Hartmut Halfter

Mutations in SEPT9 cause hereditary neuralgic amyotrophy

Hereditary neuralgic amyotrophy (HNA) is an autosomal dominant recurrent neuropathy affecting the brachial plexus. HNA is triggered by environmental factors such as infection or parturition. We report three mutations in the gene septin 9 (SEPT9) in six families with HNA linked to chromosome 17q25. HNA is the first monogenetic disease caused by mutations in a gene of the septin family. Septins are implicated in formation of the cytoskeleton, cell division and tumorigenesis.

E‐cadherin controls adherens junctions in the epidermis and the renewal of hair follicles

E‐cadherin is thought to mediate intercellular adhesion in the mammalian epidermis and in hair follicles as the adhesive component of adherens junctions. We have tested this role of E‐cadherin directly by conditional gene ablation in the mouse. We show that postnatal loss of E‐cadherin in keratinocytes leads to a loss of adherens junctions and altered epidermal differentiation without accompanying signs of inflammation. Overall tissue integrity and desmosomal structures were maintained, but skin hair follicles were progressively lost. Tumors were not observed and β‐catenin levels were not strongly altered in the mutant skin. We conclude that E‐cadherin is required for maintaining the adhesive properties of adherens junctions in keratinocytes and proper skin differentiation. Furthermore, continuous hair follicle cycling is dependent on E‐cadherin.

Inhibition of Growth and Induction of Differentiation of Glioma Cell Lines by Oncostatin M (OSM)

The neuropoietic cytokines of the interleukin-6 family are a group of structurally and functionally related polypeptides. We studied the effect of the multifunctional neuropoietic cytokines, including oncostatin M (OSM), leukemia inhibitory factor (LIF) and interleukin-6 (IL-6), on anaplastic glioma cell lines. Growth and morphology of the glioma cell lines were affected differently. While IL-6 and LIF exerted no or only small minor morphological changes and growth retardation, OSM induced a marked change in morphology and a strong suppression of growth. OSM treated cells were characterized by enlargement and the formation of multiple, thin processes thus resembling mature cultured astrocytes. The growth inhibitory effects were dose dependent with a maximum exerted by addition of 50 ng/ml OSM. The inhibition of DNA synthesis by OSM could be abolished by antibodies blocking either the activity of OSM or the OSM-receptor component, gp130.

Sodium-dependent vitamin C transporter 2 (SVCT2) is necessary for the uptake of L-ascorbic acid into Schwann cells

Ascorbic acid has been shown to be an essential component for in vitro myelination and to improve the clinical and pathological phenotype of a mouse model of Charcot–Marie‐tooth disease 1A. The mechanism of ascorbic acid uptake into peripheral nerves, however, has not been addressed so far. Hence, we studied the expression and activity of sodium‐dependent vitamin C transporters 1 and 2 (SVCT1 and 2) in the peripheral nervous system. Using immunohistochemistry, immunoblotting, and reverse transcription PCR, we could show that SVCT1 and 2 were differentially expressed in myelinated peripheral nerve fibers and Schwann cell (SC) cultures. SVCT1 was expressed at very low levels confined to the axons, whereas SVCT2 was highly expressed both in the axons and in the SCs. SVCT2 was localized particularly in SC compartments of uncompacted myelin. Uptake assays using 14C‐labeled ascorbic acid showed transport of ascorbic acid into SC cultures. Ascorbic acid transport was dependent on the concentration of sodium, magnesium, and calcium in the extracellular medium. Treatment with the flavonoid phloretin, a known inhibitor of SVCT1 and 2, and specific RNA interference with SVCT2 caused significant reductions in ascorbic acid uptake into SCs. Phloretin‐inhibited uptake of ascorbic acid was further shown in freshly dissected, cell‐culture‐naïve rat sciatic nerves. These results provide evidence for the first time that uptake of ascorbic acid in the peripheral nervous system is crucially dependent on the expression and activity of SVCT2.

Inhibition of N‐cadherin and β‐catenin function reduces axon‐induced Schwann cell proliferation

N‐cadherin and β‐catenin are involved in cell adhesion and cell cycle in tumor cells and neural crest. Both are expressed at key stages of Schwann cell (SC) development, but little is known about their function in the SC lineage. We studied the role of these molecules in adult rat derived SC‐embryonic dorsal root ganglion cocultures by using low‐Ca2+ conditions and specific blocking antibodies to interfere with N‐cadherin function and by using small interfering RNA (siRNA) to decrease β‐catenin expression in both SC‐neuron cocultures and adult rat‐derived SC monocultures. N‐cadherin blocking conditions decreased SC‐axon association and reduced axon‐induced SC proliferation. In SC monocultures, β‐catenin reduction diminished the proliferative response of SCs to the mitogen β1‐heregulin, and, in SC‐DRG cocultures, β‐catenin reduction inhibited axon‐contact‐dependent SC proliferation. Stimulation of SC cultures with β1‐heregulin increased total β‐catenin protein amount, phosphorylation of GSK‐3β and β‐catenin presence in nuclear extracts. In conclusion, our findings suggest a previously unrecognized contribution of β‐catenin and N‐cadherin to axon‐induced SC proliferation.

Sodium-dependent vitamin C transporter 2 deficiency causes hypomyelination and extracellular matrix defects in the peripheral nervous system.

Ascorbic acid (vitamin C) is necessary for myelination of Schwann cell/neuron cocultures and has shown beneficial effects in the treatment of a Charcot-Marie-Tooth neuropathy 1A (CMT1A) mouse model. Although clinical studies revealed that ascorbic acid treatment had no impact on CMT1A, it is assumed to have an important function in peripheral nerve myelination and possibly in remyelination. However, the transport pathway of ascorbic acid into peripheral nerves and the mechanism of ascorbic acid function in peripheral nerves in vivo remained unclear. In this study, we used sodium-dependent vitamin C transporter 2-heterozygous (SVCT2+/−) mice to elucidate the functions of SVCT2 and ascorbic acid in the murine peripheral nervous system. SVCT2 and ascorbic acid levels were reduced in SVCT2+/− peripheral nerves. Morphometry of sciatic nerve fibers revealed a decrease in myelin thickness and an increase in G-ratios in SVCT2+/− mice. Nerve conduction velocities and sensorimotor performance in functional tests were reduced in SVCT2+/− mice. To investigate the mechanism of ascorbic acid function, we studied the expression of collagens in the extracellular matrix of peripheral nerves. Here, we show that expression of various collagen types was reduced in sciatic nerves of SVCT2+/− mice. We found that collagen gene transcription was reduced in SVCT2+/− mice but hydroxyproline levels were not, indicating that collagen formation was regulated on the transcriptional and not the posttranslational level. These results help to clarify the transport pathway and mechanism of action of ascorbic acid in the peripheral nervous system and may lead to novel therapeutic approaches to peripheral neuropathies by manipulation of SVCT2 function.

Kinetics of 3-[123I]iodo-l-α-methyltyrosine transport in rat C6 glioma cells

Abstract. 3-[123I]Iodo-l-α-methyltyrosine (123I-IMT) is used for the diagnosis and monitoring of brain tumours by means of single-photon emission tomography (SPET). To date, little has been known about the system for the transport of 123I-IMT into brain tumour cells. It is assumed that 123I-IMT is transported by a specific carrier for large, neutral amino acids (L-system). In this study, rat C6 glioma cells were used to characterize the uptake system of 123I-IMT and to investigate its precise kinetics. The time course of 123I-IMT uptake into the cells was examined for a range of 1–60 min. 123I-IMT uptake rates with varying concentrations of 123I-IMT (2.5–50 µM) in the medium were quantified to assess the kinetic parameters of 123I-IMT transport. Furthermore, competition of 123I-IMT with other amino acids was investigated to identify the distinct transport systems involved in 123I-IMT uptake. 123I-IMT uptake into C6 glioma cells was linear for approximately 10 min and reached a steady-state level within 30 min. The analysis of the rate of uptake of 123I-IMT at different concentrations was concordant with the predominance of a single uptake system. The apparent Michaelis constant (Km) of 123I-IMT was 26.2±1.9 µM, and the maximum transport velocity (Vmax) was 35.4±1.7 nmol/mg protein per 10 min. 77%±10% of 123I-IMT transport was sodium independent and 23%±3% was sodium dependent. Competitive inhibition of 123I-IMT uptake by 2-aminobicyclo[2.2.1]heptane-2-carboxylic acid, α-(methylamino)isobutyric acid and naturally occurring amino acids revealed a major 123I-IMT transport via the sodium-independent system L (72%) and a minor uptake via the sodium-dependent system B0,+ (17%). Our results show that 123I-IMT transport into C6 glioma cells is principally mediated by the L-system and to a minor extent by the B0,+-system. The kinetic parameters of 123I-IMT uptake are in the range of those of naturally occurring amino acids.

The Gdap1 knockout mouse mechanistically links redox control to Charcot–Marie–Tooth disease

Mutations in the mitochondrial fission factor GDAP1 are associated with severe peripheral neuropathies, but why the CNS remains unaffected is unclear. Using a Gdap1−/− mouse, Niemann et al. demonstrate that a CNS-expressed Gdap1 paralogue changes its subcellular localisation under oxidative stress conditions to also act as a mitochondrial fission factor.

HSJ1-related hereditary neuropathies Novel mutations and extended clinical spectrum

Objectives: To determine the nature and frequency of HSJ1 mutations in patients with hereditary motor and hereditary motor and sensory neuropathies. Methods: Patients were screened for mutations by genome-wide or targeted linkage and homozygosity studies, whole-exome sequencing, and Sanger sequencing. RNA and protein studies of skin fibroblasts were used for functional characterization. Results: We describe 2 additional mutations in the HSJ1 gene in a cohort of 90 patients with autosomal recessive distal hereditary motor neuropathy (dHMN) and Charcot-Marie-Tooth disease type 2 (CMT2). One family with a dHMN phenotype showed the homozygous splice-site mutation c.229+1G>A, which leads to retention of intron 4 in the HSJ1 messenger RNA with a premature stop codon and loss of protein expression. Another family, presenting with a CMT2 phenotype, carried the homozygous missense mutation c.14A>G (p.Tyr5Cys). This mutation was classified as likely disease-related by several automatic algorithms for prediction of possible impact of an amino acid substitution on the structure and function of proteins. Both mutations cosegregated with autosomal recessive inheritance of the disease and were absent from the general population. Conclusions: Taken together, in our cohort of 90 probands, we confirm that HSJ1 mutations are a rare but detectable cause of autosomal recessive dHMN and CMT2. We provide clinical and functional information on an HSJ1 splice-site mutation and report the detailed phenotype of 2 patients with CMT2, broadening the phenotypic spectrum of HSJ1-related neuropathies.

Autosomal-dominant striatal degeneration is caused by a mutation in the phosphodiesterase 8B gene.

Autosomal-dominant striatal degeneration (ADSD) is an autosomal-dominant movement disorder affecting the striatal part of the basal ganglia. ADSD is characterized by bradykinesia, dysarthria, and muscle rigidity. These symptoms resemble idiopathic Parkinson disease, but tremor is not present. Using genetic linkage analysis, we have mapped the causative genetic defect to a 3.25 megabase candidate region on chromosome 5q13.3-q14.1. A maximum LOD score of 4.1 (Theta = 0) was obtained at marker D5S1962. Here we show that ADSD is caused by a complex frameshift mutation (c.94G>C+c.95delT) in the phosphodiesterase 8B (PDE8B) gene, which results in a loss of enzymatic phosphodiesterase activity. We found that PDE8B is highly expressed in the brain, especially in the putamen, which is affected by ADSD. PDE8B degrades cyclic AMP, a second messenger implied in dopamine signaling. Dopamine is one of the main neurotransmitters involved in movement control and is deficient in Parkinson disease. We believe that the functional analysis of PDE8B will help to further elucidate the pathomechanism of ADSD as well as contribute to a better understanding of movement disorders.