Oprisoreanu, A.-M. | bioRxiv Science

bioRxiv2026

Tachykinin neuropeptides are involved in axonal and synaptic differentiation of the pioneer motor axon in zebrafish

In contrast to many other developing systems, in which axon pathfinding and synaptogenesis are separated in time, the pioneering axon of the individually identifiable caudal primary motor neuron in embryonic zebrafish forms en passant synapses during its stereotypical ventral growth. How simultaneous synaptic differentiation and axon pathfinding are coordinated is not fully understood. Here we ask what the role of the tac1 gene, coding for the synaptic tachykinin neuropeptides, is in this unique axon differentiation process. The gene is expressed during axon outgrowth and its disruption results in increased branch length of CaP axons and subtle morphological defects of the pre-synapse. These abnormalities are accompanied by a robust [~]1.5-fold increase in motor neuron activity and in spontaneous early contractions in tac1-deficient embryos. Furthermore, pharmacological inhibition of the tachykinin receptor (Tacr1) leads to altered CaP axonal morphology, mimicking the axonal phenotype observed in tac1-deficient zebrafish. These findings suggest that tachykinin neuropeptides modulate formation and activity of en passant synapses and prevent aberrant axon branching during growth of zebrafish motor axons. HIGHLIGHTS- tac1 refines CaP primary motor axon development in zebrafish - Loss of tac1 disrupts presynaptic maturation at the horizontal myoseptum - tac1 mutants show elevated motor neuron activity and spontaneous contractions

neuroscience↗

bioRxiv2025

Altered lysosomal biology impairs motor neuron survival via TFEB dysregulation in spinal muscular atrophy

Spinal muscular atrophy (SMA) is a devastating motor neuron disease, caused by recessive mutations or deletions of the SMN1 gene, representing the leading genetic cause of infant mortality. Available therapies, aimed at increasing SMN protein levels, can only partially halt motor neuron (MN) degeneration in a select number of patients, reinforcing the need for combinatorial treatments to improve clinical outcomes. We previously showed that mTORC1 overactivation and impaired autophagosome clearance in SMA MNs lead to the accumulation of protein aggregates, contributing to MN degeneration. However, the mechanistic link between SMN protein deficiency and autophagy-lysosomal dysfunction remained unknown. Here, using patient iPSC-derived MNs along with isogenic and healthy controls, we show that SMA MNs exhibit reduced lysosome numbers and impaired functionality. Furthermore, the master regulator of lysosomal biogenesis and autophagy, TFEB, is downregulated, and its nuclear translocation compromised upon SMN deficiency. We further propose the upregulation of the mTORC1 positive modulator TPT1 as contributor to TFEB dysregulation. Notably, TFEB overexpression ameliorates protein aggregate accumulation in SMA MNs and enhances MN survival both in vitro and in a zebrafish SMA model. Our findings identify lysosomal dysfunction as a key player in SMA pathology and highlight TFEB activation as a potential therapeutic strategy for SMA treatment. One Sentence SummaryTFEB activation restores lysosomal function and improves motor neuron survival in SMA, highlighting its potential as a therapeutic target.

neuroscience↗