Abstract
SummaryObjectiveThe anterior cruciate ligament transection (ACLT) rabbit osteoarthritis (OA) model confers permanent knee instability and induces joint degeneration. The degeneration process is complex, but includes chondrocyte apoptosis and OA-like loss of cartilage integrity. Previously, we reported that activation of a volume-sensitive Cl− current (ICl,vol) can mediate cell shrinkage and apoptosis in rabbit articular chondrocytes. Our objective was therefore to investigate whether ICl,vol was activated in the early stages of the rabbit ACLT OA model.DesignAdult Rabbits underwent unilateral ACLT and contralateral arthrotomy (sham) surgery. Rabbits were euthanized at 2 or 4 weeks. Samples were analyzed histologically and with assays of cell volume, apoptosis and electrophysiological characterization of ICl,vol.ResultsAt 2 and 4 weeks post ACLT cartilage appeared histologically normal, nevertheless cell swelling and caspase 3/7 activity were both significantly increased compared to sham controls. In cell-volume experiments, exposure of chondrocytes to hypotonic solution led to a greater increase in cell size in ACLT compared to controls. Caspase-3/7 activity, an indicator of apoptosis, was elevated in both ACLT 2wk and 4wk. Whole-cell currents were recorded with patch clamp of chondrocytes in iso-osmotic and hypo-osmotic external solutions under conditions where Na+, K+ and Ca2+ currents were minimized. ACLT treatment resulted in a large increase in hypotonic-activated chloride conductance.ConclusionChanges in chondrocyte ion channels take place prior to the onset of apparent cartilage loss in the ACLT rabbit model of OA. Further studies are needed to investigate if pharmacological inhibition of ICl,vol decreases progression of OA in animal models.
Highlights
Chondrocyte apoptosis is an important contributor to the development and growth of healthy articular cartilage[1]
This study used a knee instability anterior cruciate ligament tear rabbit model (ACLT) which induces OA-like degradative changes in the joint[2,3] and investigated changes in chondrocyte physiology taking place prior to apparent macroscopic changes of the cartilage
Our results show that ICl,vol, cell-volume control and caspase activity are all altered following anterior cruciate ligament transection (ACLT), but prior to the manifestation of histologically detectable OA
Summary
Chondrocyte apoptosis is an important contributor to the development and growth of healthy articular cartilage[1]. Apoptosis during aging is responsible for the hypocellularity associated with degradation and/or pathological remodeling of the cartilage matrix, and exacerbates the risk of degenerative joint diseases such as osteoarthritis (OA)[1] Study of this in biopsies from OA patients can suffer from the limitation that OA is typically presented at an advanced stage. Aberrant activation of ICl,vol under iso-osmotic conditions contributes to the cellshrinkage associated with induction of apoptosis in cells including chondrocytes[12,13]. We calculated conductance plots using Boltzmann transformations of data to separate the underlying whole-cell ion conductance from the Ohmic driving force for ion flow These data were fit by Boltzmann equations[17] as follows: gðVmÞ. Where g is the maximal conductance, Vh is the Vm at which the conductance is half-activated (“midpoint”) and k is the slope of activation
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