ObjectiveTo assess the role of the glycoprotein PRG4 in joint lubrication and chondroprotection by measuring friction, stiffness, surface topography, and subsurface histology of the hip joints of Prg4−/− and wild-type (WT) mice.MethodsFriction and elastic modulus were measured in cartilage from the femoral heads of Prg4−/− and WT mice ages 2, 4, 10, and 16 weeks using atomic force microscopy, and the surface microstructure was imaged. Histologic sections of each femoral head were stained and graded.ResultsHistologic analysis of the joints of Prg4−/− mice showed an enlarged, fragmented surface layer of variable thickness with Safranin O–positive formations sometimes present, a roughened underlying articular cartilage surface, and a progressive loss of pericellular proteoglycans. Friction was significantly higher on cartilage of Prg4−/− mice at age 16 weeks, but statistically significant differences in friction were not detected at younger ages. The elastic modulus of the cartilage was similar between cartilage surfaces of Prg4−/− and WT mice at young ages, but cartilage of WT mice showed increasing stiffness with age, with significantly higher moduli than cartilage of Prg4−/− mice at older ages.ConclusionDeletion of the gene Prg4 results in significant structural and biomechanical changes in the articular cartilage with age, some of which are consistent with osteoarthritic degeneration. These findings suggest that PRG4 plays a significant role in preserving normal joint structure and function.

Articular cartilage provides a low-friction, wear-resistant surface for the motion of diarthrodial joints. The objective of this study was to develop a method for in situ friction measurement of murine cartilage using a colloidal probe attached to the cantilever of an atomic force microscope. Sliding friction was measured between a chemically functionalized microsphere and the cartilage of the murine femoral head. Friction was measured at normal loads ranging incrementally from 20 to 100 nN with a sliding speed of 40 microm/s and sliding distance of 64 microm. Under these test conditions, hydrostatic pressurization and biphasic load support in the cartilage were minimized, providing frictional measurements that predominantly reflect boundary lubrication properties. Friction coefficients measured on murine tissue (0.25+/-0.11) were similar to those measured on porcine tissue (0.23+/-0.09) and were in general agreement with measurements of boundary friction on cartilage by other researchers. Using the colloidal probe as an indenter, the elastic mechanical properties and surface roughness were measured in the same configuration. Interfacial shear was found to be the principal mechanism of friction generation, with little to no friction resulting from plowing forces, collision forces, or energy losses due to normal deformation. This measurement technique can be applied to future studies of cartilage friction and mechanical properties on genetically altered mice or other small animals.

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The rate of uptake of 2-aminoisobutyrate (AIB), a non-metabolizable analogue of alanine, by Leishmania donovani was investigated as a function of culture age and osmolality in the presence of protein kinase inhibitors and of glucose, glutamate or proline. Hyperosmolality inhibited AIB uptake by cells of both growth stages, but to a greater extent for log than for stationary cells. Staurosporine, a protein kinase C inhibitor, had no effect on AIB uptake by cells of either culture age, but it reduced the rate of AIB release in response to hypo-osmolality, and more so in stationary cells than in log cells. Genistein, a protein-tyrosine kinase inhibitor, caused a small increase in AIB uptake by log cells under both iso- and hyperosmotic conditions, but had no effect on AIB uptake by stationary cells. Genistein also caused a small increase in the rate of release of AIB in response to a decrease in osmolality. Brief preincubation with glucose, glutamate or proline of cells that had been depleted of their osmolytes by prior exposure to hypo-osmolality caused an increase in AIB release by log cells, but a decrease by stationary cells. Similarly, whereas glucose and glutamate increased the rate of AIB uptake by log cells, and proline inhibited it, they had no effect on AIB uptake by stationary cells. Thus AIB uptake and release are sensitive to changes in osmolality, to protein kinase inhibitors, and to certain nutrients in a manner that changes markedly with culture age.