Spinal Muscolar Atrophy (SMA) is the most common motor neuron disease in children.

ABSTRACT

Spinal Muscolar Atrophy (SMA) is the most common motor neuron disease in children.

SMA is a neurogenetic autosomal recessive disorder characterized by degeneration of the motor neurons of spinal cord, associated with muscular atrophy and paralysis. Based on aged of onset and disease severity, SMA have been subdivided into four clinical types (I-IV). At present, the molecular mechanisms of the disease remain unknown and no theraphies exist. All SMA types are caused by reduced levels of the Survival Motor Neuron (SMN) protein. In humans this protein derives from two highly homologous genes (SMN1 and SMN2) placed on chromosome 5. These two genes differs only for five nucleotides, and one of them is localized on exon 7. This nucleotide is responsible for alternative splicing of this exon. SMN1 encodes for a full lenght SMN protein, while SMN2 encodes for a full lenght protein only in 10% of cases. In 90% of cases SMN2 encodes for an highly ustable protein, lacking the portion encoded by exon 7. In the 98% of SMA cases SMN1 gene presents mutations, while SMN2 gene is unchanged in all patients. However, the protein encoded by SMN2 gene is unable to compensate the SMN full lenght’s deficit.

SMN is the most important therapeutic target for development of an effective treatment for SMA. For example, in vitro studies have demonstrated that valproic acid, which is a mood stabilizing drug and which inhibits glycogen synthase kinase 3beta (GSK-3beta) protein, increases SMN levels in fibroblasts of SMA patients. In vivo, valproic acid increases SMN levels in spinal cords of mice models of SMA, and delays disease progression.

In this study we investigated the effect of lithium (another mood stabilizing drug that

inhibits GSK-3beta) on the disease progression in a mice model of SMA-III.

In order to investigate the protective effect of lithium we used a mouse model of slow desease progression (SMA-III; SMN1A2G

SMN2

SMN

, knock out double transgenic).

We treated SMA-III mice with lithium (1 mEq/kg) every other days for about 15 months. During this time interval behavioural tests were performed in order to evaluate lithium effects on muscular strength of mice.

Mice were sacrificed and spinal cord analysed by means light microscopy, transmission electron microscopy and Western Blot analysis.

We found that lithium delayed disease progression. Moreover, lithium reduced motor neurons loss and it preserved neuron size. Immunocytochemistry and Western Blot investigations confirmed an increase of SMN levels in spinal cord after lithium treatment.

These results demonstrate that lithium has protective role in contrasting disease progression. Furthermore, modulation of SMN expression by lithium has been evaluated in PC12 cells, which represents a simple model of neuron. We demonstrated that SMN levels are incresed by lithium treatment also in PC12 cells.

These data indicated that lithium treatment induced an increase of SMN concomitantly

with a reduction of neurological impairment. This effect suggests that SMN levels are

increased by inhibition of GSK-3beta and lithium is known to possess such an effect.