Importance for humans of recently discovered protein compounds - yolkin and yolk glycopeptide 40, present in the plasma of hen egg yolk

Abstract

Vitellogenin (Vt) is considered the primary protein precursor of egg yolk, serving as a source of protein- and lipid-rich nutrients for the developing embryo. However, recent research has revealed that the functions of Vt and Vt-derived polypeptides, such as yolkin (Y) and yolk glycopeptide 40 (YGP40), extend beyond their nutritional roles as a source of amino acids. Emerging evidence has demonstrated that both Y and YGP40 possess immunomodulatory properties and can contribute to host immune defenses. Additionally, Y polypeptides have been shown to exhibit neuroprotective activity, participating in the modulation of neurons' survival and activity, inhibiting neurodegeneration processes, and improving cognitive functions in rats. These non-nutritional functions not only enhance our understanding of the physiological roles of these molecules during embryonic development but also offer a promising basis for the potential application of these proteins in human health.

Keywords: immunomodulatory activity; neuroprotective activity; vitellogenin; yolk glycopeptide 40; yolkin.

Figure 1

Schematic representation of the precursor-product relationship between vitellogenin II (Vt II) and the yolk proteins. In the process of egg formation, Vtg II (from hen's blood plasma) is proteolytically cleaved to the main proteins of the yolk, which are accumulated in immature egg cells (oocytes) located in the antral ovarian follicles. Abbreviations: LvH, Lipovitellin 1; LvL, Lipovitellin 2; Pv, Phosvitin; Vwfd, D domain of the von Willebrand factor; YGP 40, yolk glycopeptide 40.

Figure 2

Mechanisms of immunomodulatory and neuroprotective activity of yolkin (Y). Y polypeptide complex isolated from hen egg yolk keeps the immune response balance of human whole blood cells by induction of IFN and TNF-alpha (Th1 cytokines), and IL-6 and IL-10 (Th2 cytokines). It may also play an important role in the induction of maturation and stimulatory signals in immature T and B cells. Y regulates both M1 and M2 macrophage profiles. Significantly down-regulates macrophage proliferation and simultaneously increases the cell surface marker expression of CD80/CD86 characterized M1 macrophage phenotype. Y is recognized by, and activated TLR2 and TLR4 receptors, and up-regulates mRNA expression and production of proinflammatory factors (type one IFNs, TNF-α, IL-6, iNOS/NO). Besides, Y initiates anti-inflammatory M2-dependent IL-10 production and inhibits LPS–induced nitric oxide production in macrophages. The neuroprotective activity of Y is connected to its impact on BDNF expression/secretion, and reduction of ROS generation in neurons, resulting in improvement of the cognitive functions.