Skeletal phenotype amelioration in mucopolysaccharidosis VI requires intervention at the earliest stages of postnatal development.

Mucopolysaccharidosis VI (MPS VI) is a rare lysosomal disease arising from impaired function of the enzyme arylsulfatase B (ARSB). This impairment causes aberrant accumulation of dermatan sulfate, a glycosaminoglycan (GAG) abundant in cartilage. While clinical severity varies along with age at first symptom manifestation, MPS VI usually presents early and strongly affects the skeleton. Current enzyme replacement therapy (ERT) does not provide effective treatment for the skeletal manifestations of MPS VI. This lack of efficacy may be due to an inability of ERT to reach affected cells or to the irreversibility of the disease. To address the question of reversibility of skeletal phenotypes, we generated a conditional by inversion (COIN) mouse model of MPS VI, ArsbCOIN/COIN, wherein Arsb is initially null and can be restored to WT using Cre. We restored Arsb at different times during postnatal development, using a tamoxifen-dependent global Cre driver. By restoring Arsb at P7, P21, and P56-P70, we determined that skeletal phenotypes can be fully rescued if Arsb restoration occurs at P7, while only achieving partial rescue at P21 and no significant rescue at P56-P70. This work has highlighted the importance of early intervention in patients with MPS VI to maximize therapeutic impact.

Cell type-selective targeted delivery of a recombinant lysosomal enzyme for enzyme therapies.

Lysosomal diseases are a class of genetic disorders predominantly caused by loss of lysosomal hydrolases, leading to lysosomal and cellular dysfunction. Enzyme replacement therapy (ERT), where recombinant enzyme is given intravenously, internalized by cells, and trafficked to the lysosome, has been applied to treat several lysosomal diseases. However, current ERT regimens do not correct disease phenotypes in all affected organs because the biodistribution of enzyme uptake does not match that of the affected cells that require the enzyme. We present here targeted ERT, an approach that utilizes antibody-enzyme fusion proteins to target the enzyme to specific cell types. The antibody moiety recognizes transmembrane proteins involved in lysosomal trafficking and that are also preferentially expressed in those cells most affected in disease. Using Pompe disease (PD) as an example, we show that targeted ERT is superior to ERT in treating the skeletal muscle phenotypes of PD mice both as a protein replacement therapeutic and as a gene therapy.

Mechanically induced Ca2+ oscillations in osteocytes release extracellular vesicles and enhance bone formation.

The vast osteocytic network is believed to orchestrate bone metabolic activity in response to mechanical stimuli through production of sclerostin, RANKL, and osteoprotegerin (OPG). However, the mechanisms of osteocyte mechanotransduction remain poorly understood. We've previously shown that osteocyte mechanosensitivity is encoded through unique intracellular calcium (Ca2+) dynamics. Here, by simultaneously monitoring Ca2+ and actin dynamics in single cells exposed to fluid shear flow, we detected actin network contractions immediately upon onset of flow-induced Ca2+ transients, which were facilitated by smooth muscle myosin and further confirmed in native osteocytes ex vivo. Actomyosin contractions have been linked to the secretion of extracellular vesicles (EVs), and our studies demonstrate that mechanical stimulation upregulates EV production in osteocytes through immunostaining for the secretory vesicle marker Lysosomal-associated membrane protein 1 (LAMP1) and quantifying EV release in conditioned medium, both of which are blunted when Ca2+ signaling was inhibited by neomycin. Axial tibia compression was used to induce anabolic bone formation responses in mice, revealing upregulated LAMP1 and expected downregulation of sclerostin in vivo. This load-related increase in LAMP1 expression was inhibited in neomycin-injected mice compared to vehicle. Micro-computed tomography revealed significant load-related increases in both trabecular bone volume fraction and cortical thickness after two weeks of loading, which were blunted by neomycin treatment. In summary, we found mechanical stimulation of osteocytes activates Ca2+-dependent contractions and enhances the production and release of EVs containing bone regulatory proteins. Further, blocking Ca2+ signaling significantly attenuates adaptation to mechanical loading in vivo, suggesting a critical role for Ca2+-mediated signaling in bone adaptation.

A noninvasive approach to determine viscoelastic properties of an individual adherent cell under fluid flow.

Mechanical properties of cells play an important role in their interaction with the extracellular matrix as well as the mechanotransduction process. Several in vitro techniques have been developed to determine the mechanical properties of cells, but none of them can measure the viscoelastic properties of an individual adherent cell in fluid flow non-invasively. In this study, techniques of fluid-structure interaction (FSI) finite element method and quasi-3-dimensional (quasi-3D) cell microscopy were innovatively applied to the frequently used flow chamber experiment, where an adherent cell was subjected to fluid flow. A new non-invasive approach, with cells at close to physiological conditions, was established to determine the viscoelastic properties of individual cells. The results showed an instantaneous modulus of osteocytes of 0.49 ± 0.11 kPa, an equilibrium modulus of 0.31 ± 0.044 kPa, and an apparent viscosity coefficient of 4.07 ± 1.23 kPas. This new quantitative approach not only provides an excellent means to measure cell mechanical properties, but also may help to elucidate the mechanotransduction mechanisms for a variety of cells under fluid flow stimulation.

In situ intracellular calcium oscillations in osteocytes in intact mouse long bones under dynamic mechanical loading.

Osteocytes have been hypothesized to be the major mechanosensors in bone. How in situ osteocytes respond to mechanical stimuli is still unclear because of technical difficulties. In vitro studies have shown that osteocytes exhibited unique calcium (Ca(2+)) oscillations to fluid shear. However, whether this mechanotransduction phenomenon holds for in situ osteocytes embedded within a mineralized bone matrix under dynamic loading remains unknown. Using a novel synchronized loading/imaging technique, we successfully visualized in real time and quantified Ca(2+) responses in osteocytes and bone surface cells in situ under controlled dynamic loading on intact mouse tibia. The resultant fluid-induced shear stress on the osteocyte in the lacunocanalicular system (LCS) was also quantified. Osteocytes, but not surface cells, displayed repetitive Ca(2+) spikes in response to dynamic loading, with spike frequency and magnitude dependent on load magnitude, tissue strain, and shear stress in the LCS. The Ca(2+) oscillations were significantly reduced by endoplasmic reticulum (ER) depletion and P2 purinergic receptor (P2R)/phospholipase C (PLC) inhibition. This study provides direct evidence that osteocytes respond to in situ mechanical loading by Ca(2+) oscillations, which are dependent on the P2R/PLC/inositol trisphosphate/ER pathway. This study develops a novel approach in skeletal mechanobiology and also advances our fundamental knowledge of bone mechanotransduction.

Spreading area and shape regulate apoptosis and differentiation of osteoblasts.

The in vivo observations have indicated that at the remodeling sites of bone, the spreading area or shape of preosteoblasts is confined by the mineralized matrix. But it remains unknown whether this spreading confinement regulates the differentiation or apoptosis of osteoblasts. In the present study, osteoblast-like cells (MC3T3-E1) were seeded on micropatterned islands with different area and shape. The expression of three osteogenic differentiation markers was measured by immunofluorescence staining and apoptotic cells were detected using a terminal deoxyribonucleotidyl transferase-mediated dUTP nick end labelling assay kit. The membrane fluorescence staining results showed that the actual spreading area of micropatterned osteoblasts coincided with the designed value. When the area of a micropatterned cell was confined as 314 or 615 µm(2), which was lower than that of freely spreading osteoblasts, the circular shape promoted the expression of osteogenic differentiation markers and the percentage of apoptotic osteoblasts compared with the branched shape. This shape-regulated differentiation and apoptosis of osteoblasts with confined spreading area were abolished when actin polymerization was inhibited by cytochalasin D. The present study gives an insight into the roles of spreading morphology on osteoblastic differentiation and apoptosis.

Simultaneous tracking of 3D actin and microtubule strains in individual MLO-Y4 osteocytes under oscillatory flow.

Osteocytes in vivo experience complex fluid shear flow patterns to activate mechanotransduction pathways. The actin and microtubule (MT) cytoskeletons have been shown to play an important role in the osteocyte's biochemical response to fluid shear loading. The dynamic nature of physiologically relevant fluid flow profiles (i.e., 1Hz oscillatory flow) impedes the ability to image and study both actin and MT cytoskeletons simultaneously in the same cell with high spatiotemporal resolution. To overcome these limitations, a multi-channel quasi-3D microscopy technique was developed to track the actin and MT networks simultaneously under steady and oscillatory flow. Cells displayed high intercellular variability and intracellular cytoskeletal variability in strain profiles. Shear Exz was the predominant strain in both steady and oscillatory flows in the form of viscoelastic creep and elastic oscillations, respectively. Dramatic differences were seen in oscillatory flow, however. The actin strains displayed an oscillatory strain profile more often than the MT networks in all the strains tested and had a higher peak-to-trough strain magnitude. Taken together, the actin networks are the more responsive cytoskeletal networks in osteocytes under oscillatory flow and may play a bigger role in mechanotransduction pathway activation and regulation.

Theoretical Analysis of Novel Quasi-3D Microscopy of Cell Deformation.

A novel quasi-three-dimensional (quasi-3D) microscopy technique has been developed to enable visualization of a cell under dynamic loading in two orthogonal planes simultaneously. The three-dimensional (3D) dynamics of the mechanical behavior of a cell under fluid flow can be examined at a high temporal resolution. In this study, a numerical model of a fluorescently dyed cell was created in 3D space, and the cell was subjected to uniaxial deformation or unidirectional fluid shear flow via finite element analysis (FEA). Therefore, the intracellular deformation in the simulated cells was exactly prescribed. Two-dimensional fluorescent images simulating the quasi-3D technique were created from the cell and its deformed states in 3D space using a point-spread function (PSF) and a convolution operation. These simulated original and deformed images were processed by a digital image correlation technique to calculate quasi-3D-based intracellular strains. The calculated strains were compared to the prescribed strains, thus providing a theoretical basis for the measurement of the accuracy of quasi-3D and wide-field microscopy-based intracellular strain measurements against the true 3D strains. The signal-to-noise ratio (SNR) of the simulated quasi-3D images was also modulated using additive Gaussian noise, and a minimum SNR of 12 was needed to recover the prescribed strains using digital image correlation. Our computational study demonstrated that quasi-3D strain measurements closely recovered the true 3D strains in uniform and fluid flow cellular strain states to within 5% strain error.

Quasi-3D cytoskeletal dynamics of osteocytes under fluid flow.

Osteocytes respond to dynamic fluid shear loading by activating various biochemical pathways, mediating a dynamic process of bone formation and resorption. Whole-cell deformation and regional deformation of the cytoskeleton may be able to directly regulate this process. Attempts to image cellular deformation by conventional microscopy techniques have been hindered by low temporal or spatial resolution. In this study, we developed a quasi-three-dimensional microscopy technique that enabled us to simultaneously visualize an osteocyte's traditional bottom-view profile and a side-view profile at high temporal resolution. Quantitative analysis of the plasma membrane and either the intracellular actin or microtubule (MT) cytoskeletal networks provided characterization of their deformations over time. Although no volumetric dilatation of the whole cell was observed under flow, both the actin and MT networks experienced primarily tensile strains in all measured strain components. Regional heterogeneity in the strain field of normal strains was observed in the actin networks, especially in the leading edge to flow, but not in the MT networks. In contrast, side-view shear strains exhibited similar subcellular distribution patterns in both networks. Disruption of MT networks caused actin normal strains to decrease, whereas actin disruption had little effect on the MT network strains, highlighting the networks' mechanical interactions in osteocytes.

A Trabecular Bone Explant Model of Osteocyte-Osteoblast Co-Culture for Bone Mechanobiology.

The osteocyte network is recognized as the major mechanical sensor in the bone remodeling process, and osteocyte-osteoblast communication acts as an important mediator in the coordination of bone formation and turnover. In this study, we developed a novel 3D trabecular bone explant co-culture model that allows live osteocytes situated in their native extracellular matrix environment to be interconnected with seeded osteoblasts on the bone surface. Using a low-level medium perfusion system, the viability of in situ osteocytes in bone explants was maintained for up to 4 weeks, and functional gap junction intercellular communication (GJIC) was successfully established between osteocytes and seeded primary osteoblasts. Using this novel co-culture model, the effects of dynamic deformational loading, GJIC, and prostaglandin E(2) (PGE(2)) release on functional bone adaptation were further investigated. The results showed that dynamical deformational loading can significantly increase the PGE(2) release by bone cells, bone formation, and the apparent elastic modulus of bone explants. However, the inhibition of gap junctions or the PGE(2) pathway dramatically attenuated the effects of mechanical loading. This 3D trabecular bone explant co-culture model has great potential to fill in the critical gap in knowledge regarding the role of osteocytes as a mechano-sensor and how osteocytes transmit signals to regulate osteoblasts function and skeletal integrity as reflected in its mechanical properties.

Over-expression of alpha-actinin with a GFP fusion protein is sufficient to increase whole-cell stiffness in human osteoblasts.

Osteoblasts respond to shear stress by simultaneously increasing their whole-cell stiffness and up-regulating the cytoskeletal crosslinking protein alpha-actinin. The stiffness of reconstituted cytoskeletal networks increases following the addition of alpha-actinin, but the effect of alpha-actinin on whole-cell mechanical behavior has not been investigated. The hypothesis of this study was that increasing alpha-actinin in the cytoskeleton would be sufficient to increase whole-cell stiffness. hFOB osteoblasts were transfected with a plasmid for GFP-tagged alpha-actinin, resulting in a 150% increase in the amount of alpha-actinin. The GFP-alpha-actinin fusion protein co-fractionated with the cytoskeleton and co-localized to the same regions of the cytoskeleton as endogenous alpha-actinin. Whole-cell mechanical behavior was measured by atomic force microscopy using a 25 mum diameter microsphere as an indenter. The whole-cell stiffness of cells over-expressing GFP-alpha-actinin was 60% higher than cells expressing only endogenous alpha-actinin (p < 0.002), which was within the range of mechanical behavior observed in osteoblastic cells exposed to 1 and 2 Pa of fluid shear. These results indicate that the up-regulation of alpha-actinin synthesis in osteoblasts is sufficient to alter the whole-cell mechanical behavior and highlights the potential role of alpha-actinin to reinforce cells against mechanical loads.