An ideal therapeutics delivery system transports right amount, at right place and for right amount of time with less side effects. Scientists made significant progress by introducing novel drug delivery systems time and again with desired properties. Recently, bacterial contractile injection system (CIS) emerged as potential therapeutics delivery device. Researchers re-programmed this system to deliver desired proteins to human cells.
CIS are syringe-like cell-puncturing machines. Bacteria deploy it for various natural functions including cell-cell communications, defence, and in endosymbionts to interact with the hosts. CIS translocate effectors molecules (toxins or proteins) into the target cell by puncturing its cell membrane. Within the target cell, effectors modulate cell components such as cytoskeleton and DNA.
CIS can be assembled on the cytoplasmic membrane and act as contact dependent system, known as Type VI secretion systems (T6SS). It can exist as free complexes in the cytoplasm, and is released into the extracellular space, known as extracellular CIS (eCIS).
eCIS resembles a bacteriophage (bacteria attacking viruses) without a head. It comprises inner tube, surrounded by an outer contractile sheath, anchored with a base plate. Other end of base plate has tail fibre and spikes. Tail fibre recognises the specific receptor on cell surface and spikes help to bind.
One of the eCIS system is Photorhabdus virulence cassettes (PVC) from Photorhabdus asymbiotica. Photorhabdus genus is known to be insect pathogens and naturally translocate toxins into insect cells. Not long ago, researchers re-engineered eCIS to target mouse cells (not a natural target). It raised possibility to use these sophisticated nanomachines in humans as well. Recently, scientists re-engineered PVC type of eCIS to deliver proteins into the human cells and live mice.
How were they re-engineered for human cell?
eCIS are highly specific in target identification and delivery, thus, attracted attention as potential therapeutics delivery device. How eCIS recognises and delivers to the target is not known. But researchers took the clues from close resembling contractile tail phages (for which mechanism is known). A phage uses its tail fibres to recognise and attach to the target. It brings down the base plate (inner tube, sheath) closer to target and initiate the attachment and injection. Modifying a tail-fibre altered the target specificity in the phages. With similar approach, they modified tail fibres of PVC to target the human cells. Using an AI tool AlphaFold - which predicts the 3D structures of protein from amino acid sequences- they predicted shape of the tail fibre, which recognised human cell rather than insect cell. With another modification, the system delivered proteins of choice.
How efficient this system is?
Re-engineered eCIS that targeted the cancer cells expressing epidermal growth factor receptor (EGFR) only killed cells expressing receptor and with 100% efficiency. In another experiment, engineered eCIS that delivered protein into the brain of live mice provoked no immune responses. This ensures how safe, specific and efficient the system is.
What potential it holds?
Re-engineered eCIS are versatile as could deliver variety of proteins including cancer killing toxins, base editor protein (mediate single nucleotide change in DNA) and Cas9 (DNA cutting enzyme in gene therapy).
Further research would test the technology on different tissues and for different diseases. A better understanding of eCIS would improve its design to be more potent and less immunogenic. The system is versatile (with proteins), having them to deliver DNA or RNA in future would be advantageous. Still early, but can become a powerful tool in cancer and gene therapy!
Kreitz, J., Friedrich, M. J., Guru, A., Lash, B., Saito, M., Macrae, R. K., & Zhang, F. (2023). Programmable protein delivery with a bacterial contractile injection system. Nature, 616 (7956), 357–364. https://doi.org/10.1038/s41586-023-05870-7
Benjamin T., Nick P. H. (29 march 2023) Bacterial ‘syringes’ could inject drugs directly into human cells. Nature podcast https://www.nature.com/articles/d41586-023-00926-0
Heiman, C. M., Vacheron, J., & Keel, C. (2023). Evolutionary and ecological role of extracellular contractile injection systems: from threat to weapon. Frontiers in microbiology, 14, 1264877. https://doi.org/10.3389/fmicb.2023.1264877