<div>Mesenchymal stem cells (MSCs) are responsive to mechanical stimuli that play an essential role in directing their differentiation to the chondrogenic lineage. A better</div><div>understanding of the mechanisms that allow MSCs to respond to mechanical stimuli is important to improving cartilage tissue engineering and regenerative medicine. Hydrostatic pressure (HP) in particular is known to be a primary mechanical force in joints. However, little is known about the underlying mechanisms that facilitate HP</div><div>mechanotransduction. Understanding the signaling pathways in MSCs in transducing HP to a beneficial biologic response and their interrelationship were the focus of this thesis. Studies used porcine marrow-derived MSCs seeded in agarose gel. Calcium ion Ca++ signaling, focal adhesion kinase (FAK) involvement, and sirtuin1 activity were investigated in conjunction with HP application.</div><div><br></div><div><div>Intracellular Ca++ concentration was previously shown to be changed with HP application. In our study a bioreactor was used to apply a single application of HP to the MSC-seeded gel structures and observe Ca++ signaling via live imaging of a fluorescent calcium indicator in cells. However, no fluctuations in Ca++ concentrations were observed with 10 minutes loading of HP. Additionally a problem with the biore actor design was discovered. First the gel was floating around in the bioreactor even without loading. After stabilizing the gel and stopping it from floating, there were still about 16 µm of movement and deformation in the system. The movement and deformation was analyzed for the gel structure and different parts of the bioreactor. </div><div><br></div><div>Furthermore, we investigated the role of FAK in early and late chondrogenesis and also its involvement in HP mechanotransduction. A FAK inhibitor was used on MSCs from day 1 to 21 and showed a dose-dependent suppression of chondrogenesis. However, when low doses of FAK inhibitor added to the MSC culture from day 21 to 42, chondrogenesis was not inhibited. With 4 hour cyclic HP, FAK phosphorylation increased. The beneficial effect of HP was suppressed with overnight addition of the</div></div><div><div>FAK inhibitor to MSC medium, suggesting FAK involvement in HP mechanotransducation by MSCs.</div></div><div><br></div><div>Moreover, sirtuin1 participation in MSC chondrogenesis and mechanotransduction was also explored. The results indicated that overnight sirtuin1 inhibition increased chondrogenic gene expression (Agc, Col2, and Sox9) in MSCs. Additionally, the activity of sirtuin1 was decreased with both 4 hour cyclic hydrostatic pressure and inhibitor application. These two together demonstrated that sirtuin1 inhibition enhances chondrogenesis.</div><div><br></div><div><div>In this research we have investigated the role of Ca++ signaling, FAK involvement, and sirtuin1 activity in the mechanotransduction of HP in MSCs. These understandings about the mechanisms regulating the chondrogenesis with respect to HP could have important implications for cartilage tissue engineering and regenerative studies.</div></div>

Les cellules souches de glioblastomes (GSC) sont sensibles aux propriétés mécaniques de leur microenvironnement et utilisent la rigidité pour favoriser leur invasion.Dans ce contexte, nous avons développé une matrice 3D fibrillaire artificielle permettant de récapituler in vitro les comportements migratoires observés in vivo. Cette matrice étant hautement plastique, nous avons pu moduler la rigidité, la chimie de surface ou encore l'alignement des fibres.Dans un premier temps nous avons modifié leur chimie de surface grâce à l’ajout de protéines de la matrice extracellulaire (MEC) puis déposé des neurosphères (NS) de GSC sur ce tissu artificiel. L’ajout de laminine nous a permis d’observer le passage d’un comportement migratoire collectif à individuel.Dans un deuxième temps nous avons modulé la rigidité des fibres. Après cinq jours de migration des GSC dans différentes conditions de rigidités, nous avons constaté une augmentation de la vitesse de migration à la rigidité intermédiaire de 166kPa par rapport à la condition plus souple de 3,2 kPa et à la condition plus rigides de 1260kPa. Cette capacité migratoire maximale dans nos conditions est associée à des changements de morphologie des GSC, à une augmentation de l'expression des protéines associée à la transition épithélio-mésenchymateuse (EMT) et à une modification de la régulation des protéines des adhérences focales.La surexpression de Mannoside acetyl glucosaminyltransférase 5 (MGAT5) est associée aux tumeurs malignes et est impliquée dans la formation de regroupement de protéines membranaires grâce à la formation d’un treillis. Elle favorise également la maturation des adhérences focales, la migration cellulaire, l'invasion et l’EMT, entraînant ainsi des avantages fonctionnels pour les cellules tumorales. Au vu des résultats précédents sur l'expression des protéines EMT et des modifications de la régulation des protéines des adhérences focales, nous avons généré des GSC n’exprimant plus la MGAT5 grâce à la technique CRISPR Cas9 et les avons placées en NS sur les matrices 3D présentant différentes rigidité. Nous avons alors observé une diminution de la vitesse de migration à 166kPa par rapport aux GSC contrôles, associée à une diminution de l'EMT et de la maturation des adhérences focales. Par conséquent, cette étude démontre l'implication de la glycosylation et plus particulièrement de la glycosylation médiée par la MGAT5 sur la mécanotransduction des GSC. / Glioblastoma stem cells (CSC) have been reported to be sensitive to the mechanical properties of the surrounding tissue/microenvironment and to use the microenvironment stiffness to enhance invasion.In this context, we developed a 3D artificial fibrillary tissue which can allow in vitro recapitulation of the migration behavior observed in vivo. This 3D matrix is highly plastic which allows for modulation of stiffness, surface chemistry and fibers alignment.On the first hand, we functionalized the fibers with extracellular matrix proteins and then we plated GSCs neurospheres (NS) on the developed 3D artificial tissue. The addition of laminin modulates the migration behaviour from single cell to collective mode.On the second hand we have modified stiffness of the fibers. After five days of GSCs NS migration on 3D electrospun fibers of different stiffnesses, we have seen an increase in migration velocity for an intermediate stiffness of 166kPa in comparison with our softest 3.2 kPa and stiffest stiffnesses (1260 kPa). This maximum migration rate is associated with changes in cell shape, increase of EMT proteins expressions and modifications of focal adhesion proteins regulation.Mannoside acetyl glucosaminyltransferase 5 (MGAT5) overexpression is associated with malignant tumors and it is implicated in the clustering of membrane proteins through lattice formation, focal adhesions, cell migration, invasion, and epithelial-to-mesenchymal transition (EMT), resulting in functional advantages for tumor cells.In light of previous results about of EMT proteins expressions and modifications of focal adhesion proteins regulation, we generated MGAT5 CRISPR Cas9 GSCs and placed it on 3D matrix with different stiffnesses. We observe a decrease in migration velocity at the intermediate stiffness in comparison with GSCs WT associated with a decrease in focal adhesion maturation and EMT. This study demonstrates the implication of glycosylation and more particularly MGAT5-mediated glycosylation on GSCs mechanotransduction.

L'homéostasie épidermique repose sur différents paramètres dont les propriétés mécaniques du tissu de soutien, le derme, et les tensions intrinsèques dans le tissu épithélial. Lors de la cicatrisation, l'augmentation de la rigidité cutanée résultante peut perturber les conditions initiales de l'homéostasie. Afin de mieux comprendre les mécanismes cellulaires dans ce contexte physiopathologique, et l'incidence des propriétés mécaniques du derme sur le comportement du kératinocyte humain, des cellules primaires ont été cultivés à la surface d'hydrogels de polyacrylamide de différentes rigidités, et sur le plastique (> GPa), condition in vitro classiquement utilisée. Nos résultats ont mis en évidence que les substrats les plus mous favorisent un arrêt de prolifération et un profil phénotypique similaire à un kératinocyte différencié, alors que les substrats les plus rigides facilitent l'adhérence et la prolifération au détriment de la capacité de différenciation, et ce de façon graduelle. L'analyse transcriptomique par séquençage haut débit a permis d'identifier un récepteur membranaire orphelin couplé à la protéine G, GPRC5A (G Protein-Coupled Receptor Class C Group 5 Member A) et une protéine du cytosquelette, la spectrine beta III, dont les augmentations d'expression sont corrélées à l'augmentation de la rigidité. In vivo, GPRC5A est exclusivement localisé dans les berges de la plaie, lieu des kératinocytes en migration. Ces observations ont été confirmées par l'utilisation d'outils d'ARN interférence (siRNA et shRNA) dirigés contre GPRC5A dans les kératinocytes humains, montrant l'implication de ce récepteur dans l'adhérence et la migration cellulaire. De plus, les résultats montrent que la diminution de GPRC5A entraine un défaut de différenciation et d'organisation du feuillet épidermique, conduisant notamment à une mort cellulaire accrue, dans un modèle d'épiderme reconstruit. En parallèle, par des approches similaires d'ARN interférence dirigée contre le gène SPTBN2 (spectrin beta non-erythrocytic 2) codant pour la spectrine beta III, les résultats mettent en évidence un rôle fonctionnel de la spectrine beta III dans la prolifération cellulaire, l'étalement, la migration des kératinocytes et participe à la mécanotransduction en réponse à la rigidité en permettant la survie cellulaire. L'ensemble des résultats de la thèse montrent une adaptation phénotypique des kératinocytes dépendante de la rigidité du substrat sur lequel ils se trouvent et positionne GPRC5A et la spectrine beta III comme des acteurs clés dans la réponse du kératinocyte primaire humain aux changements mécaniques. Ces résultats permettent d'ouvrir de nouvelles voies stratégiques pour le traitement de la cicatrisation et plus largement dans les pathologies affectant la mécanique cutanée / Epidermal homeostasis is determined by several characteristics, including dermis mechanical properties. During skin wound healing, dermis mechanical properties are modified and can alter epidermal integrity. Accordingly, it is essential to understand how keratinocytes respond and adapt to mechanical changes; however, these mechanisms remain unclear. To investigate how mechanical properties of cell microenvironment affect the human keratinocyte, primary cells were seeded on polyacrylamide hydrogels of different compliances (soft: 4 kPa, medium: 14 kPa, rigid: 45 kPa) in comparison with glass coverslip (> GPa). The results showed that on the softer hydrogel, keratinocyte spreading and proliferation were strongly decreased, while a strong increase in the expression of late differentiation markers was observed. On the contrary, the stiffer substrates promote adhesion and proliferation. Moreover, a transcriptomic profiling analysis reveals G protein-coupled receptor, class C, group 5, member A (GPRC5A) and spectrin beta non-erythrocytic 2 (SPTBN2) as potential mechanosensors for substrate adaptation of the keratinocyte. Actually, GPRC5A and SPTBN2 gene expression levels are associated with stiffness increase. We have characterized a dynamic relocation of GPRC5A during keratinocyte adhesion. Furthermore, the knockdown of GPRC5A increased cell adhesion and reduced cell migration. These results were confirmed by GPRC5A immunostaining in healing wounds from mice skin, showing a transient and specific expression of GPRC5A at wound leading edges, a site of cell migration, thus strengthening a role of GPRC5A in the keratinocyte migration process. Concerning SPTBN2, the knockdown of SPTBN2 decreased cell proliferation, spreading and migration, indicating a functional role in keratinocyte processes. Moreover, SPTBN2 knockdown induced keratinocyte death on soft substrate. In conclusion, we demonstrated an adaptive response of keratinocytes depending on substrate stiffness. We also found a new function of GPRC5A in keratinocyte process during skin wound healing and a mechanosensor role of SPTBN2 in keratinocyte. All of these results place GPRC5A and SPTBN2 as key regulators in the response to mechanical changes. These results pave new strategic pathways in skin wound healing treatment, and more widely in the pathologies affecting cutaneous mechanics

La circulation du flux sanguin à sens unique dans le système cardiovasculaire des vertébrés est assurée par les valves cardiaques. Leur formation est très contrôlée au cours du développement embryonnaire. Cependant, il arrive que celle-ci soit défectueuse, et donc à l’origine de maladies cardiaques congénitales. Ces maladies représentent une des causes majeures de décès à la naissance. L’étude de la formation des valves cardiaques constitue donc un champ de recherche majeur. Dans cette thèse, nous avons utilisé le poisson zèbre, comme animal d’étude modèle pour étudier la formation des valves atrio-ventriculaires. Les forces mécaniques générées par le flux sanguin constituent un signal modulant le programme génétique valvulaire. Elles initient la formation des valves en contraignant l’expression du facteur de transcription, Klf2a, à un groupe de cellules endothéliales du canal atrio-ventriculaire. Nos travaux ont démontré l’activation d’un autre facteur, Egr1, dans cette même région dans le même lapse de temps. Notre étude a cherché à élucider le réseau génétique impliquant klf2a, son paralogue klf2b, et egr1 en combinant une analyse pangénomique de l’expression génique et des sites accessibles de la chromatine avec une approche d’imagerie haute résolution in vivo. Nous avons déterminé les interactions entre ces facteurs et les réseaux qu’ils régulent. Cette étude a finalement démontré qu’egr1, klf2a/klf2b modulent la morphogénèse des valves cardiaques en contrôlant en particulier flt1, has2 et wnt9b. / Cardiac valves are necessary for maintaining a unidirectional blood flow in the cardiovascular system of vertebrates. Their efficient gating function requires a highly controlled developmental program. However, this program may be impaired and thus leading to defective valves. In fact, congenital heart valve diseases represent the most common form of birth defects. Therefore, cardiac valve development studies constitute a challenging research field. In this thesis, we used the zebrafish as a model organism for studying the formation of atrioventricular valves. To date, it is known that mechanical forces generated by blood flow constitute key modulators dictating valve formation. In particular, they initiate valvulogenesis by restricting the expression of the transcription factor Klf2a in a subset of endocardial cells of the atrio-ventricular canal. Our work demonstrated the activation of another transcription factor, Egr1, in this same region and within the same time window. We aimed at deciphering the mechanosentitive gene network involving klf2a, its paralog klf2b as well as egr1, by combining genome-wide analysis of gene expression and chromatin accessibility with live imaging. We addressed the potential interactions of these factors and studied their downstream signalling pathways. Finally, we demonstrated that egr1, klf2a/klf2b modulates valve morphogenesis by specifically controlling flt1, has2 and wnt9b expression.

La morphogenèse épithéliale est essentielle à la formation des organes. J'utilise le follicule ovarien de Drosophila comme modèle d'étude de l’aplatissement des cellules. Un follicule est composé de cellules germinales en croissance entourées d'une monocouche de cellules épithéliales cuboïdes. À un stade de développement spécifique, une part de ces cellules s'aplatit en suivant une vague régulée. Cet aplatissement est en partie contrôlé par un gradient de pression provenant d’un groupe de cellules germinales (les cellules nourricières). Toutes les cellules germinales sont connectées via des ponts cytoplasmiques. Cette thèse étudie les mécanismes conduisant à la génération du gradient de pression, et à la modulation moléculaire induite par cette force mécanique pour permettre l'aplatissement. J'ai montré que le nombre et le diamètre des ponts cytoplasmiques influaient sur la pression. En utilisant des reconstructions tridimensionnelles de follicules, j’ai étudié le rôle de la croissance différentielle des cellules nourricières en mesurant le changement de volume des cellules germinales lors de l'aplatissement des cellules. Enfin, j’ai cherché le mécanisme moléculaire conduisant à l’aplatissement des cellules et influencé par un stimulus mécanique à partir de la pression germinale, en proposant un rôle de la voie Hippo dans ce processus. En conclusion, nous proposons que la croissance des cellules germinales influe de manière mécanique et génétique sur les cellules épithéliales pour permettre l’élongation, et donc l'acquisition de la forme finale du follicule. / Epithelial morphogenesis is essential for organ formation. I use the Drosophila ovarian follicle as a model for studying cell flattening. A follicle is composed of growing germ cells surrounded by a monolayer of cuboidal epithelial cells. At a specific stage of development, some of these cells flatten out following a regulated wave. This flattening is partly controlled by a pressure gradient from part of the germ cells (the nurse cells). All germ cells are connected via cytoplasmic bridges. This thesis studies the mechanisms leading to the generation of the gradient of pressure, and to the molecular modulation induced by this mechanical force to allow flattening. I have shown that the number and diameter of cytoplasmic bridges affect the pressure. Using three-dimensional reconstructions of follicles, I studied the role of differential growth of nurse cells by measuring the change in germ cell volume during epithelial cell flattening. Finally, I looked for the molecular mechanism leading to the flattening and influenced by a mechanical stimulus from the germinal pressure, supporting a role of the Hippo pathway in this process. In conclusion, we propose that germ cell growth mechanically and genetically influences epithelial cells to allow elongation, and thus the acquisition of the final form of the follicle.

This thesis takes a fresh approach to cochlear mechanics. Over the last quarter of a century, we have learnt that the cochlea is active and highly tuned, observations suggesting that something may be resonating. Rather than accepting the standard traveling wave interpretation, here I investigate whether a resonance theory of some kind can be applied to this remarkable behaviour.¶ A historical survey of resonance theories is first conducted, and advantages and drawbacks examined. A corresponding look at the traveling wave theory includes a listing of its short-comings.¶ A new model of the cochlea is put forward that exhibits inherently high tuning. The surface acoustic wave (SAW) model suggests that the three rows of outer hair cells (OHCs) interact in a similar way to the interdigital transducers of an electronic SAW device. Analytic equations are developed to describe the conjectured interactions between rows of active OHCs in which each cell is treated as a point source of expanding wavefronts. Motion of a cell launches a wave that is sensed by the stereocilia of neighbouring cells, producing positive feedback. Numerical calculations confirm that this arrangement provides sharp tuning when the feedback gain is set just below oscillation threshold.¶ A major requirement of the SAW model is that the waves carrying the feedback have slow speed (5-200 mm/s) and high dispersion. A wave type with the required properties is identified - a symmetric Lloyd-Redwood wave (or squirting wave) - and the physical properties of the organ of Corti are shown to well match those required by theory.¶ The squirting wave mechanism may provide a second filter for a primary traveling wave stimulus, or stand-alone tuning in a pure resonance model. In both, cyclic activity of squirting waves leads to standing waves, and this provides a physical rendering of the cochlear amplifier. In keeping with pure resonance, this thesis proposes that OHCs react to the fast pressure wave rather than to bending of stereocilia induced by a traveling wave. Investigation of literature on OHC ultrastructure reveals anatomical features consistent with them being pressure detectors: they possess a cuticular pore (a small compliant spot in an otherwise rigid cell body) and a spherical body within (Hensens body) that could be compressible. I conclude that OHCs are dual detectors, sensing displacement at high intensities and pressure at low. Thus, the conventional traveling wave could operate at high levels and resonance at levels dominated by the cochlear amplifier. ¶ The latter picture accords with the description due to Gold (1987) that the cochlea is an ‘underwater piano’ - a bank of strings that are highly tuned despite immersion in liquid.¶ An autocorrelation analysis of the distinctive outer hair cell geometry shows trends that support the SAW model. In particular, it explains why maximum distortion occurs at a ratio of the two primaries of about 1.2. This ratio also produces near-integer ratios in certain hair-cell alignments, suggesting that music may have a cochlear basis.¶ The thesis concludes with an evaluation and proposals to experimentally test its validity.

An emerging non-pharmacological treatment for bone degenerative diseases is whole body vibration (WBV), a mechanical signal composed of low-magnitude, high-frequency (LMHF) vibrations that when applied to bone, have osteogenic and anti-resorptive effects. Currently, the cellular and molecular mechanism underlying the effect of WBV on bone is unclear. In this study, we investigated the response of osteocytes, the putative mechanosensor in bone, under LMHF vibration. As bone cells differentiate from mesenchymal stromal cells (MSCs), we also studied the osteogenic differentiation of rat MSCs in the presence of vibration loading. We found that vibrated osteocytes show gene and protein expression changes suggestive of an anti-osteoclastogenic response, and secrete soluble factors that inhibit osteoclast formation and activity. In contrast, rat MSCs showed moderate to no response to LMHF vibration during osteogenic differentiation. Our data suggest that in vivo effects of LMHF vibration are mediated through mechanosensing and biochemical responses by osteocytes.

Nano-FET biosensors are an emerging nanoelectronic technology capable of real-time and label-free quantification of soluble biological molecules. This technology promises to enable novel in vitro experimental approaches for investigating complex biological systems. In this study, we first explored osteocyte mechanosensitivity under different mechanical stimuli and found that osteocytes are exquisitely sensitive to different oscillatory fluid flow conditions. We therefore aimed to characterize protein-mediated intercellular communication between mechanically-stimulated osteocytes and other bone cell populations in vitro to elucidate the underlying mechanisms of load-induced bone remodeling. To this end, we devised a novel nano-manipulation based fabrication method for manufacturing nano-FET biosensors with precisely controlled device parameters, and further investigated the effect of these parameters on sensor performance.

An emerging non-pharmacological treatment for bone degenerative diseases is whole body vibration (WBV), a mechanical signal composed of low-magnitude, high-frequency (LMHF) vibrations that when applied to bone, have osteogenic and anti-resorptive effects. Currently, the cellular and molecular mechanism underlying the effect of WBV on bone is unclear. In this study, we investigated the response of osteocytes, the putative mechanosensor in bone, under LMHF vibration. As bone cells differentiate from mesenchymal stromal cells (MSCs), we also studied the osteogenic differentiation of rat MSCs in the presence of vibration loading. We found that vibrated osteocytes show gene and protein expression changes suggestive of an anti-osteoclastogenic response, and secrete soluble factors that inhibit osteoclast formation and activity. In contrast, rat MSCs showed moderate to no response to LMHF vibration during osteogenic differentiation. Our data suggest that in vivo effects of LMHF vibration are mediated through mechanosensing and biochemical responses by osteocytes.

Nano-FET biosensors are an emerging nanoelectronic technology capable of real-time and label-free quantification of soluble biological molecules. This technology promises to enable novel in vitro experimental approaches for investigating complex biological systems. In this study, we first explored osteocyte mechanosensitivity under different mechanical stimuli and found that osteocytes are exquisitely sensitive to different oscillatory fluid flow conditions. We therefore aimed to characterize protein-mediated intercellular communication between mechanically-stimulated osteocytes and other bone cell populations in vitro to elucidate the underlying mechanisms of load-induced bone remodeling. To this end, we devised a novel nano-manipulation based fabrication method for manufacturing nano-FET biosensors with precisely controlled device parameters, and further investigated the effect of these parameters on sensor performance.