Mutations in the netrin-1 gene cause congenital mirror movements.

Netrin-1 is a secreted protein that was first identified 20 years ago as an axon guidance molecule that regulates midline crossing in the CNS. It plays critical roles in various tissues throughout development and is implicated in tumorigenesis and inflammation in adulthood. Despite extensive studies, no inherited human disease has been directly associated with mutations in NTN1, the gene coding for netrin-1. Here, we have identified 3 mutations in exon 7 of NTN1 in 2 unrelated families and 1 sporadic case with isolated congenital mirror movements (CMM), a disorder characterized by involuntary movements of one hand that mirror intentional movements of the opposite hand. Given the diverse roles of netrin-1, the absence of manifestations other than CMM in NTN1 mutation carriers was unexpected. Using multimodal approaches, we discovered that the anatomy of the corticospinal tract (CST) is abnormal in patients with NTN1-mutant CMM. When expressed in HEK293 or stable HeLa cells, the 3 mutated netrin-1 proteins were almost exclusively detected in the intracellular compartment, contrary to WT netrin-1, which is detected in both intracellular and extracellular compartments. Since netrin-1 is a diffusible extracellular cue, the pathophysiology likely involves its loss of function and subsequent disruption of axon guidance, resulting in abnormal decussation of the CST.

Non cell-autonomous role of DCC in the guidance of the corticospinal tract at the midline.

DCC, a NETRIN-1 receptor, is considered as a cell-autonomous regulator for midline guidance of many commissural populations in the central nervous system. The corticospinal tract (CST), the principal motor pathway for voluntary movements, crosses the anatomic midline at the pyramidal decussation. CST fails to cross the midline in Kanga mice expressing a truncated DCC protein. Humans with heterozygous DCC mutations have congenital mirror movements (CMM). As CMM has been associated, in some cases, with malformations of the pyramidal decussation, DCC might also be involved in this process in human. Here, we investigated the role of DCC in CST midline crossing both in human and mice. First, we demonstrate by multimodal approaches, that patients with CMM due to DCC mutations have an increased proportion of ipsilateral CST projections. Second, we show that in contrast to Kanga mice, the anatomy of the CST is not altered in mice with a deletion of DCC in the CST. Altogether, these results indicate that DCC controls CST midline crossing in both humans and mice, and that this process is non cell-autonomous in mice. Our data unravel a new level of complexity in the role of DCC in CST guidance at the midline.

Specific inhibition of the JNK pathway promotes locomotor recovery and neuroprotection after mouse spinal cord injury.

Limiting the development of secondary damage represents one of the major goals of neuroprotective therapies after spinal cord injury. Here, we demonstrate that specific JNK inhibition via a single intraperitoneal injection of the cell permeable peptide D-JNKI1 6h after lesion improves locomotor recovery assessed by both the footprint and the BMS tests up to 4 months post-injury in mice. JNK inhibition prevents c-jun phosphorylation and caspase-3 cleavage, has neuroprotective effects and results in an increased sparing of white matter at the lesion site. Lastly, D-JNKI1 treated animals show a lower increase of erythrocyte extravasation and blood brain barrier permeability, thus indicating protection of the vascular system. In total, these results clearly point out JNK inhibition as a promising neuroprotective strategy for preventing the evolution of secondary damage after spinal cord injury.

RAD51 haploinsufficiency causes congenital mirror movements in humans.

Congenital mirror movements (CMM) are characterized by involuntary movements of one side of the body that mirror intentional movements on the opposite side. CMM reflect dysfunctions and structural abnormalities of the motor network and are mainly inherited in an autosomal-dominant fashion. Recently, heterozygous mutations in DCC, the gene encoding the receptor for netrin 1 and involved in the guidance of developing axons toward the midline, have been identified but CMM are genetically heterogeneous. By combining genome-wide linkage analysis and exome sequencing, we identified heterozygous mutations introducing premature termination codons in RAD51 in two families with CMM. RAD51 mRNA was significantly downregulated in individuals with CMM resulting from the degradation of the mutated mRNA by nonsense-mediated decay. RAD51 was specifically present in the developing mouse cortex and, more particularly, in a subpopulation of corticospinal axons at the pyramidal decussation. The identification of mutations in RAD51, known for its key role in the repair of DNA double-strand breaks through homologous recombination, in individuals with CMM reveals a totally unexpected role of RAD51 in neurodevelopment. These findings open a new field of investigation for researchers attempting to unravel the molecular pathways underlying bimanual motor control in humans.

Specific JNK inhibition by D-JNKI1 protects Purkinje cells from cell death in Lurcher mutant mouse.

In the Lurcher mutant mouse (+/Lc), Purkinje cells (PCs) selectively die due to the mutation that converts alanine to threonine in the glutamate ionotropic receptor GRID 2, thus resulting in a constitutively leaky cation channel. This intrinsic cell death determines a target-dependent cell death of granule cells and olivary neurons and cerebellum cytoarchitecture is severely disrupted in the adult Lurcher mutant. Although the +/Lc mutant has been widely characterized, less is known about the molecules involved in +/Lc PC death. We, here, used organotypic cerebellar slice cultures from P0 mice to investigate the role of c-jun N-terminal kinase (JNK) in +/Lc PC death by using D-JNKI1 as very specific tool to inhibit its action. Our results showed that D-JNKI1 treatment increased the number of +/Lc PC at 14 DIV of 3.6-fold. Conversely, this specific JNK inhibitor cell permeable peptide did not increase PC number in +/+ treated versus untreated cultures. These results clearly indicate that JNK plays an important role in +/Lc PC mechanism of cell death.

Expression of netrin-1, slit-1 and slit-3 but not of slit-2 after cerebellar and spinal cord lesions.

To determine whether members of the Netrin-1 and Slit families and their receptors are expressed after central nervous system (CNS) injury, we performed in situ hybridization for netrin-1, slit-1, 2 and 3, and their receptors (dcc, unc5h-1, 2 and 3, robo-1, 2 and 3) 8 days, 2-3 months and 12-18 months after traumatic lesions of rat cerebellum. The expression pattern of these molecules was unchanged in axotomized Purkinje cells, whereas unc5h3 expression was upregulated in deafferented granule cells. Cells expressing slit-2 or dcc were never detected at the lesion site. By contrast, cells expressing netrin-1, slit-1 and slit-3, unc5h-1, 2 and 3, and robo-1, 2 and 3 (rig-1) could be detected at the cerebellar lesion site as soon as 8 days after injury. Expression of unc5h-2, robo-1, robo-2, slit-1 and slit-3 at the lesion site was maintained until 3 months, and up to 12-18 months for unc5h-1 and 3 and robo-3. Likewise, in the mouse spinal cord, netrin-1, slit-1 and slit-3 were also expressed at the lesion site 8 days after injury. Most of the cells expressing these mRNAs were located at the centre of the lesions, suggesting that they are macrophages/activated microglial cells (macrophagic cells) or meningeal fibroblastic cells. The macrophagic nature of most Netrin-1-positive cells and the macrophagic or fibroblastic nature of Robo-1-positive cells were corroborated by double staining. Thus, Netrin-1, Slits and their receptors may contribute to the regenerative failure of axons in the adult CNS by inhibiting axon outgrowth or by participating in the formation of the CNS scar.

Microglia promote the death of developing Purkinje cells.

The loss of neuronal cells, a prominent event in the development of the nervous system, involves regulated triggering of programmed cell death, followed by efficient removal of cell corpses. Professional phagocytes, such as microglia, contribute to the elimination of dead cells. Here we provide evidence that, in addition to their phagocytic activity, microglia promote the death of developing neurons engaged in synaptogenesis. In the developing mouse cerebellum, Purkinje cells die, and 60% of these neurons that already expressed activated caspase-3 were engulfed or contacted by spreading processes emitted by microglial cells. Apoptosis of Purkinje cells in cerebellar slices was strongly reduced by selective elimination of microglia. Superoxide ions produced by microglial respiratory bursts played a major role in this Purkinje cell death. Our study illustrates a mammalian form of engulfment-promoted cell death that links the execution of neuron death to the scavenging of dead cells.

Sprouting of adult Purkinje cell axons in lesioned mouse cerebellum: "non-permissive" versus "permissive" environment.

Adult rat Purkinje cells are extremely resistant to axotomy and, although they lack spontaneous regeneration, are able to sprout. Axon sprouting is a late process that occurs mainly 6 to 18 months after the lesion and results from an interplay between Purkinje cell intrinsic properties and chemical remodeling of the glial scar. To better appraise the role of the local environment in the late sprouting, we performed new axotomy experiments in mice. In this species, unlike the rat, there is no cavitation because the post-lesional necrotic tissue is invaded by astrocytes and incorporated into the glial scar. In this scarring tissue, chondroitin sulfate proteoglycans (CS-PGs) and PSA-NCAM are present one week after the lesion, but the time courses of their expression differ: the former are transiently expressed and rapidly disappears (by one month), thus preventing early sprouting and providing a negative spatiotemporal correlation with the late sprouting. PSA-NCAM expression, which is maintained up till 12 months, is by itself not sufficient to attract the sprouts, since the core of the glial scar--which exhibits high level of PSA-NCAM--is always devoid of them. Finally, by using a double experimental approach (lesion and graft) aimed at providing a permissive environment to the terminal bulbs of axotomized Purkinje cells, we show that the presence of grafted cerebellum at the lesion site neither changes the time course of the sprouting nor enhances the Purkinje cell axonal regeneration. Nevertheless, these experiments have revealed a new type of altered Purkinje cells, the "irritated" Purkinje cells with a high potentiality for axon sprouting.