Beat W. Schäfer

The S100 family of EF-hand calcium-binding proteins: functions and pathology

Calcium lons as second messengers control many biological processes, at least in part, via interaction with a large number of Ca(2+)-binding proteins. One class of these proteins shares a common Ca(2+)-binding motif, the EF-hand, Here, we describe some functional aspects of EF-hand proteins, which have been found recently in different cellular compartments. Novel links between EF-hand proteins, particularly S100 proteins, and specific diseases are now emerging.

S100 proteins: structure, functions and pathology.

S100 proteins regulate intracellular processes such as cell growth and motility, cell cycle regulation, transcription and differentiation. Twenty members have been identified so far, and altogether, S100 proteins represent the largest subgroup in the EF-hand Ca2+ -binding protein family. A unique feature of these proteins is that individual members are localized in specific cellular compartments from which some are able to relocate upon Ca2+ activation, transducing the Ca2+ signal in a temporal and spacial manner by interacting with different targets specific for each S100 protein. Some members are even secreted from cells exerting extracellular, cytokine-like activities partially via the surface receptor RAGE (receptor for advanced glycation endproducts) with paracrine effects e.g. on neurons, promoting their survival during development or after injury. Another important aspect is that 14 bona fide S100 genes are found in a gene cluster on human chromosome 1q21 where a number of chromosomal abnormalities occur. This results in a deregulated expression of some S100 genes associated with neoplasias. Recently, S100 proteins have received increasing attention due to their close association with several human diseases including cardiomyopathy, neurodegenerative disorders and cancer. They have also been proven to be valuable in the diagnostic of these diseases, as predictive markers of improving clinical management, outcome and survival of patients and are considered having a potential as drug targets to improve therapies.

Minimal residual disease-directed risk stratification using real-time quantitative PCR analysis of immunoglobulin and T-cell receptor gene rearrangements in the international multicenter trial AIEOP-BFM ALL 2000 for childhood acute lymphoblastic leukemia

Detection of minimal residual disease (MRD) is the most sensitive method to evaluate treatment response and one of the strongest predictors of outcome in childhood acute lymphoblastic leukemia (ALL). The 10-year update on the I-BFM-SG MRD study 91 demonstrates stable results (event-free survival), that is, standard risk group (MRD-SR) 93%, intermediate risk group (MRD-IR) 74%, and high risk group (MRD-HR) 16%. In multicenter trial AIEOP-BFM ALL 2000, patients were stratified by MRD detection using quantitative PCR after induction (TP1) and consolidation treatment (TP2). From 1 July 2000 to 31 October 2004, PCR target identification was performed in 3341 patients: 2365 (71%) patients had two or more sensitive targets (⩽10−4), 671 (20%) patients revealed only one sensitive target, 217 (6%) patients had targets with lower sensitivity, and 88 (3%) patients had no targets. MRD-based risk group assignment was feasible in 2594 (78%) patients: 40% were classified as MRD-SR (two sensitive targets, MRD negativity at both time points), 8% as MRD-HR (MRD ⩾10−3 at TP2), and 52% as MRD-IR. The remaining 823 patients were stratified according to clinical risk features: HR (n=108) and IR (n=715). In conclusion, MRD-PCR-based stratification using stringent criteria is feasible in almost 80% of patients in an international multicenter trial.

Isolation of a YAC clone covering a cluster of nine S100 genes on human chromosome 1q21: Rationale for a new nomenclature of the S100 calcium-binding protein family

S100 proteins are low-molecular-weight calcium-binding proteins of the EF-hand superfamily and appear to be involved in the regulation of a number of cellular processes such as cell cycle progression and differentiation. More than 10 members of the S100 protein family have been described from human sources so far. We have now isolated a YAC clone from human chromosome 1q21, on which 9 different genes coding for S100 calcium-binding proteins could be localized. Moreover, we have mapped the gene coding for S100P to human chromosome 4p16 and thereby completed the chromosomal assignments of all known human S100 genes. The clustered organization of S100 genes in the 1q21 region allows us to introduce a new logical nomenclature for these genes, which is based on the physical arrangement on the chromosome. The new nomenclature should facilitate and further the understanding of this protein family and be easily expandable to other species.

The MLL recombinome of acute leukemias

Chromosomal rearrangements of the human MLL gene are a hallmark for aggressive (high-risk) pediatric, adult and therapy-associated acute leukemias. These patients need to be identified in order to subject these patients to appropriate therapy regimen. A recently developed long-distance inverse PCR method was applied to genomic DNA isolated from individual acute leukemia patients in order to identify chromosomal rearrangements of the human MLL gene. We present data of the molecular characterization of 414 samples obtained from 272 pediatric and 142 adult leukemia patients. The precise localization of genomic breakpoints within the MLL gene and the involved translocation partner genes (TPGs) was determined and several new TPGs were identified. The combined data of our study and published data revealed a total of 87 different MLL rearrangements of which 51 TPGs are now characterized at the molecular level. Interestingly, the four most frequently found TPGs (AF4, AF9, ENL and AF10) encode nuclear proteins that are part of a protein network involved in histone H3K79 methylation. Thus, translocations of the MLL gene, by itself coding for a histone H3K4 methyltransferase, are presumably not randomly chosen, rather functionally selected.

Induction of autophagy-dependent necroptosis is required for childhood acute lymphoblastic leukemia cells to overcome glucocorticoid resistance

In vivo resistance to first-line chemotherapy, including to glucocorticoids, is a strong predictor of poor outcome in children with acute lymphoblastic leukemia (ALL). Modulation of cell death regulators represents an attractive strategy for subverting such drug resistance. Here we report complete resensitization of multidrug-resistant childhood ALL cells to glucocorticoids and other cytotoxic agents with subcytotoxic concentrations of obatoclax, a putative antagonist of BCL-2 family members. The reversal of glucocorticoid resistance occurred through rapid activation of autophagy-dependent necroptosis, which bypassed the block in mitochondrial apoptosis. This effect was associated with dissociation of the autophagy inducer beclin-1 from the antiapoptotic BCL-2 family member myeloid cell leukemia sequence 1 (MCL-1) and with a marked decrease in mammalian target of rapamycin (mTOR) activity. Consistent with a protective role for mTOR in glucocorticoid resistance in childhood ALL, combination of rapamycin with the glucocorticoid dexamethasone triggered autophagy-dependent cell death, with characteristic features of necroptosis. Execution of cell death, but not induction of autophagy, was strictly dependent on expression of receptor-interacting protein (RIP-1) kinase and cylindromatosis (turban tumor syndrome) (CYLD), two key regulators of necroptosis. Accordingly, both inhibition of RIP-1 and interference with CYLD restored glucocorticoid resistance completely. Together with evidence for a chemosensitizing activity of obatoclax in vivo, our data provide a compelling rationale for clinical translation of this pharmacological approach into treatments for patients with refractory ALL.

Subcellular targeting of metabolic enzymes to titin in heart muscle may be mediated by DRAL/FHL-2

During sarcomere contraction skeletal and cardiac muscle cells consume large amounts of energy. To satisfy this demand, metabolic enzymes are associated with distinct regions of the sarcomeres in the I-band and in the M-band, where they help to maintain high local concentrations of ATP. To date, the mechanism by which metabolic enzymes are coupled to the sarcomere has not been elucidated. Here, we show that the four and a half LIM-only protein DRAL/FHL-2 mediates targeting of the metabolic enzymes creatine kinase, adenylate kinase and phosphofructokinase by interaction with the elastic filament protein titin in cardiomyocytes. Using yeast two-hybrid assays, colocalisation experiments, co-immunoprecipitation and protein pull-down assays, we show that DRAL/FHL-2 is bound to two distinct sites on titin. One binding site is situated in the N2B region, a cardiac-specific insertion in the I-band part of titin, and the other is located in the is2 region of M-band titin. We also show that DRAL/FHL-2 binds to the metabolic enzymes creatine kinase, adenylate kinase and phosphofructokinase and might target these enzymes to the N2B and is2 regions in titin. We propose that DRAL/FHL-2 acts as a specific adaptor protein to couple metabolic enzymes to sites of high energy consumption in the cardiac sarcomere.

Gene Expression Signatures Identify Rhabdomyosarcoma Subtypes and Detect a Novel t(2;2)(q35;p23) Translocation Fusing PAX3 to NCOA1

Rhabdomyosarcoma is a pediatric tumor type, which is classified based on histological criteria into two major subgroups, namely embryonal rhabdomyosarcoma and alveolar rhabdomyosarcoma. The majority, but not all, alveolar rhabdomyosarcoma carry the specific PAX3(7)/FKHR-translocation, whereas there is no consistent genetic abnormality recognized in embryonal rhabdomyosarcoma. To gain additional insight into the genetic characteristics of these subtypes, we used oligonucleotide microarrays to measure the expression profiles of a group of 29 rhabdomyosarcoma biopsy samples (15 embryonal rhabdomyosarcoma, and 10 translocation-positive and 4 translocation-negative alveolar rhabdomyosarcoma). Hierarchical clustering revealed expression signatures clearly discriminating all three of the subgroups. Differentially expressed genes included several tyrosine kinases and G protein-coupled receptors, which might be amenable to pharmacological intervention. In addition, the alveolar rhabdomyosarcoma signature was used to classify an additional alveolar rhabdomyosarcoma case lacking any known PAX3 or PAX7 fusion as belonging to the translocation-positive group, leading to the identification of a novel translocation t(2;2)(q35;p23), which generates a fusion protein composed of PAX3 and the nuclear receptor coactivator NCOA1, having similar transactivation properties as PAX3/FKHR. These experiments demonstrate for the first time that gene expression profiling is capable of identifying novel chromosomal translocations.

Altered expression of the Ca2+-binding protein S100A1 in human cardiomyopathy

The Ca(2+)-binding protein S100A1 displays a tissue-specific expression pattern with highest levels in myocardium and has been shown to interact with SR-proteins regulating the Ca(2+)-induced Ca(2+)-release. We, therefore, hypothesized that changes in S100A1 gene expression might correlate with the pathognomonic finding of altered SR Ca(2+)-transients in human end stage heart failure. To test this hypothesis, we established a specific and sensitive method to analyse S100A1 expression in cardiac tissues by employing hydrophobic interaction-chromatography and reversed-phase high performance liquid chromatography (RP-HPLC) coupled with Electron-Ionisation-Mass-Spectrometry (ESI-MS). Porcine myocardium showed a differential expression of S100A1 with relative protein concentrations of 62 +/- 8% in the right ventricle (RV), 57 +/- 9% in the right atrium (RA), and 25 +/- 15% in the left atrium (LA) as compared to the left ventricle (LV) (100 +/- 10%; P < 0.001). Northern blot analyses confirmed a likewise distribution of porcine S100A1 mRNA implying a regulation on the transcriptional level. Analyses of left ventricular specimen of patients with end stage heart failure (CHF, n = 6; CHD, n = 6) revealed significantly reduced S100A1 protein levels, while integration of S100A1 peaks after RP-HPLC yielded two groups of patients with < 76% (69 +/- 7%, n = 6) and < 35% (23 +/- 12%, n = 6) respectively as compared to controls (100 +/- 8%, n = 3). These data demonstrate for the first time that S100A1 is differentially expressed in myocardium and that in human cardiomyopathy a reduced expression of S100A1 may contribute to a compromised contractility.

Calcium regulation of Ndr protein kinase mediated by S100 calcium-binding proteins

Ndr is a nuclear serine/threonine protein kinase that belongs to a subfamily of kinases identified as being critical for the regulation of cell division and cell morphology. The regulatory mechanisms that control Ndr activity have not been characterized previously. In this paper, we present evidence that Ndr is regulated by EF‐hand calcium‐binding proteins of the S100 family, in response to changes in the intracellular calcium concentration. In vitro, S100B binds directly to and activates Ndr in a Ca2+‐dependent manner. Moreover, Ndr is recovered from cell lysates in anti‐S100B immunoprecipitates. The region of Ndr responsible for interaction with Ca2+/S100B is a basic/hydrophobic motif within the N‐terminal regulatory domain of Ndr, and activation of Ndr by Ca2+/S100B is inhibited by a synthetic peptide derived from this region. In cultured cells, Ndr is rapidly activated following treatment with Ca2+ ionophore, and this activation is dependent upon the identified Ca2+/S100B‐binding domain. Finally, Ndr activity is inhibited by W‐7 in melanoma cells overexpressing S100B, but is unaffected by W‐7 in melanoma cells that lack S100B. These results suggest that Ndr is regulated at least in part by changes in the intracellular calcium concentration, through binding of S100 proteins to its N‐terminal regulatory domain.