Axonal Transport in SMALED, a Motor Neuron Disease – ATOMy

Axonal transport defects are observed in many neurological disorders. Analysis of animal models showed that impaired transport can cause neurodegeneration or impinge on the development of neural circuits. In support of such a role in human, mutations in DYNC1H1, that encodes the heavy chain of dynein, and in its partner BICD2, two core components of the intracellular transport machinery, cause a rare neurodevelopmental motor neuron (MN) disease termed Spinal Muscular Atrophy with Lower Extremity Dominance (SMALED) in which leg-innervating MNs are preponderantly affected. This observation suggests a graded reliance on axonal transport among neuronal subtypes and that MN, and even MN subtypes, might be the most vulnerable cells to defects in this pathway. As axonal transport defects are observed in many neurological disorders, approaching the consequences of mutations in genes coding for core components of the axonal transport machinery on MN subtype development might shed lights not only on rare MN diseases but more broadly provide information on the specific roles of transport in disease. However, the full clinical spectrum of SMALED and its natural history remain unknown causing important limitations for genetic counselling and future therapeutic strategies. Furthermore, it remains undetermined i) whether these mutations affect the axonal transport of cargoes and ii) how transport defects selectively impact leg-MN development. Such a lack of knowledge is a significant setback for the development of therapeutic strategies for SMALED in part due to the difficulties in accessing differentially affected human MN subtypes bearing SMALED-causing mutations for comparative cell biology and transcriptomic studies.
We thus developed a consortium composed of clinicians, cell and stem cell biologists to study the clinical, cellular and molecular basis of SMALED. We identified the largest European cohort of SMALED patients. We will determine the spectrum and the natural history of SMALED, clarify the phenotypic heterogeneity of early-onset SMALED and the genotype-phenotype correlations (Aim1). We derived human induced pluripotent stem cells (hiPS) from patients with BICD2 and DYNC1H1 SMALED mutations and developed new methods to differentiate them in the most affected leg-innervating MNs. Based on our expertise in live imaging of intracellular trafficking, we will determine whether SMALED mutations induce defects in vesicular axonal trafficking, in particular in the retrograde signalling of target derived-neurotrophic factors that normally supports motor circuit development and MN survival. We will test if these defects can be rescued by drugs modulating dynein activity (Aim2). Using iPS derived-MNs and a new SMALED mouse model, we will test whether SMALED mutations impact selectively on leg-MN specification, outgrowth and/or survival. We will define the deregulated pathways in MN subpopulations using single cell transcriptomic and test whether modulating deregulated genes can alleviate cell phenotypes. Finally, we will test whether inhibition of dynein can alleviate the symptoms of the mouse model (Aim 3).
The overarching aim of this study is to determine how mutations in core components of the transport machinery impact on axonal transport and leads to the selective vulnerability of leg-innervating MN. The refined cellular and molecular models generated will then be instrumental to develop future drug screening assays before testing potential therapeutic strategies. Importantly, our study will bring basic information on human MN diversity as well as conceptual framework and methods to study this diversity in health and disease. Considering that the differential sensitivity of MN subtypes is a hallmark of all MN diseases and that defect in axonal transport is observed in many neurological disorders, the clinical, technical and biological answers that we will bring will be applicable beyond SMALED diseases.