Amyotrophic lateral sclerosis (ALS) also called as Lou Gehrig's disease or Maladie de Charcot, is a chronic, progressive and invariably fatal motor neuron disease that damages both the upper as well as lower motor neurons. (Here: A-myo-trophic=No-muscle-nourishment, Lateral refers to the region of the spinal cord where the nerve cells controlling the skeletal muscles are located and sclerosis, due to the consequent scarring and hardening as the damage progresses). In ALS, due to enervation the muscles gradually weaken, waste away and develop fasciculation, eventually leading to loss of control over voluntary movements. Patients become paralysed and often require ventilation.
Epidemiology
Prevalence rate is about 4 per 100,000 and incidence rate is 1 per 100,000. The male to female ratio is about 2:1. Although it is seen in all age groups, it is more common in the middle age (55-75 years). The disease has no racial, socioeconomic, or ethnic boundaries.
Types and variants of ALS
Familial ALS is autosomal less often recessive while sporadic ALS is not associated with family history. Classical ALS, a distinct syndrome characterised by a combination of UMN and LMN problems. Progressive bulbar palsy symptoms begin in muscles innervated by the lower brainstem that control articulation, chewing and swallowing. Usually it progresses to ALS. In benign ALS very slow progression of ALS symptoms occurs. Parkinson's like illness, it is postulated to be caused by a toxin from cycad nut. Primary lateral sclerosis is progressive weakness in voluntary muscles.
Multifocal motor neuropathy is characterised by a slowly progressive, asymmetric weakness of the limbs without sensory loss. Cervical spondylitis produces ALS like symptoms, but this one can be ruled out using specific medical testing. Spastic paraparesis, progressive muscular atrophy, spinal bulbar muscular atrophy, many of these in reality are the same disease form, just representing differing progressive stages.
Symptoms and diagnosis
The symptoms reflect both upper motor neurons damage which includes stiff muscles (spasticity), exaggerated reflexes, as well as lower motor neurons damage which includes muscle weakness and atrophy, muscle cramps, and fleeting twitches of muscles under the skin.
Early symptoms include twitching, cramping, or stiffness of muscles, slurred and nasal speech, difficulty chewing or swallowing, muscle weakness affecting an arm or a leg. These further spread onto other regions (including respiratory muscles) resulting in a generalised muscular weakness or atrophy. There are no definitive tests for the diagnosis of ALS except an autopsy. It is generally done by eliminating the possibilities of other neuromuscular degenerative diseases and although this is inadvisably risky. Autopsy investigations reveal that the neuropathological findings of long duration ALS are same as that in early illness.
Tests include knee-jerk reaction which is abnormally quick, Nerve conduction velocity (NCV), Electromyography (EMG), MRI, analysis of CSF and blood tests. NCV detects, amplifies and displays muscle response. In ALS there is a marked decrease in compound muscle action potential and the speed of the nerves. EMG measures nerve impulses within muscles and in ALS the responses are abnormal.
ALS is a multifactorial disease with several aspects contributing to its pathogenesis. Mutation in SOD1 gene: Responsible for 1/5th of the cases. Free radicals are highly reactive molecules that damage the structural proteins and DNA within the cell and hence have to neutralise. The enzyme Cu-Zn superoxide dismutase (SOD1) occurs abundantly and is a powerful antioxidant that protects the body from the damage of free radicals. A mutation of the gene that codes for this enzyme (on chromosome 21) or malfunction of this enzyme is postulated to cause ALS due accumulation of free radicals.
Higher levels of glutamate
Glutamate is one of the neurotransmitters in the brain. An exposure to this over long periods can result in excitotoxicity and so neurons begin to die off. ALS patients are found to have higher levels of glutamate in serum and spinal fluid as compared to healthy people. The mechanism behind the build up of glutamate and how it contributes to development of ALS is still being studied. Evidences indicate that alterations in AMPA receptor subunits (Glu receptor 2) are involved in the motor neuron loss of ALS patients and transgenic mice; also it accelerates the disease progression in SOD1 mutant mice.
Other factors
" Mitochondrial alteration may induce cell-death pathways in motor neurons through the activation of a caspase-mediated cascade. Mitochondrial alteration invariably leads to the generation of reactive oxygen species (ROS) which are markers of oxidative stress, found in-vitro and in-vivo models SOD1 linked ALS and ALS patients.
" Mutant SOD1 proteins can form aberrant aggregates in the mitochondria, causing damage through interactions with specific proteins such as the anti-apoptotic protein Bcl-2 and Cytochrome C. This mechanism occurs in other neurodegenerative disorders that can be prevented by activation of heat shock proteins (HSPs).
" Autoimmune responses that occur when the body's immune system attacks normal cells have been suggested as possible causes for motor neuron degeneration in ALS.
" There may be a possible role of certain environmental factors (exposure to toxic or infectious agents) or a dietary deficiency or trauma. Lack of neurotropic support might be a key factor in pathogenesis of ALS.
" Oxidative stress enhanced by tumour necrosis factor (TNF-?) along with nitric oxide and ROS secreted by reactive glial cells, propagates motor neuron damage in ALS.
" Defects in axonal transport are also thought to have an important role in neurodegeneration.
Strategies in treatment of ALS
In preclinical tests, treatments for SOD1 transgenic animals are judged in terms of their ability to prolong survival, to maintain and/or improve motor performance and to protect motor neurons. As such there is no cure for ALS and treatment focuses on relieving symptoms and maintaining optimal quality of life.
Anti-glutamate strategies
Riluzole: It limits synaptic glutamate release and thus is relatively effective at increasing the lifespan of both mouse models and patients. Riluzole does not reverse the damage already done to motor neurons. Side effects include dizziness, elevated liver enzymes, reduced leukocytes in the blood (granulocytopenia) and weakness (asthenia).
Ceftriaxone: It apparently activates EAAT2 expression and may be neuroprotective for motor neurons.
Cannabinoid: It has anti-glutamate properties. In ALS mice, it delayed motor impairment and prolonged survival, and has been proposed as a potential therapeutic agent for ALS.
AMPA antagonists: These significantly delay motor function impairment and death in SOD1 mutant mice.
Anti-oxidant strategies
Creatine: It improves energy metabolism in neurons and muscle and also improves motor function in transgenic SOD1 animal models of ALS.
Edaravone: It a potent free radical scavenger which decreases oxidative stress and reduces the damage of DNA by free radicals.
Manganese porphyrin (AEOL 10150): It is a potent antioxidant which markedly prolonged survival in some ALS transgenic mouse models
Co-enzyme Q10: It is an anti-oxidant which improves mitochondrial membrane and cellular energy production.
Neurotrophic factor strategies (IGF-1)
Xaliproden: It is a novel small peptide with both neurotropic and neuroprotectant properties and good CNS penetration.
Anti-inflammatory strategies
Celecoxib: One of the COX-2 inhibitors which treats inflammation. Proinflammatory cytokine levels are increased and microglias are activated in ALS patients and ALS mice.
Anti-apoptotic strategies
Evidence suggests that apoptotic pathways (caspase cascades) are activated in ALS.
Minocycline: Minocycline which inhibits microglia activation and the apoptotic cascade has shown moderate benefits in ALS mice.
TCH346: It binds glyceraldehyde-3-phosphate dehydrogenase and prevents p53-related neuronal apoptosis. TCH346 treatment in an animal model slowed disease onset.
Methyl cobalamin: It is a vitamin B12 derivative, but recently it has been found to have anti-apoptotic properties.
Other neuroprotection strategies
Treatment with insulin-like growth factor (IGF)-1 or, to a lesser extent, glial-cell-line-derived neurotrophic factor (GDNF) helps to preserve the morphology of motoneurons, decrease gliosis and increase the lifespan of SOD1 transgenic mice. Vascular endothelial growth factor (VEGF) also has a neuroprotective effect on motoneurons and increases the lifespan of SOD1 mutant mice and rats; furthermore, VEGF has been shown to be a modifier of human (and mouse) ALS.
ONO-2506: It is a glial cell modulating factor and appears to have neuroprotective effects via its action on glial cells.
Pentoxyfilline: It increases cellular cyclic AMP and GMP, which are considered to function as neuroprotective agents in degenerating neurons.
Tamoxifen: It is a well-known anti-breast cancer agent and a protein kinase C inhibitor. In a patient with breast cancer and ALS who was treated with tamoxifen experienced marked slowing in progression of the ALS. Abnormally activated protein kinase C and abnormally phosphorylated proteins have been found in degenerating motor neurons.
Arimoclomol: It upregulates HSPs and was recently reported to have beneficial effects in ALS transgenic mice by preventing formation of protein aggregates.
Hyperbaric oxygen: The theory behind hyperbaric oxygen treatment in ALS is based on mitochondrial abnormalities found in ALS motor neurons. Patients received hyperbaric 100% oxygen at 2 atmospheres for 60 minutes daily for 5 days a week for 4 weeks. Isometric muscle strength measurements revealed that strength markedly improved (up to 97%) in almost all muscles tested.
Neurovaccination strategies
Vaccination regulates or suppresses inflammatory and non-inflammatory processes that damage tissues. Vaccination strategies may be used to induce nonpathogenic T-cell responses, including activation of anti-inflammatory TH2 cells.
Glatiramer acetate: It is a mixture of synthetic polypeptides composed of four amino acids, L-alanine, L-glutamate, L-lysine and L-tyrosine in a molar ratio of 0.43: 0.14: 0.33: 0.1 and provides significant neuroprotection in certain ALS transgenic mouse models.
Gene therapy
Deletion of the survival motor neuron 1 (SMN1) gene has been found in spinal muscular atrophy (SMA) and conversion from SMN1 to SMN2 is associated with a milder form of SMA. 4-phenylbutyrate (PBA), a histone deacetylase inhibitor, increases gene activation.
A study of adeno-associated virus (AAV) transfer of the IGF-I gene in ALS mice gave exciting results. When the genetically modified AAV was infected in the skeletal muscles of ALS mice after symptom onset, it resulted in remarkably prolonged survival and maintenance of motor function.
Symptomatic treatment
Dextromethorphan: It is an NMDA receptor antagonist and a sigma 1 receptor agonist (NMDA receptor modulator), was used to treat patients with ALS. Pseudobulbar symptoms (uncontrolled laughter or crying) were improved.
New devices and technologies
High-frequency chest wall oscillation is an airway-clearing method. The pulmonary assistive device (the Vest system) used for such airway clearing has been the standard of care for children with cystic fibrosis and lung donors.
Physical therapy
Passive stretching helps to avoid permanent contraction of muscles (contractures) that may cause joint problems. ALS patients require a diet of high-energy foods that are easy to swallow.
Need for multi drug therapy: ALS is not only a multifactorial disease but also a multisystemic disease that affects several cell types. Use of a multi-drug regimen could enable an effective treatment effect by addressing multiple disease mechanisms simultaneously.
Approach in multi drug therapy: Apart from considerations of possible negative interactions between drugs an approach is made towards simultaneous treatment. For instance, molecules that rectify glutamate-mediated transmission, combined with anti-inflammatory drugs and muscle-derived trophic factors that are delivered using viral vectors. Animal studies have shown that combination therapies often have synergistic effects, and such a treatment would intercept damage signals from the three cell types that are involved in ALS. In a mouse model of ALS (SOD1G37R mice) a combination approach was tested by administering in diet from late presymptomatic stage (8-9 months), consisting of three drugs for distinct targets in the complex pathway to neuronal death: Minocycline, an antimicrobial agent that inhibits microglial activation, Riluzole, a glutamate antagonist, and Nimodipine, a voltage-gated calcium channel blocker. This proved efficacious as it delayed the onset of disease, slowed the loss of muscle strength, and increased the average longevity of mSOD1 mice by 6 weeks. Another example would be of minocycline producing an additional beneficial effect in mSOD1 mice when administered with creatine (which improves energy metabolism in neurons and muscle).
Non-pharmaceutical intervention can also be utilised. One approach is to silence the synthesis of specific crucial targets. For instance, silencing mSOD1 using small interfering RNA increases the lifespan of SOD1G93A transgenic mice markedly. Also treatment with IGF-1 or VEGF retrogradely transported in motor neurons through viral vectors that were injected into muscles produced best results in mice models even at overt disease symptoms. Combining these strategies with approaches aimed at intercepting mechanisms that are activated by cells surrounding motor neurons might be even more effective at halting the progression of ALS in mouse models and, also in ALS patients. Anti-glutamate agents are thought to affect the initiation but not the propagation of motor neuron degeneration. The converse is true for antioxidant agents. Albuterol may have a selective effect on neck flexor and respiratory muscles whereas creatine may benefit appendicular muscles. One approach might include riluzole, several antioxidants like co-enzyme Q10 or creatine.
To date multiple clinical trials have used only individual agents while combinations have been tried only using animal models.
Disadvantages of multi drug therapy (drug cocktail)
It is not wise to test a drug cocktail comprised of drugs with unproven activity in ALS. One reason is that, if such a cocktail proved efficacious, it would be impossible to determine which component or components provided the positive effect. However, all drugs have adverse effects, and combinations of agents will magnify the frequency of such effects. It is therefore important to simplify any regimen as much as possible to increase tolerability of the treatment. Second, drugs are expensive, and it is unjustifiable from a health policy perspective to promote a therapy in which some of the agents used are likely ineffective. Third, multiple agents can interact with each other to negate a potentially important benefit. Thus, combining agents with unclear interactions may lead to failure to detect a potential therapeutic agent. Some agents may have individual negative effects, thus cancelling out potential beneficial effects of other agents in the mixture.
Other approaches
As an alternative to pharmacological treatments, the recent developments in stem-cell research provide possibilities for neural implantation therapy in patients with ALS. Stem cells could help to replace dead motoneurons or could protect those that remain by releasing neurotrophic factors or by enhancing glutamate uptake, if the donor cells are integrated functionally into the recipient CNS circuitry. Clearly, the use of other stem-cell types as 'nurse cells' to support the function of motoneurons and/or alternative delivery protocols must be examined in preclinical contexts to exploit the potential benefits of this novel therapeutic approach in ALS. Other non-pharmaceutical approaches like moderate exercise are beneficial in transgenic mSOD1 mice. Moderate activity increases IGF-1 uptake in the brain and induces the anti-apoptotic protein Bcl-2. Finally, it is indicated that diet is also important as energetic metabolism is observed in ALS patients. Indeed, compensating the energetic imbalance with a highly energetic diet extends mean survival by 20% in an mSOD1 mouse model.
(The authors Anantha Nagappa Naik, Vidhan Chandra Roy, Supraja Madhavan are with Manipal College of Pharmacuetical Sciences, Manipal 576 104 and Ravikumar Vyas D is with Birla Institute of Technology and Science, Pilani 333 031)