Glutamate is the principal excitatory neurotransmitter in the nervous system. Inactivation of synaptic glutamate is handled by the glutamate transporter GLT1 (also known as EAAT2; refs 1, 2), the physiologically dominant astroglial protein. In spite of its critical importance in normal and abnormal synaptic activity, no practical pharmaceutical can positively modulate this protein. Animal studies show that the protein is important for normal excitatory synaptic transmission, while its dysfunction is implicated in acute and chronic neurological disorders, including amyotrophic lateral sclerosis (ALS), stroke, brain tumours and epilepsy. Using a blinded screen of 1,040 FDA-approved drugs and nutritionals, we discovered that many beta-lactam antibiotics are potent stimulators of GLT1 expression. Furthermore, this action appears to be mediated through increased transcription of the GLT1 gene. beta-Lactams and various semi-synthetic derivatives are potent antibiotics that act to inhibit bacterial synthetic pathways. When delivered to animals, the beta-lactam ceftriaxone increased both brain expression of GLT1 and its biochemical and functional activity. Glutamate transporters are important in preventing glutamate neurotoxicity. Ceftriaxone was neuroprotective in vitro when used in models of ischaemic injury and motor neuron degeneration, both based in part on glutamate toxicity. When used in an animal model of the fatal disease ALS, the drug delayed loss of neurons and muscle strength, and increased mouse survival. Thus these studies provide a class of potential neurotherapeutics that act to modulate the expression of glutamate neurotransmitter transporters via gene activation.

Although Charcot described amyotrophic lateral sclerosis (ALS) more than 130 years ago, the mechanism underlying the characteristic selective degeneration and death of motor neurons in this common adult motor neuron disease has remained a mystery. There is no effective remedy for this progressive, fatal disorder. Modern genetics has now identified mutations in one gene [Cu/Zn superoxide dismutase (SOD1)] as a primary cause and implicated others [encoding neurofilaments, cytoplasmic dynein and its processivity factor dynactin, and vascular endothelial growth factor (VEGF)] as contributors to, or causes of, motor neuron diseases. These insights have enabled development of model systems to test hypotheses of disease mechanism and potential therapies. Along with errors in the handling of synaptic glutamate and the potential excitotoxic response this provokes, these model systems highlight the involvement of nonneuronal cells in disease progression and provide new therapeutic strategies.

High levels of familial Amyotrophic Lateral Sclerosis (ALS)-linked SOD1 mutants G93A and G37R were previously shown to mediate disease in mice through an acquired toxic property. We report here that even low levels of another mutant, G85R, cause motor neuron disease characterized by an extremely rapid clinical progression, without changes in SOD1 activity. Initial indicators of disease are astrocytic inclusions that stain intensely with SOD1 antibodies and ubiquitin and SOD1-containing aggregates in motor neurons, features common with some cases of SOD1 mutant-mediated ALS. Astrocytic inclusions escalate markedly as disease progresses, concomitant with a decrease in the glial glutamate transporter (GLT-1). Thus, the G85R SOD1 mutant mediates direct damage to astrocytes, which may promote the nearly synchronous degeneration of motor neurons.

Analysis of transgenic mice expressing familial amyotrophic lateral sclerosis (ALS)-linked mutations in the enzyme superoxide dismutase (SOD1) have shown that motor neuron death arises from a mutant-mediated toxic property or properties. In testing the disease mechanism, both elimination and elevation of wild-type SOD1 were found to have no effect on mutant-mediated disease, which demonstrates that the use of SOD mimetics is unlikely to be an effective therapy and raises the question of whether toxicity arises from superoxide-mediated oxidative stress. Aggregates containing SOD1 were common to disease caused by different mutants, implying that coaggregation of an unidentified essential component or components or aberrant catalysis by misfolded mutants underlies a portion of mutant-mediated toxicity.

Amyotrophic lateral sclerosis (ALS) is increasingly recognized to be a multisystem disorder which includes both clinical and neuropathological features of a frontotemporal lobar degeneration (FTLD). In order to provide a common framework within which to discuss the characteristics of the cognitive and behavioural syndromes of ALS, and with which to conduct clinical and neuropathological research, an international research workshop on frontotemporal dementia (FTD) and ALS was held in London, Canada in June 2007. The recommendations arising from this research workshop address the requirement for a concise clinical diagnosis of the underlying motor neuron disease (Axis I), defining the cognitive and behavioural dysfunction (Axis II), describing additional non-motor manifestations (Axis III) and identifying the presence of disease modifiers (Axis IV).

To identify novel genes associated with ALS, we undertook two lines of investigation. We carried out a genome-wide association study comparing 20,806 ALS cases and 59,804 controls. Independently, we performed a rare variant burden analysis comparing 1,138 index familial ALS cases and 19,494 controls. Through both approaches, we identified kinesin family member 5A (KIF5A) as a novel gene associated with ALS. Interestingly, mutations predominantly in the N-terminal motor domain of KIF5A are causative for two neurodegenerative diseases: hereditary spastic paraplegia (SPG10) and Charcot-Marie-Tooth type 2 (CMT2). In contrast, ALS-associated mutations are primarily located at the C-terminal cargo-binding tail domain and patients harboring loss-of-function mutations displayed an extended survival relative to typical ALS cases. Taken together, these results broaden the phenotype spectrum resulting from mutations in KIF5A and strengthen the role of cytoskeletal defects in the pathogenesis of ALS.

Directing the differentiation of induced pluripotent stem cells into motor neurons has allowed investigators to develop novel models of ALS. However, techniques vary between laboratories and the cells do not appear to mature into fully functional adult motor neurons. Here we discuss common developmental principles of both lower and upper motor neuron development that have led to specific derivation techniques. We then suggest how these motor neurons may be matured further either through direct expression or administration of specific factors or co-culture approaches with other tissues. Ultimately, through a greater understanding of motor neuron biology, it will be possible to establish more reliable models of ALS. These in turn will have a greater chance of validating new drugs that may be effective for the disease.

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