Impairment of the neurotrophic signaling hub B-Raf contributes to motoneuron degeneration in spinal muscular atrophy

Abstract

Spinal muscular atrophy (SMA) is a motoneuron disease caused by deletions of the Survival of Motoneuron 1 gene (SMN1) and low SMN protein levels. SMN restoration is the concept behind a number of recently approved drugs which result in impressive yet limited effects. Since SMN has already been enhanced in treated patients, complementary SMN-independent approaches are needed. Previously, a number of altered signaling pathways which regulate motoneuron degeneration have been identified as candidate targets. However, signaling pathways form networks, and their connectivity is still unknown in SMA. Here, we used presymptomatic SMA mice to elucidate the network of altered signaling in SMA. The SMA network is structured in two clusters with AKT and 14-3-3 ζ/δ in their centers. Both clusters are connected by B-Raf as a major signaling hub. The direct interaction of B-Raf with 14-3-3 ζ/δ is important for an efficient neurotrophic activation of the MEK/ERK pathway and crucial for motoneuron survival. Further analyses in SMA mice revealed that both proteins were down-regulated in motoneurons and the spinal cord with B-Raf being reduced at presymptomatic stages. Primary fibroblasts and iPSC-derived motoneurons from SMA patients both showed the same pattern of down-regulation. This mechanism is conserved across species since a Caenorhabditis elegans SMA model showed less expression of the B-Raf homolog lin-45. Accordingly, motoneuron survival was rescued by a cell autonomous lin-45 expression in a C. elegans SMA model resulting in improved motor functions. This rescue was effective even after the onset of motoneuron degeneration and mediated by the MEK/ERK pathway.

What Genetics Has Told Us and How It Can Inform Future Experiments for Spinal Muscular Atrophy, a Perspective

“…In addition, while sensory defects have been reported in SMA mice, it is not clear there is death of sensory neurons in mice or SMA patients. Lastly, the importance of the B-Raf pathway was confirmed using the C. elegans model of SMA and has yet to be replicated in mice or iPS cells [194]. We stress the importance of translating these experiments into mammalian models.…”

“…Currently we would say that targets are ill-defined and unclear in all cases. An example is the recently reported B-Raf which is implied to be critically impaired and result in motor neuron death [194]. The primary evidence given is that the neurotrophic pathway that supports motor neurons is reduced in presymptomatic mice.…”

Protein Network Analysis Reveals a Functional Connectivity of Dysregulated Processes in ALS and SMA

“… 8 , 9 Thus, we searched for studies which matched with the following terms: “ motoneuron degeneration ” AND “ rescue ” AND “ SMN-independent .” Two results were identified in PubMed search, including one paper with an unbiased approach focusing on alterations and rescue approaches in several SMA models. 22 , 46 In SMA, neurotrophic signaling is dysregulated in a network, in which the serine/threonine kinase B-Raf is central. 22 Next to B-Raf, its binding partner 14-3-3 ζ/δ, which was also shown to be downregulated in SMA, was added to the protein network list.…”

“… 22 Next to B-Raf, its binding partner 14-3-3 ζ/δ, which was also shown to be downregulated in SMA, was added to the protein network list. 22 …”

High Concentration of an ISS-N1-Targeting Antisense Oligonucleotide Causes Massive Perturbation of the Transcriptome

“…In more than 95% of instances, SMA is caused by low levels of the Survival Motor Neuron (SMN) protein due to deletions of or mutations in the SMN1 gene [9]. SMN is a multifunctional protein involved in most aspects of cellular metabolism, such as DNA damage repair, transcription, snRNP biogenesis, splicing, translation, selenoprotein synthesis, stress granule formation, macromolecular trafficking, signaling pathways, and cytoskeletal dynamics [10][11][12][13][14][15][16]. Therefore, low levels of SMN affect most tissues, including bone, brain, gastrointestinal tract, heart, kidney, liver, lung, muscle, ovary, spleen, spinal cord, and testis [8].…”