Tau Clearance Mechanisms and Their Possible Role in the Pathogenesis of Alzheimer Disease

One of the defining pathological features of Alzheimer disease (AD) is the intraneuronal accumulation of tau. The tau that forms these accumulations is altered both posttranslationally and conformationally, and there is now significant evidence that soluble forms of these modified tau species are the toxic entities rather than the insoluble neurofibrillary tangles. However there is still noteworthy debate concerning which specific pathological forms of tau are the contributors to neuronal dysfunction and death in AD. Given that increases in aberrant forms of tau play a role in the neurodegeneration process in AD, there is growing interest in understanding the degradative pathways that remove tau from the cell, and the selectivity of these different pathways for various forms of tau. Indeed, one can speculate that deficits in a pathway that selectively removes certain pathological forms of tau could play a pivotal role in AD. In this review we will discuss the different proteolytic and degradative machineries that may be involved in removing tau from the cell. How deficits in these different degradative pathways may contribute to abnormal accumulation of tau in AD will also be considered. In addition, the issue of the selective targeting of specific tau species to a given degradative pathway for clearance from the cell will be addressed

D-β-Hydroxybutyrate Is Protective in Mouse Models of Huntington's Disease

Abnormalities in mitochondrial function and epigenetic regulation are thought to be instrumental in Huntington's disease (HD), a fatal genetic disorder caused by an expanded polyglutamine track in the protein huntingtin. Given the lack of effective therapies for HD, we sought to assess the neuroprotective properties of the mitochondrial energizing ketone body, D-β-hydroxybutyrate (DβHB), in the 3-nitropropionic acid (3-NP) toxic and the R6/2 genetic model of HD. In mice treated with 3-NP, a complex II inhibitor, infusion of DβHB attenuates motor deficits, striatal lesions, and microgliosis in this model of toxin induced-striatal neurodegeneration. In transgenic R6/2 mice, infusion of DβHB extends life span, attenuates motor deficits, and prevents striatal histone deacetylation. In PC12 cells with inducible expression of mutant huntingtin protein, we further demonstrate that DβHB prevents histone deacetylation via a mechanism independent of its mitochondrial effects and independent of histone deacetylase inhibition. These pre-clinical findings suggest that by simultaneously targeting the mitochondrial and the epigenetic abnormalities associated with mutant huntingtin, DβHB may be a valuable therapeutic agent for HD

Selectively Enhancing the Clearance of Pathological Tau Through the Use of Phytochemical Compounds

Thesis (Ph.D.)--University of Rochester. School of Medicine & Dentistry. Dept. of Program in Neuroscience, 2015.Alzheimer disease (AD) is the leading cause of neurodegeneration worldwide. Unfortunately, there are limited therapeutic options, due in part to an incomplete understanding of disease pathogenesis. One pathological hallmark of AD is the intracellular accumulation of hyperphosphorylated and truncated forms of the microtubule associated protein tau. These pathological forms of tau cannot appropriately interact with microtubules and also cause neuronal damage. Impaired protein degradation is likely a factor in tau accumulation in AD. Therefore, interventions that selectively facilitate the clearance of pathological tau are of significant interest. Evidence suggests some modified forms of tau are preferentially cleared via the autophagy pathway. One aim of this project was to further clarify how different forms of tau are cleared through the use of novel stable cell lines expressing one of several known AD-relevant modifications: either truncation at aspartate 421 or phosphorylation at threonine 231 or serine 262. An additional aim was to investigate potential ways to enhance the clearance of phosphorylated tau. Phytochemicals are bioactive compounds found in common foods like green tea and cruciferous vegetables. Because of their ready availability and low toxicity profile, phytochemicals are attractive to explore as novel therapeutics. Several phytochemicals are able to activate the protective Nrf2 pathway, and these Nrf2 activators have shown promise in mitigating amyloid pathology in vitro and in vivo. A recent study showed the isothiocyanate sulforaphane (SFN) can reduce phosphorylated tau in an autophagy-dependent manner. The flavonoid epigallocatechin-3-gallate (EGCG) has shown anti-amyloidogenic properties, but its effect on tau pathology has not been assessed. This study demonstrated that EGCG can enhance the clearance of phosphorylated tau species in a primary neuron culture model. Interestingly, the protein clearance effect appeared to be independent of the Nrf2 pathway, as EGCG had limited Nrf2 activator activity in primary neurons. Additionally, EGCG did not enhance autophagy as a general process, but did significantly increase mRNA expression of the key autophagy adaptor proteins NDP52 and p62. Taken together, these results demonstrate EGCG has the ability to clear phosphorylated tau species in a highly specific manner, likely through increasing in adaptor protein expression

Metabolic pathways of DβHB in mitochondria.

<p>The metabolism of DβHB in mitochondria is stereospecific to the D isoform. Under normal physiological conditions, the level of DβHB is low but becomes dramatically elevated during starvation, increased fatty acid metabolism or pathological conditions such as diabetes. From its site of production in the liver, DβHB is released into the blood and circulated for utilization by other tissues. In general, the rate of ketone body usage in the brain is proportional to the concentration in the circulation <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0024620#pone.0024620-Sokoloff1" target="_blank">[59]</a>. Circulating DβHB readily crosses the blood-brain barrier and enters brain mitochondria where it is metabolized by mitochondrial β-hydroxybutyrate dehydrogenase to acetoacetate, which is subsequently converted to acetyl-coenzyme A to feed into the tricarboxylic acid cycle (TCA) cycle. The intermediate products generated from this cycle, NADH and succinate, in turn feed into the electron transport chain to subsequently generate ATP at complex V. Through this metabolic pathway, DβHB is an excellent alternative source of energy in the brain when glycolysis is not operative or when glucose supply is depleted such as during starvation <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0024620#pone.0024620-Sokoloff1" target="_blank">[59]</a>.</p

DβHB attenuates striatal lesions induced by 3-NP.

<p>Male C57Bl/6 mice (∼21 weeks-old) was infused subcutaneously (1 µl/h) with either DβHB (1.6 mmol/kg in saline) or vehicle (saline) using implanted osmotic minipumps. One day after surgery, each animal group was further subdivided into two groups to receive nine i.p. injections of either saline or 3-NP (50 mg/kg) at 12 h intervals. Five hours after the last 3-NP injection, animals were intracardially perfused with 4% paraformaldehyde. As compared to control (<b>A</b>), 3-NP induced significant cell loss (pale region) as seen here with Nissl stain (<b>B</b>). This lesion was not detectable in the group that received DβHB infusion (<b>C</b>). DβHB by itself did not affect the baseline level (not shown) of Nissl staining. Immunoreactivity for MAC-1/CD11b was observed after 3-NP treatment (<b>E</b>, <b>H</b>) in the lesioned area (outlined) as compared to the saline control group (<b>D</b>, <b>G</b>). Although 3-NP also induced discrete regions of increased microglia reaction in some animals that received DβHB (<b>F</b>, <b>I</b>), the immunoreactivity was not as dramatic as compared to the 3-NP treated group (<b>E</b>, <b>H</b>). <b>Panel J</b> summarizes the death rate and numbers of animals with striatal lesions induced by 3-NP in the groups that received either saline or DβHB infusion. <b>Panel K</b> demonstrates comparable magnitude of complex II inhibition in the two 3-NP treated groups, indicating that DβHB did not interfere with the function of complex II <i>per se</i>.</p

DβHB extends life expectancy and stabilizes glucose levels of Tg-R6/2 mice.

<p>Animals from <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0024620#pone-0024620-g004" target="_blank">Figure 4</a> were monitored twice daily for their survival rate (<b>A</b>). These mice were considered to reach their end stage if they were unable to right themselves after being placed on their back or died overnight. DβHB significantly extended the survival time in Tg-R6/2 mice. <i>n</i> = 12 (Ntg-saline), <i>n</i> = 7 (Tg-saline), <i>n</i> = 7 (Tg-DβHB). <i>p</i><0.001 Kaplan-Meier analysis between Tg-sal and Tg- DβHB groups. These animals were also assessed weekly for glucose levels after six hours of fasting (<b>B</b>). One animal from the Tg-saline group survived up to 14 weeks and exhibited an elevated glucose level.</p

DβHB and LβHB prevent histone deacetylation induced by mhtt.

<p>(<b>Panel A</b>): Dose-response study of DβHB and sodium butyrate (0.25 mM-4 mM) in PC12 cells with inducible expression of Htt^Q103 upon addition of tebufenozide (1 µM) using immunoblotting. Two microgram of histones extracted from Hela cells treated with 5 mM sodium butyrate (obtained from Upstate Biotechnology Inc.) was used as a positive control (“+C”). Levels of histone acetylation were quantified in cells with mutant (<b>Panel B</b>, Htt^Q103) or normal (<b>Panel C</b>, Htt^Q25) huntingtin 48 h after induction in the absence (“0”) or presence of equimolar concentrations (4 mM) of DβHB (“D”), LβHB (“L”) or sodium butyrate (“Na”) and compared with the non-tebufenozide treated control group (“C”). Data represent <i>n</i> = 6–10 per group (pooled from three separate experiments). *<i>p</i><0.01 as compared to the control group (“C”) without tebufenozide treatment.</p

DβHB and LβHB do not inhibit HDAC activities.

<p>Hela nuclear extract was used as a source of class I and II HDAC activity (<b>Panel A</b>) where as recombinant human Sir1 was used as a source of class III HDAC activity (<b>Panel B</b>). HDAC activities were assessed in the absence (control, “C”) or presence of 4 mM DβHB (“D”), LβHB (“L”) or sodium butyrate (“Na”, an HDAC I and II inhibitor), 1 µM trichostatin A (“TA”, an HDAC I and II inhibitor) or 1 mM nicotinamide (“Nic”, an HDAC III inhibitor). Activities were quantified based on fluorescence intensity and expressed as arbitrary fluorescence unit (AFU). <i>n</i> = 6–16 per group from 5 independent experiments.</p