Neuronal Activity and Glutamate Uptake Decrease Mitochondrial Mobility in Astrocytes and Position Mitochondria Near Glutamate Transporters

Abstract

Within neurons, mitochondria are nonuniformly distributed and are retained at sites of high activity and metabolic demand. Glutamate transport and the concomitant activation of the Na ϩ /K ϩ -ATPase represent a substantial energetic demand on astrocytes. We hypothesized that mitochondrial mobility within astrocytic processes might be regulated by neuronal activity and glutamate transport. We imaged organotypic hippocampal slice cultures of rat, in which astrocytes maintain their highly branched morphologies and express glutamate transporters. Using time-lapse confocal microscopy, the mobility of mitochondria within individual astrocytic processes and neuronal dendrites was tracked. Within neurons, a greater percentage of mitochondria were mobile than in astrocytes. Furthermore, they moved faster and farther than in astrocytes. Inhibiting neuronal activity with tetrodotoxin (TTX) increased the percentage of mobile mitochondria in astrocytes. Mitochondrial movement in astrocytes was inhibited by vinblastine and cytochalasin D, demonstrating that this mobility depends on both the microtubule and actin cytoskeletons. Inhibition of glutamate transport tripled the percentage of mobile mitochondria in astrocytes. Conversely, application of the transporter substrate D-aspartate reversed the TTX-induced increase in the percentage of mobile mitochondria. Inhibition of reversed Na ϩ /Ca 2ϩ exchange also increased the percentage of mitochondria that were mobile. Last, we demonstrated that neuronal activity increases the probability that mitochondria appose GLT-1 particles within astrocyte processes, without changing the proximity of GLT-1 particles to VGLUT1. These results imply that neuronal activity and the resulting clearance of glutamate by astrocytes regulate the movement of astrocytic mitochondria and suggest a mechanism by which glutamate transporters might retain mitochondria at sites of glutamate uptake.

Mitochondrial dynamics in astrocytes

“…Although significant progress has been made to elucidate the mechanisms that regulate mitochondrial trafficking dynamics in neurons, far less is known about the mechanisms regulating mitochondrial dynamics in astrocytes. Recently, Jackson et al [42] revealed that mitochondria are non-uniformly distributed along the fine processes of astrocytes (<600 nm in diameter) in organotypic slices, confirming previous results from astrocyte cultures [43,44] and in vivo [45,46]. Their trafficking was bidirectional with 44 % of mitochondria moving in the retrograde direction (towards the cell body) and 56 % moving in the anterograde direction (away from the cell body).…”

Morphological plasticity of astroglia: Understanding synaptic microenvironment

“…This does not necessarily imply that elegant experimental manipulations with astroglial Ca 2+ within a certain dynamic range by triggering certain cellular cascades should reproduce such effects (Agulhon et al, 2010; Fiacco et al, 2007; Petravicz et al, 2008) (see (Rusakov et al, 2014; Volterra et al, 2014) for discussion). In addition to the much debated astrocyte‐neuron exchange, Ca 2+ rises in astrocytes could also boost the expression level of glutamate transporters (Devaraju et al, 2013), re‐position mitochondria closer to glutamate transporters (Jackson et al, 2014; Ugbode et al, 2014), and regulate neuro‐metabolic coupling with neurons (Bernardinelli et al, 2004; Porras et al, 2008). Recent findings suggest that such Ca 2+ signals could be required for morphological changes in PAPs (Molotkov et al, 2013; Tanaka et al, 2013).…”

Fluctuations in Cytosolic Calcium Regulate the Neuronal Malate–Aspartate NADH Shuttle: Implications for Neuronal Energy Metabolism

“…-induced inhibition of MAS. We know that mitochondria are heterogeneous in terms of their proteome and function among different organs, within organs, and likely within the same cell [38][39][40][41]. Is it likely that mitochondria within the same neuron are different, or that they experience differences in their cellular environment that causes them to respond distinctly to different stimuli?…”