL1ß were considered that were experimentally shown to critically affect CA. To this end, a Nucleus Pulposus multicellular system was modelled. It integrated experimental findings from in vitro studies of human or bovine Nucleus Pulposus cells, to relate the individual effects of targeted stimuli to alterations in CA. Unknown stimulus-CA relationships were obtained through own experimental 3D cultures of bovine Nucleus Pulposus cells in alginate beads. Translation of experimental findings into suitable parameters for network modelling approaches was achieved thanks to a new numerical approach to estimate the individual sensitivity of a CA to each stimulus type. Hence, the effect of each stimulus type on a specific CA was assessed and integrated to approximate a multifactorial stimulus environment. Tackled CA were the mRNA expressions of Aggrecan, Collagen types I & II, MMP3, and ADAMTS4. CA was assessed for four different proinflammatory cell states; non-inflamed and inflamed for IL1ß, TNF-α or both IL1ß&TNF-α. Inflamed cell clusters were eventually predicted in a multicellular 3D agent-based model. Experimental results showed that glucose had no significant impact on proinflammatory cytokine or ADAMTS4 mRNA expression, whereas TNF-α caused a significant catabolic shift in most explored CA. In silico results showed that the presented methodology to estimate the sensitivity of a CA to a stimulus type importantly improved qualitative model predictions. However, more stimuli and/or further experimental knowledge need to be integrated, especially regarding predictions about the possible progression of inflammatory environments under adverse nutritional conditions. Tackling the multicellular level is a new and promising approach to estimate manifold responses of intervertebral disc cells. Such a top-down high-level network modelling approach allows to obtain information about relevant stimulus environments for a specific CA and could be shown to be suitable to tackle complex biological systems, including different proinflammatory cell states. The development of this methodology required a close interaction with experimental research. Thereby, specific experimental needs were derived from systematic in silico approaches and obtained results were directly used to enhance model predictions, which reflects a novelty in this research field. Eventually, the presented methodology provides modelling solutions suitable for multiscale approaches to contribute to a better understanding about dynamics over multiple spatial scales. Future work should focus on an amplification of the stimulus environment by integrating more key relevant stimuli, such as mechanical loading parameters, in order to better approximate native physiological environments.

[Enoxaparin and tinzaparin in pediatrics: Impact of recommendation on prescription quality and anti-Xa levels]. / Énoxaparine et tinzaparine en pédiatrie : impact de recommandations sur la qualité des prescriptions et l'activité anti-Xa.

OBJECTIVES: A protocol has been written and distributed in May 2017 to all prescribers in a pediatric hospital to standardize and to secure the prescriptions of enoxaparin and tinzaparin considered as two high risk medications. The aim of this study is to evaluate the impact of the protocol on those prescriptions in a pediatric population. METHODS: This is a monocentric retrospective study comparing prescriptions of this two low-molecular-weight heparins for patients under 18 years old in 2016 and 2018, thus before and after the protocol redaction. RESULTS: In 2016, 2246 prescriptions of enoxaparin and tinzaparin were analyzed for 627 patients. Among them, 142 (22.6%) patients have had at least one anti-Xa level dosed. On the other hand, in 2018, 2061 prescriptions were written for 628 patients including 96 (15.3%) who have had at least one anti-Xa level dosed. The conformity rate of the first dose in IU/kg/administration of the first enoxaparin prescription goes from 36.3% before protocol to 52.1% after (P=0.03*). Concerning tinzaparin, the conformity rate goes from 69.2% to 83.3%. (P=0.19). The rate of first anti-Xa level in the range 0.4 to 1.2 IU/ml increase between 2016 and 2018 from 27.7% to 43.8% (P<0.001*). CONCLUSION: This protocol enabled to improve the quality of prescriptions in terms of: dosage written in IU/kg/administration, frequency of administration, dilution conformity, and result of the first anti-Xa level. Some efforts must be made in writing the dose in IU not in mg or ml.

Simulating intervertebral disc cell behaviour within 3D multifactorial environments.

MOTIVATION: Low back pain is responsible for more global disability than any other condition. Its incidence is closely related to intervertebral disc (IVD) failure, which is likely caused by an accumulation of microtrauma within the IVD. Crucial factors in microtrauma development are not entirely known yet, probably because their exploration in vivo or in vitro remains tremendously challenging. In silico modelling is, therefore, definitively appealing, and shall include approaches to integrate influences of multiple cell stimuli at the microscale. Accordingly, this study introduces a hybrid Agent-based (AB) model in IVD research and exploits network modelling solutions in systems biology to mimic the cellular behaviour of Nucleus Pulposus cells exposed to a 3D multifactorial biochemical environment, based on mathematical integrations of existing experimental knowledge. Cellular activity reflected by mRNA expression of Aggrecan, Collagen type I, Collagen type II, MMP-3 and ADAMTS were calculated for inflamed and non-inflamed cells. mRNA expression over long periods of time is additionally determined including cell viability estimations. Model predictions were eventually validated with independent experimental data. RESULTS: As it combines experimental data to simulate cell behaviour exposed to a multifactorial environment, the present methodology was able to reproduce cell death within 3 days under glucose deprivation and a 50% decrease in cell viability after 7 days in an acidic environment. Cellular mRNA expression under non-inflamed conditions simulated a quantifiable catabolic shift under an adverse cell environment, and model predictions of mRNA expression of inflamed cells provide new explanation possibilities for unexpected results achieved in experimental research. AVAILABILITYAND IMPLEMENTATION: The AB model as well as used mathematical functions were built with open source software. Final functions implemented in the AB model and complete AB model parameters are provided as Supplementary Material. Experimental input and validation data were provided through referenced, published papers. The code corresponding to the model can be shared upon request and shall be reused after proper training. SUPPLEMENTARY INFORMATION: Supplementary data are available at Bioinformatics online.

Modeling the impact of prostate edema on LDR brachytherapy: a Monte Carlo dosimetry study based on a 3D biphasic finite element biomechanical model.

Prostate volume changes due to edema occurrence during transperineal permanent brachytherapy should be taken under consideration to ensure optimal dose delivery. Available edema models, based on prostate volume observations, face several limitations. Therefore, patient-specific models need to be developed to accurately account for the impact of edema. In this study we present a biomechanical model developed to reproduce edema resolution patterns documented in the literature. Using the biphasic mixture theory and finite element analysis, the proposed model takes into consideration the mechanical properties of the pubic area tissues in the evolution of prostate edema. The model's computed deformations are incorporated in a Monte Carlo simulation to investigate their effect on post-operative dosimetry. The comparison of Day1 and Day30 dosimetry results demonstrates the capability of the proposed model for patient-specific dosimetry improvements, considering the edema dynamics. The proposed model shows excellent ability to reproduce previously described edema resolution patterns and was validated based on previous findings. According to our results, for a prostate volume increase of 10-20% the Day30 urethra D10 dose metric is higher by 4.2%-10.5% compared to the Day1 value. The introduction of the edema dynamics in Day30 dosimetry shows a significant global dose overestimation identified on the conventional static Day30 dosimetry. In conclusion, the proposed edema biomechanical model can improve the treatment planning of transperineal permanent brachytherapy accounting for post-implant dose alterations during the planning procedure.

Generation of stable orthogonal gradients of chemical concentration and substrate stiffness in a microfluidic device.

Cellular responses to chemical cues are at the core of a myriad of fundamental biological processes ranging from embryonic development to cancer metastasis. Most of these biological processes are also influenced by mechanical cues such as the stiffness of the extracellular matrix. How a biological function is influenced by a synergy between chemical concentration and extracellular matrix stiffness is largely unknown, however, because no current strategy enables the integration of both types of cues in a single experiment. Here we present a robust microfluidic device that generates a stable, linear and diffusive chemical gradient over a biocompatible hydrogel with a well-defined stiffness gradient. Device fabrication relies on patterned PSA (Pressure Sensitive Adhesive) stacks that can be implemented with minimal cost and lab equipment. This technique is suitable for long-term observation of cell migration and application of traction force microscopy. We validate our device by testing MDCK cell scattering in response to perpendicular gradients of hepatocyte growth factor (HGF) and substrate stiffness.

The role of endplate poromechanical properties on the nutrient availability in the intervertebral disc.

OBJECTIVE: To investigate the relevance of the human vertebral endplate poromechanics on the fluid and metabolic transport from and to the intervertebral disc (IVD) based on educated estimations of the poromechanical parameter values of the bony endplate (BEP). METHODS: 50 micro-models of different BEP samples were generated from µCTs of lumbar vertebrae and allowed direct determination of porosity values. Permeability values were calculated by using the micro-models, through the simulation of permeation via computational fluid dynamics. These educated ranges of porosity and permeability values were used as inputs for mechano-transport simulations to assess their effect on both the distributions of metabolites within an IVD model and the poromechanical calculations within the cartilaginous part of the endplate i.e., the cartilage endplate (CEP). RESULTS: BEP effective permeability was highly correlated to local variations of porosity (R(2) ≈ 0.88). Universal patterns between bone volume fraction and permeability arose from these results and from other experimental data in the literature. These variations in BEP permeability and porosity had negligible effects on the distributions of metabolites within the disc. In the CEP, the variability of the poromechanical properties of the BEP did not affect the predicted consolidation but induced higher fluid velocities. CONCLUSIONS: The present paper provides the first sets of thoroughly identified BEP parameter values that can be further used in patient-specific poromechanical studies. Representing BEP structural changes through variations in poromechanical properties did not affect the diffusion of metabolites. However, attention might be paid to alterations in fluid velocities and cell mechano-sensing within the CEP.

Material property discontinuities in intervertebral disc porohyperelastic finite element models generate numerical instabilities due to volumetric strain variations.

Numerical studies of the intervertebral disc (IVD) are important to better understand the load transfer and the mechanobiological processes within the disc. Among the relevant calculations, fluid-related outputs are critical to describe and explore accurately the tissue properties. Porohyperelastic finite element models of IVD can describe accurately the disc behaviour at the organ level and allow the inclusion of fluid effects. However, results may be affected by numerical instabilities when fast load rates are applied. We hypothesized that such instabilities would appear preferentially at material discontinuities such as the annulus-nucleus boundary and should be considered when testing mesh convergence. A L4-L5 IVD model including the nucleus, annulus and cartilage endplates were tested under pure rotational loads, with different levels of mesh refinement. The effect of load relaxation and swelling were also studied. Simulations indicated that fluid velocity oscillations appeared due to numerical instability of the pore pressure spatial derivative at material discontinuities. Applying local refinement only was not enough to eliminate these oscillations. In fact, mesh refinements had to be local, material-dependent, and supplemented by the creation of a material transition zone, including interpolated material properties. Results also indicated that oscillations vanished along load relaxation, and faster attenuation occurred with the incorporation of the osmotic pressure. We concluded that material discontinuities are a major cause of instability for poromechanical calculations in multi-tissue models when load velocities are simulated. A strategy was presented to address these instabilities and recommendations on the use of IVD porohyperelastic models were given.

Directing bone marrow-derived stromal cell function with mechanics.

Because bone marrow-derived stromal cells (BMSCs) are able to generate many cell types, they are envisioned as source of regenerative cells to repair numerous tissues, including bone, cartilage, and ligaments. Success of BMSC-based therapies, however, relies on a number of methodological improvements, among which better understanding and control of the BMSC differentiation pathways. Since many years, the biochemical environment is known to govern BMSC differentiation, but more recent evidences show that the biomechanical environment is also directing cell functions. Using in vitro systems that aim to reproduce selected components of the in vivo mechanical environment, it was demonstrated that mechanical loadings can affect BMSC proliferation and improve the osteogenic, chondrogenic, or myogenic phenotype of BMSCs. These effects, however, seem to be modulated by parameters other than mechanics, such as substrate nature or soluble biochemical environment. This paper reviews and discusses recent experimental data showing that despite some knowledge limitation, mechanical stimulation already constitutes an additional and efficient tool to drive BMSC differentiation.

The effect of bone graft geometry on spinal fusion vertebral stresses.

Spinal fusion is a frequent surgical technique in which the success is uncertain due to post-operative changes in the biomechanics of the spine. Bone grafts are good candidates for disc and vertebra substitutes due to their similar bone properties and their good osteogenic properties. However, the effect of the anatomic harvest location of the bone graft on the load transfer is unknown. A physiologic three-dimensional (3D) finite element model of a lumbar spine was modified to model spinal fusion with a fixator and a bone graft. Bone grafts were taken either from the femur, the tibia, or from the fibula in a configuration of three or six fragments. The configurations were submitted to physiological loadings, and strain and stress distributions were calculated within the vertebrae, the fixator and the bone grafts. Quantitative differences were found from one type of bone graft to another. It was found that fibula bone grafts provided better stability by carrying a large part of the load. However, femoral and tibial bone grafts provided a more similar strain distribution within the vertebrae compared to the physiologic model. For tibial bone grafts, load transfer was found to be sensitive to the orientation used during the surgery. The use of a femoral bone graft to replace one vertebra and two intervertebral discs was found to give a better biomechanical function than using a tibial or fibula bone graft. This surgical technique is proposed to be beneficial in the case of severe spinal trauma providing good interface is obtained between the bone graft and the vertebrae.