Mezagitamab in systemic lupus erythematosus: clinical and mechanistic findings of CD38 inhibition in an autoimmune disease

OBJECTIVE

To evaluate safety and mechanism of action of mezagitamab (TAK-079), an anti-CD38 monoclonal antibody, in patients with moderate to severe systemic lupus erythematosus (SLE).

METHODS

A phase 1b double-blind, placebo-controlled, multicentre study was conducted in patients with SLE receiving standard background therapy. Eligible patients were adults who met the 2012 SLICC or ACR criteria for diagnosis, had a baseline SLE Disease Activity Index 2000 (SLEDAI-2K) score of ≥6 and were positive for anti-double-stranded DNA antibodies and/or anti-extractable nuclear antigens antibodies. Patients received 45 mg, 90 mg or 135 mg of mezagitamab or placebo every 3 weeks over 12 weeks. Primary endpoints were safety and tolerability. Secondary endpoints included pharmacokinetics and pharmacodynamics. Exploratory assessments included disease activity scales, deep immune profiling and interferon pathway analysis.

RESULTS

22 patients received at least one dose of either mezagitamab or placebo. In patients exposed to mezagitamab (n=17), drug was well tolerated. Adverse event (AEs) were balanced across treatment groups, with no treatment emergent AEs exceeding grade 2. Responder analyses for Cutaneous Lupus Erythematosus Disease Area and Severity Index (CLASI) and SLEDAI-2K did not reveal any observable differences across treatment groups. However, there was a trend for more profound skin responses among patients with higher CLASI scores (>10) at baseline. Pharmacodynamic analysis showed median CD38 receptor occupancy up to 88.4% on CD38+ natural killer cells with concurrent depletion of these cells up to 90% in the 135 mg group. Mean reductions in IgG and autoantibodies were less than 20% in all dose groups. Cytometry by time of flight and type 1 interferon gene analysis revealed unique fingerprints that are indicative of a broad immune landscape shift following CD38 targeting.

CONCLUSIONS

Mezagitamab had a favourable safety profile in patients with moderate to severe SLE and elicited a pharmacodynamic effect consistent with CD38+ cell depletion. These findings reveal novel insights into the drug's mechanism of action and support the continued investigation of mezagitamab in autoimmune diseases.

Abstract 3928: The pan-CLK/DYRK inhibitor cirtuvivint selectively disrupts alternative splicing and has broad anti-tumor activity in preclinical models

Dysregulated alternative pre-mRNA splicing (AS) is commonly associated with human malignancies, producing pathological proteomes that underlie disease initiation, progression, and therapeutic resistance. The CDC2-like kinases (CLKs) and dual-specificity tyrosine-regulated kinases (DYRKs) are thought to govern AS efficiency and specificity by directly phosphorylating splicing factors that influence pre-mRNA splice junction selection. Cirtuvivint (SM08502) is a first-in-class small molecule ATP-competitive inhibitor of CLK/DYRK kinases. To evaluate the contribution of these kinases to AS profiles, high-depth RNAseq analysis was performed across 39 tumor cell lines (representing 9 lineages) and compared to 6 non-cancerous cell lines after treatment with cirtuvivint. Both baseline and drug-induced changes in AS events (ASEs) were measured using a multivariate analysis of transcript splicing (rMATS). Pan-CLK/DYRK inhibition was found to affect a minority of baseline ASEs, leaving the majority of spliceosome activity intact in all samples; however, the magnitude and quantity of detected drug-induced alterations were larger in tumor compared to non-cancerous cell lines. The majority of ASEs resulting from pan-CLK/DYRK inhibition were enriched for exon skipping events and were tumor type-specific. Moreover, drug-induced ASEs were significantly associated with disease-promoting biology across lineage and oncogenic driver contexts, including perturbed splicing of the AR-V7 variant in treatment-resistant prostate cancer and MDM2 in p53 wild-type cancers.To assess the breadth and depth of CLK/DYRK dependencies in human cancers, the effects of pan-CLK/DYRK inhibition on growth and survival of 154 cancer cell line models, 46 PDX models, and 43 CDX models were tested. EC50s in cell viability assays ranged from 0.014 to 0.73 μM in culture with strong effects observed in subsets of cell lines across all lineages tested. Multiomic data integration revealed sensitivity to cirtuvivint was associated with alterations in splicing genes. In vivo, tumor growth inhibition studies testing cirtuvivint at exposures ~2X below the MTD resulted in significant disease control (≥50% TGI) in 38/46 PDX and 38/43 CDX models. These observations indicate broad cancer relevance, at least in the preclinical setting, and expose vulnerabilities to CLK-DYRK-regulated splicing that can potentially be therapeutically addressed with pan CLK/DYRK inhibitors.

Citation Format: Elizabeth A. McMillan, Carine Bossard, Emily Creger, Carsten Merkwirth, Raffaella Pippa, Matthew Jarvis, Krishna Sriram, Melinda Pedraza, Maureen Ibanez, Deepti Bhat, Carolyn Lai, Josh Stewart, Brian Eastman, Chi-Ching Mak, Michael A. White, Darrin Beaupre. The pan-CLK/DYRK inhibitor cirtuvivint selectively disrupts alternative splicing and has broad anti-tumor activity in preclinical models [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 3928.

Molecular characterization of type I IFN-induced cytotoxicity in bladder cancer cells reveals biomarkers of resistance

Although anti-tumor activities of type I interferons (IFNs) have been recognized for decades, the molecular mechanisms contributing to clinical response remain poorly understood. The complex functions of these pleiotropic cytokines include stimulation of innate and adaptive immune responses against tumors as well as direct inhibition of tumor cells. In high-grade, Bacillus Calmette-Guérin (BCG)-unresponsive non-muscle-invasive bladder cancer, nadofaragene firadenovec, a non-replicating adenovirus administered locally to express the IFNα2b transgene, embodies a novel approach to deploy the therapeutic activity of type I IFNs while minimizing systemic toxicities. Deciphering which functions of type I IFN are required for clinical activity will bolster efforts to maximize the efficacy of nadofaragene firadenovec and other type I IFN-based therapies, and inform strategies to address resistance. As such, we characterized the phenotypic and molecular response of human bladder cancer cell lines to IFNα delivered in multiple contexts, including adenoviral delivery. We found that constitutive activation of the type I IFN signaling pathway is a biomarker for resistance to both transcriptional response and direct cytotoxic effects of IFNα. We present several genes that discriminate between sensitive and resistant tumor cells, suggesting they should be explored for utility as biomarkers in future clinical trials of type I IFN-based anti-tumor therapies.

Abstract P242: Preclinical efficacy landscape of the pan-CLK/DYRK inhibitor Cirtuvivint (SM08502)

A monumental challenge facing cancer therapy is the vast mechanistic diversity of tumors among and even within cancer patients. This severely limits the number of patients that respond to any given therapy and the durability of responses that do occur. Deep phenotypic profiling of almost 9000 patient samples indicates perturbation of alternative pre-mRNA splicing (AS) is often a root cause of cancer. This offers an extraordinary opportunity to restore health through normalization of AS in diseased tissue. The bottleneck to progress has been finding and drugging the right targets. The CLK/DYRK kinases modulate AS efficiency and specificity, and therefore, targeting them offers the potential to address this bottleneck. The isoquinoline cirtuvivint is a potent ATP-competitive inhibitor of the Cdc2-like kinases (CLK1-4) and the dual-specificity tyrosine phosphorylation-regulated kinases (DYRK1-4), with activity against only a minimal number of the remaining members of the CMGC-family kinases or the kinome as a whole. Cellular target engagement assays indicated biological IC50s below 0.06 μM across the CLK/DYRK target class. Thus, cirtuvivint has utility for robust chemical evaluation of the overarching contribution of CLK/DYRK family activity to tumor biology in general and context- dependent AS in particular. To help assess the breadth and depth of CLK/DYRK dependencies in human cancers, the consequences of pan-CLK/DYRK inhibition on growth and survival of 154 cancer cell line models, 46 PDX models, and 43 CDX models were tested. EC50s in cell viability assays ranged from 0.014 to 0.73 μM in culture with strong effects observed in subsets of cell lines across all lineages tested. Molecular profiles associated with high sensitivity to cirtuvivint included somatic mutations in the RBM10 splicing factor. Tumor growth inhibition assays with daily dosing at exposures about 2X below the MTD resulted in significant disease control (≥75% TGI) in 15/46 PDX and 18/43 CDX models. Together, these results indicate broad cancer relevance, at least in the preclinical setting, and are consistent with a common reliance on CLK/DYRK-dependent alternative splicing among otherwise highly mechanistically heterogeneous disease.

Citation Format: Carine Bossard, Emily Creger, Elizabeth A. McMillan, Carsten Merkwirth, Maureen Ibanez, Deepti Bhat, Josh Stewart, Brian Eastman, Chi-Ching Mak, Vishal Deshmukh, Michael A. White. Preclinical efficacy landscape of the pan-CLK/DYRK inhibitor Cirtuvivint (SM08502) [abstract]. In: Proceedings of the AACR-NCI-EORTC Virtual International Conference on Molecular Targets and Cancer Therapeutics; 2021 Oct 7-10. Philadelphia (PA): AACR; Mol Cancer Ther 2021;20(12 Suppl):Abstract nr P242.

Abstract P231: Multigenomic characterization of context-dependent alternative splicing in normal and neoplastic cells

Alternative pre-mRNA splicing (AS) supports the dynamic and regulated diversification of cells by allowing the production of multiple distinct proteins from individual genes. Dysregulated AS is commonly associated with human malignancies, producing pathological proteomes that underpin disease initiation, progression, and emergence of therapy resistance. The CDC2-like kinases (CLKs) and dual-specificity tyrosine-regulated kinases (DYRKs) are thought to govern the efficiency and specificity of AS by directly phosphorylating serine/arginine-rich splicing factors (SRSFs) and thereby influencing pre-mRNA splice junction selection. Cirtuvivint (SM08502) is a first-in-class small molecule ATP-competitive inhibitor of CLK/DYRK kinases. To directly evaluate the contribution of these kinases to AS profiles, changes in AS following treatment with cirtuvivint followed by high-depth RNAseq analysis across >25 cell lines representing 7 tumor lineages were evaluated. Both baseline and drug-induced changes in AS events (ASEs) were measured using a multivariate analysis of transcript splicing (rMATS). Pan-CLK/DYRK inhibition was found to affect a minority of baseline ASEs, leaving the majority of spliceosome activity intact in all samples. However, the magnitude and quantity of detected drug-induced alterations were larger in a sample of tumor cells from a patient compared with adjacent non-tumorigenic cells. Moreover, most ASEs sensitive to pan-CLK/DYRK inhibition were tumor type-specific irrespective of selective presence at the gene level. Multi-omics data integration revealed sensitivity to cirtuvivint was associated with alterations in splicing genes and that drug-induced ASEs were significantly associated with disease-promoting biology across lineage and oncogenic driver contexts. Perturbed splicing of the AR-V7 variant in treatment-resistant prostate cancer and MDM2 in p53 wild-type cancers were prominent examples. These observations indicate vulnerabilities to CLK-DYRK regulated splicing span a wide range of oncogenic contexts with potential to be therapeutically addressed with pan CLK/DYRK inhibitors.

Citation Format: Elizabeth A. McMillan, Krishna Sriram, Emily Creger, Carsten Merkwirth, Raffaella Pippa, Melinda Pedraza, Long Do, Vishal Deshmukh, Carine Bossard, Michael A. White. Multigenomic characterization of context-dependent alternative splicing in normal and neoplastic cells [abstract]. In: Proceedings of the AACR-NCI-EORTC Virtual International Conference on Molecular Targets and Cancer Therapeutics; 2021 Oct 7-10. Philadelphia (PA): AACR; Mol Cancer Ther 2021;20(12 Suppl):Abstract nr P231.

Signaling Networks Determining Life Span

The health of an organism is orchestrated by a multitude of molecular and biochemical networks responsible for ensuring homeostasis within cells and tissues. However, upon aging, a progressive failure in the maintenance of this homeostatic balance occurs in response to a variety of endogenous and environmental stresses, allowing the accumulation of damage, the physiological decline of individual tissues, and susceptibility to diseases. What are the molecular and cellular signaling events that control the aging process and how can this knowledge help design therapeutic strategies to combat age-associated diseases? Here we provide a comprehensive overview of the evolutionarily conserved biological processes that alter the rate of aging and discuss their link to disease prevention and the extension of healthy life span.

Prohibitin-2 Depletion Unravels Extra-Mitochondrial Functions at the Kidney Filtration Barrier

Mitochondrial fusion is essential for maintenance of mitochondrial function and requires the prohibitin ring complex subunit prohibitin-2 (PHB2) at the mitochondrial inner membrane. Loss of the stomatin/PHB/flotillin/HflK/C (SPFH) domain containing protein PHB2 causes mitochondrial dysfunction and defective mitochondria-mediated signaling, which is implicated in a variety of human diseases, including progressive renal disease. Here, we provide evidence of additional, extra-mitochondrial functions of this membrane-anchored protein. Immunofluorescence and immunogold labeling detected PHB2 at mitochondrial membranes and at the slit diaphragm, a specialized cell junction at the filtration slit of glomerular podocytes. PHB2 coprecipitated with podocin, another SPFH domain-containing protein, essential for the assembly of the slit diaphragm protein-lipid supercomplex. Consistent with an evolutionarily conserved extra-mitochondrial function, the ortholog of PHB2 in Caenorhabditis elegans was also not restricted to mitochondria but colocalized with the mechanosensory complex that requires the podocin ortholog MEC2 for assembly. Knockdown of phb-2 partially phenocopied loss of mec-2 in touch neurons of the nematode, resulting in impaired gentle touch sensitivity. Collectively, these data indicate that, besides its established role in mitochondria, PHB2 may have an additional function in conserved protein-lipid complexes at the plasma membrane.

Loss of OMA1 delays neurodegeneration by preventing stress-induced OPA1 processing in mitochondria

Loss of OMA1 in a mouse model of neurodegeneration stabilizes fusion-active L-OPA1, which supports neuronal survival by preventing apoptosis, independent of its effects on cristae morphogenesis.

Proteolytic cleavage of the dynamin-like guanosine triphosphatase OPA1 in mitochondria is emerging as a central regulatory hub that determines mitochondrial morphology under stress and in disease. Stress-induced OPA1 processing by OMA1 triggersmitochondrial fragmentation, which is associated with mitophagy and apoptosis in vitro. Here, we identify OMA1 as a critical regulator of neuronal survival in vivo and demonstrate that stress-induced OPA1 processing by OMA1 promotes neuronal death and neuroinflammatory responses. Using mice lacking prohibitin membrane scaffolds as a model of neurodegeneration, we demonstrate that additional ablation of Oma1 delays neuronal loss and prolongs lifespan. This is accompanied by the accumulation of fusion-active, long OPA1 forms, which stabilize the mitochondrial genome but do not preserve mitochondrial cristae or respiratory chain supercomplex assembly in prohibitin-depleted neurons. Thus, long OPA1 forms can promote neuronal survival independently of cristae shape, whereas stress-induced OMA1 activation and OPA1 cleavage limit mitochondrial fusion and promote neuronal death.

Mitochondrial UPR: A Double-Edged Sword

The mitochondrial unfolded protein response (UPR(mt)) promotes the recovery of dysfunctional mitochondria. Surprisingly, UPR(mt) activation inadvertently maintains and propagates the deleterious mtDNA in a heteroplasmic Caenorhabditis elegans strain, with detrimental consequences. This study extends our understanding of the UPR(mt) and provides a possible therapeutic target for diseases associated with mtDNA mutations.

Two Conserved Histone Demethylases Regulate Mitochondrial Stress-Induced Longevity

Across eukaryotic species, mild mitochondrial stress can have beneficial effects on the lifespan of organisms. Mitochondrial dysfunction activates an unfolded protein response (UPR(mt)), a stress signaling mechanism designed to ensure mitochondrial homeostasis. Perturbation of mitochondria during larval development in C. elegans not only delays aging but also maintains UPR(mt) signaling, suggesting an epigenetic mechanism that modulates both longevity and mitochondrial proteostasis throughout life. We identify the conserved histone lysine demethylases jmjd-1.2/PHF8 and jmjd-3.1/JMJD3 as positive regulators of lifespan in response to mitochondrial dysfunction across species. Reduction of function of the demethylases potently suppresses longevity and UPR(mt) induction, while gain of function is sufficient to extend lifespan in a UPR(mt)-dependent manner. A systems genetics approach in the BXD mouse reference population further indicates conserved roles of the mammalian orthologs in longevity and UPR(mt) signaling. These findings illustrate an evolutionary conserved epigenetic mechanism that determines the rate of aging downstream of mitochondrial perturbations.

Imbalanced OPA1 processing and mitochondrial fragmentation cause heart failure in mice

Mitochondrial morphology is shaped by fusion and division of their membranes. Here, we found that adult myocardial function depends on balanced mitochondrial fusion and fission, maintained by processing of the dynamin-like guanosine triphosphatase OPA1 by the mitochondrial peptidases YME1L and OMA1. Cardiac-specific ablation of Yme1l in mice activated OMA1 and accelerated OPA1 proteolysis, which triggered mitochondrial fragmentation and altered cardiac metabolism. This caused dilated cardiomyopathy and heart failure. Cardiac function and mitochondrial morphology were rescued by Oma1 deletion, which prevented OPA1 cleavage. Feeding mice a high-fat diet or ablating Yme1l in skeletal muscle restored cardiac metabolism and preserved heart function without suppressing mitochondrial fragmentation. Thus, unprocessed OPA1 is sufficient to maintain heart function, OMA1 is a critical regulator of cardiomyocyte survival, and mitochondrial morphology and cardiac metabolism are intimately linked.

Inhibition of insulin/IGF-1 receptor signaling protects from mitochondria-mediated kidney failure

Mitochondrial dysfunction and alterations in energy metabolism have been implicated in a variety of human diseases. Mitochondrial fusion is essential for maintenance of mitochondrial function and requires the prohibitin ring complex subunit prohibitin-2 (PHB2) at the mitochondrial inner membrane. Here, we provide a link between PHB2 deficiency and hyperactive insulin/IGF-1 signaling. Deletion of PHB2 in podocytes of mice, terminally differentiated cells at the kidney filtration barrier, caused progressive proteinuria, kidney failure, and death of the animals and resulted in hyperphosphorylation of S6 ribosomal protein (S6RP), a known mediator of the mTOR signaling pathway. Inhibition of the insulin/IGF-1 signaling system through genetic deletion of the insulin receptor alone or in combination with the IGF-1 receptor or treatment with rapamycin prevented hyperphosphorylation of S6RP without affecting the mitochondrial structural defect, alleviated renal disease, and delayed the onset of kidney failure in PHB2-deficient animals. Evidently, perturbation of insulin/IGF-1 receptor signaling contributes to tissue damage in mitochondrial disease, which may allow therapeutic intervention against a wide spectrum of diseases.

TRPV1 pain receptors regulate longevity and metabolism by neuropeptide signaling

The sensation of pain is associated with increased mortality, but it is unknown whether pain perception can directly affect aging. We find that mice lacking TRPV1 pain receptors are long-lived, displaying a youthful metabolic profile at old age. Loss of TRPV1 inactivates a calcium-signaling cascade that ends in the nuclear exclusion of the CREB-regulated transcriptional coactivator CRTC1 within pain sensory neurons originating from the spinal cord. In long-lived TRPV1 knockout mice, CRTC1 nuclear exclusion decreases production of the neuropeptide CGRP from sensory endings innervating the pancreatic islets, subsequently promoting insulin secretion and metabolic health. In contrast, CGRP homeostasis is disrupted with age in wild-type mice, resulting in metabolic decline. We show that pharmacologic inactivation of CGRP receptors in old wild-type animals can restore metabolic health. These data suggest that ablation of select pain sensory receptors or the inhibition of CGRP are associated with increased metabolic health and control longevity.

Loss of prohibitin induces mitochondrial damages altering β-cell function and survival and is responsible for gradual diabetes development

Prohibitins are highly conserved proteins mainly implicated in the maintenance of mitochondrial function and architecture. Their dysfunctions are associated with aging, cancer, obesity, and inflammation. However, their possible role in pancreatic β-cells remains unknown. The current study documents the expression of prohibitins in human and rodent islets and their key role for β-cell function and survival. Ablation of Phb2 in mouse β-cells sequentially resulted in impairment of mitochondrial function and insulin secretion, loss of β-cells, progressive alteration of glucose homeostasis, and, ultimately, severe diabetes. Remarkably, these events progressed over a 3-week period of time after weaning. Defective insulin supply in β-Phb2(-/-) mice was contributed by both β-cell dysfunction and apoptosis, temporarily compensated by increased β-cell proliferation. At the molecular level, we observed that deletion of Phb2 caused mitochondrial abnormalities, including reduction of mitochondrial DNA copy number and respiratory chain complex IV levels, altered mitochondrial activity, cleavage of L-optic atrophy 1, and mitochondrial fragmentation. Overall, our data demonstrate that Phb2 is essential for metabolic activation of mitochondria and, as a consequence, for function and survival of β-cells.

FOXO4 is necessary for neural differentiation of human embryonic stem cells

Proteostasis is critical for maintaining cell function and proteome stability may play an important role in human embryonic stem cell (hESC) immortality. Notably, hESC populations exhibit a high assembly of active proteasomes, a key node of the proteostasis network. FOXO4, an insulin/IGF-1 responsive transcription factor, regulates proteasome activity in hESCs. We find that loss of FOXO4 reduces the potential of hESCs to differentiate into neural lineages. Therefore, FOXO4 crosses evolutionary boundaries and links hESC function to invertebrate longevity modulation.

The mitochondrial electron transport chain is dispensable for proliferation and differentiation of epidermal progenitor cells

Tissue stem cells and germ line or embryonic stem cells were shown to have reduced oxidative metabolism, which was proposed to be an adaptive mechanism to reduce damage accumulation caused by reactive oxygen species. However, an alternate explanation is that stem cells are less dependent on specialized cytoplasmic functions compared with differentiated cells, therefore, having a high nuclear-to-cytoplasmic volume ratio and consequently a low mitochondrial content. To determine whether stem cells rely or not on mitochondrial respiration, we selectively ablated the electron transport chain in the basal layer of the epidermis, which includes the epidermal progenitor/stem cells (EPSCs). This was achieved using a loxP-flanked mitochondrial transcription factor A (Tfam) allele in conjunction with a keratin 14 Cre transgene. The epidermis of these animals (Tfam(EKO)) showed a profound depletion of mitochondrial DNA and complete absence of respiratory chain complexes. However, despite a short lifespan due to malnutrition, epidermal development and skin barrier function were not impaired. Differentiation of epidermal layers was normal and no proliferation defect or major increase of apoptosis could be observed. In contrast, mice with an epidermal ablation of prohibitin-2, a scaffold protein in the inner mitochondrial membrane, displayed a dramatic phenotype observable already in utero, with severely impaired skin architecture and barrier function, ultimately causing death from dehydration shortly after birth. In conclusion, we here provide unequivocal evidence that EPSCs, and probably tissue stem cells in general, are independent of the mitochondrial respiratory chain, but still require a functional dynamic mitochondrial compartment.

Obesity-induced overexpression of miRNA-143 inhibits insulin-stimulated AKT activation and impairs glucose metabolism

The contribution of altered post-transcriptional gene silencing to the development of insulin resistance and type 2 diabetes mellitus so far remains elusive. Here, we demonstrate that expression of microRNA (miR)-143 and 145 is upregulated in the liver of genetic and dietary mouse models of obesity. Induced transgenic overexpression of miR-143, but not miR-145, impairs insulin-stimulated AKT activation and glucose homeostasis. Conversely, mice deficient for the miR-143-145 cluster are protected from the development of obesity-associated insulin resistance. Quantitative-mass-spectrometry-based analysis of hepatic protein expression in miR-143-overexpressing mice revealed miR-143-dependent downregulation of oxysterol-binding-protein-related protein (ORP) 8. Reduced ORP8 expression in cultured liver cells impairs the ability of insulin to induce AKT activation, revealing an ORP8-dependent mechanism of AKT regulation. Our experiments provide direct evidence that dysregulated post-transcriptional gene silencing contributes to the development of obesity-induced insulin resistance, and characterize the miR-143-ORP8 pathway as a potential target for the treatment of obesity-associated diabetes.

Prohibitins and the functional compartmentalization of mitochondrial membranes

Prohibitins constitute an evolutionarily conserved and ubiquitously expressed family of membrane proteins that are essential for cell proliferation and development in higher eukaryotes. Roles for prohibitins in cell signaling at the plasma membrane and in transcriptional regulation in the nucleus have been proposed, but pleiotropic defects associated with the loss of prohibitin genes can be largely attributed to a dysfunction of mitochondria. Two closely related proteins, prohibitin-1 (PHB1) and prohibitin-2 (PHB2), form large, multimeric ring complexes in the inner membrane of mitochondria. The absence of prohibitins leads to an increased generation of reactive oxygen species, disorganized mitochondrial nucleoids, abnormal cristae morphology and an increased sensitivity towards stimuli-elicited apoptosis. It has been found that the processing of the dynamin-like GTPase OPA1, which regulates mitochondrial fusion and cristae morphogenesis, is a key process regulated by prohibitins. Furthermore, genetic analyses in yeast have revealed an intimate functional link between prohibitin complexes and the membrane phospholipids cardiolipin and phosphatidylethanolamine. In light of these findings, it is emerging that prohibitin complexes can function as protein and lipid scaffolds that ensure the integrity and functionality of the mitochondrial inner membrane.

Prohibitin function within mitochondria: essential roles for cell proliferation and cristae morphogenesis

Prohibitins comprise an evolutionary conserved and ubiquitously expressed family of membrane proteins. Various roles in different cellular compartments have been proposed for prohibitin proteins. Recent experiments, however, identify large assemblies of two homologous prohibitin subunits, PHB1 and PHB2, in the inner membrane of mitochondria as the physiologically active structure. Mitochondrial prohibitin complexes control cell proliferation, cristae morphogenesis and the functional integrity of mitochondria. The processing of the dynamin-like GTPase OPA1, a core component of the mitochondrial fusion machinery, has been defined as a key process affected by prohibitins. The molecular mechanism of prohibitin function, however, remained elusive. The ring-like assembly of prohibitins and their sequence similarity with lipid raft-associated SPFH-family members suggests a scaffolding function of prohibitins, which may lead to functional compartmentalization in the inner membrane of mitochondria.

Prohibitins control cell proliferation and apoptosis by regulating OPA1-dependent cristae morphogenesis in mitochondria

Prohibitins comprise an evolutionarily conserved and ubiquitously expressed family of membrane proteins with poorly described functions. Large assemblies of PHB1 and PHB2 subunits are localized in the inner membrane of mitochondria, but various roles in other cellular compartments have also been proposed for both proteins. Here, we used conditional gene targeting of murine Phb2 to define cellular activities of prohibitins. Our experiments restrict the function of prohibitins to mitochondria and identify the processing of the dynamin-like GTPase OPA1, an essential component of the mitochondrial fusion machinery, as the central cellular process controlled by prohibitins. Deletion of Phb2 leads to the selective loss of long isoforms of OPA1. This results in an aberrant cristae morphogenesis and an impaired cellular proliferation and resistance toward apoptosis. Expression of a long OPA1 isoform in PHB2-deficient cells suppresses these defects, identifying impaired OPA1 processing as the primary cellular defect in the absence of prohibitins. Our results therefore assign an essential function for the formation of mitochondrial cristae to prohibitins and suggest a coupling of cell proliferation to mitochondrial morphogenesis.

Regulation of B cell homeostasis and activation by the tumor suppressor gene CYLD

B cell homeostasis is regulated by multiple signaling processes, including nuclear factor-κB (NF-κB), BAFF-, and B cell receptor signaling. Conditional disruption of genes involved in these pathways has shed light on the mechanisms governing signaling from the cell surface to the nucleus. We describe a novel mouse strain that expresses solely and excessively a naturally occurring splice variant of CYLD (CYLD^ex7/8 mice), which is a deubiquitinating enzyme that is integral to NF-κB signaling. This shorter CYLD protein lacks the TRAF2 and NEMO binding sites present in full-length CYLD. A dramatic expansion of mature B lymphocyte populations in all peripheral lymphoid organs occurs in this strain. The B lymphocytes themselves exhibit prolonged survival and manifest a variety of signaling disarrangements that do not occur in mice with a complete deletion of CYLD. Although both the full-length and the mutant CYLD are able to interact with Bcl-3, a predominant nuclear accumulation of Bcl-3 occurs in the CYLD mutant B cells. More dramatic, however, is the accumulation of the NF-κB proteins p100 and RelB in CYLD^ex7/8 B cells, which, presumably in combination with nuclear Bcl-3, results in increased levels of Bcl-2 expression. These findings suggest that CYLD can both positively and negatively regulate signal transduction and homeostasis of B cells in vivo, depending on the expression of CYLD splice variants.

Fast nearest-neighbor searching for nonlinear signal processing

A fast algorithm for exact and approximate nearest-neighbor searching is presented that is suitable for tasks encountered in nonlinear signal processing. Empirical benchmarks show that the algorithm's performance depends mainly on the (fractal) dimension D(d) of the data set, which is usually smaller than the dimension D(s) of the vector space in which the data points are embedded. We also compare the running time of our algorithm with those of two previously proposed algorithms for nearest-neighbor searching.

Prediction of spatiotemporal time series based on reconstructed local states

Spatiotemporal time series are analyzed and predicted using reconstructed local states. As numerical examples the evolution of a Kuramoto-Sivashinsky equation and a coupled map lattice are predicted from previously sampled data.