Should nagarse be used during the isolation of brain mitochondria?

Unravelling the role of epigenetics in reproductive adaptations to early-life environment

Reproductive function adjusts in response to environmental conditions in order to optimize success. In humans, this plasticity includes age of pubertal onset, hormone levels and age at menopause. These reproductive characteristics vary across populations with distinct lifestyles and following specific childhood events, and point to a role for the early-life environment in shaping adult reproductive trajectories. Epigenetic mechanisms respond to external signals, exert long-term effects on gene expression and have been shown in animal and cellular studies to regulate normal reproductive function, strongly implicating their role in these adaptations. Moreover, human cohort data have revealed differential DNA methylation signatures in proxy tissues that are associated with reproductive phenotypic variation, although the cause-effect relationships are difficult to discern, calling for additional complementary approaches to establish functionality. In this Review, we summarize how adult reproductive function can be shaped by childhood events. We discuss why the influence of the childhood environment on adult reproductive function is an important consideration in understanding how reproduction is regulated and necessitates consideration by clinicians treating women with diverse life histories. The resolution of the molecular mechanisms responsible for human reproductive plasticity could also lead to new approaches for intervention by targeting these epigenetic modifications.

Mitochondria: isolation, structure and function

Mitochondria are complex organelles constantly undergoing processes of fusion and fission, processes that not only modulate their morphology, but also their function. Yet the assessment of mitochondrial function in skeletal muscle often involves mechanical isolation of the mitochondria, a process which disrupts their normally heterogeneous branching structure and yields relatively homogeneous spherical organelles. Alternatively, methods have been used where the sarcolemma is permeabilized and mitochondrial morphology is preserved, but both methods face the downside that they remove potential influences of the intracellular milieu on mitochondrial function. Importantly, recent evidence shows that the fragmented mitochondrial morphology resulting from routine mitochondrial isolation procedures used with skeletal muscle alters key indices of function in a manner qualitatively similar to mitochondria undergoing fission in vivo. Although these results warrant caution when interpreting data obtained with mitochondria isolated from skeletal muscle, they also suggest that isolated mitochondrial preparations might present a useful way of interrogating the stress resistance of mitochondria. More importantly, these new findings underscore the empirical value of studying mitochondrial function in minimally disruptive experimental preparations. In this review, we briefly discuss several considerations and hypotheses emerging from this work.

Mitochondrial structure and function are disrupted by standard isolation methods

Mitochondria regulate critical components of cellular function via ATP production, reactive oxygen species production, Ca²⁺ handling and apoptotic signaling. Two classical methods exist to study mitochondrial function of skeletal muscles: isolated mitochondria and permeabilized myofibers. Whereas mitochondrial isolation removes a portion of the mitochondria from their cellular environment, myofiber permeabilization preserves mitochondrial morphology and functional interactions with other intracellular components. Despite this, isolated mitochondria remain the most commonly used method to infer in vivo mitochondrial function. In this study, we directly compared measures of several key aspects of mitochondrial function in both isolated mitochondria and permeabilized myofibers of rat gastrocnemius muscle. Here we show that mitochondrial isolation i) induced fragmented organelle morphology; ii) dramatically sensitized the permeability transition pore sensitivity to a Ca²⁺ challenge; iii) differentially altered mitochondrial respiration depending upon the respiratory conditions; and iv) dramatically increased H₂O₂ production. These alterations are qualitatively similar to the changes in mitochondrial structure and function observed in vivo after cellular stress-induced mitochondrial fragmentation, but are generally of much greater magnitude. Furthermore, mitochondrial isolation markedly altered electron transport chain protein stoichiometry. Collectively, our results demonstrate that isolated mitochondria possess functional characteristics that differ fundamentally from those of intact mitochondria in permeabilized myofibers. Our work and that of others underscores the importance of studying mitochondrial function in tissue preparations where mitochondrial structure is preserved and all mitochondria are represented.

Lower respiratory capacity in extraocular muscle mitochondria: evidence for intrinsic differences in mitochondrial composition and function

PURPOSE

The constant activity of the extraocular muscles is supported by abundant mitochondria. These organelles may enhance energy production by increasing the content of respiratory complexes. The authors tested the hypothesis that extraocular muscle mitochondria respire faster than do mitochondria from limb muscles because of the higher content of respiratory complexes.

METHODS

Inner mitochondrial membrane density was determined by stereological analysis of triceps surae (a limb muscle) and extraocular muscles of adult male Sprague-Dawley rats. The authors measured respiration rates of isolated mitochondria using a Clark-type electrode. The activity of respiratory complexes I, II, and IV was determined by spectrophotometry. The content of respiratory complexes was estimated by Western blot.

RESULTS

States 3, 4, and 5 respiration rates in extraocular muscle mitochondria were 40% to 60% lower than in limb muscle mitochondria. Extraocular muscle inner mitochondrial membrane density was similar to that of other skeletal muscles. Activity of complexes I and IV was lower in extraocular muscle mitochondria (approximately 50% the activity in triceps), but their content was approximately 15% to 30% higher. There was no difference in complex II content or activity or complex III content. Finally, complex V was less abundant in extraocular muscle mitochondria.

CONCLUSIONS

The results demonstrate that extraocular muscle mitochondria respire at slower rates than mitochondria from limb muscles, despite similar mitochondrial ultrastructure. Instead, differences were found in the activity (I, IV) and content (I, IV, V) of electron transport chain complexes. The discrepancy between activity and content of some complexes is suggestive of alternative subunit isoform expression in the extraocular muscles compared with limb muscles.

Evidence for the mitochondrial lactate oxidation complex in rat neurons: demonstration of an essential component of brain lactate shuttles

To evaluate the presence of components of a putative Intracellular Lactate Shuttle (ILS) in neurons, we attempted to determine if monocarboxylate (e.g. lactate) transporter isoforms (MCT1 and -2) and lactate dehydrogenase (LDH) are coexpressed in neuronal mitochondria of rat brains. Immunohistochemical analyses of rat brain cross-sections showed MCT1, MCT2, and LDH to colocalize with the mitochondrial inner membrane marker cytochrome oxidase (COX) in cortical, hippocampal, and thalamic neurons. Immunoblotting after immunoprecipitation (IP) of mitochondria from brain homogenates supported the histochemical observations by demonstrating that COX coprecipitated MCT1, MCT2, and LDH. Additionally, using primary cultures from rat cortex and hippocampus as well as immunohistochemistry and immunocoprecipitation techniques, we demonstrated that MCT2 and LDH are coexpressed in mitochondria of cultured neurons. These findings can be interpreted to mean that, as in skeletal muscle, neurons contain a mitochondrial lactate oxidation complex (mLOC) that has the potential to facilitate both intracellular and cell-cell lactate shuttles in brain.

Is the first site of phosphorylation operative in rat brain mitochondria in early neonatal life? A critical re-evaluation

The age-dependent changes in oxidative phosphorylation in rat brain mitochondria were studied in order to ascertain if the efficiency of phosphorylation with NAD(+)-linked substrates increases during the first month after birth. The state 3 respiration rates with all substrates tested increased with age but with distinctive developmental profiles for each substrate; the extent of increase was also substrate-specific. The ADP/O ratios obtained with all substrates, including NAD(+)-linked ones were comparable with the value for adult mitochondria right from the first postnatal week. The intramitochondrial cytochrome contents also increased with age; the different classes of cytochromes exhibiting different developmental patterns. Simultaneously there was an elevation in the serum concentrations of thyroid hormones up to the third postnatal week following which they declined. The increase in respiration rates was paralleled by decrease in the ratio of T4/T3 in the serum, thus suggesting that this ratio is a sensitive index for developmental profiles of substrate oxidation.

Mitochondrial metabolism following traumatic brain injury in rats

Although a number of studies of traumatic brain injury have implicated mitochondrial dysfunction as a cause of altered posttraumatic energy metabolism, no studies to date have isolated mitochondria and measured their respiratory capacity following trauma. The present study sought to determine whether mitochondrial capacity for oxidative phosphorylation is adversely affected by fluid-percussion-induced traumatic brain injury in rats. Prior to brain injury, the mitochondrial respiratory control ratio was 4.3 +/- 0.2 and the ratio of nmoles of ADP phosphorylated per natom oxygen consumed (ADP/O ratio) was 2.66 +/- 0.09. After injury (2.8 atm; t = 4 h), there were slight but not significant alterations in ADP/O ratio (2.41 +/- 0.07) and state 3 respiratory rate (ADP stimulated); however, there were no changes in the respiratory control ratio. These data suggest that traumatic brain injury, unlike ischemia, does not cause uncoupling of ATP synthesis from respiration, and that brain mitochondria are quite resistant to trauma-induced injury.

Fractionation and analysis of mitochondria with polycarbonate membrane filters

Polycarbonate membrane filters were used to fractionate mitochondrial populations depending on their aggregation or association with other subcellular structures. Isolated rat liver mitochondria penetrated through filters which have pore sizes larger than 1 micron. In contrast, mitochondria which were induced to aggregate in vitro by incubation at low pH were retained by the filters and thus could be separated from the single or small aggregates of mitochondria. Use of this membrane filtration method to analyze release of mitochondria from isolated hepatocytes showed that treatment with digitonin at concentrations only sufficient to lyse the plasma membrane did not release mitochondria. Homogenization or sonication following digitonin treatment released 25-50% of the mitochondria, but only a small fraction was intact. A high yield of intact mitochondria was released from digitonin-treated cells by a brief treatment with a low concentration of the proteolytic enzyme nagarse. Thus, this membrane filtration method provides a simple and rapid approach to analyze the extent of mitochondrial aggregation and association with other subcellular structures.