Probing the structure of the pericellular matrix via novel biophysical assays

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McLane, Louis T.
Curtis, Jennifer E.
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The pericellular matrix (PCM) is a voluminous polymer network adhered to and surrounding many different types of mammalian cells, and which extends out into the environment outside the cell for distances ranging up to twenty microns. It is comprised of very long flexible polymers (hyaluronan) which are tethered to the cell surface and which have binding sites for large, highly charged bottle brush proteoglycans (aggrecan). The PCM plays an important role in many cell functions such as cell proliferation, cell adhesion, cell migration, and cancer development, however the precise way it influences these processes remains unclear. Three original biophysical tools are developed in this thesis in order to study the PCM: the quantitative particle exclusion assay (qPEA), optical force probe assay (OFPA), and exogenous fluorescent aggrecan mapping assays. These tools are used to measure the polymeric and biophysical properties of the matrix in order to make further advancements in the understanding the PCMs role in adhesion, transport to and from the cell surface, its purported function as a chemical micro-reservoir, as well as basic studies on the kinetics of its formation, turnover and maintenance. The qPEAs measure the penetration and distribution of sub-micron particles after they diffuse into the cell coat, where their distribution maps the interior structure of the PCM. The qPEA assays reveal that the PCM acts a sieve, separating incoming particles by their size, preventing micron sized particles from entering the PCM while allowing sub 100 nm particles to pass to the cell surface. The OFPA uses an optically-trapped bead to study the force response of the matrix as it encounters the probe. The assay not only reveals new details about the PCM such as the fact that it is larger than initially thought, having a two layer structure, but when combined with a polymer physics model which relates the observed equilibrium forces to an existing osmotic pressure gradient within the PCM, the OFPA studies produce the first discovery and measurement of the correlation length distribution in the cell coat. The OFPA and qPEA assays are also performed on cells modified with exogenous aggrecan, resulting in a model for possible proteoglycan mediated cell coat transformations. The fluorescent exogenous aggrecan assays measure the dynamics of the exogenous aggrecan binding to and releasing from the coat, revealing that the PCM can be rapidly modified by a changing environment, and quantitatively measure how the exogenous aggrecan modifies the existing PCM. Together, these assays provide an unprecedented look into the interior structure of the PCM, and the mechanisms responsible both for this structure and its modification.
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