Age-related downregulation of CCN2 is regulated by cell size in a YAP/TAZ-dependent manner in human dermal fibroblasts: impact on dermal aging

CCN Proteins as Matricellular Regulators of Bone in Aging and Disease

PURPOSE OF REVIEW

This review explores the role of cell communication network (CCN) proteins in regulating skeletal physiology, aging, and disease, particularly within the context of balanced bone remodeling.

RECENT FINDINGS

Recent conceptualization of paracrine and endocrine networks in bone marrow as a form of osteoimmunological crosstalk suggests a significant role for matricellular signaling in regulating bone homeostasis. As multifunctional adapters of cell-matrix interactions, CCNs are emerging as a focal point for parathyroid hormone (PTH) signaling and regulation of the RANKL/RANK/OPG axis in skeletal aging. Altered bone marrow CCN expression creates a permissive environment for accelerated postmenopausal bone loss and may contribute to the pathogenesis of osteoporosis and other diseases related to skeletal aging. CCNs modulate fundamental signaling mechanisms in bone development, homeostasis and repair. During aging, dysregulation of CCNs may negatively affect skeletal health and contribute to disease progression. As a result, CCNs may constitute promising therapeutic targets for improving and maintaining aging bone health.

Integrin α11β1 as a Key Collagen Receptor in Human Skin Dermis: Insight into Fibroblast Function and Skin Dermal Aging

Collagen-binding integrins play a crucial role in facilitating fibroblast-collagen interactions and regulating cellular functions. In this study, we identified that among 4 collagen-binding integrins, integrin α11 was the predominant type in human skin dermal fibroblasts and that loss of integrin α11 expression contributed to skin dermal aging. Integrin α11β1 was critical for regulating fibroblast-collagen interactions, including cell adhesion, spreading, morphology, mechanical tension, and the production of collagenous extracellular matrix. TGF-β was recognized as the primary regulator of integrin α11 expression. Notably, dermal fibroblasts in aged human skin demonstrated impaired TGF-β signaling, which coincided with a loss of integrin α11 expression, whereas the expression of other collagen-binding integrins remained unchanged. Similarly, in senescent dermal fibroblasts in vitro, impaired TGF-β signaling was associated with a significant reduction in integrin α11 expression, whereas other collagen-binding integrins were upregulated or unaffected. Furthermore, collapsed dermal fibroblasts, a key characteristic of dermal fibroblasts in aged human skin, specifically downregulated integrin α11, whereas other collagen-binding integrins were upregulated or remained unchanged. These findings suggest a negative feedback loop in which an impaired TGF-β-integrin α11β1 axis and fibroblast collapse promote dermal aging in human skin. This self-reinforcing cycle reflects the progressive and unidirectional nature of biological aging.

CCN2 functions as a modulator of cell cycle regulation in human dermal fibroblasts

CCN2 is widely regarded as a profibrotic factor involved in fibrotic disorders by regulating extracellular matrix (ECM). We report here that CCN2 functions as a critical cell cycle regulator in primary human dermal fibroblasts (HDFs). siRNA-mediated knockdown of CCN2 halted proliferation of primary HDFs, which was rescued by a siRNA-resistant CCN2 expression vector. Furthermore, CCN2 knockdown caused a significant accumulation of cells in G1/G0 phase and blocked entry into S-phase. Mechanistically, CCN2 knockdown blocked cyclin E and CDK4/cyclin D nuclear translocation, and abrogated CDK2 activity. Markedly, CCN2 translocated to the nucleus and co-localized with cyclin D1 upon cell cycle stimulation. Finally, we show that CCN2, a bona fide YAP/TAZ target gene, partially mediates YAP/TAZ-dependent proliferation of primary HDFs. These data provide evidence of a novel CCN2 function as a cell cycle regulator in primary HDFs proliferation, in addition to its known role in ECM regulation.

Remodeling of cytoskeleton, chromatin, and gene expression during mechanical rejuvenation of aged human dermal fibroblasts

Aging is associated with a progressive decline in cellular function. To reset the aged cellular phenotype, various reprogramming approaches, including mechanical routes, have been explored. However, the epigenetic mechanisms underlying cellular rejuvenation are poorly understood. Here, we studied the cytoskeletal, genome-wide chromatin and transcriptional changes in young, aged, and mechanically rejuvenated fibroblasts using immunofluorescence, RNA sequencing, and Hi-C experiments. The mechanically rejuvenated aged fibroblasts, that had partially reset their transcription to a younger cell state, showed a local reorganization of the interchromosomal contacts and lamina-associated domains. Interestingly, the observed chromatin reorganization correlated with the transcriptional changes. Immunofluorescence experiments in the rejuvenated state confirmed increased actomyosin contractility like younger fibroblasts. In addition, the rejuvenated contractile properties were maintained over multiple cell passages. Overall, our results give an overview of how changes in the cytoskeleton, chromatin, and gene activity are connected to aging and rejuvenation.

Involvement of Matricellular Proteins in Cellular Senescence: Potential Therapeutic Targets for Age-Related Diseases

Senescence is a physiological and pathological cellular program triggered by various types of cellular stress. Senescent cells exhibit multiple characteristic changes. Among them, the characteristic flattened and enlarged morphology exhibited in senescent cells is observed regardless of the stimuli causing the senescence. Several studies have provided important insights into pro-adhesive properties of cellular senescence, suggesting that cell adhesion to the extracellular matrix (ECM), which is involved in characteristic morphological changes, may play pivotal roles in cellular senescence. Matricellular proteins, a group of structurally unrelated ECM molecules that are secreted into the extracellular environment, have the unique ability to control cell adhesion to the ECM by binding to cell adhesion receptors, including integrins. Recent reports have certified that matricellular proteins are closely involved in cellular senescence. Through this biological function, matricellular proteins are thought to play important roles in the pathogenesis of age-related diseases, including fibrosis, osteoarthritis, intervertebral disc degeneration, atherosclerosis, and cancer. This review outlines recent studies on the role of matricellular proteins in inducing cellular senescence. We highlight the role of integrin-mediated signaling in inducing cellular senescence and provide new therapeutic options for age-related diseases targeting matricellular proteins and integrins.

Fibroblast Yap/Taz Signaling in Extracellular Matrix Homeostasis and Tissue Fibrosis

Tissue fibrosis represents a complex pathological condition characterized by the excessive accumulation of collagenous extracellular matrix (ECM) components, resulting in impaired organ function. Fibroblasts are central to the fibrotic process and crucially involved in producing and depositing collagen-rich ECM. Apart from their primary function in ECM synthesis, fibroblasts engage in diverse activities such as inflammation and shaping the tissue microenvironment, which significantly influence cellular and tissue functions. This review explores the role of Yes-associated protein (Yap) and Transcriptional co-activator with PDZ-binding motif (Taz) in fibroblast signaling and their impact on tissue fibrosis. Gaining a comprehensive understanding of the intricate molecular mechanisms of Yap/Taz signaling in fibroblasts may reveal novel therapeutic targets for fibrotic diseases.

Emerging Perspectives of YAP/TAZ in Human Skin Epidermal and Dermal Aging

Yes-associated protein (YAP) and transcriptional coactivator with PDZ-binding motif (TAZ) are key downstream effectors of the Hippo signaling pathway, which plays a central role in tissue homeostasis, organ development, and regeneration. While the dysregulation of YAP/TAZ has been linked to various human diseases, their involvement in the aging of human skin has only recently begun to manifest. In the skin, the YAP/TAZ effectors emerge as central regulators in maintaining homeostasis of epidermal stem cells and dermal extracellular matrix, and thus intimately linked to skin aging processes. This review underscores recent molecular breakthroughs highlighting how age-related decline of YAP/TAZ activity impacts human epidermal and dermal aging. Gaining insight into the evolving roles of YAP/TAZ in human skin aging presents a promising avenue for the development of innovative therapeutic approaches aimed at enhancing skin health and addressing age-related skin conditions.

YAP prevents senescence of dermal fibroblast and inhibits melanogenesis via paracrine effect of DKK1

Senile skin hyperpigmentation displays remarkable histopathological features of dermal aging. The crosstalk between melanocytes and dermal fibroblasts plays crucial roles in aging-related pigmentation. While senescent fibroblasts can upregulate pro-melanogenic factors, the role of anti-melanogenic factors, such as dickkopf1 (DKK1), and the upstream regulatory mechanism during aging remain obscure. This study investigated the roles of yes-associated protein (YAP) and DKK1 in the regulation of dermal fibroblast senescence and melanogenesis. Our findings demonstrated decreased YAP activity and DKK1 levels in intrinsic and extrinsic senescent fibroblasts. YAP depletion induced fibroblast senescence and downregulated the expression and secretion of DKK1, whereas YAP overexpression partially reversed the effect. The transcriptional regulation of DKK1 by YAP was supported by dual-luciferase reporter and chromatin immunoprecipitation assays. Moreover, YAP depletion in fibroblasts upregulated Wnt/β-catenin in melanocytes and stimulated melanogenesis, which was partially rescued by the re-supplementation of DKK1. Conversely, overexpression of YAP in senescent fibroblasts decreased Wnt/β-catenin levels in melanocytes and inhibited melanogenesis. Additionally, reduced levels of YAP and DKK1 were verified in the dermis of solar lentigines. These findings suggest that, during skin aging, epidermal pigmentation may be influenced by YAP in the dermal microenvironment via the paracrine effect of DKK1.

Age-related changes in dermal collagen physical properties in human skin

Collagen is the major structural protein in the skin. Fragmentation and disorganization of the collagen fibrils are the hallmarks of the aged human skin dermis. These age-related alterations of collagen fibrils impair skin structural integrity and make the tissue microenvironment more prone to skin disorders. As the biological function of collagen lies predominantly in its physical properties, we applied atomic force microscopy (AFM) and nanoindentation to evaluate the physical properties (surface roughness, stiffness, and hardness) of dermal collagen in young (25±5 years, N = 6) and aged (75±6 years, N = 6) healthy sun-protected hip skin. We observed that in the aged dermis, the surface of collagen fibrils was rougher, and fiber bundles were stiffer and harder, compared to young dermal collagen. Mechanistically, the age-related elevation of matrix metalloproteinase-1 (MMP-1) and advanced glycation end products (AGEs) are responsible for rougher and stiffer/harder dermal collagen, respectively. Analyzing the physical properties of dermal collagen as a function of age revealed that alterations of the physical properties of collagen fibrils changed with age (22-89 years, N = 18). We also observed that the reticular dermis is rougher and mechanically stiffer and harder compared to the papillary dermis in human skin. These data extend the current understanding of collagen beyond biological entities to include biophysical properties.

Molecular insights of human skin epidermal and dermal aging

Human skin is the most widespread and abundant type of tissue in the human body. With the passage of time, most of our organs, including a substantial part of the skin, tend to undergo a gradual thinning or decrease in size. As we age, there is a gradual and progressive reduction in the thickness of both the epidermis and dermis layers of our skin. This is primarily attributed to the decline of epidermal stem cells and the loss of dermal collagen, which is the most abundant protein in the human body. Age-related alterations of the epidermis and dermis impair skin structure/function and create a tissue microenvironment that promotes age-related skin diseases, such as impaired skin barrier, delayed wound healing, and skin cancer development. This review will examine the current body of literature pertaining to our knowledge of skin epidermal and dermal aging.

Matricellular Proteins in the Homeostasis, Regeneration, and Aging of Skin

Matricellular proteins are secreted extracellular proteins that bear no primary structural functions but play crucial roles in tissue remodeling during development, homeostasis, and aging. Despite their low expression after birth, matricellular proteins within skin compartments support the structural function of many extracellular matrix proteins, such as collagens. In this review, we summarize the function of matricellular proteins in skin stem cell niches that influence stem cells' fate and self-renewal ability. In the epidermal stem cell niche, fibulin 7 promotes epidermal stem cells' heterogeneity and fitness into old age, and the transforming growth factor-β-induced protein ig-h3 (TGFBI)-enhances epidermal stem cell growth and wound healing. In the hair follicle stem cell niche, matricellular proteins such as periostin, tenascin C, SPARC, fibulin 1, CCN2, and R-Spondin 2 and 3 modulate stem cell activity during the hair cycle and may stabilize arrector pili muscle attachment to the hair follicle during piloerections (goosebumps). In skin wound healing, matricellular proteins are upregulated, and their functions have been examined in various gain-and-loss-of-function studies. However, much remains unknown concerning whether these proteins modulate skin stem cell behavior, plasticity, or cell-cell communications during wound healing and aging, leaving a new avenue for future studies.

CCN proteins: opportunities for clinical studies-a personal perspective

The diverse members of the CCN family now designated as CCN1(CYR61), CCN2 (CTGF), CCN3(NOV), CCN4(WISP1), CCN5(WISP2), CCN6(WISP3) are a conserved matricellular family of proteins exhibiting a spectrum of functional properties throughout all organs in the body. Interaction with cell membrane receptors such as integrins trigger intracellular signaling pathways. Proteolytically cleaved fragments (constituting the active domains) can be transported to the nucleus and perform transcriptional relevant functional activities. Notably, as also found in other protein families some members act opposite to others creating a system of functionally relevant checks and balances. It has become apparent that these proteins are secreted into the circulation, are quantifiable, and can serve as disease biomarkers. How they might also serve as homeostatic regulators is just becoming appreciated. In this review I have attempted to highlight the most recent evidence under the subcategories of cancer and non-cancer relevant that could lead to potential therapeutic approaches or ideas that can be factored into clinical advances. I have added my own personal perspective on feasibility.

Adding a new dimension: Multi-level structure and organization of mixed-species Pseudomonas aeruginosa and Staphylococcus aureus biofilms in a 4-D wound microenvironment

Biofilms in wounds typically consist of aggregates of bacteria, most often Pseudomonas aeruginosa and Staphylococcus aureus, in close association with each other and the host microenvironment. Given this, the interplay across host and microbial elements, including the biochemical and nutrient profile of the microenvironment, likely influences the structure and organization of wound biofilms. While clinical studies, in vivo and ex vivo model systems have provided insights into the distribution of P. aeruginosa and S. aureus in wounds, they are limited in their ability to provide a detailed characterization of biofilm structure and organization across the host-microbial interface. On the other hand, biomimetic in vitro systems, such as host cell surfaces and simulant media conditions, albeit reductionist, have been shown to support the co-existence of P. aeruginosa and S. aureus biofilms, with species-dependent localization patterns and interspecies interactions. Therefore, composite in vitro models that bring together key features of the wound microenvironment could provide unprecedented insights into the structure and organization of mixed-species biofilms. We have built a four-dimensional (4-D) wound microenvironment consisting of a 3-D host cell scaffold of co-cultured human epidermal keratinocytes and dermal fibroblasts, and an in vitro wound milieu (IVWM); the IVWM provides the fourth dimension that represents the biochemical and nutrient profile of the wound infection state. We leveraged this 4-D wound microenvironment, in comparison with biofilms in IVWM alone and standard laboratory media, to probe the structure of mixed-species P. aeruginosa and S. aureus biofilms across multiple levels of organization such as aggregate dimensions and biomass thickness, species co-localization and spatial organization within the biomass, overall biomass composition and interspecies interactions. In doing so, the 4-D wound microenvironment platform provides multi-level insights into the structure of mixed-species biofilms, which we incorporate into the current understanding of P. aeruginosa and S. aureus organization in the wound bed.