The spindle matrix through the cell cycle in Drosophila

A spindle matrix has long been proposed to provide structural support for counterbalancing force production and a substrate for essential mitotic factors. For years the molecular identity of such a structure remained elusive. Recently a complex of nuclear proteins that reorganize into a spindle-like structure during prophase through metaphase that shows characteristics of a spindle matrix has been identified in Drosophila. We review how these results support the concept of a spindle matrix and discuss its possible function(s) during mitosis. Importantly, these molecules also appear to play critical roles during interphase in nuclear organization and function. Given that during cell division the entire nucleus undergoes a dynamic and tightly orchestrated reorganization, the reorganization of spindle matrix components during mitosis may comprise one phase of a continuum of "nuclear architectural remodeling events" that can be considered to extend throughout the entire cell cycle, even in the absence of a defined nucleus.     

Nuclear envelope dynamics.

The nuclear envelope (NE) provides a semi permeable barrier between the nucleus and cytoplasm and plays a central role in the regulation of macromolecular trafficking between these two compartments. In addition to this transport function, the NE is a key determinant of interphase nuclear architecture. Defects in NE proteins such as A-type lamins and the inner nuclear membrane protein, emerin, result in several human diseases that include cardiac and skeletal myopathies as well as lipodystrophy. Certain disease-linked A-type lamin defects cause profound changes in nuclear organization such as loss of peripheral heterochromatin and redistribution of other nuclear envelope components. While clearly essential in maintenance of nuclear integrity, the NE is a highly dynamic organelle. In interphase it is constantly remodeled to accommodate nuclear growth. During mitosis it must be completely dispersed so that the condensed chromosomes may gain access to the mitotic spindle. Upon completion of mitosis, dispersed NE components are reutilized in the assembly of nuclei within each daughter cell. These complex NE rearrangements are under precise temporal and spatial control and involve interactions with microtubules, chromatin, and a variety of cell-cycle regulatory molecules.     

Spatial organization of interphase chromosomes and the role of chromatin fibril dynamics in the positioning of genome elements

Many studies are devoted to the analysis of interphase chromosome architecture due to the evidence of the functional-dependent spatial organization of the genome. These studies are based on classical cytological methods, as well as on biochemical approaches (3C, 4C, 5C, Hi-C), which allow one to detect long-range interactions between fragments of chromatin fibril, including the genome-wide interactions. In this review, we discuss the results of these projects, which allow us to explain the functional basis of nucleus multilevel compartmentalization and to identify the principles of high-level chromatin organization. Special attention is paid to the enhancer-promoter interactions, which are important for the regulation of gene expression. In this regard, we provide a new interpretation to the model of an active chromatin hub and to the alternative model of an active chromatin compartment, which was proposed during reconsideration of some steps of the 3C procedure.     

Structural-functional organization of DNA in the interphase nucleus. Structural aspects

The role of residual nuclear structures (structures persisting upon the treatment of nuclei with a non-ionic detergent, nucleases and 2 M NaCl) in the spatial organization of DNA in the interphase nucleus has been considered. Experimental works that have engendered the concept of loop level of DNA organization in the nucleus are discussed. A comparison is made of the loop-domain and rosette-like patterns of DNA organization in the interphase nucleus.     

Arrangement of chromosomes in the interphase nucleus of plants

Chromosomal arrangement in the interphase nucleus has two main aspects: (1) arrangement of chromosomes with respect to nuclear polarity and to other nuclear components, and (2) arrangement of chromosomes with respect to one another. The latter aspect consists of two main types of spatial relationships; (a) relationships between different members of one chromosomal set, (b) relationships between different chromosomal sets. Data concerning various aspects of chromosomal arrangement in the interphase nucleus are presented and discussed and the genetic control as well as subcellular mechanisms which are involvled in nuclear organization, are elucidated. Evidence is presented indicating that, in common wheat, the gene system that determines the specific pattern of chromosomal arrangement in the nucleus is operating via the microtubular elements of the spindle system. The significance of ordered arrangement of chromosomes in the nucleus for the regularity of genetic activity and chromosomal behavior, is pointed out.Supported in part by a grant from the Stiftung Volkswagenwerk AZ I/34 075/76     

A Crowdsourced nucleus: Understanding nuclear organization in terms of dynamically networked protein function

The spatial organization of the nucleus results in a compartmentalized structure that affects all aspects of nuclear function. This compartmentalization involves genome organization as well as the formation of nuclear bodies and plays a role in many functions, including gene regulation, genome stability, replication, and RNA processing. Here we review the recent findings associated with the spatial organization of the nucleus and reveal that a common theme for nuclear proteins is their ability to participate in a variety of functions and pathways. We consider this multiplicity of function in terms of Crowdsourcing, a recent phenomenon in the world of information technology, and suggest that this model provides a novel way to synthesize the many intersections between nuclear organization and function. This article is part of a Special Issue entitled: Chromatin and epigenetic regulation of animal development.     

Structure and function in the budding yeast nucleus

Budding yeast, like other eukaryotes, carries its genetic information on chromosomes that are sequestered from other cellular constituents by a double membrane, which forms the nucleus. An elaborate molecular machinery forms large pores that span the double membrane and regulate the traffic of macromolecules into and out of the nucleus. In multicellular eukaryotes, an intermediate filament meshwork formed of lamin proteins bridges from pore to pore and helps the nucleus reform after mitosis. Yeast, however, lacks lamins, and the nuclear envelope is not disrupted during yeast mitosis. The mitotic spindle nucleates from the nucleoplasmic face of the spindle pole body, which is embedded in the nuclear envelope. Surprisingly, the kinetochores remain attached to short microtubules throughout interphase, influencing the position of centromeres in the interphase nucleus, and telomeres are found clustered in foci at the nuclear periphery. In addition to this chromosomal organization, the yeast nucleus is functionally compartmentalized to allow efficient gene expression, repression, RNA processing, genomic replication, and repair. The formation of functional subcompartments is achieved in the nucleus without intranuclear membranes and depends instead on sequence elements, protein-protein interactions, specific anchorage sites at the nuclear envelope or at pores, and long-range contacts between specific chromosomal loci, such as telomeres. Here we review the spatial organization of the budding yeast nucleus, the proteins involved in forming nuclear subcompartments, and evidence suggesting that the spatial organization of the nucleus is important for nuclear function.     

Species-specific reorganization of the interphase chromosome architecture in generative tissue as a special type of chromosomal mutations associated with speciation

Epigenetic mechanisms of speciation are considered, including heterochromatic modifications and changes in spatial chromosome organization in the generative cell systems. The value of lamina, topoisomerase II, and a DNA polypurine tract in the attachment of chromosomes to the nuclear envelope is discussed. It is postulated that the main event leading to species-specific fixation of gene mutations, chromosomal mutations, and heterochromatin modifications in speciation is the rearrangement of spatial chromosome organization in the nucleus. The change in interchromosomal relationships associated with the reorganization of the system of chromosomal contacts with the nuclear envelope and the rearrangement of the chromocenter apparatus of the interphase nucleus is estimated as a systemic mutation directly related to speciation.     

Juxtaposition of the generative cell and vegetative nucleus in the mature pollen grain of amaryllis (Hippeastrum vitatum)

Summary Computer-generated, three-dimensional reconstructions from serial ultrathin sections were used to investigate the spatial organization and extent of association between the generative cell and vegetative nucleus within the mature pollen grain of amaryllis. In all cases examined, the highly lobed vegetative nucleus was found in close proximity and positioned laterally to the elongated, oval shaped generative cell. Numerous projections of the vegetative nucleus come to within 53 nm of the inner vegetative cell plasma membrane which surrounds the generative cell. These areas of close association may continue transversely around the generative cell for a distance of up to 4 m. Although an association exists between the generative cell and vegetative nucleus of the mature pollen grain, it is apparent that several changes must take place after pollination in order to achieve the high amount of close contact that occurs between the vegetative nucleus and the numerous terminal cell extensions of the leading sperm in the pollen tube of amaryllis (Mogensen 1986). Thus, this study demonstrates that the spatial organization among components of the male germ unit in the mature pollen grain does not necessarily reflect relationships that ultimately exist among these components within the pollen tube.     

Nuclear matrix and structural and functional compartmentalization of the eucaryotic cell nucleus

Becoming popular at the end of the 20th century, the concept of the nuclear matrix implies the existence of a nuclear skeleton that organizes functional elements in the cell nucleus. This review presents a critical analysis of the results obtained in the study of nuclear matrix in the light of current views on the organization of the cell nucleus. Numerous studies of nuclear matrix have failed to provide evidence of the existence of such a structure. Moreover, the existence of a filamentous structure that supports the nuclear compartmentalization appears to be unnecessary, since this function is performed by the folded genome itself.     

Review: The Dynamics of the Nuclear Lamins during the Cell Cycle— Relationship between Structure and Function

The nuclear lamins are members of the intermediate filament (IF) family of proteins. The lamins have an essential role in maintaining nuclear integrity, as do the other IF family members in the cytoplasm. Also like cytoplasmic IFs, the organization of lamins is dynamic. The lamins are found not only at the nuclear periphery but also in the interior of the nucleus, as distinct nucleoplasmic foci and possibly as a network throughout the nucleus. Nuclear processes such as DNA replication may be organized around these structures. In this review, we discuss changes in the structure and organization of the nuclear lamins during the cell cycle and during cell differentiation. These changes are correlated with changes in nuclear structure and function. For example, the interactions of lamins with chromatin and nuclear envelope components occur very early during nuclear assembly following mitosis. During S-phase, the lamins colocalize with markers of DNA replication, and proper lamin organization must be maintained for replication to proceed. When cells differentiate, the expression pattern of lamin isotypes changes. In addition, changes in lamin organization and expression patterns accompany the nuclear alterations observed in transformed cells. These lamin structures may modulate nuclear function in each of these processes.