Dendritic Spine Density Scales with Microtubule Number in Rat Hippocampal Dendrites

• Published in: Neuroscience Vol. 489; pp. 84 - 97
• Main Authors: Harris, Kristen M., Hubbard, Dusten D., Kuwajima, Masaaki, Abraham, Wickliffe C., Bourne, Jennifer N., Bowden, Jared B., Haessly, Andrea, Mendenhall, John M., Parker, Patrick H., Shi, Bitao, Spacek, Josef
• Format: Journal Article
• Language: English
• Published: United States Elsevier Ltd 01-05-2022
• Subjects: 3D-reconstruction; Animals; Dendrites - physiology; Dendritic Spines; Hippocampus; Microtubules; Pyramidal Cells - physiology; Rats; spine density; synapses; Synapses - physiology; ultrastructure; spine density; synapses; microtubules; ultrastructure; 3D-reconstruction
• ISSN: 0306-4522; 1873-7544
• DOI: 10.1016/j.neuroscience.2022.02.021

•3DEM reveals dendrite diameter and spine density correlate with microtubule number.•In thick dendrites, microtubules support both dendritic branches and spines.•In thin unbranched dendrites, microtubules need only support local dendritic spines.•Existing dendritic microtubules are sufficient to supply synapses after plasticity.•Resources are delivered to and from dendrites in proportion to synapse load.
Microtubules deliver essential resources to and from synapses. Three-dimensional reconstructions in rat hippocampus reveal a sampling bias regarding spine density that needs to be controlled for dendrite caliber and resource delivery based on microtubule number. The strength of this relationship varies across dendritic arbors, as illustrated for area CA1 and dentate gyrus. In both regions, proximal dendrites had more microtubules than distal dendrites. For CA1 pyramidal cells, spine density was greater on thicker than thinner dendrites in stratum radiatum, or on the more uniformly thin terminal dendrites in stratum lacunosum moleculare. In contrast, spine density was constant across the cone shaped arbor of tapering dendrites from dentate granule cells. These differences suggest that thicker dendrites supply microtubules to subsequent dendritic branches and local dendritic spines, whereas microtubules in thinner dendrites need only provide resources to local spines. Most microtubules ran parallel to dendrite length and associated with long, presumably stable mitochondria, which occasionally branched into lateral dendritic branches. Short, presumably mobile, mitochondria were tethered to microtubules that bent and appeared to direct them into a thin lateral branch. Prior work showed that dendritic segments with the same number of microtubules had elevated resources in subregions of their dendritic shafts where spine synapses had enlarged, and spine clusters had formed. Thus, additional microtubules were not required for redistribution of resources locally to growing spines or synapses. These results provide new understanding about the potential for microtubules to regulate resource delivery to and from dendritic branches and locally among dendritic spines.