Aquaporin-1: a potential membrane channel for facilitating the adaptability of rabbit nucleus pulposus cells to an extracellular matrix environment

Achilles' Heel-The Significance of Maintaining Microenvironmental Homeostasis in the Nucleus Pulposus for Intervertebral Discs

The dysregulation of intracellular and extracellular environments as well as the aberrant expression of ion channels on the cell membrane are intricately linked to a diverse array of degenerative disorders, including intervertebral disc degeneration. This condition is a significant contributor to low back pain, which poses a substantial burden on both personal quality of life and societal economics. Changes in the number and function of ion channels can disrupt the water and ion balance both inside and outside cells, thereby impacting the physiological functions of tissues and organs. Therefore, maintaining ion homeostasis and stable expression of ion channels within the cellular microenvironment may prove beneficial in the treatment of disc degeneration. Aquaporin (AQP), calcium ion channels, and acid-sensitive ion channels (ASIC) play crucial roles in regulating water, calcium ions, and hydrogen ions levels. These channels have significant effects on physiological and pathological processes such as cellular aging, inflammatory response, stromal decomposition, endoplasmic reticulum stress, and accumulation of cell metabolites. Additionally, Piezo 1, transient receptor potential vanilloid type 4 (TRPV4), tension response enhancer binding protein (TonEBP), potassium ions, zinc ions, and tungsten all play a role in the process of intervertebral disc degeneration. This review endeavors to elucidate alterations in the microenvironment of the nucleus pulposus during intervertebral disc degeneration (IVDD), with a view to offer novel insights and approaches for exploring therapeutic interventions against disc degeneration.

The TLR-2/TonEBP signaling pathway regulates 29-kDa fibronectin fragment-dependent expression of matrix metalloproteinases

Tonicity-responsive enhancer-binding protein (TonEBP; nuclear factor of activated T cells 5) is a transcription factor that responds to changes in osmolality. However, recent studies have shown that it also modulates immune responses under inflammatory conditions independently of hyperosmolality. Fibronectin fragments (FN-fs), which are abundant in the synovial fluid of patients with osteoarthritis (OA), induce expression of matrix metalloproteinases (MMPs) via the toll-like receptor-2 (TLR-2) signaling pathway. In this study we examined whether TonEBP is involved in 29-kDa FN-f-induced expression of MMPs. The expression of TonEBP was significantly higher in human osteoarthritis compared with normal cartilage samples. 29-kDa FN-f affected the expression of MMPs 1, 3, and 13 via TonEBP, and expression and nuclear accumulation of TonEBP were induced by activation of the phospholipase C/NF-κB/MAPK signaling pathway and, in particular, modulated by TLR-2. In addition, 29-kDa FN-f induced the expression of osmoregulatory genes, including Tau-T, SMIT, and AR, as well as voltage-dependent calcium channels via the TonEBP/TLR-2 signaling pathway. These results show that 29-kDa FN-f upregulates MMPs in chondrocytes via the TLR-2/TonEBP signaling pathway.

TonEBP regulates the hyperosmotic expression of aquaporin 1 and 5 in the intervertebral disc

The central region of the intervertebral disc (IVD) is rich in proteoglycans, leading to a hyperosmotic environment, which fluctuates with daily loading. The cells of the nucleus pulposus (NP cells) have adapted to this environment via the function of tonicity enhancer binding protein (TonEBP), and NP cells have been shown to express several water channels known as aquaporins (AQP). We have previously shown that AQP1 and 5 decrease during IVD degeneration. Here, the regulation of AQP1 and 5 by hyperosmotic conditions and the role of TonEBP in this regulation was investigated. AQP1 and 5 gene expression was upregulated by hyperosmotic conditions mimicking the osmolality of the healthy IVD, which was abrogated by TonEBP knockdown. Furthermore, AQP1 and 5 immunopositivity was significantly reduced in TonEBP^Δ/Δ E17.5 mice when compared with wildtype controls, indicating in vivo expression of AQP1 and 5 is controlled at least in part by TonEBP. This hyperosmotic regulation of AQP1 and 5 could help to explain the decreased AQP1 and 5 expression during degeneration, when the osmolality of the NP decreases. Together this data suggests that TonEBP-regulated osmo-adaptation may be disrupted during IVD degeneration when the expression of both AQPs is reduced.

Hyper-osmolarity environment-induced oxidative stress injury promotes nucleus pulposus cell senescence in vitro

Nucleus pulposus (NP) cell senescence is involved in disc degeneration. The in situ osmolarity within the NP region is an important regulator of disc cell's biology. However, its effects on NP cell senescence remain unclear. The present study was aimed to investigate the effects and mechanism of hyper-osmolarity on NP cell senescence. Rat NP cells were cultured in the in situ-osmolarity medium and hyper-osmolarity medium. The reactive oxygen species (ROS) scavenger N-acetylcysteine (NAC) was added along with the medium to investigate the role of oxidative injury. Cell cycle, cell proliferation, senescence associated β-galactosidase (SA-β-Gal) activity, telomerase activity, expression of senescence markers (p16 and p53) and matrix molecules (aggrecan and collagen II) were tested to assess NP cell senescence. Compared with the in situ-osmolarity culture, hyper-osmolarity culture significantly decreased cell proliferation and telomerase activity, increased SA-β-Gal activity and cell fraction in the G0/G1 phase, up-regulated expression of senescence markers (p16 and p53) and down-regulated expression of matrix molecules (aggrecan and collagen II), and increased intracellular ROS accumulation. However, addition of NAC partly reversed these effects of hyper-osmolarity culture on cellular senescence and decreased ROS content in NP cells. In conclusion, a hyper-osmolarity culture promotes NP cell senescence through inducing oxidative stress injury. The present study provides new knowledge on NP cell senescence and helps us to better understand the mechanism of disc degeneration.

Aquaporin expression in the human and canine intervertebral disc during maturation and degeneration

The intervertebral disc (IVD) is a highly hydrated tissue, the rich proteoglycan matrix imbibes water, enabling the disc to withstand compressive loads. During aging and degeneration increased matrix degradation leads to dehydration and loss of function. Aquaporins (AQP) 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. Here, the expression of all 13 AQPs at gene and protein level was investigated in human and canine nondegenerate and degenerate IVDs to develop an understanding of the role of AQPs during degeneration. Furthermore, in order to explore the transition of notochordal cells (NCs) towards nucleus pulposus (NP) cells, AQP expression was investigated in canine IVDs enriched in NCs to understand the role of AQPs in IVD maturation. AQP0, 1, 2, 3, 4, 5, 6, 7, and 9 were expressed at gene and protein level in both nondegenerate and degenerate human NP tissue. AQP2 and 7 immunopositivity increased with degeneration in human NP tissue, whereas AQP4 expression decreased with degeneration in a similar way to AQP1 and 5 shown previously. All AQP proteins that were identified in human NP tissue were also expressed in canine NP tissue. AQP2, 5, 6, and 9 were found to localize to vacuole-like membranes and cell membranes in NC cells. In conclusion, AQPs were abundantly expressed in human and canine IVDs. The expression of many AQP isotypes potentially alludes to multifaceted functions related to adaption of NP cells to the conditions they encounter within their microenvironment in health and degeneration. The presence of AQPs within the IVD may suggest an adaptive role for these water channels during the development and maintenance of the healthy, mature IVD.

Verkman A, Virani R, Kim B, Niu T, Lu DC. Aquaporin-1 Expression in Herniated Human Lumbar Intervertebral Discs

STUDY DESIGN

Case series.

OBJECTIVE

Intervertebral disc (IVD) degeneration is the cause of spondylosis. The pathogenesis is poorly understood, but disc dehydration often plays a role. In this study, we aim to identify and quantify aquaporin-1 (AQP1) in ex vivo human degenerated IVDs obtained intraoperatively and to investigate the relationship between AQP1 levels and magnetic resonance imaging (MRI) T2 intensity of the disc.

METHODS

Ex vivo samples of nucleus pulposus (NP) tissue from lumbar IVDs were obtained from 18 consecutive patients who underwent surgery for disc herniation at L4/5 and L5/S1 level. Immunohistochemistry was performed to determine the presence of AQP1 expression, and this was quantified by Western blot analysis. AQP1 expression was compared to preoperative IVD signal intensity on T2-weighted MRI.

RESULTS

NP tissue was obtained from 18 patients (9 for L4/5 level and 9 for L5/S1 level). AQP1 expression was detected in all samples by Western blot and immunohistochemistry. AQP1 expression had a linear correlation with the preoperative IVD signal intensity on T2-weighted MRI at L4/5 level (R² = 0.90) and at L5/S1 level (R² = 0.92). AQP1 expression was 52.2 ± 59.0 at L5/S1 level and 15.9 ± 20.6 at L4/5 (P = .10).

CONCLUSIONS

Our results show that AQP1 can be detected in IVD obtained from live human subjects. Increased AQP1 expression is associated with greater disc hydration as measured by signal intensity on T2-weighted MRI. AQP1 may have a role in the dehydration associated with disc degeneration.

Aquaporin 1 and 5 expression decreases during human intervertebral disc degeneration: Novel HIF-1-mediated regulation of aquaporins in NP cells

Objectives of this study were to investigate whether AQP1 and AQP5 expression is altered during intervertebral disc degeneration and if hypoxia and HIF-1 regulate their expression in NP cells. AQP expression was measured in human tissues from different degenerative grades; regulation by hypoxia and HIF-1 was studied using promoter analysis and gain- and loss-of-function experiments. We show that both AQPs are expressed in the disc and that mRNA and protein levels decline with human disease severity. Bioinformatic analyses of AQP promoters showed multiple evolutionarily conserved HREs. Surprisingly, hypoxia failed to induce promoter activity or expression of either AQP. While genomic chromatin immunoprecipitation showed limited binding of HIF-1α to conserved HREs, their mutation did not suppress promoter activities. Stable HIF-1α suppression significantly decreased mRNA and protein levels of both AQPs, but HIF-1α failed to induce AQP levels following accumulation. Together, our results demonstrate that AQP1 and AQP5 expression is sensitive to human disc degeneration and that HIF-1α uniquely maintains basal expression of both AQPs in NP cells, independent of oxemic tension and HIF-1 binding to promoter HREs. Diminished HIF-1 activity during degeneration may suppress AQP levels in NP cells, compromising their ability to respond to extracellular osmolarity changes.

Strategies to minimize hypertrophy in cartilage engineering and regeneration

Due to a blood supply shortage, articular cartilage has a limited capacity for self-healing once damaged. Articular chondrocytes, cartilage progenitor cells, embryonic stem cells, and mesenchymal stem cells are candidate cells for cartilage regeneration. Significant current attention is paid to improving chondrogenic differentiation capacity; unfortunately, the potential chondrogenic hypertrophy of differentiated cells is largely overlooked. Consequently, the engineered tissue is actually a transient cartilage rather than a permanent one. The development of hypertrophic cartilage ends with the onset of endochondral bone formation which has inferior mechanical properties. In this review, current strategies for inhibition of chondrogenic hypertrophy are comprehensively summarized; the impact of cell source options is discussed; and potential mechanisms underlying these strategies are also categorized. This paper aims to provide guidelines for the prevention of hypertrophy in the regeneration of cartilage tissue. This knowledge may also facilitate the retardation of osteophytes in the treatment of osteoarthritis.

Reconstruction of an in vitro niche for the transition from intervertebral disc development to nucleus pulposus regeneration

The nucleus pulposus (NP) plays a prominent role in both the onset and progression of intervertebral disc degeneration. While autologous repair strategies have demonstrated some success, their in vitro culture system is outdated and insufficient for maintaining optimally functioning cells through the required extensive passaging. Consequently, the final population of cells may be unsuitable for the overwhelming task of repairing tissue in vivo and could result in subpar clinical outcomes. Recent work has identified synovium-derived stem cells (SDSCs) as a potentially important new candidate. This population of precursors can promote matrix regeneration and additionally restore the balance of catabolic and anabolic metabolism of surrounding cells. Another promising application is their ability to produce an extracellular matrix in vitro that can be modified via decellularization to produce a tissue-specific substrate for efficient cell expansion, while retaining chondrogenic potential. When combined with hypoxia, soluble factors, and other environmental regulators, the resultant complex microenvironment will more closely resemble the in vivo niche, which further improves the cell capacity, even after extensive passaging. In this review, the adaptive mechanisms NP cells utilize in vivo are considered for insight into what factors are important for constructing a tissue-specific in vitro niche. Evidence for the use of SDSCs for NP regeneration is also discussed. Many aspects of NP behavior are still unknown, which could lead to future work yielding key information on producing sufficient numbers of a high-quality NP-specific population that is able to regenerate deteriorated NP in vivo.

Aquaporin-1 and aquaporin-3 expressions in the intervertebral disc of rats with aging

OBJECTIVE

The intervertebral disc (IVD) undergoes biochemical and morphologic degenerative changes during the process of aging. Aquaporins (AQPs) are a family of water channel proteins that facilitate water and small solute movement in tissues and may have a potential role in the aging degeneration of IVDs. One of the important problems in understanding disc degeneration is to find cellular molecules which contribute to the pathogenesis of IVDs. XThe aim of this study was to demonstrate the expression of aquaporin 1 and 3 in nucleus pulposus (NP), annulus fibrosus (AF) cells of rat lumbar intervertebral discs from both young and aged animals using immunohistochemistry.

MATERIAL AND METHODS

Twenty Wistar-albino rats were included in the study. The rats were separated into two groups: 2-month-old rats (n=10) as the young group, 18-month-old rats (n=10) as the old group. The intervertebral disc tissues obtained from the lumbar spine (L1-L4, 4 discs) were used for immunohistochemical staining of AQP-1 and 3.

RESULTS

This study demonstrated that AQP-1 and AQP-3 immunoreactivity significantly decreased in NP and AF of aged rats compared to the young rats.

CONCLUSION

We suggest that AQP-1 and 3 may contribute to the age related degeneration of the intervertebral disc.

Aquaporin 1 expression in human temporomandibular disc

Aquaporins (AQPs) are a family of hydrophobic membrane channel proteins. The expression of several AQP isoforms has been investigated in different human tissues, including the orofacial region. However, information on the role and localization of AQP1 in joints is limited, and no data are available on aquaporins in the normal temporomandibular joint (TMJ) disc. Sixteen human TMJ discs without degenerative changes were taken from fresh cadavers to investigate the presence and distribution of AQP1 by immunohistochemistry. The aim of the study was to gain additional insights into the biomolecular composition of aquaporins and their role in homeostasis of the TMJ. Porcine TMJ discs were also studied by Western blotting for comparison. Scattered AQP1 immunoexpression was detected in human disc cells, documenting its constitutive expression, but differences amongst the three disc regions were not significant. AQP1 expression was demonstrated in porcine TMJ disc by Western blotting. Our findings suggest that AQP1 is normally expressed in the TMJ disc and confirm a role for it in the maintenance of TMJ homeostasis. Further studies are needed to elucidate expression patterns of aquaporins in diseased TMJ discs.