New spindle morphogenesis model by Dynein, Nudel, and the spindle matrix

A computational model for the formation of lamin-B mitotic spindle envelope and matrix

Recent reports show that, after nuclear envelope breakdown, lamin-B, a component of the nuclear lamina in interphase, localizes around the mitotic spindle as a membranous network. How this process occurs, however, and how it influences mitotic spindle morphogenesis is unclear. Here, we develop a computational model based on a continuum description to represent the abundance and location of various molecular species involved during mitosis, and use the model to test a number of hypotheses regarding the formation of the mitotic matrix. Our model illustrates that freely diffusible nuclear proteins can be captured and transported to the spindle poles by minus-end-directed microtubule (MT) motors. Moreover, simulations show that these proteins can be used to build a shell-like region that envelopes the mitotic spindle, which helps to improve the focusing of the mitotic spindle by spatially restricting MT polymerization and limiting the effective diffusion of the free MTs. Simulations also confirm that spatially dependent regulation of the spindle network through the Ran system improves spindle focusing and morphology. Our results agree with experimental observations that lamin-B reorganizes around the spindle and helps to maintain spindle morphology.

Premature senescence in cells from patients with autosomal recessive hypercholesterolemia (ARH): evidence for a role for ARH in mitosis

OBJECTIVE

The defective gene causing autosomal recessive hypercholesterolemia (ARH) encodes ARH, a clathrin-associated adaptor protein required for low-density-lipoprotein receptor endocytosis in most cells but not in skin fibroblasts. The aim here was to elucidate why ARH fibroblasts grow slowly and undergo premature senescence.

METHODS AND RESULTS

Knockdown of ARH by RNA interference in IMR90 cells produces the same phenotype, indicated by increased p16 expression, γ-H2AX-positive foci, and enlarged flattened morphology. We showed that ARH contributes to several aspects of mitosis: it localizes to mitotic microtubules, with lamin B1 on the nuclear envelope and spindle matrix, and with clathrin heavy chain on mitotic spindles. Second, ARH is phosphorylated in G(2)/M phase by a roscovitine-sensitive kinase, probably cdc2. Third, cells lacking ARH show disfigured nuclei and defective mitotic spindles. Defects are most marked in ARH W22X cells, where translation starts at Met46, so the protein lacks a phosphorylation site at Ser14, identified by mass spectrometry of wild-type ARH.

CONCLUSIONS

The ARH protein is involved in cell cycle progression, possibly by affecting nuclear membrane formation through interaction with lamin B1 or other mitotic proteins, and its absence affects cell proliferation and induces premature senescence, which may play a role in the development of atherosclerosis in ARH.