In the developing embryo, motor neurons develop and nearly half preferentially die prior to birth (Henderson, et al., 1997, "Hepatocyte growth factor (HGF/SF) is a muscle-derived survival factor for a subpopulation of embryonic motoneurons"). As shown in Forger, et al., 2001 ("Blockade of Endogenous Neurotrophic Factors Prevents the Androgenic Rescue of Rat Spinal Motoneurons"), loss of muscular targets also leads to post-natal motor neuron degeneration. Post-natal mice engineered to have degenerated muscle spindles exhibit ataxia and resting tremors, indicating a decrease in proprioception due to loss of sensory-motor synapses (Frank, et al., 2002, "Muscle Spindle-Derived Neurotrophin 3 Regulates Synaptic Connectivity between Muscle Sensory and Motor Neurons").
One interesting factor seems to suggest a link with testosterone in preserving motor neurons, which could be a possible explanation for the statistically higher numbers of men affected in middle-age or above, and that of women in post-menopause, when hormone levels experience radical shift. Indeed, Cilliary Neurotrophic Factor, a potent motor neuron trophic factor, is regulated by gonadal hormones (Forger, et al., 1998, "Ciliary Neurotrophic Factor Receptor in Spinal Motoneurons is Regulated by Gonadal Hormones").
Leaving aside the question of hormone levels, there is much evidence that muscle-derived neurotrophic factors are necessary for the health and survival of the motor neurons. One in particular, Motoneuronotrophic Factor 1 (MNTF1), appears essential to this critical process. Experiments in Wobbler mice show that motor neuron disease increases as MNTF1 levels decrease (http://www.ncbi.nlm.nih.gov/pubmed/10453487). MNTF1 was first described in the early 90s, and the human form was successfully cloned as an artificial protein. Various fragments were extracted and shown to have neurotrophic effect.
Two overlapping domains of a 33 amino acid fragment of MNTF1, dubbed the WMLSAFS and FSRYAR domains, are sufficient to stimulate motor neuroprotection in a manner similar to the whole 33 amino acid MNTF1 fragment. The FSRYAR domain is sufficient to direct selective reinnervation of muscle targets by motor neurons in vivo in a manner similar to the 33 amino acid MNTF1 fragment. A recombinant protein containing the FSRYAR domain maintained motoneuron viability, increased neurite outgrowth, reduced motoneuron cell death/apoptosis and supported the growth and spreading of motoneurons into giant, active neurons with extended growth cone-containing axons.
For those curious about the amino acids in each domain, please refer to the image below:
Genervon has patented these fragments and is using them in a Phase 2-A clinical trial in ALS. From the above it is quite possible that at least some forms of ALS are caused by a sort of a muscular dystrophy (not to be confused with the distinct condition by that name). It therefore stands to reason that there is reason for hope that some will benefit. The standard caveat of basic and preclinical research often not translating to human trials obviously applies. However, we are entering an exciting time where extremely potent shots are being taken at more fundamental aspects of ALS. One or a combination seem likely to have the effect we have been waiting for.