. Cell-type-specific profiling of brain mitochondria reveals functional and molecular diversity. Nat Neurosci. 2019 Oct;22(10):1731-1742. Epub 2019 Sep 9 PubMed.

Recommends

Please login to recommend the paper.

Comments

  1. In this very nice study, Misgeld’s group has engineered a set of MitoTag mice expressing GFP in the outer mitochondrial membrane (OMM) in a cell-type-specific manner. Importantly, they demonstrate that GFP-OMM-tagged mitochondria can be purified in separate fractions and that it is possible to, for example, study the mitochondrial proteome in specific cell populations. I think the most interesting is to see that mitochondrial proteins are differently expressed depending on cell type, revealing that mitochondria are not all the same but have diverse functions in different cell types.

    Using this method, it is also possible to determine cell-specific mitochondrial markers and changes in these markers related to neurodegeneration. In terms of function, mitochondria in cerebellar granule cells show a significantly higher expression of the mitochondria calcium uniporter (Mcu) as compared to Purkinje cell mitochondria. This is also reflected by the cell-type-specific mitochondria’s capacity to buffer calcium and their sensitivity to Mcu ablation. Mitochondria from granule cells buffer calcium better and are more sensitive to loss of Mcu as compared to inhibitory Purkinje cells (PC), very much in line with the fact that granule cells are excitatory and signal via glutamate receptors and calcium influx.

    We have for several years studied the endoplasmic reticulum (ER)-mitochondria interface and how the interplay between these two organelles is altered in Alzheimer’s disease (Hedskog et al., 2013; Leal et al., 2016; Leal et al., 2018; Schreiner et al., 2015; Filadi et al., 2018). Intriguingly, Misgeld and colleagues identify Rmdn3 (PTPIP51) in the proteomic profiling and show that Rmdn3 is enriched in PC mitochondria. Rmdn3 was first identified by Miller’s group as an OMM protein interacting with the ER protein VAPB. The Rmdn3 (PTPIP51)-VAPB complex is established as a scaffold between mitochondria and ER (DeVos et al., 2011) at specialized contact points referred to as mitochondria-associated membranes (MAM) (Vance, 1990). Here it is shown that PC have increased ER-mitochondria contacts, suggesting that such cells mainly handle calcium buffering via ER- mitochondria shuttling.

    Using human brain tissue, AD mouse models, and cells we have shown a correlation between Aβ levels and increased ER-mitochondria contact as well as increased calcium shuttling between the two organelles. Currently, partly based on RNA-Seq data, we hypothesize that in AD, increased ER-mitochondria contact is an initial stress response and a way for the cell to support mitochondrial functions and thus synaptic activity. As AD progresses, the dysregulation of organelle contact may turn into negative effects and thus negatively affect neuronal function and survival.

    References:

    . Alterations in mitochondria-endoplasmic reticulum connectivity in human brain biopsies from idiopathic normal pressure hydrocephalus patients. Acta Neuropathol Commun. 2018 Oct 1;6(1):102. PubMed.

    . TOM70 Sustains Cell Bioenergetics by Promoting IP3R3-Mediated ER to Mitochondria Ca2+ Transfer. Curr Biol. 2018 Feb 5;28(3):369-382.e6. Epub 2018 Jan 27 PubMed.

    . Amyloid-β peptides are generated in mitochondria-associated endoplasmic reticulum membranes. J Alzheimers Dis. 2015;43(2):369-74. PubMed.

    . Mitofusin-2 knockdown increases ER-mitochondria contact and decreases amyloid β-peptide production. J Cell Mol Med. 2016 Sep;20(9):1686-95. Epub 2016 May 20 PubMed.

    . Modulation of the endoplasmic reticulum-mitochondria interface in Alzheimer's disease and related models. Proc Natl Acad Sci U S A. 2013 May 7;110(19):7916-21. PubMed.

    . VAPB interacts with the mitochondrial protein PTPIP51 to regulate calcium homeostasis. Hum Mol Genet. 2011 Dec 13; PubMed.

    . Phospholipid synthesis in a membrane fraction associated with mitochondria. J Biol Chem. 1990 May 5;265(13):7248-56. PubMed.

    View all comments by Maria Ankarcrona
  2. The work from Fecher et al. is a very innovative solution to the older problem of estimating if mitochondria in major cell types of the mammalian brain are different. Based on their differing physiology and energetic requirements, it is gratifying, if expected, that there were differentially expressed mitochondrial proteins between neurons and astrocytes. The additional differences in mitochondrial proteomes between neuronal subtypes also reflects important physiological differences, as the authors point out in their discussion, around Purkinje cells’ calcium handling.

    While there is some exploration of AD and ALS mice in this study, one can see a number of additional future applications in the context of disease modeling. For example, several groups have now proposed mechanisms by which PINK1 and parkin, both associated with early onset recessive parkinsonism, affect mitochondria in vivo but with different predicted effects on the mitochondrial proteome. Richard Youle’s lab has proposed that loss of parkin results in a failure of control of pathogenic mtDNA mutations (Pickrell et al., 2015), whereas more recently, Matheoud et al. showed that PINK1 is critical for mitochondrial antigen presentation in the context of bacterial infections (Matheoud et al., 2019). It would be of huge interest to express the MitoTag in different cells in PINK1 and parkin-deficient mice and see how the proteome reshapes under different conditions.

    References:

    . Intestinal infection triggers Parkinson's disease-like symptoms in Pink1-/- mice. Nature. 2019 Jul;571(7766):565-569. Epub 2019 Jul 17 PubMed.

    . Endogenous Parkin Preserves Dopaminergic Substantia Nigral Neurons following Mitochondrial DNA Mutagenic Stress. Neuron. 2015 Jul 15;87(2):371-81. PubMed.

    View all comments by Mark Cookson
  3. This is a very creative methods-development paper that will likely fill a much-needed niche in the mitochondrial research field. Over the past few decades it has increasingly become clear that mitochondria are not equivalent between cell types. For example, we’ve come to realize mitochondrial proteomes differ between different tissues. But what about mitochondria within different cell types within a single tissue? This issue is critical in studies of the brain, since the brain is not a homogeneous organ. It contains different cell types, and it has become increasingly appreciated that mitochondria in neurons differ functionally and even structurally from mitochondria in astrocytes.

    Unfortunately, the techniques used to isolate mitochondria from the different cell types within the brain are cumbersome and can disrupt mitochondrial integrity. In defining an approach that leverages a transgenic mouse that can strategically tag cell-type-specific mitochondria, with subsequent immunocapture of those mitochondria, the authors show they can effectively and efficiently enrich for intact, cell-type-specific brain mitochondria.

    As an investigator who labors over separating neurons, astrocytes, microglia, and endothelial cells from mouse brains in order to get a better grasp of what their mitochondria are up to, the potential of this approach to facilitate studies of brain mitochondria is obvious. From the perspective of someone who studies mitochondria in Alzheimer’s disease, I would hope this will allow for, and facilitate, new insights into how mitochondria and their function pertain to the disease. The one caveat is that while this technique will benefit studies of AD mouse model mitochondria, we will still need to not overlook the general limitations of these models.      

    View all comments by Russell Swerdlow

Make a Comment

To make a comment you must login or register.

This paper appears in the following:

News

  1. Hello Mitochondriomics: Tagging Method Identifies Different Subtypes