Application of Peptides in the Management of Alzheimer's Disease

2025-03-25

Alzheimer's disease (AD) is a form of dementia and the most common progressive neurodegenerative disease (ND). Strategies targeting β-amyloid (Aβ) aggregation are one of the most widely used methods for managing AD, and global efforts are underway to develop peptidyl compounds for early diagnosis and treatment of AD. Here, we briefly discuss the use of peptidyl compounds in the early diagnosis and treatment of AD, as well as the use of peptidyl inhibitors against various Aβ aggregation checkpoints. In addition, we briefly discuss the latest applications of peptidyl inhibitors targeting various AD targets, including β-amyloid, β-amyloid precursor protein lyase 1 (BACE1), glyceraldehyde-3-phosphate dehydrogenase (GAPDH), tyrosine phosphatase (TP), and potassium channel KV1.3.

Introduction to Peptides in Alzheimer's Disease

Alzheimer's disease is the most common fatal neurodegenerative disease characterized by the loss of structure and function of neurons. Over the past few decades, Alzheimer's disease and its associated risk factors have become a major healthcare issue in most developed countries. In addition, Alzheimer's disease has been reported to be the fifth leading cause of death in people over 65 years of age, with an incidence of more than 5 million cases per year in the United States (Alzheimer's Association, 2017). The World Health Organization (WHO) estimates that the global prevalence of Alzheimer's disease will quadruple to about 114 million cases by 2050.

Pathologic features of Alzheimer's disease include the gradual accumulation of extraneuronal amyloid plaques (extracellular amyloid plaques) and neurofibrillary tangles (hyperphosphorylated tau aggregations) within neurons. These pathological changes gradually lead to neuronal loss and eventually neuronal death. Although the etiology and pathogenesis of Alzheimer's disease are not well understood, the amyloid cascade hypothesis is widely accepted and supported by several studies. This hypothesis is further confirmed by the discovery of a protective mutation near the β cleavage site of the amyloid precursor protein (APP) that prevents the development of late-onset dementia.

β-Amyloid, a peptide composed of 39 to 43 amino acid residues, is a key component of extracellular amyloid plaques, and its expression is thought to be a key event in the progression of Alzheimer's disease. Aβ is a peptide product produced by sequential proteolytic cleavage of APP, a type I transmembrane protein, by sequential β- and γ-secretases. These proteolytic cleavage processes produce two types of Aβ isoforms (Aβ40 and Aβ42), and although Aβ40 is present in higher levels of human fluids than Aβ42, Aβ42 aggregates more rapidly and is thought to be more harmful to neuronal survival. Diffusible Aβ oligomers, such as fibrils, prefibrous aggregates, and Aβ-derived diffusible ligands (ADDLs), have been reported to be major toxic factors during the development and progression of AD. Numerous clinical trials of amyloid-reducing therapies (ART) have failed to deliver the desired clinical improvement in AD patients, and these failures have raised real concerns about the validity of the amyloid cascade hypothesis and the value of further study of ART.

Table.1 β-Amyloid and fragments at Creative Peptides.

Today, peptide drugs have become an important class of therapeutic drugs thanks to the tireless efforts of researchers and pharmaceutical companies, and as a result, their therapeutic use has attracted significant attention from the scientific community and industry over the past decade. Such research is still ongoing, and many natural and synthetic therapeutic peptides are currently undergoing clinical trials. Overall, these drugs have several advantages over small molecule therapeutics, especially in terms of efficacy and fewer side effects.

Many other deadly neurodegenerative diseases, such as Parkinson's disease (PD), amyotrophic lateral sclerosis (ALS), prion disease, and Huntington's disease (HD), are characterized by protein misfolded aggregation in AD. Peptides have proven to be an important tool in the study of neurodegenerative diseases and can be used to study the properties of misfolded proteins and/or peptides. In this review article, we discuss some of the peptidyl-based therapies that can be used to treat AD, the use of peptide inhibitors against different aspects of AD, and the use of peptides in the diagnosis and early detection of AD.

Application of Peptides as Diagnostic Probes

Currently, there are limited in vivo diagnostic techniques available to detect Alzheimer's disease, but early detection of the disease is essential for effective treatment, as preventive treatment may control or eliminate the early disease. Studies in patients with AD have shown that amyloid plaques appear years before cognitive symptoms appear. Therefore, from a prognostic perspective, early detection and quantification of amyloid in the brain is considered very important and critical for assessing treatment efficacy. Several molecular imaging techniques, including magnetic resonance imaging (MRI), positron emission tomography (PET), and single-photon emission computed tomography (SPECT), provide the means to achieve this. These molecular imaging techniques have been reported to be useful for detecting biomarkers of AD (primarily Aβ) and monitoring Aβ expression, thus attracting considerable attention in the field of AD research. In addition, amyloid ligands have been used as contrast agents to estimate amyloid plaque burden and have been shown to specifically stain amyloid plaques in brain tissue of patients with Alzheimer's disease, making these peptides suitable probes for in vivo imaging.

D-enantiomer peptides (also known as ACI-80 derivatives) are another group of peptides that have been reported to bind to Aβ1-42. In one study, ACI-80 was also found to bind to Aβ1-42 with high affinity (in the submicromolar range) and is currently being used as a molecular probe to monitor Aβ1-42 plaque load in living brains. The findings suggest that when ACI-80 is injected into the brain, it specifically binds to Aβ1-42 and stains dense amyloid deposits in the brain, but not diffuse plaques, making it a suitable molecular probe for in vivo imaging of Alzheimer's disease. In addition, recent studies have described a series of D-enantiomer peptides that also specifically bind to aggregated Aβ1-42 but have higher stability and Aβ-binding properties than ACI-80. To confirm the binding of these peptide derivatives, in vitro immunochemistry experiments were performed using an AD transgenic mouse model, and the ACI-80 derivatives ACI-87-Kφ, ACI-88-Kφ, and ACI-89-Kφ had a higher binding affinity with aggregated Aβ1-42 than the parent ACI-80. These findings suggest that these compounds may be useful probes for specific types of Aβ aggregation and plaque in vivo.

Larbanoix et al. (2010) employed phage display techniques to search for peptide ligands with carrieration capabilities to target contrast agents against amyloid (AP) and identified 12 peptide ligands with high affinity for Aβ42 from 22 sequenced phage clones. Two peptides (C-FRHMTEQ-C and C-IPLPFYN-C) were selected because they occur in multiple clones and have an affinity constant (Kd value) in the nanomolar range for Aβ42. For example, C-IPLPFYN-C has a Kd value of 2.2×10-10 M for mouse Aβ42 and C-frhmteq-C has a Kd value of 5.45×10-10 M for mouse Aβ42. The two peptides were also tested for human Aβ42, with C-FRHMTEQ-C exhibiting a similar affinity for human Aβ42, while C-IPLPFYN-C showed a higher affinity for human Aβ42 than for mouse Aβ42. In addition, in preliminary in vivo magnetic resonance imaging (MRI) studies in transgenic mouse models of Alzheimer's disease, both C-FRHMTEQ-C and C-IPLPFYN-C demonstrated excellent contrast agent performance.

Application of Peptides as Inhibitors

Peptide Inhibitors Targeting β Amyloid

The accumulation of amyloid plaques and Aβ fibrosis are clinical features of Alzheimer's disease, and as such, inhibition of amyloid aggregation has been the focus of research over the past two decades, and the use of peptide-based inhibitors is an important part of these efforts. Much research has focused on designing peptide fragments that bind to key regions of Aβ aggregation. These designed peptides prevent the formation of monomers/oligomers by binding to Aβ to prevent fiber formation or Aβ elongation. A large number of peptides have been devised specifically to target Aβ, but here, we discuss some of the recently reported Aβ inhibitor peptides. Table 1 shows some of the important AD inhibitor peptides described in the literature. Peptide inhibitors with similar hydrophobic sequences to Aβ alter Aβ aggregation and reduce the cytotoxicity of Aβ. Two Aβ inhibitor peptides, RGKLVFFGR (OR1) and RGKLVFFGR-NH2 (OR2), are described, which are produced by the introduction of RG−/−GR residues at the N- and C-terminus of the KLVFF amino acid sequence of Aβ. Both inhibitors have been reported to be effective in inhibiting the formation of Aβ fibers.

Table.2 Peptides in alzheimer's disease at Creative Peptides.

In another study, Wei et al. (2011) reported a number of novel peptide inhibitors based on the modification of the hydrophobic KLVFF amino acid sequence of Aβ and synthesized a conjugate of the pentapeptide KLVFF with a ferrocene group (Fc) to improve the lipophilicity and antiproteolytic stability of the peptide inhibitor. In another study, Rangachari et al. (2009) designed two α-containing peptides β-dehydroalanine (ΔAla) based on KLVFF and found that two novel ΔAla-containing peptides (KLVF-ΔA-I-ΔA and KF-ΔA-ΔA-ΔA-F) disrupt Aβ aggregation, albeit with distinct mechanisms of action.

Researchers have designed and synthesized a decapeptide inhibitor for Aβ1-40 aggregation. This inhibitor, named RR (RYYAAFFARR), differs from other inhibitors by targeting an extended region of Aβ (Aβ11-23), which includes a GAG-binding site, a hydrophobic core, and a salt bridge region. RR exhibited a high binding affinity for Aβ1-40 (Kd = 1.10 μM), significantly surpassing the affinity of the known β-sheet disrupting peptide iAβ5 (Kd = 156 μM).

Research has shown that replacing L-type amino acids with D-type amino acids can enhance peptide stability and therapeutic potential. The enantiomeric inversion (D-enantiomer) effect of five peptides (KKLVFFA, KLVFFA, KIVFFA, KFVFFA, and KVVFFA) was evaluated, revealing that D-enantiomers exhibited greater inhibition of Aβ aggregation compared to their L-type counterparts. Notably, the D-enantiomer of KKLVFFA demonstrated a significantly stronger ability to suppress the neurotoxic effects of Aβ than its L-enantiomer. 

A study synthesized three short D-type peptides—KKLVFFARRRRA, PGKLVYA, and KKLVFFA—based on residues from the Aβ central hydrophobic core (residues 16 to 20) and examined their effects on Aβ aggregation. These D-type peptides were found to effectively inhibit Aβ fibril formation, with KKLVFFA and PGKLVYA also demonstrating the ability to improve the survival of transgenic Caenorhabditis elegans.

Peptide Inhibitors Targeting BACE1

β-site amyloid precursor protein lyase 1 (BACE1) is a human aspartic acid protease that is thought to play a key role in the production of β-amyloid peptide (Aβ) in Alzheimer's disease. BACE1, which has a typical two-leaf structure, is a membrane-bound aspartate protease with an open active site, is more hydrophilic than other aspartate proteases, and can accommodate up to 11 substrate residues. AD patients have elevated levels of BACE1 in brain tissue, and its overexpression in cerebrospinal fluid may be a biomarker for early disease. When BACE1 is overexpressed, it competes with γ-secretase to initiate cleavage at the β site of the amyloid precursor protein (APP). In addition, inhibition of BACE1 prevents the formation of Aβ in the first step of the amyloid production process in APP. BACE1 consists of two characteristic peptides, DTGS at positions 93 - 96 and DSGT at positions 289 - 292, which together form the active site and have the ability to inhibit APP. In addition, peptides derived from BACE1 sequences have also been reported to inhibit APP processing. BACE1 has a catalytic domain with an active site containing a pair of aspartic acid residues, and by inhibiting BACE1, it blocks Aβ production, which reduces Aβ production by reducing C99 as a substrate for γ-secretase. A decrease in Aβ production was observed in BACE1-deficient mice, which supports the idea that BACE1 inhibitors reduce Aβ levels.

Peptide Inhibitors Targeting GAPDH

Glyceraldehyde 3-phosphate dehydrogenase (GAPDH) is a glycolytic enzyme that has attracted much attention in neurodegenerative disease research, especially in Alzheimer's disease research. Recent studies have shown that GAPDH interacts with APP known to be associated with AD. GAPDH can undergo a variety of oxidative modifications that control its structure, function, and activity, and in AD, when exposed to oxidative stress, Aβ forms amyloid-like aggregates, reducing the number of neurons and synapses. In addition, the insoluble aggregates of GAPDH accelerate the formation of Aβ amyloid and neuronal cell death in vitro and in vivo, while the aggregation of GAPDH formed by cysteine oxidation and intermolecular disulfide bonds decreases its catalytic activity. Glycyl-L-histayl-L-lysine-copper (GHK-Cu) is a naturally occurring peptide in human plasma with an amazing array of effects that appear to be able to combat aging-related diseases and conditions. In plasma, the concentration of GHK-Cu is about 200 ng/mL (10-7 mol/L) at age 20 but drops to 80 ng/mL by age 60, and interestingly, the enzymes primarily involved in GAPDH gene silencing also belong to the histone deacetylase (HDACs) protein family. Selective histone deacetylase (HDAC) inhibitors have been shown to have neuroprotective properties in animal models of brain diseases and are being suggested as potential therapies for Alzheimer's disease. Glycylprolyl glutamate (GPE) is present in plasma and brain tissue and has been shown to have neuroprotective effects in animal models of neurodegenerative diseases (NDs) such as Huntington's disease, Parkinson's disease, and Alzheimer's disease. Underlying structural studies have shown that GPE can interact with single or multiple glutamate receptor types and bind to N-methyl-D-aspartate (NMDA) receptors. In addition, the C-terminus (glutamate) of the GPE is required for NMDA receptor binding, which induces dopamine release from potassium ions at dopaminergic terminals of the nigrastriatal body via an unknown mechanism as well as acetylcholine release.

Peptide Inhibitors Targeting Tyrosine Phosphatase (TP)

Striatum-rich tyrosine phosphatase (STEP) is expressed in neurons of the striatum, neocortex, hippocampus, and related structures, and acts on signaling pathways at the postsynaptic terminal of excitatory glutamatergic cells. STEP regulates a variety of synaptic activity, including the transport of glutamate receptors, which plays a key role in learning and memory. Interestingly, recent studies have shown that STEP is overactive in Alzheimer's disease and schizophrenia. Endogenous STEP levels also affect neuronal susceptibility to excitotoxicity and alter synaptic conductivity by simultaneous dephosphorylation of multiple substrates that regulate synaptic plasticity, thereby influencing the regulation of synaptic proteins. STEP is an intracellular tyrosine phosphatase (TP) encoded by the ptpn5 gene, which contains a characteristic consensus sequence [I/V] HCxAGXXR[S/T] G essential for its catalytic activity at the C-terminus, and an active motif (I/V) HCXAGXGR(S/T), also known as the P-loop, which houses the catalytic cysteine for nucleophilic challenge, a distinctive feature of the PTP superfamily. In addition, an endogenous tripeptide GPE with neuroprotective effects has been reported to protect cells from Aβ-induced death, and multiple modifications of GPE, including on proline and/or glutamate residues, have been synthesized and evaluated. The binding of NNZ-2566 (glycyl-L-2-methylprolyl-L-glutamate) analogue to Aβ showed better neuroprotective effects in young rats compared to GPE.

Peptide Inhibitors Targeting Potassium Channel KV1.3

The potassium channel KV1.3 was recently identified as a potential target for Alzheimer's disease. Rangaraju et al. (2015) conducted a study in AD patients and non-AD patients and found that Kv1.3 was overexpressed in the frontal cortex of AD patients, suggesting that the potassium channel KV1.3 could be used as a therapeutic target for AD. BmKTX-R11-T28-H33 (ADWX-1), OsK1-K16-D20, and HsTx1[R14A] examples of peptides subsequently designed to target Kv1.3.

Summary

Numerous research efforts have led to advances in the diagnosis and treatment of Alzheimer's disease, and most researchers believe that the cause of the disease can be explained by the amyloid cascade hypothesis. In addition, it is widely accepted that early detection of Alzheimer's disease can help with advanced treatments, and that the use of peptides to Alzheimer's disease is an effective tool. However, these peptide-based approaches still need to be further developed to achieve effective diagnosis and appropriate treatment of early Alzheimer's disease.

References

  1. Larbanoix, Lionel, et al., Potential amyloid plaque-specific peptides for the diagnosis of Alzheimer's disease. Neurobiology of aging 31.10 (2010): 1679-1689.
  2. Wei, Chuan-Wan, et al., Synthesis and evaluation of ferrocenoyl pentapeptide (Fc-KLVFF) as an inhibitor of Alzheimer's Aβ1–42 fibril formation in vitro. Bioorganic & medicinal chemistry letters 21.19 (2011): 5818-5821.
  3. Rangachari, Vijayaraghavan, et al., Rationally designed dehydroalanine (ΔAla)‐containing peptides inhibit amyloid‐β (Aβ) peptide aggregation. Biopolymers: Original Research on Biomolecules 91.6 (2009): 456-465.
  4. Rangaraju, Srikant, et al., Potassium channel Kv1. 3 is highly expressed by microglia in human Alzheimer's disease. Journal of Alzheimer's disease 44.3 (2015): 797-808.

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