Browsing by Author "Xie, Zhihui"

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    Cell Mechanosensitivity to Extremely Low Magnitude Signals is Enabled by a LINCed Nucleus
    (Wiley, 2015-06) Uzer, Gunes; Thompson, William R.; Sen, Buer; Xie, Zhihui; Yen, Sherwin S.; Miller, Sean; Bas, Guniz; Styner, Maya; Rubin, Clinton T.; Judex, Stefan; Burridge, Keith; Rubin, Janet; Physical Therapy, School of Health and Rehabilitation Sciences
    A cell's ability to recognize and adapt to the physical environment is central to its survival and function, but how mechanical cues are perceived and transduced into intracellular signals remains unclear. In mesenchymal stem cells (MSCs), high-magnitude substrate strain (HMS, ≥2%) effectively suppresses adipogenesis via induction of focal adhesion (FA) kinase (FAK)/mTORC2/Akt signaling generated at FAs. Physiologic systems also rely on a persistent barrage of low-level signals to regulate behavior. Exposing MSC to extremely low-magnitude mechanical signals (LMS) suppresses adipocyte formation despite the virtual absence of substrate strain (<0.001%), suggesting that LMS-induced dynamic accelerations can generate force within the cell. Here, we show that MSC response to LMS is enabled through mechanical coupling between the cytoskeleton and the nucleus, in turn activating FAK and Akt signaling followed by FAK-dependent induction of RhoA. While LMS and HMS synergistically regulated FAK activity at the FAs, LMS-induced actin remodeling was concentrated at the perinuclear domain. Preventing nuclear-actin cytoskeleton mechanocoupling by disrupting linker of nucleoskeleton and cytoskeleton (LINC) complexes inhibited these LMS-induced signals as well as prevented LMS repression of adipogenic differentiation, highlighting that LINC connections are critical for sensing LMS. In contrast, FAK activation by HMS was unaffected by LINC decoupling, consistent with signal initiation at the FA mechanosome. These results indicate that the MSC responds to its dynamic physical environment not only with "outside-in" signaling initiated by substrate strain, but vibratory signals enacted through the LINC complex enable matrix independent "inside-inside" signaling.
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    LARG GEF and ARHGAP18 orchestrate RhoA activity to control mesenchymal stem cell lineage
    (Elsevier, 2018-02) Thompson, William R.; Yen, Sherwin S.; Uzer, Gunes; Xie, Zhihui; Sen, Buer; Styner, Maya; Burridge, Keith; Rubin, Janet; Physical Therapy, School of Health and Rehabilitation Sciences
    The quantity and quality of bone depends on osteoblastic differentiation of mesenchymal stem cells (MSCs), where adipogenic commitment depletes the available pool for osteogenesis. Cell architecture influences lineage decisions, where interfering with cytoskeletal structure promotes adipogenesis. Mechanical strain suppresses MSC adipogenesis partially through RhoA driven enhancement of cytoskeletal structure. To understand the basis of force-driven RhoA activation, we considered critical GEFs (activators) and GAPs (inactivators) on bone marrow MSC lineage fate. Knockdown of LARG accelerated adipogenesis and repressed basal RhoA activity. Importantly, mechanical activation of RhoA was almost entirely inhibited following LARG depletion, and the ability of strain to inhibit adipogenesis was impaired. Knockdown of ARHGAP18 increased basal RhoA activity and actin stress fiber formation, but did not enhance mechanical strain activation of RhoA. ARHGAP18 null MSCs exhibited suppressed adipogenesis assessed by Oil-Red-O staining and Western blot of adipogenic markers. Furthermore, ARHGAP18 knockdown enhanced osteogenic commitment, confirmed by alkaline phosphatase staining and qPCR of Sp7, Alpl, and Bglap genes. This suggests that ARHGAP18 conveys tonic inhibition of MSC cytoskeletal assembly, returning RhoA to an “off state” and affecting cell lineage in the static state. In contrast, LARG is recruited during dynamic mechanical strain, and is necessary for mechanical suppression of adipogenesis. In summary, mechanical activation of RhoA in mesenchymal progenitors is dependent on LARG, while ARHGAP18 limits RhoA delineated cytoskeletal structure in static cultures. Thus, on and off GTP exchangers work through RhoA to influence MSC fate and responses to static and dynamic physical factors in the microenvironment.
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    Osteocyte specific responses to soluble and mechanical stimuli in a stem cell derived culture model
    (Nature Publishing Group, 2015-06-09) Thompson, William R.; Uzer, Gunes; Brobst, Kaitlyn E.; Xie, Zhihui; Sen, Buer; Yen, Sherwin S.; Styner, Maya; Rubin, Janet; Department of Physical Therapy, IU School of Health and Rehabilitation Sciences
    Studying osteocyte behavior in culture has proven difficult because these embedded cells require spatially coordinated interactions with the matrix and surrounding cells to achieve the osteocyte phenotype. Using an easily attainable source of bone marrow mesenchymal stem cells, we generated cells with the osteocyte phenotype within two weeks. These "stem cell derived-osteocytes" (SCD-O) displayed stellate morphology and lacunocanalicular ultrastructure. Osteocytic genes Sost, Dmp1, E11, and Fgf23 were maximally expressed at 15 days and responded to PTH and 1,25(OH)2D3. Production of sclerostin mRNA and protein, within 15 days of culture makes the SCD-O model ideal for elucidating regulatory mechanisms. We found sclerostin to be regulated by mechanical factors, where low intensity vibration significantly reduced Sost expression. Additionally, this model recapitulates sclerostin production in response to osteoactive hormones, as PTH or LIV repressed secretion of sclerostin, significantly impacting Wnt-mediated Axin2 expression, via β-catenin signaling. In summary, SCD-O cells produce abundant matrix, rapidly attain the osteocyte phenotype, and secrete functional factors including sclerostin under non-immortalized conditions. This culture model enables ex vivo observations of osteocyte behavior while preserving an organ-like environment. Furthermore, as marrow-derived mesenchymal stem cells can be obtained from transgenic animals; our model enables study of genetic control of osteocyte behaviors.