It is a fact that skin changes when growing older, as a result of a lifetime of exposure to environmental agents, personal habits such as smoking and diet, and modifications that happen in the normal process of cellular aging. The skin starts appearing less smooth and tight than before, which can lead to visible sagginess, lines, and wrinkles, among other evidence.
What are skin sagginess inducers and what opposes them?
Several external factors are known to exacerbate the natural aging process like UV exposure or extreme temperatures, but gravity plays a particularly relevant role. As it is a constant force pulling downwards, the skin needs to be firm and elastic to counteract its pressure and stay in place. Although this attraction force cannot be avoided, the elements that provide skin firmness and elasticity can be certainly enhanced. This is the case with elastin and collagen, both key proteins of the dermal Extracellular Matrix (ECM) that provide the foundation for the skin, working in tandem.
Collagen cushions and supports the epidermis while elastin allows the skin to stretch and flex smoothly. Thus, they confer cohesion and elasticity to the skin, as well as the capacity to recoil after common facial movements like smiling, laughing, drinking, or crying.
Additionally, several intracellular elements help to reinforce these skin properties and oppose flaccidity. It is agreed that the functionally active form of both proteins is reduced with increasing age as well as their correct assembly, diminishing the skin's capacity to deal with deforming agents and avoid their visible consequences in facial look. Stimulating the adequate mechanisms that strengthen skin cohesion and tightness would reduce sagginess induced by gravity for instance.
What are the factors of skin aging?
Like in other organs of the body, the physiological functions and structures within the skin continuously decline with advancing age. Skin aging results from the deterioration of its structures and the slowing of its functions, caused by many factors and origins, which may be included into different categories: intrinsic, mechanical, and extrinsic aging.
Biological or intrinsic aging is the result of often genetically-determined changes that occur naturally within the body from the mid-20s onwards, despite their later evident effects. The biological clock or chronological age determined by genetics also applies to the skin, which gradually loses its ability to function as it once did. This deterioration occurs due to a gradual shift in the balance of certain messenger molecules excreted within the body that lead to natural changes manifesting in outward signs of aging.
Understood as the consequence of continually repeated muscle movements like smiling or frowning, mechanical aging also contributes to skin deterioration day by day, exacerbating expression lines.
Environmental or extrinsic aging takes place due to daily exposure to external sources in the environment: ultraviolet rays, pollution, smoke, harsh weather, gravity, and external stress. These agents limit the ability of cells to function properly and alter the integrity of overall cell composition. Years of accumulated environmental stress on skin cellular structures translate into premature aging. UV damage from sun exposure accounts for 90% of premature skin aging. The damage to skin components caused by both prolonged and incidental sun exposure is called photoaging, which is a process that occurs over a period of years (the effects are cumulative) and can lead the skin to lose its repairing ability.
All aging types cause alterations in the skin which include a slower cellular turnover, reduced collagen production, elastin disturbances, and skin thinning. Additionally, UV radiation disturbs melanocytes and the moisture barrier and accelerates collagen and elastin loss as well as their fibers breakdown in the skin.
The alteration of collagen and elastin is controlled by the activity of Matrix Metalloproteinase (MMP) enzymes known as collagenase and elastase, respectively, both of them being activated by UV radiation. Long-term elevation of the MMPs results in disorganized and clumped collagen and elastin, which is characteristic of photodamaged skin. Thus, there are differences between intrinsic and extrinsic aging effects on the skin, and obviously versus young skin.
If we look at intrinsic or chronologically aged skin, without environmental influences, it is smooth, generally unblemished, and with some exaggerated expression lines, but the skin is well preserved in general, despite an inner flattening of the epidermal-dermal interface and some disruption of the dermal tissue. In direct contrast, extrinsically-aged skin (mainly the face, hands, and chest) presents wrinkles, hyper/hypopigmentation, sallow areas, increased fragility, roughness, and a loss of tonicity and elasticity, due to more fragmented and thick collagen and elastin.
Elastin fibers are present in adult skin in various stages of maturity, forming a distinctive arrangement within the papillary (outer) and reticular (inner) dermis. The least mature fibers course perpendicularly from the dermal-epidermal junction to the top of the reticular dermis (oxytalan fibers), whereas more mature fibers containing added deposited elastin are horizontally arranged (elaunin fibers). Both elastin fibers are connected as oxytalan merges from elaunin fibers. The most mature fibers are found deeper in the reticular dermis.
Changes in elastin fibers are so distinctive in photoaged skin that the condition known as elastosis is considered one of its hallmarks. This describes an accumulation of amorphous elastin protein and a breakdown in the typical structural layout, which results in reduced skin elasticity and tensile strength. This phenomenon accounts for why more photo-exposed skin takes longer to assume its original position when extended or pulled.
All aging types alter elastin fibers, but in different ways. Biologically, our body naturally diminishes elastin production within fibroblasts as we age, so fewer fibers are created and the skin loses resilience. Environmentally, UV rays can penetrate skin layers to damage elastin-producing fibroblasts. Also, as skin cell renewal decreases, the skin thins becoming more susceptible to environmental damage. Finally, mechanical stress can permanently stretch out elastin fibers. Aging alters skin structure and inner components like elastin and collagen, which need to be enhanced to reduce skin flaccidity.
Which are the key elements combating skin flaccidity?
In order to protect skin from external and internal agents that reduce its capacity to stay firm, it is first important to know the current mechanisms and elements that manage to do so, preserving skin elasticity and firmness until the passing years reduce this intrinsic capacity. Although elastin and collagen are the most well-known proteins that provide firmness, other elements located in the ECM and inside skin cells contribute to such important tasks. Altogether, they capacitate the skin to be firm and elastic at the same time, avoiding sagginess.
With an estimated molecular mass of 64-66 kDa, elastin is a protein found in any elastic connective tissue and, in the skin, it is mainly located in the dermis, responsible for cutaneous critical properties. The unique gene responsible for elastin production is mainly expressed before birth and in the first years of life, but it is substantially turned down with the passing years, so most of the elastin found in adults comes from this initial production.
Although elastin has proved to be the longest-lasting protein in the body with a half-life of 70-74 years approximately, its slower adult production may lead to a non-complete repair when the passing years damage it, a fact that implies a reduction of skin elasticity. Elastin and microfibrils are the two components of the elastic fibers, which represent the largest structure of the ECM. The major and core component is elastin, which is formed in the process of elastogenesis through the assembly and cross-linking of its precursor protein known as tropoelastin.
In fibroblasts, the expression of the elastin gene results in the intracellular formation of soluble tropoelastin monomers of 60-70 kDa, which may be different in length due to undergoing alternative splicing, and their secretion to the cell surface. Mature tropoelastin monomers are able to self-assemble and aggregate by coacervation, which implies tropoelastin is more concentrated and aligned for subsequent cross-linking.
Coacervated tropoelastin (micro-assembly) is then deposited onto long linear microfibrils (10-15 nm) of the ECM (macro-assembly), which serve as a scaffold to guide cross-linking. In tissues, microfibrils form packed parallel bundles close to the cell surface and their main structural elements are fibrillins, which are large glycoproteins (by 350 kDa) that form their backbone.
Being a product of fibroblasts and keratinocytes, fibrillin-1 appears as the principal component of these microfibrils in adults. It is agreed that specific elements are needed for this last step of the elastin fiber formation to occur properly, Fibulin 5 (FBLN5/DANCE) and Lysyl Oxidase-Like 1 (LOXL1) being key players for a proper assembly.
FBLN5 and LOXL1 role
Fibulins are a family of ECM glycoproteins between 50-200 kDa that are associated with the stabilization of structures like elastic fibers, binding to tropoelastin with different affinities. FBLN5 (66 kDa) is one of the five members of this family and it is thought to be essential in 7 elastin fiber organization as it is colocalized with such fibers, its overexpression increases their assembly and its decrease and absence cause their defective development, and disorganization, as it happens when aging.
This glycoprotein is recognized as a bridge molecule because it binds not only to tropoelastin but also to LOXL1, fibrillin-1, and integrins, all of their necessary components for the adequate assembly of elastin fibers. LOXL1 is one of the members of the lysyl oxidase family (LOX), which comprises LOX and LOX-like proteins from 1-4.
Secreted to the ECM, this enzyme catalyzes the formation of covalent cross-links between two adjacent tropoelastin molecules, ensuring spatially defined deposition of elastin and originating the insoluble elastin polymer. Thus, it is found in sites of elastogenesis, where FBLN5 could be responsible for its binding and activation as well. As it occurs with FBLN5, LOXL1 levels decrease with advanced age.
Apart from binding to FBLN5, fibrillin-1 from the microfibrils also binds to integrins, which are transmembrane receptors that externally bind to the ECM and internally to the contractile cytoskeleton. Therefore, the complex forming the mature elastin fibers gets linked to cells due to FBLN5 and to fibrillin-1 as both elements connect with this family of receptors, which in turn join other important elements for skin firmness like type I, IV, and VI collagen.
Collagen cohesive function
When the term collagen appears, it is usually applied to type I collagen, which is the most abundant protein in the ECM and in the human body. Actually, this type is the principal collagen in the skin, first found as procollagen before it is cleaved and assembled into collagen fibril polymers first and then aggregated into larger bundles as collagen fibers.
It offers the major platform for cell attachment and anchorage to macromolecules, providing cutaneous structural support. Conversely, type IV collagen is a specific non-fibrillar type found in the basement membrane, which serves as a structural barrier and substrate for cellular interactions.
Additionally, type IV collagen is able to join type I and VI collagen, among other compounds, and form supramolecular networks that bind to collagen fibrils and elastic fibers as well, providing cohesion to the fibrillar components of the dermis and influencing cellular adhesion and migration, all of them fundamental for the integrity and function of membrane basements under mechanical demands. Furthermore, it seems that type VI collagen is assembled into microfibrils distributed in elastic and non-elastic tissues, where the microfibrils function as essential structural elements.
Type XIV collagen is localized near the surface of collagen fibrils, regulating the fibrillogenesis process in sites with high mechanical demand, like the skin, its alteration leading to skin laxity and flaccidity. Moreover, it is thought that this collagen type is associated with type I collagen fibrils and also with cellular adhesion mechanisms.
Cellular adhesion to the ECM can occur due to cell surface integrins that get to link intra- and extra-cellular components via multiprotein complexes, called Focal Adhesions (FAs). This bond needs the coordinated binding of integrins receptors to adhesive domains in ECM ligands but also FAs assembly and their interactions with the cytoskeleton of actin (a key protein in cellular movement, contraction, and shape maintenance). Thus, FAs act as an interface between the actin cytoskeleton and the ECM compounds.
Upon cellular adhesion, one of the numerous structural elements of the FAs, known as talin, rapidly accumulates in focal contacts and is able to directly bind to integrins. Talin is a high-molecular-weight protein with binding sites for actin but also for vinculin, another protein that stabilizes cell-cell and cell-matrix junctions.
Tensile and mechanical forces acting on talin activate its union with vinculin, enhance FA assembly and increase the strength of the linkage between integrins and the actin cytoskeleton. Mechanical forces also induce the recruitment of zyxin, a protein that facilitates actin filament assembly and may be involved in adhesion-stimulated changes in gene expression and the cytoskeletal organization of actin bundles, which translates into the reinforcement of the final binding function of the FAs. Actinin is a necessary protein as it links the actin filaments to zyxin, associating these filaments with the membrane.
In conclusion, FBLN5 and LOXL1 are key components for maintaining elastin fibers properly assembled and functioning. Together with collagen molecules and elements of the FAs like talin and zyxin, they are responsible for the firmness and resistance of the skin.