Osteocytic perilacunar/canalicular turnover in hemodialysis patients with high and low serum PTH levels

Osteocytic perilacunar/canalicular turnover in hemodialysis patients has not yet been reported. Osteocyte lacunae in lamellar bone and woven bone were classified as eroded surface-, osteoid surface-, and quiescent surface-predominant osteocyte lacunae (ES-Lc, OS-Lc, QS-Lc, respectively) in 55 hemodialysis patients with either high- (n = 45) or low- (n = 10) parathyroid hormone levels, and 19 control subjects without chronic kidney disease. We calculated the area and number of ES-Lc, OS-Lc, and QS-Lc. The mineralized surface on the osteocyte lacunar walls was measured in each group, and compared among the three groups. The shapes of the osteocyte lacunar walls were validated by backscattered electron microscopy. While the number of ES-Lc per bone area (N.ES-Lc/B.Ar) was higher than the number of OS-Lc per bone area (N.OS-Lc/B.Ar) in all groups, N.ES-Lc/B.Ar and N.OS-Lc/B.Ar were greater in high-parathyroid hormone group than in low-parathyroid hormone and control groups. The total volume of ES-Lc per bone area (ES-Lc.Ar/B.Ar) was greater than the total volume of OS-Lc per bone area (OS-Lc.Ar/B.Ar) in both parathyroid hormone groups. However, both lacunar erosion and lacunar formation increased proportionally, suggesting that global coupling between them was maintained. N.ES-Lc/B.Ar was higher in woven bone than in lamellar bone. The rate of OS-Lc stained by tetracycline hydrochloride, the mineralized lacunar surface and the mean area of OS-Lc with Tc obtained from both parathyroid hormone groups were greater than those in the control group. We conclude that osteocytic perilacunar/canalicular turnover is increased in hemodialysis patients with high parathyroid hormone levels. Osteocytic perilacunar/canalicular turnover depends, at least in part, on serum parathyroid hormone level. However, the ideal PTH level for osteocytic perilacunar/canalicular turnover could not be determined but osteocytic osteolysis was predominant in both the high- and low-PTH groups in this study. Thus, attention should be paid to bone loss from the viewpoint of osteocytic perilacunar/canalicular turnover in hemodialysis patients.

Polymer-tethered lipid multi-bilayers: a biomembrane-mimicking cell substrate to probe cellular mechano-sensing

Cells tiptoe through their environment forming highly localized and dynamic focal contacts. Experiments on polymeric gels of adjustable elasticity have shown that cells probe the viscoelasticity of their environment through an adaptive process of focal contact assembly/disassembly that critically affects cell adhesion, morphology, and motility. However, the specific mechanisms of this process have not yet been fully revealed. Here we report, for the first time, that fibroblast adhesion, morphology, and migration can also be controlled by altering the number of bilayers in a stack of multiple polymer-tethered lipid bilayers stabilized via maleimide-sulfhydral coupling chemistry. The observed changes in cell morphology, migration, and cytoskeletal organization in response to bilayer stacking correspond well with those previously observed on polymeric substrates of different polymer crosslinking density suggesting that variations in bilayer stacking are associated with changes in substrate viscoelasticity. This is in conceptual agreement with the existing knowledge about the structural, dynamic, and mechanical properties of polymer-lipid composite materials. Several distinct features, such as the lateral mobility of individual cell linkers and the immobilization of linker clusters, make the described substrates highly attractive tools for the study of dynamic, mechano-regulated cell linkages and cellular mechano-sensing.

Iterative layer-by-layer assembly of polymer-tethered multi-bilayers using maleimide–thiol coupling chemistry

The current study reports on the layer-by-layer assembly of a polymer-tethered lipid multi-bilayer stack using the iterative addition and roll out of giant unilamellar vesicles (GUVs) containing constituents with thiol and maleimide functional groups, respectively. Confocal microscopy and photobleaching experiments confirm stack integrity and stability over time, as well as the lateral fluidity of individual bilayers within the stacks. Complementary wide-field single molecule fluorescence microscopy and atomic force microscopy experiments show that increasing bilayer-substrate distances are associated with changes in lipid lateral mobility and bilayer morphology. Importantly, the described iterative approach can be employed to assemble multi-bilayer stacks with more than two bilayers, thus further reducing the influence of the underlying solid substrate on membrane behavior. Furthermore, the presence of lipopolymers within the multi-bilayer stacks results in fascinating membrane dynamics and organization properties, with interesting parallels to those found in plasma membranes. In that sense, the described multi-bilayer architecture represents an attractive model membrane platform for a variety of different biophysical studies.

Design of quantum dot-conjugated lipids for long-term, high-speed tracking experiments on cell surfaces

The current study reports the facile design of quantum dot (QD)-conjugated lipids and their application to high-speed tracking experiments on cell surfaces. CdSe/ZnS core/shell QDs with two types of hydrophilic coatings, 2-(2-aminoethoxy)ethanol (AEE) and a 60:40 molar mixture of 1,2-dipalmitoyl- sn-glycero-3-phosphocholine and 1,2-dipalmitoyl- sn-glycero-3-phosphoethanolamine- N-[methoxy(polyethylene glycol-2000], are conjugated to sulfhydryl lipids via maleimide reactive groups on the QD surface. Prior to lipid conjugation, the colloidal stability of both types of coated QDs in aqueous solution is confirmed using fluorescence correlation spectroscopy. A sensitive assay based on single lipid tracking experiments on a planar solid-supported phospholipid bilayer is presented that establishes conditions of monovalent conjugation of QDs to lipids. The QD-lipids are then employed as single-molecule tracking probes in plasma membranes of several cell types. Initial tracking experiments at a frame rate of 30 frames/s corroborate that QD-lipids diffuse like dye-labeled lipids in the plasma membrane of COS-7, HEK-293, 3T3, and NRK cells, thus confirming monovalent labeling. Finally, QD-lipids are applied for the first time to high-speed single-molecule imaging by tracking their lateral mobility in the plasma membrane of NRK fibroblasts with up to 1000 frames/s. Our high-speed tracking data, which are in excellent agreement with previous tracking experiments that used larger (40 nm) Au labels, not only push the time resolution in long-time, continuous fluorescence-based single-molecule tracking but also show that highly photostable, photoluminescent nanoprobes of 10 nm size can be employed (AEE-coated QDs). These probes are also attractive because, unlike Au nanoparticles, they facilitate complex multicolor experiments.