Comparative analysis of motoneuron loss and functional deficits in PMN mice: implications for human motoneuron disease

Pegylated Insulin-Like Growth Factor 1 attenuates Hair Cell Loss and promotes Presynaptic Maintenance of Medial Olivocochlear Cholinergic Fibers in the Cochlea of the Progressive Motor Neuropathy Mouse

The progressive motor neuropathy (PMN) mouse is a model of an inherited motor neuropathy disease with progressive neurodegeneration. Axon degeneration associates with homozygous mutations of the TBCE gene encoding the tubulin chaperone E protein. TBCE is responsible for the correct dimerization of alpha and beta-tubulin. Strikingly, the PMN mouse also develops a progressive hearing loss after normal hearing onset, characterized by degeneration of the auditory nerve and outer hair cell (OHC) loss. However, the development of this neuronal and cochlear pathology is not fully understood yet. Previous studies with pegylated insulin-like growth factor 1 (peg-IGF-1) treatment in this mouse model have been shown to expand lifespan, weight, muscle strength, and motor coordination. Accordingly, peg-IGF-1 was evaluated for an otoprotective effect. We investigated the effect of peg-IGF-1 on the auditory system by treatment starting at postnatal day 15 (p15). Histological analysis revealed positive effects on OHC synapses of medial olivocochlear (MOC) neuronal fibers and a short-term attenuation of OHC loss. Peg-IGF-1 was able to conditionally restore the disorganization of OHC synapses and maintain the provision of cholinergic acetyltransferase in presynapses. To assess auditory function, frequency-specific auditory brainstem responses and distortion product otoacoustic emissions were recorded in animals on p21 and p28. However, despite the positive effect on MOC fibers and OHC, no restoration of hearing could be achieved. The present work demonstrates that the synaptic pathology of efferent MOC fibers in PMN mice represents a particular form of “efferent auditory neuropathy.” Peg-IGF-1 showed an otoprotective effect by preventing the degeneration of OHCs and efferent synapses. However, enhanced efforts are needed to optimize the treatment to obtain detectable improvements in hearing performances.

Mutation of the TBCE gene causes disturbance of microtubules in the auditory nerve and cochlear outer hair cell degeneration accompanied by progressive hearing loss in the pmn/pmn mouse

The progressive motor neuronopathy (pmn/pmn) mouse, an animal model for a fast developing human motor neuron disorder, is additionally characterized by simultaneous progressive sensorineural hearing loss. The gene defect in the pmn/pmn mouse is localized to a missense mutation in the tubulin-specific chaperone E (TBCE) gene on mouse chromosome 13, which is one of the five tubulin-specific chaperons involved in tubulin folding and dimerization. The missense mutation leads to a disturbance of tubulin structures in the auditory nerve and a progressive outer hair cell loss due to apoptosis, which is accompanied by highly elevated ABR-thresholds and loss of DPOAEs. In addition the TBCE protein is selectively expressed in the outer hair cells and the transcellular processes of the inner pillar cells in the cochlea of control and pmn/pmn mouse. We conclude from our study that the mutation of the TBCE gene affects the auditory nerve and the cochlear hair cells simultaneously, leading to progressive hearing loss. This animal model will give the chance to test possible therapeutic strategies in special forms of hearing loss, in which the auditory nerve and the cochlear hair cells are simultaneously affected.

Fast motor axon loss in SMARD1 does not correspond to morphological and functional alterations of the NMJ

Spinal muscular atrophy with respiratory distress type 1 (SMARD1) is a childhood motoneuron disease caused by mutations in the gene encoding for IGHMBP2, an ATPase/Helicase. Paralysis of the diaphragm is an early and prominent clinical sign resulting both from denervation and myopathy. In skeletal muscles, muscle atrophy mainly results from loss of motoneuron cell bodies and axonal degeneration. Although it is well known that loss of motoneurons at the lumbar spinal cord is an early event in the pathogenesis of the disease, it is not clear whether the corresponding proximal axons and NMJs are also early affected. In order to address this question, we have investigated the time course of the disease progression at the level of the motoneuron cell body, proximal axon (ventral root), distal axon (sciatic nerve), NMJ, and muscle fiber in Nmd(2J) mice, a mouse model for SMARD1. Our results show an early and apparently parallel loss of motoneurons, proximal axons, and NMJs. In affected muscles, however, denervated fibers coexist with NMJs with normal morphology and unaltered neurotransmission. Furthermore, unaffected axons are able to sprout and reinnervate muscle fibers, suggesting selective vulnerability of neurons to Ighmbp2 deficiency. The preservation of the NMJ morphology and neurotransmission in the Nmd(2J) mouse until motor axon loss takes place, differs from that observed in SMA mouse models in which NMJ impairment is an early and more general phenomenon in affected muscles.

SOD1 targeted to the mitochondrial intermembrane space prevents motor neuropathy in the Sod1 knockout mouse

Motor axon degeneration is a critical but poorly understood event leading to weakness and muscle atrophy in motor neuron diseases. Here, we investigated oxidative stress-mediated axonal degeneration in mice lacking the antioxidant enzyme, Cu,Zn superoxide dismutase (SOD1). We demonstrate a progressive motor axonopathy in these mice and show that Sod1(-/-) primary motor neurons extend short axons in vitro with reduced mitochondrial density. Sod1(-/-) neurons also show oxidation of mitochondrial--but not cytosolic--thioredoxin, suggesting that loss of SOD1 causes preferential oxidative stress in mitochondria, a primary source of superoxide in cells. SOD1 is widely regarded as the cytosolic isoform of superoxide dismutase, but is also found in the mitochondrial intermembrane space. The functional significance of SOD1 in the intermembrane space is unknown. We used a transgenic approach to express SOD1 exclusively in the intermembrane space and found that mitochondrial SOD1 is sufficient to prevent biochemical and morphological defects in the Sod1(-/-) model, and to rescue the motor phenotype of these mice when followed to 12 months of age. These results suggest that SOD1 in the mitochondrial intermembrane space is fundamental for motor axon maintenance, and implicate oxidative damage initiated at mitochondrial sites in the pathogenesis of motor axon degeneration.

Auditory development in progressive motor neuronopathy mouse mutants

The present study was performed to elucidate the hearing development in the progressive motor neuronopathy (pmn) mouse mutant. This mouse has been used as a model for human motoneuron disease. A missense mutation in the tubulin-specific chaperon E (Tbce) gene on mouse chromosome 13 was localized as the underlying genetic defect. The protein encoded by the Tbce gene is essential for the formation of primary tubulin complexes. Studies on motoneurons show disorganization in microtubules and disturbed axonal transport, followed by retrograde degeneration of the motoneurons. A similar pathomechanism is also possible for hearing disorders where disrupted microtubules could cause functional deficits in spiral ganglion neurons or in cochlear hair cells. Click auditory brainstem response (ABR) audiometry in homozygous pmn mutants showed a normal onset of hearing, but an increasing hearing threshold from postnatal day 26 (P26) on to death, compared to heterozygous mutants and wild-type mice. Histological sections of the cochlea at different ages showed a regular morphology. Additionally, spiral ganglion explants from mutant and wild-type mice were cultured. The neurite length from pmn mutants was shorter than in wild-type mice, and the neurite number/explant was significantly decreased in pmn mutants. We show that the pmn mouse mutant is a model for a progressive rapid hearing loss from P26 on, after initially normal hearing development. Heterozygous mice are not affected by this defect. With the knowledge of the well-known pathomechanism of this defect in motoneurons, a dysfunction of cellular mechanisms regulating tubulin assembling suggests that tubulin assembling plays an essential role in hearing function and maintenance.

Axonal degeneration in motor neuron disease

Growing evidence from animal models and patients with amyotrophic lateral sclerosis (ALS) suggests that distal axonal degeneration begins very early in this disease, long before symptom onset and motor neuron death. The cause of axonal degeneration is unknown, and may involve local axonal damage, withdrawal of trophic support from a diseased cell body, or both. It is increasingly clear that axons are not passive extensions of their parent cell bodies, and may die by mechanisms independent of cell death. This is supported by studies in which protection of motor neurons in models of ALS did not significantly improve symptoms or prolong lifespan, likely due to a failure to protect axons. Here, we will review the evidence for early axonal degeneration in ALS, and discuss possible mechanisms by which it might occur, with a focus on oxidative stress. We contend that axonal degeneration may be a primary feature in the pathogenesis of motor neuron disease, and that preventing axonal degeneration represents an important therapeutic target that deserves increased attention.

Impairments in fast axonal transport and motor neuron deficits in transgenic mice expressing familial Alzheimer's disease-linked mutant presenilin 1

Presenilins (PS) play a central role in gamma-secretase-mediated processing of beta-amyloid precursor protein (APP) and numerous type I transmembrane proteins. Expression of mutant PS1 variants causes familial forms of Alzheimer's disease (FAD). In cultured mammalian cells that express FAD-linked PS1 variants, the intracellular trafficking of several type 1 membrane proteins is altered. We now report that the anterograde fast axonal transport (FAT) of APP and Trk receptors is impaired in the sciatic nerves of transgenic mice expressing two independent FAD-linked PS1 variants. Furthermore, FAD-linked PS1 mice exhibit a significant increase in phosphorylation of the cytoskeletal proteins tau and neurofilaments in the spinal cord. Reductions in FAT and phosphorylation abnormalities correlated with motor neuron functional deficits. Together, our data suggests that defects in anterograde FAT may underlie FAD-linked PS1-mediated neurodegeneration through a mechanism involving impairments in neurotrophin signaling and synaptic dysfunction.

Axonal defects in mouse models of motoneuron disease

Human motoneuron disease is characterized by loss of motor endplates, axonal degeneration, and cell death of motoneurons. The identification of the underlying gene defects for familial ALS, spinal muscular atrophy (SMA), and spinal muscular atrophy with respiratory distress (SMARD) has pointed to distinct pathophysiological mechanisms that are responsible for the various forms of the disease. Accumulating evidence from mouse models suggests that enhanced vulnerability and sensitivity to proapoptotic stimuli is only responsible for some but not all forms of motoneuron disease. Mechanisms that modulate microtubule assembly and the axonal transport machinery are defective in several spontaneous and ENU (ethylnitrososurea) mutagenized mouse models but also in patients with mutations in the p150 subunit of dynactin. Recent evidence suggests that axonal growth defects contribute significantly to the pathophysiology of spinal muscular atrophy. Reduced levels of the survival motoneuron protein that are responsible for SMA lead to disturbed RNA processing in motoneurons. This could also affect axonal transport of mRNAs for beta-actin and other proteins that play an essential role in axon growth and synaptic function. The local translation of specific proteins might be affected, because developing motoneurons contain ribosome-like structures in distal axons and growth cones. Altogether, the evidence from these mouse models and the new genetic data from patients suggest that axon growth and maintenance involves a variety of mechanisms, including microtubule assembly and axonal transport of proteins and ribonucleoproteins (RNPs). Thus, defects in axon maintenance could play a leading role in the development of several forms of human motoneuron disease.

A transgene carrying an A2G missense mutation in the SMN gene modulates phenotypic severity in mice with severe (type I) spinal muscular atrophy

5q spinal muscular atrophy (SMA) is a common autosomal recessive disorder in humans and the leading genetic cause of infantile death. Patients lack a functional survival of motor neurons (SMN1) gene, but carry one or more copies of the highly homologous SMN2 gene. A homozygous knockout of the single murine Smn gene is embryonic lethal. Here we report that in the absence of the SMN2 gene, a mutant SMN A2G transgene is unable to rescue the embryonic lethality. In its presence, the A2G transgene delays the onset of motor neuron loss, resulting in mice with mild SMA. We suggest that only in the presence of low levels of full-length SMN is the A2G transgene able to form partially functional higher order SMN complexes essential for its functions. Mild SMA mice exhibit motor neuron degeneration, muscle atrophy, and abnormal EMGs. Animals homozygous for the mutant transgene are less severely affected than heterozygotes. This demonstrates the importance of SMN levels in SMA even if the protein is expressed from a mutant allele. Our mild SMA mice will be useful in (a) determining the effect of missense mutations in vivo and in motor neurons and (b) testing potential therapies in SMA.

Differential vulnerability of cranial motoneurons in mouse models with motor neuron degeneration

Amyotrophic lateral sclerosis (ALS) is characterized by the progressive degeneration of selective motoneuron populations, yet it remains unclear why some groups of motoneurons are more vulnerable than others. Our aim was to compare the motoneuron loss in five cranial nuclei at different stages of the disease in three mouse models of ALS: two naturally occurring murine models (progressive motor neuronopathy (pmn) and wobbler) and a transgenic mouse model with a human G93A mutation in the superoxide dismutase-1 (SOD1) gene. By quantifying these different motoneuron populations we report that the degree of degeneration in the various cranial motoneuron nuclei depends on the mouse model and the stage of the disease. The biologically most significant difference between the mutations occurs in the oculomotor/trochlear nucleus which is affected in the pmn mouse but not in the wobbler and SOD G93A mice. These results suggest that there is a selective degeneration of cranial motoneurons in these mouse models as in ALS patients.

Conditional gene ablation of Stat3 reveals differential signaling requirements for survival of motoneurons during development and after nerve injury in the adult

Members of the ciliary neurotrophic factor (CNTF)/leukemia inhibitory factor (LIF)/cardiotrophin gene family are potent survival factors for embryonic and lesioned motoneurons. These factors act via receptor complexes involving gp130 and LIFR-beta and ligand binding leads to activation of various signaling pathways, including phosphorylation of Stat3. The role of Stat3 in neuronal survival was investigated in mice by Cre-mediated gene ablation in motoneurons. Cre is expressed under the neurofilament light chain (NF-L) promoter, starting around E12 when these neurons become dependent on neurotrophic support. Loss of motoneurons during the embryonic period of naturally occurring cell death is not enhanced in NF-L-Cre; Stat3(flox/KO) mice although motoneurons isolated from these mice need higher concentrations of CNTF for maximal survival in culture. In contrast, motoneuron survival is significantly reduced after facial nerve lesion in the adult. These neurons, however, can be rescued by the addition of neurotrophic factors, including CNTF. Stat3 is essential for upregulation of Reg-2 and Bcl-xl expression in lesioned motoneurons. Our data show that Stat3 activation plays an essential role for motoneuron survival after nerve lesion in postnatal life but not during embryonic development, indicating that signaling requirements for motoneuron survival change during maturation.