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The mammalian target of rapamycin (mTOR) pathway integrates multiple signals and regulates crucial cell functions via the molecular complexes mTORC1 and mTORC2. These complexes are functionally dependent on their raptor (mTORC1) or rictor (mTORC2) subunits. mTOR has been associated with oligodendrocyte differentiation and myelination downstream of the PI3K/Akt pathway, but the functional contributions of individual complexes are largely unknown. We show, by oligodendrocyte-specific genetic deletion of Rptor and/or Rictor in the mouse, that CNS myelination is mainly dependent on mTORC1 function, with minor mTORC2 contributions. Myelinassociated lipogenesis and protein gene regulation are strongly reliant on mTORC1. We found that also oligodendrocyte-specific overactivation of mTORC1, via ablation of tuberous sclerosis complex 1 (TSC1), causes hypomyelination characterized by downregulation of Akt signaling and lipogenic pathways. Our data demonstrate that a delicately balanced regulation of mTORC1 activation and action in oligodendrocytes is essential for CNS myelination, which has practical overtones for understanding CNS myelin disorders.

The central hypothesis of excitotoxicity is that excessive stimulation of neuronal NMDA-sensitive glutamate receptors is harmful to neurons and contributes to a variety of neurological disorders. Glial cells have been proposed to participate in excitotoxic neuronal loss, but their precise role is defined poorly. In this in vivo study, we show that NMDA induces profound nuclear factor B (NF-B) activation in Müller glia but not in retinal neurons. Intriguingly, NMDA-induced death of retinal neurons is effectively blocked by inhibitors of NF-B activity. We demonstrate that tumor necrosis factor ␣ (TNF␣) protein produced in Müller glial cells via an NMDA-induced NF-B-dependent pathway plays a crucial role in excitotoxic loss of retinal neurons. This cell loss occurs mainly through a TNF␣-dependent increase in Ca 2ϩ -permeable AMPA receptors on susceptible neurons. Thus, our data reveal a novel non-cell-autonomous mechanism by which glial cells can profoundly exacerbate neuronal death following excitotoxic injury.

Myelin formation during peripheral nervous system (PNS) development, and reformation after injury and in disease, requires multiple intrinsic and extrinsic signals. Akt/mTOR signaling has emerged as a major player involved, but the molecular mechanisms and downstream effectors are virtually unknown. Here, we have used Schwann-cell-specific conditional gene ablation of raptor and rictor, which encode essential components of the mTOR complexes 1 (mTORC1) and 2 (mTORC2), respectively, to demonstrate that mTORC1 controls PNS myelination during development. In this process, mTORC1 regulates lipid biosynthesis via sterol regulatory element-binding proteins (SREBPs). This course of action is mediated by the nuclear receptor RXRγ, which transcriptionally regulates SREBP1c downstream of mTORC1. Absence of mTORC1 causes delayed myelination initiation as well as hypomyelination, together with abnormal lipid composition and decreased nerve conduction velocity. Thus, we have identified the mTORC1-RXRγ-SREBP axis controlling lipid biosynthesis as a major contributor to proper peripheral nerve function.

Neurotrophin binding to the p75 neurotrophin receptor (p75 NTR ) activates neuronal apoptosis following adult central nervous system injury, but the underlying cellular mechanisms remain poorly defined. In this study, we show that the proform of nerve growth factor (proNGF) induces death of retinal ganglion cells in adult rodents via a p75 NTR -dependent signaling mechanism. Expression of p75 NTR in the adult retina is confined to Müller glial cells; therefore we tested the hypothesis that proNGF activates a non-cellautonomous signaling pathway to induce retinal ganglion cell (RGC) death. Consistent with this, we show that proNGF induced robust expression of tumor necrosis factor alpha (TNFα) in Müller cells and that genetic or biochemical ablation of TNFα blocked proNGF-induced death of retinal neurons. Mice rendered null for p75 NTR , its coreceptor sortilin, or the adaptor protein NRAGE were defective in proNGF-induced glial TNFα production and did not undergo proNGF-induced retinal ganglion cell death. We conclude that proNGF activates a non-cell-autonomous signaling pathway that causes TNFα-dependent death of retinal neurons in vivo.T he four mammalian neurotrophins comprise a family of related secreted factors that are required for differentiation, survival, development, and death of specific populations of neurons and nonneuronal cells. Neurotrophins are produced as proforms of ∼240 amino acids that are cleaved by furins and proconvertases to yield products of ∼120 amino acids. Recent studies have indicated that nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) can be secreted as proforms in the central nervous system (CNS) (1-3) and demonstrated that proneurotrophins can function as potent apoptosis-inducing ligands both in vitro and in vivo (4). However, the precise mechanisms by which proneurotrophins lead to neuronal death are poorly defined.The biological effects of neurotrophins are mediated by binding to TrkA, TrkB, and TrkC receptor tyrosine kinases and to the p75 neurotrophin receptor (p75 NTR ). Trk receptors respond preferentially to mature neurotrophins whereas proneurotrophins exert their apoptotic effect via a receptor complex that contains p75 NTR and sortilin (5). The precise signaling cascades evoked by occupancy of the p75 NTR -sortilin complex remain to be elucidated, but several lines of evidence indicate that NRIF and NRAGE adaptor proteins play key roles in death signaling cascades evoked by p75 NTR (6, 7).Previous studies have shown that neurotrophins induce cell death via p75 NTR during early retinal development (8). p75 NTR has also been implicated in light-induced photoreceptor death in adult rodents in vivo (9) and a proNGF-p75 NTR link has been proposed to facilitate apoptosis in a retinal cell line (10). Here, we investigate the role of proNGF in the adult retina and demonstrate that proNGF promotes death of retinal ganglion cells (RGCs) in vivo. Importantly, proNGF-induced RGC loss is indirect and requires the p75 NTRdependent production of tumor necrosis...

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Mutations in the mitochondrial fission factor GDAP1 are associated with severe peripheral neuropathies, but why the CNS remains unaffected is unclear. Using a Gdap1−/− mouse, Niemann et al. demonstrate that a CNS-expressed Gdap1 paralogue changes its subcellular localisation under oxidative stress conditions to also act as a mitochondrial fission factor.

Glaucoma is defined as a chronic and progressive optic nerve neuropathy, characterized by apoptosis of retinal ganglion cells (RGC) that leads to irreversible blindness. Ocular hypertension is a major risk factor, but in glaucoma RGC death can persist after ocular hypertension is normalized. To understand the mechanism underlying chronic RGC death we identified and characterized a gene product, ␣2-macroglobulin (␣2M), whose expression is up-regulated early in ocular hypertension and remains up-regulated long after ocular hypertension is normalized. In ocular hypertension retinal glia up-regulate ␣2M, which binds to low-density lipoprotein receptor-related protein-1 receptors in RGCs, and is neurotoxic in a paracrine fashion. Neutralization of ␣2M delayed RGC loss during ocular hypertension; whereas delivery of ␣2M to normal eyes caused progressive apoptosis of RGC mimicking glaucoma without ocular hypertension. This work adds to our understanding of the pathology and molecular mechanisms of glaucoma, and illustrates emerging paradigms for studying chronic neurodegeneration in glaucoma and perhaps other disorders.Vision impairment due to glaucoma affects 50 million people worldwide. In open angle glaucoma, visual field loss is caused by retinal ganglion cell (RGC) 3 apoptosis, concomitant with elevated intraocular pressure (IOP). Current treatments are limited to reduction of high IOP. Unfortunately, whereas these treatments are often successful at normalizing IOP, progressive RGC death and visual field loss often continue (1-3). In addition ϳ20% of patients are affected by normal tension glaucoma, a distinct optic nerve neuropathy in the absence of high IOP.Proposed mechanisms of RGC apoptosis in glaucoma include: mechanical compression of the optic nerve head preventing axonal transport of neurotrophins required for RGC survival (also known as "physiologic axotomy") (4), excitotoxic damage by hyperactive NMDA receptors, elevated glutamate, Ca 2ϩ fluxes, and nitric oxide (5, 6); ischemic and other retinal injury leading to activation of microglia (7), ␤-amyloid toxicity (8), and inflammatory damage through tumor necrosis factor-␣ (TNF␣) (9, 10). However, none of these mechanisms explain two key issues. First, all the cells in the inner retina are exposed to these deleterious effects, thus it is puzzling that RGCs are preferentially susceptible to apoptosis. Second, normalization of pressure often does not result in the complete arrest of RGC death, which continues chronically.To address these questions, we hypothesized that a short span of ocular hypertension can trigger long-lived changes in retinal gene expression, changes that are deleterious to RGCs. Five unique criteria were set to identify intraocular pressure regulated early gene (IPREG) candidates. Altered gene expression should: (i) occur specifically in the retina and be caused specifically by ocular hypertension; (ii) occur relatively early following ocular hypertension and prior to RGC damage; (iii) be long-lived; (iv) independent of the continuous p...