Plastin 3 ameliorates spinal muscular atrophy via delayed axon pruning and improves neuromuscular junction functionality.

F-actin bundling plastin 3 (PLS3) is a fully protective modifier of the neuromuscular disease spinal muscular atrophy (SMA), the most common genetic cause of infant death. The generation of a conditional PLS3-over-expressing mouse and its breeding into an SMA background allowed us to decipher the exact biological mechanism underlying PLS3-mediated SMA protection. We show that PLS3 is a key regulator that restores main processes depending on actin dynamics in SMA motor neurons (MNs). MN soma size significantly increased and a higher number of afferent proprioceptive inputs were counted in SMAPLS3 compared with SMA mice. PLS3 increased presynaptic F-actin amount, rescued synaptic vesicle and active zones content, restored the organization of readily releasable pool of vesicles and increased the quantal content of the neuromuscular junctions (NMJs). Most remarkably, PLS3 over-expression led to a stabilization of axons which, in turn, resulted in a significant delay of axon pruning, counteracting poor axonal connectivity at SMA NMJs. These findings together with the observation of increased endplate and muscle fiber size upon MN-specific PLS3 over-expression suggest that PLS3 significantly improves neuromuscular transmission. Indeed, ubiquitous over-expression moderately improved survival and motor function in SMA mice. As PLS3 seems to act independently of Smn, PLS3 might be a potential therapeutic target not only in SMA but also in other MN diseases.

Severe SMA mice show organ impairment that cannot be rescued by therapy with the HDACi JNJ-26481585.

Spinal muscular atrophy (SMA) is the leading genetic cause of early childhood death worldwide and no therapy is available today. Many drugs, especially histone deacetylase inhibitors (HDACi), increase SMN levels. As all HDACi tested so far only mildly ameliorate the SMA phenotype or are unsuitable for use in humans, there is still need to identify more potent drugs. Here, we assessed the therapeutic power of the pan-HDACi JNJ-26481585 for SMA, which is currently used in various clinical cancer trials. When administered for 64 h at 100 nM, JNJ-26481585 upregulated SMN levels in SMA fibroblast cell lines, including those from non-responders to valproic acid. Oral treatment of Taiwanese SMA mice and control littermates starting at P0 showed no overt extension of lifespan, despite mild improvements in motor abilities and weight progression. Many treated and untreated animals showed a very rapid decline or unexpected sudden death. We performed exploratory autopsy and histological assessment at different disease stages and found consistent abnormalities in the intestine, heart and lung and skeletal muscle vasculature of SMA animals, which were not prevented by JNJ-26481585 treatment. Interestingly, some of these features may be only indirectly caused by α-motoneuron function loss but may be major life-limiting factors in the course of disease. A better understanding of - primary or secondary - non-neuromuscular organ involvement in SMA patients may improve standard of care and may lead to reassessment of how to investigate SMA patients clinically.

SAHA ameliorates the SMA phenotype in two mouse models for spinal muscular atrophy.

Proximal spinal muscular atrophy (SMA) is a common autosomal recessively inherited neuromuscular disorder determined by functional impairment of alpha-motor neurons within the spinal cord. SMA is caused by functional loss of the survival motor neuron gene 1 (SMN1), whereas disease severity is mainly influenced by the number of SMN2 copies. SMN2, which produces only low levels of full-length mRNA/protein, can be modulated by small molecules and drugs, thus offering a unique possibility for SMA therapy. Here, we analysed suberoylanilide hydroxamic acid (SAHA), a FDA-approved histone deacetylase inhibitor, as potential drug in two severe SMA mouse models each carrying two SMN2 transgenes: US-SMA mice with one SMN2 per allele (Smn(-/-);SMN2(tg/tg)) and Taiwanese-SMA mice with two SMN2 per allele (Smn(-/-);SMN2(tg/wt)), both on pure FVB/N background. The US-SMA mice were embryonically lethal with heterozygous males showing significantly reduced fertility. SAHA treatment of pregnant mothers rescued the embryonic lethality giving rise to SMA offspring. By using a novel breeding strategy for the Taiwanese model (Smn(-/-);SMN2(tg/tg) x Smn(-/+) mice), we obtained 50% SMA offspring that survive approximately 10 days and 50% control carriers in each litter. Treatment with 25 mg/kg twice daily SAHA increased lifespan of SMA mice by 30%, significantly improved motor function abilities, reduced degeneration of motor neurons within the spinal cord and increased the size of neuromuscular junctions and muscle fibers compared with vehicle-treated SMA mice. SMN RNA and protein levels were significantly elevated in various tissues including spinal cord and muscle. Hence, SAHA, which lessens the progression of SMA, might be suitable for SMA therapy.

her1 and her13.2 are jointly required for somitic border specification along the entire axis of the fish embryo.

Delta-Notch and FGF signaling are involved in the control of somitogenesis in zebrafish. her genes are generally known as downstream targets of Delta-Notch signaling, but the her13.2 gene from zebrafish has recently been shown to depend on FGF signaling only. We have here studied the functional role of her13.2 in conjunction with her genes that are under Delta-Notch control. We show that joint inactivation of her1 and her13.2 leads to a complete loss of all somitic borders, including the most anterior ones. This somitic phenotype is much stronger than would be expected from the effects of the inactivation of either gene alone. A joint inactivation of her13.2 and her7, which is a paralogue of her1, does not show this enhanced effect. Thus, our results confirm inferences from in vitro studies that her1 and her13.2 form specific heterodimers, which may directly be required for regulating further target genes. These two her genes thus constitute the link between Delta-Notch pathway and FGF signaling during entire somitogenesis. We show that this interaction is conserved in the rice fish medaka, as a joint inactivation of the respective orthologues leads also to the same phenotype as in zebrafish. In addition, our results suggest that the mechanisms for anterior and posterior somite formation are not principally different, although the anterior somites often seem more refractory to genetic perturbations.