Download PAPER Glucosensing neurons do more than just sense glucose

International Journal of Obesity (2001) 25, Suppl 5, S68–S72
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PAPER
Glucosensing neurons do more than just sense
glucose
BE Levin1*
1
Neurology Service, VA Medical Center, E Orange, and Department of Neurosciences, New Jersey Medical School, Newark,
New Jersey, USA
The brain regulates energy homeostasis by balancing energy intake, expenditure and storage. To accomplish this, it has evolved
specialized neurons that receive and integrate afferent neural and metabolic signals conveying information about the energy
status of the body. These sensor – integrator – effector neurons are located in brain areas involved in homeostatic functions such
as the hypothalamus, locus coeruleus, basal ganglia, limbic system and nucleus tractus solitarius. The ability to sense and
regulate glucose metabolism is critical because of glucose’s primacy as a metabolic substrate for neural function. Most neurons
use glucose as an energy substrate, but glucosensing neurons also use glucose as a signaling molecule to regulate neuronal firing
and transmitter release. There are two types of glucosensing neurons that either increase (glucose responsive, GR) or decrease
(glucose sensitive, GS) their firing rate as brain glucose levels rise. Little is known about the mechanism by which GS neurons
sense glucose. However, GR neurons appear to function much like the pancreatic b-cell where glycolysis regulates the activity of
an ATP-sensitive Kþ (KATP) channel. The KATP channel is composed of four pore-forming units (Kir6.2) and four sulfonylurea
binding sites (SUR). Glucokinase (GK) appears to modulate KATP channel activity via its gatekeeper role in the glycolytic
production of ATP. Thus, GK may serve as a marker for GR neurons. Neuropeptide Y (NPY) and pro-opiomelanocortin (POMC)
neurons in the hypothalamic arcuate nucleus are critical components of the energy homeostasis pathways in the brain. Both
express Kir6.2 and GK, as well as leptin receptors. They also receive visceral neural and intrinsic neuropeptide and transmitter
inputs. Such metabolism-related signals can summate upon KATP channel activity which then alters membrane potential,
neuronal firing rate and peptide=transmitter release. The outputs of these neurons are integral components of effector systems
which regulate energy homeostasis. Thus, arcuate NPY and POMC neurons are probably prototypes of this important class of
sensor – integrator – effector neurons.
International Journal of Obesity (2001) 25, Suppl 5, S68 – S72
Keywords: glucose responsive; glucose sensitive; neuropeptide Y; POMC; glucokinase; SUR; KATP
Through evolution it has become necessary for organisms
to sense the state of their energy homeostasis in order to
maintain a proper balance during times of famine and
plenty. Although many sites within the body can respond
to differences in energy balance, the brain stands out as the
primary site for sensing, integrating and effecting signals
essential for maintenance of homeostasis. Since prolonged
energy deficit is the most life-threatening, the body has
evolved numerous overlapping and redundant systems to
be sure that the organism is driven to seek food when bodily
*Correspondence: BE Levin, Neurology Service (127C), VA Medical
Center, 385 Tremont Ave, E Orange, NJ 07018-1095, USA.
E-mail: [email protected]
energy stores become depleted. There are systems that signal
both short- and long-term changes in homeostatic balance.
Glucose is a prototypical short-term signal since its stores in
the body are limited and the brain and other organs have a
critical dependence upon it as a primary energy source. Thus,
significant deviations from a fairly narrow range of plasma
glucose levels lead to significant counter-regulatory
responses designed to restore glucose availability. While it
is clear that the brain can both sense and respond to severe
hyper- or hypoglycemia,1 – 5 it is less certain that minor
ultradian variations in glucose can serve as a signal to
modulate physiologic functions such as food intake.6,7 In
fact, glucosensing neurons in the brain do seem capable of
responding to relatively small changes in ambient glucose
concentrations by altering their membrane potential and
firing rate. This defines a ‘glucosensing neuron’, ie a
Glucosensing neurons
BE Levin
S69
neuron that uses glucose as a signaling molecule to alter cell
function and neuronal activity. This distinguishes glucosensing neurons from the majority of neurons which utilize
glucose simply as a metabolic substrate to fuel increases in
neuronal activity and metabolic demands.
As it turns out, glucosensing neurons respond to more
than just short-term alterations in glucose availability. In
brain areas such as the hypothalamus, glucosensing neurons
also contain receptors for insulin, leptin, monoamines and
other transmitters and peptides involved in energy homeostasis.8 – 12 Thus, many or all glucosensing neurons respond
to both short- and long-term signals relating to both the
physical and affective components of energy homeostasis.
These same neurons are generally connected to efferent
pathways involved in regulating both energy intake and
expenditure and so fulfill the criteria for a true metabolic
sensor – integrator – effector. While such sensors may also be
located in the periphery, the best characterized of these lie in
brain areas such as the hypothalamus, caudal medulla, substantia nigra and locus coeruleus.812 – 16 Of these, the neuropeptide Y and proopiomelanocortin neurons of the
hypothalamic arcuate nucleus appear to be prototypic examples of metabolic sensor – integrator – effector neurons.5,10,17
In general, there are two classes of glucosensing neurons.
Neuronal activity in glucose sensitive (GS) neurons is the
reciprocal of ambient glucose levels. In glucose responsive
(GR) neurons, activity parallels changes in ambient glucose.8,12 – 16 Thus, as glucose levels rise, GR neurons fire
more rapidly and GS neurons decrease their firing rate.
Although first described more than 25 y ago,14 almost nothing is known about the mechanism used by GS neurons to
sense glucose. On the other hand, GR neurons appear to
function much like the pancreatic b-cell which uses an ATPsensitive Kþ (KATP) channel to sense glucose.10,16,18 When the
intracellular ATP=ADP ratio is increased by metabolism of
glucose, ATP is bound to the KATP channel causing it to
become inactive (closed). This raises intracellular Kþ levels
leading to membrane depolarization, opening of a voltagedependent Ca2þ channel and cell firing (Figures 1 and 2).
KATP channels also reside on axon terminals of some gammaaminobutyric acid (GABA) and glutamate neurons.12,19,20
Raising ambient glucose levels around these terminals leads
to transmitter release. In fact, many published studies
describing neurons as ‘GS’ or ‘GR’ based on changes in
their firing rates when ambient glucose levels were changed
did not account for the fact that glucose can both stimulate
GR neurons to fire and simultaneously release GABA or
glutamate onto both these same GR neurons and onto
non-GR neurons. Thus, the resultant activity of a given
neuron in the face of changing glucose levels does not
guarantee that it has intrinsic glucosensing properties nor
that the resulting firing rate is necessarily due to an effect of
glucose solely on that cell.12 Rather, the resultant firing rate
of a given neuron is the sum of inhibitory and excitatory
inputs produced by the direct and indirect effects of glucose
acting through the KATP channel.
The KATP channel is an octomeric structure containing
four inwardly rectifying pore-forming units (Kir6.2) and 4
sulfonylurea receptors (SUR).21 Channel function requires
the presence of both Kir6.2 and SUR subunits. However,
there are at least three SUR isoforms. SUR1 is a high-affinity
receptor found in b-cells and some neurons.5,21,22 SUR2A and
2B are low affinity and are found in neurons, cardiac, skeletal
and smooth muscle.21 The grouping of Kir6.2 with various
combinations of SUR1 or SUR2 isoforms may play a major
role in determining the characteristics of the KATP channel
on a given cell or axon terminal.23 This may explain the
highly pleomorphic responses of individual neurons when
ambient concentrations of glucose and=or sulfonylureas are
changed.15,23
While the KATP channel has a clear role in modulating
membrane potential in GR neurons, it is less certain how
ATP=ADP ratios at the channel are modulated within the
narrow range needed to act as a regulator of channel function. In the b-cell, both the GLUT2 glucose transporter and
glucokinase (GK) have been proposed as candidates for such
a regulatory role.24 In the brain, glucose levels are generally
about 20% of those in the plasma, ie in the low mM range.15
Under these conditions, GLUT3, which is found on most
neurons, is generally saturated because of its low Km.5 Similarly, hexokinase I is present in most neurons and its Km is
outside the usual physiologic range of brain glucose levels
making it an unlikely regulator of the rate of neuronal
glycolytic production of ATP.5 However, we17 and others16
have shown that GK is also present in the brain. Moreover, it
is expressed in relatively few sites in the brain and many of
those contain populations of known glucosensing neurons.17 Among the neurons which express GK are neuropeptide Y (NPY) and pro-opiomelanocortin (POMC) neurons in
the arcuate nucleus17 and GABA neurons in the ventromedial nucleus, while orexin neurons in the lateral hypothalamus (where GS neurons predominate) do not contain GK
(unpublished results). Similarly, SUR2 binding25 and mRNA
expression (unpublished data) are highly localized to these
same hypothalamic nuclei which are known to contain GR
neurons with KATP channels. Both Kir6.2 and SUR1 are
widely distributed in neurons throughout the brain (22,
10) while GK and SUR2 appear to have a much more limited
distribution.17 Thus, one or both of these may be critical
components of true GR neurons. Other neurons containing
the KATP channel may utilize it as a neuroprotective device
by which severe limitations of energy availability as occur
during hypoxia or hypoglycemia would lower intracellular
ATP, hyperpolarize the neuron by activating (opening) the
KATP channel and prevent neurotoxic cell death found under
these conditions.4,5 On the other hand, the requirement for
GR neurons to be highly sensitive to small changes in
ambient glucose may have necessitated them to abrogate
the neuroprotective function of the KATP channel. This may
explain why presumed GR neurons in the arcuate nucleus
appear to undergo apoptotic cell death during transient
bouts of non-coma-producing hypoglycemia.4
International Journal of Obesity
Glucosensing neurons
BE Levin
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Figure 1 Model of a KATP channel on a brain glucose responsive neuron which utilizes the KATP channel to sense glucose and other metabolic and neural
signals. When glucose is at low levels, relatively low levels of intracellular ATP are available to act at the KATP channel. Tyrosine kinase receptors for insulin
or JAK=STAT mediated receptors for leptin or cytokines activate intracellular phosphoinositide 3 kinase (PI3-K) leading to phosphorylation of
phosphatidylinositol-4-phosphate (PIP) to phosphatidylinositol-4, 5-bisphosphate (PIP 2) and phosphatidylinositol-3,4,5-triphosphate (PIP 3). PIP 2
and PIP 3 stabilize the KATP channel in the activated (open) state, possibly by causing disaggregation of actin filaments associated with the channel and by
lowering the sensitivity of the channel to ATP. Kþ leaves the neuron causing membrane hyperpolarization and reduced cell firing. (Adapted from Harvey
et al27,28 and Baukrowitz and Fakler.29)
The exact role of such neurons in modulating physiologic
functions remains in question. While small changes in
plasma glucose have been shown to occur preceding
meals,6 it is uncertain whether the brain or periphery has
sensing mechanisms sensitive enough to respond to such
small changes. Clearly, severe hypoglycemia can be sensed
by the brain2 – 4 and this leads to a profound counterregulatory sympathoadrenal response which mobilizes remaining
available glucose within the body. Similarly, relatively large
increases in plasma glucose can selectively activate the
sympathetic nervous system.1,26 While extremes of glucose
availability evoke physiologic responses, it has been extremely difficult to establish a definitive connection between
true physiologic changes in cell firing and homeostatic
functions such as changes in food intake, energy expenditure
and storage.
Despite these difficulties, mounting evidence suggests
that glucosensing neurons in some brain areas such as the
hypothalamus are equipped to monitor more than just
ambient glucose levels. Neurons containing the KATP channel are responsive to leptin, insulin, neurotransmitters and
peptides.8,9,11 Several types of evidence in cultured cell lines
point to a role for phosphatidyl inositol 3,4 phosphate (PIP
International Journal of Obesity
2) and 3,4,5 phosphate (PIP 3) in modulating KATP channel
function21,27 – 29 as a result of activation of G proteincoupled, JAK-STAT or tyrosine kinase-coupled receptors29
(Figures 1 and 2). Normally, ATP derived from GK-mediated
glycolysis of glucose binds to and inactivates the KATP channel (Figure 1). Similarly, binding of sulfonylureas to SUR
inactivates the channel. Both these events lead to membrane
depolarization and increase neuronal firing. Generally, conditions that increase intracellular phosphorylation stabilize
the channel in the activated state (open) and counteract the
effects of either ATP or sulfonylurea binding.30 Similarly,
conditions that decrease phosphorylation tend to inactivate
the channel. Thus, agonists of G-protein-coupled receptors
linked to activation of phospholipase C and=or PI 3 phosphatase reduce phosphorylation of PIP 2 and PIP 3, making
the inactive state of the KATP channel more stable. This
provides a theoretical mechanism for monoamines and
neuropeptides which act on this pathway to promote inactivation of the channel and increased cell firing. On the other
hand, activation of the JAK-STAT- or tyrosine kinase-coupled
pathways by leptin, insulin or cytokines can activate phosphoinositide 3 kinase (PIP 3) kinase. This leads to phosphorylation of PIP to PIP 2 and PIP 3. Both of these can inhibit
Glucosensing neurons
BE Levin
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Figure 2 Inactivation of the KATP channel on a glucose responsive neuron by increased extracellular glucose and=or application of sulfonylureas. The
ATP=ADP ratio is raised by glycolysis mediated by glucokinase (GK). ATP is bound to the KATP channel and sulfonylureas to the sulfonylurea receptor (SUR),
both of which inactivate (close) the channel. This leads to increased intracellular Kþ leading to membrane depolarization with opening of a voltage-gated
Ca2þ channel, influx of extracellular Ca2þ and neuronal firing. Neurotransmitters and peptides which act at a G protein-coupled receptor (GPCR) can lead
to activation of phospholipase C (PLC) and=or phosphoinositide 3 phosphatase (PI3-P). Both of these enzymes lead to lowering of PIP 2 and PIP 3, which
tends to stabilize actin filaments associated with the KATP channel enhancing the inactivation of the channel caused by ATP and sulfonylurea binding to the
channel. Transmitters or peptides which inhibit PLC or PI3-P through their actions on the GPCR would tend to oppose the actions of glucose or
sulfonylureas. (Adapted from Harvey et al27,28 and Baukrowitz and Fakler.29)
ATP binding which would favor the active configuration of
the KATP channel.29 This process appears to involve a structural disaggregation of actin filaments associated with the
channel.28 Although such data were derived from in vitro cell
systems, it is quite possible that leptin and insulin could
utilize a similar mechanism in neurons to hyperpolarize the
cell membrane in GR neurons. Thus, the KATP channel on GR
neurons might provide a central focus for the actions of
multiple metabolic and neural signals from the periphery
and surrounding the brain, allowing such neurons to monitor and regulate energy homeostasis. While this does not
exclude other mechanisms for control of energy homeostasis, it does provide a model system to explore the concept of
a central metabolic sensor – integrator – effector neuron.
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