Plasma (the solution fraction of blood after removal of blood cells) or serum (the solution fraction of blood after removal of blood cells and clotting factors) is perhaps the most complex protein mixture containing
- May be greater than 100,000 proteins and proteins variants if post translation modifications (PTMs), truncation, splice variants, degradation products, precursor and mature proteins and finally all the immunoglobulin variants are taken into account
- Protein concentration range of 10 orders of magnitude: Albumin is present at 50mg/ml whereas interleukins are present in sub pg/ml range
Change in abundance of plasma/serum protein level and/or protein structural changes are responsible for majority if not all of the human diseases. Considering the importance of plasma/serum proteome (collection of all the proteins in plasma/serum) a simple, inexpensive and easy to implement detection method for quantitation of proteins in plasma/serum will be very attractive for diagnostics. Prof. Rotello and his group have come up with a proof-of-principle concept for serum protein sensor by combining gold nanoparticles and Green fluorescent proteins (GFPs). Concept is simple and elegant: when gold nanoparticles (NPs) decorated with positively charged ligands are added to the GFP proteins solution (pI=5.92 hence negatively charged at physiological pH) the fluorescence of GFPs is quenched due to the formation of NP-GFP complex (because of electrostatic attraction).
When other proteins are added to NP-GFP complex the GFP is either released from NP-GFP complex and result in increase in fluorescence or added proteins complex with GFPs further reducing the fluorescence. As it turns out the increase and decrease in fluorescence is protein dependent as well as dependent on the ligands decorating gold NPs. When specific proteins are mixed with a panel of five gold NPs decorated with different ligands each increase and decrease of fluorescence create a pattern/signature that is specific for each protein. Using this approach, researchers demonstrated that they could detect five most abundant serum proteins in buffer as well as in serum. A similar trick was used by them to create a ‘chemical nose’ to sniff out cancer cells.
This technology has the potential to be implemented as a bed side diagnostic device but several challenges remains especially
- Detection of low abundant proteins. Cytokines in serum are important indicators of inflammations among other things and are typically present in pg/mL range
- Can the sensor distinguish between native proteins or proteins modified by PTMs, truncation, and degradation?
- Cost and multiplexing capabilities
Will keep an eye on next generation of “chemical noses”!