Browsing by Subject "Integrin"

Intervertebral disc (IVD) disorders can cause pain and disability for affected individuals. A subset of IVD disorders are thought to originate in the nucleus pulposus (NP) region of the IVD, due to alterations in tissue structure and composition with age and injury. Cells of the NP undergo a phenotypic and behavioral shift with age, changes that are thought to disrupt tissue homeostasis and lead to disc degeneration. There is significant interest in developing biomaterials and strategies to revert adult degenerate NP cells to healthy, young NP cell phenotypes with increased biosynthesis and cell clustering. Studies demonstrate that healthy porcine NP cells interact with laminin proteins through specific integrin subunits on soft substrates in a process that regulates cell morphology and biosynthesis. The central hypothesis of this dissertation is that the engagement of cell-surface adhesion receptors, using laminin-mimetic peptides on a controlled stiffness material, can revert adult degenerate NP cellular phenotype and behaviors to their healthy, biosynthetically active form.

In the first part of this dissertation, integrin subunits used by adult degenerate human NP cells to attach to laminin were revealed using flow cytometric analyses, function blocking antibodies, and immunohistochemistry. Secondly, NP cell recognition peptides were identified by screening laminin-mimetic peptides for cell attachment. Finally, human NP cells were cultured on peptide functionalized polyacrylamide gels to examine the effect of ligand and substrate stiffness in regulating adult human NP cell phenotype and biosynthesis.

Findings reveal that adult human NP cells express and utilize integrin subunits α3, α5, and β1 to attach to laminins, in contrast to integrin α6β1 found previously for healthy porcine NP cells. This difference between current and previous findings likely arises from aging-associated difference in NP cells between human and porcine tissues. Select laminin-mimetic peptides, chosen from the literature and informed by NP cell integrin expression, were found to promote significant NP cell attachment in a concentration dependent manner. Finally, a subset of laminin mimetic peptides were found to promote a young NP cell phenotype by increasing cell clustering on soft (0.3 kPa) and stiff (14 kPa) substrates as well as increasing proteoglycan synthesis on soft substrates.

The studies presented in this dissertation demonstrate that adult degenerate human NP cells attach to laminin proteins in an integrin dependent manner. Furthermore, laminin-mimetic peptides are able to mediate human NP cell attachment at levels comparable to full-length laminin, increase cell clustering when presented on soft and stiff substrates, and can increase NP cell biosynthesis when presented on soft substrates. Utilizing laminin-mimetic peptides may allow for the design of biomaterials that promote a healthy young NP phenotype for a variety of disc therapies.

Intervertebral disc (IVD) disorders are a major contributor to disability and health costs. Disc disorders and resulting pain may be preceded by changes which first occur in the nucleus pulposus (NP) region of the IVD, where significant alterations in tissue cellularity, composition, and structure begin early in human life and continue with increasing age and degeneration. These changes coincide with the loss of a distinct cell population, notochordally-derived immature NP cells, which may play a key role in the generation and maintenance of this tissue. These cells reside in a gelatinous, highly-hydrated extracellular matrix (ECM) environment and exhibit in situ cell-matrix and cell-cell interactions which are quite distinct from cells in other regions of the disc or in other cartilagenous, including expression of laminin cell-matrix receptors and cell-associated laminin proteins. The ECM environment is known to be a key regulator of cellular behaviors, with ECM ligands and elasticity modulating cell adhesion, organization, differentiation, and phenotype. The primary motivating hypothesis of this thesis is that the unique ECM environment of the NP plays a key role in modulating immature NP cell behaviors, and that laminin ligands, in combination with ECM elasticity, modulate immature NP cell behaviors including adhesion, organization, and phenotype.

To investigate this hypothesis, flow cytometric analyses were performed to examine IVD cell integrin receptor expression over time in culture, including assessment of key laminin-binding integrin subunits. The roles of specific integrin receptors modulating NP cell adhesion to ECM proteins were identified in studies utilizing function-blocking antibodies. NP cell adhesion, spreading, and relative cell adhesion strength was assessed on various ECM constituents, including specific isoforms of laminin. Additionally, studies were performed to examine the roles of ECM ligand and substrate stiffness in modulating NP cellular organization, utilizing polyacrylamide gel substrates with tunable mechanical properties and functionalized with different ECM ligands. Finally, the role of ECM environment was examined on one key measure of NP cell function, proteoglycan production, over time in culture.

NP cells isolated from immature NP tissues were found to express high levels of the laminin-binding integrin subunit alpha 6 ex situ and maintain this expression over time in culture. Function blocking studies revealed this receptor to be a key regulator of NP cell adhesion to laminin, in contrast to cells from the adjacent AF region, which did not express this receptor nor adhere to laminin. Cell adhesion studies demonstrated that NP cells preferentially interact with two laminin isoforms, LM-511 and LM-332, in comparison to other ECM proteins, with enhanced cell attachment, spreading, and adhesion strength on surfaces coated with these ligands. These findings correspond with laminin isoform and receptor expression patterns identified in immature NP tissues. Additionally, NP cell-cell interactions were found to be modulated by both ECM ligand and substrate stiffness, with soft, laminin-functionalized substrates promoting self-assembly of NP cells into cell clusters with morphologies similar to those identified in immature NP tissues. Finally, culture of immature NP cells on soft, laminin-rich substrates was found to promote a key measure of NP cell function, proteoglycan synthesis.

The studies presented here demonstrate that immature NP cells interact uniquely with laminin extracellular matrix proteins, and that laminin ligands and matrix elasticity are two key regulators of NP cell organization and phenotype in the IVD. These findings suggest that alterations in one or both of these factors during IVD aging or degeneration may contribute to the differentiation or loss of this unique cell population. Additionally, these results indicate that soft, laminin-functionalized biomaterials may be appropriate for in vitro culture and expansion of immature NP cells, as well as for use in NP tissue engineering strategies.