Open Access

Identification of Tmem10/Opalin as a novel marker for oligodendrocytes using gene expression profiling

  • Angelika Kippert1, 2,
  • Katarina Trajkovic1, 2,
  • Dirk Fitzner1, 2, 3,
  • Lennart Opitz2 and
  • Mikael Simons1, 2, 3Email author
BMC Neuroscience20089:40

DOI: 10.1186/1471-2202-9-40

Received: 26 October 2007

Accepted: 25 April 2008

Published: 25 April 2008

Abstract

Background

During the development of the central nervous system, oligodendrocytes generate large amounts of myelin, a multilayered insulating membrane that ensheathes axons, thereby allowing the fast conduction of the action potential and maintaining axonal integrity. Differentiation of oligodendrocytes to myelin-forming cells requires the downregulation of RhoA GTPase activity.

Results

To gain insights into the molecular mechanisms of oligodendrocyte differentiation, we performed microarray expression profiling of the oligodendroglial cell line, Oli-neu, treated with the Rho kinase (ROCK) inhibitor, Y-27632 or with conditioned neuronal medium. This resulted in the identification of the transmembrane protein 10 (Tmem10/Opalin), a novel type I transmembrane protein enriched in differentiating oligodendrocytes. In primary cultures, Tmem10 was abundantly expressed in O4-positive oligodendrocytes, but not in oligodendroglial precursor cells, astrocytes, microglia or neurons. In mature oligodendrocytes Tmem10 was enriched in the rims and processes of the cells and was only found to a lesser extent in the membrane sheets.

Conclusion

Together, our results demonstrate that Tmem10 is a novel marker for in vitro generated oligodendrocytes.

Background

Oligodendrocytes are specialized cells of the central nervous system that produce myelin, a multilayered membrane spirally ensheathing axons and facilitating rapid nerve conduction [1, 2]. The development of oligodendrocytes is a gradual process, in which each step of the differentiation process is characterized by stage specific markers [36]. Oligodendrocytes originate from oligodendrocyte precursor cells that arise from multiple foci along the neuronal tube and migrate into the future white matter of the brain. After reaching their final position, they develop into mature post-mitotic cells that produce the myelin sheaths [7]. During the progression through the oligodendroglial lineage, the cells loose their migratory and proliferative capacities and undergo dramatic changes in their morphology by the formation of a highly branched network of processes. This transformation is accompanied by the expression of a number of gene products that are highly enriched or even specific to oligodendrocytes such as the myelin basic protein (MBP), proteolipid proteins (PLP/DM20), myelin-associated glycoprotein (MAG), cyclic nucleotide phosphodiesterase (CNP) and the glycolipids, galactosylceramide and sulfatide. The capacity of oligodendrocyte precursor cells to differentiate into oligodendrocytes that express these different gene products is intrinsic to the lineage and occurs even in the absence of neurons [4, 8]. Oligodendrocytes need to provide specific sorting and transport mechanisms to enable the synthesis of an extensive amount of myelin membrane in a very short time [3, 9]. Since oligodendrocytes must produce myelin at the appropriate time of neuronal development, a number of reciprocal signalling systems are likely to operate to coordinate the organisation of axonal domains and the biogenesis of myelin [1015]. A number of recent studies have shown that neuronal-derived signalling molecules control the development of myelin-forming glial cells [1621]. We have recently shown that neurons regulate membrane trafficking in oligodendrocytes [19]. In the absence of neurons, the major myelin protein, PLP, is internalized and stored in late endosomes. After receiving an unknown soluble signal from neurons, oligodendrocytes reduce the rate of endocytosis and increase the retrograde transport of PLP from late endosomes to the plasma membrane. A fraction of PLP is released in association with exosomes [22, 23]. Our previous work shows that changes in Rho GTPase activity were responsible for switching between these two modes of transport [24]. Inactivation of Rho GTPase activity reduced the transport of cargo to late endosomes and at the same time increased the mobilization of membrane from late endosomes. We found that a neuronal soluble factor was responsible for the downregulation of RhoA GTPase activity in the oligodendroglial cell line, Oli-neu [24]. The downregulation of RhoA function during morphological differentiation of oligodendrocytes is supported by a number of other studies [18, 25]. In this study, we were interested in the transcriptional changes that occur after differentiation of Oli-neu cells by conditioned neuronal medium or by inactivation of Rho GTPase function. This effort led to the identification of the transmembrane protein 10 (Tmem10/Opalin) as a novel marker for oligodendrocytes. The transmembrane protein 10 is known as Tmem10/TMEM10 in mice, rats and humans, with the synonyms TMP10 or HTMP10. Recently four Tmem10 homologs of prosimian species (Eulemur macaco, Lemur catta, Microcebus murinus and Otolemur garnetti) have been named Opalin [26]. In this work the human, rat and mouse transmembrane protein 10 will be referred to as Tmem10.

Results and Discussion

As a cellular model for oligodendrocyte differentiation we use the oligodendroglial cell line, Oli-neu. The advantage of this system is that morphological differentiation of a pure oligodendroglial culture can be triggered synchronously by adding conditioned medium from primary neuronal cultures to the cells. To characterize the gene changes that occur after incubation of Oli-neu cells with conditioned neuronal medium, we used Affymetrix microarrays. Oli-neu cells were incubated for 16 hours with conditioned neuronal medium and compared to untreated parallel cultures. Cell fractions were used to purify mRNA for microarray analysis (the data is available at NCBI Geo accession number GSE10291). Using a 1.7 fold cut-off, we found that 716 genes were up-regulated, whereas 386 genes were down-regulated by conditioned neuronal medium as compared to the control. The eighty most highly induced genes are shown in Table 1. We have previously shown that incubation of Oli-neu cells with conditioned neuronal medium leads to RhoA GTPase inactivation. We, therefore, performed gene expression profiling analysis after treating cells with the Rho kinase (ROCK) inhibitor, Y27632 for 16 hours and compared the transcriptional changes to the ones obtained after incubation with conditioned neuronal medium. Interestingly, 70% of the genes that were controlled by both conditioned neuronal medium and by treatment with Y27632 were regulated into the same direction, indicating that these treatments affected the fate of the cells into a common path (Table 2). Some of these genes such as the UDP galactosyltransferase 8a and CNP are known to be upregulated during the progression through the oligodendroglial lineage [4], whereas known housekeeping genes such as the ATP synthase served as internal controls and were not differentially expressed after incubation with conditioned neuronal medium or Y27632 (data not shown). One so far not characterized, but highly upregulated gene is the brain-specifically expressed, Tmem10 [27]. For our further study, we decided to focus on Tmem10 for a number of reasons. Tmem10 was the strongest up-regulated gene in our analysis of transcriptional changes induced by conditioned neuronal medium and as well highly up-regulated after treatment with Y27632. The analysis of the average fold up-regulation in both conditions shows that Tmem10 is the most up-regulated gene together with Cyp2c39 (cytochrome P450, family2, polypeptide 39) (Table 3). In addition, a previous microarray analysis identified a 23.75 fold up-regulation of Tmem10 during oligodendrocyte differentiation [28] and in situ expression data of Tmem10 in the Allen Brain Atlas http://www.brain-map.org suggested enrichment in the white matter of the brain.
Table 1

Top 80 upregulated genes after addition of conditioned neuronal medium to Oli-neu cells

Probe set ID

Fold change

p-value

Gene name

Gene symbol

A_52_P624415

7.12

0.00015

transmembrane protein 10

Tmem10

A_51_P225761

6.96

0.00020

ESTs, no homologies found

 

A_52_P225856

6.76

0.00013

ESTs, no homologies found

 

A_52_P329250

6.51

0.00000

chromodomain helicase DNA binding protein 1

Chd1

A_51_P304109

6.41

0.00007

cytochrome P450, family 2, subfamily c, polypeptide 39

Cyp2c39

A_52_P160518

6.23

0.00005

Scm-like with four mbt domains 1

Sfmbt1

A_52_P771513

6.22

0.00027

ESTs, no homologies found

 

A_52_P61864

6.14

0.00023

wingless-related MMTV integration site 2

Wnt2

A_51_P370640

6.05

0.00013

zinc finger, CCHC domain containing 5

Zcchc5

A_51_P186092

6

0.00017

male sterility domain containing 2

Mlstd2

A_52_P24076

5.95

0.00026

myotubularin related protein 7

Mtmr7

A_52_P448870

5.9

0.00025

RAB26, member RAS oncogene family

Rab26

A_52_P193256

5.81

0.00032

DNA segment, Chr 10, Brigham & Women's Genetics 0791

D10Bwg0791e

A_52_P350750

5.81

0.00017

cholinergic receptor, nicotinic, alpha polypeptide 4

Chrna4

A_52_P391098

5.74

0.00022

cAMP responsive element modulator

Crem

A_51_P127035

5.72

0.00082

RIKEN cDNA 4432405B04 gene

4432405B04Rik

A_52_P600304

5.69

0.00013

RIKEN cDNA 1200007B05 gene

1200007B05Rik

A_51_P448632

5.68

0.00033

RIKEN cDNA C030022K24 gene

C030022K24Rik

A_52_P188593

5.68

0.00017

hypothetical gene supported by AK049058; BC025881

LOC433886

A_51_P359002

5.59

0.00028

ESTs, no homologies found

 

A_52_P302587

5.55

0.00021

chimerin (chimaerin) 2

Chn2

A_51_P444502

5.44

0.00029

immunoglobulin kappa light chain variable region Vk23

LOC381783

A_51_P461404

5.42

0.00014

SWI/SNF related, actin dependent regulator of chromatin

Smarca1

A_52_P577329

5.37

0.00021

RIKEN cDNA A230069A22 gene

A230069A22Rik

A_51_P334449

5.37

0.00033

olfactory receptor 50

Olfr50

A_52_P354306

5.32

0.00013

peroxisome biogenesis factor 26

Pex26

A_51_P462978

5.31

0.00033

membrane protein, palmitoylated 2

Mpp2

A_52_P661972

5.3

0.00021

RIKEN cDNA 9230112E08 gene

9230112E08Rik

A_52_P1133703

5.29

0.00014

CD47 antigen (Rh-related antigen)

Cd47

A_51_P169617

5.28

0.00041

TAF3 RNA polymerase II, TATA box binding protein

Taf3

A_51_P339934

5.24

0.00022

neurofilament, light polypeptide

Nefl

A_51_P472113

5.23

0.00060

ESTs, no homologies found

 

A_51_P262563

5.22

0.00031

ESTs, no homologies found

 

A_51_P284486

5.2

0.00030

glutathione S-transferase, mu 2

Gstm2

A_51_P103706

5.17

0.00054

cytochrome P450, family 2, subfamily c, polypeptide 29

Cyp2c29

A_52_P223626

5.17

0.00011

oligodendrocyte transcription factor 2

Olig2

A_51_P392209

5.17

0.00019

zinc finger protein 482

Zfp482

A_51_P283499

5.17

0.00022

dopamine receptor 4

Drd4

A_52_P229052

5.17

0.00019

transmembrane prot. with EGF-like and two follistatin-like

Tmeff2

A_52_P337910

5.15

0.00015

RIKEN cDNA E130114P18 gene

E130114P18Rik

A_51_P129108

5.11

0.00019

activating transcription factor 6

Atf6

A_51_P393934

5.11

0.00028

CD82 antigen

Cd82

A_51_P413005

5.08

0.00021

chimerin (chimaerin) 2

Chn2

A_51_P394574

5.05

0.00019

ESTs, no homologies found

 

A_51_P478003

5.04

0.00046

poly(A) polymerase gamma

Papolg

A_52_P516733

5.02

0.00086

DNA segment, Chr 15, ERATO Doi 621, expressed

D15Ertd621e

A_52_P127776

5.01

0.00030

ESTs, no homologies found

 

A_52_P384479

4.95

0.00014

leucine rich repeat and fibronectin type III domain

Lrfn5

A_52_P118323

4.93

0.00042

ESTs, no homologies found

 

A_52_P685963

4.91

0.00041

tenascin R

Tnr

A_52_P313068

4.83

0.00016

RIKEN cDNA 8030462N17 gene

8030462N17Rik

A_51_P489107

4.83

0.00013

pleckstrin homology domain-containing, family A, memb. 2

Plekha2

A_51_P454008

4.82

0.00027

lipopolysaccharide binding protein

Lbp

A_52_P418956

4.81

0.00059

RIKEN cDNA 4933431E20 gene

4933431E20Rik

A_51_P244453

4.8

0.00059

potassium channel tetramerisation domain containing 3

Kctd3

A_51_P342206

4.79

0.00890

cytochrome P450, family 2, subfamily c, polypeptide 38

Cyp2c38

A_51_P270899

4.78

0.00019

zinc finger protein 61

Zfp61

A_52_P370162

4.78

0.00034

G protein-coupled receptor 23

Gpr23

A_52_P356170

4.77

0.00024

glyceraldehyde-3-phosphate dehydrogenase, spermatogenic

Gapdhs

A_51_P130254

4.75

0.00021

pleckstrin and Sec7 domain containing 3

Psd3

A_51_P363461

4.73

0.00022

ESTs, no homologies found

 

A_52_P502838

4.72

0.00033

mannoside acetylglucosaminyltransferase 5

Mgat5

A_51_P494122

4.69

0.00050

RIKEN cDNA 1810009K13 gene

1810009K13Rik

A_52_P285194

4.69

0.00021

ESTs, no homologies found

 

A_51_P169087

4.67

0.00035

ESTs, no homologies found

 

A_51_P506822

4.65

0.00015

UDP galactosyltransferase 8A

Ugt8a

A_52_P164709

4.64

0.00030

WD repeat domain 51A

Wdr51a

A_51_P226269

4.62

0.00022

RIKEN cDNA 1190002H23 gene

1190002H23Rik

r60_a9

4.55

0.00003

fibronectin 1

Fn1

A_51_P111233

4.49

0.00013

dopamine receptor 2

Drd2

A_52_P265556

4.49

0.00042

predicted gene, ENSMUSG00000056850

ENSMUSG00000056850

A_52_P417654

4.47

0.00022

transcription elongation factor A (SII) 1

Tcea1

A_52_P625249

4.44

0.00021

cytochrome P450, family 2. subfamily c, polypeptide 37

Cyp2c37

A_52_P603038

4.44

0.00021

oligodendrocyte transcription factor 1

Olig1

A_51_P199199

4.42

0.00024

phosphoinositide-3-kinase adaptor protein 1

Pik3ap1

A_52_P510706

4.41

0.00030

DnaJ (Hsp40) homolog, subfamily A, member 2

Dnaja2

A_52_P57416

4.39

0.00044

ESTs, no homologies found

 

A_52_P322639

4.38

0.00031

ESTs, no homologies found

 

A_51_P232901

4.37

0.00007

cyclic nucleotide phosphodiesterase 1

Cnp1

Table 2

Genes regulated in the same direction after treatment with conditioned neuronal medium (cnm) or Y27632

 

Cnm

Y27632

  

Probe set ID

Fold change

p-val

Fold change

p-val

Gene name

Gene symbol

A_52_P624415

7,12

0,00015

2,2

0,00054

transmembrane protein 10

Tmem10

A_51_P304109

6,41

0,00007

2,96

0,00127

cytochrome P450, family 2, subfamily c, polypept. 39

Cyp2c39

A_52_P302587

5,55

0,00021

1,48

0,00111

chimerin (chimaerin) 2

Chn2

A_52_P229052

5,17

0,00019

2,05

0,00067

transmembr. prot. with EGF-like, dom. 2

Tmeff2

A_51_P103706

5,17

0,00054

1,29

0,00429

cytochrome P450, family 2, subfamily c, polypept. 29

Cyp2c29

A_51_P413005

5,08

0,00021

1,43

0,00086

chimerin (chimaerin) 2

Chn2

A_51_P454008

4,82

0,00027

1,37

0,00043

lipopolysaccharide binding protein

Lbp

A_51_P506822

4,65

0,00015

1,25

0,00296

UDP galactosyltransferase 8A

Ugt8a

A_51_P232901

4,37

0,00007

1,47

0,00096

cyclic nucleotide phosphodiesterase 1

Cnp1

A_52_P661327

4,31

0,00018

1,3

0,00029

phytanoyl-CoA hydroxylase interacting protein-like

Phyhipl

A_51_P433194

3,43

0,00024

1,46

0,00218

breast carcinoma amplified sequence 1

Bcas1

A_51_P437079

3,07

0,00013

1,44

0,00068

RIKEN cDNA 5730559C18 gene

5730559C18Rik

A_52_P269003

2,53

0,00062

1,45

0,00147

Neogenin

Neo1

A_51_P259975

2,48

0,00007

1,54

0,00051

aspartoacylase

Aspa

A_52_P493854

2,42

0,0002

2,18

0,00084

potassium channel tetramerisation domain

Kctd4

A_52_P493857

2,39

0,00023

2,31

0,00111

potassium channel tetramerisation domain

Kctd4

A_51_P354354

2,29

0,00032

2,12

0,00172

galactose-3-O-sulfotransferase 1

Gal3st1

A_51_P112308

2,25

0,00014

1,26

0,00057

RIKEN cDNA 1810011O10 gene

1810011O10Rik

A_51_P413721

2,18

0,00025

1,88

0,00036

gap junction membrane channel prot. epsilon 1

Gje1

A_51_P145376

2,12

0,00024

1,48

0,00033

OTU domain containing 7B

Otud7b

A_52_P168953

2,07

0,00037

1,38

0,0073

Versican

Vcan

A_52_P376169

2

0,00016

1,23

0,00125

LY6/PLAUR domain containing 6

Lypd6

A_51_P196596

1,95

0,00022

2,42

0,00046

tripartite motif protein 2

Trim2

A_51_P159453

1,94

0,00026

1,68

0,00202

serine (or cystein) peptidase inhib. 3n

Serpina3n

A_52_P149801

1,92

0,00044

1,9

0,0007

phosphodiesterase 4B, cAMP specific

Pde4b

A_52_P121502

1,87

0,00045

1,34

0,00159

plasma membrane proteolipid

Pllp

A_52_P465012

1,76

0,00081

2,75

0,00029

protein phosphatase 2, SU B (PR 52), beta isoform

Ppp2r2b

A_51_P512119

1,73

0,00067

1,31

0,00255

cDNA sequence AF067063

AF067063

A_52_P213932

-4,18

0,00021

-1,85

0,00255

metallopeptidase with thrombospondin type 1

Adamts1

A_51_P426754

-4,17

0,00203

-2,89

0,00059

annexin A5

Anxa5

A_52_P520495

-4,09

0,00022

-3,34

0,0005

vascular cell adhesion molecule 1

Vcam1

A_51_P115462

-3,73

0,00027

-2,57

0,00091

spermatogen. associat. glut. (E)-rich prot. 6, ps 1

Speer6-ps1

A_52_P433119

-3,55

0,00313

-2,14

0,00092

spermatogenesis associat. glut. (E)-rich prot. 2

Speer2

A_51_P183571

-3,31

0,00022

-1,64

0,0013

serine (or cysteine) peptidase inhib. 1

Serpine1

A_52_P62037

-3,18

0,00032

-1,47

0,00052

annexin A2

Anxa2

A_52_P63948

-3,13

0,0002

-2,44

0,00058

cDNA sequence BC048651

BC048651

A_52_P148703

-3,09

0,00013

-1,66

0,00059

fer-1-like 3, myoferlin (C. elegans)

Fer1l3

A_51_P131408

-2,99

0,00014

-1,33

0,00168

tumor necrosis factor receptor superfamily,12a

Tnfrsf12a

A_51_P282584

-2,96

0,0002

-1,39

0,00137

Olfactomedin-like 2B

Olfml2b

A_51_P165342

-2,87

0,00035

-1,84

0,00135

annexin A2

Anxa2

A_52_P518949

-2,84

0,00044

-2,3

0,00035

similar to spermatogen. associat. glut. (E)-rich prot. 2

LOC381612

A_52_P771912

-2,84

0,00019

-2,18

0,00091

lymphocyte antigen 6 complex, locus C

Ly6c

A_52_P360440

-2,81

0,00063

-1,39

0,00052

fer-1-like 3, myoferlin (C. elegans)

Fer1l3

A_51_P182116

-2,79

0,00021

-2,97

0,00033

Down syndrome critical region homolog 1 (human)

Dscr1

A_52_P628885

-2,77

0,00027

-2,25

0,00072

brain and acute leukemia, cytoplasmic

Baalc

A_52_P385546

-2,64

0,0002

-1,32

0,00158

C1q-like 3

C1ql3

A_52_P594584

-2,59

0,00179

-2,09

0,00208

spermatogenesis associat. glut. (E)-rich protein 2

Speer2

A_51_P228472

-2,48

0,00028

-1,94

0,00069

insulin-like growth factor binding protein 3

Igfbp3

A_52_P196732

-2,43

0,00039

-1,57

0,00151

NIMA-related expressed kinase 6

Nek6

A_51_P181595

-2,38

0,00024

-1,77

0,00058

spermatogen. associat. glut. (E)-rich prot. 1, ps 1

Speer1-ps1

A_51_P435023

-2,37

0,00007

-1,44

0,0003

Ras association (RalGDS/AF-6) domain family 1

Rassf1

A_52_P661565

-2,36

0,00034

-3,42

0,00068

chloride intracellular channel 4 (mitochondrial)

Clic4

A_52_P230938

-2,34

0,00082

-2,02

0,00771

lymphocyte antigen 6 complex, locus C

Ly6c

A_52_P93256

-2,25

0,00047

-1,49

0,00054

angiopoietin-like 2

Angptl2

A_51_P384968

-2,18

0,00046

-1,34

0,00158

nerve growth factor recept. (TNFR superfam., m16)

Ngfr

A_52_P427640

-2,14

0,00009

-1,84

0,00035

serine (or cysteine) proteinase inhib. 3m

Serpina3m

A_51_P351896

-2,08

0,00022

-1,29

0,00214

RIKEN cDNA 1110032E23 gene

1110032E23Rik

A_51_P427663

-2,08

0,00022

-1,61

0,00043

calponin 2

Cnn2

A_51_P248441

-2,07

0,00061

-1,23

0,00047

ubiquitin-conjugating enzyme E2G 2

Ube2g2

A_51_P344263

-2,01

0,00079

-1,47

0,00115

brain and acute leukemia, cytoplasmic

Baalc

A_51_P411253

-2

0,00014

-1,73

0,00062

phosphoprotein enriched in astrocytes 15A

Pea15a

A_52_P201206

-2

0,00022

-1,31

0,00102

secernin 1

Scrn1

A_51_P517843

-1,94

0,00697

-1,43

0,00125

GLI pathogenesis-related 2

Glipr2

A_51_P503162

-1,9

0,00064

-1,48

0,00099

Kruppel-like factor 6

Klf6

A_52_P359088

-1,9

0,00704

-1,4

0,00314

solute carrier family 25 (mitochond., phosphat.)

Slc25a25

A_52_P617327

-1,76

0,00042

-1,54

0,00049

Down syndrome critical region homolog 1 (human)

Dscr1

A_52_P403157

-1,73

0,00131

-4,01

0,00029

sorbin and SH3 domain containing 2

Sorbs2

A_52_P246698

-1,72

0,00036

-1,6

0,00091

down-regulated by Ctnnb1, a

Drctnnb1a

A_51_P103819

-1,71

0,00173

-1,2

0,00036

similar to Tribbles homolog 2 (predicted)

RGD1564451_pred.

Table 3

Top 10 up-regulated genes co-regulated after treatment with conditioned neuronal medium (cnm) or ROCK inhibitor (Y27632)

 

cnm

Y27632

(cnm+Y27632)/2

  

Probe set ID

Fold change

Fold change

Avgerage Fold-change

Gene name

Gene symbol

A_52_P624415

7,12

2,2

4,7

transmembrane protein 10

Tmem10

A_51_P304109

6,41

2,96

4,7

cytochrome P450, family 2, subfamily c, polypept. 39

Cyp2c39

A_52_P302587

5,55

1,48

3,5

chimerin (chimaerin) 2

Chn2

A_52_P229052

5,17

2,05

3,6

transmembr. prot. with EGF-like, dom. 2

Tmeff2

A_51_P103706

5,17

1,29

3,2

cytochrome P450, family 2, subfamily c, polypept. 29

Cyp2c29

A_51_P413005

5,08

1,43

3,3

chimerin (chimaerin) 2

Chn2

A_51_P454008

4,82

1,37

3,1

lipopolysaccharide binding protein

Lbp

A_51_P506822

4,65

1,25

3,0

UDP galactosyltransferase 8A

Ugt8a

A_51_P232901

4,37

1,47

2,9

cyclic nucleotide phosphodiesterase 1

Cnp1

A_52_P661327

4,31

1,3

2,8

phytanoyl-CoA hydroxylase interacting protein-like

Phyhipl

To begin our characterization of Tmem10 we raised an antibody against the C-terminal part of the protein and performed immunofluorescence analysis on primary cultures of oligodendrocytes. We did not detect Tmem10 on NG2-positive oligodendrocyte precursor cells, whereas pre-oligodendrocytes that were still NG2-positive, but also contained O4 started to express Tmem10 (Fig 1A, B). Higher expression of Tmem10 was identified on NG2-negative and O4-positive or O1-positive oligodendrocytes (Fig 1A, C). Low levels of Tmem10 expression can be detected in A2B5-positive oligodendrocyte progenitors (Fig 1D). Colocalization studies of Tmem10 with MBP indicated that Tmem10 was present in mature oligodendrocytes where it was enriched in the rims and processes of the cells and was found only to a lesser extent in the membrane sheets (Fig. 1E). Comparisons of Tmem10 expression in O4-positive/MBP-negative and MBP-positive cells shows that Tmem10 is redistributed to the rims of the membrane sheets, but the expression level does not change significantly compared to O4-positive oligodendrocytes (Fig 1B).
https://static-content.springer.com/image/art%3A10.1186%2F1471-2202-9-40/MediaObjects/12868_2007_Article_723_Fig1_HTML.jpg
Figure 1

Tmem10 is expressed during oligodendrocyte differentiation. (A) Primary oligodendrocytes were cultured for 2–3 days and levels of Tmem10 (green) expression were determined by immunofluorescence. The developmental stage of the cells was determined by costaining for NG2 (red) or O4 (blue). (B) Quantitative analysis of the Tmem10 immunofluorescence intensities. Values represent the mean ± SEM (n > 20 cells, ***p < 0,001). (C) Tmem10 (green) is expressed in O1 (red) positive oligodendrocytes (day 2) and (D) Tmem10 (green) is weakly expressed in A2B5 (red) positive cells (day 2). (E) Tmem10 (green) localizes to processes and rims of mature oligodendrocytes, expressing MBP (red) (day 5). Scale bars, 10 μm.

Interestingly, double labelling of Tmem10 with GFAP or neuron specific βIII Tubulin showed that Tmem10 could not be detected on GFAP-positive astrocytes or neurons, which are present as a minor cell population in the same cultures, indicating a specific expression of Tmem10 in differentiating oligodendrocytes (Fig. 2A, B). Additionally, we analysed primary cultures of astrocytes and microglia cultures, but could not detect Tmem10 on either of these cell types (Fig. 2C, D).
https://static-content.springer.com/image/art%3A10.1186%2F1471-2202-9-40/MediaObjects/12868_2007_Article_723_Fig2_HTML.jpg
Figure 2

Tmem10 is not detected on neurons, astrocytes and microglia. (A) Tmem10 (red) is specifically expressed in oligodendrocytes, but absent form GFAP-positive astrocytes (GFAP, green). (B) Tmem10 (green) is enriched in oligodendrocytes, labelled with O4 (blue) compared to neurons stained for neuronal βIII Tubulin (red). Tmem10 (green) is not detectable on (C) astrocytes (GFAP, red) and (D) microglia (stained with isolectin IB4 conjugated to Alexa Fluor 568, red) in cultures enriched for these cells types. Scale bars, 10 μm.

The analysis of the primary amino acid sequence of Tmem10 for protein domains and functional sites with InterProScan [29] revealed a predicted signal peptide (amino acid 1–15 in the mouse sequence) and a putative transmembrane domain (amino acid 31–51 in the mouse sequence) (Fig. 3A). To further characterize the protein structure and the membrane orientation of Tmem10, we used N-terminal ECFP- and C-terminal EYFP-fusion proteins of Tmem10. Oli-neu cells were transfected with either of the fusion proteins and live staining was performed at 4°C with anti-GFP antibody to specifically label the proteins at the cell surface. We found that only the N-terminal ECFP-fusion protein was detectable by surface staining, whereas premeabilization of the cells uncovered both fusion proteins (Fig. 3B). These results show that Tmem10, as predicted from its primary amino acid sequence, is a type I membrane protein.
https://static-content.springer.com/image/art%3A10.1186%2F1471-2202-9-40/MediaObjects/12868_2007_Article_723_Fig3_HTML.jpg
Figure 3

Tmem10 is a type I transmembrane protein. (A) Alignment of mouse, rat and human Tmem10 amino acid sequence. Black/grey box indicates the predicted position of the transmembrane domain. (B) Oli-neu cells were transfected to express either N-terminal ECFP-tagged or C-terminal EYFP-tagged Tmem10. Subsequent staining with anti-GFP antibody was performed either on living, unpermeabilized (surface anti-GFP) or on fixed, permeabilized (after permeabilization anti-GFP) cells. Scale bars, 10 μm.

After ectopic expression of both Tmem10 fusion proteins and an untagged expression construct, we found that it was mainly localized at the plasma membrane with only very little intracellular staining. Tmem10 appeared to be enriched in actin-rich membrane ruffles at the cell surface as shown by its colocalization with rhodamine-phalloidin (Fig 4A). Additionally, comparison of Tmem10 transfected Oli-neu with untransfected control cells shows that our anti-Tmem10 antibody specifically recognizes Tmem10 (Fig 4A).
https://static-content.springer.com/image/art%3A10.1186%2F1471-2202-9-40/MediaObjects/12868_2007_Article_723_Fig4_HTML.jpg
Figure 4

Tmem10 colocalizes with actin in processes and membrane ruffles. (A) Oli-neu cells were transfected with a plasmid encoding for rat Tmem10 and subsequently stained for Tmem10 (green) and Actin (red) with rhodamin-labelled phalloidin. Note the colocalization of Tmem10 with Actin in processes and membrane ruffles. The absence of Tmem10 labelling in untransfected cells shows the specificity of the generated anti-Tmem10 antiserum. (B, C) Oli-neu cells were transfected with Tmem10-EYFP and treated with 2 μM latrunculin A for 30 min 16 h after transfection. (B) FRAP was measured by bleaching a squared region of interest within the cell body and fluorescence recovery in this regions was examined. Average FRAP tracings for 15 cells form 2 independent experiments are shown. (C) Tmem10-EYFP (green) accumulates in intracellular sites (arrow head) after disruption of the F-actin (red) cytoskeleton with latrunculin A. Scale bar, 10 μm.

To investigate the role of the actin cytoskeleton on the lateral mobility of Tmem10 we performed fluorescence recovery after photobleaching (FRAP) experiments in the presence or absence of the F-actin disrupting drug, latrunculin A. The mobile fraction and the half time of fluorescence recovery did not change after treatment with latrunculin A (Fig 4B), indicating that the lateral mobility of Tmem10 within the plasma membrane is not dependent on a functional cortical actin cytoskeleton. Interestingly, we did observe a redistribution of a fraction of plasma membrane Tmem10 into intracellular sites after treatment with latrunculin A (Fig 4C), suggesting a role of the actin cytoskeleton in keeping Tmem10 at the cell surface.

In summary, we have applied an expression profiling approach to identify genes upregulated during Oli-neu cell differentiation. We used two different experimental approaches – incubation with conditioned neuronal medium or treatment with the Rho kinase (ROCK) inhibitor, Y-27632. This procedure led to the identification of Tmem10, a protein that appears during oligodendrocyte differentiation. In cultured oligodendrocytes, Tmem10 was absent from bipolar precursor cells and started to be expressed after the cells had acquired the O4 epitope.

A previous comparative genome analysis suggested that Tmem10 is a mammalian-specific gene [26]. Interestingly, the comparison of the genome structure of the Tmem10 gene and its flanking region identified an evolutionary conserved region within the first intron that functions as an oligodendrocyte-specific enhancer. This domain contains binding sites for Myt1 and cAMP-response element binding protein (CREB) and the treatment of Oli-neu cells with cAMP enhanced the expression of Tmem10 [26]. Previous studies have already shown that cAMP regulates the expression of several other oligodendroglial-specific genes [30], suggesting a general role for cAMP dependent signalling in the differentiation of oligodendrocytes into myelin-forming cells. Another factor that appears to regulate Tmem10 gene expression was the leukaemia inhibitory factor (LIF) [26], which seems to be released by astrocytes in response to ATP secreted by neurons and to promote myelination by mature oligodendrocytes [31]. Together, these data suggest a function for Tmem10 in an oligodendroglial specific process. Our finding that Tmem10 colocalizes with F-actin in plasma membrane ruffles and in F-actin-rich processes, points to a role in the regulation of the oligodendroglial actin cytoskeleton. This is reminiscent to another oligodendroglial-specific protein, Ermin, which has been implicated in the regulation of cell morphology by modulating the actin cytoskeleton [32]. The localization of Tmem10 to the leading edge of myelin sheets in mature oligodendrocytes suggests a role for Tmem10 in myelin membrane sheet extension. As Tmem10 localizes to the growing tip of the myelin sheet it could also be involved in the process of recognition or adhesion to potential axonal targets. Further analysis will be required to elucidate these issues.

Conclusion

During the development of the nervous system oligodendrocytes form a highly branched network of processes and several oligodendroglial-specific genes such as Ermin, CNP and Tmem10 are expressed during this process. Gene profiling using microarrays is a useful starting point to identify genes relevant to oligodendrocyte differentiation and myelination [28, 33, 34]. Functional analysis of these proteins, as performed for CNP [35], will be required to elucidate their exact biological function in the generation of myelin-forming processes.

Methods

Cell culture, transfections and immunofluorescence

Primary cultures of mouse oligodendrocytes were prepared as described previously [36]. In brief, cells were plated in MEM containing B27 supplement, 1% horse serum, L-thyroxine, tri-iodo-thyronine, glucose, glutamine, gentamycine, pyruvate, and bicarbonate on poly-L-lysine coated glass-coverslips after shaking. The minor population of neurons and astrocytes which arise together with the oligodendrocytes in the mixed brain cultures were used to assess Tmem10 expression in other cell types. Primary cultures of microglia and astrocytes were prepared as described previously [37]. In brief, microglial cells were shaken off, centrifuged and plated on poly-L-lysine coated coverslips in D-MEM containing 10% FCS, glutamine, penicillin and streptomycin. The remaining astrocytes were trypsinized, centrifuged and plated on poly-L-lysine coated coverslips in D-MEM containing 10% FCS, glutamine, penicillin and streptomycin. The oligodendroglial cell line Oli-neu was cultured as described previously [38]. For microarray experiments we used Oli-neu cells stably expressing PLP-EGFP [19]. Cells were treated for 16 h with conditioned neuronal medium or Y27632 (Calbiochem). Conditioned neuronal medium was obtained from primary cultures enriched in neurons after culturing for 2 weeks and used directly as described previously [24]. Transient transfections were performed using FuGENE transfection reagent (Roche Diagnostics, Basel, Switzerland) according to the manufacturer's protocol. Immunofluorescence was performed as described previously [19]. For surface labelling of ECFP-Tmem10 or Tmem10-EYFP transfected, living cells were incubated with anti-GFP antibody in medium for 10 min at 4°C, washed, fixed and labelled with secondary antibody. Disruption of the actin cytoskeleton was done 16 h after transfection with 2 μM latrunculin A for 30 min at 37°C.

Antibodies and plasmids

The following plasmids were used: human Tmem10 cDNA C-terminally fused with EYFP or N-terminally fused with ECFP [39] generated from modified pECFP-C1 or pEYFP-N1 expression vectors (Clonetech, Heidelberg, Germany), rat Tmem10 cDNA subcloned in pExpress-1 expression vector purchased form RZPD (Deutsches Ressourcenzentrum für Genomforschung GmbH, Berlin, Germany).

Anti-Tmem10 antiserum was induced in rabbits against the C-terminal Tmem10 sequence LERRRGLWWLVPSLSLE and the affinity purified IgG fraction was used. Peptid synthesis, immunization of the rabbit and affinity purification was carried out by Davids Biotechnology (Regensburg, Germany). Further the following primary antibodies were used: A2B5 (mouse monoclonal IgM, Chemicon (Millipore)), GFAP (mouse monoclonal IgG1; Vision BioSystems Novocastra, New Castle upon Tyne, UK), GFP (rabbit polyclonal IgG; Abcam, Cambridge, UK), MBP (mouse monoclonal IgG1; Sternberger Inc., Lutherville, MD), NG2 (rat IgG) [40], O1 (monoclonal IgM) [41], O4 (monoclonal IgM) [41], Tmem10 (rabbit polyclonal IgG fraction), neuron specific βIII Tubulin (mouse monoclonal IgG1; Promega, Madison, WI). Microglial cells were stained with isolectin IB4 conjugated to Alexa Fluor 568 from Molecular Probes (Invitrogen, Carlsbad, CA). Secondary antibodies were purchased from Dianova (Hamburg, Germany) and rhodamin-labelled phalloidin from Molecular Probes (Invitrogen, Carlsbad, CA).

RNA isolation

RNA isolation was performed using the Trizol (Invitrogen, Carlsbad, CA) method according to the manufacturer's recommendations and stored at -80°C. Afterwards, the samples were DNAse I treated in order to remove genomic DNA contaminations. RNA quality was determined using the Agilent 2100 Bioanalyzer (Agilent Technologies) microfluidic electrophoresis. Only sample pairs with comparable RNA integrity numbers were selected for microarray analysis.

Experimental design and sample preparation for 2 colour-microarrays

For gene expression profiling, a two-colour 1 × 2 design including a dye swap using 6 arrays was applied, comparing Oli-neu cells stably expressing PLP-EGFP treated for 16 h with conditioned neuronal medium or Y27632, respectively, to untreated parallel cultures.

The samples for hybridization were prepared from total RNA according to the Atlas SMART Fluorescent Probe Amplification Kit (Clonetech-Takara Bio Europe) protocol, except, that the RNA template was hydrolyzed under alkaline conditions before cDNA purification, and the PCR amplification process was monitored and stopped in the exponential phase. Quantity and Cy-dye incorporation rates of the generated target material were assessed using a NanoDrop ND-100. Cy3- and Cy5-labelled cDNA fragments, respectively, were hybridized to Agilent Technologies 44 K Mouse Whole Genome Microarrays (G4112A) for 17 h at 65°C. Post-processing washes were done according to the Agilent Technologies SSPE protocol (v2.1), replacing wash solution 3 by acetonitril, followed by immediate scanning using an Agilent G2505B scanner. Intensity data were extracted using the software 'Automatic Image processing for Microarrays'.

Statistical analysis

Normalization of the raw intensity data was done with a non-linear loess regression [42].

Uni- and multivariate designs

Differentially expressed genes were identified by an ANOVA fixed effects model [43]. The resulting P-values were adjusted with the Benjamini-Hochberg method to control the False-Discovery-Rate [44]. Normalization and statistical computation was done for two independent datasets derived from a high gain and a low gain scan, allowing replacement of saturated features in the high gain scan with data from the low gain measurement.

Sequence analysis

The prediction of the transmembrane domain of Tmem10 was done using the InterProScan algorithm provided at EMBL-EBI [29].

Microscopy and analysis

Fluorescence images were acquired on a confocal laser scanning microscope (TCS SP equipped with AOBS, Leica) with a 40× or 63× oil plan-apochromat objective (Leica). Image processing and analysis was performed using Meta Imaging Series 6.1 software (Universal Imaging Corporation). Quantification of fluorescence intensities was performed as described previously [19]. FRAP experiments were done as described in [20].

Declarations

Acknowledgements

We thank S. Bechtel and S. Wiemann (DKFZ, Heidelberg, Germany) for providing us with the ECFP-Tmem10 and Tmem10-EYFP plasmids and J. Trotter (University of Mainz, Germany) for antibodies. The work was supported by the Deutsche Forschungsgemeinschaft (SFB 523).

Authors’ Affiliations

(1)
Centre for Biochemistry and Molecular Cell Biology, University of Göttingen
(2)
Max-Planck-Institute for Experimental Medicine
(3)
Department of Neurology, University of Göttingen

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