REVIEW ARTICLE: Pathways and Pathology in Cell Nuclear Migration and Anchorage

Written by Dmitry Ratner

Introduction

The nucleus of a cell is not sedentary, but occupies different locations in the cell at different times for different purposes. The proper migration and anchoring of the nucleus during development in polarized cells (e.g. neurons, muscle cells) becomes physiologically critical for signal transduction, structural integrity, and other functions in multicellular organisms. Current studies show that failure of nuclei to migrate and anchor properly in different cells of the body may result in a range of physiological disorders including ataxias (Gros-Louis et al., 2007), where the affected individual suffers from poor coordination and an unsteady gait; muscular dystrophy (Bione et al., 1994; Burke et al., 2001; Burton and Davies, 2002, Hutchison et al., 2001), which is characterized by progressive deterioration of skeletal muscles, eventually leading to complete physical disability; and lissencephaly (Hong et al., 2000), a condition where the folds in the gray matter of the brain form improperly, resulting in severe physical and mental retardation. All of these disorders are traced to defects in the components of the nuclear envelope – one of the most neglected areas of study in cell biology until a few years ago.

Over the past few years, a revived interest in the functions of the nuclear envelope has warranted active research on the biochemical pathways of the nuclear envelope, particularly the pathways of nuclear migration and anchorage. Most of the focus has been on identifying proteins associated with the nuclear envelope, as well as characterizing their topology (e.g. their structure, function, and interaction with other proteins). This review explores recent findings in scientific literature on the role of the nuclear envelope in nuclear migration and anchorage, and it evaluates the relevance and implications of these findings for human disease.

Discussion Migration

In mammals, the consequences of most nuclear migration defects are so severe that they usually result in pre-natal death. Lissencephaly is a rare example of a nuclear migration defect in the neurons of the brain, where the affected individual may survive for many years. It is characterized by the improper folding (or complete lack of folding) of the brain's gray matter, which results in severe mental and physical retardation. Hong et al. (2000) determined the root cause of lissencephaly to be a mutation in the RELN gene, whose protein product reelin is required for the nuclear migration in cortical neurons. However, aside from lissencephaly, most nuclear migration defects are too severe in mammals to permit meaningful research on the mechanism. It is for this reason that nuclear migration is mostly studied in more simple organisms.

Indeed, most of what we currently know about nuclear migration is based on models that came from studying biochemical pathways in the model organism Caenorhabditis elegans, a nematode whose simple organization and high rate of reproduction makes it feasible and efficient to study under the microscope. An important breakthrough came when Starr et al. (2001) discovered the outer-nuclear membrane protein Unc-83 and determined that it is necessary for nuclear migration in C. elegans. Unc-83 is heavily expressed in migratory cells, and it localizes to the outer-nuclear membrane. McGee et al. (2007) confirmed that Unc-83 is required for nuclear migration, and showed that it interacts via its KASH domain with the SUN domain of inner-nuclear membrane protein Unc-84. However, it is still unknown what Unc-83 binds to on the cytosolic side.

As of yet, there is no definitive model for how nuclear migration occurs in humans and other mammals, yet it appears that cell biologists are taking note of the clues from the C. elegans system to guide their research of the pathway. Notably, Wilhelmsen et al. (2005) discovered a new outer nuclear-membrane protein, Nesprin-3, and determined that it indirectly connects to intermediate filaments in the cytoskeleton via the cross-linker protein plectin. The function of Nesprin-3 is still under speculation, but there is much excitement over its potential involvement in nuclear migration because of its homology to the better-studied protein Unc-83 in C. elegans. With the advent of Nesprin-3, it becomes plausible to hypothesize that Unc-83 binds to plectin in the cytosol, and that Nesprin-3 binds to a SUN protein at the nuclear envelope. Thus, homologies of proteins between different model organisms are proving very useful to researchers by giving them hints about the kinds of proteins that may be involved in nuclear migration, and how these proteins may interact. Nevertheless, the triggering events, coordination, and other details of the mechanism of migration have yet to be determined.

Anchorage

Once the nucleus of a cell migrates, it must securely anchor in place and not float away. Again, much of our current knowledge about nuclear anchorage comes from the C. elegans system. Starr and Han (2002) determined that in C. elegans, the nucleus is tethered to the actin cytoskeleton by the large dystrophin-like protein, Anc-1. Similar to the migration model, Anc-1 interacts via its KASH domain with the SUN domain of inner-nuclear membrane protein, Unc-84. The authors showed that anc-1 mutants displayed a strongly uncoordinated phenotype, indicating that nuclear anchorage is a physiologically important process even in simple organisms like C. elegans.

As the study of nuclear anchorage is expanding to mammalian systems, it is becoming apparent that the mammalian model shares many similarities with the C. elegansmodel. In the mammalian model, outer nuclear membrane proteins Syne-1 and Syne-2 (also known as Nesprin-1 and Nesprin-2) anchor the nucleus to the actin cytoskeleton by binding to F-actin on the cytosolic end, and SUN proteins of the inner nuclear membrane on the other end (Grady et al., 2005; Haque et al. 2006; Zhang et al., 2007). Both Syne-1 and Syne-2 are homologues of Anc-1 with a well-conserved KASH domain, while SUN proteins are similar in size to Unc-84 and also contain the well-conserved SUN domain. These similarities show that nuclear anchorage pathways are well-conserved among species, suggesting that this is indeed a critical mechanism in simple and complex organisms.

A number of studies have shown that Syne proteins play a crucial role in nuclear anchorage, and their proper expression in cells is physiologically important in organisms. Grady et al. (2005) showed via fluorescent microscopy that either Syne-1 or Syne-2 is required for the anchorage of myonuclei at the neuromuscular junction (NMJ) of muscle tissue in mice. Recently, Zhang et al. (2007) supported Grady et al.'s findings, and also confirmed the necessity of the KASH domain in both Syne-1 and Syne-2 for proper anchorage of both synaptic and non-synaptic nuclei at the NMJ. Zhang et al. found that disruption in anchorage by a genome-wide SYNE1 knockout causes respiratory failure in mice within 20 minutes of birth, indicating that Syne-1 is vital for respiration and motor neuron innervation. Mutations in SYNE1 have also been implicated in cerebellar ataxia in humans (Gros-Louis et al., 2007), where nuclei fail to anchor in Purkinje cells and the individual suffers from an unsteady gait. These studies all confirm that Syne-1 is necessary for the proper anchorage of nuclei at synaptic junctions, and they demonstrate the range of pathology resulting from disruptions in this pathway.

It appears that proper binding of inner nuclear membrane proteins to the nuclear lamina is also a crucial part of anchorage. Perhaps this is because the chromatin (which is attached to the lamina) must move smoothly along with the nuclear envelope to retain its position within the nucleus. Four separate studies (Bione et al., 1994; Burke et al., 2001; Burton and Davies, 2002; Hutchison et al., 2001) showed that mutations in genes coding for emerin, lamin A, and lamin C are implicated in Emery-Dreifuss muscular dystrophy, possibly through the disruption of nuclear anchorage. This pathway is not entirely understood in mammals, as there appears to be contradictory evidence between two equally convincing studies: Lee et al. (2002) discovered that the homologue of SUN1 in C. elegans, unc-84, requires lamin for localization at the inner nuclear membrane, yet Haque et al. (2006) found that in mammals SUN1 binds to lamin A but does not require it for localization. These findings not only raise questions about the role of lamins in nuclear anchorage, but also they suggest that there appears at least sometimes an unbroken chain of proteins from chromatin to the cytoskeleton (and therefore to the plasma membrane).

Topology of Key Nuclear Envelope Proteins

Syne-1 has important homologues in other model organisms which have aided researchers in studying nuclear anchorage. Two important homologues with similar function are MSP-300 in Drosophila melanogaster(Rosenberg-Hasson et al., 1996), and ANC-1 in C. elegans (Starr and Han, 2002). All three of these proteins have a well-conserved Klarsicht/ANC/Syne Homology (KASH) domain at the C-terminus which appears to be a unifying feature of all cytosolic proteins that localize to the outer nuclear envelope (Wilhelmsen et al., 2006), regardless of whether they are involved in migration or anchorage. Because it is so well conserved, many believe that KASH proteins may be universal "adaptors" that regulate nuclear migration and anchorage in all eukaryotes. Some have also proposed that proteins with KASH-like domains may be involved in the migration and anchorage of other organelles such as mitochondria and the golgi apparatus (Starr & Fischer, 2005). However, in the context of nuclear migration and anchorage, KASH domains are significant because they associate exclusively with the SUN domains of inner-nuclear membrane proteins. This will hopefully help to identify future proteins involved in migration and anchorage. Also, the high degree of conservation over different phylogenies demonstrates that nuclear migration and anchorage are extremely important cellular processes, and that the genes controlling these processes are under strong evolutionary control.

Conclusion

In the past five years, studies of the nuclear envelope have revealed a surprisingly complex array of proteins for what does not appear at first glance to be a complex or significant process. However, an increasing number of diseases and syndromes are being traced to defects in nuclear migration and anchorage, warranting more research in this area of cell biology. The next step is to propose a biochemical mechanism for how migration actually occurs, how the correct position of the nucleus is determined, and finally how the cell switches from the migration pathway to the anchorage pathway.

Thanks to studies in non-mammalian model organisms, researchers have been able to identify important domains of the proteins involved in nuclear migration and anchorage based on conserved homologies across species. As more nuclear envelope proteins continue to be discovered, we will gain better insight into the mechanisms that govern these processes. In future studies, it is likely that the current model for migration and anchorage will be extrapolated to include other organelles of the cell, as there are promising hints that similar mechanisms are involved. As we uncover more components of the unbroken chain of proteins extending from the nucleus to the plasma membrane, we will learn how the cell is able to alter the shape and position of its organelles. Ultimately, we should hope that future studies of these nanoscopic processes will lead to a better understanding of how their disruption leads to maladies on the macroscopic level.

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