These authors contributed equally to this work
The radical response of peripheral nerves to injury (Wallerian degeneration) is the cornerstone of nerve repair. We show that activation of the transcription factor c-Jun in Schwann cells is a global regulator of Wallerian degeneration. c-Jun governs major aspects of the injury response, determines the expression of trophic factors, adhesion molecules, the formation of regeneration tracks and myelin clearance and controls the distinctive regenerative potential of peripheral nerves. A key function of c-Jun is the activation of a repair program in Schwann cells and the creation of a cell specialized to support regeneration. We show that absence of c-Jun results in the formation of a dysfunctional repair cell, striking failure of functional recovery, and neuronal death. We conclude that a single glial transcription factor is essential for restoration of damaged nerves, acting to control the transdifferentiation of myelin and Remak Schwann cells to dedicated repair cells in damaged tissue.
► Schwann cell c-Jun is a master regulator of the PNS injury response ► c-Jun activates a defined repair program in Schwann cells of damaged nerves ► c-Jun controls transdifferentiation of differentiated Schwann cells to repair cells ► Schwann cell c-Jun is essential for neuronal survival and functional recovery
Unlike the central nervous system, injured peripheral nerves regenerate to restore function after injury. Arthur-Farraj et al. show that this repair potential depends on glial (Schwann) cell expression of the transcription factor c-Jun.
Published: August 22, 2012
How transcription factors control cellular plasticity and maintain differentiation is currently of great interest, inspired by the success of experimental reprogramming, where remarkable phenotypic transitions can be induced by enforced expression of fate determining factors (
The striking regenerative capacity of the PNS rests on the surprising plasticity of Schwann cells, and the ability of these cells to switch between differentiation states, a feature that is highly unusual in mammals (
In a process reminiscent of the radical injury responses of zebrafish cardiomyocytes or pigment cells of the newt iris, nerve injury, and loss of axonal contact causes mammalian Schwann cells to lose their differentiated morphology, downregulate myelin genes, upregulate markers of immature Schwann cells, and re-enter the cell cycle. This radical process of natural dedifferentiation has few if any parallels in mammalian systems.
At the same time as Schwann cells dedifferentiate, they upregulate genes implicated in promoting axon growth, neuronal survival, and macrophage invasion, and activate mechanisms to break down their myelin sheaths and transform morphologically into cells with long, parallel processes. This allows them to form uninterrupted regeneration tracks (Bands of Bungner) that guide axons back to their targets (
Little is known about the transcriptional control of changes in adult differentiation states, including natural dedifferentiation and transdifferentiation, in any system (
Here, we use mice with selective inactivation of the transcription factor c-Jun in Schwann cells to show that c-Jun is a global regulator of the Schwann cell injury response that specifies the characteristic gene expression, structure, and function of the denervated Schwann cell, a cell that is essential for nerve repair. Consequently, axonal regeneration and functional repair are strikingly compromised or absent when Schwann cell c-Jun is inactivated. Notably, the effects of c-Jun are injury specific, since c-Jun inactivation has no significant effects on nerve development or adult nerve function.
These observations provide a molecular basis for understanding Schwann cell plasticity, show that c-Jun is a key regulator of Wallerian degeneration, and offer conclusive support for the notion that glial cells control repair in the PNS. They also show that the Schwann cell injury response has much in common with transdifferentiation, since it represents the generation, by dedicated transcriptional controls, of a distinct Schwann cell repair phenotype, specialized for supporting axon growth and neuronal survival in injured nerves. Because these cells form the regeneration tracks called Bungner’s bands, we will refer to them as Bungner cells.
Earlier, we found that neonatal mice with conditional deletion of c-Jun in Schwann cells (c-Jun mutant mice) show delayed loss of myelin proteins and mRNA after nerve injury (
Before injury, the nerves of adult c-Jun mutant mice were normal on the basis of a number of criteria. Thus, the numbers of myelinated and unmyelinated axons (see
The close similarity between WT and mutant nerves was confirmed by the Affymetrix screen (
We selected 32 of the 172 disregulated genes for further analysis by RT-QPCR. In every case this confirmed the disregulation shown by the microarray data (
Lastly, we found that three proteins implicated in regeneration, N-cadherin, p75NTR, and NCAM, were disregulated in cut mutant nerves, although their mRNAs were normally expressed. Injured mutant nerves expressed strongly reduced N-cadherin and p75NTR but elevated levels of NCAM (
Denervated Schwann cells in injured adult nerves are often considered similar to immature Schwann cells in developing nerves. However, the immature cells for instance do not share the axon guidance, myelin breakdown and macrophage recruitment functions of denervated cells, and these cells differ in molecular expression (
Together these results show that c-Jun controls the molecular reprogramming that transforms mature Schwann cells to the denervated cell phenotype following injury. This includes the regulation of genes that differentiate denervated from immature cells and extends to the posttranscriptional control of protein expression.
Denervated Schwann cells form cellular columns that replace the axon-Schwann cell units of intact nerves and serve as substrate for growing axons. We examined these structures by electron microscopy in the distal stump 4 weeks after cut. Because these cells have been without axonal contact for 4 weeks they are comparable to the cells encountered by growing axons in distal parts of crushed nerves in the c-Jun mutant where regeneration is delayed beyond the normal 3–4 week period, while at this time WT nerves have just reached their targets. We found that the structure of these regeneration tracks is strikingly abnormal in c-Jun mutants (
Thus, c-Jun is an cell-intrinsic determinant of Schwann cell morphology that controls the structure of the essential regeneration tracks that guide growing axons back to correct targets.
c-Jun specification of gene expression and morphology of denervated cells suggested that Schwann cell c-Jun might exert a decisive control over nerve repair. Because survival of injured neurons is the basis for repair, we measured the survival of small and large dorsal root sensory (DRG) neurons following sciatic nerve crush at the sciatic notch. We counted axons in L4 dorsal roots (
Axon counts in WT dorsal roots showed that 20%–25% of the unmyelinated axons were lost following crush, as expected (
The number of myelinated axons in dorsal roots remained unchanged in injured WT mice as expected (
Consistent with neuronal death, the number of myelinated axons in the mutant tibial nerve 10 weeks after crush was reduced by about 35% and unmyelinated axons were reduced by about 65%, both compared to crushed WT controls (
These experiments show that without Schwann cell c-Jun, small, unmyelinated DRG neurons are about twice as likely to die following axonal damage. Significantly, about a third of the large, myelinated DRG neurons also die in crushed c-Jun mutants, although none die in injured WT controls, and in other studies these cells are resistant to death following axonal damage (
The number of myelinated axons in ventral roots of both WT and mutant mice remained unchanged following sciatic nerve crush (
To analyze regeneration, we examined sciatic nerves 4 days after crush, using the nerve pinch test and by quantifying the number and length of axons in longitudinal sections immunolabeled by CGRP or galanin antibodies to label regenerating DRG and motoneurons. This showed a strong decrease in axon outgrowth in the mutants compared to WT (
Regeneration in the mouse sciatic nerve is independent of Schwann cell proliferation (
Together this shows that in the absence of Schwann cell c-Jun, the regeneration of axons from surviving neurons is severely reduced, leading to a permanent deficit in the number of neurons that reconnect with denervated targets. The observation that that Schwann cell numbers in regenerating mutant nerves are elevated up to 5-fold compared to uninjured nerves, together with the independence of regeneration from elevated Schwann cell numbers (
A nerve is a complex cell community. We therefore used microfluidic chambers containing neurons and purified Schwann cells to test whether the poor axon growth in mutants was caused by disturbance of direct axon-Schwann cell interactions or whether the effect depended on other cells.
Axon regeneration by axotomized, adult WT DRG neurons was strongly stimulated by control Schwann cells relative to laminin substrate alone, as expected (
These experiments show that injury-activated Schwann cell c-Jun controls direct communication between Schwann cells and growing neurites.
We have shown that c-Jun controls three important functions of denervated Schwann cells, formation of regeneration tracks, support of neuronal survival, and promotion of axon regrowth. A fourth major role classically ascribed to these cells is removal of myelin and associated growth inhibitors, a task they accomplish by breaking down myelin early after injury and indirectly by instructing macrophages to complete myelin clearance (
We found that myelin clearance was substantially delayed in mutants. Four weeks after sciatic nerve transection (without regeneration), the distal stump of WT nerves was translucent, while mutant nerves remained gray/white (
Electron microscopy revealed that although transected mutant nerves did not contain intact myelin, many Schwann cells contained lipid droplets, a late product of myelin breakdown (
These findings show that c-Jun mutant Schwann cells are deficient in their ability to break down myelin.
Surprisingly, abnormalities in myelin breakdown extended to the macrophage compartment, although the macrophages are genetically normal (
Macrophage numbers in the distal stump were strongly elevated in both WT and c-Jun mutant nerves after injury. Three days after cut, their number close to the injury site was significantly higher in WT mice, and a migration assay using Boyden chambers showed that WT nerves attracted more macrophages than mutant nerves (
These results show that injured c-Jun mutant nerves develop substantial problems with myelin clearance. This is evident not only in Schwann cells but also in macrophages, an observation that suggests a role for Schwann cells in the control of macrophage activation and myelin degradation.
Previous sections show that injury-activated Schwann cell c-Jun controls the generation of the denervated Schwann cell, and controls key cellular interactions during Wallerian degeneration and nerve repair. The end result of this process is functional recovery. This is remarkably effective in rodents, where full recovery is seen 3–4 weeks after crush. We found that this essential feature of peripheral nerves was abolished or strikingly compromised in c-Jun mutant mice.
To measure sensory function, we used the following: (1) toe pinching (pressure), (2) Von Frey hairs (light touch), and (3) the Hargreaves test (temperature).
In WT and c-Jun mutants, sciatic nerve crush abolished the response to toe pinching and decreased the responses to light touch and heat to the minimum measurable by these assays. Control mice recovered in 3–4 weeks as expected, using the toe-pinching test. c-Jun mutants, however, showed minimal recovery, even after extensive (up to 70 day) periods (
To measure motor function we used the toe-spreading reflex. We found that toe extension, abolished by nerve crush, recovered on schedule in WT controls but failed to recover even at 70 days in mutants (
To measure sensory motor coordination we measured the sciatic functional index (SFI;
Having shown that c-Jun activation is necessary for the conversion of injured nerves to an environment that supports repair, we tested whether c-Jun activation alone was sufficient for this critical transformation.
We took advantage of
This shows that c-Jun is not only necessary but also sufficient for the generation of a growth supporting environment in injured nerves. This observation also confirms that regeneration failure in
Identification of transcription factors that define cell type, control transit between differentiation states, and enable tissues to repair is a central issue in regenerative biology. The response of Schwann cells to injury provides an exceptionally striking example of a phenotypic transition by adult, differentiated cells. This process is also the basis for the singular regenerative power of peripheral nerves. We show that the Schwann cell injury response represents a c-Jun dependent natural reprogramming of differentiated Schwann cells to generate the repair cell, a distinct Schwann cell state (Bungner cell, since they form Bungner’s bands) specialized to promote regeneration. In mice without c-Jun in Schwann cells, activation of the repair program fails. The disregulated repair cell formed in these mutants is unable to support normal axonal regeneration, neuronal survival, myelin clearance, and macrophage activity. The result is a striking failure of functional recovery.
In addition to the c-Jun mutant,
It is important to note that although the Bungner cells generated in the mutants are dysfunctional, other Schwann cell functions are normal. Thus, mutant cells remyelinated those axons that regenerated, Schwann cell development appeared normal, and Schwann cells and nerve function in uninjured adults were normal. Although 172 genes were disregulated in the distal stump of the c-Jun mutants, the large majority of the ∼4,000 genes regulated in injured WT nerves remained normally regulated. Therefore, the absence of c-Jun does not have a general impact on the Schwann cell phenotype. Instead, c-Jun appears to have a specific function in adult cells, where it is required for activation of the repair program and timely suppression of the myelin program.
The Schwann cell response to injury is commonly referred to as dedifferentiation, implying that adult denervated cells revert to an earlier stage resembling the immature Schwann cells of perinatal nerves (
In injured nerves, myelinating Schwann cells, that are specialized to support fast conduction of action potentials, transform to Bungner cells that are specialized for the unrelated task of organizing nerve repair. This represents an unambiguous change of function, brought about by the combination of dedifferentiation and activation of an alternative differentiation program, the c-Jun dependent Schwann cell repair program. Transitions that share this set of features have been described in other systems, where they are generally referred to as transdifferentiation (
The regeneration defects in the c-Jun mutant are substantially more severe than those reported for other mouse mutants, in spite of the fact that the genetic defect is restricted to Schwann cells. The likely reason is the number and diversity of the molecules controlled by this single transcription factor. Among the 172 molecules that are abnormally expressed in the mutant are growth factors, adhesion molecules, growth-associated proteins, and transcription factors. This allows c-Jun to integrate a broad collection of functions that support nerve regeneration, and therefore to act as a global regulator of the Schwann cell repair program. This program involves regulation of molecules that have been directly implicated in repair such as the surface proteins N-cadherin, p75NTR, and NCAM, and the signaling molecules GDNF, artemin, sonic hedgehog, and BDNF. It also includes the morphogenetic processes that change myelinating cells to process bearing cells forming regeneration tracks, and the conversion of Schwann cells to cells equipped to rapidly clear myelin from injured nerves (
The exceptional repair potential of peripheral nerves is likely due to the coordinated functions of the repair program. Yet individual factors can also be presumed to play a prominent role, as exemplified by the enhanced regeneration seen when GDNF and artemin levels are increased in c-Jun mutant facial nerves (
c-Jun is absent from Schwann cell precursors, expressed in immature cells in vivo and in cultured Schwann cells, suppressed by Krox-20 on myelination, but rapidly re-expressed at high levels in Schwann cells of injured nerves (
Among potential intracellular activators of c-Jun is the AP-1 transcription complex, of which c-Jun is a key component. AP-1 activity, in turn, is controlled by numerous signals, including the major MAPK pathways Erk1/2, JNK, and p38. These are all activated in injured nerves and therefore potential upstream regulators of c-Jun (
We described previously that c-Jun shows cross-inhibitory interactions with the pro-myelin transcription factor Krox20 (
The long term persistence of Schwann cell lipid droplets and large multivacuolated (foamy) macrophages in transected mutant nerves suggests problems with lipid clearance and macrophage activation and exit. Recent evidence indicates that failure of lipid breakdown may delay regeneration (
The reduced macrophage numbers in the mutant early after injury is unlikely to contribute substantially to the regeneration problems, a conclusion supported by the microfluidic chamber experiments, where axon growth fails in the presence of mutant Schwann cells, even in the absence of macrophages. Even severe depletion of invading macrophages has no effect on the number of myelinated axons in dorsal roots following nerve injury (
The disregulated mutant Bungner cell not only fails to support axon regeneration, but also fails to rescue injured neurons from death. In the mutants, injured type B DRG neurons are about twice as likely to die as in WT mice. Even more notable is the death of about a third of type A neurons, because we find no death of these cells in WT animals, in agreement with previous work in mice and other species (
The observation that denervated adult Schwann cells acquire the ability to generate melanocytes, a property of Schwann cell precursors but not of immature Schwann cells (
c-Jun is not significantly expressed in Schwann cell precursors (D.K.W., unpublished). It is therefore possible that the unique identity of the Bungner repair cell in adult nerves consists of a c-Jun-activated repair program in a cell that in significant other aspects has dedifferentiated more completely than hitherto envisaged.
It is clear that the transdifferentiation of myelinating cells to Bungner cells is central to nerve repair. But much remains to be learned about the twin components of this process, the dedifferentiation and repair programs, and about the molecular links that integrate them. This includes issues of practical importance such as the identification of methods to sustain expression of the repair program over the long periods required for nerve repair in humans, and the question of whether the repair program can be activated in other glial cells.
Animal experiments conformed to UK Home Office guidelines.
Sciatic nerves of adult mice were cut or crushed at the sciatic notch.
RNA was extracted, cDNA generated and applied to Mouse 430 2.0 array (Affymetrix, MA). Significantly different genes were selected with Bayes’ t test. After control for false discovery rate, genes with a p value of less than 0.05 were filtered out. The microarray data are MIAME compliant.
This was performed as described (
QPCR was performed with Sybrgreen SYBR Green JumpStart (Sigma) and carried out using Chromo4 Real Time Detector (Bio-Rad). For primers see
Adenovirus expressing
Nerve segments, spinal cords or Schwann cell cultures were fixed in paraformaldehyde (PF)/PBS for 10 min–2 hr. Sections were fixed in 2% or 4% PF/PBS for 10 min or methanol for 30 min prior to immunolabeling. Alternatively, nerves were fixed in PF/PBS for 24 hr and wax embedded. Four micrometer sections were deparaffinized and antigen retrieved prior to immunolabeling. Blocking solution was used before incubation with primary antibodies overnight followed by secondary antibodies for 30 min to 1 hr. The first layer was omitted as a control.
The nerve pinch test was used to assess axonal regeneration distance in vivo. Sensory motor coordination was assessed using mouse footprints to calculate the sciatic functional index. Sensory function was assessed by Von Frey Hair analysis, the Hargreaves test and response to toe pinching. Motor function was analyzed by observing toe spread (see
True Blue (2 μl) was injected into the tibialis anterior muscle at three sites to label motor neurons in spinal cord segments L2 to L6. Seven days later, mice were perfused. Serial 30 μm sections were collected and the number of labeled neurons was counted (
The L4 DRG was cryosectioned. DRG neurons (nuclei) were counted as described (
Following PF fixation, 10 μm sections were treated with 2% OsO4-PBS solution overnight. Percentage stained nerve area relative to that in uninjured nerves was quantified using NIH ImageJ.
Frozen nerve samples or cell lysates were blotted as described (
Using a three-compartment microfuidic chamber (
Photographs (15–20) were taken at 2500× of transverse ultrathin sections of tibial nerve 5 mm from the sciatic notch. The g ratio was calculated for each myelinated fiber (axon diameter divided by diameter of the axon and myelin sheath). Statistical difference was measured using Mann Whitney U test.
A montage of ultrathin sections (×1000) was made and the number of myelinated fibers counted. For unmyelinated fibers, 30%–40% of each nerve was photographed (×5000). The ratio myelinated:unmyelinated fibers was measured and the total for each nerve/root was multiplied by total myelin fiber count, as described elsewhere (
Macrophage migration was assessed using 6.5 mm Transwells with 5 μm pores (Corning Costar; see
Data are presented as arithmetic mean ± standard error of the mean (SEM) unless otherwise stated. Statistical significance was estimated by Student’s t test, two-way ANOVA, or Mann-Whitney U-test.
The 172 genes that are disregulated in the distal stump of c-Jun mutant nerves (cKO) compared to the distal stump of WT nerves (cKO/WT). The data were obtained by microarray 7 days after transection without regeneration. Red type indicates genes expressed at higher levels in cut c-Jun mutant nerves compared to cut WT nerves (66 genes), while blue type indicates those genes that are expressed at lower levels in cut c-Jun mutant nerves compared to cut WT nerves (106 genes).
QT-RT-QPCR analysis of the regulation of the gene subset shown in Figure 1D. The table shows expression levels in cut relative to uncut nerves in mutant and WT mice and shows statistical significance of each comparison. Error bars ±SEM; (n = 3–6 pools of animals with 3 animals in each pool; see figure legend).
This work was funded by Wellcome Trust Program and project grants to K.R.J. and R. Mirsky and an MRC project grant to K.R.J. and R. Mirsky. The research leading to these results has received funding from the European Community’s Seventh Framework Program (FP7/2007-2013) under grant agreement No. HEALTH-F2-2008-201535. P.J.A.-F. and B.J. were funded by PhD studentships from the MRC and Wellcome Trust, respectively. A.W. is supported by a Ramon y Cajal fellowship from the Spanish Ministry of Science and a PFIS grant (PS09/00094) from the Instituto Carlos III. The London Research Institute (A. Behrens) is funded by Cancer Research UK. G.R is supported by BBSRC grants (S20299, B/D009537/1) and Wellcome Trust grant (WT088646MA). We thank P.N. Anderson for help with in vivo injections of
Supplemental Information includes six figures, five tables, and Supplemental Experimental Procedures and can be found with this article online at
c-Jun Controls the Molecular Reprogramming of Schwann Cells in Injured Nerves
(A) The number of genes in wild-type (WT) and mutant (cKO) nerves that show significant change in expression levels 7 days post-nerve cut, determined by microarray. The number of up- and downregulated genes is also indicated. Only genes that showed ≥1.5-fold change compared to uninjured nerves were considered. The gene screen data are an average of two independent experiments, each involving nerves pooled from 7–8 animals.
(B) Heatmap showing expression of the 172 genes differentially regulated (≥1.5-fold threshold; microarray) in the distal stump of cut WT and c-Jun mutant mice. Comparison of cut WT and cut mutant nerves shows four main types of disregulation in c-Jun mutants (categories 1–4 indicated on the heatmap): (1) enhanced activation, (2) failure of activation, (3) enhanced downregulation, (4) failure of downregulation.
(C) Enriched gene ontology (GO) categories for the 172 differentially regulated genes.
(D) Subset of the 172 differentially regulated genes chosen for further analysis. This includes the 10 most upregulated and 10 most downregulated genes comparing the distal stump of WT and mutants, and 14 other genes chosen for potential relevance in nerve injury. Genes expressed at higher levels in the distal stump of mutants versus WT are highlighted in red. Genes expressed at lower levels in mutants versus WT are highlighted in blue. FC (cKO/WT) indicates fold change in expression levels in mutant versus WT distal stumps determined by microarray.
(E) Genes overexpressed in the mutant: RT-QPCR determination of genes from (D) (red upper panel) showing fold increase in expression in the distal stump of mutants versus WT. Error bars: ± SEM; n = 3–6 pools of 3 animals/pool.
(F) Genes underexpressed in mutants: RT-QPCR determination of genes from (D) (blue lower panel) showing fold decrease in expression in the distal stumps of mutants versus WT. Error bars: ± SEM; n = 3–6 pools of 3 animals/pool.
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c-Jun-Dependent Regulation of Schwann Cell Genes and Proteins
(A) In purified Schwann cell cultures, c-Jun suppresses myelin genes but activates genes of denervated cells. Graphs show RT-QPCR measurements of six differentially regulated genes in cells from control mice with physiological levels of c-Jun (WT), in cells from mutants lacking c-Jun (cKO), and in mutant cells infected with c-Jun adenovirus to re-express c-Jun (cKO + c-Jun). The y axis shows fold difference in expression levels. Error bars: ± SEM; ∗p < 0.05, n = 4.
(B and C) Posttranscriptional control of protein expression by c-Jun. (B) Immunolabeling of Schwann cell cultures (5 days in vitro) from p8 WT and mutant nerves. Three proteins characteristically expressed by denervated Schwann cells are shown. Note suppression of N-cadherin (N-cad) and p75NTR but overexpression of NCAM in mutant cells. (Bar: 50 μm). (C) Western blots of distal stump nerve extracts from WT and mutants (cKO) 7 days after cut show a similar expression pattern. Lower panel: quantitation. Error bars: ± SEM; ∗p < 0.05; ∗∗∗p < 0.001, n = 3.
(D and E) Immature and denervated cells differ in gene expression. (D) In situ hybridization using probes against
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c-Jun Controls the Structure of Denervated Schwann Cells In Vivo and in Culture
(A) Electron micrographs of distal stumps of WT and c-Jun mutant (cKO) sciatic nerve 28 days after transection (no regeneration). The WT nerve contains classic regeneration tracks (Bands of Bungner; an example is bracketed, showing several Schwann cell processes within a basal lamina tube). These do not form in the mutant, which instead contains irregular and flattened cellular profiles. Bar: 1 μm.
(B) The number of cellular profiles per regeneration track is sharply reduced in the mutant. Error bars: ± SEM; p < 0.05, n = 4.
(C) Mutant cellular profiles are flatter (lower roundness index). Error bars: ± SEM; p < 0.05, n = 4.
(D and E) In vitro, WT Schwann cells from neonatal nerves show typical bi- or tripolar morphology, but mutant cells (cKO) are flat, irrespective of whether they are taken from c-Jun mutants (D) or obtained by infecting c-Junf/f cells with CRE-adenovirus (E). Cells are labeled with β1 integrin antibodies. Bar: 20 μm.
Axonal Injury Results in Extensive Neuron Death in c-Jun Mutants
(A and B) Number of unmyelinated axons in L4 dorsal roots (A) and number of small neuronal cell body profiles (B cells) in corresponding DRGs (B), expressed relative to number of B cells in uninjured WT mice. The data show before (0) and at different times after injury. Error bars: ± SEM; ∗p < 0.05, n = 4).
(C and D) Number of myelinated axons in L4 dorsal roots (C) and large neuronal cell body profiles (A cells) in corresponding DRGs (D), expressed relative to the number of A cells in uninjured WT mice. The data show before (0) and at different times after injury. Error bars: ± SEM; ∗p < 0.05, n = 4.
(E and F) Number of myelinated (E) and unmyelinated (F) axons in tibial nerves (midthigh level) before injury and 10 weeks postcrush in WT and mutant mice. Error bars: ± SEM; ∗p < 0.05, n = 4. Note that axon and neuron numbers are normal in c-Jun mutants before injury, but significantly reduced post nerve crush.
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Axonal Regeneration Failure in c-Jun Mutants
(A) A reduced number of backfilled motoneurons is seen in the ventral horn of c-Jun mutants after True Blue injection into the tibialis anterior muscle 10 weeks post-nerve crush. Bar: 100 μm.
(B) Quantification of backfilled neurons 5 and 10 weeks after crush. Abercrombie correction applied. Note reduced number of labeled neurons in the L2–L6 region of the spinal cord. Error bars: ± SEM; ∗∗p < 0.01, n = 4.
(C) Regeneration failure in mutants judged by nerve pinch test 4 days post sciatic nerve crush. Error bar: ± SEM; n = 4.
(D–H) CGRP+ or galanin+ regenerating axon numbers are reduced in mutants 4 days postcrush. Micrograph (G) shows CGRP labeling of nerve fronts (arrows; bar: 1 mm) in WT and mutant nerves; (H) shows axons 3 mm from the crush site (bar: 100 μm). Regeneration delay is quantified by measuring the distance from crush traveled by the longest axon (D), or by counting how many axons extend 2 or 3 mm from the crush site (E and F). For (D) ∗p < 0.05; ∗∗p < 0.01, n = 4; for (E) and (F) ∗∗p < 0.01, n = 4.
(I) In microfluidic chambers, WT Schwann cells and mutant cells with enforced c-Jun expression promote axon growth relative to no cells or mutant cells. Each trace shows axon growth into a side chamber from a central compartment containing neuronal cell bodies. The four types of side chamber are as follows: (no SC) chamber with no Schwann cells; (WT SC) chamber with WT Schwann cells with normal constitutive c-Jun expression; (cKO SC) chamber with cells from c-Jun mutants (no c-Jun), and (cKO SC + c-Jun) chamber with mutant cells infected with c-Jun adenovirus to re-express c-Jun. Bar: 250 μm.
(J) Quantification of the number of axons longer than 50 μm growing into the side compartment in all conditions shown in (I).
(K) Quantification of the total area covered by axons in the side compartment in all conditions depicted in (I). Error bars: ± SEM; ∗∗p < 0.01, n = 3.
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Myelin Clearance Is Slow in Injured c-Jun Mutant Nerves
(A) Unlike WT nerves, mutant nerves (cKO) are not translucent 4 weeks postcut, indicating lipid retention. Bar: 2 mm.
(B) Lipid persistence in mutant nerves shown by quantification of osmium stained area in transverse sections of sciatic nerve 4 weeks post cut (no regeneration). Error bars: ± SEM; p < 0.001, n = 4.
(C) Electron micrograph showing lipid droplets in denervated Schwann cells of mutant nerves 4 weeks post cut. Bar: 2 μm.
(D) Electron micrograph showing relative preservation of intact myelin sheaths in mutants 3 days postcut, 3 mm from cut site. Right, counts of intact sheaths in tibial nerves of WT and mutants 3 days after cut. Error bars: ± SEM; p < 0.05, n = 5. Bar: 20 μm.
(E) Myelin sheath counts in tibial nerves following axotomy in vivo and in nerve segments in vitro, as indicated. Note that sheath preservation in the mutant does not depend on blood-born macrophages. Error bars: ± SEM; p < 0.05, n = 4.
(F) Normal myelin breakdown fails in mutant Schwann cells, shown by persistence of myelin debris in mutant cells in phase-contrast micrographs of cultures from p8 WT and mutant nerves and maintained 6 days in vitro. Bar: 50 μm.
(G) Immunolabeling of mutant Schwann cells (green; S100 antibodies), bloated with myelin debris (red; MBP antibodies) (arrows indicate two cells). Bar: 50 μm.
(H) Counts of cells containing the myelin proteins MPZ and MBP at different times after plating cells from WT and mutant p8 nerves show delay in myelin protein clearance by mutant Schwann cells. 0 = 3 hr after plating. Differences between cKO and WT were significant at all time points. Error bars: ±SEM; p < 0.001, (two way ANOVA), n = 5.
(I) Electron micrograph shows the persistence in mutant nerves of large, foamy macrophages (example arrowed). Bar: 5 μm.
(J) Lipid droplet counts in macrophages in tibial nerve sections 28 days postcut (no regeneration), show a strong delay in lipid clearance by macrophages in the mutant. Error bars: ±SEM; p < 0.05, n = 4.
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Functional Recovery Fails in c-Jun Mutants
(A) Percentage of mice responding to pinching of distal parts of toes 3, 4, and 5 after nerve crush. The difference between WT and mutant (cKO) mice was significant from day 21 to day 70, p < 0.01 (two-way ANOVA), n = 5.
(B and C) Sensitivity to heat (B), and light touch (C) quantified in mice before and after nerve crush. In animals assayed 7 days after crush, the assay was terminated at 20 s (Hargreaves test) and limited to the use of a hair weight of 8 g (Von Frey test). Mutants show normal sensation when uninjured, but no recovery after injury. Error bars: ±SEM; ∗p < 0.05, n = 4.
(D) Percentage of mice showing normal toe spreading reflex (score 0: no toe extension; score 2: full normal extension) after crush. The difference between WT and mutant mice was significant from day 12 to day 70. p < 0.001 (two-way ANOVA), n = 5.
(E) Quantification of sensory-motor function in WT and mutants. Note permanent failure of recovery in mutants, although mutant and WT SFIs are similar before and immediately after injury. The difference between WT and mutant mice was significant from day 12 to day 72. Error bars: ± SEM; p < 0.01, n = 4.
(F) Increased regeneration in
(G and H) Regeneration failure in
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