. Motor Cortex Layer V Pyramidal Neurons Exhibit Dendritic Regression, Spine Loss, and Increased Synaptic Excitation in the Presymptomatic hSOD1G93A Mouse Model of Amyotrophic Lateral Sclerosis. J Neurosci. 2015 Jan 14;35(2):643-7. PubMed.

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  1. The paper is succinct and reveals an important message. I want to applaud the authors for revealing very early spine defects in the apical dendrites of corticospinal motor neurons.

    There is growing evidence revealing the importance of corticospinal motor neurons (CSMN) to disease pathology in ALS, and this paper shows that spine density and morphology changes occur as early as 21 days old, and that there potentially could be cortical connectivity defects, as well. This is of great interest because we were told for many years that the upper motor neurons were not critical for ALS pathology, or that their degeneration was secondary to spinal motor neuron loss. However, many new findings show that the cortex may actually be the starting point for defects in the motor neuron circuitry.

    This paper builds upon previous findings emerging from our lab and from others suggesting that upper motor neuron degeneration is an early event in ALS. It is very important that they show the cellular defects occur at a very early age, even before the spinal motor neuron loss occurs. While this brief communication may not be strong enough to change the field, it is strong enough to add one more twist—it is now even more interesting to find the intrinsic and extrinsic factors that contribute to CSMN vulnerability, and how they relate to disease pathology.

    That motor neuron death is early and not secondary to spinal motor neuron loss is a very important concept. We should begin to appreciate the importance of the cortex and the cortical component of motor neuron circuitry. The tools, reporter lines, and animal models to investigate this neuron population in detail are now available. It will be interesting to see if ALS could initiate from the cortex after all.

  2. Where does ALS begin? Despite Charcot's initial observation of concomitant upper (UMN) and lower motor neuron (LMN) pathological changes in ALS, the question of where ALS begins has not been definitively established. Resolution of this question would enhance understanding of the pathophysiology of ALS, with diagnostic and therapeutic importance.

    The dying-forward hypothesis proposes that ALS is mainly a disorder of corticomotoneurons, which connect monosynaptically with anterior horn cells, mediating anterograde degeneration of anterior horn cells via glutamate excitotoxicity. Support for a dying-forward hypothesis includes results from transcranial magnetic stimulation studies documenting that cortical hyperexcitability is an early feature in patients with sporadic ALS and precedes the clinical onset of familial ALS.

    Clinical support comes from observations that motor neurons without a monosynaptic connection with corticomotoneurons, such as the oculomotor, abducens, and Onuf's nuclei, are typically spared in ALS; and that pure LMN forms of ALS are rare, whereas subclinical UMN involvement is invariably detected.

    Further support has now been provided by Bellingham and colleagues, who identify changes consistent with very early degeneration of the upper motor neuron in the SOD1 mouse model, with loss of dendritic spines at 3 weeks. These changes are associated with the development of hyperexcitability and the advent of neurodegeneration.  

    While these animal studies are now consistent with clinical findings and experimental observations, an ongoing concern relates to the suitability of the SOD1 model for the human disease of ALS. Until recently, this mouse model was considered by some as the benchmark for testing potential neuroprotectants in ALS. Unfortunately the positive results from trials undertaken in the SOD1 mouse model have not translated to therapy for patients diagnosed with ALS.

    Why has this mismatch occurred? In part, given that SOD1 mutations account for about 2 percent of all ALS cases, it may be argued that this model might have less relevance to human sporadic disease. Furthermore, the SOD1 model undergoes a series of stereotypical changes that begin with hind limb weakness.

    In terms of future approaches, strategies aimed at modulating gene expression are now emerging as potential novel therapeutic options, particularly in light of significant advances in the understanding of the genetic causes of ALS.  One such approach involves the use of antisense oligonucleotides which, when delivered intrathecally, reduce mRNA and SOD1 protein concentrations in brain and spinal cord in the SOD1 mouse and prolongs survival.

    Separately, the recent development of mouse models with mutations in the gene encoding TDP-43 is a further potential advance in therapeutic development for ALS, providing basic scientists with a new, perhaps more relevant, platform for studying novel therapies.

  3. Cortical abnormalities in hSOD1G93A mouse model confirm a key role of synaptic dysfunction in ALS

    Amyotrophic lateral sclerosis (ALS) is characterized by the concomitant dysfunction of corticospinal motoneurons (CSMNs) in the primary motor cortex, and motoneurons of the spinal cord and brainstem that CSMNs activate. Postmortem examination of CSMNs in patients after Golgi staining reveals vast morphological abnormalities, including stunted basal dendritic arborization, gnarled apical dendrites, and spine loss (Hammer et al., 1979). Furthermore, CSMN hyperexcitability has been detected in sporadic ALS patients and in presymptomatic familial ALS patients (Eisen et al., 1993; Vucic et al., 2008). These and other studies suggest that loss of voluntary motor function in ALS patients is initiated by cortical dysfunction (hyperexcitability of CSMNs), and subsequently spreads to spinal motoneurons (Braak et al., 2013). Moreover, one theory on why some spinal motoneurons (SMNs) are selectively vulnerable in ALS while others are completely unaffected focuses on the CSMNs and their downstream synaptic targets. The subsets of motoneurons which are innervated by CSMNs in humans are generally most vulnerable to degeneration in ALS patients, while those with little CSMN input are spared (Eisen et al., 1992). Hence it has been proposed that synaptic connectivity from the CSMNs could be the key to selective vulnerability in ALS.

    It is therefore puzzling that in the hSOD1G93A mouse model of ALS, dysfunction in SMNs has been more thoroughly characterized than in CSMNs. That is, until recently: Several exciting studies have now revealed insights into early pathological events in CSMNs. The first report, in 2002, showed a loss of corticospinal projections in the hSOD1G93A mouse before overt symptom onset (projections lost by postnatal day [P] 60, while tremor and weakness appear in hindlimbs around P90) (Zang and Cheema, 2002). Then, in 2009, electrophysiological abnormalities in cultured embryonic cortical neurons were described by the Zona laboratory at the University of Rome (Pieri et al., 2009). Coincidentally, electrical changes in cultured cortical neurons mirrored many of the changes observed in cultured embryonic SMNs, including increased firing frequencies of action potentials and a larger-amplitude persistent inward current (Pieri et al., 2003; Kuo et al., 2004; Kuo et al., 2005). In addition to those abnormalities, studies of hSOD1G93A brainstem and spinal motoneurons have shown altered dendritic arborization (reduced in embryonic spinal and postnatal brainstem motoneurons, and expanded postnatally in spinal neurons) (Martin et al., 2013; van Zundert et al., 2008; Amendola and Durand, 2008; Filipchuk and Durand, 2012), increased persistent inward currents throughout development (Kuo et al., 2005; van Zundert et al., 2008; Quinlan et al., 2011; Delestree et al., 2014), and eventual progression of motoneurons to hypoexcitability just preceding symptom onset (Delestree et al., 2014). Additional dysfunction has been documented presymptomatically in axon transport (Bilsland et al., 2010; De Vos et al., 2007; Williamson and Cleveland, 1999; Zhang et al., 1997; Kieran et al., 2005; Warita et al., 1999), mitochondrial function (Li et al., 2010; Jaiswal and Keller, 2009; Mattiazzi et al., 2002; Damiano et al., 2006; Nguyen et al., 2009; Bilsland et al., 2008), protein handling and ER stress (Saxena et al., 2009), all in brainstem and spinal motoneurons. Research on the pathophysiology of CSMNs is finally catching up. Recent studies now show that, presymptomatically, CSMNs are just as perturbed as spinal motoneurons, if not more so. Ozdinler and colleagues at Harvard and Northwestern demonstrate degeneration in the apical dendrite of CSMNs, decreased spine density, and decreased numbers of CSMNs from P60 (Jara et al., 2012; Ozdinler et al., 2011).

    Now, further inroads are reported by Mark Bellingham's laboratory at the University of Queensland. Fogarty and colleagues demonstrate an even earlier onset of pathology in layer V pyramidal neurons, a group of cortical neurons that includes CSMNs. The authors show apical dendrite degeneration starting from P28, and even earlier spine loss along the apical dendrite, from P21. Anatomical changes in the spines are likely driven synaptically, and Fogarty et al. give electrophysiological evidence of this: Spine loss is accompanied by an increased frequency of excitatory postsynaptic currents (EPSCs) in layer V pyramidal neurons (Fogarty et al., 2015). These results constitute the first evidence of a disturbance in the cortical circuitry in ALS starting at P21, an age at which the mice are just weaned, long before symptom onset.

    Another study out last month in Cerebral Cortex from the Zona laboratory (Saba et al., 2015) presented other clues to the overactive EPSCs. In addition to showing the same increased frequency of EPSCs in layer V pyramidal neurons in hSOD1G93A mice at P26-31, they showed EPSCs were mediated via non-NMDA type glutamate receptors, and even when action potential-mediated neurotransmission was blocked, an increased frequency in mini EPSCs remained. Through a combination of immunohistochemistry, western blotting, and quantitative PCR, they demonstrated enhanced vesicular glutamate transporter 2 (VGlut2) expression. Specifically, more glutamatergic contacts formed on layer V pyramidal neurons in layer II/III and V of primary motor cortex. Thalamocortical projections to layer II/III and V of the motor cortex are VGlut2+ and are a potential source of the EPSCs barraging layer V pyramidal neurons (Kuramoto et al., 2009).

    The evidence suggests that instead of cortical neurons initiating symptoms of ALS, they could just be on the receiving end of faulty synaptic drive. Rather than vulnerable neuronal populations explaining ALS, perhaps the problem is vulnerable circuits. It would be interesting to see future studies probe deeper into possible dysfunction in the cortical circuitry in ALS. Meanwhile, advances in the pathophysiology of brainstem and spinal motoneurons in ALS also lead to questions of whether the circuitry is perturbed there as well (van Zundert et al., 2008; Saxena et al., 2013; Bories et al., 2007; Vinsant et al., 2013; Vinsant et al., 2013; Casas et al., 2013; Martin et al., 2007; Chang and Martin, 2009; Wootz et al., 2013).

    Interestingly, even though the hSOD1G93A mouse faithfully recapitulates numerous features of ALS, including CSMN degeneration, muscle weakness, muscle atrophy, spinal motor neuron loss and progressive paralysis (Gurney, 1994), a valid criticism of this model is that rodents generally lack direct synaptic inputs from CSMNs to SMNs (Alstermark et al., 2004; Yang and Lemon, 2003). However, with concomitant degeneration now demonstrated in both CSMNs and SMNs in the mouse model, this may indicate that these specific synaptic connections are less important in conferring dysfunction than overall homeostasis of the motor control network. To support this, the sole FDA-approved drug treatment for ALS, riluzole, seems to exert its beneficial effects both by dampening cellular excitability and network excitability (Bellingham, 2011; Miller et al., 2012). Although the hSOD1G93A mouse is not a perfect model of ALS, it remains a useful tool, as these studies demonstrate. This model, along with more recently developed animal models of ALS using mutations in the UBQLN2 (Gorrie et al., 2014), TARDP (Wegorzewska et al., 2009; Wils et al., 2010; Xu et al., 2010) and FUS/TLS (Huang et al., 2011) genes, as well as induced pluripotent stem cells from ALS patients (Dimos et al., 2008; Devlin et al., 2015), provide us with the means to study the mechanisms of disease in ALS at all levels, from individual cells to neural circuits, in search of more accessible and effective targets for treatment.

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