Primary cilia of Systemic Sclerosis dermal fibroblasts are disrupted by downregulation of Caveolin-1 and aberrant activity of Aurora A kinase, independently of Transforming Growth Factor Beta

Systemic sclerosis (SSc) is a rare autoimmune disease of unknown aetiology characterised by chronic and progressive fibrosis of the skin, and of the internal organs in more advanced disease. Aberrant activation of the Transforming Growth Factor Beta (TGFβ) signalling pathway and downregulation of the cell membrane protein Caveolin-1 (CAV1) have been implicated in disease progression. Both TGFβ and CAV1 have roles at the primary cilium, a sensory organelle which forms a signalling hub for pathways involved in cellular homeostasis. Several of the pathways coordinated at the cilium, which include the TGFβ pathway itself, are dysregulated in SSc. Preliminary observations indicated that primary cilia of SSc dermal fibroblasts appeared to be abnormally short, which could hypothetically explain the dysregulation of key profibrotic pathways.

This study aimed to investigate in detail cilia morphology in SSc dermal fibroblasts, the effects of TGFβ activation and Caveolin-1 downregulation on cilia length, and the mechanism by which cilia of SSc dermal fibroblasts are shortened, with the aim of identifying targets to induce cilia elongation and normalise signalling.

Dermal fibroblasts from SSc patients and from very early diagnosis of SSc (VEDOSS) patients displayed shorter cilia compared to dermal fibroblasts isolated from healthy controls (HC). Silencing of CAV1 in shCAV1 fibroblasts also resulted in shorter cilia compared to control shSCR fibroblasts, suggesting a role in establishment of the phenotype in SSc. Additionally, TGFβ treatment reduced cilium length. This effect was fully abolished by TGFβRI inhibition and partially blocked by ROCK2 inhibition, but was unaffected by inhibition of SMAD3, PI3K-AKT, or the AURKA-HDAC6 axis, a known negative regulator of cilium length. Inhibition of the TGFβ signalling pathway did not restore SSc cilium length, but inhibition of the AURKA-HDAC6 axis restored SSc and shCAV1 cilia length to that of controls and reduced TGFβ-induced contractility. In addition to shorter cilia, we identified a loss of SPAG17 localisation at the base of the cilium in SSc, shCAV1, and TGFβ-treated dermal fibroblasts, consistent with recent findings of a role for SPAG17 in regulating cilia length, and profibrotic signalling in SSc.

This study reveals that primary cilium length is stably reduced in SSc dermal fibroblasts, even in early disease prior to clinically evident fibrosis. The results suggest that CAV1 downregulation and aberrant AURKA activity may contribute to aberrantly short cilia and profibrotic activation in SSc dermal fibroblasts. While TGFβ activation can shorten cilia in a mechanism at least partially dependent on ROCK2, TGFβ signalling is not responsible for the stably shortened cilia phenotype in SSc. These findings implicate the cilium as a potential therapeutic target for SSc, a disease for which there is currently no cure, and limited treatment options, as well as other fibrotic disorders.