Ralf Baumeister

Perception of UV-B by the Arabidopsis UVR8 Protein

A plant ultraviolet-B photoreceptor uses a tryptophan-based chromophore. To optimize their growth and survival, plants perceive and respond to ultraviolet-B (UV-B) radiation. However, neither the molecular identity of the UV-B photoreceptor nor the photoperception mechanism is known. Here we show that dimers of the UVR8 protein perceive UV-B, probably by a tryptophan-based mechanism. Absorption of UV-B induces instant monomerization of the photoreceptor and interaction with COP1, the central regulator of light signaling. Thereby this signaling cascade controlled by UVR8 mediates UV-B photomorphogenic responses securing plant acclimation and thus promotes survival in sunlight.

Direct Inhibition of the Longevity-Promoting Factor SKN-1 by Insulin-like Signaling in C. elegans

Insulin/IGF-1-like signaling (IIS) is central to growth and metabolism and has a conserved role in aging. In C. elegans, reductions in IIS increase stress resistance and longevity, effects that require the IIS-inhibited FOXO protein DAF-16. The C. elegans transcription factor SKN-1 also defends against oxidative stress by mobilizing the conserved phase 2 detoxification response. Here we show that IIS not only opposes DAF-16 but also directly inhibits SKN-1 in parallel. The IIS kinases AKT-1, -2, and SGK-1 phosphorylate SKN-1, and reduced IIS leads to constitutive SKN-1 nuclear accumulation in the intestine and SKN-1 target gene activation. SKN-1 contributes to the increased stress tolerance and longevity resulting from reduced IIS and delays aging when expressed transgenically. Furthermore, SKN-1 that is constitutively active increases life span independently of DAF-16. Our findings indicate that the transcription network regulated by SKN-1 promotes longevity and is an important direct target of IIS.

C. elegans SGK-1 is the critical component in the Akt/PKB kinase complex to control stress response and life span.

The DAF-2 insulin receptor-like signaling pathway controls metabolism, development, longevity, and stress response in C. elegans. Here we show that SGK-1, the C. elegans homolog of the serum- and glucocorticoid-inducible kinase SGK, acts in parallel to the AKT kinases to mediate DAF-2 signaling. Loss of sgk-1 results in defective egg-laying, extended generation time, increased stress resistance, and an extension of life span. SGK-1 forms a protein complex with the AKT kinases, and is activated by and strictly depends on PDK-1. All three kinases of this complex are able to directly phosphorylate DAF-16/FKHRL1, yet have different functions in DAF-2 signaling. Whereas AKT-1 and AKT-2 are more important for regulating dauer formation, SGK-1 is the crucial factor for the control of development, stress response, and longevity. Our data also suggest the existence of a second pathway from DAF-2 to DAF-16 that does not depend on AKT-1, AKT-2, and SGK-1.

A loss of function mutation of presenilin-2 interferes with amyloid beta-peptide production and notch signaling.

Presenilin-1 (PS1) facilitates γ-secretase cleavage of the β-amyloid precursor protein and the intramembraneous cleavage of Notch1. Although Alzheimer’s disease-associated mutations in the homologous presenilin (PS2) gene elevate amyloid β-peptide (Aβ42) production like PS1 mutations, here we demonstrate that a gene ablation of PS2 (unlike that of PS1) in mice does not result in a severe phenotype resembling that of Notch-ablated animals. To investigate the amyloidogenic function of PS2 more directly, we mutagenized a conserved aspartate at position 366 to alanine, because the corresponding residue of PS1 is known to be required for its amyloidogenic function. Cells expressing the PS2 D366A mutation exhibit significant deficits in proteolytic processing of β-amyloid precursor protein indicating a defect in γ-secretase activity. The reduced γ-secretase activity results in the almost complete inhibition of Aβ and p3 production in cells stably expressing PS2 D366A, whereas cells overexpressing the wild-type PS2 cDNA produce robust levels of Aβ and p3. Using highly sensitive in vivo assays, we demonstrate that the PS2 D366A mutation not only blocks γ-secretase activity but also inactivates PS2 activity in Notch signaling by inhibiting the proteolytic release of the cytoplasmic Notch1 domain. These data suggest that PS2 is functionally involved in Aβ production and Notch signaling by facilitating similar proteolytic cleavages.

Glycine 384 is required for presenilin-1 function and is conserved in bacterial polytopic aspartyl proteases.

Endoproteolysis of β-amyloid precursor protein (βAPP) and Notch requires conserved aspartate residues in presenilins 1 and 2 (PS1 and PS2). Although PS1 and PS2 have therefore been proposed to be aspartyl proteases, no homology to other aspartyl proteases has been found. Here we identify homology between the presenilin active site and polytopic aspartyl proteases of bacterial origin, thus supporting the hypothesis that presenilins are novel aspartyl proteases.

Presenilin-1 differentially facilitates endoproteolysis of the β-amyloid precursor protein and Notch

Mutations in the presenilin-1 (PS1) gene are associated with Alzheimer’s disease and cause increased secretion of the neurotoxic amyloid-β peptide (Aβ). Critical intramembraneous aspartates at residues 257 and 385 are required for the function of PS1 protein. Here we investigate the biological function of a naturally occurring PS1 splice variant (PS1 Δexon 8), which lacks the critical aspartate 257. Cell lines that stably express PS1 Δexon 8 or a PS1 protein in which aspartate residue 257 is mutated secrete significant levels of Aβ, whereas Aβ generation is severely reduced in cells transfected with PS1 containing a mutation of aspartate 385. In contrast, endoproteolytic processing of Notch is almost completely inhibited in cell lines expressing any of the PS1 variants that lack one of the critical aspartates. These data indicate that PS1 may differentially facilitate γ-secretase-mediated generation of Aβ and endoproteolysis of Notch.

Caenorhabditits elegans LRK-1 and PINK-1 Act Antagonistically in Stress Response and Neurite Outgrowth

Mutations in two genes encoding the putative kinases LRRK2 and PINK1 have been associated with inherited variants of Parkinson disease. The physiological role of both proteins is not known at present, but studies in model organisms have linked their mutants to distinct aspects of mitochondrial dysfunction, increased vulnerability to oxidative and endoplasmic reticulum stress, and intracellular protein sorting. Here, we show that a mutation in the Caenorhabditits elegans homologue of the PTEN-induced kinase pink-1 gene resulted in reduced mitochondrial cristae length and increased paraquat sensitivity of the nematode. Moreover, the mutants also displayed defects in axonal outgrowth of a pair of canal-associated neurons. We demonstrate that in the absence of lrk-1, the C. elegans homologue of human LRRK2, all phenotypic aspects of pink-1 loss-of-function mutants were suppressed. Conversely, the hypersensitivity of lrk-1 mutant animals to the endoplasmic reticulum stressor tunicamycin was reduced in a pink-1 mutant background. These results provide the first evidence of an antagonistic role of PINK-1 and LRK-1. Due to the similarity of the C. elegans proteins to human LRRK2 and PINK1, we suggest a common role of both factors in cellular functions including stress response and regulation of neurite outgrowth. This study might help to link pink-1/PINK1 and lrk-1/LRRK2 function to the pathological processes resulting from Parkinson disease-related mutants in both genes, the first manifestations of which are cytoskeletal defects in affected neurons.

Caenorhabditis elegans MPP+ Model of Parkinson’s Disease for High-Throughput Drug Screenings

The neurotoxin MPTP and its active metabolite MPP+ cause Parkinson’s disease (PD)-like symptoms in vertebrates by selectively destroying dopaminergic neurons in the substantia nigra. MPTP/MPP+ models have been established in rodents to screen for pharmacologically active compounds. In addition to being costly and time consuming, these animal models are not suitable for large scale testings using compound libraries. We present a novel MPP+-based model for high-throughput screenings using the nematode Caenorhabditis elegans. Incubation of C. elegans with MPTP or its active metabolite MPP+ resulted in strong symptomatic defects including reduced mobility and increased lethality, and is correlated with a specific degeneration of the dopaminergic neurons. The phenotypic consequences of MPTP/MPP+ treatments were recorded using automated hardware and software for quantification. Incubation of C. elegans with a variety of pharmacologically active components used in PD treatment reduced the MPP+-induced defects. Our data suggest that the C. elegans MPTP/MPP+ model can be used for the quantitative evaluation of anti-PD drugs.

Regulation of the myosin-directed chaperone UNC-45 by a novel E3/E4-multiubiquitylation complex in C. elegans

The organization of the motor protein myosin into motile cellular structures requires precise temporal and spatial control. Caenorhabditis elegans UNC-45 facilitates this by functioning both as a chaperone and as a Hsp90 cochaperone for myosin during thick filament assembly. Consequently, mutations in C. elegans unc-45 result in paralyzed animals with severe myofibril disorganization in striated body wall muscles. Here, we report a new E3/E4 complex, formed by CHN-1, the C. elegans ortholog of CHIP (carboxyl terminus of Hsc70-interacting protein), and UFD-2, an enzyme known to have ubiquitin conjugating E4 activity in yeast, as necessary and sufficient to multiubiquitylate UNC-45 in vitro. The phenotype of unc-45 temperature-sensitive animals is partially suppressed by chn-1 loss of function, while UNC-45 overexpression in worms deficient for chn-1 results in severely disorganized muscle cells. These results identify CHN-1 and UFD-2 as a functional E3/E4 complex and UNC-45 as its physiologically relevant substrate.

Bile proteomic profiles differentiate cholangiocarcinoma from primary sclerosing cholangitis and choledocholithiasis

Early detection of malignant biliary tract diseases, especially cholangiocarcinoma (CC) in patients with primary sclerosing cholangitis (PSC), is very difficult and often comes too late to give the patient a therapeutic benefit. We hypothesize that bile proteomic analysis distinguishes CC from nonmalignant lesions. We used capillary electrophoresis mass spectrometry (CE‐MS) to identify disease‐specific peptide patterns in patients with choledocholithiasis (n = 16), PSC (n = 18), and CC (n = 16) in a training set. A model for differentiation of choledocholithiasis from PSC and CC (PSC/CC model) and another model distinguishing CC from PSC (CC model) were subsequently validated in independent cohorts (choledocholithiasis [n = 14], PSC [n = 18] and CC [n = 25]). Peptides were characterized by sequencing. Application of the PSC/CC model in the independent test cohort resulted in correct exclusion of 12/14 bile samples from patients with choledocholithiasis and identification of 40/43 patients with PSC or CC (86% specificity, 93% sensitivity). The corresponding receiver operating characteristic (ROC) analysis revealed an area under the curve (AUC) of 0.93 (95% confidence interval [CI]: 0.82‐0.98, P = 0.0001). The CC model succeeded in an accurate detection of 14/18 bile samples from patients with PSC and 21/25 samples with CC (78% specificity, 84% sensitivity) in the independent cohort, resulting in an AUC value of 0.87 (95% CI: 0.73‐0.95, P = 0.0001) in ROC analysis. Eight out of 10 samples of patients with CC complicating PSC were identified. Conclusion: Bile proteomic analysis discriminates benign conditions from CC accurately. This method may become a diagnostic tool in future as it offers a new possibility to diagnose malignant bile duct disease and thus enables efficient therapy particularly in patients with PSC. (HEPATOLOGY 2010;)