ECM stiffness modulates the proliferation but not the motility of primary corneal keratocytes in response to PDGF-BB

The mechano-chemical circuit in fibroblasts and dendritic cells drives basal cell proliferation in psoriasis

Psoriasis is an intractable immune-mediated disorder that disrupts the skin barrier. While studies have dissected the mechanism by which immune cells directly regulate epidermal cell proliferation, the involvement of dermal fibroblasts in the progression of psoriasis remains unclear. Here, we identified that signals from dendritic cells (DCs) that migrate to the dermal-epidermal junction region enhance dermal stiffness by increasing extracellular matrix (ECM) expression, which further promotes basal epidermal cell hyperproliferation. We analyzed cell-cell interactions and observed stronger interactions between DCs and fibroblasts than between DCs and epidermal cells. Using single-cell RNA (scRNA) sequencing, spatial transcriptomics, immunostaining, and stiffness measurement, we found that DC-secreted LGALS9 can be received by CD44+ dermal fibroblasts, leading to increased ECM expression that creates a stiffer dermal environment. By employing mouse psoriasis and skin organoid models, we discovered a mechano-chemical signaling pathway that originates from DCs, extends to dermal fibroblasts, and ultimately enhances basal cell proliferation in psoriatic skin.

Corneal keratocytes, fibroblasts, and myofibroblasts exhibit distinct transcriptional profiles in vitro

Purpose

After stromal injury to the cornea, the release of growth factors and pro-inflammatory cytokines promotes the activation of quiescent keratocytes into a migratory fibroblast and/or fibrotic myofibroblast phenotype. Persistence of the myofibroblast phenotype can lead to corneal fibrosis and scarring, which are leading causes of blindness worldwide. This study aims to establish comprehensive transcriptional profiles for cultured corneal keratocytes, fibroblasts, and myofibroblasts to gain insights into the mechanisms through which these phenotypic changes occur.

Methods

Primary rabbit corneal keratocytes were cultured in either defined serum-free media (SF), fetal bovine serum (FBS) containing media, or in the presence of TGF-β1 to induce keratocyte, fibroblast, or myofibroblast phenotypes, respectively. Bulk RNA sequencing followed by bioinformatic analyses was performed to identify significant differentially expressed genes (DEGs) and enriched biological pathways for each phenotype.

Results

Genes commonly associated with keratocytes, fibroblasts, or myofibroblasts showed high relative expression in SF, FBS, or TGF-β1 culture conditions, respectively. Differential expression and functional analyses revealed novel DEGs for each cell type, as well as enriched pathways indicative of differences in proliferation, apoptosis, extracellular matrix (ECM) synthesis, cell-ECM interactions, cytokine signaling, and cell mechanics.

Conclusions

Overall, these data demonstrate distinct transcriptional differences among cultured corneal keratocytes, fibroblasts, and myofibroblasts. We have identified genes and signaling pathways that may play important roles in keratocyte differentiation, including many related to mechanotransduction and ECM biology. Our findings have revealed novel molecular markers for each cell type, as well as possible targets for modulating cell behavior and promoting physiological corneal wound healing.

Treatment with both TGF-β1 and PDGF-BB disrupts the stiffness-dependent myofibroblast differentiation of corneal keratocytes

During corneal wound healing, stromal keratocytes transform into a repair phenotype that is driven by the release of cytokines, like transforming growth factor-beta 1 (TGF-β1) and platelet-derived growth factor-BB (PDGF-BB). Previous work has shown that TGF-β1 promotes the myofibroblast differentiation of corneal keratocytes in a manner that depends on PDGF signaling. In addition, changes in mechanical properties are known to regulate the TGF-β1-mediated differentiation of cultured keratocytes. While PDGF signaling acts synergistically with TGF-β1 during myofibroblast differentiation, how treatment with multiple growth factors affects stiffness-dependent differences in keratocyte behavior is unknown. Here, we treated primary corneal keratocytes with PDGF-BB and TGF-β1 and cultured them on polyacrylamide (PA) substrata of different stiffnesses. In the presence of TGF-β1 alone, the cells underwent stiffness-dependent myofibroblast differentiation. On stiff substrata, the cells developed robust stress fibers, exhibited high levels of ⍺-SMA staining, formed large focal adhesions (FAs), and exerted elevated contractile forces, whereas cells in a compliant microenvironment showed low levels of ⍺-SMA immunofluorescence, formed smaller focal adhesions, and exerted decreased contractile forces. When the cultured keratocytes were treated simultaneously with PDGF-BB however, increased levels of ⍺-SMA staining and stress fiber formation were observed on compliant substrata, even though the cells did not exhibit elevated contractility or focal adhesion size. Pharmacological inhibition of PDGF signaling disrupted the myofibroblast differentiation of cells cultured on substrata of all stiffnesses. These results indicate that treatment with PDGF-BB can decouple molecular markers of myofibroblast differentiation from the elevated contractile phenotype otherwise associated with these cells, suggesting that crosstalk in the mechanotransductive signaling pathways downstream of TGF-β1 and PDGF-BB can regulate the stiffness-dependent differentiation of cultured keratocytes.

Statement of Significance

In vitro experiments have shown that changes in ECM stiffness can regulate the differentiation of myofibroblasts. Typically, these assays involve the use of individual growth factors, but it is unclear how stiffness-dependent differences in cell behavior are affected by multiple cytokines. Here, we used primary corneal keratocytes to show that treatment with both TGF-β1 and PDGF-BB disrupts the dependency of myofibroblast differentiation on substratum stiffness. In the presence of both growth factors, keratocytes on soft substrates exhibited elevated ⍺-SMA immunofluorescence without a corresponding increase in contractility or focal adhesion formation. This result suggests that molecular markers of myofibroblast differentiation can be dissociated from the elevated contractile behavior associated with the myofibroblast phenotype, suggesting potential crosstalk in mechanotransductive signaling pathways downstream of TGF-β1 and PDGF-BB.

Light-modulation of gel stiffness: a glyconucleoside based bolaamphiphile as a photo-cleavable low molecular weight gelator

Photo-cleavable glyconucleoside bolaamphiphiles containing a nitrophenyl unit feature gelation abilities in aqueous media. The stiffness of the resulting gels can be modulated upon light irradiation thanks to the photocleavage reaction of nitrophenyl moieties.

The impact of UV cross-linking on corneal stromal cell migration, differentiation and patterning

Previous studies have demonstrated that UV cross-linking (CXL) increases stromal stiffness and produces alterations in extracellular matrix (ECM) microstructure. In order to investigate how CXL impacts both keratocyte differentiation and patterning within the stroma, and fibroblast migration and myofibroblast differentiation on top of the stroma, we combined CXL with superficial phototherapeutic keratectomy (PTK) in a rabbit model. Twenty-six rabbits underwent a 6 mm diameter, 70 μm deep phototherapeutic keratectomy (PTK) with an excimer laser to remove the epithelium and anterior basement membrane. In 14 rabbits, standard CXL was performed in the same eye immediately after PTK. Contralateral eyes served as controls. In vivo confocal microscopy through focusing (CMTF) was used to analyze corneal epithelial and stromal thickness, as well as stromal keratocyte activation and corneal haze. CMTF scans were collected pre-operatively, and from 7 to 120 days after the procedure. A subset of rabbits was sacrificed at each time point, and corneas were fixed and labeled in situ for multiphoton fluorescence microscopy and second harmonic generation imaging. In vivo and in situ imaging demonstrated that haze after PTK was primarily derived from a layer of myofibroblasts that formed on top of the native stroma. Over time, this fibrotic layer was remodeled into more transparent stromal lamellae, and quiescent cells replaced myofibroblasts. Migrating cells within the native stroma underneath the photoablated area were elongated, co-aligned with collagen, and lacked stress fibers. In contrast, following PTK + CXL, haze was derived primarily from highly reflective necrotic "ghost cells" in the anterior stroma, and fibrosis on top of the photoablated stroma was not observed at any time point evaluated. Cells formed clusters as they migrated into the cross-linked stromal tissue and expressed stress fibers; some cells at the edge of the CXL area also expressed α-SM actin, suggesting myofibroblast transformation. Stromal thickness increased significantly between 21 and 90 days after PTK + CXL (P < 0.001) and was over 35 μm higher than baseline at Day 90 (P < 0.05). Overall, these data suggest that cross-linking inhibits interlamellar cell movement, and that these changes lead to a disruption of normal keratocyte patterning and increased activation during stromal repopulation. Interestingly, CXL also prevents PTK-induced fibrosis on top of the stroma, and results in long term increases in stromal thickness in the rabbit model.

Extracellular-Matrix Mechanics Regulate the Ocular Physiological and Pathological Activities

The extracellular matrix (ECM) is a noncellular structure that plays an indispensable role in a series of cell life activities. Accumulating studies have demonstrated that ECM stiffness, a type of mechanical forces, exerts a pivotal influence on regulating organogenesis, tissue homeostasis, and the occurrence and development of miscellaneous diseases. Nevertheless, the role of ECM stiffness in ophthalmology is rarely discussed. In this review, we focus on describing the important role of ECM stiffness and its composition in multiple ocular structures (including cornea, retina, optic nerve, trabecular reticulum, and vitreous) from a new perspective. The abnormal changes in ECM can trigger physiological and pathological activities of the eye, suggesting that compared with different biochemical factors, the transmission and transduction of force signals triggered by mechanical cues such as ECM stiffness are also universal in different ocular cells. We expect that targeting ECM as a therapeutic approach or designing advanced ECM-based technologies will have a broader application prospect in ophthalmology.

Effects of Topography and PDGF on the Response of Corneal Keratocytes to Fibronectin-Coated Surfaces

During corneal wound healing, corneal keratocytes are exposed to both biophysical and soluble cues that cause them to transform from a quiescent state to a repair phenotype. How keratocytes integrate these multiple cues simultaneously is not well understood. To investigate this process, primary rabbit corneal keratocytes were cultured on substrates patterned with aligned collagen fibrils and coated with adsorbed fibronectin. After 2 or 5 days of culture, keratocytes were fixed and stained to assess changes in cell morphology and markers of myofibroblastic activation by fluorescence microscopy. Initially, adsorbed fibronectin had an activating effect on the keratocytes as evidenced by changes in cell shape, stress fiber formation, and expression of alpha-smooth muscle actin (α-SMA). The magnitude of these effects depended upon substrate topography (i.e., flat substrate vs aligned collagen fibrils) and decreased with culture time. When keratocytes were simultaneously exposed to adsorbed fibronectin and soluble platelet-derived growth factor-BB (PDGF-BB), the cells elongated and had reduced expression of stress fibers and α-SMA. In the presence of PDGF-BB, keratocytes plated on the aligned collagen fibrils elongated in the direction of the fibrils. These results provide new information on how keratocytes respond to multiple simultaneous cues and how the anisotropic topography of aligned collagen fibrils influences keratocyte behavior.

Unraveling the mechanobiology of cornea: From bench side to the clinic

The cornea is a transparent, dome-shaped structure on the front part of the eye that serves as a major optic element and a protector from the external environment. Recent evidence shows aberrant alterations of the corneal mechano-environment in development and progression of various corneal diseases. It is, thus, critical to understand how corneal cells sense and respond to mechanical signals in physiological and pathological conditions. In this review, we summarize the corneal mechano-environment and discuss the impact of these mechanical cues on cellular functions from the bench side (in a laboratory research setting). From a clinical perspective, we comprehensively review the mechanical changes of corneal tissue in several cornea-related diseases, including keratoconus, myopia, and keratectasia, following refractive surgery. The findings from the bench side and clinic underscore the involvement of mechanical cues in corneal disorders, which may open a new avenue for development of novel therapeutic strategies by targeting corneal mechanics.