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La célula. 4. Nucleus.

NUCLEAR ENVELOPE

« Nucleus Nuclear pores »

The nuclear envelope separates nucleoplasm from cytoplasm.

It is composed of outer membrane, inner membrane, intermembrane space, nuclear pores and nuclear lamina.

Nuclear envelope is necessary for maintaining the organization and size of the nucleus, mainly because of the nuclear lamina.

Nuclear pores control the trafficking of molecules between nucleoplasm and cytoplasm.

The outer membrane of the nuclear envelope is continuous with the membrane of the endoplasmic reticulum..

In the late XIX century, a barrier limiting the nucleus was suggested, which was later confirmed by electron microscopy. The nuclear envelope is a physical barrier that separates nucleoplasm (chromatin and the rest of the molecular content of the nucleus) from cytoplasm, controls the communication between both, i.e. the movement of molecules, is in charge of the nuclear morphology, and contributes to the organization of the chromatin, providing the anchoring points where chromatin is attached.

The nuclear envelope is composed of outer membrane, inner membrane, and intermembrane space (25-40 nm in height) between them. All together they form the so called perinuclear cisterna. The outer membrane is continuous with the inner membrane and with the endoplasmic reticulum, and attached ribosomes can be observed. Membrane continuity allows a direct communication between the lumen of endoplasmic reticulum and the lumen of perinuclear cisternae. For example, nuclear envelope may work as calcium storing compartment, along with endoplasmic reticulum. Nuclear envelope may interact with cytoskeleton filaments, such as microtubules and actin filaments, which determine the location of the nucleus inside the cell.

The inner membrane of the nuclear envelope has a distinct molecular composition. For example, there are transmembrane proteins linked to chromatin and to the nuclear lamina. Nuclear lamina is another component of the nuclear envelope. The inner and outer membranes are continuous in the nuclear pore regions. How is it posible that both membranes have different molecular composition? A retaining molecular mechanism in the inner membrane has been suggested. Transmembrane proteins are synthesized in the outer membrane of the nuclear envelope or in the rough endoplasmic reticulum, and arrive to the inner membrane by lateral diffusion, but those that become linked to chromatin or to the nuclear lamina are retained in the inner membrane of the nuclear envelope.

Nuclear envelope

Organization of the nuclear envelope. It is composed of outer membrane, intermembrane space, inner membrane, and nuclear lamina. The outer membrane is continuous with the endoplasmic reticulum membrane. Nuclear pores are another component of the nuclear envelope.

In animal cells, nuclear lamina is a molecular network located between the inner membrane and chromatin. It is linked to the inner membrane by one side and to the chromatin by the other. The attachment between the inner membrane and the nuclear lamina is mediated by at least 20 different types of proteins inserted in the inner membrane. In mammals, nuclear lamina is around 20 - 25 nm thick. The main components of nuclear lamina are known as laminas, which are grouped in type A (A and C laminas) and type B (B1 and B2/B3 laminas). All of them belong to the intermediate filaments family. One of the main functions of the nuclear lamina is to contribute to keep the shape of the nucleus, as well as the organization of the nuclear envelope. Laminopathies are disorders caused by mutations in lamina genes, which lead to nuclear disorganization, weaker nuclear envelope structure, and eventually cell death. During mitosis, nuclear envelope should be disassembled and assembled again. This process is mediated by enzymatic action over the laminas that causes breakdown of nuclear lamina. The nuclear shape changes along with the expression of the proteins of the nuclear lamina, which clearly occurs during embryonic development, cell differentiation, and in some cell pathologies. Another function of nuclear lamina is to provide physical support for chromatin, which affects gen expression. For example, chromatin anchored to nuclear lamina is not usually transcribed, although some genes can do it. Furthermore, these chromatin anchored regions are different depending on the cell type and differentiation stage of the cell. It is suggested that nuclear lamina-chromatin interactions are regulatory elements of gene expression. Nuclear lamina is also a place for anchoring the nucleus to cytoskeleton, allowing to place the nucleus in a precise location of the cell, and also to move the nucleus from one place to another.

Nuclear pores are inserted in the nuclear envelope. They are in charge of the trafficking between the cytoplasm and nucleoplasm (see next page). Since it involves a considerable amount of resources, why do eukaryotic cells need to separate the DNA from cytoplasm? Some of the reasons are the following:

a) Gene stability: confining the gnome inside one compartment helps to maintain the stability of genes, which is higher than in prokaryotes; it should keep in mind that it is a huge amount of DNA.

b) Gene regulation: separation allows the gene regulation at a level that prokaryotes will never reach. For example, it prevents or allows the access of trasncription factors to DNA. Transcription factors are proteins that regulate gene expression and are synthesized in the cytoplasm. They must cross the nuclear envelope to work on the DNA. The molecular mechanisms that allow a transcription factor to enter the nucleus is usually the result of a chain of molecules, which may start with the activation of one receptor located in the plasma membrane. If some step of this chain is stopped, the gene will not be expressed.

c) Eukaryotic genes contain exons and introns, meaning that a maturation process of the primary mRNA is needed. It is dangerous to translate an unprocessed mRNA because it will produce malformed proteins, that may even cause pathologies. This mRNA processing is done in the nucleoplasm and only mature mRNA is allowed to cross the nuclear envelope.

d) Transcription and translation taking place in separate compartments (nucleoplasm and cytosol, respectively) provide an additional tool for regulation the flow of information from DNA to proteins. In this way, the transcription of a gene to mRNA does not mean inmediate translation. For example, preventing the crossing of the nuclear envelope by a mRNA avoids the synthesis of this particular protein at this moment. When needed, the synthesis of this protein will be very quick because the mRNA was already synthesized and cross the nuclear envelope is the only thing to be done to be translated in ribosomes.

In most of eukaryotic cells, the nuclear envelope is broken in little pieces during the mitotic prophase. Then, microtubules can gain acces and make contact with chromosomes. Once chromosomes are segregated, nuclear envelope is assembled again during telophase to form the nuclei of the two new cells. In yeasts, however, the integrity of nuclear envelope is maintained and new nuclei are formed by a process of strangulation, as during cytokinesis. This is because yeast are able to build a mitotic spindle inside the nucleus.

Bibliography

Rowat AC, Lammerding J, Herrmann H, Aebi U . Towards and integrated understanding of the structure and mechanics of the cell nucleus. 2008. BioEssays 30: 226-236.

Guo T, Fang Y. . Functional organization and dynamics of the cell nucleus. 2014. Frontiers in plant biology. vol 5. Artículo 378. doi: 10.3389/fpls.2014.00378


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Actualizado: 05-05-2016. 13:34