Gas Uptake and Thermodynamics in Porous Liquids Elucidated by 129Xe NMR

We exploited 129Xe NMR to investigate xenon gas uptake and dynamics in a porous liquid formed by dissolving porous organic cages in a cavity-excluded solvent. Quantitative 129Xe NMR shows that when the amount of xenon added to the sample is lower than the amount of cages present (subsaturation), the porous liquid absorbs almost all xenon atoms from the gas phase, with 30% of the cages occupied with a Xe atom. A simple two-site exchange model enables an estimate of the chemical shift of 129Xe in the cages, which is in good agreement with the value provided by first-principles modeling. T2 relaxation times allow the determination of the exchange rate of Xe between the solvent and cage sites as well as the activation energies of the exchange. The 129Xe NMR analysis also enables determination of the free energy of confinement, and it shows that Xe binding is predominantly enthalpy-driven.

Melting of aqueous NaCl solutions in porous materials: shifted phase transition distribution (SIDI) approach for determining NMR cryoporometry pore size distributions

Nuclear magnetic resonance cryoporometry (NMRC) and differential scanning calorimetry thermoporometry (DSC-TPM) are powerful methods for measuring mesopore size distributions. The methods are based on the fact that, according to the Gibbs-Thomson equation, the melting point depression of a liquid confined to a pore is inversely proportional to the pore size. However, aqueous salt solutions, which inherently exist in a broad range of biological porous materials as well as technological applications such as electrolytes, do not melt at a single temperature. This causes artefacts in the pore size distributions extracted by traditional Gibbs-Thomson analysis of NMRC and DSC-TPM data. Bulk aqueous NaCl solutions are known to have a broad distribution of melting points between the eutectic and pure water phase transition points (252-273 K). Here, we hypothesize that, when aqueous NaCl solution (saline) is confined to a small pore, the whole melting point distribution is shifted toward lower temperatures by the value predicted by the Gibbs-Thomson equation. We show that this so-called shifted phase transition distribution (SIDI) approach removes the artefacts arising from the traditional Gibbs-Thomson analysis and gives correct pore size distributions for saline saturated mesoporous silica gel and controlled pore materials analyzed by NMR cryoporometry. Furthermore, we demonstrate that the method can be used for determining pore sizes in collagen-chondroitin sulphate hydrogels resembling the composition of the extracellular matrix of articular cartilage. It is straightforward to apply the SIDI analysis for DSC-TMP data as well.

SPICY: a method for single scan rotating frame relaxometry

T _1ρ is an NMR relaxation mode that is sensitive to low frequency molecular motions, making it an especially valuable tool in biomolecular research. Here, we introduce a new method, SPICY, for measuring T_1ρ relaxation times. In contrast to conventional T_1ρ experiments, in which the sequence is repeated many times to determine the T_1ρ time, the SPICY sequence allows determination of T_1ρ within a single scan, shortening the experiment time remarkably. We demonstrate the method using ¹H T_1ρ relaxation dispersion experiments. Additionally, we combine the sequence with spatial encoding to produce 1D images in a single scan. We show that T_1ρ relaxation times obtained using the single scan approach are in good agreement with those obtained using the traditional experiments.

The TAT Protein Transduction Domain as an Intra-Articular Drug Delivery Technology

OBJECTIVE

Intra-articular drug delivery holds great promise for the treatment of joint diseases such as osteoarthritis. The objective of this study was to evaluate the TAT peptide transduction domain (TAT-PTD) as a potential intra-articular drug delivery technology for synovial joints.

DESIGN

Experiments examined the ability of TAT conjugates to associate with primary chondrocytes and alter cellular function both in vitro and in vivo. Further experiments examined the ability of the TAT-PTD to bind to human osteoarthritic cartilage.

RESULTS

The results show that the TAT-PTD associates with chondrocytes, is capable of delivering siRNA for chondrocyte gene knockdown, and that the recombinant enzyme TAT-Cre is capable of inducing in vivo genetic recombination within the knee joint in a reporter mouse model. Last, binding studies show that osteoarthritic cartilage preferentially uptakes the TAT-PTD from solution.

CONCLUSIONS

The results suggest that the TAT-PTD is a promising delivery strategy for intra-articular therapeutics.

Quantifying NMR relaxation correlation and exchange in articular cartilage with time domain analysis

Measured nuclear magnetic resonance (NMR) transverse relaxation data in articular cartilage has been shown to be multi-exponential and correlated to the health of the tissue. The observed relaxation rates are dependent on experimental parameters such as solvent, data acquisition methods, data analysis methods, and alignment to the magnetic field. In this study, we show that diffusive exchange occurs in porcine articular cartilage and impacts the observed relaxation rates in T₁-T₂ correlation experiments. By using time domain analysis of T₂-T₂ exchange spectroscopy, the diffusive exchange time can be quantified by measurements that use a single mixing time. Measured characteristic times for exchange are commensurate with T₁ in this material and so impacts the observed T₁ behavior. The approach used here allows for reliable quantification of NMR relaxation behavior in cartilage in the presence of diffusive fluid exchange between two environments.

Non-Invasive Quantification of Cartilage Using a Novel In Vivo Bioluminescent Reporter Mouse

Mouse models are common tools for examining post-traumatic osteoarthritis (OA), which involves cartilage deterioration following injury or stress. One challenge to current mouse models is longitudinal monitoring of the cartilage deterioration in vivo in the same mouse during an experiment. The objective of this study was to assess the feasibility for using a novel transgenic mouse for non-invasive quantification of cartilage. Chondrocytes are defined by expression of the matrix protein aggrecan, and we developed a novel mouse containing a reporter luciferase cassette under the inducible control of the endogenous aggrecan promoter. We generated these mice by crossing a Cre-dependent luciferase reporter allele with an aggrecan creERT2 knockin allele. The advantage of this design is that the targeted knockin retains the intact endogenous aggrecan locus and expresses the tamoxifen-inducible CreERT2 protein from a second IRES-driven open reading frame. These mice display bioluminescence in the joints, tail, and trachea, consistent with patterns of aggrecan expression. To evaluate this mouse as a technology for non-invasive quantification of cartilage loss, we characterized the relationship between loss of bioluminescence and loss of cartilage after induction with (i) ex vivo collagenase digestion, (ii) an in vivo OA model utilizing treadmill running, and (iii) age. Ex vivo experiments revealed that collagenase digestion of the femur reduced both luciferase signal intensity and pixel area, demonstrating a link between cartilage degradation and bioluminescence. In an in vivo model of experimental OA, we found decreased bioluminescent signal and pixel area, which correlated with pathological disease. We detected a decrease in both bioluminescent signal intensity and area with natural aging from 2 to 13 months of age. These results indicate that the bioluminescent signal from this mouse may be used as a non-invasive quantitative measure of cartilage. Future studies may use this reporter mouse to advance basic and preclinical studies of murine experimental OA with applications in synovial joint biology, disease pathogenesis, and drug delivery.