Chondrocyte primary cilia shorten in response to osmotic challenge and are sites for endocytosis.

OBJECTIVE: The purpose of this study was to examine the influence of cartilage site and osmolarity on primary cilia incidence, length and orientation in live chondrocytes in undisturbed cartilage. Additionally, we imaged endocytotic markers to test our hypothesis that the ciliary pocket is a site for endocytosis.
MATERIALS AND METHODS: We measured primary cilia incidence, length and orientation in the coronal plane using ex vivo live cell confocal imaging of intact murine femoral chondrocytes. Measurements were taken from five regions of the medial and lateral condyles of the left and right femur and also after one minute of osmotic challenge. Transmission electron microscopy and immunocytochemistry were used to characterize the orientation and position of chondrocyte primary cilia in the saggital plane and to determine the colocalization of clathrin coated vesicles, endosomal and lysosomal proteins and CD44 with the ciliary pocket.
RESULTS: Chondrocyte primary cilia length decreased significantly after a one minute hypo- or hyper-osmotic challenge and varied between condyles and across the surface of each condyle. The majority of the length of the chondrocyte primary cilia was positioned within a membranous invagination rather than projecting out from the cell membrane and clathrin coated vesicles, endosomal proteins and CD44 colocalised with the ciliary pocket.
CONCLUSIONS: We demonstrate that live ex vivo chondrocyte primary cilia are capable of shortening within minutes in response to osmotic challenge and provide subcellular and cellular evidence that chondrocyte primary cilia are deeply invaginated in a ciliary pocket which contains sites for endocytosis.