Design of a novel quantitative PCR (QPCR)-based protocol for genotyping mice carrying the neuroprotective Wallerian degeneration slow (Wlds) gene

Microarray profiling emphasizes transcriptomic differences between hippocampal in vivo tissue and in vitro cultures

Primary hippocampal cell cultures are routinely used as an experimentally accessible model platform for the hippocampus and brain tissue in general. Containing multiple cell types including neurons, astrocytes and microglia in a state that can be readily analysed optically, biochemically and electrophysiologically, such cultures have been used in many in vitro studies. To what extent the in vivo environment is recapitulated in primary cultures is an on-going question. Here, we compare the transcriptomic profiles of primary hippocampal cell cultures and intact hippocampal tissue. In addition, by comparing profiles from wild type and the PrP 101LL transgenic model of prion disease, we also demonstrate that gene conservation is predominantly conserved across genetically altered lines.

Neuroprotection by WldS depends on retinal ganglion cell type and age in glaucoma

BACKGROUND

Early challenges to axonal physiology, active transport, and ultrastructure are endemic to age-related neurodegenerative disorders, including those affecting the optic nerve. Chief among these, glaucoma causes irreversible vision loss through sensitivity to intraocular pressure (IOP) that challenges retinal ganglion cell (RGC) axons, which comprise the optic nerve. Early RGC axonopathy includes distal to proximal progression that implicates a slow form of Wallerian degeneration. In multiple disease models, including inducible glaucoma, expression of the slow Wallerian degeneration (WldS) allele slows axon degeneration and confers protection to cell bodies.

METHODS

Using an inducible model of glaucoma along with whole-cell patch clamp electrophysiology and morphological analysis, we tested if WldS also protects RGC light responses and dendrites and, if so, whether this protection depends upon RGC type. We induced glaucoma in young and aged mice to determine if neuroprotection by WldS on anterograde axonal transport and spatial contrast acuity depends on age.

RESULTS

We found WldS protects dendritic morphology and light-evoked responses of RGCs that signal light onset (αON-Sustained) during IOP elevation. However, IOP elevation significantly reduces dendritic complexity and light responses of RGCs that respond to light offset (αOFF-Sustained) regardless of WldS. As expected, WldS preserves anterograde axon transport and spatial acuity in young adult mice, but its protection is significantly limited in aged mice.

CONCLUSION

The efficacy of WldS in conferring protection to neurons and their axons varies by cell type and diminishes with age.

Initiation of CNS Myelination in the Optic Nerve Is Dependent on Axon Caliber

Emerging evidence suggests that neuronal signaling is important for oligodendrocyte myelination; however, the necessity of this signaling during development is unclear. By eliminating dynamic neuronal signaling along the developing optic nerve, we find that oligodendrocyte differentiation is not dependent on neuronal signaling and that the initiation of myelination is dependent on a permissive substrate, namely supra-threshold axon caliber. Furthermore, we show that loss of dynamic neuronal signaling results in hypermyelination of axons. We propose that oligodendrocyte differentiation is regulated by non-neuronal factors during optic nerve development, whereas myelination is sensitive to the biophysical properties of axonal diameter.

Mayoral et al. show that elimination of neuronal signaling via enucleation of the developing optic nerve of Wlds mice results in normal oligodendrocyte differentiation but disrupted myelination. Myelination is rescued when axons are enlarged prior to enucleation, showing that supra-threshold axon caliber, but not neuronal signaling, is necessary for myelination.

Altered mitochondrial bioenergetics are responsible for the delay in Wallerian degeneration observed in neonatal mice

Neurodegenerative and neuromuscular disorders can manifest throughout the lifespan of an individual, from infant to elderly individuals. Axonal and synaptic degeneration are early and critical elements of nearly all human neurodegenerative diseases and neural injury, however the molecular mechanisms which regulate this process are yet to be fully elucidated. Furthermore, how the molecular mechanisms governing degeneration are impacted by the age of the individual is poorly understood. Interestingly, in mice which are under 3 weeks of age, the degeneration of axons and synapses following hypoxic or traumatic injury is significantly slower. This process, known as Wallerian degeneration (WD), is a molecularly and morphologically distinct subtype of neurodegeneration by which axons and synapses undergo distinct fragmentation and death following a range of stimuli. In this study, we first use an ex-vivo model of axon injury to confirm the significant delay in WD in neonatal mice. We apply tandem mass-tagging quantitative proteomics to profile both nerve and muscle between P12 and P24 inclusive. Application of unbiased in silico workflows to relevant protein identifications highlights a steady elevation in oxidative phosphorylation cascades corresponding to the accelerated degeneration rate. We demonstrate that inhibition of Complex I prevents the axotomy-induced rise in reactive oxygen species and protects axons following injury. Furthermore, we reveal that pharmacological activation of oxidative phosphorylation significantly accelerates degeneration at the neuromuscular junction in neonatal mice. In summary, we reveal dramatic changes in the neuromuscular proteome during post-natal maturation of the neuromuscular system, and demonstrate that endogenous dynamics in mitochondrial bioenergetics during this time window have a functional impact upon regulating the stability of the neuromuscular system.

In vivo characterization of the role of tissue-specific translation elongation factor 1A2 in protein synthesis reveals insights into muscle atrophy

Translation elongation factor 1A2 (eEF1A2), uniquely among translation factors, is expressed specifically in neurons and muscle. eEF1A2‐null mutant wasted mice develop an aggressive, early‐onset form of neurodegeneration, but it is unknown whether the wasting results from denervation of the muscles, or whether the mice have a primary myopathy resulting from loss of translation activity in muscle. We set out to establish the relative contributions of loss of eEF1A2 in the different tissues to this postnatal lethal phenotype. We used tissue‐specific transgenesis to show that correction of eEF1A2 levels in muscle fails to ameliorate the overt phenotypic abnormalities or time of death of wasted mice. Molecular markers of muscle atrophy such as Fbxo32 were dramatically upregulated at the RNA level in wasted mice, both in the presence and in the absence of muscle‐specific expression of eEF1A2, but the degree of upregulation at the protein level was significantly lower in those wasted mice without transgene‐derived expression of eEF1A2 in muscle. This provides the first in vivo confirmation that eEF1A2 plays an important role in translation. In spite of the inability of the nontransgenic wasted mice to upregulate key atrogenes at the protein level in response to denervation to the same degree as their transgenic counterparts, there were no measurable differences between transgenic and nontransgenic wasted mice in terms of weight loss, grip strength, or muscle pathology. This suggests that a compromised ability fully to execute the atrogene pathway in denervated muscle does not affect the process of muscle atrophy in the short term.

Mutations in NMNAT1 cause Leber congenital amaurosis and identify a new disease pathway for retinal degeneration

Leber congenital amaurosis (LCA) is a blinding retinal disease that presents within the first year after birth. Using exome sequencing, we identified mutations in the nicotinamide adenine dinucleotide (NAD) synthase gene NMNAT1 encoding nicotinamide mononucleotide adenylyltransferase 1 in eight families with LCA, including the family in which LCA was originally linked to the LCA9 locus. Notably, all individuals with NMNAT1 mutations also have macular colobomas, which are severe degenerative entities of the central retina (fovea) devoid of tissue and photoreceptors. Functional assays of the proteins encoded by the mutant alleles identified in our study showed that the mutations reduce the enzymatic activity of NMNAT1 in NAD biosynthesis and affect protein folding. Of note, recent characterization of the slow Wallerian degeneration (Wld(s)) mouse model, in which prolonged axonal survival after injury is observed, identified NMNAT1 as a neuroprotective protein when ectopically expressed. Our findings identify a new disease mechanism underlying LCA and provide the first link between endogenous NMNAT1 dysfunction and a human nervous system disorder.

Synaptic protection in the brain of WldS mice occurs independently of age but is sensitive to gene-dose

BACKGROUND

Disruption of synaptic connectivity is a significant early event in many neurodegenerative conditions affecting the aging CNS, including Alzheimer's disease and Parkinson's disease. Therapeutic approaches that protect synapses from degeneration in the aging brain offer the potential to slow or halt the progression of such conditions. A range of animal models expressing the slow Wallerian Degeneration (Wld(S)) gene show robust neuroprotection of synapses and axons from a wide variety of traumatic and genetic neurodegenerative stimuli in both the central and peripheral nervous systems, raising that possibility that Wld(S) may be useful as a neuroprotective agent in diseases with synaptic pathology. However, previous studies of neuromuscular junctions revealed significant negative effects of increasing age and positive effects of gene-dose on Wld(S)-mediated synaptic protection in the peripheral nervous system, raising doubts as to whether Wld(S) is capable of directly conferring synapse protection in the aging brain.

METHODOLOGY/PRINCIPAL FINDINGS

We examined the influence of age and gene-dose on synaptic protection in the brain of mice expressing the Wld(S) gene using an established cortical lesion model to induce synaptic degeneration in the striatum. Synaptic protection was found to be sensitive to Wld(S) gene-dose, with heterozygous Wld(S) mice showing approximately half the level of protection observed in homozygous Wld(S) mice. Increasing age had no influence on levels of synaptic protection. In contrast to previous findings in the periphery, synapses in the brain of old Wld(S) mice were just as strongly protected as those in young mice.

CONCLUSIONS/SIGNIFICANCE

Our study demonstrates that Wld(S)-mediated synaptic protection in the CNS occurs independently of age, but is sensitive to gene dose. This suggests that the Wld(S) gene, and in particular its downstream endogenous effector pathways, may be potentially useful therapeutic agents for conferring synaptic protection in the aging brain.

Expression of the neuroprotective slow Wallerian degeneration (WldS) gene in non-neuronal tissues

Background

The slow Wallerian Degeneration (Wld^S) gene specifically protects axonal and synaptic compartments of neurons from a wide variety of degeneration-inducing stimuli, including; traumatic injury, Parkinson's disease, demyelinating neuropathies, some forms of motor neuron disease and global cerebral ischemia. The Wld^S gene encodes a novel Ube4b-Nmnat1 chimeric protein (Wld^S protein) that is responsible for conferring the neuroprotective phenotype. How the chimeric Wld^S protein confers neuroprotection remains controversial, but several studies have shown that expression in neurons in vivo and in vitro modifies key cellular pathways, including; NAD biosynthesis, ubiquitination, the mitochondrial proteome, cell cycle status and cell stress. Whether similar changes are induced in non-neuronal tissue and organs at a basal level in vivo remains to be determined. This may be of particular importance for the development and application of neuroprotective therapeutic strategies based around Wld^S-mediated pathways designed for use in human patients.

Results

We have undertaken a detailed analysis of non-neuronal Wld^S expression in Wld^S mice, alongside gravimetric and histological analyses, to examine the influence of Wld^S expression in non-neuronal tissues. We show that expression of Wld^S RNA and protein are not restricted to neuronal tissue, but that the relative RNA and protein expression levels rarely correlate in these non-neuronal tissues. We show that Wld^S mice have normal body weight and growth characteristics as well as gravimetrically and histologically normal organs, regardless of Wld^S protein levels. Finally, we demonstrate that previously reported Wld^S-induced changes in cell cycle and cell stress status are neuronal-specific, not recapitulated in non-neuronal tissues at a basal level.

Conclusions

We conclude that expression of Wld^S protein has no adverse effects on non-neuronal tissue at a basal level in vivo, supporting the possibility of its safe use in future therapeutic strategies targeting axonal and/or synaptic compartments in patients with neurodegenerative disease. Future experiments determining whether Wld^S protein can modify responses to injury in non-neuronal tissue are now required.

Axonal and neuromuscular synaptic phenotypes in Wld(S), SOD1(G93A) and ostes mutant mice identified by fiber-optic confocal microendoscopy

We used live imaging by fiber-optic confocal microendoscopy (CME) of yellow fluorescent protein (YFP) expression in motor neurons to observe and monitor axonal and neuromuscular synaptic phenotypes in mutant mice. First, we visualized slow degeneration of axons and motor nerve terminals at neuromuscular junctions following sciatic nerve injury in Wld(S) mice with slow Wallerian degeneration. Protection of axotomized motor nerve terminals was much weaker in Wld(S) heterozygotes than in homozygotes. We then induced covert modifiers of axonal and synaptic degeneration in heterozygous Wld(S) mice, by N-ethyl-N-nitrosourea (ENU) mutagenesis, and used CME to identify candidate mutants that either enhanced or suppressed axonal or synaptic degeneration. From 219 of the F1 progeny of ENU-mutagenized BALB/c mice and thy1.2-YFP16/Wld(S) mice, CME revealed six phenodeviants with suppression of synaptic degeneration. Inheritance of synaptic protection was confirmed in three of these founders, with evidence of Mendelian inheritance of a dominant mutation in one of them (designated CEMOP_S5). We next applied CME repeatedly to living Wld(S) mice and to SOD1(G93A) mice, an animal model of motor neuron disease, and observed degeneration of identified neuromuscular synapses over a 1-4day period in both of these mutant lines. Finally, we used CME to observe slow axonal regeneration in the ENU-mutant ostes mouse strain. The data show that CME can be used to monitor covert axonal and neuromuscular synaptic pathology and, when combined with mutagenesis, to identify genetic modifiers of its progression in vivo.

The Wallerian degeneration slow (Wld(s)) gene does not attenuate disease in a mouse model of spinal muscular atrophy

Spinal muscular atrophy (SMA) is a severe neuromuscular disease characterized by loss of spinal alpha-motor neurons, resulting in the paralysis of skeletal muscle. SMA is caused by deficiency of survival motor neuron (SMN) protein levels. Recent evidence has highlighted an axon-specific role for SMN protein, raising the possibility that axon degeneration may be an early event in SMA pathogenesis. The Wallerian degeneration slow (Wld(s)) gene is a spontaneous dominant mutation in mice that delays axon degeneration by approximately 2-3 weeks. We set out to examine the effect of Wld(s) on the phenotype of a mouse model of SMA. We found that Wld(s) does not alter the SMA phenotype, indicating that Wallerian degeneration does not directly contribute to the pathogenesis of SMA development.