Rescue of Glaucomatous Neurodegeneration by Differentially Modulating Neuronal Endoplasmic Reticulum Stress Molecules.

UNLABELLED: Axon injury is an early event in neurodegenerative diseases that often leads to retrograde neuronal cell death and progressive permanent loss of vital neuronal functions. The connection of these two obviously sequential degenerative events, however, is elusive. Deciphering the upstream signals that trigger the neurodegeneration cascades in both neuronal soma and axon would be a key step toward developing the effective neuroprotectants that are greatly needed in the clinic. We showed previously that optic nerve injury-induced neuronal endoplasmic reticulum (ER) stress plays an important role in retinal ganglion cell (RGC) death. Using two in vivo mouse models of optic neuropathies (traumatic optic nerve injury and glaucoma) and adeno-associated virus-mediated RGC-specific gene targeting, we now show that differential manipulation of unfolded protein response pathways in opposite directions-inhibition of eukaryotic translation initiation factor 2α-C/EBP homologous protein and activation of X-box binding protein 1-promotes both RGC axons and somata survival and preserves visual function. Our results indicate that axon injury-induced neuronal ER stress plays an important role in both axon degeneration and neuron soma death. Neuronal ER stress is therefore a promising therapeutic target for glaucoma and potentially other types of neurodegeneration. SIGNIFICANCE STATEMENT: Neuron soma and axon degeneration have distinct molecular mechanisms although they are clearly connected after axon injury. We previously demonstrated that axon injury induces neuronal endoplasmic reticulum (ER) stress and that manipulation of ER stress molecules synergistically promotes neuron cell body survival. Here we investigated the possibility that ER stress also plays a role in axon degeneration and whether ER stress modulation preserves neuronal function in neurodegenerative diseases. Our results suggest that neuronal ER stress is a general mechanism of degeneration for both neuronal cell body and axon, and that therapeutic targeting of ER stress produces significant functional recovery.

A fly model for the CCUG-repeat expansion of myotonic dystrophy type 2 reveals a novel interaction with MBNL1.

Expanded non-coding RNA repeats of CUG and CCUG are the underlying genetic causes for myotonic dystrophy type 1 (DM1) and type 2 (DM2), respectively. A gain-of-function of these pathogenic repeat expansions is mediated at least in part by their abnormal interactions with RNA-binding proteins such as MBNL1 and resultant loss of activity of these proteins. To study pathogenic mechanisms of CCUG-repeat expansions in an animal model, we created a fly model of DM2 that expresses pure, uninterrupted CCUG-repeat expansions ranging from 16 to 720 repeats in length. We show that this fly model for DM2 recapitulates key features of human DM2 including RNA repeat-induced toxicity, ribonuclear foci formation and changes in alternative splicing. Interestingly, expression of two isoforms of MBNL1, MBNL135 and MBNL140, leads to cleavage and concurrent upregulation of the levels of the RNA-repeat transcripts, with MBNL140 having more significant effects than MBNL135. This property is shared with a fly CUG-repeat expansion model. Our results suggest a novel mechanism for interaction between the pathogenic RNA repeat expansions of myotonic dystrophy and MBNL1.

RNA toxicity is a component of ataxin-3 degeneration in Drosophila.

Polyglutamine (polyQ) diseases are a class of dominantly inherited neurodegenerative disorders caused by the expansion of a CAG repeat encoding glutamine within the coding region of the respective genes. The molecular and cellular pathways underlying polyQ-induced neurodegeneration are the focus of much research, and it is widely considered that toxic activities of the protein, resulting from the abnormally long polyQ tract, cause pathogenesis. Here we provide evidence for a pathogenic role of the CAG repeat RNA in polyQ toxicity using Drosophila. In a Drosophila screen for modifiers of polyQ degeneration induced by the spinocerebellar ataxia type 3 (SCA3) protein ataxin-3, we isolated an upregulation allele of muscleblind (mbl), a gene implicated in the RNA toxicity of CUG expansion diseases. Further analysis indicated that there may be a toxic role of the RNA in polyQ-induced degeneration. We tested the role of the RNA by altering the CAG repeat sequence to an interrupted CAACAG repeat within the polyQ-encoding region; this dramatically mitigated toxicity. In addition, expression of an untranslated CAG repeat of pathogenic length conferred neuronal degeneration. These studies reveal a role for the RNA in polyQ toxicity, highlighting common components in RNA-based and polyQ-protein-based trinucleotide repeat expansion diseases.

Triplet repeat-derived siRNAs enhance RNA-mediated toxicity in a Drosophila model for myotonic dystrophy.

More than 20 human neurological and neurodegenerative diseases are caused by simple DNA repeat expansions; among these, non-coding CTG repeat expansions are the basis of myotonic dystrophy (DM1). Recent work, however, has also revealed that many human genes have anti-sense transcripts, raising the possibility that human trinucleotide expansion diseases may be comprised of pathogenic activities due both to a sense expanded-repeat transcript and to an anti-sense expanded-repeat transcript. We established a Drosophila model for DM1 and tested the role of interactions between expanded CTG transcripts and expanded CAG repeat transcripts. These studies revealed dramatically enhanced toxicity in flies co-expressing CTG with CAG expanded repeats. Expression of the two transcripts led to novel pathogenesis with the generation of dcr-2 and ago2-dependent 21-nt triplet repeat-derived siRNAs. These small RNAs targeted the expression of CAG-containing genes, such as Ataxin-2 and TATA binding protein (TBP), which bear long CAG repeats in both fly and man. These findings indicate that the generation of triplet repeat-derived siRNAs may dramatically enhance toxicity in human repeat expansion diseases in which anti-sense transcription occurs.

Neuroprotection by eIF2α-CHOP inhibition and XBP-1 activation in EAE/optic neuritiss.

No therapies exist to prevent neuronal deficits in multiple sclerosis (MS), because the molecular mechanism responsible for the progressive neurodegeneration is unknown. We previously showed that axon injury-induced neuronal endoplasmic reticulum (ER) stress plays an important role in retinal ganglion cell (RGC) death and optic nerve degeneration in traumatic and glaucomatous optic neuropathies. Optic neuritis, one of the most common clinical manifestations of MS, is readily modeled by experimental autoimmune encephalomyelitis (EAE) in mouse. Using this in vivo model, we now show that ER stress is induced early in EAE and that modulation of ER stress by inhibition of eIF2α-CHOP and activation of XBP-1 in RGC specifically, protects RGC somata and axons and preserves visual function. This finding adds to the evidence that ER stress is a general upstream mechanism for neurodegeneration and suggests that targeting ER stress molecules is a promising therapeutic strategy for neuroprotection in MS.

Abnormal mesenchymal differentiation in the superior semicircular canal of brn4/pou3f4 knockout mice.

OBJECTIVE: To examine the developmental time course of the mutant phenotype and cellular mechanisms that result in malformations of the superior semicircular canal (SSCC) in Brn4 knockout mice. Mutations in the Brn4/Pou3f4 gene result in characteristic inner ear abnormalities in mutant mouse pedigrees, and the findings in these mice are similar to those in human X-linked deafness type III. DESIGN: Mutant and control mice were killed at various neonatal time points to assess the development of the SSCC. Measurements of SSCC diameter were made on paint-perfused specimens at postnatal day (P) 0, P7, P10, and P14. Histologic evaluation of the SSCC was made on hematoxylin-eosin-stained sections at P10. RESULTS: A dysmorphic constriction of the superior arc of the SSCC in Brn4 knockout mice was initially detectable at P14. Interestingly, the mutant SSCC is indistinguishable from control mice at earlier neonatal time points. In mutant neonates, there is persistence of immature woven bone with high cellularity surrounding the perilymphatic space of the SSCC. These findings are not present in control animal specimens, which demonstrate appropriate lamellar bony architecture. CONCLUSIONS: In Brn4 knockout mice, constriction of the SSCC with narrowing of the bony labyrinth develops in the postnatal period at approximately P14. The persistence of immature bone in affected mice indicates that signaling abnormalities disrupt normal mesenchymal differentiation in the SSCC.

The mTORC1 effectors S6K1 and 4E-BP play different roles in CNS axon regeneration.

Using mouse optic nerve (ON) crush as a CNS injury model, we and others have found that activation of the mammalian target of rapamycin complex 1 (mTORC1) in mature retinal ganglion cells by deletion of the negative regulators, phosphatase and tensin homologue (PTEN), and tuberous sclerosis 1 promotes ON regeneration. mTORC1 activation inhibits eukaryotic translation initiation factor 4E-binding protein (4E-BP) and activates ribosomal protein S6 kinase 1 (S6K1), both of which stimulate translation. We reasoned that mTORC1's regeneration-promoting effects might be separable from its deleterious effects by differential manipulation of its downstream effectors. Here we show that S6K1 activation, but not 4E-BP inhibition, is sufficient to promote axon regeneration. However, inhibition of 4E-BP is required for PTEN deletion-induced axon regeneration. Both activation and inhibition of S6K1 decrease the effect of PTEN deletion on axon regeneration, implicating a dual role of S6K1 in regulating axon growth.

A novel Drosophila model of nerve injury reveals an essential role of Nmnat in maintaining axonal integrity.

Axons damaged by acute injury, toxic insults, or during neurodegenerative diseases undergo Wallerian or Wallerian-like degeneration, which is an active and orderly cellular process, but the underlying mechanisms are poorly understood. Drosophila has been proven to be a successful system for modeling human neurodegenerative diseases. In this study, we established a novel in vivo model of axon injury using the adult fly wing. The wing nerve highlighted by fluorescent protein markers can be directly visualized in living animals and be precisely severed by a simple wing cut, making it highly suitable for large-scale screening. Using this model, we confirmed an axonal protective function of Wld(S) and nicotinamide mononucleotide adenylyltransferase (Nmnat). We further revealed that knockdown of endogenous Nmnat triggered spontaneous, dying-back axon degeneration in vivo. Intriguingly, axonal mitochondria were rapidly depleted upon axotomy or downregulation of Nmnat. The injury-induced mitochondrial loss was dramatically suppressed by upregulation of Nmnat, which also protected severed axons from degeneration. However, when mitochondria were genetically eliminated from axons, upregulation of Nmnat was no longer effective to suppress axon degeneration. Together, these findings demonstrate an essential role of endogenous Nmnat in maintaining axonal integrity that may rely on and function by stabilizing mitochondria.