Functional Interactions between BK_Caα-Subunit and Annexin A5: Implications in Apoptosis

Area of Science:

• Biochemistry
• Cell Biology
• Ion Channel Physiology

Background:

• Proteomic studies suggest an interaction between the BKCaα and ANXA5.
• ANXA5 translocates to the outer plasma membrane leaflet during apoptosis.
• BKCaα's C-terminus contains Ca2+-binding and potential ANXA5-binding sites.

Purpose of the Study:

• To investigate the functional consequences of the BKCaα-ANXA5 interaction in apoptosis.
• To elucidate the role of ANXA5 in regulating BKCaα channel activity during cell death.
• To explore the impact of this partnership on caspase activation.

Main Methods:

• Coimmunoprecipitation and double labeling immunocytochemistry to confirm protein interaction.
• Membrane biotinylation to track ANXA5 translocation during apoptosis.
• Patch-clamp electrophysiology to measure BKCa channel Ca2+-sensitivity.
• HEK 293 cells were used to study serum deprivation-induced apoptosis and caspase activation.

Main Results:

• ANXA5 was observed to flip to the outer membrane leaflet early in serum deprivation-induced apoptosis in HEK 293 cells.
• Serum deprivation led to caspase-3/7 activation, which was enhanced in cells expressing BKCaα.
• ANXA5 knockdown increased BKCa channel Ca2+-sensitivity, while ANXA5 overexpression decreased it.

Conclusions:

• Membrane flippage of ANXA5 during apoptosis removes its inhibitory effect on BKCaα channel activity.
• This removal amplifies Ca2+ influx, leading to augmented caspase activation and promoting cell death.
• A novel model for early apoptosis stages involving ANXA5 and BKCaα is proposed.