Modulating fibroblasts in hard-to-heal wounds and skin ageing : roles of circRNAs and dermal injectables

Abstract

Human skin wound healing is a complex and tightly regulated biological process comprising four sequential yet overlapping phases: hemostasis, inflammation, proliferation and remodeling. The process is initiated by tissue injury and bleeding, followed by clot formation to rapidly restore vascular integrity. Subsequently, the inflammation phase is characterized by the recruitment of immune cells to the wound environment, driven by the release of pro-inflammatory mediators. This phase is followed by the proliferation phase, during which fibroblasts, key effector cells of the skin dermis, become activated, accompanied by keratinocyte migration, neovascularization and angiogenesis. The final remodeling phase culminates in wound closure and tissue maturation, involving reorganization of the extracellular matrix, scar formation and a gradual restoration of tensile strength. Despite this highly coordinated process, wound healing can become impaired or dysregulated, resulting in delayed repair and the development of chronic, hard- to-heal wounds. In parallel, physiological ageing is associated with a progressive decline in skin integrity, elasticity and regenerative capacity.Owing to an incomplete understanding of the underlying pathophysiological mechanisms governing chronic wounds, effective therapeutic strategies remain limited. In recent years, circular RNA (circRNA), a distinct class of non-coding RNA, have emerged as important regulators of gene expression. Their covalently closed structure confers stability, rendering them as promising candidates for novel therapeutics. In contrast, dermal injectables are widely used in clinical and aesthetic medicine to restore lost volume and counteract visible signs of skin ageing, although their molecular and cellular effects on skin homeostasis remains incompletely characterized. Accordingly, this doctoral thesis aims to investigate circular RNAs as potential therapeutic targets in chronic wound healing, while also exploring the differential cellular and tissue-level effects of dermal injectables in the context of skin ageing. To this end, the work is based on analyses of healthy skin and acute wound tissue obtained from healthy volunteers, as well as biopsies derived from patients with chronic wounds. Through this approach, the thesis seeks to advance the understanding of non-coding RNA-mediated mechanisms in skin repair and to provide insights into innovative therapeutic strategies for both impaired wound healing and skin ageing.In Paper I, we investigate CircGLIS3(2), a circular RNA with a unique ability to undergo translation, prompting an exploration of how its coding and non-coding function influence human dermal fibroblasts behavior. CircGLIS3(2) is transiently upregulated in wound fibroblasts in response to injury-associated cues, including IL-la, TGF-B, hypoxia, and ER stress, which collectively drive its expression and translation. Functionally, CircGLIS3(2) exerts a dual role: its RNA promotes fibroblast activation by stabilizing the protein PCOLCE, while its translated protein enhances fibroblast proliferation through interaction with BTF3. Together, CircGLIS3(2) RNA and protein facilitates key reparative processes and ultimately improves wound healing.Paper II centers on mitochondria encoded circular RNA (mecciRNA) CircMT- RNR2, in the context of diabetic foot ulcers (DFU), where its expression is markedly reduced. Given the well-established association between mitochondrial dysfunction, oxidative stress, and DFUs, we identify CircMT-RNR2 as a key regulator of mitochondrial redox homeostasis in dermal fibroblasts. CircMT-RNR2 enhances fibroblast migration, proliferation and extracellular matrix production. Mechanistically, this mecciRNA exerts its protective effects through direct interaction with PRDX3, a mitochondrial antioxidant protein, thereby stabilizing redox balance. Collectively, these findings position CircMT-RNR2 as a potential therapeutic target to improve wound repair in DFU.In Paper III, we explore the role of CircASH1L(4,5) in wound healing, with a particular focus on its interaction with the microRNA (miRNA) miR-129-5p. We demonstrate that CircASH1L(4,5) enhances the stability of miR-129-5p by protecting it from target-directed miRNA degradation. Functionally, the coordinated actions of CircASH1L(4,5) and miR-129-5p enhance epidermal keratinocyte growth and motility, facilitating efficient wound re-epithelialization. This study uncovers a previously unrecognized function of circRNAs as stabilizing scaffolds for miRNAs and highlights their importance in skin repair.In Paper IV, the focus shifts to combating skin ageing through the use of dermal injectables. In this comparative study, we investigate the cellular and molecular effects of two poly-L-lactic acid (PLLA) dermal injectables with distinct particle design, PLLA microspheres and PLLA microflakes, in human dermal fibroblasts or skin tissue. Our findings demonstrate that particle design is a critical determinant of biological outcome. Specifically, PLLA microspheres enhance fibroblast function by promoting cell migration, extracellular matrix (ECM) production and wound contraction, whereas PLLA microflakes trigger a pronounced inflammatory response. Transcriptomic profiling further revealed that PLLA microspheres upregulate genes associated with adipogenic differentiation, while PLLA microflakes suppress regenerative signaling pathways. Taken together, the results provide a foundation for the rational design of more efficient and safer dermal injectables for skin rejuvenation.Collectively, the findings presented in this doctoral thesis provide novel insights into the role of circular RNAs and dermal injectables as modulators of fibroblast function in hard-to-heal wounds and skin ageing. This deepens our understanding on wound repair and skin regeneration, paving the way for future clinical interventions aimed at improving wound healing and promoting skin health.List of scientific papersI. Collaborative duality of CircGLIS3(2) RNA and protein in human wound repair. Guanglin Niu*, Maria A. Toma*, Jennifer Geara, Xiaowei Bian, Yongjian Chen, Lihua Luo, Qizhang Wang, Yunting Xiao, Manika Vij, Minna Piipponen, Zhuang Liu, Sho Oasa, Letian Zhang, Dörte Schlesinger, Ákos Végvári, Dongjing Li, Aoxue Wang, Vladana Vukojevíc, Simon J Elsässer, Pehr Sommar, and Ning Xu Landen Advanced Science, 2025 Jul; 12(25): e2416784. https://doi.org/10.1002/advs.202416784II. Mitochondrial CircRNA CircMT-RNR2 safeguards antioxidant defense to support fibroblast functions in wound repair Guanglin Niu*, Jennifer Geara*, Yongjian Chen, Yanwei Xiao, Zhuang Liu, Pehr Sommar, Aoxue Wang, Xiaowei Zheng and Ning Xu Landen Advanced Science, 2026 Feb 8: e17141. https://doi.org/10.1002/advs.202517141III. Circular RNA CircASH1L(4,5) protects microRNA-129-5p from target-directed microRNA degradation in human skin wound healing Qizhang Wang, Guanglin Niu, Zhuang Liu, Maria A. Toma, Jennifer Geara, Xiaowei Bian, Letian Zhang, Minna Piipponen, Dongqing Li, Aoxue Wang, Pehr Sommar, and Ning Xu Landen British Journal of Dermatology, 2025 Feb 18; 192(3): 468-480. https://doi.org/10.1093/bjd/ljae405IV. Poly-L-Lactic Acid Microspheres Promote Skin Rejuvenation via Enhanced Fibroblast Function Jennifer Geara, Lihua Luo, Onur Parlak, Pehr Sommar, and Ning Xu Landén Journal of Biomedical Materials Research Part A, 2025 Nov; 113(11): e38017. https://doi.org/10.1002/jbm.a.38017*Co-first authors</p