: Naturally occurring peptides found in the human skin can serve particular biological activities and play roles as signaling molecules of various physiological processes such as homeostasis, growth, defense or immunity. Their great biological activity resulted in a growing interest in the pharmaceutical industry. Researchers consider peptides either promising compounds with potential application for human diagnosis, therapy or cosmetics. Peptides are becoming interesting cosmetic ingredients with the functions to reduce premature skin aging, improve the barrier function of skin, moisturize the skin, protect it from UV damage, and anti-inflammatory properties that alleviate acne and irritation. Till now, peptides of different origins were investigated in formulation developed to enhance collagen or elastin production, increase fibroblast proliferation, improve wound healing or skin condition. Most of them are obtained by chemical synthesis or by partial digestion of animal proteins. Short and easily synthesized peptides with alternative amino acid sequence, and combinations have created a new field of molecules inspired by nature and implemented in the cosmetic industry. Nowadays, peptides are cheaper and easier to produce in large quantities. The efficient process development methods allow obtaining nearly unlimited sequences, which makes them functionally preferred. Generally, cosmetic peptides are categorized as carrier peptides, neurotransmitter- affecting peptides, enzyme inhibitor peptides and signal peptides. The use of signal peptides in cosmetics increased over a few years. These molecules trigger a signaling cascade and stimulate fibroblast collagen production, the proliferation of elastin, fibronectin, laminin, etc. Thus, a literature search on a topical application of the most common signal peptides; and their current status in the cosmetic industry was carried out.

Peptides have revealed a large range of biological activities with high selectivity and efficiency for the development of new drugs, including neuroprotective agents. Therefore, this work investigates the neuroprotective properties of naturally occurring peptides, endomorphin-1 (EM-1), endomorphin-2 (EM-2), rubiscolin-5 (R-5), and rubiscolin-6 (R-6). We aimed at answering the question of whether well-known opioid peptides can counteract cell injury in a common in vitro model of Parkinson’s disease (PD). Antioxidant activity of these four peptides was evaluated by the 2-diphenyl-1-picrylhydrazyl radical (DPPH) scavenging activity, oxygen radical absorbance capacity (ORAC), and ferric-reducing antioxidant power (FRAP) assays, while neuroprotective effects were assessed in a neurotoxic model induced by 6-hydroxydopamine (6-OHDA) in a human neuroblastoma cell line (SH-SY5Y). The mechanisms associated with neuroprotection were investigated by the determination of mitochondrial membrane potential (MMP), reactive oxygen species (ROS) production, and Caspase-3 activity. Among the tested peptides, endomorphins significantly prevented neuronal death induced by 6-OHDA treatment, decreasing MMP (EM-1) or Caspase-3 activity (EM-2). Meanwhile, R-6 showed antioxidant potential by FRAP assay and exhibited the highest capacity to recover the neurotoxicity induced by 6-OHDA via attenuation of ROS levels and mitochondrial dysfunction. Generally, we hypothesize that peptides’ ability to suppress the toxic effect induced by 6-OHDA may be mediated by different cellular mechanisms. The protective effect caused by endomorphins results in an antiapoptotic effect (mitochondrial protection and decrease in Caspase-3 activity), while R-6 potency to increase a cell’s viability seems to be mediated by reducing oxidative stress. Our results may provide new insight into neurodegeneration and support the short peptides as a potent drug candidate to treat PD. However, further studies should be conducted on the detailed mechanisms of how tested peptides could suppress neuronal injuries.

Background: To identify drug candidates that reduce cellular stress, linear peptides known as endomorphin (EM) analogs containing proline surrogates in position 2 were tested in in vitro injury models induced by corticosterone (CORT). Methods: In this study, neuroblastoma (SH-SY5Y) cells were treated with CORT and synthesized peptides, and then the cell viability and morphology, reactive oxygen species production (ROS), mitochondrial membrane potential (ΔΨm), adenosine triphosphate (ATP), and intracellular calcium ion [Ca²⁺]_i levels were evaluated. We also conducted an in-depth analysis of the apoptosis markers using quantitative real-time PCR (qPCR). Finally, we explore the brain-derived neurotrophic factor (BDNF) expression (qPCR) and protein levels (ELI-SA and Western blot). Results: The strongest neuroprotective effect in the CORT-induced stress model was shown by peptide 3 and peptide 7 (in the following sequence Tyr-Inp-Trp-Phe-NH2 and Tyr-Inp-Phe-Phe-NH2, respectively). These peptides significantly improved cell viability and reduced oxidative stress in CORT-treated cells. Conclusions: Their neuroprotective potential appears linked to anti-apoptotic effects, along with in-creased BDNF expression. Moreover, in the lipopolysaccharide (LPS)- and interferon-γ (IFN-γ)-induced damage model in macrophage RAW 264.7 cells, these two peptides reduced the secretion of inflammatory mediators nitric oxide (NO), tumor necrosis factor-α (TNF-α), and interleukin-6 (IL-6). Peptides exhibiting both neuroprotective and anti-inflammatory properties warrant further investigation as potential therapeutic agents.