Fred W. van Leeuwen

Dot1p Modulates Silencing in Yeast by Methylation of the Nucleosome Core

DOT1 was originally identified as a gene affecting telomeric silencing in S. cerevisiae. We now find that Dot1p methylates histone H3 on lysine 79, which maps to the top and bottom of the nucleosome core. Methylation occurs only when histone H3 is assembled in chromatin. In vivo, Dot1p is solely responsible for this methylation and methylates approximately 90% of histone H3. In dot1delta cells, silencing is compromised and silencing proteins become redistributed at the expense of normally silenced loci. We suggest that methylation of histone H3 lysine 79 limits silencing to discrete loci by preventing the binding of Sir proteins elsewhere along the genome. Because Dot1p and histone H3 are conserved, similar mechanisms are likely at work in other eukaryotes.

Vasopressin-immunoreactive cell bodies in the bed nucleus of the stria terminalis of the rat

SummaryIn the dorsal and ventral portions of the bed nucleus of the stria terminalis of the rat numerous cell bodies immunoreactive for vasopressin and neurophysin II were found after colchicin pretreatment. These cells are predominantly multipolar but sometimes also bipolar, and have a width and length of approximately 9 and 16 μm, respectively. In the homozygous Brattleboro rat, which is deficient in vasopressin, no immunoreactive vasopressin was found in these cells. Following incubation with anti-oxytocin and anti-bovine neurophysin I, only magnocellular immunoreactive cell bodies were found in the septal region. The consequences of these results concerning the vasopressin fiber pathways in the brain are discussed.

Mutant ubiquitin found in neurodegenerative disorders is a ubiquitin fusion degradation substrate that blocks proteasomal degradation

Loss of neurons in neurodegenerative diseases is usually preceded by the accumulation of protein deposits that contain components of the ubiquitin/proteasome system. Affected neurons in Alzheimers disease often accumulate UBB+1, a mutant ubiquitin carrying a 19–amino acid C-terminal extension generated by a transcriptional dinucleotide deletion. Here we show that UBB+1 is a potent inhibitor of ubiquitin-dependent proteolysis in neuronal cells, and that this inhibitory activity correlates with induction of cell cycle arrest. Surprisingly, UBB+1 is recognized as a ubiquitin fusion degradation (UFD) proteasome substrate and ubiquitinated at Lys29 and Lys48. Full blockade of proteolysis requires both ubiquitination sites. Moreover, the inhibitory effect was enhanced by the introduction of multiple UFD signals. Our findings suggest that the inhibitory activity of UBB+1 may be an important determinant of neurotoxicity and contribute to an environment that favors the accumulation of misfolded proteins.

Antigenic variation in malaria

Like most protozoan infections malaria is usually chronic. Even in the absence of persistent exoerythrocytic infection parasites may remain in the blood for months after antiplasmodial antibody is detectable in the serum.

β-d-glucosyl-hydroxymethyluracil: A novel modified base present in the DNA of the parasitic protozoan T. brucei

We have previously shown that the DNA of the unicellular eukaryote T. brucei contains about 0.1% of a novel modified base, called J. The presence of J correlates with a DNA modification associated with the silencing of telomeric expression sites for the variant surface antigens of trypanosomes. Here we show that J is 5-((beta-D-glucopyranosyloxy)-methyl)-uracil (shortened to beta-D-glucosyl-hydroxymethyluracil), a base not previously found in DNA. We discuss putative pathways for the introduction of this base modification at specific positions in the DNA and the possible contribution of this modification to repression of surface antigen gene expression.

Disease-specific accumulation of mutant ubiquitin as a marker for proteasomal dysfunction in the brain

Molecular misreading of the ubiquitin‐B (UBB) gene results in a dinucleotide deletion in UBB mRNA. The resulting mutant protein, UBB+1, accumulates in the neuropathological hallmarks of Alzheimer disease. In vitro, UBB+1 inhibits proteasomal proteolysis, although it is also an ubiquitin fusion degradation substrate for the proteasome. Using the ligase chain reaction to detect dinucleotide deletions, we report here that UBB+1 transcripts are present in each neurodegenerative disease studied (tauo‐ and synucleinopathies) and even in control brain samples. In contrast to UBB+1 transcripts, UBB+1 protein accumulation in the ubiquitin‐containing neuropathological hallmarks is restricted to the tauopathies such as Pick disease, frontotemporal dementia, progressive supranuclear palsy, and argyrophilic grain disease. Remarkably, UBB+1 protein is not detected in the major forms of synucleinopathies (Lewy body disease and multiple system atrophy). The neurologically intact brain can cope with UBB+1 as lentivirally delivered UBB+1 protein is rapidly degraded in rat hippocampus, whereas the K29,48R mutant of UBB+1, which is not ubiquitinated, is abundantly expressed. The finding that UBB+1 protein only accumulates in tauopathies thus implies that the ubiquitin‐proteasome system is impaired specifically in this group of neurodegenerative diseases and not in synucleinopathies and that the presence of UBB+1 protein reports proteasomal dysfunction in the brain.—Fischer, D. F., de Vos, R. A. I., van Dijk, R., de Vrij, F. M. S., Proper, E. A., Sonnemans, M. A. F., Verhage, M. C., Sluijs, J. A., Hobo, B., Zouambia, M., Jansen Steur, E. N. H., Kamphorst, W., Hol, E. M., van Leeuwen, F. W. Disease‐specific accumulation of mutant ubiquitin as a marker for proteasomal dysfunction in the brain. FASEB J. 17, 2014–2024 (2003)

Localized H3K36 methylation states define histone H4K16 acetylation during transcriptional elongation in Drosophila.

Post‐translational modifications of histones are involved in transcript initiation and elongation. Methylation of lysine 36 of histone H3 (H3K36me) resides promoter distal at transcribed regions in Saccharomyces cerevisiae and is thought to prevent spurious initiation through recruitment of histone‐deacetylase activity. Here, we report surprising complexity in distribution, regulation and readout of H3K36me in Drosophila involving two histone methyltransferases (HMTases). Dimethylation of H3K36 peaks adjacent to promoters and requires dMes‐4, whereas trimethylation accumulates toward the 3′ end of genes and relies on dHypb. Reduction of H3K36me3 is lethal in Drosophila larvae and leads to elevated levels of acetylation, specifically at lysine 16 of histone H4 (H4K16ac). In contrast, reduction of both di‐ and trimethylation decreases lysine 16 acetylation. Thus di‐ and trimethylation of H3K36 have opposite effects on H4K16 acetylation, which we propose enable dynamic changes in chromatin compaction during transcript elongation.

Adult human subventricular, subgranular, and subpial zones contain astrocytes with a specialized intermediate filament cytoskeleton.

Human glial fibrillary acidic protein‐δ (GFAP‐δ) is a GFAP protein isoform that is encoded by an alternative splice variant of the GFAP‐gene. As a result, GFAP‐δ protein differs from the predominant splice form, GFAP‐α, by its C‐terminal protein sequence. In this study, we show that GFAP‐δ protein is not expressed by all GFAP‐expressing astrocytes but specifically by a subpopulation located in the subpial zone of the cerebral cortex, the subgranular zone of the hippocampus, and, most intensely, by a ribbon of astrocytes following the ependymal layer of the cerebral ventricles. Therefore, at least in the sub ventricular zone (SVZ), GFAP‐δ specifically marks the population of astrocytes that contain the neural stem cells in the adult human brain. Interestingly, the SVZ astrocytes actively splice GFAP‐δ transcripts, in contrast to astrocytes adjacent to this layer. Furthermore, we show that GFAP‐δ protein, unlike GFAP‐α, is not upregulated in astrogliosis. Our data therefore indicate a different functional role for GFAP‐δ in astrocyte physiology. Finally, transfection studies showed that GFAP‐δ protein expression has a negative effect on GFAP filament formation, and therefore could be important for modulating intermediate filament cytoskeletal properties, possibly facilitating astrocyte motility. Further studies on GFAP‐δ and the cells that express it are important for gaining insights into its function during differentiation, migration and during health and disease.

Mutant ubiquitin expressed in Alzheimer’s disease causes neuronal death1

Ubiquitin‐B+1 (UBB+1) is a mutant ubiquitin that accumulates in the neurones of patients with Alzheimers disease (AD). Here we report on the biochemical and functional differences between ubiquitin and UBB+1 and the effect of the mutant protein on neuronal cells. UBB+1 lacks the capacity to ubiquiti‐nate, and although it is ubiquitinated itself, UBB+1 is not degraded by the ubiquitin‐proteasomal system and is quite stable in neuronal cells. Overexpression of UBB+1 in neuroblastoma cells significantly induces nuclear fragmentation and cell death. Our results demonstrate that accumulation of UBB+1 in neurones is detrimental and may contribute to neuronal dysfunction in AD patients.—de Vrij, F. M. S., Sluijs, J. A., Gregori, L., Fischer, D. F., Hermens, W. T. J. M. C., Goldgaber, D., Verhaagen, J., van Leeuwen, F. W., Hol, E. M. Mutant ubiquitin expressed in Alzheimers disease causes neuronal death. FASEB J. 15, 2680–2688 (2001)

Nonprocessive methylation by Dot1 leads to functional redundancy of histone H3K79 methylation states

Whereas mono-, di- and trimethylation states of lysines on histones typically have specific functions, no specific functions have been attributed so far to the different methylation states of histone H3 Lysine 79 (H3K79) generated by Dot1. Here we show that Dot1, in contrast to other known histone methyltransferases, introduces multiple methyl groups via a nonprocessive mechanism. The kinetic mechanism implies that the H3K79 methylation states cannot be generated independently, suggesting functional redundancy. Indeed, gene silencing in yeast, which is dependent on Dot1, relied on global H3K79 methylation levels and not on one specific methylation state. Furthermore, our findings suggest that histone H2B ubiquitination affects H3K79 trimethylation by enhancing synthesis of all H3K79 methylation states. Our results suggest that multiple methylation of H3K79 leads to a binary code, which is expected to limit the possibilities for regulation by putative demethylases or binding proteins.