Decoding how receptor tyrosine kinases (RTKs) mediate nuclear calcium signaling

Calcium (Ca2+) is a highly versatile intracellular messenger that regulates several cellular processes. Although it is unclear how a single-second messenger coordinates various effects within a cell, there is growing evidence that spatial patterns of Ca2+ signals play an essential role in determining their specificity. Ca2+ signaling patterns can differ in various cell regions, and Ca2+ signals in the nuclear and cytoplasmic compartments have been observed to occur independently. The initiation and function of Ca2+ signaling within the nucleus are not yet fully understood. Receptor tyrosine kinases (RTKs) induce Ca2+ signaling resulting from phosphatidylinositol 4,5-bisphosphate (PIP2) hydrolysis and inositol 1,4,5-trisphosphate (InsP3) formation within the nucleus. This signaling mechanism may be responsible for the effects of specific growth factors on cell proliferation and gene transcription. This review highlights the recent advances in RTK trafficking to the nucleus and explains how these receptors initiate nuclear calcium signaling.

Expansion microscopy at the nanoscale: The nuclear pore complex as a fiducial landmark

Expansion microscopy (ExM) is a magnification method that allows achieving super-resolved images using a conventional light microscope. In ExM, biomolecules, fluorescent proteins, and dyes are functionalized with specific handles to link a dense polyelectrolyte hydrogel, which can achieve an isotropic expansion of 4.5-fold in water. The use of ExM coupled with STED nanoscopy allows examining macromolecular machinery in life science, like the nuclear pore complex (NPC). In particular, in this chapter, we show a general protocol for labeling one of its subunit, i.e. the Nup153. Such method shows the nanoscale isotropy of the expansion process and enables precise measurement of the expansion factor. Finally, we used ExM for the visualization of a peculiar nuclear invagination in normal and aged cells.

Unr defines a novel class of nucleoplasmic reticulum involved in mRNA translation

Unr (officially known as CSDE1) is a cytoplasmic RNA-binding protein with roles in the regulation of mRNA stability and translation. In this study, we identified a novel function for Unr, which acts as a positive regulator of placental development. Unr expression studies in the developing placenta revealed the presence of Unr-rich foci that are apparently located in the nuclei of trophoblast giant cells (TGCs). We determined that what we initially thought to be foci, were actually cross sections of a network of double-wall nuclear membrane invaginations that contain a cytoplasmic core related to the nucleoplasmic reticulum (NR). We named them, accordingly, Unr-NRs. Unr-NRs constitute a novel type of NR because they contain high levels of poly(A) RNA and translation factors, and are sites of active translation. In murine tissues, Unr-NRs are only found in two polyploid cell types, in TGCs and hepatocytes. In vitro, their formation is linked to stress and polyploidy because, in three cancer cell lines, cytotoxic drugs that are known to promote polyploidization induce their formation. Finally, we show that Unr is required in vivo for the formation of Unr-containing NRs because these structures are absent in Unr-null TGCs.

Deep nuclear invaginations are linked to cytoskeletal filaments - integrated bioimaging of epithelial cells in 3D culture

The importance of context in regulation of gene expression is now an accepted principle; yet the mechanism by which the microenvironment communicates with the nucleus and chromatin in healthy tissues is poorly understood. A functional role for nuclear and cytoskeletal architecture is suggested by the phenotypic differences observed between epithelial and mesenchymal cells. Capitalizing on recent advances in cryogenic techniques, volume electron microscopy and super-resolution light microscopy, we studied human mammary epithelial cells in three-dimensional (3D) cultures forming growth-arrested acini. Intriguingly, we found deep nuclear invaginations and tunnels traversing the nucleus, encasing cytoskeletal actin and/or intermediate filaments, which connect to the outer nuclear envelope. The cytoskeleton is also connected both to other cells through desmosome adhesion complexes and to the extracellular matrix through hemidesmosomes. This finding supports a physical and/or mechanical link from the desmosomes and hemidesmosomes to the nucleus, which had previously been hypothesized but now is visualized for the first time. These unique structures, including the nuclear invaginations and the cytoskeletal connectivity to the cell nucleus, are consistent with a dynamic reciprocity between the nucleus and the outside of epithelial cells and tissues.

Methods for Single-Cell Pulse-Chase Analysis of Nuclear Components

Pulse-chase methods offer powerful tools for following the evolution of a biological system over time, but are usually limited to ensemble measurements of the average behavior of very large numbers of cells. Here we describe three methods ranging from a true pulse-chase, through selective regional photoactivation, to pharmacological induction of an altered protein state, which can be applied to time-dependent studies at the single-cell level. These methods are exemplified by experimental protocols to follow region-selective nuclear envelope targeting of nascent phospholipids, a nascent nuclear lamin protein (lamin B1), and an immature lamin precursor (prelamin A).