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Colloidal structure of honey and its influence on antibacterial activity

Honey colloidal structure emerges as a new trend in research on honey functions since it became recognized as a major factor altering bioactivity of honey compounds. In honey complex matrix, macromolecules self-associate to colloidal particles at the critical concentration, driven by honey viscosity. Sequestration of macromolecules into colloids changes their activities and affects honey antibacterial function. This review fills the 80-year-old gap in research on honey colloidal structure. It summarizes past and current status of the research on honey colloids and describes physicochemical properties and the mechanisms of colloid formation and their dissociation upon honey dilution. The experimental observations are explained in the context of theoretical background of colloidal science. The functional changes and bioactivity of honey macromolecules bound to colloidal particles are illustrated here by the production of H₂ O₂ by glucose oxidase and the effect they have on antibacterial activity of honey. The changes in the production of H₂ O₂ and antibacterial activity of honey were coordinated with the changes in the aggregation-dissociation states of honey colloidal particles upon dilution. In all cases, these changes were nonlinear, assuming an inverted U-shaped dose-response curve. At the curve maximum, the production of H₂ O₂ and antibacterial activity reached the peak. The curve maximum signaled the minimum honey concentration required for the phase separation. With phase transition from two-phase colloidal condense state to dilute state dispersion, the change to opposite effects of dilution on these honey's activities occurred. Thus, the colloidal structure strongly influences bioactivity of honey compounds and affects its antibacterial activity.

Structure of native glycolipoprotein filaments in honeybee royal jelly

Royal jelly (RJ) is produced by honeybees (Apis mellifera) as nutrition during larval development. The high viscosity of RJ originates from high concentrations of long lipoprotein filaments that include the glycosylated major royal jelly protein 1 (MRJP1), the small protein apisimin and insect lipids. Using cryo-electron microscopy we reveal the architecture and the composition of RJ filaments, in which the MRJP1 forms the outer shell of the assembly, surrounding stacked apisimin tetramers harbouring tightly packed lipids in the centre. The structural data rationalize the pH-dependent disassembly of RJ filaments in the gut of the larvae.

Architecture of the native major royal jelly protein 1 oligomer

Honeybee caste development is nutritionally regulated by royal jelly (RJ). Major royal jelly protein 1 (MRJP1), the most abundant glycoprotein among soluble royal jelly proteins, plays pivotal roles in honeybee nutrition and larvae development, and exhibits broad pharmacological activities in humans. However, its structure has long remained unknown. Herein, we identify and report a 16-molecule architecture of native MRJP1 oligomer containing four MRJP1, four apisimin, and eight unanticipated 24-methylenecholesterol molecules at 2.65 Å resolution. MRJP1 has a unique six-bladed β-propeller fold with three disulfide bonds, and it interacts with apisimin mainly by hydrophobic interaction. Every four 24-methylenecholesterol molecules are packaged by two MRJP1 and two apisimin molecules. This assembly dimerizes to form an H-shaped MRJP1₄-apisimin₄-24-methylenecholesterol₈ complex via apisimin in a conserved and pH-dependent fashion. Our findings offer a structural basis for understanding the pharmacological effects of MRJPs and 24-methylenecholesterol, and provide insights into their unique physiological roles in bees.

Shotgun proteomics deciphered age/division of labor-related functional specification of three honeybee (Apis mellifera L.) exocrine glands

The honeybee (Apis mellifera L.) uses various chemical signals produced by the worker exocrine glands to maintain the functioning of its colony. The roles of worker postcerebral glands (PcGs), thoracic glands (TGs), and mandibular glands (MGs) and the functional changes they undergo according to the division of labor from nursing to foraging are not as well studied. To comprehensively characterize the molecular roles of these glands in workers and their changes according to the division of labor of workers, we analyzed the proteomes of PcGs, TGs, and MGs from nurse bees and foragers using shotgun proteomics technology. We identified approximately 2000 proteins from each of the nurse bee or forager glands and highlighted the features of these glands at the molecular level by semiquantitative enrichment analyses of frequently detected, gland-selective, and labor-selective proteins. First, we found the high potential to produce lipids in PcGs and MGs, suggesting their relation to pheromone production. Second, we also found the proton pumps abundant in TGs and propose some transporters possibly related to the saliva production. Finally, our data unveiled candidate enzymes involved in labor-dependent acid production in MGs.

Beyond Royalactin and a master inducer explanation of phenotypic plasticity in honey bees

Distinct female castes produced from one genotype are the trademark of a successful evolutionary invention in eusocial insects known as reproductive division of labour. In honey bees, fertile queens develop from larvae fed a complex diet called royal jelly. Recently, one protein in royal jelly, dubbed Royalactin, was deemed to be the exclusive driver of queen bee determination. However, this notion has not been universally accepted. Here I critically evaluate this line of research and argue that the sheer complexity of creating alternate phenotypes from one genotype cannot be reduced to a single dietary component. An acceptable model of environmentally driven caste differentiation should include the facets of dynamic thinking, such as the concepts of attractor states and genetic hierarchical networks.

In honeybees, genotypically identical females develop into queens or sterile workers, depending on their diets. In this review, Ryszard Maleszka discusses the controversial role of the royal jelly protein Royalactin in caste determination and provides a framework for moving beyond the master inducer concept.

The honeybee as a model insect for developmental genetics

Honeybees are an important component of modern agricultural systems, and a fascinating and scientifically engrossing insect. Honeybees are not commonly used as model systems for understanding development in insects despite their importance in agriculture. Honeybee embryogenesis, while being superficially similar to Drosophila, is molecularly very different, especially in axis formation and sex determination. In later development, much of honeybee biology is modified by caste development, an as yet poorly understood, but excellent, system to study developmental plasticity. In adult stages, developmental plasticity of the ovaries, related to reproductive constraint exhibits another aspect of plasticity. Here they review the tools, current knowledge and opportunities in honeybee developmental biology, and provide an updated embryonic staging scheme to support future studies.

Longevity extension of worker honey bees (Apis mellifera) by royal jelly: optimal dose and active ingredient

In the Western honey bee, Apis mellifera, queens and workers have different longevity although they share the same genome. Queens consume royal jelly (RJ) as the main food throughout their life, including as adults, but workers only eat worker jelly when they are larvae less than 3 days old. In order to explore the effect of RJ and the components affecting longevity of worker honey bees, we first determined the optimal dose for prolonging longevity of workers as 4% RJ in 50% sucrose solution, and developed a method of obtaining long lived workers. We then compared the effects of longevity extension by RJ 4% with bee-collected pollen from rapeseed (Brassica napus). Lastly, we determined that a water soluble RJ protein obtained by precipitation with 60% ammonium sulfate (RJP₆₀) contained the main component for longevity extension after comparing the effects of RJ crude protein extract (RJCP), RJP₃₀ (obtained by precipitation with 30% ammonium sulfate), and RJ ethanol extract (RJEE). Understanding what regulates worker longevity has potential to help increase colony productivity and improve crop pollination efficiency.

Characterizing the Structure and Oligomerization of Major Royal Jelly Protein 1 (MRJP1) by Mass Spectrometry and Complementary Biophysical Tools

Royal jelly (RJ) triggers the development of female honeybee larvae into queens. This effect has been attributed to the presence of major royal jelly protein 1 (MRJP1) in RJ. MRJP1 isolated from royal jelly is tightly associated with apisimin, a 54-residue α-helical peptide that promotes the noncovalent assembly of MRJP1 into multimers. No high-resolution structural data are available for these complexes, and their binding stoichiometry remains uncertain. We examined MRJP1/apisimin using a range of biophysical techniques. We also investigated the behavior of deglycosylated samples, as well as samples with reduced apisimin content. Our mass spectrometry (MS) data demonstrate that the native complexes predominantly exist in a (MRJP1₄ apisimin₄) stoichiometry. Hydrogen/deuterium exchange MS reveals that MRJP1 within these complexes is extensively disordered in the range of residues 20-265. Marginally stable secondary structure (likely antiparallel β-sheet) exists around residues 266-432. These weakly structured regions interchange with conformers that are extensively unfolded, giving rise to bimodal (EX1) isotope distributions. We propose that the native complexes have a "dimer of dimers" quaternary structure in which MRJP1 chains are bridged by apisimin. Specifically, our data suggest that apisimin acts as a linker that forms hydrophobic contacts involving the MRJP1 segment ³¹⁶VLFFGLV³²². Deglycosylation produces large soluble aggregates, highlighting the role of glycans as aggregation inhibitors. Samples with reduced apisimin content form dimeric complexes with a (MRJP1₂ apisimin₁) stoichiometry. The information uncovered in this work will help pave the way toward a better understanding of the unique physiological role played by MRJP1 during queen differentiation.

Oral transfer of chemical cues, growth proteins and hormones in social insects

Social insects frequently engage in oral fluid exchange - trophallaxis - between adults, and between adults and larvae. Although trophallaxis is widely considered a food-sharing mechanism, we hypothesized that endogenous components of this fluid might underlie a novel means of chemical communication between colony members. Through protein and small-molecule mass spectrometry and RNA sequencing, we found that trophallactic fluid in the ant Camponotus floridanus contains a set of specific digestion- and non-digestion related proteins, as well as hydrocarbons, microRNAs, and a key developmental regulator, juvenile hormone. When C. floridanus workers' food was supplemented with this hormone, the larvae they reared via trophallaxis were twice as likely to complete metamorphosis and became larger workers. Comparison of trophallactic fluid proteins across social insect species revealed that many are regulators of growth, development and behavioral maturation. These results suggest that trophallaxis plays previously unsuspected roles in communication and enables communal control of colony phenotypes.