Brain
and Muscle Energy Group &
Synaptic
Neurochemistry Laboratory –
Energy Supply and
Synthesis, Recycling, Packaging and Action of Neuro- and Gliotransmitters
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Professor of Physiology,
University of Oslo, Norway Professor Neurobiology of Aging, University of Copenhagen, Denmark
Professor of Physiology Department
of Oral Biology (DOB) Group Leader Centre
for Molecular Biology and Neuroscience (CMBN) / SERTA Healthy
Brain Aging University
of Copenhagen, Denmark Nansen
Neuroscience Network Post: PO Box 1105
Blindern, 0317 Oslo, Norway Visit: Domus Medica, Room
1293, Sognsvannsveien 9, Oslo, Norway Telephone: +47 22851496 / Mobile telephone: +47 97032049 Email: l.h.bergersen@medisin.uio.no |
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Current SN-Lab
Members: Linda Hildegard Bergersen (Head)
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Glio-
and Neurotransmitters
Vidar Gundersen (Head, Linjeleder)
Maja Amedjkouh Puchades (Postdoc)
Cecilie Morland (Postdoc)
Lasse Ormel (PhD student)
Kaja Nordengen (Medical research student)
Mats Julius Stensrud
(Medical research student)
Carl Johan Sogn
(Medical research student)
Grazyna Babinska
(Technician)
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Funding: Work in the lab is currently funded by an
RCN researcher grant to LHB, an Oslo University Hospital researcher grant to
VG, an RCN postdoctoral grant to JHR, a UiO PhD student grant to LHB, a
Nasjonalforeningen for folkehelsen (Norwegian National Association for Public
Health) (first PhD grant in dementia research) to CR/LHB, two medical
research student grants to VG, a joint RCN/UiO researcher grant to LHB/JSM, a RCN/Oslo University Hospital
UNIKARD grant to Attramadal/LHB/JSM, a Lundbeck Foundation PhD student grant
to SHH/LHB. |
Photo:
Elmer Laahne, for Nasjonalt
medisinsk museum/Norsk teknisk museum
Overview
The function of both brain and
muscle depends crucially on their energy supply. The brain uses most of its
energy on electrical signalling, along and between neurons. Brain neurons
contact each other at synapses. A synapse consists of a presynaptic terminal
and a postsynaptic dendrite, separated by a 20 nm wide synaptic cleft, into
which the neurotransmitter (eg, glutamate and GABA) is released from
presynaptic vesicle stores to activate receptor proteins at outward facing
binding sites. Glial cells, the astrocytes, tightly surround the synapse.
Information transmission at synapses is based on ion fluxes through the
postsynaptic cell membrane. The ions that enter or leave the cell must later be
pumped out or in again, consuming energy. Whereas glutamate and GABA exert
their primary effects at the synapse, part of the released transmitter
molecules escape the synaptic gap and diffuse to deliver their signal via
extracellular receptors in a larger volume of brain tissue surrounding the
active synapse. This phenomenon is known as ‘volume transmission’ as opposed to
the classical ‘wiring transmission’ restricted to the synapse. We have recently
proposed that even the metabolic intermediate lactate can exert a ‘volume
transmitter’ function, transmitting signals about the energy state of active
brain cells, in addition to being consumed as a fuel.
Electron microscopic immunogold quantification
Our main research tool is
quantitative immunogold electron microscopy (Bergersen et al. 2008 Nature
Protoc) to decipher the microstructural localization of molecules, such as
transporters and receptors that mediate ion movements and synaptic and
extrasynaptic signalling, terminate signals, and provide energy to the cells.
![nprot.2007.525-F2[2]](index_files/image017.jpg)
Our current research
focuses on the following projects
(Only select references to own
work are cited, see below for extensive publication list linked from PubMed)
Lactate transporters and roles of
lactate
Energy. Lactate is an important metabolic
intermediate in all cells, including neurons in the brain. Although glucose is the
major energy substrate for the central nervous system, lactate is produced
within the brain itself and this lactate may be used by brain cells to produce
energy. Unresolved questions are to what extent lactate is a required energy
substrate for brain cells and which role lactate plays in disease states
(Review: Bergersen 2007 Neuroscience).
Cellular differences. According to a current idea lactate
is produced primarily in astrocytes,
from which it is exported for uptake and oxidation in neurons. This lactate flux is thought to be coupled to
glutamatergic transmission, with release of lactate from astrocytes being
stimulated by uptake of glutamate. We provided early evidence for the
localization of MCTs in the brain in line with this notion (Bergersen 1999
Neuroscience; 2001 Exp Brain Res; 2002 Neurochem Res; 2003 Neuroscience). We
found that MCT2 is located at fast acting glutamatergic synapses and that MCT4
is in the surrounding astroglia, compatible with a flux of lactate from
astrocytes to metabolically active neurons. On the other hand, MCT4
localization in the perivascular glial endfeet and MCT1 in endothelial cells
provide means for exchange of lactate between blood and brain and rationalize
the finding that lactate exits from the brain only at rest and is taken up from
the bloodstream when blood lactate rises, such as during exercise.
Like lactate produced in fast,
glycolytic muscle cell is oxidized in enduring, oxidative muscle cells, lactate
is produced and oxidized in different components of brain tissue, and indeed of
tumours, and excess lactate produced in muscle can be oxidized in heart and
brain.
Activity dependence. We have taken advantage of the fact
that brain and the experimentally more easily accessible skeletal muscle share
features of lactate production and consumption: we compare the localization of
MCTs in brain and muscle to study how lactate and other
monocarboxylates can influence energy supply under physiological and
pathological conditions. Thus we have shown, by cross-reinnervation experiments
of slow oxidative and fast glycolytic muscle, that the type and quantity of MCT
expressed, is governed by the nerve, like is the contraction speed (Bergersen
et al. 2006 Neuroscience). The idea that the activity pattern is a general
regulator of MCT expression gains support from our observation that MCT2 is
concentrated at fast acting synapses in the brain (Bergersen et al. 2005 Cereb
Cortex).
Lactate,
a ’volume transmitter’. While the preferential
location of MCT2 at the postsynaptic membrane of excitatory synapses (Bergersen
et al. 2001 Exp Brain Res; 2005 Cereb Cortex) is consistent with the
requirement for high flux rates at this site of particular energy expenditure,
the location at the synapse together with glutamate receptors may also be
indicative of signalling functions. Indeed, lactate is known to carry vasomotor
signals and beneficial effects of lactate in ischemic and excitotoxic brain
insults may be more readily explained by signalling than by metabolism. The
existence of G-protein coupled receptors for lactate (GPR81, aka HCA1 receptor) and other
hydroxyl-carboxylic acids is evidence of such signalling function. In situ
hybridization data (Allen Brain Atlas http://www.brain-map.org)
indicate GPR81 mRNA to be expressed in the principal neurons in mouse brain.
Immunocytochemical investigations will determine the precise sites of action
(Lauritzen KH et al. in preparation). Receptor-like actions can also be exerted
by lactate modifying the NADH/NAD+ ratio, and its equilibration over cell
membranes combined with diffusion through the extracellular space and inside
the gap-junction connected astrocytic network makes lactate able to spread in a
‘volume transmitter’ fashion from a site of generation to surrounding receptor
sites (Bergersen & Gjedde 2012 Front Neuroenergetics).
Fig 2. Electron microscopic immunogold localization and
quantification of MCT1 in hippocampal blood vessel endothelium in human
surgical specimens from patients with mesial temporal lobe epilepsy (MTLE)
and non-MTLE. Less gold particle labeling for MCT1 is found on CA1
microvessel plasma membranes in MTLE, compared to non-MTLE. The reduced
particle density in MTLE is evident both at abluminal (arrows in A, B) and
luminal (arrows in C, D) plasma membranes. Abbreviations: bl, basal lamina;
ec, endothelial cell; lu, blood vessel lumen; nu, nucleus; p, pericyte. The
bar diagrams depict mean±SE, *Pb0.05. Scale bar: 500 nm. From Lauritzen F,
de Lanerolle NC, Lee TS, Spencer DD, Kim JH, Bergersen LH, Eid T (2011)
Monocarboxylate transporter 1 is deficient on microvessels in the human
epileptogenic hippocampus. Neurobiol Dis 41(2):577-84
Disease states. In experimental heart failure in
rat we observed a dramatic upregulation of MCT1 in the cardiomyocytes,
concomitant with increased lactate uptake for oxidation and presumably
alleviation of energy deficit (Jóhannsson E et al. 2001 Circulation). Energy
deficit has been invoked as contributing to the pathogenetic mechanisms of a
variety of brain diseases, including epilepsy. We have recently shown that in
human mesial temporal lobe epilepsy as well as in several rat models of the
disease, MCT1 is downregulated in the vascular endothelium and upregulated in
the astrocytic network in the epileptogenic hippocampus (Lauritzen F et al.
2011 Neurobiol Dis; 2012 Neurobiol Dis; 2012 Glia).
Myelin. In addition to regulatory roles and
being an energy fuel, lactate may serve as a precursor of materials needed to
build components of the brain. Thus lactate can be converted to lipid building
blocks of myelin, the insulating sheaths that protect axons and boost their
conduction velocity, essential for efficient communication between widely
separated brain regions. We recently showed that lactate, taken up into
oligodendrocytes through MCT1, is required to sustain oligodendrocytes and
myelin formation. In CNS diseases involving energy deprivation at times of
myelination or remyelination, such as periventricular leukomalacia leading to
cerebral palsy, stroke, and secondary ischemia after spinal cord injury,
lactate transporters in oligodendrocytes may play an important role in
minimizing the inhibition of myelination that occurs (Rinholm et al. 2011 J
Neurosci). Oligodendrocytes may even supply axons with lactate for fuel
(Rinholm & Bergersen 2012 Nature (News&Views)).
Neurotransmitter receptor actions on
neurons and glia
Glutamate-mediated damage to oligodendrocytes
contributes to mental or physical impairment in periventricular leukomalacia
(pre- or perinatal white matter injury leading to cerebral palsy), spinal cord
injury, multiple sclerosis and stroke. Unlike neurons, white matter
oligodendrocytes reportedly lack NMDA (N-methyl-D-aspartate)
type glutamate receptors. In an interdisciplinary study with David Attwell,
University College London, we showed that precursor, immature and mature
oligodendrocytes in the white matter of the cerebellum and corpus callosum
exhibit NMDA-evoked currents, mediated by receptors that are blocked only
weakly by Mg2+ and that may contain NR1, NR2C and
NR3 NMDA receptor subunits. NMDA receptors are present in the myelinating
processes of oligodendrocytes, where the small intracellular space could lead
to a large rise in intracellular ion concentration in response to NMDA receptor
activation. Simulating ischaemia led to development of an inward current in
oligodendrocytes, which was partly mediated by NMDA receptors. These results
point to NMDA receptors of unusual subunit composition as a potential
therapeutic target for preventing white matter damage in a variety of diseases
(Káradóttir et al. 2005 Nature).
GABA actions in myelin. GABAA receptor subunits are expressed throughout
compact myelin in vivo, at a density comparable to that at inhibitory synapses.
GABA release might contribute to oligodendrocyte depolarization during energy
deprivation (Attwell et al. In preparation).
Attention-deficit/hyperactivity disorder (ADHD) is the
most common neurobehavioural disorder among children. ADHD children are
hyperactive, impulsive and have problems with sustained attention. These
cardinal features are also present in the best validated animal model of ADHD,
the spontaneously hypertensive rat (SHR), which is derived from the Wistar
Kyoto rat (WKY) (Sagvolden et al. 2009 Neuropharmacology). Current theories of
ADHD relate symptom development to factors that alter learning. N-methyl-D-aspartate receptor (NMDAR)
dependent long term changes in synaptic efficacy in the mammalian CNS are
thought to represent underlying cellular mechanisms for some forms of learning.
We therefore hypothesized that synaptic abnormality in excitatory,
glutamatergic synaptic transmission might contribute to the altered behavior in
SHRs. In interdisciplinary studies with the groups of Terje Sagvolden (deceased
2011) and Øyvind Hvalby, Dept of Physiology, we examined physiological and
anatomical aspects of hippocampal CA3-to-CA1 synapses in age-matched SHR and
WKY (controls). The results indicate that functionally altered NMDA-type
glutamate receptors in with a subunit composition characteristic of early
developmental stages causes functional impairments in glutamatergic synaptic
transmission in SHR. This may be one of the underlying mechanisms leading to
the abnormal behavior in SHR, and possibly in human ADHD (Jensen V, Rinholm et
al. 2009 Neuroscience).
Transmitter release from astrocytes
There is growing evidence that
astrocytes, like neurons, can release transmitters.
Glutamate has been shown to be released from
astrocytes in a vast number of studies. Although astrocytic glutamate may be
released by several mechanisms, the evidence in favour of exocytosis is most
compelling. Astrocytes may respond to neuronal activity by such exocytotic
release of glutamate. The astrocyte derived glutamate can in turn activate
neuronal glutamate receptors, in particular N-methyl-D-aspartate
(NMDA) receptors. Morphological and functional data show that astrocytes
possess the machinery for exocytosis of glutamate and contribute to the control
of synaptic strength (Jourdain, Bergersen et al. 2007 Nature Neurosci). We
describe the presence of small synaptic-like microvesicles, SNARE proteins and
vesicular glutamate transporters (VGLUTs) in astrocytes, as well as NMDA
receptors (Fig 3) situated in the vicinity of the astrocytic vesicles
(Bergersen & Gundersen 2009 Neuroscience). The content of VGLUT harbouring
synaptic-like microvesicles appears to be a property of astrocytes in multiple
brain regions (Ormel et al. 2012 Glia).
Fig 3. Glutamate from astrocytes
stimulates nerve endings in hippocampus. Electron micrograph showing NMDA
type glutamate receptor subunits NR2B (immunogold particles) at the synapse
as well as in extrasynaptic membranes (arrows) of nerve terminals (ter)
that synapse on dendritic spines (sp). NR2Bs face astrocytic processes
(ast) containing glutamate laden synaptic-like microvesicles (arrowheads in
enlarged inset). Scale bars, 100 nm. From: Jourdain P, Bergersen LH,
Bhaukaurally K, Bezzi P, Santello M, Domercq M, Matute C, Tonello F,
Gundersen V, Volterra A (2007) Glutamate exocytosis from astrocytes
controls synaptic strength. Nature Neuroscience 10(3):331-9
D-serine is present in brain in substantial quantities and
this unusual amino acid is a ligand of the glycine binding site of the NMDA
receptor, which must be occupied for the NMDA receptor to be activated by
glutamate. We have shown that D-serine, like glutamate, is concentrated in
clusters of small synaptic-like microvesicles forming microdomains with
endoplasmic reticulum cisterns in the perisynaptic processes of astrocytes (Fig
4). The results indicate that D-serine may be co-released with glutamate to
cause the astrocyte dependent activation of NMDA receptors at synapses
(Bergersen et al. 2012 Cereb Cortex).
Fig 4. An astrocyte process (Ast) has
synaptic-like microvesicles (red arrowheads) containing D-serine (small
particles); the Ast is identified as having glutamate transporter EAAT2 in
its plasma membrane (large particles) and is adjacent to a nerve ending
(Term) synapsing on a spine (Sp).
From: Bergersen LH, Morland C, Ormel L, Rinholm JE, Larsson M, Wold
JF, Røe AT, Stranna A, Santello M, Bouvier D, Ottersen OP, Volterra A,
Gundersen V (2012) Immunogold detection of L-glutamate and D-serine in small
synaptic-like microvesicles in adult hippocampal astrocytes. Cereb Cortex 22(7):1690-7
Neurotransmitter transporters
The vertebrate neuromuscular
junction (NMJ) is
known to be a cholinergic synapse at which acetylcholine (ACh) is released from
the presynaptic terminal to act on postsynaptic nicotinic ACh receptors. There
is now growing evidence that glutamate, which is the main excitatory
transmitter in the CNS and at invertebrate NMJs, may have a signalling function
together with ACh also at the vertebrate NMJ. In the CNS, the extracellular
concentration of glutamate is kept at a subtoxic level by Na+-driven high-affinity glutamate
transporters such as GLT and GLAST located in plasma membranes of astrocytes
and neurons. The glutamate transporters are also pivotal for shaping glutamate
receptor responses at synapses. Analyses in rat showed that GLAST and GLT are
enriched in the junctional folds of the postsynaptic membrane at the NMJ. GLT
was relatively higher in the slow-twitch muscle soleus than in the fast-twitch
muscle extensor digitorum longus, whereas GLAST was relatively higher in
extensor digitorum longus than in soleus. The findings show - together with previous
demonstration of vesicular glutamate, a vesicular glutamate transporter and
glutamate receptors - that mammalian NMJs contain the machinery required for
synaptic release and action of glutamate. This indicates a signalling role for
glutamate at the normal NMJ and provides a basis for the ability of denervated
muscle to be reinnervated by glutamatergic axons from the CNS (Rinholm JE et
al. 2007 Neuroscience).
Mitochondria
Mitochondria trafficking. Energy use, mainly to reverse ion
movements in neurons, is a fundamental constraint on brain information
processing. Because nerve cells are so large, they need special mechanisms to
locate mitochondria at locations where the energy is needed. Synapses are sites
of particularly large energy consumption. Mitochondria can be moved around
cells by kinesin motors on intracellular microtubule tracks (for a movie visit http://www.ucl.ac.uk/npp/jk.html and look at the bottom of the
page). The Group member, Johanne Egge Rinholm (then PhD student of LHB),
collaborated with the groups of David Attwell and Josef Kittler at the
University College London discovering that the linkage of mitochondria to these
motors is mediated by a protein called Miro. Importantly, this linkage is
uncoupled by calcium ions, which enter through NMDA receptors when synapses are
activated. The resulting uncoupling of mitochondria from their motors leads to
them being "parked" near the active synapses where they are needed to
generate energy. This work advances our understanding of how neurons are
powered. It also explains why in pathological conditions like stroke, which
dramatically raise the internal calcium concentration in nerve cells, all
movements of mitochondria within the cell halt (Macaskill AF, Rinholm JE et al.
2009 Neuron).
Mitochondria in myelin? JE Rinholm, now a Postdoc in the
Group (RCN fellowship 2012-14 as one of four awarded among 64 applicants),
pursues this line of research along with the complementary one on lactate and
myelination (see above: Rinholm et al. 2011 J Neurosci) to solve the puzzle of
energy supply in myelin and myelinated axons. For this we collaborate with the
labs of David A Clayton and Eric Betzig (Howard Hughes Medical Institute,
Janelia Farm, Ashburn, Virginia), where Rinholm will use newly developed
super-resolution imaging technique called structured illumination microscopy
(SIM) to test her hypothesis that mitochondria are located in cytoplasmic loops
in myelin, as suggested by her preliminary observations with confocal imaging
(Rinholm JE et al. in preparation).
Mitochondrial dysfunction. Mitochondrial dysfunction is
considered to underly changes in neurodegenerative diseases and is often
associated with apoptosis and a progressive loss of neurons. Such pathologies
are proposed to be caused by damage to the mitochondrial genome. In a current
collaborative study with Arne Klungland, Rikshospitalet, we have designed a
mouse model that allows us to specifically induce mitochondrial DNA toxicity in
forebrain structures. We demonstrate that a continual generation of
apyrimidinic sites causes apoptosis, neuronal death and progressive atrophy in
the hippocampus leading to decreased synaptic efficiency associated with a
reduction in the size of the postsynaptic membrane (ie overlying the
postsynaptic density, PSD) on dendritic spines in hippocampal CA1 pyramidal
cells. The mice subsequently show abnormal behaviour, including increased
activity, reduced cognitive abilities and lack of anxiety responses (Lauritzen
KH et al. 2010 Mol Cell Biol). Further studies suggested that the accumulating
pathology was caused by reduced mtDNA copy number and transcription, increased
oxidative stress, and disturbed mitochondrial trafficking, thus mimicking
changes observed in neurodegenerative diseases (Lauritzen KH et al. 2011 DNA
Repair (Amst)).
Impact of gene
repair on brain structure and function
Accumulation of oxidative DNA
damage, not only in mtDNA but also in nuclear DNA, has been proposed as a
potential cause of age-related cognitive decline. The major pathway for removal
of oxidative base lesions is base excision repair, which is initiated by DNA
glycosylases. In mice, Neil3 is the main DNA glycosylase for repair of
hydantoin lesions in single stranded DNA of neural stem/progenitor cells
(NSPC), that sustain neurogenesis. Adult neurogenesis is considered crucial for
maintenance of hippocampus dependent functions involved in normal behavior. In
Niel3-deficient mice, we demonstrated
learning and memory deficits and reduced anxiety-like behavior. NSPCs
from aged Neil3-/- mice showed impaired proliferative capacity. Further,
hippocampal neurons in aged Neil3-/- mice displayed synaptic irregularities,
comprising reduced synaptic membrane size, and selective increase (NR1, NR2A/B,
GABAAα1) or reduction (GluR2/3) of transmitter receptor subunits. The results
indicate that repair by Neil3 of oxidative DNA damage in NSPCs is required for
the maintenance of adult neurogenesis to counteract age associated synaptic
changes and deterioration of cognitive performance (Regnell et al. 2012 Cell
Rep).
Collaborations
We currently have
active collaborations including the labs of:
·
David
Attwell (UK) http://www.ucl.ac.uk/npp/research/da
·
Magnar
Bjørås (NO) http://www.cmbn.no/group-bjoras.html & http://www.ous-research.no/bjoras/
·
David
A. Clayton (USA) http://www.hhmi.org/research/groupleaders/clayton_bio.html
· Tore
Eid (USA) http://labmed.yale.edu/people/tore_eid.profile
· Albert Gjedde (DK & CA) http://forskning.ku.dk/search/profil/?id=52543
· Øyvind
Hvalby (NO) http://www.med.uio.no/imb/english/people/aca/oivind/index.html
· Tim
Karl (AU) http://www.neura.edu.au/research/themes/karl-group
· Arne
Klungland (NO) http://www.cmbn.no/group-klungland.html & http://www.ous-research.no/klungland/
· Erik
Pettersen (NO) http://www.mn.uio.no/fysikk/english/people/aca/erikop/index.html
· Lene
Juul Rasmussen (DK) http://icmm.ku.dk/english/icmm-staff/lene_rasmussen/
·
Andrea
Volterra (CH) http://www.unil.ch/fbm/page28867_en.html
Publications listed
in PubMed
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Citations
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19
Thomson
Reuters ISI Web of Knowledge, All Databases (10th December 2012)
Author=(Bergersen
L*) minus 3 papers by extraneous
‘Bergersen L’
Fourteen of my publications have been cited more than
30 times, two more than 200 times.
Updated 21th
February 2013 by lindabe