Amyotrophic lateral sclerosis (ALS) is a relentless neurodegenerative disease caused by the selective destruction of motor neurons in the motor cortex, brainstem and spinal cord. The steady progressive loss of motor neurons throughout the neuraxis causes muscle atrophy, weakness and immobility. Premature death, typically within 5 years of diagnosis, is inevitable and is most often caused by paralysis of respiratory muscles with subsequent respiratory failure. Currently ALS is incurable; treatment consists of therapies aimed at symptomatic relief and Rilutek® – the only US FDA-approved disease-modifying drug for ALS – provides modest life-prolonging benefit. Development of relevant therapies has been challenging and elusive, complicated by the lack of understanding of the underlying inciting pathophysiology.
Amyotrophic lateral sclerosis is largely a sporadic disease with an unclear cause. However, approximately 5–10% of patients diagnosed with ALS have an inherited, familial ALS form of the disease, which shares nearly identical clinical and histopathologic hallmarks with sporadic ALS [76
]. While many disease-causing mutations have been identified, the most common are point mutations within the gene encoding for Cu/Zn superoxide dismutase (SOD)1. This clear genetic link has led to the development of transgenic rodent models carrying various mutant human SOD1
genes (i.e., point mutations with amino acid substitutions G37R, G85R and G93A), which cause clinical manifestations, mimicking both sporadic ALS and familial ALS. Since their development, better understanding of disease pathophysiology has ensued with mounting evidence to support the concept of a multifactorial disease process culminating in apoptosis of motor neurons. Mechanisms implicated in this process include: glutamate excitotoxicity, oxidative damage, cytoskeletal abnormalities, endoplasmic reticulum stress from abnormal cellular protein products, mitochondrial dysfunction, abnormal microglial and astrocyte function, and impaired neurotrophic support [77
Stem cell therapies hold significant promise for clinical benefit by offering both the possibility of cellular replacement, as well as targeted gene modification and neurotrophic factor delivery to interrupt these abnormal mechanisms. Based on their unique properties, MSCs are playing a key role in developing treatment strategies. Both murine MSCs and hMSCs have been delivered with varying techniques to transgenic SOD1 ALS rodent models to evaluate safety, effectiveness and disease-altering properties. Zhao and colleagues performed intravenous injection of hMSCs into presymptomatic irradiated G93A mice [82
]. They reported that the hMSCs survived over 20 weeks in the recipient mice, integrated into the parenchyma of both the brain and spinal cord. The transplanted mice had both delayed onset and slower disease progression with an increased lifespan when compared with the untreated mice [82
Several groups have demonstrated that intraparenchymal delivery of hMSCs is safe and can delay loss of motor neurons in rodents. Vercelli et al.
transplanted hMSCs directly into the lumbar spinal cords of transgenic SOD1 mice. The MSCs migrated throughout the spinal cord and delayed loss of motor neurons, prolonging motor performance [83
]. Another study compared the efficacy of transplanting olfactory ensheathing cells and rat MSCs intrathecally through the fourth ventricle in the spinal cord. Although the olfactory ensheathing cells distributed widely, no significant changes in clinical outcomes were observed until after MSC transplantation, when female ALS mice showed statistically longer disease duration than males and control mice [84
Quantitative pathological analysis has been carried out to examine the neuromuscular junctions, ventral root and spinal cord at multiple ages in the G93A mouse model. Fischer et al.
reported histopathologic abnormalities of the neuromuscular junctions as the first sign of disease onset [85
]. Approximately 60% of the ventral roots suffered damage, with decline of the neuromuscular junctions, prior to any development of abnormalities in the motor neuron cell body or neuroglia [85
]. GDNF is a promising factor in that it has a high affinity for motor neurons and can prevent their death. The protein is large and does not cross the blood–brain barrier, so it is difficult to directly administer to the brain. Svendsen's group demonstrated that human neural progenitors isolated from the cortex and modified to secrete GDNF survived up to 11 weeks in the lumbar spinal cord of rats overexpressing the G93A SOD1
mutation. Cellular integration into both gray and white matter was observed with secretion of GDNF within the region of cell survival. Fibers upregulated cholinergic markers in response to GDNF, indicating that it was physiologically and locally active [86
]. Central implantation of GDNF-secreting neural precursor cells by the same group improved maintenance of spinal motor neurons but failed to improve hindlimb function [87
]. Suzuki et al.
delivered intramuscular hMSCs that secreted GDNF [88
]. The MSCs survived within the muscle, provided continuous neurotrophic support, increased the number of neuromuscular junctions, ameliorated loss of motor neurons within the spinal cord, and improved survival and function in ALS rats [88
]. Taken together, these intriguing results provide support for the role of multitargeted treatment strategies and the potential for MSCs to deliver augmented neurotrophic support.
Many trophic factors have been studied using adeno-associated viral (AAV)-mediated delivery in ALS. AAV GDNF, IGF-1 and VEGF have demonstrated promising effects in rodent models, by increasing axonal outgrowth, blocking neuronal apoptosis and promoting neurogenesis. AAV-delivered HGF retards the progression of disease in the transgenic SOD1
mouse model. In addition to direct neurotrophic activities, HGF functions on the astrocytes of G93A mice to maintain levels of EAAT2, a glial-specific glutamate transporter that might be responsible for the reduction of glutamatergic neurotoxicity of motor neurons. In addition, HGF is capable of reducing astrocytosis and microglial accumulation [89
]. However, AAV-mediated delivery presents clinical challenges for treatment in humans and there are some safety concerns [91
Neurotrophic factors that provided benefit in the murine model have had mixed results in humans. A total of three Phase III clinical trials using IGF-1 protein have failed to produce consistent meaningful effects when delivered systemically through subcutaneous injection. After intrathecal IGF-1 delivery showed promise in the SOD1
G93A mice [92
], Nagano et al.
completed a small double-blind clinical trial to assess the effect of intrathecal administration of IGF-1 on disease progression in nine patients with ALS [93
]. They received either high-dose (3 μg/kg of bodyweight) or low-dose (0.5 μg/kg of bodyweight) IGF-1 every 2 weeks for 40 weeks. The high-dose treatment slowed the decline of motor functions, but not bulbar function or vital capacity [93
]. This may be caused by gravitational effects on the medication and the spinal level of intrathecal delivery. MSCs that have been genetically engineered to produce IGF-1 may provide superior efficacy, as it is well established that MSCs migrate through the parenchyma, homing to areas of cellular distress, providing the opportunity for discrete targeted delivery. Efforts from our laboratory are currently exploring the effects of IGF-1 expression from hMSCs implanted into both the peripheral muscle and spinal cord of SOD1 G93A mice [Joyce n et al.
A human cellular therapy trial has already demonstrated progress in the treatment of ALS by intraspinal injection. After characterization of bone marrow-derived MSCs, Mazzini and colleagues transplanted the autologous MSCs into the thoracic spinal cord of nine patients with ALS [94
]. No significant acute or late side effects were reported and four of the patients showed significant slowing of the linear decline of forced vital capacity and the ALS-Functional Rating Scale score [94
]. A Phase II clinical trial using MSCs is underway in Europe, and the FDA has recently approved a Phase I trial in the USA. The need for safe and effective cellular treatments is great in ALS. These therapies offer hope to patients and their families struggling with this devastating disease.