Download A plastic axonal hotspot

NEWS & VIEWS
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nEUrosciEncE
A plastic axonal hotspot
Jan gründemann and Michael Häusser
Neurons generate their output signal — the action potential — in a distinct
region of the axon called the initial segment. The location and extent of this
trigger zone can be modified by neural activity to control excitability.
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where, is crucial for determining the selectivity
and impact of the excitability change.
Over the past decade, activity-dependent
changes in the density and function of many
voltage-gated channels have been reported7–10.
By being restricted to subneuronal compartments, such as individual segments of
dendrites11,12 — short, branching neuronal
processes that receive synaptic input — such
changes can provide some degree of specificity.
But typically they are widespread, being found
mainly in the neuron’s cell body and dendrites.
Grubb and Burrone1 (page 1070) and Kuba
et al.2 (page 1075) implicate a specific axonal
structure in intrinsic plasticity — the axon
initial segment (AIS), a highly organized, ionchannel-enriched matrix of proteins that is
situated close to the cell body. Significantly,
the fact that the action potential is initiated in
the AIS13 would allow this form of plasticity
Neuron
Cell body
AIS
Axon
c Regulation of excitability
ChR2+ a Increased
activity
Light
burst
b Hearing
loss
Time
↑ Spacing
line
b
Neuronal output
The brain changes with experience. But where
are these changes, which ultimately result in
altered patterns of activity, stored in neural
circuits? Although the plasticity of the synaptic connections between neurons has received
much attention, the intrinsic excitability of a
neuron — its responsiveness to synaptic input
— can also be markedly altered by experience.
In this issue, two groups (Grubb and Burrone1 and Kuba et al.2) identify a new target
of intrinsic plasticity in the axon, the output
structure of the neuron. They show that the
proximal region of the axon known as the
axon initial segment, the initiation site of the
action potential (the neuron’s output signal),
can be modified to make the cell more or less
responsive to inputs.
Activity-dependent changes in neural circuits are crucial for brain development and
memory formation. Deprivation of a sensory
modality such as visual input causes long-term
changes in sensory circuits3. The conventional
view is that such activity-dependent plasticity
involves changes in the strength of synapses.
This idea has tremendous appeal, given that
synaptic changes can be highly specific and
show a wide dynamic range, and that, given
their vast numbers, synapses offer enormous
capacity for information storage.
Changing the overall intrinsic excitability
of a neuron is an alternative, complementary
mechanism for information storage. This
widespread strategy, which is not specific
to individual synapses, is of immense value
in neural circuits. It allows multi-purpose
circuits to switch between activity modes4,
enables adjustments to be made in the firing
rate of neurons during sensorimotor learning5,
and provides a powerful mechanism for the
homeostatic regulation of activity6. Intrinsic
plasticity can come about through the altered
composition, density and distribution of ion
channels in the neuronal membrane that are
activated by changes in voltage (voltage-gated
ion channels). Which channels are altered, and
to regulate the final site of integration of the
synaptic input directly.
To investigate how the location of the AIS
might depend on activity, Grubb and Burrone
use cultures of neurons from the hippocampus. They find that when extracellular potassium levels are chronically elevated, mimicking
increased neuronal activity, the AIS shifts away
from the cell body. This movement involves
wholesale translocation of several types of
AIS-specific protein, thereby creating a nonexcitable ‘spacer’ region — 21 micrometres
long — between the AIS and the cell body.
This shift further isolates the action-potential
trigger site from the synaptic input to the dendrites and thereby reduces the ability of the
input to trigger action potentials. Consequently,
neurons with a more distal AIS are less excitable
and require stronger stimulation to fire.
To achieve more precise control over neuronal activity, Grubb and Burrone manipulated
their cultures so that the neurons expressed a
membrane protein called channelrhodopsin-2
(ChR2), which is a light-activated ion channel14
(Fig. 1a). They could then use light stimuli to
directly trigger spiking with precise temporal
control. Long-term, regular, low-frequency
light stimulation at 1 hertz had little effect on
AIS location. But when the neurons were activated by high-frequency bursts of light, the AIS
shifted significantly.
What mechanisms underlie these activitydependent changes? Many forms of neuronal plasticity are triggered by increases
in intracellular calcium-ion concentration.
Indeed, Grubb and Burrone1 show that blocking T- and L-type calcium channels prevents
the AIS from moving. This suggests that
activity-dependent calcium signals provide
a read-out of the pattern of neuronal activity, which triggers structural plasticity at the
AIS. But these experiments were carried out
Base
1. Shen, B., Makley, D. M. & Johnston, J. N. Nature 465,
1027–1032 (2010).
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(1985).
NATURE|Vol 465|24 June 2010
a
↑ Length
Input strength
↓ Excitability
↑ Excitability
Figure 1 | intrinsic plasticity, courtesy of the axon initial segment (Ais). a, Grubb and Burrone1
show that, in cultured hippocampal neurons expressing the light-activated channel ChR2, bursts of
activity triggered by light lead to a calcium-dependent movement of the AIS away from the cell body.
Consequently, neuronal excitability is reduced. b, Kuba et al.2 find that, in neurons from the nucleus
magnocellularis of chicks, deafness — and thus loss of sensory input — caused by removal of the
cochlea increases the length of the AIS, leading to a corresponding compensatory increase in neuronal
excitability. c, Activity-dependent structural reorganization of the AIS can therefore shift the strength
of the synaptic input required to produce a particular neuronal output — a homeostatic mechanism.
© 2010 Macmillan Publishers Limited. All rights reserved
NEWS & VIEWS
NATURE|Vol 465|24 June 2010
on neurons in cell culture, raising the question
of whether this form of homeostatic axonal
plasticity occurs in the intact brain.
Kuba et al.2 neatly answer this question. They
had previously found that, in brainstem neurons
responsible for encoding sound, the precise
location and length of the AIS depend on the
characteristic sound frequency that each neuron processes15. Now they have tested the effects
of hearing loss on the AIS location (Fig. 1b).
Removing the cochlea, the auditory part of
the inner ear, from one-day-old chicks causes
loss of synaptic input to neurons in the nucleus
magnocellularis — an essential relay station in
the auditory pathway. Kuba and co-workers
find that such input deprivation has a dramatic
effect on the AIS of these neurons: following
hearing loss, the AIS is elongated by roughly
70%. These nucleus magnocellularis neurons
lack dendrites, confirming that the excitability
changes are indeed restricted to the axon.
Intriguingly, although Kuba et al. did not
detect changes in the density and subtype
composition of sodium channels in the AIS,
hearing deprivation increased total sodium
currents in the axons, and this could be attributed to the expansion of the AIS. Consequently,
smaller current injections were sufficient to
trigger action potentials after hearing deprivation, indicating that neuronal excitability
had increased to compensate for the reduced
synaptic drive.
Together, these papers1,2 show that the AIS
is a powerful target for homeostatic plasticity
mechanisms. Notably, the changes in the AIS
are bidirectional and reversible — properties
crucial for the proposed homeostatic role of
such mechanisms.
The two studies also open the door to
exploring how AIS plasticity occurs in different
types of neuron under various conditions that
affect neural circuits. For instance, both teams
used relatively crude methods to alter neuronal activity: long-term increases in neuronal
activity or sensory deprivation. More subtle
and controlled manipulations, over shorter
timescales, could reveal whether AIS plasticity
is restricted to developmental and pathological conditions, or whether it is a normal physiological mechanism that could dynamically
regulate excitability.
The studies identify distinct mechanisms for
modulating neuronal excitability — either displacement or extension of the AIS (Fig. 1 a, b).
It will therefore be necessary to determine
which prevails in different neuronal network
states and brain areas. Neither group directly
addressed how the changes in the AIS alter the
integration of synaptic input by the neurons.
This is particularly relevant for inputs mediated by the neurotransmitter γ-aminobutyric
acid (GABAergic inputs), which cluster at the
AIS16. Do these GABAergic synapses move
with the AIS, and, if not, how does this affect
their ability to influence the generation of
action potentials? Finally, if the molecular
mechanisms underlying this form of plasticity
are identified, they could provide targets for
manipulating neuronal excitability in various
disease states that involve altered excitability,
particularly epilepsy.
■
Jan Gründemann and Michael Häusser are at the
Wolfson Institute for Biomedical Research, and
the Department of Physiology, Pharmacology
and Neuroscience, University College London,
Gower Street, London WC1E 6BT, UK.
e-mail: [email protected]
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DnA rEPAir
how to accurately bypass damage
Suse Broyde and Dinshaw J. Patel
Ultraviolet radiation can cause cancer through DNA damage — specifically,
by linking adjacent thymine bases. Crystal structures show how the enzyme
DNA polymerase η accurately bypasses such lesions, offering protection.
In this issue, two papers — by Silverstein et al.1
(page 1039) and Biertümpfel et al.2 (page 1044)
— describe the crystal structure of the enzyme
DNA polymerase η (Polη), which can efficiently and accurately overcome DNA damage caused by ultraviolet radiation. These data
are of particular interest because they elucidate
how the inactivation of Polη leads to XPV, a
variant form of xeroderma pigmentosum — a
type of severe skin cancer in humans.
DNA polymerase enzymes mediate DNA
replication and repair. In eukaryotes (organisms such as animals, plants and yeast), at
least 14 such polymerases, with diverse functions, have been identified3. For instance,
DNA polymerases of the Y family specialize in DNA-lesion bypass. According to the
polymerase-switch model4, when a high-fidelity replicative polymerase encounters a DNAdistorting lesion, it stalls and is replaced by one
or more lesion-bypass polymerases. The bypass
polymerases transit the lesion and extend the
DNA until the perturbation has been passed.
The replicative polymerase then resumes its
task of rapidly synthesizing the growing DNA
chain.
Humans have four Y-family DNA polymerases: Polι, Polκ, REV1 and Polη. The crystal
structures of ternary complexes — containing
one of these polymerases, together with template and primer DNA, and the deoxyribonucleoside 5ʹ-triphosphate (dNTP) positioned
ready for addition to the growing primer chain
— have been solved for Polι (ref. 5), Polκ (ref. 6)
and REV1 (ref. 7). Together with the structures
of the non-human Y-family polymerases Dpo4
© 2010 Macmillan Publishers Limited. All rights reserved
(ref. 8) and Dbh (ref. 9), these structures have
revealed that some features are unique to a particular Y-family member, whereas others are
universal to all members.
For example, like other polymerases, all
Y-family polymerases have a hand-like shape,
with palm, fingers and thumb domains10, as
well as an active site with evolutionarily conserved carboxylate-containing amino-acid
residues (aspartic acid and glutamic acid) and
two divalent metal ions, usually magnesium
ions (Mg2+). In addition, being generally of low
fidelity, Y-family polymerases have a spacious
and solvent-accessible active site to facilitate
lesion bypass11. This feature is in contrast to
that of high-fidelity polymerases, the fingers
of which close tightly on the nascent base pair
to promote accurate replication10,12. What’s
more, Y-family polymerases have a domain
termed the little finger, or PAD, which aids
in gripping the DNA11,13. None of the human
Y-family polymerases was found to be highly
specialized for the error-free bypass of specific
types of DNA damage, although the structures
and biochemical evidence provided clues to
the nature of the lesions that the enzymes are
designed to process5–7,11,13,14.
The most elusive and intriguing of the
human bypass polymerases is Polη. On a
functional level, it is essential for error-free
bypass of a highly prevalent lesion that results
from exposure to ultraviolet radiation, including sunlight11,13. The lesion is called a cis–syn
thymine dimer (Fig. 1a, overleaf): two adjacent
thymine bases on the same strand form two
covalent bonds to produce an open-book-like
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