Abstract
Peroxisomes can be frequently found in proximity to other subcellular organelles such as the endoplasmic reticulum (ER), mitochondria or lysosomes. The tail-anchored protein ACBD5 was recently identified as part of a tethering complex at peroxisome–ER contact sites, interacting with the ER resident protein VAPB. Contact site disruption was found to significantly increase peroxisome motility, apparently interfering with intracellular positioning systems. Unlike other somatic cells, neurons have to distribute organelles across relatively long distances in order to maintain their extraordinary cellular polarity. Using confocal live imaging microscopy in cultured hippocampal neurons we observed that peroxisomes and mitochondria show a strikingly similar motility with approximately 10% performing microtubule-driven long range movements. In order to investigate if ER contacts influence overall peroxisome motility and cellular distribution patterns, hippocampal neurons were transfected with plasmids encoding ACBD5 to stimulate peroxisome–ER interactions. Overexpression of ACBD5 reduced peroxisomal long range movements in the neurites of the hippocampal cells by 70%, implying that ER attachment counteracts microtubule-driven peroxisome transport, while mitochondrial motility was unaffected. Moreover, the analyses of peroxisome distribution in fixed neurons unveiled a significant redistribution of peroxisomes towards the periphery of the perikaryon underneath the plasma membrane and into neurites, where peroxisomes are frequently found in close proximity to mitochondria. Surprisingly, further analysis of peroxisome and VAPB distribution upon ACBD5 expression did not reveal a substantial colocalization, implying this effect may be independent of VAPB. In line with these findings, expression of an ACBD5 variant unable to bind to VAPB still altered the localization of peroxisomes in the same way as the wild-type ACBD5. Thus, we conclude, that the VAPB-ACBD5 facilitated peroxisome-ER interaction is not responsible for the observed organelle redistribution in neurons. Rather, we suggest that additional ACBD5-binding proteins in neurons may tether peroxisomes to contact sites at or near the plasma membrane of neurons.
Highlights
In order to maintain their extraordinary cellular asymmetry, neurons have to closely coordinate transport processes into dendritic and axonal compartments
POs share an intricate relationship with the endoplasmic reticulum (ER) as well as mitochondria, which is morphologically documented by close intracellular proximities, implying the existence of specific contact sites between the different organelles [26, 27]
To evaluate how POs in neurons distribute in relation to their major organellar interaction partners, hippocampal neurons were transfected with plasmids expressing EGFP-SKL-PO, RFP-KDEL-ER, and mPlum-Mito3 to serve as PO, ER and mitochondrial marker proteins, respectively
Summary
In order to maintain their extraordinary cellular asymmetry, neurons have to closely coordinate transport processes into dendritic and axonal compartments. To this end, long distance trafficking and local distribution of organelles are determined via microtubule polarity along axons and dendrites [1]. Mitochondria, for example, have to be precisely immobilized at energydemanding cellular regions such as synapses [2] For such intracellular positioning, the organelles require specific docking proteins. One example is syntaphilin, which has been recently reported to reside on axonal mitochondria and work as a stationary docking factor, immobilizing the organelles on the microtubule cytoskeleton [3]. Lippincott-Schwartz and coworkers observed that a functional microtubule cytoskeleton appears to generally influence organelle contact site formation, implying a causal relationship between organelle motility and contact site formation [4]
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