User:RostyslavZvanych/The a-ketoglutarate dehydrogenase complex in Alzheimer’s disease

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Alzheimer’s disease (AD) is a complex neurodegenerative disorder, characterized by a neuronal loss, consequent cognitive declines, memory and judgment deteriorations. A partial cause of such loss is the reduction of the brain metabolism, which mainly originates from the diminished activity of the key metabolic enzyme in the tricaboxylic acid (TCA) cycle - α-ketoglutarate dehydrogenase complex (α-KGDHC). Several symptoms of AD were found to correlate with deficiency of the α-KGDHC, which raises many questions about the relationship between the α-KGDHC and AD, one of which is: how does the α-KGDHC-related metabolism change in AD patients and which specific subunit(s) of α-KGDHC is(are) responsible for such change? This wikipedia page presents the correlation of α-KGDHC to the phenotypes of AD and brain metabolism based on studies about the enzyme's activities in conjunction with its cofactor thiamine pyrophosphate (TPP) and subunits E2k, E3 in different regions in AD brain. Overall, highest reduction of α-KGDHC activity was observed in seven areas of the AD brain in absence of TPP, and this absence also initiated oxidative stress. This result was further supported by a higher decrease in α-KGDHC protein level and transcription, especially E2k and E3, was observed in regions of the brain vulnerable to thiamine deficiency. In addition, thiamine deficiency and oxidative stress can be linked to the formation of the beta-amyloid(Aβ) plaques, and this formation was further accelerated by E2k deficiency in α-KGDHC, along with acceleration of other AD phenotypes including Aβ oligomers formation and memory deficiency. Further correlation of the α-KGDHC activity and malfunction of brain metabolism in AD showed that deficiency of the α-KGDHC may result in reduced ATP production and neuronal death as the consequence. Moreover, a positive feedback phenomenon was observed between adenosine triphosphate (ATP) and TPP. In brief, these results provided an insight into the complexity of this disorder and implicitly suggested the potential direction of the research in Alzheimer’s field.

Main Primary Articles

1. "Brain α-ketoglutarate dehydrogenase complex activity in Alzheimer’s disease" by Mastrogiacoma, F., Bergeron, C. and Kish, S.J. [1]

2. "Responses of the mitochondrial alpha-ketoglutarate dehydrogenase complex to thiamine deficiency may contribute to regional selective vulnerability" by Shi, Q., Karuppagounder, S. S., Xu, H., Pechman, D., Chen, H., and Gibson, G. E. [2]

3. "Mitochondrial dihydrolipoyl succinyltransferase deficiency accelerates amyloid pathology and memory deficit in a transgenic mouse model of amyloid deposition." by Dumont, M. et al. [3]

Lists of Abbreviation

α-KGDHC - α-ketoglutarate dehydrogenase complex

Aβ - Beta-amyloid

AD - Alzheimer's disease

APP - Amyloid precursor protein

ATP - Adenosine triphosphate

CoA - Coenzyme A

FAD/FADH2 - Flavin adenine dinucleotide

NAD+/NADH - Nicotinamide adenine dinucleotide

NFTs - Neuro-fibrillary tangles

ROS - Reactive oxygen species

SmTN - Sub-medial thalamic nucleus

TCA - Tricarboxylic acid cycle

TPP - Thiamine pyrophosphate


Alzheimer’s Disease[edit]

File:PPlaques and tangles bor3der-1.jpg
Figure 1: The resulted neuritic plaques of beta-amyloid (Aβ) peptide and neurofibrillary tangles of tau protein in brain tissues [4].
File:Untitled23.jpg
Figure 3: Different multiple assembly states of Aβ peptides: Aβ monomers (Aβ peptides), oligomers, protofibrils and fibrils. Aβ fibrils can eventually aggregate into insoluble Aβ peptide plaque. [5]

Alzheimer’s disease (AD) is a progressive neurodegenerative disorder characterized by a loss of cognitive function [6]. The hallmarks of AD are neuritic plaques of beta-amyloid (Aβ) peptide and neurofibrillary tangles (NFTs) of tau protein in brain tissue, which result in a significant loss of neurons, shrinkage of neuronal volume and loss of synaptic connections (Figure 1).

Aβ peptide is a 39–43 amino acids long peptide that can exist in multiple assembly states: monomers (peptides), oligomers, protofibrils and fibrils (Figure 2) [7] [8]. In general, Aβ peptide is produced by the cleavage of the parental amyloid precursor protein (APP) that is located at plasma membrane, trans-Golgi network, endoplasmic reticulum and endosomal, lysosomal and mitochondrial membranes (Figure 3). Soluble Aβ oligomers formed are found to be toxic since they can bind to different neurotransmitter receptors and thus stimulate kinase dysfunction, oxidative stress, and synapse loss. All these effects eventually lead to the loss of neurons. On the other hand, Aβ peptides can also spontaneously aggregate into fibrils, which eventually lead to the formation of insoluble Aβ plaques that are toxic to the neurons.

Currently, the etiology of AD is not well understood; however, it is thought to be caused by both genetic and environmental factors. The most significant risk factor for developing neurodegenerative disorders such as AD, Parkinson’s disease and Huntington’s disease is advanced age. A study in Europe shows the incidence of AD in 60-year olds is about 1%; this increases to approximately 30% for those 85 years old [9].


Figure 2: The formation of Aβ peptide, which can eventually aggregate into soluble Aβ oligomer and insoluble Aβ peptide plaque. [10]






Oxidative Stress[edit]

Oxidative stress is a process characterized by a production of reactive oxygen species (ROS), that are partially reduced derivatives of molecular oxygen (O2) [11]. These oxygen species can react with many cellular constituents including proteins, lipids and DNA. The extent of oxidative stress varies depending on the cell type and its metabolic state, but the substantial source of ROS is mitochondria. Within any healthy living cells, this process is an unavoidable consequence of metabolism. More specifically, the exact ROS sources are Complex I of the electron transport chain and the tricarboxylic acid (TCA) cycle enzyme α-KGDHC [12]. Interestingly, an accumulation of Aβ peptides is linked to mitochondrial dysfunction and increased production of ROS [13]. The highest increase in oxidative markers occurred at the early stage of AD and kept increasing with ages. Oxidative stress has also been shown to increase Aβ peptides formation and other hallmarks of the disease such as increased tau phosphorylation, and the onset of behavioral abnormalities. In addition, the activity of α-KGDHC is reduced in brains from AD patients, thus its deficiency is correlated with occurrence of AD.



Brain Areas affected by Alzheimer’s Disease[edit]

The brain consists of three parts: the cerebrum, the cerebellum, and the brain stem (Figure 4) [14]. Each part is responsible for different brain functions, although there is often some overlap. In AD, the cerebellum is the part that affected by AD. The cerebrum is the largest area of the brain. The cerebrum can be anatomically divided into two parts: the right and the left hemispheres. The right hemisphere controls the left side of the body, whereas the left hemisphere controls the right side. Each hemisphere contains a caudate nucleus and is divided into four sections, the frontal, parietal, temporal, and the occipital lobes.

Area Responsibilities
Caudate nucleus Learning and memory system
Frontal lobe Speech, emotion, behavior, movement, and planning
Parietal lobe Object identification, pain, pressure, and other physical sensations
Occipital lobe Visual stimuli and allows the brain to process light and objects
Temporal lobe Memory, personality, language, smells and sounds

In the temporal lobe, the hippocampus and amygdala (Figure 5) are the parts that are most affected by AD. The hippocampus is responsible for memory. In AD, the Hippocampus is one of the first regions of the brain to suffer damage; memory problems and disorientation appear among the first symptoms. The Amygdala (Figure 5) has important role on the mediation and control of such activities and feelings as love, friendship, affection, and expression of mood. The amygdala is also the center for identification of danger and is fundamental for self preservation.













The α-ketoglutarate dehydrogenase complex[edit]

The α-KGDHC is found in the tricarboxylic acid cycle (TCA), which situated between glycolysis and the electron transport chain (Figure 6). The TCA cycle, also referred to as the citric acid or Krebs cycle, is a series of enzyme-catalyzed chemical reactions that occurs in the matrix of the mitochondria. The citric acid cycle is a cyclic metabolic pathway involved in cellular respiration, which eventually converts carbohydrates, fats and proteins into carbon dioxide and water to generate ATP. The citric acid cycle occurs in between glycolysis and oxidative phosphorylation (i.e. electron transport chain). In addition, it provides precursors for many compounds including some amino acids. α-KGDHC is an enzyme that is part of the citric acid cycle. Basically, it catalyzes the second oxidative decarboxylation of the TCA cycle, which produces NADH, CO2 and a high energy thioester bond to CoA. This enzyme consists of three subunits which possess different functions. [15].


Structures and Components[edit]

The α-ketoglutarate dehydrogenase complex (KGDHC) is an enzyme involved in the citric acid cycle that consists of three subunits: E1k, E2k and E3 [16] (Figure 7).

File:Structure of alpha-KGDHC.jpg
Figure 7: Structure of the alpha-KGDHC subunits (A) Ribbon diagram of the homo-dimer structure of the E. coli E1k subunit. Cofactor TPP and AMP are shown in cyan and blue, respectively. [17] (B) Diagram of the 12 subunit catalytic core of the cubic 24 subunit human E2k. [18] (C) Ribbon diagram of the 8 Monomer E3 subunit. Enzyme is shown bound to FAD and NADH as brown and cyan spheres, respectively. [19]


The E1k (2-oxoglutarate dehydrogenase, thiamine pyrophosphate cofactor (Figure 8), E.C. 1.2.4.2,) subunit is the largest in size and forms a 962 amino acids long homodimer [17] [20]. It is a thiamine-pyrophosphate–dependent protein that catalyzes the oxidative decarboxylation of α-ketoglutarate substrate and passes the resulted succinic acid moiety to the next subunit in the complex. The E1k is encoded on the OGDH gene, located in chromosome 7p13-p14 [21].

The E2k (dihydrolipoamide acyltransferase, lipoic acid and CoASH cofactors, EC 2.3.1.61) subunit is a core protein and is the smallest subunit, only 453 amino acids long [22]. The E2k is encoded in the DLST gene, located on the chromosome 14q24.3. It is known to receive the succinic acid product from E1k and catalyze its transfer to the coenzyme A, yielding succinyl-CoA [23]. The enzyme consists of 24 subunits forming a cubic structure. The 12 subunit catalytic core of the complex contains 5 α-helical segments and 10 β-strands [18]. The E2k component contains the lipoic moiety that transports the substrate between the subunits of KGDHC [24].

Unlike the E1k and E2k subunits, which are specific for KGDHC, the E3 (dihydrolipoyl dehydrogenase, FAD and NAD+ cofactors, EC 1.8.1.4) subunit is not. It has been found to be a component of other systems like pyruvate dehydrogenase and branched-chain dehydrogenase [25]. This subunit is 464 amino acids long and is encoded on the DLD gene, located on chromosome 7q31.1-7q32 [26]. It is an 8 monomer that can be divided into 4 domains; the FAD-binding domain (residues 1–149), the NAD+-binding domain (residues 150–282), the central domain (residues 283–350), and the interface domain (residues 351–474) [19] [27]. It is a flavoprotein that catalyzes the reaction of two-electron transfer of reducing equivalents from E2k that yields NADH and a proton, completing the catalytic conversion of α-ketoglutarate substrate [25].




Reaction Catalyzed[edit]

The α-KGDHC catalyzes the second oxidative decarboxylation of the TCA cycle [15], which produces NADH, CO2 and a high energy thioester bond to CoA (Figure 9). This reaction consists of three steps:

  1. Decarboxylation of α-ketoglutarate (catalyzed by E1k)
  2. Reduction of NAD+ to NADH (catalyzed by E2k)
  3. Subsequent transfer to CoA, which forms the end product, succinyl CoA (catalyzed by E3)


Regulation of KGDHC Activity[edit]

The reaction catalyzed by the α-KGDHC produces large negative ΔG values (-43.9 kJ/mol) under mitochondrial conditions and are thus one of the primary sites of regulation in the cycle (Figure 6). The α-KGDHC is activated by AMP and inhibited by NADH and succinyl-CoA. NADH and succinyl CoA can be directed to produce NAD+ and CoA. The direction of this reaction is dictated by the concentration of reactants versus products. This allows control of the enzyme complex, via product inhibition as NADH and succinyl CoA compete with NAD+ and CoA for binding sites on their respective enzymes. They also drive the reversible E2k and E3 reactions backwards. When concentrations of NADH and succinyl CoA are high, the reversible reactions catalyzed by E2k and E3 are driven backwards so inhibiting further formation of acetyl CoA. E1k is not reversible, so cannot accept more α-ketoglutarate till it is binding sites are emptied [15].

The α-KGDHC in Alzheimer’s Disease[edit]

Similarly to any neurodegenerative disease, Alzheimer’s is a complex disorder and could be characterized from various angles with several contributing, potentially malfunctioning, aspects. It is worth pointing out that the changes in α-KGDHC is only one piece of the Alzheimer’s puzzle, nonetheless it is an important aspect in this disorder. The complexity of this multi-subunit enzyme complicates the search for the factors that cause its metabolic activity to diminish. Several studies provided solid evidence that diminished activities of α-KGDHC are directly caused by the deficiency in its thiamine pyrophosphate (TPP) cofactor. However, the exact changes that lead to this decrease in activity, as well as its relation to the AD are difficult to study. Some of the in vitro studies determined that selective neurodegeneration observed in AD is directly related to the vulnerability of the brain areas to the thiamine deficiency, which then leads to diminished α-KGDHC activities and consequent oxidative stress. Multiple studies cited in this page were geared towards the understanding of the underlining mechanisms of the exact effect thiamine deficiency has on the α-KGDHC activity. These studies show that α-KGDHC activity is diminished due to the decreased mRNA, protein levels, as well as post-secondary modifications of its subunits, underlining how crucial the cofactor is for its proper functioning. Other studies demonstrated the exclusive contribution of the E2k deficiency to the overall diminished activity of α-KGDHC. More specifically, E2k defciency is strongly correlated with the increase in Aβ plaques deposition, Aβ oligomers formation, and elevated levels of oxidative stress markers, together with deficits in spatial learning and memory. Interestingly, gender seems to have an affect on these complex phenomena. Additional studies eliminated few possible candidates that could also contribute to the diminished activities. These are pH, lactate levels, changes in other metabolically relevant enzymes form the TCA cycle (citrate synthase, fumarase and succinate dehydrogenase). Low levels of ATP production are logically attributed to deficiencies in metabolically relevant enzymes, such as α-KGDHC. Despite the logic of this assumption, decreased ATP levels could also be caused by the low glucose utilization, often observed in AD patients, thus α-KGDHC activity is not the sole factor for such phenomenon.

The intricate complexity of the Alzheimer's disease is incredible. The changes in the α-KGDHC activity alone seem to have multiple entrance routes. Branching off the main disorder seems to reveal new riddles about its origin. Nonetheless, α-KGDHC does have a substantial impact on AD and gives a more wide insight into this disease, suggesting an additional rout to be considered in search for therapeutics.

The Enzyme Cofactor Thiamine Pyrophosphate and Brain Anatomy[edit]

Reduction in the α-KGDHC activity is a generalized brain phenomenon in AD patients. In a study done by Mastrogiacoma et al [1], the enzyme activity in seven brain areas including four cerebral cortex areas (frontal, temporal, parietal, occipital) and three subcortical areas (hippocampus, amygdala and caudate nucleus) was measured by monitoring the formation of NADH (Figure 9) in the presence and absence of the enzyme cofactor TPP (Figure 6). The results showed a non-uniform reduction in different regions of AD brains (Figure 10). A reduced mean α-KGDHC activity was observed in seven examined brain areas of AD patients. In the presence of exogenously administered TPP, significant reduction of the enzyme activity was found in the frontal, temporal and parietal cortices of the AD as compared to the control group, whereas in the absence of TPP, substantial reductions were found in five of the seven examined AD brain areas, with the most reduction in cerebral cortex and hippocampus. In the AD group, the magnitude of the enzyme reduction was approximately 10-23% less than that in the absence of TPP in seven examined AD brain areas (Figure 11). These observations show that TPP cofactor is crucial for the α-KGDHC activity and the enzyme activity is reduced in all of the AD brain areas.

File:Activity of KGDHC.jpg
Figure 10: Activity of the α-KGDHC in cerebral cortex (frontal, temporal, parietal, and occipital) , hippocampus, amygdala, and caudate nucleus. Data are mean ± SE (bars) values [1].
File:TPP stimulation of KGDHC activity.jpg
Figure 11:TPP stimulation of KGDHC activity in four cerebral cortex, hippocampus, amygdala, and caudate nucleus of controls and AD patients. Data are mean ± SE (bars) values, expresses as percent TPP stimulation of α-KGDHC activity [1].

TPP levels are reduced in postmortem AD brains since TPP is able to stimulate the α-KGDHC activity. The enzyme-stimulating role of TPP depends on the enzyme itself due to the α-KGDHC activity remains reduced even with the maximal addition of exogenous TPP in an in vitro assay. This result suggests that reduction in α-KGDHC activity may be related to a possible cofactor stabilization role of TPP for its dependent enzyme. This possibility was examined via measuring the rate of inactivation of α-KGDHC when incubated in both the presence and absence of TPP. A slightly slower rate of α-KGDHC inactivation was observed when incubated in the presence rather than in the absence of TPP. Thus, irreversible inactivation of α-KGDHC could be due to lower TPP levels in AD brain, which may be a result of elevated TPP degradation and/or reduced TPP synthesis, reduced thiamine transport across the blood-brain barrier, or a general lack of dietary thiamine intake and/or absorption across the intestinal wall. However, the blood thiamine levels in AD have been reported normal. Therefore, reduction in α-KGDHC activity in AD brain is more likely to be related to altered TPP chemistry. Therefore, exogenously administered TTP has a greater stimulatory effect on brain α-KGDHC activity in AD group, as compared with the controls [1]. This ability of TPP to stimulate enzyme activity is regarded as a reliable and sensitive criterion for the diagnosis of thiamine deficiency [28].

File:Untitled322.jpg
Figure 12 : In situ KGDHC activity in SmTN and cortex of mice from control and thiamine deficient groups. KGDHC activity was measured by an in situ histochemistry activity stain at 4, 8 and 10 days of TD. The change in activity was more pronounced in the SmTN, as it was in the cortex [2].

Thiamine Defficiency-induced Oxidative Stress and α-KGDHC Activity[edit]

Evidence suggests that deficiency in the TPP cofactor causes diminished activities of thiamine dependent-enzymes, such as α-KGDHC, and induces oxidative stress. [29]. Under thiamine deficient conditions, the E3 subunit of α-KGDHC, which is responsible for NADH formation by transferring the electrons to NAD+, is more prone to transfer electrons to O2, leading to the formation of reaction oxygen species (ROS) [30]. Interestingly, thiamine deficiency, and consequent oxidative stress, was linked to the formation of the Aβ plaques in the Alzheimer’s mouse model. [31]. Moreover, the metabolic activities of α-KGDHC and its response to the thiamine deficiency are strongly attributed to the onset of neurodegeneration [2]. A research study conducted by Shi, Q. et al compares the regions of the brain that are affected by selective neurodegeneration (Figure 12) [2]. The exact mechanism of changes in the α-KGDHC activity due to the thiamine deficiency was studied in the sub-medial thalamic nucleus (SmTN), region of the brain vulnerable to thiamine deficiency, and relatively tolerant cortex region.



Changes in α-KGDHC Subunits and Potential Causes for Selective Neurodegeneration[edit]

The changes in the α-KGDHC enzyme levels, the subunit activities and their mRNA levels have all been studies in detail under thiamine deficient conditions [2]. These holistically approached experiments were crucial to be able to confirm that there is a regional selectivity for neurodegeneration, as a single parameter (i.e. the mRNA levels) could not be reliable enough to draw such conclusion. Regions of the brain that are thiamine deficiency vulnerable, like sub-medial thalamic nucleus (SmTN) are severely affected (Figure 12 ). Detailed analysis revealed several causes for such significant decrease in α-KGDHC activity. The first factor was a decrease in transcription (mRNA levels) of the subunits comprising α-KGDHC. As shown in Figure 13, after ten days of thiamine deficiency, the mRNA levels in SmTN of mice were much lower, when compared to the control (no thiamine deficiency). The E2k and E3 subunits were affected the most. On the other hand, the same was not true for the cortex region, where the mRNA levels remained constant (Figure 13). But these regional differences were expected based on the thiamine deficiency tolerance.

Another factor contributing to the decreased α-KGDHC activity in this region was the decreased protein levels for each subunit. (Figure 14) [2]. Declining protein levels were the most evident for the E1k and E2k subunits, when compared to the control. On contrary, as the case was with mRNA levels, there was no significant variation between the protein levels in the mice under experiment and control in the cortex region (Figure 14).

As determined by the immunocytochemistry assay, after ten days of thiamine deficiency, the immunoreactivity of each subunit was diminished only in the thiamine deficiency vulnerable SmTN, and remained unaffected in the cortex, confirming the vital role of TPP in this region. (Figure 15). [2].

File:Untitled31234.jpg
Figure 13: The changes in the mRNA levels for each α-KGDHC subunit in the sub-medial thalamic nucleus (vulnerable to thiamine deficiency) and the cortex (non-vulnerable to thiamine deficiency) regions of the brain [2].
File:Untitledsd4.jpg
Figure 14: The changes in the protein levels for each α-KGDHC subunit in the sub-medial thalamic nucleus (vulnerable to thiamine deficiency) and the cortex (non-vulnerable to thiamine deficiency) regions of the brain [2].
File:The changes in immunoreactivity.jpg
Figure 15: The changes in the immunoreactivity for each α-KGDHC subunit in the sub-medial thalamic nucleus (vulnerable to thiamine deficiency) and the cortex (non-vulnerable to thiamine deficiency) regions of the brain after 10 days for thiamine deficiency. (TD – thiamine deficiency, CON – control) [2].
File:Figure 1 Summary.jpg
Figure 16: The change in α-ketoglutarate dehydrogenase activity in mice with E2k deficiency and AD genetic mutations (Tg19959-DLST+/−), mice with E2k deficiency only (DLST+/−), healthy mice (WT), and mice with AD genetic mutations only (Tg19959). [3]




Additionally, in the study done by Dumont et al. [3], E2k deficiency was shown to accelerate the onset of AD pathogenesis. Four characteristics of AD were measured in mice with both E2k deficiency and AD genetic mutations including Aβ plaques deposition (pathological feature in AD), Aβ oligomers formation (pathological feature in AD), nitrotyrosine levels (a marker for measuring the level of oxidative stress) and the occurrence of spatial learning and memory deficits, an important clinical feature of AD (5). The occurrence of spatial learning and memory deficits were measured using a behavioral Morris water maze test.

Another experiment verified that E2k deficiency was sufficient enough to cause a deficiency in the whole α-KGDHC (Figure 16). Higher level of Aβ plaques was found in the cortex and the hippocampus of the mice with E2k deficiency and AD genetic mutations in comparison to mice with only the AD genetic mutations. The highest level of Aβ plaques was found in the cortex area of female mice with both E2k deficiency and AD genetic mutations (Figure 17).

Furthermore, different levels of Aβ oligomers formation were compared. Similar levels of Aβ oligomers formation was observed in the cortex and hippocampus of the male mice with AD genetic mutations only and male mice with both E2k deficiency and AD genetic mutations. However, a higher level of Aβ oligomers was observed in the cortex and hippocampus of female mice with both E2k deficiency and AD genetic mutations in comparison to female mice with only AD genetic mutations (Figure 18).

File:Figure 2 Summary.jpg
Figure 17: Aβ plaque number (A for males and C for females) and percentage area covered by Aβ plaques (B for males and D for females) in the cortex and the hippocampus of mice with AD genetic mutations (Tg19959) and mice with both E2k deficiency and AD genetic mutations (Tg19959-DLST+/−). Pictures (E) of female brain sections labeled with Aβ peptide bind antibody are shown to highlight Aβ plaque formed (black arrows). [3]
File:Figure 3 Summary.jpg
Figure 18: Aβ oligomer number (A for males and C for females) and percentage area covered by Aβ oligomers (B for males and D for females) in the cortex and the hippocampus of mice with AD genetic mutations (Tg19959) and mice with both E2k deficiency and AD genetic mutations (Tg19959-DLST+/−). Pictures (E) of female brain sections labeled with Aβ peptide bind antibody are shown to highlight Aβ oligomers formed (black arrows). [3]





File:Figure 4 Summary.png
Figure 19: Dot-blots of nitrotyrosine (marker of oxidative stress) in (A) male and (B) female mice. The ratios of nitrotyrosine to tubulin relative to healthy mice was plotted against (A) male and (B) female mice with both E2k deficiency and AD genetic mutations (Tg19959-DLST+/−), healthy mice (WT), mice with E2k deficiency only (DLST+/−) and mice with AD genetic mutations only (Tg19959). Pictures (C) showed female brain sections labeled with nitrotyrosine bind antibody (black arrows), with an amplification of the cortex area. [3]

Oxidative stress levels were found to be similar in the brains of healthy male mice, male mice with AD genetic mutations only, male mice with E2k deficiency only, and male mice with both E2k deficiency and AD genetic mutations. However, a much higher level of oxidative stress was noticed in female mice with both E2k deficiency and AD genetic mutations in comparison to healthy female mice, female mice with AD genetic mutations only and female mice with E2k deficiency only (Figure 19).

Female mice with both E2k deficiency and AD genetic mutations was found to display severe spatial learning and memory retention deficits in comparison to healthy female mice, female mice with AD genetic mutations only and female mice with E2k deficiency only. There was no behavioral difference observed for different groups of male mice.

For most of the tests, only female mice with both E2k deficiency and AD genetic mutations were found to have significant AD-like phenotype appearance (5). Therefore, gender difference may play a role concerning thes effect of E2k deficiency on AD related phenotype.


Other Factors[edit]

Unlike the thiamine deficiency, pH and lactate level are found to have no effect on α-KGDHC activity in AD brain. This conclusion was drawn by Mastrogiacoma et al, as the mean brain pH and lactate levels were not significantly different between the AD and the control groups [1]. Also, the α-KGDHC activity reduction is unlikely to be due to the loss of other mitochondrial enzymes, because the decreased enzyme activity of α-KGDHC in AD cerebral cortex exceeds the reduction in frontal cortical activity of citrate synthase (one of the control mitochondrial enzymes) [1]. Other researchers have reported normal or near-normal activities of two other Krebs cycle enzymes - fumarase [32] and succinate dehydrogenase [33] in AD brain, which supports the specificity of the α-KGDHC change in AD brain.

A detailed neuropathological examination was performed on all AD cases [1]. The density of both neuritic plaques and neurofibrillary tangles (NFTs) were measured in both neocortex and hippocampus, because they are the criteria for the diagnosis of AD in the absence of any other degenerative process. Negative correlation was observed between α-KGDHC activity and NFT count in AD parietal cortex, the brain area showing the greatest reduction in α-KGDHC activity. However, no correlation between neuritic plaques and the enzyme activity was recorded. These results along with the finding that NFTs and not neuritic plaques parallel to the duration and severity of AD [34] suggests that the reduction in α-KGDHC activity is related to the development and/or severity of the disease via a reduced energy metabolism in AD brain [1].


ATP Metabolism[edit]

From a logical standpoint, a deficiency of the α-KGDHC should result in reduced ATP energy stores, which, if severe enough, would lead to neuronal death, since α-KGDHC is a rate-limiting enzyme in the TCA cycle. Furthermore, since ATP is a pyrophosphatase donor in the TPP synthesis process, reduced ATP levels would lower the TPP production, leading to further decrease in α-KGDHC activity and consequently, lowered ATP levels as a positive feedback phenomenon. However, the reduced energy metabolism in AD brain is suggested to be directly caused by a decrease in brain glucose utilization in living patients [35]. Additionally, an increase in CO2 production from glycolysis and only a slight reduction in ATP levels were observed in an in vitro study of biopsied neocortex from AD patients, suggesting a partial uncoupling of oxidation to ATP production [36]. Taken all of this into account, ATP metabolism changes could not be solely attributed to the diminished activity of α-KGDHC.









Conclusion[edit]

α-KGDHC deficiency was linked to the occurrence of AD. This correlation was further enhanced by studying cofactor TPP, subunits E2k, E3 and its involvement in phenotypes of AD. Highest reduction of the α-KGDHC activity was found in seven areas of the AD brain in absence of TPP including four cerebral cortex (frontal, temporal, parietal, occipital) and three subcortical (hippocampus, amygdala and caudate nucleus). This result was explained by stabilization of α-KGDHC through TPP, which further supported by a slower rate of α-KGDHC inactivation in the presence of TPP. Also deficiency in the TPP cofactor was found to induce oxidative stress due to electron transport from E3 subunit of α-KGDHC to the oxygen atom instead of NAD+. This theory was supported by a higher decrease in α-KGDHC activity was observed in regions of the brain vulnerable to thiamine deficiency such as sub-medial thalamic nucleus (SmTN). Together, thiamine deficiency and oxidative stress can be linked to the formation of the Aβ plaques in AD. In addition, decrease in α-KGDHC subunits transcription, especially E2k and E3, was also found in this region. This result was further confirmed by another research study, which shows that E2k deficiency accelerates the onset of AD as a higher level of Aβ plaques, Aβ oligomers, oxidative stress and memory deficiency were observed in the cortex and the hippocampus of the mice with E2k deficiency and AD genetic mutations. The changes in ATP metabolism are not entirely caused by the decrease in α-KGDHC activity. Nonetheless, it does contribute to the reduced levels of ATP production. The decreased ATP levels get involved in a positive feedback with TPP production (lower its production), which in turn further decreases α-KGDHC activity and ATP levels, eventually leading to the neuronal death.

Alzheimer's disease is truly a very complex disorder and although the changes in α-KGDHC activity are not a sole reason for its onset, they do play an inevitable role and should not be disregarded. The results of several studies presented in this wikipedia page suggest the potential routes for the future research in Alzheimer's field. More specifically, knowing that thiamine deficiency is the major cause of decreased metabolic activity of α-KGDHC and possesses regional selectivity in the brain, research should be done on the factors causing this selective deficiency. When the mechanisms of this selective deficiency are fully established, therapeutics could be designed to target these brain areas selectively, decreasing the chances of the onset of this disease. The α-KGDHC could potentially become a biomarker for AD, and detecting any disruptions in its metabolic activity could help identify this disorder more easily, and perhaps even prior to its genesis.

References[edit]

  1. ^ a b c d e f g h i Mastrogiacoma, F., Bergeron, C. and Kish, S.J., (1993) Brain α-ketoglutarate dehydrogenase complex activity in Alzheimer’s disease. J. Neurochem. 61, 2007–2014.
  2. ^ a b c d e f g h i j Shi, Q., Karuppagounder, S. S., Xu, H., Pechman, D., Chen, H., and Gibson, G. E. (2007) Responses of the Mitochondrial Alpha-Ketoglutarate Dehydrogenase Complex to Thiamine Deficiency may Contribute to Regional Selective Vulnerability. Neurochem. Int. 50, 921-931.
  3. ^ a b c d e f Dumont, M. et al. (2009) Mitochondrial dihydrolipoyl succinyltransferase deficiency accelerates amyloid pathology and memory deficit in a transgenic mouse model of amyloid deposition. Free Radical Biology & Medicine 47, 1019–1027.
  4. ^ Obtained on Feb 19, 2011 from http://www.ahaf.org/alzheimers/about/understanding/plaques-and-tangles.html
  5. ^ LaFerla, F.M., Green, K.N. and Oddo,S. (2007) Intracellular amyloid- in Alzheimer's disease. Nature Reviews Neuroscience 8, 499-509).
  6. ^ Lee, D.S. (2009) Age-related differences in in-vitro sensitivity to inhibition of human red blood cell (rbc) acetylcholinesterase (ache) and plasma butyrylcholinesterase (buche) by the cholinesterase (che) inhibitors physostigmine (phys), pyridostigmine (pyr), donepezil (don) and galantamine (gal), pp 1-10, Virginia Commonwealth University, Virginia.
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Category:Alzheimer's disease