Identification of hair shaft progenitors that create a niche for hair pigmentation.

Hair differentiates from follicle stem cells through progenitor cells in the matrix. In contrast to stem cells in the bulge, the identities of the progenitors and the mechanisms by which they regulate hair shaft components are poorly understood. Hair is also pigmented by melanocytes in the follicle. However, the niche that regulates follicular melanocytes is not well characterized. Here, we report the identification of hair shaft progenitors in the matrix that are differentiated from follicular epithelial cells expressing transcription factor KROX20. Depletion of Krox20 lineage cells results in arrest of hair growth, confirming the critical role of KROX20+ cells as antecedents of structural cells found in hair. Expression of stem cell factor (SCF) by these cells is necessary for the maintenance of differentiated melanocytes and for hair pigmentation. Our findings reveal the identities of hair matrix progenitors that regulate hair growth and pigmentation, partly by creating an SCF-dependent niche for follicular melanocytes.

Identifying the niche controlling melanocyte differentiation.

Melanocytes present in hair follicles are responsible for their pigmentation. Melanocyte differentiation and hair pigmentation depend on the stem cell factor (SCF)/c-Kit signaling pathway, but the niche that regulates melanocyte differentiation is not well characterized. In this issue of Genes & Development, Liao and colleagues (pp. 744-756) identify Krox20+-derived cells of the hair shaft as the niche and the essential source of SCF required for melanocyte maturation. This study delineates the niche factors regulating melanocyte differentiation and hair pigmentation and opens up new avenues to further characterize the cross-talk between the hair follicle and melanocytes that controls melanocyte maintenance and differentiation.

Loss of prohibitin 2 in Schwann cells dysregulates key transcription factors controlling developmental myelination.

Schwann cells are critical for the proper development and function of the peripheral nervous system (PNS), where they form a collaborative relationship with axons. Past studies highlighted that a pair of proteins called the prohibitins play major roles in Schwann cell biology. Prohibitins are ubiquitously expressed and versatile proteins. We have previously shown that while prohibitins play a crucial role in Schwann cell mitochondria for long-term myelin maintenance and axon health, they may also be present at the Schwann cell-axon interface during development. Here, we expand on this, showing that drug-mediated modulation of prohibitins in vitro disrupts myelination and confirming that Schwann cell-specific ablation of prohibitin 2 (Phb2) in vivo results in severe defects in radial sorting and myelination. We show in vivo that Phb2-null Schwann cells cannot effectively proliferate and the transcription factors EGR2 (KROX20), POU3F1 (OCT6), and POU3F2 (BRN2), necessary for proper Schwann cell maturation, are dysregulated. Schwann cell-specific deletion of Jun, a transcription factor associated with negative regulation of myelination, confers partial rescue of the developmental defect seen in mice lacking Schwann cell Phb2. Finally, we identify a pool of candidate PHB2 interactors that change their interaction with PHB2 depending on neuronal signals, and thus are potential mediators of PHB2-associated developmental defects. This work develops our understanding of Schwann cell biology, revealing that Phb2 may modulate the timely expression of transcription factors necessary for proper PNS development, and proposing candidates that may play a role in PHB2-mediated integration of axon signals in the Schwann cell.

COUP-TFII plays a role in cAMP-induced Schwann cell differentiation and in vitro myelination by up-regulating Krox20.

Schwann cells (SCs) are known to produce myelin for saltatory nerve conduction in the peripheral nervous system (PNS). Schwann cell differentiation and myelination processes are controlled by several transcription factors including Sox10, Oct6/Pou3f1, and Krox20/Egr2. Chicken ovalbumin upstream promoter-transcription factor II (COUP-TFII/NR2F2) is an orphan receptor that plays a role in the development and differentiation. However, the role of COUP-TFII in the transcriptional regulatory network of SC differentiation has not been fully identified yet. Thus, the objective of this study was to investigate the role and molecular hierarchy of COUP-TFII during cAMP-induced SC differentiation. Our results showed that dibutyryl-cAMP (db-cAMP) increased expression levels of COUP-TFII along with the expressions of Oct6, Krox20, and myelin-related genes known to be related to SC differentiation. Our mechanistic studies showed that COUP-TFII acted downstream of Hsp90/ErbB2/Gab1/ERK-AKT pathway during db-cAMP-induced SC differentiation. In addition, we found that COUP-TFII induced Krox20 expression by directly binding to Krox20-MSE8 as revealed by chromatin immunoprecipitation assay and promoter activity assay. In line with this, the expression of COUP-TFII was increased before up-regulation of Oct6, Krox20, and myelin-related genes in the sciatic nerves during early postnatal myelination period. Finally, COUP-TFII knockdown by COUP-TFII siRNA or via AAV-COUP-TFII shRNA in SCs inhibited db-cAMP-induced SC differentiation and in vitro myelination of sensory axons, respectively. Taken together, these findings indicate that COUP-TFII might be involved in postnatal myelination through induction of Krox20 in SCs. Our results present a new insight into the transcriptional regulatory mechanism in SC differentiation and myelination.

Krox20 defines a subpopulation of cardiac neural crest cells contributing to arterial valves and bicuspid aortic valve.

Although cardiac neural crest cells are required at early stages of arterial valve development, their contribution during valvular leaflet maturation remains poorly understood. Here, we show in mouse that neural crest cells from pre-otic and post-otic regions make distinct contributions to the arterial valve leaflets. Genetic fate-mapping analysis of Krox20-expressing neural crest cells shows a large contribution to the borders and the interleaflet triangles of the arterial valves. Loss of Krox20 function results in hyperplastic aortic valve and partially penetrant bicuspid aortic valve formation. Similar defects are observed in neural crest Krox20-deficient embryos. Genetic lineage tracing in Krox20-/- mutant mice shows that endothelial-derived cells are normal, whereas neural crest-derived cells are abnormally increased in number and misplaced in the valve leaflets. In contrast, genetic ablation of Krox20-expressing cells is not sufficient to cause an aortic valve defect, suggesting that adjacent cells can compensate this depletion. Our findings demonstrate a crucial role for Krox20 in arterial valve development and reveal that an excess of neural crest cells may be associated with bicuspid aortic valve.

Effects of Theranekron and alpha-lipoic acid combined treatment on GAP-43 and Krox-20 gene expressions and inflammation markers in peripheral nerve injury.

Peripheral nerve injury (PNI) is a major health problem that results in loss of motor and sensory functions. In treatment of PNI, various methods such as anastomosis, nerve grafts, nonneural tissue grafts, and nerve conduits are applied. In the present study, it was aimed to investigate the effects of Theranekron and Alpha-lipoic acid (ALA) combined treatment on nerve healing in experimental PNI by using histomorphometric, electron microscopic, immunohistochemical and molecular biological methods. Sixty-two Wistar rats were divided into six groups; the normal control group, sham operation group, experimental control group having a crush type injury with no treatment, Theranekron treatment group, ALA treatment group and Theranekron+ALA combined treatment group. Sciatic nerve tissue samples were obtained on days 1, 7 and 14 following injury in all groups. GAP-43 expression was upregulated in all PNI received groups compared to the control group. Krox-20 expression was downregulated in all groups that received PNI compared to the control group. While intensely positive TNF-α and IL-6 expressions were observed up to the 1st to the 14th day for the experimental control group, these expressions were seen as "weakly positive" in the treatment groups from the 1st day to the 14th day. The number of myelinated fibers was higher in the control and sham operation groups. Additionally, the number of myelinated nerve fibers increased in the combined treatment group. In conclusion, these findings suggest that combined therapy of Theranekron and ALA promotes structural recovery and it should be considered as an effective treatment protocol following PNI.

Molecular dissection of segment formation in the developing hindbrain.

Although many components of the genetic pathways that provide positional information during embryogenesis have been identified, it remains unclear how these signals are integrated to specify discrete tissue territories. Here, we investigate the molecular mechanisms underlying the formation of one of the hindbrain segments, rhombomere (r) 3, specified by the expression of the gene krox20. Dissecting krox20 transcriptional regulation has identified several input pathways: Hox paralogous 1 (PG1) factors, which both directly activate krox20 and indirectly repress it via Nlz factors, and the molecular components of an Fgf-dependent effector pathway. These different inputs are channelled through a single initiator enhancer element to shape krox20 initial transcriptional response: Hox PG1 and Nlz factors define the anterior-posterior extent of the enhancer's domain of activity, whereas Fgf signalling modulates the magnitude of activity in a spatially uniform manner. Final positioning of r3 boundaries requires interpretation of this initial pattern by a krox20 positive-feedback loop, orchestrated by another enhancer. Overall, this study shows how positional information provided by different patterning mechanisms is integrated through a gene regulatory network involving two cis-acting elements operating on the same gene, thus offering a comprehensive view of the delimitation of a territory.

Human epidermal neural crest stem cells as a source of Schwann cells.

We show that highly pure populations of human Schwann cells can be derived rapidly and in a straightforward way, without the need for genetic manipulation, from human epidermal neural crest stem cells [hEPI-NCSC(s)] present in the bulge of hair follicles. These human Schwann cells promise to be a useful tool for cell-based therapies, disease modelling and drug discovery. Schwann cells are glia that support axons of peripheral nerves and are direct descendants of the embryonic neural crest. Peripheral nerves are damaged in various conditions, including through trauma or tumour-related surgery, and Schwann cells are required for their repair and regeneration. Schwann cells also promise to be useful for treating spinal cord injuries. Ex vivo expansion of hEPI-NCSC isolated from hair bulge explants, manipulating the WNT, sonic hedgehog and TGFß signalling pathways, and exposure of the cells to pertinent growth factors led to the expression of the Schwann cell markers SOX10, KROX20 (EGR2), p75NTR (NGFR), MBP and S100B by day 4 in virtually all cells, and maturation was completed by 2 weeks of differentiation. Gene expression profiling demonstrated expression of transcripts for neurotrophic and angiogenic factors, as well as JUN, all of which are essential for nerve regeneration. Co-culture of hEPI-NCSC-derived human Schwann cells with rodent dorsal root ganglia showed interaction of the Schwann cells with axons, providing evidence of Schwann cell functionality. We conclude that hEPI-NCSCs are a biologically relevant source for generating large and highly pure populations of human Schwann cells.

EGR2 is a gonadotropin-induced survival factor that controls the expression of IER3 in ovarian granulosa cells.

Pituitary gonadotropins are key hormones that orchestrate the growth and development of ovarian follicles. However, limited information is available on intra-ovarian factors that mediate the actions of gonadotropins. In this study, we identified that the early growth response 2 gene (EGR2) is a gonadotropin-inducible gene in granulosa cells of rats and humans. Analysis of consensus EGR-binding elements (EBEs) showed that the immediate early response 3 gene (IER3) is a novel transcriptional target gene of EGR2 as confirmed by the luciferase assay, electrophoretic mobility-shift assay (EMSA), chromatin immunoprecipitation (ChIP), and western blot analysis. Overexpression of EGR2 promoted survival of KGN human granulosa-derived cells in which IER3 acts as a mediator; knockdown of EGR2 induced death in KGN cells. Additionally, EGR2 was found to regulate the expression of myeloid cell leukemia 1 (MCL-1), which belongs to the BCL-2 family of proteins regulating cell survival. Thus, this study identified a novel signaling axis, comprised of gonadotropins-EGR2-IER3, which is important for the survival of granulosa cells during folliculogenesis.

Nonuniform molecular features of myelinating Schwann cells in models for CMT1: distinct disease patterns are associated with NCAM and c-Jun upregulation.

We investigated three models for Charcot-Marie-Tooth type 1 (CMT1) neuropathy, comprising mice lacking connexin 32 (Cx32def), mice with reduced myelin protein zero (P0) expression (P0het) and transgenic mouse mutants overexpressing peripheral myelin protein 22 (PMP22tg), with regard of the expression of the developmentally regulated molecules NCAM, L1, the low-affinity NGF-receptor p75 (p75(NTR) ) and the transcription factor component c-Jun. We found that all molecules were uniformly expressed by myelin deficient and supernumerary Schwann cells. The mutant myelinating Schwann cells of PMP22tg mice showed a robust NCAM-immunoreactivity in Schmidt-Lanterman incisures (SLI) that accompanies other early onset abnormalities, such as the presence of supernumerary Schwann cells and impaired myelin formation in some fibers. In line with this, Cx32def and P0het mice, which represent demyelinating models, only rarely express NCAM in SLI. Surprisingly, c-Jun immunoreactivity displayed a mosaic-like pattern with mostly negative and some weakly or moderately positive nuclei both in myelinating Schwann cells and Remak cells of wildtype (wt), P0het and PMP22tg mice. However, c-Jun expression was substantially upregulated in myelinating Schwann cells of Cx32def mice and spatially associated with axon perturbation, a typical predemyelinating feature of Cx32 deficiency. Additionally, c-Jun upregulation was correlated with an elevated level of GDNF, possibly causally linked to the typical compensatory sprouting of axons in Cx32def mice and CMT1X patients. Our findings suggest that in myelinating Schwann cells of distinct models of CMT1, c-Jun upregulation is a marker for predemyelinating axonal perturbation while myelin-related NCAM expression is indicative for early Schwann cell abnormalities.

SP1 and KROX20 Regulate the Proliferation of Dermal Papilla Cells and Target the CUX1 Gene.

Previous studies have demonstrated that CUX1 could contribute to the proliferation of DPCs in vitro, but the upstream transcriptional regulatory mechanisms of CUX1 remain largely unknown. This study aimed to investigate the upstream transcriptional regulators of CUX1 to enhance our comprehension of the mechanism of action of the CUX1 gene in ovine DPCs. Initially, the JASPAR (2024) software was used to predict the upstream target transcription factors for the CUX1 gene. Subsequently, through RT-qPCR and a double luciferase reporter assay, the interaction between SP1, KROX20, and CUX1 was established, respectively. The results indicated that SP1 and KROX20 were two highly reliable upstream transcription regulators for the CUX1 gene. Additionally, we found that SP1 promoted the proliferation of DPCs by overexpressing SP1 in DPCs, and KROX20 inhibited the proliferation of DPCs by overexpressing KROX20 in DPCs. These findings are also consistent with the transcriptional regulation of CUX1 by SP1 and KROX20, respectively. This study suggests that the effect of DPC proliferation in vitro by CUX1 may regulated by the transcription factors SP1 and KROX20.

Loss of prohibitin 2 in Schwann cells dysregulates key transcription factors controlling developmental myelination.

Schwann cells are critical for the proper development and function of the peripheral nervous system, where they form a mutually beneficial relationship with axons. Past studies have highlighted that a pair of proteins called the prohibitins play major roles in Schwann cell biology. Prohibitins are ubiquitously expressed and versatile proteins. We have previously shown that while prohibitins play a crucial role in Schwann cell mitochondria for long-term myelin maintenance and axon health, they may also be present at the Schwann cell-axon interface during development. Here, we expand on this work, showing that drug-mediated modulation of prohibitins in vitro disrupts myelination and confirming that Schwann cell-specific ablation of prohibitin 2 (Phb2) in vivo results in early and severe defects in peripheral nerve development. Using a proteomic approach in vitro, we identify a pool of candidate PHB2 interactors that change their interaction with PHB2 depending on the presence of axonal signals. Furthermore, we show in vivo that loss of Phb2 in mouse Schwann cells causes ineffective proliferation and dysregulation of transcription factors EGR2 (KROX20), POU3F1 (OCT6) and POU3F2 (BRN2) that are necessary for proper Schwann cell maturation. Schwann cell-specific deletion of Jun, a transcription factor associated with negative regulation of myelination, confers partial rescue of the development defect seen in mice lacking Schwann cell Phb2. This work develops our understanding of Schwann cell biology, revealing that Phb2 may directly or indirectly modulate the timely expression of transcription factors necessary for proper peripheral nervous system development, and proposing candidates that may play a role in PHB2-mediated integration of axon signals in the Schwann cell.

Krox20 expression in abnormal scars: An immunohistochemical study.

BACKGROUND: Scars are the end outcome of healing. They are grouped into several types, the common of which are keloids, hypertrophic, and atrophic scars. The role of Krox20 in skin and hair physiology and pathology had emerged. Overexpression of Krox20 was sufficient to stimulate collagen gene expression and myofibroblast differentiation and is necessary for transforming growth factor-ß (TGF-ß) induced profibrotic responses. OBJECTIVE: To investigate the role of Krox20 in abnormal scar pathogenesis. Hopefully, this insight can set the route for newer therapeutic approaches. MATERIALS AND METHOD: This study was carried out on 30 cases (10 cases of keloids, 10 cases of atrophic scars, and 10 cases with hypertrophic scars [HTS]) and 10 age and gender-matched apparently healthy subjects as a control group. Thirty biopsies were taken from perilesional areas. Evaluation of Krox20 expression was done using standard immunohistochemical technique. RESULTS: Krox20 was downregulated in epidermis of scar biopsies compared with perilesional and normal skin (p = 0.02) while it was overexpressed in fibroblasts in lesional scar biopsies compared with perilesional and normal skin (p < 0.001). Keloid cases have significantly higher Krox20 expression in fibroblasts compared with HTS cases (p < 0.001). Krox20 had significantly nucleocytoplasmic pattern of staining in scar cases compared with normal skin (p < 0.001). CONCLUSION: Krox20 overexpression may have a role in scar pathogenesis through upregulation of multiple genes associated with tissue remodeling and wound healing. This may open an avenue for research for new therapies based on Krox20 inhibition.

Axon-dependent expression of YAP/TAZ mediates Schwann cell remyelination but not proliferation after nerve injury.

Previously we showed that YAP/TAZ promote not only proliferation but also differentiation of immature Schwann cells (SCs), thereby forming and maintaining the myelin sheath around peripheral axons (Grove et al., 2017). Here we show that YAP/TAZ are required for mature SCs to restore peripheral myelination, but not to proliferate, after nerve injury. We find that YAP/TAZ dramatically disappear from SCs of adult mice concurrent with axon degeneration after nerve injury. They reappear in SCs only if axons regenerate. YAP/TAZ ablation does not impair SC proliferation or transdifferentiation into growth promoting repair SCs. SCs lacking YAP/TAZ, however, fail to upregulate myelin-associated genes and completely fail to remyelinate regenerated axons. We also show that both YAP and TAZ are redundantly required for optimal remyelination. These findings suggest that axons regulate transcriptional activity of YAP/TAZ in adult SCs and that YAP/TAZ are essential for functional regeneration of peripheral nerve.

Krox20 Regulates Endothelial Nitric Oxide Signaling in Aortic Valve Development and Disease.

Among the aortic valve diseases, the bicuspid aortic valve (BAV) occurs when the aortic valve has two leaflets (cusps), rather than three, and represents the most common form of congenital cardiac malformation, affecting 1-2% of the population. Despite recent advances, the etiology of BAV is poorly understood. We have recently shown that Krox20 is expressed in endothelial and cardiac neural crest derivatives that normally contribute to aortic valve development and that lack of Krox20 in these cells leads to aortic valve defects including partially penetrant BAV formation. Dysregulated expression of endothelial nitric oxide synthase (Nos3) is associated with BAV. To investigate the relationship between Krox20 and Nos3 during aortic valve development, we performed inter-genetic cross. While single heterozygous mice had normal valve formation, the compound Krox20+/-;Nos3+/- mice had BAV malformations displaying an in vivo genetic interaction between these genes for normal valve morphogenesis. Moreover, in vivo and in vitro experiments demonstrate that Krox20 directly binds to Nos3 proximal promoter to activate its expression. Our data suggests that Krox20 is a regulator of nitric oxide in endothelial-derived cells in the development of the aortic valve and concludes on the interaction of Krox20 and Nos3 in BAV formation.

Scalable Differentiation and Dedifferentiation Assays Using Neuron-Free Schwann Cell Cultures.

This chapter describes protocols to establish simplified in vitro assays of Schwann cell (SC) differentiation in the absence of neurons. The assays are based on the capacity of isolated primary SCs to increase or decrease the expression of myelination-associated genes in response to the presence or absence of cell permeable analogs of cyclic adenosine monophosphate (cAMP). No special conditions of media or substrates beyond the administration or removal of cAMP analogs are required to obtain a synchronous response on differentiation and dedifferentiation. The assays are cost-effective and far easier to implement than traditional myelinating SC-neuron cultures. They are scalable to a variety of plate formats suited for downstream experimentation and analysis. These cell-based assays can be used as drug discovery platforms for the evaluation of novel agents controlling the onset, maintenance, and reversal of the differentiated state using any typical adherent SC population.

Distinct Roles of Transcription Factors KLF4, Krox20, and Peroxisome Proliferator-Activated Receptor γ in Adipogenesis.

Much of our knowledge on adipogenesis comes from cell culture models of preadipocyte differentiation. Adipogenesis is induced by treating confluent preadipocytes with the adipogenic cocktail, which activates transcription factors (TFs) glucocorticoid receptor (GR) and CREB within minutes and increases expression of TFs C/EBPß, C/EBPδ, KLF4, and Krox20 within hours. All of these TFs have been shown to be capable of promoting adipogenesis in culture when they are overexpressed. However, it has remained unclear whether endogenous KLF4 and Krox20 are required for adipogenesis in culture and in vivo Using conditional knockout mice and derived white and brown preadipocytes, we show that endogenous KLF4 and Krox20 are dispensable for adipogenesis in culture and for brown adipose tissue development in mice. In contrast, the master adipogenic TF peroxisome proliferator-activated receptor γ (PPARγ) is essential. These results challenge the existing model on transcriptional regulation in the early phase of adipogenesis and highlight the need of studying adipogenesis in vivo.

Lithium Reversibly Inhibits Schwann Cell Proliferation and Differentiation Without Inducing Myelin Loss.

This study was undertaken to examine the bioactivity, specificity, and reversibility of lithium's action on the growth, survival, proliferation, and differentiation of cultured Schwann cells (SCs). In isolated SCs, lithium promoted a state of cell cycle arrest that featured extensive cell enlargement and c-Jun downregulation in the absence of increased expression of myelin-associated markers. In addition, lithium effectively prevented mitogen-induced S-phase entry without impairing cell viability. When lithium was administered together with differentiating concentrations of cyclic adenosine monophosphate (cAMP) analogs, a dramatic inhibition of the expression of the master regulator of myelination Krox-20 was observed. Likewise, lithium antagonized the cAMP-dependent expression of various myelin markers such as protein zero, periaxin, and galactocerebroside and allowed SCs to maintain high levels of expression of immature SC markers even in the presence of high levels of cAMP and low levels of c-Jun. Most importantly, the inhibitory action of lithium on SC proliferation and differentiation was shown to be dose dependent, specific, and reversible upon removal of lithium compounds. In SC-neuron cultures, lithium suppressed myelin sheath formation while preserving axonal integrity, SC-axon contact, and basal lamina formation. Lithium was unique in its ability to prevent the onset of myelination without promoting myelin degradation or SC dedifferentiation. To conclude, our results underscored an unexpected antagonistic action of lithium on SC mitogenesis and myelin gene expression. We suggest that lithium represents an attractive pharmacological agent to safely and reversibly suppress the onset of SC proliferation, differentiation, and myelination while maintaining the integrity of pre-existing myelinated fibers.

XRCC1 mediated the development of cervival cancer through a novel Sp1/Krox-20 swich.

Cervical cancer is the second leading cause of mortality among women. Impairment of the base excision repair (BER) pathway is one of the major causes of the initiation and progression of cervical cancer. However, whether the polymorphisms of the BER pathway components (i.e., HOGG1, XRCC1, ADPRT, and APE1) can affect the risk of cervical cancer remains unknown. Herein, we applied a hospital-based case-control study covering two independent cohorts and a subsequent functional assay to determine the roles of the single nucleotide polymorphisms (SNPs) of the BER pathway genes in cervical cancer. Results indicated that the XRCC1 rs3213245 (-77TC) TT genotype was associated with an increased risk of cervical cancer. The immunohistochemistry assay showed that XRCC1 protein expression levels were upregulated in cervical cancer patients with the XRCC1 rs3213245 CC genotype compared with the CT or TT genotypes. Further, results from ChIP assay showed that Sp1 could bind to the -77 site and that the rs3213245 C genotype promoted the binding of Sp1 to the XRCC1 promoter. Moreover, ChIP/Re-ChIP assays revealed that transcription factor Krox-20 was recruited to the XRCC1 rs3213245 mutation region and regulated the transcription of the XRCC1 gene by interacting with Sp1, ultimately mediated cervical cancer development. In summary, the findings indicated that the functional XRCC1 SNP rs3213245 was associated with the risk of cervical cancer based on the Sp1/Krox-20 switch.

Age-Dependent Schwann Cell Phenotype Regulation Following Peripheral Nerve Injury.

BACKGROUND: Schwann cells are integral to the regenerative capacity of the peripheral nervous system, which declines after adolescence. The mechanisms underlying this decline are poorly understood. This study sought to compare the protein expression of Notch, c-Jun, and Krox-20 after nerve crush injury in adolescent and young adult rats. We hypothesized that these Schwann cell myelinating regulatory factors are down-regulated after nerve injury in an age-dependent fashion. METHODS: Adolescent (2 months old) and young adult (12 months old) rats (n = 48) underwent sciatic nerve crush injury. Protein expression of Notch, c-Jun, and Krox-20 was quantified by Western blot analysis at 1, 3, and 7 days post-injury. Functional recovery was assessed in a separate group of animals (n = 8) by gait analysis (sciatic functional index) and electromyography (compound motor action potential) over an 8-week post-injury period. RESULTS: Young adult rats demonstrated a trend of delayed onset of the dedifferentiating regulatory factors, Notch and c-Jun, corresponding to the delayed functional recovery observed in young adult rats compared to adolescent rats. Compound motor action potential area was significantly greater in adolescent rats relative to young adult rats, while amplitude and velocity trended toward statistical significance. CONCLUSIONS: The process of Schwann cell dedifferentiation following peripheral nerve injury shows different trends with age. These trends of delayed onset of key regulatory factors responsible for Schwann cell myelination may be one of many possible factors mediating the significant differences in functional recovery between adolescent and young adult rats following peripheral nerve injury.