Biogenesis of nuclear bodies

Abstract

The nucleus is unique amongst cellular organelles in that it contains a myriad of discrete suborganelles. These nuclear bodies are morphologically and molecularly distinct entities, and they host specific nuclear processes. Although the mode of biogenesis appears to differ widely between individual nuclear bodies, several common design principles are emerging, particularly, the ability of nuclear bodies to form de novo, a role of RNA as a structural element and self-organization as a mode of formation. The controlled biogenesis of nuclear bodies is essential for faithful maintenance of nuclear architecture during the cell cycle and is an important part of cellular responses to intra- and extracellular events.

Figure 1.

Structure and maintenance of nuclear bodies. The appearance and number of nuclear bodies is highly variable. Prominent examples include (A) the nucleolus (RNA polymerase I subunit RPA43 in green, rRNA processing enzyme fibrillarin in red); (B) nuclear speckles (splicing factor SC35 in red; DNA in blue); and (C) Cajal bodies (coilin in red) and histone locus bodies (histone genes transcription factor NPAT in green), which are closely associated with each other (arrow). Inset: higher magnification view. Scale bars: 2 µm. (D) Nuclear bodies are dynamic, steady-state structures exposed to a continuous flow of material in and out. The ratio of on/off rate determines the size, and possibly the shape, of the body. Minor changes in on/off-rate ratio can strongly influence nuclear body morphology.

Figure 2.

Classification of nuclear bodies based on activity. Nuclear bodies may either form (A) associated with a nuclear function, such as transcription (yellow), making their appearance dependent on the ongoing activity or (B) they may form independently of a function and subsequently associate with a particular nuclear site, such as an active gene locus (yellow).

Figure 3.

Theoretical nuclear body assembly pathways. Assembly of nuclear bodies may either involve a hierarchical and ordered assembly pathway in which assembly occurs by association of nuclear body components in a strictly defined sequence of steps or, alternatively, in a stochastic manner in which components assemble in random order.