Investigation of the determinants of nuclear pore number

Abstract

To assess the functional significance of nuclear pore complexes, we have investigated whether the number of pores per nucleus is determined by such factors as the nuclear volume, nuclear surface area, DNA content, or aspects of nuclear activity. Comparisons were made between cell types chosen to permit observation of differences in nuclear pore number as a function of differences in the other qualities measured. The number of nuclear pores was determined by freeze-etching and measurements of nuclear surface and nuclear volume by electron and light microscopy. Pairs of cell strains in culture that contained different numbers of chromosome sets were investigated to examine the relation of pore number to total DNA content. Tetraploid cells of the rat kangaroo (Potorous tridactylus) have almost exactly twice the number of pores found in the parental diploid strain. However, the pore number in diploid grassfrog (Rana pipiens) cells was only 65% greater than in the parental haploid cells. In addition, a polyploid series of nucleated RBC had a 62% pore number increase with each successive increase in ploidy. Diploid cell strains from the canyon mouse (Peromyscus crinitus) and from the cactus mouse (P. eremicus) were compared to test whether a difference reflecting the 36% additional DNA in cells of the latter, associated with extra heterochromatin, existed. Although both were found to have the same number of pores and nuclear surface area, the cells differed in nuclear volume. These observations suggest that the number of nuclear pores is independent of the total amount of nuclear DNA, the nuclear surface area (and, thus, presumably the fraction of DNA that is bound to the nuclear membrane), the nuclear volume, and the size of the genome. Rather, the number of nuclear pores appears to be associated with some aspect of nuclear metabolic activity, e.g., transcriptional capacity or release of products to the cytoplasm. Further evidence for such a view comes from studies of chick embryo erythroblasts. In these, nuclear pore number was found to be lower in association with the decreasing nuclear transcriptional activity and longer generation times that characterize the successive cell divisions leading to the fully differentiated state. The number of pore complexes reconstructed in the last cell cycles declined in a manner consistent with reutilization of previously formed pores in the absence of new pore synthesis. Challenging this interpretation is the increase in pore number at lower metabolic activity when Xenopus laevis cells are grown at different temperatures. The speculation that pore complexes have a longer half-life in cooler grown Xenopus cells could resolve the discrepancy.

Pore-linked filaments in anura spermatocyte nuclei

“…The data showed that the number of NPCs is not directly proportional to the DNA content but to the nuclear surface which in the 4n is only 1.6 times larger than in the 2n. This fact suggested that the number of NPCs could be related to transcription activity or to the transport of products to the cytoplasm (Maul et al 1980). The intranuclear fi laments here described in both 2n and 4n species assemble in a network that connect NPCs and are associated with chromatin fi bers, spheroidal structures and RNP particles.…”

“…A possible difference among these species is the common presence of aggregates of pores connected by intranuclear fi laments in the tetraploid animals. Previous data showed that the frequency of clusters with four pores increases in these polyploid anurans (Maul et al 1980). We do not know whether the complex aggregates of neighboring NPCs associated with the intranuclear fi laments obtained in the spreads might correspond to the pore clustering described in these 4n frogs using freeze-etching experiments (Maul et al 1980).…”

“…Previous data showed that the frequency of clusters with four pores increases in these polyploid anurans (Maul et al 1980). We do not know whether the complex aggregates of neighboring NPCs associated with the intranuclear fi laments obtained in the spreads might correspond to the pore clustering described in these 4n frogs using freeze-etching experiments (Maul et al 1980). This is an interesting question considering that the number of NPCs in the 4n frogs with reduced metabolism seems to be related to transcription activity or to the export (Maul et al 1980).…”

“…We do not know whether the complex aggregates of neighboring NPCs associated with the intranuclear fi laments obtained in the spreads might correspond to the pore clustering described in these 4n frogs using freeze-etching experiments (Maul et al 1980). This is an interesting question considering that the number of NPCs in the 4n frogs with reduced metabolism seems to be related to transcription activity or to the export (Maul et al 1980). Moreover, pore clustering is either an early feature of apoptotic cells or of Nup 153 depleted cells with altered import mechanisms (Reipert et al 1999, Walther et al 2001.…”

Ceramide regulation of nuclear protein import

“…Although overt changes in nucleocytoplasmic trafficking will dramatically affect cellular homeostasis, it is unclear whether subtle stimuli can sufficiently alter transport to effect physiological and pathological changes in cell growth. Although metabolism plays an important role in modifying nuclear protein import (31), it currently remains unknown whether a lipid metabolite can inhibit nuclear transport. This suggestion was based upon an association of nuclear pore density with cellular transcriptional capacity and "release of products to the cytoplasm" (31).…”