IGF-I and FGF-2 coordinately enhance cyclin D1 and cyclin E-cdk2 association and activity to promote G1 progression in oligodendrocyte progenitor cells

Insulin-like growth factor-1 promotes G(1)/S cell cycle progression through bidirectional regulation of cyclins and cyclin-dependent kinase inhibitors via the phosphatidylinositol 3-kinase/Akt pathway in developing rat cerebral cortex

Although survival-promoting effects of insulin-like growth factor-1 (IGF-1) during neurogenesis are well characterized, mitogenic effects remain less well substantiated. Here, we characterize cell cycle regulators and signaling pathways underlying IGF-1 effects on embryonic cortical precursor proliferation in vitro and in vivo. In vitro, IGF-1 stimulated cell cycle progression and increased cell number without promoting cell survival. IGF-1 induced rapid increases in cyclin D1 and D3 protein levels at 4 h and cyclin E at 8 h. Moreover, p27(KIP1) and p57(KIP2) expression were reduced, suggesting downregulation of negative regulators contributes to mitogenesis. Furthermore, the phosphatidylinositol 3-kinase (PI3K)/Akt pathway specifically underlies IGF-1 activity, because blocking this pathway, but not MEK (mitogen-activated protein kinase kinase)/ERK (extracellular signal-regulated kinase), prevented mitogenesis. To determine whether mechanisms defined in culture relate to corticogenesis in vivo, we performed transuterine intracerebroventricular injections. Whereas blockade of endogenous factor with anti-IGF-1 antibody decreased DNA synthesis, IGF-1 injection stimulated DNA synthesis and increased the number of S-phase cells in the ventricular zone. IGF-1 treatment increased phospho-Akt fourfold at 30 min, cyclins D1 and E by 6 h, and decreased p27(KIP1) and p57(KIP2) expression. Moreover, blockade of the PI3K/Akt pathway in vivo decreased DNA synthesis and cyclin E, increased p27(KIP1) and p57(KIP2) expression, and prevented IGF-1-induced cyclin E mRNA upregulation. Finally, IGF-1 injection in embryos increased postnatal day 10 brain DNA content by 28%, suggesting a role for IGF-1 in brain growth control. These results demonstrate a mitogenic role for IGF-1 that tightly controls both positive and negative cell cycle regulators, and indicate that the PI3K/Akt pathway mediates IGF-1 mitogenic signaling during corticogenesis.

Gene expression changes in dorsal root ganglion of rat experimental lumber disc herniation models

STUDY DESIGN

Comprehensive overviews of gene expression changes in dorsal root ganglion (DRG) were obtained using microarrays in 2 rat models of experimental lumbar disc herniation (LDH).

OBJECTIVE

To clarify the mechanisms of painful radiculopathy caused by LDH from the viewpoint of gene expression changes in DRG of 2 rat models of LDH.

SUMMARY AND BACKGROUND DATA

Mechanical compression and chemical irritation are considered to be the 2 major causative factors of radiculopathy associated with LDH. Several basic studies have revealed histologic and functional changes in the nerve root and DRG induced by mechanical compression or application of nucleus pulposus. However, the effects of the 2 major factors have not been investigated in detail. METHODS.: The effects of mechanical and chemical factors were assessed in 2 models and in sham-operated rats. The mechanical compression model had a stainless steel rod inserted through a drill hole in the L5 lamina; the nucleus pulposus model had autologous nucleus pulposus placed in the drill hole, and only a drill hole was made in the L5 lamina of sham-operated rats. Samples from the left L5 DRG were harvested from the models with mechanical allodynia and from sham rats and analyzed using microarrays at 3 and 7 days after surgery.

RESULTS

The gene expression profiles differed in the 2 models at 7 days, but were similar at 3 days after surgery. Expression of the growth factor gene, insulin-like growth factor 1, and of the cyclinD1, cell division cycle 2 homolog A, and cyclinA2 genes related to the cell cycle was significantly upregulated in the DRG of the mechanical compression group at 7 days after surgery.

CONCLUSION

Mechanical and chemical factors caused altered gene expression in the DRG at 7 days after surgery, suggesting that the mechanisms of nerve injury induced by these factors differ. The upregulation of IGF-1 might be a key factor in painful radiculopathy induced by mechanical factors.

Diet controls normal and tumorous germline stem cells via insulin-dependent and -independent mechanisms in Drosophila

The external environment influences stem cells, but this process is poorly understood. Our previous work showed that germline stem cells (GSCs) respond to diet via neural insulin-like peptides (DILPs) that act directly on the germ line to upregulate stem cell division and cyst growth under a protein-rich diet in Drosophila. Here, we report that DILPs specifically control the G2 phase of the GSC cell cycle via phosphoinositide-3 kinase (PI3K) and dFOXO, and that a separate diet mediator regulates the G1 phase. Furthermore, GSC tumors, which escape the normal stem cell regulatory microenvironment, or niche, still respond to diet via both mechanisms, indicating that niche signals are not required for GSCs to sense or respond to diet. Our results document the effects of diet and insulin-like signals on the cell cycle of stem cells within an intact organism and demonstrate that the response to diet requires multiple signals. Moreover, the retained ability of GSC tumors to respond to diet parallels the long known connections between diet, insulin signaling, and cancer risk in humans.

Delayed IGF-1 administration rescues oligodendrocyte progenitors from glutamate-induced cell death and hypoxic-ischemic brain damage

We previously demonstrated that IGF-1 blocks glutamate-mediated death of late oligodendrocyte progenitors (OPs) by preventing Bax translocation, mitochondrial cytochrome c release and cleavage of caspases 9 and 3. Here, we demonstrate that IGF-1 prevents caspase 3 activation in late OPs when administered up to 16 h following exposure to glutamate. Moreover, late addition of IGF-1 to OPs previously exposed to toxic levels of glutamate promotes oligodendrocyte maturation as measured by myelin basic protein expression. We also demonstrate that intraventricularly administered IGF-1 retains OPs in the perinatal white matter after hypoxia-ischemia when given after insult. These results suggest that delayed administration of IGF-1 will rescue OPs in the immature white matter and promote myelination following hypoxia-ischemia.

Effects of acute and repeated exposure to lipopolysaccharide on cytokine and corticosterone production during remyelination

Chronic exposure to the copper-chelating agent, cuprizone (CPZ), is an increasingly popular model for producing demyelination. More importantly, cessation of cuprizone exposure allows for full remyelination, which represents a window of opportunity for determining the influence of environmental factors on regenerative processes. In the present study, CPZ-treated animals were assessed for functional status of systemic and central cytokine responsiveness to LPS, as well as assessment for signs of body weight changes. Exposure of male C57BL/6J mice to 5 weeks of 0.2% CPZ in the diet was optimal in producing demyelination and microglial activation, as measured by myelin basic protein, CD11b, and CD45 immunohistochemistry. Acute challenge with LPS at the end of 5 weeks CPZ treatment did not alter IL-1beta, IL-6, nor TNFalpha responses in the spleen and corpus callosum. Similarly, repeated exposure to LPS during the remyelination phase (CPZ removal) did not influence these measures to LPS. Plasma corticosterone was unaffected following acute challenge of CPZ-pretreated animals, but after repeated LPS treatment, there was a significant augmentation of the corticosterone response in CPZ-pretreated mice. Interestingly, the basal concentration of IL-1beta in the corpus callosum of CPZ treated animals was significantly increased, which was in keeping with the increase in activated microglial cells. In conclusion, the cuprizone model of demyelination and remyelination does not appear to influence the systemic nor central IL-1, IL-6, and TNF responses to acute nor repeated LPS. This opens up the possibility for studying the contribution of systemic inflammatory processes on remyelination after cessation of CPZ treatment.

Synergistic induction of cyclin D1 in oligodendrocyte progenitor cells by IGF-I and FGF-2 requires differential stimulation of multiple signaling pathways

D-type cyclins are direct targets of extracellular signals and critical regulators of G(1) progression. Our previous data demonstrated that IGF-I and FGF-2 synergize to enhance cyclin D1 expression, cyclin E/cdk2 complex activation, and S-phase entry in OP cells. Here, we provide a mechanistic explanation for how two growth factor signaling pathways converge on a major cell cycle regulator. IGF-I and FGF-2 differentially activate signaling pathways to coordinately promote cyclin D1 expression. We show that the p44/p42 MAPK signaling pathway is essential for FGF-2 induction of cyclin D1 mRNA. In contrast, blocking the PI3-Kinase pathway results in loss of IGF-I/FGF-2 synergistic induction of cyclin D1 protein levels. Moreover, the presence of IGF-I significantly enhances nuclear localization of cyclin D1, which also requires PI3K signaling. GSK-3beta, a downstream target of the PI3K/Akt pathway, is phosphorylated in the presence of IGF-I in OPs. Consistent with a known role for GSK-3beta in cyclin D1 degradation, we show that proteasome inhibition in OPs exposed to FGF-2 increased cyclin D1 levels, equivalent to levels seen in IGF-I/FGF-2 treated cells. Thus, we provide a model for cyclin D1 coordinate regulation where FGF-2 stimulation of the MAPK pathway promotes cyclin D1 mRNA expression while IGF-I activation of the PI3K pathway inhibits proteasome degradation of cyclin D1 and enhances nuclear localization of cyclin D1.

The cyclolignan picropodophyllin attenuates intimal hyperplasia after rat carotid balloon injury by blocking insulin-like growth factor-1 receptor signaling

OBJECTIVE

Smooth muscle cell proliferation (SMC) is a pivotal factor in the development of intimal hyperplasia after vascular injury. A number of growth factors, including insulin-like growth factor-1 (IGF-1), have been shown to be involved in SMC proliferation. We evaluated the effect of picropodophyllin (PPP), a new IGF-1 receptor inhibitor, in the prevention of SMC proliferation and development of intimal hyperplasia after vascular injury.

METHODS

The effects of systemic administration of PPP on intimal hyperplasia were studied in a balloon rat carotid injury model. Lesions were quantified by morphometry and SMC proliferation and apoptosis was studied by immunohistochemical staining for proliferating cell nuclear antigen (PCNA) and activated caspase 3, respectively. The effect of PPP on rat aortic SMC proliferation and apoptosis was studied in vitro by using cell counting, 3[H]-thymidine incorporation, and a flow cytometry assay for annexin V. Phosphorylation of the IGF-1 receptor, protein kinase B (Akt), and extracellular signal-regulated kinase 1/2 (ERK1/2) in vitro and in vivo were analyzed by using Western blotting.

RESULTS

PPP inhibited IGF-1-mediated SMC proliferation in vitro but no significant increase in apoptosis was detected. In rats treated with PPP, a more than a twofold reduction in carotid intima area was observed 2 weeks after balloon injury, a significant decrease in PCNA staining was demonstrated in early lesions, but activated caspase 3 was not detected. In addition, PPP attenuated phosphorylation of the IGF-1 receptor, Akt, and ERK1/2 in IGF-1-stimulated SMCs in vitro, and a reduced phosphorylation of the IGF-1 receptor and Akt was found in balloon-injured carotid arteries in rats treated with PPP.

CONCLUSION

These results show that PPP potently blocks IGF-1-mediated phosphorylation of the IGF-1 receptor in SMCs, decreases downstream Akt and ERK1/2 activation, inhibits SMC replication, and subsequently attenuates intimal hyperplasia after balloon injury of rat carotid arteries.

Mechanism of insulin-like growth factor I-mediated proliferation of adult neural progenitor cells: role of Akt

Insulin-like growth factor I (IGF-I) is involved in the proliferation and differentiation of adult neural progenitor cells; however, the underlying mechanism is not clear. We analysed the involvement of the phosphatidylinositol 3-kinase/Akt and MEK/extracellular signal-regulated kinase (ERK) pathways in the IGF-I-mediated proliferation of rat neural progenitor cells. Stimulation of neural progenitor cells with IGF-I enhanced the phosphorylation of Akt but not ERK. Cell proliferation assay demonstrated that 2-(4-morpholinyl)-8-phenyl-4H-1-benzopyran-4-one (phosphoinositide 3-kinase inhibitor) but not 1,4-diamino-2,3-dicyano-1,4-bis(2-aminophenylthio)-butadiene (U0126) (ERK inhibitor) inhibited the IGF-I-induced survival of cells, whereas fibroblast growth factor 2 (FGF-2) enhanced the IGF-I-mediated survival of cells. Consistent with the cell proliferation assay, 5'bromo-2-deoxy-uridine incorporation studies established a negative role for IGF-I in proliferation. However, FGF-2 (ERK activator) in the presence of IGF-I (Akt activator) increased the proliferation of cells. Accordingly, stimulation of the ERK pathway by FGF-2 induced the expression of cyclin D1, which is essential for the entry of cells into cell cycle, and IGF-I in the presence of FGF-2 up-regulated the expression of cyclin D1. IGF-I in the absence or presence of FGF-2 increased the phosphorylation of glycogen synthase kinase, thus supporting its role in the survival of neural progenitor cells. To further confirm the role of ERK activation in the proliferation, we cultured cells in FGF-2 + IGF-I-containing medium in the presence and absence of U0126 (ERK inhibitor), and showed the inhibition of nestin expression in U0126-treated cells. The decrease in the cyclin D1 content in conjunction with the inhibition of nestin expression by ERK inhibitor confirms the role of ERK in the proliferation of cells.

IGF-I-induced oligodendrocyte progenitor proliferation requires PI3K/Akt, MEK/ERK, and Src-like tyrosine kinases

Insulin-like growth factor-I (IGF-I) is required for the growth of oligodendrocytes, although the underlying mechanisms are not fully understood. Our aim was to investigate the role of phosphatidylinositol 3-kinase (PI3K), mitogen-activated protein kinase kinase (MEK1), and Src family tyrosine kinases in IGF-I-stimulated proliferation of oligodendrocyte progenitors. IGF-I treatment increased the proliferation of cultured oligodendrocyte progenitors as determined by measuring incorporation of [(3)H]-thymidine and bromodeoxy-uridine (BrdU). IGF-I stimulated a transient phosphorylation of 3-phosphoinositide-dependent kinase-1 (PDK1) and extracellular signal-regulated kinases (ERK1/2) (targets of MEK1), as well as a rapid and sustained activation of Akt (a target of PI3K). Furthermore, inhibitors of PI3K (LY294002 and Wortmannin), MEK1 (PD98059 and U0126), and Src family tyrosine kinases (PP2) decreased IGF-I-induced proliferation, and blocked ERK1/2 activation. LY294002, Wortmannin and PP2 also blocked Akt activation. To further determine whether Akt is required for IGF-I stimulated oligodendrocyte progenitor proliferation, cultures were infected with adenovirus vectors expressing dominant-negative mutants of Akt or treated with pharmacological inhibitors of Akt. All treatments reduced IGF-I-induced oligodendrocyte progenitor proliferation. Our data indicate that stimulation of oligodendrocyte progenitor proliferation by IGF-I requires Src-like tyrosine kinases as well as the PI3K/Akt and MEK1/ERK signaling pathways.

Requirement for Id1 in opioid-induced oligodendrogenesis in cultured adult rat hippocampal progenitors

Growth factors and peptides playing important roles during early development of the central nervous system have also been shown to maintain their regulation of cell genesis in the adult brain. We have previously described that endogenous opioids, expressed in the developing hippocampus, regulate proliferation and differentiation in the adult rat hippocampus. The aim of this study was to investigate the effects of the opioid beta-endorphin on gene expression and glial differentiation in cultures of adult rat hippocampal progenitors (AHPs). Changes in gene expression after stimulation of AHPs with beta-endorphin for 48 h were investigated using cDNA arrays. Confirmation experiments verified that stimulation with beta-endorphin increased the mRNA levels of myelin basic protein, glutathione S-transferase pi, c-junD and rab16 (P < 0.05), genes that are associated with oligodendrogenesis. Furthermore, beta-endorphin increased the levels of Id1, but not Id3, mRNA on the arrays. Incubation of AHPs with beta-endorphin resulted in a threefold increase in oligodendrogenesis (P < 0.01) but no significant change in astrogliogenesis. No effect on oligodendrogenesis was observed in the presence of the opioid antagonist naloxone. Coincubation of beta-endorphin with Id1 antisense oligonucleotides for 10 days also entirely blocked the induced oligodendrogenesis in our AHP cultures. Moreover, a subpopulation of AHPs (25%) showed nuclear expression of the proneural transcriptional activator Mash1 that was reduced to approximately 5% of the cells when exposed to beta-endorphin. We suggest a requirement for Id1 in opioid-induced oligodendrogenesis in cultured AHPs possibly acting on opioid-responsive AHPs expressing the proneural transcriptional activator Mash1.

Trophic factors counteract elevated FGF-2-induced inhibition of adult neurogenesis

The dentate gyrus of adult mammalian brain contains neural progenitor cells with self-renewal and multi-lineage potential. The lineage and maturation of the neural progenitors are determined by the composition and levels of the trophic factors in their microenvironment. In Alzheimer disease (AD) brain, especially the hippocampus, the level of basic fibroblast growth factor (FGF-2) is markedly elevated. Here we show that elevated FGF-2 enhances the division and nestin levels of cultured adult rat hippocampal progenitors but impairs neuronal lineage determination and maturation of these cells in culture. The trophic factors ciliary neurotrophic factor (CNTF), glial-derived neurotrophic factor (GDNF), and insulin-like growth factors-1 and -2 (IGF-1, IGF-2) as well as an Alzheimer peptidergic drug, Cerebrolysin((R)) (CL), in which we found these neurotrophic activities, counteract the effect of FGF-2 in inducing neuronal lineage (early neurogenesis). Whereas CNTF is the most active of the neurotrophic factors studied in promoting neurogenesis, CL, probably because of a combined effect of these factors, induces similar changes but without inhibiting cell proliferation. These findings suggest that CNTF, GDNF, IGF-1, and IGF-2 are promising therapeutic targets for AD and other diseases in which neurogenesis is probably inhibited.

Insulin-like growth factor actions during development of neural stem cells and progenitors in the central nervous system

Insulin-like growth factor-I (IGF-I) plays a key role in normal development. Recent studies show that IGF-I exerts a wide variety actions in the central nervous system during development as well as in adulthood. This report reviews recent developments on IGF-I actions and its mechanisms in the central nervous system, with a focus on its actions during the development of neural stem cells and progenitors. Available data strongly indicate that IGF-I shortens the length of the cell cycle in neuron progenitors during embryonic life and has an influence on the growth of all neural cell types. The phosphatidylinositol-3 kinase/Akt and mitogen-activated protein kinase pathways seem to be the predominant mediators of IGF-I-stimulated neural cell proliferation and survival. IGF-I actions, however, likely depend on cell type, developmental stage, and microenvironmental milieu.

IGF1 gene polymorphism and risk for hereditary nonpolyposis colorectal cancer

Hereditary nonpolyposis colorectal cancer (HNPCC) is an autosomal dominant disorder caused by germline mutations in DNA mismatch repair (MMR) genes. Insulin-like growth factor-I (IGF-I) is involved in colorectal carcinogenesis, and elevated plasma IGF-I levels are associated with sporadic colorectal cancer (CRC) risk. We investigated the relationship between IGF1 promoter cytosine-adenine (CA) dinucleotide-repeat polymorphism length and CRC risk in 121 MMR gene mutation carriers using Cox regression and Kaplan-Meier analysis. All statistical tests were two-sided. Time to onset for CRC increased for each decrease in CA-repeat number (median = 19 repeats, range = 12-22 repeats; hazard ratio [HR] = 1.17, 95% confidence interval [CI] = 1.05 to 1.31; P = .006). Patients carrying a CA(< or = 17) repeat allele had a statistically significantly higher CRC risk (HR = 2.36; 95% CI = 1.28 to 4.36; P = .006) than all others and were younger at onset (44 years versus 56.5 years; P = .023). These findings indicate a statistically significant association between shorter IGF1 CA-repeat lengths and increased risk for CRC in HNPCC. This is the first report, to our knowledge, to show that IGF1 variant genotypes modify risk of a hereditary form of cancer.

Macrocephaly and the control of brain growth in autistic disorders

Autism is a childhood-onset neuropsychiatric disorder characterized by marked impairments in social interactions and communication, with restricted stereotypic and repetitive patterns of behavior, interests, and activities. Genetic epidemiology studies indicate that a strong genetic component exists to this disease, but these same studies also implicate significant environmental influence. The disorder also displays symptomatologic heterogeneity, with broad individual differences and severity on a graded continuum. In the search for phenotypes to resolve heterogeneity and better grasp autism's underlying biology, investigators have noted a statistical overrepresentation of macrocephaly, an indicator of enlarged brain volume. This feature is one of the most widely replicated biological findings in autism. What then does brain enlargement signify? One hypothesis invoked for the origin of macrocephaly is a reduction in neuronal pruning and consolidation of synapses during development resulting in an overabundance of neurites. An increase in generation of cells is an additional mechanism for macrocephaly, though it is less frequently discussed in the literature. Here, we review neurodevelopmental mechanisms regulating brain growth and highlight one underconsidered potential causal mechanism for autism and macrocephaly--an increase in neurogenesis and/or gliogenesis. We review factors known to control these processes with an emphasis on nuclear receptor activation as one signaling control that may be abnormal and contribute to increased brain volume in autistic disorders.

S-phase entry of oligodendrocyte lineage cells is associated with increased levels of p21Cip1

The mechanisms regulating the number of myelinating cells in the central nervous system are crucial for both normal development and repair in pathological conditions. Among relevant growth factors involved in this process, fibroblast growth factor-2 (FGF2) induces oligodendrocyte progenitors (OLPs) to proliferate and stimulates mature oligodendrocytes (OLs) to reenter the S-phase of the cell cycle. S-phase entry is modulated by the formation of complexes between cyclins and cyclin-dependent kinases (CDKs), on one hand, and by their interactions with cell cycle inhibitors (e.g., p18INK, p27Kip1, p21Cip1), on the other. Although the roles of cyclin E/CDK2 complexes and the inhibitor p27Kip1 have been extensively investigated relative to proliferation and differentiation in the OL lineage, less is known about the regulation of the formation of cyclin D1/CDK4 complexes and the role of p21Cip1 in these events. In this study, we show that the FGF2-mediated increase in bromodeoxyuridine (BrdU) incorporation into OL progenitors and mature OLs occurs concomitantly with increase in the levels of p21Cip1 and the formation of p21Cip1/cyclin D1/CDK4 ternary complexes. These complexes are functionally active is indicated by the ensuing FGF2-dependent hyperphosphorylation of the downstream target Rb. In untreated mature OLs that do not incorporate BrdU, the levels of p21Cip1 are low, and the level of the inhibitor p18INK is high. Furthermore, p18INK sequesters CDK2 into binary complexes, precluding the formation of p21Cip1/cyclin D1/CDK4 ternary complexes in these cells. Therefore, we propose that p21Cip1 is acting as a positive regulator, rather than an inhibitor, of cell cycle entry by favoring the assembly of active cyclin D1/CDK4 complexes.

Basic fibroblast growth factor exhibits dual and rapid regulation of cyclin D1 and p27 to stimulate proliferation of rat cerebral cortical precursors

While extracellular signals play a major role in brain neurogenesis, little is known about the cell cycle machinery underlying mitogen stimulation of precursor proliferation. Current models suggest that the D cyclins function as primary sensors of extracellular mitogens. Here we define the mechanisms by which basic fibroblast growth factor (bFGF) stimulates cortical precursors, with particular attention to the responses of cell cycle promitogenic and antimitogenic regulators. bFGF produced a 4-fold increase in DNA synthesis and a 3-fold rise in bromodeoxyuridine labeling, suggesting that the factor promotes the G1/S transition. There was also a 3-fold increase in cyclin-dependent kinase 2 (CDK2) kinase activity, which is critical for S phase entry. CDK2 activation was apparently cyclin E dependent, since only its protein and mRNA levels were elevated at 24 h, whereas CDK2, p27KIP1 and p57KIP2 levels were unaltered. Late G1 phase CDK2/cyclin E activity depends on early G1 D cyclin function. Indeed, cyclin D1, but not cyclin D3, was upregulated selectively at 8 h after bFGF treatment, a time when cyclin E was unchanged. The sequential activation of cyclin D1 and cyclin E supports the idea that cyclin E gene transcription is regulated by cyclin-D/CDK4/6-mediated pRb phosphorylation and subsequent E2F transcription factor release. However, in addition to increased D1 cyclin, we unexpectedly detected a 75% reduction in p27KIP1 protein at 8 h, suggesting that both pro- and antimitogenic regulators are targets of extracellular mitogens during brain development.

In vivo analysis of oligodendrocyte lineage development in postnatal FGF2 null mice

Analysis of fibroblast growth factor 2 null (FGF2-/-) and wild-type (FGF2+/+) mice was used to interpret the potential in vivo role of endogenous FGF2 on oligodendrocyte lineage cell (OLC) responses during oligodendrogenesis and myelination. In wild-type mouse spinal cord, FGF2 levels increased approximately threefold between the first and second postnatal weeks, a period corresponding with the peak of oligodendrogenesis. Absence of this developmental FGF2 elevation in FGF2-/- mice eliminated the transient overproduction of oligodendrocytes that is known to occur at the peak of oligodendrogenesis in wild-type mice. Absence of FGF2 did not affect oligodendrocyte progenitor (OP) density or proliferation, based on BrdU incorporation, and also did not alter survival, based on TUNEL analysis. To examine OLC differentiation in vivo, retrovirus encoding-enhanced green fluorescent protein (GFP) was injected into the spinal cord to heritably label endogenous cycling cells in the white matter at postnatal day 7 and then identify the generated cells at postnatal day 28. Phenotypes of cells expressing GFP were identified by morphology and immunolabeling, using CC1 for oligodendrocytes and NG2 combined with platelet-derived growth factor alpha receptor for OPs. Within the population of GFP-labeled cells, the proportion of oligodendrocytes was higher in FGF2-/- mice, indicating that endogenous FGF2 inhibited OLC differentiation in wild-type mice. Furthermore, in FGF2-/- mice fewer cells appeared to be generated from an initial retrovirus-labeled cell, consistent with more frequent differentiation into post-mitotic oligodendrocytes. This in vivo analysis demonstrates that the predominant role of endogenous FGF2 on OLCs in development is inhibition of differentiation.

Developmental abnormalities of myelin basic protein expression in fyn knock-out brain reveal a role of Fyn in posttranscriptional regulation

Fyn protein-tyrosine kinase (PTK), a member of the Src-PTK family, is essential for myelin development in the central nervous system (CNS). The absence of Fyn activity results in defects in the morphogenesis of oligodendrocyte precursors (OPCs) and CNS hypomyelination. However, molecular mechanisms for Fyn to control CNS myelinogenesis remain elusive. Here we show that Fyn-PTK is significantly up-regulated in early OPC differentiation, concentrated in the compact myelin, and declines during myelin development. Despite the high levels of Fyn-PTK expression during early OPC differentiation, Fyn deficiency does not affect the expression of mRNAs that encode myelin structural proteins, including that for the myelin basic protein (MBP), until postnatal day 13 (P13). However, the accumulation rate of MBP mRNA is significantly attenuated during the most active period of myelinogenesis (P13 and P20). Interestingly, the absence of Fyn causes a preferential reduction of the exon-2 containing MBP mRNA isoforms derived from alternative splicing, providing the first evidence that Fyn is required for posttranscriptional regulation of MBP. Consistent with this idea, Fyn phosphorylates the selective RNA-binding protein QKI, which likely modulates the activity of QKI in binding and stabilizing the MBP mRNA. Furthermore, Fyn deficiency exerts an opposing influence on MBP isoform patterning in comparison to that by QKI deficiency. These observations collectively suggest that Fyn plays critical roles in promoting accelerated MBP expression during myelinogenesis in a MBP isoform-preferential manner, and QKI may act in the same pathway downstream of Fyn for MBP mRNA homeostasis.