Nuclear speckles

Abstract

Nuclear speckles, also known as interchromatin granule clusters, are nuclear domains enriched in pre-mRNA splicing factors, located in the interchromatin regions of the nucleoplasm of mammalian cells. When observed by immunofluorescence microscopy, they usually appear as 20-50 irregularly shaped structures that vary in size. Speckles are dynamic structures, and their constituents can exchange continuously with the nucleoplasm and other nuclear locations, including active transcription sites. Studies on the composition, structure, and dynamics of speckles have provided an important paradigm for understanding the functional organization of the nucleus and the dynamics of the gene expression machinery.

Figure 1.

Speckles form in the interchromatin space. HeLa cells showing splicing factors localized in a speckled pattern as well as being diffusely distributed throughout the nucleoplasm. Bar = 5 µm.

Figure 2.

Structured illumination microscopy, using the OMX system (Applied Precision, Issaqua, Washington), of a HeLa cell expressing SC35-EYFP. At 100 nm resolution substructure can be observed within speckles. In addition, the diffuse population of SC35-EYFP is resolved as a granular distribution. Projection of twelve 0.125 µm optical sections through the center of a nucleus encompassing 1.5 µm. Image provided by Zsolt Lazar and R. Ileng Kumaran. Bar = 2 µm.

Figure 3.

Nuclear speckles are equivalent to interchromatin granule clusters. Immunoelectron microscopy using a primary antibody against SC35 and a secondary antibody conjugated to 15 nm colloidal gold. IGCs are composed of a series of particles measuring 20–25 nm in diameter that are connected in places by a thin fibril resulting in a beaded chain appearance. Bar = 500 nm.