Introduction

The intervertebral disc (IVD) is a highly hydrated and hyperosmotic tissue, water and salt content fluctuate daily due to mechanical loading. Resident IVD cells must adapt to this ever-changing osmotic environment, to maintain normal behaviour. However, during IVD degeneration the disc becomes permanently dehydrated and cells can no longer perform their correct function. Here, we investigated how human nucleus pulposus (NP) cells respond to altered osmolality with regards to cell size and the rate of water permeability, along with the potential involvement of aquaporins (AQPs) and transient receptor potential vanilloid (TRPV) membrane channels.

The IVD is a highly hydrated, hyperosmolar tissue that allows the correct biomechanical function of the spine. When degenerated, water and ions are lost from the disc, especially within the central nucleus pulposus (NP), producing a hypoosmotic environment in which the resident cells can no longer function correctly, exacerbating the degenerative cascade. One potential way that IVD cells may adapt to their environment is through the expression and regulation of aquaporin (AQP) channels that control the movement of water in and out of cells. During human IVD degeneration AQP1 and 5 expression is decreased, highlighting AQPs may be of importance for the correct function of NP cells. The regulation of AQPs in NP cells by healthy and degenerate conditions, and the potential underlying molecular mechanisms, were investigated in both human and rat IVD cells. The gene and protein expression of AQP1 and AQP5 was upregulated by hyperosmotic conditions (425mOsm/kg H₂O) in rat and human NP cells. Lentiviral knockdown of tonicity enhancer binding protein (TonEBP), a transcription factor responsible for maintaining the function of NP cells, resulted in the loss of AQP1 and 5 gene expression under hyperosmotic conditions. The maintenance of the IVD environment and adaptation of cells is vital for the function of the IVD. The regulation of AQPs by physiological conditions and TonEBP suggests a role for these water channels related to the adaptation of disc cells to their environment, which is dysregulated during degeneration.

Introduction

During development the central disc contains large, vacuolated notochordal (NC) cells which in humans are replaced by mature nucleus pulposus (NP) cells during aging, but are maintained in certain breeds of dogs. During degeneration the disc becomes less hydrated which affects its normal function. Aquaporins (AQP) are a family of 13 transmembrane channel proteins that allow passage of water and are responsible for maintaining water homeostasis. AQP1, 2, 3 and 5 have been identified in the intervertebral disc (IVD). Here, expression of AQPs in human and canine IVDs to determine expression in NC v/s NP cells and whether expression changes during degeneration.

Introduction

Within the intervertebral disc (IVD), nucleus pulposus (NP) cells reside within a unique microenvironment. Factors such as hypoxia, osmolality, pH and the presence of cytokines all dictate the function of NP cells and as such the cells must adapt to their environment to survive. Previously we have identified the expression of aquaporins (AQP) within human IVD tissue. AQPs allow the movement of water across the cell membrane and are important in cellular homeostasis. Here we investigated how AQP gene expression was regulated by the microenvironment of the IVD.

Introduction

The intervertebral disc (IVD) is a highly hydrated tissue which is reduced during degeneration leading to loss of function. Aquaporins (AQP) are a family of 13 (AQP0-12) transmembrane channel proteins that selectively allow the passage of water and other small molecules in and out of cells and are responsible for maintaining water homeostasis. AQP1, 2, 3 and 5 have been identified in the IVD. Here gene and protein expression of all 13 AQPs was investigated in a large cohort of human IVDs to investigate expression during IVD degeneration.