Studies on the Histone Methyltransferase G9a

The size and complexity of eukaryotic genomes require that specific mechanisms exist for ensuring both the stability as well as the accessibility of DNA. One such mechanism is the association of DNA with histones to form chromatin, the physiological substrate of gene expression. An important means by which histones impact transcriptional activity is through site-specific enzymatic modification of the amino terminal histone “tailsâ€, which can alter the spectrum of chromatin-associated proteins and hence transcriptional states. Among the known modifications of histones, lysine methylation has been proposed to represent a relatively stable mark which might mediate stable activation or repression, depending upon the site modified. The immune system provides an ideal system in which to test the physiological functions of particular chromatin-modifying activities, since proper lymphocyte development and function requires integration of multiple signals, both cell autonomous and receptor-mediated, with complex DNA recombination reactions which are unique to lymphocytes. We have exploited these features to explore the possible functions of histone 3, lysine 3 (H3K9) methylation in the immune system through conditional inactivation of the H3K9-specific methyltransferases G9a and GLP. These studies demonstrate that G9a is essential for B cell development in the mouse, but is dispensable for T cell development. The defect in B lymphopoiesis in the absence of G9a is caused by a block in development at the pro-B cell stage, corresponding to the onset of immunoglobulin heavy chain recombination. The overall normal behavior of G9a-deficient peripheral B cells argues in favor of the specificity of this effect. Furthermore, through analysis of G9a and GLP protein sequences, we have identified a novel mechanism by which chromatin-modifying complexes can be regulated. We find that both G9a and GLP contain conserved H3K9-like motifs, on which the main biochemical features of H3K9 itself are recapitulated. Considering both the sequence and functional conservation between these sites and histones, we term these motifs in nonhistone proteins “histone mimicsâ€. Our initial analysis indicates that many chromatinassociated proteins potentially contain H3K9-type histone mimics, and that this phenomenon is therefore likely to be a general one

Magnetic Resonance Imaging of the Elbow: A Structured Approach

Context: The elbow is a complex joint and commonly injured in athletes. Evaluation of the elbow by magnetic resonance imaging (MRI) is an important adjunct to the physical examination. To facilitate accurate diagnosis, a concise structured approach to evaluation of the elbow by MRI is presented. Evidence Acquisition: A PubMed search was performed using the terms elbow and MR imaging. No limits were set on the range of years searched. Articles were reviewed for relevance with an emphasis of the MRI appearance of normal anatomy and common pathology of the elbow. Results: The spectrum of common elbow disorders varies from obvious acute fractures to chronic overuse injuries whose imaging manifestations can be subtle. MRI evaluation should include bones; lateral, medial, anterior, and posterior muscle groups; the ulnar and radial collateral ligaments; as well as nerves, synovium, and bursae. Special attention should be paid to the valgus extension overload syndrome and the MRI appearance of associated injuries when evaluating throwing athletes. Conclusion: MRI evaluation of the elbow should follow a structured approach to facilitate thoroughness, accuracy, and speed. Such an approach should cover bone, cartilage, muscle, tendons, ligaments, synovium, bursae, and nerves

G9a co-suppresses LINE1 elements in spermatogonia

BACKGROUND: Repression of retrotransposons is essential for genome integrity and the development of germ cells. Among retrotransposons, the establishment of CpG DNA methylation and epigenetic silencing of LINE1 (L1) elements and the intracisternal A particle (IAP) endogenous retrovirus (ERV) is dependent upon the piRNA pathway during embryonic germ cell reprogramming. Furthermore, the Piwi protein Mili, guided by piRNAs, cleaves expressed L1 transcripts to post-transcriptionally enforce L1 silencing in meiotic cells. The loss of both DNA methylation and the Mili piRNA pathway does not affect L1 silencing in the mitotic spermatogonia where histone H3 lysine 9 dimethylation (H3K9me2) is postulated to co-repress these elements. RESULTS: Here we show that the histone H3 lysine 9 dimethyltransferase G9a co-suppresses L1 elements in spermatogonia. In the absence of both a functional piRNA pathway and L1 DNA methylation, G9a is both essential and sufficient to silence L1 elements. In contrast, H3K9me2 alone is insufficient to maintain IAP silencing in spermatogonia. The loss of all three repressive mechanisms has a major impact on spermatogonial populations inclusive of spermatogonial stem cells, with the loss of all germ cells observed in a high portion of seminiferous tubules. CONCLUSIONS: Our study identifies G9a-mediated H3K9me2 as a novel and important L1 repressive mechanism in the germ line. We also demonstrate fundamental differences in the requirements for the maintenance of L1 and IAP silencing during adult spermatogenesis, where H3K9me2 is sufficient to maintain L1 but not IAP silencing. Finally, we demonstrate that repression of retrotransposon activation in spermatogonia is important for the survival of this population and testicular homeostasis

Heterochromatin-Mediated Gene Silencing Facilitates the Diversification of Olfactory Neurons

An astounding property of the nervous system is its cellular diversity. This diversity, which was initially realized by morphological and electrophysiological differences, is ultimately produced by variations in gene-expression programs. In most cases, these variations are determined by external cues. However, a growing number of neuronal types have been identified in which inductive signals cannot explain the few but decisive transcriptional differences that cause cell diversification. Here, we show that heterochromatic silencing, which we find is governed by histone methyltransferases G9a (KMT1C) and GLP (KMT1D), is essential for stochastic and singular olfactory receptor (OR) expression. Deletion of G9a and GLP dramatically reduces the complexity of the OR transcriptome, resulting in transcriptional domination by a few ORs and loss of singularity in OR expression. Thus, our data suggest that, in addition to its previously known functions, heterochromatin creates an epigenetic platform that affords stochastic, mutually exclusive gene choices and promotes cellular diversity

Myoblast fusion confusion: the resolution begins

The fusion of muscle precursor cells is a required event for proper skeletal muscle development and regeneration. Numerous proteins have been implicated to function in myoblast fusion, however the majority are expressed in diverse tissues and regulate numerous cellular processes. How myoblast fusion is triggered and coordinated in a muscle-specific manner has remained a mystery for decades. Through the discovery of two muscle-specific fusion proteins, Myomaker and Myomerger/Minion, we are now primed to make significant advances in our knowledge of myoblast fusion. This article reviews the latest findings regarding the biology of Myomaker and Minion/Myomerger, places these findings in the context of known pathways in mammalian myoblast fusion, and highlights areas that require further investigation. As our understanding of myoblast fusion matures so does our potential ability to manipulate cell fusion for therapeutic purposes

MRI and Ultrasound of the Wrist Tendons

Wrist pain is common, and with diagnostic imaging often readily available, more patients are getting scans to find the etiology of pain. Both magnetic resonance imaging (MRI) and ultrasound are useful tools for evaluating the wrist. Ultrasound is a suitable imaging modality for the wrist tendons as it is fast and relatively inexpensive. Since the wrist tendons are superficial, the probe can be directly placed over them, thus acquiring excellent resolution and anatomic detail. MRI also provides high quality images of the tendons with the added bonus of detecting other abnormalities of the bones or cartilage which may not be apparent on ultrasound. In this paper we will review the normal anatomy and appearance of the tendons of the wrist and then present examples of common pathologies on MRI and ultrasound

Myoblast fusion confusion: the resolution begins

Abstract The fusion of muscle precursor cells is a required event for proper skeletal muscle development and regeneration. Numerous proteins have been implicated to function in myoblast fusion; however, the majority are expressed in diverse tissues and regulate numerous cellular processes. How myoblast fusion is triggered and coordinated in a muscle-specific manner has remained a mystery for decades. Through the discovery of two muscle-specific fusion proteins, Myomaker and Myomerger–Minion, we are now primed to make significant advances in our knowledge of myoblast fusion. This article reviews the latest findings regarding the biology of Myomaker and Minion–Myomerger, places these findings in the context of known pathways in mammalian myoblast fusion, and highlights areas that require further investigation. As our understanding of myoblast fusion matures so does our potential ability to manipulate cell fusion for therapeutic purposes

State of the Art: Imaging of Brown Adipose Tissue

The rates of diabetes, obesity, and metabolic disease have reached epidemic proportions worldwide. In recent years there has been renewed interest in combating these diseases not only by modifying energy intake and lifestyle factors, but also by promoting endogenous energy expenditure. This approach has largely been prompted by the recent recognition that brown adipose tissue (BAT)—long known to initiate heat production and energy expenditure in infants and hibernating mammals—also exists in adult humans. This landmark finding relied upon the use of clinical 18F-FDG-PET/CT and imaging techniques continue to play a critical and increasingly central role in understanding BAT physiology and function. Here, we review the origins of BAT imaging, discuss current preclinical and clinical strategies for imaging BAT, and discuss novel imaging methods that will provide crucial insight into metabolic disease and how it may be treated through modulation of BAT activity

WebPresent- A World Wide Web based tele-presentation tool for physicians

In this paper, we present the design architecture and the implementation status of WebPresent- a World Wide Web (WWW) based tele-presentation tool. This tool allows a physician to use a conference server workstation and make a presentation of patient cases to a geographically distributed audience. The audience consists of other physicians collaborating on patients ' health care management and physicians participating in continuing medical education. These physicians are at several locations with networks of different bandwidth and capabilities connecting them. Audiences also receive the patient case information on different computers ranging from highend display workstations to laptops with low-resolution displays. WebPresent is a scalable networked multimedia tool which supports the presentation of hypertext, images, audio, video, and a white-board to remote physicians with hospital Intranet access. WebPresent allows the audience to receive customized information. The data received can differ in resolution and bandwidth, depending on the availability of resources such as display resolution and network bandwidth