Regulation of axonal mitochondrial transport and its impact on synaptic transmission

Abstract

Mitochondria are essential organelles for neuronal survival and play important roles in ATP generation, calcium buffering, and apoptotic signaling. Due to their extreme polarity, neurons utilize specialized mechanisms to regulate mitochondrial transport and retention along axons and near synaptic terminals where energy supply and calcium homeostasis are in high demand. Axonal mitochondria undergo saltatory and bidirectional movement and display complex mobility patterns. In cultured neurons, approximately one-third of axonal mitochondria are mobile, while the rest remain stationary. Stationary mitochondria at synapses serve as local energy stations that produce ATP to support synaptic function. In addition, axonal mitochondria maintain local Ca²+ homeostasis at presynaptic boutons. The balance between mobile and stationary mitochondria is dynamic and responds quickly to changes in axonal and synaptic physiology. The coordination of mitochondrial mobility and synaptic activity is crucial for neuronal function synaptic plasticity. In this update article, we introduce recent advances in our understanding of the motor-adaptor complexes and docking machinery that mediate mitochondrial transport and axonal distribution. We will also discuss the molecular mechanisms underlying the complex mobility patterns of axonal mitochondria and how mitochondrial mobility impacts the physiology and function of synapses.

Figure 1. Axonal mitochondrial transport

In axons, MTs are uniformly organized with the plus (+) ends facing toward the axonal terminals and the minus (−) ends toward the cell body. While kinesin motors are mostly plus-end directed, dyneins travel toward the minus ends of MTs. Therefore, kinesin motors generally mediate anterograde axonal transport of mitochondria and dynein drives retrograde axonal transport of mitochondria. (Adapted with permission from Qian Cai, Zu-Hang Sheng. Mitochondrial transport and docking in axons. Experimental Neurology 218, 257–267, 2009).

Figure 2. Structure of motor proteins

(A) KIF5 motors form homodimers through the coiled-coil region in the stalk domains. While KIF5 possesses motor function, it also binds to the kinesin-1 light chain (KLC) through its stalk and tail domains. The specific association of KIF5 with cargoes or organelles can be mediated directly through the cargo-binding region in its tail domain or indirectly via the COOH-terminal domains of KLC, indicating the existence of two forms of KIF5 motor-cargo coupling. (B) Cytoplasmic dyneins consist of heavy chains (DHC), intermediate chains (DIC), light intermediate chains (DLIH), and light chains (DLC). To transport cargoes, cytoplasmic dynein also bind to the dynactin complex (not shown).

Figure 3. KIF5 motor adaptors for mitochondrial transport

(A) Milton-Miro adaptor complex. Miro is a member of the mitochondrial Rho-GTPase family and a mitochondrial outer membrane protein. Milton attaches indirectly to mitochondria via an interaction with Miro and recruits KIF5 to mitochondria independently of KLC. Two mammalian Milton orthologues, TRAK1 and TRAK2, can form complexes with the two mammalian orthologues of dMiro (Miro1 and Miro2). (B) Syntabulin is a KIF5 adaptor that targets to mitochondria via its carboxyl-terminal transmembrane domain. It provides a link between KIF5 and mitochondria that mediates mitochondrial anterograde transport. In addition, FEZ1 was reported to serve as a candidate kinesin motor adaptor.

Figure 4. Syntaphilin acts as a receptor for docking/anchoring axonal mitochondria

(A) Syntaphilin (SNPH) targets to axonal mitochondria in cultured hippocampal neurons. Hippocampal neurons at DIV14 were co-immunostained for SNPH and mitochondrial marker cytochrome c. Arrows point to SNPH-enriched mitochondria, and arrowheads indicate mitochondria poorly labeled by SNPH within an axonal process. Scale bars, 10 μm. (B) SNPH immobilizes axonal mitochondria, while deletion of the snph gene in mice robustly increases axonal mitochondrial mobility. Axonal mitochondrial mobility was observed in live neurons one week after transfection. Motion data are presented in kymograph, in which vertical lines represent stationary mitochondria and slant or curved lines indicate mobile ones. Left panel: wild-type neurons co-transfected at DIV6 with DsRed-mito (red) and GFP-SNPH (green); middle panel: wild-type neurons transfected with DsRed-Mito alone; right panel: snph^−/− neurons transfected with DsRed-Mito alone. (C) SNPH acts as a receptor for docking/anchoring mitochondria in axons and is required for maintaining a large stationary axonal mitochondrial pool by interacting with the MT-based cytoskeleton. (Images in A are adapted with permission from Jian-Sheng Kang, Jin-Hua Tian, Philip Zald, Ping-Yue Pan, Cuiling Li, Chuxia Deng, and Zu-Hang Sheng. Docking of axonal mitochondria by syntaphilin controls their mobility and affects short-term facilitation. Cell 132, 137–148, 2008. Images in B and schematic diagram in C are adapted with permission from Qian Cai, Zu-Hang Sheng. Mitochondrial transport and docking in axons. Experimental Neurology 218, 257–267, 2009).

Figure 5. Two proposed models of Miro’s role as a Ca²⁺ sensor that regulates mitochondrial mobility

Miro contains two GTPase domains and calcium-binding EF-hand motifs. Thus, Miro regulates axonal mitochondrial mobility by either GTP hydrolysis or calcium binding in response to calcium signals and synaptic activity. (A) Ca²⁺-binding turns “off” KIF5 engagement with MTs. The tail of KIF5 links to Miro via Milton in a Ca²⁺-independent manner, thus leaving its motor domain free to engage with MTs. Ca²⁺-binding to the EF-hands triggers the direct interaction of the motor domain with Miro, which prevents the motor from engaging MTs (Wang and Schwarz, 2009). (B) Ca²⁺-binding detaches KIF5 from mitochondria. Mitochondrial transport is mediated by Miro/KIF5 linkage. Ca²⁺-binding to the EF-hands dissociates Miro from KIF5 while KIF5-binding protein GRIF-1/TRAK2 (a mammalian homologue of Milton) remains bound to Miro1 (Macaskill et al., 2008; 2009). (The model diagram is adapted with permission from Qian Cai and Zu-Hang Sheng. Moving or stopping mitochondria: Miro as a traffic cop by sensing calcium. Neuron 61, 493–496. 2009).