Metabolic Syndrome and Neurological Disorders

24 IN VIVO EVIDENCE OF THE CONVERGENCE OF TYPE 2 DIABETES AND ALZHEIMER DISEASE SUN AH PARK Department of Neurology, Soonchunhyang University College of Medicine, Bucheon Hospital, Bucheon, Korea Abstract: Various animal model studies have provided potential mechanisms shared by type-2 diabetes (T2DM) and Alzheimer disease (AD). The investigations started from observations of AD pathology in diabetic animals, and extended into T2DM induction or genetic modification of diabetes-related target molecules in AD models to further understand the common pathomechanisms and therapeutic implications. Tau pathology is the most prominent AD pathology found in T2DM models. However, aggravation of other major AD pathologies is consistently noted in AD-T2DM double disease models. Recent therapeutic trials targeting T2DM in AD models demonstrate the possibility that this novel approach can provide clues for development of efficient AD treatment. 24.1 INTRODUCTION Alzheimer disease (AD) is the most common neurodegenerative disorder characterized by progressive cognitive dysfunction. Based on results from large numbers of studies using AD animal models with familial AD mutations, clinical trials directly targeting amyloid ␤ protein (A␤) and its pathway have been attempted, but have failed. Aging is the strongest risk factor for AD; thus, factors intimately related with aging are very likely to be major players in AD pathogenesis [1]. Type 2 diabetes (T2DM) and the prediabetic state with insulin resistance exponentially increase with age, and these conditions are closely associated with cognitive impair- ment and neurodegeneration [2, 3]. Epidemiological, clinical, and pathological studies support the connection between T2DM and AD. The risk of AD is 1.4 to 4.3 times higher in patients with T2DM [4–9]. Pathologic markers of insulin resistance are commonly found in AD brains. Furthermore, these show close negative correlation with cognitive function [10–12]. In this chapter, the various animal studies investigating the relationship between T2DM and AD, which support the close relationship between these two common disorders, are summarized. The aim is to provide a comprehensive understanding of the shared mechanisms of AD and T2DM, thereby contributing to the development of efficient therapeutic strategies for AD. Metabolic Syndrome and Neurological Disorders, First Edition. Edited by Tahira Farooqui and Akhlaq A. Farooqui.  C 2013 John Wiley & Sons, Inc. Published 2013 by John Wiley & Sons, Inc. 395 396 METABOLIC SYNDROME AND NEUROLOGICAL DISORDERS TABLE 24.1 Animal studies using diabetes mellitus (DM) models DM models AD-related pathologic changes Behavior changes STZ systemic injection to C57BL/6J, JAX mice [14] ↑ p-Tau at S199/202 & T231, No tau cleavage Less prominent tau pathology than T2DM models (db/db mice), which were compared at the same time ND STZ systemic injection to Swiss Webster mice [16] ↑ p-Tau at T231 ↑ A␤, but similar level of APP & CTF ♣ Partial reversal of these changes by insulin Impaired learning on Barnes circular-maze task STZ systemic injection to C57BL/6NJcl mice [15] ↑ p-Tau at S199, S396, S404 ⇒↓Tau binding to microtubules p-Tau expression is limited to the neuropil & axons, No difference in APP, CTF, & A␤ levels ♣ Prevention of tau pathology by insulin ND STZ systemic injection to Wistar rats [20] ↑AGEs-RAGE complexes ⇒ ↑BACE1 (both protein & mRNA) through ↑NF-kB ND STZ i.c.v. injection to Wistar rats [19] ↑ p-Tau at S199, T212, S396 in the cerebrum, but not at S202, T205, S214, S217, S262, S422 ↑ Phosphorylation of neurofilaments ↓ Microtubule-binding activity of tau ↑ Neurofibrillary degeneration ND STZ i.c.v. injection to Wistar rats [18] ↑ Congo-red-positive aggregates in the brain capillaries, suggesting A␤ peptide-like aggregates ↓Memory function on Morris water-maze test STZ i.c.v. (3rd ventricle) injection into Wistar rat [21] ↓ Phosphorylation of CREB ↓ Akt, ↓IDE, ↑A␤ in hippocampus ↓ Performances on Morris water-maze test BBZDR/Wor rats (T2DM) compared with BB/Wor (T1DM) [22] Neuronal loss in both, but worse in BBZDR/Wor Gliosis in both, but more in BBZDR/Wor ↓ Synaptophysin stain ↑ Dystrophic neurites, but worse in BBZDR/Wor ↑ APP, ␤-secretase, A␤, but more in BBZDR/Wor ↑ p-Tau in both, but more in BBZDR/Wor ♣ Prevention by insulinomimetic C-peptide ND db/db mice (T2DM) compared with STZ-injection model [14] ↑Tau cleavage in db/db ↑ More prominent p-Tau at more sites, S199/202, T231, & Ser396 in db/db ND Otsuka Long-Evans Tokushima Fatty (OLETF) rats [23] ↑3R Tau isoform with alteration of splicing factors, ↑Total Tau, ↑p-tau, ↑Tau cleavage, ↑Tau aggregates, ↓Synaptophysin ND STZ-injection models Spontaneous DM models IN VIVO EVIDENCE OF THE CONVERGENCE OF TYPE 2 DIABETES AND ALZHEIMER DISEASE TABLE 24.1 397 (Continued) DM models AD-related pathologic changes Behavior changes Genetically engineered models targeting insulin signaling Neuron-specific insulin receptor KO mice [26] Complete loss of activation of PI-3K and Akt ↓ p-Akt & p-GSK3␤ ↑ p-Tau at T231 but not at S202 No neurofibrillary tangles No alteration in neuronal proliferation/survival No memory impairments No change in brain glucose metabolism on PET IRS-2-deletion mice [28] ↓ Neuronal proliferation only during development by 50%, but no increase in apoptosis ↑p-Tau at S202 in cytoplasmic deposits ND ♣, Therapeutic trials; A␤, amyloid ␤ protein; AD, Alzheimer disease; APP, amyloid precursor protein; BBZDR, Bio-Breeding Zucker diabetic rat; CTF, C-terminal fragment; CREB, cAMP response element-binding; i.c.v., intracerebroventricular; IDE, insulin-degrading enzyme; IRS-2, insulin receptor substrate 2; KO, knockout; ND, no test for cognition or behavior; OLETF, Otsuka Long-Evans Tokushima Fatty; PET, positron-emission tomography; p-GSK-3␤, phosphorylated glycogen synthase kinase-3␤; PI-3K, phosphatidylinositol 3-kinase; PP2A, protein phosphatase 2A; p-Tau, phosphorylated tau; RAGE receptor for advanced glycation end products; STZ, streptozotocin; T1DM, type-1 DM; T2DM, type-2 DM; Tg, transgenic. Modified from Park (2011) [61] with permission. 24.2 STUDIES IN DIABETES MELLITUS MODELS (TABLE 24.1) 24.2.1 Streptozotocin-Injection Models Streptozotocin [STZ; 2-deoxy-2-(3-(methyl-3-nitrosoureido)-D-glucopyranose], a betacytotoxic drug, selectively destroys insulin-secreting pancreatic ␤ cells, resulting in insulin-dependent type-1 diabetes mellitus (T1DM) [13]. STZ cannot cross the blood– brain barrier (BBB); thus, systemic injections of STZ allow only the brain to be exposed to systemic hypoinsulinemic and hyperglycemic conditions. This approach has been widely used to examine the relationship between DM and AD. Increased tau phosphorylation with decreased binding to microtubules is the most consistent AD-related pathologic finding in this model [14–16]. STZ injection-induced ADrelated pathology was partially reversed by insulin supplements [16]. Intracerebroventricular (i.c.v.) injection can selectively destroy heterogeneously distributed glucose transporter 2 (GLUT2) [17] in brain, thereby decreasing insulin levels without disturbing systemic concentrations of insulin and glucose. The i.c.v. STZ injection model is much closer to the condition of AD brains because the markers of insulin resistance, reduced insulin, and IGF-1 receptors are evident in brain compared with the systemic injection model [18, 19]. Tau phosphorylation in brain is increased after i.c.v. injection and is usually more extensive, involving more phosphorylation sites, than in the systemic injection model. These data suggest that insulin resistance of the brain is more important in inducing tau hyperphosphorylations than the systemic conditions of hyperinsulinemia or hyperglycemia. Overactivation of glycogen synthase kinase-3␤ (GSK3␤) [14], decreased O-linked N-acetylglucosamine glycosylation (O-GlcNAcylation) [19], and phosphatase activity [15] resulting from impaired insulin signaling are thought to induce tau hyperphosphorylation. In contrast to tau, A␤ pathology is rarely noted in STZ injection models. One study reported the levels of BACE1 and c-terminal fragment of amyloid precursor protein (APP) were increased in both systemic and i.c.v. STZ injection models [20], providing evidence of increased amyloidogenic processing of APP in STZ models. The tested performances of these animals were severely impaired, and sometimes accompanied by decreased expression of pCREB (phospho-cyclic AMP-responsive element binding protein) and Akt [21]. 24.2.2 Spontaneous DM Models Through inbreeding, several different lines of spontaneous DM animals have been developed. The benefit of these animals is that they permit chronic 398 METABOLIC SYNDROME AND NEUROLOGICAL DISORDERS exposure to DM, and are therefore closer to the condition of diabetic patients. Several spontaneous DM models, such as BB/Wor rats (T1DM) [22], Bio-Breeding Zucker diabetic rat (BBZDR)/Wor rats (T2DM) [22], db/db mice (T2DM) [14], and Otsuka Long-Evans Tokushima Fatty (OLETF) rats, (T2DM) [23] have been studied in relation to AD pathology. Increased tau phosphorylation is the prominent finding, similar to STZ injection models. However, A␤ pathologies, such as increased APP, ␤-secretase, and A␤s, are common in spontaneous DM models [22]. When T1DM and T2DM spontaneous models were compared, diabetes-induced tau hyperphosphorylation was more profound in T2DM [22], consistent with the results from STZ injection models where insulin resistance (i.c.v. injection models) is more connected with tau hyperphosphorylation than insulin deficits (systemic injection models). Moreover, enhanced tau cleavage, which makes tau more toxic to neurons and aggregates to form neurofibrillary tangles [24], is exclusively noted in T2DM models [14, 23]. Disturbed tau splicing, which increases the 3R tau isoform, was found in spontaneous T2DM rats with obesity [23] and made tau more soluble and phosphorylated. Together, T2DM-induced tau pathologies are suggested to contribute to neurodegenerative changes, as verified by synaptophysin staining and the number of dystrophic neurites in spontaneous T2DM animals [22, 23]. 24.2.3 Genetic Models Targeting Insulin Signaling These studies consistently suggest that the pathogenic factors related to T2DM are more important in inducing AD pathology than T1DM. Insulin resistance and the consequent impaired glucose metabolism/ utilization characterize T2DM. Considering that insulin resistance is frequently observed in AD patients and their brains [25], impaired insulin signaling is very likely to be the key linker between T2DM and AD. Based on this hypothesis, many studies were performed using genetically modified animals targeting insulin signaling. When neuron-specific insulin receptor was selectively abolished, downstream signaling of insulin receptor, phosphatidylinositol 3kinase (PI-3K), and Akt was also completely inhibited [26]. Active Akt decreases tau phosphory- lation via inhibiting GSK3␤ activity. However, when tau hyperphosphorylation was explored at GSK3␤dependent sites, phosphorylation was increased at Thr 231 but not at Ser 202. Moreover, changes in neurodegeneration, cognitive performance, and glucose metabolism as measured by positron-emission tomography (PET) scans were not noted. These results demonstrate that disturbing insulin receptors and their direct downstream signaling pathways alone cannot result in AD. The controversial effect of IR deletion on AD was also observed in a recent study using AD mice, hAPP/IrP1195L/wt [27], which will be discussed in later sections. There is another study directly targeting insulin signaling. IRS-2 is the molecule connecting the receptors for insulin and IGF-1 to various downstream effectors, including PI-3K and Akt as well as extracellular signal-regulated kinase (ERK) cascades. When insulin receptor substrate (IRS)-2 was knocked out, highly increased tau phosphorylation forming cytoplasmic deposits was noted. IRS-2 deletion profoundly inhibited neuronal proliferation during development, but evidence of mature neuronal cell loss was not observed [28]. 24.3 STUDIES WITH AD MODELS (TABLE 24.2) Recent investigations of the linkage of T2DM and AD have moved toward the use of AD models with induced diabetes. The results from a series of studies with DM-AD double diseases models are helpful to understand the current status and future directions of research related to this topic. 24.3.1 The effect of STZ injection in transgenic mouse models of AD The effects of T1DM resulting from systemic STZinduced insulin deficiency were evaluated in several AD transgenic models: pR5 mice with P301L tau [29] and triple transgenic (3 × TG) AD mice [30], double APP mutant mice with London (V717I) and Swedish (K670M/N671L) mutation [31], APP/PS1 mutant mice [32], and 5 × FAD mice [33]. Along with remarkable tau pathology showing increased phosphorylation and insoluble fraction [29,31], the increase of A␤ level by enhanced BACE1 expression IN VIVO EVIDENCE OF THE CONVERGENCE OF TYPE 2 DIABETES AND ALZHEIMER DISEASE TABLE 24.2 399 Animal studies using AD transgenic animal models AD models AD-related pathologic changes Behavior changes STZ systemic injection to AD TG models STZ systemic injection to pR5 mice (P301L tau) [29] Aggravation of tau pathology by DM; ↑p-Tau, ↑Insoluble tau, ↑Neurofibrillary tangles ND STZ systemic injection to 3xTG AD mice [30] ↑Soluble A␤ & APP, but no change in total tau ♣ Exendin-4, GLP-1 receptor stimulator, reversed these pathologies ND STZ systemic injection to mutant APP mice (V717I/K670M/N671L) [31] ↑Activity of GSK-3␤ (↑p at Tyr216, ↓ p at Ser9), ↑p-Tau at Thr231 & Ser199, ↑ A␤42, ↑ A␤ plaque ↓Memory on Barnes maze test STZ systemic injection to APP/PS1 mice [32] ↑Glucose, ↓p-IR, ↑Activity of GSK-3␣/␤, ↑pJNK, ↑BACE1, ↑A␤, ↑␤-cleavage of APP, ↑A␤ plaque ↓Spatial memory STZ systemic injection to 5xFAD mice [33] ↑BACE1, but normal BACE1 mRNA, ↑APP, ↑␤-CTF, ↑p-elF2␣ ND i.c.v. STZ injection into AD TG models STZ i.c.v. injection to TG2576 (APP695swe) [34] ↑Level of A␤ & diffuse A␤ plaques ↑Total tau, ↑total GSK3␣/␤, but normal p-tau ↓Phosphorylated/total GSK3␣/␤ ratio A linear negative correlation between A␤42 & cognition, & between GSK3 ␣/␤ ratio (phosphorylated/total) ↓Spatial memory, ↓Spatial cognitive performance APP/PS1 mice & A␤O i.c.v. injected primates [35] ↑Activation JNK/TNF-a pathway, ↑p- IRS-1 (Ser636), ↓Insulin signaling ♣ GLP-1 receptor stimulation with exendin-4 ⇒ ↓ p-JNK & p-IRS-1, ↓soluble A␤, ↓ A␤ plaque ↓Spatial memory, memory retention ↑Spatial memory, memory retention In TG2576 [37] ↑A␤ generation, ↑A␤ plaque burden, & ↑␥ -secretase activity, ↓Phosphorylation of GSK3␣ & -␤ ↓Spatial learning In 3xTG AD mice [38] ↑A␤40 & A␤42 in detergent-insoluble extracts ↑Soluble tau ↓Postsynaptic marker, drebrin ND High-fat diet in AD TG models High-sucrose diet in AD TG models In 3xTG AD mice [39] ↑ A␤, Impaired mitochondrial respiratory chain, oxidative phosphorylation system, & Ca2 + homeostasis ND (Continued) 400 METABOLIC SYNDROME AND NEUROLOGICAL DISORDERS TABLE 24.2 (Continued) AD models AD-related pathologic changes Behavior changes Cross mated transgenic models between DM and AD APP23−ob/ob by cross-mating and APP23-NSY by pronuclear microinjection [40] ↓Brain weight of APP−ob/ob ↑Inflammatory molecules, IL-6, TNF-␣ ↑Astrogliosis, ↓CHAT-immunoreactive fibers Dense amyloid deposits in small arteries of APP−ob/ob , but only faint amyloid plaques in entorhinal cortex of APP or APP−ob/ob mice Early markedly increased RAGE immunoreactivity in blood vessels of APP−ob/ob mice Early learning deficit, Poor performance in visible-platform test Genetic modulation of insulin signaling in AD TG models Tg2576 crossed with IRS-2(-/-), nIGF-1R(-/-) or nIR(-/-) [43] IGF-1R /IRS-2 deficiency ⇒ prevent premature mortality in Tg2576, ↓APP processing and A␤ accumulation IR deficiency ⇒ no effect ND Double Tg2576/Irs2−/− mice [42] ↓Amyloid deposition ↑ p-Tau (at S409, S396/404, S235, S202) with reduced tau phosphatase PP2A Improved Behavioral deficits hAPP/IrP1195L/wt mice, J20 crossed by IR mutant mice (Ir p1195L/wt ) [27] No change in plaque formation, & A␤ level No effect on cognitive function ♣, Therapeutic trials; A␤, amyloid ␤ protein; A␤O, amyloid ␤ protein oligomer; AD, Alzheimer disease; APP, amyloid precursor protein; CHAT, choline acetyltransferase; CTF, C-terminal fragment; GLP-1, glucagon-like peptide-1; GSK-3, glycogen synthase kinase 3; i.c.v., intracerebroventricular; IL, interleukin; IGF, insulin-like growth factor, IR, insulin receptor; IRS, insulin receptor substrate; JNK, Jun N-terminal kinases; NSY, Nagoya-Shibata-Yasuda; p, phosphorylation; RAGE, receptor for advanced glycation end products; PS1, presenilin1; STZ, streptozotocin; 3xTG, triple transgenic; TG, transgenic; TNF-␣, tumor necrosis factor alpha. Modified from Park (2011) [62] with permission. and ␤-cleavage was also evident in the double disease models [31–33]. I.c.v. injection of STZ resulting in an insulinresistant state restricted in the brain was attempted in TG2576 mice, APP/PS1 mice, and old primates [34, 35]. The effect of i.c.v. injection was similar to that of systemic injection in AD models, demonstrating increased concentrations of A␤, A␤ deposits, and total tau proteins. These pathologic changes showed a negative linear correlation with cognitive decline [34]. Activation of Jun N-terminal kinase (JNK) and tumor necrosis factor-␣ (TNF-␣) signaling pathways correlate with impaired insulin signaling and decreased performance on memory tasks in APP/PS1 mice and A␤ oligomer injected primates [35]. The results demonstrating that both systemic and selective injection of STZ to brain aggravate AD pathologies imply that both insulin deficiency and insulin resistance are important in accelerating preexisting AD pathology. 24.3.2 Transgenic Mouse Models of AD with a High-Calorie-Diet Induced T2DM Chronic ingestion of high-fat or high-sucrose diets induce insulin resistance and T2DM. TG 2576 and 3 × TG-AD mice fed a high-fat diet showed an increased concentration of A␤ and the number of A␤ plaques, along with enhanced ␥ -secretase activity and decreased IDE activities [36–39]. Disturbed insulin signaling, characterized by decreased IR-␤ autophosphorylation and reduced PI-3K-Akt signaling, was evident in these models. Impaired inhibition of GSK-3␣/␤ activity via phosphorylation IN VIVO EVIDENCE OF THE CONVERGENCE OF TYPE 2 DIABETES AND ALZHEIMER DISEASE subsequently increased tau phosphorylation. Long periods of a high-sucrose diet impaired mitochondrial function, the oxidative phosphorylation system, and Ca2 + homeostasis in brain [39]. As demonstrated in 3 × TG-AD mice, mitochondrial dysfunction due to a high sucrose diet was closely associated with significant increases in A␤ levels. Taken together, these data show that a high calorie diet aggravated AD pathology under the genetic background of AD, which suggests the importance of diet-induced T2DM and metabolic syndrome in AD pathogenesis. 24.3.3 Double-Transgenic Models of AD and T2DM APP23 mice overexpressing the Swedish mutation of APP were crossed with ob/ob mice, (leptin-deficient T2DM mice) or NSY mice (polygenic T2DM mice) [40]. These T2DM–AD double-transgenic mice exhibit an earlier onset and more severe phenotype of both diabetes and AD, clearly showing the synergistic effect of the two disorders. In the double disease models, instead of the usual findings of increased A␤ levels, interestingly, prominent microvascular changes, characterized by increased inflammation with upregulated RAGE expression and amyloidosis in vascular walls, were noted. These signs correlate well with neurodegenerative changes, such as reactive gliosis and decreased cholinergic fibers. Remarkable vascular inflammation with deposition of RAGE-amyloid complexes is thought to aggravate neurodegeneration in AD under T2DM conditions [41]. This study provides direct in vivo evidence that cerebrovascular alterations play a critical role linking T2DM and AD, which have been established by the clinical data. 24.3.4 Genetic Modulation of Insulin Signaling in Transgenic AD Models As previously described, single targeting of IR or IRS-1 by genetic engineering was not sufficient to induce AD pathology in induced conditions of insulin resistance [26,28]. Consistently, the results from double mutant mice models, such as hAPP TG mice (line J20) with IR mutation (Ir P1195L/wt ) [27] or Tg2576 mice with IRS-2 deletion (Tg2576/Irs2-/-) [42], have been negative or controversial. Although signs of impaired downstream signaling, such as the 401 PI-3K–Akt pathway, were evident, AD phenotypes validated by A␤ level, plaque numbers, or cognitive impairments were unchanged or diminished after IR or IRS-2 deletion. In another study [43], IGF-1 or its downstream IRS-2 deletion prevented premature death and delayed amyloid accumulation by altering APP processing in Tg2576 mice. In summary, in AD animal models, the effects of genetic modulation by singly targeting IR or IRS-2 in AD-related pathologies have been controversial and even contradictory to the expects. Further studies are needed to determine the importance of insulin signaling in AD and evaluate its possibility as a therapeutic target. 24.4 THERAPEUTIC TRIALS WITH ANTI-DIABETIC AGENTS WITH AD MODELS (TABLE 24.3) Despite the fact that insulin resistance has been posited as the most important mechanism linking AD and T2DM [44], trials in animal models targeting IR and its direct downstream molecules have not offered evidence for its therapeutic implications. However, results from many trials using antidiabetic agents in AD models have drawn attention. Initial modulation of AD pathology was attempted with insulin and insulinomimetic C-peptide in STZ injection T1DM models [14–16], where insulin deficiency caused DM. The AD-related pathologies induced by STZ injection, including neurodegeneration and increased A␤ levels, were reduced by insulinomimetic C-peptide supplementation. These results suggest that the modality targeting insulin deficiency can be applied for AD treatment. However, systemic application of insulin inevitably results in hypoglycemia, chronic desensitization of the insulin receptor, and decrease of free IDE for A␤ to bind; thus, non-systemic approaches or other agents that act like insulin without these negative effects are to be administered. Glucagon-like peptide 1 (GLP-1) increases cell growth and proliferation, inhibits apoptosis, and also regulates postprandial glucose metabolism like insulin [45]. GLP-1 analogs, including anti-diabetic agents, liraglutide (Victoza) and exendin-4 (Exenatide, Byetta, Bydureon), have been demonstrated to lower the level of Aß in various AD mice TABLE 24.3 Therapeutic trials of anti-diabetics in AD animal models Substrates Models Effects Insulin, Insulin-mimetic C-peptide BBZDR/Wor rats, T2DM model [22] Prevention of T2DM induced-AD pathology (neuronal loss, gliosis, synaptophsin decrease, & increase of APP, ␤-secretase & A␤) GGLP-1 analoque: Liraglutide, exendin-4, Val(8)GLP-1 analogue STZ systemic injection to 3xTG AD mice [30] ↓The diabetic effect of STZ; ↑ insulin, ↓glucose, ↓HbA1C ↓Soluble A␤, ↓APP, No change in total tau APP/PS1 mice [48] ↑Memory ↓Synapse loss, ↑Synaptic plasticity in hippocampus ↓ A␤ plaques, ↓Soluble A␤O, ↓Activated microglia. A␤40 i.c.v, injected Wistar rats [47] ↓A␤-induced LTP detriments Restore A␤-induced impairment of spatial learning & memory APP/PS1 mice & A␤O i.c.v. injected primates [35] ↓ Phosphorylation of JNK & IRS-1, ↓Soluble A␤, ↓A␤ plaque, ↑Spatial memory, memory retention Recovery of A␤-induced memory impairment APP/PS1-21 mice [49] Protect LTP, ↓ Age-related synaptic degeneration, ↓Dense-core plaque number Leptin CRND8 (K670N & M671L & V717F) mice [60] ↓A␤1-40, ↓Amyloid burden in hippocampus, ↓C99-CTF, ↓␤-secretase activity maybe through PPAR gamma agonistic effect ↓P-tau at AT8 & Ser396 Improve cognitive performance in novel object recognition & fear conditioning tests Rosiglitazone APPV717I mice [53] ↓Reactive astrogliosis & microglial activation, ↓COX-2, ↓iNOS and inflammatory markers, ↓BACE-1 mRNA & protein, ↓Soluble A␤42, ↓A␤42 deposits Tg2576 mice [54] ↑ Spatial learning, ↓Serum corticosterone level Attenuate reduction in IDE mRNA and activity ↓ A␤42 without affecting deposition PDAPP (J20) mice [55] ↑ A␤ clearance, ↓ A␤ aggregates & A␤O ↓ Neuropil thread containing phosphorylated tau ↓ Proinflammatory markers ↑ Object recognition & spatial memory APPswe/PS1E9 mice [57, 58] ↓ Insoluble A␤42, ↓Reactive astrogliosis & microglial activation Improved behavioral deficits PDAPP (J20) mice [56] ↑Cerebrovascular reactivity, ↑Cerebral glucose utilization Reverse SOD2 increase, ↓Reactive astrogliosis, ↑Fibers of cholinergic neurons, No change in A␤ plaque and spatial memory Pioglitazone A␤, amyloid ␤ protein; A␤O, amyloid ␤ protein oligomer; AD, Alzheimer disease; APP, amyloid precursor protein; BBZDR, Bio-Breeding Zucker diabetic rat; COX-2, cyclooxygenase-2; CTF, C-terminal fragment; GLP-1, glucagon-like peptide-1; i.c.v., intracerebroventricular; IDE, insulin-degrading enzyme; iNOS, inducible-nitric oxide synthase; IRS, insulin receptor substrate; JNK, Jun N-terminal kinases; LTP, long-term potentiation; PPAR, peroxisome proliferator-activated receptor; PS1, presenilin1; SOD2, superoxide dismutase 2; STZ, streptozotocin; 3xTG, triple transgenic; TG, transgenic; T2DM, type 2 DM IN VIVO EVIDENCE OF THE CONVERGENCE OF TYPE 2 DIABETES AND ALZHEIMER DISEASE models, highlighting its availability as an AD therapeutic [30, 35, 46–49]. Furthermore, GLP-1 analogs also rescue decreased synaptic activity, neurodegeneration, and neuroinflammation in a mouse model of AD [35, 48, 49]. Peroxisome proliferator-activated receptor gamma (PPAR␥ ) activator thiazolidinediones, rosiglitazone or pioglitazone, are useful to restore peripheral insulin sensitivity, which is known to be beneficial in AD patients [50–52]. AD patients lacking the ApoEε4 allele are selectively sensitive to the beneficial effects of chronic treatment with rosiglitazone [51]. Additionally, in AD animal models, APPV717I, Tg2576, and 3 × TG AD mice, rosiglitazone or pioglitazone have been shown to decrease A␤ deposition [53, 54], dystrophic neurites containing phophorylated tau [55], and inflammation [53, 55–58]. Furthermore, these relieving effects on ADrelated pathology are followed by improvement of cognitive function [54,55, 58]. Activating the nuclear 403 receptor of PPAR-␥ decreases inflammation and modulates ␥ -cleavage of APP. However, when different start time points were compared, a certain age (9 months old) was more responsive to rosiglitazone therapy than the other ages (5 or 13 months old in Tg2576 mice) [59]. Leptin was tried in the TgCRND8 (K670N/ M671L/V717F) AD model [60], and effectively reduced AD pathologies by decreasing A␤ level, plaque number, ␤-cleavage of APP, and tau phosphorylation. All these changes cooperatively improved cognitive performance. 24.5 CONCLUSION Data from animal studies illustrate the intimate relationship between T2DM and AD and provide insights into the mechanisms shared by these conditions. The various mechanisms are suggested to constitute the Fig. 24.1 The molecular mechanisms linking T2DM and AD. (1) Insulin resistance chronically induces decreased expression of glucose transporters in the brain, thereby impairing glucose metabolism in the brain, resulting in neuronal cell dysfunction and decrease of O-GlcNAcylation. (2) Impaired insulin signaling pathways inhibit phosphatase activity but increase GSK-3␤ activity. These cooperatively increase tau phosphorylation. (3) When insulin levels are increased, more insulin binds to IDE, which impairs A␤ clearance by IDE. (4) Chronic hyperglycemia enhances formation of AGEs and activation of its receptor, RAGE. These pathways increase inflammation, oxidative stress, and mitochondrial dysfunction, all of which make the brain more susceptible to toxic materials. (5) Cerebrovascular insufficiency due to T2DM disturbs the export of A␤ to systemic circulation through cerebral vessels, resulting in increased A␤ levels in the brain. All these T2DM-altered pathways are synergistic in inducing AD pathology. 404 METABOLIC SYNDROME AND NEUROLOGICAL DISORDERS molecular links and mutually intensify T2DM and AD (Fig. 24.1). Pathogenic alterations in insulin signaling, A␤ clearance by IDE, glucose metabolism, OGlcNAcylation, inflammation, oxidative stress, circulating cortisol, and cerebrovascular integrity are considered pathogenic factors in both T2DM and AD. The incidence of comorbidity of T2DM and AD increases with aging. Therefore, this common pathophysiology is likely to constitute a major underpinning of late-onset sporadic AD, and a novel therapeutic approach targeting this pathological process could contribute to the development of more efficient and effective treatments for AD. 24.6 ACKNOWLEDGMENTS This study was supported by a grant of the Korean Health Technology Research and Development Project, Ministry for Health, Welfare, and Family Affairs, Republic of Korea (A092004). The author has no conflict of interest. REFERENCES 1. de la Monte SM. Contributions of brain insulin resistance and deficiency in amyloid-related neurodegeneration in Alzheimer’s disease. Drugs 2012; 72:49–66. 2. 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