F-ACTIN DISTRIBUTION OF THE CHONDROCYTE CYTOSKELETON AND ITS ASSOCIATION WITH EARLY OSTEOARTHRITIC CHONDROCYTE PROPERTIES
Authors: Algarni Mohammed , Amin Anish , Hall Andrew 
Published: November 2023

Objectives

Osteoarthritis (OA) is a painful and debilitating disorder of diarthroidal joints. Progressive degeneration of the cartilage extracellular matrix (ECM) together with abnormal chondrocyte characteristics occur leading to a switch to a fibroblast-like phenotype and production of mechanically-weak cartilage. Early changes to chondrocytes within human cartilage have been observed including chondrocyte swelling[1] together with the development of thin cytoplasmic processes which increase in number and length with degeneration[2]. Changes to chondrocyte phenotype in degenerate cartilage are associated with F-actin redistribution and stress fibres (SF) formation, leading to morphologically-dedifferentiated (fibroblast-like) chondrocytes[3,4]. It is unclear if these processes are a consequence of ‘passive’ cell swelling into a defective ECM or an ‘active’ event driven by changes in cell metabolism resulting in alterations to cell shape. To address this, we have quantified and compared the distribution and levels of F-actin, a key cytoskeletal protein involved in the formation of cytoplasmic processes, within in situ chondrocytes in non-degenerate and mildly degenerate human cartilage.

Methods

Human femoral head cartilage was obtained from 21 patients [15 females, 6 males, average age 69.6yrs, (range 47–90yrs)] following femoral neck fracture, with Ethical Approval and patient's permission. Cartilage explants were removed from areas graded non-degenerate grade 0 (G0) or mildly degenerate grade 1 (G1) and cultured for up to 3wks in Dulbecco's Modified Eagle's Medium (DMEM) +/− 25% human serum (HS). In situ chondrocytes were stained with CMFDA (5-chloromethylfluoresceindiacetate, Cell-Tracker Green®) and phalloidin (F-actin labelling) and imaged by confocal microscopy and analysed quantitatively using ImageJ and Imaris® software.

Results

There were significant increases in the total amount (TA) of F-actin and its distribution [intense punctuate (IP) and intense areas (IA)] between the whole chondrocyte populations of G0 and G1 cartilage (P=0.0356; 0.0112; 0.016, respectively). Where the volume of chondrocytes was divided into normal (<1000 µm³) and swollen (≥1000 µm³) cells, F-actin TA increased in swollen cells (P=0.036 within G0 and G1, and P=0.0009 between grades) compared to chondrocytes of normal volume in each grade. Moreover, IP and IA within and between G0 and G1 were higher compared to normal chondrocytes (with P<0.0001 for IP and P<0.001 for IA). In addition, tissue culture experiments demonstrated that 90% of chondrocytes with cytoplasmic processes had strong F-actin intensity (either IP or IA with P<0.0001). Furthermore, 83% of this F-actin was associated with cytoplasmic processes, with >65% situated at the base of the process (P<0.0001).

Conclusions

The increases in chondrocyte F-actin levels (TA) and its localisation (IP, IA) appear to be associated with cell swelling and development of cytoplasmic processes, which are both characteristics of early OA cartilage[1]. This suggests the formation of chondrocyte cytoplasmic processes is an ‘active’ event potentially involving changes to matrix metabolism rather than a ‘passive’ cell swelling into a defective extracellular matrix.

Declaration of Interest

(b) declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research reported:I declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research project.