The mechanical microenvironment is linked to the sclerostin level in single osteocytes in an in vivo model of bone adaptation

The Faculty of Informatics is pleased to announce a seminar given by Andreas J. Trüssel

DATE: Tuesday, August 20th 2013
PLACE: USI Lugano Campus, room A12, Red building (Via G. Buffi 13)
TIME: 14.30

ABSTRACT:
Mechanical loads are one major factor controlling bone mass in adults. Embedded deeply within the mineralized bone matrix, osteocytes are thought to be the key mediators of the load induced remodelling process. Owing to their inaccessible location, the molecular mechanisms by which mechanical forces are translated into metabolic signals remain poorly understood. Sclerostin (scl) has recently been identified as an anti-anabolic signal secreted by osteocytes that reduces the bone forming activity of osteoblasts1. We determined the role of scl in mechanical induced bone remodelling in trabecular bone in vivo. We therefore established a working protocol to i) measure the scl level of single osteocytes in histological sections, to ii) map these osteocytes into their three dimensional mechanical micro-environment and to iii) determine bone formation and resorption activity in their close neighbourhood. The 6th caudal vertebrae (CV 6) of 2 adult female C57BL/6 mice were dynamically loaded2 and scanned by in vivo micro computed tomography (μCT) at week 0, 2 and 4. The 3D dynamic morphometric parameters were assessed using advanced registration techniques3. Micro finite element (μFE) analysis was used to calculate the strain energy density (SED) on the local tissue level. Samples were harvested one day after the last loading cycle, decalcified, embedded in paraffin, sectioned (8µm) and stained for scl using immunohistochemistry1. To quantify scl levels, the number of positive stained pixels inside each osteocyte was counted. Osteocytes were mapped into the in vivo 3D μCT volumes and the corresponding μFE models. They were grouped according to regions of formation, quiescence and resorption. SED was quantified around each osteocyte location (sphere with 40μm radius) prior to both loading and sacrifice. The change in SED (ΔSED) was compared to scl level. The results demonstrated that scl level was lower in osteocytes in regions of bone formation than in regions of bone resorption confirming our hypothesis. No correlation between scl level and SED was found. However, when comparing scl level with ΔSED, significantly decreased scl levels were found in osteocytes in volumes with increased ΔSED. Therefore osteocytes seem to respond to changes in their mechanical microenvironment rather than to the mechanical microenvironment itself. Moreover they seem to enhance bone formation locally by reducing scl concentration in areas with increasing strains. This is the first study to show that scl is locally regulated in the mechanically induced trabecular bone adaptation process. Using the methodology developed here we will be able to identify and investigate further key molecules and thereby extend our molecular understanding of the local remodelling processes in bone.

(in collaboration with D.J. Webster, M. Salis-Soglio, G.A. Kuhn and R. Müller)

BIO:
Andreas Trüssel studied Nanoscience at the Swiss Nanoscience Institute at the University of Basel (2005-2010). During his study, he completed two student research projects, one at the Institute of Physics in Basel (2008) "Dye sensitized nanocrystalline solar cells" and the other abroad at the Nanobiomechanical Laboratory at National University of Singapore (2009) "A Model for Adhesion Force Study". In May 2010, he finished his Master Thesis "The Influence of Substrate Elasticity on Adhesion and Phenotype of re-differentiating Human Articular Chondrocytes" at the University Hospital in Basel. After his study he did an internship at the Biomaterial group at Synthes GmbH in Oberdorf, where he investigated mechanical properties of bone cements. In January 2011 Andreas Truessel joined Prof. Ralph Müller's Group at the Institute for Biomechanics at ETH Zurich as a PhD student. His research focuses on the adaptation process of bone after altered mechanical environment. He has been working on qualitative and quantitative methodologies using bioimaging, visualization techniques, computational analyses, biomechanical testing, single-cell RT-PCR and microfluidics to assess the anabolic response of trabecular bone in response to mechanical stimulation. His research has already successfully brought engineering methods to cell and molecular biology, potentially enabling the development of novel strategies for the management of osteoporosis.

HOST: Prof. Rolf Krause, Dr. Roberto Croce