Ultrastructural study of mitochondria and their cristae in embryonic rats and primate (N. nemistrina)

Sodium-Glucose Cotransporter 2 (SGLT2) Inhibitors: Guardians against Mitochondrial Dysfunction and Endoplasmic Reticulum Stress in Heart Diseases

Sodium-glucose cotransporter 2 (SGLT2) inhibitors are an innovative class of antidiabetic drugs that provide cardiovascular benefits to both diabetic and nondiabetic patients, surpassing those of other antidiabetic drugs. Although the roles of mitochondria and endoplasmic reticulum (ER) in cardiovascular research are increasingly recognized as promising therapeutic targets, the exact molecular mechanisms by which SGLT2 inhibitors influence mitochondrial and ER homeostasis in the heart remain incompletely elucidated. This review comprehensively summarizes and discusses the impacts of SGLT2 inhibitors on mitochondrial dysfunction and ER stress in heart diseases including heart failure, ischemic heart disease/myocardial infarction, and arrhythmia from preclinical and clinical studies. Based on the existing evidence, the effects of SGLT2 inhibitors may potentially involve the restoration of mitochondrial biogenesis and alleviation of ER stress. Such consequences are achieved by enhancing adenosine triphosphate (ATP) production, preserving mitochondrial membrane potential, improving the activity of electron transport chain complexes, maintaining mitochondrial dynamics, mitigating oxidative stress and apoptosis, influencing cellular calcium and sodium handling, and targeting the unfolded protein response (UPR) through three signaling pathways including inositol requiring enzyme 1α (IRE1α), protein kinase R like endoplasmic reticulum kinase (PERK), and activating transcription factor 6 (ATF6). Therefore, SGLT2 inhibitors have emerged as a promising target for treating heart diseases due to their potential to improve mitochondrial functions and ER stress.

Cardiac Metabolism

The heart is composed of a heterogeneous mixture of cellular components perfectly intermingled and able to integrate common environmental signals to ensure proper cardiac function and performance. Metabolism defines a cell context-dependent signature that plays a critical role in survival, proliferation, or differentiation, being a recognized master piece of organ biology, modulating homeostasis, disease progression, and adaptation to tissue damage. The heart is a highly demanding organ, and adult cardiomyocytes require large amount of energy to fulfill adequate contractility. However, functioning under oxidative mitochondrial metabolism is accompanied with a concomitant elevation of harmful reactive oxygen species that indeed contributes to the progression of several cardiovascular pathologies and hampers the regenerative capacity of the mammalian heart. Cardiac metabolism is dynamic along embryonic development and substantially changes as cardiomyocytes mature and differentiate within the first days after birth. During early stages of cardiogenesis, anaerobic glycolysis is the main energetic program, while a progressive switch toward oxidative phosphorylation is a hallmark of myocardium differentiation. In response to cardiac injury, different signaling pathways participate in a metabolic rewiring to reactivate embryonic bioenergetic programs or the utilization of alternative substrates, reflecting the flexibility of heart metabolism and its central role in organ adaptation to external factors. Despite the well-established metabolic pattern of fetal, neonatal, and adult cardiomyocytes, our knowledge about the bioenergetics of other cardiac populations like endothelial cells, cardiac fibroblasts, or immune cells is limited. Considering the close intercellular communication and the influence of nonautonomous cues during heart development and after cardiac damage, it will be fundamental to better understand the metabolic programs in different cardiac cells in order to develop novel interventional opportunities based on metabolic rewiring to prevent heart failure and improve the limited regenerative capacity of the mammalian heart.

Metabolic Control of Cardiomyocyte Cell Cycle

Current therapies for heart failure aim to prevent the deleterious remodeling that occurs after MI injury, but currently no therapies are available to replace lost cardiomyocytes. Several organisms now being studied are capable of regenerating their myocardium by the proliferation of existing cardiomyocytes. In this review, we summarize the main metabolic pathways of the mammalian heart and how modulation of these metabolic pathways through genetic and pharmacological approaches influences cardiomyocyte proliferation and heart regeneration.

Synergistic effects of hormones on structural and functional maturation of cardiomyocytes and implications for heart regeneration

The limited endogenous regenerative capacity of the human heart renders cardiovascular diseases a major health threat, thus motivating intense research on in vitro heart cell generation and cell replacement therapies. However, so far, in vitro-generated cardiomyocytes share a rather fetal phenotype, limiting their utility for drug testing and cell-based heart repair. Various strategies to foster cellular maturation provide some success, but fully matured cardiomyocytes are still to be achieved. Today, several hormones are recognized for their effects on cardiomyocyte proliferation, differentiation, and function. Here, we will discuss how the endocrine system impacts cardiomyocyte maturation. After detailing which features characterize a mature phenotype, we will contemplate hormones most promising to induce such a phenotype, the routes of their action, and experimental evidence for their significance in this process. Due to their pleiotropic effects, hormones might be not only valuable to improve in vitro heart cell generation but also beneficial for in vivo heart regeneration. Accordingly, we will also contemplate how the presented hormones might be exploited for hormone-based regenerative therapies.

Mitochondrial maturation in the trophectoderm and inner cell mass regions of bovine blastocysts

Cellular differentiation induces various morphological changes, including elongation, in mitochondria. Preimplantation embryos have round-shaped mitochondria, characteristic of undifferentiated cells. However, there is controversy regarding the precise mitochondrial morphology in blastocyst embryos, which are generated from two cell lineages: undifferentiated inner cell mass (ICM) and differentiated trophectoderm (TE). This study attempted to precisely determine mitochondrial morphology in these two blastocyst regions. Transmission electron microscopy analyses were conducted using more than 1000 mitochondria from blastocyst embryos. No significant differences were observed in the configuration of mitochondrial cristae and frequencies of hooded mitochondria, which are specific to embryos of livestock animals, between the ICM and TE. To accurately compare mitochondrial roundness between the ICM and TE, oblateness was calculated based on both the major and minor axes. Average oblateness was significantly greater in the TE than in the ICM (P < 0.01). These results indicate tissue-specific mitochondrial maturation with complete elongation in the TE at the blastocyst stage. Since mitochondrial elongation is closely associated with cellular metabolism and differentiation, the present study provides new insights for better understanding of early embryonic development in cattle.

Mitochondria: A worthwhile object for ultrastructural qualitative characterization and quantification of cells at physiological and pathophysiological states using conventional transmission electron microscopy

Mitochondria are highly dynamic intracellular organelles with ultrastructural heterogeneity reflecting the behaviour and functions of the cells. The ultrastructural remodelling, performed by the counteracting active processes of mitochondrial fusion and fission, enables the organelles to respond to diverse cellular requirements and cues. It is also an important part of mechanisms underlying adaptation of mitochondria to pathophysiological conditions that challenge the cell homeostasis. However, if the stressor is constantly acting, the adaptive capacity of the cell can be exceeded and defective changes in mitochondrial morphology (indicating the insufficient functionality of mitochondria or development of mitochondrial disorders) may appear. Beside qualitative description of mitochondrial ultrastructure, stereological principles concerning the estimation of alterations in mitochondrial volume density or surface density are invaluable approaches for unbiased quantification of cells under physiological or pathophysiological conditions. In order to improve our understanding of cellular functions and dysfunctions, transmission electron microscopy (TEM) still remains a gold standard for qualitative and quantitative ultrastructural examination of mitochondria from various cell types, as well as from those experienced to different stimuli or toxicity-inducing factors. In the current study, general morphological and functional features of mitochondria, and their ultrastructural heterogeneity related to physiological and pathophysiological states of the cells are reviewed. Moreover, stereological approaches for accurate quantification of mitochondrial ultrastructure from electron micrographs taken from TEM are described in detail.

Role of Tafazzin in Mitochondrial Function, Development and Disease

Tafazzin, an enzyme associated with the rare inherited x-linked disorder Barth Syndrome, is a nuclear encoded mitochondrial transacylase that is highly conserved across multiple species and plays an important role in mitochondrial function. Numerous studies have elucidated the mechanisms by which Tafazzin affects mitochondrial function, but its effects on development and susceptibility to adult disease are incompletely understood. The purpose of this review is to highlight previous functional studies across a variety of model organisms, introduce recent studies that show an important role in development, and also to provide an update on the role of Tafazzin in human disease. The profound effects of Tafazzin on cardiac development and adult cardiac homeostasis will be emphasized. These studies underscore the importance of mitochondrial function in cardiac development and disease, and also introduce the concept of Tafazzin as a potential therapeutic modality.

The Redox Theory of Development

Significance: The geological record shows that as atmospheric O2 levels increased, it concomitantly coincided with the evolution of metazoans. More complex, higher organisms contain a more cysteine-rich proteome, potentially as a means to regulate homeostatic responses in a more O2-rich environment. Regulation of redox-sensitive processes to control development is likely to be evolutionarily conserved. Recent Advances: During early embryonic development, the conceptus is exposed to varying levels of O2. Oxygen and redox-sensitive elements can be regulated to promote normal development, defined as changes to cellular mass, morphology, biochemistry, and function, suggesting that O2 is a developmental morphogen. During periods of O2 fluctuation, embryos are "reprogrammed," on the genomic and metabolic levels. Reprogramming imparts changes to particular redox couples (nodes) that would support specific post-translational modifications (PTMs), targeting the cysteine proteome to regulate protein function and development. Critical Issues: Major developmental events such as stem cell expansion, proliferation, differentiation, migration, and cell fate decisions are controlled through oxidative PTMs of cysteine-based redox nodes. As such, timely coordinated redox regulation of these events yields normal developmental outcomes and viable species reproduction. Disruption of normal redox signaling can produce adverse developmental outcomes. Future Directions: Furthering our understanding of the redox-sensitive processes/pathways, the nature of the regulatory PTMs involved in development and periods of activation/sensitivity to specific developmental pathways would greatly support the theory of redox regulation of development, and would also provide rationale and direction to more fully comprehend poor developmental outcomes, such as dysmorphogenesis, functional deficits, and preterm embryonic death.

Maternal valproic acid exposure leads to neurogenesis defects and autism-like behaviors in non-human primates

Despite the substantial progress made in identifying genetic defects in autism spectrum disorder (ASD), the etiology for majority of ASD individuals remains elusive. Maternal exposure to valproic acid (VPA), a commonly prescribed antiepileptic drug during pregnancy in human, has long been considered a risk factor to contribute to ASD susceptibility in offspring from epidemiological studies in humans. The similar exposures in murine models have provided tentative evidence to support the finding from human epidemiology. However, the apparent difference between rodent and human poses a significant challenge to extrapolate the findings from rodent models to humans. Here we report for the first time the neurodevelopmental and behavioral outcomes of maternal VPA exposure in non-human primates. Monkey offspring from the early maternal VPA exposure have significantly reduced NeuN-positive mature neurons in prefrontal cortex (PFC) and cerebellum and the Ki67-positive proliferating neuronal precursors in the cerebellar external granular layer, but increased GFAP-positive astrocytes in PFC. Transcriptome analyses revealed that maternal VPA exposure disrupted the expression of genes associated with neurodevelopment in embryonic brain in offspring. VPA-exposed juvenile offspring have variable presentations of impaired social interaction, pronounced stereotypies, and more attention on nonsocial stimuli by eye tracking analysis. Our findings in non-human primates provide the best evidence so far to support causal link between maternal VPA exposure and neurodevelopmental defects and ASD susceptibility in humans.

Rewiring of embryonic glucose metabolism via suppression of PFK-1 and aldolase during mouse chorioallantoic branching

Adapting the energy metabolism state to changing bioenergetic demands is essential for mammalian development accompanying massive cell proliferation and cell differentiation. However, it remains unclear how developing embryos meet the changing bioenergetic demands during the chorioallantoic branching (CB) stage, when the maternal-fetal exchange of gases and nutrients is promoted. In this study, using metabolome analysis with mass-labeled glucose, we found that developing embryos redirected glucose carbon flow into the pentose phosphate pathway via suppression of the key glycolytic enzymes PFK-1 and aldolase during CB. Concomitantly, embryos exhibited an increase in lactate pool size and in the fractional contribution of glycolysis to lactate biosynthesis. Imaging mass spectrometry visualized lactate-rich tissues, such as the dorsal or posterior neural tube, somites and head mesenchyme. Furthermore, we found that the heterochronic gene Lin28a could act as a regulator of the metabolic changes observed during CB. Perturbation of glucose metabolism rewiring by suppressing Lin28a downregulation resulted in perinatal lethality. Thus, our work demonstrates that developing embryos rewire glucose metabolism following CB for normal development.

Highlighted article: Metabolic remodelling during E8.5 to E10.5 in mouse redirects glucose carbon into the pentose phosphate pathway, a process partially regulated by Lin28a.

Effect of Hyperglycemia on Gene Expression during Early Organogenesis in Mice

Background

Cardiovascular and neural malformations are common sequels of diabetic pregnancies, but the underlying molecular mechanisms remain unknown. We hypothesized that maternal hyperglycemia would affect the embryos most shortly after the glucose-sensitive time window at embryonic day (ED) 7.5 in mice.

Methods

Mice were made diabetic with streptozotocin, treated with slow-release insulin implants and mated. Pregnancy aggravated hyperglycemia. Gene expression profiles were determined in ED8.5 and ED9.5 embryos from diabetic and control mice using Serial Analysis of Gene Expression and deep sequencing.

Results

Maternal hyperglycemia induced differential regulation of 1,024 and 2,148 unique functional genes on ED8.5 and ED9.5, respectively, mostly in downward direction. Pathway analysis showed that ED8.5 embryos suffered mainly from impaired cell proliferation, and ED9.5 embryos from impaired cytoskeletal remodeling and oxidative phosphorylation (all P ≤ E-5). A query of the Mouse Genome Database showed that 20–25% of the differentially expressed genes were caused by cardiovascular and/or neural malformations, if deficient. Despite high glucose levels in embryos with maternal hyperglycemia and a ~150-fold higher rate of ATP production from glycolysis than from oxidative phosphorylation on ED9.5, ATP production from both glycolysis and oxidative phosphorylation was reduced to ~70% of controls, implying a shortage of energy production in hyperglycemic embryos.

Conclusion

Maternal hyperglycemia suppressed cell proliferation during gastrulation and cytoskeletal remodeling during early organogenesis. 20–25% of the genes that were differentially regulated by hyperglycemia were associated with relevant congenital malformations. Unexpectedly, maternal hyperglycemia also endangered the energy supply of the embryo by suppressing its glycolytic capacity.

Mechanism of Developmental Effects in Rats Caused by an N-Phenylimide Herbicide: Transient Fetal Anemia and Sequelae during Mid-to-Late Gestation

BACKGROUND

Rat developmental toxicity including embryolethality and teratogenicity (mainly ventricular septal defects [VSDs] and wavy ribs) was produced by an N-phenylimide herbicide that inhibits protoporphyrinogen oxidase (PPO) common to chlorophyll and heme biosynthesis. Major characteristics of the developmental toxicity included species difference between rats and rabbits, compound-specific difference among structurally similar herbicides, and sensitive period. Protoporphyrin accumulation in treated fetuses closely correlated with the major characteristics. Iron deposits in erythroblastic mitochondria and degeneration of erythroblasts were observed in treated rat fetuses. In this study we investigated fetal anemia and subsequent developmental effects in rats, and inhibition of PPO in rats, rabbits, and humans by the herbicides in vitro.

METHODS

Fetuses were treated on gestational day (GD) 12 and removed on GDs 13 through 20. All litters were examined externally. One half of litters were examined for blood and skeletal development, and the other half for interventricular foramen closure. Effects on PPO were determined in mitochondria from embryos and adult livers.

RESULTS

Fetal anemia in rats was evident on GDs 13 through 16. Subsequently, enlarged heart, delayed closure of the foramen, reduced serum protein, and retarded rib ossification were observed. In vitro PPO inhibition exhibited species- and compound-specific differences corresponding to the developmental toxicity.

CONCLUSION

We propose that developmental toxicity results from PPO inhibition in primitive erythroblasts, causing transient fetal anemia followed by death. Compensatory enlargement of the fetal heart results in failure of interventricular foramen closure and VSD. Reduced serum protein leads to delayed ossification and wavy ribs.

Ketamine-induced oxidative stress at different developmental stages of zebrafish (Danio rerio) embryos

The changes induced by ketamine exposure were developmental stage-dependent, and related with the gradual development of the antioxidant defense system of the embryo, which is dependent on changes in energy-sensing pathways.

Improvement of bovine in vitro embryo production by vitamin K₂ supplementation

Mitochondria play an important role during early development in mammalian embryos. It has been shown that properly controlled follicular preparation increases the likelihood of in-vitro-produced bovine embryos reaching the blastocyst stage and that competent embryos exhibit heightened expression of genes associated with mitochondrial function. We hypothesized that apparently incompetent embryos could be rescued by restoring mitochondrial function. It has been shown that vitamin K2 (a membrane-bound electron carrier similar to ubiquinone) can restore mitochondrial dysfunction in eukaryotic cells. The aim of this study was therefore to investigate the effects of vitamin K2 on bovine embryonic development in vitro. The vitamin was found most effective when added 72 h after fertilization. It produced a significant (P<0.05) increase in the percentage of blastocysts (+8.6%), more expanded blastocysts (+7.8%), and embryos of better morphological quality. It improved the mitochondrial activity significantly and had a measurable impact on gene expression. This is the first demonstration that current standard conditions of in vitro production of bovine embryos may be inadequate due to the lack of support for mitochondrial function and may be improved significantly by supplementing the culture medium with vitamin K2.

Engineering adolescence: maturation of human pluripotent stem cell-derived cardiomyocytes

The discovery of human pluripotent stem cells (hPSCs), including both human embryonic stem cells and human-induced pluripotent stem cells, has opened up novel paths for a wide range of scientific studies. The capability to direct the differentiation of hPSCs into functional cardiomyocytes has provided a platform for regenerative medicine, development, tissue engineering, disease modeling, and drug toxicity testing. Despite exciting progress, achieving the optimal benefits has been hampered by the immature nature of these cardiomyocytes. Cardiac maturation has long been studied in vivo using animal models; however, finding ways to mature hPSC cardiomyocytes is only in its initial stages. In this review, we discuss progress in promoting the maturation of the hPSC cardiomyocytes, in the context of our current knowledge of developmental cardiac maturation and in relation to in vitro model systems such as rodent ventricular myocytes. Promising approaches that have begun to be examined in hPSC cardiomyocytes include long-term culturing, 3-dimensional tissue engineering, mechanical loading, electric stimulation, modulation of substrate stiffness, and treatment with neurohormonal factors. Future studies will benefit from the combinatorial use of different approaches that more closely mimic nature's diverse cues, which may result in broader changes in structure, function, and therapeutic applicability.

Crosstalk between mitochondrial (dys)function and mitochondrial abundance

A controlled regulation of mitochondrial mass through either the production (biogenesis) or the degradation (mitochondrial quality control) of the organelle represents a crucial step for proper mitochondrial and cell function. Key steps of mitochondrial biogenesis and quality control are overviewed, with an emphasis on the role of mitochondrial chaperones and proteases that keep mitochondria fully functional, provided the mitochondrial activity impairment is not excessive. In this case, the whole organelle is degraded by mitochondrial autophagy or "mitophagy." Beside the maintenance of adequate mitochondrial abundance and functions for cell homeostasis, mitochondrial biogenesis might be enhanced, through discussed signaling pathways, in response to various physiological stimuli, like contractile activity, exposure to low temperatures, caloric restriction, and stem cells differentiation. In addition, mitochondrial dysfunction might also initiate a retrograde response, enabling cell adaptation through increased mitochondrial biogenesis.

The permeability transition pore controls cardiac mitochondrial maturation and myocyte differentiation

Although mature myocytes rely on mitochondria as the primary source of energy, the role of mitochondria in the developing heart is not well known. Here, we find that closure of the mitochondrial permeability transition pore (mPTP) drives maturation of mitochondrial structure and function and myocyte differentiation. Cardiomyocytes at embryonic day (E) 9.5, when compared to E13.5, displayed fragmented mitochondria with few cristae, a less-polarized mitochondrial membrane potential, higher reactive oxygen species (ROS) levels, and an open mPTP. Pharmacologic and genetic closing of the mPTP yielded maturation of mitochondrial structure and function, lowered ROS, and increased myocyte differentiation (measured by counting Z bands). Furthermore, myocyte differentiation was inhibited and enhanced with oxidant and antioxidant treatment, respectively, suggesting that redox-signaling pathways lie downstream of mitochondria to regulate cardiac myocyte differentiation.

Bioenergetics, mitochondria, and cardiac myocyte differentiation

Cardiac metabolism is finely tuned, and disruption of myocardial bioenergetics can be clinically devastating. Many cardiomyopathies that present early in life are due to disruption of the maturation of these metabolic pathways. However, this bioenergetic maturation begins well before birth, when the embryonic heart is first beginning to beat, and continues into the mature animal. Thus, the changes in energy production seen after birth are actually part of a continuum that coincides with the structural and functional changes that occur as the cardiac myocyte differentiates and the heart undergoes morphogenesis. Therefore, although bioenergetics and mitochondrial biology have not been studied in great detail in the developing heart, bioenergetic maturation should be considered an important component of normal myocyte differentiation.Although events occurring after birth will be discussed, this review will focus on the changes in bioenergetics and mitochondrial biology that coincide with myocyte differentiation and cardiac morphogenesis. The relationship of these changes to the etiology and presentation of cardiomyopathies will be used as a starting point for this discussion. Then, after reviewing cardiac development and mitochondrial biology, the published data on bioenergetics and mitochondrial structure and function in the developing heart will be presented. Finally, the case will be made that mitochondria may be critical regulators of cardiac myocyte differentiation and cardiac development.

Teratogen-induced oxidative stress targets glyceraldehyde-3-phosphate dehydrogenase in the organogenesis stage mouse embryo

Exposure during the organogenesis stage of the mouse embryo to the model teratogen, hydroxyurea (HU), induces curly tail and limb malformations. Oxidative stress contributes to the developmental toxicity of HU. Reactive oxygen species (ROS) interact with polyunsaturated bilipid membranes to form α,β-unsaturated reactive aldehydes; 4-hydroxy-2-nonenal (4-HNE), one of the most cytotoxic of these aldehydes, covalently adducts with proteins, lipids, and nucleic acids. The goal of the current study is to determine if HU exposure of CD1 mice on gestation day 9 generates region-specific 4-HNE-protein adducts in the embryo and to identify the proteins targeted. The formation of 4-HNE-protein adducts was elevated in the caudal region of control embryos; HU exposure further increased 4-HNE-protein adduct formation in this area. Interestingly, three of the 4-HNE–modified proteins, glyceraldehyde-3-phosphate dehydrogenase (GAPDH), glutamate oxaloacetate transaminase 2, and aldolase 1, A isoform, are involved in energy metabolism. The formation of 4-HNE-GAPDH protein adducts reduced GAPDH enzymatic activity by 20% and attenuated lactate production by 40%. Furthermore, HU exposure induced the nuclear translocation of GAPDH in the caudal region of exposed embryos; this nuclear translocation may be associated with the reactivation of oxidized proteins involved in DNA repair, such as apurinic/apyrimidinic endonuclease-1, and the stimulation of E1A-associated P300 protein/creb-binding protein (p300/CBP) activity, initiating cell death in a p53-dependent pathway. We propose that GAPDH is a redox-sensitive target in the embryo and may play a role in a stress response during development.

Mitochondrial DNA content and expression of genes involved in mtDNA transcription, regulation and maintenance during human fetal development

The mitochondrial biogenesis and adequate energy production are important for fetal growth and early postnatal adaptation. The aim of the study was to characterize mitochondrial DNA (mtDNA) content and expression patterns of POLG, TFAM, NRF1,NRF2 and PGC1 family of regulated coactivators (PGC1A, PGC1B and PRC) involved in the mtDNA transcription, regulation and maintenance in human fetal tissues during second trimester of gestation. Further the mRNA expression profiles of selected cytochrome c oxidase (COX) subunits were analysed. Moreover enzyme activities of COX and CS and protein levels of COX subunits were analysed. DNA, RNA and proteins were isolated from 26 pairs of fetal liver and muscle samples obtained at autopsy after termination of pregnancy for genetic indications unrelated to OXPHOS deficiency between 13th and 28th week of gestation. This work offers a broad view on the mtDNA content changes in two different tissues during the second trimester of gestation and in the corresponding tissues after birth. The important differences in expression of POLG, TFAM, NRF2 genes and family PGC1 coactivators were found between the fetal tissues. The significant tissue-specific changes in expression of selected COX subunits on mRNA level (COX4 and MTCO2) were observed. Further the considerable differences in enzyme activities of COX and CS are demonstrated between fetal and postnatal phase. In conclusion our study indicates that the fetal developing tissues might differ in the control of mitochondrial biogenesis depending on their energy demand and the age of gestation. Moreover the gene expression is changed mainly on transcriptional level through fetal period.

Arsenate-induced maternal glucose intolerance and neural tube defects in a mouse model

BACKGROUND

Epidemiological studies have linked environmental arsenic (As) exposure to increased type 2 diabetes risk. Periconceptional hyperglycemia is a significant risk factor for neural tube defects (NTDs), the second most common structural birth defect. A suspected teratogen, arsenic (As) induces NTDs in laboratory animals.

OBJECTIVES

We investigated whether maternal glucose homeostasis disruption was responsible for arsenate-induced NTDs in a well-established dosing regimen used in studies of arsenic's teratogenicity in early neurodevelopment.

METHODS

We evaluated maternal intraperitoneal (IP) exposure to As 9.6 mg/kg (as sodium arsenate) in LM/Bc/Fnn mice for teratogenicity and disruption of maternal plasma glucose and insulin levels. Selected compounds (insulin pellet, sodium selenate (SS), N-acetyl cysteine (NAC), l-methionine (L-Met), N-tert-Butyl-alpha-phenylnitrone (PBN)) were investigated for their potential to mitigate arsenate's effects.

RESULTS

Arsenate caused significant glucose elevation during an IP glucose tolerance test (IPGTT). Insulin levels were not different between arsenate and control dams before (arsenate, 0.55 ng/dl; control, 0.48 ng/dl) or after glucose challenge (arsenate, 1.09 ng/dl; control, 0.81 ng/dl). HOMA-IR index was higher for arsenate (3.9) vs control (2.5) dams (p=0.0260). Arsenate caused NTDs (100%, p<0.0001). Insulin pellet and NAC were the most successful rescue agents, reducing NTD rates to 45% and 35%.

CONCLUSIONS

IPGTT, insulin assay, and HOMA-IR results suggest a modest failure of glucose stimulated insulin secretion and insulin resistance characteristic of glucose intolerance. Insulin's success in preventing arsenate-induced NTDs provides evidence that these arsenate-induced NTDs are secondary to elevated maternal glucose. The NAC rescue, which did not restore maternal glucose or insulin levels, suggests oxidative disruption plays a role.

Morphological dynamics of mitochondria--a special emphasis on cardiac muscle cells

Mitochondria play a critical role in cellular energy metabolism, Ca(2+) homeostasis, reactive oxygen species generation, apoptosis, aging, and development. Many recent publications have shown that a continuous balance of fusion and fission of these organelles is important in maintaining their proper function. Therefore, there is a steep correlation between the form and function of mitochondria. Many major proteins involved in mitochondrial fusion and fission have been identified in different cell types, including heart. However, the functional role of mitochondrial dynamics in the heart remains, for the most part, unexplored. In this review we will cover the recent field of mitochondrial dynamics and its physiological and pathological implications, with a particular emphasis on the experimental and theoretical basis of mitochondrial dynamics in the heart.

Prenatal hypoxia induces increased cardiac contractility on a background of decreased capillary density

Background

Chronic hypoxia in utero (CHU) is one of the most common insults to fetal development and may be associated with poor cardiac recovery from ischaemia-reperfusion injury, yet the effects on normal cardiac mechanical performance are poorly understood.

Methods

Pregnant female wistar rats were exposed to hypoxia (12% oxygen, balance nitrogen) for days 10–20 of pregnancy. Pups were born into normal room air and weaned normally. At 10 weeks of age, hearts were excised under anaesthesia and underwent retrograde 'Langendorff' perfusion. Mechanical performance was measured at constant filling pressure (100 cm H₂O) with intraventricular balloon. Left ventricular free wall was dissected away and capillary density estimated following alkaline phosphatase staining. Expression of SERCA2a and Nitric Oxide Synthases (NOS) proteins were estimated by immunoblotting.

Results

CHU significantly increased body mass (P < 0.001) compared with age-matched control rats but was without effect on relative cardiac mass. For incremental increases in left ventricular balloon volume, diastolic pressure was preserved. However, systolic pressure was significantly greater following CHU for balloon volume = 50 μl (P < 0.01) and up to 200 μl (P < 0.05). For higher balloon volumes systolic pressure was not significantly different from control. Developed pressures were correspondingly increased relative to controls for balloon volumes up to 250 μl (P < 0.05). Left ventricular free wall capillary density was significantly decreased in both epicardium (18%; P < 0.05) and endocardium (11%; P < 0.05) despite preserved coronary flow. Western blot analysis revealed no change to the expression of SERCA2a or nNOS but immuno-detectable eNOS protein was significantly decreased (P < 0.001) in cardiac tissue following chronic hypoxia in utero.

Conclusion

These data offer potential mechanisms for poor recovery following ischaemia, including decreased coronary flow reserve and impaired angiogenesis with subsequent detrimental effects of post-natal cardiac performance.

Role of glucose in cloned mouse embryo development

Cloned mouse embryos display a marked preference for glucose-containing culture medium, with enhanced development to the blastocyst stage in glucose-containing medium attributable mainly to an early beneficial effect during the first cell cycle. This early beneficial effect of glucose is not displayed by parthenogenetic, fertilized, or tetraploid nuclear transfer control embryos, indicating that it is specific to diploid clones. Precocious localization of the glucose transporter SLC2A1 to the cell surface, as well as increased expression of glucose transporters and increased uptake of glucose at the one- and two-cell stages, is also seen in cloned embryos. To examine the role of glucose in early cloned embryo development, we examined glucose metabolism and associated metabolites, as well as mitochondrial ultrastructure, distribution, and number. Clones prepared with cumulus cell nuclei displayed significantly enhanced glucose metabolism at the two-cell stage relative to parthenogenetic controls. Despite the increase in metabolism, ATP content was reduced in clones relative to parthenotes and fertilized controls. Clones at both stages displayed elevated concentrations of glycogen compared with parthenogenetic controls. There was no difference in the number of mitochondria, but clone mitochondria displayed ultrastructural alterations. Interestingly, glucose availability positively affected mitochondrial structure and localization. We conclude that cloned embryos may be severely compromised in terms of ATP-dependent processes during the first two cell cycles and that glucose may exert its early beneficial effects via positive effects on the mitochondria.

Clk-1 deficiency induces apoptosis associated with mitochondrial dysfunction in mouse embryos

Clk-1 gene encodes demethoxyubiquinone hydroxylase that catalyzes the production of coenzyme Q (CoQ) in mitochondria. Clk-1-deficient mice that lack CoQ fail to survive beyond the embryonic day 10.5 (E10.5). However, the relationship between the clk-1-deficiency and embryonic lethality remains unclear. We show in this study that TUNEL-positive cells are frequently observed in whole bodies of clk-1-deficient mouse embryos at E10.5. In addition, dissociated cells from the embryos exhibited characteristic features of apoptosis, such as externalization of phosphatidylserine on the plasma membrane, caspase-3 activation, and the release of cytochrome c from mitochondria into the cytoplasm, as the first sign of mitochondria-mediated apoptosis. In embryonic cells, the mitochondrial functions such as maintenance of the mitochondrial membrane potential and intracellular ATP level were impaired. Since exogenous CoQ10 rescued the mitochondrial dysfunction and suppressed apoptosis in clk-1-deficient cells, we propose that clk-1-deficiency induces apoptosis associated with mitochondrial dysfunction due to a lack of CoQ, which may lead to embryonic lethality in mice around E10.5.

The effect of hypoxia in development

There is increasing evidence that the oxygen supply to the human embryo in the first trimester is tightly controlled, suggesting that too much oxygen may interfere with development. The use of hypoxia probes in mammalian embryos during the organogenic period indicates that the embryo is normally in a state of partial hypoxia, and this may be essential to control cardiovascular development, perhaps under the control of hypoxia-inducible factor (HIF). A consequence of this state of partial hypoxia is that disturbances in the oxygen supply can more easily lead to a damaging degree of hypoxia. Experimental mammalian embryos show a surprising degree of resilience to hypoxia, with many organogenic stage embryos able to survive 30-60 min of anoxia. However, in some embryos this degree of hypoxia causes abnormal development, particularly transverse limb reduction defects. These abnormalities are preceded by hemorrhage/edema and tissue necrosis. Other parts of the embryo are also susceptible to this hypoxia-induced damage and include the genital tubercle, the developing nose, the tail, and the central nervous system. Other frequently observed defects in animal models of prenatal hypoxia include cleft lip, maxillary hypoplasia, and heart defects. Animal studies indicate that hypoxic episodes in the first trimester of human pregnancy could occur by temporary constriction of the uterine arteries. This could be a consequence of exposure to cocaine, misoprostol, or severe shock, and there is evidence that these exposures have resulted in hypoxia-related malformations in the human. Exposure to drugs that block the potassium current (IKr) can cause severe slowing and arrhythmia of the mammalian embryonic heart and consequently hypoxia in the embryo. These drugs are highly teratogenic in experimental animals. There is evidence that drugs with IKr blockade as a side effect, for example phenytoin, may cause birth defects in the human by causing periods of embryonic hypoxia. The strongest evidence of hypoxia causing birth defects in the human comes from studies of fetuses lacking hemoglobin (Hb) F. These fetuses are thought to be hypoxic from about the middle of the first trimester and show a range of birth defects, particularly transverse limb reduction defects.

Responses of mitochondrial biogenesis and function to maternal diabetes in rat embryo during the placentation period

Mitochondria are cellular organelles that have been reported to be altered in diabetes, being closely related to its associated complications. Moreover, mitochondrial biogenesis and function are essential for proper embryo development throughout the placentation period, occurring during organogenesis, when a great rate of congenital malformations have been associated with diabetic pregnancy. Thus, the aim of the current work was to investigate the effect of the diabetic environment on mitochondrial function and biogenesis during the placentation period. For this purpose, we studied the oxidative phosphorylation system (OXPHOS) enzymatic activities as well as the expression of genes involved in the coordinated regulation of both mitochondrial and nuclear genome (PGC-1alpha, NRF-1, NRF-2alpha, mtSSB, and TFAM) and mitochondrial function (COX-IV, COX-I, and beta-ATPase) in rat embryos from control and streptozotocin-induced diabetic mothers. Our results reflected that diabetic pregnancy retarded and altered embryo growth. The embryos from diabetic mothers showing normal morphology presented a reduced content of proteins regulated through the PGC-1alpha mitochondriogenic pathway on gestational day 12. This fact was accompanied by several responses that entailed the activation of OXPHOS activities on the same day and the recovery of the content of the studied proteins to control levels on day 13. As a result, the mitochondria of these embryos would reach a situation close to control on day 13 that could allow them to follow the normal mitochondriogenic schedule throughout a gestational period in which the mitochondrial differentiation process is critical. Nevertheless, malformed embryos from diabetic mothers seemed to show a lower adaptation capability, which could exacerbate their maldevelopment.

Mitochondrial differentiation and oxidative phosphorylation system capacity in rat embryo during placentation period

Mitochondrial biogenesis and function are essential for proper embryo development; however, these processes have not been further studied during the placentation period, when important oxidative metabolism activation is taking place. Thus, the aim of the present study was to investigate the oxidative phosphorylation system (OXPHOS) enzymatic activities as well as the expression of genes involved in the coordinated regulation of both mitochondrial and nuclear genomes (peroxisome proliferator-activated receptor-γ coactivator-1α, nuclear respiratory factors 1 and 2, mitochondrial single-strand DNA-binding protein, mitochondrial transcription factor A), and mitochondrial function (cytochromecoxidase subunit IV, cytochromecoxidase subunit I and β-ATP phosphohydrolase) in rat embryo throughout the placentation period (gestational days 11, 12 and 13). Our results reflect that embryo mitochondria were enhancing their OXPHOS potential capacities, pointing out that embryo mitochondria become more differentiated during the placentation period. Besides, the current findings show that the mRNAs of the nuclear genes involved in mitochondrial biogenesis were downregulated, whereas their protein content together with the mitochondrial DNA expression were upregulated throughout the period studied. These data indicate that the molecular regulation of the mitochondrial differentiation process during placentation involves a post-transcriptional activation of the nuclear-encoded genes that would lead to an increase in both the nuclear- and mitochondrial-encoded proteins responsible for the mitochondrial biogenic process. As a result, embryo mitochondria would reach a more differentiated stage with a more efficient oxidative metabolism that would facilitate the important embryo growth during the second half of the pregnancy.

Somatic cell nuclei in cloning: strangers traveling in a foreign land

The recent successes in producing cloned offspring by somatic cell nuclear transfer are nothing short of remarkable. This process requires the somatic cell chromatin to substitute functionally for both the egg and the sperm genomes, and indeed the processing of the transferred nuclei shares aspects in common with processing of both parental genomes in normal fertilized embryos. Recent studies have yielded new information about the degree to which this substitution is accomplished. Overall, it has become evident that multiple aspects of genome processing and function are aberrant, indicating that the somatic cell chromatin only infrequently manages the successful transition to a competent surrogate for gamete genomes. This review focuses on recent results revealing these limitations and how they might be overcome.

BCS1L is expressed in critical regions for neural development during ontogenesis in mice

BCS1L is a chaperone necessary for the incorporation of Rieske FeS and Qcr10p into complex III (CIII) of the respiratory chain. Mutations in the BCS1L gene cause early fetal growth restriction and a lethal neonatal disease in humans, however, the pathogenesis remains unclear. Here, we analysed the expression of BCS1L during mouse embryonic development and compared its expression with that of the mitochondrial markers Porin, GRIM19, Core I, and Rieske FeS. BCS1L was strongly expressed in embryonic tissues already at embryonic days 7 (E7) and 9 whereas the expression of Porin and Rieske FeS was not as evident at this time point. At E11, BCS1L, Porin, and Rieske FeS had overlapping expression patterns in organs known to contain high numbers of mitochondria such as heart, liver and somites. In contrast, BCS1L was differently distributed compared to the mitochondrial proteins Porin, Rieske FeS, Core I and Grim 19 in the floor plate of the E11, E12 and E13 neural tube. These results show that the expression pattern of BCS1L only partially overlaps with the expression of Porin and Rieske FeS. Thus, BCS1L alone or in cooperation with Rieske FES may during development have previously unknown functions beside its role in assembly of complex III. The floor plate of the neural tube is essential for dorsal ventral patterning and the guidance of the developing neurons to their targets. The predominant expression of BCS1L in this region, together with its presence in peripheral ganglia from E13 onwards, indicates a role for BCS1L in the development of neural structures.

Tough beginnings: alterations in the transcriptome of cloned embryos during the first two cell cycles

Cloned embryos produced by somatic cell nuclear transfer (SCNT) display a plethora of phenotypic characteristics that make them different from fertilized embryos, indicating defects in the process of nuclear reprogramming by the recipient ooplasm. To elucidate the extent and timing of nuclear reprogramming, we used microarrays to analyze the transcriptome of mouse SCNT embryos during the first two cell cycles. We identified a large number of genes mis-expressed in SCNT embryos. We found that genes involved in transcription and regulation of transcription are prominent among affected genes, and thus may be particularly difficult to reprogram, and these likely cause a ripple effect that alters the transcriptome of many other functions, including oxidative phosphorylation, transport across membrane, and mRNA transport and processing. Interestingly, we also uncovered widespread alterations in the maternal (i.e., non-transcribed) mRNA population of SCNT embryos. We conclude that gene expression in early SCNT embryos is grossly abnormal, and that this is at least in part the result of incomplete reprogramming of transcription factor genes.

Nitric oxide and mitochondrial biogenesis

The characteristic structural organization of mitochondria is the product of synthesis of macromolecules within the mitochondria together with the import of proteins and lipids synthesized outside the organelle. Synthetic and import processes are required for mitochondrial proliferation and might also facilitate the growth of pre-existing mitochondria. Recent evidence indicates that these events are regulated in a complex way by several agonists and environmental conditions, through activation of specific signaling pathways and transcription factors. A newly discovered role of this organelle in retrograde intracellular signaling back to the nucleus has also emerged. This is likely to have far-reaching implications in development, aging, disease and environmental adaptation. Generation of nitric oxide (NO) appears to be an important player in these processes, possibly acting as a unifying molecular switch to trigger the whole mitochondrial biogenesis process. High levels of NO acutely inhibit cell respiration by binding to cytochrome c oxidase. Conversely, chronic, smaller increases in NO levels stimulate mitochondrial biogenesis in diverse cell types. NO-induced mitochondrial biogenesis seems to be linked to proliferation and differentiation of normal and tumor cells, as well as in aging.

Mitochondrial dysmorphology in the neuroepithelium of rat embryos following a single dose of maternal hyperthermia during gestation

Hyperthermia is teratogenic to human and animal embryos and induces mainly anomalies of the nervous system. However, the teratogenic mechanism is poorly understood. Mammalian embryos are known to switch from anaerobic to aerobic metabolism around the time of neural tube closure. This critical event might be sensitive to hyperthermia. The objective of the present study was to evaluate the ultrastructural changes of the mitochondria of the neuroepithelium (NE) of rat embryos following maternal exposure to hyperthermia. Pregnant rats were heat stressed for an hour on gestation day (GD) 9 and embryos were examined by electron microscopy on GD 10. NE presented extensive apoptosis. Intercellular junctions were weakened and copious cellular debris projected into the ventricle. The mitochondria were of diverse size and shape. Most of them were swollen and had short cristae and electron dense matrix. Hydropic changes were also observed in numerous mitochondria. Lipid-laden mitochondria were found in the apical portions of neuroblasts. The mesenchyme (ME) of heat-treated embryos showed paucity of cells and only as frequent apoptosis as the controls. Their mitochondria also showed changes similar to those of the NE. Additionally extensive lipid accumulation was observed in and in the vicinity of mitochondria, often surrounded by short strands of endoplasmic reticulum. Whereas mitochondrial pathology was associated with profound apoptosis in the NE, growth restriction and lipid accumulation accompanied mitochondrial changes in the ME. The results of this study indicate that the embryonic response to maternal heat shock is tissue-specific and morphologically distinct in this species.

Role of the mitochondrial genome in preimplantation development and assisted reproductive technologies

Our fascination for mitochondria relates to their origin as symbiotic, semi-independent organisms on which we, as eukaryotic beings, rely nearly exclusively to produce energy for every cell function. Therefore, it is not surprising that these organelles play an essential role in many events during early development and in artificial reproductive technologies (ARTs) applied to humans and domestic animals. However, much needs to be learned about the interactions between the nucleus and the mitochondrial genome (mtDNA), particularly with respect to the control of transcription, replication and segregation during preimplantation. Nuclear-encoded factors that control transcription and replication are expressed during preimplantation development in mice and are followed by mtDNA transcription, but these result in no change in mtDNA copy number. However, in cattle, mtDNA copy number increases during blastocyst expansion and hatching. Nuclear genes influence the mtDNA segregation patterns in heteroplasmic animals. Because many ARTs markedly modify the mtDNA content in embryos, it is essential that their application is preceded by careful experimental scrutiny, using suitable animal models.

Mitochondrial transcription factor A (TFAM) is increased in rat embryo during placentation and associated with mitochondrial differentiation

In the current study, the mitochondrial proliferationdifferentiation process was investigated in rat embryo during the placentation process, straight after organogenesis, when there is an important oxidative metabolism activation. For this purpose, on gestational days 11, 12 and 13 we studied the mitochondrial DNA (mtDNA) content and the relative gene expression of proteins involved in mtDNA replication (mitochondrial single strand DNA binding protein (mtSSB)), mtDNA transcription (mitochondrial transcription factor A (TFAM)), as well as in mitochondrial function (cytochrome c oxidase subunit I (COXI)). The results indicated that during placentation important changes in mitochondrial proliferation-differentiation process take place in rat embryo. There is a great decrease in cellular mtDNA content and a rise in the ratio between TFAM and mtDNA accompanied by an increase in COXI gene expression. Thus, we can conclude that on gestational day 13 mitochondrial differentiation predominates over mitochondrial proliferation in embryo cells. Besides, our work reveals that in a physiological condition such as embryonic development the TFAM levels change in order to regulate the transcriptional activity of mtDNA.

Estrogen, mitochondria, and growth of cancer and non-cancer cells

In this review, we discuss estrogen actions on mitochondrial function and the possible implications on cell growth. Mitochondria are important targets of estrogen action. Therefore, an in-depth analysis of interaction between estrogen and mitochondria; and mitochondrial signaling to nucleus are pertinent to the development of new therapy strategies for the treatment of estrogen-dependent diseases related to mitochondrial disorders, including cancer.

Effects of soft diet on rat masseter muscle mitochondrial development

There is not fine information regarding the influence of diet on development of masseter muscle and its mitochondria. Objective of this study is to compare mitochondrial enzyme activity, and morphology of masseter muscle fiber cells and its mitochondria prepared from rats fed soft diet to those fed hard diet. Cross-sectional area of fiber cells and number of mitochondria per unit area prepared from rats fed hard diet were greater than those of animals fed soft diet on postnatal day 60, and these differences decreased under both feeding conditions on days 120. Structure of mitochondrial cristae of rat fed hard diet was clear but that of fed soft diet ambiguous and contains many halos. Mitochondrial succinate-O2 and NADH-O2 oxidoreductase activities isolated from rats fed hard diet were higher than those of soft diet group over the period from days 30, and differences in both diet groups became the largest on days 60, and decreased on days 120. Our results indicated that development of masseter muscle fiber cells and mitochondria is hindered when rats are fed soft diet, but recover partially later, and hard diet is required for normal development of masseter muscle.

Alterations in mitochondrial morphology are associated with hyperthermia-induced apoptosis in early postimplantation mouse embryos

BACKGROUND

Previously, we showed that teratogens such as hyperthermia activate the mitochondrial apoptotic pathway in day nine mouse embryos. Activation of this pathway involves an initial release of cytochrome c from intermembranous spaces of the mitochondria into the cytoplasm. Cytoplasmic cytochrome c then activates a caspase cascade resulting in the orderly demise of the cell. In addition, we showed that teratogens activate the mitochondrial pathway in cells of the neuroepithelium, but not the heart.

METHODS

To further investigate the role of the mitochondrion in teratogen-induced apoptosis, we used transmission electron microscopy (TEM) to compare mitochondrial morphology in cells of the neuroepithelium and heart of control and hyperthermia-treated embryos. Because we know that the apoptotic pathway is activated some time during the first 5 hr after teratogen exposure is initiated, we assessed mitochondrial morphology at 1, 2.5, and 5 hr after day nine mouse embryos were exposed to hyperthermia (43 degrees C, 15 min).

RESULTS

In neuroepithelial cells of the prosencephalon, abnormally-shaped mitochondria were observed at the 1 hr time point and thereafter, whereas loss of cristae and shrunken mitochondria were noted at the 5 hr time point. In contrast, no obvious changes in mitochondria of heart cells were observed at any of the time points monitored.

CONCLUSIONS

These results indicate that teratogen-induced cell death in neuroepithelial cells is temporally correlated with alterations in mitochondrial morphology, whereas the absence of cell death in the heart is correlated with a corresponding lack of change in mitochondrial morphology. Birth Defects Research (Part A), 2003.

Calcineurin signaling and NFAT activation in cardiovascular and skeletal muscle development

Calcineurin signaling has been implicated in a broad spectrum of developmental processes in a variety of organ systems. Calcineurin is a calmodulin-dependent, calcium-activated protein phosphatase composed of catalytic and regulatory subunits. The serine/threonine-specific phosphatase functions within a signal transduction pathway that regulates gene expression and biological responses in many developmentally important cell types. Calcineurin signaling was first defined in T lymphocytes as a regulator of nuclear factor of activated T cells (NFAT) transcription factor nuclear translocation and activation. Recent studies have demonstrated the vital nature of calcium/calcineurin/NFAT signaling in cardiovascular and skeletal muscle development in vertebrates. Inhibition, mutation, or forced expression of calcineurin pathway genes result in defects or alterations in cardiomyocyte maturation, heart valve formation, vascular development, skeletal muscle differentiation and fiber-type switching, and cardiac and skeletal muscle hypertrophy. Conserved calcineurin genes are found in invertebrates such as Drosophila and Caenorhabditis elegans, and genetic studies have demonstrated specific myogenic functions for the phosphatase in their development. The ability to investigate calcineurin signaling pathways in vertebrates and model genetic organisms provides a great potential to more fully comprehend the functions of calcineurin and its interacting genes in heart, blood vessel, and muscle development.

Mitochondrial biogenesis as a cellular signaling framework

The identification, more than 50 years ago, of mitochondria as the site of oxidative energy metabolism has prompted studies that have unraveled the complexity of the numerous biosynthetic and degradative reactions, fundamental to cell function, carried out by these organelles. These activities depend on a distinctive mitochondrial structure, with different enzymes and reactions localized in discrete membranes and aqueous compartments. The characteristic mitochondrial structural organization is the product of both synthesis of macromolecules within the mitochondria and the import of proteins and lipids synthesized outside the organelle. Synthesis and import of mitochondrial components are required for mitochondrial proliferation, but rather than producing new organelles, these processes may facilitate the growth of pre-existing mitochondria. Recent evidence indicates that these events are regulated in a complex way by several agonists and environmental conditions, through activation of specific transcription factors and signaling pathways. Some of these are now being elucidated. Generation of nitric oxide (NO) appears to be a novel player in this scenario, possibly acting as a unifying molecular switch to trigger the whole mitochondriogenic process.

Can endogenous gaseous messengers control mitochondrial biogenesis in mammalian cells?

Mitochondria have been identified as the site of oxidative energy metabolism and of numerous biosynthetic and degradative reactions, which depend on a distinctive mitochondrial structure, with different enzymes and reactions localised in discrete membranes and aqueous compartments. Synthesis and import of mitochondrial components are required for mitochondrial proliferation, but rather than producing new organelles, these processes may facilitate the growth of preexisting mitochondria. Recent evidence indicates that these events are regulated in a complex way by several agonists and environmental conditions, through activation of specific transcription factors and signaling pathways. Some of these are now being elucidated. Generation of nitric oxide (NO) appears to be a novel player in this scenario, possibly acting as a unifying molecular switch to trigger the whole process of the mitochondrial biogenesis.

NFATc3 and NFATc4 are required for cardiac development and mitochondrial function

Activation of the nuclear factor of activated T-cell (NFAT) family of transcription factors is associated with changes in gene expression and myocyte function in adult cardiac and skeletal muscle. However, the role of NFATs in normal embryonic heart development is not well characterized. In this report, the function of NFATc3 and NFATc4 in embryonic heart development was examined in mice with targeted disruption of both nfatc3 and nfatc4 genes. The nfatc3-/-nfatc4-/- mice demonstrate embryonic lethality after embryonic day 10.5 and have thin ventricles, pericardial effusion, and a reduction in ventricular myocyte proliferation. Cardiac mitochondria are swollen with abnormal cristae, indicative of metabolic failure, but hallmarks of apoptosis are not evident. Furthermore, enzymatic activity of complex II and IV of the respiratory chain and mitochondrial oxidative activity are reduced in nfatc3-/-nfatc4-/- cardiomyocytes. Cardiac-specific expression of constitutively active NFATc4 in nfatc3-/-nfatc4-/- embryos prolongs embryonic viability to embryonic day 12 and preserves ventricular myocyte proliferation, compact zone density, and trabecular formation. The rescued embryos also maintain cardiac mitochondrial ultrastructure and complex II enzyme activity. Together, these data support the hypothesis that loss of NFAT activity in the heart results in a deficiency in mitochondrial energy metabolism required for cardiac morphogenesis and function.

The absence of mitochondrial thioredoxin 2 causes massive apoptosis, exencephaly, and early embryonic lethality in homozygous mice

Thioredoxin 2 (Trx-2) is a small redox protein containing the thioredoxin active site Trp-Cys-Gly-Pro-Cys that is localized to the mitochondria by a mitochondrial leader sequence and encoded by a nuclear gene (Trx-2). Trx-2 plays an important role in cell viability and the regulation of apoptosis in vitro. To investigate the role of Trx-2 in mouse development, we studied the phenotype of mice that have the Trx-2 gene silenced by mutational insertion. Homozygous mutant embryos do not survive to birth and die after implantation at Theiler stage 15/16. The homozygous mutant embryos display an open anterior neural tube and show massively increased apoptosis at 10.5 days postcoitus and are not present by 12.5 days postcoitus. The timing of the embryonic lethality coincides with the maturation of the mitochondria, since they begin oxidative phosphorylation during this stage of embryogenesis. In addition, embryonic fibroblasts cultured from homozygous Trx-2-null embryos were not viable. Heterozygous mice are fertile and have no discernible phenotype visible by external observation, despite having decreased Trx-2 mRNA and protein. These results show that the mitochondrial redox protein Trx-2 is required for normal development of the mouse embryo and for actively respiring cells.

Involvement of mitochondria and other free radical sources in normal and abnormal fetal development

Shepard and Mackler have documented quantitative increases in mitochondrial cristae between gestational days 10 and 14 in rats accompanied by decreased glucose utilization and increased NADH oxidase activity. Findings show a shift from glycolytic to oxidative metabolism starting at around the time of implantation. Exposure to many substances that cause transient uteroplacental hypoperfusion, including cocaine, phenytoin, calcium channel blockers, and nitric oxide synthase (NOS) inhibitors, induce limb and central nervous system (CNS) malformations while sparing the heart. We have reported that isolated electron transport particles prepared from sensitive tissues show reduced NADH oxidase activities compared with insensitive heart. They also have significantly greater superoxide formation in association with significantly reduced superoxide dismutase activities. NOS inhibitors induce severe limb reductions in late gestation. Exposure is associated with hemorrhage and nitrotyrosine (NT) formation shortly after treatment. Hemorrhage, malformations, and NT formation can be significantly reduced by coadministration of PBN, allopurinol, or aminoguanidine. On the basis of these findings, we have proposed a role for the formation of reactive oxygen species (ROS) and reactive nitrogen species (RNS) in the genesis of limb reduction defects. It is important to note that limb reduction defects occur in humans ( approximately 0.22/1000) and have been associated with the agents listed above.