New insights into the role of mitochondrial calcium homeostasis in cell migration

Abstract

Mitochondria are dynamic organelles involved in numerous physiological functions. Beyond their function in ATP production, mitochondria regulate cell death, reactive oxygen species (ROS) generation, immunity and metabolism. Mitochondria also play a key role in the buffering of cytosolic calcium, and calcium transported into the matrix regulates mitochondrial metabolism. Recently, the identification of the mitochondrial calcium uniporter (MCU) and associated regulators has allowed the characterization of new physiological roles for calcium in both mitochondrial and cellular homeostasis. Indeed, recent work has highlighted the importance of mitochondrial calcium homeostasis in regulating cell migration. Cell migration is a property common to all metazoans and is critical to embryogenesis, cancer progression, wound-healing and immune surveillance. Previous work has established that cytoplasmic calcium is a key regulator of cell migration, as oscillations in cytosolic calcium activate cytoskeletal remodelling, actin contraction and focal adhesion (FA) turnover necessary for cell movement. Recent work using animal models and in cellulo experiments to genetically modulate MCU and partners have shed new light on the role of mitochondrial calcium dynamics in cytoskeletal remodelling through the modulation of ATP and ROS production, as well as intracellular calcium signalling. This review focuses on MCU and its regulators in cell migration during physiological and pathophysiological processes including development and cancer. We also present hypotheses to explain the molecular mechanisms by which MCU may regulate mitochondrial dynamics and motility to drive cell migration.

Keywords: Calcium; Cell migration; MCU; Mitochondria.

Fig. 1

Calcium homeostasis at the ER-mitochondria contact sites. Under stimulation of the G protein-coupled receptor at the plasma membrane (PM), the phospholipase C (PLC) hydrolyses phosphatidylinositol 4,5-biphosphate (PIP2) into inositol 1,4,5-triphosphate inositol (IP3) and diacylgycerol (DAG). IP3 binds and activates the IP3 receptor (IP3R) leading to endoplasmic reticulum (ER) Ca²⁺ release in the cytosol or into neighbouring organelles. Due to the close proximity of the ER and the mitochondria, ensured by membrane tethering, highly localized and concentrated Ca²⁺ microdomains are specifically formed facilitating Ca²⁺ transfer to the mitochondria. Ca²⁺ first enters the mitochondria through the voltage-dependent anion channel (VDAC) at the OMM and then the mitochondrial calcium uniporter (MCU) transports it across the IMM. MCU is part of a complex, the MCU machinery (MCUM) composed of a negative regulator, MCUb, and EMRE, an essential IMM component required for the uniporter minimal activity. MCU is mainly regulated by members of the MICU family of proteins localized in the IMS, including MICU1 and MICU2 (because of the unknown function of MICU3 and its specific expression neuronal tissues, the latter is not represented in the model). MICU1 is considered as the MCU gatekeeper; at low cytosolic [Ca²⁺] MICU1 inhibits MCU activity whereas at high cytosolic [Ca²⁺], the binding of Ca²⁺ on MICU1 EF-hand leads to its conformational change and MCU channel activation. Ca²⁺ is extruded from the mitochondrial matrix by the IMM resident NCLX, which exchanges 1 Ca²⁺ for 3 Na⁺. The sarco/endoplasmic reticulum Ca²⁺ ATPase (SERCA) ensures the ER-refilling in Ca²⁺.

Fig. 2

Schematic representation of calcium signalling in a migrating cell. During migration, cells exhibit a typical rear-to-front polarization. The cell migration machinery, including actin polarization and focal adhesion (FA) dynamics, is spatio-temporally regulated by cytosolic Ca²⁺. A [Ca²⁺] gradient is observed in the polarized cell, with a high [Ca²⁺] at the back required for calpain-dependent FA disassembly and with low [Ca²⁺] at the leading edge facilitating functional local Ca²⁺ pulses formation. This [Ca²⁺] gradient is ensured by the accumulation of the plasma membrane Ca²⁺ ATPase (PMCA) pump at the cell leading edge leading to Ca²⁺ extrusion. Local Ca²⁺ flickers/pulses at the leading edge are established by intracellular Ca²⁺ entry controlled by the transient receptor potential (TRP) channel or by a SOCE-STIM1/ORAI1-dependent mechanism. These localized Ca²⁺ microdomains allow actin-myosin contraction and FA assembly dynamics for cell migration. Mitochondrial drp1-dependent fission allows their relocalization to the leading edge, in order to generate ATP and ROS required for cytoskeleton remodelling. Mitochondria at the leading edge may also control the intracellular Ca²⁺ signalling, including store-operated calcium entry (SOCE) or ER Ca²⁺ release required for proper cell migration.

Fig. 3

Proposed models for the role of mitochondria in SOCE regulation. SOCE is characterized by the extracellular Ca²⁺ entry controlled by ER Ca²⁺ store depletion. At low ER [Ca²⁺], Ca²⁺ dissociates form the ER-resident Stromal interacting molecule 1 (STIM1) allowing its oligomerization and relocalization at ER-PM contact sites. At these sites, STIM1 interacts with ORAI1 and activates the channel allowing Ca²⁺ entry. During this process, PCMA extrudes Ca²⁺ in the extracellular space and SERCA constantly refills the ER. In non-excitable cells, the contribution of mitochondria to SOCE regulation remains controversial. Mitochondria can be involved in SOCE regulation: (A) The immune cell model: During T-cell activation, mitochondria relocalize at the PM where they directly buffer Ca²⁺ entry. This reduces [Ca²⁺] at ER-PM contact sites and prevents the slow inactivation of ORAI1 by Ca²⁺ (Red circle arrow). (B) Alternative model: Due to steric hindrance at the ER-PM contact sites, mitochondria cannot directly buffer Ca²⁺ at these sites. Mitochondria contribute to ER Ca²⁺ store depletion by directly uptaking Ca²⁺ from the ER at the mitochondria-ER contact sites, contributing indirectly to SOCE activation.