The Wlds mutation affords protection of retinal ganglion cell (RGC) axons in retinal ischemia and in inducible and hereditary preclinical models of glaucoma. We undertook the present study to determine whether the Nmnat1 portion of the chimeric protein provides axonal and somatic protection of RGCs in models of ischemia and glaucoma, particularly when localized to nonnuclear regions of the cell.
The survival and integrity of RGC axons and soma from transgenic mice with confirmed cytoplasmic overexpression of Nmnat1 in retina and optic nerve (cytNmnat1-Tg mice) were examined in the retina and postlaminar optic nerve 4 days following acute retinal ischemia, and 3 weeks following the chronic elevation of intraocular pressure.
Ischemia- and glaucoma-induced disruptions of proximal segments of RGC axons that comprise the nerve fiber layer in wild-type mice were both robustly abrogated in cytNmnat1-Tg mice. More distal portions of RGC axons within the optic nerve were also protected from glaucomatous disruption in the transgenic mice. In both disease models, Nmnat1 overexpression in extranuclear locations significantly enhanced the survival of RGC soma.
Overexpression of Nmnat1 in the cytoplasm and axons of RGCs robustly protected against both ischemic and glaucomatous loss of RGC axonal integrity, as well as loss of RGC soma. These findings reflect the more pan-cellular protection of CNS neurons that is realized by cytoplasmic Nmnat1 expression, and thus provide a therapeutic strategy for protecting against retinal neurodegeneration, and perhaps other CNS neurodegenerative diseases as well.
This study demonstrates that Nmnat1 overexpression targeted to the cytoplasm affords protection of retinal ganglion cell soma and axons from acute ischemic injury and chronic glaucomatous neurodegeneration; thus, this enzyme could be a therapeutic target for treating these diseases.
Here we demonstrate that the dual leucine zipper kinase (DLK) promotes robust regeneration of peripheral axons following nerve injury in mice. Peripheral axon regeneration is accelerated by prior injury, however DLK KO neurons do not respond to a preconditioning lesion with enhanced regeneration in vivo or in vitro. Assays for activation of transcription factors in injury-induced pro-regenerative pathways reveal that loss of DLK abolishes upregulation of p-STAT3 and p-cJun in the cell body following axonal injury. DLK is not required for the phosphorylation of STAT3 at the site of nerve injury, but is necessary for retrograde transport of p-STAT3 to the cell body. These data demonstrate that DLK enhances regeneration by promoting a retrograde injury signal that is required for the activation of the neuronal pro-regenerative program.
Axon degeneration is an evolutionarily conserved process that drives the loss of damaged axons and is an early event in many neurological disorders, and so it is important to identify the molecular constituents of this poorly understood mechanism. Here we demonstrate that the Phr1 E3 ubiquitin ligase is a central component of this axon degeneration program. Loss of Phr1 results in prolonged survival of severed axons both in the peripheral and central nervous systems, as well as preservation of motor and sensory nerve terminals. Phr1 depletion increases the axonal level of the axon survival molecule nicotinamide mononucleotide adenyltransferase 2 (NMNAT2), and NMNAT2 is necessary to mediate Phr1-dependent axon stability. The profound long-term protection of peripheral and central mammalian axons following Phr1 deletion suggests that pharmacological inhibition of Phr1 function may be an attractive therapeutic candidate for ameliorating axon loss in neurological disease.
The regenerative response of Schwann cells after peripheral nerve injury is a critical process directly related to the pathophysiology of a number of neurodegenerative diseases. This SC injury response is dependent on an intricate gene regulatory program coordinated by a number of transcription factors and microRNAs, but the interactions among them remain largely unknown. Uncovering the transcriptional and post-transcriptional regulatory networks governing the Schwann cell injury response is a key step towards a better understanding of Schwann cell biology and may help develop novel therapies for related diseases. Performing such comprehensive network analysis requires systematic bioinformatics methods to integrate multiple genomic datasets.
In this study we present a computational pipeline to infer transcription factor and microRNA regulatory networks. Our approach combined mRNA and microRNA expression profiling data, ChIP-Seq data of transcription factors, and computational transcription factor and microRNA target prediction. Using mRNA and microRNA expression data collected in a Schwann cell injury model, we constructed a regulatory network and studied regulatory pathways involved in Schwann cell response to injury. Furthermore, we analyzed network motifs and obtained insights on cooperative regulation of transcription factors and microRNAs in Schwann cell injury recovery.
This work demonstrates a systematic method for gene regulatory network inference that may be used to gain new information on gene regulation by transcription factors and microRNAs.
Transcriptional regulatory network; MicroRNA regulatory network; Myelination; Schwann cells
We have investigated the source(s) and targeting of components to PNS nodes of Ranvier. We show adhesion molecules are diffusible within the axon membrane and accumulate at forming nodes from local axonal sources. In contrast, ion channels and cytoskeletal components exhibit limited planar mobility and require transport to the node. We have characterized further targeting of NF186, an axonal adhesion molecule, which pioneers node formation. NF186 redistributes to nascent nodes from a mobile, surface pool. This initial accumulation, and clearance from the internode require extracellular interactions whereas subsequent targeting to mature nodes, i.e. those flanked by paranodal junctions, requires intracellular interactions. After incorporation into the node, NF186 is immobile, stable, and promotes node integrity. Thus nodes assemble from two sources: adhesion molecules, which initiate assembly, accumulate by diffusion trapping via interactions with Schwann cells whereas ion channels and cytoskeletal components accumulate via subsequent transport. In mature nodes, components turnover slowly and are replenished via transport.
AMP activated protein kinase (AMPK) plays a key role in the regulatory network responsible for maintaining systemic energy homeostasis during exercise or nutrient deprivation. To understand the function of the regulatory β2 subunit of AMPK in systemic energy metabolism, we characterized β2 subunit-deficient mice. Using these mutant mice, we demonstrated that the β2 subunit plays an important role in regulating glucose, glycogen, and lipid metabolism during metabolic stress. The β2 mutant animals failed to maintain euglycemia and muscle ATP levels during fasting. In addition, β2-deficient animals showed classic symptoms of metabolic syndrome, including hyperglycemia, glucose intolerance, and insulin resistance when maintained on a high-fat diet (HFD), and were unable to maintain muscle ATP levels during exercise. Cell surface-associated glucose transporter levels were reduced in skeletal muscle from β2 mutant animals on an HFD. In addition, they displayed poor exercise performance and impaired muscle glycogen metabolism. These mutant mice had decreased activation of AMPK and deficits in PGC1α-mediated transcription in skeletal muscle. Our results highlight specific roles of AMPK complexes containing the β2 subunit and suggest the potential utility of AMPK isoform-specific pharmacological modulators for treatment of metabolic, cardiac, and neurological disorders.
Alterations in mitochondrial dynamics (fission, fusion and movement) are implicated in many neurodegenerative diseases, from rare genetic disorders such as Charcot-Marie-Tooth disease, to common conditions including Alzheimer’s disease. However, the relationship between altered mitochondrial dynamics and neurodegeneration is incompletely understood. Here we show that disease associated MFN2 proteins suppressed both mitochondrial fusion and transport, and produced classic features of segmental axonal degeneration without cell body death, including neurofilament filled swellings, loss of calcium homeostasis, and accumulation of reactive oxygen species. By contrast, depletion of Opa1 suppressed mitochondrial fusion while sparing transport, and did not induce axonal degeneration. Axon degeneration induced by mutant MFN2 proteins correlated with the disruption of the proper mitochondrial positioning within axons, rather than loss of overall mitochondrial movement, or global mitochondrial dysfunction. We also found that augmenting expression of MFN1 rescued the axonal degeneration caused by MFN2 mutants, suggesting a possible therapeutic strategy for Charcot-Marie-Tooth disease. These experiments provide evidence that the ability of mitochondria to sense energy requirements and localize properly within axons is key to maintaining axonal integrity, and may be a common pathway by which disruptions in axonal transport contribute to neurodegeneration.
mitochondria; neurodegeneration; Charcot-Marie-Tooth Disease; axonal transport; mitofusin 2
In the peripheral nervous system, Schwann cells (SCs) surrounding damaged axons undergo an injury response that is driven by an intricate transcriptional program and is critical for nerve regeneration. To examine whether these injury-induced changes in SCs are also regulated posttranscriptionally by miRNAs, we performed miRNA expression profiling of mouse sciatic nerve distal segment after crush injury. We also characterized the SC injury response in mice containing SCs with disrupted miRNA processing due to loss of Dicer. We identified 87 miRNAs that were expressed in mouse adult peripheral nerve, 48 of which were dynamically regulated after nerve injury. Most of these injury-regulated SC miRNAs were computationally predicted to inhibit drivers of SC dedifferentiation/proliferation and thereby re-enforce the transcriptional program driving SC remyelination. SCs deficient in miRNAs manifested a delay in the transition between the distinct differentiation states required to support peripheral nerve regeneration. Among the miRNAs expressed in adult mouse SCs, miR-34a and miR-140 were identified as functional regulators of SC dedifferentiation/proliferation and remyelination, respectively. We found that miR-34a interacted with positive regulators of dedifferentiation and proliferation such as Notch1 and Ccnd1 to control cell cycle dynamics in SCs. miR-140 targeted the transcription factor Egr2, a master regulator of myelination, and modulated myelination in DRG/SC cocultures. Together, these results demonstrate that SC miRNAs are important modulators of the SC regenerative response after nerve damage.
Mitochondrial dysfunction is a common cause of peripheral neuropathies. While the role of neuron and axonal mitochondria in peripheral nerve disease is well appreciated, whether Schwann cell (SC) mitochondrial deficits contribute to peripheral neuropathies is unclear. Here we examine how SC mitochondrial dysfunction affects axonal survival and contributes to the decline of peripheral nerve function by generating mice with SC-specific mitochondrial deficits. These mice (Tfam-SCKOs) were produced through the tissue-specific deletion of the mitochondrial transcription factor A gene (Tfam), which is essential for mitochondrial DNA (mtDNA) transcription and maintenance. Tfam-SCKOs were viable but, as they aged, they developed a progressive peripheral neuropathy characterized by nerve conduction abnormalities as well as extensive muscle denervation. Morphological examination of Tfam-SCKO nerves revealed early preferential loss of small unmyelinated fibers followed by prominent demyelination and degeneration of larger-caliber axons. Tfam-SCKOs displayed sensory and motor deficits consistent with this pathology. Remarkably, the severe mtDNA depletion and respiratory chain abnormalities in Tfam-SCKO mice did not affect SC proliferation or survival. Mitochondrial function in SCs is therefore essential for maintenance of axonal survival and normal peripheral nerve function, suggesting that SC mitochondrial dysfunction contributes to human peripheral neuropathies.
TFAM; metabolism; mitochondrial dysfunction; axonal degeneration
Mitochondrial dysfunction is a central mediator of disease progression in diverse neurodegenerative diseases that often present with prominent gastrointestinal abnormalities. Gastrointestinal dysfunction in these disorders is related, at least in part, to defects in the enteric nervous system (ENS). The role of mitochondrial deficits in ENS neurodegeneration and their relative contribution to gastrointestinal dysfunction, however, are unclear. To better understand how mitochondrial abnormalities in the ENS influence enteric neurodegeneration and affect intestinal function, we generated mice (Tfam-ENSKOs) with impaired mitochondrial metabolism in enteric neurons and glia through the targeted deletion of the mitochondrial transcription factor A gene (Tfam). Tfam-ENSKO mice were initially viable but, at an early age, they developed severe gastrointestinal motility problems characterized by intestinal pseudo-obstruction resulting in premature death. This gastrointestinal dysfunction was caused by extensive, progressive neurodegeneration of the ENS involving both neurons and glia. Interestingly, mitochondrial defects differentially affected specific subpopulations of enteric neurons and regions of the gastrointestinal tract. Mitochondrial deficiency-related neuronal and glial loss was most prominent in the proximal small intestine, but the first affected neurons, nitrergic inhibitory neurons, had the greatest losses in the distal small intestine. This regional and subtype-specific variability in susceptibility to mitochondrial defects resulted in an imbalance of inhibitory and excitatory neurons that likely accounts for the observed phenotype in Tfam-ENSKO mice. Mitochondrial dysfunction, therefore, is likely to be an important driving force of neurodegeneration in the ENS and contribute to gastrointestinal symptoms in people with neurodegenerative disorders.
Axonal degeneration is a hallmark of many neurological disorders. Studies in animal models of neurodegenerative diseases indicate that axonal degeneration is an early event in the disease process, and delaying this process can lead to decreased progression of the disease and survival extension. Overexpression of the Wallerian degeneration slow (Wlds) protein can delay axonal degeneration initiated via axotomy, chemotherapeutic agents, or genetic mutations. The Wlds protein consists of the N-terminal portion of the ubiquitination factor Ube4b fused to the nicotinamide adenine dinucleotide (NAD+) biosynthetic enzyme nicotinamide mononucleotide adenylyl transferase 1 (Nmnat1). We previously showed that the Nmnat1 portion of this fusion protein was the critical moiety for Wlds-mediated axonal protection. Here, we describe the development of an automated quantitative assay for assessing axonal degeneration. This method successfully showed that Nmnat1 enzymatic activity is important for axonal protection as mutants with reduced enzymatic activity lacked axon protective activity. We also found that Nmnat enzymes with diverse sequences and structures from various species, including Drosophila melanogaster, Saccharomyces cerevisiae, and archaebacterium Methanocaldococcus jannaschii, which encodes a protein with no homology to eukaryotic Nmnat enzymes, all mediate robust axonal protection after axotomy. Besides the importance of Nmnat enzymatic activity, we did not observe changes in the steady-state NAD+ level, and we found that inhibition of nicotinamide phosphoribosyltransferase (Nampt), which synthesizes substrate for Nmnat in mammalian cells, did not affect the protective activity of Nmnat1. These results provide the possibility of a role for new Nmnat enzymatic activity in axonal protection in addition to NAD+ synthesis.
Axonal degeneration is a hallmark of many debilitating neurological disorders and is thought to be regulated by mechanisms distinct from those governing cell body death. Recently, caspase 6 activation via APP cleavage and activation of DR6 was discovered to induce axon degeneration after NGF withdrawal. We tested whether this pathway is involved in axonal degeneration caused by withdrawal of other trophic support, axotomy or vincristine exposure. Neurturin deprivation, like NGF withdrawal activated this APP/DR6/caspase 6 pathway and resulted in axonal degeneration, however, APP cleavage and caspase 6 activation were not involved in axonal degeneration induced by mechanical or toxic insults. However, loss of surface APP (sAPP) and caspase 6 activation were observed during axonal degeneration induced by dynactin 1(Dctn1) dysfunction, which disrupts axonal transport. Mutations in Dctn1 are associated with motor neuron disease and frontal temporal dementia, thus suggesting that the APP/caspase 6 pathway could be important in specific types of disease-associated axonal degeneration. The NGF deprivation paradigm, with its defined molecular pathway, was used to examine the context of Nmnat-mediated axonal protection. We found that although Nmnat blocks axonal degeneration after trophic factor withdrawal, it did not prevent loss of axon sAPP or caspase 6 activation within the axon, suggesting it acts downstream of caspase 6. These results indicate that diverse insults induce axonal degeneration via multiple pathways and that these degeneration signals converge on a common, Nmnat-sensitive program that is uniquely involved in axonal, but not cell body, degeneration.
axonal degeneration; Nmnat; Wlds; NGF; apoptosis
RNA interference (RNAi) is a useful in vitro research tool, but its application as a safe and effective therapeutic agent may benefit from improved understanding of mechanisms of exogenous siRNA delivery, including cell trafficking and sorting patterns. We report the development of a transfection reagent for siRNA delivery which employs a distinctive non-digestive mode of particle-cell membrane interaction through the formation of a hemifusion complex resulting in lipid-raft transport of cargo to the cytosol, bypassing the usual endosomal nanoparticle uptake pathway. We further demonstrate markedly enhanced efficacy over conventional transfection agents for suppressing endothelial cell expression of upregulated vascular adhesion molecules.
Mitofusins (Mfn1 and Mfn2) are outer mitochondrial membrane proteins involved in regulating mitochondrial dynamics. Mutations in Mfn2 cause Charcot-Marie-Tooth disease (CMT) type 2A, an inherited disease characterized by degeneration of long peripheral axons, but the nature of this tissue selectivity remains unknown. Here we present evidence that Mfn2 is directly involved in and required for axonal mitochondrial transport, distinct from its role in mitochondrial fusion. Live imaging of neurons cultured from Mfn2 knockout mice, or neurons expressing Mfn2 disease mutants, show that axonal mitochondria spend more time paused and undergo slower anterograde and retrograde movements, indicating an alteration in attachment to microtubule based transport systems. Furthermore, Mfn2 disruption altered mitochondrial movement selectively, leaving transport of other organelles intact. Importantly, both Mfn1 and Mfn2 interact with mammalian Miro (Miro1/Miro2) and Milton (OIP106/GRIF1) proteins, members of the molecular complex that link mitochondria to kinesin motors. Knockdown of Miro2 in cultured neurons produced transport deficits identical to loss of Mfn2, indicating that both proteins must be present at the outer membrane to mediate axonal mitochondrial transport. In contrast, disruption of mitochondrial fusion via knockdown of the inner mitochondrial membrane protein Opa1 had no effect on mitochondrial motility, indicating that loss of fusion does not inherently alter mitochondrial transport. These experiments identify a role for mitofusins in directly regulating mitochondrial transport, and offer important insight into the cell type specificity and molecular mechanisms of axonal degeneration in CMT2A and dominant optic atrophy.
mitochondria; Charcot-Marie-Tooth Disease; axonal transport; axonopathy; mitofusin 2; axon
Small unmyelinated sensory neurons classified as nociceptors are divided into two subpopulations based on phenotypic differences including expression of neurotrophic factor receptors. Approximately half of unmyelinated nociceptors express the NGF receptor TrkA and half express the GDNF Family Ligand (GFL) receptor Ret. The function of NGF/TrkA signaling in the TrkA population of nociceptors has been extensively studied and NGF/TrkA signaling is a well established mediator of pain. The GFLs are analgesic in models of neuropathic pain emphasizing the importance of understanding the physiological function of GFL/Ret signaling in nociceptors. However, perinatal lethality of Ret-null mice has precluded the study of the physiological role of GFL/Ret signaling in the survival, maintenance and function of nociceptors in viable mice. We deleted Ret exclusively in nociceptors by crossing nociceptor-specific Nav1.8 Cre and Ret conditional mice to produce Ret-Nav1.8 conditional knock out (CKO) mice. Loss of Ret exclusively in nociceptors results in a reduction in nociceptor number and size indicating Ret signaling is important for the survival and trophic support of these cells. Ret-Nav1.8 CKO mice exhibit reduced epidermal innervation, but normal central projections. In addition, Ret-Nav1.8 CKO mice have increased sensitivity to cold and increased formalin-induced pain, demonstrating that Ret signaling modulates the function of nociceptors in vivo. Enhanced inflammation-induced pain may be mediated by decreased Prostatic Acid Phosphatase (PAP) as PAP levels are markedly reduced in Ret-Nav1.8 CKO mice. The results of this study identify the physiological role of endogenous Ret signaling in the survival and function of nociceptors.
Ret; neurotrophic factor; GDNF; pain; inflammation; nociceptor
In mammals, the first step in the perception of form and texture is the activation of trigeminal or dorsal root ganglion (DRG) mechanosensory neurons, which are classified as either rapidly (RA) or slowly adapting (SA) according to their rates of adaptation to sustained stimuli. The molecular identities and mechanisms of development of RA and SA mechanoreceptors are largely unknown. We found that the “early Ret+” DRG neurons are RA mechanoreceptors, which form Meissner corpuscles, Pacinian corpuscles and longitudinal lanceolate endings. The central projections of these RA mechanoreceptors innervate layers III through V of the spinal cord and terminate within discrete subdomains of the dorsal column nuclei. Moreover, mice lacking Ret signaling components are devoid of Pacinian corpuscles and exhibit a dramatic disruption of RA mechanoreceptor projections to both the spinal cord and medulla. Thus, the early Ret+ neurons are RA mechanoreceptors and Ret signaling is required for the assembly of neural circuits underlying touch perception.
Axonal degeneration is a key component of many neurodegenerative diseases. Injured axons undergo a program of self-destruction termed Wallerian degeneration that is an active, well-regulated process. The pathways leading to axon fragmentation are uncharacterized, but experiments with wlds mutant mice led to the discovery that overexpression of nicotinamide mononucleotide adenylyltransferase (Nmnat1) or treatment with NAD+ can inhibit axonal degeneration. Here, we show that the purine nucleosides adenosine and guanosine, but not inosine, inhibit injury-induced axonal degeneration in cultured DRG neurons. Axons can be preserved by adding adenosine within 6 hr of the axonal injury. The presence of adenosine was required continuously after the injury to maintain axonal protection. Together these results suggest that adenosine does not alter the neuronal response to injury, but instead inhibits a local axonal pathway necessary for the commitment and/or execution of the axon destructive program.
Axonal Degeneration; adenosine; guanosine; purine nucleosides; Wallerian degeneration
The expression of NKX3.1, a transcriptional regulator and tumor suppressor gene in prostate cancer, is downregulated during early stages of prostate tumorigenesis. However, little is known of the alterations in gene expression that occur as a result of this event. We combined laser capture microdissection and gene expression profiling to analyze the molecular consequences of Nkx3.1 loss during prostate cancer initiation using Nkx3.1-deficient mice. This analysis identified a cohort of genes (loss-of-Nkx3.1 signature) that are aberrantly overexpressed during loss-of-Nkx3.1 driven tumor initiation. We studied the expression of these same genes in independent loss-of-Pten and c-myc overexpression prostate adenocarcinoma mouse models. Nkx3.1 expression is lost in prostate epithelial proliferation in both of these mouse models. However, Nkx3.1 loss is an early event of tumor development in the loss-of-Pten model, whereas it occurs at later stages in c-myc transgenic mice. A member of genes of the loss-of-Nkx3.1 signature, like clusterin and quiescin Q6, are highly expressed in prostatic hyperplasia and intraepithelial neoplasia (PIN) lesions that also lack Nkx3.1 in the Pten-deficient prostate, but not in similar lesions in the c-myc transgenic model. Meta-analysis of multiple prostate cancer gene expression datasets including those from loss-of-Nkx3.1, loss-of-Pten, c-myc overexpression, and constitutively active Akt prostate cancer models, further confirmed that genes associated with the loss-of-Nkx3.1 signature integrate with Pten-Akt signaling pathways, but do not overlap with molecular changes associated with the c-myc signaling pathway. In human prostate tissue samples, loss of NKX3.1 expression and corresponding clusterin overexpression are co-localized at sites of prostatic inflammatory atrophy, a possible very early stage of human prostate tumorigenesis. Collectively, these results suggest that the molecular consequences of NKX3.1 loss depend on the epithelial proliferative stage at which its expression is lost and, that alterations in the Pten-Akt-Nkx3.1 axis are important for prostate cancer initiation.
The receptor tyrosine kinase ret protooncogene (RET) is implicated in the pathogenesis of several diseases and in several developmental defects, particularly those in neural crest–derived structures and the genitourinary system. In order to further elucidate RET-mediated mechanisms that contribute to these diseases and decipher the basis for specificity in the pleiotropic effects of RET, we characterized development of the enteric and autonomic nervous systems in mice expressing RET9 or RET51 isoforms harboring mutations in tyrosine residues that act as docking sites for the adaptors Plcγ, Src, Shc, and Grb2. Using this approach, we found that development of the genitourinary system and the enteric and autonomic nervous systems is dependent on distinct RET-stimulated signaling pathways. Thus, mutation of RET51 at Y1062, a docking site for multiple adaptor proteins including Shc, caused distal colon aganglionosis reminiscent of Hirschsprung disease (HSCR). On the other hand, this mutation in RET9, which encodes an isoform that lacks the Grb2 docking site present in RET51, produced severe abnormalities in multiple organs. Mutations that abrogate RET-Plcγ binding, previously shown to produce features reminiscent of congenital anomalies of kidneys or urinary tract (CAKUT) syndrome, produced only minor abnormalities in the nervous system. Abrogating RET51-Src binding produced no major defects in these systems. These studies provide insight into the basis of organotypic specificity and redundancy in RET signaling within these unique systems and in diseases such as HSCR and CAKUT.
AMP activated protein kinase (AMPK) is an evolutionary conserved metabolic sensor that responds to alterations in cellular energy levels to maintain energy balance. While its role in metabolic homeostasis is well documented, its role in mammalian development is less clear. Here we demonstrate that mutant mice lacking the regulatory AMPK β1 subunit have profound brain abnormalities. The β1−/− mice show atrophy of the dentate gyrus and cerebellum, severe loss of neurons, oligodendrocytes and myelination throughout the CNS. These abnormalities stem from reduced AMPK activity with ensuing cell cycle defects in neural stem and progenitor cells (NPC). The β1−/− NPC deficits result from hypophosphorylation of the retinoblastoma protein (Rb), which is directly phosphorylated by AMPK at Ser804. The AMPK-Rb axis is utilized by both growth factors and energy restriction to increase NPC growth. Together, our results reveal that the metabolic sensor AMPK integrates growth factor signaling with cell cycle control to regulate brain development.
Axonal degeneration is a key component of a variety of neurological diseases. Studies using wlds mutant mice have demonstrated that delaying axonal degeneration slows disease course and prolongs survival in neurodegenerative disease models. The Wlds protein is normally localized to the nucleus, and contains the N-terminus of ubiquitination factor Ube4b fused to full length Nmnat1, an NAD biosynthetic enzyme. While Nmnat enzymatic activity is necessary for Wlds-mediated axonal protection, several important questions remain including whether the Ube4b component of Wlds also plays a role, and in which cellular compartment (nucleus vs. cytosol) the axonal protective effects of Nmnat activity are mediated. While Nmnat alone is clearly sufficient to delay axonal degeneration in cultured neurons, we sought to determine whether it was also sufficient to promote axonal protection in vivo. Using cytNmnat1, an engineered mutant of Nmnat1 localized only to the cytoplasm and axon, that provides more potent axonal protection than that afforded by Wlds or Nmnat1, we generated transgenic mice using the prion protein promoter (PrP). The sciatic nerve of these cytNmnat1 transgenic mice was transected and microscopic analysis of the distal nerve segment 7 days later revealed no evidence of axonal loss or myelin debris, indicating that Nmnat alone, without any other Wlds sequences, is all that is required to delay axonal degeneration in vivo. These results highlight the importance of understanding the mechanism of Nmnat-mediated axonal protection for the development of new treatment strategies for neurological disorders.
Nmnat; axonal degeneration; Wlds; transgenic mouse; NAD; neuromuscular junction
By stimulating collagen synthesis and myofibroblasts differentiation, transforming growth factor-β (TGF- β) plays a pivotal role in tissue repair and fibrosis. The early growth response-1 (Egr-1) transcription factor mediates profibrotic TGF-β responses, and its expression is elevated in biopsies from patients with scleroderma. NGF1-A-binding protein 2 (Nab2) is a conserved transcriptional cofactor that directly binds to Egr-1 and positively or negatively modulates Egr-1 target gene transcription. Despite the recognized importance of Nab2 in governing the intensity of Egr-1-dependent responses, the regulation and function of Nab2 in the context of fibrotic TGF-β signaling is unknown. Here we show that TGF-β caused a time-dependent stimulation of Nab2 protein and mRNA in normal fibroblasts. Ectopic expression of Nab2 in these cells blocked Egr-1-dependent transcriptional responses, and abrogated TGF-β-induced stimulation of collagen synthesis and myofibroblasts differentiation. These inhibitory effects of Nab2 involved recruitment of the NuRD chromatin remodeling complex to the COL1A2 promoter and were accompanied by reduced histone H4 acetylation. Mice with targeted deletion of Nab2 displayed increased collagen accumulation in the dermis, and genetic or siRNA-mediated loss of Nab2 in fibroblasts was associated with constitutively elevated collagen synthesis and accentuation of Egr-1-dependent TGF-β responses in vitro. Expression of Nab2 was markedly up-regulated in skin biopsies from patients with scleroderma, and was localized primarily to epidermal keratinocytes. In contrast, little Nab2 could be detected in dermal fibroblasts. These results identify Nab2 as a novel endogenous negative regulator of Egr-1-dependent TGF-β signaling responsible for setting the intensity of fibrotic responses. Defective Nab2 expression or function in dermal fibroblasts might play a role in persistent fibrotic responses in scleroderma.
Axon degeneration underlies many common neurological disorders, but the signaling pathways that orchestrate axon degeneration are unknown. We demonstrate that the dual leucine kinase (DLK) promotes degeneration of severed axons in Drosophila and mice, and its target JNK promotes degeneration locally in axons as they commit to degenerate. This pathway also promotes degeneration after chemotherapy exposure, and thus may be a component of a general axon self-destruction program.
Mouse models of human disease are helpful for understanding the pathogenesis of the disorder, and ultimately for testing potential therapeutic agents. Here we describe the engineering and characterization of a mouse carrying the I268N mutation in Egr2, observed in patients with recessively inherited Charcot-Marie-Tooth disease type 4E, which is predicted to alter the ability of Egr2 to interact with the Nab transcriptional coregulatory proteins. Mice homozygous for Egr2I268N develop a congenital hypomyelinating neuropathy similar to their human counterparts. Egr2I268N is expressed at normal levels in developing nerve, but is unable to interact with Nab proteins or to properly activate transcription of target genes critical for proper peripheral myelin development. Interestingly Egr2I268N/I268N mutant mice maintain normal weight and have only mild tremor until 2 weeks after birth, at which point they rapidly develop worsening weakness and uniformly die within several days. Nerve electrophysiology revealed conduction block, and neuromuscular junctions showed marked terminal sprouting similar to that seen in animals with pharmacologically induced blockade of action potentials or neuromuscular transmission. These studies describe a unique animal model of CMT whereby weakness is due to conduction block or neuromuscular junction failure rather than secondary axon loss, and demonstrate that the Egr2-Nab complex is critical for proper peripheral nerve myelination.
Myelin; peripheral nerve; Axon; neuromuscular junction; Nerve; Neuropathy
The GDNF family ligands (GFLs: GDNF, NRTN, ARTN, and PSPN), interact with GDNF family receptors (GFRαs) and activate intracellular signaling through the Ret receptor tyrosine kinase. To characterize the role of Ret signaling in retinal activity, we examined Ret hypomorphic and Ret conditional mice using electroretinography. We found that aberrant Ret function resulted in markedly diminished scotopic and photopic responses. Using mice deficient in individual GFLs, we found that only NRTN deficiency led to reduced retinal activity. To determine the potential target cell-type for NRTN, we examined the retinal expression of its coreceptors (GFRα1 and GFRα2) and Ret using mice expressing fluorescence reporter EGFP from their respective loci. We found robust GFRα1 and Ret expression in horizontal, amacrine, and ganglion cells, whereas GFRα2 expression was only detected in a subset of amacrine and ganglion cells. In contrast to previous studies, no expression of GFRα1, GFRα2, or Ret was detected in photoreceptors or Müller cells, suggesting these cells are not directly affected by Ret. Finally, detailed morphologic analyses of retinas from NRTN- and Ret-deficient mice demonstrated a reduction in normal horizontal cell dendrites and axons, abnormal extensions of horizontal cell and bipolar cell processes into the outer nuclear layer, and mislocalized synaptic complexes. These anatomic abnormalities indicate a possible basis for the abnormal retinal activity in the Ret and NRTN mutant mice.
RET; Neurturin; retina; ERG; horizontal; outer plexiform layer