Protein composition of the muscle mitochondrial reticulum during postnatal development

Developmental dynamics of mitochondrial fission and fusion proteins in functionally divergent skeletal muscles of goat

During skeletal muscle development, the intricate mitochondrial network formation relies on continuous fission and fusion. This process in larger mammals differs from rodents, the most used animal models. However, the expression pattern of proteins regulating mitochondrial dynamics in developing skeletal muscle remains unexplored in larger mammals. Therefore, we characterized the cellular expression and tissue-level distribution of these proteins during development taking goat as a model. We have performed histological and immunohistochemical analyses to study metabolic features in various muscles. Neonatal muscles display uniform distribution of mitochondrial activity. In contrast, adult muscles exhibit clear distinctions based on their function, whether dedicated for posture maintenance or facilitating locomotion. Mitochondrial fission proteins like DRP-1, MFF, and fusion proteins like MFN-1 and 2 are abundantly expressed in neonatal muscles. Fission proteins exhibit drastic downregulation with limited peripheral expression, whereas fusion proteins continue to express in a fiber-specific manner during adulthood. Locomotory muscles exhibit different fibers based on mitochondrial activity and peripheralization with high SDH activity. The proximity ligation assay between MFN1 and MFN2 demonstrates that their interaction is restricted to subsarcolemmal mitochondria in adult fibers while distributed evenly in neonatal fibers. These differences between postural and locomotory muscles suggest their physiological and metabolic properties are different.

Reorganization of mitochondria-organelle interactions during postnatal development in skeletal muscle

Skeletal muscle cellular development requires the integrated assembly of mitochondria and other organelles adjacent to the sarcomere in support of muscle contractile performance. However, it remains unclear how interactions among organelles and with the sarcomere relates to the development of muscle cell function. Here, we combine 3D volume electron microscopy, proteomic analyses, and live cell functional imaging to investigate the postnatal reorganization of mitochondria-organelle interactions in skeletal muscle. We show that while mitochondrial networks are disorganized and loosely associated with the contractile apparatus at birth, contact sites among mitochondria, lipid droplets and the sarcoplasmic reticulum are highly abundant in neonatal muscles. The maturation process is characterized by a transition to highly organized mitochondrial networks wrapped tightly around the muscle sarcomere but also to less frequent interactions with both lipid droplets and the sarcoplasmic reticulum. Concomitantly, expression of proteins involved in mitochondria-organelle membrane contact sites decreases during postnatal development in tandem with a decrease in abundance of proteins associated with sarcomere assembly despite an overall increase in contractile protein abundance. Functionally, parallel measures of mitochondrial membrane potential, NADH redox status, and NADH flux within intact cells revealed that mitochondria in adult skeletal muscle fibres maintain a more activated electron transport chain compared with neonatal muscle mitochondria. These data demonstrate a developmental redesign reflecting a shift from muscle cell assembly and frequent inter-organelle communication toward a muscle fibre with mitochondrial structure, interactions, composition and function specialized to support contractile function. KEY POINTS: Mitochondrial network organization is remodelled during skeletal muscle postnatal development. The mitochondrial outer membrane is in frequent contact with other organelles at birth and transitions to more close associations with the contractile apparatus in mature muscles. Mitochondrial energy metabolism becomes more activated during postnatal development. Understanding the developmental redesign process within skeletal muscle cells may help pinpoint specific areas of deficit in muscles with developmental disorders.

Loss of mitochondrial Ca2+ uptake protein 3 impairs skeletal muscle calcium handling and exercise capacity

Mitochondrial calcium concentration ([Ca2+ ]m ) plays an essential role in bioenergetics, and loss of [Ca2+ ]m homeostasis can trigger diseases and cell death in numerous cell types. Ca2+ uptake into mitochondria occurs via the mitochondrial Ca2+ uniporter (MCU), which is regulated by three mitochondrial Ca2+ uptake (MICU) proteins localized in the intermembrane space, MICU1, 2, and 3. We generated a mouse model of systemic MICU3 ablation and examined its physiological role in skeletal muscle. We found that loss of MICU3 led to impaired exercise capacity. When the muscles were directly stimulated there was a decrease in time to fatigue. MICU3 ablation significantly increased the maximal force of the KO muscle and altered fibre type composition with an increase in the ratio of type IIb (low oxidative capacity) to type IIa (high oxidative capacity) fibres. Furthermore, MICU3-KO mitochondria have reduced uptake of Ca2+ and increased phosphorylation of pyruvate dehydrogenase, indicating that KO animals contain less Ca2+ in their mitochondria. Skeletal muscle from MICU3-KO mice exhibited lower net oxidation of NADH during electrically stimulated muscle contraction compared with wild-type. These data demonstrate that MICU3 plays a role in skeletal muscle physiology by setting the proper threshold for mitochondrial Ca2+ uptake, which is important for matching energy demand and supply in muscle. KEY POINTS: Mitochondrial calcium uptake is an important regulator of bioenergetics and cell death and is regulated by the mitochondrial calcium uniporter (MCU) and three calcium sensitive regulatory proteins (MICU1, 2 and 3). Loss of MICU3 leads to impaired exercise capacity and decreased time to skeletal muscle fatigue. Skeletal muscle from MICU3-KO mice exhibits a net oxidation of NADH during electrically stimulated muscle contractions, suggesting that MICU3 plays a role in skeletal muscle physiology by matching energy demand and supply.

Characterization of differential distribution patterns between mitofusin isoforms and their interaction in developing skeletal muscles of rat

Skeletal muscle during postnatal development undergoes several structural and biochemical modifications. It is proposed that these changes are closely intertwined with the increase in load-bearing capacity of the muscle (i.e., myofibrils) and molecular machinery to support the energy demand (i.e., mitochondria). Concomitant establishment of the sarcoplasmic reticulum (SR) and mitochondrial network seems to be a major developmental adjustment of skeletal muscle leading to adult phenotype. Here, we have studied oxidativeness, vascularization, and the changes in mitofusins (Mfn) 1-Mfn 2 expression and interaction in the due course of muscle development. Toward this, we used a series of histochemical techniques to compare neonatal and adult limb muscles (Gastrocnemius and Quadriceps) of Wistar rat (Rattus norvegicus). Additionally, we probed the proximity between Mfn 1 and Mfn 2 using a highly sensitive antibody-based proximity ligation assay indicating the change in mitochondrial fusion pattern or mitochondria-SR interaction. The results show that neonatal fibers bear a uniform distribution of mitochondria while a differential pattern of distribution is seen in adults. The distribution of the blood vessels is also quite distinct in adult muscles with a well-formed capillary network but in neonates, only central blood vessels are seen. Interestingly, our Mfn 1-Mfn 2 interaction data show that this interaction is uniformly distributed throughout the neonatal fibers, while it becomes peripherally localized in fibers of adult muscles. This peripheralization of Mfn 1-Mfn 2 interaction must be an important event of muscle development and might be critical to cater to the metabolic needs of adult muscle.

M1BP is an essential transcriptional activator of oxidative metabolism during Drosophila development

Oxidative metabolism is the predominant energy source for aerobic muscle contraction in adult animals. How the cellular and molecular components that support aerobic muscle physiology are put in place during development through their transcriptional regulation is not well understood. Using the Drosophila flight muscle model, we show that the formation of mitochondria cristae harbouring the respiratory chain is concomitant with a large-scale transcriptional upregulation of genes linked with oxidative phosphorylation (OXPHOS) during specific stages of flight muscle development. We further demonstrate using high-resolution imaging, transcriptomic and biochemical analyses that Motif-1-binding protein (M1BP) transcriptionally regulates the expression of genes encoding critical components for OXPHOS complex assembly and integrity. In the absence of M1BP function, the quantity of assembled mitochondrial respiratory complexes is reduced and OXPHOS proteins aggregate in the mitochondrial matrix, triggering a strong protein quality control response. This results in isolation of the aggregate from the rest of the matrix by multiple layers of the inner mitochondrial membrane, representing a previously undocumented mitochondrial stress response mechanism. Together, this study provides mechanistic insight into the transcriptional regulation of oxidative metabolism during Drosophila development and identifies M1BP as a critical player in this process.

Identification of evolutionarily conserved regulators of muscle mitochondrial network organization

Mitochondrial networks provide coordinated energy distribution throughout muscle cells. However, pathways specifying mitochondrial networks are incompletely understood and it is unclear how they might affect contractile fiber-type. Here, we show that natural energetic demands placed on Drosophila melanogaster muscles yield native cell-types among which contractile and mitochondrial network-types are regulated differentially. Proteomic analyses of indirect flight, jump, and leg muscles, together with muscles misexpressing known fiber-type specification factor salm, identified transcription factors H15 and cut as potential mitochondrial network regulators. We demonstrate H15 operates downstream of salm regulating flight muscle contractile and mitochondrial network-type. Conversely, H15 regulates mitochondrial network configuration but not contractile type in jump and leg muscles. Further, we find that cut regulates salm expression in flight muscles and mitochondrial network configuration in leg muscles. These data indicate cell type-specific regulation of muscle mitochondrial network organization through evolutionarily conserved transcription factors cut, salm, and H15.

Mouse models of SMA show divergent patterns of neuronal vulnerability and resilience

Background

Spinal muscular atrophy (SMA) is a form of motor neuron disease affecting primarily children characterised by the loss of lower motor neurons (MNs). Breakdown of the neuromuscular junctions (NMJs) is an early pathological event in SMA. However, not all motor neurons are equally vulnerable, with some populations being lost early in the disease while others remain intact at the disease end-stage. A thorough understanding of the basis of this selective vulnerability will give critical insight into the factors which prohibit pathology in certain motor neuron populations and consequently help identify novel neuroprotective strategies.

Methods

To retrieve a comprehensive understanding of motor neuron susceptibility in SMA, we mapped NMJ pathology in 20 muscles from the Smn^2B/- SMA mouse model and cross-compared these data with published data from three other commonly used mouse models. To gain insight into the molecular mechanisms regulating selective resilience and vulnerability, we analysed published RNA sequencing data acquired from differentially vulnerable motor neurons from two different SMA mouse models.

Results

In the Smn^2B/- mouse model of SMA, we identified substantial NMJ loss in the muscles from the core, neck, proximal hind limbs and proximal forelimbs, with a marked reduction in denervation in the distal limbs and head. Motor neuron cell body loss was greater at T5 and T11 compared with L5. We subsequently show that although widespread denervation is observed in each SMA mouse model (with the notable exception of the Taiwanese model), all models have a distinct pattern of selective vulnerability. A comparison of previously published data sets reveals novel transcripts upregulated with a disease in selectively resistant motor neurons, including genes involved in axonal transport, RNA processing and mitochondrial bioenergetics.

Conclusions

Our work demonstrates that the Smn^2B/- mouse model shows a pattern of selective vulnerability which bears resemblance to the regional pathology observed in SMA patients. We found drastic differences in patterns of selective vulnerability across the four SMA mouse models, which is critical to consider during experimental design. We also identified transcript groups that potentially contribute to the protection of certain motor neurons in SMA mouse models.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13395-022-00305-9.

Energy transfer between the mitochondrial network and lipid droplets in insulin resistant skeletal muscle

Mitochondria and lipid droplets in the insulin resistant skeletal muscle of type 2 diabetic individuals have both been heavily investigated independently and are characterized by more fragmented, dysfunctional mitochondrial networks and larger lipid droplets compared to skeletal muscle of healthy individuals. Specialized contacts between mitochondrial and lipid droplet membranes are known to decrease in diabetic muscle, though it remains unclear how energy transfer at the remaining mitochondria-lipid droplet contact sites may be altered by type 2 diabetes. The purpose of this review is to highlight recent data on mitochondrial structure and function and lipid droplet dynamics in type 2 diabetic skeletal muscle and to underscore the need for more detailed investigations into the functional nature of mitochondria-lipid droplet interactions in type 2 diabetes.

Energy metabolism design of the striated muscle cell

The design of the energy metabolism system in striated muscle remains a major area of investigation. Here, we review our current understanding and emerging hypotheses regarding the metabolic support of muscle contraction. Maintenance of ATP free energy, so called energy homeostasis, via mitochondrial oxidative phosphorylation is critical to sustained contractile activity, and this major design criterion is the focus of this review. Cell volume invested in mitochondria reduces the space available for generating contractile force, and this spatial balance between mitochondria acontractile elements to meet the varying sustained power demands across muscle types is another important design criterion. This is accomplished with remarkably similar mass-specific mitochondrial protein composition across muscle types, implying that it is the organization of mitochondria within the muscle cell that is critical to supporting sustained muscle function. Beyond the production of ATP, ubiquitous distribution of ATPases throughout the muscle requires rapid distribution of potential energy across these large cells. Distribution of potential energy has long been thought to occur primarily through facilitated metabolite diffusion, but recent analysis has questioned the importance of this process under normal physiological conditions. Recent structural and functional studies have supported the hypothesis that the mitochondrial reticulum provides a rapid energy distribution system via the conduction of the mitochondrial membrane potential to maintain metabolic homeostasis during contractile activity. We extensively review this aspect of the energy metabolism design contrasting it with metabolite diffusion models and how mitochondrial structure can play a role in the delivery of energy in the striated muscle.

Microarray and qPCR Analysis of Mitochondrial Metabolism Activation during Prenatal and Early Postnatal Development in Rats and Humans with Emphasis on CoQ10 Biosynthesis

At the end of the mammalian intra-uterine foetal development, a rapid switch from glycolytic to oxidative metabolism must proceed. Using microarray techniques, qPCR, enzyme activities and coenzyme Q content measurements, we describe perinatal mitochondrial metabolism acceleration in rat liver and skeletal muscle during the perinatal period and correlate the results with those in humans. Out of 1546 mitochondrial genes, we found significant changes in expression in 1119 and 827 genes in rat liver and skeletal muscle, respectively. The most remarkable expression shift occurred in the rat liver at least two days before birth. Coenzyme Q-based evaluation in both the rat model and human tissues showed the same trend: the total CoQ content is low prenatally, significantly increasing after birth in both the liver and skeletal muscle. We propose that an important regulator of rat coenzyme Q biosynthesis might be COQ8A, an atypical kinase involved in the biosynthesis of coenzyme Q. Our microarray data, a total of 16,557 RefSeq (Entrez) genes, have been deposited in NCBI’s Gene Expression Omnibus and are freely available to the broad scientific community. Our microarray data could serve as a suitable background for finding key factors regulating mitochondrial metabolism and the preparation of the foetus for the transition to extra-uterine conditions.

Marf-mediated mitochondrial fusion is imperative for the development and functioning of indirect flight muscles (IFMs) in drosophila

Dynamic changes in mitochondrial shape and size are vital for mitochondrial health and for tissue development and function. Adult Drosophila indirect flight muscles contain densely packed mitochondria. We show here that mitochondrial fusion is critical during early muscle development (in pupa) and that silencing of the outer mitochondrial membrane fusion gene, Marf, in muscles results in smaller mitochondria that are functionally defective. This leads to abnormal muscle development resulting in muscle dysfunction in adult flies. However, post-developmental silencing of Marf has no obvious effects on mitochondrial and muscle phenotype in adult flies, indicating the importance of mitochondrial fusion during early muscle development.

Temperament influences mitochondrial capacity in skeletal muscle from 8 through 18 mo of age in Brahman heifers

Temperamental cattle tend to yield carcasses of poorer quality, and Brahman cattle are reportedly more temperamental than non-indicus cattle breeds. A potential link between temperament and product quality may be mitochondrial activity. We hypothesized that mitochondrial measures would be greater in temperamental compared with calm heifers and that the relationships between temperament and mitochondria would persist as heifers age. Serum cortisol and skeletal muscle (longissimus thoracis [LT] and trapezius [TRAP]) mitochondrial profiles and antioxidant activities were quantified from the same calm (n = 6) and temperamental (n = 6) Brahman heifers at 8, 12, and 18 mo of age. Data were analyzed using a mixed model ANOVA in SAS (9.4) with repeated measures. Serum cortisol was greater in temperamental compared with calm heifers throughout the study (P = 0.02). Mitochondrial volume density (citrate synthase [CS] activity) increased over time (P < 0.0001) but was similar between temperament and muscle groups. Mitochondrial function (cytochrome c oxidase activity) was greatest in the temperamental LT at 8 mo of age (P ≤ 0.0006), greatest in the temperamental TRAP at 18 mo of age (P ≤ 0.003), and did not differ by temperament at 12 mo of age. Integrative (relative to tissue wet weight) mitochondrial oxidative phosphorylation capacity with complex I substrates (PCI), PCI plus complex II substrate (PCI+II), noncoupled electron transfer system capacity (ECI+II), and E with functional complex II only (ECII) were greater in the TRAP than LT for calm heifers at all ages (P ≤ 0.002), but were similar between muscle groups in temperamental heifers. Overall, calm heifers tended to have greater intrinsic (relative to CS activity) PCI and flux control of PCI+II (P ≤ 0.1) than temperamental heifers, indicating greater utilization of complex I paired with greater coupling efficiency in calm heifers. Within the LT, integrative PCI+II was greater (P = 0.05) and ECI+II tended to be greater (P = 0.06) in temperamental compared with calm heifers. From 8- to 18-mo old, glutathione peroxidase (GPx) activity decreased (P < 0.0001) and superoxide dismutase activity increased (P = 0.02), and both were similar between muscle groups. The activity of GPx was greater in temperamental compared with calm heifers at 8 (P = 0.004) but not at 12 or 18 mo of age. These results detail divergent skeletal muscle mitochondrial characteristics of live Brahman heifers according to temperament, which should be further investigated as a potential link between temperament and product quality.

Cell energy metabolism: An update

In living cells, growth is the result of coupling between substrate catabolism and multiple metabolic processes that take place during net biomass formation and maintenance processes. During growth, both ATP/ADP and NADH/NAD⁺ molecules play a key role. Cell energy metabolism hence refers to metabolic pathways involved in ATP synthesis linked to NADH turnover. Two main pathways are thus involved in cell energy metabolism: glycolysis/fermentation and oxidative phosphorylation. Glycolysis and mitochondrial oxidative phosphorylation are intertwined through thermodynamic and kinetic constraints that are reviewed herein. Further, our current knowledge of short-term and long term regulation of cell energy metabolism will be reviewed using examples such as the Crabtree and the Warburg effect.

Mitochondrial light switches: optogenetic approaches to control metabolism

Developing new technologies to study metabolism is increasingly important as metabolic disease prevalence increases. Mitochondria control cellular metabolism and dynamic changes in mitochondrial function are associated with metabolic abnormalities in cardiovascular disease, cancer, and obesity. However, a lack of precise and reversible methods to control mitochondrial function has prevented moving from association to causation. Recent advances in optogenetics have addressed this challenge, and mitochondrial function can now be precisely controlled in vivo using light. A class of genetically encoded, light-activated membrane channels and pumps has addressed mechanistic questions that promise to provide new insights into how cellular metabolism downstream of mitochondrial function contributes to disease. Here, we highlight emerging reagents-mitochondria-targeted light-activated cation channels or proton pumps-to decrease or increase mitochondrial activity upon light exposure, a technique we refer to as mitochondrial light switches, or mt^SWITCH . The mt^SWITCH technique is broadly applicable, as energy availability and metabolic signaling are conserved aspects of cellular function and health. Here, we outline the use of these tools in diverse cellular models of disease. We review the molecular details of each optogenetic tool, summarize the results obtained with each, and outline best practices for using optogenetic approaches to control mitochondrial function and downstream metabolism.

Synaptic withdrawal following nerve injury is influenced by postnatal maturity, muscle-specific properties, and the presence of underlying pathology in mice

Axonal and synaptic degeneration occur following nerve injury and during disease. Traumatic nerve injury results in rapid fragmentation of the distal axon and loss of synaptic terminals, in a process known as Wallerian degeneration (WD). Identifying and understanding factors that influence the rate of WD is of significant biological and clinical importance, as it will facilitate understanding of the mechanisms of neurodegeneration and identification of novel therapeutic targets. Here, we investigate levels of synaptic loss following nerve injury under a range of conditions, including during postnatal development, in a range of anatomically distinct muscles and in a mouse model of motor neuron disease. By utilising an ex vivo model of nerve injury, we show that synaptic withdrawal is slower during early postnatal development. Significantly more neuromuscular junctions remained fully innervated in the cranial nerve/muscle preparations analysed at P15 than at P25. Furthermore, we demonstrate variability in the level of synaptic withdrawal in response to injury in different muscles, with retraction being slower in abdominal preparations than in cranial muscles across all time points analysed. Importantly, differences between the cranial and thoracoabdominal musculature seen here are not consistent with differences in muscle vulnerability that have been previously reported in mouse models of the childhood motor neuron disease, spinal muscular atrophy (SMA), caused by depletion of survival motor neuron protein (Smn). To further investigate the relationship between synaptic degeneration in SMA and WD, we induced WD in preparations from the Smn^2B/− mouse model of SMA. In a disease‐resistant muscle (rostral band of levator auris longus), where there is minimal denervation, there was no change in the level of synaptic loss, which suggests that the process of synaptic withdrawal following injury is Smn‐independent. However, in a muscle with ongoing degeneration (transvs. abdominis), the level of synaptic loss significantly increased, with the percentage of denervated endplates increasing by 33% following injury, compared to disease alone. We therefore conclude that the presence of a die‐back can accelerate synaptic loss after injury in Smn^2B/− mice.

Estrogen receptor-α in female skeletal muscle is not required for regulation of muscle insulin sensitivity and mitochondrial regulation

OBJECTIVE

Estrogen receptor-α (ERα) is a nuclear receptor family member thought to substantially contribute to the metabolic regulation of skeletal muscle. However, previous mouse models utilized to assess the necessity of ERα signaling in skeletal muscle were confounded by altered developmental programming and/or influenced by secondary effects, making it difficult to assign a causal role for ERα. The objective of this study was to determine the role of skeletal muscle ERα in regulating metabolism in the absence of confounding factors of development.

METHODS

A novel mouse model was developed allowing for induced deletion of ERα in adult female skeletal muscle (ERαKOism). ERαshRNA was also used to knockdown ERα (ERαKD) in human myotubes cultured from primary human skeletal muscle cells isolated from muscle biopsies from healthy and obese insulin-resistant women.

RESULTS

Twelve weeks of HFD exposure had no differential effects on body composition, VO2, VCO2, RER, energy expenditure, and activity counts across genotypes. Although ERαKOism mice exhibited greater glucose intolerance than wild-type (WT) mice after chronic HFD, ex vivo skeletal muscle glucose uptake was not impaired in the ERαKOism mice. Expression of pro-inflammatory genes was altered in the skeletal muscle of the ERαKOism, but the concentrations of these inflammatory markers in the systemic circulation were either lower or remained similar to the WT mice. Finally, skeletal muscle mitochondrial respiratory capacity, oxidative phosphorylation efficiency, and H2O2 emission potential was not affected in the ERαKOism mice. ERαKD in human skeletal muscle cells neither altered differentiation capacity nor caused severe deficits in mitochondrial respiratory capacity.

CONCLUSIONS

Collectively, these results suggest that ERα function is superfluous in protecting against HFD-induced skeletal muscle metabolic derangements after postnatal development is complete.

Visualizing Mitochondrial Form and Function within the Cell

The specific cellular role of mitochondria is influenced by the surrounding environment because effective mitochondrial function requires the delivery of inputs (e.g., oxygen) and export of products (e.g., signaling molecules) to and from other cellular components, respectively. Recent technological developments in mitochondrial imaging have led to a more precise and comprehensive understanding of the spatial relationships governing the function of this complex organelle, opening a new era of mitochondrial research. Here, I highlight current imaging approaches for visualizing mitochondrial form and function within complex cellular environments. Increasing clarity of mitochondrial behavior within cells will continue to lend mechanistic insights into the role of mitochondria under normal and pathological conditions and point to spatially regulated processes that can be targeted to improve cellular function.