|B. Amyotrophic Lateral Sclerosis||Alzheimer|
Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disorder that affects motor neurons in the motor cortex, brainstem, and spinal cord. These motor neurons control voluntary muscles so that patients have difficulty in walking, speaking or other forms of voluntary movement. Pathologically, ALS is characterized by the inclusion bodies comprising cleaved or abnormally phosphorylated TDP-43 - a protein involved in the biogenesis of microRNA (Figure B-1).
TDP-43 Pathology in Motor Neurons
Compelling evidence supports the hypothesis that motor neuron degeneration results from glutamate-induced excitotoxicity (van Zundert et al., 2012). There are three classes of ionotropic glutamate receptors (which form ion channels): NMDA, AMPA and Kainate. Among them, the NMDA receptor has high Ca2+ permeability. The AMPA receptor is composed of four types of subunits, designated as GluR1 - GluR4. GluR2 is critical in determining whether or not the AMPA receptor is permeable to Ca2+. Since Ca2+ overload is toxic to neurons, both AMPA and NMDA receptors are implicated in ALS (Kawahara et al., 2004; Spalloni et al., 2013). Moreover, decreased uptake of extrasynaptic glutamate by reduced EAAT2/GLT1 transporter activity may also contribute to motor neuron degeneration (Lin et al., 1998).
In many types of cells, the toxicity of calcium overload can be alleviated by Ca2+ buffering proteins which are a special class of Ca2+ binding proteins that bind transiently with Ca2+ ions, preventing further Ca2+ signaling (Schwaller, 2010). In motor neurons, the expression level of Ca2+ buffering proteins (e.g., calbindin and parvalbumin) is low (Jaiswal, 2014). Hence, a significant portion of excess Ca2+ ions may enter mitochondria, resulting in oxidative stress (Peng and Jou, 2010). Furthermore, large amount of Ca2+ ions may activate calpain to cleave TDP-43 (Yamashita et al., 2012), or promote TDP-43 phosphorylation by stimulating protein phosphatase 2B (PP2B or calcineurin) to dephosphorylate and activate casein kinase-1 (CK1) (Liu et al., 2002; Nakano et al., 2010). The activated CK1 can directly phosphorylate TDP-43 (Hasegawa et al., 2008).
Neurofilaments are a major component of the neuronal cytoskeleton, located primarily in the axon. Their dysregulation, which impairs axonal transport, has been shown to play an important role in the pathogenesis of ALS (Collard et al., 1995). In patients with ALS, the level of neurofilaments in the cerebrospinal fluid (CSF) is significantly elevated (Xu et al., 2016). TDP-43 promotes the biogenesis of several microRNAs, including miR-9 (Zhang et al., 2013) which targets neurofilaments. A neurofilament is composed of polypeptide chains (subunits), which are classified into light, medium or heavy chains, on the basis of their molecular weight. The mRNA of the neurofilament light chain contains a single miR-9 binding site, but the heavy chain mRNA includes nine miR-9 binding sites (Haramati et al., 2010). It has been shown that TDP-43 mutations result in miR-9 down-regulation (Zhang et al., 2013), which may account for neurofilament dysregulation in TDP-43 pathology. The pathogenic cascade in motor neurons is summarized in Figure B-2.
Hyperexcitability in Beta Neurons
In most cases, TDP-43 pathology occurs earlier in upper motor neurons (in motor cortex) than lower motor neurons (in brainstem and spinal cord), suggesting that ALS may begin in the motor cortex, and then spread to brainstem and spinal cord (Braak et al., 2013; Braak et al., 2017). Furthermore, the Ca2+ overload in the upper motor neurons, known as "Betz cells", may not originate from over-expression of their glutamate receptors, but from hyperexcitability of the neurons that release excess glutamate to over-stimulate Betz cells (Sasaki and Maruyama, 1994; Pieri et al., 2009; van Zundert et al., 2012; Geevasinga et al., 2016). These glutamatergic neurons will be referred to as "beta neurons" because they are likely to oscillate at the beta band (15-30 Hz) which is known to control motor timing. Experiments have demonstrated the link between altered beta rhythms and motor neuron degeneration in ALS (Proudfoot et al., 2017).
The neurons in entorhinal cortex (EC), hippocampus and amygdala that display Tau pathology (neurofibrillary tangles) oscillate at the theta band (Bienvenu et al., 2012). As discussed in Chapter 12, their hyperexcitability may originate from BDNF deficiency, resulting in miR-132 down-regulation and consequently Tau overproduction. Incidentally, ALS is also associated with BDNF deficiency (He et al., 2013) and miR-132 down-regulation (Freischmidt et al., 2013). Moreover, the ultimate risk factor (mTOR) is also implicated in ALS:
These observations indicate that ALS and Alzheimer's disease may share the same pathogenic cascade from BDNF deficiency to hyperexcitability (Chapter 12). They differentiate at the neuronal types that exhibit hyperexcitability: hyperexcitable theta neurons lead to Alzheimer's disease while hyperexcitable beta neurons in the motor cortex cause ALS. Both theta and beta neurons are likely to display Tau pathology because their axon initial segment (AIS) may contain high level of voltage-gated calcium channels (e.g., T-type) to regulate microtubule dynamics for long range synchronization. The Ca2+ overload via AIS is prone to induce Tau pathology as Tau proteins are normally present in the axon. By contrast, the Ca2+ overload via dendritic spines is less vulnerable to Tau pathology since normal Tau proteins are absent in dendrites. Motor neurons do not participate in long range synchronization. Their Ca2+ overload occurs mainly through glutamate receptors at the spines. This may explain why the brainstem and spinal cord rarely display Tau pathology compared to many other regions that are likely involved in long range synchronization (Mimuro et al., 2007; Yang and Strong, 2012). Interestingly, astrocytes are also susceptible to Tau pathology (Yang and Strong, 2012). This could be due to over-stimulation of their Ca2+-permeable NMDA receptors (Orellana and Stehberg, 2014) and the presence of Tau proteins (Shin et al., 1991).
The C9ORF72 Pathology
The most common cause of familial ALS and frontotemporal dementia (FTD) arises from mutations in the C9ORF72 gene. In a normal person, the gene contains less than 15 GGGGCC repeats, but in ALS/FTD patients, the number of GGGGCC repeats may expand to over 200 (Renton et al., 2011). The C9ORF72 pathology is characterized by the "dipeptide repeat inclusion" which is composed of dipeptide repeat proteins (translated from GGGGCC repeats), ubiquilin and p62 (ubiquilin-binding protein), but rarely TDP-43 (Al-Sarraj et al., 2011).
The biological function of C9ORF72 is not clear. However, it is unlikely to play a role in motor functions, as knock-out of the C9ORF72 gene in mice does not cause motor neuron degeneration or motor deficits (Koppers et al., 2015). On the other hand, expression of the GGGGCC expanded C9ORF72 in mice recapitulates disease features such as dipeptide repeat inclusions, TDP-43 pathology, and behavioral abnormalities (Chew et al., 2015). These results demonstrate that the toxicity of C9ORF72 mutation is not due to the loss of its normal function, but arises from the gain of abnormal processes.
In many cases, Drosha proteins are also recruited to the dipeptide repeat inclusions (Porta et al., 2015). Since Drosha is critical for microRNA biogenesis, the mutant C9ORF72 may cause ALS/FTD by binding to Drosha, thereby disrupting the biogenesis of essential microRNAs. Indeed, the C9ORF72 mutation has been shown to result in miR-132 down-regulation (Freischmidt et al., 2013) and hyperexcitability (Geevasinga et al., 2015; Schanz et al., 2016).
The Guam-type ALS
The Guam-type ALS, also known as ALS/parkinsonism-dementia complex (ALS/PDC), exhibits Tau pathology, TDP-43 pathology and the α-synuclein pathology that characterizes Parkinson's disease (Miklossy et al., 2008). Neurofibrillary tangles are abundant not only in the primary motor cortex (Hof and Perl, 2002), but also in the entorhinal, frontal and temporal cortex (Hof et al., 1991; Hof et al., 1994). ALS/PDC was originally discovered on the island of Guam in the Pacific (Arnold et al., 1953). It is caused by a non-protein amino acid, β-N-methylamino-L-alanine (BMAA), which can be misincorporated into normal proteins, resulting in protein misfolding (Dunlop et al., 2013). Mounting evidence suggests that protein misfolding may promote mTOR activation, and consequently hyperexcitability (see Epilepsy and mTOR). In line with this notion, a single large dose of BMAA is sufficient to cause seizures in newborn mice (Ross and Spencer, 1987).
Figure B-3 illustrates the BMAA-induced pathogenic cascade. This figure does not imply that enhanced mTOR activity will definitely lead to all of these pathologies. Rather, mTOR activity simply affects the probability of developing these pathologies. Many other factors are also involved in neurodegeneration, such as the expression level of miR-132, TDP-43, α-synuclein, calpain, GSK-3, voltage-gated calcium channels, Ca2+-permeable glutamate receptors, etc. The BMAA-enhanced mTOR activity could be overwhelming. In comparison, the mTOR activity enhanced by SOD1 mutations is mild. It does not lead to TDP-43 pathology (Mackenzie et al., 2007) or miR-132 down-regulation (Freischmidt et al., 2013).
In Appendix A, we have seen that hyperexcitability caused by huntingtin mutation in Huntington's disease may lead to symptoms akin to other neurodegenerative disorders. Here the hyperexcitability caused by BMAA may result in all three neurodegenerative pathologies: Tau, TDP-43 and α-synuclein. This demonstrates the common origin of neurodegeneration.
Author: Frank Lee