Nuclear actin and myosin in chromatin regulation and maintenance of genome integrity.

Cytoskeletal proteins are beginning to be considered as key regulators of nuclear function. Among them, actin and myosin have been implicated in numerous tasks, including chromatin regulation, transcription and assembly of nascent ribonucleoprotein complexes. We also know from work performed by several labs that influx of actin and myosin into the nucleus and out of the nucleus is tightly regulated. In particular, in the case of actin, its nucleocytoplasmic import/export cycle is controlled by the importin/exportin system and it correlates with the transcriptional state of the cell. These basic molecular functions of both actin and myosin seem to impact key cellular functions, including development and differentiation as well as the cellular response to DNA damage by directly affecting transcriptional reprograming. These observations are beginning to suggest that actin and myosin could play an important role in consolidating the organization of the mammalian genome and that loss of actin and myosin likely leads to a general instability of the genome. In this chapter, we provide a general background on evidence that actin and myosin are important in key nuclear functions. Following this, we will focus on evidence supporting of a role in genome organization and finally we will discuss increasingly striking results on the role of actin and myosin in the maintenance of genome integrity.

Emerging roles of cytoskeletal proteins in regulating gene expression and genome organization during differentiation.

In the eukaryotic cell nucleus, cytoskeletal proteins are emerging as essential players in nuclear function. In particular, actin regulates chromatin as part of ATP-dependent chromatin remodeling complexes, it modulates transcription and it is incorporated into nascent ribonucleoprotein complexes, accompanying them from the site of transcription to polyribosomes. The nuclear actin pool is undistinguishable from the cytoplasmic one in terms of its ability to undergo polymerization and it has also been implicated in the dynamics of chromatin, regulating heterochromatin segregation at the nuclear lamina and maintaining heterochromatin levels in the nuclear interiors. One of the next frontiers is, therefore, to determine a possible involvement of nuclear actin in the functional architecture of the cell nucleus by regulating the hierarchical organization of chromatin and, thus, genome organization. Here, we discuss the repertoire of these potential actin functions and how they are likely to play a role in the context of cellular differentiation.