skip to main content

Title: Morphological principles of neuronal mitochondria
In the highly dynamic metabolic landscape of a neuron, mitochondrial membrane architectures can provide critical insight into the unique energy balance of the cell. Current theoretical calculations of functional outputs like ATP and heat often represent mitochondria as idealized geometries and therefore can miscalculate the metabolic fluxes. To analyze mitochondrial morphology in neurons of mouse cerebellum neuropil, 3D tracings of complete synaptic and axonal mitochondria were constructed using a database of serial TEM tomographyimages and converted to watertight meshes with minimal distortion of the original microscopy volumes with agranularity of 1.6 nanometer isotropic voxels. The resulting in silico representations were subsequently quantified by differential geometry methods in terms of the mean and Gaussian curvatures, surface areas, volumes, and membrane motifs, all of which can alter the metabolic output of the organelle. Finally, we identify structural motifs that are present across this population of mitochondria; observations which may contribute to future modeling studies of mitochondrial physiology and metabolism in neurons.
Authors:
; ; ; ; ; ; ; ; ; ; ; ; ; ;
Award ID(s):
2014862
Publication Date:
NSF-PAR ID:
10250923
Journal Name:
bioRxiv
ISSN:
2692-8205
Sponsoring Org:
National Science Foundation
More Like this
  1. Beard, D (Ed.)
    Neurons rely on localized mitochondria to fulfill spatially heterogeneous metabolic demands. Mitochondrial aging occurs on timescales shorter than the neuronal lifespan, necessitating transport of fresh material from the soma. Maintaining an optimal distribution of healthy mitochondria requires an interplay between a stationary pool localized to sites of high metabolic demand and a motile pool capable of delivering new material. Interchange between these pools can occur via transient fusion / fission events or by halting and restarting entire mitochondria. Our quantitative model of neuronal mitostasis identifies key parameters that govern steady-state mitochondrial health at discrete locations. Very infrequent exchange between stationarymore »and motile pools optimizes this system. Exchange via transient fusion allows for robust maintenance, which can be further improved by selective recycling through mitophagy. These results provide a framework for quantifying how perturbations in organelle transport and interactions affect mitochondrial homeostasis in neurons, a key aspect underlying many neurodegenerative disorders.« less
  2. Mitochondrial cristae are dynamic invaginations of the inner membrane and play a key role in its metabolic capacity to produce ATP. Structural alterations caused by either genetic abnormalities or detrimental environmental factors impede mitochondrial metabolic fluxes and lead to a decrease in their ability to meet metabolic energy requirements. While some of the key proteins associated with mitochondrial cristae are known, very little is known about how the inner membrane dynamics are involved in energy metabolism. In this study, we present a computational strategy to understand how cristae are formed using a phase-based separation approach of both the inner membranemore »space and matrix space, which are explicitly modeled using the Cahn–Hilliard equation. We show that cristae are formed as a consequence of minimizing an energy function associated with phase interactions which are subject to geometric boundary constraints. We then extended the model to explore how the presence of calcium phosphate granules, entities that form in calcium overload conditions, exert a devastating inner membrane remodeling response that reduces the capacity for mitochondria to produce ATP. This modeling approach can be extended to include arbitrary geometrical constraints, the spatial heterogeneity of enzymes, and electrostatic effects to mechanize the impact of ultrastructural changes on energy metabolism.« less
  3. Mitochondria are dynamic organelles that undergo fission and fusion. While they are essential for cellular metabolism, the effect of dysregulated mitochondrial dynamics on cellular metabolism is not fully understood. We previously found that transmembrane protein 135 ( Tmem135) plays a role in the regulation of mitochondrial dynamics in mice. Mice homozygous for a Tmem135 mutation ( Tmem135 FUN025/FUN025 ) display accelerated aging and age-related disease pathologies in the retina including the retinal pigment epithelium (RPE). We also generated a transgenic mouse line globally overexpressing the Tmem135 gene ( Tmem135 TG). In several tissues and cells that we studied such asmore »the retina, heart, and fibroblast cells, we observed that the Tmem135 mutation causes elongated mitochondria, while overexpression of Tmem135 results in fragmented mitochondria. To investigate how abnormal mitochondrial dynamics affect metabolic signatures of tissues and cells, we identified metabolic changes in primary RPE cell cultures as well as heart, cerebellum, and hippocampus isolated from Tmem135 FUN025/FUN025 mice (fusion > fission) and Tmem135 TG mice (fusion < fission) using nuclear magnetic resonance spectroscopy. Metabolomics analysis revealed a tissue-dependent response to Tmem135 alterations, whereby significant metabolic changes were observed in the heart of both Tmem135 mutant and TG mice as compared to wild-type, while negligible effects were observed in the cerebellum and hippocampus. We also observed changes in Tmem135 FUN025/FUN025 and Tmem135 TG RPE cells associated with osmosis and glucose and phospholipid metabolism. We observed depletion of NAD + in both Tmem135 FUN025/FUN025 and Tmem135 TG RPE cells, indicating that imbalance in mitochondrial dynamics to both directions lowers the cellular NAD + level. Metabolic changes identified in this study might be associated with imbalanced mitochondrial dynamics in heart tissue and RPE cells which can likely lead to functional abnormalities. Impact statement Mitochondria are dynamic organelles undergoing fission and fusion. Proper regulation of this process is important for healthy aging process, as aberrant mitochondrial dynamics are associated with several age-related diseases/pathologies. However, it is not well understood how imbalanced mitochondrial dynamics may lead to those diseases and pathologies. Here, we aimed to determine metabolic alterations in tissues and cells from mouse models with over-fused (fusion > fission) and over-fragmented (fusion < fission) mitochondria that display age-related disease pathologies. Our results indicated tissue-dependent sensitivity to these mitochondrial changes, and metabolic pathways likely affected by aberrant mitochondrial dynamics. This study provides new insights into how dysregulated mitochondrial dynamics could lead to functional abnormalities of tissues and cells.« less
  4. Abstract Mitochondria play important roles in ovarian follicle development. Mitochondrial dysfunction, including mitochondrial gene deficiency, impairs ovarian development. Here, we explored the role and mechanism of mitochondrial inner membrane gene Immp2l in ovarian follicle growth and development. Our results revealed that female Immp2l-/- mice were infertile, whereas Immp2l+/- mice were normal. Body and ovarian weights were reduced in the female Immp2l-/- mice, ovarian follicle growth and development were stunted in the secondary follicle stage. Although a few ovarian follicles were ovulated, the oocytes were not fertilized because of mitochondrial dysfunction. Increased oxidative stress, decreased estrogen levels, and altered genes expressionmore »of Wnt/β-catenin and steroid hormone synthesis pathways were observed in 28-day-old Immp2l-/- mice. The Immp2l mutation accelerated ovarian aging process, as no ovarian follicles were detected by age 5 months in Immp2l-/- mice. All the aforementioned changes in the Immp2l-/- mice were reversed by administration of antioxidant melatonin to the Immp2l-/- mice. Furthermore, our in vitro study using Immp2l knockdown granulosa cells confirmed that the Immp2l downregulation induced granulosa cell aging by enhancing reactive oxygen species (ROS) levels, suppressing Wnt16, increasing β-catenin, and decreasing steroid hormone synthesis gene cyp19a1 and estrogen levels, accompanied by an increase in the aging phenotype of granulosa cells. Melatonin treatment delayed granulosa cell aging progression. Taken together, Immp2l causes ovarian aging through the ROS-Wnt/β-catenin-estrogen (cyp19a1) pathway, which can be reversed by melatonin treatment.« less
  5. Abstract STUDY QUESTION

    Is the combined use of fluorescence lifetime imaging microscopy (FLIM)-based metabolic imaging and second harmonic generation (SHG) spindle imaging a feasible and safe approach for noninvasive embryo assessment?

    SUMMARY ANSWER

    Metabolic imaging can sensitively detect meaningful metabolic changes in embryos, SHG produces high-quality images of spindles and the methods do not significantly impair embryo viability.

    WHAT IS KNOWN ALREADY

    Proper metabolism is essential for embryo viability. Metabolic imaging is a well-tested method for measuring metabolism of cells and tissues, but it is unclear if it is sensitive enough and safe enough for use in embryo assessment.

    STUDY DESIGN, SIZE, DURATION

    Thismore »study consisted of time-course experiments and control versus treatment experiments. We monitored the metabolism of 25 mouse oocytes with a noninvasive metabolic imaging system while exposing them to oxamate (cytoplasmic lactate dehydrogenase inhibitor) and rotenone (mitochondrial oxidative phosphorylation inhibitor) in series. Mouse embryos (n = 39) were measured every 2 h from the one-cell stage to blastocyst in order to characterize metabolic changes occurring during pre-implantation development. To assess the safety of FLIM illumination, n = 144 illuminated embryos were implanted into n = 12 mice, and n = 108 nonilluminated embryos were implanted into n = 9 mice.

    PARTICIPANTS/MATERIALS, SETTING, METHODS

    Experiments were performed in mouse embryos and oocytes. Samples were monitored with noninvasive, FLIM-based metabolic imaging of nicotinamide adenine dinucleotide (NADH) and flavin adenine dinucleotide (FAD) autofluorescence. Between NADH cytoplasm, NADH mitochondria and FAD mitochondria, a single metabolic measurement produces up to 12 quantitative parameters for characterizing the metabolic state of an embryo. For safety experiments, live birth rates and pup weights (mean ± SEM) were used as endpoints. For all test conditions, the level of significance was set at P < 0.05.

    MAIN RESULTS AND THE ROLE OF CHANCE

    Measured FLIM parameters were highly sensitive to metabolic changes due to both metabolic perturbations and embryo development. For oocytes, metabolic parameter values were compared before and after exposure to oxamate and rotenone. The metabolic measurements provided a basis for complete separation of the data sets. For embryos, metabolic parameter values were compared between the first division and morula stages, morula and blastocyst and first division and blastocyst. The metabolic measurements again completely separated the data sets. Exposure of embryos to excessive illumination dosages (24 measurements) had no significant effect on live birth rate (5.1 ± 0.94 pups/mouse for illuminated group; 5.7 ± 1.74 pups/mouse for control group) or pup weights (1.88 ± 0.10 g for illuminated group; 1.89 ± 0.11 g for control group).

    LIMITATIONS, REASONS FOR CAUTION

    The study was performed using a mouse model, so conclusions concerning sensitivity and safety may not generalize to human embryos. A limitation of the live birth data is also that although cages were routinely monitored, we could not preclude that some runt pups may have been eaten.

    WIDER IMPLICATIONS OF THE FINDINGS

    Promising proof-of-concept results demonstrate that FLIM with SHG provide detailed biological information that may be valuable for the assessment of embryo and oocyte quality. Live birth experiments support the method’s safety, arguing for further studies of the clinical utility of these techniques.

    STUDY FUNDING/COMPETING INTEREST(S)

    Supported by the Blavatnik Biomedical Accelerator Grant at Harvard University and by the Harvard Catalyst/The Harvard Clinical and Translational Science Center (National Institutes of Health Award UL1 TR001102), by NSF grants DMR-0820484 and PFI-TT-1827309 and by NIH grant R01HD092550-01. T.S. was supported by a National Science Foundation Postdoctoral Research Fellowship in Biology grant (1308878). S.F. and S.A. were supported by NSF MRSEC DMR-1420382. Becker and Hickl GmbH sponsored the research with the loaning of equipment for FLIM. T.S. and D.N. are cofounders and shareholders of LuminOva, Inc., and co-hold patents (US20150346100A1 and US20170039415A1) for metabolic imaging methods. D.S. is on the scientific advisory board for Cooper Surgical and has stock options with LuminOva, Inc.

    « less