Characterization of the mitochondrial active-site serine protein LACTB: filaments in the mitochondrial intermembrane space

Mitochondria have evolved from endosymbiotic alpha-proteobacteria. During the endosymbiotic process early eukaryotes dumped the major component of the bacterial cell wall, the peptidoglycan layer. Peptidoglycan is synthesized and maintained by active-site serine enzymes belonging to the penicillin-binding protein and the β-lactamase superfamily. Mammals harbor a protein named LACTB that shares sequence similarity with bacterial penicillin-binding proteins and β-lactamases. Since eukaryotes lack the synthesis machinery for peptidoglycan, the physiological role of LACTB is intriguing. Recently, LACTB has been validated in vivo to be causative for obesity, suggesting that LACTB is implicated in metabolic processes. The aim of this study was to investigate the phylogeny, structure, biochemistry and cell biology of LACTB in order to elucidate its physiological function. Phylogenetic analysis revealed that LACTB has evolved from penicillin binding-proteins present in the bacterial periplasmic space. A structural model of LACTB indicates that LACTB shares characteristic features common to all penicillin-binding proteins and β-lactamases. Recombinat LACTB protein expressed in E. coli was recovered in significant quantities. Biochemical and cell biology studies showed that LACTB is a soluble protein localized in the mitochondrial intermembrane space. Further analysis showed that LACTB preprotein underwent proteolytic processing disclosing an N-terminal tetrapeptide motif also found in a set of cell death-inducing proteins. Electron microscopy structural studies revealed that LACTB can polymerize to form stable filaments with lengths ranging from twenty to several hundred nanometers. These data suggest that LACTB filaments define a distinct microdomain in the intermembrane space. A possible role of LACTB filaments is proposed in the intramitochondrial membrane organization and microcompartmentation. The implications of these findings offer novel insight into the evolution of mitochondria. Further studies of the LACTB function might provide a tool to treat mitochondria-related metabolic diseases.

Tonically Active Kainate Receptors (tKARs) : A Novel Mechanism Regulating Neuronal Function in the Brain

Fast excitatory transmission between neurons in the central nervous system is mainly mediated by L-glutamate acting on ligand gated (ionotropic) receptors. These are further categorized according to their pharmacological properties to AMPA (2-amino-3-(5-methyl-3-oxo-1,2- oxazol-4-yl)propanoic acid), NMDA (N-Methyl-D-aspartic acid) and kainate (KAR) subclasses. In the rat and the mouse hippocampus, development of glutamatergic transmission is most dynamic during the first postnatal weeks. This coincides with the declining developmental expression of the GluK1 subunit-containing KARs. However, the function of KARs during early development of the brain is poorly understood. The present study reveals novel types of tonically active KARs (hereafter referred to as tKARs) which play a central role in functional development of the hippocampal CA3-CA1 network. The study shows for the first time how concomitant pre- and postsynaptic KAR function contributes to development of CA3-CA1 circuitry by regulating transmitter release and interneuron excitability. Moreover, the tKAR-dependent regulation of transmitter release provides a novel mechanism for silencing and unsilencing early synapses and thus shaping the early synaptic connectivity. The role of GluK1-containing KARs was studied in area CA3 of the neonatal hippocampus. The data demonstrate that presynaptic KARs in excitatory synapses to both pyramidal cells and interneurons are tonically activated by ambient glutamate and that they regulate glutamate release differentially, depending on target cell type. At synapses to pyramidal cells these tKARs inhibit glutamate release in a G-protein dependent manner but in contrast, at synapses to interneurons, tKARs facilitate glutamate release. On the network level these mechanisms act together upregulating activity of GABAergic microcircuits and promoting endogenous hippocampal network oscillations. By virtue of this, tKARs are likely to have an instrumental role in the functional development of the hippocampal circuitry. The next step was to investigate the role of GluK1 -containing receptors in the regulation of interneuron excitability. The spontaneous firing of interneurons in the CA3 stratum lucidum is markedly decreased during development. The shift involves tKARs that inhibit medium-duration afterhyperpolarization (mAHP) in these neurons during the first postnatal week. This promotes burst spiking of interneurons and thereby increases GABAergic activity in the network synergistically with the tKAR-mediated facilitation of their excitatory drive. During development the amplitude of evoked medium afterhyperpolarizing current (ImAHP) is dramatically increased due to decoupling tKAR activation and ImAHP modulation. These changes take place at the same time when the endogeneous network oscillations disappear. These tKAR-driven mechanisms in the CA3 area regulate both GABAergic and glutamatergic transmission and thus gate the feedforward excitatory drive to the area CA1. Here presynaptic tKARs to CA1 pyramidal cells suppress glutamate release and enable strong facilitation in response to high-frequency input. Therefore, CA1 synapses are finely tuned to high-frequency transmission; an activity pattern that is common in neonatal CA3-CA1 circuitry both in vivo and in vitro. The tKAR-regulated release probability acts as a novel presynaptic silencing mechanism that can be unsilenced in response to Hebbian activity. The present results shed new light on the mechanisms modulating the early network activity that paves the way for oscillations lying behind cognitive tasks such as learning and memory. Kainate receptor antagonists are already being developed for therapeutic use for instance against pain and migraine. Because of these modulatory actions, tKARs also represent an attractive candidate for therapeutic treatment of developmentally related complications such as learning disabilities.

Koti keskellä kaupunkia : Keskusta lapsiperheen asuinalueena, esimerkkeinä Tukholma ja Helsinki

Families with children have traditionally moved to suburbs. In the last 20 years a modest counter process has however been recognized. Families with an urban lifestyle stay in the city centres. This study looks at the phenomenon through two cases, Stockholm and Helsinki. In the first case it has already been observed that the city centre has grown in popularity among families with children. Therefore it serves as a basis for the study and as well as a point of comparison. Stockholm’s city centre is expanding as new neighbourhoods have been built and are being planned. In the city centre of Helsinki the building of two large neighbourhoods for 30 000 inhabitants will start in a few years. The first aim of the study is to look closer at what has really happened in the city centre of Stockholm, why families choose to live there with their children and how the City of Stockholm has reacted to the change. The main sources of information are secondary sources, statistics and interviews with planners, politicians and experts in the field. The main object is to look at the situation in the city centre of Helsinki. Can a preference for urban residential environments be observed in Helsinki? What are the reasons for a family to choose the city centre as a living place? How does the everyday life of a family in the city centre appear? How are these families taken into account in the planning of the city? The main sources of information here are statistics, interviews with dwellers in the neighbourhood Kruununhaka and interviews with planners. In Stockholm the birth rate has grown constantly during the 2000s and is highest in the city centre. Some of the families still move elsewhere, but many of them do not. One of the most important reasons for living in the city centre is short working distances which give working parents more time with their children. Another reason is a preference of an urban, active lifestyle. Families prefer to live close to everything, childcare, schools, shops and entertainments. The popularity of the city centre among families with children has taken politicians and planners by surprise. Helsinki has not experienced a baby boom like Stockholm. However the negative changes in the birth rate have been more modest in the central areas than in the suburbs. Statistics show, that many families move away from the city centre as the children grow. Families who stay in the city centre especially appreciate closeness to public and private services and good public transportation which means that they are not dependent on using the car. Further they find that the city centre has a tolerant climate and is a safe and beautiful place to live in. The families enjoy the social life of the neighbourhood and feel that it makes a good climate to raise children in. However they are concerned with traffic safety and the lack of stimulus in the playgrounds of the neighbourhood parks. Two large neighbourhoods with homes for about 30 000 inhabitants are now planned in the former Port Districts in the city centre of Helsinki. The other one, Jätkäsaari has been planned to become an attractive alternative for families with children. Traffic safety has been one of the main objects for the planning. The other, Kalasatama, has been planned to attract all groups in society.