CHPG enhances BDNF and myelination in cuprizone-treated mice through astrocytic metabotropic glutamate receptor 5.

It is well recognized that astrocytes can produce factors known to affect the myelination process. One such factor, brain-derived neurotrophic factor (BDNF), can enhance the differentiation of oligodendrocyte lineage cells following a demyelinating lesion. Our previous work indicated that enhancing astrocyte-derived BDNF via injection of a general agonist of Group I/II metabotropic glutamate receptors (mGluRs) into the lesion increased myelin proteins in the cuprizone model of demyelination after 4 hr. To determine if this observation has potential therapeutic significance, we now use a more specific mGluR agonist, 2-chloro-5-hydroxyphenylglycine (CHPG), which binds to mGluR5, to examine effects on myelination through the clinically relevant approach of a peripheral injection. In initial studies, intraperitoneal injection of CHPG resulted in an increase in myelin proteins within the lesioned corpus callosum. These effects were blocked when either BDNF or the CHPG receptor, mGluR5, was deleted from glial fibrillary acidic protein (GFAP)+ astrocytes or when the BDNF receptor, tropomyosin receptor kinase B (TrkB), was deleted from proteolipid protein (PLP)+ oligodendrocytes. Moreover, injection of CHPG over 2 weeks not only elevated BDNF and myelin proteins, but also enhanced myelination and reversed behavioral deficits. Interestingly, effects on myelin and myelin proteins were not seen in the control animals, indicating that a lesion is critical in eliciting effects. Taken together, the data suggest that the mGluR agonist CHPG may be a potential therapeutic strategy for treating demyelinating diseases and that it works by enhancing the release of BDNF from astrocytes.

The trophic role of oligodendrocytes in the basal forebrain.

Traditionally, the primary function of oligodendrocytes (OLGs) in the CNS has been considered to be myelination. Here, we investigated whether OLGs may play a trophic role, particularly during development. Neurotrophin expression was assessed in postnatal day 7 basal forebrain (BF) OLGs, using in situ hybridization and detection of myelin basic protein. Nerve growth factor, brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3) mRNAs were revealed in OLGs in vivo and in culture. To determine whether OLGs support nearby neurons, we examined the influence of OLGs on BF cholinergic neurons. Neuronal function was enhanced by cocultured OLGs and OLG conditioned medium. Moreover, trophic effects of OLG conditioned medium were partially blocked by K252a, a trk tyrosine kinase inhibitor, and by neutralizing anti-BDNF or anti-NT-3 antisera, indicating that neurotrophins may mediate these effects, perhaps in concert with other signals. Our studies support a novel role for OLGs in providing local trophic support for neurons in the CNS.

Glutamate elicits release of BDNF from basal forebrain astrocytes in a process dependent on metabotropic receptors and the PLC pathway.

A key neurotrophin responsible for the survival and function of basal forebrain (BF) cholinergic neurons is brain-derived neurotrophic factor (BDNF). A number of studies now indicate that a source of this factor may be BF astrocytes. This study was designed to define the role of BF-astrocyte-derived BDNF on cholinergic neurons. Moreover, it investigated regulatory events that modulate BDNF content and release. In initial work BDNF derived from BF-astrocyte-conditioned medium (ACM) was found to increase both numbers of BF acetylcholinesterase (AChE+) cholinergic neurons and the cholinergic synthetic enzyme choline acetyltransferase (ChAT). Western blots, immunocytochemistry and pharmacological inhibition studies revealed that glutamate, through group I metabotropic glutamate receptors (mGluR), increases the intracellular levels of BDNF in BF astrocytes in culture, as well as its release. Furthermore, the release of BDNF is mediated by the actions of PLC, IP3 and internal stores of Ca2+. These results suggest that BF astrocytes serve as local sources of BDNF for cholinergic neurons, and that they may be regulated as such by the neuronal signal, glutamate, through the mediation of group I metabotropic receptors and the PLC pathway.

Mitogen-activated protein kinase pathway mediates effects of brain-derived neurotrophic factor on differentiation of basal forebrain oligodendrocytes.

Previous studies indicate that brain-derived neurotrophic factor (BDNF), through the mediation of the trkB receptor, modulates the expression of differentiated traits in basal forebrain (BF) oligodendrocytes (OLGs). Specifically, BDNF up-regulates the expression of myelin basic protein (MBP), proteolipid protein (PLP), and myelin associated glycoprotein (MAG; Du et al. [2006] Mol. Cell. Neurosci. 31:366-375). However, the signaling cascades mediating the effects of BDNF have not been defined. The current study employs biochemical and molecular biological approaches to examine the involvements of the mitogen-activated protein kinase (MAPK) pathway, the phosphatidylinositol-3 kinase (PI3K) pathway, and the phospholipase C-gamma (PLC-gamma) pathway. Our results indicate that, in BF OLGs, BDNF activates the MAPK pathway and the PLC-gamma pathway but not the PI3K-Akt signaling cascade. By using specific inhibitors and mutated dominant negative or constitutively active forms of MAPK kinase, we demonstrate that the MAPK pathway is mediating the effects of BDNF on expression of differentiated traits in BF OLGs.

Distinct effects of p75 in mediating actions of neurotrophins on basal forebrain oligodendrocytes.

Previous studies indicate that brain-derived neurotrophic factor (BDNF), nerve growth factor (NGF) and neurotrophin-3 (NT-3) increase myelin basic protein, (MBP) in differentiating basal forebrain (BF) oligodendrocytes (OLGs) (Du, Y., Fischer, T.Z., Lee, L.N., Lercher, L.D., Dreyfus, C. F., 2003. Regionally specific effects of BDNF on oligodendrocytes. Dev. Neurosci. 25, 116-126). While receptors, trk and p75, are expressed by subsets of oligodendrocytes (Du, Y., Fischer, T.Z., Lee, L.N., Lercher, L.D., Dreyfus, C. F., 2003. Regionally specific effects of BDNF on oligodendrocytes. Dev. Neurosci. 25, 116-126), those responsible for affecting differentiation have not been defined. In contrast, studies of peripheral Schwann cells reported that myelination is enhanced by BDNF working through p75, and diminished by trkC mediated processes (Cosgaya, J.M., Chan, J.R., Shooter, E.M., 2002. The neurotrophin receptor p75NTR as a positive modulator of myelination. Science 298, 1245-1248). To define receptors affecting central oligodendrocyte MBP, p75 knockout animals, p75 blocking antibodies, and an inhibitor of neurotrophin binding to p75, PD90780, were utilized. While p75 was implicated in the actions of NGF and NT-3, it did not affect actions of BDNF. On the other hand, K252a, an inhibitor of trk receptors, abolished the effects of the neurotrophins, including BDNF. All neurotrophins activated their respective trk receptors.

Regionally specific effects of BDNF on oligodendrocytes.

To define the effects of neurotrophins on oligodendrocytes, we monitored NGF, BDNF and NT-3 actions on basal forebrain (BF) and cortical populations. NGF, BDNF and NT-3 applied to BF oligodendrocytes elicited increases in expression of myelin basic protein (MBP) and enhanced the numbers of MBP+ cells, without affecting total cell numbers. In the cortex, however, while NGF and NT-3 influenced MBP expression, BDNF was without effect. To explore this apparent regional difference in BDNF action, we compared expression of the neurotrophin receptors trkA, trkB and trkC. While BF cells expressed all three trks, cortical cells did not express the full-length BDNF receptor, trkB. Interestingly, in no case was any receptor expressed by all oligodendrocytes, indicating that oligodendrocytes may be heterogeneous within a brain region. The data suggest that BF oligodendrocytes are influenced by BDNF to express MBP and are distinct in this ability from cortical cells.