Cellular effects of BAPTA: Are they only about Ca chelation?

Intracellular Ca signals play a vital role in a broad range of cell biological and physiological processes in all eukaryotic cell types. Dysregulation of Ca signaling has been implicated in numerous human diseases. Over the past four decades, the understanding of how cells use Ca as a messenger has flourished, largely because of the development of reporters that enable visualization of Ca signals in different cellular compartments, and tools that can modulate cellular Ca signaling. One such tool that is frequently used is BAPTA; a fast, high-affinity Ca-chelating molecule. By making use of a cell-permeable acetoxymethyl ester (AM) variant, BAPTA can be readily loaded into the cytosol of cells (referred to as BAPTAi), where it is trapped and able to buffer changes in cytosolic Ca. Due to the ease of loading of the AM version of BAPTA, this reagent has been used in hundreds of studies to probe the role of Ca signaling in specific processes. As such, for decades, researchers have almost universally attributed changes in biological processes caused by BAPTAi to the involvement of Ca signaling. However, BAPTAi has often been used without any form of control, and in many cases has neither been shown to be retained in cells for the duration of experiments nor to buffer any Ca signals. Moreover, increasing evidence points to off-target cellular effects of BAPTA that are clearly not related to Ca chelation. Here, we briefly introduce Ca signaling and the history of Ca chelators and fluorescent Ca indicators. We highlight Ca-independent effects of BAPTAi on a broad range of molecular targets and describe some of BAPTAi's impacts on cell functions that occur independently of its Ca-chelating properties. Finally, we propose strategies for determining whether Ca chelation, the binding of other metal ions, or off-target interactions with cell components are responsible for BAPTAi's effect on a particular process and suggest some future research directions.