Relationship of the Cajal Body to the Nucleolus

Abstract

Since its discovery over 100 years ago, the nucleoplasmic Cajal body (CB) has been linked to the nucleolus by both physical proximity and shared components. The CB contains factors necessary for small nuclear ribonucleoprotein (snRNP) and small nucleolar ribonucleoprotein (snoRNP) biogenesis. As snoRNPs are needed for ribosomal RNA processing, the CB can be considered to play a supportive role for nucleolar activity. Not all cell types contain CBs, but this structure is found in cells that are transcriptionally active, such as neuronal and cancer cells. Live-cell studies have shown that CBs can move to, associate with, and bud from nucleoli. Furthermore, CBs, or components therein, accumulate at the nucleolar surface or inside the nucleolus in response to various stresses, such as transcription inhibition, disruption of ribonucleoprotein (RNP) biogenesis, or DNA damage induced by neurodegeneration. Therefore, the CB and the nucleolus share a similar response to stress, and this arrangement may coordinate the level of ribosomal DNA transcription with the appropriate amount of snoRNP resources. These new findings demonstrate another facet of the functional relationship between the CB and the nucleolus.