Large Cytoplasmic Vacuoles within Notochordal Nucleus Pulposus Cells: A Possible Regulator of Intracellular Pressure That Shapes the Cytoskeleton and Controls Proliferation

Abstract

Degeneration of the intervertebral disc, which is closely associated with the loss of vacuolated notochordal nucleus pulposus cells (NNPC), remains a major cause of lower-back pain and motor deficiency. Being the most defining characteristic of NNPC, large cytoplasmic vacuoles not only modulate the cytoskeleton and shape cell morphology but they also respond to the disc microenvironment and regulate the biological behavior of vacuolated cells as a potent reporter of the histocytological changes that occur at the beginning of disc aging and degeneration. Here we hypothesize a model in which large cytoplasmic vacuoles primarily function to maintain a reasonable intracellular pressure (P_v) that facilitates NNPC in resisting the extracellular mechanical loading (P_e), part of which is absorbed by the extracellular matrix (P_m), forming the equation P_e = P_m + P_v. By mimicking a situation of contact-induced growth inhibition, the crowded cytoplasmic vacuoles slow down the proliferation of NNPC and restrain the generation of nonvacuolated chondrocytic nucleus pulposus cells (CNPC), whereas increased mechanical loading (↑P_e) alters cytoskeletons and breaches cytoplasmic vacuoles, which in turn weakens the vacuoles-mediated proliferation check, increases the generation of CNPC that accumulates fibrocartilaginous matrix, and rebalances the increased loading with elevated P_m (↑P_m) and lowered P_v (↓P_v), equating to ↑P_e = ↑P_m + ↓P_v. By depicting the biological function and the disappearance of the cytoplasmic vacuoles, our model highlights a mechanical exhaustion of the notochordal cell resources, which might help to elucidate the histocytological changes that initiate disc aging and degeneration.

Bioactive in situ crosslinkable polymer-peptide hydrogel for cell delivery to the intervertebral disc in a rat model

“…2 ). This observation is of interest because AHA staining within vacuolar structures appeared modest at best, which may support the hypothesis that vacuoles play a more important role in regulation of intracellular pressure than in molecular transport [ 65 – 67 ]. However, further assessments need to be conducted to better understand these findings and their implications.…”

Notochordal Cell-Based Treatment Strategies and Their Potential in Intervertebral Disc Regeneration

“…The NC vacuoles are presumably mechanoprotected by the caveolae, flask-shaped structures of the plasma membrane that express caveolins ( Nixon et al, 2007 ; Sinha et al, 2011 ; Bach et al, 2016a ) and by the ECM of the notochordal sheath, rich in collagens, proteoglycans (mostly chondroitin sulfate and heparan sulfate), laminins, and fibronectins ( Yasuoka, 2020 ). Since the vacuoles play an important biomechanical role in the developing embryo, they might also have a biomechanical role in the postnatal IVD by maintaining intracellular pressure to resist mechanical loading ( Hong et al, 2018 ). Since NC vacuoles are important organelles in the embryonic notochord and postnatal NP, they might also be involved in the NC-mediated regenerative effects.…”

“…In addition to the cargo content, also the self-renewal ability of the NCs is a topic of debate. There is evidence that the presence of large intracytoplamic vacuoles is a limiting factor for cell proliferation ( Joo Han Kim et al, 2009 ; Hong et al, 2018 ), which is important to consider when aiming for NC-based regenerative therapies. Taken together, studying the NC vacuoles may provide novel information, possibly enabling guidance for NC-based IVD regeneration.…”

Notochordal cells: A potential therapeutic option for intervertebral disc degeneration

“…The ion pump on the vacuole membrane generates a hypotonic solution in the vacuole, which can be released into the cytoplasm to dilute under osmotic pressure changes. This not only balances the osmotic pressure across the cell membrane, 70 but also regulates cell volume and tension during rapid osmotic pressure changes, protecting NCs from damage caused by swelling and rupture 71 . Therefore, the increase in vacuoles within NCs may indirectly indicate an elevation of pressure within the NCs.…”