Motor Neurons Selectively Degenerate in ALS Patients
Lesions to motor structures in humans and experimental animals lead to impairments in normal movement. In ALS, as MNs degenerate, the ability to control movement of the muscles is progressively lost. Specific MNs in the brain, brainstem and spinal cord are selectively targeted, and pathology appears first in these restricted MN populations. In fact, the name “Amyotrophic Lateral Sclerosis” reflects the strikingly selective degeneration of MNs in ALS. It is derived from a combination of three words; “Lateral” refers to the lateral spinal cord, given that corticospinal MNs are particularly vulnerable to degeneration; “Amyotrophic” is from the Greek “amyotrophia,” meaning lacking muscle nourishment; and “Sclerosis” (fibrosis) refers to gliosis of the crossed corticospinal tract in the dorsolateral quadrant of the spinal cord (Charcot, 1874; Frey et al., 2000; Pun et al., 2006). In the brain, UMNs in the primary cortex are also amongst the first to degenerate in ALS, and similarly, in the brainstem, the hypoglossal MNs that innervate the muscles of the tongue involved in swallowing and breathing, are also targeted early in disease course. In the brainstem, ALS can also affect trigeminal MNs, the facial MNs and ambiguous MNs. However, other MN subgroups within this region are relatively resistant to degeneration, including MNs of the oculomotor (III), trochlear (IV) and abducens (VI) nuclei, innervating the extraocular muscles (Mannen et al., 1977; Schrøder and Reske-Nielsen, 1984). Hence, eye movements remain relatively preserved throughout disease course (Kanning et al., 2010) and as a consequence, eye tracking devices are often used to aid communication in the later stages of ALS (Caligari et al., 2013). Whilst it has been reported that oculomotor neurons may be affected at disease end stage, this was recently attributed to dysfunction of the dorsolateral prefrontal cortex, the frontal eye field and the supplementary eye field, confirming the relative resistance of pure oculomotor functions in ALS (Shaunak et al., 1995; Proudfoot et al., 2015). Widespread loss of GABAergic interneurons has also been described in ALS, in both the cortex (Stephens et al., 2001; Maekawa et al., 2004) and the spinal cord (Stephens et al., 2006; Hossaini et al., 2011).
MRI studies of ALS patients has revealed that very specific neuronal networks are vulnerable to degeneration in ALS (Bede et al., 2016). However, whilst TDP-43 pathology is the signature pathological hallmark of almost all ALS cases, it can arise in areas of the CNS that are not particularly vulnerable to degeneration (Geser et al., 2008). Significant TDP-43 pathology is present in the substantia nigra and basal ganglia, which are not affected in ALS, as well as in the motor gyrus, midbrain and spinal cord. Curiously, pathological forms of TDP-43 are also detectable in the occipital lobe, amygdala, orbital gyrus and hippocampus (Geser et al., 2008). Hence, whilst major degeneration of corticobulbar, LMN, pyramidal and frontotemporal networks underlie the widespread clinical symptoms of ALS, it remains unclear how other circuits, such as the visual, sensory, autonomic and auditory systems, remain relatively protected in ALS. These unaffected networks, however, have not been well studied in ALS patients.