Miro1 Regulates Activity-Driven Positioning of Mitochondria within Astrocytic Processes Apposed to Synapses to Regulate Intracellular Calcium Signaling

Abstract

It is fast emerging that maintaining mitochondrial function is important for regulating astrocyte function, although the specific mechanisms that govern astrocyte mitochondrial trafficking and positioning remain poorly understood. The mitochondrial Rho-GTPase 1 protein (Miro1) regulates mitochondrial trafficking and detachment from the microtubule transport network to control activity-dependent mitochondrial positioning in neurons. However, whether Miro proteins are important for regulating signaling-dependent mitochondrial dynamics in astrocytic processes remains unclear. Using live-cell confocal microscopy of rat organotypic hippocampal slices, we find that enhancing neuronal activity induces transient mitochondrial remodeling in astrocytes, with a concomitant, transient reduction in mitochondrial trafficking, mediated by elevations in intracellular Ca2+. Stimulating neuronal activity also induced mitochondrial confinement within astrocytic processes in close proximity to synapses. Furthermore, we show that the Ca2+-sensing EF-hand domains of Miro1 are important for regulating mitochondrial trafficking in astrocytes and required for activity-driven mitochondrial confinement near synapses. Additionally, activity-dependent mitochondrial positioning by Miro1 reciprocally regulates the levels of intracellular Ca2+in astrocytic processes. Thus, the regulation of intracellular Ca2+signaling, dependent on Miro1-mediated mitochondrial positioning, could have important consequences for astrocyte Ca2+wave propagation, gliotransmission, and ultimately neuronal function.SIGNIFICANCE STATEMENTMitochondria are key cellular organelles that play important roles in providing cellular energy and buffering intracellular calcium ions. The mechanisms that control mitochondrial distribution within the processes of glial cells called astrocytes and the impact this may have on calcium signaling remains unclear. We show that activation of glutamate receptors or increased neuronal activity leads to the altered transport of mitochondria and their positioning at synapses dependent on a key mitochondrial trafficking protein called Miro1. We also show that, the control of mitochondrial movement and stopping by Miro plays an important role in regulating astrocyte calcium responses. Thus the regulation of intracellular calcium signaling, by Miro-mediated mitochondrial positioning, could have important consequences for astrocyte signaling and neuron–glial interactions.

Mitochondrial dynamics in neuronal injury, development and plasticity

“…Here, the now stationary mitochondria help return Ca 2+ to baseline levels by directly taking up Ca 2+ and by providing ATP for the pumps that either extrude Ca 2+ or shunt Ca 2+ into the endoplasmic reticulum (Zündorf and Reiser, 2011). Mitochondrial Ca 2+ uptake further boosts ATP synthesis, which is necessary for high-frequency neurotransmitter release (Verstreken et al, 2005;Sheng, 2014;Rangaraju et al, 2014;Stephen et al, 2015).…”

Connecting mitochondrial dynamics and life-or-death events via Bcl-2 family proteins

“…Beyond physical principles of cellular mechanics, mitochondrial distribution must be actively controlled to meet the needs of the cell. In that respect, mitochondrial enrichment has been described at subcellular areas with high ATP consumption, including axonal growth cones (Morris and Hollenbeck, 1993), presynaptic terminals, astrocytic processes near synapses (Stephen et al, 2015), viral assembly sites in ASFV (African swine fever virus)-infected cells (Rojo et al, 1998), and in cortical cytoskeleton of tumor cells (Desai et al, 2013) and oocytes (Guillemin et al, 2009; Van Blerkom and Davis, 2006). …”

“…For example, astrocyte mitochondria move slower than neuronal ones, perhaps pertaining to cell type-specific differences in the trafficking machinery (Fiacco and McCarthy, 2004). However astrocyte mitochondria are still capable of covering the same distances as neuronal mitochondria (Stephen et al, 2015). Hence, in addition to mitochondrial velocity, assessment of the distance covered by moving mitochondria may be of interest.…”

Astroglial glutamate transporters coordinate excitatory signaling and brain energetics

“…While GLT-1 (GLAST, glutamine synthetase, Na + /K + -ATPase, etc) have been localized to the fine processes of astrocytes (Chaudhry et al, 1995; Cholet et al, 2002; Norenberg and Martinez-Hernandez, 1979), there was a general belief that mitochondria were too large to fit into these fine processes (in many other cells mitochondria are 1 μm in diameter or larger). In fact, several studies have demonstrated the presence of mitochondria in fine astrocytic processes (Aoki et al, 1987; Derouiche et al, 2015; Fernandez et al, 1983; Genda et al, 2011; Jackson et al, 2014; Lovatt et al, 2007; Mathiisen et al, 2010; Motori et al, 2013; Mugnaini, 1964; Oberheim et al, 2009; Stephen et al, 2015; Xu et al, 2003 for review, see Stephen et al, 2014). In organotypic hippocampal slice cultures, mitochondria occupy 45% of the average astrocytic process and overlap with GLT-1 puncta occurs more often than would occur by chance (~70% of the time) (Genda et al, 2011).…”

“…In organotypic cultures of rat hippocampus, astrocytes maintain their highly branched morphology (Benediktsson et al, 2005). Using this system and biolistically introduced fluorescent reporter proteins, we and others have recently demonstrated that between 15 and 30% of mitochondria are mobile in astrocytic processes (Jackson et al, 2014; Stephen et al, 2015). In some of our analyses, the percentage of mobile mitochondria was as low as 15%.…”