Mesenchymal Stem Cells as Trojan Horses for GDNF Delivery in ALS

Amyotrophic lateral sclerosis (ALS), also known as Lou Gehrig's disease, is one of the most destructive adult-onset degenerative diseases to strike the central nervous system. In this disease, motor neurons perish, leading to difficulty in motor function. As the disease progresses in a relentless manner, motor neurons innervating respiratory muscles die, leading to respiratory failure and ultimately death, typically within 3 to 5 years after diagnosis. The prevalence of the disease is 2 to 3 per 100,000 individuals.1Cleveland DW Rothstein JD From Charcot to Lou Gehrig: deciphering selective motor neuron death in ALS.Nat Rev. 2001; 2: 806-819Crossref Scopus (1167) Google Scholar Approximately 5–10% of cases are hereditary, and the causes of the other sporadic cases remain unknown.2Rowland LP Shneider NA Amyotrophic lateral sclerosis.N Engl J Med. 2001; 344: 1688-1700Crossref PubMed Scopus (1560) Google Scholar Despite significant research since the first description of the disease by Charcot in 1874 and numerous clinical trials over many decades, the development of effective human therapies to prevent or delay disease progression has not been forthcoming. However, significant advances have been made in understanding some of the cellular components involved in the disease, such as the initiation of disease onset in motor neurons, along with deciphering of cellular types involved in disease progression, such as astrocytes and microglia.3Clement AM Nguyen MD Roberts EA Garcia ML Boillee S Rule M et al.Wild-type nonneuronal cells extend survival of SOD1 mutant motor neurons in ALS mice.Science. 2003; 302: 113-117Crossref PubMed Scopus (896) Google Scholar,4Boillee S Vande Velde C Cleveland DW ALS: a disease of motor neurons and their nonneuronal neighbors.Neuron. 2006; 52: 39-59Abstract Full Text Full Text PDF PubMed Scopus (1129) Google Scholar,5Boillee S Yamanaka K Lobsiger CS Copeland NG Jenkins NA Kassiotis G et al.Onset and progression in inherited ALS determined by motor neurons and microglia.Science. 2006; 312: 1389-1392Crossref PubMed Scopus (1262) Google Scholar,6Miller TM Kim SH Yamanaka K Hester M Umapathi P Arnson H et al.Gene transfer demonstrates that muscle is not a primary target for non-cell-autonomous toxicity in familial amyotrophic lateral sclerosis.Proc Natl Acad Sci USA. 2006; 103: 19546-19551Crossref PubMed Scopus (132) Google Scholar,7Yamanaka K Boillee S Roberts EA Garcia ML McAlonis-Downes M Mikse OR et al.Mutant SOD1 in cell types other than motor neurons and oligodendrocytes accelerates onset of disease in ALS mice.Proc Natl Acad Sci USA. 2008; 105: 7594-7599Crossref PubMed Scopus (217) Google Scholar,8Yamanaka K Chun SJ Boillee S Fujimori-Tonou N Yamashita H Gutmann DH et al.Astrocytes as determinants of disease progression in inherited amyotrophic lateral sclerosis.Nat Neurosci. 2008; 11: 251-253Crossref PubMed Scopus (855) Google Scholar Several mouse and rat models have been developed that express mutations in the gene encoding superoxide dismutase-1, demonstrating phenotypes similar to human familial ALS.9Gurney ME Pu H Chiu AY Dal Canto MC Polchow CY Alexander DD et al.Motor neuron degeneration in mice that express a human Cu, Zn superoxide dismutase mutation.Science. 1994; 264: 1772-1775Crossref PubMed Scopus (3419) Google Scholar,10Howland DS Liu J She Y Goad B Maragakis NJ Kim B et al.Focal loss of the glutamate transporter EAAT2 in a transgenic rat model of SOD1 mutant-mediated amyotrophic lateral sclerosis (ALS).Proc Natl Acad Sci USA. 2002; 99: 1604-1609Crossref PubMed Scopus (693) Google Scholar These models have been the predominant in vivo model for studying disease mechanisms and therapeutic candidates.Neurotrophic and trophic factors (such as brain-derived neurotrophic factor, glial cell line–derived neurotrophic factor (GDNF), ciliary neurotrophic factor, vascular endothelial-derived growth factor, and insulin-like growth factor-1 (IGF-1)) have demonstrated promise in familial ALS models by delaying onset and/or progression of disease. However, clinical trials using neurotrophic factors have been unsuccessful thus far because of problems with drug pharmacokinetics, dose-limiting toxicities, and the potential for antibody inactivation. Therefore, significant efforts have focused on optimal delivery methods for these factors. Studies have investigated pump-based delivery of recombinant proteins to the brain, directed gene delivery by viral and nonviral vectors, and transplantation of cells expressing these factors, which is the focus of this Commentary.In this issue of Molecular Therapy, Suzuki et al.11Suzuki M McHugh J Tork C Shelley B Hayes A Bellantuono I et al.Direct muscle delivery of GDNF with human mesenchymal stem cells improves motor neuron survival and function in a rat model of familial ALS. 2008; (Mol Ther 16: 2002–2010.)Google Scholar report the use of human mesenchymal stem cells (MSCs) to deliver trophic factors to various muscles in a rat model of familial ALS. MSCs, found in the bone marrow of adults, are capable of differentiating into a variety of tissues, including bone, heart, muscle, and other organ tissues. They have been explored for therapeutic development for numerous disorders, including neurodegenerative diseases. However, a major challenge has been delivering MSCs efficiently to a target tissue such as skeletal muscle for optimal cell survival, migration, and incorporation. In this study, Suzuki and colleagues have partly overcome this problem by optimizing the engraftment of the cells. They induced focal injury in the muscle by intramuscular injection of a local anesthetic to encourage engraftment. They also demonstrated that some of the injected cells acquired a skeletal muscle phenotype. The authors next used their optimized MSC delivery approach to transplant MSCs expressing the neurotrophic factor GDNF into the rat model of familial ALS. MSCs were transduced with a lentivirus encoding GDNF and were confirmed to express GDNF at high levels. An exciting finding was that animals treated with MSCs expressing GDNF experienced therapeutic benefit. In treated animals, the transplanted cells abolished neuromuscular junction denervation, provided motor neuron protection, improved motor function, and, importantly, increased survival.Interestingly, Suzuki et al. demonstrated that MSCs not genetically altered to express a neurotrophic factor also showed a trend for increased neuromuscular function and increased survival, potentially through trophic factors that MSCs naturally provide. However, the benefit was greatest when MSCs were expressing GDNF. In animals that showed slower disease progression, the effects on survival were much greater, showing a median increase in survival of 28 days, which is substantial in the rat model of this disease. These animals also showed improvements in motor function as assessed by limb-function testing using the Basso–Beatti–Bresnahan locomotor rating test.These results are exciting for many reasons. First, the authors have demonstrated and confirmed that neurotrophic factors such as GDNF are effective in sparing motor neuron death and increase survival in a very rapidly progressing animal model of ALS, thus bringing neurotrophic factors back into the spotlight for this disease. Second, the authors showed profound extension of survival in this model, with an increase in life span of several weeks to a month. Additionally, they highlight a relatively new delivery approach in ALS that seems to be well tolerated and have lasting effects.Indeed, GDNF has demonstrated significant effects in this disease model in numerous studies. Previous work from these authors and by others has shown that GDNF expression in the spinal cord, delivered by neural progenitor cells, protected dying motor neurons but not their projections to muscles, in the same rat model of familial ALS used in this study.12Klein SM Behrstock S McHugh J Hoffmann K Wallace K Suzuki M et al.GDNF delivery using human neural progenitor cells in a rat model of ALS.Hum Gene Ther. 2005; 16: 509-521Crossref PubMed Scopus (228) Google Scholar,13Suzuki M McHugh J Tork C Shelley B Klein SM Aebischer P et al.GDNF secreting human neural progenitor cells protect dying motor neurons, but not their projection to muscle, in a rat model of familial ALS.PLoS ONE. 2007; 2: e689Crossref PubMed Scopus (245) Google Scholar,14Li W Brakefield D Pan Y Hunter D Myckatyn TM Parsadanian A Muscle-derived but not centrally derived transgene GDNF is neuroprotective in G93A-SOD1 mouse model of ALS.Exp Neurol. 2007; 203: 457-471Crossref PubMed Scopus (93) Google Scholar Several reports have already tested GDNF expression in the muscle, along with muscle and spinal cord delivery, and demonstrated that GDNF was neuroprotective and could prolong life span in the ALS models.15Acsadi G Anguelov RA Yang H Toth G Thomas R Jani A et al.Increased survival and function of SOD1 mice after glial cell-derived neurotrophic factor gene therapy.Hum Gene Ther. 2002; 13: 1047-1059Crossref PubMed Scopus (147) Google Scholar,16Kaspar BK Llado J Sherkat N Rothstein JD Gage FH Retrograde viral delivery of IGF-1 prolongs survival in a mouse ALS model.Science. 2003; 301: 839-842Crossref PubMed Scopus (741) Google Scholar Furthermore, the concept of transplanting cells expressing GDNF to muscle in ALS was first tested by Mohajeri and colleagues.17Mohajeri M.H Figlewicz DA Bohn MC Intramuscular grafts of myoblasts genetically modified to secrete glial cell line-derived neurotrophic factor prevent motoneuron loss and disease progression in a mouse model of familial amyotrophic lateral sclerosis.Hum Gene Ther. 1999; 10: 1853-1866Crossref PubMed Scopus (122) Google Scholar GDNF-expressing myoblasts were injected into muscles of ALS mice before disease onset, resulting in prevention of motor neuron loss and an increase in survival of treated mice.17Mohajeri M.H Figlewicz DA Bohn MC Intramuscular grafts of myoblasts genetically modified to secrete glial cell line-derived neurotrophic factor prevent motoneuron loss and disease progression in a mouse model of familial amyotrophic lateral sclerosis.Hum Gene Ther. 1999; 10: 1853-1866Crossref PubMed Scopus (122) Google Scholar Myoblast transplantation has not been further explored in ALS, possibly because of the difficulty of isolating and propagating such cells, and also because of poor engraftment, as highlighted by Suzuki and colleagues.Although various trophic factors have demonstrated promise in ALS, several studies have demonstrated that GDNF seems to influence primarily disease onset and not progression, making it a difficult therapeutic candidate for individuals with more advanced ALS.15Acsadi G Anguelov RA Yang H Toth G Thomas R Jani A et al.Increased survival and function of SOD1 mice after glial cell-derived neurotrophic factor gene therapy.Hum Gene Ther. 2002; 13: 1047-1059Crossref PubMed Scopus (147) Google Scholar,16Kaspar BK Llado J Sherkat N Rothstein JD Gage FH Retrograde viral delivery of IGF-1 prolongs survival in a mouse ALS model.Science. 2003; 301: 839-842Crossref PubMed Scopus (741) Google Scholar,18Wang LJ Lu YY Muramatsu S Ikeguchi K Fujimoto K Okada T et al.Neuroprotective effects of glial cell line-derived neurotrophic factor mediated by an adeno-associated virus vector in a transgenic animal model of amyotrophic lateral sclerosis.J Neurosci. 2002; 22: 6920-6928Crossref PubMed Google Scholar The study by Suzuki et al. demonstrates the effects of GDNF in presymptomatic animals, with the greatest effects in the animals in which disease progressed slowly. Their results, while impressive and of clinical importance in ALS to target motor neurons in which disease may not have initiated or progressed, are somewhat hampered by GDNF's failure to slow disease progression. Although motor neurons were spared, there was no reduction in activated astroglial or microglial cells, two cell types that have been strongly implicated in disease progression.3Clement AM Nguyen MD Roberts EA Garcia ML Boillee S Rule M et al.Wild-type nonneuronal cells extend survival of SOD1 mutant motor neurons in ALS mice.Science. 2003; 302: 113-117Crossref PubMed Scopus (896) Google Scholar,4Boillee S Vande Velde C Cleveland DW ALS: a disease of motor neurons and their nonneuronal neighbors.Neuron. 2006; 52: 39-59Abstract Full Text Full Text PDF PubMed Scopus (1129) Google Scholar,5Boillee S Yamanaka K Lobsiger CS Copeland NG Jenkins NA Kassiotis G et al.Onset and progression in inherited ALS determined by motor neurons and microglia.Science. 2006; 312: 1389-1392Crossref PubMed Scopus (1262) Google Scholar,6Miller TM Kim SH Yamanaka K Hester M Umapathi P Arnson H et al.Gene transfer demonstrates that muscle is not a primary target for non-cell-autonomous toxicity in familial amyotrophic lateral sclerosis.Proc Natl Acad Sci USA. 2006; 103: 19546-19551Crossref PubMed Scopus (132) Google Scholar,7Yamanaka K Boillee S Roberts EA Garcia ML McAlonis-Downes M Mikse OR et al.Mutant SOD1 in cell types other than motor neurons and oligodendrocytes accelerates onset of disease in ALS mice.Proc Natl Acad Sci USA. 2008; 105: 7594-7599Crossref PubMed Scopus (217) Google Scholar,8Yamanaka K Chun SJ Boillee S Fujimori-Tonou N Yamashita H Gutmann DH et al.Astrocytes as determinants of disease progression in inherited amyotrophic lateral sclerosis.Nat Neurosci. 2008; 11: 251-253Crossref PubMed Scopus (855) Google Scholar One might have hoped that not only would GDNF be neuroprotective but also that it could reverse or repair the damage in motor neurons that seems to trigger aberrant glial activity. This does not seem to be the case for all trophic factors, however. We have recently reported that IGF-1 has direct effects to attenuate glial cell activation in the familial mouse model of familial ALS.19Dodge JC Haidet AM Yang W Passini MA Hester M Clarke J et al.Delivery of AAV-IGF-1 to the CNS extends survival in ALS mice through modification of aberrant glial cell activity.Mol Ther. 2008; 16: 1056-1064Abstract Full Text Full Text PDF PubMed Scopus (127) Google Scholar Perhaps the information gleaned from these studies is that no single trophic factor will provide a one-shot approach to treating this disease, setting the stage for a combination approach that may entail cellular transplants at various targets within the neuromuscular circuit as well as gene delivery. MSCs as Trojan horses to deliver therapeutic factors are one step forward for gene delivery in ALS. Further study of these cells based on these results is certainly warranted to advance their clinical development in individuals with ALS. Several key questions arise: How many injections will be needed in an ALS patient? Will the therapy be a one-time delivery? Will the cells survive, integrate, and express for long periods of time in human muscles? Will the cells migrate to other organs?In summary, one could imagine that the optimal therapy for this devastating disease includes cellular transplants of new motor neurons and astrocytes in the spinal cord, the addition of new muscle cells, and delivery of trophic factors by transplantation of cells such as MSCs, along with gene therapy targeting of the muscle and central nervous system. This new work represents an additional step forward in the ongoing development of therapeutic approaches for this devastating disease. Amyotrophic lateral sclerosis (ALS), also known as Lou Gehrig's disease, is one of the most destructive adult-onset degenerative diseases to strike the central nervous system. In this disease, motor neurons perish, leading to difficulty in motor function. As the disease progresses in a relentless manner, motor neurons innervating respiratory muscles die, leading to respiratory failure and ultimately death, typically within 3 to 5 years after diagnosis. The prevalence of the disease is 2 to 3 per 100,000 individuals.1Cleveland DW Rothstein JD From Charcot to Lou Gehrig: deciphering selective motor neuron death in ALS.Nat Rev. 2001; 2: 806-819Crossref Scopus (1167) Google Scholar Approximately 5–10% of cases are hereditary, and the causes of the other sporadic cases remain unknown.2Rowland LP Shneider NA Amyotrophic lateral sclerosis.N Engl J Med. 2001; 344: 1688-1700Crossref PubMed Scopus (1560) Google Scholar Despite significant research since the first description of the disease by Charcot in 1874 and numerous clinical trials over many decades, the development of effective human therapies to prevent or delay disease progression has not been forthcoming. However, significant advances have been made in understanding some of the cellular components involved in the disease, such as the initiation of disease onset in motor neurons, along with deciphering of cellular types involved in disease progression, such as astrocytes and microglia.3Clement AM Nguyen MD Roberts EA Garcia ML Boillee S Rule M et al.Wild-type nonneuronal cells extend survival of SOD1 mutant motor neurons in ALS mice.Science. 2003; 302: 113-117Crossref PubMed Scopus (896) Google Scholar,4Boillee S Vande Velde C Cleveland DW ALS: a disease of motor neurons and their nonneuronal neighbors.Neuron. 2006; 52: 39-59Abstract Full Text Full Text PDF PubMed Scopus (1129) Google Scholar,5Boillee S Yamanaka K Lobsiger CS Copeland NG Jenkins NA Kassiotis G et al.Onset and progression in inherited ALS determined by motor neurons and microglia.Science. 2006; 312: 1389-1392Crossref PubMed Scopus (1262) Google Scholar,6Miller TM Kim SH Yamanaka K Hester M Umapathi P Arnson H et al.Gene transfer demonstrates that muscle is not a primary target for non-cell-autonomous toxicity in familial amyotrophic lateral sclerosis.Proc Natl Acad Sci USA. 2006; 103: 19546-19551Crossref PubMed Scopus (132) Google Scholar,7Yamanaka K Boillee S Roberts EA Garcia ML McAlonis-Downes M Mikse OR et al.Mutant SOD1 in cell types other than motor neurons and oligodendrocytes accelerates onset of disease in ALS mice.Proc Natl Acad Sci USA. 2008; 105: 7594-7599Crossref PubMed Scopus (217) Google Scholar,8Yamanaka K Chun SJ Boillee S Fujimori-Tonou N Yamashita H Gutmann DH et al.Astrocytes as determinants of disease progression in inherited amyotrophic lateral sclerosis.Nat Neurosci. 2008; 11: 251-253Crossref PubMed Scopus (855) Google Scholar Several mouse and rat models have been developed that express mutations in the gene encoding superoxide dismutase-1, demonstrating phenotypes similar to human familial ALS.9Gurney ME Pu H Chiu AY Dal Canto MC Polchow CY Alexander DD et al.Motor neuron degeneration in mice that express a human Cu, Zn superoxide dismutase mutation.Science. 1994; 264: 1772-1775Crossref PubMed Scopus (3419) Google Scholar,10Howland DS Liu J She Y Goad B Maragakis NJ Kim B et al.Focal loss of the glutamate transporter EAAT2 in a transgenic rat model of SOD1 mutant-mediated amyotrophic lateral sclerosis (ALS).Proc Natl Acad Sci USA. 2002; 99: 1604-1609Crossref PubMed Scopus (693) Google Scholar These models have been the predominant in vivo model for studying disease mechanisms and therapeutic candidates. Neurotrophic and trophic factors (such as brain-derived neurotrophic factor, glial cell line–derived neurotrophic factor (GDNF), ciliary neurotrophic factor, vascular endothelial-derived growth factor, and insulin-like growth factor-1 (IGF-1)) have demonstrated promise in familial ALS models by delaying onset and/or progression of disease. However, clinical trials using neurotrophic factors have been unsuccessful thus far because of problems with drug pharmacokinetics, dose-limiting toxicities, and the potential for antibody inactivation. Therefore, significant efforts have focused on optimal delivery methods for these factors. Studies have investigated pump-based delivery of recombinant proteins to the brain, directed gene delivery by viral and nonviral vectors, and transplantation of cells expressing these factors, which is the focus of this Commentary. In this issue of Molecular Therapy, Suzuki et al.11Suzuki M McHugh J Tork C Shelley B Hayes A Bellantuono I et al.Direct muscle delivery of GDNF with human mesenchymal stem cells improves motor neuron survival and function in a rat model of familial ALS. 2008; (Mol Ther 16: 2002–2010.)Google Scholar report the use of human mesenchymal stem cells (MSCs) to deliver trophic factors to various muscles in a rat model of familial ALS. MSCs, found in the bone marrow of adults, are capable of differentiating into a variety of tissues, including bone, heart, muscle, and other organ tissues. They have been explored for therapeutic development for numerous disorders, including neurodegenerative diseases. However, a major challenge has been delivering MSCs efficiently to a target tissue such as skeletal muscle for optimal cell survival, migration, and incorporation. In this study, Suzuki and colleagues have partly overcome this problem by optimizing the engraftment of the cells. They induced focal injury in the muscle by intramuscular injection of a local anesthetic to encourage engraftment. They also demonstrated that some of the injected cells acquired a skeletal muscle phenotype. The authors next used their optimized MSC delivery approach to transplant MSCs expressing the neurotrophic factor GDNF into the rat model of familial ALS. MSCs were transduced with a lentivirus encoding GDNF and were confirmed to express GDNF at high levels. An exciting finding was that animals treated with MSCs expressing GDNF experienced therapeutic benefit. In treated animals, the transplanted cells abolished neuromuscular junction denervation, provided motor neuron protection, improved motor function, and, importantly, increased survival. Interestingly, Suzuki et al. demonstrated that MSCs not genetically altered to express a neurotrophic factor also showed a trend for increased neuromuscular function and increased survival, potentially through trophic factors that MSCs naturally provide. However, the benefit was greatest when MSCs were expressing GDNF. In animals that showed slower disease progression, the effects on survival were much greater, showing a median increase in survival of 28 days, which is substantial in the rat model of this disease. These animals also showed improvements in motor function as assessed by limb-function testing using the Basso–Beatti–Bresnahan locomotor rating test. These results are exciting for many reasons. First, the authors have demonstrated and confirmed that neurotrophic factors such as GDNF are effective in sparing motor neuron death and increase survival in a very rapidly progressing animal model of ALS, thus bringing neurotrophic factors back into the spotlight for this disease. Second, the authors showed profound extension of survival in this model, with an increase in life span of several weeks to a month. Additionally, they highlight a relatively new delivery approach in ALS that seems to be well tolerated and have lasting effects. Indeed, GDNF has demonstrated significant effects in this disease model in numerous studies. Previous work from these authors and by others has shown that GDNF expression in the spinal cord, delivered by neural progenitor cells, protected dying motor neurons but not their projections to muscles, in the same rat model of familial ALS used in this study.12Klein SM Behrstock S McHugh J Hoffmann K Wallace K Suzuki M et al.GDNF delivery using human neural progenitor cells in a rat model of ALS.Hum Gene Ther. 2005; 16: 509-521Crossref PubMed Scopus (228) Google Scholar,13Suzuki M McHugh J Tork C Shelley B Klein SM Aebischer P et al.GDNF secreting human neural progenitor cells protect dying motor neurons, but not their projection to muscle, in a rat model of familial ALS.PLoS ONE. 2007; 2: e689Crossref PubMed Scopus (245) Google Scholar,14Li W Brakefield D Pan Y Hunter D Myckatyn TM Parsadanian A Muscle-derived but not centrally derived transgene GDNF is neuroprotective in G93A-SOD1 mouse model of ALS.Exp Neurol. 2007; 203: 457-471Crossref PubMed Scopus (93) Google Scholar Several reports have already tested GDNF expression in the muscle, along with muscle and spinal cord delivery, and demonstrated that GDNF was neuroprotective and could prolong life span in the ALS models.15Acsadi G Anguelov RA Yang H Toth G Thomas R Jani A et al.Increased survival and function of SOD1 mice after glial cell-derived neurotrophic factor gene therapy.Hum Gene Ther. 2002; 13: 1047-1059Crossref PubMed Scopus (147) Google Scholar,16Kaspar BK Llado J Sherkat N Rothstein JD Gage FH Retrograde viral delivery of IGF-1 prolongs survival in a mouse ALS model.Science. 2003; 301: 839-842Crossref PubMed Scopus (741) Google Scholar Furthermore, the concept of transplanting cells expressing GDNF to muscle in ALS was first tested by Mohajeri and colleagues.17Mohajeri M.H Figlewicz DA Bohn MC Intramuscular grafts of myoblasts genetically modified to secrete glial cell line-derived neurotrophic factor prevent motoneuron loss and disease progression in a mouse model of familial amyotrophic lateral sclerosis.Hum Gene Ther. 1999; 10: 1853-1866Crossref PubMed Scopus (122) Google Scholar GDNF-expressing myoblasts were injected into muscles of ALS mice before disease onset, resulting in prevention of motor neuron loss and an increase in survival of treated mice.17Mohajeri M.H Figlewicz DA Bohn MC Intramuscular grafts of myoblasts genetically modified to secrete glial cell line-derived neurotrophic factor prevent motoneuron loss and disease progression in a mouse model of familial amyotrophic lateral sclerosis.Hum Gene Ther. 1999; 10: 1853-1866Crossref PubMed Scopus (122) Google Scholar Myoblast transplantation has not been further explored in ALS, possibly because of the difficulty of isolating and propagating such cells, and also because of poor engraftment, as highlighted by Suzuki and colleagues. Although various trophic factors have demonstrated promise in ALS, several studies have demonstrated that GDNF seems to influence primarily disease onset and not progression, making it a difficult therapeutic candidate for individuals with more advanced ALS.15Acsadi G Anguelov RA Yang H Toth G Thomas R Jani A et al.Increased survival and function of SOD1 mice after glial cell-derived neurotrophic factor gene therapy.Hum Gene Ther. 2002; 13: 1047-1059Crossref PubMed Scopus (147) Google Scholar,16Kaspar BK Llado J Sherkat N Rothstein JD Gage FH Retrograde viral delivery of IGF-1 prolongs survival in a mouse ALS model.Science. 2003; 301: 839-842Crossref PubMed Scopus (741) Google Scholar,18Wang LJ Lu YY Muramatsu S Ikeguchi K Fujimoto K Okada T et al.Neuroprotective effects of glial cell line-derived neurotrophic factor mediated by an adeno-associated virus vector in a transgenic animal model of amyotrophic lateral sclerosis.J Neurosci. 2002; 22: 6920-6928Crossref PubMed Google Scholar The study by Suzuki et al. demonstrates the effects of GDNF in presymptomatic animals, with the greatest effects in the animals in which disease progressed slowly. Their results, while impressive and of clinical importance in ALS to target motor neurons in which disease may not have initiated or progressed, are somewhat hampered by GDNF's failure to slow disease progression. Although motor neurons were spared, there was no reduction in activated astroglial or microglial cells, two cell types that have been strongly implicated in disease progression.3Clement AM Nguyen MD Roberts EA Garcia ML Boillee S Rule M et al.Wild-type nonneuronal cells extend survival of SOD1 mutant motor neurons in ALS mice.Science. 2003; 302: 113-117Crossref PubMed Scopus (896) Google Scholar,4Boillee S Vande Velde C Cleveland DW ALS: a disease of motor neurons and their nonneuronal neighbors.Neuron. 2006; 52: 39-59Abstract Full Text Full Text PDF PubMed Scopus (1129) Google Scholar,5Boillee S Yamanaka K Lobsiger CS Copeland NG Jenkins NA Kassiotis G et al.Onset and progression in inherited ALS determined by motor neurons and microglia.Science. 2006; 312: 1389-1392Crossref PubMed Scopus (1262) Google Scholar,6Miller TM Kim SH Yamanaka K Hester M Umapathi P Arnson H et al.Gene transfer demonstrates that muscle is not a primary target for non-cell-autonomous toxicity in familial amyotrophic lateral sclerosis.Proc Natl Acad Sci USA. 2006; 103: 19546-19551Crossref PubMed Scopus (132) Google Scholar,7Yamanaka K Boillee S Roberts EA Garcia ML McAlonis-Downes M Mikse OR et al.Mutant SOD1 in cell types other than motor neurons and oligodendrocytes accelerates onset of disease in ALS mice.Proc Natl Acad Sci USA. 2008; 105: 7594-7599Crossref PubMed Scopus (217) Google Scholar,8Yamanaka K Chun SJ Boillee S Fujimori-Tonou N Yamashita H Gutmann DH et al.Astrocytes as determinants of disease progression in inherited amyotrophic lateral sclerosis.Nat Neurosci. 2008; 11: 251-253Crossref PubMed Scopus (855) Google Scholar One might have hoped that not only would GDNF be neuroprotective but also that it could reverse or repair the damage in motor neurons that seems to trigger aberrant glial activity. This does not seem to be the case for all trophic factors, however. We have recently reported that IGF-1 has direct effects to attenuate glial cell activation in the familial mouse model of familial ALS.19Dodge JC Haidet AM Yang W Passini MA Hester M Clarke J et al.Delivery of AAV-IGF-1 to the CNS extends survival in ALS mice through modification of aberrant glial cell activity.Mol Ther. 2008; 16: 1056-1064Abstract Full Text Full Text PDF PubMed Scopus (127) Google Scholar Perhaps the information gleaned from these studies is that no single trophic factor will provide a one-shot approach to treating this disease, setting the stage for a combination approach that may entail cellular transplants at various targets within the neuromuscular circuit as well as gene delivery. MSCs as Trojan horses to deliver therapeutic factors are one step forward for gene delivery in ALS. Further study of these cells based on these results is certainly warranted to advance their clinical development in individuals with ALS. Several key questions arise: How many injections will be needed in an ALS patient? Will the therapy be a one-time delivery? Will the cells survive, integrate, and express for long periods of time in human muscles? Will the cells migrate to other organs? In summary, one could imagine that the optimal therapy for this devastating disease includes cellular transplants of new motor neurons and astrocytes in the spinal cord, the addition of new muscle cells, and delivery of trophic factors by transplantation of cells such as MSCs, along with gene therapy targeting of the muscle and central nervous system. This new work represents an additional step forward in the ongoing development of therapeutic approaches for this devastating disease.