Disruption of the gene encoding 3b-hydroxysterol D 14 -reductase (Tm7sf2) in mice does not impair cholesterol biosynthesis Anna M. Bennati 1 , Gianluca Schiavoni 1 , Sebastian Franken 2 , Danilo Piobbico 3 , Maria A. Della Fazia 3 , Donatella Caruso 4 , Emma De Fabiani 4 , Laura Benedetti 5 , Maria G. Cusella De Angelis 5 , Volkmar Gieselmann 2 , Giuseppe Servillo 3 , Tommaso Beccari 1 and Rita Roberti 1 1 Department of Internal Medicine, University of Perugia, Italy 2 Institut fu ¨ r Physiologische Chemie, Rheinische Friedrich-Wilhelms-Universita ¨ t, Bonn, Germany 3 Department of Clinical and Experimental Medicine, University of Perugia, Italy 4 Department of Pharmacological Sciences, University of Milan, Italy 5 Department of Experimental Medicine, University of Pavia, Italy In cholesterol biosynthesis, lanosterol undergoes removal of the methyl group at C14, leading to the formation of C14–C15 unsaturated sterol intermedi- ates. The enzymatic activity responsible for the reduc- tion of the introduced double-bond, 3b-hydroxysterol D 14 -reductase (EC 1.3.1.70), is carried out by the endoplasmic reticulum (ER) protein delta14-sterol reductase (C14SR) encoded by the TM7SF2 gene Keywords 3beta-hydroxysterol delta14-reductase; cholesterol biosynthesis; gene expression; lamin B receptor; Tm7sf2 Correspondence R. Roberti, Department of Internal Medicine, Laboratory of Biochemistry, University of Perugia, Via del Giochetto, 06122 Perugia, Italy Fax: +39 075 585 7428 Tel: +39 075 585 7426 E-mail: roberti@unipg.it (Received 24 May 2008, accepted 11 August 2008) doi:10.1111/j.1742-4658.2008.06637.x Tm7sf2 gene encodes 3b-hydroxysterol D 14 -reductase (C14SR, DHCR14), an endoplasmic reticulum enzyme acting on D 14 -unsaturated sterol interme- diates during the conversion of lanosterol to cholesterol. The C-terminal domain of lamin B receptor, a protein of the inner nuclear membrane mainly involved in heterochromatin organization, also possesses sterol D 14 -reductase activity. The subcellular localization suggests a primary role of C14SR in cholesterol biosynthesis. To investigate the role of C14SR and lamin B receptor as 3b-hydroxysterol D 14 -reductases, Tm7sf2 knockout mice were generated and their biochemical characterization was performed. No Tm7sf2 mRNA was detected in the liver of knockout mice. Neither C14SR protein nor 3b-hydroxysterol D 14 -reductase activity were detectable in liver microsomes of Tm7sf2 () ⁄ )) mice, confirming the effectiveness of gene inactivation. C14SR protein and its enzymatic activity were about half of control levels in the liver of heterozygous mice. Normal cholesterol levels in liver membranes and in plasma indicated that, despite the lack of C14SR, Tm7sf2 () ⁄ )) mice are able to perform cholesterol biosynthesis. Lamin B receptor 3b-hydroxysterol D 14 -reductase activity determined in liver nuclei showed comparable values in wild-type and knockout mice. These results suggest that lamin B receptor, although residing in nuclear membranes, may contribute to cholesterol biosynthesis in Tm7sf2 () ⁄ )) mice. Affymetrix microarray analysis of gene expression revealed that several genes involved in cell-cycle progression are downregulated in the liver of Tm7sf2 () ⁄ )) mice, whereas genes involved in xenobiotic metabolism are upregulated. Abbreviations C14SR ⁄ DHCR14, 3b-hydroxysterol D 14 -reductase; C27D 8 ,5a-cholesta-8(9)-en-3b-ol; C27D 8,14 ,5a-cholesta-8(9),14-dien-3b-ol; C29D 8 , 4,4-dimethyl-5a-cholesta-8(9)-en-3b-ol; C29D 8,14 , 4,4-dimethyl-5a-cholesta-8(9),14-dien-3b-ol; ER, endoplasmic reticulum; HEM, Hydrops- Ectopic calcification-Moth-eaten skeletal dysplasia; LBR, lamin B receptor. 5034 FEBS Journal 275 (2008) 5034–5047 ª 2008 The Authors Journal compilation ª 2008 FEBS [1,2]. A second protein of the inner nuclear membrane, the lamin B receptor (LBR), catalyzes the 3b-hydroxys- terol D 14 -reductase reaction, as demonstrated by its ability to complement C14SR-deficient yeast strains [3,4] and by enzymatic assay of the protein overexpres- sed in transfected COS-1 cells [5]. Recently, the mouse gene encoding 3b-hydroxysterol D 14 -reductase has been termed Dhcr14 [6]; in this study the former gene name Tm7sf2 will be used. Tm7sf2 is located on chromosome 19A. The expression of cholesterol biosynthesis genes is regulated by cell sterol levels through the action of the transcription factor SREBP-2 [7,8]. In HepG2 hepa- toma cells, sterol starvation results in induction of the TM7SF2 gene, C14SR protein and 3b-hydroxysterol D 14 -reductase activity. In addition, human TM7SF2 promoter is regulated by SREBP-2 [5]. Therefore, the adaptability of TM7SF2 gene to the needs of choles- terol biosynthesis appears well established. On the con- trary, LBR gene expression is not responsive to sterol starvation conditions [5] and its importance in choles- terol biosynthesis remains unravelled. The lack of 3b-hydroxysterol D 14 -reductase caused by mutations of the LBR gene was previously indi- cated as responsible for Hydrops-Ectopic calcification- Moth-eaten skeletal dysplasia (HEM or Greenberg dysplasia) [9–11]. The severe phenotype of natural Lbr () ⁄ )) mutants, ichthyosis mice, has been described, but no information on cholesterol levels and ⁄ or sterol intermediate accumulation has been reported [12]. Therefore, inactivation of Tm7sf2 gene in mice would provide insights into the role of both genes encoding 3b-hydroxysterol D 14 -reductase, Tm7sf2 and Lbr,in cholesterol biosynthesis. While Tm7sf2 () ⁄ )) mice generated in our laboratory were under characterization, a paper was published describing mice defective for Lbr (Lbr () ⁄ )) , ichthyosis mice), defective for Dhcr14 ⁄ Tm7sf2 (Dhcr14 (D4-7 ⁄ D4-7) ) or defective for Dhcr14 ⁄ Tm7sf2 and heterozygous for Lbr (Lbr (+ ⁄ )) :Dhcr14 (D4-7 ⁄ D4-7) ) [6]. The paper states that HEM dysplasia is a laminopathy not caused by 3b-hydroxysterol D 14 -reductase deficiency. Mutants have distinct physical and biochemical phenotypes, but no sterol abnormalities were detected in liver, whereas marked elevations of D 14 -sterols were seen in brain of Lbr (+ ⁄ )) :Dhcr14 (D4-7 ⁄ D4-7) mice. Here, we describe the generation of Tm7sf2 () ⁄ )) mice and their biochemical characterization. Determi- nation of Tm7sf2 and Lbr mRNA expression in differ- ent mouse tissues, expression of C14SR and LBR proteins in liver and a measure of their 3b-hydroxys- terol D 14 -reductase activity are reported. Despite the lack of C14SR, Tm7sf2 () ⁄ )) mice are apparently healthy and have normal cholesterol levels in liver membranes and in plasma, suggesting that LBR can function as 3b-hydroxysterol D 14 -reductase in vivo. Furthermore, microarray analysis of gene expression in liver comparing wild-type and Tm7sf2 () ⁄ )) mice has been performed. Results Identification of mouse Tm7sf2 gene Genomic clones of Tm7sf2 were isolated by screening a mouse 129 ⁄ SvJ genomic library, subcloned and sequenced. Comparison of the mouse genomic and the cDNA sequences revealed that the exon–intron organi- zation of the mouse Tm7sf2 gene is highly similar to the homologous human gene [1]. The gene spans  5 kb and consists of 10 exons and 9 introns. Table 1 shows the size of the exons and introns and the sequence of the exon–intron junctions. All 5¢ donor and 3¢ acceptor splice sites conformed to the consensus Table 1. Exon–intron organization of mouse Tm7sf2 gene. Exon and intron length is reported in parentheses. Exon Intron Sequence of exon–intron junction 5¢ splice donor–3¢ splice acceptor 1 (143 bp) 1 (349 bp) GGGCCGTTGG gtaaatggga–––ctctttccag GCGTCGCGGC 2 (197 bp) 2 (100 bp) CTGCACGAAG gtgtgtgatc–––gtacccgcag GTGGCCGAAG 3 (55 bp) 3 (274 bp) CCTATTAATG gtgactgggg–––tgtggttcag GCTTCCAGGC 4 (195 bp) 4 (84 bp) GGAAACTCAG gtgagaaggg–––ttgttcccag GAAATTCCAT 5 (104 bp) 5 (2112 bp) CATTGGCTGG gtatgctgac–––acttctttag GTTTTCATTA 6 (120 bp) 6 (88 bp) CTGGTATGAG gtgagactgg–––gttcctgcag GAGTCTGTCC 7 (169 bp) 7 (214 bp) CTCCTTAAGG gtcagtagga–––cttccctcag TTATTGGTTA 8 (81 bp) 8 (80 bp) AGCGTGGCTG gtaagctggg–––gtatttctag GTCTTGAGAC 9 (123 bp) 9 (250 bp) TTGCCCTGTG gtgagtgggt–––ttccctccag GGCTATCCCA 10 (253 bp) CTATCCCATC––– A. M. Bennati et al. Tm7sf2 knockout mice FEBS Journal 275 (2008) 5034–5047 ª 2008 The Authors Journal compilation ª 2008 FEBS 5035 GT–AG rule. The transcription initiation site, deter- mined by RACE, was located at )91 bp upstream the ATG start codon. A polyadenylation signal (AATAAA) is present 49 bp downstream of the stop codon. The genomic sequence has been submitted to GenBank under accession number EU672836. Tm7sf2 and Lbr expression in mouse tissues Tm7sf2 and Lbr relative mRNA expression was mea- sured in adrenal, brain, heart, kidney, liver, lung, ovary and testis of 8-week-old mice using qRT-PCR. The highest Tm7sf2 mRNA abundance was found in liver, followed by ovary, testis, kidney and brain (Fig. 1A). Testis and lung showed the highest Lbr gene expression, followed by heart, ovary, kidney and liver (Fig. 1B). Tm7sf2 versus Lbr expression was determined by using Lbr as the internal calibrator for each tissue. Table 2 shows comparable expression of the two genes in ovary, kidney and adrenal gland. Compared with Lbr,  8- and 2.5-fold higher Tm7sf2 expression was found in liver and brain, respectively. By contrast, Lbr versus Tm7sf2 expression was  5-, 12- and 16-fold higher in testis, heart and lung, respectively. Generation of Tm7sf2-null mice Tm7sf2 gene was disrupted in HM1 mouse embryonic stem cells using a targeting vector in which exon 5 was interrupted by a neomycin resistance cassette (Fig. 2B). The homologous recombination between the targeting vector and the endogenous Tm7sf2 allele resulted in insertion of the neomycin phosphotransferase gene into exon 5. Of 96 ES cell clones surviving G418 selection, four targeted clones were identified by PCR analysis and by Southern blot of EcoRI-digested genomic DNA and two of them were injected into blastocysts. Only clone E-53 generated germline transmitting chi- meric male founders, which were intercrossed with C57 ⁄ B6 females to generate outbred strains. Offspring were genotyped by PCR, which produced the expected 230 and 388 bp fragments from wild-type and dis- rupted allele, respectively (Fig. 2B,C). Genotyping was confirmed by Southern blot of EcoRI-digested DNA. Labelled 8.9 and 7.8 kb fragments were obtained from wild-type and mutated allele, respectively (Fig. 2B,D). Offspring from heterozygous intercrosses showed the typical Mendelian distribution of wild-type, heterozy- gous and homozygous. Tm7sf2 () ⁄ )) mice develop normally, appear healthy and are fertile. Histopathological analysis of liver, kid- ney, adrenal and brain did not reveal differences between control and mutant mice (data not shown). Followed over a 3-month period, Tm7sf2 () ⁄ )) mice grow at the same rate as littermate control mice. Groups of control and Tm7sf2 () ⁄ )) female weighed at 14 months of age were 27.7 ± 1.9 g (n = 9) and 30.9 ± 1.9 g (n = 7), respectively. No apparent age- dependent problems were observed in females or males over a 14-month period. These results confirm previ- ously reported data [6]. 0.0 0.2 0.4 0.6 0.8 1.0 1.2 A B Tm7sf2 relative mRNA expression Adrenal Brain Heart Kidney Liver Lung Ovary Testis Adrenal Brain Heart Kidney Liver Lung Ovary Testis 0.0 0.2 0.4 0.6 0.8 1.0 1.2 Lbr relative mRNA expression Fig. 1. Expression of Tm7sf2 and Lbr mRNA in mouse tissues. RNA was extracted from pooled tissues of three 8-week-old male mice (females were used for ovary). Relative mRNA expression was determined by qRT-PCR using liver and testis as internal cali- brators for Tm7sf2 (A) and Lbr (B) genes, respectively. Experiments were performed in triplicate and repeated twice with different RNA preparations. Reported data are mean ± SD. Tm7sf2 knockout mice A. M. Bennati et al. 5036 FEBS Journal 275 (2008) 5034–5047 ª 2008 The Authors Journal compilation ª 2008 FEBS To test whether the mutation abolishes the expres- sion of C14SR, RNA was extracted from liver and the cDNA was synthesized by RT-PCR using the forward and reverse primers that amplify the entire cDNA. Figure 3A shows that the cDNA is absent in Tm7sf2 () ⁄ )) mice, whereas the expected 1.3 kb frag- ment is obtained in control mice. Although no quanti- tative PCR was performed, the cDNA was about half of the control in heterozygous mice. Gapdh was ampli- fied in parallel as housekeeping gene. Western blot analysis of microsomes prepared from liver was performed using anti-(bovine C14SR) serum, which cross-reacts with the mouse protein. Figure 3B shows that C14SR protein is absent in Tm7sf2 () ⁄ )) mice, whereas the band intensity is about half of control (0.55 ± 0.09) in heterozygous mice, thus con- firming the results obtained with the cDNA. To investigate whether disruption of the Tm7sf2 gene modifies Lbr mRNA expression, qRT-PCR was performed in tissues of 8-week-old wild-type and Tm7sf2 () ⁄ )) mice, using the wild-type as internal cali- brator for each tissue. No significant differences of Lbr mRNA expression in Tm7sf2 () ⁄ )) mice, compared with that of wild-type mice, were found in the exam- ined tissues (adrenal, brain, heart, kidney, liver, lung, ovary and testis) (data not shown). Western blot analysis of nuclear membranes from liver confirmed that LBR is expressed at about the same level of control in Tm7sf2 () ⁄ )) mice (Fig. 3C). A B C D Fig. 2. Structure and targeted disruption of Tm7sf2 gene. (A) The conversion of 4,4- dimethyl-5a-cholesta-8(9),14-dien-3b-ol (C29D 8,14 ) to 4,4-dimethyl-5a-cholesta-8(9)- en-3b-ol (C29D 8 ) is catalysed by C14SR and LBR. (B) Tm7sf2 genomic structure, target- ing vector, and disrupted allele. Exon (filled box) and intron lengths are approximately to scale. PCR primers a, b, and neo are indi- cated by arrows. The 3¢-probe used in Southern blot experiments spans exon 10. The size of EcoRI-digested wild-type and disrupted allele is reported. (C) PCR geno- typing of heterozygous Tm7sf2 intercross. Primers a and b (B) amplify a 230-bp frag- ment from the wild-type allele. Primers a and neo (B) amplify a 388-bp fragment from the disrupted allele. (D) Southern blot analy- sis of mouse tail DNA isolated from the progeny of a mating between heterozygous parents. DNAs were digested with EcoRI and hybridized with the 3¢-probe indicated in (B). Table 2. Tm7sf2 and Lbr expression in mouse tissues. Total RNA was extracted from tissues of 8-week-old mice and retrotranscribed as reported in Experimental procedures. Tm7sf2 and Lbr mRNA expression was measured by qRT-PCR using the specific primers (see Experimental procedures) and Lbr as internal calibrator for each tissue. Hprt was used as the reference gene for sample nor- malization. Data are mean ± SD of two experiments performed in triplicate. Tissue Tm7sf2 ⁄ Lbr ratio Adrenal 0.68 ± 0.06 Brain 2.40 ± 0.51 Heart 0.09 ± 0.03 Kidney 0.90 ± 0.09 Liver 7.95 ± 0.71 Lung 0.06 ± 0.01 Ovary 1.06 ± 0.13 Testis 0.19 ± 0.05 A. M. Bennati et al. Tm7sf2 knockout mice FEBS Journal 275 (2008) 5034–5047 ª 2008 The Authors Journal compilation ª 2008 FEBS 5037 3b-Hydroxysterol D 14 -reductase activity 3b-Hydroxysterol D 14 -reductase activity in microsomes or in intact nuclei prepared from liver of 6-week-old mice was determined in vitro by incubating the enzyme source in the presence of the C27D 8,14 sterol substrate. Activity was measured on the basis of C27D 8 forma- tion and C27D 8,14 disappearance, evaluated as the peak area ratio between the individual sterol and cholestane, the internal standard. No enzymatic activity was detected in liver micro- somes of Tm7sf2 () ⁄ )) mice, whereas reduced activity was detected in heterozygous compared with wild-type mice (Fig. 4). The data are in accordance with C14SR expression detected by western blot. The enzymatic activity measured in intact nuclei, which can be referred as LBR activity, was comparable in Tm7sf2 () ⁄ )) and Tm7sf2 (+ ⁄ +) mice (Fig. 4). The contribution of C14SR and LBR to the 3b-hy- droxysterol D 14 -reductase reaction in liver from wild- type mice can be evaluated. On the basis of the amount of incubated proteins (see legend in Fig. 4) in the experimental conditions used, C14SR-specific activ- ity was approximately eightfold higher than that of LBR. Because 4.4 and 1.5 mg proteinÆg )1 of fresh tis- sue were recovered as microsomes and nuclei, respec- tively, C14SR enzymatic activity is >20-fold higher than LBR enzymatic activity. This result is in accor- dance with the high Tm7sf2 mRNA expression in liver, compared with Lbr (Table 2). Sterol determinations Cholesterol concentration in liver microsomal mem- branes of 6-week-old mice was measured both by GC-MS analysis and by densitometry analysis of mem- brane lipids separated by TLC. Despite the lack of C14SR activity in microsomes, normal cholesterol levels were found in these membranes (Table 3). No differences were found between male and female mice. Cholesterol biosynthetic precursors, including C29D 8,14 , were not detectable by GC-MS analysis of microsomal sterols, indicating that C29D 8,14 inter- mediate was not accumulated in Tm7sf2 () ⁄ )) mice. kb +/+ +/– –/– Gapdh 1.5 Tm7sf2 cDNAABC kDa +/+ –/– 66 45 31 +/– Anti-C14SR Microsomes 66 kDa +/+ –/– Anti-LBR Nuclear membranes Fig. 3. C14SR and LBR expression in liver. (A) PCR analysis of Tm7sf2 mRNA in the liver of Tm7sf2 () ⁄ )) , heterozygous and wild-type mice. The primers used (see Experimental procedures) amplify a 1285-bp fragment. (B) Western blot analysis of liver microsomes. The anti-(bovine C14SR) serum recognises a protein band with an apparent molecular mass of  40 kDa. Equal amounts of protein (30 l g) were loaded in each lane and checked by Ponceau staining of poly(vinylidene difluoride) membranes after protein transfer (data not shown). (C) Western blot analysis of liver nuclear membranes. The anti-(human LBR) serum recognizes a protein band with an apparent molecular mass of  66 kDa. Equal amounts of proteins (50 lg) were loaded in each lane and checked by Ponceau staining of poly(vinylidene difluoride) mem- branes after protein transfer (data not shown). Experiments were repeated at least three times with different RNA (A) or membrane (B and C) preparations. One representative experiment is reported. 0.0 0.5 1.0 1.5 2.0 2.5 Sterol/cholestane (peak area ratio) Nuclei t 0 +/+ +/– –/– +/+ –/– Microsomes Fig. 4. 3b-Hydroxysterol D 14 -reductase activity. Microsomes (0.25 mg protein) and intact nuclei (0.5 mg protein) prepared from liver of wild-type, heterozygous and Tm7sf2 () ⁄ )) mice were assayed for 3b-hydroxysterol D 14 -reductase activity by incubation for 30 min with C27D 8,14 in the conditions described in Experimen- tal procedures. Enzymatic activity was evaluated on the basis of the decrease of peak area ratio between m ⁄ z 426 and m ⁄ z 372 ions (C27D 8,14 ⁄ 5a-cholestane, filled columns) and the increase of peak area ratio between m ⁄ z 428 and m ⁄ z 372 ions (C27D 8 ⁄ 5a- cholestane, open columns) at the expected retention time. C27D 8 was absent at zero incubation time both in the microsomes and in nuclei. Data shown are mean ± SD (n = 3). Tm7sf2 knockout mice A. M. Bennati et al. 5038 FEBS Journal 275 (2008) 5034–5047 ª 2008 The Authors Journal compilation ª 2008 FEBS Plasma cholesterol was comparable between 6-week- old Tm7sf2 (+ ⁄ +) and Tm7sf2 () ⁄ )) mice, both in males and females (Table 3). Groups of 14-month-old female mice were also analysed, and comparable plasma cholesterol levels were seen in wild-type and knockout mice (Table 3). Affymetrix oligonucleotide array hybridization and data analysis The expression of 45 101 transcripts represented on the Affymetrix murine GeneChipMOE430 2.0 was quantified in liver from wild-type and Tm7sf2 () ⁄ )) mice. Duplicate hybridizations were performed for each sample, using the wild-type condition as a control to measure fold change of gene expression in knockout mice. Expression values were calculated using the Robust Multi-Array Analysis (RMA) plugin [13] of the genespring 7.3 software (Agilent Technologies, Milan, Italy). Using this analysis, 1119 transcripts with a SD < 0.2 were selected in the comparison knock- out ⁄ wild-type mice. Volcano Plot analysis of this tran- script list was performed to identify transcripts with a defined minimal fold change and statistically significant P-value for a t-test of differences between samples. By selecting a fold change > 1.5 and a P-value < 0.01, 66 transcripts were identified as increased (Table 4) and 41 as decreased (Table 5) in livers of Tm7sf2 () ⁄ )) mice. Table 4 shows that several transcripts of oxidoreduc- tases are increased, including members of cyt p450 families. Glutathione S-transferase, involved in xenobi- otic metabolism, is also increased. Some genes involved in cell proliferation and cell-cycle progression show decreased transcripts (Table 5). qRT-PCR analysis was applied to some of the genes that show the highest up- or down expression comparing knockout to the wild-type mice. Although with different fold changes, the results obtained in the microarray experiment were confirmed (Tables 4 and 5). The complete panel of genes upstream and down- stream Tm7sf2 in the cholesterol biosynthetic pathway and Lbr were analysed carefully using less stringent parameters (P-value for the t-test < 0.05). Neverthe- less, no difference in their expression was found in the liver of Tm7sf2 () ⁄ )) mice. Discussion The discovery that inborn errors in cholesterol biosyn- thesis are the cause of human malformation syndromes characterized by severe developmental abnormalities [14] highlights the role of cholesterol in mammalian embryonic development. Indeed, cholesterol is a cova- lent ligand of hedgehog morphogenic signalling pro- teins [15]. Normal cholesterol biosynthesis is required for cell membrane assembly and for the biosynthesis of different biologically active sterol molecules. Further- more, the cholesterol biosynthetic pathway furnishes a variety of intermediate molecules involved in several cell processes. In light of this, it is predictable that mouse models bearing inactivation of any of the genes of cholesterol biosynthesis are characterized by impaired cell functions and developmental abnormali- ties incompatible with life. Indeed, several mutant mice lacking genes of the pre- or post-squalene segment of the cholesterol biosynthetic pathway, that are lethal to various degrees, have been described [16–23]. The mutant mouse model generated in our labora- tory by inactivation of the Tm7sf2 gene lacks the ER 3b-hydroxysterol D 14 -reductase, C14SR. Nevertheless, mice are viable and do not exhibit a pathological phe- notype, as previously reported for Dhcr14 (D4-7 ⁄ D4-7) mice [6]. Using the antibody raised in our laboratory against bovine liver C14SR, we were able to verify a reduction and the complete absence of C14SR expres- sion in liver microsomes of Tm7sf2 (+ ⁄ )) and Tm7sf2 () ⁄ )) mice, respectively. In accordance with these data, no 3b-hydroxysterol D 14 -reductase activity is detectable in liver microsomes of Tm7sf2 () ⁄ )) mice. Despite the lack of 3b-hydroxysterol D 14 -reductase activity of C14SR, normal cholesterol biosynthesis occurs in Tm7sf2 () ⁄ )) mice. Indeed, the level of Table 3. Cholesterol in plasma and liver microsomal membranes. Total plasma cholesterol was measured using a commercial kit. For micro- somal membrane cholesterol determination, samples were saponified and cholesterol was measured by GC-MS and by densitometric analy- sis of lipids separated by TLC and stained as described in Experimental procedures. Plasma cholesterol (mgÆdL )1 ) Microsomal membrane cholesterol (nmolÆmg )1 protein) Tm7sf2 (+ ⁄ +) Tm7sf2 () ⁄ )) Tm7sf2 (+ ⁄ +) Tm7sf2 () ⁄ )) Male (6 weeks) 82.2 ± 10.3 (n = 8) 77.6 ± 14.5 (n = 12) 73.9 ± 5.8 (n = 6) 71.5 ± 6.5 (n =6) Female (6 weeks) 70.7 ± 12.9 (n = 15) 66.0 ± 11.8 (n = 10) Female (14 months) 65.6 ± 21.1 (n = 9) 81.7 ± 20.2 (n =7) A. M. Bennati et al. Tm7sf2 knockout mice FEBS Journal 275 (2008) 5034–5047 ª 2008 The Authors Journal compilation ª 2008 FEBS 5039 Table 4. Genes whose transcripts increase in liver of Tm7sf2 () ⁄ )) mice. Total RNA from the pooled livers of three 6-week-old mice was pre- pared for Affymetrix oligonucleotide hybridization as described in Experimental procedures. The relative mRNA expression of each gene in livers of knockout mice compared with wild-type control is shown. Parameters for Volcano plot statistical analysis were fold change > 1.5 and P-value for the t-test < 0.01. Results from duplicate hybridizations are listed by fold change. Affymetrix probeset identification GenBank Gene symbol Gene name Fold change 1438211_s_at NM_016974 Dbp D-site albumin promoter binding protein 6.21 ⁄ 4.05 a 1448793_a_at NM_011521 Sdc4 syndecan 4 3.43 ⁄ 4.80 a 1420603_s_at NM_009016, NM_009017, NM_009018, NM_020030, NM_198193, XM_001006217 Raet1a, Raet1b, Raet1c, Raet1d, Raet1e retinoic acid early transcript 1alpha, beta, gamma, delta, 1E 3.18 1449347_a_at NM_001081642, NM_021365, NM_183094, XM_001471704, XM_001471888, XM_001475552, XM_001487778, XM_978371, XR_035676, XR_035679 LOC100044048, LOC100044049, LOC100046087, Xlr4a, Xlr4b, Xlr4c, Xlr4e X-linked lymphocyte-regulated 4A, 4B, 4C, 4E 3.02 1444438_at XM_356089, XM_904518 Cib3 calcium and integrin binding family member 3 2.98 ⁄ 1.66 a 1424853_s_at NM_010011, NM_201640, XM_001471913 Cyp4a10, Cyp4a31, LOC100044218 cytochrome P450, family 4, subfamily a, polypeptides 10&31 2.68 ⁄ 1.83 a 1416318_at NM_025429 Serpinb1a serine (or cysteine) peptidase inhibitor, clade B, member 1a 2.65 1444810_at – – – 2.55 1421363_at NM_010003 Cyp2c39 cytochrome P450, family 2, subfamily c, polypeptide 39 2.53 ⁄ 17.2 a 1439560_x_at XM_488763, XM_489668 EG432995 predicted gene, EG432995 2.49 1419700_a_at NM_008935 Prom1 prominin 1 2.38 1453345_at NM_001081205 Npal1 NIPA-like domain containing 1 2.33 1421040_a_at NM_008182 Gsta2 glutathione S-transferase, alpha 2 (Yc2) 2.32 1435459_at NM_018881 Fmo2 flavin containing monooxygenase 2 2.26 1435893_at NM_013703 Vldlr very low density lipoprotein receptor 2.14 1423875_at NM_145505, XM_001473083 AI450540, LOC100044843 expressed sequence AI450540 2.13 1431240_at NM_053165 Clec2h C-type lectin domain family 2, member h 2.13 1421041_s_at NM_008181, NM_008182, XM_001478795 Gsta1, Gsta2, LOC100042295 glutathione S-transferase, alpha 1 (Ya); glutathione S-transferase, alpha 2 (Yc2) 2.06 1438194_at – 2900019G14Rik RIKEN cDNA 2900019G14 gene 2.06 1456973_at – – – 2.05 1452501_at NM_010002 Cyp2c38 cytochrome P450, family 2, subfamily c, polypeptide 38 1.98 1422903_at NM_010745 Ly86 lymphocyte antigen 86 1.93 1415932_x_at NM_015731 Atp9a ATPase, class II, type 9A 1.92 1418213_at NM_033373 Krt23 keratin 23 1.92 1444706_at – Nav2 neuron navigator 2 1.91 1423627_at NM_008706 Nqo1 NAD(P)H dehydrogenase, quinone 1 1.88 1450505_a_at NM_001034851, NM_025459 1810015C04Rik RIKEN cDNA 1810015C04 gene 1.85 1450648_s_at NM_207105 H2-Ab1 histocompatibility 2, class II antigen A, beta1 1.84 1417900_a_at NM_013703 Vldlr very low density lipoprotein receptor 1.83 1455316_x_at XM_915804 ENSMUSG00000073624 predicted gene, ENSMUSG00000073624 1.81 1458585_at – – – 1.80 1447643_x_at NM_011415 Snai2 snail homolog 2 (Drosophila) 1.78 Tm7sf2 knockout mice A. M. Bennati et al. 5040 FEBS Journal 275 (2008) 5034–5047 ª 2008 The Authors Journal compilation ª 2008 FEBS cholesterol in microsomes from liver, as well as plasma cholesterol, were comparable between wild-type and Tm7sf2 () ⁄ )) mice. Plasma cholesterol was still normal in a group of 14-month-old female mice. In addition to C14SR, the inner nuclear membrane protein LBR exhibits 3b-hydroxysterol D 14 -reductase activity [5]. Reciprocal increase of Lbr and Tm7sf2 gene expression was detected in the liver of 1-day-old Dhcr14 (D4-7 ⁄ D4-7) and Lbr () ⁄ )) mice, respectively [6]. These results, together with the absence of D 14 -sterols accumulation, supported the hypothesis that C14SR and LBR provide redundancy with respect to 3b-hydroxy- sterol D 14 -reductase activity [6]. We analysed Lbr expression in several adult mice tissues, including liver, Table 4. Continued Affymetrix probeset identification GenBank Gene symbol Gene name Fold change 1427604_a_at NM_015731 Atp9a ATPase, class II, type 9A 1.78 1439293_at NM_153584 BC031353 cDNA sequence BC031353 1.76 1419430_at NM_007811 Cyp26a1 cytochrome P450, family 26, subfamily a, polypeptide 1 1.75 1450884_at NM_007643 Cd36 CD36 antigen 1.74 1420879_a_at NM_018753 Ywhab tyrosine 3-monooxygenase ⁄ tryptophan 5-monooxygenase activation protein, beta polypeptide 1.73 1429831_at NM_031376 Pik3ap1 phosphoinositide-3-kinase adaptor protein 1 1.72 1418710_at NM_007652 Cd59a CD59a antigen 1.71 1448978_at NM_019867 Ngef neuronal guanine nucleotide exchange factor 1.68 1446731_at – A730016A17 Fanconi anemia, complementation group F 1.68 1417025_at NM_010382 H2-Eb1 histocompatibility 2, class II antigen E beta 1.68 1422975_at NM_008604 Mme membrane metallo endopeptidase 1.63 AFFX-r2-Bs- thr-M_s_at – – – 1.63 1417629_at NM_011172 Prodh proline dehydrogenase 1.63 1417017_at NM_007809 Cyp17a1 cytochrome P450, family 17, subfamily a, polypeptide 1 1.63 AFFX-ThrX-M_at – – – 1.62 1431916_at NM_001012306 Hsd3b3 hydroxy-delta-5-steroid dehydrogenase, 3 beta- and steroid delta-isomerase 3 1.62 1417828_at NM_007474 Aqp8 aquaporin 8 1.61 1448595_a_at NM_009052 Bex1 brain expressed gene 1 1.60 1428083_at NR_003513, XR_035481, XR_035482 2310043N10Rik RIKEN cDNA 2310043N10 gene 1.60 1448568_a_at NM_015747 Slc20a1 solute carrier family 20, member 1 1.60 1420549_at NM_010259 Gbp1 guanylate nucleotide binding protein 1 1.59 1453109_at NM_029847 Arsk arylsulfatase K 1.59 1416193_at NM_001083957, NM_009799 Car1 carbonic anhydrase 1 1.58 1415822_at NM_009128 Scd2 stearoyl-Coenzyme A desaturase 2 1.58 1445862_at NM_001081154 4921513D23Rik RIKEN cDNA 4921513D23 gene 1.58 1424683_at NM_001034851, NM_025459 1810015C04Rik RIKEN cDNA 1810015C04 gene 1.57 1429104_at NM_172397 Limd2 LIM domain containing 2 1.57 1449067_at NM_031197 Slc2a2 solute carrier family 2 (facilitated glucose transporter), member 2 1.53 1422479_at NM_019811 Acss2 acyl-CoA synthetase short-chain family member 2 1.52 1443056_at – – – 1.52 1442418_at – B930096F20Rik RIKEN cDNA B930096F20 gene 1.51 1457760_at – A930004J17Rik RIKEN cDNA A930004J17 gene 1.50 1456225_x_at NM_175093 Trib3 tribbles homolog 3 (Drosophila) 1.50 1437176_at NM_001033207 AI451557 expressed sequence AI451557 1.50 a Fold change determined by qRT-PCR. A. M. Bennati et al. Tm7sf2 knockout mice FEBS Journal 275 (2008) 5034–5047 ª 2008 The Authors Journal compilation ª 2008 FEBS 5041 Table 5. Genes whose transcripts decrease in liver of Tm7sf2 () ⁄ )) mice. See legend to Table 4. Affymetrix probeset identification GenBank Gene symbol Gene name Fold change 1444297_at NR_002861, XM_001471933 LOC100044164, Serpina4-ps1 serine (or cysteine) peptidase inhibitor, clade A, member 4, pseudogene 1 )5.26 1444296_a_at NR_002861, XM_001471933 LOC100044164 Serpina4-ps1 serine (or cysteine) peptidase inhibitor, clade A, member 4, pseudogene 1 )4.05 1448092_x_at NR_002861, XM_001471933 LOC100044164 Serpina4-ps1 serine (or cysteine) peptidase inhibitor, clade A, member 4, pseudogene 1 )4.02 1427797_s_at NM_007799 Ctse cathepsin E )3.92- 1416664_at NM_023223 Cdc20 b cell division cycle 20 homolog (S. cerevisiae) )3.60 ⁄ )10.8 a 1424638_at NM_007669 Cdkn1a b cyclin-dependent kinase inhibitor 1A (P21) )3.38 ⁄ )8.9 a 1417764_at NM_025965, XM_911969 LOC636537, Ssr1 signal sequence receptor, alpha )3.30 1420451_at NM_021370 Accn5 amiloride-sensitive cation channel 5, intestinal )3.23 ⁄ )4.1 a 1421447_at NM_008262, XM_001480325 LOC100048479, Onecut1 (Hnf6) b one cut domain, family member 1 )3.02 ⁄ )6.1 a 1424278_a_at NM_001012273, NM_009689 Birc5 (survivin40) b baculoviral IAP repeat-containing 5 )2.98 1450252_at NM_008262 Onecut1 (Hnf6) b one cut domain, family member 1 )2.40 1425948_a_at NM_026232 Slc25a30 solute carrier family 25, member 30 )2.35 1452754_at NM_029720 Creld2 cysteine-rich with EGF-like domains 2 )2.29 1456974_at NM_008262 Onecut1 (Hnf6) b one cut domain, family member 1 )2.27 1420836_at NM_026232 Slc25a30 solute carrier family 25, member 30 )2.20 1425127_at NM_153193 Hsd3b2 hydroxy-delta-5-steroid dehydrogenase, 3 beta- and steroid delta-isomerase2 )2.17 1416076_at NM_172301, XM_001005050, XM_485921, XM_900988, XR_030797, XR_033681 Ccnb1 b , Ccnb1-rs1, EG434175, LOC667005 cyclin B1; cyclin B1, related sequence 1; predicted gene, EG434175 )2.13 1422001_at NM_010565 Inhbc inhibin beta-C )2.10 1423397_at NM_133894 Ugt2b38 UDP glucuronosyltransferase 2 family, polypeptide B38 )2.08 1419669_at NM_011178 Prtn3 proteinase 3 )1.98 1417370_at NM_011575 Tff3 trefoil factor 3, intestinal )1.96 1424695_at NM_025912 2010011I20Rik RIKEN cDNA 2010011I20 gene )1.93 1442051_at NM_013549 Hist2h2aa1 histone cluster 2, H2aa1 )1.92 1437073_x_at – AV025504 expressed sequence AV025504 )1.89 1450440_at NM_010279 Gfra1 glial cell line derived neurotrophic factor family receptor alpha 1 )1.84 1424118_a_at NM_025565 Spc25 b SPC25, NDC80 kinetochore complex component, homolog (S. cerevisiae) )1.83 1416299_at NM_011369 Shcbp1 b Shc SH2-domain binding protein 1 )1.82 1433955_at NM_145125 Brwd1 b bromodomain and WD repeat domain containing 1 )1.77 1419319_at NM_011316 Saa4 serum amyloid A 4 )1.77 1448314_at NM_007659 Cdc2a b cell division cycle 2 homolog A (S. pombe) )1.76 1425993_a_at NM_013559 Hsp110 heat shock 105kDa ⁄ 110kDa protein 1 )1.76 1416757_at NM_026507 Zwilch b zwilch, kinetochore associated, homolog (Drosophila) )1.75 1424684_at NM_024456 Rab5c RAB5C, member RAS oncogene family )1.73 1417991_at NM_007860 Dio1 deiodinase, iodothyronine, type I )1.67 1425282_at NM_146042 Ibrdc2 b ring finger protein 144B )1.67 1455892_x_at – – – )1.67 1425107_a_at NM_013584 Lifr b leukemia inhibitory factor receptor )1.66 1449824_at XM_355243 Prg4 proteoglycan 4 (megakaryocyte stimulating factor, articular superficial zone protein) )1.66 1448756_at NM_009114 S100a9 S100 calcium binding protein A9 (calgranulin B) )1.66 1429379_at NM_053247 Lyve1 lymphatic vessel endothelial hyaluronan receptor 1 )1.64 1439695_a_at NM_183046 Mphosph1 b kinesin family member 20B ) 1.64 a Fold change determined by qRT-PCR. b Genes involved in cell proliferation and cell-cycle progression. Tm7sf2 knockout mice A. M. Bennati et al. 5042 FEBS Journal 275 (2008) 5034–5047 ª 2008 The Authors Journal compilation ª 2008 FEBS and we did not find significant differences between wild- type and Tm7sf2 () ⁄ )) mice. In the liver, the result obtained by qRT-PCR was further confirmed by the Affymetrix oligonucleotide array hybridization experi- ment. In addition, the expression of LBR protein in nuclei from liver and its enzymatic activity were compa- rable in wild-type and Tm7sf2 () ⁄ )) mice. These results indicate that, at least in adult Tm7sf2 () ⁄ )) mice, LBR can account for normal cholesterol biosynthesis without increasing its expression. The discrepancy in Lbr gene expression between 1-day-old Dhcr14 (D4-7 ⁄ D4-7) and adult Tm7sf2 () ⁄ )) mice could be related to higher cho- lesterol biosynthetic activity during development, com- pared with adult mice. In vitro, C14SR exhibits higher cholesterol biosynthetic capacity than LBR. Although it cannot be excluded that this is due to the experimental conditions used for enzymatic activity determination (substrate and ⁄ or cofactors), this result is in accordance with higher Tm7sf2 gene expression in liver, compared with Lbr. In vivo, the regulation of Tm7sf2 gene expres- sion by cholesterol levels could represent a mechanism to adapt 3b-hydroxysterol D 14 -reductase activity to increased cholesterol needs. The post-squalene segment of cholesterol biosynthe- sis takes place in the ER. Among the enzymes involved in this pathway, LBR is the unique residing in the inner nuclear membranes. The N-terminal nucleoplas- mic domain of LBR is involved in heterochromatin organization [24], whereas the transmembrane domain is responsible for 3b-hydroxysterol D 14 -reductase activ- ity. Following its synthesis, LBR diffuses laterally from the ER throughout the nuclear pore membranes and it is retained in the inner nuclear membranes by its bind- ing to the lamina [25]. It has been speculated whether the ability of LBR to synthesize cholesterol in vivo is restricted to its presence in the ER or in the nuclear envelope [26]. Deficiency of LBR during granulopoiesis results in hypolobulation of the mature neutrophil nucleus, suggesting that LBR helps make the extra nuclear membranes required during the nuclear lobula- tion of granulopiesis [27]. Although LBR was not detectable in liver microsomes, it cannot be excluded that in vivo LBR supports cholesterol biosynthesis when present in the ER. Alternatively, the mobility of sterol intermediates in the lateral plane of the mem- brane to reach enzymes differently compartmentalized should occur. The role of cholesterol and intermediates of its bio- synthesis in cell growth and division is well known [28,29]. The stringency of the requirement for choles- terol during proliferation and cell-cycle progression was investigated in promyelocytic HL-60 cells by com- parison with other sterols of the biosynthetic pathway. In the absence of exogenous cholesterol, accumulation of intermediate sterols upstream 7-dehydrocholesterol, including C29D 8,14 , resulted in the inhibition of cell proliferation and cell cycle arrest in G2 ⁄ M phase [30]. Affymetrix oligonucleotide array analysis showed that several genes involved in cell proliferation and cell- cycle progression have decreased expression in the liver of Tm7sf2 () ⁄ )) mice. Although no altered phenotype has been observed in Tm7sf2 () ⁄ )) mice so far, we could speculate that an impaired response of liver cells to proliferative stress is conceivable in these mice. The evaluation of C14SR and LBR expression and the determination of their enzymatic activity in the liver of wild-type and Tm7sf2 () ⁄ )) mice reinforce the hypothesis that LBR and C14SR provide enzymatic redundancy with respect to cholesterol synthesis [6]. Indeed, no sterol abnormalities were detected in Tm7sf2 () ⁄ )) mice. The significance of this enzymatic redundancy is not clear. Even though it can be hypoth- esized that multiple mechanisms have been developed during evolution to ensure cholesterol biosynthesis, it should be considered that the 3b-hydroxysterol D 14 - reductase reaction is the only one in the post-squalene pathway to be catalysed by two different enzymes. It is worth noting that Drosophila LBR lacks sterol reduc- tase activity, which could have been lost during evolu- tion [31]. Different roles for Tm7sf2 and Lbr genes during development or in tissues can be expected, rais- ing the question of how 3b-hydroxysterol D 14 -reductase activity can be switched between C14SR and LBR and which regulatory mechanisms are involved. Experimental procedures Materials Cholesterol, 5a-cholestane, and commercial antibodies were purchased from Sigma (Milan, Italy). The polyclonal anti- (bovine C14SR) serum was raised in our laboratory as previously described [2]. The polyclonal anti-(human LBR) serum was a kind gift of H. Hermann (German Cancer Research Center, Heidelberg, Germany) [32]. Complete pro- tease inhibitor cocktail tablets were from Roche Diagnos- tics (Milan, Italy). RNAlater RNA Stabilization Reagent, Qiazol Lysis Reagent, and RNeasy Mini Kit were from Qiagen (Milan, Italy). QuickChange Site-Directed Muta- genesis Kit, AffinityScript Multiple Temperature Reverse Transcriptase, and Brilliant Ò SYBR Ò Green QPCR Master Mix were purchased from Stratagene (La Jolla, CA, USA). RiboLock RNase inhibitor, random hexamer primers, Taq DNA polymerase, and restriction enzymes were from Fermentas (St Leon-Rot, Germany). 5a-Cholesta-8,14-dien- 3b-ol was synthesized as previously described [2]. A. M. Bennati et al. Tm7sf2 knockout mice FEBS Journal 275 (2008) 5034–5047 ª 2008 The Authors Journal compilation ª 2008 FEBS 5043 [...]... from the Tm7sf2 genomic clones were subcloned into pBlueScript vector and sequenced Generation of Tm7sf2 knockout mice A Tm7sf2 targeting vector was generated by subcloning into pBluescript vector a 5380 bp KpnI ⁄ XbaI fragment, containing exons 1–5 of the gene (Fig 2B) The neomycin phosphotransferase gene (neo) was inserted in a SalI restriction site generated by in vitro mutagenesis in exon 5 The. .. dynamics and reassembly in living cells: targeting of an inner nuclear membrane protein in interphase and mitosis J Cell Biol 138, 1193–1206 26 Hoffmann K, Sperling K, Olins AL & Olins DE (2007) The granulocyte nucleus and lamin B receptor: avoiding the ovoid Chromosoma 116, 227–235 27 Zwerger M, Herrmann H, Gaines P, Olins AL & Olins DE (2008) Granulocytic nuclear differentiation of lamin B receptor-deficient... descendants (F1) of these animals Backcross into C57 ⁄ B6 was also carried out for four generations to obtain mice with > 90% C57 ⁄ B6 genetic background All experiments involving animals were 5044 conducted according to protocols approved by the Bioethics Committee of University of Perugia Mice were genotyped for the introduced Tm7sf2 mutated gene by PCR analysis of tail genomic DNA using three specific... membranes after protein transfer Labelled proteins were detected by the enhanced chemiluminescence assay, images were acquired using the VersaDoc Imaging System, and signals were quantified using quantity one software (Bio-Rad, Milan, Italy) 3b-Hydroxysterol D14-reductase activity Determination of 3b-hydroxysterol D14-reductase activity was performed by incubating the enzyme source for 30 min at 37 °C with... set according to Affymetrix Two copies of the complete genechipmoe430 2.0 were hybridized with each biotin-labelled target Images were scanned using an Affymetrix GeneChip Scanner3000 7G, using default parameters The resulting images were analysed using genechip operating software v1.4 (GCOS1.4) Some differentially expressed genes were analysed by qRT-PCR as described above using the following primers... cycling conditions were: 15 min denaturation at 95 °C followed by 45 cycles of 30 s at 95 °C, 45 s at 64 °C, 3 min at 72 °C, and a final extension of 10 min at 72 °C The positive clones were subjected to Southern blotting after EcoRI genomic DNA digestion, using a 450-bp probe located to the 3¢-end of the gene and external to the targeting construct (Fig 2B) The probe detects an 8.9 kb EcoRI fragment in. .. concentration was determined by the method of Lowry [38] Western blotting Liver microsomal or nuclear membrane proteins were analysed by western blotting using polyclonal rabbit anti(bovine C14SR) serum or guinea-pig anti-(human LBR) serum as previously described [5] Equivalent protein loading was checked by Ponceau staining of poly(vinylidene FEBS Journal 275 (2008) 5034–5047 ª 2008 The Authors Journal... ATCAGGACATAGCGTTGGC-3¢; see Fig 2B) PCR cycling conditions were: 2 min denaturation at 95 °C followed by 35 cycles of 30 s at 95 °C, 45 s at 65 °C, 30 s at 72 °C, and a final extension of 5 min at 72 °C Mouse genotype was confirmed by Southern blotting analysis 5¢ RACE The transcription start site of the mouse Tm7sf2 gene was determined by 5¢-RACE [34] Mouse liver RNA was reverse transcribed using the primer 5¢-AGGAGCTACCAAAGC... Unit (Campus IFOM IEO, Milan, Italy) Biotin-labelled cRNA targets were synthesized starting from 3 lg of total RNA Double stranded cDNA synthesis was performed with One-Cycle cDNA Synthesis Kit, and biotin-labelled antisense RNA was transcribed in vitro using IVT Labelling Kit All steps of the labelling protocol were performed with One-Cycle Eukaryotic Target Labelling Assay as suggested by Affymetrix... CTTCG-3¢ (nucleotides +468 to +449 from the ATG start codon) A homopolymeric A-tail was then added to the 3¢-end using terminal transferase and dATP The tailing product was purified and amplified using the nested reverse primer 5¢-CGACTCTTGTCCTTCAGTTCC-3¢ (nucleotides +284 to +264) and an oligo(dT) forward primer The product of amplification was amplified again using the nested reverse primer 5¢-GTGCAGGCAGCAAATAGA . Disruption of the gene encoding 3b-hydroxysterol D 14 -reductase (Tm7sf2) in mice does not impair cholesterol biosynthesis Anna M. Bennati 1 ,. during the conversion of lanosterol to cholesterol. The C-terminal domain of lamin B receptor, a protein of the inner nuclear membrane mainly involved in heterochromatin