Accumulation of distinct prelamin A variants in human diploid fibroblasts differentially affects cell homeostasis.
Lamin A is a component of the nuclear lamina that plays a major role in the structural organization and function of the nucleus. Lamin A is synthesized as a prelamin A precursor which undergoes four sequential post-translational modifications to generate mature lamin A. Significantly, a large number of point mutations in the LMNA gene cause a range of distinct human disorders collectively known as laminopathies. The mechanisms by which mutations in lamin A affect cell function and cause disease are unclear. Interestingly, recent studies have suggested that alterations in the normal lamin A pathway can contribute to cellular dysfunction. Specifically, we and others have shown, at the cellular level, that in the absence of mutations or altered splicing events, increased expression of wild-type prelamin A results in a growth defective phenotype that resembles that of cells expressing the mutant form of lamin A, termed progerin, associated with Hutchinson-Gilford Progeria syndrome (HGPS). Remarkably, the phenotypes of cells expressing elevated levels of wild-type prelamin A can be reversed by either treatment with farnesyltransferase inhibitors or overexpression of ZMPSTE24, a critical prelamin A processing enzyme, suggesting that minor increases in the steady-state levels of one or more prelamin A intermediates is sufficient to induce cellular toxicity. Here, to investigate the molecular basis of the lamin A pathway toxicity, we characterized the phenotypic changes occurring in cells expressing distinct prelamin A variants mimicking specific prelamin A processing intermediates. This analysis demonstrates that distinct prelamin A variants differentially affect cell growth, nuclear membrane morphology, nuclear distribution of lamin A and the fundamental process of transcription. Expression of prelamin A variants that are constitutively farnesylated induced the formation of lamin A aggregates and dramatic changes in nuclear membrane morphology, which led to reduced levels of the basal transcription factor TATA-binding protein (TBP) and global transcription, and severely limited cell growth. Expression of a prelamin A variant that cannot be farnesylated, although did not appreciably influence cell growth, resulted in the formation of lamin A nucleoplasmic foci and caused, in a minor subpopulation of cells, changes in nuclear morphology that were accompanied by reduced levels of TBP and transcription. In contrast, expression of mature lamin A did not affect any of these parameters. These data demonstrate that accumulation of any partially processed prelamin A protein alters cellular homeostasis to some degree, even though the most dramatic effects are caused by variants with a permanently farnesylated carboxyl-terminal tail.
Candelario
Jose
J
Department of Molecular Microbiology and Immunology, Institute for Genetic Medicine, Keck School of Medicine, University of Southern California, Los Angeles, CA 90033, USA.
Borrego
Stacey
S
Reddy
Sita
S
Comai
Lucio
L
D004171:Diploidy; D005347:Fibroblasts / Q000378:metabolism* / Q000502:physiology; D014644:Genetic Variation; D006706:Homeostasis; D006801:Humans; D034904:Lamin Type A / Q000235:genetics; D009154:Mutation; D009687:Nuclear Proteins / Q000235:genetics / Q000378:metabolism; D011371:Progeria; D011498:Protein Precursors / Q000235:genetics / Q000378:metabolism
D034904:Lamin Type A; D009687:Nuclear Proteins; D011498:Protein Precursors; C090764:prelamin A