Park, Chang Seo (CTO, LCS Biotech)
Types and functions of skin barrier ceramide
Ceramide is a major compositional lipid that accounts for more than 40–50% of the lipids of the skin barrier, and its importance is further emphasized as the main cause of the decrease in ceramide content in various skin diseases such as atopic dermatitis (1). Since 1985, when ceramides were first discovered by Wertz et al. in the human skin, the structure and function of the skin barrier described as "Brick and Mortar" by the P. Elias group was established in the early 1990s. From this point on, cosmetic companies began to use ceramide as a key ingredient for moisturizers (2). Ceramides were seven species at the time of their initial publication, and have been named Ceramides 1, 2, and 3, according to the sequence in which they were deployed on the TLC. Since then, new ceramides have been discovered along with the development of geological analysis technology, with at least 16 classes reported so far. Meanwhile, four types of sphingoids, sphingosine, phytosphingosine, sphinganine, 6-hydroxysphingosine, and the type of fatty acid that binds them, were introduced as a method of classifying ceramide (Table 1). For example, ceramide NP (Ceramide 3) is a combination of phytospingosine with nonhydroxy fatty acid, such as stearinic acid, and ceramide AP (Ceramide 6) is a combination of phytospingosine with a-hydroxy fatty acid.
There are several types of ceramides in the skin barrier of the human body, even within the 16 classes identified so far, resulting in more than 200 types of ceramides. This is because the fatty acids combined in each ceramide class range from C16 to more than C24 super-long fatty acids. The length of N-acylated fatty acids has drawn attention because of studies reporting that the length of fatty acids in ceramides separated from the skin of atopic patients has been shorter than that of normal skin (7, 8). Several researchers have since demonstrated a correlation between the length of fatty acids bound to ceramide and the permeability of the skin barrier (7). Thus, the functional study of dermal barrier ceramide is forced to consider the problem of its composition and length at the same time. Therefore, the latest ceramide research trends are focused on finding answers to ‘why human skin barriers need ceramides with various structures?’. For example, the functional differences between ceramide NP and NS, or the effects of NP and AP inter-ratio changes on lipid membrane structures (8, 13). The importance of ω-esterified ceramide, such as ceramide EOS, EOP, and EODS, is emphasized (11), as it is not only necessary for the formation of LPP (long periodicity phases) in the lipid membrane structure, but also contributes to the stabilization of multilayer lipid membranes with rivet-like effects. Meanwhile, 1-O-acylceramide, a newly discovered ceramide that combines fatty acids into the hydroxyl group of head groups, was first reported in 2013 and was named Ceramide ENS and ENDS (6). Although the role of multi-layer lipid membranes in skin barriers has yet to be identified, it is expected that the multi-layer lipid membranes will provide structural stability by capturing adjacent lipid membranes (Table 1 and 2).
Problems with the composition and length of the skin barrier ceramide
The permeable membrane function, which is the main function of skin barrier function, is closely correlated with the ceramide content. The presence is confirmed by various research reports. Imokawa et al. reported a general decrease in ceramide content in the lesions of atopic patients, especially ceramide 1 and 3 by 60% and 42%, respectively (1). However, recent studies have identified unusual increases or decreases depending on the type of ceramide. In other words, ceramide NP, NH, EOS, EOP, etc. are significantly reduced while ceramide NS, AP, etc. are found to increase in atopic skin, so each ceramide composition ratio is considered important for normal skin barrier function (3, 11). The composition of the skin barrier ceramide reported so far and the fatty acids combined here are shown in Figure 1. Ceramide NP, or phytosphingosin, is the most common form of ceramide combined with regular fatty acids, accounting for 22.1%, while ceramide NS is relatively low at 7.4%. The ceramide of the sphingosin system is thought to have a significant impact on the formation of lamellae phase, with 6.5% of the ceramide EOS. In terms of the length of fatty acids, C24-Cer is the most common at 16.3%. However, significant amounts of ceramides with short fatty acids such as 19.3% C16–C18–Cer and 10.4% C19–C23–Cer are also present. In the meantime, more than 60 percent of the ceramide's fatty acids made up of skin barrier were saturated fatty acids. However, it is noteworthy that ceramide, which has unsaturated fatty acids, is more than 20 percent.
The conclusion drawn from the latest findings is that in order for the intercellular multilayer lipid membrane responsible for the permeability barrier function of the skin barrier to be formed and maintained normally, the following three conditions must be met:
First, the optical isomeric structure of all ceramides should be of type D-erythro. The interaction between cholesterol and ceramide, which is important in the formation of multilayer lipid membranes while being present in the same level as ceramide, is known to be caused by hydrogen bonding between the amino group of ceramide and the 3-hydroxyl group of cholesterol (9).
Second, as previously described, it is an appropriate composition ratio among several classes of ceramides, including ceramide NP, NS, AP, AH, and EOS. This composition ratio represents an unusual composition ratio depending on the location of the skin. For example, it has been reported that the composition of ceramide in the face and arms is different (3)
Third, the length of the ceramide also varies significantly depending on the skin area. In the case of skin inside the arm with high skin barrier function, the rate of ceramide with ultra-long length is significantly higher than that of the face skin (3).
In conclusion, an ideal moisturizing formulation similar to human skin could be developed if a customized ceramide can be used with the same D-erythro type as human ceramide, as well as the appropriate ceramide composition ratio and fatty acid length for each skin region.
Ceramide present and future of the research and development
The development of ideal moisturizers, as described above, requires the reproduction of the ceramide present in the human skin barrier. However, it is not possible at the current level of technology and, in addition, it is difficult to obtain large quantities of C24 super-branch fatty acids from plant resources. Ceramide NP, made of phytospingosine, a yeast fermentation product that is currently widely used as a cosmetics material, was commercialized in the mid-1990s and is the most frequently used ceramide to date. The same type of human D-erythro ceramide currently commercialized and registered with INCI includes Ceramide NP, AP, NS, NDS, and EOP. However, they are not all ceramides made of single fatty acids and have different lengths, such as human skin. Evonik of Germany succeeded in improving phytospingosine production yeast into a strain that produced sphinganin and sphingosine (4, 5). Although sphinganine-based ceramides have been commercialized, no practical application of sphingosine-based ceramides has been reported. However, in the near future, sphingoid bases such as sphinganin, sphingosin, and even 6-hydroxy sphingosin will be produced in large quantities, allowing ceramide NP, NS, NDS and NH to be used as cosmetic materials.
For the sphingoid bases listed above, the production of various lengths of ceramide using C16–C26 fatty acids that bind to them is a separate problem, even if mass production is possible due to yeast fermentation. Recently, the manufacturing technology of ceramide NP with various lengths of fatty acids was developed by LCS Biotech and commercialized as EcoCeramideTM. It was confirmed that applying this to the skin improves the skin barrier function, the persistence of skin moisturization, and ultimately the constancy of the epidermis. This has been demonstrated (10) that ceramide NP complexes of different lengths similar to the human body have better skin barrier function than ceramide NP of single-length and molecules (10). Subsequent studies have shown that EcoCeramideTM has physiological activity that allows skin cells to biosynthesize super-long-chain ceramide on their own or facilitates key gene expression needed for NMF biosynthesis. However, conventional single-ceramide NPs have been found to have no such physiological activity (12). We confirm that simply giving length diversity to phytospingosine-based ceramide NP improves skin barrier function. Supposedly, the use of ceramide ENP, ENS, and ENDS, which are newly discovered 1-O-Aclyceramide, combined with other sphingoid bases such as sphingosine and sphinganine, will enable the development of ceramide-based moisturizers almost similar to the human skin. Currently, LCS Biotech participated in the 'Skin Science Applied Materials Leadership Technology Development Project' by the Ministry of Health and Welfare to carry out the 'Global Universal Materials Technology Development and Commercialization based on Human Body Uniform Ceramide' and its main goal is to produce ceramide.
Meanwhile, the Ministry of Commerce, Industry and Energy is participating in 'High-tech Core Technology Development Project' and is carrying out 'Development and Productization of Microbiological-Based Sphingo New Material Technology'. Its main goal is to commercialize the sphingosine-based ceramide through the improvement of sphingosine-producing yeast strains. In conjunction with the performance of the Ministry of Health and Welfare's project, it is expected that various ceramides of the same type of human body will be practical by solving the problem of the composition and length of ceramides listed above. In particular, it is the world's first development of ceramide ENPs, and it is believed that a new moisturizing theory of ceramide can be established once the proposed "Anchor bolt–like mode of action" is identified through molecular modeling, etc. (Figure 3).