GTPase of the immune-associated nucleotide-binding protein 5 (GIMAP5) regulates calcium influx in T-lymphocytes by promoting mitochondrial calcium accumulation

Mature T-lymphocytes undergo spontaneous apoptosis in the biobreeding diabetes-prone strain of rats due to the loss of the functional GIMAP5 (GTPase of the immune-associated nucleotide-binding protein 5) protein. The mechanisms underlying the pro-survival function of GIMAP5 in T-cells have not yet been elucidated. We have previously shown that GIMAP5 deficiency in T-cells impairs Ca²⁺ entry via plasma membrane channels following exposure to thapsigargin or stimulation of the T-cell antigen receptor. In the present study we report that this reduced Ca²⁺ influx in GIMAP5-deficient T-cells is associated with the inability of their mitochondria to sequester Ca²⁺ following capacitative entry, which is required for sustained Ca²⁺ influx via the plasma membrane channels. Consistent with a role for GIMAP5 in regulating mitochondrial Ca²⁺, overexpression of GIMAP5 in HEK (human embryonic kidney)-293 cells resulted in increased Ca²⁺ accumulation within the mitochondria. Disruption of microtubules, but not the actin cytoskeleton, abrogated mitochondrial Ca²⁺ sequestration in primary rat T-cells, whereas both microtubules and actin cytoskeleton were needed for the GIMAP5-mediated increase in mitochondrial Ca²⁺ in HEK-293 cells. Moreover, GIMAP5 showed partial colocalization with tubulin in HEK-293 cells. On the basis of these findings, we propose that the pro-survival function of GIMAP5 in T-lymphocytes may be linked to its requirement to facilitate microtubule-dependent mitochondrial buffering of Ca²⁺ following capacitative entry.