Jürgen Hench

Brain homogenates from human tauopathies induce tau inclusions in mouse brain.

Filamentous inclusions made of hyperphosphorylated tau are characteristic of numerous human neurodegenerative diseases, including Alzheimer’s disease, tangle-only dementia, Pick disease, argyrophilic grain disease (AGD), progressive supranuclear palsy, and corticobasal degeneration. In Alzheimer’s disease and AGD, it has been shown that filamentous tau appears to spread in a stereotypic manner as the disease progresses. We previously demonstrated that the injection of brain extracts from human mutant P301S tau-expressing transgenic mice into the brains of mice transgenic for wild-type human tau (line ALZ17) resulted in the assembly of wild-type human tau into filaments and the spreading of tau inclusions from the injection sites to anatomically connected brain regions. Here we injected brain extracts from humans who had died with various tauopathies into the hippocampus and cerebral cortex of ALZ17 mice. Argyrophilic tau inclusions formed in all cases and following the injection of the corresponding brain extracts, we recapitulated the hallmark lesions of AGD, PSP and CBD. Similar inclusions also formed after intracerebral injection of brain homogenates from human tauopathies into nontransgenic mice. Moreover, the induced formation of tau aggregates could be propagated between mouse brains. These findings suggest that once tau aggregates have formed in discrete brain areas, they become self-propagating and spread in a prion-like manner.

IDH/MGMT-driven molecular classification of low-grade glioma is a strong predictor for long-term survival

BACKGROUND Low-grade gliomas (LGGs) are rare brain neoplasms, with survival spanning up to a few decades. Thus, accurate evaluations on how biomarkers impact survival among patients with LGG require long-term studies on samples prospectively collected over a long period. METHODS The 210 adult LGGs collected in our databank were screened for IDH1 and IDH2 mutations (IDHmut), MGMT gene promoter methylation (MGMTmet), 1p/19q loss of heterozygosity (1p19qloh), and nuclear TP53 immunopositivity (TP53pos). Multivariate survival analyses with multiple imputation of missing data were performed using either histopathology or molecular markers. Both models were compared using Akaikes information criterion (AIC). The molecular model was reduced by stepwise model selection to filter out the most critical predictors. A third model was generated to assess for various marker combinations. RESULTS Molecular parameters were better survival predictors than histology (ΔAIC = 12.5, P< .001). Forty-five percent of studied patients died. MGMTmet was positively associated with IDHmut (P< .001). In the molecular model with marker combinations, IDHmut/MGMTmet combined status had a favorable impact on overall survival, compared with IDHwt (hazard ratio [HR] = 0.33, P< .01), and even more so the triple combination, IDHmut/MGMTmet/1p19qloh (HR = 0.18, P< .001). Furthermore, IDHmut/MGMTmet/TP53pos triple combination was a significant risk factor for malignant transformation (HR = 2.75, P< .05). CONCLUSION By integrating networks of activated molecular glioma pathways, the model based on genotype better predicts prognosis than histology and, therefore, provides a more reliable tool for standardizing future treatment strategies.

Invited review: Prion-like transmission and spreading of tau pathology.

Filaments made of hyperphosphorylated tau protein are encountered in a number of neurodegenerative diseases referred to as ‘tauopathies’. In the most prevalent tauopathy, Alzheimers disease, tau pathology progresses in a stereotypical manner with the first lesions appearing in the locus coeruleus and the entorhinal cortex from where they appear to spread to the hippocampus and neocortex. Propagation of tau pathology is also characteristic of argyrophilic grain disease, where the tau lesions appear to spread throughout distinct regions of the limbic system. These findings strongly implicate neurone‐to‐neurone propagation of tau aggregates. Isoform composition and morphology of tau filaments can differ between tauopathies suggesting the existence of conformationally diverse tau strains. Altogether, this points to prion‐like mechanisms in the pathogenesis of tauopathies.

Mitochondrial DNA level, but not active replicase, is essential for Caenorhabditis elegans development.

A number of studies showed that the development and the lifespan of Caenorhabditis elegans is dependent on mitochondrial function. In this study, we addressed the role of mitochondrial DNA levels and mtDNA maintenance in development of C. elegans by analyzing deletion mutants for mitochondrial polymerase gamma (polg-1(ok1548)). Surprisingly, even though previous studies in other model organisms showed necessity of polymerase gamma for embryonic development, homozygous polg-1(ok1548) mutants had normal development and reached adulthood without any morphological defects. However, polg-1 deficient animals have a seriously compromised gonadal function as a result of severe mitochondrial depletion, leading to sterility and shortened lifespan. Our results indicate that the gonad is the primary site of mtDNA replication, whilst the mtDNA of adult somatic tissues mainly stems from the developing embryo. Furthermore, we show that the mtDNA copy number shows great plasticity as it can be almost tripled as a response to the environmental stimuli. Finally, we show that the mtDNA copy number is an essential limiting factor for the worm development and therefore, a number of mechanisms set to maintain mtDNA levels exist, ensuring a normal development of C. elegans even in the absence of the mitochondrial replicase.

The small GTPase Arf1 modulates mitochondrial morphology and function

The small GTPase Arf1 plays critical roles in membrane traffic by initiating the recruitment of coat proteins and by modulating the activity of lipid‐modifying enzymes. Here, we report an unexpected but evolutionarily conserved role for Arf1 and the ArfGEF GBF1 at mitochondria. Loss of function of ARF‐1 or GBF‐1 impaired mitochondrial morphology and activity in Caenorhabditis elegans. Similarly, mitochondrial defects were observed in mammalian and yeast cells. In Saccharomyces cerevisiae, aberrant clusters of the mitofusin Fzo1 accumulated in arf1‐11 mutants and were resolved by overexpression of Cdc48, an AAA‐ATPase involved in ER and mitochondria‐associated degradation processes. Yeast Arf1 co‐fractionated with ER and mitochondrial membranes and interacted genetically with the contact site component Gem1. Furthermore, similar mitochondrial abnormalities resulted from knockdown of either GBF‐1 or contact site components in worms, suggesting that the role of Arf1 in mitochondrial functioning is linked to ER–mitochondrial contacts. Thus, Arf1 is involved in mitochondrial homeostasis and dynamics, independent of its role in vesicular traffic.

Impaired complex IV activity in response to loss of LRPPRC function can be compensated by mitochondrial hyperfusion

Significance Mitochondria, the powerhouses of the cell, constantly change their shape by fusing and dividing. How these two opposite processes are controlled remains unclear. In our study, we identified the Caenorhabditis elegans homolog of the human mitochondrial protein LRPPRC (leucine-rich pentatricopeptide repeat containing), which has been previously associated with the neurodegenerative French Canadian Leigh Syndrome. Analysis of this protein revealed an evolutionary conserved pathway that regulates mitochondrial shape. Specifically, we show that mitochondria transiently form a highly connected network to compensate for a decrease of the activity of the complex IV of the electron transport chain. Mitochondrial morphology changes in response to various stimuli but the significance of this is unclear. In a screen for mutants with abnormal mitochondrial morphology, we identified MMA-1, the Caenorhabditis elegans homolog of the French Canadian Leigh Syndrome protein LRPPRC (leucine-rich pentatricopeptide repeat containing). We demonstrate that reducing mma-1 or LRPPRC function causes mitochondrial hyperfusion. Reducing mma-1/LRPPRC function also decreases the activity of complex IV of the electron transport chain, however without affecting cellular ATP levels. Preventing mitochondrial hyperfusion in mma-1 animals causes larval arrest and embryonic lethality. Furthermore, prolonged LRPPRC knock-down in mammalian cells leads to mitochondrial fragmentation and decreased levels of ATP. These findings indicate that in a mma-1/LRPPRC–deficient background, hyperfusion allows mitochondria to maintain their functions despite a reduction in complex IV activity. Our data reveal an evolutionary conserved mechanism that is triggered by reduced complex IV function and that induces mitochondrial hyperfusion to transiently compensate for a drop in the activity of the electron transport chain.

Extending two-dimensional histology into the third dimension through conventional micro computed tomography.

Histological examination achieves sub-micrometer resolution laterally. In the third dimension, however, resolution is limited to section thickness. In addition, histological sectioning and mounting sections on glass slides introduce tissue-dependent stress and strain. In contrast, state-of-the-art hard X-ray micro computed tomography (μCT) systems provide isotropic sub-micrometer resolution and avoid sectioning artefacts. The drawback of μCT in the absorption contrast mode for visualising physically soft tissue is a low attenuation difference between anatomical features. In this communication, we demonstrate that formalin-fixed paraffin-embedded human cerebellum yields appropriate absorption contrast in laboratory-based μCT data, comparable to conventional histological sections. Purkinje cells, for example, are readily visible. In order to investigate the pros and cons of complementary approaches, two- and three-dimensional data were manually and automatically registered. The joint histogram of histology and the related μCT slice allows for a detailed discussion on how to integrate two-dimensional information from histology into a three-dimensional tomography dataset. This methodology is not only rewarding for the analysis of the human cerebellum, but it also has relevance for investigations of tissue biopsies and post-mortem applications. Our data indicate that laboratory-based μCT as a modality can fill the gap between synchrotron radiation-based μCT and histology for a variety of tissues. As the information from haematoxylin and eosin (H&E) stained sections and μCT data is related, one can colourise local X-ray absorption values according to the H&E stain. Hence, μCT data can correlate and virtually extend two-dimensional (2D) histology data into the third dimension.

Synaptic dysfunction, memory deficits and hippocampal atrophy due to ablation of mitochondrial fission in adult forebrain neurons

Well-balanced mitochondrial fission and fusion processes are essential for nervous system development. Loss of function of the main mitochondrial fission mediator, dynamin-related protein 1 (Drp1), is lethal early during embryonic development or around birth, but the role of mitochondrial fission in adult neurons remains unclear. Here we show that inducible Drp1 ablation in neurons of the adult mouse forebrain results in progressive, neuronal subtype-specific alterations of mitochondrial morphology in the hippocampus that are marginally responsive to antioxidant treatment. Furthermore, DRP1 loss affects synaptic transmission and memory function. Although these changes culminate in hippocampal atrophy, they are not sufficient to cause neuronal cell death within 10 weeks of genetic Drp1 ablation. Collectively, our in vivo observations clarify the role of mitochondrial fission in neurons, demonstrating that Drp1 ablation in adult forebrain neurons compromises critical neuronal functions without causing overt neurodegeneration.

Tomographic brain imaging with nucleolar detail and automatic cell counting

Brain tissue evaluation is essential for gaining in-depth insight into its diseases and disorders. Imaging the human brain in three dimensions has always been a challenge on the cell level. In vivo methods lack spatial resolution, and optical microscopy has a limited penetration depth. Herein, we show that hard X-ray phase tomography can visualise a volume of up to 43 mm3 of human post mortem or biopsy brain samples, by demonstrating the method on the cerebellum. We automatically identified 5,000 Purkinje cells with an error of less than 5% at their layer and determined the local surface density to 165 cells per mm2 on average. Moreover, we highlight that three-dimensional data allows for the segmentation of sub-cellular structures, including dendritic tree and Purkinje cell nucleoli, without dedicated staining. The method suggests that automatic cell feature quantification of human tissues is feasible in phase tomograms obtained with isotropic resolution in a label-free manner.

Succinate Dehydrogenase Upregulation Destabilize Complex I and Limits the Lifespan of gas-1 Mutant

Many Caenorhabditis elegans mutants with dysfunctional mitochondrial electron transport chain are surprisingly long lived. Both short-lived (gas-1(fc21)) and long-lived (nuo-6(qm200)) mutants of mitochondrial complex I have been identified. However, it is not clear what are the pathways determining the difference in longevity. We show that even in a short-lived gas-1(fc21) mutant, many longevity assurance pathways, shown to be important for lifespan prolongation in long-lived mutants, are active. Beside similar dependence on alternative metabolic pathways, short-lived gas-1(fc21) mutants and long-lived nuo-6(qm200) mutants also activate hypoxia-inducible factor –1α (HIF-1α) stress pathway and mitochondrial unfolded protein response (UPRmt). The major difference that we detected between mutants of different longevity, is in the massive loss of complex I accompanied by upregulation of complex II levels, only in short-lived, gas-1(fc21) mutant. We show that high levels of complex II negatively regulate longevity in gas-1(fc21) mutant by decreasing the stability of complex I. Furthermore, our results demonstrate that increase in complex I stability, improves mitochondrial function and decreases mitochondrial stress, putting it inside a “window” of mitochondrial dysfunction that allows lifespan prolongation.