Cilia protein IFT88 regulates extracellular protease activity by optimizing LRP‐1–mediated endocytosis

Abstract

Matrix protease activity is fundamental to developmental tissue patterning and remains influential in adult homeostasis. In cartilage, the principal matrix proteoglycan is aggrecan, the protease-mediated catabolism of which defines arthritis; however, the pathophysiologic mechanisms that drive aberrant aggrecanolytic activity remain unclear. Human ciliopathies exhibit altered matrix, which has been proposed to be the result of dysregulated hedgehog signaling that is tuned within the primary cilium. Here, we report that disruption of intraflagellar transport protein 88 (IFT88), a core ciliary trafficking protein, increases chondrocyte aggrecanase activity in vitro. We find that the receptor for protease endocytosis in chondrocytes, LDL receptor–related protein 1 (LRP-1), is unevenly distributed over the cell membrane, often concentrated at the site of cilia assembly. Hypomorphic mutation of IFT88 disturbs this apparent hot spot for protease uptake, increases receptor shedding, and results in a reduced rate of protease clearance from the extracellular space. We propose that IFT88 and/or the cilium regulates the extracellular remodeling of matrix—independently of Hedgehog regulation—by enabling rapid LRP-1–mediated endocytosis of proteases, potentially by supporting the creation of a ciliary pocket. This result highlights new roles for the cilium’s machinery in matrix turnover and LRP-1 function, with potential relevance in a range of diseases.—Coveney, C. R., Collins, I., Mc Fie, M., Chanalaris, A., Yamamoto, K., Wann, A. K. T. Cilia protein IFT88 regulates extracellular protease activity by optimizing LRP-1–mediated endocytosis.

The ciliary protein IFT88 controls post-natal cartilage thickness and influences development of osteoarthritis

“…We previously linked IFT88 to aggrecanase activity in vitro [16], but here found no correlations in gene expression to link observed atrophy to altered expression of proteases or LRP-1 (receptor for protease clearance). There were also no apparent increases in aggrecanase-mediated aggrecan neoepitope in situ.…”

“…The primary cilium has also been coined a 'mechanosensor', bending in response to fluid flow within the kidney, initiating downstream signaling pathways [14]. In vitro experiments in chondrocytes have implicated ciliary IFT88 both with compressioninduced production of extracellular matrix proteins [15] and impaired clearance of aggrecanases, resulting in exacerbated degradation of aggrecan [16].…”

Regulation of the Extracellular Matrix by Ciliary Machinery

“…Research in our own lab showed that LRP-1-mediated endocytosis of proteases is impaired in chondrocytes derived from the ORPK mutant mice. Co-culture of these cells with the purified matrix component aggrecan, and subsequent detection of protease-generated aggrecan neoepitopes, showed that ORPK chondrocytes have constitutively increased protease activity in vitro [108]. This increased activity was not associated with increased Hh signaling or increased mRNA levels of aggrecan-degrading proteases.…”

Primary cilia: Versatile regulator in cartilage development

“…Furthermore, the ciliary functional intraflagellar transport protein IFT20 is essential in regulating Hedgehog signalling transduction and affecting Golgi size to maintain a cartilaginous matrix and maintain the homeostasis of condylar cartilage (Figure ) . Disruption of ciliary trafficking protein IFT88 can also affect the extracellular protease activity and regulate the extracellular matrix remodelling of articular chondrocyte via regulating LRP‐1‐mediated endocytosis …”