Expert Insights Into the Relationship Between the Hippo Pathway and Extensive Fibrosis in HS

Hurley Stage II hidradenitis suppurativa

Image courtesy of DermNet

A recent study published in the Journal of Clinical Investigation demonstrated that the Hippo pathway plays a central role in promoting extensive fibrosis in hidradenitis suppurativa (HS). Researchers from the University of Michigan and Almirall found evidence that the profibrotic fibroblast response in HS can be modulated through inhibition of the Hippo pathway, providing further insights into the pathogenesis of HS.

Straalen et al used single-cell RNA sequencing (scRNA-Seq) and spatial transcriptomics to define the cellular composition and spatial architecture of the infiltrate in chronic HS lesions. “Our results provide an unprecedented view of HS pathology, demonstrating how stromal-immune cell interactions contribute to the inflammatory network at the site of disease and identifying a pathway implicated in HS fibrosis that may serve as a potential target for future therapeutic interventions,” wrote the study authors.

Through scRNA-Seq, spatial transcriptomics, and immunostaining, Straalen et al demonstrated that 2 stromal subtypes enriched in HS lesional skin, CXCL13+ and SFRP4+ fibroblasts (FBs), play a key role in “shaping and perpetuating the inflammatory response in HS through secretion of chemokines that recruit B cells and myeloid cells, as well as driving the extensive fibrosis that is characteristic of long-standing HS.”

Ex-vivo experiments using primary dermal fibroblasts taken from chronic HS lesions were used to further study the functional role of Hippo signaling in HS. FBs were stimulated with TRULI (blocks YAP phosphorylation, which activates YAP-mediated transcriptional activation) or verteporfin (disrupts YAP-TEAD interaction, which causes YAP target inhibition).

Verteporfin significantly reduced both protein and RNA expression of collagen I and reduced smooth muscle actin (SMA/ACTA2), less than protein and RNA expression, in HS FBs. The stimulation of YAP transcriptional activity with TRULI resulted in a nonsignificant increase in RNA expression of smooth muscle actin (ACTA2) and collagen I (COL1A1).

Straalen et al examined the expression of multiple cytokines and chemokines after TRULI and verteporfin stimulation alone or in combination with single cytokine stimulations to assess the relevance of the Hippo pathway to proinflammatory characteristics of HS FBs.

The study authors found that neither TRULI nor verteporfin significantly affected the expression of CCL2, CCL5, CXCL1, CXCL8, or IL6 in HS FBs in response to stimulation with the previously identified upstream regulators IL-1β, TNF, or IFN-γ.

“These experiments indicate that the Hippo pathway is involved in HS myofibroblast differentiation but dispensable for the HS-specific CXCL13+ FB phenotype. These data support a role for the Hippo pathway in promoting the extensive fibrosis of HS and demonstrate that inhibition of this pathway can modulate the profibrotic characteristics of HS FBs, independent of their proinflammatory characteristics,” wrote Straalen et al.

Expert Insights

Özge Uluçkan, PhD (left), Marta Calbet, PhD (right)

Image courtesy of Almirall

To further review the findings of the relationship between the Hippo pathway and HS, as well as the real-world implications of the study findings, Dermatology Times spoke to 2 of the study authors, Özge Uluçkan, PhD, the senior director of drug discovery project leaders at Almirall, and Marta Calbet, PhD, a principal investigator and project leader at Almirall.

Q&A

Dermatology Times: What motivated your research into the pathogenesis of chronic hidradenitis suppurativa?

A: At Almirall, we are committed to helping patients with skin diseases as these can have a severe impact on their lives. One of our therapeutic focus areas is immune inflammatory diseases of the skin. Hidradenitis suppurativa (HS) is a debilitating inflammatory skin disease that has a very complex, under-studied pathology. To be able to innovate novel treatment options for patients suffering from this disease, we need to generate more fundamental disease understanding.

Towards this goal, to further understand this disease at the clinical and molecular level, we collaborated with Prof. Johann Gudjonsson at the University of Michigan. He is a world-renowned expert on inflammatory skin diseases, and his lab is at the forefront of state-of-the art techniques in analyzing skin at the single-cell level.

Dermatology Times: Can you explain the methodology used in integrating single-cell RNA sequencing, spatial transcriptomics, and immunostaining to study the cellular players and interactions in HS?

A: To understand cell composition in HS compared to healthy skin, single-cell RNA sequencing studies were performed on cells isolated from chronic lesional skin of five HS patients and eight healthy donors. Results show an increased ratio of several resident cells such as keratinocytes and fibroblasts that also show a clear transcriptomic shift compared to healthy skin and increased infiltration of immune cells including T-cells, B-cells, plasma cells, and myeloid cells in HS.

Integrating this data with the results obtained from spatial transcriptomics studies on four samples helped to elucidate the spatial organization of the identified cell types showing an interesting, layered architecture of the HS skin. Spatial transcriptomics allows us to create a map of the molecular changes onto the histological architecture of the tissue.

Some of these findings at the RNA level were further validated at a protein level using Immunostaining.

Dermatology Times: What were the main findings regarding the cellular players and their interactions in the chronic HS infiltrate?

A: We found that the cellular infiltrate in chronic HS is highly layered. We could show that two fibroblast subtypes, previously undescribed in HS, orchestrate the immune responses, recruiting both innate and adaptive immune cells, and creating the fibrosis observed in HS. In addition, we have clearly mapped the interaction of the different cell networks through ligand-receptor interaction analysis.

Dermatology Times: How do these findings contribute to our understanding of HS pathogenesis?

A: These findings highlight the complex immune infiltrate and cellular interactions orchestrating the heavy disease burden on patients. Through such integrated analyses, we start to understand the role that each cell type and its mediators play in these chronic HS lesions. Fibroblasts have not been at the center of discussion for this indication before, especially given the potential role they can play in orchestrating the immune reaction. We are pleased that we were able to show that fibroblasts play a key role in HS, and further describe the two pathological subtypes.

Dermatology Times: How do you envision these findings being translated into clinical practice or improving patient care for individuals with HS?

A: This is a first step to generate a better understanding of the pathology of HS at a molecular level. We need to keep building on these findings to generate novel hypotheses for drug discovery in the path to finding novel effective treatment options for patients suffering from HS.

Reference

van Straalen KR, Ma F, Tsou PS, et al. Single-cell sequencing reveals Hippo signaling as a driver of fibrosis in hidradenitis suppurativa. J Clin Invest. 2024;134(3):e169225. Published 2024 Feb 1. doi:10.1172/JCI169225