Dementia with Lewy Bodies: Molecular Pathology in the Frontal Cortex in Typical and Rapidly Progressive Forms

Abstract

Objectives: The goal of this study was to assess mitochondrial function, energy, and purine metabolism, protein synthesis machinery from the nucleolus to the ribosome, inflammation, and expression of newly identified ectopic olfactory receptors (ORs) and taste receptors (TASRs) in the frontal cortex of typical cases of dementia with Lewy bodies (DLB) and cases with rapid clinical course (rpDLB: 2 years or less) compared with middle-aged non-affected individuals, in order to learn about the biochemical abnormalities underlying Lewy body pathology.

Methods: Real-time quantitative PCR, mitochondrial enzymatic assays, and analysis of β-amyloid, tau, and synuclein species were used.

Results: The main alterations in DLB and rpDLB, which are more marked in the rapidly progressive forms, include (i) deregulated expression of several mRNAs and proteins of mitochondrial subunits, and reduced activity of complexes I, II, III, and IV of the mitochondrial respiratory chain; (ii) reduced expression of selected molecules involved in energy metabolism and increased expression of enzymes involved in purine metabolism; (iii) abnormal expression of nucleolar proteins, rRNA18S, genes encoding ribosomal proteins, and initiation factors of the transcription at the ribosome; (iv) discrete inflammation; and (v) marked deregulation of brain ORs and TASRs, respectively. Severe mitochondrial dysfunction involving activity of four complexes, minimal inflammatory responses, and dramatic altered expression of ORs and TASRs discriminate DLB from Alzheimer's disease. Altered solubility and aggregation of α-synuclein, increased β-amyloid bound to membranes, and absence of soluble tau oligomers are common in DLB and rpDLB. Low levels of soluble β-amyloid are found in DLB. However, increased soluble β-amyloid 1-40 and β-amyloid 1-42, and increased TNFα mRNA and protein expression, distinguish rpDLB.

Conclusion: Molecular alterations in frontal cortex in DLB involve key biochemical pathways such as mitochondria and energy metabolism, protein synthesis, purine metabolism, among others and are accompanied by discrete innate inflammatory response.

Keywords: Alzheimer’s disease; dementia with Lewy bodies; inflammation; mitochondria; olfactory receptors; protein synthesis; α-synuclein; β-amyloid.

Figure 1

Protein expression in middle-aged (MA) (n = 12), dementia with Lewy bodies (DLB) (n = 9) and rapid DLB (rpDLB) (n = 4) of subunits of mitochondrial complexes I (NDUFFA7, NDUFA10, NDUFB10, NDUFS8, NDUFB8), II (SDHB), III (UQCRC2), IV (MTCO1), and V (ATP5A, ATP50) normalized with both β-actin- and voltage-dependent anion channel (VDAC). Three representative cases are shown in western blots. Diagrams show quantitative values of all assessed cases. Significant decrease in the expression levels of the majority of these subunits is seen in the frontal cortex in DLB and less markedly in rpDLB when compared with MA cases when normalized with β-actin and in most cases with VDAC. Decreased expression of VDAC in DLB probably reflects decrease in the number of mitochondria: *p < 0.05, **p < 0.01, and ***p < 0.001.

Figure 2

Mitochondrial enzymatic activities in complex I, II, III, IV, and V in middle-aged (MA), dementia with Lewy bodies (DLB), and rapid DLB (rpDLB). All the mitochondrial activities are corrected with the appropriate values of citrate synthase for each sample. Significant decreased activity of complex I, II, III, and IV is observed in DLB and rpDLB when compared with MA. Complex V activity showed a trend toward reduction in DLB and rpDLB: *p < 0.05, **p < 0.01, ***p < 0.001.

Figure 3

Protein expression, as revealed by western blotting, of initiation and elongation factors of protein transcription at the ribosome in MA (n = 12), dementia with Lewy bodies (DLB) (n = 9) and rapid DLB (rpDLB) (n = 4) using β-actin for normalization. Reduced expression of initiation factors is more marked in rpDLB than in DLB, whereas the expression of elongation factors eEF1A and eEF2 is not modified in DLB and rpDLB: *p < 0.05, **p < 0.01, and ***p < 0.001.

Figure 4

TNFα, GFAP, and Iba-1 protein levels in middle-aged (MA), dementia with Lewy bodies (DLB), and rapid DLB (rpDLB) as revealed by western blotting using β-actin for normalization. Significant increased GFAP expression occurs in DLB and rpDLB when compared with MA (p < 0.01). A significant increase also occurs in TNFα levels in rpDLB when compared with MA (p < 0.05): *p < 0.05, **p < 0.01, and ***p < 0.001.

Figure 5

Soluble Aβ40 and Aβ42, membrane-associated β-amyloid, tau oligomers, and α-synuclein oligomeric species in total homogenate fractions in middle-aged (MA) (n = 10), dementia with Lewy bodies (DLB) (n = 10) and rapid DLB (rpDLB) (n = 4). (A) Soluble Aβ40 and Aβ42 levels are similar in MA and DLB cases but soluble Aβ40 and Aβ42 are significantly increased in rpDLB when compared with MA and DLB; (B) membrane-associated β-amyloid is detected in DLB and rpDLB but not in MA cases as revealed with human amyloid-beta protein antibodies 4G8 and 6E10; (C) no tau oligomers are detected in DLB and rpDLB even after membrane over-exposition; (D) α-synuclein oligomers are present equally in DLB and rpDLB, which is in contrast with the lack of α-synuclein oligomers in MA. The figures are representative of four MA, four DLB, and four rpDLB; statistical values represent the totality of samples: *p < 0.05, **p < 0.01, and ***p < 0.001.