Étude de la mécanotransduction dans la scoliose idiopathique de l’adolescence (SIA)

Adolescent idiopathic scoliosis (AIS) is the most commonly occurring musculoskeletal deformity among children today. It is generally well accepted in scoliosis research that mechanical forces, especially the internal biomechanical forces of the musculoskeletal system, could well have a major role in the induction and pathogenesis of the disease. However, the process by which mechanical loads or stimuli are converted into biochemical activity (mechanotransduction) has not been explored so deeply.  This emerging facet of research in AIS holds much promise for new insights into the disease. Here, we aim to characterize mechanotransduction in scoliosis patients using some novel techniques at both the in vivo and in vitro levels.  Previously in our lab, we demonstrated that the level of plasma osteopontin (OPN) and sCD44 in the human body is a strong indicator of disease progression and severity, and that these changes are observable before scoliosis onset. In the literature, OPN in vitro is known to be mechanosensitive, showing upregulation in response to mechanical stress in a variety of cell types across many species. However, to the best of the author’s knowledge, no literature exists as to whether this behaviour carries over in vivo in humans.  A major difficulty in AIS research is the heterogeneity of the disease, both physically and biochemically. Because of this, many times it is difficult to find results with wide applicability to patients. Study of biomechanical factors in AIS is no exception. We believe, however, that study of fluid shear stress in the musculoskeletal system may be able to solve this problem for mechanotransduction-related issues in AIS. Native physiological fluid shear stresses in humans are experienced in the musculoskeletal system, caused by fluid movement over cells therein. These fluid shear stresses are omnipresent and universal in all humans, regardless of age, gender, fitness level, etc., which means that studying it could very well go a long way towards establishing a fundamental basis of understanding the differences as to mechanotransduction in scoliosis patients as opposed to normal cases.  In this project, then, we advanced the hypothesis that AIS patients are distinguishable in the way they respond to mechanical force at both the cellular level (in terms of gene expression) as well as globally at the in vivo level (in terms of the scoliosis marker OPN and its receptor sCD44). To test the in vivo portion of our hypothesis, we recruited a cohort of patients between the ages of 9-17, each one of which fell into one of 4 subject groups: i) surgical cases (pre-surgery, Cobb angle > 45°), ii) moderately affected cases (Cobb angle 10-44°), iii) controls, or iv) asymptomatic children at risk of developing scoliosis matched for age and gender against healthy controls. A dynamic, pulsatile, compressive pressure of variable amplitude from 0-4 psi at 0.006 Hz was applied to the arm of each subject for a period of 90 minutes. Initially and at intervals of 30 minutes after the start of force application, blood samples were taken in order to monitor circulating plasma OPN and sCD44 levels in subjects. We found that the change of circulating OPN levels, but not sCD44 levels, measured in vivo in response to our mechanical stimulation was statistically significantly correlated to status of spinal deformity severity, with more severely affected subjects demonstrating lower magnitudes of ΔOPN.  To test the cellular portion of our hypothesis, osteoblasts from severely affected AIS patients and unaffected controls were cultured for use with our parallel plate flow chamber (PPFC) apparatus setup, which permits application of fluid shear stress patterns to cells in a predictable, controllable manner. Subjects were all females who fell into the 11-17 years age range, with scoliotic patients presenting with double major curves. A dynamic, sinusoidal and oscillatory fluid shear stress pattern was applied to osteoblasts at 2 Pa, 0.5 Hz for 90 minutes. Overall gene expression changes across RNA samples as a result of our stimulation were measured using microarray and qRT-PCR approaches. In response, only a very small number of genes are either mutually differentially expressed or statistically unchanged across all functional scoliotic subgroups while having the opposite condition in the control group, indicating a great degree of difference in terms of mechanotransductive response as compared internally between AIS functional subgroups, as well as between control and AIS patients.  Globally, this project’s work may contribute to the development of innovative diagnostic tools to identify asymptomatic children at risk of developing scoliosis, and to assess the risk of curve progression at an early stage in those already affected. Also, in years to come, the mechanotransductive profile of a patient could be another integral factor to weigh, clinically, when considering or designing treatment plans for affected persons.

Keyphrases: parallel plate flow chamber, sCD44, mechanotransduction, idiopathic scoliosis, osteopontin, fluid shear stress, biomechanical, diagnostic tools