The nuclear tubular invaginations are dynamic structures inside the nucleus of HeLa cells

Nuclear tubules (NTs) were found in the nucleus of HeLa cells. Although no function has been ascribed to these structures, our previous data has shown that they are the sites of Ca(2+) release with mitochondria shuttled around. In the present study, we further characterized these NTs through different fluorescent dye-labeling and red fluorescent protein transfection experiments. We found that doxorubicin (Dox) is a good indicator to demonstrate the NTs since Dox is fluorescent and DNA is able to quench its fluorescence. By using confocal and electron microscopy, we show that the number and nature of the NTs in HeLa vary from cell to cell, ranging from tubular to intricately branched structures. Additionally, these NTs are double-membrane invaginations of the nuclear envelope and usually lie close to nucleolus. At rest, NTs appeared to be stable and their mouths are always closed. Upon Ca(2+) ionomycin stimulation, various forms of dynamism, including membrane protrusion to the nucleus, enlargement and shrinkage of the NTs, and distortion of the nuclear envelope and NTs were observed over a time scale of minutes. These observations suggest that the NT represents a specialized and dynamic compartment inside the nucleus under the control of Ca(2+).