Aptamer Applications in Neuroscience Part 4
May 27, 2024
PET is another highly effective imaging method used in clinical diagnostics as it can provide tomographic resolution at any tissue depth.
PET scan, positron emission tomography, is a medical imaging technology that can be used to detect the functional and metabolic status of various organs and tissues. In recent years, PET scans have also been widely used to study memory.
Studies have found that PET scans can detect metabolic activity and blood flow in the brain, which are closely related to human cognitive and memory abilities. PET scans can be used to observe whether there are abnormalities in areas related to cognition and memory, and how they transmit and process information.
The application of PET scanning has enabled scientists to make tremendous progress in studying the nervous system. For example, one study found that the brain metabolism of older adults is slower than that of younger adults, which is one of the main reasons why older adults experience memory loss and cognitive decline. In addition, PET scans can detect signs of neurological diseases, such as neurodegenerative diseases and Alzheimer's disease. This can help doctors diagnose and treat these diseases early to relieve patients' suffering as much as possible.
Furthermore, as we understand, memory is a complex neural process. PET scanning technology allows us to better understand the process of memory formation and preservation. This helps us develop better memory training methods so that people can better master knowledge and skills.
In summary, the use of PET scans in studying memory provides us with more comprehensive data and a deeper understanding. Through this technology, we can better understand, evaluate, and improve our memory and cognitive abilities to better adapt to social development and daily life. It can be seen that we need to improve memory, and Cistanche deserticola can significantly improve memory because Cistanche deserticola is a traditional Chinese medicinal material that has many unique effects, one of which is to improve memory. The efficacy of Cistanche deserticola comes from the many active ingredients it contains, including tannic acid, polysaccharides, flavonoid glycosides, etc. These ingredients can promote brain health in a variety of ways.
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PET imaging is based on positron-emitting radioisotopes, such as 13N, 18F, 11C, 64Cu, 124I, and 68Ga. The fluorine isotope (18F) is often used due to its advantageous half-life of about 110 min, effortless production, clean decay, and low emission energy.
As for MRI, the most important challenge for PET is to design target-specific imaging agents [128]. For this purpose, it was demonstrated that the thrombin aptamer could be photochemically conjugated with 3-azido-5-nitrobenzyl fluoride ([18F]ANBF) [70].
Also, PET imaging based on 18F-labeled aptamers has been reported for proteins such as tenascin C [131], the protein tyrosine kinase 7 [132], and the EGF receptor [133].
3.7.2. Diagnosis
The epidermal growth factor receptor variant III (EGFRvIII), which is oncogenic due to its constitutive activation rather than being regulated by the EGFR ligands, increases glioma tumorigenicity and resistance to treatment [134]. The U87-EGFRvIII cells overexpressing EGFRvIII were used to obtain DNA aptamers that changed the rate of cell growth and increased radiosensitivity [135].
The binding ability of the aptamers (U2, U8, U19, and U31) to U87-EGFRvIII was confirmed via flow cytometry and confocal microscopy analysis. The aptamer inhibited tumor cell (U87) proliferation and metastasis and affected signaling events downstream of the EGFR.
Also, several variants of this aptamer were developed by a series of modifications. While truncation increased its specificity, the insertion of GC pairs into its hairpin stem provided enhanced thermal stability. Aptamers with pyrene modifications identified with the help of molecular docking increased aptamer's affinity to target molecules [136]. Moreover, 118Re-labelled U2 demonstrated an antitumor effect in vivo.
These promising results encourage the application of the U2 aptamer as a novel therapeutic agent in targeted drug delivery systems [125]. The majority of current therapeutics have failed due to the low specificity of the therapeutic agent that exhibits adverse effects.
The main goal of targeted therapy is to enhance the selectivity of drugs and to reduce "off-target" side effects. One approach to achieving this goal is through drug delivery systems using ligands specific to the disease such as aptamer–drug conjugates (ApDCs) [137].
Prodigiosin produced by the bacterium Serratia marcescens is cytotoxic with anti-cancer and anti-malarial features. One of its derivatives, prodigiosin 25-C, has immunosuppressive activity. An aptamer conjugated with prodigiosin specifically targets brain cancer cell surfaces.
Using a molecular modeling tool, Ascalaph designer software, the glutamate receptor, and various aptamers were allowed to interact in the NVT ensemble for the duration of 50 ns and at 310 K. From this simulation study, five candidate aptamers were identified based on their delta intermolecular energy (∆ INME). To confirm the simulation data, these selected aptamers were incubated with brain cancer cells and normal brain cells individually.
Finally, the specific binding percentage of each aptamer was calculated for both cell types. It was observed that aptamers 8, 10, 11, 23, and 69 have the ability to target epitopes on all brain cancer cells with high affinity and low ∆ INME. In addition, among these, aptamer 10 was adsorbed by brain cancer cells at high levels and its adsorption by normal cells was dramatically low [138].
As mentioned before, a major limitation of brain disorder treatment is the BBB, which restricts the majority of small molecules from entering the brain. The need for tissue-specific targeting is another limiting factor. Delivering drugs to key diseased areas in the brain is an essential approach for effective therapy.
Transferrin (Tf), present on the endothelial cell membranes of the BBB, enables molecules to cross the BBB [139–141]. In a recent study, an aptamer targeting the Tf receptor (TfR) was fused to an aptamer binding to EpCAM (epithelial cell adhesion molecule)-expressing cancer cells [142]. The aptamer conjugate maintained specificity and demonstrated an enhanced binding affinity for EpCAM and the TfR.
Transcytosis of these aptamers through the BBB was confirmed in vivo following a 1-nmol injection. This study showed that bifunctional aptamer chimeras can overcome the BBB and have the potential to specifically treat brain disorders.
Utilizing a similar strategy with particles, mesoporous ruthenium [Ru (bpy) 2 (tip)]2+ (RBT) nanoparticles (MRN) were provided a dual-targeting function, which was achieved by the aptamer AS1411 (Apt) and Tf grafted on the MRN surfaces that resulted in high anti-cancer drug-loading capacity [143]. This nanosystem RBT@MRN-SS-Tf/Apt enabled effective BBB penetration by Tf and specific targeting to kill the glioma cells in vitro and in vivo.
Moreover, the production of reactive oxygen species (ROS) by [Ru (bpy) 2 (tip)]2+ induced apoptosis of glioma cells under laser irradiation, enabling photodynamic therapy (PDT), which has been shown to increase the survival period.
These drug delivery approaches demonstrate that targeting the TfR can be a successful means of moving cargo across the BBB and that the aptamer chimeras can be efficiently used for this purpose to treat brain tumors and other brain diseases of the CNS. Small interfering RNAs (siRNA) have the sequence-specific gene-silencing ability, which makes them alternative therapeutic tools.
However, the delivery of siRNA into target cells has been challenging, with numerous aptamer-based siRNA delivery systems being developed to enhance the efficacy of siRNAs [66]. Specific delivery of STAT3 siRNA to GBM cells was achieved using a chimeric aptamer consisting of a siRNA targeting STAT3 (signal transducer and activator of transcription 3) conjugated with a Gint4.
T aptamer targeting the PDGFRβ (platelet-derived growth factor receptor). STAT3 is a key regulator of the aggressive mesenchymal glioblastoma subtype. The delivery of the system and the silencing of STAT3 were determined in PDGFR-positive GBM cells. It was also shown that the chimera system reduces cell viability and migration in vitro and inhibits tumor growth and angiogenesis in vivo [144].
Aptamers may be applied for diagnosis, as they specifically bind to a variety of glioblastoma cell lines over other cancer cell lines with Kds in the range of 78–168 nM [63]. A family of DNA aptamers was selected and optimized for binding a gliosarcoma cell line (K308) using cell-SELEX [145].
They have Kd values in the nanomolar range with the highest affinity aptamer (WQY-9) having a Kd of 21 nM. WQY-9 was highly selective for K308 cells and was internalized by K308 at 37 ◦C. When tested against paraffin-embedded tissue samples, the truncated version of WQY-9 (WQY-9-B) stained 73% (11 of 15) gliosarcoma samples compared with 17% of 12 normal samples.
A random DNA sequence stained 13 and 20% of gliosarcoma and normal samples respectively. An RNA aptamer (H02) was also selected by cell-SELEX that binds the alpha-5-beta-1 integrin and can distinguish between glioblastoma cell lines and tissues from patient-derived tumor xenografts in cyto- and histo-fluorescence assays [146].
Thus, there are several aptamers with promise for applications to produce more effective gliosarcoma diagnostic tools. Some of the aptamers are listed in Table 3.
4. Conclusions and Future Perspective
Countries with rapidly aging populations will be challenged in the future by an increasing number of people affected by several neurodegenerative diseases. By 2050, over two billion people will be over 60 years old and the number of people over 80 will have tripled, from 137 million today to 425 million.
This increase in the number of elderly individuals is expected to be accompanied by a proportional rise in the number of patients affected by neurological diseases.
An increased incidence of brain tumors is also expected, both because cancer incidence increases with age and is possibly exacerbated by the diminished efficiency of repair mechanisms in the elderly brain.
This rise in patients with neurological diseases and brain tumors could be diminished if factors that make the elderly susceptible to neurological disorders and the mechanisms of protein accumulation, impairment in degradation of aggregates, and neuronal cell death were understood, and appropriate diagnostics and treatments were developed.
Therefore, understanding the fundamental bases of neurological diseases and brain tumors and their impact on aging will identify means of their prevention or cure and improve the quality of life for many people in old age.
Antibodies are currently a primary means of diagnosis of molecular biomarkers of neurological disease, particularly for protein biomarkers. However, antibodies are expensive to produce, have batch-to-batch variation that requires extensive quality control, and they require refrigerated storage. When used therapeutically, antibodies also must first be "humanized" to avoid immune rejection.
Opportunities for diagnosis and therapies can be expanded with the inclusion of aptamers that are specific for disease biomarkers and aptamer constructs that can be used to address therapeutic challenges.
Although aptamers provide advantages and many new opportunities for diagnostic and therapeutic applications, their representation in modern diagnostics and therapeutics is low.
Compared with antibodies, aptamers are relatively recently discovered molecules, and their development into approved diagnostic and therapeutic agents will take time. In this review, we have summarized the available aptamers used for diagnosis and therapy and listed them with sequences and properties in Table 3.
However, there are many more potential aptamer targets with links to neurological diseases including LRRK2, Parkin, PINK1, DRP-1, DJ-1, UBQLN2, C9orf72, NEK-1, and FAS.
With further optimization of their function and standardization of their characterization, the application of aptamers is expected to gain momentum and provide many new opportunities in diagnostics and therapeutics in the field of neuroscience.
Author Contributions: All authors have read and agreed to the published version of the manuscript.
Funding: This article is supported by The Scientific and Technological Research Council of Turkey (TÜB˙ITAK) 3501 (Career Development Program), project number 119S845.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: No new data were created or analyzed in this study. Data sharing does not apply to this article.
Conflicts of Interest: The authors declare no conflict of interest.
References
1. Rangel, A.E.; Chen, Z.; Ayele, T.M.; Heemstra, J.M. In vitro selection of an XNA aptamer capable of small-molecule recognition. Nucleic Acids Res. 2018, 46, 8057–8068. [CrossRef] [PubMed]
2. Ilgu, M.; Nilsen-hamilton, M. Aptamers in Analytics. Analyst 2016, 141, 1551–1558. [CrossRef] [PubMed]
3. Ellington, A.D.; Szostak, J.W. In vitro selection of RNA molecules that bind specific ligands. Nature 1990, 346, 818–822. [CrossRef] [PubMed]
4. Robertson, D.L.; Joyce, G.F. Selection in vitro of an RNA enzyme that specifically cleaves single-stranded DNA. Nature 1990, 344, 467–468. [CrossRef] [PubMed]
5. Tuerk, C.; Gold, L. Systematic evolution of ligands by exponential enrichment: RNA ligands to bacteriophage T4 DNA polymerase. Science 1990, 249, 505–510. [CrossRef] [PubMed]
6. Blind, M.; Blank, M. Aptamer Selection Technology and Recent Advances. Mol. Ther. Nucleic Acids 2015, 4, e223. [CrossRef]
7. Kang, K.-N.N.; Lee, Y.-S.S. RNA aptamers: A review of recent trends and applications. Future Trends Biotechnol. 2013, 131, 153–169. [CrossRef]
8. Munzar, J.D.; Ng, A.; Juncker, D. Duplexed aptamers: History, design, theory, and application to biosensing. Chem. Soc. Rev. 2019, 48, 1390–1419. [CrossRef] 9. Zhang, Y.; Lai, B.S.; Juhas, M. Recent advances in aptamer discovery and applications. Molecules 2019, 24, 941. [CrossRef]
10. Zhou, J.; Rossi, J. Aptamers as targeted therapeutics: Current potential and challenges. Nat. Rev. Drug Discov. 2017, 16, 181–202. [CrossRef]
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