Aquaporin Expression, Regulation and Function in the Intervertebral Disc

Intervertebral disc degeneration-associated low back pain is a debilitating condition with no current treatments directed towards halting or reversing the degenerative cascade at a cellular level. The lack of such treatments is in part due to an incomplete knowledge of the molecular mechanisms that govern IVD function in health and degeneration. Due to the unique location and role of the IVD within the spine, many factors contribute to the microenvironment that cells reside within. The survival and function of cells has been irrefutably linked to their ability to adapt to the microenvironment in which they live. However, it is not completely understood how IVD cells have been able to survive and adapt to their environment. The intervertebral disc is a highly hydrated tissue; the rich proteoglycan matrix imbibes water, enabling the disc to withstand compressive loads. During ageing and degeneration increased matrix degradation leads to dehydration and loss of function. Aquaporins are a family of transmembrane channel proteins that selectively allow the passage of water in and out of cells and are responsible for maintaining water homeostasis in many tissues; hence many AQPs are potentially expressed by cells within the intervertebral disc to enable their adaptation to this highly hydrated tissue. The aim of this thesis was to investigate the expression, regulation and function of AQP transmembrane water channels within the IVD and how they potentially contribute to the adaptation of cells to their environment. Results have highlighted NP cells express many AQP water channels in vivo, whose expression may be altered between disc development and degeneration. AQP1 and 5 were found to be upregulated by TonEBP in hyperosmotic conditions, a transcription factor controlling osmotic adaptation and matrix expression, which may implicate them in the adaptation of NP cells to their environment, which becomes unachievable when AQP1 and 5 are decreased during degeneration. AQP4 and TRPV4 function in NP cells was required for fundamental cellular processes such as cell volume regulation and water permeability, enabling adaptation to their osmotically fluxing environment. Finally, it was identified that other microenvironmental factors also contribute to AQP expression in NP cells, indicating the regulation and function of these water channels is potentially very complex. Together, investigations presented in this thesis have demonstrated many AQPs are expressed by the intervertebral disc and enable NP cells to respond and adapt to their environment, ultimately contributing to the overall function of the tissue. Their regulation by multiple environmental factors may signify that AQPs have diverse roles within the IVD, which remain to be elucidated. Importantly, this body of work has contributed novel findings, increased knowledge and opened new avenues of research in the field of IVD and spine biology.