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Endoplasmic Reticulum Identified as Site of Gene Silencing
Ribonucleic acid (RNA) is generally known for its role in the production of protein. Messenger RNA (mRNA) acts as a middle-man between DNA and the protein building ribosomal RNA and transfer RNA molecules. mRNA is transcribed from the directions provided by DNA. It then moves from the nucleus of the cell to the ribosomes, which are located in the cytoplasm, to be used as a template for protein translation. This process of going from DNA to RNA to protein is termed the central dogma of molecular biology, and describes some of the most important processes common to all living organisms.

However, some short stretches of single stranded RNA, termed micro RNA (miRNA) are also involved in silencing gene expression. miRNA functions by finding long single strands of RNA in the cytoplasm. When the miRNA is able to complimentary base pair with a molecule of single stranded RNA, it prevents the gene from being expressed. miRNA mediated gene silencing can occur by either targeting the RNA for degradation, or by preventing translational machinery from accessing the RNA and making protein. This is a defense mechanism in the cell that helps protect against viral infections.

Gene silencing using miRNAs is a popular area of study, with high hopes for treating diseases and as a possible gene therapy tool. In many diseases, the expression of a particular gene may either be increased or decreased. Using gene silencing could provide a method to control the expression of these vital genes, thereby limiting symptoms and disease severity. In addition to potential therapeutic use, gene silencing is often used as a tool in scientific research, to determine cellular responses in the absence of a particular gene. miRNA are transferred into cells to help determine the function of a specific gene. Scrambled miRNA is an easily available negative control that allows the researchers to determine if the transfer protocol has affected the cells in any way. Gene silencing is an efficient, easily controlled method to inhibit gene expression in both primary cells as well as immortalized cell lines.

It is not know how precisely gene silencing works in the cell. It is suspected that binding of miRNA to single stranded RNA may simply block the ribosome, the organelle that builds proteins, from binding to the messenger RNA, thus blocking translation. However, targeted degredation of RNA is less well understood. Researchers do not yet understand how the miRNA brings machinery to the RNA to break it down. It is known that certain molecules seem to help cut up the single stranded RNA, which prevents translation of the gene.

While the general mechanism for gene silencing is understood, the location in the cell and specific details of gene silencing by miRNA has only been recently examined. Researchers at the University of California at Riverside have recently identified the membrane of the endoplasmic reticulum (ER) as the location of gene silencing. The ER is a network of membranes within the cell that is used for transport and production of organic molecules such as proteins and lipids. The researchers determined that miRNA mediated gene silencing requires a membrane protein called AMP-1, which is found in both the cell membrane as well as the membrane of the ER in both plant and animal cells. This was a surprising location for gene silencing to occur. The researchers hypothesize that most gene silencing occurs in the rough ER, which contains protein building ribosomes. Indeed, many secretory proteins are built in the rough ER so they can be packaged in transport vesicles and sent to their final destination. In addition, the protein AMP-1 is found primarily in the membrane of the rough ER.

The researchers obtained this data from studies conducted using plant cells; however, since AMP-1 is found in both plant and animal cells, they are confident the results will be similar in plant cells. Knowing where to look in the cell for miRNA mediated gene silencing will allow researchers to further dissect the mechanisms of gene silencing. The team plans to determine how the ER is involved in miRNA mediated RNA degredation, as well as determining how miRNA is targeted to the ER. This information will be valuable as more therapies are designed using gene silencing in the treatment of diseases.

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