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Mol. Cell. Neurosci. 30 (2005) 137 – 148

Cell adhesion and neurite outgrowth are promoted by neurofascin

NF155 and inhibited by NF186

Darshan Koticha,1 Joanne Babiarz,1 Noriko Kane-Goldsmith, Jeffrey Jacob,2

Karthik Raju, and Martin Grumet*

W. M. Keck Center for Collaborative Neuroscience, Rutgers, State University of New Jersey, 604 Allison Road, Piscataway, NJ 08854-8082, USA

Department of Cell Biology and Neuroscience, Rutgers University, Piscataway, NJ 08854-8082, USA

Received 9 December 2004; revised 10 June 2005; accepted 28 June 2005

Neurofascin (NF) is a neural cell adhesion molecule in the L1-family

containing six Ig domains and multiple fibronectin type III (FnIII)

repeats in its extracellular region. NF has many splicing variants and

two of these are exemplars that have different cellular patterns of

expression during development. NF186, which is expressed on neurons,

contains an unusual mucin-like region and NF155, which is expressed

on glia, contains a unique FnIII repeat with an RGD motif. Analysis of

Fc fusion proteins representing different extracellular regions of NF

indicate that NF186 inhibits cell adhesion and neurite outgrowth, and

the inhibition is associated with the region containing the mucin-like

domain. NF155 promotes neural cell adhesion and neurite outgrowth,

and the RGD motif in its third FnIII repeat is critical for cell spreading

and neurite outgrowth. The results suggest that different splicing

variants of NF expressed on neurons and glia play distinct roles during

neural development.

D 2005 Elsevier Inc. All rights reserved.

Introduction

NF is a member of the L1-subgroup of cell adhesion molecules

(CAMs), belonging to the Ig superfamily of proteins (reviewed in

Brummendorf and Rathjen, 1995; Grumet, 1991). Other members

of the L1-family have been implicated in multiple processes during

nervous system development, including cell adhesion, cell migra-

tion, neurite outgrowth, and neuronal fasciculation (Brummendorf

and Rathjen, 1995; Hortsch, 2000). L1-family members interact

with extracellular proteins (Grumet et al., 1996) including other

CAMs (Brummendorf and Rathjen, 1995) as well as cytoplasmic

1044-7431/$ - see front matter D 2005 Elsevier Inc. All rights reserved.

doi:10.1016/j.mcn.2005.06.007

* Corresponding author. W. M. Keck Center for Collaborative Neuro-

science, Rutgers, State University of New Jersey, 604 Allison Road,

Piscataway, NJ 08854-8082, USA. Fax: +1 732 445 2063.

E-mail address: mgrumet@rci.rutgers.edu (M. Grumet).1 These authors contributed equally to this work.2 Present address: Mayo Medical School, Rochester, MN 55905, USA.

Available online on ScienceDirect (www.sciencedirect.com).

proteins (Bennett and Lambert, 1999), which can control receptor

organization.

Like other L1-family members, NF consists of six Ig domains

followed by a stretch of fibronectin type three (FnIII) repeats

linked by a transmembrane domain to a cytoplasmic region

(Brummendorf and Rathjen, 1995; Grumet, 1991). NF is distin-

guished among the Ig superfamily in the large number of splicing

variants identified including an unusual one that has a mucin-like

proline-, alanine-, threonine-rich domain (PAT domain), which is

O-glycosylated (Davis et al., 1996; Volkmer et al., 1992). In

addition, the third FnIII repeat, which is also alternatively spliced,

contains an arginine, glycine, aspartic acid (RGD) motif that in

many proteins mediates interactions with integrins (Reichardt

et al., 1991). NF splicing variants exhibit complex spatial and

temporal localization in different cells during neural development

(Collinson et al., 1998; Hassel et al., 1997). These are

exemplified by an axonal isoform NF186 and a glial isoform

NF155 (Davis et al., 1996; Tait et al., 2000). The axonal isoform

is present throughout development at initial segments and

becomes clustered very early at nodes of Ranvier during their

formation, whereas the glial isoform is expressed during

myelination on glia and becomes concentrated in the paranodal

loops and microvilli (Bennett et al., 1997; Kordeli et al., 1995;

Tait et al., 2000). The axonal isoform is distinguished by the

presence of the PAT domain followed by a fifth FnIII domain.

The glial isoform lacks the PAT domain, but contains a different

alternatively spliced FnIII repeat with an RGD motif. Antibodies

against NF were initially found to inhibit neurite fasciculation

(Chang et al., 1987) and recent studies indicate that antibodies

against NF and a chimeric NF–Fc fusion protein inhibit

myelination in central (CNS) (Charles et al., 2002) and peripheral

nervous system cultures (Koticha et al., in press).

NF binds heterophilically in vitro to other CAMs including Nr-

CAM, axonin-1, and contactin (Volkmer et al., 1996), to tenascin-

R (Volkmer et al., 1998), to sodium channels (Ratcliffe et al.,

2001), and intracellularly to ankyrinG (Davis et al., 1993) and a

PDZ domain protein syntenin-1 (Koroll et al., 2001). NF was also

shown to multimerize on cell surfaces by interacting with ankyrinG

D. Koticha et al. / Mol. Cell. Neurosci. 30 (2005) 137–148138

and this process is regulated by phosphorylation (Garver et al.,

1997). Thus, a significant amount of information is known about

the function of the intracellular region of NF. However, the

function of the complex extracellular region of NF in cell adhesion

and neurite outgrowth has not been explored extensively. Our

study describes functions of the extracellular domains of NF

including novel roles of the region containing the mucin-like

domain as an inhibitor of cell adhesion and the positive role of the

RGD in the third FnIII repeat.

Fig. 2. NF186 inhibits cell adhesion. Reaggregated cerebellar cells isolated

from post-natal day 3 mice were incubated for 16–18 h on petri dishes

spotted with 30 Ag/ml of the indicated Fc fusion proteins and imaged before

(NF186-Fc), or after fixation (NF155-Fc, L1, BSA). Note that the

aggregates adhered to all substrates except the NF186-Fc and therefore in

this case images were collected before fixation and washing. Scale bar =

100 Am.

Results

Two major isoforms of NF exhibit opposite activities on cell

adhesion

To analyze functions of the extracellular region of NF, initially

two constructs were prepared as Fc fusion proteins corresponding

to the major forms of the molecule (Davis et al., 1996). One of

these represents the 155-kDa form, called NF155-Fc, which is

found primarily on glia (Collinson et al., 1998; Tait et al., 2000)

and includes the six Ig domains and FnIII repeats 1–4 (Fig. 1A).

The other represents the 186-kDa form, called NF186, found

primarily on neurons (Tait et al., 2000), which differs from

NF155-Fc primarily in that it lacks the third FnIII repeat and has a

mucin-like region, rich in proline, alanine, and threonine residues

that are O-glycosylated (Volkmer et al., 1992), followed by a fifth

FnIII repeat (Fig. 1A). These constructs were transfected into HEK

Fig. 1. Model and analysis of neurofascin proteins. (A) The extracellular

domains of rat NF isoforms NF155 and NF186 fused to human Fc (gray)

are shown. NF155-Fc contains six Ig domains and four FnIII repeats

linked to the Fc protein. NF186 contains six Ig domains, FnIII repeats 1,

2, and 4 but lacking FnIII repeat 3, followed by a mucin-like (m) PAT

domain and a fifth FnIII repeat revealed by the conserved Domain

Architecture Retrieval Tool (cDART, NCBI, www.ncbi.nlm.nih.gov). (B)

The Fc fusion proteins were purified from culture supernatants of HEK

293 cells and electrophoresed on a 7.5% SDS-PAGE gels and stained with

SYPRO-Orange.

293 cells and cell lines stably expressing these proteins were

selected. Proteins were purified by affinity chromatography on

protein A columns and their purity was determined following

electrophoresis on SDS-PAGE gels (Fig. 1B). As expected,

NF186-Fc had a higher apparent molecular weight than NF155-

Fc and the identity of the proteins was confirmed following

immunoblotting with antibodies specific to NF and to human Fc

(data not shown).

Given that other CAMs such as L1 can promote neurite

outgrowth as Fc fusion proteins (Haspel and Grumet, 2003;

Haspel et al., 2000) and that NF has been implicated in

interactions among neurites (Chang et al., 1987), we tested the

efficacy of the NF proteins on cell adhesion and neurite

outgrowth. Purified Fc fusion proteins were spotted on petri

dishes and reaggregates of rat cerebellar cells were incubated on

these substrates (Fig. 2). Aggregates adhered to L1-Fc and showed

robust outgrowth of fine processes as expected (Haspel et al.,

2000); in some experiments, staining with TuJ1 confirmed that the

fine processes were neurites (data not shown and see Fig. 6). Cell

adhesion and neurite outgrowth were also observed from

aggregates on NF155-Fc but the neurites were more fasciculated,

and many non-neuronal cells were also observed to migrate out

from the aggregates. Aggregates attached to BSA substrates but

extended only few neurites that were very short. Surprisingly,

substrates coated with NF186-Fc (Fig. 2) did not permit adherence

of aggregates. The results suggest that NF186-Fc is an inhibitor of

neuronal adhesion, in contrast to NF155-Fc, which is permissive

for cell adhesion and neurite outgrowth.

NF186 inhibits L1-Fc-mediated cell adhesion and neurite

outgrowth

The inhibition of cell adhesion was further studied by growing

aggregates on mixtures of proteins with L1-Fc or laminin, both of

which are potent promoters of neurite outgrowth in vitro (Fig. 3).

NF155-Fc and human Ig as a control did not interfere with L1-Fc-

Fig. 3. NF186 inhibits L1-mediated cell adhesion and neurite outgrowth.

(A) Reaggregated cerebellar neurons were incubated on petri dishes spotted

with mixtures of 30 Ag/ml of L1 (black) or laminin (gray) plus 10 Ag/ml of

the indicated proteins. The lengths of the third longest neurites were

measured and averages of three experiments are shown with standard

errors.

Fig. 4. Model and analysis of neurofascin 186 deletion mutants. (A) The

upper schematic is a diagram of NF186-Fc and below it are NF(186-M)-Fc,

which represent the six Ig domains and FnIII repeats 1, 2, and 4 fused to Fc,

and NFM-Fc, which represents the amino-terminal signal sequence linked

to the mucin-like domain and fifth FnIII repeat fused to Fc. (B) Fc fusion

proteins were purified from culture supernatants of HEK 293 cells and

electrophoresed on a 7.5% reducing SDS-PAGE gel stained with SYPRO-

Orange. (C) NFM-Fc was incubated with (+) or without (�) O-glycosidase,

resolved on a 7.5% reducing SDS-PAGE and immunoblotted with anti-

human Ig.

D. Koticha et al. / Mol. Cell. Neurosci. 30 (2005) 137–148 139

or laminin-mediated neurite outgrowth (Fig. 3). In contrast,

NF186-Fc clearly inhibited L1-Fc-mediated cell adhesion, but

had no effect on laminin-coated surfaces (Fig. 3), consistent with

previous findings that antibodies against NF blocked neurite

outgrowth on axonal substrates that express L1, but did not block

growth on laminin (Chang et al., 1987). To analyze neurite

outgrowth, NF186-Fc was varied over a tenfold range of concen-

trations while keeping L1-Fc constant. We found near maximal

inhibition of cell adhesion at a 2:1 molar ratio of L1-Fc to NF186-

Fc. Therefore, to study the inhibitory effect of NF186-Fc on cell

adhesion and neurite outgrowth, we used a 3:1 molar ratio of L1-Fc

to NF186-Fc since at this ratio aggregates adhered to the substrate

and extended very short, highly fasciculated neurites. When lengths

of the neurites were measured, we found that they were tenfold

shorter on L1-Fc when combined with NF186-Fc, but they were not

shorter when laminin was combined with NF186-Fc (Fig. 3).

However, the average lengths of neurites were not statistically

different on L1-Fc and laminin, either alone or in combination with

each other or when combined with NF155-Fc or Ig as a control

(Fig. 3). These results confirm that the extracellular region of

NF186 is an inhibitor of neuronal adhesion and neurite outgrowth

when used at a 1:3 molar ratio to L1-Fc. At this ratio, NF186-Fc did

not compete with L1-Fc for binding to the substrate insofar as the

amount of L1-Fc on the substrate was not diminished as detected by

a colorimetric assay in which the plates were stained with an

antibody to L1 (data not shown).

Extracellular regions of NF186 that inhibit cell adhesion and

neurite outgrowth

Given the extensive similarity in the amino-terminal Ig and

FnIII domains of NF186 and NF155, we hypothesized that the

inhibitory affects are related to the mucin-like region, which

represents one of the major differences between these two proteins.

To test this idea, we constructed two deletion mutants of NF186-Fc

with or without the M region, defined as the Fmucin-like_ domain

(m) plus the fifth FnIII repeat (Fig. 4A). NFM-Fc represents the

amino-terminal signal sequence plus the M region fused to the Fc

protein sequence, whereas NF(186-M)-Fc comprises the comple-

mentary extracellular region of NF186-Fc consisting of six Ig

domains followed by FnIII repeats 1, 2, and 4, but lacks the amino

acids comprising the M region (Fig. 4A); we have designated the

M region to include the mucin and FnIII repeat 5, and deleted both

because the latter is missing from a majority of NF186-like

isoforms known to be expressed in vivo (Collinson et al., 1998;

Hassel et al., 1997). These two constructs were transfected in HEK

293 cells and cell lines expressing proteins stably were used to

produce Fc fusion proteins that were purified by protein A affinity

chromatography (Fig. 4B). The mucin-like domain was predicted

to be heavily glycosylated (Volkmer et al., 1992) and the SYPRO-

D. Koticha et al. / Mol. Cell. Neurosci. 30 (2005) 137–148140

stained gel shows a diffuse pattern for NFM-Fc in comparison to a

more compact band for NF(186-M)-Fc. Treatment of the purified

protein with O-glycosidase reduced its heterogeneity and yielded a

sharper band upon immunoblotting, providing evidence for the

presence of O-linked glycosides in NFM-Fc (data not shown) as

indicated previously for NF186 (Davis et al., 1996).

To test the activities of different regions of NF, cerebellar

aggregates were grown on substrates coated with different Fc

fusion proteins (data not shown). When used at 30 Ag/ml to coat

substrates, NFM-Fc completely inhibited cell adhesion similar to

the effect of NF186-Fc. In contrast, NF(186-M)-Fc was permis-

sive for cell adhesion and neurite outgrowth, similar to NF155-

Fc. To analyze further the inhibitory activity of NFM-Fc, we

tested its effects on L1-Fc-mediated neurite outgrowth (Fig. 5A).

There is no evidence for direct binding between NF and L1

(Jacob et al., 2002); thus, they should bind only to the substrate

Fig. 5. NFM-Fc inhibits neurite outgrowth. (A) Reaggregated cerebellar

neurons were incubated on petri dishes spotted with mixtures of 3:1 molar

ratios of L1: Fc fusion proteins as indicated and imaged after 16–18 h.

Note the long fine neurites with L1 substrates containing NF155-Fc and

NF(186-M)-Fc but not with NF186-Fc or NFM-Fc. Scale bar = 100 Am. (B)

Lengths of the tenth-longest neurites extending away from each aggregate

were measured and averages from three independent experiments are

shown. (C) Equivalent aliquots of the same 3:1 mixtures of proteins that

were used for neurite outgrowth were electrophoresed on a 7.5% reducing

SDS-PAGE gel and immunoblotted with an HRP conjugated anti-human Fc

antibody to compare their relative amounts after scanning (see text and

Experimental methods).

and not to each other. Mixtures of NFM-Fc and L1-Fc at 1:3

molar ratios supported aggregate adhesion to the substrate but

there was little or no neurite outgrowth, similar to the effect of

NF186-Fc plus L1-Fc (Fig. 5A). In contrast, NF(186-M)-Fc did

not disrupt L1-Fc-mediated adhesion and neurite outgrowth

similar to results obtained with NF155-Fc. Note that only short,

highly fasciculated neurites were detected on NFM-Fc and

NF186-Fc in contrast to the long thin fibers observed on L1-Fc

with NF155-Fc or NF(186-M)-Fc. The neurite lengths on NFM-

Fc and NF186-Fc were found to be ¨tenfold shorter than those

on NF155-Fc and NF(186-M)-Fc (Fig. 5B). The ratios of the

protein mixtures used for these experiments were analyzed by

immunoblotting with anti-Fc antibody and quantitative analysis

verified that the test proteins were at ¨3-fold lower molar ratios

when compared to L1-Fc (Fig. 5C and data not shown). These

results indicate that the inhibitory activity of NF186-Fc is

associated with the M region but not with the remaining

extracellular region of NF186-Fc.

The results described above indicate that NF186-Fc and NFM-

Fc are non-permissive substrates for cell adhesion and neurite

outgrowth both in the absence and presence of L1-Fc. To

determine more directly whether NF186-Fc and NFM-Fc inhibit

neurite outgrowth, we designed a neurite outgrowth assay that

allows neurons to adhere to certain regions of the dish and extend

their neurites to contact bordering regions containing test proteins.

To establish borders of proteins, a circular spot was coated with

L1-Fc, blocked with BSA-Alexa488 to demarcate the initial spot,

then a surrounding ring was coated with the test protein and finally

a larger ring was coated with L1-Fc. When dissociated neurons

were added, they adhered to the L1-Fc-coated spot and to the outer

ring in all cases, and to the test proteins when they were L1-Fc or

NF155-Fc (Fig. 6). However, very few neurons adhered to the

inner ring (IR) when the test proteins were NF186-Fc or NFM-Fc

(Figs. 6B and F). Moreover, neurites did not cross the inner or

outer borders when the test proteins were NF186-Fc or NFM-Fc

and there were no apparent border effects with L1-Fc or NF155-Fc

(Figs. 6D and H). These results indicate that NF186-Fc and NFM-

Fc are not permissive for neurite outgrowth even when the

neuronal cell bodies adhere in permissive regions. Moreover,

addition of L1-Fc to inner rings coated with NF186-Fc or NFM-Fc

did not reverse the inhibition of cell adhesion and neurite

outgrowth, consistent with the idea that L1-Fc does not bind to

NF (Jacob et al., 2002). The combined results indicate that the

inhibition of cell adhesion and neurite outgrowth by NF186-Fc is

mediated by its M region and not by the Ig and FnIII domains that

are common to NF155-Fc.

Cells expressing NF186 inhibit neurite outgrowth

Considering that NF is expressed as a cell surface protein, it

was of interest to confirm the inhibitory effects of NF186 when

expressed on cells. Adenoviral vectors encoding GFP or NF186-

GFP were used to infect COS7 cells in monolayers and DRG

neurons were added 2–3 days later. Following incubation for 16

h, most DRG neurons extended two processes on control (data

not shown) or GFP monolayers (Fig. 7A, right) but fewer did so

on NF186-GFP monolayers (Fig. 7A, left). More DRG neurons

did not extend any processes on NF186-GFP monolayers (31%)

in comparison to the control GFP monolayers (17%) (Fig. 7B).

When composite images of the neurites on the GFP (control)

monolayers were analyzed, many neurites appeared to grow

Fig. 6. Inhibition of neurite outgrowth across borders of NF186-Fc and

NFM-Fc. Spots containing L1-Fc and BSA-Alex488 were surrounded with

20 Ag/ml of different proteins (NFM-Fc, NF155-Fc, NF186-Fc, and L1 as

indicated in the figure) to create an inner ring (IR) and outer ring (OR) (panel

A) as described in Experimental methods. DRG neurons adhered to the L1-

coated spot and to the OR in all cases and to the test protein when it was L1

(G, H) or NF155-Fc (C, D) but not when it was NFM-Fc (A, B) and NF186-

Fc (E, F). After incubation for 18 h, the neurons were immunostained with

TuJ1; images are overlays of the green fluorescing BSA and red TuJ1 except

in panels B and F that only show the red. Dashed lines in panels B and D

represent the border of the spot of fluorescent BSA-Alex488 that is not

shown to allow better visualization of the neurites. Note that neurites did not

enter the rings containing NFM-Fc and NF186-Fc either from the central

spot containing L1 or from the OR, but crossed the borders with NF155-Fc

(arrows) without any apparent disruptions (D). Similar results were observed

in three independent experiments. Scale bars = 500 Am (A, C, E, G) and

100 Am (B, D, F, H).

D. Koticha et al. / Mol. Cell. Neurosci. 30 (2005) 137–148 141

over the green GFP-labeled cells yielding a yellow color, in

contrast to the NF186-GFP monolayers where there was little

overlap (Fig. 7A). In addition, on the NF186-GFP monolayers,

neurites rarely contacted each other while they did so more

frequently on the GFP monolayers. Measurement of lengths of

the neurites indicated that the average lengths were shorter on

the NF186-GFP than on the GFP monolayers (Fig. 7C). The

results suggest that DRG neurites avoided cells expressing

NF186 resulting in shorter processes and fewer processes per

DRG neuron.

NF186 inhibits adhesion of neural and non-neural cells

Whereas the most striking effect of NF186-Fc was on neuronal

adhesion and neurite outgrowth, it also appeared to inhibit

adhesion of non-neuronal cells that were present in the aggregates

(Fig. 2B). To test the specificity of this effect, proteins were spotted

on petri dishes and populations of rat Schwann cells, astrocytes,

and CRL-1213 fibroblasts were incubated on the dishes in the

presence of serum for 18 h. NF155-Fc and NF(186-M)-Fc were

permissive for cell adhesion but we observed a zone of clearance

corresponding to spots of NF186-Fc and NFM-Fc for all three cell

types tested (Fig. 8). These results indicate that the inhibitory effect

of NF186-Fc and the M region is not restricted to neuronal cells,

but includes glia and fibroblasts.

NF155 supports neural cell adhesion and its third FnIII repeat

promotes cell spreading and neurite outgrowth

Comparison of cell adhesion to NF155-Fc vs. NF(186-M)-Fc

(Fig. 8) suggested that glial cells adhered more extensively to

NF155-Fc than to NF(186-M)-Fc in the presence of serum.

Considering that the major difference between these NF proteins

is the third FnIII repeat in NF155, which contains an RGD motif

known to interact with integrins and mediate cell adhesion

(Previtali et al., 2001; Reichardt and Tomaselli, 1991), it was of

interest to test whether the RGD sequence contributed to the

function of NF155. It has been well established that the effect of

RGD motifs can be neutralized by a hexapeptide such as GRGDSP

containing an RGD sequence while a closely related peptide with

the aspartate (D) changed to a glutamate (E) has no effect. We

tested adhesion in the absence of serum since serum contains

multiple cell adhesion and growth factors. Cells were detached

gently from the substrate using 5 mM EDTA to preserve cell

surface proteins, washed to remove traces of EDTA and serum, and

incubated over spots of NF155-Fc or NF(186-M)-Fc in the

presence of 200 Ag/ml of RGD or RGE control peptide. After a

2-h incubation, more Schwann cells adhered to NF155-Fc than to

NF(186-M)-Fc (Figs. 9A and B). The RGD peptide inhibited cell

adhesion (Fig. 9B) and spreading (Fig. 9A) on NF155-Fc but had

no significant effect on NF(186-M)-Fc where there was little or no

cell spreading. A similar effect was found for astrocytes (data not

shown) and DRG neurons but not for CRL-1213 fibroblasts (Fig.

9A). The lack of fibroblast adhesion indicates that NF155 acts

selectively to promote adhesion of neural cells.

Interestingly, Schwann cells and DRG neurons adhered and

spread out processes on NF155-Fc but not on NF(186-M)-Fc

and the adhesion was unaffected by addition of the control

peptide RGE (Fig. 9A and data not shown). Both glial

spreading and neurite outgrowth were inhibited significantly

by the RGD peptide, suggesting involvement of h1 integrins as

receptors. Laminin is known to promote cell adhesion through a

variety of RGD-independent pathways (Previtali et al., 2001;

Reichardt and Tomaselli, 1991) and cell adhesion on laminin

was not affected by the RGD peptide as expected (Figs. 9A and

C). To confirm the involvement of integrins in NF155-Fc

binding, the same assay was performed in the presence of an

antibody to rat h1 integrin. The h1 antibody, but not control

antibody, blocked Schwann cell spreading and DRG neurite

outgrowth on NF155-Fc, suggesting the involvement of h1integrin as a receptor for cell spreading mediated by NF155-Fc

(data not shown). Taken together, these results indicate that

Fig. 7. Outgrowth of neurites on cell monolayers expressing NF186. DRG neurons were grown for 18 h on monolayers of COS7 cells (A) expressing NF186-

GFP or GFP alone, fixed, and stained with TuJ1. DRG neurons extending two (arrow), one (arrowhead), or no (asterisk) processes were marked. Numbers of

DRG neurons extending two (blue), one (red), or no (yellow) processes were quantitated (B). The lengths of individual neurites extending two processes (blue)

or one process (red) were measured and plotted. These results are from an average of triplicate measurements from three separate experiments. Statistically

significant differences using paired t tests are indicated; numbers of neurons were 238 for NF186-GFP and 130 for the GFP control. Scale bar = 100 Am.

D. Koticha et al. / Mol. Cell. Neurosci. 30 (2005) 137–148142

interaction of neural cells with the third FnIII repeat of NF155

involves h1 integrins.

Discussion

NF exhibits very complex patterns of splicing (Hassel et al.,

1997) and here we describe novel functions of the extracellular

region for the two major spliced variants, NF155 and NF186. We

found that an alternatively spliced region in NF186 including the

region containing the mucin-like domain is an inhibitor of cell

adhesion and neurite outgrowth. The inhibition was also observed

with dissociated neurons or in aggregates in the presence of L1-Fc

as a promoter of neurite outgrowth, both when the proteins were

applied together or adsorbed separately to adjacent regions on the

substrates, and in addition when NF186 was presented on cells. In

addition, the alternatively spliced third FnIII repeat in NF155

contains an RGD sequence that promotes neural cell spreading and

neurite outgrowth.

NF155 is a glial isoform that promotes cell adhesion

The cell adhesion studies with different NF constructs suggest

that amino-terminal regions including the six Ig domains and two

FnIII repeats are sufficient to promote adhesion but without process

extension for neural cells including Schwann cells, DRG neurons,

and astrocytes. Interestingly, an isoform corresponding to NF186-

M, which shares aminal terminal domains with NF155, has been

detected in the chick early during development prior to myelination

(Hassel et al., 1997). Previous observations indicate that NF155 can

promote neurite outgrowth in a manner that is dependent on either

Nr-CAM or axonin-1 (Volkmer et al., 1998). The present results

raise the possibility that h1 integrins on axons may promote

interaction of Schwann cells with axons by binding to NF155.

Oligodendrocytes in the CNSmay act similarly and the involvement

of NF155 in CNS myelination was suggested from inhibition of

myelination in culture by anti-NF155 antibodies and NF-Fc

(Charles et al., 2002). These observations are consistent with the

idea that NF155 on glia is involved in interactions with neurons.

Fig. 8. NFM-Fc inhibits adhesion of glia and fibroblasts. Primary rat Schwann cells and astrocytes, and CRL-1213 fibroblasts were cultured for 18 h in the

presence of serum on petri dishes spotted with the proteins indicated and images were captured. Note the lack of adhesion to circular regions coated with

NF186-Fc and NFM-Fc whereas cells adhered to spots coated with NF155-Fc and NF(186-M)-Fc. Scale bar = 250 Am.

D. Koticha et al. / Mol. Cell. Neurosci. 30 (2005) 137–148 143

NF can mediate both homophilic and heterophilic interactions

between cells (Garver et al., 1997; Sakurai et al., 1997; Volkmer

et al., 1996). The observation that several types of cells including

fibroblasts bind to NF155 and NF(186-M)-Fc but not to NF186-

Fc or NFM-Fc suggests that cells that do not express NF may

interact via heterophilic binding partners for NF. The restricted

expression of NF to the nervous system limits its possible

interactions with cells and the expression of the mucin domain in

NF186 on neurons may further limit interactions. The ability of

NF155 to bind to NF 186 on beads (Jacob et al., 2002) (data not

shown) may allow Schwann cells to establish initial interactions

with axons via homophilic NF binding given that myelinating

Schwann cells upregulate NF155 just prior to myelination (S.

Basak and M.G., unpublished observations). With node forma-

tion, however, glial NF155 becomes localized in separate spatial

domains (e.g., paranodes) from NF186 that clusters at the node of

Ranvier on apposing axons (Salzer, 2003), suggesting that

homophilic NF interactions might only occur transiently during

neural development prior to protein segregation in the nodal

region.

NF186 is a neuronal isoform that can inhibit cell adhesion and

neurite outgrowth

The discovery of an inhibitory region in NF186 was particularly

interesting since CAMs of the Ig superfamily have been found

primarily to promote cell adhesion and neurite growth (Brummen-

dorf and Rathjen, 1995). Previous studies have implicated the Ig

domains of NF in interactions with Nr-CAM (Volkmer et al.,

1996), contactin, and axonin-1 (Volkmer et al., 1998). In addition,

the first Ig domain as well as the second FnIII repeat of NF186 are

involved in binding to the h1 subunit of the sodium channel

(Ratcliffe et al., 2001). mRNA splicing in NF may provide

additional degrees of freedom to allow folding of the protein into

different forms (Volkmer et al., 1998). For instance, different

functional states of the proteins can result from splicing of a short

sequence upstream of the first Ig domain (De Angelis et al., 2001;

Haspel and Grumet, 2003; Jacob et al., 2002). NF contains a splice

site in a comparable location (Davis et al., 1996; Hassel et al.,

1997) that may allow it to exhibit similar quaternary folding

patterns that modulate its various interactions.

The mucin-like domain in NF186 is a relatively large and

highly charged domain that may alter the protein’s structure as well

as its function by steric hindrance or charge repulsion. This may be

somewhat analogous to the effect of polysialic acid in N-CAM

(Rutishauser, 1992), which modulates binding of other CAMs

(e.g., L1) locally. Binding mediated by amino-terminal regions of

NF186 may be modulated by glycosylation in the mucin domain,

and it will be interesting to determine whether this glycosylation is

regulated developmentally. Amino-terminal regions of NF186 are

necessary for its binding to Schwann cells and its heterophilic

binding to NF-negative Schwann cells in vitro is weaker when the

mucin-like domain is present (Koticha et al., in press). Thus,

mRNA splicing and post-translational modifications may deter-

mine whether NF186 on neurons is a promoter or inhibitor of cell

adhesion by regulating splicing of mucin and its glycosylation.

NF is involved in neurite fasciculation possibly acting by

homophilic binding of NF186 on neurons (Chang et al., 1987). L1,

which has been implicated in homophilic binding and neurite

fasciculation, is widely expressed on axons and its activity can be

modulated by interactions with other CAMs in the plasma

membrane (Brummendorf and Rathjen, 1995). Given that neurons

may express different levels of various forms of NF, it is possible

that the inhibitory activity of the mucin-like region may not be

evident in the presence of excess levels of promoters of adhesion

(e.g., L1) unless its relative concentration exceeds a threshold

D. Koticha et al. / Mol. Cell. Neurosci. 30 (2005) 137–148144

D. Koticha et al. / Mol. Cell. Neurosci. 30 (2005) 137–148 145

level; therefore, the inhibitory activity may not have been detected

previously. In support of this, we found that the effects of NF186-

Fc and NFM-Fc are dose-dependent and the inhibition diminished

significantly with increasing proportions of L1-Fc. Whereas

NF186 inhibited L1-mediated cell adhesion and neurite outgrowth,

it had no detectable effect on laminin, suggesting that NF affects

the L1 pathway but not the laminin pathway (Chang et al., 1987) or

much higher relative levels of NF186 are needed to affect laminin-

mediated adhesion.

NF186 and development of the node of Ranvier

NF and Nr-CAM are prime candidates to promote node

formation since both are localized to nodes very early during their

development (Custer et al., 2003; Jenkins and Bennett, 2002;

Kazarinova-Noyes and Shrager, 2002; Koticha et al., in press;

Lambert et al., 1997). Both CAMs bind intracellularly to ankyrinG(Garver et al., 1997), which also binds to sodium channels and

thereby may serve in organizing these proteins in the node (Bennett

et al., 1997; Zhou et al., 1998). In addition, NF binds to sodium

channels extracellularly (Ratcliffe et al., 2001). Thus, there are at

least two mechanisms to cluster NF186 with sodium channels;

these receptors in the axonal plasma membrane can bind to each

other, and both can be cross-linked intracellularly by ankyrinG and

associated cytoskeletal proteins. Moreover, Nr-CAM also binds to

NF (Volkmer et al., 1996) and ankyrin (Davis and Bennett, 1994),

allowing it to co-cluster with the sodium channels at the node.

The accumulation of NF186 on the axonal membrane at initial

segments and developing nodes of Ranvier (Bennett et al., 1997)

raises the possibility that at such sites its local density may be high

enough to inhibit cell adhesion. Thus, as glial cells ensheath and

extend along axons, their leading processes may push clusters of

sodium channels, NF, and Nr-CAM along the axonal surface

(Dugandzija-Novakovic et al., 1995; Pedraza et al., 2001) but fail

to form stable contacts with the clusters because of inhibition by

NF186. Schwann cell microvilli are associated with the node but

their interaction (e.g., spreading) may be limited because of

inhibition by NF186. Although receptors on microvilli, possibly

linked to ezrin–moesin–radixin (ERM) proteins intracellularly,

have yet to be identified (Melendez-Vasquez et al., 2001; Scherer

et al., 2001), Peles recently discovered a Schwann cell ligand for

NF-Fc called gliomedin (Accession # AAP22419) (Soc. for

Neurosci. Abstracts, 2004). Considering the inhibitory activity of

NF186, it may inhibit spreading of Schwann cell microvilli at the

node of Ranvier where it becomes concentrated.

Analyses of region-specific knockouts of ankyrinG demonstra-

ted that it is critical for clustering receptors at nodes (Zhou et al.,

1998). In contrast, Nr-CAM does not appear to be obligatory for

node formation inasmuch as Nr-CAM null mice develop nodes but

in a delayed manner (Custer et al., 2003). These results combined

with the Fc perturbation experiments indicate a role for Nr-CAM in

node formation (Lustig et al., 2001). However, NF may play a

more central role in node formation given the nearly complete

Fig. 9. The third FnIII repeat in NF155 mediates cell adhesion and neurite outgrow

1213 fibroblasts were cultured for 2 h in serum-free media in the presence of 200 Awith the indicated proteins and imaged using phase-contrast optics. DRG neuron

serum-free media, fixed, stained with a mouse monoclonal antibody to rat h-IIIneurites, and images were captured. Scale bar = 100 Am. (B) The numbers of cell

experiments that yielded qualitatively similar results. (C) Averages of lengths o

representative experiments with similar results. In panels B and C, the control da

block of node formation in the presence of NF–Fc fusion proteins

(Koticha et al., in press).

On glia apposing axons, NF155 becomes restricted to the

paranodes during myelination. While Caspr/paranodin and con-

tactin on the axon are essential for paranode formation, clusters of

sodium channels can form even when paranodes are disrupted

suggesting that node formation may be quite independent of

paranode formation (Arroyo et al., 2002; Jenkins and Bennett,

2002; Melendez-Vasquez et al., 2001). The discovery of a

functional RGD motif in NF155 suggests that it might be involved

in initial axoglial contacts acting through integrin receptors in

addition to existing evidence for a role of h1 integrin in axoglial

interactions (Feltri et al., 2002).

In conclusion, the different isoforms of NF have different

functions in neurons and glia. Further elucidation of NF receptors

is likely to reveal additional details about various stages of nerve

development and node formation. The novel activities of the

different forms of NF also provide clues to explain why such high

levels of expression persist in the adult (Davis et al., 1993) in

comparison to related CAMs that are developmentally upregulated

but decrease with maturation (Daniloff et al., 1986).

Experimental methods

Materials

The Fail-Safe PCR kit was used for generating mutant proteins

(Epicentre, Madison, WI) in combination with Pwo polymerase

(Eurogentec, Belgium). T4 DNA ligase was from Promega

(Madison, WI). Rabbit polyclonal anti-L1 antibody was described

previously (Friedlander et al., 1994) and rabbit anti-NF antibody

was a kind gift from Dr. Elior Peles (Weizmann Institute of

Science, Rehovot, Israel). Other antibodies were purchased

including mouse monoclonal anti-h-III tubulin antibody (Chem-

icon, Temecula, CA), and HRP- and alkaline phosphatase-

conjugated goat anti-human Fc antibodies and rhodamine-con-

jugated anti-mouse antibody were from Jackson Immunoresearch

Laboratories (West Grove, PA). Laminin was from GIBCO/BRL

(Gaithersburg, MD), Enhanced chemiluminescence detection kits

were purchased from NEN Life Science Products (Boston, MA).

Rat Schwann cells were a kind gift from Dr. Mary Bunge

(University of Miami, Miami FL) and were cultured in DMEM

(Biowhittaker, Fisher Scientific, Fairlawn, NJ)/10% fetal bovine

serum (FBS) (Invitrogen, Carlsbad, CA)/4 AM forskolin (SIGMA,

St. Louis, MO)/10 ng/ml Bovine Pituitary extract (SIGMA) and l-

glutamine (SIGMA). Human embryonic kidney 293 (HEK 293)

cells, CRL-1213 fibroblasts (ATCC, Manassas, VA), and rat

primary astrocytes were cultured in DMEM + 10% FBS. Rat

dorsal root ganglion neurons were grown in MEM/F12 medium

supplemented with N2, vitamins (Invitrogen), 2 mM glutamine, 8

mg/ml glucose, 0.1% BSA, and 30 ng/ml NGF (Karagogeos et al.,

1991).

th involving the RGD sequence. (A) Primary rat Schwann cells and CRL-

g/ml of GRDGSP (RGD) or GRGESP (RGE) peptides on petri dishes coated

al populations were cultured on identically treated petri dishes for 18 h in

tubulin and a rhodamine-conjugated anti-mouse Ig antibody to visualize

s bound in three independent 20� fields were averaged; this is one of four

f individual neurites from triplicate spots are shown; this is one of four

ta were calculated using the GRGESP control peptide.

D. Koticha et al. / Mol. Cell. Neurosci. 30 (2005) 137–148146

Constructs

Plasmids pCR-NF155-Fc and pCR-NF186-Fc expressing the

extracellular domains of rat NF155 and NF186 fused to human Fc,

were prepared from the corresponding rat NF cDNAs (Davis et al.,

1996) kindly provided by Dr. Steve Lambert (Univ. of Mass.

Medical Center, Worcester, MA). PCR products encoding extrac-

ellular regions of NF155 (through nucleotide 4147; numbering

according to accession #L11002) and of NF186 (through nucleo-

tide 4654) (Fig. 1) were cloned in frame into the EcoRV site of

pCR3.1/Fc (Haspel et al., 2000). The resulting Fc fusion proteins

were called NF155-Fc and NF186-Fc, respectively. Constructs

pCR-NFM-Fc lacking the six Ig and three FnIII repeats of NF186,

and pCR-NF186-M lacking the mucin-like domain (nucleotides

4148–4366) and FnIII repeat number 5 (nucleotides 4367–4654)

in NF186 (see Fig. 4), were prepared by performing FLong-RangeInverse PCR_ (Koticha et al., 1999) on pCR-NF186-Fc using the

following primer pairs—5VGAT CCTCCCACATTGCCCCC and

5V-CATCGGAATCTCAATGGCCCC for pCR-NFM-Fc and 5VTGCAGAGTTCGATCCCGAGGAGCCCAAATCT and 5V-GATATCTCGGCTGCAGTTGGAGTAGCTTCATTT for pCR-

NF186-M. The sequence of each construct was confirmed by

DNA sequencing at the UMDNJ DNA Core facility (UMDNJ,

Piscataway, NJ).

Full-length NF186 was made by amplifying the transmembrane

and cytoplasmic domains of NF155 (Davis et al., 1996) with the

primers 5V TTTTTCTCAGAATTCGGCTGGTTCATCGGGCT-

CAT and 5VCGGAATAGATGGCATTGACTGGAGAT, digestingthe PCR product with EcoRI and ligating into the EcoRI and

SmaI-cut pEGFP.NI vector (BD Biosciences, Franklin Lakes, NJ).

This construct was subsequently digested with EcoRI and the

extracellular region of NF186 was excised from pCR-NF186-Fc

with EcoRI and inserted to generate pNF186-EGFP. This construct

expressed the full-length NF186 fused to the N-terminus of EGFP.

The protein expressed by this construct was validated by

sequencing and Western blotting. The adenoviral expression vector

was generated using the BD-Creator Technology. The NF186-

EGFP coding region was excised from pNF186-EGFP and

subcloned into pDNR-CMV (BD Biosciences) using KpnI and

XbaI to generate pDNR-NF186-EGFP. This plasmid was recom-

bined with pAdeno-X2 plasmid using Cre recombinase to generate

pAdeno-X2-NF186-EGFP plasmid. This plasmid was linearized

and transfected into HEK 293 cells and recombinant adenovirus

was harvested according to the manufacturer’s protocol.

Protein production, purification, and analysis

HEK 293 cells were transfected with plasmids expressing

recombinant proteins using Lipofectamine 2000 (Invitrogen),

according to the manufacturer’s protocol and cell lines expressing

recombinant Fc fusion proteins stably, were selected using G418

(Invitrogen). For production of Fc fusion proteins, HEK 293 cell

lines were grown in DMEM/1% low Ig FBS (Invitrogen) for 4

weeks, with weekly collection of culture supernatants. Recombi-

nant proteins including L1-16-Fc (called L1-Fc in this study) were

purified (Haspel et al., 2000) using affinity chromatography with

Ultra-link Protein-A beads (Pierce, Rockford, IL). NF155-Fc and

NFM-Fc were eluted from Protein A columns using 200 mM

Glycine–HCl, pH 2.5 and NF186-Fc and NF(186-M)-Fc were

eluted using Pierce gentle Ag/Ab elution buffer according to

manufacturer’s protocol. Proteins were dialyzed extensively

against HEPES buffered saline (Biowhittaker) at 4-C and stored

at �70-C. Protein concentrations were estimated by the Bradford

assay (BioRad kit, Hercules, CA) and ELISA assay using HRP-

anti-Human Fc antibody. The purity of the proteins was

determined by electrophoresis on 7.5% SDS-PAGE gels and

staining with SYPRO-Orange (Molecular Probes, Eugene, OR).

The identity of the proteins was further confirmed by transferring

them to nitrocellulose membranes and immunoblotting with

antibodies to NF and with HRP-anti-human Fc antibody.

Adsorption of Fc fusion proteins to substrates was confirmed by

staining coated petri dishes with alkaline phosphatase-conjugated

anti-human Fc antibody (Sakurai et al., 1997) and scanned on a

HP ScanJet 5300C with Precision Scan Pro and intensities were

measured in ImageQuant; L1-Fc protein was detected specifically

by reaction with rabbit anti-L1 antibody (Friedlander et al., 1994)

using L1-Fc (1–40 Ag/ml) as a standard. For glycosidase

treatment of NFM-Fc protein, 0.1 Ag was treated with 190

milliUnits of O-glycosidase (SIGMA), corresponding to 3.6 Agenzyme, for 4 h at 37-C according to manufacturer’s protocol;

treated proteins were analyzed by immunoblotting with HRP-anti-

human Fc antibody. CDART analysis was determined by

accessing www.ncbi.nlm.nih.gov.

Neurite outgrowth assays

Assays were performed as described by Haspel et al. (2000);

briefly, cerebellar neurons were isolated from post-natal day 3 or

4 mice and cultured as hanging drops to allow the neurons to

aggregate overnight. The aggregates were incubated on petri

dishes spotted with Fc fusion proteins for 16–18 h and imaged

pre- or post-fixation with 4% paraformaldehyde (Fisher Scien-

tific). Images were taken on a Nikon Diaphot microscope (Nikon,

Japan) using phase-contrast optics with a SPOT camera (Diag-

nostic Instruments, Sterling Heights, MI). In some experiments,

neurites were identified by staining with TuJ1 (Chemicon; 1:500)

and neurite lengths were measured using Scion Image (NIH,

Bethesda, MD) as described (Haspel et al., 2000). Composite

images were made using Adobe Photoshop version 6 (Adobe

Systems, San Jose, CA).

To test neurite outgrowth at borders, proteins were spotted onto

60-mm Falcon petri dishes #351007 within a scored 20-mm central

zone as follows. A 5-Al drop of L1-Fc (10 Ag/ml) was applied for 1

h followed by the addition of 1 Al of BSA (1 mg/ml) conjugated to

Alexa488 (Molecular Probes) for 30 min in a humid chamber. The

spots (¨1–2 mm diameter) were carefully aspirated and washed 3

times with Leibovitz’s L15 medium. Then, a 20-Al drop of the test

protein (20 Ag/ml) was applied over the center of the L1-Fc/BSA-

Alexa488 spot and allowed to coat a larger area (¨4–5 mm

diameter) for 1 h. Test proteins were then aspirated off and the

spots were washed 3 times with L15 medium. A final coating was

performed with 10 Ag/ml L1-Fc over the entire 20-mm region.

After washing, the entire region was flooded with L15 medium and

105 dissociated DRG neurons were added in N3 media (Karago-

geos et al., 1991). Cultures were incubated within a humid

chamber at 37-C for 16–18 h and then fixed by adding an equal

volume of 8% paraformaldehyde/0.2 M PO4 for 15 min at room

temperature. After washing 3� with PBS and quenching with 50

mM glycine/PBS, the cultures were rinsed with TBS, and treated

with 0.1% Triton X-100 and immunostained with TuJ1 and

Rhodamine-goat anti-mouse Ig (1:200). Fluorescent and phase-

contrast images were captured on a Nikon Diaphot microscope.

D. Koticha et al. / Mol. Cell. Neurosci. 30 (2005) 137–148 147

Cell adhesion assay

For adhesion assays, proteins were spotted in a circular pattern

on 3.5-cm petri dishes (Falcon, BD Biosciences) at 40 Ag/ml

unless otherwise indicated (Haspel et al., 2000). After incubating

at room temperature for 60 min, spots were individually washed

three times with PBS and the plate was blocked with PBS/1%

BSA (SIGMA) for a further 60 min at room temperature. Cells

were detached either with trypsin or EDTA treatment, washed

once with DMEM/10% FBS, and plated at a density of 150,000

per dish in DMEM/10% FBS. After incubation at 37-C for 18 h,

cells were washed four times with PBS and observed for adhesion

to the substrate before or after fixation in PBS/4% paraformalde-

hyde (SIGMA) for 15 min. To assay integrin involvement in

adhesion of cells to substrates, plates were spotted with the

indicated substrates. Cells were detached using PBS/2% FBS/5

mM EDTA to preserve cell surface proteins. Cells were washed in

serum-free Leibovitz’s L-15 media (Invitrogen) and plated at a

density of 60,000 per plate, in serum-free L-15 media containing

200 Ag/ml GRGDSP or GRGESP peptides (a kind gift from Dr.

Bruce Babiarz, Rutgers University) or 50 Ag/ml anti-h1 integrin

antibodies (anti-CD29 monoclonal antibody, BD Biosciences) for

2 h at 37-C. Cells were then fixed and images were captured

using the Nikon Diaphot microscope for phase-contrast optics or

Zeiss Axiovision on an Axiovert 200 (Thornwood, NY) for

fluorescent microscopy. DRG neurons were permeabilized and

stained with anti-hIII tubulin (TuJ1) and a rhodamine-labeled

anti-mouse antibody.

Growth of DRG neurons on COS7 cell monolayers

COS7 cells were seeded onto coverslips in a 24-well tissue

culture dish at 40,000 per well and infected with Adenovirus-

NF186-EGFP at a multiplicity of infection (MOI) of 10,

yielding an infection rate of about 50%. After 48 h, 10,000

dissociated DRG neurons isolated from E15 rat embryos were

seeded on each monolayer of cells in DMEM/5% FBS. The co-

cultures were fixed 18 h later and stained with the antibody to

h-III tubulin and secondary anti-mouse IgG conjugated to

Rhodamine red. The images were acquired on the Axiophot

microscope using constant parameters and images of identical

fields were superimposed in Photoshop. In control experiments,

the Adenovirus expressing EGFP (Qbiogene, Carlsbad, CA) was

used to infect COS7 cell monolayers at a comparable MOI.

This adenovirus was prepared similar to the Adeno-NF186-

EGFP virus. The numbers of neurons (h-III tubulin+) containingtwo, one, or no processes were counted, and the lengths of the

processes were measured; processes <5 Am were not included in

the analysis.

Acknowledgments

We thank Dr. Steve Lambert for providing NF cDNAs and

assistance in assembling Fc constructs, Dr. Bruce Babiarz for

supplying mice and RGD peptides, Dr. Elior Peles for antibodies to

NF, Emily Su for providing DRG neurons, Tejal Mehta for

producing L1-Fc proteins, and Drs. Jeffrey Haspel, Takeshi

Sakurai, Elior Peles, and Jim Salzer for stimulating discussions,

comments, and technical help. This work was supported by grants

from NIH and NMSS.

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