- Composition of the Cell Nucleus
- Prestressesed nuclear organization
- What properties of the nucleus make it substrate for mechanotransduction?
- The dynamic cytoskeleton actively couples the cell surface to the nucleus
- The nucleoskeleton imparts mechanical stability to the nucleus
- Chromatin condensation forces keep the nucleus compact
The nucleus is an organelle found in most eukaryotic cells, the exception being red blood cells. In animal cells it is both the largest and stiffest organelle and is easily identifiable by light microscopy. The average mammalian nucleus has a diameter of ~6µm and occupies about 10% of the total cell volume.
The primary functions of the nucleus are to store the cell’s DNA, maintain its integrity, and facilitate its transcription and replication. The nuclear contents, which include the genetic material and the many proteins required for its processing, are enclosed within a double membrane known as the nuclear envelope, but remain functionally connected to the cytoplasm via nuclear pores. It is through these pores that RNA can be transported to the cytoplasm for further processing.
Deeper inside the nucleus resides the DNA, which usually exists in the form of interphase chromosomes. Being an extremely long molecule (~2 meters for mammals) DNA must be packaged extensively to fit inside the relatively small space of the nucleus. This is achieved via an energy dependent process that involves numerous proteins and ultimately gives rise to a structure known as chromatin.
Despite the importance of DNA packaging, it is equally important that the DNA sequence remains accessible to repair and transcription machinery. This accessibility is highly dependent on the extent of compaction and this is reflected in the two types of chromatin that exist in the nucleus; euchromatin and heterochromatin (reviewed in 4). Euchromatin is less compact than heterochromatin and is more transcriptionally active.
Another prominent structure found in the nucleus is the nucleolus. This is often seen as a distinctly dense body and is sometimes referred to as a sub-organelle (reviewed in ), although it is not bound by membrane. The nucleolus is enriched with tandem repeats of rDNA (regions of DNA that encode rRNA or ribosomal RNA). The major role of the nucleolus therefore, is the synthesis of rRNA and the assembly of ribosomes - the protein synthesis machinery of the cell.
Along with these major structural and functional components of the nucleus, dozens of additional smaller assemblies are often observed. These include cajal bodies, promyelocytic leukemia bodies, nuclear speckles, etc (reviewed in [6, 7]). While the functions of these structures remain unclear, their presence clearly indicates a high level of functional compartmentalization and organization within the nucleus.
Further, the plasticity of stem cell nucleus is attributed to enhanced collisions between chromosome interfaces due to lack of spatial organization. Differentiated cells reveal precise cell-type specific positional coordinates for each chromosome through physical anchoring to other chromosomes or scaffolding proteins [18, 31, 32, 33]. Such well-defined interfaces are brought about by the orderly assembly of nuclear structural proteins upon activation of gene expression programs [31, 34].
Microrheology studies have also demonstrated a higher viscoelastic modulus for the nucleoplasm relative to cytoplasm, arising primarily due to the heterogeneous chromatin organization [35, 36, 37]. The nuclear envelope behaves like an elastic sac with gel-like viscous contents [38, 39]. The importance of nuclear mechanics is reflected in many disease conditions.
While actin filaments and microtubules constantly undergo remodeling by a contractile mechanism and dynamic instability respectively  domain proteins and the microtubule associated motor protein, dynein, thus providing structural integrity to the nucleus. From inside, the nuclear lamins and chromatin are anchored to the inner nuclear membrane through adaptor transmembrane SUN (Sad1p, UNC-84) proteins, which in turn are connected to KASH proteins . Hence, though physically separated by the nuclear membrane (~50nm), the cytoplasm and nucleoplasm are linked by these evolutionarily conserved proteins, that mediate force transmission. Together these proteins are known as the Linker of Nucleoskeleton and Cytoskeleton (LINC) complex, [43, 44]. The localization of KASH domain proteins like nesprin at outer nuclear envelope is affected by depletion of SUNs, which in turn depend on nuclear lamins [45, 46, 47]. These links are emerging to be pivotal in various physiological processes including cell migration and cytoskeletal integrity. Together, this network helps the cell cope with mechanical stress .
Work by Mazumder et al. ascertained the active involvement of cytoskeletal forces in determining nuclear morphology. Change in nuclear size upon perturbation of actomyosin and microtubules affirmed their roles in exerting tensile and compressive forces respectively on the nucleus, correlating with their functions in the cellular context [49, 27, 12].
Furthermore, the 'perinuclear cap', which is composed of contractile actin bundles that bridge focal adhesions on either side of the nucleus, has been shown to tightly regulate the nuclear geometry . These bundles pass apically to form a dome covering the top of nucleus and are connected to the nucleus through the LINC complexes. They are completely absent in pluripotent cells whereas during differentiation, their formation accompanies expression and assembly of lamin A/C as well as the LINC complexes on the nuclear envelope . As a result, the nuclear height and shape are under their control, suggesting a role in mediating mechanosensitive processes such as motility and polarization .
Besides nuclear morphology, cytoplasmic forces also govern nuclear positioning in the cell by regulating the translational and rotational dynamics [53, 54]. Positioning is accomplished by the physical connection by nuclear envelope proteins SUN-KASH-lma1 between centromeric heterochromatin regions and the microtubule network . With the centromere providing tensional force on the microtubules that undergo dynamic instability, dynein motors mediate the rotation [56, 57]. Actin links via SUN-nesprin are implicated in force transduction for nuclear movement during cell migration . Regulation of nuclear position and orientation is critical in many cellular processes such as migration, cell division, polarization, fertilization and differentiation [54, 56].