Morphometric comparison of the vulnerability of peripheral motor and sensory neurons in amyotrophic lateral sclerosis

Schwann cell JUN expression worsens motor performance in an amyotrophic lateral sclerosis mouse model

Amyotrophic lateral sclerosis is a devastating neurodegenerative disease characterized by motor neuron death and distal axonopathy. Despite its clinical severity and profound impact in the patients and their families, many questions about its pathogenesis remain still unclear, including the role of Schwann cells and axon-glial signaling in disease progression. Upon axonal injury, upregulation of JUN transcription factor promotes Schwann cell reprogramming into a repair phenotype that favors axon regrowth and neuronal survival. To study the potential role of repair Schwann cells on motoneuron survival in amyotrophic lateral sclerosis, we generated a mouse line that over-expresses JUN in the Schwann cells of the SOD1G93A mutant, a mouse model of this disease. Then, we explored disease progression by evaluating survival, motor performance and histology of peripheral nerves and spinal cord of these mice. We found that Schwann cell JUN overexpression does not prevent axon degeneration neither motor neuron death in the SOD1G93A mice. Instead, it induces a partial demyelination of medium and large size axons, worsening motor performance and resulting in more aggressive disease phenotype.

Instrumented assessment of lower and upper motor neuron signs in amyotrophic lateral sclerosis using robotic manipulation: an explorative study

BACKGROUND

Amyotrophic lateral sclerosis (ALS) is a lethal progressive neurodegenerative disease characterized by upper motor neuron (UMN) and lower motor neuron (LMN) involvement. Their varying degree of involvement results in a clinical heterogenous picture, making clinical assessments of UMN signs in patients with ALS often challenging. We therefore explored whether instrumented assessment using robotic manipulation could potentially be a valuable tool to study signs of UMN involvement.

METHODS

We examined the dynamics of the wrist joint of 15 patients with ALS and 15 healthy controls using a Wristalyzer single-axis robotic manipulator and electromyography (EMG) recordings in the flexor and extensor muscles in the forearm. Multi-sinusoidal torque perturbations were applied, during which participants were asked to either relax, comply or resist. A neuromuscular model was used to study muscle viscoelasticity, e.g. stiffness (k) and viscosity (b), and reflexive properties, such as velocity, position and force feedback gains (kv, kp and kf, respectively) that dominated the responses. We further obtained clinical signs of LMN (muscle strength) and UMN (e.g. reflexes, spasticity) dysfunction, and evaluated their relation with the estimated neuromuscular model parameters.

RESULTS

Only force feedback gains (kf) were elevated in patients (p = 0.033) compared to controls. Higher kf, as well as the resulting reflexive torque (Tref), were both associated with more severe UMN dysfunction in the examined arm (p = 0.040 and p < 0.001). Patients with UMN symptoms in the examined arm had increased kf and Tref compared to controls (both p = 0.037). Neither of these measures was related to muscle strength, but muscle stiffness (k) was lower in weaker patients (p = 0.012). All these findings were obtained from the relaxed test. No differences were observed during the instructions comply and resist.

CONCLUSIONS

This findings are proof-of-concept that instrumented assessment using robotic manipulation is a feasible technique in ALS, which may provide quantitative, operator-independent measures relating to UMN symptoms. Elevated force feedback gains, driving larger reflexive muscle torques, appear to be particularly indicative of clinically established levels of UMN dysfunction in the examined arm.

Heterozygous knockout of Synaptotagmin13 phenocopies ALS features and TP53 activation in human motor neurons

Spinal motor neurons (MNs) represent a highly vulnerable cellular population, which is affected in fatal neurodegenerative diseases such as amyotrophic lateral sclerosis (ALS) and spinal muscular atrophy (SMA). In this study, we show that the heterozygous loss of SYT13 is sufficient to trigger a neurodegenerative phenotype resembling those observed in ALS and SMA. SYT13+/- hiPSC-derived MNs displayed a progressive manifestation of typical neurodegenerative hallmarks such as loss of synaptic contacts and accumulation of aberrant aggregates. Moreover, analysis of the SYT13+/- transcriptome revealed a significant impairment in biological mechanisms involved in motoneuron specification and spinal cord differentiation. This transcriptional portrait also strikingly correlated with ALS signatures, displaying a significant convergence toward the expression of pro-apoptotic and pro-inflammatory genes, which are controlled by the transcription factor TP53. Our data show for the first time that the heterozygous loss of a single member of the synaptotagmin family, SYT13, is sufficient to trigger a series of abnormal alterations leading to MN sufferance, thus revealing novel insights into the selective vulnerability of this cell population.

Cellular and axonal transport phenotypes due to the C9ORF72 HRE in iPSC motor and sensory neurons

Induced pluripotent stem cell (iPSC)-derived motor neurons (MNs) from patients with amyotrophic lateral sclerosis (ALS) and the C9ORF72 hexanucleotide repeat expansion (HRE) have multiple cellular phenotypes, but which of these accurately reflect the biology underlying the cell-specific vulnerability of ALS is uncertain. We therefore compared phenotypes due to the C9ORF72 HRE in MNs with sensory neurons (SNs), which are relatively spared in ALS. The iPSC models were able to partially reproduce the differential gene expression seen between adult SNs and MNs. We demonstrated that the typical hallmarks of C9ORF72-ALS, including RNA foci and dipeptide formation, as well as specific axonal transport defects, occurred equally in MNs and SNs, suggesting that these in vitro phenotypes are not sufficient to explain the cell-type selectivity of ALS in isolation.

Selective vulnerability of motor neuron types and functional groups to degeneration in amyotrophic lateral sclerosis: review of the neurobiological mechanisms and functional correlates

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative condition characterised by a progressive loss of motor neurons controlling voluntary muscle activity. The disease manifests through a variety of motor dysfunctions related to the extent of damage and loss of neurons at different anatomical locations. Despite extensive research, it remains unclear why some motor neurons are especially susceptible to the disease, while others are affected less or even spared. In this article, we review the neurobiological mechanisms, neurochemical profiles, and morpho-functional characteristics of various motor neuron groups and types of motor units implicated in their differential exposure to degeneration. We discuss specific cell-autonomous (intrinsic) and extrinsic factors influencing the vulnerability gradient of motor units and motor neuron types to ALS, with their impact on disease manifestation, course, and prognosis, as revealed in preclinical and clinical studies. We consider the outstanding challenges and emerging opportunities for interpreting the phenotypic and mechanistic variability of the disease to identify targets for clinical interventions.

iPSC motor neurons, but not other derived cell types, capture gene expression changes in postmortem sporadic ALS motor neurons

Motor neuron degeneration, the defining feature of amyotrophic lateral sclerosis (ALS), is a primary example of cell-type specificity in neurodegenerative diseases. Using isogenic pairs of induced pluripotent stem cells (iPSCs) harboring different familial ALS mutations, we assess the capacity of iPSC-derived lower motor neurons, sensory neurons, astrocytes, and superficial cortical neurons to capture disease features including transcriptional and splicing dysregulation observed in human postmortem neurons. At early time points, differentially regulated genes in iPSC-derived lower motor neurons, but not other cell types, overlap with one-third of the differentially regulated genes in laser-dissected motor neurons from ALS compared with control postmortem spinal cords. For genes altered in both the iPSC model and bona fide human lower motor neurons, expression changes correlate between the two populations. In iPSC-derived lower motor neurons, but not other derived cell types, we detect the downregulation of genes affected by TDP-43-dependent splicing. This reduction takes place exclusively within genotypes known to involve TDP-43 pathology.

A cellular taxonomy of the adult human spinal cord

The mammalian spinal cord functions as a community of cell types for sensory processing, autonomic control, and movement. While animal models have advanced our understanding of spinal cellular diversity, characterizing human biology directly is important to uncover specialized features of basic function and human pathology. Here, we present a cellular taxonomy of the adult human spinal cord using single-nucleus RNA sequencing with spatial transcriptomics and antibody validation. We identified 29 glial clusters and 35 neuronal clusters, organized principally by anatomical location. To demonstrate the relevance of this resource to human disease, we analyzed spinal motoneurons, which degenerate in amyotrophic lateral sclerosis (ALS) and other diseases. We found that compared with other spinal neurons, human motoneurons are defined by genes related to cell size, cytoskeletal structure, and ALS, suggesting a specialized molecular repertoire underlying their selective vulnerability. We include a web resource to facilitate further investigations into human spinal cord biology.

Sensory Involvement in Amyotrophic Lateral Sclerosis

Although amyotrophic lateral sclerosis (ALS) is pre-eminently a motor disease, the existence of non-motor manifestations, including sensory involvement, has been described in the last few years. Although from a clinical perspective, sensory symptoms are overshadowed by their motor manifestations, this does not mean that their pathological significance is not relevant. In this review, we have made an extensive description of the involvement of sensory and autonomic systems described to date in ALS, from clinical, neurophysiological, neuroimaging, neuropathological, functional, and molecular perspectives.

From Physiological Properties to Selective Vulnerability of Motor Units in Amyotrophic Lateral Sclerosis

Spinal alpha-motoneurons are classified in several types depending on the contractile properties of the innervated muscle fibers. This diversity is further displayed in different levels of vulnerability of distinct motor units to neurodegenerative diseases such as Amyotrophic Lateral Sclerosis (ALS). We summarize recent data suggesting that, contrary to the excitotoxicity hypothesis, the most vulnerable motor units are hypoexcitable and experience a reduction in their firing prior to symptoms onset in ALS. We suggest that a dysregulation of activity-dependent transcriptional programs in these motoneurons alter crucial cellular functions such as mitochondrial biogenesis, autophagy, axonal sprouting capability and re-innervation of neuromuscular junctions.

Neural mechanisms of psychosis vulnerability and perceptual abnormalities in the ALS-FTD spectrum

OBJECTIVE

The aims of this study were to (i) explore psychotic experiences across the entire amyotrophic lateral sclerosis-frontotemporal dementia (ALS-FTD) spectrum from a clinical and genetic perspective, (ii) determine the rate of abnormal perceptual experiences across the five sensory modalities and (iii) explore the neurobiological factors that lead to psychosis vulnerability in ALS-FTD.

METHODS

In a prospective case-controlled study design, 100 participants were enrolled including ALS (n = 37, 24% satisfied criteria for ALS-Plus), ALS-FTD (n = 11), bvFTD (n = 27) and healthy controls (n = 25). Psychotic experiences, perceptual abnormalities and psychosocial factors were determined by means of the clinical interview and carer and patient reports. Voxel-based morphometry analyses determined atrophy patterns in patients experiencing psychosis-like experiences and other perceptual abnormalities.

RESULTS

The rates of psychotic experiences and abnormalities of perception in each sensory modality were high across the entire ALS-FTD continuum. The rate was highest in those with C9orf72 expansions. Rates were also high in patients with pure ALS including psychosis measured by carer-based reports (18%) and self-report measures of psychotic-like experiences (21%). In an ENTER regression model, social anxiety and ACE-III scores were the best predictors of psychosis proneness, accounting for 44% of the score variance. Psychosis-like experiences and perceptual abnormalities were associated with a predominantly frontal and temporal pattern of atrophy that extended to the cerebellum and centred on the anterior thalamus.

INTERPRETATION

The model for psychosis proneness in ALS-FTD likely includes complex interactions between cognitive, social and neurobiological factors that determine vulnerability to psychosis and that may have relevance for individualised patient management.

Delayed Onset Muscle Soreness (DOMS): The Repeated Bout Effect and Chemotherapy-Induced Axonopathy May Help Explain the Dying-Back Mechanism in Amyotrophic Lateral Sclerosis and Other Neurodegenerative Diseases

Delayed onset muscle soreness (DOMS) is hypothesized to be caused by glutamate excitotoxicity-induced acute compression axonopathy of the sensory afferents in the muscle spindle. Degeneration of the same sensory afferents is implicated in the disease onset and progression of amyotrophic lateral sclerosis (ALS). A series of “silent” acute compression proprioceptive axonopathies with underlying genetic/environmental factors, damaging eccentric contractions and the non-resolving neuroinflammatory process of aging could lead to ALS disease progression. Since the sensory terminals in the muscle spindle could not regenerate from the micro-damage in ALS, unlike in DOMS, the induced protective microcircuits and their long-term functional plasticity (the equivalent of the repeated bout effect in DOMS) will be dysfunctional. The acute stress invoking osteocalcin, bradykinin, COX1, COX2, GDNF, PGE2, NGF, glutamate and N-methyl-D-aspartate (NMDA) receptors are suggested to be the critical signalers of this theory. The repeated bout effect of DOMS and the dysfunctional microcircuits in ALS are suggested to involve several dimensions of memory and learning, like pain memory, inflammation, working and episodic memory. The spatial encoding of these memory dimensions is compromised in ALS due to blunt position sense from the degenerating proprioceptive axon terminals of the affected muscle spindles. Dysfunctional microcircuits progressively and irreversibly interfere with postural control, with motor command and locomotor circuits, deplete the neuroenergetic system, and ultimately interfere with life-sustaining central pattern generators in ALS. The activated NMDA receptor is suggested to serve the “gate control” function in DOMS and ALS in line with the gate control theory of pain. Circumvention of muscle spindle-loading could be a choice of exercise therapy in muscle spindle-affected neurodegenerative diseases.

Exciting Complexity: The Role of Motor Circuit Elements in ALS Pathophysiology

Amyotrophic lateral sclerosis (ALS) is a fatal disease, characterized by the degeneration of both upper and lower motor neurons. Despite decades of research, we still to date lack a cure or disease modifying treatment, emphasizing the need for a much-improved insight into disease mechanisms and cell type vulnerability. Altered neuronal excitability is a common phenomenon reported in ALS patients, as well as in animal models of the disease, but the cellular and circuit processes involved, as well as the causal relevance of those observations to molecular alterations and final cell death, remain poorly understood. Here, we review evidence from clinical studies, cell type-specific electrophysiology, genetic manipulations and molecular characterizations in animal models and culture experiments, which argue for a causal involvement of complex alterations of structure, function and connectivity of different neuronal subtypes within the cortical and spinal cord motor circuitries. We also summarize the current knowledge regarding the detrimental role of astrocytes and reassess the frequently proposed hypothesis of glutamate-mediated excitotoxicity with respect to changes in neuronal excitability. Together, these findings suggest multifaceted cell type-, brain area- and disease stage- specific disturbances of the excitation/inhibition balance as a cardinal aspect of ALS pathophysiology.

A nerve conduction study predicts the prognosis of sporadic amyotrophic lateral sclerosis

OBJECTIVE

To clarify the relationship between nerve conduction study (NCS) and prognosis in patients with amyotrophic lateral sclerosis (ALS).

METHODS

We included 190 patients with sporadic ALS. We used onset age, sex, onset site (bulbar vs. spinal), revised El Escorial criteria category (definite vs. others), and the King's clinical systems, and the Milano-Torino (MiToS) functional staging systems, and decline rates of revised ALS functional rating scale (ALSFRS-R) as known prognostic factors. An NCS was performed on the median, ulnar, tibial, and sural nerves. The endpoint was death or the introduction of tracheostomy positive-pressure ventilation. Multivariate analysis for each NCS variable, known prognostic factors was performed using Cox stepwise proportional hazards analysis. Univariate analysis was performed for NCS variables that showed a significant association with prognosis in multivariate analysis. Survival was analyzed with a Kaplan-Meier curve and log-rank test.

RESULTS

The Cox model identified the compound muscle action potential (CMAP) and sensory nerve action potential (SNAP) amplitudes of the median nerve as prognostic factors. In the log-rank test, patients with higher median nerve CMAP amplitude had a significantly better prognosis than those with lower amplitude, regardless of age. And prognosis was better in the group with lower median nerve SNAP amplitude only in patients younger than the 25th percentile (~ 57 years).

CONCLUSIONS

CMAP and SNAP amplitudes of the median nerve are considered to be independent prognostic factors of sporadic ALS.

Neurodegenerative Diseases - Is Metabolic Deficiency the Root Cause?

Neurodegenerative diseases, including Alzheimer, Parkinson, Huntington, and amyotrophic lateral sclerosis, are a prominent class of neurological diseases currently without a cure. They are characterized by an inexorable loss of a specific type of neurons. The selective vulnerability of specific neuronal clusters (typically a subcortical cluster) in the early stages, followed by the spread of the disease to higher cortical areas, is a typical pattern of disease progression. Neurodegenerative diseases share a range of molecular and cellular pathologies, including protein aggregation, mitochondrial dysfunction, glutamate toxicity, calcium load, proteolytic stress, oxidative stress, neuroinflammation, and aging, which contribute to neuronal death. Efforts to treat these diseases are often limited by the fact that they tend to address any one of the above pathological changes while ignoring others. Lack of clarity regarding a possible root cause that underlies all the above pathologies poses a significant challenge. In search of an integrative theory for neurodegenerative pathology, we hypothesize that metabolic deficiency in certain vulnerable neuronal clusters is the common underlying thread that links many dimensions of the disease. The current review aims to present an outline of such an integrative theory. We present a new perspective of neurodegenerative diseases as metabolic disorders at molecular, cellular, and systems levels. This helps to understand a common underlying mechanism of the many facets of the disease and may lead to more promising disease-modifying therapeutic interventions. Here, we briefly discuss the selective metabolic vulnerability of specific neuronal clusters and also the involvement of glia and vascular dysfunctions. Any failure in satisfaction of the metabolic demand by the neurons triggers a chain of events that precipitate various manifestations of neurodegenerative pathology.

Facial nerve atrophy in patients with amyotrophic lateral sclerosis: Evaluation with fast imaging employing steady-state acquisition (FIESTA)

BACKGROUND

In amyotrophic lateral sclerosis (ALS), motor neurons in the brainstem markedly deplete, whereas sensory neurons are less severely affected.

PURPOSE

To determine whether facial nerve (FN) measurement on 3D fast imaging employing steady-state acquisition (FIESTA) is useful for ALS diagnosis.

STUDY TYPE

Retrospective.

SUBJECTS

Fifteen ALS patients and 16 controls.

FIELD STRENGTH/SEQUENCE

3T FIESTA MR.

ASSESSMENT

The cross-sectional area of the FN and cochlear nerve (CN) were measured, and the FN/CN ratio (FCR) was assessed. For qualitative assessment, the FN cross-sectional area was compared with that of the CN and the following scores were assigned: score 1 (large), the FN is larger than the CN; score 2 (almost equal), the size difference between the FN and CN is within 10%; score 3 (small), the FN is smaller than the CN (10-50%); score 4 (significantly small), size of the FN is less than half the size of the CN.

STATISTICAL TESTS

The differences in FCR between the ALS patients and the controls were tested using the Wilcoxon Mann-Whitney U-test. For the qualitative and quantitative assessments, we performed a receiver operating characteristic analysis for the diagnosis of ALS with an abnormal finding as score 3 or 4.

RESULTS

The mean FCR was significantly smaller for ALS patients (0.71 ± 0.17) than for controls (0.95 ± 0.08) (P < 0.001) and the area under the curve was 0.93. When an FN score was 3 or 4, indicative of FN atrophy, the sensitivity and specificity values of FIESTA for discriminating ALS patients from controls were 93.3% (14/15) and 90.0% (18/20), respectively.

DATA CONCLUSION

The FN atrophy revealed on FIESTA, which may reflect lower motor neuron impairment in ALS, allowed us to distinguish ALS patients from controls with a high degree of accuracy.

LEVEL OF EVIDENCE

3 Technical Efficacy: Stage 2 J. Magn. Reson. Imaging 2020;51:757-766.

The Peripheral Nervous System in Amyotrophic Lateral Sclerosis: Opportunities for Translational Research

Although amyotrophic lateral sclerosis (ALS) has been considered as a disorder of the motor neuron (MN) cell body, recent evidences show the non-cell-autonomous pathogenic nature of the disease. Axonal degeneration, loss of peripheral axons and destruction of nerve terminals are early events in the disease pathogenic cascade, anticipating MN degeneration, and the onset of clinical symptoms. Therefore, although ALS and peripheral axonal neuropathies should be differentiated in clinical practice, they also share damage to common molecular pathways, including axonal transport, RNA metabolism and proteostasis. Thus, an extensive evaluation of the molecular events occurring in the peripheral nervous system (PNS) could be fundamental to understand the pathogenic mechanisms of ALS, favoring the discovery of potential disease biomarkers, and new therapeutic targets.

The ALS-inducing factors, TDP43A315T and SOD1G93A, directly affect and sensitize sensory neurons to stress

There is increased recognition that sensory neurons located in dorsal root ganglia (DRG) are affected in amyotrophic lateral sclerosis (ALS). However, it remains unknown whether ALS-inducing factors, other than mutant superoxide dismutase 1 (SOD1G93A), directly affect sensory neurons. Here, we examined the effect of mutant TAR DNA-binding protein 1 (TDP43A315T) on sensory neurons in culture and in vivo. In parallel, we reevaluated sensory neurons expressing SOD1G93A. We found that cultured sensory neurons harboring either TDP43A315T or SOD1G93A grow neurites at a slower rate and elaborate fewer neuritic branches compared to control neurons. The presence of either ALS-causing mutant gene also sensitizes sensory neurons to vincristine, a microtubule inhibitor that causes axonal degeneration. Interestingly, these experiments revealed that cultured sensory neurons harboring TDP43A315T elaborate shorter and less complex neurites, and are more sensitive to vincristine compared to controls and to SOD1G93A expressing sensory neurons. Additionally, levels of two molecules involved in stress responses, ATF3 and PERK are significantly different between sensory neurons harboring TDP43A315T to those with SOD1G93A in vitro and in vivo. These findings demonstrate that sensory neurons are directly affected by two ALS-inducing factors, suggesting important roles for this neuronal subpopulation in ALS-related pathogenesis.

Ultra-High Field Diffusion MRI Reveals Early Axonal Pathology in Spinal Cord of ALS mice

Background

Amyotrophic lateral sclerosis (ALS) is a disease characterized by a progressive degeneration of motor neurons leading to paralysis. Our previous MRI diffusion tensor imaging studies detected early white matter changes in the spinal cords of mice carrying the G93A-SOD1 mutation. Here, we extend those studies using ultra-high field MRI (17.6 T) and fluorescent microscopy to investigate the appearance of early structural and connectivity changes in the spinal cords of ALS mice.

Methods

The spinal cords from presymptomatic and symptomatic mice (80 to 120 days of age) were scanned (ex-vivo) using diffusion-weighted MRI. The fractional anisotropy (FA), axial (AD) and radial (RD) diffusivities were calculated for axial slices from the thoracic, cervical and lumbar regions of the spinal cords. The diffusion parameters were compared with fluorescence microscopy and membrane cellular markers from the same tissue regions.

Results

At early stages of the disease (day 80) in the lumbar region, we found, a 19% decrease in FA, a 9% decrease in AD and a 35% increase in RD. Similar changes were observed in cervical and thoracic spinal cord regions. Differences between control and ALS mice groups at the symptomatic stages (day 120) were larger. Quantitative fluorescence microscopy at 80 days, demonstrated a 22% reduction in axonal area and a 22% increase in axonal density. Tractography and quantitative connectome analyses measured by edge weights showed a 52% decrease in the lumbar regions of the spinal cords of this ALS mice group. A significant increase in ADC (23.3%) in the ALS mice group was related to an increase in aquaporin markers.

Conclusions

These findings suggest that the combination of ultra-high field diffusion MRI with fluorescent ALS mice reporters is a useful approach to detect and characterize presymptomatic white matter micro-ultrastructural changes and axonal connectivity anomalies in ALS.

Excitability of sensory axons in amyotrophic lateral sclerosis

OBJECTIVE

To evaluate the excitability of sensory axons in patients with amyotrophic lateral sclerosis (ALS).

METHODS

Comprehensive sensory nerve excitability studies were prospectively performed on 28 sporadic ALS patients, compared to age-matched controls. Sensory nerve action potentials were recorded from digit 2 following median nerve stimulation at the wrist. Disease severity was measured using motor unit number estimation (MUNE), the revised ALS Functional Rating Scale (ALSFRS-R) and the MRC scale.

RESULTS

There were no significant differences in standard and extended measures of nerve excitability between ALS patients and controls. These unchanged excitability measures included accommodation to long-lasting hyperpolarization and the threshold changes after two supramaximal stimuli during the recovery cycle. Excitability parameters did not correlate with MUNE, ALSFRS-R, APB MRC scale or disease duration.

CONCLUSIONS

This cross-sectional study has identified normal axonal membrane properties in myelinated sensory axons of ALS patients. Previously described sensory abnormalities could be the result of axonal fallout, possibly due to a ganglionopathy, or to involvement of central sensory pathways rostral to gracile and cuneate nuclei.

SIGNIFICANCE

These results demonstrate the absence of generalized dysfunction of the membrane properties of sensory axons in ALS in the face of substantial deficits in motor function.

Escape from homeostasis: spinal microcircuits and progression of amyotrophic lateral sclerosis

In amyotrophic lateral sclerosis (ALS), loss of motoneuron function leads to weakness and, ultimately, respiratory failure and death. Regardless of the initial pathogenic factors, motoneuron loss follows a specific pattern: the largest α-motoneurons die before smaller α-motoneurons, and γ-motoneurons are spared. In this article, we examine how homeostatic responses to this orderly progression could lead to local microcircuit dysfunction that in turn propagates motoneuron dysfunction and death. We first review motoneuron diversity and the principle of α-γ coactivation and then discuss two specific spinal motoneuron microcircuits: those involving proprioceptive afferents and those involving Renshaw cells. Next, we propose that the overall homeostatic response of the nervous system is aimed at maintaining force output. Thus motoneuron degeneration would lead to an increase in inputs to motoneurons, and, because of the pattern of neuronal degeneration, would result in an imbalance in local microcircuit activity that would overwhelm initial homeostatic responses. We suggest that this activity would ultimately lead to excitotoxicity of motoneurons, which would hasten the progression of disease. Finally, we propose that should this be the case, new therapies targeted toward microcircuit dysfunction could slow the course of ALS.

Motor neurone disease

Motor neurone disease (MND) patients exhibit poor gait, balance, and postural control, all of which significantly increases their risk of falling. Falls are frequent in the MND population, and are associated with an increased burden of disease. The complex interplay of both motor and extramotor manifestations in this disease contributes to the heterogeneous and multifactorial causes of such dysfunction. This review highlights the pathophysiologic influence of motor degeneration in gait disturbance, but also the additional influence on postural instability from other inputs such as cognitive impairment, autonomic dysregulation, cerebellar dysfunction, sensory impairment, and extrapyramidal involvement. In various combinations, these impairments are responsible for reduced gait speed and alteration in gait cycle, as well as structurally more variable and disorganized gait patterns. Based on these features, this chapter will also provide disease-specific interventions to assess, manage, and prevent falls in the MND cohort.

Abnormal cortical brain integration of somatosensory afferents in ALS

OBJECTIVES

Infraclinical sensory alterations have been reported at early stages of amyotrophic lateral sclerosis (ALS). While previous studies mainly focused on early somatosensory evoked potentials (SEPs), late SEPs, which reflect on cortical pathways involved in cognitive-motor functions, are relatively underinvestigated. Early and late SEPs were compared to assess their alterations in ALS.

METHODS

Median and ulnar nerves were electrically stimulated at the wrist, at 9 times the perceptual threshold, in 21 ALS patients without clinical evidence of sensory deficits, and 21 age- and gender-matched controls. SEPs were recorded at the Erb point using surface electrodes and using a needle inserted in the scalp, in front of the primary somatosensory area (with reference electrode on the ear lobe).

RESULTS

Compared to controls, ALS patients showed comparable peripheral (N9) and early cortical component (N20, P25, N30) reductions, while the late cortical components (N60, P100) were more depressed than the early ones.

CONCLUSIONS

The peripheral sensory alteration likely contributed to late SEP depression to a lesser extent than that of early SEPs.

SIGNIFICANCE

Late SEPs may provide new insights on abnormal cortical excitability affecting brain areas involved in cognitive-motor functions.

Motor neuron vulnerability and resistance in amyotrophic lateral sclerosis

In the fatal disease-amyotrophic lateral sclerosis (ALS)-upper (corticospinal) motor neurons (MNs) and lower somatic MNs, which innervate voluntary muscles, degenerate. Importantly, certain lower MN subgroups are relatively resistant to degeneration, even though pathogenic proteins are typically ubiquitously expressed. Ocular MNs (OMNs), including the oculomotor, trochlear and abducens nuclei (CNIII, IV and VI), which regulate eye movement, persist throughout the disease. Consequently, eye-tracking devices are used to enable paralysed ALS patients (who can no longer speak) to communicate. Additionally, there is a gradient of vulnerability among spinal MNs. Those innervating fast-twitch muscle are most severely affected and degenerate first. MNs innervating slow-twitch muscle can compensate temporarily for the loss of their neighbours by re-innervating denervated muscle until later in disease these too degenerate. The resistant OMNs and the associated extraocular muscles (EOMs) are anatomically and functionally very different from other motor units. The EOMs have a unique set of myosin heavy chains, placing them outside the classical characterization spectrum of all skeletal muscle. Moreover, EOMs have multiple neuromuscular innervation sites per single myofibre. Spinal fast and slow motor units show differences in their dendritic arborisations and the number of myofibres they innervate. These motor units also differ in their functionality and excitability. Identifying the molecular basis of cell-intrinsic pathways that are differentially activated between resistant and vulnerable MNs could reveal mechanisms of selective neuronal resistance, degeneration and regeneration and lead to therapies preventing progressive MN loss in ALS. Illustrating this, overexpression of OMN-enriched genes in spinal MNs, as well as suppression of fast spinal MN-enriched genes can increase the lifespan of ALS mice. Here, we discuss the pattern of lower MN degeneration in ALS and review the current literature on OMN resistance in ALS and differential spinal MN vulnerability. We also reflect upon the non-cell autonomous components that are involved in lower MN degeneration in ALS.

Differential effects of myostatin deficiency on motor and sensory axons

INTRODUCTION

Deletion of myostatin in mice (MSTN^-/- ) alters structural properties of peripheral axons. However, properties like axon diameter and myelin thickness were analyzed in mixed nerves, so it is unclear whether loss of myostatin affects motor, sensory, or both types of axons.

METHODS

Using the MSTN^-/- mouse model, we analyzed the effects of increasing the number of muscle fibers on axon diameter, myelin thickness, and internode length in motor and sensory axons.

RESULTS

Axon diameter and myelin thickness were increased in motor axons of MSTN^-/- mice without affecting internode length or axon number. The number of sensory axons was increased without affecting their structural properties.

DISCUSSION

These results suggest that motor and sensory axons establish structural properties by independent mechanisms. Moreover, in motor axons, instructive cues from the neuromuscular junction may play a role in co-regulating axon diameter and myelin thickness, whereas internode length is established independently. Muscle Nerve 56: E100-E107, 2017.

Modeling molecular mechanisms in the axon

Axons are living systems that display highly dynamic changes in stiffness, viscosity, and internal stress. However, the mechanistic origin of these phenomenological properties remains elusive. Here we establish a computational mechanics model that interprets cellular-level characteristics as emergent properties from molecular-level events. We create an axon model of discrete microtubules, which are connected to neighboring microtubules via discrete crosslinking mechanisms that obey a set of simple rules. We explore two types of mechanisms: passive and active crosslinking. Our passive and active simulations suggest that the stiffness and viscosity of the axon increase linearly with the crosslink density, and that both are highly sensitive to the crosslink detachment and reattachment times. Our model explains how active crosslinking with dynein motors generates internal stresses and actively drives axon elongation. We anticipate that our model will allow us to probe a wide variety of molecular phenomena-both in isolation and in interaction-to explore emergent cellular-level features under physiological and pathological conditions.

Extra-motor abnormalities in amyotrophic lateral sclerosis: another layer of heterogeneity

INTRODUCTION

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease defined by the presence of muscle weakness. The motor features of disease are heterogeneous in site of onset and progression. There are also extra-motor features in some patients. The genetic basis for extra-motor features is uncertain. The heterogeneity of ALS is an issue for clinical trials. Areas covered: This paper reviews the range and prevalence of extra-motor features associated with ALS, and highlights the current information about genetic associations with extra-motor features. Expert commentary: There are extra-motor features of ALS, but these are not found in all patients. The most common is cognitive abnormality. More data is required to ascertain whether extra-motor features arise with progression of disease. Extra-motor features are reported in patients with a range of causative genetic mutations, but are not found in all patients with these mutations. Further studies are required of the heterogeneity of ALS, and genotype/phenotype correlations are required, taking note of extra-motor features.

Gamma motor neurons survive and exacerbate alpha motor neuron degeneration in ALS

The molecular and cellular basis of selective motor neuron (MN) vulnerability in amyotrophic lateral sclerosis (ALS) is not known. In genetically distinct mouse models of familial ALS expressing mutant superoxide dismutase-1 (SOD1), TAR DNA-binding protein 43 (TDP-43), and fused in sarcoma (FUS), we demonstrate selective degeneration of alpha MNs (α-MNs) and complete sparing of gamma MNs (γ-MNs), which selectively innervate muscle spindles. Resistant γ-MNs are distinct from vulnerable α-MNs in that they lack synaptic contacts from primary afferent (I_A) fibers. Elimination of these synapses protects α-MNs in the SOD1 mutant, implicating this excitatory input in MN degeneration. Moreover, reduced I_A activation by targeted reduction of γ-MNs in SOD1^G93A mutants delays symptom onset and prolongs lifespan, demonstrating a pathogenic role of surviving γ-MNs in ALS. This study establishes the resistance of γ-MNs as a general feature of ALS mouse models and demonstrates that synaptic excitation of MNs within a complex circuit is an important determinant of relative vulnerability in ALS.

Sensory neuropathy in progressive motor neuronopathy (pmn) mice is associated with defects in microtubule polymerization and axonal transport

Motor neuron diseases such as amyotrophic lateral sclerosis (ALS) are now recognized as multi-system disorders also involving various non-motor neuronal cell types. The precise extent and mechanistic basis of non-motor neuron damage in human ALS and ALS animal models remain however unclear. To address this, we here studied progressive motor neuronopathy (pmn) mice carrying a missense loss-of-function mutation in tubulin binding cofactor E (TBCE). These mice manifest a particularly aggressive form of motor axon dying back and display a microtubule loss, similar to that induced by human ALS-linked TUBA4A mutations. Using whole nerve confocal imaging of pmn × thy1.2-YFP16 fluorescent reporter mice and electron microscopy, we demonstrate axonal discontinuities, bead-like spheroids and ovoids in pmn suralis nerves indicating prominent sensory neuropathy. The axonal alterations qualitatively resemble those in phrenic motor nerves but do not culminate in the loss of myelinated fibers. We further show that the pmn mutation decreases the level of TBCE, impedes microtubule polymerization in dorsal root ganglion (DRG) neurons and causes progressive loss of microtubules in large and small caliber suralis axons. Live imaging of axonal transport using GFP-tagged tetanus toxin C-fragment (GFP-TTC) demonstrates defects in microtubule-based transport in pmn DRG neurons, providing a potential explanation for the axonal alterations in sensory nerves. This study unravels sensory neuropathy as a pathological feature of mouse pmn, and discusses the potential contribution of cytoskeletal defects to sensory neuropathy in human motor neuron disease.

Amyotrophic lateral sclerosis: Current perspectives from basic research to the clinic

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease characterized by progressive degeneration of upper and lower motoneurons, leading to muscle weakness and paralysis, and finally death. Considerable recent advances have been made in basic research and preclinical therapeutic attempts using experimental models, leading to increasing clinical and translational research in the context of this disease. In this review we aim to summarize the most relevant findings from a variety of aspects about ALS, including evaluation methods, animal models, pathophysiology, and clinical findings, with particular emphasis in understanding the role of every contributing mechanism to the disease for elucidating the causes underlying degeneration of motoneurons and the development of new therapeutic strategies.

Visualization of Sensory Neurons and Their Projections in an Upper Motor Neuron Reporter Line

Visualization of peripheral nervous system axons and cell bodies is important to understand their development, target recognition, and integration into complex circuitries. Numerous studies have used protein gene product (PGP) 9.5 [a.k.a. ubiquitin carboxy-terminal hydrolase L1 (UCHL1)] expression as a marker to label sensory neurons and their axons. Enhanced green fluorescent protein (eGFP) expression, under the control of UCHL1 promoter, is stable and long lasting in the UCHL1-eGFP reporter line. In addition to the genetic labeling of corticospinal motor neurons in the motor cortex and degeneration-resistant spinal motor neurons in the spinal cord, here we report that neurons of the peripheral nervous system are also fluorescently labeled in the UCHL1-eGFP reporter line. eGFP expression is turned on at embryonic ages and lasts through adulthood, allowing detailed studies of cell bodies, axons and target innervation patterns of all sensory neurons in vivo. In addition, visualization of both the sensory and the motor neurons in the same animal offers many advantages. In this report, we used UCHL1-eGFP reporter line in two different disease paradigms: diabetes and motor neuron disease. eGFP expression in sensory axons helped determine changes in epidermal nerve fiber density in a high-fat diet induced diabetes model. Our findings corroborate previous studies, and suggest that more than five months is required for significant skin denervation. Crossing UCHL1-eGFP with hSOD1^G93A mice generated hSOD1^G93A-UeGFP reporter line of amyotrophic lateral sclerosis, and revealed sensory nervous system defects, especially towards disease end-stage. Our studies not only emphasize the complexity of the disease in ALS, but also reveal that UCHL1-eGFP reporter line would be a valuable tool to visualize and study various aspects of sensory nervous system development and degeneration in the context of numerous diseases.

Amyotrophic lateral sclerosis: Is the spinal fluid pathway involved in seeding and spread?

Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative disorder manifested primarily by loss of upper and lower motor neurons. Current explanations for disease progression invoke regional spread attributed to the transfer of pathogenic factors among physically contiguous neurons. However, this explanation incompletely explains certain clinical and in vitro data. Considering this, we propose that the cerebrospinal fluid (CSF) pathway is likely to be a key vector for seeding local and distal disease. Subsequent disease progression would be expected to occur independently via either axonal or CSF transmission. If one accepts the hypothesis that the CSF pathway is involved in ALS progression, it follows that the choroid plexus (CP) might well be a driver of the disease process. In support of this, we briefly review the anatomical and physiological features of the CSF pathway and the choroid plexus responsible for secreting CSF. In addition, we draw attention to the interface of the CP and CSF with the immune system. We then summarize both clinical and cell culture research that supports a key role of the CSF in the establishment and inter-neuronal spread of ALS, and which suggest directions for translational research.

Electrophysiological and spinal imaging evidences for sensory dysfunction in amyotrophic lateral sclerosis

OBJECTIVES

The prevalence of sensory impairment at an early stage of amyotrophic lateral sclerosis (ALS) is still debated. The study aim was to investigate the anatomofunctional properties of sensory pathways in patients with ALS, combining spinal diffusion tensor imaging (DTI) and somatosensory evoked potentials (SEPs).

DESIGN

Case-control study.

SETTINGS

ALS referral centre and laboratory of biomedical imaging (Paris, France).

PARTICIPANTS

Well-characterised group of 21 patients with ALS with moderate disability (mean amyotrophic lateral sclerosis Functional Rating Scale (ALSFRS) score 39.3±1.0) and no clinical sensory signs and control group of 21 gender and age-matched healthy subjects.

OUTCOME MEASURES

Fractional anisotropy and diffusivity of the dorsal columns at C5-T1 levels (DTI metrics) and SEPs after median and ulnar nerve stimulations (latency and amplitude of N9 and N20 components).

RESULTS

Abnormal DTI metrics indicated anatomical damages of ascending sensory fibres in ∼60% of patients (p<0.05). Raw SEPs (μV) were smaller in ∼40% of patients but the difference with healthy subjects was not significant (p>0.16). Their normalisation to prestimulus activity strengthened the difference between groups (p<0.05) and allowed identification of ∼60% of patients with abnormal values. According to N9 latency, the peripheral conduction time was normal in patients (p>0.32) but based on N20 latency, the central conduction time (between spinal cord and parietal cortex) was found to be slower (p<0.05). Significant correlation was found between DTI metrics and N9 amplitude (p<0.05). Altered SEPs were also correlated with the disease duration (p<0.05). Taken together, spinal imaging and electrophysiology helped to identify ∼85% of patients with subclinical sensory defect while separated methods revealed abnormal values in ∼60%.

CONCLUSIONS

Sensory impairments have been underestimated at early stages of ALS. These results show for the first time the interest to combine electrophysiology and imaging to assess non-motor system involvement in ALS.

TRIAL REGISTRATION NUMBER

IDRCB2012-A00016-37.

Corneal confocal microscopy reveals trigeminal small sensory fiber neuropathy in amyotrophic lateral sclerosis

Although subclinical involvement of sensory neurons in amyotrophic lateral sclerosis (ALS) has been previously demonstrated, corneal small fiber sensory neuropathy has not been reported to-date. We examined a group of sporadic ALS patients with corneal confocal microscopy, a recently developed imaging technique allowing in vivo observation of corneal small sensory fibers. Corneal confocal microscopy (CCM) examination revealed a reduction of corneal small fiber sensory nerve number and branching in ALS patients. Quantitative analysis demonstrated an increase in tortuosity and reduction in length and fractal dimension of ALS patients’ corneal nerve fibers compared to age-matched controls. Moreover, bulbar function disability scores were significantly related to measures of corneal nerve fibers anatomical damage. Our study demonstrates for the first time a corneal small fiber sensory neuropathy in ALS patients. This finding further suggests a link between sporadic ALS and facial-onset sensory and motor neuronopathy (FOSMN) syndrome, a rare condition characterized by early sensory symptoms (with trigeminal nerve distribution), followed by wasting and weakness of bulbar and upper limb muscles. In addition, the finding supports a model of neurodegeneration in ALS as a focally advancing process.

Directed midbrain and spinal cord neurogenesis from pluripotent stem cells to model development and disease in a dish

Induction of specific neuronal fates is restricted in time and space in the developing CNS through integration of extrinsic morphogen signals and intrinsic determinants. Morphogens impose regional characteristics on neural progenitors and establish distinct progenitor domains. Such domains are defined by unique expression patterns of fate determining transcription factors. These processes of neuronal fate specification can be recapitulated in vitro using pluripotent stem cells. In this review, we focus on the generation of dopamine neurons and motor neurons, which are induced at ventral positions of the neural tube through Sonic hedgehog (Shh) signaling, and defined at anteroposterior positions by fibroblast growth factor (Fgf) 8, Wnt1, and retinoic acid (RA). In vitro utilization of these morphogenic signals typically results in the generation of multiple neuronal cell types, which are defined at the intersection of these signals. If the purpose of in vitro neurogenesis is to generate one cell type only, further lineage restriction can be accomplished by forced expression of specific transcription factors in a permissive environment. Alternatively, cell-sorting strategies allow for selection of neuronal progenitors or mature neurons. However, modeling development, disease and prospective therapies in a dish could benefit from structured heterogeneity, where desired neurons are appropriately synaptically connected and thus better reflect the three-dimensional structure of that region. By modulating the extrinsic environment to direct sequential generation of neural progenitors within a domain, followed by self-organization and synaptic establishment, a reductionist model of that brain region could be created. Here we review recent advances in neuronal fate induction in vitro, with a focus on the interplay between cell intrinsic and extrinsic factors, and discuss the implications for studying development and disease in a dish.

Accumulation of misfolded SOD1 in dorsal root ganglion degenerating proprioceptive sensory neurons of transgenic mice with amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is an adult-onset progressive neurodegenerative disease affecting upper and lower motoneurons (MNs). Although the motor phenotype is a hallmark for ALS, there is increasing evidence that systems other than the efferent MN system can be involved. Mutations of superoxide dismutase 1 (SOD1) gene cause a proportion of familial forms of this disease. Misfolding and aggregation of mutant SOD1 exert neurotoxicity in a noncell autonomous manner, as evidenced in studies using transgenic mouse models. Here, we used the SOD1^G93A mouse model for ALS to detect, by means of conformational-specific anti-SOD1 antibodies, whether misfolded SOD1-mediated neurotoxicity extended to neuronal types other than MNs. We report that large dorsal root ganglion (DRG) proprioceptive neurons accumulate misfolded SOD1 and suffer a degenerative process involving the inflammatory recruitment of macrophagic cells. Degenerating sensory axons were also detected in association with activated microglial cells in the spinal cord dorsal horn of diseased animals. As large proprioceptive DRG neurons project monosynaptically to ventral horn MNs, we hypothesise that a prion-like mechanism may be responsible for the transsynaptic propagation of SOD1 misfolding from ventral horn MNs to DRG sensory neurons.

Neuroimaging to investigate multisystem involvement and provide biomarkers in amyotrophic lateral sclerosis

Neuroimaging allows investigating the extent of neurological systems degeneration in amyotrophic lateral sclerosis (ALS). Advanced MRI methods can detect changes related to the degeneration of upper motor neurons but have also demonstrated the participation of other systems such as the sensory system or basal ganglia, demonstrating in vivo that ALS is a multisystem disorder. Structural and functional imaging also allows studying dysfunction of brain areas associated with cognitive signs. From a biomarker perspective, numerous studies using diffusion tensor imaging showed a decrease of fractional anisotropy in the intracranial portion of the corticospinal tract but its diagnostic value at the individual level remains limited. A multiparametric approach will be required to use MRI in the diagnostic workup of ALS. A promising avenue is the new methodological developments of spinal cord imaging that has the advantage to investigate the two motor system components that are involved in ALS, that is, the lower and upper motor neuron. For all neuroimaging modalities, due to the intrinsic heterogeneity of ALS, larger pooled banks of images with standardized image acquisition and analysis procedures are needed. In this paper, we will review the main findings obtained with MRI, PET, SPECT, and nuclear magnetic resonance spectroscopy in ALS.

Therapeutic targets for amyotrophic lateral sclerosis: current treatments and prospects for more effective therapies

Although amyotrophic lateral sclerosis (ALS) was described more than 130 years ago, the cause(s) of most cases of this adult motor neuron disease remains a mystery. With the discovery of mutations in one gene (Cu/Zn superoxide dismutase) as a primary cause of some forms of ALS, model systems have been developed that have helped us begin to understand mechanisms involved in motor neuron death and enabled testing of potential new therapies. Several other genes have been implicated as risk factors in motor neuron diseases, including neurofilaments, cytoplasmic dynein and dynactin, vascular endothelial growth factor, and angiogenin. With advances in the basic research of the disease, many hypotheses accounting for motor neuron death are being explored, including loss of trophic support, protein mishandling, mitochondrial dysfunction, excitotoxicity, axonal abnormalities and inflammation. Many of these mechanisms are the focus of research in other neurodegenerative disorders, such as Parkinson's, Alzheimer's and Huntington's disease.

Characterization of thoracic motor and sensory neurons and spinal nerve roots in canine degenerative myelopathy, a potential disease model of amyotrophic lateral sclerosis

Canine degenerative myelopathy (DM) is a progressive, adult-onset, multisystem degenerative disease with many features in common with amyotrophic lateral sclerosis (ALS). As with some forms of ALS, DM is associated with mutations in superoxide dismutase 1 (SOD1). Clinical signs include general proprioceptive ataxia and spastic upper motor neuron paresis in pelvic limbs, which progress to flaccid tetraplegia and dysphagia. The purpose of this study was to characterize DM as a potential disease model for ALS. We previously reported that intercostal muscle atrophy develops in dogs with advanced-stage DM. To determine whether other components of the thoracic motor unit (MU) also demonstrated morphological changes consistent with dysfunction, histopathologic and morphometric analyses were conducted on thoracic spinal motor neurons (MNs) and dorsal root ganglia (DRG) and in motor and sensory nerve root axons from DM-affected boxers and Pembroke Welsh corgis (PWCs). No alterations in MNs or motor root axons were observed in either breed. However, advanced-stage PWCs exhibited significant losses of sensory root axons, and numerous DRG sensory neurons displayed evidence of degeneration. These results indicate that intercostal muscle atrophy in DM is not preceded by physical loss of the motor neurons innervating these muscles, nor of their axons. Axonal loss in thoracic sensory roots and sensory neuron death suggest that sensory involvement may play an important role in DM disease progression. Further analysis of the mechanisms responsible for these morphological findings would aid in the development of therapeutic intervention for DM and some forms of ALS.

Moving forward in clinical trials for ALS: motor neurons lead the way please

Amyotrophic lateral sclerosis (ALS) is one of the most complex motor neuron diseases. Even though scientific discoveries are accelerating with an unprecedented pace, to date more than 30 clinical trials have ended with failure and staggering frustration. There are too many compounds that increase life span in mice, but too little evidence that they will improve human condition. Increasing the chances of success for future clinical trials requires advancement of preclinical tests. Recent developments, which enable the visualization of diseased motor neurons, have the potential to bring novel insight. As we change our focus from mice to motor neurons, it is possible to foster a new vision that translates into effective and long-term treatment strategies in ALS and related motor neuron disorders (MND).

eGFP expression under UCHL1 promoter genetically labels corticospinal motor neurons and a subpopulation of degeneration-resistant spinal motor neurons in an ALS mouse model

Understanding mechanisms that lead to selective motor neuron degeneration requires visualization and cellular identification of vulnerable neurons. Here we report generation and characterization of UCHL1-eGFP and hSOD1(G93A)-UeGFP mice, novel reporter lines for cortical and spinal motor neurons. Corticospinal motor neurons (CSMN) and a subset of spinal motor neurons (SMN) are genetically labeled in UCHL1-eGFP mice, which express eGFP under the UCHL1 promoter. eGFP expression is stable and continues through P800 in vivo. Retrograde labeling, molecular marker expression, electrophysiological analysis, and cortical circuit mapping confirmed CSMN identity of eGFP(+) neurons in the motor cortex. Anatomy, molecular marker expression, and electrophysiological analysis revealed that the eGFP expression is restricted to a subset of small-size SMN that are slow-twitch α and γ motor neurons. Crossbreeding of UCHL1-eGFP and hSOD1(G93A) lines generated hSOD1(G93A)-UeGFP mice, which displayed the disease phenotype observed in a hSOD1(G93A) mouse model of ALS. eGFP(+) SMN showed resistance to degeneration in hSOD1(G93A)-UeGFP mice, and their slow-twitch α and γ motor neuron identity was confirmed. In contrast, eGFP(+) neurons in the motor cortex of hSOD1(G93A)-UeGFP mice recapitulated previously reported progressive CSMN loss and apical dendrite degeneration. Our findings using these two novel reporter lines revealed accumulation of autophagosomes along the apical dendrites of vulnerable CSMN at P60, early symptomatic stage, suggesting autophagy as a potential intrinsic mechanism for CSMN apical dendrite degeneration.

The complex molecular biology of amyotrophic lateral sclerosis (ALS)

Amyotrophic lateral sclerosis (ALS) is an adult-onset neurodegenerative disorder that causes selective death of motor neurons followed by paralysis and death. A subset of ALS cases is caused by mutations in the gene for Cu, Zn superoxide dismutase (SOD1), which impart a toxic gain of function to this antioxidant enzyme. This neurotoxic property is widely believed to stem from an increased propensity to misfold and aggregate caused by decreased stability of the native homodimer or a tendency to lose stabilizing posttranslational modifications. Study of the molecular mechanisms of SOD1-related ALS has revealed a complex array of interconnected pathological processes, including glutamate excitotoxicity, dysregulation of neurotrophic factors and axon guidance proteins, axonal transport defects, mitochondrial dysfunction, deficient protein quality control, and aberrant RNA processing. Many of these pathologies are directly exacerbated by misfolded and aggregated SOD1 and/or cytosolic calcium overload, suggesting the primacy of these events in disease etiology and their potential as targets for therapeutic intervention.