Cx43 hemichannels contribute to astrocyte-mediated toxicity in sporadic and familial ALS.

Connexin 43 (Cx43) gap junctions and hemichannels mediate astrocyte intercellular communication in the central nervous system under normal conditions and contribute to astrocyte-mediated neurotoxicity in amyotrophic lateral sclerosis (ALS). Here, we show that astrocyte-specific knockout of Cx43 in a mouse model of ALS slows disease progression both spatially and temporally, provides motor neuron (MN) protection, and improves survival. In addition, Cx43 expression is up-regulated in human postmortem tissue and cerebrospinal fluid from ALS patients. Using human induced pluripotent stem cell­derived astrocytes (hiPSC-A) from both familial and sporadic ALS, we establish that Cx43 is up-regulated and that Cx43-hemichannels are enriched at the astrocyte membrane. We also demonstrate that the pharmacological blockade of Cx43-hemichannels in ALS astrocytes using GAP 19, a mimetic peptide blocker, and tonabersat, a clinically tested small molecule, provides neuroprotection of hiPSC-MN and reduces ALS astrocyte-mediated neuronal hyperexcitability. Extending the in vitro application of tonabersat with chronic administration to SOD1G93A mice results in MN protection with a reduction in reactive astrocytosis and microgliosis. Taking these data together, our studies identify Cx43 hemichannels as conduits of astrocyte-mediated disease progression and a pharmacological target for disease-modifying ALS therapies.

Connexin 43 in astrocytes contributes to motor neuron toxicity in amyotrophic lateral sclerosis.

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease characterized by progressive loss of motor neurons in the CNS. Astrocytes play a critical role in disease progression of ALS. Astrocytes are interconnected through a family of gap junction proteins known as connexins (Cx). Cx43 is a major astrocyte connexin conducting crucial homeostatic functions in the CNS. Under pathological conditions, connexin expression and functions are altered. Here we report that an abnormal increase in Cx43 expression serves as one of the mechanisms for astrocyte-mediated toxicity in ALS. We observed a progressive increase in Cx43 expression in the SOD1(G93A) mouse model of ALS during the disease course. Notably, this increase in Cx43 was also detected in the motor cortex and spinal cord of ALS patients. Astrocytes isolated from SOD1(G93A) mice as well as human induced pluripotent stem cell (iPSC)-derived astrocytes showed an increase in Cx43 protein, which was found to be an endogenous phenomenon independent of neuronal co-culture. Increased Cx43 expression led to important functional consequences when tested in SOD1(G93A) astrocytes when compared to control astrocytes over-expressing wild-type SOD1 (SOD1(WT) ). We observed SOD1(G93A) astrocytes exhibited enhanced gap junction coupling, increased hemichannel-mediated activity, and elevated intracellular calcium levels. Finally, we tested the impact of increased expression of Cx43 on MN survival and observed that use of both a pan Cx43 blocker and Cx43 hemichannel blocker conferred neuroprotection to MNs cultured with SOD1(G93A) astrocytes. These novel findings show a previously unrecognized role of Cx43 in ALS-related motor neuron loss. GLIA 2016;64:1154-1169.

Focal and dose-dependent neuroprotection in ALS mice following AAV2-neurturin delivery.

Neurotrophic factors as candidates for ALS therapeutics have previously been studied in the context of attempts to slow disease progression. For a variety of reasons, clinical trials of neurotrophic factors have failed to show efficacy in ALS patients. Previous studies in Parkinson's Disease (PD) models have shown promise with the use of recombinant adeno-associated virus serotype-2 (rAAV2)-neurturin (NRTN) [AAV2-NRTN] providing neuroprotection and behavioral improvements in preclinical models which subsequently resulted in several clinical studies in patients with PD. Given that this neurotrophic compound has not been studied in the context of ALS, we conducted a study of AAV2-NRTN to assess the preclinical safety, tolerability, biodistribution, and efficacy of this compound in an ALS mouse model. SOD1G93A mice were injected with AAV2-NRTN intraspinally at several doses into the cervical spinal cord at 60 days of age. NRTN expression was noted in motor neurons (MNs) of the targeted cervical spinal cord as well as in their neuromuscular junction projections but not in the lumbar spinal cord, which was not targeted. Neuropathologically, a dose-dependent neuroprotective effect was seen in cervical MNs and neuromuscular junctions that was reflected in a slowing of forelimb grip strength decline. As expected, this neuroprotection was found to be focal and was not seen beyond the immediate region of injection. Overall, there were no increases in morbidity, changes in serum chemistries or blood counts and no cases of drug-related mortality. Because there is a broad clinical experience for this compound, these data provide evidence to support further investigation of AAV2-NRTN as a potential ALS therapeutic.

Role of Human-Induced Pluripotent Stem Cell-Derived Spinal Cord Astrocytes in the Functional Maturation of Motor Neurons in a Multielectrode Array System.

The ability to generate human-induced pluripotent stem cell (hiPSC)-derived neural cells displaying region-specific phenotypes is of particular interest for modeling central nervous system biology in vitro. We describe a unique method by which spinal cord hiPSC-derived astrocytes (hiPSC-A) are cultured with spinal cord hiPSC-derived motor neurons (hiPSC-MN) in a multielectrode array (MEA) system to record electrophysiological activity over time. We show that hiPSC-A enhance hiPSC-MN electrophysiological maturation in a time-dependent fashion. The sequence of plating, density, and age in which hiPSC-A are cocultured with MN, but not their respective hiPSC line origin, are factors that influence neuronal electrophysiology. When compared to coculture with mouse primary spinal cord astrocytes, we observe an earlier and more robust electrophysiological maturation in the fully human cultures, suggesting that the human origin is relevant to the recapitulation of astrocyte/motor neuron crosstalk. Finally, we test pharmacological compounds on our MEA platform and observe changes in electrophysiological activity, which confirm hiPSC-MN maturation. These findings are supported by immunocytochemistry and real-time PCR studies in parallel cultures demonstrating human astrocyte mediated changes in the structural maturation and protein expression profiles of the neurons. Interestingly, this relationship is reciprocal and coculture with neurons influences astrocyte maturation as well. Taken together, these data indicate that in a human in vitro spinal cord culture system, astrocytes support hiPSC-MN maturation in a time-dependent and species-specific manner and suggest a closer approximation of in vivo conditions. Stem Cells Translational Medicine 2019;8:1272&1285.

Serial in vivo imaging of transplanted allogeneic neural stem cell survival in a mouse model of amyotrophic lateral sclerosis.

Neural stem cells (NSCs) are being investigated as a possible treatment for amyotrophic lateral sclerosis (ALS) through intraspinal transplantation, but no longitudinal imaging studies exist that describe the survival of engrafted cells over time. Allogeneic firefly luciferase-expressing murine NSCs (Luc+-NSCs) were transplanted bilaterally (100,000 cells/2µl) into the cervical spinal cord (C5) parenchyma of pre-symptomatic (63day-old) SOD1G93A ALS mice (n=14) and wild-type age-matched littermates (n=14). Six control SOD1G93A ALS mice were injected with saline. Mice were immunosuppressed using a combination of tacrolimus+sirolimus (1mg/kg each, i.p.) daily. Compared to saline-injected SOD1G93A ALS control mice, a transient improvement (p<0.05) in motor performance (rotarod test) was observed after NSC transplantation only at the early disease stage (weeks 2 and 3 post-transplantation). Compared to day one post-transplantation, there was a significant decline in bioluminescent imaging (BLI) signal in SOD1G93A ALS mice at the time of disease onset (71.7±17.9% at 4weeks post-transplantation, p<0.05), with a complete loss of BLI signal at endpoint (120day-old mice). In contrast, BLI signal intensity was observed in wild-type littermates throughout the entire study period, with only a 41.4±8.7% decline at the endpoint. In SOD1G93A ALS mice, poor cell survival was accompanied by accumulation of mature macrophages and the presence of astrogliosis and microgliosis. We conclude that the disease progression adversely affects the survival of engrafted murine Luc+-NSCs in SOD1G93A ALS mice as a result of the hostile ALS spinal cord microenvironment, further emphasizing the challenges that face successful cell therapy of ALS.

Human glial progenitor engraftment and gene expression is independent of the ALS environment.

Although Amyotrophic Lateral Sclerosis (ALS) is a motor neuron disease, basic research studies have highlighted that astrocytes contribute to the disease process. Therefore, strategies which replace the diseased astrocyte population with healthy astrocytes may protect against motor neuron degeneration. Our studies have sought to evaluate astrocyte replacement using glial-restricted progenitors (GRPs), which are lineage-restricted precursors capable of differentiating into astrocytes after transplantation. The goal of our current study was to evaluate how transplantation to the diseased ALS spinal cord versus a healthy, wild-type spinal cord may affect human GRP engraftment and selected gene expression. Human GRPs were transplanted into the spinal cord of either an ALS mouse model or wild-type littermate mice. Mice were sacrificed for analysis at either the onset of disease course or at the endstage of disease. The transplanted GRPs were analyzed by immunohistochemistry and NanoString gene profiling which showed no gross differences in the engraftment or gene expression of the cells. Our data indicate that human glial progenitor engraftment and gene expression is independent of the neurodegenerative ALS spinal cord environment. These findings are of interest given that human GRPs are currently in clinical development for spinal cord transplantation into ALS patients.

Gene profiling of human induced pluripotent stem cell-derived astrocyte progenitors following spinal cord engraftment.

The generation of human induced pluripotent stem cells (hiPSCs) represents an exciting advancement with promise for stem cell transplantation therapies as well as for neurological disease modeling. Based on the emerging roles for astrocytes in neurological disorders, we investigated whether hiPSC-derived astrocyte progenitors could be engrafted to the rodent spinal cord and how the characteristics of these cells changed between in vitro culture and after transplantation to the in vivo spinal cord environment. Our results show that human embryonic stem cell- and hiPSC-derived astrocyte progenitors survive long-term after spinal cord engraftment and differentiate to astrocytes in vivo with few cells from other lineages present. Gene profiling of the transplanted cells demonstrates the astrocyte progenitors continue to mature in vivo and upregulate a variety of astrocyte-specific genes. Given this mature astrocyte gene profile, this work highlights hiPSCs as a tool to investigate disease-related astrocyte biology using in vivo disease modeling with significant implications for human neurological diseases currently lacking animal models.

Altered astrocytic expression of TDP-43 does not influence motor neuron survival.

The role of glia as a contributing factor to motor neuron (MN) death in amyotrophic lateral sclerosis (ALS) is becoming increasingly appreciated. However, most studies implicating astrocytes have focused solely on models of ALS caused by superoxide dismutase 1 (SOD1) mutations. The goal of our study was to determine whether astrocytes contribute to wild-type MN death in the case of ALS caused by mutations in tar-DNA binding protein 43 (TDP-43). Since it is currently unknown how TDP-43 mutations cause disease, we derived astrocytes for study from both gain and loss of function mouse models of TDP-43. Astrocytes overexpressing mutant TDP-43(A315T) as well as astrocytes lacking TDP-43 were morphologically indistinguishable from wild-type astrocytes in vitro. Furthermore, astrocytes with these TDP-43 alterations did not cause the death of wild-type MNs in co-culture. To investigate the in vivo effects of TDP-43 alterations in astrocytes, glial-restricted precursors were transplanted to the wild-type rat spinal cord where they differentiated into astrocytes and interacted with host MNs. Astrocytes with TDP-43 alterations did not cause host wild-type MN damage although they were capable of engrafting and interacting with host MNs with the same efficiency as wild-type astrocytes. These data indicate that astrocytes do not adopt the same toxic phenotype as mutant SOD1 astrocytes when TDP-43 is mutated or expression levels are modified. Our study reinforces the heterogeneity in ALS disease mechanisms and highlights the potential for future screening subsets of ALS patients prior to treatment with cell type-directed therapies.