Golgi Antiapoptotic proteins, or GAAPs (not to be confused with GAPs…) are a highly conserved family of membrane proteins that as their name suggests inhibit apoptosis and promote Ca2+ release from intracellular stores in the Golgi. The v-GAAP (viral GAAP) was first characterised in camelpox and vaccinia only 5 years ago, modulating virulence (as infectious agents don’t themselves undergo apoptosis).
The mitochondrion reigns supreme over apoptosis regulation, but endoplasmic reticulum (ER), lysosomes, nucleus, and Golgi apparatus can all transmit stress signals to mitochondria, and thus induce apoptosis.
Calcium ions are tasked with diverse functions in the cell, but notably in its adhesion and motility. The authors of a study in this week’s JCB hypothesise that human GAAP proteins (hGAAPs) may be implicated in these events.
Their work found that hGAAP increased store-operated Ca2+ entry (SOCE) and thereby the activity of calpain [cysteine proteases] at newly forming protrusions.
These hGAAP-dependent effects regulated focal adhesion dynamics and cell migration. Indeed, inhibition or knockdown of calpain 2 abrogated the effects of hGAAP on cell spreading and migration. Our data reveal that hGAAP is a novel regulator of focal adhesion dynamics, cell adhesion, and migration by controlling localized Ca2+ dependent activation of calpain.
The lab used a FRET biosensor comprising CFP and YFP (Cyan and Yellow Fluorescent Proteins) to report the spatial distribution of calpain protease activity near the plasma membrane in live cells. CFP is separated from YFP by a peptide sequence that is cleaved by calpain and thus abolishing FRET (which requires the 2 to be at a short distance).
Interference reflection miscopy (IRM) was used in parallel to define the adhesion areas where cells contacted the substrate. Following the cell revealed calpain activity rose near the plasma membrane in hGAAP-overexpressing cells, and the precision of IRM measurements helped show the overlap in space and time between the increased calpain activity & focal adhesion disassembly.
In the image above, a colour scale from red (low FRET / high calpain activity) through green to blue (high FRET / non-cleaved biosensor) the cell on the right is visibly in the red at its edges, indicating high rates of protein breakdown.
Manganese ions, which like calcium are also divalent (2+), confirmed that hGAAP stimulated entry of cations across the plasma membrane thanks to its quenching of intracellular fura-2 fluorescence.
The group conclude:
hGAAP enhances SOCE, which then stimulates localized activation of calpain near the plasma membrane and thereby increased cleavage of focaladhesion proteins such as FAK. Results with both small molecule inhibitors and siRNA confirm that stimulation of calpain 2 activity is required for hGAAP to promote cleavage of FAK, cell spreading and enhanced migration.
The ER and Golgi usually contain relatively high concentrations of Ca2+. Targeted aequorins have shown previously that hGAAP reduces luminal Ca2+ concentrations in both the ER and Golgi. Constitutive and bidirectional cycling of membranous compartments occurs between the ER and Golgi. Depletion of luminal Ca2+ in one organelle may therefore affect the luminal Ca2+ concentration in the other.
Apoptosis and survival signals are intimately linked to the ability of cells to adhere either to other cells or to the extracellular matrix. In many cases, this is orchestrated by integrin-dependent survival signals that signal via FAK. The hGAAP-dependent inhibition of apoptosis may, therefore, be directly linked with the new roles for GAAP in cell adhesion that we describe here.
Saraiva et al (2013) hGAAP promotes cell adhesion and migration via the stimulation of store-operated Ca2+ entry and calpain 2. Journal of Cell Biology, 202: 699—713
■ English and Voeltz (2013) Endoplasmic Reticulum Structure and Interconnections with Other Organelles. Cold Spring Harbor Perspectives in Biology, 5:a013227