Neurocalcin Delta Suppression Protects against Spinal Muscular Atrophy in Humans and across Species by Restoring Impaired Endocytosis

Abstract

Homozygous SMN1 loss causes spinal muscular atrophy (SMA), the most common lethal genetic childhood motor neuron disease. SMN1 encodes SMN, a ubiquitous housekeeping protein, which makes the primarily motor neuron-specific phenotype rather unexpected. SMA-affected individuals harbor low SMN expression from one to six SMN2 copies, which is insufficient to functionally compensate for SMN1 loss. However, rarely individuals with homozygous absence of SMN1 and only three to four SMN2 copies are fully asymptomatic, suggesting protection through genetic modifier(s). Previously, we identified plastin 3 (PLS3) overexpression as an SMA protective modifier in humans and showed that SMN deficit impairs endocytosis, which is rescued by elevated PLS3 levels. Here, we identify reduction of the neuronal calcium sensor Neurocalcin delta (NCALD) as a protective SMA modifier in five asymptomatic SMN1-deleted individuals carrying only four SMN2 copies. We demonstrate that NCALD is a Ca-dependent negative regulator of endocytosis, as NCALD knockdown improves endocytosis in SMA models and ameliorates pharmacologically induced endocytosis defects in zebrafish. Importantly, NCALD knockdown effectively ameliorates SMA-associated pathological defects across species, including worm, zebrafish, and mouse. In conclusion, our study identifies a previously unknown protective SMA modifier in humans, demonstrates modifier impact in three different SMA animal models, and suggests a potential combinatorial therapeutic strategy to efficiently treat SMA. Since both protective modifiers restore endocytosis, our results confirm that endocytosis is a major cellular mechanism perturbed in SMA and emphasize the power of protective modifiers for understanding disease mechanism and developing therapies.

Infants Diagnosed with Spinal Muscular Atrophy and 4 SMN2 Copies through Newborn Screening – Opportunity or Burden?1

“…Whereas most patients with 4 and more copies of SMN2 develop either type 2 or type 3 SMA, there are 32 reported individuals belonging to 22 families in the literature who show no signs despite a homozygous deletion in the SMN1 gene [32][33][34][35][36][37][38]. Most of these patients have 3-5 SMN2 copies and other known but separate SMA modifiers as well.…”

Stasimon Contributes to the Loss of Sensory Synapses and Motor Neuron Death in a Mouse Model of Spinal Muscular Atrophy

“…Elucidation of the specific cellular deficits underlying Stasimon-dependent motor circuit pathology in SMA will require greater knowledge of the full breadth of Stasimon functions. It will also need to integrate previous, seemingly unrelated, findings linking dysregulation of endocytosis (Ackermann et al, 2013;Hosseinibarkooie et al, 2016;Janzen et al, 2018;Riessland et al, 2017) and UBA1/ GARS-dependent pathways (Shorrock et al, 2018) to the loss of SMA proprioceptive synapses into a coherent model. In summary, our work identifies Stasimon as a determinant of defective synaptic connectivity and function in the SMA sensorymotor circuit as well as an upstream trigger of a signaling cascade that feeds into p53 and the motor neuron cell death pathway through p38 MAPK activation.…”

Stasimon contributes to the loss of sensory synapses and motor neuron death in a mouse model of spinal muscular atrophy

“…Elucidation of the downstream molecular mechanisms underlying dysfunction and loss of proprioceptive synapses in SMA will require greater knowledge of the full breadth of Stasimon functions. It will also need to integrate previous, seemingly unrelated findings implicating dysregulation of endocytosis (Ackermann et al, 2013;Hosseinibarkooie et al, 2016;Janzen et al, 2018;Riessland et al, 2017) and UBA1/GARS-dependent pathways (Shorrock et al, 2018) in the loss of SMA proprioceptive synapses into a coherent model.…”