Ca²⁺ -dependent interactions between lipids and the tumor-targeting peptide pHLIP

Area of Science:

• Biophysics
• Molecular Biology
• Cancer Research

Background:

• Cancerous tissues exhibit altered extracellular environments, including changes in pH, Ca2+ concentration, and phosphatidylserine (PS) exposure.
• These cellular changes influence molecular interactions with the plasma membrane, impacting the design of targeted therapies.
• Understanding these interactions is key for developing novel cancer-targeting agents like pH-low insertion peptides (pHLIP).

Purpose of the Study:

• To investigate the influence of PS, Ca2+, and peptide protonation state on pHLIP interactions with lipid membranes.
• To elucidate the molecular mechanisms underlying pHLIP targeting and insertion into cancer cell membranes.
• To explore the role of extracellular Ca2+ in modulating pHLIP's selective binding to cancer cells.

Main Methods:

• Cellular studies to assess pHLIP targeting in the presence of exposed PS.
• Fluorescence and circular dichroism experiments using model membranes.
• Microsecond-timescale all-atom molecular dynamics simulations to analyze peptide-lipid interactions.

Main Results:

• Exposed PS on cancer cell membranes significantly promotes pHLIP targeting, with effect magnitude dependent on extracellular Ca2+ concentration.
• Ca2+ ions mediate the coupling between pHLIP and lipid membranes in its unprotonated transmembrane state.
• pH-induced pHLIP insertion depletes surrounding PS, whereas Ca2+-induced insertion increases PS proximity, revealing distinct targeting mechanisms.

Conclusions:

• Extracellular Ca2+ levels are critical for linking cellular membrane changes to selective pHLIP targeting and insertion.
• The Ca2+-dependent coupling between pHLIP and PS provides atomistic insights into lipid-coupled regulation of protein-membrane insertion.
• This study advances the knowledge-based design of molecular tools for cancer therapy by detailing Ca2+-mediated pHLIP-membrane interactions.