Mitochondria and diabetes. Genetic, biochemical, and clinical implications of the cellular energy circuit

Physiologically, a postprandial glucose rise induces metabolic signal sequences that use several steps in common in both the pancreas and peripheral tissues but result in different events due to specialized tissue functions. Glucose transport performed by tissue-specific glucose transporters is, in general, not rate limiting. The next step is phosphorylation of glucose by cell-specific hexokinases. In the beta-cell, glucokinase (or hexokinase IV) is activated upon binding to a pore protein in the outer mitochondrial membrane at contact sites between outer and inner membranes. The same mechanism applies for hexokinase II in skeletal muscle and adipose tissue. The activation of hexokinases depends on a contact site-specific structure of the pore, which is voltage-dependent and influenced by the electric potential of the inner mitochondrial membrane. Mitochondria lacking a membrane potential because of defects in the respiratory chain would thus not be able to increase the glucose-phosphorylating enzyme activity over basal state. Binding and activation of hexokinases to mitochondrial contact sites lead to an acceleration of the formation of both ADP and glucose-6-phosphate (G-6-P). ADP directly enters the mitochondrion and stimulates mitochondrial oxidative phosphorylation. G-6-P is an important intermediate of energy metabolism at the switch position between glycolysis, glycogen synthesis, and the pentose-phosphate shunt. Initiated by blood glucose elevation, mitochondrial oxidative phosphorylation is accelerated in a concerted action coupling glycolysis to mitochondrial metabolism at three different points: first, through NADH transfer to the respiratory chain complex I via the malate/aspartate shuttle; second, by providing FADH2 to complex II through the glycerol-phosphate/dihydroxy-acetone-phosphate cycle; and third, by the action of hexo(gluco)kinases providing ADP for complex V, the ATP synthetase. As cytosolic and mitochondrial isozymes of creatine kinase (CK) are observed in insulinoma cells, the phosphocreatine (CrP) shuttle, working in brain and muscle, may also be involved in signaling glucose-induced insulin secretion in beta-cells. An interplay between the plasma membrane-bound CK and the mitochondrial CK could provide a mechanism to increase ATP locally at the KATP channels, coordinated to the activity of mitochondrial CrP production. Closure of the KATP channels by ATP would lead to an increase of cytosolic and, even more, mitochondrial calcium and finally to insulin secretion. Thus in beta-cells, glucose, via bound glucokinase, stimulates mitochondrial CrP synthesis. The same signaling sequence is used in the opposite direction in muscle during exercise when high ATP turnover increases the creatine level that stimulates mitochondrial ATP synthesis and glucose phosphorylation via hexokinase. Furthermore, this cytosolic/mitochondrial cross-talk is also involved in activation of muscle glycogen synthesis by glucose. The activity of mitochondrially bound hexokinase provides G-6-P and stimulates UTP production through mitochondrial nucleoside diphosphate kinase. Pathophysiologically, there are at least two genetically different forms of diabetes linked to energy metabolism: the first example is one form of maturity-onset diabetes of the young (MODY2), an autosomal dominant disorder caused by point mutations of the glucokinase gene; the second example is several forms of mitochondrial diabetes caused by point and length mutations of the mitochondrial DNA (mtDNA) that encodes several subunits of the respiratory chain complexes. Because the mtDNA is vulnerable and accumulates point and length mutations during aging, it is likely to contribute to the manifestation of some forms of NIDDM.(ABSTRACT TRUNCATED)     

Learning Situation Models in a Smart Home

This paper addresses the problem of learning situation models for providing context-aware services. Context for modeling human behavior in a smart environment is represented by a situation model describing environment, users, and their activities. A framework for acquiring and evolving different layers of a situation model in a smart environment is proposed. Different learning methods are presented as part of this framework: role detection per entity, unsupervised extraction of situations from multimodal data, supervised learning of situation representations, and evolution of a predefined situation model with feedback. The situation model serves as frame and support for the different methods, permitting to stay in an intuitive declarative framework. The proposed methods have been integrated into a whole system for smart home environment. The implementation is detailed, and two evaluations are conducted in the smart home environment. The obtained results validate the proposed approach.     

The Routineness of Routines: Measuring Rhythms of Media Interaction

The routines of information work are commonplace yet difficult to characterize. Although cognitive models have successfully characterized routine tasks within which there is little variation, a large body of ethnomethodological research has identified the inherent nonroutineness of routines in information work. We argue that work does not fall into discrete classes of routine versus nonroutine; rather, task performance lies on a continuum of routineness, and routineness metrics are important to the understanding of workplace multitasking.

In a study of 10 information workers shadowed for 3 whole working days each, we utilize the construct of working sphere to model projects/tasks as a network of humans and artifacts. Employing a statistical technique called T-pattern analysis, we derive measures of routineness from these real-world data. In terms of routineness, we show that information workers experience archetypes of working spheres. The results indicate that T-patterns of interactions with information and computational media are important indicators of facets of routineness and that these measures are correlated with workers' affective states. Our results are some of the first to demonstrate how regular temporal patterns of media interaction in tasks are related to stress. These results suggest that designs of systems to facilitate so-called routine work should consider the degrees to which a person's working spheres fall along varying facets of routineness.     

Detecting Human Behavior Models From Multimodal Observation in a Smart Home

This paper addresses learning and recognition of human behavior models from multimodal observation in a smart home environment. The proposed approach is part of a framework for acquiring a high-level contextual model for human behavior in an augmented environment. A 3-D video tracking system creates and tracks entities (persons) in the scene. Further, a speech activity detector analyzes audio streams coming from head set microphones and determines for each entity, whether the entity speaks or not. An ambient sound detector detects noises in the environment. An individual role detector derives basic activity like ldquowalkingrdquo or ldquointeracting with tablerdquo from the extracted entity properties of the 3-D tracker. From the derived multimodal observations, different situations like ldquoaperitifrdquo or ldquopresentationrdquo are learned and detected using statistical models (HMMs). The objective of the proposed general framework is two-fold: the automatic offline analysis of human behavior recordings and the online detection of learned human behavior models. To evaluate the proposed approach, several multimodal recordings showing different situations have been conducted. The obtained results, in particular for offline analysis, are very good, showing that multimodality as well as multiperson observation generation are beneficial for situation recognition.     

Context-aware environments: from specification to implementation

Abstract: This paper deals with the problem of implementing a context model for a smart environment. The problem has already been addressed several times using many different data- or problem-driven methods. In order to separate the modelling phase from implementation, we first represent the context model by a network of situations. Then, different implementations can be automatically generated from this context model depending on user needs and underlying perceptual components. Two different implementations are proposed in this paper: a deterministic one based on Petri nets and a probabilistic one based on hidden Markov models. Both implementations are illustrated and applied to real-world problems.     

Mitochondrial intermembrane inclusion bodies: the common denominator between human mitochondrial myopathies and creatine depletion, due to impairment of cellular energetics

Mitochondrial inclusion bodies are often described in skeletal muscle of patients suffering diseases termed mitochondrial myopathies. A major component of these structures was discovered as being mitochondrial creatine kinase. Similar creatine kinase enriched inclusion bodies in the mitochondria of creatine depleted adult rat cardiomyocytes have been demonstrated. Structurally similar inclusion bodies are observed in mitochondria of ischemic and creatine depleted rat skeletal muscle. This paper describes the various methods for inducing mitochondrial inclusion bodies in rodent skeletal muscle, and compares their effects on muscle metabolism to the metabolic defects of mitochondrial myopathy muscle. We fed rats with a creatine analogue guanidino propionic acid and checked their solei for mitochondrial inclusion bodies, with the electron microscope. The activity of creatine kinase was analysed by measuring creatine stimulated oxidative phosphorylation in soleus skinned fibres using an oxygen electrode. The guanidino propionic acid-rat soleus mitochondria displayed no creatine stimulation, whereas control soleus did, even though the GPA solei had a five fold increase in creatine kinase protein per mitochondrial protein. The significance of these results in light of their relevance to human mitochondrial myopathies and the importance of altered cell energetics and metabolism in the formation of these crystalline structures are discussed.     

The permeability transition pore complex: a target for apoptosis regulation by caspases and bcl-2-related proteins

Early in programmed cell death (apoptosis), mitochondrial membrane permeability increases. This is at least in part due to opening of the permeability transition (PT) pore, a multiprotein complex built up at the contact site between the inner and the outer mitochondrial membranes. The PT pore has been previously implicated in clinically relevant massive cell death induced by toxins, anoxia, reactive oxygen species, and calcium overload. Here we show that PT pore complexes reconstituted in liposomes exhibit a functional behavior comparable with that of the natural PT pore present in intact mitochondria. The PT pore complex is regulated by thiol-reactive agents, calcium, cyclophilin D ligands (cyclosporin A and a nonimmunosuppressive cyclosporin A derivative), ligands of the adenine nucleotide translocator, apoptosis-related endoproteases (caspases), and Bcl-2-like proteins. Although calcium, prooxidants, and several recombinant caspases (caspases 1, 2, 3, 4, and 6) enhance the permeability of PT pore-containing liposomes, recombinant Bcl-2 or Bcl-XL augment the resistance of the reconstituted PT pore complex to pore opening. Mutated Bcl-2 proteins that have lost their cytoprotective potential also lose their PT modulatory capacity. In conclusion, the PT pore complex may constitute a crossroad of apoptosis regulation by caspases and members of the Bcl-2 family.     

The molecular structure of mitochondrial contact sites. Their role in regulation of energy metabolism and permeability transition

Contact sites between the outer and peripheral inner membrane of mitochondria are involved in protein precursor uptake and energy transfer. Hexokinase and mitochondrial creatine kinase could be attributed by different techniques to the energy transfer contacts. Kinetic analyses suggested a functional interaction between the kinases, outer membrane pore protein, and inner membrane adenylate translocator (ANT). This suggestion was strongly supported by isolation of hexokinase and creatine kinase complexes that were constituted of kinase oligomers, porin and ANT. Phospholipid vesicles carrying reconstituted kinase-porin-ANT complexes enclosed internal ATP in contrast to vesicles containing free porin only. This indicated that unspecific transport through porin was regulated by its interaction with a specific antiporter, ANT. A direct interaction between porin and ANT in the hexokinase complex conferred the reconstituted system with permeability properties reminiscent of the mitochondrial permeability transition (PT) pore. In the creatine kinase complex this interaction between porin and ANT was replaced by contact of both with the kinase octamer. Thus PT-pore-like functions were not observed unless the creatine kinase octamer was dissociated, suggesting that the ANT was locked in the antiporter state by interaction with the octamer. Indeed, reconstituted pure ANT showed PT-pore-like properties concerning Ca2+ sensitivity. However, as cyclophilin was missing, sensitivity against cyclosporin was not observed.     

Is monoamine oxidase activity in the outer mitochondrial membrane influenced by the mitochondrial respiratory state?

Monoamine oxidase activity was measured in isolated rat liver mitochondria using the radiochemical assay with [14C]tyramine as substrate. With toluene as the extracting solvent the apparent activity in the resting state (State 4) was much higher than in the active state (State 3) in agreement with Smith and Reid (Smith, G.S. and Reid, R.A. (1978) Biochem. J. 176, 1011-1014). However, with ethyl acetate or diethyl ether as extracting solvents, the activity in both states was almost identical and several times higher than that measured with toluene. p-Hydroxyphenylacetaldehyde, p-hydroxyphenylacetalcohol and p-hydroxyphenylacetic acid were identified as final reaction products, the latter one being hardly extractable with toluene. It is concluded that monoamine oxidase activity is not influenced by the respiratory state of mitochondria and that differences found by Smith and Reid are due to different extractability of secondary reaction products. NADPH-dependent aldehyde reductase was tentatively identified in rat liver mitochondria, its specific activity amounting to about one fourth of that in the cytosol.