Study finds amphipathic peptide that safely opens endosomal membrane

The cell plasma membrane and the derived endosomal membranes are both greasy barriers that prevent the inflow of water-soluble macromolecules like nucleic acids and proteins into cells.

Breaking “doors” on the plasma membrane directly can destroy the cell easily, however opening inner “doors” on the endosomal membrane was proven to be difficult, despite being safe.

Recent research reported that a peptide that can securely open up an endosomal membrane. This facilitates cargos-like proteins to be delivered into the cytoplasm and at the same time, leaves the plasma membrane intact. The research was published in the Journal of Biological Chemistry.

The study was headed by Professor Hao Fei from the Suzhou Institute of Nano-tech and Nano-Bionics of the Chinese Academy of Sciences. The researchers employed a strategy of hydrophobicity-tuning on a set of artificial peptides and found an optimized peptide specifically responsive to the acidic environment on the endosomal membrane.

These peptides normally called “amphipathic peptide”, have both polar (water-attracting or hydrophilic) and greasy (water-repelling or hydrophobic) ends. The greasy end facilitates the insertion of the peptide into membranes. The polar end aids the peptide to persist as water-soluble and later develop helical shapes to create holes in the membranes.

These peptides are positively charged or “cationic”. This is due to the numerous amine-containing amino acids in their sequences and these cationic groups are drawn to the negatively charged sialic acids (a kind of sugar) and acidic phospholipids mostly seen in the cell membrane, however greatly enriched in endosomal membranes.

The scientists balanced the two major actions of the peptide by the hydrophobicity-tuning strategy. The two actions are attaching themselves to the cell membrane without resulting in cell death, and developing peptide assembled holes in the acidic endosomal membrane to facilitate cargo entry.

The resultant peptide LP6 has the required and adequate hydrophobicity to endue its helical and aggregation capability in the acidic endosomal environment, and can drastically facilitate cargo cell entry and cytosolic delivery of macromolecules like protein toxin and dextran.

From the reductionist point of view, the study used the simplest natural proteinogenic amino acids and the most essential structures of peptide, thus our finding may represent a structure-function relationship that is fundamental.”

Hao Fei, Professor, Suzhou Institute of Nano-tech and Nano-Bionics, Chinese Academy of Sciences

It is evaluated that 75% of the human proteome is intracellular and unapproachable to protein therapeutics like a monoclonal antibody. Furthermore, numerous protein targets, including mutated tumor suppressors, overexpressed oncogenic proteins, were considered “undruggable” by small molecule inhibitors.

Therefore, this research will be instructive for the rational design and modification of efficient vehicle peptides for cytosolic delivery of macromolecular therapeutics for biomedical application in the future.