Understanding the multiscale basis of solute transport in the cartilage endplate

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PROJECT ABSTRACT Low back pain is the leading cause of disability and linked to disc degeneration. Existing treatments for disc degeneration are surgical in nature, and thus, there is an unmet need for non-surgical alternatives. Intradiscal therapy looks promising but it requires high nutrition supply. Nutrients and metabolites entering and exiting the nucleus pulposus must penetrate the cartilage endplate (CEP) and its permeability depends on matrix biochemical composition (matrix quantity) and structure, for example, increased deposition of mineral, proteoglycans, or collagen limits the physical pore space available for solutes to pass, while a dense and damaged CEP matrix from degenerated discs might impair nutrient transport than an intact CEP. Indeed, a prior study from our lab showed that higher AGE content in the CEP associates with lower solute uptake for a given amount of matrix. Despite a general understanding of how matrix quantity and quality impact bulk CEP transport properties, knowledge about transport in relation to the multiscale organization of the CEP matrix constituents is lacking. These gaps motivate my main hypotheses that 1) higher glucose uptake is positively correlated with structural characteristics across multiple length scales, including greater pore network connectivity and a higher degree of matrix organization, and 2) low CEP transport properties correlate with higher AGEs, independent of matrix quantity. These hypotheses will be tested through the following two aims. In Aim 1, I will discover the structural organization of the CEP matrix across millimeter to nanometer, and how these hierarchical structure affects solute transport properties. A total of 126 samples of human CEP tissue (from 15 cadavers) will be harvested and then ordered as three groups by the rank of nutrient transport (high, middle, and low uptake). Then, these will be related to biochemical compositions, tissue hydration, and bulk material properties (porosity and thickness). Local material properties (micro- and nano- porosity and pore size/distribution) will be measured and compared with the other quanfiable data, such as pore connectivity, pore size distribution, and collagen/matrix anisotropy. A series of 2D dataset will be obtained, segmented, and reconstructed to quantify pore area/volume and in 3D. These structural outcomes will be related to biochemical composition, hydration, and porosity. In Aim 2, I will identify nanostructural and biochemical changes altered by the effect of in-vitro ribosylation assay. Previous studies showed that negatively charged sGAG attracts water, while increased levels of AGEs negatively correlated to tissue hydration. I will develop an in-vitro ribosylation assay and a glucose uptake assay to measure the variation AGE contents (control vs. incubated) of the CEP samples. Lastly, the obtained data will be related to the changes of matrix charge distribution, nanoporosity, and collagen D-period spacing. These studies will clarify and characterize the effect of hierarchical structure and matrix quality modification of CEP tissue on solute transport.