Ubiquitination Events That Regulate Recombination of Immunoglobulin Loci Gene Segments

Programed DNA mutagenesis events in the immunoglobulin (Ig) loci of developing B cells utilize the common and conserved mechanism of protein ubiquitination for subsequent proteasomal degradation to generate the required antigen-receptor diversity. Recombinase proteins RAG1 and RAG2, necessary for V(D)J recombination, and activation-induced cytidine deaminase, an essential mutator protein for catalyzing class switch recombination and somatic hypermutation, are regulated by various ubiquitination events that affect protein stability and activity. Programed DNA breaks in the Ig loci can be identified by various components of DNA repair pathways, also regulated by protein ubiquitination. Errors in the ubiquitination pathways for any of the DNA double-strand break repair proteins can lead to inefficient recombination and repair events, resulting in a compromised adaptive immune system or development of cancer

Degradation of Id2 by the anaphase-promoting complex couples cell cycle exit and axonal growth

In the developing nervous system, Id2 (inhibitor of DNA binding 2, also known as inhibitor of differentiation 2) enhances cell proliferation, promotes tumour progression and inhibits the activity of neurogenic basic helix\u2013loop\u2013helix (bHLH) transcrip- tion factors1,2. The anaphase promoting complex/cyclosome and its activator Cdh1 (APC/CCdh1) restrains axonal growth but the targets of APC/CCdh1 in neurons are unknown3\u20135. Id2 and other members of the Id family are very unstable proteins that are eliminated as cells enter the quiescent state, but how they are targeted for degradation has remained elusive6,7. Here we show that Id2 interacts with the core subunits of APC/C and Cdh1 in primary neurons. APC/CCdh1 targets Id2 for degradation through a destruction box motif (D box) that is conserved in Id1 and Id4. Depletion of Cdh1 stabilizes Id proteins in neurons, whereas Id2 D-box mutants are impaired for Cdh1 binding and remain stable in cells that exit from the cell cycle and contain active APC/CCdh1. Mutants of the Id2 D box enhance axonal growth in cerebellar granule neurons in vitro and in the context of the cerebellar cortex, and overcome the myelin inhibitory signals for growth. Conversely, activation of bHLH transcription factors induces a cluster of genes with potent axonal inhibitory functions including the gene coding for the Nogo receptor, a key transducer of myelin inhibition. Degradation of Id2 in neurons permits the accumu- lation of the Nogo receptor, thereby linking APC/CCdh1 activity with bHLH target genes for the inhibition of axonal growth. These findings indicate that deregulated Id activity might be useful to reprogramme quiescent neurons into the axonal growth mode

Expression and Function of Tetraspanins and Their Interacting Partners in B Cells

Tetraspanins are transmembrane proteins that modulate multiple diverse biological processes, including signal transduction, cell–cell communication, immunoregulation, tumorigenesis, cell adhesion, migration, and growth and differentiation. Here, we provide a systematic review of the involvement of tetraspanins and their partners in the regulation and function of B cells, including mechanisms associated with antigen presentation, antibody production, cytokine secretion, co-stimulator expression, and immunosuppression. Finally, we direct our focus to the signaling mechanisms, evolutionary conservation aspects, expression, and potential therapeutic strategies that could be based on tetraspanins and their interacting partners

E Proteins and Id2 Converge on p57(Kip2) To Regulate Cell Cycle in Neural Cells

A precise balance between proliferation and differentiation must be maintained during neural development to obtain the correct proportion of differentiated cell types in the adult nervous system. The basic helix-loop-helix (bHLH) transcription factors known as E proteins and their natural inhibitors, the Id proteins, control the timing of differentiation and terminal exit from the cell cycle. Here we show that progression into S phase of human neuroblastoma cells is prevented by E proteins and promoted by Id2. Cyclin-dependent kinase inhibitors (CKI) have been identified as key effectors of cell cycle arrest in differentiating cells. However, p57(Kip2) is the only CKI that is absolutely required for normal development. Through the use of global gene expression analysis in neuroblastoma cells engineered to acutely express the E protein E47 and Id2, we find that p57(Kip2) is a target of E47. Consistent with the role of Id proteins, Id2 prevents activation of p57(Kip2) expression, and the retinoblastoma tumor suppressor protein, a known Id2 inhibitor, counters this activity. The strong E47-mediated inhibition of entry into S phase is entirely reversed in cells in which expression of p57(Kip2) is silenced by RNA interference. During brain development, expression of p57(Kip2) is opposite that of Id2. Our findings identify p57(Kip2) as a functionally relevant target recruited by bHLH transcription factors to induce cell cycle arrest in developing neuroblasts and suggest that deregulated expression of Id proteins may be an epigenetic mechanism to silence expression of this CKI in neural tumors

Id2 Mediates Tumor Initiation, Proliferation, and Angiogenesis in Rb Mutant Mice

The inhibitor of differentiation Id2 is a target of the retinoblastoma (Rb) protein during mouse embryogenesis. In Rb(+/−) mice, LOH at the wild-type Rb allele initiates pituitary adenocarcinoma, a tumor derived from embryonic melanotropes. Here we identify a critical role for Id2 in initiation, growth, and angiogenesis of pituitary tumors from Rb(+/−) mice. We show that proliferation and differentiation are intimately coupled in Rb(+/−) pituitary cells before tumor initiation. In Id2-null pituitaries, premature activation of basic helix-loop-helix-mediated transcription and expression of the cdk inhibitor p27(Kip1) impairs the proliferation of melanotropes and tumor initiation. Without Id2, Rb(+/−) mice have fewer early tumor lesions and a markedly decreased proliferation rate of the tumor foci. Expression of Id2 by pituitary tumor cells promotes growth and angiogenesis by functioning as a master regulator of vascular endothelial growth factor (VEGF). In human neuroblastoma, the N-Myc-driven expression of Id2 is sufficient and necessary for expression of VEGF. These results establish that aberrant Id2 activity directs initiation and progression of embryonal cancer

Toward a biochemical classification of depressive disorders IX. DST results and platelet MAO activity

Post-dexamethasone cortisol and platelet monoamine oxidase (MAO) activity levels were examined in 50 depressed patients. The incidence of non-suppression was significantly greater in patients with high platelet MAO activity than in those with low activity. Similar results were obtained when males and females were analysed separately. The mean 4 p.m. post-dexamethasone cortisol level was significantly higher in those patients who had high MAO activity than in their low MAO counterparts. Moreover, a statistically significant positive correlation was observed between platelet MAO activity and 4 p.m. post-dexamethasone cortisol levels

Early onset of craniosynostosis in an Apert mouse model reveals critical features of this pathology

AbstractActivating mutations of FGFRs1–3 cause craniosynostosis (CS), the premature fusion of cranial bones, in man and mouse. The mechanisms by which such mutations lead to CS have been variously ascribed to increased osteoblast proliferation, differentiation, and apoptosis, but it is not always clear how these disturbances relate to the process of suture fusion. We have reassessed coronal suture fusion in an Apert Fgfr2 (S252W) mouse model. We find that the critical event of CS is the early loss of basal sutural mesenchyme as the osteogenic fronts, expressing activated Fgfr2, unite to form a contiguous skeletogenic membrane. A mild increase in osteoprogenitor proliferation precedes but does not accompany this event, and apoptosis is insignificant. On the other hand, the more apical coronal suture initially forms appropriately but then undergoes fusion, albeit at a slower rate, accompanied by a significant decrease in osteoprogenitor proliferation, and increased osteoblast maturation. Apoptosis now accompanies fusion, but is restricted to bone fronts in contact with one another. We correlated these in vivo observations with the intrinsic effects of the activated Fgfr2 S252W mutation in primary osteoblasts in culture, which show an increased capacity for both proliferation and differentiation. Our studies suggest that the major determinant of Fgfr2-induced craniosynostosis is the failure to respond to signals that would halt the recruitment or the advancement of osteoprogenitor cells at the sites where sutures should normally form

A role for kit receptor signaling in Leydig cell steroidogenesis

Kit and its ligand, Kitl, function in hematopoiesis, melanogenesis, and gametogenesis. In the testis, Kitl is expressed by Sertoli cells and Kit is expressed by spermatogonia and Leydig cells. Kit functions are mediated by receptor autophosphorylation and subsequent association with signaling molecules, including phosphoinositide (PI) 3-kinase. We previously characterized the reproductive consequences of blocking Kit-mediated PI 3-kinase activation in KitY719F/KitY719F knockin mutant male mice. Only gametogenesis was affected in these mice, and males are sterile because of a block in spermatogenesis during the spermatogonial stages. In the present study, we investigated effects of the KitY719F mutation on Leydig cell development and steroidogenic function. Although the seminiferous tubules in testes of mutant animals are depleted of germ cells, the testes contain normal numbers of Leydig cells and the Leydig cells in these animals appear to have undergone normal differentiation. Evaluation of steroidogenesis in mutant animals indicates that testosterone levels are not significantly reduced in the periphery but that LH levels are increased 5-fold, implying an impairment of steroidogenesis in the mutant animals. Therefore, a role for Kit signaling in steroidogenesis in Leydig cells was sought in vitro. Purified Leydig cells from C57BI6/J male mice were incubated with Kitl, and testosterone production was measured. Kitl-stimulated testosterone production was 2-fold higher than that in untreated controls. The Kitl-mediated testosterone biosynthesis in Leydig cells is PI 3-kinase dependent. In vitro, Leydig cells from mutant mice were steroidogenically more competent in response to LH than were normal Leydig cells. In contrast, Kitl-mediated testosterone production in these cells was comparable to that in normal cells. Because LH levels in mutant males are elevated and LH is known to stimulate testosterone biosynthesis, we proposed a model in which serum testosterone levels are controlled by elevated LH secretion. Leydig cells of mutant males, unable to respond effectively to Kitl stimulation, initially produce lower levels of testosterone, reducing testosterone negative feedback on the hypothalamic-pituitary axis. The consequent secretion of additional LH, under this hypothesis, causes a restoration of normal levels of serum testosterone. Kitl, acting via PI 3-kinase, is a paracrine regulator of Leydig cell steroidogenic function in vivo

Effects of senataxin and RNA exosome on B-cell chromosomal integrity

Loss of function of senataxin (SETX), a bona-fide RNA/DNA helicase, is associated with neuronal degeneration leading to Ataxia and Ocular Apraxia (AOA) in human patients. SETX is proposed to promote transcription termination, DNA replication, DNA repair, and to unwind deleterious RNA:DNA hybrids in the genome. In all the above-mentioned mechanisms, SETX unwinds transcription complex-associated nascent RNA which is then degraded by the RNA exosome complex. Here we have used B cells isolated from a SETX mutant mouse model and compared genomic instability and immunoglobulin heavy chain locus (IgH) class switch recombination (CSR) to evaluate aberrant and programmed genomic rearrangements, respectively. Similar to RNA exosome mutant primary B cells, SETX mutant primary B cells display genomic instability but a modest decrease in efficiency of CSR. Furthermore, knockdown of Setx mRNAs from CH12–F3 B-cell lines leads to a defect in IgA CSR and accumulation of aberrant patterns of mutations in IgH switch sequences. Given that SETX mutant mice do not recapitulate the AOA neurodegenerative phenotype, it is possible that some aspects of SETX biology are rescued by redundant helicases in mice. Overall, our study provides new insights into the role of the SETX/RNA exosome axis in suppressing genomic instability so that programmed DNA breaks are properly orchestrated.Biological sciences; Immunology; Genetics; Biochemistry; Molecular biology; RNA exosome, Senataxin, DNA/RNA hybrids, Class switch recombinatio

Epigenetic regulation in major depression and other stress-related disorders: molecular mechanisms, clinical relevance and therapeutic potential

Abstract Major depressive disorder (MDD) is a chronic, generally episodic and debilitating disease that affects an estimated 300 million people worldwide, but its pathogenesis is poorly understood. The heritability estimate of MDD is 30–40%, suggesting that genetics alone do not account for most of the risk of major depression. Another factor known to associate with MDD involves environmental stressors such as childhood adversity and recent life stress. Recent studies have emerged to show that the biological impact of environmental factors in MDD and other stress-related disorders is mediated by a variety of epigenetic modifications. These epigenetic modification alterations contribute to abnormal neuroendocrine responses, neuroplasticity impairment, neurotransmission and neuroglia dysfunction, which are involved in the pathophysiology of MDD. Furthermore, epigenetic marks have been associated with the diagnosis and treatment of MDD. The evaluation of epigenetic modifications holds promise for further understanding of the heterogeneous etiology and complex phenotypes of MDD, and may identify new therapeutic targets. Here, we review preclinical and clinical epigenetic findings, including DNA methylation, histone modification, noncoding RNA, RNA modification, and chromatin remodeling factor in MDD. In addition, we elaborate on the contribution of these epigenetic mechanisms to the pathological trait variability in depression and discuss how such mechanisms can be exploited for therapeutic purposes