Molecular Biology Figure 1 – The Nuclear Envelope Cytoplasm Emerin, Lap2β, and MAN1 are integral Emerin and the inner nuclear membrane (INM) proteins Mutations in the genes which code of the nuclear envelope where they Actin perform diverse functions within the for emerin have been linked to the nucleus. INM proteins bind to lamins to form a proteinaceous network called making of Microtubule development of Emery-Dreifuss the nuclear lamina that is required for nuclear structure and function. Emerin regulates gene expression by muscular dystrophy regulating chromatin architecture, by muscle binding to and regulating the activity of a number of transcription factors, and research team have found that emerin is and others have shown that emerin is able interacting with components of multiple signalling pathways. ONM, outer key to the expression of a number of genes to regulate the activity of Lmo7 through Nesprin 3/4 nuclear membrane; INM, inner nuclear important in the maintenance of muscle competitive binding, preventing the factor membrane; ER, endoplasmic reticulum. tissue; mutations in the genes which code for from initiating expression of genes in a way Nesprin 1/2 Nuclear Pore emerin have been linked to the development that is vital for correct differentiation of the cell. Complex of Emery-Dreifuss muscular dystrophy (EDMD) as well as certain cardiomyopathies (diseases Having identifiedLmo7 as a target of emerin’s of the heart muscles). How this modest protein activity, the team then demonstrated that ONM Emerin ER exerts these significant effects is the subject a mouse model which lacked the gene to Emerin of intense research by the Holaska laboratory. make Lmo7 exhibited similar symptoms to Sun 1/2 Sun LAP2 Sun 1/2 ß those of EDMD. The mice exhibited growth Emerin Emerin EMERIN AS THE ARCHITECT retardation, decreased muscle fibre size and HDAC3 Dr Holaska and his colleagues have identified impaired skeletal and cardiac muscle function. a number of ways in which emerin regulates The team were therefore able to confirm that INM HD AC3 the architecture of both the nucleus and the it is this interaction between emerin and Lmo7 Btf Repressed Emerin nucleosomes which hold the DNA within which is important for regulating the gene Repressed it. Perhaps the most important of these is transcription and signalling pathways which LMO7 LADS emerin’s regulation of histone deacetylase are the underlying mechanism of EDMD. 3 (HDAC3), one of a group of enzymes Emerin ß-Catenin which alter chromatin structure and regulate In cultures of myogenic progenitor cells TCF-LEF transcription factor access to DNA. The team (undifferentiated cells which will evolve into Chromatin GCL showed that emerin activates HDAC3 and skeletal muscle), Dr Holaska and his colleagues MAN1 localises it to the wall of the nucleus, where it have shown that the removal of emerin results E2F-DP3 is able to repress genes by restricting access in significant perturbation of four critical BAF to the DNA. The researchers have successfully signalling pathways for cell differentiation. Nucleus shown that these two molecules work They propose this is caused by emerin Lamins together to localise and activate or repress interacting with other molecules involved in specific sections of the genome during muscle the regulation of these pathways such as beta- formation and development. In investigating catenin (a dual function protein that regulates the structure of emerin itself, Dr Holaska and gene expression and cell adhesion), and his colleagues identified the regions of the microRNA (short sections of transcribed DNA protein required for specific purposes. These which interfere with other RNA to prevent The role of cellular machinery in the regulation of gene expression is IT’S ALL IN THE GENES arrangement of DNA in the nucleus (the enable the protein to be transported into the translation into proteins). The mechanisms are currently a field of intensive study. of Philadelphia’s Every cell of an organism contains the entire compartment of the cell where DNA is stored nucleus, and for it to then interact with both complex, however, and the team recognise Dr James Holaska genetic code of that organism, stored in the and managed). The entire DNA of a human the nuclear membrane and the underlying that more work is needed to fully elucidate the University of the Sciences is leading the way in uncovering the role of form of the now well-known deoxyribose cell, if stretched out, would be about two matrix of structural proteins called lamins, pathways involved. emerin in the development and maintenance of skeletal muscle and nucleic acid (DNA). However, not every cell metres long. For this reason, it is instead which supports it. This structural function pathology of muscle disease. This work has the potential to revolutionise needs all of it. Skin cells need to know how to tightly wound around support molecules has been evidenced by showing nuclear DRAWING OUT THE MERITS OF EMERIN the understanding and treatment of muscle diseases, including various make the proteins necessary for skin, nerve called histones. This histone/DNA complex is membrane dysfunction and increased cell Dr Holaska’s group continues to use the latest cells those needed to transmit signals, and called chromatin and is further packaged into death in emerin-deficient cells. molecular techniques to probe the mechanisms types of muscular dystrophy and cardiomyopathy. muscle cells those needed to make muscles units called nucleosomes. DNA cannot be underlying nuclear envelope architecture and move. For this reason, in any one cell, a vast transcribed for translation into proteins whilst EMERIN AS THE FACILITATOR the role of the proteins that interact with it. To proportion of the genome is silenced or it is stored in this way, so which sections are in The researchers have repeatedly shown that date their work has made a significant impact merin is a relatively small protein the fact that it is highly conserved (the identical repressed and will never be translated into storage and which unwound is an important emerin does not work alone, rather it recruits on the understanding of emerin’s function in made up of just 254 amino protein exists in many other species) and it proteins. How this is achieved is a major regulatory mechanism in gene transcription. and regulates a variety of other molecules. The both gene expression and the architecture of acids. Despite its small size, it has all the hallmarks of a vital element of living question for modern science, particularly most significant findings in terms of EDMD the nucleus which supports it. By observing is fundamental to the correct organisms. Knowing it is important is not considering recent work with stem cells ENTER EMERIN and related myopathies is emerin’s relationship normal function and comparing it to that in formation of muscle tissues and their enough, however, and Dr Holaska and his team (undifferentiated cells with the potential to There are many diverse proteins associated with a specific transcription factor in muscle disease states, they have contributed much to maintenance and repair. Rich in the amino acid are determined to get to the bottom of its become any cell type). with the regulation of gene expression in the cell differentiation called Lmo7. The process the understanding of EDMD in particular and Eserine, responsible for the catalytic function molecular activity and identify the mechanisms nucleus, some key members of which are of muscle cell differentiation is complex and other myopathies generally. Their research so of many enzymes, it is clearly designed to by which it exerts its influence on gene Much of the regulation is thought to be identifiable by the disorders caused when requires particular genes to be ‘switched’ on far shows that there is much more still left to interact with other molecules. Couple this to expression and molecular pathways. achieved by structural differences in the they are not correctly expressed. Dr Holaska’s and off in a time-critical manner. Dr Holaska discover in this complex field. 42 www.researchfeatures.com www.researchfeatures.com 43 Molecular Biology Detail RESEARCH OBJECTIVES Dr Holaska’s work focuses on emerin What led you to begin investigating structure of the nuclear envelope and the and its role within the cell. His work Emery-Dreifuss muscular dystrophy? biophysical behaviour of the organelle as it has multiple applications, including a When I first started my post-doctoral relates to its elasticity, rigidity, ability to resist better understanding of Emery-Dreifuss fellowship fifteen years ago, it was our initial forces when stressed and the geometrical muscular dystrophy. interest in the function of proteins within configuration of the proteins and lipids that the nuclear envelope. The nuclear envelope generate these biophysical properties. The FUNDING was then considered by many to be merely second concerns genomic or chromatin National Institutes of Health, Ellison a ‘bag’ that held the genome or a barrier architecture, whereby repressed genes Medical Foundation, American Heart that physically separated the genome from localise to specific regions within the nucleus, Association the cytoplasmic machinery. However, the including the nuclear lamina at the nuclear presence of integral membrane proteins envelope, to induce, maintain or propagate COLLABORATORS on the nuclear side of the nuclear envelope repressed chromatin. Loss of emerin causes • Elizabeth McNally, Northwestern Figure 2 – Skeletal Muscle Progenitor Cell suggested these proteins had functions the nuclei to become more easily deformed Differentiation into Mature Multinucleated University Feinberg School of Medicine analogous to proteins on the inner side and alters the biophysical properties of Myotubes • Karen Reddy, Johns Hopkins University Skeletal muscle progenitors harvested from of the plasma membrane. Evidence over the nucleus demonstrating its importance mice were cultured in a dish and induced to School of Medicine the last 20 years showed that the nuclear in nuclear structure.