RESUMEN
Transected axons typically fail to regenerate in the central nervous system (CNS), resulting in chronic neurological disability in individuals with traumatic brain or spinal cord injury, glaucoma and ischemia-reperfusion injury of the eye. Although neuroinflammation is often depicted as detrimental, there is growing evidence that alternatively activated, reparative leukocyte subsets and their products can be deployed to improve neurological outcomes. In the current study, we identify a unique granulocyte subset, with characteristics of an immature neutrophil, that had neuroprotective properties and drove CNS axon regeneration in vivo, in part via secretion of a cocktail of growth factors. This pro-regenerative neutrophil promoted repair in the optic nerve and spinal cord, demonstrating its relevance across CNS compartments and neuronal populations. Our findings could ultimately lead to the development of new immunotherapies that reverse CNS damage and restore lost neurological function across a spectrum of diseases.
Asunto(s)
Axones/metabolismo , Comunicación Celular , Sistema Nervioso Central/citología , Sistema Nervioso Central/metabolismo , Regeneración Nerviosa , Neuronas/metabolismo , Neutrófilos/metabolismo , Animales , Biomarcadores , Plasticidad de la Célula/inmunología , Supervivencia Celular/efectos de los fármacos , Supervivencia Celular/inmunología , Sistema Nervioso Central/inmunología , Péptidos y Proteínas de Señalización Intercelular/biosíntesis , Ratones , Infiltración Neutrófila/inmunología , Neutrófilos/inmunología , Nervio Óptico/inmunología , Nervio Óptico/metabolismo , Receptores de Interleucina-8B/metabolismo , Médula Espinal/citología , Médula Espinal/metabolismo , Transcriptoma , Zimosan/metabolismo , Zimosan/farmacologíaRESUMEN
The signaling lipid phosphatidylinositol 3,5-bisphosphate, PI(3,5)P2, functions in vesicular trafficking through the endo-lysosomal compartment. Cellular levels of PI(3,5)P2 are regulated by an enzyme complex comprised of the kinase PIKFYVE, the phosphatase FIG4, and the scaffold protein VAC14. Mutations of human FIG4 cause inherited disorders including Charcot-Marie-Tooth disease type 4J, polymicrogyria with epilepsy, and Yunis-Varón syndrome. Constitutive Fig4-/- mice exhibit intention tremor, spongiform degeneration of neural tissue, hypomyelination, and juvenile lethality. To determine whether PI(3,5)P2 is required in the adult, we generated Fig4flox/-; CAG-creER mice and carried out tamoxifen-induced gene ablation. Global ablation in adulthood leads to wasting, tremor, and motor impairment. Death follows within 2 months of tamoxifen treatment, demonstrating a life-long requirement for Fig4. Histological examinations of the sciatic nerve revealed profound Wallerian degeneration of myelinated fibers, but not C-fiber axons in Remak bundles. In optic nerve sections, myelinated fibers appear morphologically intact and carry compound action potentials at normal velocity and amplitude. However, when iKO mice are challenged with a chemical white matter lesion, repair of damaged CNS myelin is significantly delayed, demonstrating a novel role for Fig4 in remyelination. Thus, in the adult PNS Fig4 is required to protect myelinated axons from Wallerian degeneration. In the adult CNS, Fig4 is dispensable for fiber stability and nerve conduction, but is required for the timely repair of damaged white matter. The greater vulnerability of the PNS to Fig4 deficiency in the mouse is consistent with clinical observations in patients with Charcot-Marie-Tooth disease.
Asunto(s)
Enfermedad de Charcot-Marie-Tooth/genética , Flavoproteínas/genética , Sistema Nervioso/metabolismo , Fosfoinosítido Fosfatasas/genética , Monoéster Fosfórico Hidrolasas/genética , Animales , Axones/patología , Sistema Nervioso Central/fisiopatología , Enfermedad de Charcot-Marie-Tooth/fisiopatología , Displasia Cleidocraneal/genética , Displasia Cleidocraneal/fisiopatología , Displasia Ectodérmica/genética , Displasia Ectodérmica/fisiopatología , Humanos , Deformidades Congénitas de las Extremidades/genética , Deformidades Congénitas de las Extremidades/fisiopatología , Ratones , Ratones Transgénicos , Micrognatismo/genética , Micrognatismo/fisiopatología , Mutación , Sistema Nervioso/patología , Neuronas/patología , Sistema Nervioso Periférico/fisiopatología , Fosfatidilinositol 3-Quinasas/genética , Fosfatos de Fosfatidilinositol/genética , Fosfatos de Fosfatidilinositol/metabolismo , Polimicrogiria/genética , Polimicrogiria/fisiopatología , Nervio Ciático/fisiopatologíaRESUMEN
Although intra-axonal protein synthesis is well recognized in cultured neurons and during development in vivo, there have been few reports of mRNA localization and/or intra-axonal translation in mature CNS axons. Indeed, previous work indicated that mature CNS axons contain much lower quantities of translational machinery than PNS axons, leading to the conclusion that the capacity for intra-axonal protein synthesis is linked to the intrinsic capacity of a neuron for regeneration, with mature CNS neurons showing much less growth after injury than PNS neurons. However, when regeneration by CNS axons is facilitated, it is not known whether the intra-axonal content of translational machinery changes or whether mRNAs localize into these axons. Here, we have used a peripheral nerve segment grafted into the transected spinal cord of adult rats as a supportive environment for regeneration by ascending spinal axons. By quantitative fluorescent in situ hybridization combined with immunofluorescence to unambiguously distinguish intra-axonal mRNAs, we show that regenerating spinal cord axons contain ß-actin, GAP-43, Neuritin, Reg3a, Hamp, and Importin ß1 mRNAs. These axons also contain 5S rRNA, phosphorylated S6 ribosomal protein, eIF2α translation factor, and 4EBP1 translation factor inhibitory protein. Different levels of these mRNAs in CNS axons from regenerating PNS axons may relate to differences in the growth capacity of these neurons, although the presence of mRNA transport and likely local translation in both CNS and PNS neurons suggests an active role in the regenerative process. SIGNIFICANCE STATEMENT: Although peripheral nerve axons retain the capacity to locally synthesize proteins into adulthood, previous studies have argued that mature brain and spinal cord axons cannot synthesize proteins. Protein synthesis in peripheral nerve axons is increased during regeneration, and intra-axonally synthesized proteins have been shown to contribute to nerve regeneration. Here, we show that mRNAs and translational machinery are transported into axons regenerating from the spinal cord into the permissive environment of a peripheral nerve graft. Our data raise the possibility that spinal cord axons may make use of localized protein synthesis for regeneration.
Asunto(s)
Axones/metabolismo , Regeneración Nerviosa/fisiología , Iniciación de la Cadena Peptídica Traduccional/fisiología , Biosíntesis de Proteínas/fisiología , ARN Mensajero/metabolismo , Médula Espinal/fisiopatología , Animales , Antígenos de Neoplasias/genética , Antígenos de Neoplasias/metabolismo , Biomarcadores de Tumor/genética , Biomarcadores de Tumor/metabolismo , Células Cultivadas , Modelos Animales de Enfermedad , Femenino , Ganglios Espinales/citología , Hepcidinas/genética , Hepcidinas/metabolismo , Lectinas Tipo C/genética , Lectinas Tipo C/metabolismo , Masculino , Proteínas Asociadas a Pancreatitis , Iniciación de la Cadena Peptídica Traduccional/genética , Traumatismos de los Nervios Periféricos/patología , Ratas , Ratas Sprague-Dawley , Células Receptoras Sensoriales/metabolismo , Células Receptoras Sensoriales/patología , Traumatismos de la Médula Espinal/patologíaRESUMEN
Aberrant endoplasmic reticulum (ER) and inner nuclear membrane (INM) proteins are destroyed through ER-associated degradation (ERAD) and INM-associated degradation (INMAD). We previously showed the Hrd1, Doa10, and Asi ERAD and INMAD ubiquitin ligases (E3s) in Saccharomyces cerevisiae confer resistance to hygromycin B, which distorts the ribosome decoding center. Here, we assessed the requirement of Ubc6 and Ubc7, the primary ERAD and INMAD ubiquitin-conjugating enzymes (E2s) for hygromycin B resistance. Loss of either E2 sensitized cells to hygromycin B, with UBC7 deletion having a greater impact, consistent with characterized roles for Ubc6 and Ubc7 in ER and INM protein quality control.
RESUMEN
Upon peripheral nervous system (PNS) injury, severed axons undergo rapid SARM1-dependent Wallerian degeneration (WD). In mammals, the role of SARM1 in PNS regeneration, however, is unknown. Here we demonstrate that Sarm1 is not required for axotomy induced activation of neuron-intrinsic growth programs and axonal growth into a nerve crush site. However, in the distal nerve, Sarm1 is necessary for the timely induction of the Schwann cell (SC) repair response, nerve inflammation, myelin clearance, and regeneration of sensory and motor axons. In Sarm1-/- mice, regenerated fibers exhibit reduced axon caliber, defective nerve conduction, and recovery of motor function is delayed. The growth hostile environment of Sarm1-/- distal nerve tissue was demonstrated by grafting of Sarm1-/- nerve into WT recipients. SC lineage tracing in injured WT and Sarm1-/- mice revealed morphological differences. In the Sarm1-/- distal nerve, the appearance of p75NTR+, c-Jun+ SCs is significantly delayed. Ex vivo, p75NTR and c-Jun upregulation in Sarm1-/- nerves can be rescued by pharmacological inhibition of ErbB kinase. Together, our studies show that Sarm1 is not necessary for the activation of neuron intrinsic growth programs but in the distal nerve is required for the orchestration of cellular programs that underlie rapid axon extension.
RESUMEN
Localized translation of axonal mRNAs contributes to developmental and regenerative axon growth. Although untranslated regions (UTRs) of many different axonal mRNAs appear to drive their localization, there has been no consensus RNA structure responsible for this localization. We recently showed that limited expression of ZBP1 protein restricts axonal localization of both ß-actin and GAP-43 mRNAs. ß-actin 3'UTR has a defined element for interaction with ZBP1, but GAP-43 mRNA shows no homology to this RNA sequence. Here, we show that an AU-rich regulatory element (ARE) in GAP-43's 3'UTR is necessary and sufficient for its axonal localization. Axonal GAP-43 mRNA levels increase after in vivo injury, and GAP-43 mRNA shows an increased half-life in regenerating axons. GAP-43 mRNA interacts with both HuD and ZBP1, and HuD and ZBP1 co-immunoprecipitate in an RNA-dependent fashion. Reporter mRNA with the GAP-43 ARE competes with endogenous ß-actin mRNA for axonal localization and decreases axon length and branching similar to the ß-actin 3'UTR competing with endogenous GAP-43 mRNA. Conversely, over-expressing GAP-43 coding sequence with its 3'UTR ARE increases axonal elongation and this effect is lost when just the ARE is deleted from GAP-43's 3'UTR. We have recently found that over-expression of GAP-43 using an axonally targeted construct with the 3'UTRs of GAP-43 promoted elongating growth of axons, while restricting the mRNA to the cell body with the 3'UTR of γ-actin had minimal effect on axon length. In this study, we show that the ARE in GAP-43's 3'UTR is responsible for localization of GAP-43 mRNA into axons and is sufficient for GAP-43 protein's role in elongating axonal growth.
Asunto(s)
Regiones no Traducidas 3'/genética , Elementos Ricos en Adenilato y Uridilato/genética , Axones/metabolismo , Proteína GAP-43/genética , Proteína GAP-43/metabolismo , ARN Mensajero/genética , ARN Mensajero/metabolismo , Proteínas de Unión al ARN/genética , Proteínas de Unión al ARN/metabolismo , Actinas/biosíntesis , Actinas/genética , Animales , Axones/ultraestructura , Células Cultivadas , ADN/biosíntesis , ADN/aislamiento & purificación , Recuperación de Fluorescencia tras Fotoblanqueo , Ganglios Espinales/citología , Ganglios Espinales/metabolismo , Inmunoprecipitación , Hibridación Fluorescente in Situ , Masculino , ARN/biosíntesis , ARN/genética , Ratas , Ratas Sprague-Dawley , Elementos Reguladores de la Transcripción , Nervio Ciático/metabolismoRESUMEN
Receptor-mediated autophagic turnover of portions of the endoplasmic reticulum (ER) is mediated by macro-ER-phagy. We hypothesized macro-ER-phagy promotes proteotoxic stress resistance. We predicted Saccharomyces cerevisiae lacking macro-ER-phagy receptors would exhibit enhanced sensitivity to hygromycin B, which reduces translational fidelity and is expected to globally disrupt protein homeostasis, including at the ER. We observed that loss of either of two yeast macro-ER-phagy receptors (Atg39p or Atg40p) compromised cellular resistance to hygromycin B to a similar extent as loss of ER-associated degradation (ERAD) ubiquitin ligases Hrd1p and Doa10p. Our data are consistent with a model whereby macro-ER-phagy and ERAD collaborate to mediate ER protein quality control. Disruptions of macro-ER-phagy have been linked to neuropathy, dementia, and cancer. A dampened capacity to mediate protein quality control may contribute to these conditions.
RESUMEN
Upon trauma, the adult murine peripheral nervous system (PNS) displays a remarkable degree of spontaneous anatomical and functional regeneration. To explore extrinsic mechanisms of neural repair, we carried out single-cell analysis of naïve mouse sciatic nerve, peripheral blood mononuclear cells, and crushed sciatic nerves at 1 day, 3 days, and 7 days following injury. During the first week, monocytes and macrophages (Mo/Mac) rapidly accumulate in the injured nerve and undergo extensive metabolic reprogramming. Proinflammatory Mo/Mac with a high glycolytic flux dominate the early injury response and rapidly give way to inflammation resolving Mac, programmed toward oxidative phosphorylation. Nerve crush injury causes partial leakiness of the blood-nerve barrier, proliferation of endoneurial and perineurial stromal cells, and entry of opsonizing serum proteins. Micro-dissection of the nerve injury site and distal nerve, followed by single-cell RNA-sequencing, identified distinct immune compartments, triggered by mechanical nerve wounding and Wallerian degeneration, respectively. This finding was independently confirmed with Sarm1-/- mice, in which Wallerian degeneration is greatly delayed. Experiments with chimeric mice showed that wildtype immune cells readily enter the injury site in Sarm1-/- mice, but are sparse in the distal nerve, except for Mo. We used CellChat to explore intercellular communications in the naïve and injured PNS and report on hundreds of ligand-receptor interactions. Our longitudinal analysis represents a new resource for neural tissue regeneration, reveals location- specific immune microenvironments, and reports on large intercellular communication networks. To facilitate mining of scRNAseq datasets, we generated the injured sciatic nerve atlas (iSNAT): https://cdb-rshiny.med.umich.edu/Giger_iSNAT/.
Asunto(s)
Traumatismos de los Nervios Periféricos , Degeneración Walleriana , Ratones , Animales , Degeneración Walleriana/metabolismo , Degeneración Walleriana/patología , Leucocitos Mononucleares , Nervio Ciático/metabolismo , Degeneración Nerviosa , Compresión Nerviosa , Traumatismos de los Nervios Periféricos/metabolismo , Regeneración Nerviosa , Proteínas del Citoesqueleto/metabolismo , Proteínas del Dominio Armadillo/metabolismoRESUMEN
Neuronal injury can cause mitochondrial damage, leading to reduced energy production, decreased Ca2+ storage capacity, and increased reactive oxygen species. A new study reveals a mechanism to trigger the axonal transport of previously anchored mitochondria and promote neuroprotection and axon regeneration by replacing damaged with functional mitochondria.
Asunto(s)
Axones , Regeneración Nerviosa , Transporte Axonal , Axones/metabolismo , Neurobiología , NeuronasRESUMEN
Neurons are the cells of the nervous system and are responsible for every thought, movement and perception. Immune cells are the cells of the immune system, constantly protecting from foreign pathogens. Understanding the interaction between the two systems is especially important in disease states such as autoimmune or neurodegenerative disease. Unfortunately, this interaction is typically detrimental to the host. However, recent efforts have focused on how neurons and immune cells interact, either directly or indirectly, following traumatic injury to the nervous system. The outcome of this interaction can be beneficial - leading to successful neural repair, or detrimental - leading to functional deficits, depending on where the injury occurs. This review will discuss our understanding of neuron-immune cell interactions after traumatic injury to both the peripheral and central nervous system.
RESUMEN
Sciatic nerve crush injury triggers sterile inflammation within the distal nerve and axotomized dorsal root ganglia (DRGs). Granulocytes and pro-inflammatory Ly6Chigh monocytes infiltrate the nerve first and rapidly give way to Ly6Cnegative inflammation-resolving macrophages. In axotomized DRGs, few hematogenous leukocytes are detected and resident macrophages acquire a ramified morphology. Single-cell RNA-sequencing of injured sciatic nerve identifies five macrophage subpopulations, repair Schwann cells, and mesenchymal precursor cells. Macrophages at the nerve crush site are molecularly distinct from macrophages associated with Wallerian degeneration. In the injured nerve, macrophages 'eat' apoptotic leukocytes, a process called efferocytosis, and thereby promote an anti-inflammatory milieu. Myeloid cells in the injured nerve, but not axotomized DRGs, strongly express receptors for the cytokine GM-CSF. In GM-CSF-deficient (Csf2-/-) mice, inflammation resolution is delayed and conditioning-lesion-induced regeneration of DRG neuron central axons is abolished. Thus, carefully orchestrated inflammation resolution in the nerve is required for conditioning-lesion-induced neurorepair.
Asunto(s)
Ganglios Espinales/inmunología , Leucocitos/inmunología , Macrófagos/inmunología , Regeneración Nerviosa , Traumatismos de los Nervios Periféricos/inmunología , Fagocitosis , Nervio Ciático/inmunología , Animales , Apoptosis , Células Cultivadas , Subunidad beta Común de los Receptores de Citocinas/genética , Subunidad beta Común de los Receptores de Citocinas/metabolismo , Modelos Animales de Enfermedad , Femenino , Ganglios Espinales/metabolismo , Ganglios Espinales/patología , Regulación de la Expresión Génica , Redes Reguladoras de Genes , Factor Estimulante de Colonias de Granulocitos y Macrófagos/genética , Factor Estimulante de Colonias de Granulocitos y Macrófagos/metabolismo , Mediadores de Inflamación/metabolismo , Leucocitos/metabolismo , Leucocitos/patología , Macrófagos/metabolismo , Macrófagos/patología , Masculino , Ratones Endogámicos C57BL , Ratones Noqueados , Monocitos/inmunología , Monocitos/metabolismo , Proyección Neuronal , Traumatismos de los Nervios Periféricos/genética , Traumatismos de los Nervios Periféricos/metabolismo , Traumatismos de los Nervios Periféricos/patología , Receptores de Factor Estimulante de Colonias de Granulocitos y Macrófagos/genética , Receptores de Factor Estimulante de Colonias de Granulocitos y Macrófagos/metabolismo , Nervio Ciático/lesiones , Nervio Ciático/metabolismo , Nervio Ciático/patología , Transducción de SeñalRESUMEN
Inhibition of histone deacetylase 6 (HDAC6) was shown to support axon growth on the nonpermissive substrates myelin-associated glycoprotein (MAG) and chondroitin sulfate proteoglycans (CSPGs). Though HDAC6 deacetylates α-tubulin, we find that another HDAC6 substrate contributes to this axon growth failure. HDAC6 is known to impact transport of mitochondria, and we show that mitochondria accumulate in distal axons after HDAC6 inhibition. Miro and Milton proteins link mitochondria to motor proteins for axon transport. Exposing neurons to MAG and CSPGs decreases acetylation of Miro1 on Lysine 105 (K105) and decreases axonal mitochondrial transport. HDAC6 inhibition increases acetylated Miro1 in axons, and acetyl-mimetic Miro1 K105Q prevents CSPG-dependent decreases in mitochondrial transport and axon growth. MAG- and CSPG-dependent deacetylation of Miro1 requires RhoA/ROCK activation and downstream intracellular Ca2+ increase, and Miro1 K105Q prevents the decrease in axonal mitochondria seen with activated RhoA and elevated Ca2+ These data point to HDAC6-dependent deacetylation of Miro1 as a mediator of axon growth inhibition through decreased mitochondrial transport.
Asunto(s)
Histona Desacetilasa 6/genética , Mitocondrias/metabolismo , Neuronas/metabolismo , Proteínas de Unión al GTP rho/genética , Quinasas Asociadas a rho/genética , Acetilación/efectos de los fármacos , Animales , Transporte Axonal/efectos de los fármacos , Transporte Axonal/genética , Calcio/metabolismo , Proteoglicanos Tipo Condroitín Sulfato/farmacología , Femenino , Ganglios Espinales/citología , Ganglios Espinales/efectos de los fármacos , Ganglios Espinales/metabolismo , Regulación de la Expresión Génica , Histona Desacetilasa 6/metabolismo , Masculino , Ratones , Ratones Endogámicos C57BL , Mitocondrias/efectos de los fármacos , Glicoproteína Asociada a Mielina/farmacología , Neuronas/citología , Neuronas/efectos de los fármacos , Cultivo Primario de Células , Ratas , Ratas Sprague-Dawley , Transducción de Señal , Proteínas de Unión al GTP rho/metabolismo , Quinasas Asociadas a rho/metabolismoRESUMEN
How is protein synthesis initiated locally in neurons? We found that mTOR (mechanistic target of rapamycin) was activated and then up-regulated in injured axons, owing to local translation of mTOR messenger RNA (mRNA). This mRNA was transported into axons by the cell size-regulating RNA-binding protein nucleolin. Furthermore, mTOR controlled local translation in injured axons. This included regulation of its own translation and that of retrograde injury signaling molecules such as importin ß1 and STAT3 (signal transducer and activator of transcription 3). Deletion of the mTOR 3' untranslated region (3'UTR) in mice reduced mTOR in axons and decreased local translation after nerve injury. Both pharmacological inhibition of mTOR in axons and deletion of the mTOR 3'UTR decreased proprioceptive neuronal survival after nerve injury. Thus, mRNA localization enables spatiotemporal control of mTOR pathways regulating local translation and long-range intracellular signaling.
Asunto(s)
Axones/metabolismo , Ganglios Espinales/lesiones , Biosíntesis de Proteínas , Nervio Ciático/lesiones , Serina-Treonina Quinasas TOR/biosíntesis , Regiones no Traducidas 3' , Animales , Tamaño de la Célula , Ratones , Ratones Endogámicos , Fosfoproteínas/metabolismo , ARN Mensajero/metabolismo , Proteínas de Unión al ARN/metabolismo , Ratas , Ratas Endogámicas BB , Ratas Sprague-Dawley , Transducción de Señal , Serina-Treonina Quinasas TOR/genética , NucleolinaRESUMEN
The inhibitory environment of the spinal cord and the intrinsic properties of neurons prevent regeneration of axons following CNS injury. However, both ascending and descending axons of the injured spinal cord have been shown to regenerate into grafts of embryonic neural progenitor cells (NPCs). Previous studies have shown that grafts composed of glial-restricted progenitors (GRPs) and neural-restricted progenitors (NRPs) can provide a permissive microenvironment for axon growth. We have used cocultures of adult rat dorsal root ganglion (DRG) neurons together with NPCs, which have shown significant enhancement of axon growth by embryonic rat GRP and GRPs/NRPs, both in coculture conditions and when DRGs are exposed to conditioned medium from the NPC cultures. This growth-promoting effect of NPC-conditioned medium was also seen in injury-conditioned neurons. DRGs cocultured with GRPs/NRPs showed altered expression of regeneration-associated genes at transcriptional and post-transcriptional levels. We found that levels of GAP-43 mRNA increased in DRG cell bodies and axons. However, hepcidin antimicrobial peptide (HAMP) mRNA decreased in the cell bodies of DRGs cocultured with GRPs/NRPs, which is distinct from the increase in cell body HAMP mRNA levels seen in DRGs after injury conditioning. Endogenous GAP-43 and ß-actin mRNAs as well as reporter RNAs carrying axonally localizing 3'UTRs of these transcripts showed significantly increased levels in distal axons in the DRGs cocultured with GRPs/NRPs. These results indicate that axon growth promoted by NPCs is associated not only with enhanced transcription of growth-associated genes but also can increase localization of some mRNAs into growing axons.
Asunto(s)
Axones/metabolismo , Células-Madre Neurales/metabolismo , Proyección Neuronal/fisiología , ARN Mensajero/metabolismo , Actinas/administración & dosificación , Actinas/metabolismo , Animales , Células Cultivadas , Técnicas de Cocultivo , Medios de Cultivo Condicionados , Femenino , Proteína GAP-43/administración & dosificación , Proteína GAP-43/metabolismo , Ganglios Espinales/citología , Ganglios Espinales/metabolismo , Regulación de la Expresión Génica/fisiología , Hepcidinas/metabolismo , Masculino , Células-Madre Neurales/citología , Neuroglía/citología , Neuroglía/metabolismo , Ratas Sprague-Dawley , Médula Espinal/citología , Médula Espinal/metabolismoRESUMEN
Although initially argued to be a feature of immature neurons with incomplete polarization, there is clear evidence that neurons in the peripheral nervous system retain the capacity for intra-axonal protein synthesis well into adulthood. This localized protein synthesis has been shown to contribute to injury signaling and axon regeneration in peripheral nerves. Recent works point to potential for protein synthesis in axons of the vertebrate central nervous system. mRNAs and protein synthesis machinery have now been documented in lamprey, mouse, and rat spinal cord axons. Intra-axonal protein synthesis appears to be activated in adult vertebrate spinal cord axons when they are regeneration-competent. Rat spinal cord axons regenerating into a peripheral nerve graft contain mRNAs and markers of activated translational machinery. Indeed, levels of some growth-associated mRNAs in these spinal cord axons are comparable to the regenerating sciatic nerve. Markers of active translation tend to decrease when these axons stop growing, but can be reactivated by a second axotomy. These emerging observations raise the possibility that mRNA transport into and translation within axons could be targeted to facilitate regeneration in both the peripheral and central nervous systems.
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How can cells sense their own size to coordinate biosynthesis and metabolism with their growth needs? We recently proposed a motor-dependent bidirectional transport mechanism for axon length and cell size sensing, but the nature of the motor-transported size signals remained elusive. Here, we show that motor-dependent mRNA localization regulates neuronal growth and cycling cell size. We found that the RNA-binding protein nucleolin is associated with importin ß1 mRNA in axons. Perturbation of nucleolin association with kinesins reduces its levels in axons, with a concomitant reduction in axonal importin ß1 mRNA and protein levels. Strikingly, subcellular sequestration of nucleolin or importin ß1 enhances axonal growth and causes a subcellular shift in protein synthesis. Similar findings were obtained in fibroblasts. Thus, subcellular mRNA localization regulates size and growth in both neurons and cycling cells.
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Tamaño de la Célula , Neuronas Motoras/metabolismo , Fosfoproteínas/metabolismo , ARN Mensajero/metabolismo , Proteínas de Unión al ARN/metabolismo , Animales , Axones/metabolismo , Ratones Transgénicos , Neurogénesis , Biosíntesis de Proteínas/fisiología , NucleolinaRESUMEN
The remarkably poor regeneration of axons seen after injury of the brain and spinal cord can result in permanent loss of neural function. This failure of meaningful regeneration has been attributed to both a low intrinsic growth potential of CNS neurons and extrinsic factors that actively block axon growth in the adult CNS. Injury exacerbates this situation by increasing the expression of and exposure to proteins that actively block axonal growth in the CNS. Much experimental efforts have been aimed at overcoming the extrinsic growth inhibitory environment of the injured brain and spinal cord. A recent publication in Experimental Neurology from Kuboyama and colleagues shows that activation of protein kinase A signaling is responsible for the stalling of axon growth in gradients of CNS inhibitory molecules. This observation is unexpected given the role of cAMP signaling in supporting intrinsic growth mechanisms, emphasizing the need to consider spatial and temporal aspects of intracellular signaling in future strategies for neural repair.