Opposite impacts of tenascin-C and tenascin-R deficiencyin mice on the functional outcome of facial nerve repair

Orlando Guntinas-Lichius,1* Doychin N. Angelov,2* Fabio Morellini,3 Mithra Lenzen,2 Emmanouil Skouras,4

Melitta Schachner3 and Andrey Irintchev31Department of Oto-Rhino-Laryngology, University of Cologne, D-50924 Cologne, Germany2Anatomical Institute, University of Cologne, D-50931 Cologne, Germany3Zentrum fur Molekulare Neurobiologie, Universitat Hamburg, D-20251 Hamburg, Germany4Department of Trauma, Hand and Reconstructive Surgery, University of Cologne, D-50924 Cologne, Germany

Keywords: facial nerve, functional recovery, motion analysis, regeneration, reinnervation

Abstract

The glycoproteins tenascin-C (TNC) and tenascin-R (TNR) are extracellular matrix proteins involved in the development, plasticity and

repair of the nervous system. Altered expression patterns after nerve lesions in adult animals have suggested that these molecules

influence axonal regeneration. To test this hypothesis, we investigated adult mice constitutively deficient in the expression of TNC,

TNR or both, using the facial nerve injury paradigm. Quantitative analysis of vibrissal movements prior to nerve transection and repair

(facial–facial anastomosis) did not reveal genotype-specific differences, and thus impacts of the mutations on motor function in intact

animals. Two months after nerve repair, recovery of vibrissal whisking was poor in wild-type mice, a typical finding after facial–facial

anastomosis in rodents. Differential effects of the mutations on whisking were found: recovery of function was worse in TNC-deficient

and better in TNR null mice compared with wild-type littermates. In double-knockout animals, vibrissal performance was insufficient,

but to a lesser extent compared with TNC null mutant mice. Retrograde labelling of motoneurons in the same animals showed that

similar numbers of motoneurons had reinnervated the whisker pads in all experimental groups precluding varying extents of

motoneuron death and ⁄ or axon regeneration failures as causes for the different outcomes of nerve repair. Our results provide strong

evidence that TNC promotes and TNR impedes recovery after nerve lesion. These findings are of particular interest with regard to the

scanty knowledge about factors determining success of regeneration in the peripheral nervous system of mammals.

Introduction

Tenascin-C (TNC) and tenascin-R (TNR) are members of the tenascin

family of recognition molecules (Faissner & Schachner, 1995;

Faissner, 1997; Gotz et al., 1997; Schachner, 1997; Jones & Jones,

2000; Dityatev & Schachner, 2003; Rauch, 2004). Both molecules are

extracellular matrix glycoproteins expressed widely in the CNS with

non-overlapping functions implicated in a variety of processes ranging

from development to synaptic plasticity in the adult organism. TNC

and TNR are also expressed during the development of the peripheral

nervous system by Schwann cells and motoneurons, respectively

(Daniloff et al., 1989; Wehrle & Chiquet, 1990; Martini & Schachner,

1991; Martini, 1994; Derr et al., 1998). Postnatally, expression levels

of TNC and, less so, of TNR are reduced. Peripheral nerve injury in

adult rodents has differential effects on the expression of the two

molecules: downregulation of TNR and upregulation of TNC levels

(Daniloff et al., 1989; Martini et al., 1990; Irintchev et al., 1993;

Angelov et al., 1998). These patterns of regulation suggest that TNC

and TNR are essential for peripheral nerve regeneration.

Several studies have been performed with the aim to elucidate the

significance of TNC re-expression during peripheral nerve regeneration.

The results of these studies are largely contradictory. Langenfeld-Oster

et al. (1994) and Mege et al. (1992) have found that application of

antibodies against TNC in vivo causes delayed reinnervation consistent

with in vitro observations showing that TNC promotes neurite

outgrowth when coated as a uniform substrate (Faissner, 1997).

Morphological studies in two different TNC-deficient mouse lines,

however, have not revealed effects of TNC deficiency on nerve

regeneration (Forsberg et al., 1996; Moscoso et al., 1998). In contrast,

Cifuentes-Diaz et al. (2002) have found evidence for abnormal

reinnervation in the mouse line studied by Moscoso et al. (1998) and

generated by Saga et al. (1992). The same authors (Cifuentes-Diaz et al.,

1998) have reported that this TNC-deficient mouse has peripheral nerve

and motor endplate abnormalities, which makes interpretations of

results on reinnervation complicated.

TNR has received less attention with regard to peripheral nerve

regeneration, probably because TNR has not been observed to be

upregulated in the peripheral nerve after injury. Still, the finding of

reduced TNR expression in perineuronal nets around motoneurons

after nerve lesion, combined with results from in vitro studies showing

that TNR is anti-adhesive for microglial cells, has led to the hypothesis

that TNR may be protective against lesion-induced motoneuron death

(Angelov et al., 1998).

In this study we used a novel reliable approach for functional analysis

of vibrissal motor performance before and after surgical repair of the

facial nerve (Guntinas-Lichius et al., 2002; Tomov et al., 2002) to

evaluate the functional significance of constitutive lack of TNC and

TNR for nerve regeneration in mice. The sensitivity of this approach

allowed to reveal significant differences in the functional outcome of

nerve repair between knockout and wild-type control animals and,

Correspondence: Dr A. Irintchev, as above.

E-mail: andrey.irintchev@zmnh.uni-hamburg.de

*O.G.-L. and D.N.A. contributed equally to this work.

Received 1 July 2005, revised 16 August 2005, accepted 6 September 2005

European Journal of Neuroscience, Vol. 22, pp. 2171–2179, 2005 ª Federation of European Neuroscience Societies

doi:10.1111/j.1460-9568.2005.04424.x

interestingly, differential impacts of the two glycoproteins, or rather their

absence, on functional recovery.

Materials and methods

Animals

TNC and TNR null mutant mice described previously (Weber et al.,

1999; Evers et al., 2002)were obtained from the original breeding stocks

maintained at the animal facility of the Universitatsklinikum Hamburg

Eppendorf. Mice deficient in both TNC and TNR were generated by

breeding animals of the two mutant lines together. Male mice at the age

of about 3 months were used in all experiments. Genotyping of the mice

was performed using routine polymerase chain reaction assays. The

animals were kept under standard laboratory conditions. All experi-

ments were conducted in accordance with the German and European

Community laws on protection of experimental animals, and the

procedures used were approved by the local animal care committee at

the District Government of Cologne. Numbers of animals studied

in different experimental groups are given in the figures.

Experimental paradigm

The experimental design used in this study is well established for both

rats and mice (Guntinas-Lichius et al., 2002, 2005; Tomov et al., 2002;

Angelov et al., 2003). The experiments include analysis of vibrissal

motor performance prior to facial nerve transection and anastomosis,

and at 2 months after surgery, followed by retrograde labelling of

motoneurons by intramuscular injections of fluorescence tracers. Triple-

labelling of motoneurons projecting through different branches of the

facial nerve after regeneration allows quantitative assessment of the

extent of axonal sprouting and misdirected regrowth in the rat (Dohm

et al., 2000; Guntinas-Lichius et al., 2002, 2005; Streppel et al., 2002).

In the mouse, however, due to the small size of the animal’s head this

procedure may give misleading results because of unintended diffusion

of tracers away from the application site to other mimic muscles.

Therefore, we restricted the labelling experiments to single tracing back

from the whisker pads, which allows estimates of numbers of

motoneurons that had re-established connections with the target. A

2-month survival period was chosen for the present experiments, as this

time-period is sufficient for reaching maximum degree of recovery

(Angelov et al., 2005). To rule out pathological aberrations in the

neuromuscular system of TNC- and TNR-deficient mice, as previously

reported for another TNC-deficient mouse (Cifuentes-Diaz et al., 1998),

we performed morphological analysis of sciatic nerves, and soleus and

extensor digitorum longus (EDL) muscles.

Evaluation of vibrissal whisking

Under normal physiological conditions, the mystical vibrissae are

erect with anterior orientation. Their simultaneous sweeps, known as

‘whisking’ or ‘sniffing’ (Welker, 1964; Semba et al., 1980), occur

5–11 times per second (Komisaruk, 1970; Carvell & Simons, 1990).

The key movements of this motor activity are the protraction and

retraction of the vibrissal hairs by the piloerector muscles, all of which

are innervated by the buccal branch of the facial nerve (Dorfl, 1985).

Following transection of the facial nerve, the vibrissae acquire a

caudal orientation and remain motionless.

The video-based motion analysis of whisking has been described

previously (Guntinas-Lichius et al., 2002; Tomov et al., 2002). The

two large hairs of the C-row on each side of the face were used for

biometric analysis. Following intraperitoneal injection of ketamine

and xylazine (100 mg Ketanest�, Parke-Davis ⁄ Pfizer, Karlsruhe,

Germany, and 5 mg Rompun�, Bayer, Leverkusen, Germany, per kg

body weight), all other vibrissae were clipped with fine scissors. On

the next day, the animals were videotaped for 3–5 min during active

exploration using a digital camcorder (Panasonic NV DX-110 EG)

operating at a sampling rate of 50 frames ⁄ s. Captured video sequences

were examined at slow motion playback, and a 1.5-s-long sequence

was selected for each animal. Thereby, the stable position of the

animal’s head, the frequency of whisking and the degree of vibrissae

protraction were considered as selection criteria. Selected sequences

were captured by a 2D ⁄Manual Advanced Video System PEAK

Motus 2000 (PEAK Performance Technologies, Englewood, CO,

USA). The spatial model consisted of three reference points (tip of the

nose and the inner angles of both eyes, Fig. 1A). Each vibrissa is

represented in the spatial model by two points ) its base and a point

on the shaft 0.5 cm away from base (Fig. 1A). Using this model, the

following parameters were evaluated:

1 protraction (i.e. the forward movement of the vibrissae) measured

by the rostrally opened angle (in degrees) between the mid-sagittal

plane and the hair shaft (Fig. 1B);

2 whisking frequency as cycles of protraction and retraction (passive

backward movement) per second;

3 amplitude (the difference between maximal retraction and maximal

protraction in degrees);

4 angular velocity during protraction in degrees ⁄ s;

5 angular acceleration during protraction in degrees ⁄ s2.

Facial–facial anastomosis (FFA)

Unilateral transection and end-to-end suture of the facial nerve were

carried out under an operating microscope. Under ketamine ⁄ xylazine

anaesthesia, the trunk of the facial nerve was exposed and transected

close to its emergence from the stylomastoid foramen, but distal to the

posterior auricular nerve. The proximal stump was then microsurgi-

cally reconnected to the distal stump with 11-0 epineural sutures

(Ethicon EH 7438G, Norderstedt, Germany). Finally, the wound was

closed by three 4-0 skin sutures.

Retrograde labelling of facial motoneurons

Following the postoperative video recording (2 months after surgery),

the muscles of each whisker pad in all mice were injected with 30 lL

of 1% aqueous solution of the fluorescent retrograde tracer Fluoro-

Gold (FG, Fluorochrome, Denver, Colorado, USA) supplemented with

2% dimethylsulphoxide under diethyl ether anaesthesia. Animals were

fixed by perfusion 7 days after tracer application.

Fixation and tissue processing

Following diethyl ether anaesthesia and subsequent intraperitoneal

injection of ketamine ⁄ xylazine, all mice were transcardially perfused

with 0.9% NaCl followed by 4% formaldehyde in 0.1 m phosphate

buffer, pH 7.4, for 20 min. The brains were removed, and 50-lm-

thick coronal sections were cut through the brainstems on a vibratome.

Analysis of retrogradely labelled motoneurons

Sections were observed on a Zeiss Axioskop 50 epifluorescence

microscope (Zeiss, Oberkochen, Germany) through a custom-made

narrow band-pass filter set for FG (AHF Analysentechnik, Tubin-

gen, Germany). Employing a CCD video camera system (Optronics

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ª 2005 Federation of European Neuroscience Societies, European Journal of Neuroscience, 22, 2171–2179

Engineering Model DEI-470, Goleta, CA, USA) combined with the

image analysing software Optimas 6.5. (Optimas, Bothell, Wash-

ington, USA), the retrogradely labelled facial motoneurons were

manually counted on the computer screen (Guntinas-Lichius et al.,

2001, 2002). Employing the fractionator principle (Gundersen,

1986), all retrogradely labelled motoneurons with visible cell nuclei

in the 50-lm-thick sections were counted in every second section

through the facial nucleus on the operated and the intact

contralateral side. All counts were performed by two observers

(D. Felder and D.N. Angelov) who had no information about the

genotype of the mice.

Morphological analyses of peripheral nerves and skeletal

muscles

Five-month-old TNC- or TNR-deficient mice, and their wild-type

littermates, were fixed by transcardial perfusion as described above.

Soleus and EDL muscles, and mid-thigh segments of the sciatic nerves

were postfixed in 1% aqueous osmium tetroxide solution for 1 h at room

temperature, dehydrated and embedded in resin according to standard

protocols. Transverse 1-lm-thick sections were stained with Toluidine

Blue and used for analysis. For quantitative evaluations, overlapping

digital images were taken from whole sections through soleus muscles

Fig. 1. Spatial model used for assessment of kinematic parameters (A, B) and somatotopic organization of the facial nucleus (C and D). (A) The video frameshows a tenascin-C (TNC)-deficient mouse at a phase of maximum protraction (forward movement) of the vibrissae on both the intact (left) and operated (right)side. Two of the lines drawn in the picture (between the eyes and the sagittal one) are the coordinate lines used for estimation of vibrissal movements in time andspace. (B) The two lines oblique to the sagittal line determine the angle of maximum protraction (arcs). Note the larger angle on the operated (right) side. (C and D)Motoneurons labelled retrogradely after injection of tracer into the whisker pads on the intact (C) and operated side (D) in a TNC– ⁄ – mouse in 50-lm-thickvibratome sections. On the intact side (C), the motoneurons are localized within a circumscribed area of the facial nucleus, the lateral subnucleus. The somatotopicpattern of innervation is lost after nerve repair (D), as seen from the localization of labelled cell bodies both within (correctly projecting neurons) and outside(incorrectly projecting neurons) the lateral subnucleus. Scale bar: 100 lm (C).

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and tibial nerves of TNC-deficient mice and wild-type control animals

on an Axiophot 2 microscope equipped with a digital camera AxioCam

HRC and AxioVision software (Zeiss). The images from eachmuscle or

nerve sample were mounted together to reconstruct the whole cross-

sectional area using Adobe� Photoshop� 6.0 software (Adobe Systems,

San Jose, CA, USA). Image Tool 2.0 software (University of Texas

Health Science Center, San Antonio, TX, USA) was used for

determination of total number of myelinated axons or muscle fibres

and total cross-sectional area of the sample. Fibre density (mm)2) was

calculated as ratio of fibre number to cross-sectional area. Data files

containing the coordinates of the centres of individual axons or muscle

fibres marked during counting were used to calculate first nearest-

neighbour distances (NND) andNNDvariancewith an original software

written inDelphi (version 5, Borland Software, Scotts Valley, CA,USA)

by Alexander G. Nikonenko.

Statistical analysis

All numerical data are presented as group mean values with standard

errors of mean (SEM). Parametric tests (t-test or analysis of variance,

anova, with subsequent Tukey’s post-hoc test) were used to compare

group mean values as appropriate. Data conformed to the requirement

for normal distribution (‘normality’ test, SigmaStat 2.0, SPSS,

Chicago, IL, USA). The threshold value for acceptance of differences

between group mean values was 5%.

Results

Functional analysis

TNC-deficient mice

Vibrissal motor performance was not affected by the mutation in the

TNC gene. As shown in Fig. 2A, the values for several functional

parameters were similar in intact TNC-deficient (TNC– ⁄ –) mice and

wild-type (TNC+ ⁄ +) littermates. Two months after nerve repair,

recovery of whisking in TNC+ ⁄ + mice was poor (Fig. 2B). The

amplitude of movement, the most important parameter from a

physiological point of view, was reduced to half of the preoperative

value (compare the black bars at Ampl in Fig. 2A and B). Recovery of

function in TNC– ⁄ – mice was worse compared with TNC+ ⁄ + mice

(Fig. 2B). The values for individual parameters differed by 44–72%

between the two groups. Thus, TNC deficiency causes a severe

impairment of functional recovery after facial nerve repair.

TNR-deficient mice

The results for TNR– ⁄ – mice and their TNR+ ⁄ + littermates were

similar to those for TNC– ⁄ – and TNC+ ⁄ + mice with respect to lack of

genotype effects on vibrissal function in non-operated animals

(Fig. 3A) and degree of reduction of whisking amplitude after nerve

repair in TNR+ ⁄ + mice (compare the black bars at Ampl in Fig. 3A

and B). The finding of normal whisking behaviour in TNR– ⁄ – mice is

important because motor coordination in these animals is impaired as

seen in several motor tests such as beam walking, Rotarod and pole

test, and wire hanging test (Freitag et al., 2003; Montag-Sallaz &

Montag, 2003). Following FFA, TNR– ⁄ – mice recovered much better

than TNR+ ⁄ + animals, as indicated by the amplitude of the whisking

phases (+73%, Fig. 3B). While no differences between the groups

were present for velocity, TNR– ⁄ – mice accelerated the vibrissae after

nerve repair as fast as before operation, in large contrast to the

impaired acceleration in TNR+ ⁄ + mice (Fig. 3A and B). The overall

conclusion from these observations is that deficient expression of TNR

creates conditions favouring recovery of whisking.

Mice deficient in both TNC and TNR

Deficiency in the expression of both molecules also did not affect

motor performance in intact mice (Fig. 4A). Following nerve

anastomosis, recovery of function was worse in double-mutant mice

compared with wild-type littermates (Fig. 4B). These results are

generally similar to those for TNC– ⁄ –, mice but the differences

between the group mean values were less pronounced (compare

Fig. 2. Vibrissal function in tenascin-C (TNC)– ⁄ – and TNC+ ⁄ + mice. Shown are mean values + SEM of amplitude (Ampl, in degrees), velocity during protractionand retraction (vpro and vre, respectively, degrees ⁄ s · 0.1), and acceleration during protraction and retraction (apro and are, respectively, degrees ⁄ s

2· 0.005) during

whisking evaluated in the same animals prior to (A) and 2 months after (B) facial–facial anastomosis (FFA). Number of mice studied per group is given in thelegend in parentheses. Statistically significant differences (P < 0.05, one-way anova with Tukey post-hoc test) between values for TNC+ ⁄ + and TNC– ⁄ – mice2 months after FFA are indicated by asterisks in (B), group mean values that change significantly after operation are marked by crosshatches in (A).

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Figs 2B and 4B). Thus, we can conclude that the negative impact of

TNC deficiency dominates over the positive influence of TNR

ablation, though the latter effects are not apparently eliminated.

Analysis of motoneuron regeneration

Numbers of retrogradely labelled motoneurons on the intact side of the

unilaterally lesioned animals were similar in mice deficient in TNC,

TNR or both, and the respective groups of wild-type littermates (Intact

in Fig. 5A–C). This observation shows that the mutations do not affect

the formation and postnatal maintenance of normal motoneuron pools.

Genotype-related differences were also not found on the side of nerve

repair (Operated in Fig. 5A–C). The finding that similar numbers of

motoneurons reinnervate the whisker pads in different groups of mice

is in contrast to the largely variable functional outcome of nerve repair

(Figs 2B, 3B and 4B). However, this is not surprising as lack of

Fig. 4. Vibrissal function in tenascin-C (TNC)– ⁄ – tenascin-R (TNR)– ⁄ – and TNC+ ⁄ + TNR+ ⁄ + mice. Shown are mean values + SEM of amplitude (Ampl, indegrees), velocity during protraction and retraction (vpro and vre, respectively, degrees ⁄ s · 0.1), and acceleration during protraction and retraction (apro and are,respectively, degrees ⁄ s2 · 0.005) during whisking evaluated in the same animals prior to (A) and 2 months after (B) facial–facial anastomosis (FFA). Number ofmice studied per group is given in the legend in parentheses. Statistically significant differences (P < 0.05, one-way anova with Tukey post-hoc test) betweenvalues for TNC+ ⁄ + TNR+ ⁄ + and TNC– ⁄ – TNR– ⁄ – mice 2 months after FFA are indicated by asterisks in (B), group mean values that change significantly afteroperation are marked by crosshatches in (A). The tilde (�) in (B) indicates that the P-value for this comparison (P ¼ 0.08) was greater than the threshold value(0.05).

Fig. 3. Vibrissal function in tenascin-R (TNR)– ⁄ – and TNR+ ⁄ + mice. Shown are mean values + SEM of amplitude (Ampl, in degrees), velocity during protractionand retraction (vpro and vre, respectively, degrees ⁄ s · 0.1), and acceleration during protraction and retraction (apro and are, respectively, degrees ⁄ s

2· 0.005) during

whisking evaluated in the same animals prior to (A) and 2 months after (B) facial–facial anastomosis (FFA). Number of mice studied per group is given in thelegend in parentheses. Statistically significant differences (P < 0.05, one-way anova with Tukey post-hoc test) between values for TNR+ ⁄ + and TNR– ⁄ – mice2 months after FFA are indicated by asterisks in (B), group mean values that change significantly after operation are marked by crosshatches in (A).

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correlation between numbers of retrogradely labelled motoneurons

and functional parameters has previously been shown (Guntinas-

Lichius et al., 2005).

Numbers of labelled motoneurons on the operated side in the

different experimental groups were 32–57% lower compared with the

contralateral intact side (Fig. 5A–C). These reductions can be

explained by degeneration of axotomized motoneurons (Sendtner

et al., 1996). However, one has to consider that after nerve transection

and repair, the whisker pads are reinnervated not only by motoneurons

originally projecting to them, but also by ‘foreign’ neurons previously

innervating other muscles. This is due to axonal sprouting at the site of

nerve injury and subsequent non-selective regrowth to both proper and

improper targets (Brushart & Mesulam, 1980; Sumner, 1990; Robin-

son & Madison, 2004). The anatomical correlate of this misdirected

reinnervation, as previously shown in wild-type rats and mice, is the

loss of somatotopic organization in the facial nucleus following

reinnervation (Angelov et al., 1999, 2003; Guntinas-Lichius et al.,

2002). In this study, we observed similar loss of the somatotopic

pattern of innervation on the operated side in mice deficient in the

expression of TNC (Fig. 1C and D), and in mice of all other genotypes

studied (not shown). Therefore, genotype-related differences in

functional recovery after nerve repair cannot be explained by

differences in the degree of axonal misdirection. This finding supports

previous data from experiments with rats, indicating that precision of

Fig. 5. Number of motoneurons innervating the whisker pads. Shown are mean values + SEM of retrogradely labelled motoneurons after application of tracer intothe whisker pads on the injured side (2 months after FFA, Operated in A–C) and on the intact contralateral side (Intact) of mice of different genotypes indicated inthe figure legends (number of animals per group in parentheses). Crosshatches indicate group mean values significantly different from those on the contralateral intactside (P < 0.05, one-way anova with Tukey post-hoc test).

Fig. 6. Morphometric analysis of peripheral nerve and muscle. Shown are mean values + SEM for morphometric parameters evaluated in tissue samples from thetibial nerve (A) and soleus muscle (B) of TNC+ ⁄ +- and TNC– ⁄ –-deficient mice. The evaluated parameters were number of myelinated axons ⁄muscle fibres percomplete transverse section through the nerve ⁄muscle, area of these complete cross-sections, area density of the structures (number per unit area), first NND and itsvariance (NND var.). The NND and its variance are estimates of the spatial distribution pattern of particles. No differences between group mean values were found(two-sided t-test for independent samples). In B, the major tick interval on the vertical axis is 200.

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target reinnervation is not a major factor determining the functional

outcome of facial nerve repair (Guntinas-Lichius et al., 2005).

Structural analyses of peripheral nerves and muscles

Histopathological analysis of sciatic nerves, and soleus and EDL

muscles of TNC- and TNR-deficient mice and wild-type littermates

were performed using semithin transverse sections. In all specimens

nerve structure was normal, as indicated by the absence of axonal

degeneration, demyelination, cellular infiltrates, abnormal prevalence

of small-diameter axons or apparent increase in the myelin fibre-free

compartments. Also the appearance of the muscles was normal as

signs of acute muscle fibre necroses, previous muscle damage and

repair (‘split’ fibres, centrally located nuclei, fibrosis, Irintchev &

Wernig, 1987), denervation (atrophic muscle fibres) or inflammation

(cellular infiltrates) were not found. These observations were further

verified by quantitative analysis of tibial nerves and soleus muscles of

TNC– ⁄ – and TNC+ ⁄ + mice (Fig. 6A and B). Thus, the intact mutant

animals used in this study do not have structural aberrations in the

neuromuscular system. In contrast, another TNC-deficient mouse

(Saga et al., 1992) is characterized by numerous abnormalities in the

sciatic and facial nerves: higher axonal area density (twofold as

compared with wild-type control mice), higher area fraction occupied

by unmyelinated axons (fourfold), ‘naked’ unmyelinated axons,

axonal degeneration (affecting 10% of all axons) and disrupted

myelin sheaths (Cifuentes-Diaz et al., 1998).

Discussion

The finding that functional recovery in TNC-deficient mice is impaired

clearly indicates the significance of this glycoprotein for peripheral

nerve repair. This result does not allow conclusions as to whether the

observed effect is directly or indirectly related to the absence of TNC

protein. Observations made in other studies lend support to the idea of

direct consequences. TNC, when offered in a substrate-bound form to

neurons in culture, promotes neurite outgrowth and this effect is

counteracted by application of antibodies against TNC (Wehrle &

Chiquet, 1990; Lochter et al., 1991; Husmann et al., 1992). TNC

antibodies also delay regrowth of severed peripheral axons to motor

endplates in vivo (Langenfeld-Oster et al., 1994). Finally, retrograde

degeneration (‘dying-back’) of severed corticospinal tract axons after

spinal cord compression injury is enhanced in TNC-deficient mice

compared with wild-type control animals, and this excessive degen-

eration can be counteracted by local application of recombinant TNC

fragment (Chen, Irintchev, Loers and Schachner, unpublished obser-

vations).

One consequence of TNC absence in the distal stump of the injured

nerve is likely to be a decrease in speed of reinnervation, which is

suggested by the aforementioned results of in vitro and in vivo

experiments with antibody perturbations. Slower reinnervation, how-

ever, cannot explain the present result. In fact, the delay of

reinnervation seen in vivo is only a few days (Langenfeld-Oster

et al., 1994). A recovery period of 2 months is long enough to surpass

time-periods at which delayed reinnervation could significantly

contribute to poor function. More important is to consider that TNC

is not merely a growth substrate. It influences a variety of signal

transduction mechanisms involving, for example, integrins, syndecan

and the extracellular matrix glycoprotein fibronectin (Chiquet-Ehris-

mann & Tucker, 2004; Midwood et al., 2004; Ruiz et al., 2004). In

particular, interaction with the a7 integrin may be crucial for

peripheral nerve regeneration. This notion is supported by the findings

that TNC induces neurite outgrowth via interaction with the neuronal

a7b1 integrin (Mercado et al., 2004), and that axonal regeneration in

a7 integrin-deficient mice, evaluated in the facial nerve injury

paradigm, is impaired (Werner et al., 2000). Furthermore, it is

important to note that TNC interacts with voltage-gated sodium

channels (Srinivasan et al., 1998), and its absence may thus have an

impact on the excitability of regenerated axons. Beyond mechanisms

related to TNC deficiency in the injured nerve, we have to consider

that TNC is important for normal synaptic function and plasticity in

the CNS (Dityatev & Schachner, 2003). Cybulska-Klosowicz et al.

(2004) have recently shown that a specific form of lesion-induced

plasticity, increase in cortical whisker representation after partial

vibrissectomy, is impaired in adult TNC-deficient mice. These findings

indicate that poor recovery of vibrissal function in TNC– ⁄ – mice

observed in our study might also be related to impairments of synaptic

and circuitry remodelling, and function, induced in the CNS by

peripheral nerve injury.

Our previous work has indicated that an important factor limiting

functional recovery after facial nerve injury in the rat is polyneuronal

innervation of muscle fibres (Guntinas-Lichius et al., 2005). These

findings raised the question if different degrees of abnormal endplate

reinnervation could account for the genotype-related differences

observed in this study. We pursued to analyse quantitatively the

pattern of endplate reinnervation of target muscles using postsynaptic

acetylcholine receptor labelling combined with immunohistochemical

staining of presynaptic axon terminals as successfully done for the rat

(Guntinas-Lichius et al., 2005). Extensive pilot experiments showed

that the approach is not applicable for the mouse because of poor

penetration of the antibodies (class III b-tubulin, synaptophysin, SV2,

neurofilaments and PGP tested) into the axons after adequate

formaldehyde (4%) fixation. Application of various procedures for

enhancement of antibody penetration failed to overcome the problem.

Sufficient penetration of the antibodies was achieved after ‘mild’

fixation (1% formaldehyde), but tissue morphology after sectioning

was bad to an extent precluding reliable quantitative analysis. These

observations are consistent with previous experience showing suc-

cessful endplate visualization in ‘mildly’ fixed whole-mount mouse

muscle preparations (see, e.g. Culican et al., 1998).

The finding of better functional recovery in TNR-deficient mice is

not surprising with regard to previous results showing that TNR is

repellent for regenerating optic axons (Becker et al., 2004). Recent

observations have shown that restoration of function after compression

injury of the spinal cord is also better in TNR– ⁄ – compared with

TNR+ ⁄ + mice (Apostolova, Irintchev, Schachner, unpublished obser-

vations). All these findings are in accord with each other and with the

repellent properties of TNR observed in vitro (Pesheva et al., 1993).

However, it is not immediately clear how lack of TNR could influence

nerve regeneration as TNR expression is confined to the CNS

(Pesheva & Probstmeier, 2000). The observation that embryonic and

early postnatal Schwann cells can express TNR in vitro (Probstmeier

et al., 2001) indicates that TNR expression in the peripheral nerve will

need to be re-investigated. If TNR were indeed upregulated after nerve

injury, lack of repulsive effects mediated by TNR in the mutant mouse

would facilitate regeneration. While this possibility remains to be

proven, we have to consider another more plausible explanation. TNR

is present in perineuronal nets around motoneurons and interneurons

in the spinal cord (Murakami & Ohtsuka, 2003). If we assume

repellent functions of TNR, its absence might facilitate synaptic

remodeling, and this might be crucial for synaptic rearrangements after

injury involving the well-known phenomenon of ‘synaptic stripping’

(Moran & Graeber, 2004). In support of this notion is the finding that

TNR expression is transiently downregulated in perineuronal nets after

Tenascin-C, tenascin-R and motor recovery 2177

ª 2005 Federation of European Neuroscience Societies, European Journal of Neuroscience, 22, 2171–2179

nerve repair (Angelov et al., 1998), which can be interpreted as a

physiologically meaningful response to axotomy. Constitutive absence

of TNR may be thus advantageous. The importance of synaptic

remodelling for recovery of function is now well recognized for the

injured spinal cord where most of the recovery achieved after lesion

can be attributed to rewiring of neuronal circuitries (Rossignol et al.,

2004; Ding et al., 2005). Improved recovery from spinal cord injury

after application of chondroitinase ABC (Bradbury et al., 2002) is

possibly also attributable to enzymatic degradation of the perineuronal

nets and concomitant removal of TNR.

In this study we did not observe genotype-related differences in the

numbers of motoneurons that had reinnervated the whisker pads.

Provided we had used only this morphological approach for analysis,

we would have reached the conclusion that TNC and TNR are

dispensable for nerve regeneration. This consideration points out the

importance of using physiological assays in regeneration studies. The

motion analysis approach used in this study and a novel video-based

method for assessment of function after lesions of the femoral nerve

(Irintchev et al., 2005) provide the opportunity for rapid and reliable

estimation of motor function in two valuable injury paradigms

different with regard to connectivity, muscle function and possible

factors influencing the outcome of nerve repair.

In conclusion, TNC and TNR are apparently of functional

significance in determining the outcome of nerve repair. Future

studies on the mechanisms by which these influences are exerted will

provide valuable insights into the molecular mechanisms of repair in

the central and peripheral nervous systems.

Acknowledgements

The authors are grateful to Olga Simova for performing pilot endplate analyses,Alexander G. Nikonenko for providing software, Dirkje Felder and IlonaRohrmann for technical assistance, and Achim Dahlmann for genotyping. Thiswork was supported by grants from the Deutsche Forschungsgemeinschaft toM.S. (SCHA 185 ⁄ 42, SPP 1172 ⁄ 1) and D.N.A. (AN 331 ⁄ 2, AN 331 ⁄ 5), fromthe Koln-Fortune Forschungsprogramm to E.S. and from the Jean-UhrmacherFoundation to O.G.-L.

Abbreviations

EDL, extensor digitorum longus; FFA, facial–facial anastomosis; FG, Fluoro-Gold; NND, nearest-neighbour distance; TNC, tenascin-C; TNR, tenascin-R.

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