Soma size and Cav1.3 channel expression in vulnerable and resistant motoneuron populations of the SOD1G93A mouse model of ALS

Bifurcation analysis of motoneuronal excitability mechanisms under normal and ALS conditions

Introduction

Bifurcation analysis allows the examination of steady-state, non-linear dynamics of neurons and their effects on cell firing, yet its usage in neuroscience is limited to single-compartment models of highly reduced states. This is primarily due to the difficulty in developing high-fidelity neuronal models with 3D anatomy and multiple ion channels in XPPAUT, the primary bifurcation analysis software in neuroscience.

Methods

To facilitate bifurcation analysis of high-fidelity neuronal models under normal and disease conditions, we developed a multi-compartment model of a spinal motoneuron (MN) in XPPAUT and verified its firing accuracy against its original experimental data and against an anatomically detailed cell model that incorporates known MN non-linear firing mechanisms. We used the new model in XPPAUT to study the effects of somatic and dendritic ion channels on the MN bifurcation diagram under normal conditions and after amyotrophic lateral sclerosis (ALS) cellular changes.

Results

Our results show that somatic small-conductance Ca2+-activated K (SK) channels and dendritic L-type Ca2+ channels have the strongest effects on the bifurcation diagram of MNs under normal conditions. Specifically, somatic SK channels extend the limit cycles and generate a subcritical Hopf bifurcation node in the V-I bifurcation diagram of the MN to replace a supercritical node Hopf node, whereas L-type Ca2+ channels shift the limit cycles to negative currents. In ALS, our results show that dendritic enlargement has opposing effects on MN excitability, has a greater overall impact than somatic enlargement, and dendritic overbranching offsets the dendritic enlargement hyperexcitability effects.

Discussion

Together, the new multi-compartment model developed in XPPAUT facilitates studying neuronal excitability in health and disease using bifurcation analysis.

Motoneuron excitability dysfunction in ALS: Pseudo-mystery or authentic conundrum?

In amyotrophic lateral sclerosis (ALS), abnormalities in motoneuronal excitability are seen in early pathogenesis and throughout disease progression. Fully understanding motoneuron excitability dysfunction may lead to more effective treatments. Yet decades of research have not produced consensus on the nature, role or underlying mechanisms of motoneuron excitability dysfunction in ALS. For example, contrary to Ca excitotoxicity theory, predictions of motoneuronal hyper-excitability, normal and hypo-excitability have also been seen at various disease stages and in multiple ALS lines. Accordingly, motoneuron excitability dysfunction in ALS is a disputed topic in the field. Specifically, the form (hyper, hypo or unchanged) and what role excitability dysfunction plays in the disease (pathogenic or downstream of other pathologies; neuroprotective or detrimental) are currently unclear. Although several motoneuron properties that determine cellular excitability change in the disease, some of these changes are pro-excitable, whereas others are anti-excitable, making dynamic fluctuations in overall 'net' excitability highly probable. Because various studies assess excitability via differing methods and at differing disease stages, the conflicting reports in the literature are not surprising. Hence, the overarching process of excitability degradation and motoneuron degeneration is not fully understood. Consequently, the discrepancies on motoneuron excitability dysfunction in the literature represent a substantial barrier to our understanding of the disease. Emerging studies suggest that biological variables, variations in experimental protocols, issues of rigor and sampling/analysis strategies are key factors that may underlie conflicting data in the literature. This review highlights potential confounding factors for researchers to consider and also offers ideas on avoiding pitfalls and improving robustness of data.

Unique molecular features and cellular responses differentiate two populations of motor cortical layer 5b neurons in a preclinical model of ALS

Many neurodegenerative diseases, such as amyotrophic lateral sclerosis (ALS), lead to the selective degeneration of discrete cell types in the CNS despite the ubiquitous expression of many genes linked to disease. Therapeutic advancement depends on understanding the unique cellular adaptations that underlie pathology of vulnerable cells in the context of disease-causing mutations. Here, we employ bacTRAP molecular profiling to elucidate cell type-specific molecular responses of cortical upper motor neurons in a preclinical ALS model. Using two bacTRAP mouse lines that label distinct vulnerable or resilient projection neuron populations in motor cortex, we show that the regulation of oxidative phosphorylation (Oxphos) pathways is a common response in both cell types. However, differences in the baseline expression of genes involved in Oxphos and the handling of reactive oxygen species likely lead to the selective degeneration of the vulnerable cells. These results provide a framework to identify cell-type-specific processes in neurodegenerative disease.

Moya et al. use bacTRAP mouse lines to characterize two highly related subpopulations of layer 5b projection neurons in motor cortex that are differentially susceptible to neurodegeneration in the SOD1-G93A mouse model of ALS. They identify the regulation of genes involved in bioenergetics as a key factor regulating susceptibility.

Time Course of Alterations in Adult Spinal Motoneuron Properties in the SOD1(G93A) Mouse Model of ALS

Although amyotrophic lateral sclerosis (ALS) is an adult-onset neurodegenerative disease, motoneuron electrical properties are already altered during embryonic development. Motoneurons must therefore exhibit a remarkable capacity for homeostatic regulation to maintain a normal motor output for most of the life of the patient. In the present article, we demonstrate how maintaining homeostasis could come at a very high cost. We studied the excitability of spinal motoneurons from young adult SOD1(G93A) mice to end-stage. Initially, homeostasis is highly successful in maintaining their overall excitability. This initial success, however, is achieved by pushing some cells far above the normal range of passive and active conductances. As the disease progresses, both passive and active conductances shrink below normal values in the surviving cells. This shrinkage may thus promote survival, implying the previously large values contribute to degeneration. These results support the hypothesis that motoneuronal homeostasis may be "hypervigilant" in ALS and a source of accumulating stress.

ALS-CSF-induced structural changes in spinal motor neurons of rat pups cause deficits in motor behaviour

Amyotrophic lateral sclerosis (ALS) is a late-onset, neurodegenerative disease associated with the loss of motor neurons in the spinal cord, brain stem and primary motor cortex. Deficit in the motor function is one of the clinical features of this disease. However, the association between adverse morphological alterations in the spinal motor neurons and motor deficit in sporadic ALS (SALS) is still debated. The present study has sought to investigate the effects of serial intrathecal injections of ALS-CSF into rat pups, at post-natal (P) days 3, 9 and 14, on the motor neuronal (MN) morphology at the cervical and lumbar levels of the spinal cord at P16 and P22. The present study used Cresyl violet and Golgi-Cox staining methods to determine the progressive changes in the morphology of spinal MNs in both cervical and lumbar extensions. The study found a loss of motor neurons in the spinal cord (36% for P16 in cervical and 41.7% in P16 lumbar and 49.57% for P22 cervical and 44.63% for P22 lumbar) and reduced choline acetyl transferase (ChAT) expression after repeated infusion of ALS-CSF. Significant increase in the soma area was also found in ALS-CSF rats (around 21% in P22 cervical and 26.4% in P22 lumbar). Soma hypertrophy was associated with increased dendritic arborization of MNs at both cervical and lumbar levels of the spinal cord. The data also showed a direct correlation between ALS-CSF induced changes in the MN number in the spinal cord and motor behavioral deficits. The loss of MNs, reduced ChAT, changes in soma and dendritic morphology with declined rotarod performance, thus, confirming the pathological phenotypes as seen in ALS patients.

Altered Motoneuron Properties Contribute to Motor Deficits in a Rabbit Hypoxia-Ischemia Model of Cerebral Palsy

Cerebral palsy (CP) is caused by a variety of factors attributed to early brain damage, resulting in permanently impaired motor control, marked by weakness and muscle stiffness. To find out if altered physiology of spinal motoneurons (MNs) could contribute to movement deficits, we performed whole-cell patch-clamp in neonatal rabbit spinal cord slices after developmental injury at 79% gestation. After preterm hypoxia-ischemia (HI), rabbits are born with motor deficits consistent with a spastic phenotype including hypertonia and hyperreflexia. There is a range in severity, thus kits are classified as severely affected, mildly affected, or unaffected based on modified Ashworth scores and other behavioral tests. At postnatal day (P)0-5, we recorded electrophysiological parameters of 40 MNs in transverse spinal cord slices using whole-cell patch-clamp. We found significant differences between groups (severe, mild, unaffected and sham control MNs). Severe HI MNs showed more sustained firing patterns, depolarized resting membrane potential, and fired action potentials at a higher frequency. These properties could contribute to muscle stiffness, a hallmark of spastic CP. Interestingly altered persistent inward currents (PICs) and morphology in severe HI MNs would dampen excitability (depolarized PIC onset and increased dendritic length). In summary, changes we observed in spinal MN physiology likely contribute to the severity of the phenotype, and therapeutic strategies for CP could target the excitability of spinal MNs.

Shorter axon initial segments do not cause repetitive firing impairments in the adult presymptomatic G127X SOD-1 Amyotrophic Lateral Sclerosis mouse

Increases in axonal sodium currents in peripheral nerves are some of the earliest excitability changes observed in Amyotrophic Lateral Sclerosis (ALS) patients. Nothing is known, however, about axonal sodium channels more proximally, particularly at the action potential initiating region - the axon initial segment (AIS). Immunohistochemistry for Nav1.6 sodium channels was used to investigate parameters of AISs of spinal motoneurones in the G127X SOD1 mouse model of ALS in adult mice at presymptomatic time points (~190 days old). In vivo intracellular recordings from lumbar spinal motoneurones were used to determine the consequences of any AIS changes. AISs of both alpha and gamma motoneurones were found to be significantly shorter (by 6.6% and 11.8% respectively) in G127X mice as well as being wider by 9.8% (alpha motoneurones). Measurements from 20–23 day old mice confirmed that this represented a change during adulthood. Intracellular recordings from motoneurones in presymptomatic adult mice, however, revealed no differences in individual action potentials or the cells ability to initiate repetitive action potentials. To conclude, despite changes in AIS geometry, no evidence was found for reduced excitability within the functional working range of firing frequencies of motoneurones in this model of ALS.

Size-Dependent Vulnerability of Lumbar Motor Neuron Dendritic Degeneration in SOD1G93A Mice

The motor neuron (MN) soma surface area is correlated with motor unit type. Larger MNs innervate fast fatigue-intermediate (FInt) or fast-fatiguable (FF) muscle fibers in type FInt and FF motor units, respectively. Smaller MNs innervate slow-twitch fatigue-resistant (S) or fast fatigue-resistant (FR) muscle fibers in type S and FR motor units, respectively. In amyotrophic lateral sclerosis (ALS), FInt and FF motor units are more vulnerable, with denervation and MN death occurring for these units before the more resilient S and FR units. Abnormal MN dendritic arbors have been observed in ALS in humans and rodent models. We used a Golgi-Cox impregnation protocol to examine soma size-dependent changes in the dendritic morphology of lumbar MNs in SOD1^G93A mice, a model of ALS, at pre-symptomatic, onset and mid-disease stages. In wildtype control mice, the relationship between MN soma surface area and dendritic length or dendritic spine number was highly linear (i.e., increased MN soma size correlated with increased dendritic length and spines). By contrast, in SOD1^G93A mice, this linear relationship was lost and dendritic length reduction and spine loss were observed in larger MNs, from pre-symptomatic stages onward. These changes correlated with the neuromotor symptoms of ALS in rodent models. At presymptomatic ages, changes were restricted to the larger MNs, likely to comprise vulnerable FInt and FF motor units. Our results suggest morphological changes of MN dendrites and dendritic spines are likely to contribute ALS pathogenesis, not compensate for it. Anat Rec, 303:1455-1471, 2020. © 2019 American Association for Anatomy.

Motor Neuron Susceptibility in ALS/FTD

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease characterized by the death of both upper and lower motor neurons (MNs) in the brain, brainstem and spinal cord. The neurodegenerative mechanisms leading to MN loss in ALS are not fully understood. Importantly, the reasons why MNs are specifically targeted in this disorder are unclear, when the proteins associated genetically or pathologically with ALS are expressed ubiquitously. Furthermore, MNs themselves are not affected equally; specific MNs subpopulations are more susceptible than others in both animal models and human patients. Corticospinal MNs and lower somatic MNs, which innervate voluntary muscles, degenerate more readily than specific subgroups of lower MNs, which remain resistant to degeneration, reflecting the clinical manifestations of ALS. In this review, we discuss the possible factors intrinsic to MNs that render them uniquely susceptible to neurodegeneration in ALS. We also speculate why some MN subpopulations are more vulnerable than others, focusing on both their molecular and physiological properties. Finally, we review the anatomical network and neuronal microenvironment as determinants of MN subtype vulnerability and hence the progression of ALS.

The vulnerability of spinal motoneurons and soma size plasticity in a mouse model of amyotrophic lateral sclerosis

KEY POINTS

Motoneuron soma size is a largely plastic property that is altered during amyotrophic lateral sclerosis (ALS) progression. We report evidence of systematic spinal motoneuron soma size plasticity in mutant SOD1-G93A mice at various disease stages and across sexes, spinal regions and motoneuron types. We show that disease-vulnerable motoneurons exhibit early increased soma sizes. We show via computer simulations that the measured changes in soma size have a profound impact on the excitability of disease-vulnerable motoneurons. This study reveals a novel form of plasticity in ALS and suggests a potential target for altering motoneuron function and survival.

ABSTRACT

α-Motoneuron soma size is correlated with the cell's excitability and function, and has been posited as a plastic property that changes during cellular maturation, injury and disease. This study examined whether α-motoneuron somas change in size over disease progression in the G93A mouse model of amyotrophic lateral sclerosis (ALS), a disease characterized by progressive motoneuron death. We used 2D- and 3D-morphometric analysis of motoneuron size and measures of cell density at four key disease stages: neonatal (P10 - with earliest known disease changes); young adult (P30 - presymptomatic with early motoneuron death); symptom onset (P90 - with death of 70-80% of motoneurons); and end-stage (P120+ - with full paralysis of hindlimbs). We additionally examined differences in lumbar vs. sacral vs. cervical motoneurons; in motoneurons from male vs. female mice; and in fast vs. slow motoneurons. We present the first evidence of plastic changes in the soma size of spinal α-motoneurons occurring throughout different stages of ALS with profound effects on motoneuron excitability. Somatic changes are time dependent and are characterized by early-stage enlargement (P10 and P30); no change around symptom onset; and shrinkage at end-stage. A key finding in the study indicates that disease-vulnerable motoneurons exhibit increased soma sizes (P10 and P30). This pattern was confirmed across spinal cord regions, genders and motoneuron types. This extends the theory of motoneuron size-based vulnerability in ALS: not only are larger motoneurons more vulnerable to death in ALS, but are also enlarged further in the disease. Such information is valuable for identifying ALS pathogenesis mechanisms.

Comparison of dendritic calcium transients in juvenile wild type and SOD1(G93A) mouse lumbar motoneurons

Previous studies of spinal motoneurons in the SOD1 mouse model of amyotrophic lateral sclerosis have shown alterations long before disease onset, including increased dendritic branching, increased persistent Na⁺ and Ca²⁺ currents, and impaired axonal transport. In this study dendritic Ca²⁺ entry was investigated using two photon excitation fluorescence microscopy and whole-cell patch-clamp of juvenile (P4-11) motoneurons. Neurons were filled with both Ca²⁺ Green-1 and Texas Red dextrans, and line scans performed throughout. Steps were taken to account for different sources of variability, including (1) dye filling and laser penetration, (2) dendritic anatomy, and (3) the time elapsed from the start of recording. First, Ca²⁺ Green-1 fluorescence was normalized by Texas Red; next, neurons were reconstructed so anatomy could be evaluated; finally, time was recorded. Customized software detected the largest Ca²⁺ transients (area under the curve) from each line scan and matched it with parameters above. Overall, larger dendritic diameter and shorter path distance from the soma were significant predictors of larger transients, while time was not significant up to 2 h (data thereafter was dropped). However, Ca²⁺ transients showed additional variability. Controlling for previous factors, significant variation was found between Ca²⁺ signals from different processes of the same neuron in 3/7 neurons. This could reflect differential expression of Ca²⁺ channels, local neuromodulation or other variations. Finally, Ca²⁺ transients in SOD1^G93A motoneurons were significantly smaller than in non-transgenic motoneurons. In conclusion, motoneuron processes show highly variable Ca²⁺ transients, but these transients are smaller overall in SOD1^G93A motoneurons.