New sensor mimics cell membrane functions

Newswise — CAMBRIDGE, MA --Taking inspiration from natural sensory systems, a team led by MIT has engineered an innovative sensor capable of identifying molecules in a manner similar to naturally occurring cell receptors.

By integrating multiple cutting-edge technologies, the scientists have successfully developed a prototype sensor capable of detecting an immune molecule called CXCL12, even in concentrations as low as tens or hundreds of parts per billion. This pioneering achievement marks a significant milestone towards creating a system that could conduct routine screenings for hard-to-diagnose cancers or metastatic tumors. Moreover, the sensor holds promise as a highly biomimetic electronic "nose."

The researchers envision a user-friendly device for at-home testing, exhibiting both high specificity and sensitivity. Detecting cancer at an early stage is crucial for better treatment outcomes, making cancer diagnostics a primary focus of their efforts, as stated by Shuguang Zhang, a principal research scientist in MIT's Media Lab.

The device's design draws inspiration from cell membranes, which encompass all cells and contain numerous receptor proteins that can identify molecules in the surrounding environment. To adapt these proteins for external use, the MIT team modified some of them and anchored them in a layer of crystallized proteins situated atop an array of graphene transistors. Upon detecting the target molecule in a sample, these transistors transmit the information to a computer or smartphone, enabling efficient and accurate analysis.

According to the researchers, this sensor technology has the potential to be adapted for the analysis of various bodily fluids, including blood, tears, or saliva. Depending on the specific receptor proteins utilized, it could simultaneously screen for multiple different targets.

Rui Qing, formerly a research scientist at MIT and currently an associate professor at Shanghai Jiao Tong University, explains the process: they identify crucial receptors from biological systems and affix them to a bioelectronic interface. This allows them to capture all the biological signals and convert them into electrical outputs, which can be further analyzed and interpreted using machine-learning algorithms.

The study's lead authors are Rui Qing and Mantian Xue PhD '23, and the research has been published in Science Advances. Alongside Qing and Xue, the senior authors of the paper include Tomás Palacios, director of MIT's Microsystems Laboratory and a professor of electrical engineering and computer science, and Uwe Sleytr, an emeritus professor at the Institute of Synthetic Bioarchitectures at the University of Natural Resources and Life Sciences in Vienna.

Free from membranes

Currently, most diagnostic sensors rely on antibodies or aptamers, which are short DNA or RNA strands capable of capturing specific target molecules in bodily fluids like blood. However, these methods have their limitations. Aptamers can be easily degraded by body fluids, while ensuring consistent production of antibodies in each batch can be challenging.

As an alternative approach, scientists have been investigating the use of receptor proteins found in cell membranes. These proteins are essential for cells to monitor and respond to their surroundings. The human genome contains thousands of such receptors. However, working with these proteins proves challenging as they lose their structure when removed from the cell membrane and can only retain it when suspended in a detergent.

In 2018, Zhang, Qing, and their colleagues introduced an innovative method to transform hydrophobic proteins into water-soluble proteins. They achieved this by substituting a few hydrophobic amino acids with hydrophilic ones, following the QTY code. The code derives its name from the three hydrophilic amino acids involved: glutamine, threonine, and tyrosine, which replace the hydrophobic amino acids leucine, isoleucine, valine, and phenylalanine.

"Utilizing receptors for sensing has been attempted for many years, but its widespread application has been hindered due to the need for detergents to maintain their stability. Our breakthrough lies in rendering them water-soluble and capable of large-scale, cost-effective production," explains Zhang.

In a collaborative effort, Zhang and longtime partner Sleytr sought to attach water-soluble versions of receptor proteins to a surface, leveraging bacterial proteins that Sleytr had extensively studied. These bacterial proteins, known as S-layer proteins, naturally form the outermost surface layer of the cell envelope in various types of bacteria and archaea.

When crystallized, S-layer proteins create well-ordered monomolecular arrays on a surface. Previous research by Sleytr demonstrated that these proteins could be fused with other functional molecules like antibodies or enzymes. For their current study, the researchers, including senior scientist Andreas Breitwieser, another co-author, utilized S-layer proteins to construct a highly dense, immobilized sheet of a water-soluble version of a receptor protein known as CXCR4. This receptor binds with the target molecule CXCL12, which plays critical roles in several human diseases, including cancer, as well as with an HIV coat glycoprotein responsible for virus entry into human cells.

"We exploit these S-layer systems to facilitate the attachment of functional molecules onto a surface in a precise monomolecular array with well-defined distribution and orientation," explains Sleytr. He likens this arrangement to a chessboard, where different pieces can be positioned with exceptional accuracy.

The innovative sensing technology created by the researchers is aptly named RESENSA (Receptor S-layer Electrical Nano Sensing Array).

Sensitivity with biomimicry

The researchers found that these crystallized S-layers can be applied to virtually any surface. In this specific study, they affixed the S-layer to a chip featuring graphene-based transistor arrays, a technology previously developed in Palacios' lab. The remarkable single-atomic thickness of graphene transistors renders them highly suitable for creating incredibly sensitive detectors.

Within Palacios' lab, Xue worked on adapting the chip so that it could be coated with a dual layer of proteins – crystallized S-layer proteins combined with water-soluble receptor proteins. When a target molecule from the sample binds to a receptor protein, it induces a change in the electrical properties of the graphene, and this alteration can be easily measured and transmitted to a computer or smartphone connected to the chip.

Explaining their choice of graphene as the transducer material, Xue states, "We opted for graphene due to its exceptional electrical properties, which allow it to translate signals effectively. Additionally, its sheet-like structure of carbon atoms results in the highest surface-to-volume ratio, ensuring that any surface changes caused by protein binding events are directly translated to the entire material's bulk."

The graphene transistor chip has the remarkable capability to host S-layer-receptor proteins with an impressive density of 1 trillion receptors per square centimeter, all oriented upwards. This strategic arrangement maximizes the receptor proteins' sensitivity and ensures optimal detection of target analytes within the clinically relevant range in the human body. The chip's array incorporates over 200 devices, providing redundancy in signal detection, a crucial factor in ensuring reliable measurements, especially for rare molecules that might indicate the presence of early-stage tumors or the onset of Alzheimer's disease, as emphasized by the researchers.

By leveraging the QTY code, naturally occurring receptor proteins can be modified and harnessed to create an array of sensors within a single chip. This sensor array has the potential to screen virtually any molecule that cells are capable of detecting. The researchers envision a future in which this foundational technology can be integrated into portable devices, seamlessly connecting with cell phones and computers. This would enable individuals to conduct tests at home swiftly, obtaining essential health insights and promptly deciding whether a visit to the doctor is necessary, as articulated by Qing.

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