Lecture-hall cartoon depictions aside, the nucleus is not a balloon. Rather, its architecture is defined by a specialized cytoskeleton (the nuclear lamina) consisting of proteins known as lamins. These proteins give structure to the nuclear envelope during interphase; during cell division, a complex dance of phosphorylation and dephosphorylation causes them to disassemble (allowing for chromosome segregation into the daughter nuclei) and then reassemble following cytokinesis.
Derangement of the lamina might be expected to cause serious trouble for the cell. Indeed, an extensive body of work over the past 5 years has clearly demonstrated that a particular mutation in the lamin A gene (LMNA) results in severe nuclear malformation. In humans, this mutation gives rise to a devastating disease, Hutchinson-Gilford progeria syndrome (HGPS). (For earlier articles on HGPS, see here and here).
Two new papers, published back-to-back in the most recent issue of PNAS, describe the impact of the HGPS lamin A mutant protein (also called “progerin”) on progression of the cell cycle and the results of mitosis.
Dechat et al. find that progerin perturbs the cell cycle at multiple stages, both at entry into S phase and progression into M. The mechanisms are related: An uncleaved farnesylated moiety causes progerin to tightly associate with membranes, drastically changing its solubility profile and distribution; the mutant therefore interferes both with nuclear disassembly (during M, prior to chromosome segregation) and reassembly (after G1, before DNA synthesis begins).
Alterations in mitosis and cell cycle progression caused by a mutant lamin A known to accelerate human aging
Mutations in the gene encoding nuclear lamin A (LA) cause the premature aging disease Hutchinson–Gilford Progeria Syndrome. The most common of these mutations results in the expression of a mutant LA, with a 50-aa deletion within its C terminus. In this study, we demonstrate that this deletion leads to a stable farnesylation and carboxymethylation of the mutant LA (LAΔ50/progerin). These modifications cause an abnormal association of LAΔ50/progerin with membranes during mitosis, which delays the onset and progression of cytokinesis. Furthermore, we demonstrate that the targeting of nuclear envelope/lamina components into daughter cell nuclei in early G1 is impaired in cells expressing LAΔ50/progerin. The mutant LA also appears to be responsible for defects in the retinoblastoma protein-mediated transition into S-phase, most likely by inhibiting the hyperphosphorylation of retinoblastoma protein by cyclin D1/cdk4. These results provide insights into the mechanisms responsible for premature aging and also shed light on the role of lamins in the normal process of human aging.
Cao et al. demonstrate that the mutant protein forms aggregates during mitosis (at a time when the wildtype lamin is happily soluble, waiting for the nuclear envelope to coalesce after cytokineses). As a result, both chromosome segregation and separation of the daughter nuclei is aberrant, and binucleated cells frequently form.
A lamin A protein isoform overexpressed in Hutchinson–Gilford progeria syndrome interferes with mitosis in progeria and normal cells
Hutchinson–Gilford progeria syndrome (HGPS) is a rare genetic disorder characterized by dramatic premature aging. Classic HGPS is caused by a de novo point mutation in exon 11 (residue 1824, C -> T) of the LMNA gene, activating a cryptic splice donor and resulting in a mutant lamin A (LA) protein termed “progerin/LAΔ50″ that lacks the normal cleavage site to remove a C-terminal farnesyl group. During interphase, irreversibly farnesylated progerin/LAΔ50 anchors to the nuclear membrane and causes characteristic nuclear blebbing. Progerin/LAΔ50’s localization and behavior during mitosis, however, are completely unknown. Here, we report that progerin/LAΔ50 mislocalizes into insoluble cytoplasmic aggregates and membranes during mitosis and causes abnormal chromosome segregation and binucleation. These phenotypes are largely rescued with either farnesyltransferase inhibitors or a farnesylation-incompetent mutant progerin/LAΔ50. Furthermore, we demonstrate that small amounts of progerin/LAΔ50 exist in normal fibroblasts, and a significant percentage of these progerin/LAΔ50-expressing normal cells are binucleated, implicating progerin/LAΔ50 as causing similar mitotic defects in the normal aging process. Our findings present evidence of mitotic abnormality in HGPS and may shed light on the general phenomenon of aging.
In the penultimate sentence of that abstract lies our connection to normal aging. Progerin is generated at some rate in normal cells (probably because the splice site ablated in the mutant is not 100% efficient), and when it accumulates to a detectable level, the same cell cycle interference and mitotic failures seen in HGPS can occur in an otherwise normal cell.
If the binucleated progeny of these cell divisions escape apoptosis, they will presumably undergo some type of permanent cell cycle arrest (e.g. senescence), and persist in the tissue — unable to divide further, but certainly available to cause damage to their microenvironment and contribute to age-related decline in tissue function.
The distribution of progerin expression isn’t smooth: There’s a small population of highly progerin-positive cells, and almost no detectable expression in the vast majority of cells (see their Figure 5). If progerin expression is a truly sporadic event, one would expect to see more intermediate cases. The presence of a few very bright cells against a dark background suggests that there might be some sort of positive feedback at play: Could expression of progerin result in expression of even more expression, perhaps by interference with mRNA splicing?