1. New roles for insulin-like hormones in neuronal signalling and protection: New hopes for novel treatments of Alzheimer’s disease? 
Christian Hölscher, Lin Li 
Neurobiology of Aging 
Volume 31, Issue 9, Pages (September 2010)
DOI: /j.neurobiolaging
Copyright © 2008 Elsevier Inc. Terms and Conditions

2. Fig. 1
Effect of beta-amyloid [25-35] at 10 and 100nmol on LTP in area CA1 of the hippocampus, injected icv. 5min prior to tetanic stimulation. Following injection of 100nmol beta-amyloid [25-35], LTP was blocked and the tetanic stimulation produced only short-term potentiation (ANOVA, p<0.01). Averaged excitatory postsynaptic potentials (EPSPs) are shown recorded 5min pretetanus (1, 3, 5) and 1h posttetanus (2, 4, 6). Calibration bars: 1mV; 5ms. For details see (Freir et al., 2001).
Neurobiology of Aging  , DOI: ( /j.neurobiolaging )
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3. Fig. 2
GLP-1 or GIP injected icv. increased sub-maximum LTP. Left: A three-way ANOVA showed a difference between groups (p<0.01) and over time (p<0.001). A two-way ANOVA showed a difference between the GLP-1 group and the GLP-1(9-36) group (p<0.01) and over time (p<0.001). There was no difference between the GLP-1(9-36) group and the control group. Averaged EPSPs are shown recorded 5min pretetanus and 1h posttetanus. Calibration bars are 10ms horizontal, 1mV vertical. Right: GIP also enhanced sub-maximum LTP. An ANOVA showed a difference between the GIP group and control (p<0.01). Averaged EPSPs are shown recorded 5min pretetanus and 1h posttetanus. Calibration bars are 10ms horizontal, 1mV vertical. For details see (Gault and Holscher, 2008a,b).
Neurobiology of Aging  , DOI: ( /j.neurobiolaging )
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4. Fig. 3
(a) Injection of the protease resistant GLP-1 derivative (Val8)GLP-1 30min before beta-amyloid injection reversed the impairment of LTP. A two-way ANOVA showed a difference between the GLP-1 group and beta-amyloid (25-35) combination group, and the beta-amyloid group (p<0.001). No difference was found between the control group and the drug combination group. Averaged EPSPs are shown recorded 5min pre-HFS and 1h post-HFS. Calibration bars are 10ms horizontal, 1mV vertical. (b) Injecting N-AcGIP (15nmol icv.) 30min before beta-amyloid injection prevented the impairment of LTP. An ANOVA showed a difference between the beta-amyloid (25-35) group and control (p<0.001), and a difference between the N-AcGIP group and beta-amyloid (25-35) combination group and the beta-amyloid group (p<0.001). (c) The GIP antagonist (Pro3)GIP had been tested using a strong LTP stimulation protocol. An ANOVA showed a difference between the (Pro3)GIP group and control (p<0.001). Averaged EPSPs are shown recorded 5min pre-HFS and 1h post-HFS. Calibration bars are 10ms horizontal, 1mV vertical. For details see (Gault and Holscher, 2008a,b).
Neurobiology of Aging  , DOI: ( /j.neurobiolaging )
Copyright © 2008 Elsevier Inc. Terms and Conditions

5. Fig. 4
An overview of some of the roles and functions that insulin receptors (IR) play. Traditionally, insulin is associated with its blood glucose lowering activity. This is achieved by activating a glucose uptake transporter, e.g. GLUT-2. This function is only one of many of the IR. Recent research has uncovered a number of important roles in neuronal growth, synaptic development, and direct control of neurotransmitter release. During neuronal activity, insulin is released and binds to the α-subunit of the receptor. This activates the tyrosine kinase phosphorylation of the β-subunit. Then, several second messenger pathways can be activated: (1) activation of the insulin receptor-Shc-MAP kinase pathway activates gene expression. These code for proteins that are required for cell growth, synapse growth, and for cell repair and maintenance (Hoyer, 1997; Biessels et al., 2006). (2) IR activation has a direct effect on neurotransmission, and primes synapses for induction of long-term potentiation of neuronal transmission (LTP) (Biessels et al., 2004). This pathway most likely involves binding of insulin receptor substrate-1 (IRS1) and insulin receptor substrate-2 (IRS2) to phosphatidylinositol 3-kinase (PI3K). Then, the cyclic nucleotide phosphodiesterase 3B (cPD3B) is activated (Zhao et al., 2000). This would prime the synapse for increased neurotransmitter vesicle release (de la Monte and Wands, 2006). Modulation of neurotransmission will influence memory formation, information processing, and cognitive processes (Hölscher, 1999). (3) IR furthermore modulates neurotransmission directly by altering Glutamatergic and GABAergic receptor activity. NMDA glutamate receptors can be phosphorylated to increase the opening of the associated Ca2+ channel (Lin et al., 2000). IR activation also affects GABA transmission by recruiting functional GABA receptors to the postsynaptic site (Wan et al., 1997). (4) As a growth factor, insulin also suppresses the induction of apoptosis. This pathway involves stimulation of PI3K binding to IRS-1 and -2, activation of PI3K, PDPK, and protein kinase B (Akt/PKB), which suppresses the induction of apoptosis and thereby protects neurons (Eldar-Finkelman et al., 1999; Zhao et al., 2000; Schubert et al., 2003). For further details see (Li and Hölscher, 2007). Abbreviations: Akt/PKB=protein kinase B complex; CPD3B=cyclic phosphodiesterase 3 beta; Grb2/SOS=growth factor receptor binding protein 2/son of sevenless protein; IRS=insulin receptor substrates that get phosphorylated after activation; MAPK=mitogen activated protein kinase; PDK=phosphatidylinosite dependent kinase; Pi3K=phosphatidylinositol 3 kinase; Raf=regulation of alpha-fetoprotein; Ras=rat sarcoma virus peptide; Shc=Src homology collagen peptide.
Neurobiology of Aging  , DOI: ( /j.neurobiolaging )
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6. Fig. 5
An overview of the roles and functions of the GLP-1 receptor. Activation of the GLP-1 receptor activates a G-protein, which in turn activates the adenylyl cyclase (AC) system (Green et al., 2004). The Ga subunit of the GLP-1 receptor stimulates AC, which leads to an increase in intracellular cAMP and activation of protein kinase A (PKA). PKA activity can increase vesicle release in β-cells to enhance glucose-stimulated insulin secretion, or, as postulated in this review, can increase the release of synaptic neurotransmitter and enhance LTP formation in neurons. ADP that is also produced by AC during cAMP production acts on ATP-sensitive K+ channels. This leads to a slow depolarisation of the cell membranes following closure of K+ channels and reduced re-polarisation of neuronal membranes (Doyle and Egan, 2003). This can increase the opening of voltage-dependent L-type Ca2+ channels and increases cytosolic Ca2+, which in turn acts as a second messenger. One of many effects than increased cytosolic Ca2+ can have is the increase of transmitter release (Zucker, 1999). Activation of GLP-1 receptors also leads to an increase in cytosolic Ca2+ levels as a result of activation of voltage-dependent L-channels following phosphorylation of PKA and/or mobilisation of intracellular Ca2+ stores. An increase in phosphatidylinositol 3-kinase (PI3K) levels via the G protein activation also can activate intracellular Ca2+ stores (Fagni et al., 1991). It is proposed that the fast action of GLP-1 (and furthermore of GIP or insulin) on synaptic transmission is mediated by these fast, ion channel and vesicle release-dependent processes. The G protein dependent increase in PI3K levels also activates mitogen-activated protein kinases (MAPK). This pathway activates gene expression, which controls expression of peptides that are required for cell growth, repair, and differentiation of β-cells and also in neuronal cells. Inhibition of PI3K (with LY294002) or MAPK (with PD98059) reduced the GLP-1-stimulated neurite outgrowth (for a review see (Perry et al., 2007). GLP-1-mediated activation of PI3K and downstream transcription factors regulate expression of the genes that encode insulin, β-cell growth and differentiation phenotype (Buteau et al., 1999; Stoffers et al., 2000; Lee et al., 2002). cAMP activates multiple intracellular messenger systems via PDA or independent of activation of PKA (shown here). A PKA-independent pathway had been found in β-cells, which involves cAMP–GEFs. GEFs are activated by binding to cAMP and activation of Rap1A (Kawasaki et al., 1998). Rap1A activates PKC and B-Raf, leading to activation of MAPK (Leech et al., 2000). These metabotropic processes are much slower than the ion channel-dependent mechanisms described before, which have instantaneous effects. The processes described here activate gene expression and protein synthesis and form the growth factor like properties of insulin, GLP-1 and GIP activity. These trophic activities potentially protect and repair neurons from degenerative developments that are ongoing over time. Abbreviations: ADP=adenosine diphosphate; cAMP–GEFs=cAMP–guanine-nucleotide-exchange factors; ERK=extracellular signal-regulated kinase; MEK=MAPK kinase; other abbreviations see Fig. 4.
Neurobiology of Aging  , DOI: ( /j.neurobiolaging )
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