Early pathogenesis in the adult-onset neurodegenerative disease amyotrophic lateral sclerosis

Abstract

Amyotrophic lateral sclerosis (ALS) is a devastating paralytic disorder caused by dysfunction and degeneration of motor neurons starting in adulthood. Most of our knowledge about the pathophysiological mechanisms of ALS comes from transgenic mice models that emulate a subgroup of familial ALS cases (FALS), with mutations in the gene encoding superoxide dismutase (SOD1). In the more than 15 years since these mice were generated, a large number of abnormal cellular mechanisms underlying motor neuron degeneration have been identified, but to date this effort has led to few improvements in therapy, and no cure. Here, we consider that this surfeit of mechanisms is best interpreted by current insights that suggest a very early initiation of pathology in motor neurons, followed by a diversity of secondary cascades and compensatory mechanisms that mask symptoms for decades, until trauma and/or aging overloads their protective function. This view thus posits that adult-onset ALS is the consequence of processes initiated during early development. In fact, motor neurons in neonatal mutant SOD mice display important alterations in their intrinsic electrical properties, synaptic inputs and morphology that are accompanied by subtle behavioral abnormalities. We consider evidence that human mutant SOD1 protein in neonatal hSOD1(G93A) mice instigates motor neuron degeneration by increasing persistent sodium currents and excitability, in turn altering synaptic circuits that control excessive motor neuron firing and leads to excitotoxicity. We also discuss how therapies that are aimed at suppressing abnormal neuronal activity might effectively mitigate or prevent the onset of irreversible neuronal damage in adulthood. J. Cell. Biochem. 113: 3301-3312, 2012. © 2012 Wiley Periodicals, Inc.

Fig. 1

Model of how mutant SOD1 may underlie the etiology of ALS. Subtle synaptic dysfunction may be initiated by mutant SOD1 during development: either at early postnatal stages, or even during embryogenesis, when expression of wild-type and mutant SOD1 is first detected. Although this primary event causes little pathogenesis by itself, it may initiate a diversity of secondary cascades and compensatory mechanisms, which will influence the development and functioning of neuronal circuits and networks. Gradually (and in a vicious cycle) neuronal function is lost; however, during this early period—which can span a period of months in mouse models and decades in humans— symptoms have not yet manifested. Onset of disease may become apparent when compensatory mechanisms saturate and/or break down, either by accumulation of dysfunction or via other pathogenic events, such as trauma and environmental factors that may be aggravated by age. The mechanism(s) by which mutant SOD1 causes synaptic dysfunction is unknown, however, it could relate to the fact that ALS is at least partially a non-cell autonomous disease, and that expression of mutant SOD1 in astrocytes can induce the release of toxic factors (see text for more details).

Fig. 2

Evidence for the hypothesis that glutamate-induced excitotoxicity underlies the pathology in ALS. Schematic diagram to show how a variety of alterations in pre-synaptic neurons, post-synaptic neurons or glial cells might result in excessive Ca²⁺ entry into motor neurons. Citations listed on the diagram refer to reports that document the cellular pathophysiological processes underlying glutamate-induced excitotoxicity by analyzing fluid and tissue samples from either ALS patients (in blue) or SOD1 ALS mice models (in green). Results from cell culture-based SOD1 ALS models are also shown (in red). Studies are from high-expressor SOD1 ^G93A unless stated.* indicates that the SOD mutation was not identified in the FAIS patients. We have tried to acknowledge as many original studies as possible and apologize for those we have omitted. Reference list is shown in the supplementary data.

Fig. 3

Time-line of pathological and clinical changes in the high-expressor hSOD1^G93A line of transgenic mice. The most important and earliest abnormalities reported for the hSOD1^G93A transgenic mouse model are shown. Months before motor neurons degenerate and clinical symptoms appear (P90), widespread and early onset of pathological abnormalities are detected in this ALS mouse model. See references stated here and text for additional information. (Ref. 1 SOD1 expression is detected from embryonic (E) day 8.5 (E8.5) [Yon et al., 2008]). (Ref. 2 Temporal behavioral abnormalities during early postnatal (P) development: P2–4 mSOD1^G93A mice show reduced forelimb placing and righting capacities [van Zundert et al., 2008]. Similar reversible sensorimotor alterations are observed for neonatal (P1–P7) hSOD1^G85R mice [Amendola et al., 2004]). (Ref. 3 Electrical abnormalities in hSOD1^G93A motor neurons (MNs) at P4–P10; hypoglossal motor neurons (HMs) display increased excitability and PC_Na [van Zundert et al., 2008]. hSOD1^G93A spinal cord motor neurons also possess enhanced PC_Na (P0–P12)and PC_Ca(P6–P12) [Quinlan et al., 2011]. In addition, early (P6–P10) abnormal changes in electrical properties, including in excitability, are detected in the low-expressor hSOD1^G93A transgenic line and the hSOD1^G85R mutant mice [Bories et al., 2007; Pambo-Pambo et al., 2009]). (Ref. 4 Synaptic transmission mediated by AMPA, NMDA and glycine-receptors is altered for HMs in P4–10 hSOD1^G93A mice [van Zundert et al., 2008]). (Ref. 5 Morphological abnormalities of motor neurons of hSOD1^G93A mice at P6; precocious remodeling of HMs [van Zundert et al., 2008]. Early (P6–P10) abnormal dendritic branching is also observed for hSOD1^G85R transgenic mice [Amendola and Durand, 2008]). (Ref. 6 Genes involved in stress-related pathways are transiently upregulated between P12 and P26, in vulnerable hSOD1^G93A spinal motor neurons [Saxena et al., 2009]). (Ref. 7 Ultrastructural alterations of motor neurons in mSOD1^G93A mice, starting at P14 (2 weeks); mitochondria swellings and small vacuoles are present in distal dendrites and in the cell bodies of spinal cord motor neurons [Bendotti et al., 2001]). (Ref. 8 mutant SOD1 aggregates in spinal motor neurons of hSOD1^G93A mice at P30 [Johnston et al., 2000]). (Ref. 9 Genes involved in UPR and in the ubiquitin proteasome system (UPS) are up- and down-regulated, respectively, in vulnerable P32 spinal motor neurons of hSOD1^G93A mice [Saxena et al., 2009]). (Ref. 10 Functional loss of motor unit for fast-twitch hind-limb muscles (medial gastrocnemius [MG]) is detected starting at P40in hSOD1^G93A mice [Hegedusetal., 2007]). (Ref. 11 DNA damage (e.g., immunohistochemistry (ICH) with antibodies recognizing single-stranded breaks) is detected in motor neurons of hSOD1^G93A mice, starting at P49 (7 weeks) [Martin et al., 2007]). (Ref. 12 Structural changes in motor units of hSOD1^G93A mice starting at P50; loss of MG neuromuscular synapses and prominent vacuolation in nerve terminals [Frey et al., 2000]). [Ref. 13 Markers of apoptosis (including caspases 1 and 3) in motor neurons are detected as of P60 in spinal cord motor neurons of hSOD1^G93A mice [Li et al., 2000]. See Martin et al., [2007] for discussion on apoptotic-necrotic hybrid forms of motor neuron death in ALS]. (Ref. 14 Degeneration of motor neurons (e.g., those that display choline acetyltransferas (Chat)-positive IRR)of hSOD1^G93A mice starts at ∼P90 [Chiu et al., 1995]). (Ref. 15 Clinical symptoms (e.g., small tremors, weight lose) [Chiu et al., 1995] and more prominent behavioral abnormalities (e.g., decline in hanging test and rotarod performance) of hSOD1^G93A mice starts at ∼P90). See also additional reviews for deficits during development in other mutant SOD1 mice, including the low-expressor hSOD1^G93A, hSOD1^G85R, and hSOD1^G127X mice [Durand et al., 2006; Elbasiouny et al., 2010b; Quinlan, 2011].

Fig. 4

Model of how PC_Na might induce synaptic dysfunction in pre-symptomatic neonatal/juvenile hSOD1^G93A transgenic mice. Based on the alterations in electrophysiological properties and synaptic circuits that are detected in motor neurons and interneurons of hSOD1^G93A mice during the first days after birth, we propose a model in which small increases in PC_Na mediated by Na_v channels and PC_Ca mediated by Ca_v channels (likely Ca_v1.3) may significantly enhance neuronal excitability and increase synaptic transmission, leading to sustained toxic influxes of and Ca²⁺ (and Na⁺) through NMDA- and AMPA-receptors, and through Ca_v channels. In motor neurons, the limited expression of Ca²⁺ buffering proteins obligates the mitochondria to perform the buffering task, leading to mitochondrial dysfunction and damage. Excess Ca²⁺ also induces expression of plasticity-related genes that may, together with the imbalance between hyper-polarization and depolarization, disrupt local circuitry and networks, induce minor behavioral changes. The underlying mechanism(s) responsible for the increased PC_Na has not been determined, but could be related to the ROS/RNS produced by astrocytes that express mutations in SOD1 (not shown).