Hormonal Regulation of Nuclear Permeability

Abstract

Transport into the nucleus is critical for regulation of gene transcription and other intranuclear events. Passage of molecules into the nucleus depends in part upon their size and the presence of appropriate targeting sequences. However, little is known about the effects of hormones or their second messengers on transport across the nuclear envelope. We used localized, two-photon activation of a photoactivatable green fluorescent protein to investigate whether hormones, via their second messengers, could alter nuclear permeability. Vasopressin and other hormones that increase cytosolic Ca 2؉ and activate protein kinase C increased permeability across the nuclear membrane of SKHep1 liver cells in a rapid unidirectional manner. An increase in cytosolic Ca 2؉ was both necessary and sufficient for this process. Furthermore, localized photorelease of caged Ca 2؉ near the nuclear envelope resulted in a local increase in nuclear permeability. Neither activation nor inhibition of protein kinase C affected nuclear permeability. These findings provide evidence that hormones linking to certain G protein-coupled receptors increase nuclear permeability via cytosolic Ca 2؉ . Short term regulation of nuclear permeability may provide a novel mechanism by which such hormones permit transcription factors and other regulatory molecules to enter the nucleus, thereby regulating gene transcription in target cells.The nuclear envelope represents a structural and functional barrier to passage between the nucleus and the cytosol. Regulation of the permeability of this barrier is one potential mechanism to control access to the nucleus for proteins that affect nuclear function, including transcription factors and various kinases and phosphatases. Nucleocytosolic passage generally occurs through the nuclear pore complex (1, 2), a 125-MDa membrane-spanning protein complex consisting of eight ion channels and a large central passage (3). Movement of molecules through the nuclear pore is restricted on the basis of size and the presence or absence of appropriate localization sequences. Studies using electron scanning microscopy (4), fluorescence recovery after photo-bleaching techniques (5), and microinjection of fluorescently labeled dextrans (6) have shown that molecules up to 40 -60 kDa in size can cross the nuclear envelope without a signaling sequence. Proteins less than 4 -10 kDa in size freely pass from cytosol to nucleus (6 -8), while intermediate sized proteins (19 -40 kDa) do not need a targeting sequence to cross the nuclear pore, but the permeability of the pore to such molecules may be modulated (6 -8). One factor that appears to regulate the permeability of the nuclear pore to proteins in this intermediate size range is the amount of Ca 2ϩ within the nuclear envelope (6 -9), although there is now conflicting evidence about this (5). Little is known about the role of cytosolic factors such as second messengers in the regulation of nuclear pore permeability.The spatial pattern of second messenger signals is important for how thes...

An update on nuclear calcium signalling

“…However, a similar study examining the diffusion of GFP between the nucleoplasm and cytosol of intact Xenopus oocytes reached the opposite conclusion; the NE reduced diffusion of GFP by ~100-fold, but depletion of Ca 2+ from the lumen of the NE made no difference (Wei et al, 2003). Yet a different finding was reported in a study of the movement of photo-activated GFP across the NE in hepatocytes (O'Brien et al, 2007). In this case, a rise in cytosolic Ca 2+ levels, and not changes in NE Ca 2+ content, increased the influx (but not the efflux) of photo-activated GFP across the NE.…”

Mengovirus-Induced Rearrangement of the Nuclear Pore Complex: Hijacking Cellular Phosphorylation Machinery

“…It is appropriate to note that phosphorylation was proposed to participate in regulation of the nuclear transport machinery in uninfected cells, although appropriate mechanisms are yet to be elucidated in detail (22,35). Importantly, some Nups, including Nup62, were shown to be phosphorylated (16,28,30,34).…”

Calcium Signaling in Synapse-to-Nucleus Communication

“…Accordingly, depletion of nuclear Ca 2þ stores in a variety of cell types has been shown to induce a conformational change of the nuclear pore complex such that it excludes molecules greater in size than approximately 10 kD (Greber and Gerace 1995;Stehno-Bittel et al 1995;Perez-Terzic et al 1999;Wang and Clapham 1999;Erickson et al 2006;Stoffler et al 2006). Alternately, a recent study suggests that Ca 2þ may alter nuclear permeability cytoplasmically: stimulation of G q/11 -coupled receptors and subsequent internal Ca 2þ release were shown to rapidly and briefly increase nuclear permeability in a liver cell line (O'Brien et al 2007). Interestingly, this increase in permeability-for which an increase in [Ca 2þ ] i near the nuclear envelope was both necessary and sufficientwas unidirectional from cytosol to nucleus.…”

“…Accordingly, molecules larger in size than 40 -60 kD need a signaling sequence to cross the nuclear envelope, and molecules smaller than 4 -10 kD diffuse freely between cytosol and nucleus. Intermediate-sized molecules (10 -40 kD) do not require a targeting signal, although their passage may be regulated at the level of the nuclear pore (reviewed in O'Brien et al 2007). Two Ca 2þ -dependent mechanisms have been proposed for such regulation.…”