Hybrid fiber alterations in exercising seniors suggest contribution to fast-to-slow muscle fiber shift.

BACKGROUND: Human skeletal muscle is composed of a functional and metabolic continuum of slow (Type I) and fast fibers (IIa and IIx). Hybrid fibers co-expressing different myosin heavy chains are also present and seem to be more prominent in aging muscle. Their role is debated; hybrid fibers were reported either in a transitional state, between slow and fast fibers, or as fixed individual entities. This study examined the fate of hybrid fibers with an endurance exercise intervention in an elderly sedentary population. METHODS: Twenty-two sedentary healthy elderly men and women underwent a 16-week supervised endurance exercise intervention. Eighteen endurance-trained age- and gender-matched volunteers served as controls. Fiber type distribution was determined by immunohistochemistry on vastus lateralis muscle biopsies pre-intervention and post-intervention. RESULTS: A total of 13840 fibers were analyzed. At baseline, a Type II dominant fiber profile was observed compared with the control group, with more Type IIa (P = 0.0301) and Type IIx fibers (P = 0.0328). Hybrid fibers represented almost 5% of total muscle fibers in both groups. There was no significant difference between groups (I-IIa, P = 0.6719 and IIa-IIx, P = 0.0998). Intervention triggered qualitative dynamics towards an increase in Type I, and decrease in Type II fibers, paralleled by an increase in I-IIa hybrids (P = 0.0301). CONCLUSIONS: The present study is, to our knowledge, the first to examine hybrid muscle fiber type adaptations to an endurance exercise intervention in the elderly. Hybrid fiber proportions did not differ between chronic sedentary state and chronic endurance-trained state. Exercise intervention increased Type I-IIa hybrid fibers along with shift dynamics in other fiber types suggesting the contribution of hybrid fiber to a fast-to-slow fiber type transition, eventually serving as intermediate reservoir from one monomorphic myosin heavy chain expressing fiber type to another. This finding favours the transitional theory regarding hybrid muscle fibers and exercise, crucial to understanding reversible mechanisms of sarcopenia and development of prevention measures.

Distinct patterns of skeletal muscle mitochondria fusion, fission and mitophagy upon duration of exercise training.

AIM: Healthy ageing interventions encompass regular exercise to prevent mitochondrial dysfunction, key player in sarcopenia pathogenesis. Mitochondrial biogenesis has been well documented, but mitochondrial remodelling in response to exercise training is poorly understood. Here we investigated fusion, fission and mitophagy before and after an exercise intervention in older adults. METHODS: Skeletal muscle biopsies were collected from 22 healthy sedentary men and women before and after 4 months of supervised training. Eight lifelong trained age- and gender-matched volunteers served as positive controls. Transmission electron microscopy was used to estimate mitochondrial content. Western blotting and qRT-PCR were used to detect changes in specific proteins and transcripts. RESULTS: After intervention, mitochondrial content increased to levels of controls. While enhancement of fusion was prevalent after intervention, inhibition of fission and increased mitophagy were dominant in controls. Similarly to PARKIN, BCL2L13 content was higher in controls. The observed molecular adaptations paralleled long-term effects of training on physical fitness, exercise efficiency and oxidative capacity. CONCLUSIONS: This study describes distinct patterns of molecular adaptations in human skeletal muscle under chronic exercise training. After 16 weeks of exercise, the pattern was dominated by fusion to increase mitochondrial content to the metabolic demands of exercise. In lifelong exercise, the pattern was dominated by mitophagy synchronized with increased fusion and decreased fission, indicating an increased mitochondrial turnover. In addition to these temporally distinct adaptive mechanisms, this study suggests for the first time a specific role of BCL2L13 in chronic exercise that requires constant maintenance of mitochondrial quality.

Enhanced Respiratory Chain Supercomplex Formation in Response to Exercise in Human Skeletal Muscle.

Mitochondrial dysfunction is a hallmark of multiple metabolic complications. Physical activity is known to increase mitochondrial content in skeletal muscle, counteracting age-related decline in muscle function and protecting against metabolic and cardiovascular complications. Here, we investigated the effect of 4 months of exercise training on skeletal muscle mitochondria electron transport chain complexes and supercomplexes in 26 healthy, sedentary older adults. Exercise differentially modulated respiratory complexes. Complex I was the most upregulated complex and not stoichiometrically associated to the other complexes. In contrast to the other complexes, complex I was almost exclusively found assembled in supercomplexes in muscle mitochondria. Overall, supercomplex content was increased after exercise. In particular, complexes I, III, and IV were redistributed to supercomplexes in the form of I+III2+IV. Taken together, our results provide the first evidence that exercise affects the stoichiometry of supercomplex formation in humans and thus reveal a novel adaptive mechanism for increased energy demand.

Exercise efficiency relates with mitochondrial content and function in older adults.

Chronic aerobic exercise has been shown to increase exercise efficiency, thus allowing less energy expenditure for a similar amount of work. The extent to which skeletal muscle mitochondria play a role in this is not fully understood, particularly in an elderly population. The purpose of this study was to determine the relationship of exercise efficiency with mitochondrial content and function. We hypothesized that the greater the mitochondrial content and/or function, the greater would be the efficiencies. Thirty-eight sedentary (S, n = 23, 10F/13M) or athletic (A, n = 15, 6F/9M) older adults (66.8 ± 0.8 years) participated in this cross sectional study. V˙O2peak was measured with a cycle ergometer graded exercise protocol (GXT). Gross efficiency (GE, %) and net efficiency (NE, %) were estimated during a 1-h submaximal test (55% V˙O2peak). Delta efficiency (DE, %) was calculated from the GXT. Mitochondrial function was measured as ATPmax (mmol/L/s) during a PCr recovery protocol with (31)P-MR spectroscopy. Muscle biopsies were acquired for determination of mitochondrial volume density (MitoVd, %). Efficiencies were 17% (GE), 14% (NE), and 16% (DE) higher in A than S. MitoVD was 29% higher in A and ATPmax was 24% higher in A than in S. All efficiencies positively correlated with both ATPmax and MitoVd. Chronically trained older individuals had greater mitochondrial content and function, as well as greater exercise efficiencies. GE, NE, and DE were related to both mitochondrial content and function. This suggests a possible role of mitochondria in improving exercise efficiency in elderly athletic populations and allowing conservation of energy at moderate workloads.

Concise reviews: In vitro-produced pancreas organogenesis models in three dimensions: self-organization from few stem cells or progenitors.

Three-dimensional models of organ biogenesis have recently flourished. They promote a balance between stem/progenitor cell expansion and differentiation without the constraints of flat tissue culture vessels, allowing for autonomous self-organization of cells. Such models allow the formation of miniature organs in a dish and are emerging for the pancreas, starting from embryonic progenitors and adult cells. This review focuses on the currently available systems and how these allow new types of questions to be addressed. We discuss the expected advancements including their potential to study human pancreas development and function as well as to develop diabetes models and therapeutic cells.

In vitro pancreas organogenesis from dispersed mouse embryonic progenitors.

The pancreas is an essential organ that regulates glucose homeostasis and secretes digestive enzymes. Research on pancreas embryogenesis has led to the development of protocols to produce pancreatic cells from stem cells (1). The whole embryonic organ can be cultured at multiple stages of development (2-4). These culture methods have been useful to test drugs and to image developmental processes. However the expansion of the organ is very limited and morphogenesis is not faithfully recapitulated since the organ flattens. We propose three-dimensional (3D) culture conditions that enable the efficient expansion of dissociated mouse embryonic pancreatic progenitors. By manipulating the composition of the culture medium it is possible to generate either hollow spheres, mainly composed of pancreatic progenitors expanding in their initial state, or, complex organoids which progress to more mature expanding progenitors and differentiate into endocrine, acinar and ductal cells and which spontaneously self-organize to resemble the embryonic pancreas. We show here that the in vitro process recapitulates many aspects of natural pancreas development. This culture system is suitable to investigate how cells cooperate to form an organ by reducing its initial complexity to few progenitors. It is a model that reproduces the 3D architecture of the pancreas and that is therefore useful to study morphogenesis, including polarization of epithelial structures and branching. It is also appropriate to assess the response to mechanical cues of the niche such as stiffness and the effects on cell´s tensegrity.

Skeletal muscle mitochondria in the elderly: effects of physical fitness and exercise training.

CONTEXT: Sarcopenia is thought to be associated with mitochondrial (Mito) loss. It is unclear whether the decrease in Mito content is consequent to aging per se or to decreased physical activity. OBJECTIVES: The objective of the study was to examine the influence of fitness on Mito content and function and to assess whether exercise could improve Mito function in older adults. DESIGN AND SUBJECTS: Three distinct studies were conducted: 1) a cross-sectional observation comparing Mito content and fitness in a large heterogeneous cohort of older adults; 2) a case-control study comparing chronically endurance-trained older adults and sedentary (S) subjects matched for age and gender; and 3) a 4-month exercise intervention in S. SETTING: The study was conducted at a university-based clinical research center. OUTCOMES: Mito volume density (MitoVd) was assessed by electron microscopy from vastus lateralis biopsies, electron transport chain proteins by Western blotting, mRNAs for transcription factors involved in M biogenesis by quantitative RT-PCR, and in vivo oxidative capacity (ATPmax) by (31)P-magnetice resonance spectroscopy. Peak oxygen uptake was measured by graded exercise test. RESULTS: Peak oxygen uptake was strongly correlated with MitoVd in 80 60- to 80-year-old adults. Comparison of chronically endurance-trained older adults vs S revealed differences in MitoVd, ATPmax, and some electron transport chain protein complexes. Finally, exercise intervention confirmed that S subjects are able to recover MitoVd, ATPmax, and specific transcription factors. CONCLUSIONS: These data suggest the following: 1) aging per se is not the primary culprit leading to Mito dysfunction; 2) an aerobic exercise program, even at an older age, can ameliorate the loss in skeletal muscle Mito content and may prevent aging muscle comorbidities; and 3) the improvement of Mito function is all about content.

Artificial three-dimensional niches deconstruct pancreas development in vitro.

In the context of a cellular therapy for diabetes, methods for pancreatic progenitor expansion and subsequent differentiation into insulin-producing beta cells would be extremely valuable. Here we establish three-dimensional culture conditions in Matrigel that enable the efficient expansion of dissociated mouse embryonic pancreatic progenitors. By manipulating the medium composition we generate either hollow spheres, which are mainly composed of pancreatic progenitors, or complex organoids that spontaneously undergo pancreatic morphogenesis and differentiation. The in vitro maintenance and expansion of pancreatic progenitors require active Notch and FGF signaling, thus recapitulating in vivo niche signaling interactions. Our experiments reveal new aspects of pancreas development, such as a community effect by which small groups of cells better maintain progenitor properties and expand more efficiently than isolated cells, as well as the requirement for three-dimensionality. Finally, growth conditions in chemically defined biomaterials pave the way for testing the biophysical and biochemical properties of the niche that sustains pancreatic progenitors.

Management of tracheal intubation in the respiratory intensive care unit by pulmonary physicians.

BACKGROUND: Expert management of tracheal intubation has become fundamental to the routine practice of pulmonary physicians who work in respiratory intensive care units (ICUs). In Italy, tracheal intubation is not included as part of the training in respiratory medicine, and pulmonary physicians are usually dissuaded from managing intubations. METHODS: We prospectively studied the intubation success rate in 46 consecutive respiratory ICU patients who required either emergency or urgent intubation, conducted by 3 intubation-trained pulmonary physicians in our respiratory ICU. Intubation success was defined as successful tracheal intubation without any of 7 pre-defined complications. RESULTS: There were 17 emergency intubations and 29 urgent intubations. Intubation was successful in 43 of the 46 intubation attempts. Complications occurred in 3 cases: 2 patients needed to be intubated by an anesthesiologist, and 1 patient received fiberoptic intubation. CONCLUSIONS: Pulmonary physicians trained in tracheal intubation can have a high success rate in performing intubation in the respiratory ICU. Collaborative efforts between anesthesiologists and pulmonary physicians are necessary to optimize the training, skill-retention, and back-up for advanced airway management in the respiratory ICU.