🧬 biology

A gFET-based Sensor Equipped with a Focused Post-SELEX Aptamer Library for Specific Quantification of the Hormone-like Anti-Aging Protein Klotho in Human Serum

This study develops a highly sensitive, label-free graphene field-effect transistor (gFET) sensor equipped with a focused post-SELEX aptamer library to enable the specific and rapid quantification of the anti-aging protein Klotho and its critical KL1 domain in human serum.

Original authors: Runliu Li, Shengnan He, Daniel Gruber, Markus Krämer, Andreas Stürmer, Roger Hasler, Christoph Kleber, Wolfgang Knoll, Ann-Kathrin Kissmann, Frank Rosenau

Published 2026-08-25

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Runliu Li, Shengnan He, Daniel Gruber, Markus Krämer, Andreas Stürmer, Roger Hasler, Christoph Kleber, Wolfgang Knoll, Ann-Kathrin Kissmann, Frank Rosenau

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ⚕️ This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer

Aging is not merely the passage of time; it is a biological process that gradually alters how our tissues and organs function, often paving the way for serious conditions like heart disease, kidney failure, and memory loss. For decades, scientists have searched for a reliable way to measure the body's internal resistance to this decline. One promising candidate is a protein called Klotho, which acts as a powerful shield against aging. Found primarily in the kidneys, brain, and parathyroid glands, this protein helps regulate metabolism, reduce inflammation, and protect cells from stress. When Klotho levels drop, the risk of age-related diseases rises, and overall survival tends to decrease. However, measuring this protein in the human body has been a significant challenge. The Klotho protein exists in several forms, including a full-length version and a soluble version that floats freely in the blood. Current testing methods struggle to tell these different forms apart, leading to inconsistent results that make it difficult to use Klotho as a precise marker for health or disease.

To solve this problem, a team of researchers at Ulm University and collaborating institutions developed a new way to detect and measure the specific, active forms of Klotho circulating in human blood. Instead of using traditional antibodies, which are proteins produced by the immune system and can be difficult to manufacture consistently, the team turned to aptamers. These are short, single-stranded pieces of DNA that can be engineered to fold into specific shapes, allowing them to latch onto target molecules with high precision, much like a key fitting into a lock. The researchers began by creating a vast library of billions of these random DNA strands. They then subjected this library to a rigorous selection process known as FluMag-SELEX. In this method, the DNA strands were mixed with magnetic beads coated with the Klotho protein. The strands that failed to stick were washed away, while those that bound tightly to the protein were kept. This cycle was repeated nine times, with each round becoming more demanding, effectively filtering out the weak binders and enriching the pool with only the most effective DNA sequences.

The result of this intensive selection was a polyclonal library of aptamers that showed a remarkable ability to recognize the Klotho protein, specifically targeting a crucial section known as the KL1 domain. This domain is responsible for most of the protein's beneficial activities. The researchers verified that these DNA strands could distinguish the human Klotho protein from similar proteins, such as a related version found in mice or a different form of Klotho called beta-Klotho, which does not share the same anti-aging functions. To prove that these aptamers worked in a real-world setting, the team tested them in human serum, the liquid part of blood that contains a complex mix of salts, enzymes, and other proteins. Even in this crowded and chemically active environment, the aptamers successfully captured the Klotho protein without being confused by the surrounding biological noise. They demonstrated that the DNA strands remained stable and effective, binding to the target with a strength measured at a very low concentration, indicating a high level of sensitivity.

Building on this success, the researchers integrated these aptamers into an electronic sensor called a graphene field-effect transistor. This device uses a sheet of graphene, a material made of a single layer of carbon atoms, which is extremely sensitive to changes in its electrical environment. When the aptamers on the sensor surface caught a Klotho molecule, the electrical charge on the graphene shifted slightly. By measuring this shift, the sensor could detect the presence of the protein without needing any fluorescent labels or chemical tags. The device proved capable of detecting Klotho across a wide range of concentrations, from very low levels to high levels, covering a span of one thousand times the difference between the lowest and highest amounts. This broad range is significant because Klotho levels in humans vary widely depending on age and health status. The sensor maintained its accuracy even when tested in human serum, showing that it could reliably quantify the protein in conditions that mimic a real clinical sample.

The study confirms that this combination of DNA-based recognition and electronic sensing offers a stable, antibody-free method for measuring Klotho. Unlike previous tests that could not differentiate between the various forms of the protein, this new approach can specifically identify the active human version and its functional KL1 domain. The researchers found that the sensor could detect the protein at levels as low as 40.1 picograms per milliliter in a buffer solution, and it performed consistently well in human serum. This capability suggests that the technology could eventually be used to monitor aging and age-related diseases with greater precision than current methods allow. By providing a clear, specific, and sensitive way to measure Klotho, this work opens a path toward better understanding how the body ages and how that process might be influenced or treated in the future.

Technical Summary: A gFET-Based Sensor with a Focused Post-SELEX Aptamer Library for Klotho Quantification

Problem Statement
The Klotho (KL) protein is a critical regulator of aging, phosphate metabolism, and oxidative stress. Reduced levels of soluble $\alpha$ -Klotho in human circulation are associated with aging, Alzheimer's disease, chronic kidney disease, and diabetes. However, current detection methods, primarily Enzyme-Linked Immunosorbent Assays (ELISA), suffer from significant limitations. They often fail to distinguish between different circulating Klotho isoforms (e.g., full-length $\alpha$ -Klotho vs. its functional KL1 domain) and other homologs like $\beta$ -Klotho. Furthermore, ELISA methods typically offer limited dynamic ranges and inconsistent quantification in complex biological matrices like human serum. There is a need for a stable, antibody-free, and highly specific method capable of distinguishing Klotho subtypes and quantifying them with high sensitivity in clinical samples.

Methodology
The study employed a multi-stage approach combining advanced aptamer selection with electronic biosensing:

FluMag-SELEX Selection: The researchers utilized a Fluorescence-based Magnetic Systematic Evolution of Ligands by EXponential enrichment (FluMag-SELEX) to generate a polyclonal single-stranded DNA (ssDNA) aptamer library specific to $\alpha$ -Klotho and its KL1 domain.
- Target: $\alpha$ -Klotho immobilized on tosyl-activated magnetic beads.
- Process: Nine rounds of selection were performed with increasing stringency. This included counter-selection against unloaded beads to remove non-specific binders, increased concentrations of blocking agents (BSA and tRNA), and reduced aptamer input concentrations.
- Monitoring: Progress was tracked via quantitative PCR (qPCR) to measure enrichment and melting curve analysis to assess structural stability (melting temperature, $T_m$ ).
- Validation: Specificity was verified using fluorescence microscopy and microtiter plate assays against $\alpha$ -Klotho, $\beta$ -Klotho, mouse Klotho, and Human Serum Albumin (HSA).
Aptamer-Linked Fluorescence Assay (ALFA): A "sandwich" assay was developed where amine-labeled aptamers immobilized on magnetic beads captured Klotho from various matrices (PBS, diluted serum, total serum). A Cy5-labeled aptamer was then used to quantify the captured target. This validated the library's stability and binding capacity in complex biological fluids.
gFET Integration: The selected aptamer library was integrated into an electrolyte-gated reduced graphene oxide field-effect transistor (EG-rGO-FET).
- Functionalization: The 5'-amine-modified aptamer library was covalently coupled to a pyrene-carboxylate functionalized rGO surface via EDC-NHS chemistry.
- Detection: The sensor operated in a label-free manner. Binding of Klotho to the aptamer-functionalized channel altered the local electrostatic environment, causing measurable shifts in the source-drain current ( $I_{DS}$ ) at a fixed gate voltage ( $V_G$ ).

Key Results

Aptamer Library Evolution: The SELEX process successfully enriched a polyclonal library with high affinity. The library melting temperature rose from 62°C at the beginning of SELEX (STm) to 74°C at the end of SELEX (ETm), indicating the enrichment of aptamers with higher GC content and structural stability.
Binding Affinity and Specificity: The final Round 9 library exhibited a dissociation constant ( $K_d$ ) of $14.6 \pm 2.2$ nM. Crucially, the library demonstrated high specificity, binding strongly to human $\alpha$ -Klotho and the KL1 domain while showing negligible cross-reactivity with $\beta$ -Klotho, mouse Klotho, or HSA.
ALFA Performance: The sandwich assay confirmed that the aptamer-bead system remained stable and functional in total human serum, successfully capturing Klotho without significant loss of binding capability.
gFET Sensor Performance:
- Dynamic Range: The sensor achieved a broad linear detection range of 4 to 4000 pg/mL, covering three orders of magnitude.
- Sensitivity: The Limit of Detection (LoD) was calculated at 40.1 pg/mL for $\alpha$ -Klotho in buffer.
- Selectivity in Serum: Spike-in experiments in human serum confirmed that the sensor maintained its specificity and sensitivity in complex biological matrices, distinguishing $\alpha$ -Klotho from non-target proteins.
- Affinity in Matrix: Nonlinear regression indicated that the sensor maintained comparable affinities for $\alpha$ -Klotho in serum ( $EC_{50} \approx 2.93$ pg/mL) as in buffer, confirming the accessibility of binding sites despite the presence of interferents.

Significance and Claims
The paper claims to establish a stable, antibody-free strategy for the specific quantification of Klotho isoforms, particularly the KL1 domain, which is difficult to detect using conventional methods. By integrating a focused post-SELEX aptamer library with a graphene field-effect transistor, the authors present a platform that offers:

Isoform Discrimination: The ability to distinguish $\alpha$ -Klotho from $\beta$ -Klotho and homologous proteins, addressing a key limitation of current ELISA kits.
Serum Compatibility: Robust performance in undiluted human serum, enabling direct clinical application without extensive sample preparation.
Label-Free Detection: Fast, sensitive, and real-time quantification capabilities suitable for point-of-care diagnostics.

The authors position this technology as a promising foundation for monitoring circulating Klotho levels in the context of aging and age-related diseases, potentially facilitating the study of Klotho's role in conditions such as chronic kidney disease and metabolic dysfunction. They note that while the current study utilizes a polyclonal library without isolating individual aptamers via Next-Generation Sequencing, the library itself is sufficient for immediate functionalization and diagnostic utility.

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