8.18: Additional Subnuclear Structures

The eukaryotic nucleus is a double membrane-bound organelle that contains nearly all of the cell’s genetic material in the form of chromosomes. It is rightly called the “brain” of the cell as it shoulders the responsibility of responding to various physiological processes, stress, altered metabolic conditions, and other cellular signals.

The nucleus contains many membrane-less subnuclear organelles or nuclear bodies, such as nucleoli, Cajal bodies, speckles, paraspeckles, etc. These nuclear bodies help to spatially compartmentalize the nuclear environment and create distinct sites for some specific biological reactions. Besides, their membrane-less structure allows better exchange of their components with the nucleoplasm.

For example, Cajal bodies, which are spherical nuclear bodies inside the nucleus, contain various RNAs and proteins involved in the assembly and modification of small nuclear RNPs. It is inside these Cajal bodies that snRNPs undergo final maturation. The mature snRNPs then transit to the interchromatin granule clusters or nuclear speckles, where they are stored along with other splicing factors. The speckles are generally observed in close proximity to the active genes within the nucleus. This facilitates the recruitment of splicing factors to the sites of active transcription.

The nuclear bodies have also been shown to epigenetically regulate gene expression and hence, have implications in human diseases. For example, loss of Cajal bodies in Zebrafish embryos has been shown to arrest their development, illustrating their importance in animal development.

In a eukaryotic cell, the nucleus is one of the most complex yet highly organized organelles. 

It sustains an extremely dynamic environment with the help of many sub-nuclear structures, each of which serves a unique function inside the nucleus. 

Besides the nucleolus - which is the most prominent and well-studied organelle of the nucleus, there are several other nuclear domains or nuclear bodies that exist inside the nucleus. 

Cajal bodies and Interchromatin granule clusters are two such functionally specialized regions. 

These are both membrane-less structures that can change appearance depending on the cellular environment or the cell cycle stage. 

Cajal bodies are spherical structures enriched in proteins and RNAs. They can be found freely in the nucleoplasm or physically associated with histones and snRNA genes. 

They are the primary sites for the assembly of small nuclear ribonucleoproteins or snRNPs which are responsible for pre-mRNA splicing.

It is also inside Cajal bodies that guide RNA molecules called scaRNAs or small Cajal body-associated RNAs carry out post-transcriptional modifications on some snRNAs. 

These modifications are essential for the proper assembly and function of the snRNPs in pre-mRNA splicing. 

Interchromatin granule clusters or speckles are irregular, amorphous structures that are distributed in the nucleoplasm of a vertebrate cell nucleus. 

Speckles serve as the storehouse for fully mature snRNPs and other RNA processing components. 

The high local concentration of RNA and protein components in cajal bodies and interchromatin granule clusters helps in rapid assembly and transport of ribonucleoprotein complexes to the transcription and RNA processing sites within the nucleus.