The skin aging process in depth
The skin, the body’s largest organ, is exposed to the external environment and with age is further exposed to both internal and external aging factors. Skin ageing is characterised by external features such as wrinkles, loss of elasticity and laxity. This aging process is accompanied by phenotypic changes in skin cells as well as structural and functional changes in extracellular matrix components such as collagens and elastin. Skin ageing is induced by both internal and external factors. Internal ageing is an inevitable physiological process that results in thin, dry skin, fine wrinkles and gradual dermal atrophy, while external ageing is induced by external factors such as air pollution, smoking, and sun exposure, resulting in fine lines, wrinkles, loss of elasticity, laxity and rough textured appearance. Notably, prolonged exposure to ultraviolet solar radiation (UV) is the primary factor in aging the skin's exterior and is known as photoaging.
These aging processes are accompanied by phenotypic changes in skin cells as well as structural and functional changes in extracellular matrix components such as collagens, elastin and proteoglycans, which are necessary to provide tensile strength, elasticity and hydration to the skin, respectively.
For inherently aged skin, the most notable histological changes occur in the basal cell layer. Research shows that as a person ages, the proliferation of cells in the basal layer is reduced. The epidermis then becomes thinner and the contact surface area between the dermis and epidermis decreases, resulting in a smaller exchange surface for nutrient delivery to the epidermis and further impaired ability for basal cell proliferation. This process of reduced proliferative capacity of cells, including keratinocytes, fibroblasts and melanocytes, is called cellular senescence.
The skin, the body’s largest organ, is exposed to the external environment and with age is further exposed to both internal and external aging factors. Skin ageing is characterised by external features such as wrinkles, loss of elasticity and laxity. This aging process is accompanied by phenotypic changes in skin cells as well as structural and functional changes in extracellular matrix components such as collagens and elastin. Skin ageing is induced by both internal and external factors. Internal ageing is an inevitable physiological process that results in thin, dry skin, fine wrinkles and gradual dermal atrophy, while external ageing is induced by external factors such as air pollution, smoking, and sun exposure, resulting in fine lines, wrinkles, loss of elasticity, laxity and rough textured appearance. Notably, prolonged exposure to ultraviolet solar radiation (UV) is the primary factor in aging the skin's exterior and is known as photoaging.
These aging processes are accompanied by phenotypic changes in skin cells as well as structural and functional changes in extracellular matrix components such as collagens, elastin and proteoglycans, which are necessary to provide tensile strength, elasticity and hydration to the skin, respectively.
For inherently aged skin, the most notable histological changes occur in the basal cell layer. Research shows that as a person ages, the proliferation of cells in the basal layer is reduced. The epidermis then becomes thinner and the contact surface area between the dermis and epidermis decreases, resulting in a smaller exchange surface for nutrient delivery to the epidermis and further impaired ability for basal cell proliferation. This process of reduced proliferative capacity of cells, including keratinocytes, fibroblasts and melanocytes, is called cellular senescence.
Our skin is a dynamic and complex organ with a unique structure. The boundary between the epidermis and dermis, the epidermal-dermal junction, is a collection of proteins and structures known as the basement membrane. Beneath the basement membrane is the underlying dermis, which provides structural support, nourishment and circulation to the skin. The dermis primarily comprises fibroblasts, which produce an interconnected extracellular matrix of collagens and elastic fibers.
Aging seems to affect all skin layers and is expressed as changes in their structure and function. The aged epidermis shows a reduced capacity as a barrier function. In addition to the epidermis, both the epidermal-dermal junction and the dermis also become thinner. The flattening of the epidermal-dermal junction leads to fewer cells, less nutrition and less oxygen, resulting in wrinkle formation. The dermal extracellular matrix (ECM) also exhibits structural and functional changes in both internal and external aged skin. These include an altered accumulation of type I and type III collagens and changes in the ratio of type I/III, an impaired synthesis of these ECM molecules and changes in the elastic organization of fibers. The decrease in the number of fibroblasts also contributes to changes and degradation of the ECM, which manifests as advanced dermal thinning, increased wrinkling and loss of elasticity.
Aging cells accumulate in chronologically aged skin, as well as prematurely aged skin, and can contribute to age-related skin changes. The accumulating aged keratinocytes and fibroblasts in the skin produce cytokines, extracellular matrix-modifying enzymes and other molecules that can act at a distance and can thus exert long-term effects on the microenvironment of neighboring cells. Both internal and external factors can cause permanent aging in skin cells, as a result of telomere shortening, mitochondrial impairment and upregulation of DNA damage response signaling, finally leading to cell cycle arrest. As a result, the presence of old keratinocytes and fibroblasts causes the decline in the maintenance of skin integrity and function.
Accumulation of dysfunctional mitochondria is a major cause of elevated ROS production in aging cells. As is ROS-related mitochondrial damage in photoaging of the skin. Repeated UVA exposure has been shown to be accompanied by an increase in mitochondrial DNA mutations. In particular, photoaged skin includes up to 10 times more frequent mitochondrial DNA mutations compared to sun-protected skin. Furthermore, mitochondrial DNA mutations are associated with an increase in matrix metalloproteinase-1 (MMP-1) levels and consequent collagen degradation.
Up- and down-regulation of microRNAs in aging dermal fibroblasts also helps to reduce fibroblast attachment by down-regulating integrin. The expression of collagen XVI, a minor component of the ECM in aging fibroblasts, is directly downregulated by upregulated microRNA. UV irradiation alters microRNA expression. UVA exposure downregulates microRNA, leading to upregulation of other transcription factors that affect collagen gene activity in fibroblasts.
Our skin is a dynamic and complex organ with a unique structure. The boundary between the epidermis and dermis, the epidermal-dermal junction, is a collection of proteins and structures known as the basement membrane. Beneath the basement membrane is the underlying dermis, which provides structural support, nourishment and circulation to the skin. The dermis primarily comprises fibroblasts, which produce an interconnected extracellular matrix of collagens and elastic fibers.
Aging seems to affect all skin layers and is expressed as changes in their structure and function. The aged epidermis shows a reduced capacity as a barrier function. In addition to the epidermis, both the epidermal-dermal junction and the dermis also become thinner. The flattening of the epidermal-dermal junction leads to fewer cells, less nutrition and less oxygen, resulting in wrinkle formation. The dermal extracellular matrix (ECM) also exhibits structural and functional changes in both internal and external aged skin. These include an altered accumulation of type I and type III collagens and changes in the ratio of type I/III, an impaired synthesis of these ECM molecules and changes in the elastic organization of fibers. The decrease in the number of fibroblasts also contributes to changes and degradation of the ECM, which manifests as advanced dermal thinning, increased wrinkling and loss of elasticity.
Aging cells accumulate in chronologically aged skin, as well as prematurely aged skin, and can contribute to age-related skin changes. The accumulating aged keratinocytes and fibroblasts in the skin produce cytokines,
extracellular matrix-modifying enzymes and other molecules that can act at a distance and can thus exert long-term effects on the microenvironment of neighboring cells. Both internal and external factors can cause permanent aging in skin cells, as a result of telomere shortening, mitochondrial impairment and upregulation of DNA damage response signaling, finally leading to cell cycle arrest. As a result, the presence of old keratinocytes and fibroblasts causes the decline in the maintenance of skin integrity and function.
Accumulation of dysfunctional mitochondria is a major cause of elevated ROS production in aging cells. As is ROS-related mitochondrial damage in photoaging of the skin. Repeated UVA exposure has been shown to be accompanied by an increase in mitochondrial DNA mutations. In particular, photoaged skin includes up to 10 times more frequent mitochondrial DNA mutations compared to sun-protected skin. Furthermore, mitochondrial DNA mutations are associated with an increase in matrix metalloproteinase-1 (MMP-1) levels and consequent collagen degradation.
Up- and down-regulation of microRNAs in aging dermal fibroblasts also helps to reduce fibroblast attachment by down-regulating integrin. The expression of collagen XVI, a minor component of the ECM in aging fibroblasts, is directly downregulated by upregulated microRNA. UV irradiation alters microRNA expression. UVA exposure downregulates microRNA, leading to upregulation of other transcription factors that affect collagen gene activity in fibroblasts.
Dermis
The dermis consists primarily of the extracellular matrix (ECM) and fibroblasts. During the aging process, the dermis undergoes significant changes. Collagen, an important component of the ECM, becomes fragmented and coarsely distributed and its total amount decreases. This is mainly due to increased activity of matrix metalloproteinases and impaired transforming growth factor-β signaling induced by reactive oxygen species generated during aging. The reduction in the amount of collagen hinders the mechanical interaction between fibroblasts and ECM and consequently leads to impairment of fibroblast function and further reduction in the amount of dermal collagen. Other ECM components, including elastic fibers, glycosaminoglycans (GAGs) and proteoglycans (PGs), also change during aging, ultimately leading to a reduction in the amount of functional components. Elastic fibers decrease in inner aged skin, but abnormally accumulate in photoaged skin.
Unlike the epidermis, which is made up of densely packed keratinocytes, the dermis is primarily made up of an acellular component, the ECM. Collagen fibers are an important component of the ECM, accounting for 75% of the skin's dry weight and giving it tensile strength and elasticity. In human skin, type I collagen makes up 80 to 90% of total collagen, while type III makes up 8 to 12% and type V makes up <5%. The collagen bundles increase in size deeper in the dermis.
Elastic fibers are another fibrous element that make up the dermal ECM. The other components of the ECM are proteoglycans (PGs) and glycosaminoglycans (GAGs), which are amorphous and surround and embed the fibrous and cellular matrix elements of the dermis. Although they make up only 0.2% of the dry weight of the dermis, they absorb water up to 1,000 times their volume and have roles in regulating water binding and compressibility of the dermis.
Fibroblasts are dermal-resident cells and are differentiated from mesenchymal cells. They are responsible for the synthesis and degradation of fibrous and amorphous ECM proteins. Their function and microenvironment interactions are important for understanding the molecular mechanism behind dermal aging.
The dermis consists primarily of the extracellular matrix (ECM) and fibroblasts. During the aging process, the dermis undergoes significant changes. Collagen, an important component of the ECM, becomes fragmented and coarsely distributed and its total amount decreases. This is mainly due to increased activity of matrix metalloproteinases and impaired transforming growth factor-β signaling induced by reactive oxygen species generated during aging. The reduction in the amount of collagen hinders the mechanical interaction between fibroblasts and ECM and consequently leads to impairment of fibroblast function and further reduction in the amount of dermal collagen. Other ECM components, including elastic fibers, glycosaminoglycans (GAGs) and proteoglycans (PGs), also change during aging, ultimately leading to a reduction in the amount of functional components. Elastic fibers decrease in inner aged skin, but abnormally accumulate in photoaged skin.
Unlike the epidermis, which is made up of densely packed keratinocytes, the dermis is primarily made up of an acellular component, the ECM. Collagen fibers are an important component of the ECM, accounting for 75% of the skin's dry weight and giving it tensile strength and elasticity.
In human skin, type I collagen makes up 80 to 90% of total collagen, while type III makes up 8 to 12% and type V makes up <5%. The collagen bundles increase in size deeper in the dermis.
Elastic fibers are another fibrous element that make up the dermal ECM. The other components of the ECM are proteoglycans (PGs) and glycosaminoglycans (GAGs), which are amorphous and surround and embed the fibrous and cellular matrix elements of the dermis. Although they make up only 0.2% of the dry weight of the dermis, they absorb water up to 1,000 times their volume and have roles in regulating water binding and compressibility of the dermis.
Fibroblasts are dermal-resident cells and are differentiated from mesenchymal cells. They are responsible for the synthesis and degradation of fibrous and amorphous ECM proteins. Their function and microenvironment interactions are important for understanding the molecular mechanism behind dermal aging.
Collagen
Quantitative and structural changes in collagen fibers are the main modifications found in aged skin. Unlike those in young skin, which have abundant, densely packed and well-organized intact collagen fibres, collagen fibres in aged skin are fragmented and coarsely distributed. Increased collagen degradation and reduced collagen biosynthesis are both involved in this aberrant collagen homeostasis, resulting in a net collagen deficiency, a process seen as skin wrinkles and skin loss of elasticity, which can be seen in both naturally aged and photo-aged skin.
Reactive oxygen species (ROS) generated in the aging process induce transcription factors, including activator protein 1 (AP-1) and nuclear factor-κB (NF-κB). This activation increases matrix metalloproteinase (MMP) expression and inhibits transforming growth factor-β (TGF-β) signaling, leading to collagen fragmentation and impaired collagen biosynthesis. This hinders the mechanical interaction between fibroblasts and the extracellular matrix (ECM) and reduces the size of dermal fibroblasts. Aged fibroblasts produce a greater amount of ROS that further increases the expression of MMPs and inhibits TGF-β signaling, creating a positive feedback loop that accelerates dermal aging.
Quantitative and structural changes in collagen fibers are the main modifications found in aged skin. Unlike those in young skin, which have abundant, densely packed and well-organized intact collagen fibres, collagen fibres in aged skin are fragmented and coarsely distributed. Increased collagen degradation and reduced collagen biosynthesis are both involved in this aberrant collagen homeostasis, resulting in a net collagen deficiency, a process seen as skin wrinkles and skin loss of elasticity, which can be seen in both naturally aged and photo-aged skin.
Reactive oxygen species (ROS) generated in the aging process induce transcription factors, including activator protein 1 (AP-1) and nuclear factor-κB (NF-κB). This activation increases matrix metalloproteinase (MMP) expression and inhibits transforming growth factor-β (TGF-β) signaling, leading to collagen fragmentation and impaired collagen biosynthesis. This hinders the mechanical interaction between fibroblasts and the extracellular matrix (ECM) and reduces the size of dermal fibroblasts. Aged fibroblasts produce a greater amount of ROS that further increases the expression of MMPs and inhibits TGF-β signaling, creating a positive feedback loop that accelerates dermal aging.
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Changes in glycosaminoglycans
Glycosaminoglycans (GAGs) are large linear polysaccharides and are an important component of the ECM. There are six types of GAGs, including chondroitin sulfate (CS), dermatan sulfate (DS), keratan sulfate (KS), heparan sulfate (HS), heparin (HP) and hyaluronic acid (HA). Apart from HA, GAGs contain sulfate substituents at various positions on the chain and are highly glycosylated on their related proteoglycan (PG) core proteins. HA is not sulfated and does not bind to proteins to form proteoglycans. Instead, it binds to proteins containing the HA-binding domain. Because GAG chains contain several negatively charged carboxyl and sulfate groups, they may have important roles in maintaining water content in tissues. Cross-linking of HA with matrix proteins such as the collagen network results in the formation of supermolecular structures and increases tissue stiffness.
Dermal HA is mainly produced by fibroblasts and is abundant in the papillary dermis. It crosslinks with other ECM proteins, such as collagen, resulting in increased tissue stiffness and plays space-filling and shock-absorbing roles. In intrinsically aged skin, HA-binding proteins (HABPs) are reduced compared to young skin, while the level of HA itself is not significantly different between young and old skin. HABPs are known to trigger several intracellular signaling pathways that regulate proliferation, migration and differentiation. In contrast, dermal HA content in photoaged skin is significantly increased, especially in areas of solar elastosis.
Although UV irradiation induces HA synthase (HAS), HAS mRNA levels in aged sun-exposed skin were significantly reduced compared to those in sun-protected skin. Like solar elastosis, increased HA in photo-aged skin may be the result of abnormal accumulation of non-functional proteins.
Ambient particulate matter (PM) is a major contributor to skin damage associated with air pollution. PM in combination with UVA irradiation elevates the levels of inflammatory cytokines, IL-1β and IL-6, which increases the transcription of MMPs and thus downregulation of type I collagen. Air pollution and UVA radiation together trigger skin cell aging.
The expression of type VII collagen in keratinocytes decreases in UV-irradiated skin areas. Type VII collagen is the anchoring fibrils of the dermal-epidermal junction, abbreviated DEJ. The decrease in its production contributes to wrinkles due to the weakened connection between the dermis and epidermis. Collagen type I decreases in photoaged skin due to increased collagen degradation. Various matrix metalloproteinases (MMPs), serine proteases and other proteases participate in this degradation activity.
Enzyme-inhibiting peptides, which can reduce the activity of enzymes, are one of the keys to preventing skin ageing. Peptides are 10 times more sustainable bioactives than others and are therefore a key ingredient in many of Tromborg's anti-aging products.
The illustration shows the changes in fibroblasts, collagen and elastic fibers in the dermal decline process.
Cellular ageing includes:
1. Downregulation of genes encoding mitochondrial proteins
2. Downregulation of the protein synthesis machinery
3. Dysregulation of immune system genes
4. Reduction of growth factor signaling
5. Constitutive responses to stress and DNA damage
The most notable microstructural changes of the dermis that occur with age are a decrease in collagen and elastin content, an increase in collagen cross-links (fragmented collagen) and a decrease in the amount of proteoglycans.
Skin ageing is caused by both the passage of time (genotypic ageing, intrinsic) and cumulative exposure to external factors, such as UV light, in particular in photo-exposed areas of the skin. The extent of the resulting microstructural changes in the dermis exposed to UV light is exacerbated in photoexposed skin and depends on how early the skin is exposed to UV light and for how long.
As the skin ages, ECM integrity decreases in terms of turnover and dynamics. This happens through the accumulation of damage from collagen fragmentation, oxidation, glycosylation, cross-linking and the accumulation of protein aggregates.
Decline in ECM biosynthesis is a self-reinforcing downward spiral and causes a decline in collagen production during ageing. Collagen mass continuously decreases at a rate of 1% per year in our skin - best seen in wrinkles and sagging skin. In general, during aging there is a progressive decrease in ECM biosynthesis, accompanied by an increase in ECM degradation. Decrease in ECM biosynthesis and increase in ECM degradation accelerates skin ageing.
1. Downregulation of genes encoding mitochondrial proteins
2. Downregulation of the protein synthesis machinery
3. Dysregulation of immune system genes
4. Reduction of growth factor signaling
5. Constitutive responses to stress and DNA damage
The most notable microstructural changes of the dermis that occur with age are a decrease in collagen and elastin content, an increase in collagen cross-links (fragmented collagen) and a decrease in the amount of proteoglycans.
Skin ageing is caused by both the passage of time (genotypic ageing, intrinsic) and cumulative exposure to external factors, such as UV light, in particular in photo-exposed areas of the skin.
The extent of the resulting microstructural changes in the dermis exposed to UV light is exacerbated in photoexposed skin and depends on how early the skin is exposed to UV light and for how long.
As the skin ages, ECM integrity decreases in terms of turnover and dynamics. This happens through the accumulation of damage from collagen fragmentation, oxidation, glycosylation, cross-linking and the accumulation of protein aggregates.
Decline in ECM biosynthesis is a self-reinforcing downward spiral and causes a decline in collagen production during ageing. Collagen mass continuously decreases at a rate of 1% per year in our skin - best seen in wrinkles and sagging skin. In general, during aging there is a progressive decrease in ECM biosynthesis, accompanied by an increase in ECM degradation. Decrease in ECM biosynthesis and increase in ECM degradation accelerates skin ageing.
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The Maillard reaction is the main non-enzymatic glycosylation pathway for the formation of advanced glycosylation end products (AGEs). In simple terms, glucose interacts with lysine and arginine side chains of a protein (e.g. collagen) to form reversible Schiff base products within hours, which stabilize to ketoamine (Amadori products) within days. Within weeks to months, these products are converted in a series of chemical reactions into AGEs.
These AGEs are able to cross-link, making collagen structures less elastic and contributing to the progressive decline in ECM integrity with ageing. Aged ECMs are stiff and fragmented and lose their protective and mechanical functions that are important for cellular integrity.
The peptides in Tromborg's products provide long-term protection and slow down aging by activating collagen and ECM build-up, i.e. protecting against the progressive decline in ECM integrity.
ECM Matrisome
The matrisome consists of the core and associated matrisome. The core matrisome is the set of proteins synthesized and secreted by cells in the skin to form ECMs. The human genome encodes 44 collagen genes, 195 ECM glycoproteins (including fibronectins, laminins, etc.) and 35 proteoglycans that form the core matrisome. The "associated matrisome" consists of proteins that are secreted and either localise to the ECM or remodel the ECM.
The human matrisome is made up of 1.027 genes. These include proteins such as laminins; collagen types IV, XVII, nidogens and perlecan. All are involved in and essential for a young ECM and/or healthy ageing.
The matrisome consists of the core and associated matrisome. The core matrisome is the set of proteins synthesized and secreted by cells in the skin to form ECMs. The human genome encodes 44 collagen genes, 195 ECM glycoproteins (including fibronectins, laminins, etc.) and 35 proteoglycans that form the core matrisome.
The "associated matrisome" consists of proteins that are secreted and either localise to the ECM or remodel the ECM. The human matrisome is made up of 1.027 genes. These include proteins such as laminins; collagen types IV, XVII, nidogens and perlecan. All are involved in and essential for a young ECM and/or healthy ageing.
The dermal-epidermal junction (DEJ) is a physical and biological interface between the epidermis and dermis. In addition to providing structural integrity, the DEJ also acts as a passageway for molecular transportation. DEJ is one of the first things to affect skin ageing. Peptides act on basement membrane (BM) and DEJ. The peptides and peptide composition in Tromborg's products stimulate protein expression of collagen XVII, laminin and nidogen as well as significantly increased dermal collagen expression as well as expression of collagen XVII. The laminin protein stimulated is on the dermal side of DEJ and Tromborg's peptide complex improves the structural properties of DEJ through its ability to stimulate BM proteins.
The dermal-epidermal junction (DEJ) is a physical and biological interface between the epidermis and dermis. In addition to providing structural integrity, the DEJ also acts as a passageway for molecular transportation. DEJ is one of the first things to affect skin ageing. Peptides act on basement membrane (BM) and DEJ.
The peptides and peptide composition in Tromborg's products stimulate protein expression of collagen XVII, laminin and nidogen as well as significantly increased dermal collagen expression as well as expression of collagen XVII. The laminin protein stimulated is on the dermal side of DEJ and Tromborg's peptide complex improves the structural properties of DEJ through its ability to stimulate BM proteins.
Molecular structure of DEJ with skin in balance
DEJ's primary structural elements consist of two polymeric networks of laminin and collagen IV, which are primarily interconnected with nidogen and perlecan. Both collagen IV and laminin are trimeric proteins that self-assemble into independent networks. Laminin is the substance that holds the DEJ together, while collagen IV, the most abundant basement membrane component, is responsible for its tensile strength.
Laminin is the basic building block for the initial formation of the basement membrane. All laminins are constructed as heterotrimeric proteins consisting of an α, β and γ chain, assembled in an αβγ heterotrimer. Laminin-332 contains three subunits, α3, β3 and γ2.
The connected molecules that are central to DEJ integrity:
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Nidogen isoforms 1 and 2 connect elements between the laminin and collagen IV networks, facilitating DEJ formation and stabilization.
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Perlecan is an important proteoglycan of the basement membrane and other ECM structures and consists of an elongated core protein. Perlecan has a structural bridging function in the basement membrane and helps to assemble key components within molecular superstructures.
DEJ's primary structural elements consist of two polymeric networks of laminin and collagen IV, which are primarily interconnected with nidogen and perlecan. Both collagen IV and laminin are trimeric proteins that self-assemble into independent networks. Laminin is the substance that holds the DEJ together, while collagen IV, the most abundant basement membrane component, is responsible for its tensile strength.
Laminin is the basic building block for the initial formation of the basement membrane. All laminins are constructed as heterotrimeric proteins consisting of an α, β and γ chain, assembled in an αβγ heterotrimer. Laminin-332 contains three subunits, α3, β3 and γ2.
The connected molecules that are central to DEJ integrity:
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Nidogen isoforms 1 and 2 connect elements between the laminin and collagen IV networks, facilitating DEJ formation and stabilization.
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Perlecan is an important proteoglycan of the basement membrane and other ECM structures and consists of an elongated core protein. Perlecan has a structural bridging function in the basement membrane and helps to assemble key components within molecular superstructures.
The dermal-epidermal junction not only separates the epidermal and dermal layers of the skin, but is also a biochemical interface between the epidermis and dermis. A unique histological feature of DEJ is a wave-like structure formed by both epidermal projections into the dermis (rete ridges) and dermal projections into the epidermis (dermal papillae). In aged skin, thinning and flattening of the DEJ structure occurs, resulting in a decreased attachment of the epidermis to the dermis, which decreases mechanical stability and structural integrity of the DEJ.
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The interface area through which nutrients and signaling molecules move is also reduced. This impairment is due to elevated expressions of extracellular matrix-degrading enzymes, including matrix metalloproteinases (MMPs). These structural changes and consequent decrease in molecular exchanges between the epidermis and dermis result in various age-related changes, such as impairment of epidermal permeability barrier functions and dry skin.
In addition to the structural weakening of DEJ, age-related reductions of the basement membrane proteins comprising DEJ, including collagen I, collagen IV, collagen VII, collagen XVII, nidogen, integrin β4 and laminin-332. Collagen VII and Collagen IV, which are produced by both keratinocytes and fibroblasts, are key components in anchoring fibrils that provide mechanical support to the keratinocytes.
Collagen XVII is another structural component of the anchoring filaments, which plays important roles in the assembly and function of the cell-matrix adhesion structure, as well as roles in transmembrane signal transduction and keratinocyte differentiation. Laminins, as the most abundant glycoproteins in the basement membrane extracellular matrix, also play essential roles in supporting tissue architecture and stability. Laminins assemble with perlecan and nidogen in the extracellular matrix as a bridging protein between collagen VI and laminin to form a basement membrane.
Most DEJ components are altered during aging, including reduced expression levels of laminin-332, integrin β4, collagen IV, collagen VII, collagen XVII in the skin of older people. A clear decreased expression of epidermal perlecan occurs during skin ageing at both protein and mRNA levels. Collagen XVII expression is greatly reduced during aging, causing altered hemidesmosomes and leading to keratinocytes detaching from the DEJ and shifting the skin layers upward.
Change of DEJ during skin aging
DEJ is at the core of communication between the dermis and epidermis and performs a key biomechanical function in maintaining epidermal homeostasis. As a result of ageing, skin becomes more fragile, less resistant to shear forces and more vulnerable to damage.
With aging skin, there is a leveling of the DEJ by approximately 35% in the form of progressive loss and flattening of the epidermal rete ridges and dermal papillary projections. The rete ridge height decreases with age and the number of papillae per area decreases over time. The changes observed during aging vary according to body sites: dermal papillae are uniformly distributed in photoprotected skin, but severely reduced and non-uniformly distributed in facial skin.
DEJ is at the core of communication between the dermis and epidermis and performs a key biomechanical function in maintaining epidermal homeostasis. As a result of ageing, skin becomes more fragile, less resistant to shear forces and more vulnerable to damage. With aging skin, there is a leveling of the DEJ by approximately 35% in the form
of progressive loss and flattening of the epidermal rete ridges and dermal papillary projections. The rete ridge height decreases with age and the number of papillae per area decreases over time. The changes observed during aging vary according to body sites: dermal papillae are uniformly distributed in photoprotected skin, but severely reduced and non-uniformly distributed in facial skin.
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TGF-β is reduced in old fibroblasts, leading to reduced collagen IV synthesis. In addition, collagen IV undergoes extensive proteolysis by MMP-10 and -7, decreasing the stability of its many molecular interactions. Age-related TGF-β signaling depletion, as well as its effect on collagen VII expression, directly weakens the epidermal anchoring structures. The defect in collagen VII expression is exacerbated in photoexposed skin due to the proteolytic activity of MMP-8 and -13. Age-related changes and environmental stress cause changes in the balance between degradation and synthesis of proteins. Overexpression and activation of MMPs during aging can not only degrade collagen and elastin fibers in the dermis, but also target DEJ components, disrupting their mechanical properties that regulate the stiffness of the surrounding skin microenvironment.
Change in DEJ composition and the loss of laminin-332 expression during aging causes a weakening of the epidermal differentiation process. As laminins play an important role in basement membrane stability and are the components that hold the skin layers together, the loss of laminin-332 during skin ageing affects the stiffness of the ECM.
Based on DEJ's important roles in skin homeostasis and ageing, stimulating the basement membrane containing proteins is one way to reduce skin wrinkles. The stimulation of collagen IV and other BM proteins using a 6 and 11 double peptide complex composed in Tromborg of di-, tri-, tetrapeptides and others has proven effective in improving skin wrinkles.
Illustration depicting the molecular structure and functions of extracellular macromolecules at DEJ.
DEJ is characterised by an undulating pattern derived from epidermal rete ridges, which are downgrowths of epidermis within the papillary dermis, These ridges indent the epidermal layer and significantly increase the surface area of DEJ, strengthening the dermal-epidermal connection and keeping the dermis and epidermal layer well connected.
DEJ is characterised by an undulating pattern derived from epidermal rete ridges, which are downgrowths of epidermis within the papillary dermis.
These ridges indent the epidermal layer and significantly increase the surface area of DEJ, strengthening the dermal-epidermal connection and keeping the dermis and epidermal layer well connected.
Reduced transforming growth factor-β signaling during aging.
Transforming growth factor-β (TGF-β) is an important regulator of ECM biosynthesis. In dermal fibroblasts, TGF-β controls collagen homeostasis by regulating both collagen production and degradation via the Smad pathway. Thus, ECM collagens are directly upregulated by TGF-β/Smad signaling. In contrast, MMPs are downregulated and TIMPs are upregulated by the Smad signaling network. The TGF-β/Smad signaling pathway is essential for maintaining the structural and mechanical integrity of the dermis by increasing ECM production and inhibiting ECM degradation.
In aged skin, AP-1 induced by ROS inhibits the TGF-β signaling pathway in dermal fibroblasts. Impaired TGF-β signaling leads to decreased neocollagen synthesis and results in a reduction in the amount of net collagen in the dermis. The production of type I procollagen in aged human skin decreases due to downregulation of TGF-β/Smad signaling.
Reduced transforming growth factor-β signaling during aging.
Transforming growth factor-β (TGF-β) is an important regulator of ECM biosynthesis. In dermal fibroblasts, TGF-β controls collagen homeostasis by regulating both collagen production and degradation via the Smad pathway. Thus, ECM collagens are directly upregulated by TGF-β/Smad signaling. In contrast, MMPs are downregulated and TIMPs are upregulated by the Smad signaling network.
The TGF-β/Smad signaling pathway is essential for maintaining the structural and mechanical integrity of the dermis by increasing ECM production and inhibiting ECM degradation.
In aged skin, AP-1 induced by ROS inhibits the TGF-β signaling pathway in dermal fibroblasts. Impaired TGF-β signaling leads to decreased neocollagen synthesis and results in a reduction in the amount of net collagen in the dermis. The production of type I procollagen in aged human skin decreases due to downregulation of TGF-β/Smad signaling.
Interaction between fibroblasts and ECM
In young skin, fibroblasts adhere to the surrounding intact ECM, which is mainly composed of type I collagen. This adhesion allows fibroblasts to exert mechanical forces on the surrounding ECM and spread and maintain an elongated shape. In aged skin, fibroblast adhesion is impaired due to progressive ECM degradation, resulting in fibroblast size reduction, decreased elongation and collapsed morphology. Reduced size is a key feature of biological ageing of fibroblasts and is correlated with decreased production of ECM components. The reduction of dermal fibroblast proliferation and cell size also increases mitochondrial ROS generation. Reduced fibroblast size and mechanical forces largely mediate the reduction in TGF-β-regulated ECM production. Furthermore, the reduction in fibroblast size regulates ECM degradation through elevated MMP levels.
ROS generated in the aging process increases MMP expression and inhibits TGF-β signaling, leading to collagen fragmentation and impaired collagen biosynthesis. This hinders the mechanical interaction between fibroblasts and the ECM, and consequently leads to a reduction in the size of dermal fibroblasts. The aged fibroblasts produce more ROS, which further increases the expression of MMPs and inhibits TGF-β signaling, creating a positive feedback loop that accelerates dermal aging. Fibroblasts in aged skin maintain their appearance and function when in contact with intact ECM, suggesting that the ECM microenvironment is an important factor in fibroblast aging.
In young skin, fibroblasts adhere to the surrounding intact ECM, which is mainly composed of type I collagen. This adhesion allows fibroblasts to exert mechanical forces on the surrounding ECM and spread and maintain an elongated shape. In aged skin, fibroblast adhesion is impaired due to progressive ECM degradation, resulting in fibroblast size reduction, decreased elongation and collapsed morphology. Reduced size is a key feature of biological ageing of fibroblasts and is correlated with decreased production of ECM components. The reduction of dermal fibroblast proliferation and cell size also increases mitochondrial ROS generation. Reduced fibroblast size and mechanical forces largely mediate the reduction in TGF-β-regulated ECM production.
Furthermore, the reduction in fibroblast size regulates ECM degradation through elevated MMP levels.
ROS generated in the aging process increases MMP expression and inhibits TGF-β signaling, leading to collagen fragmentation and impaired collagen biosynthesis. This hinders the mechanical interaction between fibroblasts and the ECM, and consequently leads to a reduction in the size of dermal fibroblasts. The aged fibroblasts produce more ROS, which further increases the expression of MMPs and inhibits TGF-β signaling, creating a positive feedback loop that accelerates dermal aging. Fibroblasts in aged skin maintain their appearance and function when in contact with intact ECM, suggesting that the ECM microenvironment is an important factor in fibroblast aging.
Read about key cytokines
Key cytokines (cell messengers) in aging skin
TNF-α, a key cytokine in proinflammatory processes in the skin, inhibits collagen synthesis and induces MMP-9 elevation in chronological aging and photoaging processes, proinflammatory response.
IL-1α production in the skin decreases with age, while IL-1α, IL-1 and IL-6 induce the activation of key transcription factors in aged skin.
In aged female skin after menopause, IL-6 expression is increased and associated with wrinkle formation. IL-6 levels are increased by exposure to UV radiation.
Chronic exposure of human skin to solar ultraviolet (UV) irradiation causes premature skin aging, which is characterised by reduced type I collagen production and increased fragmentation of the dermal collagenous extracellular matrix. This imbalance of collagen homeostasis is mediated in part by elevated expression of the matricellular protein cysteine-rich protein 61 (CCN1) in dermal fibroblasts, the primary collagen-producing cell type in human skin. CCN1 is mediated by the induction of interleukin 1β (IL-1β). CCN1 and IL-1β are strikingly induced by acute UV irradiation and constitutively elevated in sun-exposed prematurely aged human skin. Elevated CCN1 rapidly induces IL-1β, inhibits type I collagen production and upregulates matrix metalloproteinase-1, which degrades collagen fibrils. Blocking IL-1β effects by IL-1 receptor antagonist largely prevents the deleterious effects of CCN1 on collagen homeostasis. Furthermore, knockdown of CCN1 significantly reduces the induction of IL-1β by UV irradiation, thereby partially preventing collagen loss. Elevated CCN1 promotes inflammation and collagen loss via induction of IL-1β, thereby contributing to premature aging in chronically sun-exposed skin.
In other words, the CCN1/CYR61 protein stimulates the production of MMP-1, thereby contributing to significant type I collagen degradation in the dermis. Furthermore, CCN1 downregulates the TGF-β type II receptor, inhibiting TGF-β signaling, which is essential for maintaining ECM homeostasis.
Reactive oxygen species (ROS) are a major driver of the increase in MMP levels in aged skin. ROS are generated in the skin from both external and internal sources, such as ultraviolet irradiation and metabolically generated pro-oxidants. Activator protein 1 (AP-1), which plays an essential role in the transcriptional regulation of MMP-1, MMP-3, MMP-9 and MMP-12. Nuclear factor-κB (NF-κB) is another transcription factor activated by ROS. In general, oxidative damage is more evident in photoaged skin, manifesting as aging features such as deep wrinkles. While the primary source of MMPs in intrinsic ageing is dermal fibroblasts, MMPs in photoaging are also produced by epidermal keratinocytes.
The increased binding of NF-κB to DNA in the nucleus is one of the main hallmarks of ageing. Various biochemical pathways have been described to influence human longevity. However, NF-κB emerges as the central factor in which all these pathways finally converge, as the signals that promote aging activate NF-κB and those that promote longevity inhibit the activation of this pathway. In fact, NF-κB is a critical transcription factor.
The effect of UV on the skin is due to the production of ROS. Excess free radicals activate the NF-κB signaling pathway and the MAPK signaling pathway, contributing to the activation of AP-1 and NF-κB. This then increases the level of TNF-α and the expression of MMPs, which induces the breakdown of the ECM and accelerates skin ageing.
MMPs are a superfamily of zinc-containing metalloproteinases that have the capacity to degrade the ECM molecules that comprise the dermal connective tissue of the skin. Notably, induction of AP-1 is elevated in MMP1 (collagenase), MMP3 (stromelysin-1) and MMP9 (92 kDa gelatinase), resulting in the degradation of ECM components of the skin in vivo. The combined effects of MMP1, MMP3 and MMP9 degrade type-I and type-III dermal collagen into fragmented, disorganized fibrils. These degraded products downregulate collagen synthesis, suggesting a negative feedback loop in collagen synthesis via collagen degradation. Furthermore, AP-1 activated by the MAPK pathway induces Smad7, which blocks Smad2/3 via the transforming growth factor beta (TGF-β) receptor, thereby regulating TGF-β signaling, inhibiting collagen production by dermal fibroblasts and reducing collagen density.
MMPs are a superfamily of zinc-containing metalloproteinases that have the capacity to degrade the ECM molecules that comprise the dermal connective tissue of the skin. Notably, induction of AP-1 is elevated in MMP1 (collagenase), MMP3 (stromelysin-1) and MMP9 (92 kDa gelatinase), resulting in the degradation of ECM components of the skin in vivo. The combined effects of MMP1, MMP3 and MMP9 degrade type-I and type-III dermal collagen into fragmented, disorganized fibrils.
These degraded products downregulate collagen synthesis, suggesting a negative feedback loop in collagen synthesis via collagen degradation. Furthermore, AP-1 activated by the MAPK pathway induces Smad7, which blocks Smad2/3 via the transforming growth factor beta (TGF-β) receptor, thereby regulating TGF-β signaling, inhibiting collagen production by dermal fibroblasts and reducing collagen density.
Read more about MMP-levels
Increased Matrix Metalloproteinase (MMP) levels
MMPs are a family of ubiquitous endopeptidases that can degrade ECM proteins. MMPs can be categorized into five main subgroups, namely: (1) collagenases (MMP-1, MMP-8 and MMP-13); (2) gelatinases (MMP-2 and MMP-9); (3) stromelysins (MMP-3, MMP-10 and MMP-11); (4) matrilysins (MMP-7 and MMP-26); and (5) membrane-type (MT) MMPs (MMP-14, MMP-15 and MMP-16) MMP-1 is the main protease that initiates fragmentation of collagen fibers, which are predominantly types I and III in human skin. After cleavage by MMP-1, collagen can be further degraded by MMP-3 and MMP-9. In the skin, the major source of MMPs is epidermal keratinocytes and dermal fibroblasts, although MMPs can also be produced by endothelial cells and immunocytes.
Physiologically, MMPs are regulated by the specific endogenous tissue inhibitors of metalloproteinases (TIMPs), which make up a family of four protease inhibitors: TIMP-1, TIMP-2, TIMP-3 and TIMP-4.
Levels of MMP-1, MMP-2, MMP-3, MMP-9, MMP-10, MMP-11, MMP-13, MMP-17, MMP-26 and MMP-27 are elevated in aged human skin. MMPs and TIMPs are often regulated in coordination to control excess MMP activity. However, elevated MMP levels in aged skin are not accompanied by a corresponding increase in the levels of endogenous MMP inhibitors. The amount of TIMP-1 in photoaged and intrinsically aged skin may even be reduced. This imbalance accelerates progressive collagen fragmentation in the dermis and accelerates skin ageing.
UVB has been shown to result in DNA damage or DNA photoproducts in the skin that trigger the signaling pathways associated with the onset of aging. UVB exposure induces dose-dependent cyclobutane pyrimidine dimers in human dermal fibroblasts. PM-induced oxidative stress results in lipid, protein and DNA damage. DNA Protection Cream from Tromborg reduces dimer formation in UV stressed skin so that DNA remains undamaged and thereby retains its super-coiled structure.
UVB has been shown to result in DNA damage or DNA photoproducts in the skin that trigger the signaling pathways associated with the onset of aging. UVB exposure induces dose-dependent cyclobutane pyrimidine dimers in human dermal fibroblasts.
PM-induced oxidative stress results in lipid, protein and DNA damage. DNA Protection Cream from Tromborg reduces dimer formation in UV stressed skin so that DNA remains undamaged and thereby retains its super-coiled structure.
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