Catherine Coffinier

A Protein Farnesyltransferase Inhibitor Ameliorates Disease in a Mouse Model of Progeria

Progerias are rare genetic diseases characterized by premature aging. Several progeroid disorders are caused by mutations that lead to the accumulation of a lipid-modified (farnesylated) form of prelamin A, a protein that contributes to the structural scaffolding for the cell nucleus. In progeria, the accumulation of farnesyl–prelamin A disrupts this scaffolding, leading to misshapen nuclei. Previous studies have shown that farnesyltransferase inhibitors (FTIs) reverse this cellular abnormality. We tested the efficacy of an FTI (ABT-100) in Zmpste24-deficient mice, a mouse model of progeria. The FTI-treated mice exhibited improved body weight, grip strength, bone integrity, and percent survival at 20 weeks of age. These results suggest that FTIs may have beneficial effects in humans with progeria.

A farnesyltransferase inhibitor improves disease phenotypes in mice with a Hutchinson-Gilford progeria syndrome mutation

Hutchinson-Gilford progeria syndrome (HGPS) is caused by the production of a truncated prelamin A, called progerin, which is farnesylated at its carboxyl terminus. Progerin is targeted to the nuclear envelope and causes misshapen nuclei. Protein farnesyltransferase inhibitors (FTI) mislocalize progerin away from the nuclear envelope and reduce the frequency of misshapen nuclei. To determine whether an FTI would ameliorate disease phenotypes in vivo, we created gene-targeted mice with an HGPS mutation (LmnaHG/+) and then examined the effect of an FTI on disease phenotypes. LmnaHG/+ mice exhibited phenotypes similar to those in human HGPS patients, including retarded growth, reduced amounts of adipose tissue, micrognathia, osteoporosis, and osteolytic lesions in bone. Osteolytic lesions in the ribs led to spontaneous bone fractures. Treatment with an FTI increased adipose tissue mass, improved body weight curves, reduced the number of rib fractures, and improved bone mineralization and bone cortical thickness. These studies suggest that FTIs could be useful for treating humans with HGPS.

Chordin-like CR domains and the regulation of evolutionarily conserved extracellular signaling systems.

In fruit flies as well as in humans the Short gastrulation (Sog)/Chordin protein functions as an antagonist of the signaling of decapentaplegic (Dpp)/bone morphogenetic protein (BMP) in the extracellular space. Such antagonism inhibits Dpp/BMP signaling by blocking its binding to the receptor. Modulation of Dpp/BMP signaling is phylogenetically conserved and is a key step for the establishment of the dorso-ventral axis in vertebrates and invertebrates. Molecular studies have shown that the inhibitory activity of Chordin on BMP resides in specific cysteine-rich (CR) domains. Interestingly, Chordin-like CR domains are present in a growing number of extracellular proteins, several of which appear to be involved in BMP signaling regulation. We review here the conservation of the Chordin and Sog proteins, and in particular their functional domain, the CR domain. We discuss how the study of CR domains may provide a general mechanism for the regulation of growth factor signaling in the extracellular space.

Abnormal development of the cerebral cortex and cerebellum in the setting of lamin B2 deficiency

Nuclear lamins are components of the nuclear lamina, a structural scaffolding for the cell nucleus. Defects in lamins A and C cause an array of human diseases, including muscular dystrophy, lipodystrophy, and progeria, but no diseases have been linked to the loss of lamins B1 or B2. To explore the functional relevance of lamin B2, we generated lamin B2-deficient mice and found that they have severe brain abnormalities resembling lissencephaly, with abnormal layering of neurons in the cerebral cortex and cerebellum. This neuronal layering abnormality is due to defective neuronal migration, a process that is dependent on the organized movement of the nucleus within the cell. These studies establish an essential function for lamin B2 in neuronal migration and brain development.

Deficiencies in lamin B1 and lamin B2 cause neurodevelopmental defects and distinct nuclear shape abnormalities in neurons

Lamin B1 is essential for neuronal migration and progenitor proliferation during the development of the cerebral cortex. The observation of distinct phenotypes of Lmnb1- and Lmnb2-knockout mice and the differences in the nuclear morphology of cortical neurons in vivo suggest that lamin B1 and lamin B2 play distinct functions in the developing brain.

Regulation of prelamin A but not lamin C by miR-9, a brain-specific microRNA

Lamins A and C, alternatively spliced products of the LMNA gene, are key components of the nuclear lamina. The two isoforms are found in similar amounts in most tissues, but we observed an unexpected pattern of expression in the brain. Western blot and immunohistochemistry studies showed that lamin C is abundant in the mouse brain, whereas lamin A and its precursor prelamin A are restricted to endothelial cells and meningeal cells and are absent in neurons and glia. Prelamin A transcript levels were low in the brain, but this finding could not be explained by alternative splicing. In lamin A-only knockin mice, where alternative splicing is absent and all the output of the gene is channeled into prelamin A transcripts, large amounts of lamin A were found in peripheral tissues, but there was very little lamin A in the brain. Also, in knockin mice expressing exclusively progerin (a toxic form of prelamin A found in Hutchinson–Gilford progeria syndrome), the levels of progerin in the brain were extremely low. Further studies showed that prelamin A expression, but not lamin C expression, is down-regulated by a brain-specific microRNA, miR-9. Expression of miR-9 in cultured cells reduced lamin A expression, and this effect was abolished when the miR-9–binding site in the prelamin A 3′ UTR was mutated. The down-regulation of prelamin A expression in the brain could explain why mouse models of Hutchinson–Gilford progeria syndrome are free of central nervous system pathology.

HIV protease inhibitors block the zinc metalloproteinase ZMPSTE24 and lead to an accumulation of prelamin A in cells

HIV protease inhibitors (HIV-PIs) target the HIV aspartyl protease, which cleaves the HIV gag-pol polyprotein into shorter proteins required for the production of new virions. HIV-PIs are a cornerstone of treatment for HIV but have been associated with lipodystrophy and other side effects. In both human and mouse fibroblasts, we show that HIV-PIs caused an accumulation of prelamin A. The prelamin A in HIV-PI-treated fibroblasts migrated more rapidly than nonfarnesylated prelamin A, comigrating with the farnesylated form of prelamin A that accumulates in ZMPSTE24-deficient fibroblasts. The accumulation of farnesyl-prelamin A in response to HIV-PI treatment was exaggerated in fibroblasts heterozygous for Zmpste24 deficiency. HIV-PIs inhibited the endoproteolytic processing of a GFP-prelamin A fusion protein. The HIV-PIs did not affect the farnesylation of HDJ-2, nor did they inhibit protein farnesyltransferase in vitro. HIV-PIs also did not inhibit the activities of the isoprenyl-cysteine carboxyl methyltransferase ICMT or the prenylprotein endoprotease RCE1 in vitro, but they did inhibit ZMPSTE24 (IC50: lopinavir, 18.4 ± 4.6 μM; tipranavir, 1.2 ± 0.4 μM). We conclude that the HIV-PIs inhibit ZMPSTE24, leading to an accumulation of farnesyl-prelamin A. The inhibition of ZMPSTE24 by HIV-PIs could play a role in the side effects of these drugs.

Crossveinless-2 Is a BMP Feedback Inhibitor that Binds Chordin/BMP to Regulate Xenopus Embryonic Patterning

Vertebrate Crossveinless-2 (CV2) is a secreted protein that can potentiate or antagonize BMP signaling. Through embryological and biochemical experiments we find that: (1) CV2 functions as a BMP4 feedback inhibitor in ventral regions of the Xenopus embryo; (2) CV2 complexes with Twisted gastrulation and BMP4; (3) CV2 is not a substrate for tolloid proteinases; (4) CV2 binds to purified Chordin protein with high affinity (K(D) in the 1 nM range); (5) CV2 binds even more strongly to Chordin proteolytic fragments resulting from Tolloid digestion or to full-length Chordin/BMP complexes; (6) CV2 depletion causes the Xenopus embryo to become hypersensitive to the anti-BMP effects of Chordin overexpression or tolloid inhibition. We propose that the CV2/Chordin interaction may help coordinate BMP diffusion to the ventral side of the embryo, ensuring that BMPs liberated from Chordin inhibition by tolloid proteolysis cause peak signaling levels.

The Posttranslational Processing of Prelamin A and Disease

Human geneticists have shown that some progeroid syndromes are caused by mutations that interfere with the conversion of farnesyl-prelamin A to mature lamin A. For example, Hutchinson-Gilford progeria syndrome is caused by LMNA mutations that lead to the accumulation of a farnesylated version of prelamin A. In this review, we discuss the posttranslational modifications of prelamin A and their relevance to the pathogenesis and treatment of progeroid syndromes.

Mouse Crossveinless-2 is the vertebrate homolog of a Drosophila extracellular regulator of BMP signaling.

The Dpp/BMP signaling pathway is highly conserved between vertebrates and invertebrates. The recent molecular characterization of the Drosophila crossveinless-2 (cv-2) mutation by Conley and colleagues introduced a novel regulatory step in the Dpp/BMP pathway (Development 127 (2000) 3945). The CV-2 protein is secreted and contains five cysteine-rich (CR) domains similar to those observed in the BMP antagonist Short gastrulation (Sog) of Drosophila and Chordin (Chd) of vertebrates. The mutant phenotype in Drosophila suggests that CV-2 is required for the differentiation of crossvein structures in the wing which require high Dpp levels. Here we present the mouse and human homologs of the Drosophila cv-2 protein. The mouse gene is located on chromosome 9A3 while the human locus maps on chromosome 7p14. CV-2 is expressed dynamically during mouse development, in particular in regions of high BMP signaling such as the posterior primitive streak, ventral tail bud and prevertebral cartilages. We conclude that CV-2 is an evolutionarily conserved extracellular regulator of the Dpp/BMP signaling pathway.