Postsynaptic mitochondria are positioned to support functional diversity of dendritic spines

Reviewer #1 (Public Review):

Summary:
Mitochondria is the power plant of the cells including neurons. Thomas et al. characterized the distribution of mitochondria in dendrites and spines of L2/3 neurons from the ferret visual cortex, for which visually driven calcium responses of individual dendritic spines were examined. The authors analyzed the relationship between the position of mitochondria and the morphology or orientation selectivity of nearby dendrite spines. They found no correlation between mitochondrion location and spine morphological parameters associated with the strength of synapses, but correlation with the spine-somatic difference of orientation preference and local heterogeneity in preferred orientation of nearby spines. Moreover, they reported that the spines that have a mitochondrion in the head or neck are larger in size and have stronger orientation selectivity. Therefore, they proposed that "mitochondria are not necessarily positioned to support the energy needs of strong spines, but rather support the structurally and functionally diverse inputs."

Strengths:
This paper attempted to address a fundamental question: whether the distribution of the mitochondria along the dendrites of visual cortical neurons is associated with the functions of the spines, postsynaptic sites of excitatory synapses. Two state of the art techniques (2 photon Ca imaging of somata and spines and EM reconstructions of cortical pyramidal neurons) had been used, which provides a great opportunity to examine and correlate the function of spine ultrastructure and spatial distribution of dendritic mitochondria.

Weaknesses:
Overall, the findings are interesting. However, the study lacks the data providing insights into either the mechanisms or the functional meaning of the pattern of mitochondrion distribution along the dendrites, which restricts the significance of the study. It also suffers from small correlation coefficients and small sample sizes (60-121 spines in 4 neurons) as well as missing some important analysis.

Reviewer #2 (Public Review):

Summary:
Mitochondria in synapses are important to support functional needs, such as local protein translation and calcium buffering. Thus, they may be strategically localized to maximize functional efficiency. In this study, the authors examine whether a correlation exists between the positioning of mitochondria and the structure or function of dendritic spines in the visual cortex of a ferret. Unexpectedly, the authors found no correlation between structural measures of synaptic strength to mitochondria positioning, which may indicate that they are not localized only because of the local energy needs. Instead, the authors discover that mitochondria are positioned preferably in spines that display heterogeneous responses, showing that they are localized to support specific functional needs probably distinct from ATP output.

Strengths:
The thorough analysis provides a yet unprecedented insight into the correlation between synaptic tuning and mitochondrial positioning in the visual cortex in vivo.

Weaknesses:
The study defined 1 μm and 5 μm as short and extended ranges relative to the synapse and examined the correlation between mitochondria volume and multiple parameters within that defined range. Results showed that mitochondria display preferences towards spines that respond differently to visual stimuli or areas with low local calcium activity. However, it is not known whether this mitochondria preference is a cause or a result of spine heterogeneity. It will be interesting to see the correlation of spine volume relative to mitochondrial positioning in 1µm and 5µm ranges around mitochondria.

Analysis of this study suggested that mitochondrial volume does not correlate with the structural measure of synaptic strength (e.g. spine volume and post-synaptic density (PSD) area). However, the authors did not examine whether mitochondrial volume correlates to synaptic transmission frequency or plasticity. It may still be possible that mitochondria are localized in positions that exhibit a high frequency of transmission or a high degree of plasticity. Future studies will have to determine the underlying cause of mitochondria positioning preference.

Reviewer #3 (Public Review):

Summary: This is a careful examination of the distribution of mitochondria in the basal dendrites of ferret visual cortex in a previously published volume electron microscopy dataset. The authors report that mitochondria are sparsely, as opposed to continuously distributed in the dendritic shafts, and that they tend to cluster near dendritic spines with heterogeneous orientation selectivity.

Strengths: Volume EM is the gold standard for quantification of organelle morphology. An unusual strength of this particular dataset is that the orientation selectivity of the dendritic spines was measured by calcium imaging prior to EM reconstruction. This allowed the authors to assess how spines with varying selectivity are organized relative to mitochondria, leading to an intriguing observation that they localize to heterogeneous spine clusters. The analysis is carefully performed.

Weaknesses: Using threshold distances between mitochondria and synapses as opposed to absolute distances may overlook important relationships in the data.