Site-Specific Onset and Spread of Neurodegeneration in ALS
The pattern of neurodegeneration in ALS/FTD is not random; it targets specific large-scale distributed networks in the brain and spinal cord. Motor manifestations begin in one region of the body in ∼98% of patients (Ravits et al., 2007) accompanied by unilateral, focal damage to MNs in the motor cortex or spinal cord, that innervate the corresponding peripheral body regions. It has been previously suggested that ALS targets specific evolutionarily linked, interdependent functions, and as the disease progresses these deficits combine into failure of specific networks (Eisen et al., 2014). More recently, several clinical studies have revealed that neurodegeneration and TDP-43 pathology spread to continuous anatomical regions during disease course (Ravits et al., 2007; Brettschneider et al., 2013; Walhout et al., 2018), and symptoms arise in the contralateral regions following a unilateral limb onset (Walhout et al., 2018). This also implies that neuronal circuitry might drive disease progression to specific MN populations in ALS/FTD. The spread of misfolded proteins from cell-to-cell, particularly TDP-43, provides a molecular explanation for the specific network and anatomical vulnerability observed in ALS. However, it must be noted that whilst contiguous spread is observed for most patients, this is not the case for all (Ravits and La Spada, 2009).
Increasing evidence suggests that ALS begins in the cortical regions of the brain, which is referred to as the “dying-forward hypothesis.” Features of cortical hyperexcitability – heralded by reduction in short interval intracortical inhibition – have been detected during the early phases of ALS in transcranial magnetic stimulation studies (Thomsen et al., 2014; Menon et al., 2015). This can precede the clinical onset of bulbar/spinal motor dysfunction by ∼3–6 months (Vucic et al., 2008; Bakulin et al., 2016). The dying forward hypothesis is consistent with Charcot, who first postulated that ALS begins in the cortex (Charcot, 1874). Clinical observations that MNs without monosynaptic connections to cortical MNs, such as the oculomotor, abducens, and Onuf’s nuclei, are spared in ALS, and that pure LMN forms of ALS are rare, also support this hypothesis. Further evidence is provided by the observation that MNs receiving direct, monosynaptic cortical input also predominantly develop TDP-43 pathology, while subcortical MNs do not (Eisen et al., 2017). Similarly, TDP-43 pathology develops in patients only in structures under the control of corticofugal projections (Brettschneider et al., 2013; Menon et al., 2015; Eisen et al., 2017)
TDP-43 pathology may then propagate through corticofugal axons to the spinal cord and regions of the brain (Braak et al., 2013; Eisen et al., 2017) in a time-dependant and region-specific manner (Brettschneider et al., 2013), consistent with the dying forward hypothesis (Figure 3). This sequential pattern of TDP-43 dissemination is consistent with the hypothesis that TDP-43 pathology is propagated synaptically from cell to cell (Brundin et al., 2010; Maniecka and Polymenidou, 2015), in a similar way to the pathogenic prion protein, a concept known as the “prion-like mechanism” (Lee and Kim, 2015; Ayers and Cashman, 2018). In this model, misfolded proteins act as template seeds to trigger aggregation of their natively folded counterparts. This results in the propagation of protein misfolding, leading to its orderly spread through the CNS (Soto, 2012; Maniecka and Polymenidou, 2015). However, the question of where disease begins remains controversial because many researchers still favor the “dying-back” hypothesis, in which ALS begins within the muscle cells or at the NMJ. This hypothesis proposes that there is a spread of pathology from LMNs to UMNs (Chou and Norris, 1993; Fischer et al., 2004; Pun et al., 2006; Turner et al., 2018), or else, a simultaneous involvement of both UMNS and LMNs (Turner et al., 2018). Whilst most of the evidence for the dying-back mechanism comes from animal models, studies of muscle biopsies from early stage ALS patients and long-term survivors have demonstrated significant morphological abnormalities and major denervation/re-innervation at the NMJ, implying that this region is targeted early in disease (Millecamps et al., 2010; reviewed in Arbour et al., 2017).
FIGURE 3  Schematic diagram representing the typical spread of neurodegeneration following an initial onset in motor neurons in ALS patients (n = 76 patients) (Brettschneider et al., 2013). Shading represents TDP-43 pathology. There is evidence to support the prion-like model in ALS. The spread of neurodegeneration through adjacent anatomical regions of the CNS resembles the orderly spread of protein misfolding in prion disease. The in vitro cell-to-cell transmission of misfolded SOD1, TDP-43 and C9orf72 di-peptide repeat proteins has been demonstrated (Grad et al., 2011, 2014; Münch et al., 2011; Nonaka et al., 2013; Feiler et al., 2015; Porta et al., 2018). Similarly, the addition of cerebrospinal fluid from ALS/FTD patients (Ding et al., 2015), detergent-insoluble fractions of ALS-disease brains (Nonaka et al., 2013) or insoluble phosphorylated TDP-43 from post-mortem brain and spinal cord tissue (Smethurst et al., 2016), results in misfolding of TDP-43 when added to human cell lines. However, so far, only misfolded SOD1 and TDP-43 transmissibility has been demonstrated in vivo (Ayers et al., 2014, 2016; Porta et al., 2018). A recent study demonstrated that injection of brain-derived extracts from FTD patients into mice promoted the spatio-temporal transmission of TDP-43 pathology via the neuroanatomical connectome, suggesting that TDP-43 travels via axonal transport through connected regions of the CNS (Porta et al., 2018). Similarly, axonal transport is implicated in the spread of mutant SOD1 in mice (Ayers et al., 2016). Overexpression of misfolded TDP-43 or SOD1 facilitated the seeding ability of each inoculum, consistent with results obtained in vitro (Nonaka et al., 2013; Feiler et al., 2015; Smethurst et al., 2016).
Whilst these animal studies demonstrate that ALS spreads within MNs that are connected synaptically, a small portion of patients do not display this contiguous spreading of pathology, however. This implies the existence of alternative mechanisms of disease progression (Fujimura-Kiyono et al., 2011; Gargiulo-Monachelli et al., 2012), such as the transfer of misfolded proteins in nanotubules or exosomes (Nonaka et al., 2013; Sundaramoorthy et al., 2013; Grad et al., 2014; Ding et al., 2015; Feiler et al., 2015; Westergard et al., 2016). Interestingly, it has been suggested that the vulnerability of specific MN populations is associated with the spread of neurodegeneration in ALS (Fu et al., 2018).