Rubiscolin-6, a δ-Opioid Peptide from Spinach RuBisCO, Exerts Antidepressant-Like Effect in Restraint-Stressed Mice

Neuroprotective and Anti-inflammatory Effects of Rubiscolin-6 Analogs with Proline Surrogates in Position 2

Naturally occurring peptides, such as rubiscolins derived from spinach leaves, have been shown to possess some interesting activities. They exerted central effects, such as antinociception, memory consolidation and anxiolytic-like activity. The fact that rubiscolins are potent even when given orally makes them very promising drug candidates. The present work tested whether rubiscolin-6 (R-6, Tyr-Pro-Leu-Asp-Leu-Phe) analogs have neuroprotective and anti-inflammatory effects. These hypotheses were tested in the 6-hydroxydopamine (6-OHDA) injury model of human neuroblastoma SH-SY5Y and lipopolysaccharide (LPS)-stimulated RAW 264.7 macrophages. The determination of reactive oxygen species (ROS), mitochondrial membrane potential (MMP), Caspase-3 activity, lipid peroxidation and nitric oxide (NO) production allowed us to determine the effects of peptides on hallmarks related to Parkinson's Disease (PD) and inflammation. Additionally, we investigated the impact of R-6 analogs on serine-threonine kinase (also known as protein kinase B, AKT) and mammalian target of rapamycin (mTOR) activation. The treatment with analogs 3 (Tyr-Inp-Leu-Asp-Leu-Phe-OH), 5 (Dmt-Inp-Leu-Asp-Leu-Phe-OH) and 7 (Tyr-Inp-Leu-Asp-Leu-Phe-NH2) most effectively prevented neuronal death via attenuation of ROS, mitochondrial dysfunction and Caspase-3 activity. Peptides 5 and 7 significantly increased the protein expression of the phosphorylated-AKT (p-AKT) and phosphorylated-mTOR (p-mTOR). Additionally, selected analogs could also ameliorate LPS-mediated inflammation in macrophages via inhibition of intracellular generation of ROS and NO production. Our findings suggest that R-6 analogs exert protective effects, possibly related to an anti-oxidation mechanism in in vitro model of PD. The data shows that the most potent peptides can inhibit 6-OHDA injury by activating the PI3-K/AKT/mTOR pathway, thus playing a neuroprotective role and may provide a rational and robust approach in the design of new therapeutics or even functional foods.

Natural Plant Materials as a Source of Neuroprotective Peptides

In many circumstances, some crucial elements of the neuronal defense system fail, slowly leading to neurodegenerative diseases. Activating this natural process by administering exogenous agents to counteract unfavourable changes seems promising. Therefore, looking for neuroprotective therapeutics, we have to focus on compounds that inhibit the primary mechanisms leading to neuronal injuries, e.g., apoptosis, excitotoxicity, oxidative stress, and inflammation. Among many compounds considered neuroprotective agents, protein hydrolysates and peptides derived from natural materials or their synthetic analogues are good candidates. They have several advantages, such as high selectivity and biological activity, a broad range of targets, and high safety profile. This review aims to provide biological activities, the mechanism of action and the functional properties of plant-derived protein hydrolysates and peptides. We focused on their significant role in human health by affecting the nervous system and having neuroprotective and brain-boosting properties, leading to memory and cognitive improving activities. We hope our observation may guide the evaluation of novel peptides with potential neuroprotective effects. Research into neuroprotective peptides may find application in different sectors as ingredients in functional foods or pharmaceuticals to improve human health and prevent diseases.

Therapeutic Potential of Orally Administered Rubiscolin-6

Rubiscolins are naturally occurring opioid peptides derived from the enzymatic digestion of the ribulose bisphosphate carboxylase/oxygenase protein in spinach leaves. They are classified into two subtypes based on amino acid sequence, namely rubiscolin-5 and rubiscolin-6. In vitro studies have determined rubiscolins as G protein-biased delta-opioid receptor agonists, and in vivo studies have demonstrated that they exert several beneficial effects via the central nervous system. The most unique and attractive advantage of rubiscolin-6 over other oligopeptides is its oral availability. Therefore, it can be considered a promising candidate for the development of a novel and safe drug. In this review, we show the therapeutic potential of rubiscolin-6, mainly focusing on its effects when orally administered based on available evidence. Additionally, we present a hypothesis for the pharmacokinetics of rubiscolin-6, focusing on its absorption in the intestinal tract and ability to cross the blood-brain barrier.

Antidepressant-like effect of tofisopam in mice: A behavioural, molecular docking and MD simulation study

BACKGROUND

Depression is a disease that affects millions of people worldwide, and the discovery and development of effective and safe antidepressant drugs is one of the important topics of psychopharmacology.

OBJECTIVES

In this study, it was aimed to investigate the antidepressant-like activity potential of tofisopam, an anxiolytic drug with 2,3-benzodiazepine structure, and to elucidate the pharmacological mechanisms mediating this effect.

METHODS

The antidepressant-like activity of tofisopam was investigated using tail suspension and modified forced swimming tests. Possible interactions of tofisopam with µ- and δ-opioid receptor subtypes were clarified by pharmacological antagonism, molecular docking and molecular dynamics simulation studies.

RESULTS

Tofisopam (50 and 100 mg/kg) significantly shortened the immobility time of mice in both the tail suspension and the modified forced swimming tests. The drug, at the same doses, prolonged the duration of swimming and climbing behaviours measured in modified forced swimming tests. A dosage of 25 mg/kg was ineffective. Mechanistic studies showed that the pretreatment with p-chlorophenylalanine methyl ester (serotonin synthesis inhibitor; 4 consecutive days, 100 mg/kg), α-methyl-para-tyrosine methyl ester (catecholamine synthesis inhibitor; 100 mg/kg), naloxonazine (selective µ-opioid receptor blocker, 7 mg/kg) and naltrindole (a selective δ-opioid receptor blocker, 0.99 mg/kg) abolished the anti-immobility effect induced by the 50 mg/kg dose of tofisopam in the tail suspension tests. Our in silico studies supported the behavioural findings that the antidepressant-like effect of tofisopam is mediated by μ- and δ-opioid receptors.

CONCLUSION

This study is the first to show that tofisopam has antidepressant-like activity mediated by the serotonergic, catecholaminergic and opioidergic systems.

Novel Pharmacological Approaches to the Treatment of Depression

Major depressive disorder is one of the most prevalent mental health disorders. Monoamine-based antidepressants were the first drugs developed to treat major depressive disorder. More recently, ketamine and other analogues were introduced as fast-acting antidepressants. Unfortunately, currently available therapeutics are inadequate; lack of efficacy, adverse effects, and risks leave patients with limited treatment options. Efforts are now focused on understanding the etiology of depression and identifying novel targets for pharmacological treatment. In this review, we discuss promising novel pharmacological targets for the treatment of major depressive disorder. Targeting receptors including N-methyl-D-aspartate receptors, peroxisome proliferator-activated receptors, G-protein-coupled receptor 39, metabotropic glutamate receptors, galanin and opioid receptors has potential antidepressant effects. Compounds targeting biological processes: inflammation, the hypothalamic-pituitary-adrenal axis, the cholesterol biosynthesis pathway, and gut microbiota have also shown therapeutic potential. Additionally, natural products including plants, herbs, and fatty acids improved depressive symptoms and behaviors. In this review, a brief history of clinically available antidepressants will be provided, with a primary focus on novel pharmaceutical approaches with promising antidepressant effects in preclinical and clinical studies.

In Vitro Analyses of Spinach-Derived Opioid Peptides, Rubiscolins: Receptor Selectivity and Intracellular Activities through G Protein- and β-Arrestin-Mediated Pathways

Activated opioid receptors transmit internal signals through two major pathways: the G-protein-mediated pathway, which exerts analgesia, and the β-arrestin-mediated pathway, which leads to unfavorable side effects. Hence, G-protein-biased opioid agonists are preferable as opioid analgesics. Rubiscolins, the spinach-derived naturally occurring opioid peptides, are selective δ opioid receptor agonists, and their p.o. administration exhibits antinociceptive effects. Although the potency and effect of rubiscolins as G-protein-biased molecules are partially confirmed, their in vitro profiles remain unclear. We, therefore, evaluated the properties of rubiscolins, in detail, through several analyses, including the CellKey^TM assay, cADDis^® cAMP assay, and PathHunter^® β-arrestin recruitment assay, using cells stably expressing µ, δ, κ, or µ/δ heteromer opioid receptors. In the CellKey^TM assay, rubiscolins showed selective agonistic effects for δ opioid receptor and little agonistic or antagonistic effects for µ and κ opioid receptors. Furthermore, rubiscolins were found to be G-protein-biased δ opioid receptor agonists based on the results obtained in cADDis^® cAMP and PathHunter^® β-arrestin recruitment assays. Finally, we found, for the first time, that they are also partially agonistic for the µ/δ dimers. In conclusion, rubiscolins could serve as attractive seeds, as δ opioid receptor-specific agonists, for the development of novel opioid analgesics with reduced side effects.

Neuromodulatory peptides: Orally active anxiolytic-like and antidepressant-like peptides derived from dietary plant proteins

Mental disorders are a severe health problem, and the number of patients is growing worldwide. Increased anxiety and decreased motivation due to excessive mental stress further accelerated the severity of the problem. Enzymatic digestion of food proteins produces bioactive peptides with various physiological functions, some of which exhibit neuromodulatory effects with oral administration. Recently, studies reported that some peptides produced from plant proteins such as soybeans, leaves, and grains exhibit emotional regulatory functions such as strong anxiolytic-like and antidepressant-like effects comparable to pharmaceuticals. Conventionally, researchers investigated bioactive peptides by fractionation of protein hydrolysates and structure-activity relationship. As a novel methodology for analyzing bioactive peptides, the information obtained by peptidomics simultaneous analysis of the digested fractions of proteins using mass spectrometry has been effectively utilized. Some small-sized peptides such as dipeptides and tripeptides released food-derived proteins show emotional regulating effects. Moreover, some middle-sized peptides produced after intestinal digestion may exhibit the emotional regulating effect via the vagus nerve, and the importance of the gut-brain axis is also focused. As the central mechanism of emotional regulation, it has been found that these plant-derived peptides regulated monoamine neurotransmitter signaling and hippocampal neurogenesis.

Isolation and functionalities of bioactive peptides from fruits and vegetables: A reviews

Bioactive peptides have recently gained more research attention as potential therapies for the management of bodily disorders and metabolic syndromes of delicate health importance. On another note, there is a rising trend on a global scale for the consumption and adoption of fruit and vegetables for the fulfilment of dietary and health needs. Furthermore, fruits and vegetables are being more studied as base materials for the isolation of biologically functional components and accordingly, they have been investigated for their concomitant bioactive peptides. This review focuses on isolation and bio-functional properties of bioactive peptides from fruits and vegetables. This manuscript is potential in serving as a material collection for fundamental consultancy on peptides derived from fruits and vegetables, and further canvasses the necessitation for the use of these food materials as primal matter for such.