,

*Department of Endocrinology, Physiopathology and Applied Biology, Universita degli Studi di Milano, Milan, Italy

�Laboratory of Molecular Neurobiology, Institute of Biophysics, Biological Research Center, Szeged, Hungary

�Center of Neuropharmacology, Department of Pharmacological Sciences, Universita degli Studi di Milano, Milan, Italy

§Department of Human Morphology and Biomedical Sciences – Citta Studi, Universita degli Studi di Milano, Milan, Italy

¶I.R.C.C.S. San Giovanni di Dio – Fatebenefratelli, Brescia, Italy

**Department of Experimental Medicine, Section of Pharmacology and Toxicology, University of Genoa, Genoa, Italy

GABA is the main inhibitory neurotransmitter in the nervoussystem which is synthesized by a one-way conversion ofglutamate to GABA. This synthesis follows a decarboxyla-tion reaction catalyzed by the enzyme glutamate decarbox-ylase (GAD; EC 4.1.1.15), which represents the rate limitingstep (Roberts & Kuriyama 1968). Mammalian speciesexpress two isoforms of GAD that differ in their cellularand subcellular distribution, the membrane anchored form of65 kDa (GAD65) and the cytoplasmic form of 67 kDa(GAD67). Early in the middle 1970s date the first studies onthe presence of GABA and its receptors outside the CNS,although their cellular distribution and the physiologicalsignificance of their presence have been underestimatedmainly in the PNS (see for review Magnaghi 2007;Magnaghi et al. 2009).

Received July 30, 2009; revised manuscript received November 21,2009; accepted November 21, 2009.Address correspondence and reprint requests to Dr Valerio Magnaghi,

Department of Endocrinology, Physiopathology and Applied Biology,University of Milan, Via G. Balzaretti 9, Milan 20133, Italy.E-mail: valerio.magnaghi@unimi.itAbbreviations used: ALLO, allopregnanolone; CaMKII, calcium/cal-

modulin-dependent protein kinase II; CRE, cAMP response element;CREB, cAMP response element binding protein; DAPI, 4¢,6-diamidino-2-phenylindole; DMEM, Dulbecco’s modified Eagle’s medium; ERK,extracellular signal-regulated MAP kinase; FCS, foetal calf serum;GABA-A and B, GABA type A and B; GAD, glutamic acid decar-boxylase; isoALLO, isoallopregnanolone or 3a-hydroxy-5a-pregnan-20-one; PBS, phosphate-buffered saline; pCREB, phospho-CREB; pERK,phospho-ERK; PMP22, peripheral myelin protein of 22 kDa.

Abstract

Recent evidence showed that neurotransmitters are syn-

thesised in glial cells, such as the Schwann cells, which form

myelin sheaths in the PNS. While the presence of GABA type

A (GABA-A) receptors has been previously demonstrated in

these cells, the evidence of GABA synthesis remained still

elusive. In an attempt to demonstrate the presence of GABA

in rat Schwann cells, we adopted a strategy, using several

integrated neurochemical, molecular as well as immunocyto-

chemical approaches. We first demonstrated the presence of

glutamic acid decarboxylase of 67 kDa (GAD67) in Schwann

cells, a crucial enzyme of the GABA synthesis mechanism.

Second, we demonstrated that GABA is synthesized and

localized in Schwann cells. As the third step we showed that

allopregnanolone (10 nM), a potent allosteric modulator of

GABA-A receptors, stimulates GABA synthesis through in-

creased levels of GAD67 in Schwann cells. Analysis of intra-

cellular signalling mechanisms revealed that the protein

kinase A pathway, through enhanced cAMP levels and cAMP

response element binding protein phosphorylation, modulates

the allosteric action of allopregnanolone at the GABA-A

receptor in Schwann cells. Our findings are the first to dem-

onstrate that this GABA mechanism is active in Schwann cells

thus establishing new potential therapeutic targets to control

Schwann cell biology, which may prove useful in the treatment

of several neurodegenerative disorders.

Keywords: cAMP response element binding protein, glu-

tamic acid decarboxylase, myelin, neuron–glia interaction,

tetrahydroprogesterone.

J. Neurochem. (2010) 112, 980–990.

JOURNAL OF NEUROCHEMISTRY | 2010 | 112 | 980–990 doi: 10.1111/j.1471-4159.2009.06512.x

980 Journal Compilation � 2009 International Society for Neurochemistry, J. Neurochem. (2010) 112, 980–990� 2009 The Authors

The neuroactive steroid allopregnanolone (ALLO), alsonamed 3a-5a-tetrahydroprogesterone, is a reduced metabo-lite of progesterone, which is synthesized ex novo mainly inglial cells of the CNS and PNS respectively (for a review, seeSchumacher et al. 2000; Mellon et al. 2001). ALLO is aneuroactive steroid and a potent modulator of all majorGABA type A (GABA-A) receptor isoforms (Lambert et al.1996; Majewska et al. 1986), whereas its action depends byits molar concentration. At low nanomolar concentrations,indeed, ALLO enhances GABA currents by acting alloster-ically on the GABA-A receptor, whereas in a submicromolar-to-micromolar range it directly activates the GABA-Areceptor channel complex (Callachan et al. 1987). Severalfactors may regulate the interaction of neuroactive steroidswith GABA-A receptors, such as the local steroid metabolism(i.e. the conversion of steroids to neuroactive compounds),the different subunit composition of GABA-A receptor aswell as their phosphorylation state (for a review see Mitchellet al. 2008). Physiologically, it has been demonstrated thatALLO influences different nervous functions spanning fromneurogenesis and synaptic stability (Wang et al. 2005;Brinton and Wang 2006), to CNS and PNS myelinogenesis(Schumacher et al. 2007; Ghoumari et al. 2003), and todifferentiation of oligodendrocytes precursors (Gago et al.2001). A high concentration of ALLO in early oligodendro-cytes pre-progenitors, in fact, may contribute to theirdevelopment and to the myelination process (Gago et al.2001). In addition, ALLO promotes the proliferation ofmultipotential precursor cells in the CNS, the early polysialicacid–neural cell adhesion molecule progenitor cells, whichnormally differentiate through glial lineages (mainly oligo-dendrocytes). Such a process appears to be mediated byGABA-A receptors as it is blocked by its specific antagonistbicuculline (Gago et al. 2004). Taken together these resultsreveal complex autocrine/paracrine loops in the control ofcentral myelination by oligodendrocytes, involving bothneuroactive steroid and GABA signalling (Gago et al. 2004).

Our previous results revealed that Schwann cells, formingmyelin peripherically, are a target for GABAs action in thePNS as shown by the presence of mRNAs coding for a varietyof GABA-A (i.e. a2, a3 and b1–3) (Magnaghi et al. 2006a;Magnaghi 2007) and GABA type B (GABA-B, i.e. -1a, -1band -2) receptor subunits (Magnaghi et al. 2006a; Magnaghi2007; Magnaghi et al. 2004a). It has also been shown thatnanomolar concentrations of ALLO modulate GABA-Areceptors in Schwann cells and increase the mRNA andprotein levels of peripheral myelin protein of 22 kDa(PMP22), one of the most important protein required for themaintenance of the peripheral myelin structure (Bronstein2000; Melcangi et al. 1999; Magnaghi et al. 2001). Thisobservation is corroborated by the evidence that muscimol, aselectiveGABA-Aagonist, similarly enhances PMP22mRNAlevels (Magnaghi et al. 2001) whereas bicuculline, a specificGABA-A antagonist, counteracts such increase (Magnaghi

et al. 2001). Taken together these observations suggest that theexpression of PMP22 in Schwann cells seems to be under thecontrol of GABA-A receptors. Adding further complexity tothe mechanism, in the same cell population, ALLO induces atransient but robust up-regulation of themRNAs coding for theGABA-B receptor subunits -1a, -1b and -2 (Magnaghi et al.2006a), which in turn control PMP22 levels (Magnaghi et al.2004a). Interestingly, these effects are partially mimicked bythe GABA-A agonist muscimol, suggesting that ALLOcontrols GABA-B receptor expression through GABA-Areceptors and revealing a complex mechanism involving bothGABA-A and GABA-B receptors in Schwann cell biology(Magnaghi et al. 2006a). Although the features of ALLO’saction are generally well known (i.e. in the nanomolar rangeALLO has an allosteric action that needs the co-presence ofendogenous GABA), the concomitant presence of GABA inSchwann cells as well as the mechanisms downstreamGABA-A receptor activation by ALLO are still debated.

To this end, we aimed at elucidating the ALLO’s mediatedmechanisms in Schwann cells, and the supposed effects onGABA synthesis, through different, integrated approachestrying to demonstrate: (i) the presence of GAD in Schwanncells, (ii) the presence and localization of GABA and (iii) theintracellular signalling pathways that might be responsiblefor the allosteric action of ALLO at the GABA-A receptor inSchwann cells.

Materials and methods

AnimalsFor the Schwann cell cultures, the minimal number (about 30

animals) of Sprague–Dawley 3-day-old rats (Charles River, Calco,

Italy) was used. For other studies, adult (3-month old) rats were

anaesthetised with an intraperitonal injection of pentobarbital

(30 mg/kg), to minimize pain and reduce discomfort and immedi-

ately killed. All experiments were performed according to institu-

tional guidelines of the Ethical Committee of our University and in

compliance with the policy on the use of animals approved by the

European Communities Council Directive (86/609/EEC).

Schwann cell cultures and treatmentsSchwann cell cultures were obtained by the method of Brockes

et al. (1979), with minor modifications (Magnaghi et al. 2006b).Sciatic nerves from 3-day-old rats were digested with 1%

collagenase and 0.25% trypsin (Sigma, Milan, Italy) then filtered

through a 30-lm nylon membrane and centrifuged for 10 min at

280 g. Pellets were solubilised in Dulbecco’s modified Eagle’s

medium (DMEM) (Serotec, Oxford, UK) supplemented with 10%

foetal calf serum (FCS; Invitrogen, San Giuliano Milanese, Italy)

and plated onto 35 mm Petri dishes; 24 h after plating, the medium

was supplemented with 10 lM arabinoside C (Sigma). Then after

48 h, the cultures were treated with a stream of cold DMEM–FCS

10%. The remaining cells were plated in DMEM–FCS 10%

supplemented with 10 lM forskolin (Sigma) and bovine pituitary

extract 200 lg/mL (Invitrogen). Cells became confluent in 10 days.

� 2009 The AuthorsJournal Compilation � 2009 International Society for Neurochemistry, J. Neurochem. (2010) 112, 980–990

GABA synthesis in Schwann cells | 981

Immunopanning for final purification was carried out incubating the

cells for 30 min with mouse anti-rat Thy1.1 antibody (Serotec,

Kidlington, UK) followed by 500 lL of baby rabbit complement

(Cedarlane, Burlington, ON, Canada). The cell suspension (3 · 105)

was plated on 6 cm Petri dishes and grown in presence of 2 lMforskolin and bovine pituitary extract 200 lg/mL. At the third

passage in vitro, Schwann cell cultures were differentiated by

treatment for 48 h with 4 lM forskolin, and then maintained in

serum-free medium 24 h before experimentation. For cell purity

assessment and other immunofluorescence analysis, cultures were

fixed in phosphate-buffered saline solution (PBS)–4% p-formalde-

hyde. The Schwann purity (more than 98%) was tested with a

specific antibody against the Schwann cell marker glycoprotein P0,

already used for other studies (Magnaghi et al. 2004a,b). Cells

immunopositive for P0 were counted under an Axioskop200

microscope (Zeiss, Arese, Italy). For RT-PCR and western analysis,

indeed, cells were treated for the proper time with ALLO 10 nM or

isoALLO (isoallopregnanolone, 3a-hydroxy-5a-pregnan-20-one;Sigma) 10 nM, mRNAs and proteins were then extracted. In the

pharmacological studies, the cells were treated with ALLO 10 nM

for the proper time and processed for GABA or cAMP assay.

RT-PCR assayTissues (brain and sciatic nerves) of 3-month-old rats and Schwann

cell cultures were homogenised in 4 M guanidine thiocyanate and

total RNA was isolated by phenol–chloroform extraction (Chom-

czynski and Sacchi 1987); 1 lg of total RNA from each sample was

treated for 15 min with deoxyribonuclease I (Invitrogen), and then

reverse transcribed at 37�C for 60 min using 200 U of Moloney

Murine Leukaemia Virus–reverse transcriptase (Invitrogen) in the

presence of random hexamer primers (Invitrogen). RT efficiency and

data normalization were measured by amplification of 18s rRNA

mouse transcript (accession number X00686). Aliquots of cDNA

were amplified using the following primer pairs: forward 5¢-AAGGGAGCTGCAGCCTTG-3¢, reverse 5¢-CTGCATCAGTCC-CTCCTCTC-3¢ for GAD65 (accession number M72422) and

forward 5¢-TGGCTTTG-GAACTGACAATG-3¢, reverse 5¢-CTG-GAAGAGGTAGCCTGCAC-3¢ for GAD67 (accession number

NM_017007). The amplification reactions were carried out, respec-

tively, for 35 cycles (annealing for 1 min at 57�C) for GAD65 and for38 cycles (annealing for 1 min at 55�C) for GAD67 in a final volume

of 25 lL containing 20 mM Tris–HCl, pH 8.3, 50 mMKCl, 1.5 mM

MgCl2, deoxynucleotides (200 lM each), 1 U of Platinum Taq DNA

polymerase (Invitrogen), 1 lM of each primer and 2 lL of cDNA as

template. Additional samples containing the same RNA, were treated

following the same amplification conditions, except that the reverse

transcriptase was omitted; these samples were used as negative

controls. The amplified products were analysed on 1.5% agarose gels

and photographed. Inverted films were acquired with a scanner and

the densitometric analysis performed with IMAGEJ 1.39t software, the

public domain image image processing system developed at NIH

(Bethesda, MD, USA).

Western blot analysisPreparation of subcellular fractions for analysis of phsophorylation

and expression of proteins were made as in Fumagalli et al. (2009).Tissues (brain and sciatic nerves) of 3-month-old rats and Schwann

cell cultures were homogenised in excess of lysis buffer (PBS, 1%

Nonidet P-40 and 1 mM EDTA) containing the Complete� protease

inhibitor cocktail (Roche, Monza, Italy). After pellet/supernatant

separation, the protein concentration of each sample fraction was

assayed according to method of Bradford (1976). Brain (10 lg of

protein), sciatic nerve (30 lg) and/or Schwann cell samples (30 lg)were dissolved in 0.1% sodium dodecyl sulphate, resolved on a 12%

sodium dodecyl sulphate–polyacrylamide gel electrophoresis and

electroblotted overnight at 30 V constant to nitrocellulose (Trans-

blot; BioRad, Milan, Italy). The membrane was blocked for 2 h at

25�C in the buffer (NaCl 135 mM, KCl 2.5 mM and Tris–HCl

25 mM, pH 7.4) containing 0.2% Nonidet P-40 and non-fat dried

milk. Filters were then incubated with specific primary antiserum.

GAD65 and 67 were detected by evaluating the specific bands

density after probing with a polyclonal rabbit anti-GAD pan-

antibody (diluted 1 : 800 in 5% non-fat dried milk; Chemicon,

Temecula, CA, USA). Total and phosphorylated cAMP response

element binding protein (pCREB), calcium/calmodulin-dependent

protein kinase II (CaMKII) and extracellular signal-regulated

mitogen-activated protein kinases (ERK)1/2 forms were detected

analysing the band density at 43 kDa for CREB, at 52 kDa for

CaMKII, at 42 kDa for ERK2 and 44 kDa for ERK1 respectively.

The conditions of primary antibodies were as follows: polyclonal

pCREB and CREB (1 : 1000, overnight, 4�C in 5% non-fat dry milk;

Cell Signalling Technology, Danvers, MA, USA), p-aCaM-

KII(Thr286) (1 : 2500 in 3% non-fat dry milk; Affinity Bioreagents,

Golden, CO, USA), aCaMKII (1 : 5000 in 3% non-fat dry milk;

Millipore Bioscience Research Reagents, Temecula, CA, USA),

polyclonal phospho-ERK (pERK1), ERK1, pERK2 and ERK2

(1 : 5000, 2 h, 25�C in 5% non-fat dry milk; all by SantaCruz

Biotechnology, Santa Cruz, CA, USA). In parallel, the top filters were

incubated with a primary monoclonal anti-a-tubulin (clone DM 1A,

diluted 1 : 1500; Sigma) or anti-actin (clone AC40 1 : 200; Sigma)

antibodies, as housekeeper proteins. Membranes were then incubated

for 1 h at 25�C, respectively, with the specific peroxidase-conjugatedsecondary antibodies (diluted 1 : 1000; Cell Signalling Technology).

The protein immunocomplexes were visualized by utilizing the

enhanced chemiluminescence western blotting kit (Amersham Life

Sciences, Monza, Italy) at different exposure times, according to the

manufacturer’s instructions. The films were acquired with a scanner

and the densitometric analysis performed with IMAGEJ 1.39t software.

Immunocytochemistry and immunofluorescenceSchwann cell cultures, plated on coverslips and coronal sections

(12 lm) of rat sciatic nerve (mounted on poly-L-lysine-coated

slides) were fixed in PBS–p-formaldehyde 4%. After washing, the

slides were incubated overnight at 4�C in PBS containing 0.25%

bovine serum albumin (Sigma), 0.1% Triton X-100 and the primary

antibodies. We used rabbit anti-P0 polyclonal antibody (diluted

1 : 500; Sigma-Genosys, Cambridge, UK), to assess Schwann cell

purity, rabbit anti-GAD (pan-antibody, diluted 1 : 250; Chemicon)

or mouse monoclonal anti-GABA (clone GB-69, diluted 1 : 200;

Sigma) respectively. The following day, cells were washed two

times with the same buffer and incubated 2 h at 25�C with the

specific secondary antibodies. We used the followings: goat anti-

rabbit Alexa-tetramethyl rhodamine iso-thiocyanate-533 (diluted

1 : 800; Molecular Probes, Invitrogen, San Giuliano Milanese,

Italy) to asses Schwann cells purity, goat anti-rabbit IgG biotinylated

(diluted 1 : 800) followed by Extravidin Peroxidase (diluted

Journal Compilation � 2009 International Society for Neurochemistry, J. Neurochem. (2010) 112, 980–990� 2009 The Authors

982 | V. Magnaghi et al.

1 : 1000; Sigma), or goat anti-rabbit Alexa-FITC-488 (diluted

1 : 800; Molecular Probes) to assess GAD presence, goat anti-

mouse IgG biotinylated (diluted 1 : 800) followed by Extravidin

Peroxidase (diluted 1 : 1000; Sigma), or goat anti-mouse Alexa-

FITC-488 (diluted 1 : 800; Molecular Probes) to assess GABA

presence. After washing in buffer, biotinylated slides were revealed

with 3-3¢-diaminobenzidine method and mounted. Otherwise, nuclei

were stained with 4¢,6-diamidino-2-phenylindole (DAPI; Sigma)

and mounted. Schwann cell purity was assessed counting cells

immunopositive for P0 on total number of cells (DAPI positive), per

Petri dish. Controls for antibodies specificity included a lack of

primary antibodies, or substitution with pre-immune serum. The

image analysis was carried out using a Zeiss Axioskop200

microscope and Adobe PHOTOSHOP 5.0.2 software (Adobe System

Software Ltd., Dublin, Ireland).

cAMP assaySchwann cell cultures maintained in serum-free medium for 24 h

were treated with DMEM containing 0.5 mM 3-isobutyl-1-methyl-

xanthine (Sigma) and incubated for 15 min at 37�C. Each Petri dish

of Schwann cells was treated with ALLO 10 nM (Sigma) for

15 min, then added with 1 mL of 70% ethanol and leaved to stand

5 min at 4�C to extract the intracellular cAMP. The supernatants

from each Petri were collected, evaporated and the cAMP levels

were measured according to the protocol of a commercial cAMP

[3H] assay system (Amersham Life Sciences). The remaining cells

attached to the bottom of each Petri dishes were scraped with NaOH

0.2 N and the protein concentration of each sample was assayed

relative to the bovine serum albumin standard, according to the

method of Bradford (1976).

HPLC and GABA determinationSchwann cell cultures were maintained 24 h in serum-free medium,

then added with vigabatrin 50 nM (Sigma) and treated with vehicle

(ethanol; control samples) or ALLO 10 nM (treated samples)

respectively. At the proper time (30, 60, 90 and 120 min) Schwann

cell samples underwent cytolysis by treating the cells with water for

5 min at 4�C, so that the cytosolic content was extracted. Cell lysateswere pelleted and protein concentration of each sample was assayed

with the method of Bradford (1976). Endogenous GABA in the

supernatants (cytosolic fraction) was measured by HPLC analysis

following pre-column derivatization with o-phthalaldehyde and

separation on a C18 reverse-phase chromatographic column coupled

with fluorometric detection (excitation wavelength 350 nm and

emission wavelength 450 nm) (Raiteri et al. 2000). Buffers and the

gradient program were as follows: solvent A, 0.1 M sodium acetate

(pH 5.8)/methanol 80 : 20; solvent B, 0.1 M sodium acetate (pH 5.8)/

methanol, 20 : 80; solvent C, 0.1 M sodium acetate (pH 6.0)/

methanol, 80 : 20; gradient program, 100% C for 4 min from the

initiation of the program; 90%Aand 10%B in 1 min; 42%Aand 58%

B in 14 min; 100%B in 1 min; isocratic flow2 min; 100%C in 3 min;

flow rate 0.9 mL/min. Homoserine was used as an internal standard.

Bioinformatics analysisThe GAD67 promoter region (GenBank ID 24379), defined as about

4.6 kb 5¢-upstream to the annotated transcription start site, was

searched for full cAMP response element (CRE) sites (TGACGT-

CA) and/or half CRE sites (TGACGCGTCA) (Zhang et al. 2005).

The analyses have been performed by different software, i.e.

ALIBABA2, MATCH (Biobase GmbH, Wolfenbuettel, Germany), which

are able to predict the consensus sequences of known transcription

factors collected in TRANSFAC� database (Wingender et al. 2001).Results have been compared to obtain the best matching sites.

Data analysisData were statistically evaluated by GRAPHPAD PRISM 4.00 (GraphPad

Software Inc., San Diego, USA), using the parametric t-test for

cAMP analysis and one-way ANOVA with Tukey’s post-test for

multiple comparison for RT-PCR/western blot analysis of GAD67,

pCREB/CREB analysis and HPLC analysis of GABA.

Results

GAD67 enzyme is present in Schwann cellsThe first step in our experiments was to support the hypothesisthat in Schwann cells thementioned action ofALLOatGABA-A receptor is an allosteric action, because it needs thecontemporary presence of GABA. To this end, we firstinvestigated whether the specific mRNAs coding for the majorGAD isoforms, GAD65 and GAD67 isoforms (Erlander et al.

(a)

(b)

Fig. 1 Presence of GAD enzyme in sciatic nerve and Schwann cell

culture. (a) RT-PCR revealed GAD65 and GAD67 mRNA transcripts.

GAD67 mRNA transcript, shown as a band of 409 bp, was present in

sciatic nerve (Sci) and Schwann cells (Schw), whereas GAD65, shown

as a band of 390 bp, was evident in sciatic nerve but not in Schwann

cells. The efficiency of the RT was shown as a band for 18s rRNA

(251 bp). mRNA from rat brain was used as a positive control. Mk,

marker; +, RNA with reverse transcriptase; ), RNA without reverse

transcriptase used as a negative control. (b) Western blot analysis of

GAD65 and GAD67 enzymes revealed both specific bands in rat brain,

used as control, although only GAD67 was present either in sciatic

nerve or in Schwann cells. Total protein loading was normalized

against a-tubulin (band of 54 kDa).

� 2009 The AuthorsJournal Compilation � 2009 International Society for Neurochemistry, J. Neurochem. (2010) 112, 980–990

GABA synthesis in Schwann cells | 983

1991), were present in Schwann cells. We found that GAD65mRNAwaspresent in the rat brain, used as positive control, andin the PNS, particularly in the sciatic nerve (Fig. 1a), accordingto previously published evidence (Hayasaki et al. 2006), butwas lacking in Schwann cells. GAD65 mRNA, in fact, wasabsent in the Schwann cells under basal culture conditions(Fig. 1a), even when a higher concentration of template(fourfold) was used (data not shown). Conversely, GAD67mRNAwas present in all the different tissues investigated, i.e.brain, sciatic nerve and Schwann cell cultures (Fig. 1a).

This pattern of GAD expression was supported by westernblot analysis using a pan-antibody able to recognize bothGAD isoforms. In fact both GAD65 and GAD67 werepresent in the brain (Fig. 1b), used as positive control,whereas only GAD67 protein levels were observed in sciaticnerve and Schwann cells, with no GAD65 protein expressionbeing detected (Fig. 1b).

Glutamic acid decarboxylase of 67 kDa distribution inSchwann cells was further confirmed by peroxidase immu-noreactivity and light microscopy (Fig. 2b) as well asimmunofluorescence analysis (Fig. 2c), showing an intensecytoplasmic staining for GAD mainly distributed in perinu-clear spaces. This is in accordance with the distribution of

GAD67 in the Golgi complex region (Dirkx et al. 1995).GAD immunolabelling was observed also in the Schwanncell elongated process, as shown at higher magnification(Fig. 2e). When the primary antibody was omitted, no signalwas observed (Fig. 2d), demonstrating antibody specificity.Schwann cell purity was found to be higher than 98% asassessed by immunofluorescence (Fig. 2a), using the anti-P0antibody which recognizes a specific protein present only inSchwann cells (Garbay et al. 2000). Additionally, also someSchwann cells surrounding the myelinated fibres in thecoronal section of rat sciatic nerve were immunopositive forGAD (Fig. 2f), probing that Schwann cells in vitro as wellin vivo express GAD enzyme.

GABA is synthesized in Schwann cellsThe presence of GAD67 in Schwann cells indicates thatGABA may be synthesized in these cells. To demonstrateGABA synthesis, we first performed an immunocytochem-ical analysis, which revealed that under basal conditionsSchwann cells are immunopositive for GABA (Fig. 3a andb). In fact, both diaminobenzidine (Fig. 3a) and immunoflu-orescence (Fig. 3b) labelling showed an intense perinuclearstaining, even distributed in some cellular processes. As

(a)

(c)

(e)

(b)

(d)

(f)

Fig. 2 Immunofluorescent localization of

GAD enzyme in Schwann cells. The purity

of Schwann cell cultures was demonstrated

by using an antibody against the specific

marker glycoprotein P0, labelled in red in

(a), whereas nuclei were stained in blue

with 4¢,6-diamidino-2-phenylindole (DAPI).

Cultures grown in DMEM–FCS 10% plus

2 lM forskolin showed purity of more than

98%. (b) Peroxidase immunoreactivity and

light microscopy and (c) immunofluores-

cence analysis showed an intense cyto-

plasmic staining for GAD67, mainly

distributed in perinuclear spaces. Nuclei

stained in blue with DAPI. GAD immunola-

beling (white arrows) is shown in the elon-

gated process of differentiated Schwann

cells at higher magnification (e). In (f) GAD

immunopositivity was also shown in some

Schwann cells (white arrows) surrounding

the axons (‘a’ in white), in coronal section

(12 lm) of rat sciatic nerve. Nuclei stained

in blue with DAPI. Controls for GAD anti-

body specificity (d), including a lack of a

primary antibody, revealed only a back-

ground signal in Schwann cells. Nuclei

stained in blue. Scale bars, 20 lm.

Journal Compilation � 2009 International Society for Neurochemistry, J. Neurochem. (2010) 112, 980–990� 2009 The Authors

984 | V. Magnaghi et al.

shown in Fig. 3c, when the primary antibody was omitted,no signal was observed, again demonstrating antibodyspecificity.

This finding has been further supported by the followingHPLC measure of GABA concentration (around 0.04 pmol/mg of protein) detected in the cytosolic fraction of Schwanncells under basal condition (see the Control levels in theHPLC analysis below).

ALLO induces up-regulation of GAD67 levels in SchwanncellsOnce established that GAD67 is present in Schwann cellsunder basal conditions, the next step was to investigate

whether ALLO might modulate GAD67 mRNA. For thispurpose, we treated Schwann cells with ALLO (10 nM) toconduct a time-dependent analysis of GAD67 mRNA levels.Our results showed that ALLO induces GAD67 mRNA, witha peak after 30 min that lasted at least 180 min aftertreatment (Fig. 4a). This effect disappeared 240 min afterstimulation (Fig. 4a). The densitometric analysis confirmedthis significant expression profile (Fig. 4b). The specificity ofthis ALLO effect was supported by the observation thatexposure to isoALLO 10 nM, the inactive isomer of ALLOat the GABA-A receptor (Lambert et al. 1995), did notmodify GAD67 mRNA levels (Fig. 4c).

To confirm the time-dependency of ALLO-inducedGAD67 mRNA augmentation we performed the westernblot analysis, which confirmed a peak in GAD67 levels30 min after ALLO (10 nM) stimulation. This effect disap-peared earlier than the mRNA levels; after 180 min in factGAD67 expression returned to control levels (Fig. 4d).Again, the densitometric analysis matches this time-depen-dent profile, revealing a significant increase of GAD67peaking 30 min after ALLO treatment (Fig. 4e). However,the apparent temporal discrepancy between ALLOs-inducedtranscription and/or translation of GAD may be attributed toa modulation of mRNA or protein stability. ALLO stimula-tion did not induce GAD65 protein at any of the time pointconsidered, confirming the absence of GAD65 in Schwanncells also under ALLO stimulation (Fig. 4d).

ALLO induces GABA synthesis in Schwann cellWe then hypothesized that, under our culture conditions,ALLO might increase GABA levels. HPLC analysis ofcytosolic fractions obtained from Schwann cells treated withALLO (10 nM) showed higher significant levels of GABA30 min after stimulation (Fig. 5a), lasting for other timepoints considered (60 and 90 min). At 120 min afterstimulation the GABA levels are back to the control levels(Fig. 5a).

ALLO induces CREB phosphorylation in Schwann cellOnce established that ALLO controls GAD67 expression andconsequently the synthesis of GABA in Schwann cells,peaking around 30 min, we furthered our investigation byexamining the intracellular signalling that might be respon-sible for ALLO effects in Schwann cells. As in Schwanncells the allosteric action of ALLO has been demonstrated toincrease PMP22 mRNA transcripts (Melcangi et al. 1999;Magnaghi et al. 2001), and previous data had also shownthat the cAMP signalling pathway is involved in themodulation of PMP22 (Saberan-Djoneidi et al. 2000), weanalysed cAMP levels following ALLO (10 nM) stimulation.Figure 5b shows that ALLO increases the levels of cAMP at15 min under our experimental conditions. We then exam-ined whether ALLO, via enhanced cAMP levels, was able totrigger a downstream increase in phosphorylation of CREB,

(a)

(c)

(b)

Fig. 3 Immunofluorescent localization of GABA in Schwann cells

under basal conditions. Both diaminobenzidine (a) and immunofluo-

rescence labelling in green (b) showed an intense perinuclear staining

for GABA neurotransmitter, even distributed in some cellular pro-

cesses. Antibody specificity was demonstrated omitting the primary

antibody, and no immunofluorescent signal was observed (c). Scale

bars, 20 lm.

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GABA synthesis in Schwann cells | 985

the main effector of the activation of cAMP-dependentprotein kinase. Figure 5c and d show that ALLO (10 nM)up-regulates CREB phosphorylation within 30 min (+80%),with no effect on the total levels of CREB protein, suggestingthat CREB activation is dependent on increased cAMPlevels. As a proof of concept, we measured other intracellularpathways that could activate CREB phosphorylation, i.e.CaMKII and ERK1/2. We did not detect any signal forphospho-CaMKII and total CaMKII, whereas pERK1 andpERK2 were significantly reduced ()80.0 ± 5.3% and)49.3 ± 10.2%, respectively, data not shown) in Schwanncells following ALLO 10 nM administration. Given thatcAMP levels were indeed enhanced by ALLO in Schwanncells, our results, albeit indirect, point to protein kinase A

pathway as the mechanism responsible for ALLO’s-inducedCREB activation.

As CREB regulates cellular gene expression by binding toa conserved CRE consensus site, that occurs either as apalindrome (TGACGTCA) or half-site (CGTCATGACG)(Mayr and Montminy 2001), we then performed a bioinfor-matic analysis searching for those CRE sequences in the ratGAD67 promoter. Table 1 reports that several putative CREsare present upstream of the GAD67 transcription start site.

Discussion

In this study, we provide evidence that (i) GABA syntheticmechanisms exist in PNS, particularly in Schwann cells; (ii)

(a)

(c)

(d)

(b)

(e)

Fig. 4 Modulation of GAD67 gene expression in Schwann cells.

GAD67 levels were up-regulated in Schwann cells. (a) RT-PCR was

performed at different times (15, 30, 60, 180 and 240 min) and showed

that allopregnanolone (ALLO; 10 nM) increased GAD67 mRNA (band

of 409 bp) throughout the time course, and peaking from 30 to

180 min after treatment. The efficiency of the RT was shown as a

band for 18s rRNA (251 bp). mRNA from rat brain was used as

a positive control. Mk, marker. (b) Densitometric analysis confirmed a

significant increase in GAD67 expression profile from 15 to 180 min

after ALLO stimulation. Time points were analysed at least in triplicate,

in three separate experiments. Data are expressed as arbitrary

densitmetric units (ADU) and statistical analysis was carried out by

one-way ANOVA with Tukey’s post-test for multiple comparison;

***p < 0.001. (c) The specificity of ALLOs effect was supported by the

observation that 1-h exposure to the inactive isomer of ALLO, called

isoallopregnanolone (isoALLO; 10 nM), did not modify GAD67 mRNA

levels. (d) Analysis of GAD67 protein levels by western blot revealed a

parallel increase of GAD67 protein, with a peak after 30 min. Total

protein loading was assayed normalizing against a-tubulin (band of

54 kDa). Experiments were repeated at least four times on different

culture preparations. (e) Densitometric analysis confirmed a significant

increase in GAD67 protein from 15 to 180 min, peaking 30 min after

ALLO stimulation. Time points were analysed at least in triplicate, in

three separate experiments. Data are expressed as ADU and statis-

tical analysis was carried out by one-way ANOVA with Tukey’s post-test

for multiple comparison; ***p < 0.001.

Journal Compilation � 2009 International Society for Neurochemistry, J. Neurochem. (2010) 112, 980–990� 2009 The Authors

986 | V. Magnaghi et al.

GABA synthesis in Schwann cells is under the control of theneuroactive steroid ALLO; and (iii) ALLO promotes GABA-mediated gene transcription through activation of the proteinkinase A pathway. Taken together, our findings suggest a pos-sible physiological role for the GABAergic system in the PNS.

Our data clearly indicate that GABA synthesis occurs inthe glial cells of the PNS, i.e. the Schwann cells. Pioneerstudies from Jessen et al. (1979) on the vertebrate peripheralautonomic nervous system suggested that GABA is aneurotransmitter in the PNS. Accordingly, myelinated andunmyelinated fibres were shown to possess GABA receptorsand GABA carriers (Brown et al. 1979; Olsen et al. 1984;Morris et al. 1983) and 3H-GABA was shown to beaccumulated in the Schwann cells of the taste buds of theamphibian Necturus maculosus (Nagai et al. 1998). How-ever, the cellular location of the GABA synthetic mechanismin mammalian PNS is still obscure. By means of differenttechniques, we here show that Schwann cells are able to

synthesize GABA in a GAD67-dependent manner. In themammalian CNS at least two forms of GAD exist, i.e.GAD65 and GAD67. Membrane associated GAD65 isthought to be responsible for the synthesis of vesicularGABA, with a role in neurotransmission, while cytoplasmicGAD67 synthesizes mainly metabolic GABA, which canfeed into the tricarboxylic acid cycle (Soghomonian andMartin 1998). GAD67 is responsible for the vast majority ofGABA synthesis in the brain (Soghomonian and Martin1998). Accordingly, our data showing higher immunoposi-tivity of GAD67 in the cytoplasmic compartment ofSchwann cells, as revealed by immunocytochemistry, sug-gest a preferential involvement of GAD67 in non-vesicularGABA release (Esclapez et al. 1994), and strengthen thehypothesis that GABA synthesis in Schwann cells maycontribute to the general metabolic activity of these cells.

A further crucial finding of this work is that theneuroactive steroid ALLO controls GABA synthesis in

(a)

(b)

(c)

(d)

Fig. 5 Modulation of GABA synthesis in Schwann cells by ALLO. (a)

HPLC analysis at different times (30, 60, 90 and 120 min) showed that

allopregnanolone (ALLO; 10 nM) increased GABA levels in the cyto-

solic fraction of Schwann cells, with a peak effect at around 30 min.

Data are expressed as pmol of GABA, normalized versus mg of total

proteins, ± SEM. Time points were analysed at least in triplicate, in

three separate experiments. Statistical analysis was carried out by

one-way ANOVA with Tukey’s post-test for multiple comparison;

*p < 0.05 (b) ALLO 10 nM induces cAMP levels in cultures of rat

Schwann cells 15 min after stimulation. Data are expressed as cAMP

concentration (pmol), normalized for mg of total protein, and represent

the mean ± SEM of 9–10 independent determinations. Control cul-

tures were treated with vehicle only. Statistical analysis was carried

out by t-test; **p < 0.01. (c) As shown in the representative western

immunoblots, ALLO (10 nM) at 30 min after stimulation up-regulated

phosphorylated CREB (43 kDa) without changes in total CREB

(43 kDa). Data are normalized versus levels of actin protein (band of

50 kDa). (d) The quantitative evaluation of western blot analysis

confirmed an increased CREB phosphorylation with no effect on

CREB protein levels. The ratio between phosphorylated and total

protein levels (pCREB/CREB, pCREB/actin and CREB/actin) was

expressed as percentage versus controls (vehicle) and the mean ±

SEM of the determinations performed. The mean value of the control

within a single experiment was set to 100; values of controls from

different experiments were within 10% of each other. Data are sta-

tistically evaluated using a one-way ANOVA and Tukey’s test for multiple

comparison; **p < 0.01 and ***p < 0.001.

� 2009 The AuthorsJournal Compilation � 2009 International Society for Neurochemistry, J. Neurochem. (2010) 112, 980–990

GABA synthesis in Schwann cells | 987

Schwann cells, suggesting a possible autocrine loop involv-ing GABA-A receptors in such control. This effect is specificas isoALLO, an ALLO’s antagonist, does not stimulate GADlevels, although we are aware that other neuroactive steroidsmay be involved in the Schwann cells modulation (Magnaghiet al. 2004b; Leonelli et al. 2007; Roglio et al. 2008). Wehave previously demonstrated that cultured Schwann cellsexpress GABA-A receptors and are responsive to ALLO andGABA-A ligands, such as muscimol or bicuculline (Magna-ghi 2007; Magnaghi et al. 2006a, 2001). Moreover, thepossibility that GABA-B receptor might be involved in theALLO’s control of the GABA system should not beexcluded. GABA-B receptor subunits, in fact, are alsoexpressed in Schwann cells and modulated by ALLO andGABA-A ligands via a GABA-A mediated mechanism(Magnaghi et al. 2006a), adding further complexity to thispicture. Interestingly, both GABA-A and GABA-B receptorscontrol the expression of PMP22 (Magnaghi et al. 2001),which is one of the most important myelin proteins of thePNS (Bronstein 2000).

We then characterized the signalling pathways down-stream of ALLO-mediated GABA-A receptor activation inSchwann cells. Our results suggested that ALLO, viaincreased intracellular cAMP levels, enhances the phosphor-ylation of the nuclear transcription factor CREB. Thismechanism might explain the increased GAD67 expressionas the bioinformatic analysis revealed several CRE elementsin the promoter region upstream of the GAD67 transcriptionstart site. Our data supported a direct relationship between

CREB and GAD, as recently showed by Yin et al. (2009),who reported that decreased phosphorylation of CREB isresponsible for amphetamine-induced inhibition of GAD67expression in the olfactory bulb. However, we cannotdisregard the evidence of Murphy and Segal (2000) whodemonstrated an indirect correlation between CREB phosp-holylation and GAD expression. Moreover, our hypothesis ofCREB mechanism downstream GABA-A receptor activationis supported by a recent manuscript investigating the CREBsignalling involvement in adult hippocampal neurogenesis.In this study, Jagasia et al. (2009) demonstrated that GABA-mediated excitation regulates CREB activation at earlydevelopmental stages in hippocampal neurons. However,we cannot rule out that other intracellular mechanisms maycome into play to modulate myelin proteins expressionthrough GABAergic activation.

As cAMP is also able to stimulate the synthesis of PMP22(Saberan-Djoneidi et al. 2000), this finding may also providea possible rationale for our previous data showing increasedexpression of myelin proteins (primarily PMP22) by ALLO(Magnaghi et al. 2001; Melcangi et al. 1999). Notably, thefunctional relevance of the putative control of myelinationexerted by ALLO is also supported by the observation thatALLO contributes to the myelination process in the CNS,because of its capability to regulate the development of earlyoligodendrocytes pre-progenitors (Gago et al. 2001).

Although speculative, the neuroactive steroid-mediatedregulation of GABAergic transmission, however, mightrepresent a mechanism involved in the pathogenesis ofperipheral nerve disorders, as decreased levels of ALLO areassociated with peripheral neuropathies (Caruso et al.2008a,b).

In conclusion, our data are the first to provide amechanistic link between the presence of GABA-A receptorsin Schwann cells, their activation by ALLO and the modu-lation of myelin protein synthesis. The identification of suchspecific intracellular mechanisms may enable the design oftargeted, interventional strategies, i.e. specific periphericallyacting neuroactive steroid or more specific GABA-A ligands,for the treatment of peripheral neuropathies.

Acknowledgements

We are grateful to Dr Astrid Willan for assistance with the

manuscript editing. The financial support of the Italian Ministry of

education (PUR 2007/2008 to VM), Compagnia di San Paolo

(Bando Programma Neuroscienze grant n� 2229/2008 to VM) and

Association Francaise contre les Myopathies (AFM grant n� 14163/2009) are gratefully acknowledged.

References

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Table 1 Putative CRE sites in the rat GAD67 promoter sequence

(GenBank ID 24379), 5¢-upstream the transcription start site

Position

5¢-3¢Sequence

Homology (%) versus

CRE consensus

sequence (TGACGTCA)

(Zhang et al. 2005)

)3488/)480 TGACGTTT 75

)2670/)2662 TGAAGTCA 87.5

)2656/)2662 TGACGCAA 75

)1895/)1887 TGATGTCA 87.5

)117/)109 TGACACCC 62.5

Position

5¢-3¢Sequence

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CRE half-site (TGACG

or CGTCA) (Zhang

et al. 2005)

)3488/)3483 TGACG 100

)2670/)2665 TGAAG 80

)2667/)2662 AGTCA 80

)2656/)2651 TGACG 100

)1895/)1890 TGATG 80

)1892/)1887 TGTCA 80

)117/)112 TGACA 80

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