Mango leaf extract improves central pathology and cognitive impairment in a type 2 diabetes mouse model

Abstract

Epidemiological studies reveal that metabolic disorders, and specifically type 2 diabetes (T2D), are relevant risk factors to develop Alzheimer's disease (AD) and vascular dementia (VaD), the most common causes of dementia. AD patients are in a tremendous need of new therapeutic options because of the limited success of available treatments. Natural polyphenols, and concretely Mangifera indica Linn extract (MGF), have been reported to have antiinflammatory, antioxidant and antidiabetic activities. The role of MGF in central complications associated with T2D, after long-term treatment of db/db mice with MGF was analyzed. Metabolic parameters (body weight, glucose and insulin levels) as well as central complications including brain atrophy, inflammatory processes, spontaneous bleeding, tau phosphorylation and cognitive function in db/db mice treated with MGF for 22 weeks were assessed. MGF limits body weight gain in obese db/db mice. Insulin and C-peptide levels, indicative of pancreatic function, were longer maintained in MGF-treated animals. MGF reduced central inflammation by lowering microglia burden, both in the cortex and the hippocampus. Likewise, central spontaneous bleeding was significantly reduced in db/db mice. Cortical and hippocampal atrophy was reduced in db/db mice and tau hyperphosphorylation was lower after MGF treatment, resulting in partial recovery of learning and memory disabilities. Altogether, the data suggested that MGF treatment may provide a useful tool to target different aspects of AD and VaD pathology, and could lead to more effective clinical therapies for the prevention of metabolic related central complications associated with AD and VaD.

Keywords: Alzheimer's disease; diabetes; hemorrhage; inflammation; mango leaf extract; vascular dementia.

Figure 1

MGF treatment improves metabolic state in db/db mice. (A) Body weight increase in db/db mice is partially controlled by MGF treatment (6, 10, 14 and 26 weeks: **p < 0.01 vs. rest of the groups, ††P < 0.0.1 vs. Control and Control‐MGF). (B) Glucose levels are not affected by MGF treatment (6, 10, 14 and 26 weeks of age ††P < 0.0.1 vs. Control and Control‐MGF). (C) Insulin levels are longer maintained in db/db mice when treated with MGF (6, 10, 14 and 26 weeks: **P < 0.01 vs. rest of the groups, ††P < 0.0.1 vs. Control and Control‐MGF). (D) A similar trend is observed for C‐peptide levels (6, 10 and 14 weeks: **P < 0.01 vs. rest of the groups, ††P < 0.0.1 vs. Control and Control‐MGF; 26 weeks: P = 0.191).

Figure 2

Long‐term MGF treatment rescues cognitive impairment. (A) While MGF treatment has a limited effect on “where” paradigm (††P < 0.01 vs. Control and Control‐MGF) a significant improvement is observed in “what” and “when” paradigms (**P < 0.01 vs. rest of the groups, ††P < 0.01 vs. Control and Control‐MGF). (B) Spatial learning impairment in the Morris water maze test is partially reverted after MGF treatment (day 1: P = 0.078, days 2, 3 and 4: **P < 0.01 vs. rest of the groups, ††P < 0.01 vs. Control and Control‐MGF, ‡‡P < 0.01 vs. Control). (C) A similar profile is observed in the retention phase of the Morris water maze test (retention 1: †P = 0.037 vs. Control and Control‐MGF); retention 2: *P = 0.032 vs. rest of the groups).

Figure 3

MGF treatment improves central atrophy in db/db mice. (A) Cortical thinning in db/db mice is limited by long‐term MGF treatment (**P < 0.01 vs. rest of the groups). A similar profile was observed in the hippocampus (P = 0.06 vs. Control). (B) Illustrative example of cresyl violet staining where cortical thinning can be observed in db/db mice while cortex is preserved in db/db‐MGF treated mice. Scale bar = 150 µm. (C) NeuN‐possitive cells are reduced in db/db mice and this effect is limited in db/db‐MGF treated mice (††P < 0.06 vs. Control and Control‐MGF). (D) Illustrative example of NeuN (red) immunostaining and DAPI (blue) counterstain: neurons are reduced in db/db mice and MGF restores this effect. Scale bar: 25 µm. (E) Caspase activation is significantly increased in the cortex from db/db mice while MGF treatment reduces this activation (*P = 0.016 vs. rest of the groups). A similar profile is observed in the hippocampus although differences do not reach statistical significance (P = 0.23). (F) Central atrophy (as final brain weight) could be predicted by metabolic parameters and significant correlations are observed (body weight: −0.548, **P < 0.01; glucose levels: −0.697, **P < 0.01; insulin levels: −0.863, **P < 0.01; C‐peptide levels: −0.689, **P < 0.01).

Figure 4

MGF treatment reduces tau hyperphosphorylation and central bleeding in db/db mice. (A) Cortical phospho‐tau/total tau ratio is increased in db/db mice and this effect is reduced after MGF treatment (†P = 0.016 vs. Control and Control‐MGF]. (B) A similar profile is observed in the hippocampus (‡P = 0.031 vs. Control). (C) Illustrative example of cortical tau phosphorylation in all groups under study. (D) Cortical hemorrhage burden is significantly increased in db/db mice and this effect is reversed after MGF treatment (**P < 0.01 vs. rest of the groups) because of a reduction in the number of hemorrhages (**P < 0.01 vs. rest of the groups), while hemorrhage size is not affected (P = 0.654). (E) Illustrative example of cortical hemorrhages stained with Prussian blue. Green arrows point at individual hemorrhages. Scale bar= 250 µm. (F) Increased hemorrhage burden in db/db hippocampus is slightly reduced after MGF treatment, although differences do not reach statistical significance (P = 0.087). Hemorrhage density is significantly reduced after treatment (**P = 0.009 vs. rest of the groups), whereas hemorrhage size is not affected (P = 0262).

Figure 5

Inflammation is reduced after MGF treatment in db/db mice. (A) Increased cortical microglia burden in db/db mice is reduced after MGF treatment (**P < 0.01 vs. rest of the groups, †† P < 0.01 vs. Control and Control‐MFG) because of a reduction of individual microglia size and density (**P < 0.01 vs. rest of the groups, †† P < 0.01 vs. Control and Control‐MFG].B) Illustrative example of cortical microglia immunostaining with anti‐IBA1 antibody (green). Scale bar= 50 µm. (C) Microglia burden in db/db mice is also reduced in the hippocampus after MGF treatment (**P < 0.01 vs. rest of the groups). Microglia size and density is also reduced (**P < 0.01 vs. rest of the groups, ‡‡ P < 0.01 vs. Control).