Oligodendrocyte progenitor cell maturation is dependent on dual function of MCT8 in the transport of thyroid hormone across brain barriers and the plasma membrane

Inactivating mutations in the thyroid hormone (TH) transporter monocarboxylate transporter 8 (MCT8) causes a rare and debilitating form of X-linked psychomotor disability known as Allan Herndon Dudley syndrome (AHDS). One of the most prominent pathophysiological symptoms of MCT8-deficiency is hypomyelination. Here, patient-derived induced pluripotent stem cells (iPSCs) were used to study the role of MCT8 and TH on the maturation of oligodendrocytes. Interestingly, neither MCT8 mutations nor reduced TH affected the in vitro differentiation of control or MCT8-deficient iPSCs into oligodendrocytes. To assess whether patient-derived iPSC-derived oligodendrocyte progenitor cells (iOPCs) could provide myelinating oligodendrocytes in vivo, cells were transplanted into the shiverer mouse corpus callosum where they survived, migrated, and matured into myelinating oligodendrocytes, though the myelination efficiency was reduced compared with control cells. When MCT8-deficient and healthy control iOPCs were transplanted into a novel hypothyroid immunodeficient triple knockout mouse (tKO, mct8^-/- ; oatp1c1^-/- ; rag2^-/- ), they failed to provide behavioral recovery and did not mature into oligodendrocytes in the hypothyroid corpus callosum, demonstrating the critical role of TH transport across brain barriers in oligodendrocyte maturation. We conclude that MCT8 plays a cell autonomous role in oligodendrocyte maturation and that functional TH transport into the central nervous system will be required for developing an effective treatment for MCT8-deficient patients.

The difficulty to model Huntington's disease in vitro using striatal medium spiny neurons differentiated from human induced pluripotent stem cells

Huntington's disease (HD) is an autosomal dominant neurodegenerative disorder caused by an expanded polyglutamine repeat in the huntingtin gene. The neuropathology of HD is characterized by the decline of a specific neuronal population within the brain, the striatal medium spiny neurons (MSNs). The origins of this extreme vulnerability remain unknown. Human induced pluripotent stem cell (hiPS cell)-derived MSNs represent a powerful tool to study this genetic disease. However, the differentiation protocols published so far show a high heterogeneity of neuronal populations in vitro. Here, we compared two previously published protocols to obtain hiPS cell-derived striatal neurons from both healthy donors and HD patients. Patch-clamp experiments, immunostaining and RT-qPCR were performed to characterize the neurons in culture. While the neurons were mature enough to fire action potentials, a majority failed to express markers typical for MSNs. Voltage-clamp experiments on voltage-gated sodium (Nav) channels revealed a large variability between the two differentiation protocols. Action potential analysis did not reveal changes induced by the HD mutation. This study attempts to demonstrate the current challenges in reproducing data of previously published differentiation protocols and in generating hiPS cell-derived striatal MSNs to model a genetic neurodegenerative disorder in vitro.

The Interaction of Aging and Cellular Stress Contributes to Pathogenesis in Mouse and Human Huntington Disease Neurons

Huntington disease (HD) is a fatal, inherited neurodegenerative disorder caused by a mutation in the huntingtin (HTT) gene. While mutant HTT is present ubiquitously throughout life, HD onset typically occurs in mid-life. Oxidative damage accumulates in the aging brain and is a feature of HD. We sought to interrogate the roles and interaction of age and oxidative stress in HD using primary Hu97/18 mouse neurons, neurons differentiated from HD patient induced pluripotent stem cells (iPSCs), and the brains of HD mice. We find that primary neurons must be matured in culture for canonical stress responses to occur. Furthermore, when aging is accelerated in mature HD neurons, mutant HTT accumulates and sensitivity to oxidative stress is selectively enhanced. Furthermore, we observe HD-specific phenotypes in neurons and mouse brains that have undergone accelerated aging, including a selective increase in DNA damage. These findings suggest a role for aging in HD pathogenesis and an interaction between the biological age of HD neurons and sensitivity to exogenous stress.

Bioenergetic deficits in Huntington's disease iPSC-derived neural cells and rescue with glycolytic metabolites

Altered cellular metabolism is believed to be an important contributor to pathogenesis of the neurodegenerative disorder Huntington's disease (HD). Research has primarily focused on mitochondrial toxicity, which can cause death of the vulnerable striatal neurons, but other aspects of metabolism have also been implicated. Most previous studies have been carried out using postmortem human brain or non-human cells. Here, we studied bioenergetics in an induced pluripotent stem cell-based model of the disease. We found decreased adenosine triphosphate (ATP) levels in HD cells compared to controls across differentiation stages and protocols. Proteomics data and multiomics network analysis revealed normal or increased levels of mitochondrial messages and proteins, but lowered expression of glycolytic enzymes. Metabolic experiments showed decreased spare glycolytic capacity in HD neurons, while maximal and spare respiratory capacities driven by oxidative phosphorylation were largely unchanged. ATP levels in HD neurons could be rescued with addition of pyruvate or late glycolytic metabolites, but not earlier glycolytic metabolites, suggesting a role for glycolytic deficits as part of the metabolic disturbance in HD neurons. Pyruvate or other related metabolic supplements could have therapeutic benefit in HD.

Characterization of Neurodevelopmental Abnormalities in iPSC-Derived Striatal Cultures from Patients with Huntington's Disease

Background:

Huntington’s disease (HD) is an inherited neurodegenerative disease and is characterized by atrophy of certain regions of the brain in a progressive manner. HD patients experience behavioral changes and uncontrolled movements which can be primarily attributed to the atrophy of striatal neurons. Previous publications describe the models of the HD striatum using induced pluripotent stem cells (iPSCs) derived from HD patients with a juvenile onset (JHD). In this model, the JHD iPSC-derived striatal cultures had altered neurodevelopment and contained a high number of nestin expressing progenitor cells at 42 days of differentiation.

Objective:

To further characterize the altered neurodevelopmental phenotype and evaluate potential phenotypic reversal.

Methods:

Differentiation of human iPSCs towards striatal fate and characterization by means of immunocytochemistry and stereological quantification.

Results:

Here this study demonstrates a distinct delay in the differentiation of the JHD neural progenitor population. However, reduction of the JHD aberrant progenitor populations can be accomplished either by targeting the canonical Notch signaling pathway or by treatment with HTT antisense oligonucleotides (ASOs).

Conclusions:

In summary, this data is postulated to reflect a potential overall developmental delay in JHD.

Huntington's Disease Patient-Derived Astrocytes Display Electrophysiological Impairments and Reduced Neuronal Support

In Huntington’s disease (HD), while the ubiquitously expressed mutant Huntingtin (mtHTT) protein primarily compromises striatal and cortical neurons, glia also undergo disease-contributing alterations. Existing HD models using human induced pluripotent stem cells (iPSCs) have not extensively characterized the role of mtHTT in patient-derived astrocytes. Here physiologically mature astrocytes are generated from HD patient iPSCs. These human astrocytes exhibit hallmark HD phenotypes that occur in mouse models, including impaired inward rectifying K⁺ currents, lengthened spontaneous Ca²⁺ waves and reduced cell membrane capacitance. HD astrocytes in co-culture provided reduced support for the maturation of iPSC-derived neurons. In addition, neurons exposed to chronic glutamate stimulation are not protected by HD astrocytes. This iPSC-based HD model demonstrates the critical effects of mtHTT on human astrocytes, which not only broadens the understanding of disease susceptibility beyond cortical and striatal neurons but also increases potential drug targets.

Inducible Expression of GDNF in Transplanted iPSC-Derived Neural Progenitor Cells

Trophic factor delivery to the brain using stem cell-derived neural progenitors is a powerful way to bypass the blood-brain barrier. Protection of diseased neurons using this technology is a promising therapy for neurodegenerative diseases. Glial cell line-derived neurotrophic factor (GDNF) has provided benefits to Parkinsonian patients and is being used in a clinical trial for amyotrophic lateral sclerosis. However, chronic trophic factor delivery prohibits dose adjustment or cessation if side effects develop. To address this, we engineered a doxycycline-regulated vector, allowing inducible and reversible expression of a therapeutic molecule. Human induced pluripotent stem cell (iPSC)-derived neural progenitors were stably transfected with the vector and transplanted into the adult mouse brain. Doxycycline can penetrate the graft, with addition and withdrawal providing inducible and reversible GDNF expression in vivo, over multiple cycles. Our findings provide proof of concept for combining gene and stem cell therapy for effective modulation of ectopic protein expression in transplanted cells.

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Created plasmid with tetracycline transactivator along with dual reporters and GDNF

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Efficient, stable transduction of human iPSC-derived neural progenitor cells

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Inducible and reversible in vivo expression of GDNF, reporter protein, and luciferase

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Promising stem cell and gene therapy strategy for neurodegenerative diseases

Optimization of trans-Splicing for Huntington's Disease RNA Therapy

Huntington's disease (HD) is a devastating neurodegenerative disorder caused by a polyglutamine (polyQ) expansion in exon 1 of the Huntingtin (HTT) gene. We have previously demonstrated that spliceosome-mediated trans-splicing is a viable molecular strategy to specifically reduce and repair mutant HTT (mtHTT). Here, the targeted tethering efficacy of the pre-mRNA trans-splicing modules (PTM) in HTT was optimized. Various PTMs that targeted the 3′ end of HTT intron 1 or the intron 1 branch point were shown trans-splice into an HTT mini-gene, as well as the endogenous HTT pre-mRNA. PTMs that specifically target the endogenous intron 1 branch point increased the trans-splicing efficacy from 1–5 to 10–15%. Furthermore, lentiviral expression of PTMs in a human HD patient iPSC-derived neural culture significantly reversed two previously established polyQ-length dependent phenotypes. These results suggest that pre-mRNA repair of mtHTT could hold therapeutic benefit and it demonstrates an alternative platform to correct the mRNA product produced by the mtHTT allele in the context of HD.

Survival of iPSC-derived grafts within the striatum of immunodeficient mice: Importance of developmental stage of both transplant and host recipient

Degeneration of the striatum can occur in multiple disorders with devastating consequences for the patients. Infantile infections with streptococcus, measles, or herpes can cause striatal necrosis associated with dystonia or dyskinesia; and in patients with Huntington's disease the striatum undergoes massive degeneration, leading to behavioral, psychological and movement issues, ultimately resulting in death. Currently, only supportive therapies are available for striatal degeneration. Clinical trials have shown some efficacy using transplantation of fetal-derived primary striatal progenitors. Large banks of fetal progenitors that give rise to medium spiny neurons (MSNs), the primary neuron of the striatum, are needed to make transplantation therapy a reality. However, fetal tissue is of limited supply, has ethical concerns, and is at risk of graft immunorejection. An alternative potential source of MSNs is induced pluripotent stem cells (iPSCs), adult somatic tissues reprogrammed back to a stem cell fate. Multiple publications have demonstrated the ability to differentiate striatal MSNs from iPSCs. Previous publications have demonstrated that the efficacy of fetal progenitor transplants is critically dependent upon the age of the donor embryo/fetus as well as the age of the transplant recipient. With the advent of iPSC technology, a question that remains unanswered concerns the graft's "age," which is crucial since transplanting pluripotent cells has an inherent risk of over proliferation and teratoma formation. Therefore, in order to also determine the effect of transplant recipient age on the graft, iPSCs were differentiated to three stages along a striatal differentiation paradigm and transplanted into the striatum of both neonatal and adult immunodeficient mice. This study demonstrated that increased murine transplant-recipient age (adult vs neonate) resulted in decreased graft survival and volume/rostro-caudal spread after six weeks in vivo, regardless of "age" of the cells transplanted. Importantly, this study implicates that the in vivo setting may provide a better neurogenic niche for iPSC-based modeling as compared to the in vitro setting. Together, these results recapitulate findings from fetal striatal progenitor transplantation studies and further demonstrate the influence of the host environment on cellular survival and maturation.

Targeting ATM ameliorates mutant Huntingtin toxicity in cell and animal models of Huntington's disease

Age-related neurodegenerative disorders including Alzheimer's disease and Huntington's disease (HD) consistently show elevated DNA damage, but the relevant molecular pathways in disease pathogenesis remain unclear. One attractive gene is that encoding the ataxia-telangiectasia mutated (ATM) protein, a kinase involved in the DNA damage response, apoptosis, and cellular homeostasis. Loss-of-function mutations in both alleles of ATM cause ataxia-telangiectasia in children, but heterozygous mutation carriers are disease-free. Persistently elevated ATM signaling has been demonstrated in Alzheimer's disease and in mouse models of other neurodegenerative diseases. We show that ATM signaling was consistently elevated in cells derived from HD mice and in brain tissue from HD mice and patients. ATM knockdown protected from toxicities induced by mutant Huntingtin (mHTT) fragments in mammalian cells and in transgenic Drosophila models. By crossing the murine Atm heterozygous null allele onto BACHD mice expressing full-length human mHTT, we show that genetic reduction of Atm gene dosage by one copy ameliorated multiple behavioral deficits and partially improved neuropathology. Small-molecule ATM inhibitors reduced mHTT-induced death of rat striatal neurons and induced pluripotent stem cells derived from HD patients. Our study provides converging genetic and pharmacological evidence that reduction of ATM signaling could ameliorate mHTT toxicity in cellular and animal models of HD, suggesting that ATM may be a useful therapeutic target for HD.

In Vivo Tracking of Human Neural Progenitor Cells in the Rat Brain Using Magnetic Resonance Imaging Is Not Enhanced by Ferritin Expression

Rapid growth in the field of stem cell research has generated a lot of interest in their therapeutic use, especially in the treatment of neurodegenerative diseases. Specifically, human neural progenitor cells (hNPCs), unique in their capability to differentiate into cells of the neural lineage, have been widely investigated due to their ability to survive, thrive, and migrate toward injured tissues. Still, one of the major roadblocks for clinical applicability arises from the inability to monitor these cells following transplantation. Molecular imaging techniques, such as magnetic resonance imaging (MRI), have been explored to assess hNPC transplant location, migration, and survival. Here we investigated whether inducing hNPCs to overexpress ferritin (hNPCs(Fer)), an iron storage protein, is sufficient to track these cells long term in the rat striatum using MRI. We found that increased hypointensity on MRI images could establish hNPC(Fer) location. Unexpectedly, however, wild-type hNPC transplants were detected in a similar manner, which is likely due to increased iron accumulation following transplantation-induced damage. Hence, we labeled hNPCs with superparamagnetic iron oxide (SPIO) nanoparticles to further increase iron content in an attempt to enhance cell contrast in MRI. SPIO-labeling of hNPCs (hNPCs-SPIO) achieved increased hypointensity, with significantly greater area of decreased T2* compared to hNPC(Fer) (p < 0.0001) and all other controls used. However, none of the techniques could be used to determine graft rejection in vivo, which is imperative for understanding cell behavior following transplantation. We conclude that in order for cell survival to be monitored in preclinical and clinical settings, another molecular imaging technique must be employed, including perhaps multimodal imaging, which would utilize MRI along with another imaging modality.

HD iPSC-derived neural progenitors accumulate in culture and are susceptible to BDNF withdrawal due to glutamate toxicity

Huntington's disease (HD) is a fatal neurodegenerative disease, caused by expansion of polyglutamine repeats in the Huntingtin gene, with longer expansions leading to earlier ages of onset. The HD iPSC Consortium has recently reported a new in vitro model of HD based on the generation of induced pluripotent stem cells (iPSCs) from HD patients and controls. The current study has furthered the disease in a dish model of HD by generating new non-integrating HD and control iPSC lines. Both HD and control iPSC lines can be efficiently differentiated into neurons/glia; however, the HD-derived cells maintained a significantly greater number of nestin-expressing neural progenitor cells compared with control cells. This cell population showed enhanced vulnerability to brain-derived neurotrophic factor (BDNF) withdrawal in the juvenile-onset HD (JHD) lines, which appeared to be CAG repeat-dependent and mediated by the loss of signaling from the TrkB receptor. It was postulated that this increased death following BDNF withdrawal may be due to glutamate toxicity, as the N-methyl-d-aspartate (NMDA) receptor subunit NR2B was up-regulated in the cultures. Indeed, blocking glutamate signaling, not just through the NMDA but also mGlu and AMPA/Kainate receptors, completely reversed the cell death phenotype. This study suggests that the pathogenesis of JHD may involve in part a population of 'persistent' neural progenitors that are selectively vulnerable to BDNF withdrawal. Similar results were seen in adult hippocampal-derived neural progenitors isolated from the BACHD model mouse. Together, these results provide important insight into HD mechanisms at early developmental time points, which may suggest novel approaches to HD therapeutics.

Neonatal immune-tolerance in mice does not prevent xenograft rejection

Assessing the efficacy of human stem cell transplantation in rodent models is complicated by the significant immune rejection that occurs. Two recent reports have shown conflicting results using neonatal tolerance to xenografts in rats. Here we extend this approach to mice and assess whether neonatal tolerance can prevent the rapid rejection of xenografts. In three strains of neonatal immune-intact mice, using two different brain transplant regimes and three independent stem cell types, we conclusively show that there is rapid rejection of the implanted cells. We also address specific challenges associated with the generation of humanized mouse models of disease.

Stimulation of GABA-induced Ca2+ influx enhances maturation of human induced pluripotent stem cell-derived neurons

Optimal use of patient-derived, induced pluripotent stem cells for modeling neuronal diseases is crucially dependent upon the proper physiological maturation of derived neurons. As a strategy to develop defined differentiation protocols that optimize electrophysiological function, we investigated the role of Ca²⁺ channel regulation by astrocyte conditioned medium in neuronal maturation, using whole-cell patch clamp and Ca²⁺ imaging. Standard control medium supported basic differentiation of induced pluripotent stem cell-derived neurons, as assayed by the ability to fire simple, single, induced action potentials. In contrast, treatment with astrocyte conditioned medium elicited complex and spontaneous neuronal activity, often with rhythmic and biphasic characteristics. Such augmented spontaneous activity correlated with astrocyte conditioned medium-evoked hyperpolarization and was dependent upon regulated function of L-, N- and R-type Ca²⁺ channels. The requirement for astrocyte conditioned medium could be substituted by simply supplementing control differentiation medium with high Ca²⁺ or γ-amino butyric acid (GABA). Importantly, even in the absence of GABA signalling, opening Ca²⁺ channels directly using Bay K8644 was able to hyperpolarise neurons and enhance excitability, producing fully functional neurons. These data provide mechanistic insight into how secreted astrocyte factors control differentiation and, importantly, suggest that pharmacological modulation of Ca²⁺ channel function leads to the development of a defined protocol for improved maturation of induced pluripotent stem cell-derived neurons.

EZ spheres: a stable and expandable culture system for the generation of pre-rosette multipotent stem cells from human ESCs and iPSCs

We have developed a simple method to generate and expand multipotent, self-renewing pre-rosette neural stem cells from both human embryonic stem cells (hESCs) and human induced pluripotent stem cells (iPSCs) without utilizing embryoid body formation, manual selection techniques, or complex combinations of small molecules. Human ESC and iPSC colonies were lifted and placed in a neural stem cell medium containing high concentrations of EGF and FGF-2. Cell aggregates (termed EZ spheres) could be expanded for long periods using a chopping method that maintained cell-cell contact. Early passage EZ spheres rapidly down-regulated OCT4 and up-regulated SOX2 and nestin expression. They retained the potential to form neural rosettes and consistently differentiated into a range of central and peripheral neural lineages. Thus, they represent a very early neural stem cell with greater differentiation flexibility than other previously described methods. As such, they will be useful for the rapidly expanding field of neurological development and disease modeling, high-content screening, and regenerative therapies based on pluripotent stem cell technology.

Analysis of a read-through promoting compound in a severe mouse model of spinal muscular atrophy

Spinal muscular atrophy (SMA) is the leading genetic cause of infantile death and caused by the loss of functional Survival Motor Neuron 1 (SMN1). The remaining copy gene, SMN2, is unable to rescue from disease because the primary gene product lacks the final coding exon, exon 7, due to an alternative splicing event. While SMNΔ7 is a rapidly degraded protein, exon 7 is not specifically required in a sequence-specific manner to confer increased functionality to this truncated protein. Based upon this molecular observation, aminoglycosides have been examined to artificially elongate the C-terminus of SMNΔ7 by "read-through" of the stop codon. An SMNΔ7 read-through event benefits intermediate mouse models of SMA. Here we demonstrate that delivery of a read-through inducing compound directly to the CNS can partially lessen the severity of a severe model of SMA (Smn(-/-); SMN2(+/+)), albeit not to the extent seen in the less severe model. This further demonstrates the utility of read-through inducing compounds in SMA.

Induced pluripotent stem cells: a new revolution for clinical neurology?

Why specific neuronal populations are uniquely susceptible in neurodegenerative diseases remains a mystery. Brain tissue samples from patients are rarely available for testing, and animal models frequently do not recapitulate all features of a specific disorder; therefore, pathophysiological investigations are difficult. An exciting new avenue for neurological research and drug development is the discovery that patients' somatic cells can be reprogrammed to a pluripotent state; these cells are known as induced pluripotent stem cells. Once pluripotency is reinstated, cell colonies can be expanded and differentiated into specific neural populations. The availability of these cells enables the monitoring in vitro of temporal features of disease initiation and progression, and testing of new drug treatments on the patient's own cells. Hence, this swiftly growing area of research has the potential to contribute greatly to our understanding of the pathophysiology of neurodegenerative and neurodevelopmental diseases.

Subcutaneous administration of TC007 reduces disease severity in an animal model of SMA

Background

Spinal Muscular Atrophy (SMA) is the leading genetic cause of infantile death. It is caused by the loss of functional Survival Motor Neuron 1 (SMN1). There is a nearly identical copy gene, SMN2, but it is unable to rescue from disease due to an alternative splicing event that excises a necessary exon (exon 7) from the majority of SMN2-derived transcripts. While SMNΔ7 protein has severely reduced functionality, the exon 7 sequences may not be specifically required for all activities. Therefore, aminoglycoside antibiotics previously shown to suppress stop codon recognition and promote translation read-through have been examined to increase the length of the SMNΔ7 C-terminus.

Results

Here we demonstrate that subcutaneous-administration of a read-through inducing compound (TC007) to an intermediate SMA model (Smn-/-; SMN2+/+; SMNΔ7) had beneficial effects on muscle fiber size and gross motor function.

Conclusion

Delivery of the read-through inducing compound TC007 reduces the disease-associated phenotype in SMA mice, however, does not significantly extend survival.

Delivery of a read-through inducing compound, TC007, lessens the severity of a spinal muscular atrophy animal model

Spinal muscular atrophy (SMA) is the leading genetic cause of infant mortality and is caused by the loss of a functional SMN1 gene. In humans, there exists a nearly-identical copy gene known as SMN2 that encodes an identical protein as SMN1, but differs by a silent C to T transition within exon 7. This single nucleotide difference produces an alternatively spliced isoform, SMNDelta7, which encodes a rapidly degraded protein. The absence of the short peptide encoded by SMN exon 7 is critical in the disease development process; however, heterologous sequences can partially compensate for the SMN exon 7 peptide in several cellular assays. Consistent with this, aminoglycosides, compounds that can suppress efficient recognition of stop codons, resulted in significantly increased levels of SMN protein in SMA patient fibroblasts. We now examine the potential therapeutic capabilities of a novel aminoglycoside, TC007. In an intermediate SMA model (Smn-/-; SMN2+/+; SMNDelta7), when delivered directly to the central nervous system (CNS), TC007 induces SMN in both the brain and spinal cord, significantly increases lifespan ( approximately 30%) and increases ventral horn cell number, consistent with its ability to increase SMN levels in induced pluripotent stem cell-derived human SMA motor neuron cultures. Collectively, these experiments are the first in vivo examination of therapeutics for SMA designed to induce read-through of the SMNDelta7 stop codon to show increased benefit by direct administration to the CNS.

Induced pluripotent stem cells from a spinal muscular atrophy patient

Spinal muscular atrophy is one of the most common inherited forms of neurological disease leading to infant mortality. Patients have selective loss of lower motor neurons resulting in muscle weakness, paralysis and often death. Although patient fibroblasts have been used extensively to study spinal muscular atrophy, motor neurons have a unique anatomy and physiology which may underlie their vulnerability to the disease process. Here we report the generation of induced pluripotent stem cells from skin fibroblast samples taken from a child with spinal muscular atrophy. These cells expanded robustly in culture, maintained the disease genotype and generated motor neurons that showed selective deficits compared to those derived from the child's unaffected mother. This is the first study to show that human induced pluripotent stem cells can be used to model the specific pathology seen in a genetically inherited disease. As such, it represents a promising resource to study disease mechanisms, screen new drug compounds and develop new therapies.

Delivery of recombinant follistatin lessens disease severity in a mouse model of spinal muscular atrophy

Spinal muscular atrophy (SMA) is the most common genetic cause of infant mortality. SMA is caused by loss of functional survival motor neuron 1 (SMN1), resulting in death of spinal motor neurons. Current therapeutic research focuses on modulating the expression of a partially functioning copy gene, SMN2, which is retained in SMA patients. However, a treatment strategy that improves the SMA phenotype by slowing or reversing the skeletal muscle atrophy may also be beneficial. Myostatin, a member of the TGF-beta super-family, is a potent negative regulator of skeletal muscle mass. Follistatin is a natural antagonist of myostatin, and over-expression of follistatin in mouse muscle leads to profound increases in skeletal muscle mass. To determine whether enhanced muscle mass impacts SMA, we administered recombinant follistatin to an SMA mouse model. Treated animals exhibited increased mass in several muscle groups, elevation in the number and cross-sectional area of ventral horn cells, gross motor function improvement and mean lifespan extension by 30%, by preventing some of the early deaths, when compared with control animals. SMN protein levels in spinal cord and muscle were unchanged in follistatin-treated SMA mice, suggesting that follistatin exerts its effect in an SMN-independent manner. Reversing muscle atrophy associated with SMA may represent an unexploited therapeutic target for the treatment of SMA.

A SMNDelta7 read-through product confers functionality to the SMNDelta7 protein

Spinal muscular atrophy (SMA) affects about 1 in every 6000 children born and is the leading genetic cause of infant death. SMA is a recessive disorder caused by the mutation or deletion of Survival Motor Neuron-1 (SMN1). SMN2, a nearly identical copy gene, has the potential to encode the same protein as SMN1 and is retained in all SMA patients. The majority of SMN2-derived transcripts are alternatively spliced and therefore encode a truncated isoform lacking exon 7 (SMNDelta7), which is a defective protein because it is unstable, has a reduced ability to self-associate and is unable to efficiently function in SMN cellular activities. However, we have shown that the SMN C-terminus functions non-specifically, since heterologous sequences can compensate for the exon 7 sequence. Several classes of compounds identified in SMN-inducing high throughput screens have been proposed to function through a read-through mechanism; however, a functional analysis of the SMNDelta7 read-through product has not been performed. In this report, the SMNDelta7 read-through product is characterized and compared to the SMNDelta7 protein. In a series of in vitro and cell based assays, SMNDelta7 read-through product is shown to increase protein stability, promote neurite outgrowths in SMN deficient neurons, and significantly elevate SMN-dependent UsnRNP assembly in extracts from SMA patient fibroblasts. Collectively, these results demonstrate that SMNDelta7 read-through product is more active than the SMNDelta7 protein and suggest that SMA therapeutics that specifically induce SMNDelta7 read-through may provide an alternative platform for drug discovery.

Novel aminoglycosides increase SMN levels in spinal muscular atrophy fibroblasts

Spinal muscular atrophy (SMA) is the leading genetic cause of infant mortality. SMA is caused by the homozygous absence of survival motor neuron-1 (SMN1). SMN2, a nearly identical copy gene, is retained in all SMA patients and encodes an identical protein as SMN1; however, SMN1 and SMN2 differ by a silent C to T transition which results in the production of an alternatively spliced isoform (SMNDelta7), which encodes a defective protein, demonstrating that the absence of the short peptide encoded by SMN exon 7 is critical in SMA development. Previously, we have shown that for some functions heterologous sequences can compensate for the exon 7 peptide, suggesting that the SMN C-terminus functions non-specifically. Consistent with this hypothesis, we now identify novel aminoglycosides that can induce SMN protein levels in patient fibroblasts. This hypothesis was supported, in part, by a novel fluorescent SMN read-through assay. Interestingly, however, through the development of a SMN exon 7-specific antibody, results suggested that levels of normal full-length SMN might also be elevated by aminoglycoside treatment. These results demonstrate that the compounds that promote read-through may provide an alternative platform for the discovery of compounds that induce SMN protein levels.