First published June 1, 2011 - More info
We have isolated progenitor cells from the stromal system of the fibrous dysplastic marrow of patients with McCune-Albright Syndrome. Analysis of the Gsalpha gene from individual colonies provided direct evidence for the presence of two different genotypes within single fibrous dysplastic lesions: marrow stromal cells containing two normal Gsalpha alleles, and those containing one normal allele and an allele with an activating mutation. Transplantation of clonal populations of normal cells into the subcutis of immunocompromised mice resulted in normal ossicle formation. In contrast, transplantation of clonal populations of mutant cells always led to the loss of transplanted cells from the transplantation site and no ossicle formation. However, transplantation of a mixture of normal and mutant cells reproduced an abnormal ectopic ossicle recapitulating human fibrous dysplasia and providing an in vivo cellular model of this disease. These results provide experimental evidence for the necessity of both normal and mutant cells in the development of McCune-Albright Syndrome fibrous dysplastic lesions in bone.
P Bianco, S A Kuznetsov, M Riminucci, L W Fisher, A M Spiegel, P G Robey
Impairment of insulin secretion due to prolonged hyperglycemia is believed to contribute to the manifestation of diabetes mellitus, often referred to as glucose toxicity of pancreatic beta cells. In addition, impaired beta cell function has been associated with elevated islet triglyceride content (lipotoxicity). Impaired functions of the transactivating factors islet duodenum homeobox-1 (IDX-1) and RIPE3b-binding proteins have been implicated in the pathological downregulation of insulin gene transcription by high glucose levels in pancreatic beta cell lines in vitro, and, similarly, the exposure of pancreatic islets to fatty acids decreases IDX-1 expression. Previously, we identified the basic leucine zipper transcription factor CCAAT/enhancer-binding protein beta (C/ EBPbeta) as an inhibitor of insulin gene transcription in pancreatic beta cells and showed that the expression of C/EBPbeta is upregulated in insulinoma-derived beta cell lines by sustained high glucose concentrations. Here we describe the regulation of the expression of IDX-1, C/EBPbeta, and insulin at the mRNA and protein levels in pancreatic islets in animal models of diabetes mellitus. Concomitant with a downregulation of IDX-1 and insulin expression, C/EBPbeta is upregulated in association with the manifestation of hyperglycemia during the development of diabetes in the Zucker diabetic fatty (fa/fa) rat and in the 90% pancreatectomy rat model of diabetes. This regulation is demonstrated to influence both the amount of cellular protein and the level of steady state messenger RNA. Our findings indicate that the differential dysregulation of both IDX-1 and C/EBPbeta, in response to sustained hyperglycemia or hyperlipidemia, may be involved in the impairment of insulin gene expression during the manifestation of diabetes mellitus.
J Seufert, G C Weir, J F Habener
Transcription intermediary factor 1 (TIF1) family members are nuclear proteins that regulate gene expression. Published data indicate that TIF1γ is important for hematopoiesis. However, in vivo data to support this are lacking, because Tif1g–/– mice die in utero. Aucagne and colleagues have now overcome this issue by generating mice lacking TIF1γ specifically in hematopoietic tissue, and they have determined that TIF1γ is a tumor suppressor in hematopoietic cells ( 2361–2370). The mice exhibited selective expansion of granulo-monocytic progenitors and, upon reaching six months of age, progressively developed a cell-autonomous myeloproliferative disorder that recapitulated the essential characteristics of chronic myelomonocytic leukemia (CMML) in humans. The clinical relevance of these data was highlighted by the authors’ observations that TIF1γ is downregulated in the leukemic cells of approximately 35% of patients with CMML. Interestingly, downregulation was a result of hypermethylation of the TIF1G promoter and not inactivating mutations. The authors therefore conclude that TIF1G is an epigenetically regulated tumor suppressor gene in hematopoietic cells. Further, they suggest that changes in TIF1γ expression could act as a biomarker of response to modifiers of chromatin structure, which are currently being developed for the treatment of CMML.
Obesity, type 2 diabetes, and the metabolic syndrome are major health problems that arise as a result of environmental and genetic factors. Studies in mice have identified the Pkcd-containing locus as contributing to both genetic and diet-induced insulin resistance. Bezy and colleagues have now extended these observations and determined that PKCδ is a regulator of hepatic insulin sensitivity and hepatosteatosis in both mice and humans ( 2504–2517). Initial analysis indicated that PKCδ expression was higher in both genetic and diet-induced mouse models of insulin resistance. Consistent with a role for PKCδ in regulating insulin sensitivity, Pkcd–/– mice and mice lacking PKCδ in the liver exhibited increased hepatic insulin signaling, accompanied by improved glucose tolerance and reduced hepatosteatosis, especially as they aged and developed features of the metabolic syndrome. Conversely, mice overexpressing PKCδ in the liver showed decreased hepatic insulin signaling and developed hepatic insulin resistance. Further analysis indicated that PKCδ expression was higher and correlated with fasting glucose levels and levels of circulating triglycerides in obese humans, leading the authors to suggest that PKCδ could be a good target for improving insulin sensitivity and preventing the development of diabetes and hepatosteatosis in individuals with diet-induced obesity.
With efforts to develop a vaccine that protects against HIV infection showing limited promise, researchers are seeking to develop alternative approaches to block HIV transmission. One such strategy is vaginal application of a microbicide. In this issue, Wheeler and colleagues report data that suggest that a CD4 aptamer (a structured RNA that binds CD4 with high affinity) fused to siRNAs targeting HIV gag or vif (which encode essential HIV proteins) or host CCR5 (which encodes a key HIV coreceptor) could be used as the active ingredient in a microbicide to prevent HIV transmission ( 2401–2412). CD4 aptamer-siRNA chimeras (CD4-AsiCs) containing siRNAs targeting gag, vif, or CCR5 were found to be specifically taken up by CD4+ cells, in which they knocked down expression of their target genes. More importantly, the CD4-AsiCs inhibited HIV infection of primary CD4+ cells in vitro and of CD4+ cells in polarized human cervicovaginal explants. Furthermore, when applied vaginally to humanized mice they protected against vaginal transmission of HIV. Although additional studies are required to determine how long gene silencing and protection lasts, these data suggest that microbicides containing CD4-AsiCs could provide a new tool in the fight against HIV/AIDS.
Clinical detection of most degenerative disorders is preceded by a nonsymptomatic phase, because the conditions arise after gradual loss of a particular cell type. Current models involve ablation of the cell type affected by a disease. However, they do not allow partial cell ablation, meaning that researchers are unable to study the early nonsymptomatic stages of the disease. In this issue, Fujioka and colleagues report their generation of a line of transgenic mice that can be crossed with mice expressing Cre in a cell-type specific manner to ablate only a fraction of the cells of a given cell type ( 2462–2469). Using these mice, the authors achieved a mosaic pattern of pancreatic β cell loss that led to impaired glucose tolerance in the presence of normoglycemia, a condition similar to the nonsymptomatic phase of type 1 diabetes. A mosaic pattern of loss of epidermal and hair follicle cells and inner ear hair cells was also achieved. Importantly, the affected tissues responded in distinct manners after partial cell loss: regeneration was observed in the islets and epidermis but not the inner ear. The authors therefore conclude that their mice will be of use for studying degenerative diseases, assessing the regenerative capacity of a tissue, and developing regenerative therapies.