Nucleocytoplasmic interaction at the nuclear envelope in post meiotic microspores of Pinus banksiana

Nuclear envelope budding and its cellular functions

The nuclear pore complex (NPC) has long been assumed to be the sole route across the nuclear envelope, and under normal homeostatic conditions it is indeed the main mechanism of nucleo-cytoplasmic transport. However, it has also been known that e.g. herpesviruses cross the nuclear envelope utilizing a pathway entitled nuclear egress or envelopment/de-envelopment. Despite this, a thread of observations suggests that mechanisms similar to viral egress may be transiently used also in healthy cells. It has since been proposed that mechanisms like nuclear envelope budding (NEB) can facilitate the transport of RNA granules, aggregated proteins, inner nuclear membrane proteins, and mis-assembled NPCs. Herein, we will summarize the known roles of NEB as a physiological and intrinsic cellular feature and highlight the many unanswered questions surrounding these intriguing nuclear events.

Nuclear envelope budding is a response to cellular stress

Nuclear envelope budding (NEB) is a recently discovered alternative pathway for nucleocytoplasmic communication distinct from the movement of material through the nuclear pore complex. Through quantitative electron microscopy and tomography, we demonstrate how NEB is evolutionarily conserved from early protists to human cells. In the yeast Saccharomyces cerevisiae, NEB events occur with higher frequency during heat shock, upon exposure to arsenite or hydrogen peroxide, and when the proteasome is inhibited. Yeast cells treated with azetidine-2-carboxylic acid, a proline analog that induces protein misfolding, display the most dramatic increase in NEB, suggesting a causal link to protein quality control. This link was further supported by both localization of ubiquitin and Hsp104 to protein aggregates and NEB events, and the evolution of these structures during heat shock. We hypothesize that NEB is part of normal cellular physiology in a vast range of species and that in S. cerevisiae NEB comprises a stress response aiding the transport of protein aggregates across the nuclear envelope.

Plant nuclei can contain extensive grooves and invaginations

Plant cells can exhibit highly complex nuclear organization. Through dye-labeling experiments in untransformed onion epidermal and tobacco culture cells and through the expression of green fluorescent protein targeted to either the nucleus or the lumen of the endoplasmic reticulum/nuclear envelope in these cells, we have visualized deep grooves and invaginations into the large nuclei of these cells. In onion, these structures, which are similar to invaginations seen in some animal cells, form tubular or planelike infoldings of the nuclear envelope. Both grooves and invaginations are stable structures, and both have cytoplasmic cores containing actin bundles that can support cytoplasmic streaming. In dividing tobacco cells, invaginations seem to form during cell division, possibly from strands of the endoplasmic reticulum trapped in the reforming nucleus. The substantial increase in nuclear surface area resulting from these grooves and invaginations, their apparent preference for association with nucleoli, and the presence in them of actin bundles that support vesicle motility suggest that the structures might function both in mRNA export from the nucleus and in protein import from the cytoplasm to the nucleus.

Post-meiotic nucleo-cytoplasmic interaction in Cosmos bipinnatus : Early events at the nuclear envelope

An interaction involving the nuclear envelope and spherical double-membrane bound inclusions takes place in the cytoplasm of post-meiotic male microspores of Cosmos (tribe Heliantheae, sub-tribe Coreopsidinae). The identity of the spherical inclusions has yet to be fully established, but they closely resemble profiles elsewhere in the cytoplasm, themselves presumably derived from the mitochondrial population of the premeiotic pollen mother cells. Both the cytoplasmic and nucleaar-associated inclusions regularly contain a central 'vesicle', formed by an ingagination of their bounding membranes. The interaction, which occurs immediately prior to the deposition of the primexine of the pollen wall, involves the adhesion of the inclusions to the nuclear surface. Experiments with osmotically disrupted cells reveal that the inclusions are firmly bound to the envelope and, at the points of contact, electron opaque granules are regularly present. Frequently elements of the chromatin may be observed in juxtapostion to these points of contact, but on the inner face of the envelope. The interaction in Cosmos is proposed to constitute part of the process by which the cytoplasm and its content are realigned to the new "gametophylic" style of growth.

Post meiotic nucleo-cytoplasmic interaction in Pinus banksiana: The secretion of RNA by the nucleus

Using digestion with specific enzymes, the chemical nature has been resolved of components of the nuclear invaginations formed in post-meiotic microspores of Pinus banksiana (Lamb.). The content of the invaginations is formed almost entirely of RNA, while the main bulk of the granular material investing the narrow part of the invaginations is also RNA. The membranes composing the narrow portion of the invaginations are themselves sensitive to RNase.These data are considered as supporting further the hypothesis that the invaginations are involved in the passage and organisation of information-carrying macromolecules.