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Neuronal survival critically depends on the integrity and functionality of mitochondria. A hierarchical system of cellular surveillance mechanisms protects mitochondria against stress, monitors mitochondrial damage and ensures the selective removal of dysfunctional mitochondrial proteins or organelles. Mitochondrial proteases emerge as central regulators that coordinate different quality control (QC) pathways within an interconnected network of mechanisms. A failure of this system causes neuronal loss in a steadily increasing number of neurodegenerative disorders, which include Parkinson's disease, spinocerebellar ataxia, spastic paraplegia and peripheral neuropathies. Here, we will discuss the role of the mitochondrial QC network for neuronal survival and neurodegeneration.

Recently, paraplegin and spastin have been found to be mutated in two autosomal forms of hereditary spastic paraplegia. Both proteins harbour a common ATPase domain that expresses a chaperone function. Paraplegin is a nuclear-encoded mitochondrial metalloprotease, while the exact role and subcellular localisation of spastin are still unclear.

The Drosophila Notch gene and its ligands, Delta and Serrate, are involved in cell fate determination in a variety of developing tissues. Recently, several Notch, Delta and Serrate homologues have been identified in vertebrates. We report here the cloning of the human and murine JAGGED2 (JAG2), a Serrate-like gene, and the analysis of its expression pattern during embryogenesis. Jag2 was found to be expressed as early as E9 in the surface ectoderm of the branchial arches and in the apical ectodermal ridge (AER) of the developing limb. At E12.5, Jag2 expression is upregulated in differentiated neurons of the central and peripheral nervous system and in the inner neuroblastic layer of the developing retina. Outside the nervous system, Jag2 is expressed in the developing vibrissae follicles, tooth buds, thymus, submandibular gland and stomach. Our findings suggest the involvement of Jagged2 in the development of the mammalian limb, branchial arches, central and peripheral nervous systems and several tissues whose development depends upon epithelial-mesenchymal interactions.

Kallmann syndrome is a genetic disorder characterized by a defect in olfactory system development, which appears to be due to an abnormality in the migration of olfactory axons and gonadotropin releasing hormone (Gn-RH) producing neurons. The X-linked Kallmann syndrome gene shares significant similarities with molecules involved in neural development. We have now isolated the evolutionarily conserved chicken homologue of the Kallmann gene. In the developing and adult chicken, high levels of expression were found in the mitral cells of the olfactory bulb (the target of olfactory axons) and in the Purkinje cells of the cerebellar cortex, both areas affected in patients with Kallmann syndrome. We propose a model in which the Kallmann syndrome gene product is a signal molecule required for neuronal targeting throughout life.

in several neurodegenerative diseases, axonal degeneration occurs before neuronal death and contributes significantly to patients' disability. Hereditary spastic paraplegia (HSP) is a genetically heterogeneous condition characterized by selective degeneration of axons of the corticospinal tracts and fasciculus gracilis. HSP may therefore be considered an exemplary disease to study the local programs mediating axonal degeneration. We have developed a mouse model for autosomal recessive HSP due to mutations in the SPG7 gene encoding the mitochondrial ATPase paraplegin. Paraplegin-deficient mice are affected by a distal axonopathy of spinal and peripheral axons, characterized by axonal swelling and degeneration. We found that mitochondrial morphological abnormalities occurred in synaptic terminals and in distal regions of axons long before the first signs of swelling and degeneration and correlated with onset of motor impairment during a rotarod test. Axonal swellings occur through massive accumulation of organelles and neurofilaments, suggesting impairment of anterograde axonal transport. Retrograde axonal transport is delayed in symptomatic mice. We speculate that local failure of mitochondrial function may affect axonal transport and cause axonal degeneration. Our data suggest that a timely therapeutic intervention may prevent the loss of axons.

We have recently reported isolation of the gene responsible for X-linked Opitz G/BBB syndrome, a defect of midline development. MID1 is located on the distal short arm of the human X chromosome (Xp22. 3) and encodes a novel member of the B box family of zinc finger proteins. We have now cloned the murine homolog of MID1 and performed preliminary expression studies during development. Mid1 expression in undifferentiated cells in the central nervous, gastrointestinal and urogenital systems suggests that abnormal cell proliferation may underlie the defect in midline development characteristic of Opitz syndrome. We have also found that Mid1 is located within the mouse pseudoautosomal region (PAR) in Mus musculus , while it seems to be X-specific in Mus spretus. Therefore, Mid1 is likely to be a recent acquisition of the M. musculus PAR. Genetic and FISH analyses also demonstrated a high frequency of unequal crossovers in the murine PAR, creating spontaneous deletion/duplication events involving Mid1. These data provide evidence for the first time that genetic instability of the PAR may affect functionally important genes. In addition, we show that MID1 is the first example of a gene subject to X-inactivation in man while escaping it in mouse. These data contribute to a better understanding of the molecular content and evolution of the rodent PAR.

Kallmann syndrome is an inherited disorder characterized by an abnormality in olfactory system development. The gene for the X-linked form of this disorder (KAL) maps to Xp22.3 and encodes a protein sharing homologies with molecules involved in neuronal migration and axonal pathfinding. Here we report the expression pattern of the KAL gene in various parts of the human fetal brain. We found KAL transcripts in granule cells of the olfactory bulb and the cerebellum, in the dorsomedial thalamus and in the developing cerebral cortex. To determine whether or not signals from the olfactory nerve are required for KAL expression in the olfactory bulb, we analyzed chick embryos in which the olfactory placode was surgically removed. Those embryos lacking an olfactory nerve had a histologically abnormal bulb which nevertheless expressed the KAL gene at high levels. These findings indicate that, while the development of the proper cytoarchitecture of the olfactory bulb requires the innervation by olfactory axons, the expression of KAL is independent of such developmental processes.

In the framework of constructing a comprehensive transcript map of the human Xp22.3 region, we identified an evolutionary conserved CpG island and cloned the corresponding gene. The predicted 760 amino acid protein encoded by this gene contains 12 hydrophobic domains and shares significant sequence and structural similarities with all the previously isolated members of a recently identified family of voltage-gated chloride channels (the 'CIC family'). This gene, termed CICN4 (Chloride Channel 4), contains at least 10 exons spanning 60 to 80 kb on the X chromosome. In contrast to most genes isolated from the human Xp22.3 region, the CICN4 gene does not share homology with the Y chromosome and it is conserved in mouse and hamster. Expression studies revealed the presence of a 7.5 kb transcript which is particularly abundant in skeletal muscle and is also detectable in brain and heart. These data suggest that we have identified a new voltage-gated chloride channel which is encoded by a gene located in the distal short arm of the X chromosome.

Kallmann syndrome is characterized by hypogonadotropic hypogonadism and anosmia and caused by a defect of migration and targeting of gonadotropin-releasing hormone-secreting neurons and olfactory axons during embryonic development. We previously cloned the gene responsible for the X-linked form of the disease encoding a 680 amino acid protein, KAL, which displays the unusual combination of a protease inhibitor domain with fibronectin type III repeats. Previous expression studies by northern blot and RNA in situ hybridization in human and chick indicated that the gene is expressed at very low levels in the olfactory bulb during development. Therefore, low abundance of the protein has hampered a detailed biochemical characterization. By overexpressing both the human and chick KAL cDNAs in eukaryotic cells, we now provide evidence that KAL is a glycosylated peripheral membrane protein with an apparent molecular weight of approximately 100 kDa. We show that this 100 kDa protein is proteolytically processed on the cell membrane to yield a 45 kDa diffusible component, which is detectable with an antisera against the C-terminal part of the protein and binds tightly to cell surfaces. These data provide a first step toward understanding KAL function in neuronal interactions and neurite extension in the olfactory bulb and suggest that KAL might be a diffusible chemoattractant molecule for olfactory axons.