Periodic Reporting for period 1 - SpecDroHuman (Impact of α-spectrin mutations on the cytoskeleton and organelle organization in neurodegeneration)
Période du rapport: 2023-09-01 au 2025-08-31
Across the European Union, approximately one million people are affected by rare inherited neurological disorders, often leading to lifelong disability and imposing a significant socio-economic burden. A common feature of many neurodegenerative diseases is the progressive loss of axonal integrity. Axons are particularly vulnerable due to their dependence on an organized cytoskeleton and efficient transport systems to maintain structure and deliver essential cellular components.
The gene SPTAN1, encoding α-II-spectrin (α-spectrin), has emerged as a key player in several neurogenetic disorders. Over 60 pathogenic variants have been linked to conditions including developmental and epileptic encephalopathies (DEE), hereditary motor neuropathy (HMN), hereditary spastic paraplegia (HSP), spinocerebellar ataxia (SCA), and distal myopathy with neurogenic features. Similar phenotypes are associated with mutations in genes encoding β-spectrin isoforms (SPTBN1, SPTBN2, SPTBN4), collectively termed spectrinopathies. Additionally, α-spectrin breakdown products have been detected in a range of neurological conditions and after traumatic brain injury, suggesting a role as biomarkers of neuronal damage. Despite this growing body of evidence, the mechanisms by which spectrin dysfunction contributes to disease remain poorly understood, and no effective treatments are available.
Spectrin, together with actin, forms a periodic, highly organized lattice beneath the axonal membrane. This cytoskeletal scaffold, visualized through super-resolution microscopy, provides mechanical stability and spatial organization for key proteins involved in signal transmission and adhesion. Notably, this structure is dynamic and remodels in response to growth factors, injury, or pharmacological stimuli—changes that may precede or drive disease onset. Spectrin proteins contain structural domains (spectrin repeats) that mediate interactions with one another and with other cytoskeletal elements. Specifically, α-spectrin and β-spectrin first form dimers, which then assemble into tetramers anchored to actin, establishing the core framework of the axonal cytoskeleton. In disorders caused by SPTAN1 mutations, we aim to elucidate how pathogenic variants affect α-spectrin’s structure, stability, and integration into these higher-order assemblies—and how these alterations impair neuronal function and contribute to disease progression.
In Drosophila melanogaster, the single α-spectrin gene is essential for neural development and synapse stability, making it a powerful tool for in vivo functional studies. In flies, synaptic retraction due to α-spectrin loss-of-function can be rescued by manipulating regulators of organelle trafficking, suggesting these phenotypes are reversible and linked to intracellular transport mechanisms. Moreover, increasing α-spectrin levels mitigates neuronal defects in a fly model of α-synuclein-induced neurodegeneration involving mitochondrial dysfunction. These findings highlight the interplay between spectrin function and organelle positioning and point to underexplored therapeutic targets. To build on these insights, we combine Drosophila with iPSC-derived neuronal models to investigate how SPTAN1 mutations disrupt α-spectrin structure and neuronal organellar quality control, aiming to uncover disease mechanisms and identify potential treatments.
Pathogenic SPTAN1 variants are linked to a broad spectrum of neurological disorders, reflecting the diverse structural and functional roles of α-spectrin across neuronal subtypes. To address this complexity, we will investigate three representative variants: (1) a recurrent HSP-associated missense mutation, (2) a loss-of-function variant causing HMN, and (3) a substitution linked to SCA and intellectual disability. These mutations, distributed across the protein, are located in domains that potentially impair oligomerization, increase susceptibility to proteolysis, and trigger nonsense-mediated mRNA decay. Together, they provide a robust platform to dissect how α-spectrin dosage, structure, and protein interactions maintain neuronal integrity.
The gene SPTAN1, encoding α-II-spectrin (α-spectrin), has emerged as a key player in several neurogenetic disorders. Over 60 pathogenic variants have been linked to conditions including developmental and epileptic encephalopathies (DEE), hereditary motor neuropathy (HMN), hereditary spastic paraplegia (HSP), spinocerebellar ataxia (SCA), and distal myopathy with neurogenic features. Similar phenotypes are associated with mutations in genes encoding β-spectrin isoforms (SPTBN1, SPTBN2, SPTBN4), collectively termed spectrinopathies. Additionally, α-spectrin breakdown products have been detected in a range of neurological conditions and after traumatic brain injury, suggesting a role as biomarkers of neuronal damage. Despite this growing body of evidence, the mechanisms by which spectrin dysfunction contributes to disease remain poorly understood, and no effective treatments are available.
Spectrin, together with actin, forms a periodic, highly organized lattice beneath the axonal membrane. This cytoskeletal scaffold, visualized through super-resolution microscopy, provides mechanical stability and spatial organization for key proteins involved in signal transmission and adhesion. Notably, this structure is dynamic and remodels in response to growth factors, injury, or pharmacological stimuli—changes that may precede or drive disease onset. Spectrin proteins contain structural domains (spectrin repeats) that mediate interactions with one another and with other cytoskeletal elements. Specifically, α-spectrin and β-spectrin first form dimers, which then assemble into tetramers anchored to actin, establishing the core framework of the axonal cytoskeleton. In disorders caused by SPTAN1 mutations, we aim to elucidate how pathogenic variants affect α-spectrin’s structure, stability, and integration into these higher-order assemblies—and how these alterations impair neuronal function and contribute to disease progression.
In Drosophila melanogaster, the single α-spectrin gene is essential for neural development and synapse stability, making it a powerful tool for in vivo functional studies. In flies, synaptic retraction due to α-spectrin loss-of-function can be rescued by manipulating regulators of organelle trafficking, suggesting these phenotypes are reversible and linked to intracellular transport mechanisms. Moreover, increasing α-spectrin levels mitigates neuronal defects in a fly model of α-synuclein-induced neurodegeneration involving mitochondrial dysfunction. These findings highlight the interplay between spectrin function and organelle positioning and point to underexplored therapeutic targets. To build on these insights, we combine Drosophila with iPSC-derived neuronal models to investigate how SPTAN1 mutations disrupt α-spectrin structure and neuronal organellar quality control, aiming to uncover disease mechanisms and identify potential treatments.
Pathogenic SPTAN1 variants are linked to a broad spectrum of neurological disorders, reflecting the diverse structural and functional roles of α-spectrin across neuronal subtypes. To address this complexity, we will investigate three representative variants: (1) a recurrent HSP-associated missense mutation, (2) a loss-of-function variant causing HMN, and (3) a substitution linked to SCA and intellectual disability. These mutations, distributed across the protein, are located in domains that potentially impair oligomerization, increase susceptibility to proteolysis, and trigger nonsense-mediated mRNA decay. Together, they provide a robust platform to dissect how α-spectrin dosage, structure, and protein interactions maintain neuronal integrity.
To investigate the impact of disease-associated SPTAN1 mutations, we generated Drosophila strains expressing wild-type and the disease-associated mutant forms of α-spectrin and used neuronal RNAi to induce loss-of-function (LoF). These models allow us to assess key aspects of neuronal health, including viability, synaptic morphology, and α-spectrin localization at nerve terminals. We further examined synaptic retraction phenotypes—an established marker of neural dysfunction in flies. We analyzed brain lysates to evaluate basal α-spectrin expression levels and the stability of mutant proteins. In parallel, we are examining the organization of other key cytoskeletal components in these neurons to determine how α-spectrin dysfunction affects broader structural integrity and intracellular transport mechanisms.
We employed CRISPR/Cas9-engineered, isogenic iPSC lines carrying heterozygous SPTAN1 variants linked to hereditary spastic paraplegia (HSP) and hereditary motor neuropathy (HMN), alongside matched controls. These lines are differentiated into motor neurons (MNs) using a small-molecule protocol that mimics in vivo developmental cues—utilizing retinoic acid (RA) for caudalization, SAG or purmorphamine to activate the Sonic Hedgehog (SHH) pathway, and BDNF, GDNF, and CNTF to promote neuronal survival and maturation. Neuronal identity is confirmed by immunocytochemistry (ICC) for markers of motor neuron specification, neuronal maturation, and cholinergic identity. We assess overall MN morphology and quantify the expression of key cytoskeletal components. Organelle distribution, morphology, and interaction with spectrin are studied in fixed cells using co-staining with organelle-specific and spectrin antibodies. In parallel, we are also investigating cytoskeletal and organellar dynamics in live cells.
Main achievements: We have established complementary in vivo (Drosophila) and in vitro (iPSC-derived motor neuron) models to study the mechanisms underlying α-spectrinopathies. We are developing robust, quantitative readouts of disease-relevant neuronal defects, which will serve as platforms for testing candidate pharmacological interventions aimed at restoring neuronal function.
We employed CRISPR/Cas9-engineered, isogenic iPSC lines carrying heterozygous SPTAN1 variants linked to hereditary spastic paraplegia (HSP) and hereditary motor neuropathy (HMN), alongside matched controls. These lines are differentiated into motor neurons (MNs) using a small-molecule protocol that mimics in vivo developmental cues—utilizing retinoic acid (RA) for caudalization, SAG or purmorphamine to activate the Sonic Hedgehog (SHH) pathway, and BDNF, GDNF, and CNTF to promote neuronal survival and maturation. Neuronal identity is confirmed by immunocytochemistry (ICC) for markers of motor neuron specification, neuronal maturation, and cholinergic identity. We assess overall MN morphology and quantify the expression of key cytoskeletal components. Organelle distribution, morphology, and interaction with spectrin are studied in fixed cells using co-staining with organelle-specific and spectrin antibodies. In parallel, we are also investigating cytoskeletal and organellar dynamics in live cells.
Main achievements: We have established complementary in vivo (Drosophila) and in vitro (iPSC-derived motor neuron) models to study the mechanisms underlying α-spectrinopathies. We are developing robust, quantitative readouts of disease-relevant neuronal defects, which will serve as platforms for testing candidate pharmacological interventions aimed at restoring neuronal function.
This project provides the first integrated Drosophila and human iPSC-based models specifically targeting α-spectrin dysfunction in SPTAN1-related disorders. By identifying and quantifying disease-relevant cytoskeletal and organellar defects at cellular and subcellular levels, we go beyond current descriptive clinical knowledge and establish a mechanistic platform for therapeutic exploration. These models offer novel avenues for screening compounds that restore spectrin function or compensate for disrupted axonal transport. To ensure further uptake and success, key next steps include expanding compound screening efforts, securing collaborations with drug discovery partners, and exploring IP protection for any promising targets or assays. Future work will also involve translating these findings into mammalian in vivo models and assessing their relevance to other forms of hereditary axonopathies.