Tryptophan-Containing Dual Neuroprotective Peptides: Prolyl Endopeptidase Inhibition and Caenorhabditis elegans Protection from β-Amyloid Peptide Toxicity

The biological role of prolyl oligopeptidase and the procognitive potential of its peptidic inhibitors from food proteins

Prolyl oligopeptidase (POP) is a conserved serine protease belonging to proline-specific peptidases. It has both enzymatic and non-enzymatic activity and is involved in numerous biological processes in the human body, playing a role in e.g., cellular growth and differentiation, inflammation, as well as the development of some neurodegenerative and neuropsychiatric disorders. This article describes the physiological and pathological aspects of POP activity and the state-of-art of its peptidic inhibitors originating from food proteins, with a particular focus on their potential as cognition-enhancing agents. Although some milk, meat, fish, and plant protein-derived peptides have the potential to be applied as natural, procognitive nutraceuticals, their effectiveness requires further evaluation, especially in clinical trials. We demonstrated that the important features of the most promising POP-inhibiting peptides are very short sequence, high content of hydrophobic amino acids, and usually the presence of proline residue.

Effects of a plant cyclotide on conformational dynamics and destabilization of β-amyloid fibrils through molecular dynamics simulations

Aggregation of β-amyloid (Aβ) peptide is one of the hallmarks of Alzheimer’s disease (AD) which results in chronic and progressive neurodegeneration of the brain. A recent study by our group have shown the ability of cyclic disulfide-rich peptides (“cyclotides”) isolated from a medicinal plant, Clitoria ternatea, to inhibit the aggregation of Aβ peptides and reduce oxidative stress caused by reactive oxygen species using in vivo models of transgenic Caenorhabditis elegans. In the present study, through extensive computational docking and multi-ns molecular dynamics (MD) simulation, we evaluated if cyclotides can stably bind to Aβ molecules and/or destabilize the Aβ fibril by preventing conformational changes from α-helical to β-sheet rich structures. We demonstrate that cyclotides bind effectively and stably to different forms of Aβ structures via hydrogen bonding and hydrophobic interactions. One of the conserved hydrophobic interface residues, Tyr10 was mutated to Ala and the impact of this virtual mutation was estimated by additional MD simulations for the wild-type (WT) and mutant protein-peptide complexes. A detailed MD simulation analyses revealed that cyclotides form hydrogen bonds with the toxic amyloid assemblies thereby weakening the inter-strand hydrogen bonds between the Aβ peptide. The φ-ѱ distribution map of residues in the cyclotide binding pocket that ideally adopt β-sheet conformation show deviation towards right-handed ɑ-helical (ɑ_R) conformation. This effect was similar to that observed for the Tyr10Ala mutant and doubly so, for the cyclotide bound form. It is therefore possible to hypothesise that the opening up of amyloid β-sheet is due to an unfolding process occurring in the Aβ caused by cyclotide binding and inhibition. Our current findings provide novel structural insights on the mode of interaction between cyclotides and Aβ fibrils and describe their anti-amyloid aggregation potential. This sheds light on the future of cyclotide-based drug design against protein aggregation, a hallmark event in many neurodegenerative diseases.

Modeling Alzheimer's Disease in Caenorhabditis elegans

Alzheimer’s disease (AD) is the most frequent cause of dementia. After decades of research, we know the importance of the accumulation of protein aggregates such as β-amyloid peptide and phosphorylated tau. We also know that mutations in certain proteins generate early-onset Alzheimer’s disease (EOAD), and many other genes modulate the disease in its sporadic form. However, the precise molecular mechanisms underlying AD pathology are still unclear. Because of ethical limitations, we need to use animal models to investigate these processes. The nematode Caenorhabditis elegans has received considerable attention in the last 25 years, since the first AD models overexpressing Aβ peptide were described. We review here the main results obtained using this model to study AD. We include works studying the basic molecular mechanisms of the disease, as well as those searching for new therapeutic targets. Although this model also has important limitations, the ability of this nematode to generate knock-out or overexpression models of any gene, single or combined, and to carry out toxicity, recovery or survival studies in short timeframes with many individuals and at low cost is difficult to overcome. We can predict that its use as a model for various diseases will certainly continue to increase.

Disulfide-Rich Cyclic Peptides from Clitoria ternatea Protect against β-Amyloid Toxicity and Oxidative Stress in Transgenic Caenorhabditis elegans

Neurotoxic aggregation of β-amyloid (Aβ) peptides is a hallmark of Alzheimer's disease and increased reactive oxygen species (ROS) is an associated process. In the present study, we report the neuroprotective effects of disulfide-rich, circular peptides from Clitoria ternatea (C. ternatea) (butterfly pea) on Aβ-induced toxicity in transgenic Caenorhabditis elegans. Cyclotides (∼30 amino acids long) are a special class of cyclic cysteine knot peptides. We show that cyclotide-rich fractions from different plant tissues delay Aβ-induced paralysis in the transgenic CL4176 strain expressing the human muscle-specific Aβ_1-42 gene. They also improved Aβ-induced chemotaxis defects in CL2355 strain expressing Aβ_1-42 in the neuronal cells. ROS assay suggests that this protection is likely mediated by the inhibition of Aβ oligomerization. Furthermore, Aβ deposits were reduced in the CL2006 strain treated with the fractions. The study shows that cyclotides from C. ternatea could be a source of a novel pharmacophore scaffold against neurodegenerative diseases.

Alkaloids of Zephyranthes citrina (Amaryllidaceae) and their implication to Alzheimer's disease: Isolation, structural elucidation and biological activity

Twenty known Amaryllidaceae alkaloids of various structural types, and one undescribed alkaloid of narcikachnine-type, named narcieliine (3), have been isolated from fresh bulbs of Zephyranthes citrina. The chemical structures of the isolated alkaloids were elucidated by a combination of MS, HRMS, 1D and 2D NMR, and CD spectroscopic techniques, and by comparison with literature data. The absolute configuration of narcieliine (3) has also been determined. Compounds isolated in a sufficient quantity were evaluated for their in vitro acetylcholinesterase (AChE; E.C. 3.1.1.7), butyrylcholinesterase (BuChE; E.C. 3.1.1.8), and prolyl oligopeptidase (POP; E.C. 3.4.21.26) inhibition activities. Significant human AChE/BuChE (hAChE/hBuChE) inhibitory activity was demonstrated by the newly described alkaloid narcieliine (3), with IC₅₀ values of 18.7 ± 2.3 µM and 1.34 ± 0.31 µM, respectively. This compound is also predicted to cross the blood-brain barrier (BBB) through passive diffusion. The in vitro data were further supported by in silico studies of 3 in the active site of hAChE/hBuChE.

Tryptophan-galactosylamine conjugates inhibit and disaggregate amyloid fibrils of Aβ42 and hIAPP peptides while reducing their toxicity

Self-assembly of proteins into amyloid fibrils is a hallmark of various diseases, including Alzheimer's disease (AD) and Type-2 diabetes Mellitus (T2DM). Aggregation of specific peptides, like Aβ42 in AD and hIAPP in T2DM, causes cellular dysfunction resulting in the respective pathology. While these amyloidogenic proteins lack sequence homology, they all contain aromatic amino acids in their hydrophobic core that play a major role in their self-assembly. Targeting these aromatic residues by small molecules may be an attractive approach for inhibiting amyloid aggregation. Here, various biochemical and biophysical techniques revealed that a panel of tryptophan-galactosylamine conjugates significantly inhibit fibril formation of Aβ42 and hIAPP, and disassemble their pre-formed fibrils in a dose-dependent manner. They are also not toxic to mammalian cells and can reduce the cytotoxicity induced by Aβ42 and hIAPP aggregates. These tryptophan-galactosylamine conjugates can therefore serve as a scaffold for the development of therapeutics towards AD and T2DM.

Improving Health-Promoting Effects of Food-Derived Bioactive Peptides through Rational Design and Oral Delivery Strategies

Over the last few decades, scientific interest in food-derived bioactive peptides has grown as an alternative to pharmacological treatments in the control of lifestyle-associated diseases, which represent a serious health problem worldwide. Interest has been directed towards the control of hypertension, the management of type 2 diabetes and oxidative stress. Many food-derived antihypertensive peptides act primarily by inhibiting angiotensin I-converting enzyme (ACE), and to a lesser extent, renin enzyme activities. Antidiabetic peptides mainly inhibit dipeptidyl peptidase-IV (DPP-IV) activity, whereas antioxidant peptides act through inactivation of reactive oxygen species, free radicals scavenging, chelation of pro-oxidative transition metals and promoting the activities of intracellular antioxidant enzymes. However, food-derived bioactive peptides have intrinsic weaknesses, including poor chemical and physical stability and a short circulating plasma half-life that must be addressed for their application as nutraceuticals or in functional foods. This review summarizes the application of common pharmaceutical approaches such as rational design and oral delivery strategies to improve the health-promoting effects of food-derived bioactive peptides. We review the structural requirements of antihypertensive, antidiabetic and antioxidant peptides established by integrated computational methods and provide relevant examples of effective oral delivery systems to enhance solubility, stability and permeability of bioactive peptides.