Spinal muscular atrophy (SMA) is an autosomal recessive disease of childhood due to loss of the telomeric survival motor neuron gene, SMN1. The general functions of the main SMN1 protein product, full-length SMN (FL-SMN), do not explain the selective motoneuronal loss of SMA. We identified axonal-SMN (a-SMN), an alternatively spliced SMN form, preferentially encoded by the SMN1 gene in humans. The a-SMN transcript and protein are down-regulated during early development in different tissues. In the spinal cord, the a-SMN protein is selectively expressed in motor neurons and mainly localized in axons. Forced expression of a-SMN stimulates motor neuron axonogenesis in a time-dependent fashion and induces axonal-like growth in non-neuronal cells. Exons 2b and 3 are essential for the axonogenic effects. This discovery indicates an unexpected complexity of the SMN gene system and may help in understanding the pathogenesis of SMA.alternative splicing ͉ neurodegeneration ͉ spinal muscular atrophy ͉ intron retention ͉ axonal sprouting S pinal muscular atrophy (SMA) is an autosomal recessive disease of childhood causing selective motor neuron death. SMA is the leading genetic cause of infant mortality, with an incidence of 1:10,000 and a carrier frequency of 1:50 (1). The telomeric survival motor neuron gene (SMN1) is the SMA disease gene, and the duplicated centromeric gene, SMN2, governs the severity of the disease (2, 3). Current knowledge suggests that SMN1 codes for a single functional protein [full-length SMN (FL-SMN)], and the major product of SMN2 is ⌬7-SMN (lacking the C terminus), an unstable protein of minor significance (4, 5).The FL-SMN protein is expressed in the cytoplasm and nucleus of all cells (6, 7) and is involved in spliceosomal assembly and pre-mRNAs maturation (8-10). Other proposed roles include general cellular functions (11-15). How reduced FL-SMN levels lead to selective degeneration of motor neurons in SMA remains undetermined. FL-SMN may serve motor neuron-specific functions, perhaps interacting with partner proteins selectively expressed in these cells. However, these functions and the partner proteins are still not known. Additional protein products of the SMN genes have been postulated (16)(17)(18)(19). The identification of SMN proteins acting specifically in motor neurons would be a major step forward in understanding SMA.We identify and characterize an SMN transcript (GenBank accession no. AY876898) originating from the retention of SMN intron 3 and encoding the axonal-SMN (a-SMN) protein. In the spinal cord, a-SMN is selectively expressed in developing motor neurons and mainly localized in axons. This is confirmed by overexpression experiments in cultured cells. These also demonstrate the following: (i) a-SMN stimulates motor neuron axonogenesis in a time-dependent fashion, and (ii) a-SMN induces axonal-like growth in non-neuronal cells such as HeLa. The specific localization and function suggest a role in motor neurons. Because human a-SMN is a specific product of the SMN1 gene, its lo...

We have generated an experimental 'double-hit' model of chronic epilepsy to recapitulate the co-existence of abnormal cortical structure and frequently recurrent seizures as observed in human focal cortical dysplasia. We induced cortical malformations by exposing rats prenatally to methylazoxymethanol acetate and triggered status epilepticus and recurrent seizures in adult methylazoxymethanol acetate rats with pilocarpine. We studied the course of epilepsy and the long-term morphologic and molecular changes induced by the occurrence of status epilepticus and subsequent chronic epilepsy in the malformed methylazoxymethanol acetate exposed brain. Behavioural and electroencephalographic analyses showed that methylazoxymethanol acetate pilocarpine rats develop more severe epilepsy than naïve rats. Morphologic and molecular analyses demonstrated that status epilepticus and subsequent seizures, but not pilocarpine treatment per se, was capable of affecting both cortical architectural and N-methyl-D-aspartate receptor abnormalities induced by methylazoxymethanol acetate. In particular, cortical thickness was further decreased and N-methyl-D-aspartate regulatory subunits were recruited at the postsynaptic membrane. In addition, methylazoxymethanol acetate pilocarpine rats showed abnormally large cortical pyramidal neurons with neurofilament over-expression. These neurons bear similarities to the hypertrophic/dysmorphic pyramidal neurons observed in acquired human focal cortical dysplasia. These data show that status epilepticus sets in motion a pathological process capable of significantly changing the cellular and molecular features of pre-existing experimental cortical malformations. They suggest that seizure recurrence in human focal cortical dysplasia might be an additional factor in establishing a pathological circuitry that favours chronic neuronal hyperexcitability.

Loss of the survival motor neuron gene (SMN1) is responsible for spinal muscular atrophy (SMA), the most common inherited cause of infant mortality. Even though the SMA phenotype is traditionally considered as related to spinal motor neuron loss, it remains debated whether the specific targeting of motor neurons could represent the best therapeutic option for the disease. We here investigated, using stereological quantification methods, the spinal cord and cerebral motor cortex of ∆7 SMA mice during development, to verify extent and selectivity of motor neuron loss. We found progressive post-natal loss of spinal motor neurons, already at pre-symptomatic stages, and a higher vulnerability of motor neurons innervating proximal and axial muscles. Larger motor neurons decreased in the course of disease, either for selective loss or specific developmental impairment. We also found a selective reduction of layer V pyramidal neurons associated with layer V gliosis in the cerebral motor cortex. Our data indicate that in the ∆7 SMA model SMN loss is critical for the spinal cord, particularly for specific motor neuron pools. Neuronal loss, however, is not selective for lower motor neurons. These data further suggest that SMA pathogenesis is likely more complex than previously anticipated. The better knowledge of SMA models might be instrumental in shaping better therapeutic options for affected patients.

To investigate hypothesized effects of severe epilepsy on malformed cortex, we analyzed surgical samples from eight patients with type IIB focal cortical dysplasia (FCD) in comparison with samples from nine non-dysplastic controls. We investigated, using stereological quantification methods, where appropriate, dysplastic neurons, neuronal density, balloon cells, glia, glutamatergic synaptic input, and the expression of N-methyl-D-aspartate (NMDA) receptor subunits and associated membrane-associated guanylate kinase (MAGUK). In all FCD patients, the dysplastic areas giving rise to epileptic discharges were characterized by larger dysmorphic neurons, reduced neuronal density, and increased glutamatergic inputs, compared to adjacent areas with normal cytology. The duration of epilepsy was found to correlate directly (a) with dysmorphic neuron size, (b) reduced neuronal cell density, and (c) extent of reactive gliosis in epileptogenic/dysplastic areas. Consistent with increased glutamatergic input, western blot revealed that NMDA regulatory subunits and related MAGUK proteins were up-regulated in epileptogenic/dysplastic areas of all FCD patients examined. Taken together, these results support the hypothesis that epilepsy itself alters morphology-and probably also function-in the malformed epileptic brain. They also suggest that glutamate/NMDA/MAGUK dysregulation might be the intracellular trigger that modifies brain morphology and induces cell death.

Background: Axonal SMN is a truncated product of the spinal muscular atrophy (SMA) disease gene SMN1.Results: Forced expression of axonal SMN in motoneuron-like NSC34 cells modulates growth, axonogenesis, and motility.Conclusion: Axonal SMN induces CCL2/CCL7 chemokines and the IGF-1 growth factor. CCL2 contributes to axonal SMN-induced motility and axonogenesis.Significance: Insights into the function and underlying mechanisms with relevance for axonal SMN in SMA are provided.