The maturation of lamin A is completed by the endoproteolytic cleavage of its farnesylated precursor protein, prelamin A. In the absence of this cleavage, prelamin A can neither give rise to lamin A nor assemble into the nuclear lamina. We call the enzyme which catalyzes this endoproteolytic step the 'prelamin A endoprotease'. In this study, we begin characterization of the regulation of prelamin A endoprotease. In particular, we address the question as to whether prelamin A endoprotease activity is constitutive in cells or responds to expression of prelamin A. To do this, we compared the activity of this novel endoprotease in cells which express prelamin A with those that do not. Our data shows that the enzymatic activity of prelamin A endoprotease is enhanced by the expression of prelamin A.

Proteins of the nuclear lamina provide structural support to the nuclear envelope and participate in a variety of cellular functions, such as chromatin organization and transcriptional regulation. One of these proteins, Lamin A (72kDa), is synthesized as a 74 kDa precursor protein, Prelamin A, which undergoes an unusual maturation pathway that requires two farnesylation-dependent endoproteolytic cleavages. The second cleavage is unique to lamin A in higher vertebrates and is specifically carried out by the endoprotease zmpste24. Although most previous studies have focused mainly on the function of mature Lamin A, recent evidence from our laboratory shows important biological functions for Prelamin A as well. Prelamin A concentration in proliferating cells is very low or undetectable. Conversely, during quiescence induced by mitogen withdrawal or contact inhibition, Prelamin A levels increase dramatically. These variations are directly regulated by changes in expression and enzymatic activity of zmpste24. The central hypothesis of this dissertation is that full-length farnesylated and carboxymethylated prelamin A (FC-PreA) antagonizes both proliferation and apoptosis, therefore playing a role in cellular quiescence/senescence. To accomplish this goal, we studied the transcriptional regulation of zmpste24 and the interaction of FC-preA with proteins that participate in cell cycle control. 1) We identified and characterized a functional site for the E2F1 transcription factor (involved in the control of cell cycle) in the proximal 5’ UTR region of zmpste24. 2) By using proximity-labeling and co-immunoprecipitation-mass spectrometry techniques, we identified a set of proteins that interact preferentially with L467R-Prelamin A (uncleavable mutant) but not with mature Lamin A. Many of these proteins function to regulate progression through cell cycle.

The discovery of mammalian protein prenylation was originally motivated by an effort to identify a nonsterol isoprenoid which indirect evidence suggested was a coregulator of isoprenoid biosynthesis and played a critical role in cellular proliferation. The first prenylated proteins to be identified were the nuclear lamin proteins-B lamins and prelamin A-which were subsequently shown to be farnesylated at a carboxyl-terminal CAAX motif. In both types of lamin, the farnesylation and carboxymethylation play a role in targeting these proteins to the nuclear envelope. The nucleus can be demonstrated to be a CAAX processing compartment for the lamins. In the case of prelamin A, there is removal of a carboxyl-terminal polypeptide which is specifically catalyzed by the enzyme Zmpste24. This processing event is necessary for assembly of lamin A into the lamina and may play a role in cell cycle control. Because the nucleus contains only one target membrane, lamin farnesylation and carboxymethylation may be sufficient to allow association with this membrane. This stands in contrast to farnesylated proteins expressed in the cytoplasm.

The nuclear lamins form a karyoskeleton providing structural rigidity to the nucleus. One member of the lamin family, lamin A, is first synthesized as a 74 kDa precursor, prelamin A. After the endopeptidase and methylation reactions which occur after farnesylation of the CAAX-box cysteine, there is a second endoproteolysis that occurs 15 amino acids upstream from the C-terminal farnesylated cysteine residue. Studies with knockout mice have implicated the enzyme Zmpste24 (Face-1) as a suitable candidate to perform one or both of these proteolytic reactions. Evidence has been presented elsewhere establishing that Zmpste24 possesses a zinc-dependent CAAX endopeptidase activity. In the present study, we confirm this CAAX endopeptidase activity with recombinant, membrane-reconstituted Zmpste24 and show that it can accept a prelamin A farnesylated tetrapeptide as substrate. To monitor the second upstream endoproteolytic cleavage of prelamin A, we expressed a 33 kDa prelamin A C-terminal tail in insect cells. We demonstrate that this purified substrate possesses a C-terminal farnesylated and carboxyl-methylated cysteine and, therefore, constitutes a valid substrate for assaying the second endoproteolytic step in lamin A maturation. With this substrate, we demonstrate that insect cell membranes bearing recombinant Zmpste24 can also catalyse the second upstream endoproteolytic cleavage.

The nuclear lamins form a karyoskeleton providing structural rigidity to the nucleus. One member of the lamin family, lamin A, is first synthesized as a 74 kDa precursor, prelamin A. Following the endopeptidase and methylation reactions which occur after farnesylation of the CAAX-box cysteine, there is a second endoproteolysis that occurs 15 amino acids upstream from the C-terminal farnesylated cysteine residue. Studies with knockout mice have implicated the enzyme Zmpste24 as a candidate to carry out one or both of these proteolytic reactions. In this body of work, the CAAX endopeptidase activity of recombinant, membrane reconstituted, Zmpste24 is demonstrated using a prelamin A farnesylated tetrapeptide as substrate. To monitor the second upstream endoproteolytic cleavage a 33 kDa prelamin A carboxyl terminal tail of prelamin A was expressed in insect cells. This purified substrate possesses a fully processed CAAX box, and, therefore, constitutes a valid substrate for assaying the second endoproteolytic step in lamin A maturation. In vitro reactions with this substrate and insect cell membranes bearing recombinant Zmpste24 demonstrate that Zmpste24 may possess the ability to directly catalyze the second endoproteolytic cleavage. Previous studies on nuclear envelope fractions have ascribed this second activity to a chymotrypsin like protease. However, Zmpste24 contains the canonical HEXXH domain, a common characteristic of zinc metalloproteinases. Experiments on Zmpste24 in this work demonstrate that inactivating the HEXXH domain by site directed mutagenesis results in a loss of the first endoproteolysis reaction, while the second endoproteolytic activity is retained. Supporting these data is the observation that a truncated mutant of Zmpste24 (residues: Met1 - Pro230) that does not contain the HEXXH motif, loses the first endoproteolytic activity while retaining the second. Furthermore, this second activity is not sensitive to the metalloproteinase inhibitors EDTA and 1,10-orthophenanthroline, but is sensitive to the chymotrypsin inhibitor TPCK and its fluorescent analogue, FFCK. The fact that Zmpste24 can be affinity labeled with FFCK suggests that this second activity is directly caused by a second, yet unidentified, active site with a chymotrypsin-like catalytic mechanism.

As the so-called “Mothership of the Human Genome,” the cell nucleus must keep all vital genetic information safe, but accessible, inside a strong protective envelope. The inner membrane of the nuclear envelope is lined by tough but adaptable proteins called lamins. While lamins polymerize into fibrous structures that hold up the “walls” of the nucleus, they also serve as an internal scaffold for the complex machinery involved in DNA replication and gene expression. It is in this later role that we have been looking for clues to premature and possibly to normal aging. One type of lamins, Lamin A is made through an unusual pathway involving a lipid dependent cleavage of a larger precursor called prelamin A. The functional significance of this processing pathway is that prelamin A cannot assemble and is inhibitory of proper lamina formation. Pathological cases of immature lamin A accumulation include Hutchinson-Gilford progeria syndrome (HGPS) or Progeria characterized by premature aging and Restrictive Dermopathy (RD), a lethal prenatal disease. We have previously shown that accumulation of prelamin A leads to cell cycle arrest and drastic changes in expression of genes involved in cell cycle control, among those, several members of the FoxO family of transcription factors. The goal of this study was to determine the mechanisms by which accumulation of uncleavable prelamin A activates FoxO-mediated cell cycle arrest. Cells expressing an uncleavable form of Lamin A in an inducible manner were used to determine subcellular distribution of FoxO3a upon accumulation of prelamin A. This was done by indirect immunofluorescence and Western blotting. The proliferation rate of these cells and controls expressing wild type Lamin A was also determined by measuring the incorporation of BrdU into DNA. During these experiments, it was hypothesized and observed that overexpression of prelamin A leads to redistribution of FoxO3a from the cytoplasm of the cell to the nucleoplasm. Expression of FoxO3a target genes was accordingly increased, leading to a decrease in cell proliferation. The information obtained from this study could not only be of interest in broadening our knowledge of the mechanisms of quiescence and aging in general, but also could inform the discussion of the use of several therapeutics for the treatment of Progeria and other diseases that result from the accumulation of prelamin A.

Failure to process prelamin A, by the enzyme ZMPSTE24, leads to the build up of farnesylated prelamin A, which has been implicated in causing the symptoms experienced in laminopathies and HIV therapy. A common feature to these conditions is the development of an irregular nuclear boundary, often including deep invaginations that form a nucleoplasmic reticulum. Additionally, dysregulated lipid synthesis is frequently associated with improper lamin A processing and I set out to address the molecular mechanisms behind these two features that could explain lipoatrophy experienced in patients. By using siRNA targeted against Zmpste24 I utilised an array of biochemical, molecular and imaging techniques to uncover a mechanism that leads to the production of a nucleoplasmic reticulum that was dependent on both the farnesylated tail of prelamin A and the phosphatidylcholine synthesising enzyme CCTα. The morphology of this structure consisted of an invagination of both the inner and outer nuclear membranes with a cytoplasmic core or just invagination of the inner nuclear membrane. Serial section dual axis electron tomography provided a new insight into the ultrastructural changes at the nuclear periphery that revealed novel structural features. The dysregulation of lipid synthesis was assessed by investigating the effects farnesylated prelamin A has on the distribution and dynamics of the transcription factor SREBP-1 and assessment of the downstream consequences this has on its targets that regulate adipocyte differentiation potential. Finally, the metabolomic profile of an HIV protease inhibitor that leads to prelamin A build up was generated and revealed increases in lipolysis, glycolysis and mediators of inflammation. The research presented offers a new insight into the development of a convoluted nuclear boundary and nucleoplasmic reticulum, in the context of lamin A mutants and how dysregulated lipid synthesis, caused by farnesylated prelamin A, leads to lipoatrophy.