Correlation between distribution of cytoskeletal proteins and release of alkaline phosphatase-rich vesicles by epiphyseal chondrocytes in primary culture

Do Media Extracellular Vesicles and Extracellular Vesicles Bound to the Extracellular Matrix Represent Distinct Types of Vesicles?

Mineralization-competent cells, including hypertrophic chondrocytes, mature osteoblasts, and osteogenic-differentiated smooth muscle cells secrete media extracellular vesicles (media vesicles) and extracellular vesicles bound to the extracellular matrix (matrix vesicles). Media vesicles are purified directly from the extracellular medium. On the other hand, matrix vesicles are purified after discarding the extracellular medium and subjecting the cells embedded in the extracellular matrix or bone or cartilage tissues to an enzymatic treatment. Several pieces of experimental evidence indicated that matrix vesicles and media vesicles isolated from the same types of mineralizing cells have distinct lipid and protein composition as well as functions. These findings support the view that matrix vesicles and media vesicles released by mineralizing cells have different functions in mineralized tissues due to their location, which is anchored to the extracellular matrix versus free-floating.

Remyelination in PNS and CNS: current and upcoming cellular and molecular strategies to treat disabling neuropathies

Myelin is a lipid-rich nerve cover that consists of glial cell's plasmalemma layers and accelerates signal conduction. Axon-myelin contact is a source for many developmental and regenerative signals of myelination. Intra- or extracellular factors including both enhancers and inhibitors are other factors affecting the myelination process. Myelin damages are observed in several congenital and hereditary diseases, physicochemical conditions, infections, or traumatic insults, and remyelination is known as an intrinsic response to injuries. Here we discuss some molecular events and conditions involved in de- and remyelination and compare the phenomena of remyelination in CNS and PNS. We have explained applying some of these molecular events in myelin restoration. Finally, the current and upcoming treatment strategies for myelin restoration are explained in three groups of immunotherapy, endogenous regeneration enhancement, and cell therapy to give a better insight for finding the more effective rehabilitation strategies considering the underlying molecular events of a lesion formation and its current condition.

Phosphate-induced autophagy counteracts vascular calcification by reducing matrix vesicle release

Autophagy is a dynamic and highly regulated process of self-digestion responsible for cell survival and reaction to oxidative stress. As oxidative stress is increased in uremia and is associated with vascular calcification, we studied the role of autophagy in vascular calcification induced by phosphate. In an in vitro phosphate-induced calcification model of vascular smooth muscle cells (VSMCs) and in an in vivo model of chronic renal failure, autophagy was inhibited by the superoxide dismutase mimic MnTMPyP, superoxide dismutase-2 overexpression, and by knockdown of the sodium-dependent phosphate cotransporter Pit1. Although phosphate-induced VSMC apoptosis was reduced by an inhibitor of autophagy (3-methyladenine) and knockdown of autophagy protein 5, calcium deposition in VSMCs was increased during inhibition of autophagy, even with the apoptosis inhibitor Z-VAD-FMK. An inducer of autophagy, valproic acid, decreased calcification. Furthermore, 3-methyladenine significantly promoted phosphate-induced matrix vesicle release with increased alkaline phosphatase activity. Thus, autophagy may be an endogenous protective mechanism counteracting phosphate-induced vascular calcification by reducing matrix vesicle release. Therapeutic agents influencing the autophagic response may be of benefit to treat aging or disease-related vascular calcification and osteoporosis.

GPM6B regulates osteoblast function and induction of mineralization by controlling cytoskeleton and matrix vesicle release

Neuronal membrane glycoprotein gene (GPM6B) encodes a membrane glycoprotein that belongs to the proteolipid protein family. We identified GPM6B as a gene that is strongly upregulated during osteoblast differentiation. To investigate the role of GPM6B in the process of bone formation, we silenced GPM6B expression during osteogenic differentiation of human mesenchymal stem cells (hMSCs). GPM6B silencing in hMSCs resulted in reduced alkaline phosphate (ALP) activity along with reduced mineralization of extracellular matrix (ECM). Microarray expression analysis of GPM6B-depleted osteogenic hMSCs revealed significant changes in genes involved in cytoskeleton organization and biogenesis. Immunocytochemistry results confirm changes in the distribution of actin filaments, as well as the shape and size of focal adhesions on GPM6B silencing. Moreover, we demonstrated that production and release of ALP-positive matrix vesicles (MVs) were reduced. In conclusion, we identified GPM6B as a novel regulator of osteoblast function and bone formation. This finding demonstrates the significance of cytoskeleton organization for MV production and subsequent mineralization.

Matrix vesicles originate from apical membrane microvilli of mineralizing osteoblast-like Saos-2 cells

In bone, mineralization is tightly regulated by osteoblasts and hypertrophic chondrocytes which release matrix vesicles (MVs) and control extracellular ionic conditions and matrix composition. MVs are the initial sites of hydroxyapatite (HA) mineral formation. Despite growing knowledge about their morphology and function, their biogenesis is not well understood. The purpose of this work was to determine the source of MVs in osteoblast lineage, Saos-2 cells, and to check whether MVs originated from microvilli. Microvilli were isolated from the apical plasma membrane of Saos-2 cells. Their morphology, structure, and function were compared with those of MVs. The role of actin network in MV release was investigated by using microfilament perturbing drugs. When examined by electron microscopy MVs and microvillar vesicles were found to exhibit similar morphology with trilaminar membranes and diameters in the same range. Both types of vesicles were able to induce HA formation. Their electrophoretic profiles displayed analogous enrichment in alkaline phosphatase, Na(+)/K(+) ATPase, and annexins A2 and A6. MVs and microvillar vesicles exhibited almost the same lipid composition with a higher content of cholesterol, sphingomyelin, and phosphatidylserine as compared to plasma membrane. Finally, cytochalasin D, which inhibits actin polymerization, was found to stimulate release of MVs. Our findings were consistent with the hypothesis that MVs originated from cell microvilli and that actin filament disassembly was involved in their biogenesis.

The effects of morphology, confluency, and phenotype on whole-cell mechanical behavior

Emerging evidence indicates that cellular mechanical behavior can be altered by disease, drug treatment, and mechanical loading. To effectively investigate how disease and mechanical or biochemical treatments influence cellular mechanical behavior, it is imperative to determine the source of large inter-cell differences in whole-cell mechanical behavior within a single cell line. In this study, we used the atomic force microscope to investigate the effects of cell morphological parameters and confluency on whole-cell mechanical behavior for osteoblastic and fibroblastic cells. For nonconfluent cells, projected nucleus area, cell area, and cell aspect ratio were not correlated with mechanical behavior (p>or=0.46), as characterized by a parallel-spring recruitment model. However, measured force-deformation responses were statistically different between osteoblastic and fibroblastic cells (p<0.001) and between confluent and nonconfluent cells (p<0.001). Osteoblastic cells were 2.3-2.8 times stiffer than fibroblastic cells, and confluent cells were 1.5-1.8 times stiffer than nonconfluent cells. The results indicate that structural differences related to phenotype and confluency affect whole-cell mechanical behavior, while structural differences related to global morphology do not. This suggests that cytoskeleton structural parameters, such as filament density, filament crosslinking, and cell-cell and cell-matrix attachments, dominate inter-cell variability in whole-cell mechanical behavior.

Ultrastructure of matrix vesicles in chick growth plate as revealed by quick freezing and freeze substitution

The ultrastructure of extracellular membrane-bound matrix vesicles (MVs), their biogenesis, and the surrounding matrix in chick tibial growth plate were studied after quick freezing and freeze substitution (FS) in an organic solvent. There were several notable differences in the ultrastructural preservation of cartilage when FS was used as compared with conventional fixation. The ultrastructural appearance of MVs after FS was extremely variable. Within the MVs, intravesicular filaments, amorphous material, and membrane-associated undercoat structures were observed. Intravesicular filaments, similar in diameter to microfilaments seen in the cytoplasm, were attached to the inside of MV membranes. This observation indicates the similarity of MV membranes and the plasma membrane. In some MVs in the proliferative zone an electron-dense material was present along the inner side of the MV membrane. In the prehypertrophic zone, crystalline material often appeared within the electron-dense material, which may be a precursor form of hydroxyapatite. The earliest crystals observed were in MVs but not in the extracellular matrix. Regarding MV formation, in addition to budding from cell surfaces and to cellular disintegration, this study also indicates that a sequential process of extrusion of preformed cytoplasmic structures may occur. Also, small MVs measuring 25-40 nm seem to arise from the disruption of large MVs. This is a previously unreported observation on MV biogenesis. FS preserves proteoglycans in the cartilage matrix as a fine, filamentous network. Initial extracellular calcification was not associated with this network.

Improved ultrastructural preservation of epiphyseal chondrocytes by the freeze-substitution method

The ultrastructure of epiphyseal chondrocytes was studied following quick-freezing and freeze-substitution, and was compared to that of cells fixed with aqueous aldehydes. The former approach provided an improved ultrastructural preservation whereby every type of chondrocyte exhibited a smoother cell contour. The plasma membrane as well as intracytoplasmic membranes revealed a trilaminar substructure. The intracytoplasmic ground substance was composed of flocculent materials which were in direct contact with the inner leaflet of the plasma membrane. Within the extracellular matrix the proteoglycan network adhered to the outer leaflet of the plasma membrane. Whenever cellular shrinkage took place, the flocculent matrix within the cytoplasm and the proteoglycan network in the pericellular matrix disappeared. The contents of the RER, the Golgi apparatus, and the intracellular vesicles and vacuoles were well retained. In the proliferative zone, the Golgi saccules of young cells contained a thread-like structure showing a clear periodicity. The cytoplasmic vesicles and vacuoles showed marked variation in their electron density. Intramitochondrial granules were sensitive to aqueous treatments, as evidenced by the observation that they disappeared after either floating on water or staining with aqueous solution. In the calcifying zone, mitochondrial granules were noted within hypertrophic chondrocytes, a feature that was not observed following conventional processing. Cytoskeletal elements were well preserved in all types of cells. A dense microfilamentous network occupied the pericellular cytoplasm. Bundles of microfilaments were seen in the cellular peripheral processes. Microtubules were distributed throughout the cytoplasm, and the Golgi complex was intimately associated with the microtubule network; it appears that the secretory processes are involved with the microtubules.

Development of a new serum-free medium, USC-HC1, for growth and normal phenotype in postembryonic chicken growth plate chondrocytes

A serum-free medium for postembryonic chicken epiphyseal growth plate chondrocytes has been developed from 104 MCDB medium. To enable these fastidious cells to survive, grow, and express normal phenotype, a substantial increase over MCDB 104 in the level of many of the amino acids was required, as well as a change in the buffer system and the addition of SerXtend, a defined, serum-free product containing various growth factors, including fibroblast growth factor. Also required was the provision of cell attachment factors, either by coating culture surfaces with type II collagen, or better, by allowing the freshly released cells to recover for several hours in a medium supplemented with 10% fetal bovine serum before plating. This new serum-free medium, which we call USC-HC1, supports growth and replication, the retention of normal polygonal morphology, the expression of significant levels of cellular alkaline phosphatase activity, the production of sulfated proteoglycans, type II collagen, and the formation of alkaline phosphatase-rich matrix vesicles by the chondrocytes. The major advantage of USC-HC1, however, is that it will provide for the first time an opportunity to examine the effects of various defined growth and hormonal factors on the phenotypic expression and differentiation of growth plate chondrocytes, in the absence of the variable (stimulatory and inhibitory) factors present in fetal bovine serum.