Search

SummaryThe incretin system is an emerging new field that might provide valuable contributions to the research of both pathophysiology and therapeutic strategies in the treatment of diabetes, obesity, and neurodegenerative disorders. This study aimed to explore the role of central glucagon-like peptide-1 (GLP-1) and gastric inhibitory polypeptide (GIP) on cell metabolism and energy in the brain as well as on the levels of these incretins, insulin and glucose, by inhibiting the central incretins’ receptors following intracerebroventricular administration of the respective antagonists in healthy rats and a streptozotocin-induced rat model of sporadic Alzheimer’s disease (sAD). Chemical ablation of the central GIP receptor (GIPR) or GLP-1 receptor (GLP-1R) in healthy and diseased animals indicated a region-dependent role of incretins in the brain cell energy and metabolism and central incretin-dependent modulation of peripheral hormone secretion, markedly after GIPR inhibition, as well as a dysregulation of the GLP-1 system in experimental sAD.

Introduction: Gastrointestinal dyshomeostasis is investigated in the context of metabolic dysfunction, systemic, and neuroinflammation in Alzheimer’s disease. Dysfunctional gastrointestinal redox homeostasis and the brain-gut incretin axis have been reported in the rat model of insulin-resistant brain state-driven neurodegeneration induced by intracerebroventricular streptozotocin (STZ-icv). We aimed to assess whether (i) the structural epithelial changes accompany duodenal oxidative stress; (ii) the brain glucose-dependent insulinotropic polypeptide receptor (GIP-R) regulates redox homeostasis of the duodenum; and (iii) the STZ-icv brain-gut axis is resistant to pharmacological inhibition of the brain GIP-R. Methods: GIP-R inhibitor [Pro3]-GIP (85 μg/kg) was administered intracerebroventricularly to the control and the STZ-icv rats 1 month after model induction. Thiobarbituric acid reactive substances (TBARSs) were measured in the plasma and duodenum, and the sections were analyzed morphometrically. Caspase-3 expression and activation were assessed by Western blot and multiplex fluorescent signal amplification. Results: Intracerebroventricular [Pro3]-GIP decreased plasma TBARSs in the control and STZ-icv animals and increased duodenal TBARSs in the controls. In the controls, inhibition of brain GIP-R affected duodenal epithelial cells, but not villus structure, while all morphometric parameters were altered in the STZ-icv-treated animals. Morphometric changes in the STZ-icv animals were accompanied by reduced levels of caspase-3. Suppression of brain GIP-R inhibited duodenal caspase-3 activation. Conclusion: Brain GIP-R seems to be involved in the regulation of duodenal redox homeostasis and epithelial cell turnover. Resistance of the brain-gut GIP axis and morphological changes indicative of abnormal epithelial cell turnover accompany duodenal oxidative stress in the STZ-icv rats.

The gastrointestinal system may be involved in the etiopathogenesis of the insulin-resistant brain state (IRBS) and Alzheimer’s disease (AD). Gastrointestinal hormone glucagon-like peptide-1 (GLP-1) is being explored as a potential therapy as activation of brain GLP-1 receptors (GLP-1R) exerts neuroprotection and controls peripheral metabolism. Intracerebroventricular administration of streptozotocin (STZ-icv) is used to model IRBS and GLP-1 dyshomeostasis seems to be involved in the development of neuropathological changes. The aim was to explore (i) gastrointestinal homeostasis in the STZ-icv model (ii) assess whether the brain GLP-1 is involved in the regulation of gastrointestinal redox homeostasis and (iii) analyze whether brain-gut GLP-1 axis is functional in the STZ-icv animals. Acute intracerebroventricular treatment with exendin-3(9-39)amide was used for pharmacological inhibition of brain GLP-1R in the control and STZ-icv rats, and oxidative stress was assessed in plasma, duodenum and ileum. Acute inhibition of brain GLP-1R increased plasma oxidative stress. TBARS were increased, and low molecular weight thiols (LMWT), protein sulfhydryls (SH), and superoxide dismutase (SOD) were decreased in the duodenum, but not in the ileum of the controls. In the STZ-icv, TBARS and CAT were increased, LMWT and SH were decreased at baseline, and no further increment of oxidative stress was observed upon central GLP-1R inhibition. The presented results indicate that (i) oxidative stress is increased in the duodenum of the STZ-icv rat model of AD, (ii) brain GLP-1R signaling is involved in systemic redox regulation, (iii) brain-gut GLP-1 axis regulates duodenal, but not ileal redox homeostasis, and iv) brain-gut GLP-1 axis is dysfunctional in the STZ-icv model.

We read with great interest a recent comprehensive review covering the knowns and the unknowns of glucagon-like peptide-1 (GLP-1) by McLean et al. (McLean et al., 2020). GLP-1 is best known for its incretin effects first described in the 1980s that provided strong foundations for the development of GLP-1 targeting drugs and revolutionized pharmacotherapy of insulin resistance in the 2000s (Drucker et al., 2017). Nevertheless, as evident from (McLean et al., 2020), we can expect that the glory days of GLP-1 are far from over and that this incretin is going to be even more important for the doctors of tomorrow based on many of its relevant actions, especially in the cardiovascular and central nervous system. A review by McLean et al. (McLean et al., 2020) drew our attention not only because it covers a timely and captivating topic, but because it clearly emphasizes serious limitations and methodological caveats regarding GLP-1 receptor (GLP-1R) expression analysis. Failure to acknowledge methodological caveats either willfully or through ignorance far too often leads to misinterpretation of the results and formation of unjustified conclusions challenging to rectify once they pass under the peer-review radar, and unfortunately – they often do. In this letter, we wish to acknowledge the intention of McLean et al. (McLean et al., 2020) to warn the readers about the methodological drawbacks, especially those regarding the specificity and sensitivity of GLP- 1R antisera and supplement the „Caveats and limitations“ section (McLean et al., 2020) with our own experience and comment regarding bs-1559R anti- GLP-1R rabbit polyclonal antibody.

BackgroundDyshomeostasis of the gastrointestinal (GI) system is investigated as a potential contributor to metabolic dysfunction, systemic and neuro-inflammation recognized as important pathophysiological drivers of neurodegeneration. Gastrointestinal redox dyshomeostasis and dysfunctional brain-gut incretin axis have been reported in the rat model of insulin-resistant brain state (IRBS)-driven neurodegeneration induced by intracerebroventricular administration of streptozotocin (STZ-icv). The aim was to assess i) whether GI oxidative stress is accompanied by structural and functional changes of the epithelial barrier; ii) whether the brain glucose-dependent insulinotropic polypeptide receptor (GIP-R) is also involved in redox regulation of the gut; and iii) whether the STZ-icv brain-gut axis is resistant to pharmacological inhibition of the brain GIP-R.MethodsForty three-month-old male Wistar rats were treated with 3mg/kg STZ-icv or vehicle. One month later the animals were randomized to receive either saline or 85 μg/kg GIP-R inhibitor [Pro3]-GIP intracerebroventricularly and sacrificed 30 minutes later. Thiobarbituric acid reactive substances (TBARS) were measured in plasma and duodenum. Duodenal sections were subjected to morphometric analysis. Caspase-3 expression and activation were analyzed by western blot and spatial signal analysis was done by multiplex fluorescent signal amplification (MFSA). Data were analyzed by linear and linear mixed modeling, and exploration was done by principal component analysis.ResultsInhibition of the brain GIP-R decreased plasma TBARS in the controls and the STZ-icv animals and increased duodenal TBARS only in the controls. Acute inhibition of brain GIP-R affects duodenal epithelial cell, but not villus structure, while all morphometric parameters were altered in the STZ-icv-treated animals. Morphometric changes in the STZ-icv animals were accompanied by reduced levels of activated and total regulator of apoptosis – caspase-3. Acute inhibition of brain GIP-R inactivated duodenal apoptosis at the level of caspase-3 activation.ConclusionsBrain GIP-R is involved in the regulation of the systemic and duodenal redox homeostasis and epithelial function. Duodenal oxidative stress in the STZ-icv rats is accompanied by the resistance of the brain-gut GIP axis and morphological changes indicative of abnormal epithelial cell turnover and dysfunctional GI barrier. Dysfunction of the brain-gut incretin axis might be an important etiopathogenetic factor in neurodegeneration and a potential pharmacological target.

Insulin resistance and metabolic dysfunction in the brain are considered to be the pathophysiological core of sporadic Alzheimer's disease (sAD). In line with that fact, nutrients that could have therapeutic effects at this level have been investigated as possible targets in AD therapy. Galactose, an epimer of glucose, may serve as an alternative source of energy, and given orally may stimulate secretion of the incretin hormone glucagon-like peptide-1 (GLP-1). Our preliminary research indicated that oral galactose might prevent development of memory impairment in a rat model of sAD generated by intracerebroventricular administration of streptozotocin (STZ-icv). Here, we explored whether chronic oral galactose treatment could have beneficial effects on cognitive deficits already manifested at the time of initiation of galactose treatment in adult STZ-icv rats (treatment initiated 1 month after STZ-icv injection). The results clearly show that a 2- month exposure to oral galactose (200 mg/kg/day administered in a drink ad libitum) normalises impaired learning and memory functions. Memory improvement was accompanied by an improvement in brain glucose hypometabolism measured by 18fluorodeoxyglucosepositron emission tomography neuroimaging and by increments in active GLP-1 plasma levels as well as by an increased expression of GLP-1 receptors in the hippocampus and hypothalamus. Our findings provide strong evidence of beneficial effects of oral galactose treatment in the STZ-icv rat model of sAD and present possible underlying mechanisms including both direct effects of galactose within the brain and indirect GLP-1- induced neuroprotective effects that might open a new, dietary-based strategy in sAD treatment.

Alzheimer's disease (AD) is the most common neurodegenerative disorder associated with insulin resistance and glucose hypometabolism in the brain. Oral administration of galactose, a nutrient that provides an alternative source of energy, prevents and ameliorates early cognitive impairment in a streptozotocin-induced model (STZ-icv) of the sporadic AD (sAD). Here we explored the influence of 2-month oral galactose treatment (200 mg/kg/day) in the familial AD (fAD) by using 5- (5M) and 10- (10M) month-old transgenic Tg2576 mice mimicking the presymptomatic and the mild stage of fAD, and compared it to that observed in 7-month old STZ-icv rats mimicking mild-to-moderate sAD. Cognitive and behavioral performance was tested by Morris Water Maze, Open Field and Elevated Plus Maze tests, and metabolic status by intraperitoneal glucose tolerance test and fluorodeoxyglucose Positron-Emission Tomography scan. The level of insulin, glucagon-like peptide-1 (GLP-1) and soluble amyloid β1-42 (sAβ1-42) was measured by ELISA and the protein expression of insulin receptor (IR), glycogen synthase kinase-3β (GSK-3β), and pre-/post-synaptic markers by Western blot analysis. Although galactose normalized alterations in cerebral glucose metabolism in all Tg2576 mice (5M+2M; 10M+2M) and STZ-icv rats, it did not improve cognitive impairment in either model. Improvement of reduced grooming behavior and normalization in reduced plasma insulin levels were seen only in 5M+2M Tg2576 mice while in 10M+2M Tg2576 mice oral galactose induced metabolic exacerbation at the level of plasma insulin, GLP-1 homeostasis and glucose intolerance, and additionally increased hippocampal sAβ1-42 level, decreased IR expression and increased GSK-3β activity. The results indicate that therapeutic potential of oral galactose seems to depend on the stage and the type/model of AD and to differ in the absence and the presence of AD-like pathology.

Basic and clinical research has demonstrated that dementia of sporadic Alzheimer's disease (sAD) type is associated with dysfunction of the insulin-receptor (IR) system followed by decreased glucose transport via glucose transporter GLUT4 and decreased glucose metabolism in brain cells. An alternative source of energy is d-galactose (the C-4-epimer of d-glucose) which is transported into the brain by insulin-independent GLUT3 transporter where it might be metabolized to glucose via the Leloir pathway. Exclusively parenteral daily injections of galactose induce memory deterioration in rodents and are used to generate animal aging model, but the effects of oral galactose treatment on cognitive functions have never been tested. We have investigated the effects of continuous daily oral galactose (200 mg/kg/day) treatment on cognitive deficits in streptozotocin-induced (STZ-icv) rat model of sAD, tested by Morris Water Maze and Passive Avoidance test, respectively. One month of oral galactose treatment initiated immediately after the STZ-icv administration, successfully prevented development of the STZ-icv-induced cognitive deficits. Beneficial effect of oral galactose was independent of the rat age and of the galactose dose ranging from 100 to 300 mg/kg/day. Additionally, oral galactose administration led to the appearance of galactose in the blood. The increase of galactose concentration in the cerebrospinal fluid was several times lower after oral than after parenteral administration of the same galactose dose. Oral galactose exposure might have beneficial effects on learning and memory ability and could be worth investigating for improvement of cognitive deficits associated with glucose hypometabolism in AD.

Hormone therapy (HT) is prescribed during or after menopausal transition to replace the decline in estrogen and progesterone levels. While some studies indicate that estrogen and progesterone depletion in postmenopausal women might carry a significant risk for developing sporadic Alzheimer's disease (sAD), which may be reduced by HT, recent clinical trials oppose this beneficial effect. This review points to possible reasons for these mixed data by considering the issues of both preclinical and clinical trials, in particular, the representativeness of animal models, timing of HT initiation, type of HT (different types of estrogen compounds, estrogen monotherapy vs. estrogen-progesterone combined therapy), mode of drug delivery (subcutaneous, transdermal, oral, or intramuscular), and hormone dosage used, as well as the heterogeneity of the postmenopausal population in clinical trials (particularly considering their sAD stage, anti-AD therapy, and hysterectomy status). Careful planning of future preclinical and clinical HT interventional studies might help to elucidate the effect of HT on cognitive status in postmenopausal women with sAD, which will eventually contribute to more effective sAD prevention and treatment.

Insulin resistance in the brain (IRB) and cerebral glucose hypometabolism are considered to be the pathophysiological core of sporadic Alzheimer's disease (sAD) and drugs aiming to improve these metabolic alterations are currently being investigated. Galactose is intracellularly converted to glucose and may act like an alternative source of energy compensating for dysfunction of insulin-dependent glucose transporters (GLUT) in IRB state. Orally given galactose prevented development of cognitive deficits in streptozotocin-induced AD rat model (STZ-icv) in our previous research. This study aimed to investigate whether galactose induced cognitive improvement in STZ-icv rat model has been mediated by changes in glucagon-like peptide 1 receptor (GLP-1R), and glucose transporters GLUT3 and GLUT4 in the brain. Methods Adult male Wistar rats were given STZ (3 mg/kg) or vehicle (controls) icv bilaterally into lateral ventricles. In the acute experiment single galactose dose was given intraperitoneally (10 and 200 mg/kg) or orally (200 and 1000 mg/kg) to control and STZ rats one month after icv-injection and animals were sacrificed 15 min after galactose administration. In the chronic experiment 2-month oral galactose treatment (200 mg/kg) was initiated 1 month after STZ-icv. Hippocampal (HPC) expression of GLP-1R, GLUT3 and GLUT4 was analyzed by Western blot and hypothalamic (HPT) expression of GLP-1R was assessed by immunohistochemistry. Data were analyzed by Kruskall-Wallis median test followed by Manne-Whitney U-test (p

AIM: Novel effective treatment is urgently needed for sporadic Alzheimer's disease (sAD). M30 ([5-(N-methyl-N-propargylaminomethyl)-8- hydroxyquinoline]) and HLA-20 (5-{; ; ; 4- propargylpiperazin-1-ylmethyl}; ; ; -8- hydroxyquinoline) are brain permeable, iron chelating compounds with antioxidant activity, showing also neuroprotective activity in animal models of neurodegeneration.Weaimed to explore their therapeutic potential in non-transgenic (non-Tg) rat model of sAD developed by intracerebroventricular administration of streptozotocin (STZ-icv). MAIN METHODS: Therapeutic effects of chronic oral M30 (2 and 10 mg/kg) and HLA20 (5 and 10 mg/kg) treatment on cognitive impairment in STZ-icv rat model were explored by Morris Water Maze (MWM) and Passive Avoidance (PA) tests in neuropreventive and neurorescue paradigms. Data were analysed by Kruskal–Wallis and Mann–Whitney U test (p b 0.05). KEY FINDINGS: Five-day oral pre- treatment with M30 and HLA20 dose-dependently prevented development of spatial memory impairment (MWM probe trial-time +116%/M30 ; +60%/HLA20) in STZ-icv rat model (p b 0.05). Eleven-week oral treatment with M30 (3×/week), initiated 8 days after STZ-icv administration dosedependently ameliorated already developed cognitive deficits in MWM test (reduced number of mistakes 3 months after the STZ-icv treatment — 59% ; p b 0.05) and fully restored them in PA test (+314% ; p b 0.05). Chronic M30 treatment fully restored (−47%/PHF1 ; −65%/AT8 ; p b 0.05) STZ-induced hyperphosphorylation of tau protein and normalized decreased expression of insulin degrading enzyme (+37% ; p b 0.05) in hippocampus. SIGNIFICANCE: The results provide first evidence of therapeutic potential of M30 and HLA20 in STZ- icv rat model of sAD with underlying molecular mechanism, further supporting the important role of multi-target ironchelators in sAD treatment.