The debut of a rational treatment for an inherited neuropathy?

Hereditary neuropathies are common neurological conditions characterized by progressive loss of motor and/or sensory function. There are no effective treatments. Among the many causes of hereditary neuropathies are dominant mutations in serine palmitoyltransferase, long chain base subunit 1 (SPTLC1), which cause hereditary sensory and autonomic neuropathy type 1 (HSAN1). By incorporating L-alanine in place of L-serine, the mutant HSAN1–associated serine palmitoyltransferase generates deoxysphingolipids, which are thought to be neurotoxic. In this issue of the JCI, Garofalo and colleagues report that oral L-serine reverses the accumulation of deoxysphingolipids in humans with HSAN1 and in a transgenic mouse model. As oral L-serine reduces the severity of neuropathy in the mouse model of HSAN1, these data suggest a rational candidate therapy for this devastating condition.

The in vivo contribution of motor neuron TrkB receptors to mutant SOD1 motor neuron disease

Brain-derived neurotrophic factor (BDNF) and its receptor tropomyosin-related kinase B (TrkB) are widely expressed in the vertebrate nervous system and play a central role in mature neuronal function. In vitro BDNF/TrkB signaling promotes neuronal survival and can help neurons resist toxic insults. Paradoxically, BDNF/TrkB signaling has also been shown, under certain in vitro circumstances, to render neurons vulnerable to insults. We show here that in vivo conditional deletion of TrkB from mature motor neurons attenuates mutant superoxide dismutase 1 (SOD1) toxicity. Mutant SOD1 mice lacking motor neuron TrkB live a month longer than controls and retain motor function for a longer period, particularly in the early phase of the disease. These effects are subserved by slowed motor neuron loss, persistence of neuromuscular junction integrity and reduced astrocytic and microglial reactivity within the spinal cord. These results suggest that manipulation of BDNF/TrkB signaling might have therapeutic efficacy in motor neuron diseases.

Cx32 and Cx47 mediate oligodendrocyte:astrocyte and oligodendrocyte:oligodendrocyte gap junction coupling

In addition to the extensive gap junction coupling between astrocytes themselves, oligodendrocytes are thought to be exclusively coupled to astrocytes (O:A coupling) via heterotypic gap junctions composed of Cx47:Cx43 and Cx32:Cx30. We used fluorescent dyes to examine functional coupling in acute slices from the cerebra of mice lacking Cx32 and/or Cx47. In the corpus callosum, unexpectedly, oligodendrocytes appeared to be directly and exclusively coupled to other oligodendrocytes (O:O coupling), and electron microscopy revealed gap junctions between adjacent oligodendrocytes. O:O coupling was more affected in mice lacking Cx32 than in mice lacking Cx47. In the neocortex, oligodendrocytes appeared to be directly and exclusively coupled to astrocytes; Cx47, but not Cx32, was required for O:A coupling.

Paranodal permeability in "myelin mutants"

Fluorescent dextran tracers of varying sizes have been used to assess paranodal permeability in myelinated sciatic nerve fibers from control and three "myelin mutant" mice, Caspr-null, cst-null, and shaking. We demonstrate that in all of these the paranode is permeable to small tracers (3 kDa and 10 kDa), which penetrate most fibers, and to larger tracers (40 kDa and 70 kDa), which penetrate far fewer fibers and move shorter distances over longer periods of time. Despite gross diminution in transverse bands (TBs) in the Caspr-null and cst-null mice, the permeability of their paranodal junctions is equivalent to that in controls. Thus, deficiency of TBs in these mutants does not increase the permeability of their paranodal junctions to the dextrans we used, moving from the perinodal space through the paranode to the internodal periaxonal space. In addition, we show that the shaking mice, which have thinner myelin and shorter paranodes, show increased permeability to the same tracers despite the presence of TBs. We conclude that the extent of penetration of these tracers does not depend on the presence or absence of TBs but does depend on the length of the paranode and, in turn, on the length of "pathway 3," the helical extracellular pathway that passes through the paranode parallel to the lateral edge of the myelin sheath.

Central nervous system dysfunction in a mouse model of FA2H deficiency

Fatty acid 2-hydroxylase (FA2H) is responsible for the synthesis of myelin galactolipids containing hydroxy fatty acid (hFA) as the N-acyl chain. Mutations in the FA2H gene cause leukodystrophy, spastic paraplegia, and neurodegeneration with brain iron accumulation. Using the Cre-lox system, we developed two types of mouse mutants, Fa2h(-/-) mice (Fa2h deleted in all cells by germline deletion) and Fa2h(flox/flox) Cnp1-Cre mice (Fa2h deleted only in oligodendrocytes and Schwann cells). We found significant demyelination, profound axonal loss, and abnormally enlarged axons in the CNS of Fa2h(-/-) mice at 12 months of age, while structure and function of peripheral nerves were largely unaffected. Fa2h(-/-) mice also exhibited histological and functional disruption in the cerebellum at 12 months of age. In a time course study, significant deterioration of cerebellar function was first detected at 7 months of age. Further behavioral assessments in water T-maze and Morris water maze tasks revealed significant deficits in spatial learning and memory at 4 months of age. These data suggest that various regions of the CNS are functionally compromised in young adult Fa2h(-/-) mice. The cerebellar deficits in 12-month-old Fa2h(flox/flox) Cnp1-Cre mice were indistinguishable from Fa2h(-/-) mice, indicating that these phenotypes likely stem from the lack of myelin hFA-galactolipids. In contrast, Fa2h(flox/flox) Cnp1-Cre mice did not show reduced performance in water maze tasks, indicating that oligodendrocytes are not involved in the learning and memory deficits found in Fa2h(-/-) mice. These findings provide the first evidence that FA2H has an important function outside of oligodendrocytes in the CNS.

Investigations of caspr2, an autoantigen of encephalitis and neuromyotonia

OBJECTIVE

To report clinical and immunological investigations of contactin-associated protein-like 2 (Caspr2), an autoantigen of encephalitis and peripheral nerve hyperexcitability (PNH) previously attributed to voltage-gated potassium channels (VGKC).

METHODS

Clinical analysis was performed on patients with encephalitis, PNH, or both. Immunoprecipitation and mass spectrometry were used to identify the antigen and to develop an assay with Caspr2-expressing cells. Immunoabsorption with Caspr2 and comparative immunostaining of brain and peripheral nerve of wild-type and Caspr2-null mice were used to assess antibody specificity.

RESULTS

Using Caspr2-expressing cells, antibodies were identified in 8 patients but not in 140 patients with several types of autoimmune or viral encephalitis, PNH, or mutations of the Caspr2-encoding gene. Patients' antibodies reacted with brain and peripheral nerve in a pattern that colocalized with Caspr2. This reactivity was abrogated after immunoabsorption with Caspr2 and was absent in tissues from Caspr2-null mice. Of the 8 patients with Caspr2 antibodies, 7 had encephalopathy or seizures, 5 neuropathy or PNH, and 1 isolated PNH. Three patients also had myasthenia gravis, bulbar weakness, or symptoms that initially suggested motor neuron disease. None of the patients had active cancer; 7 responded to immunotherapy and were healthy or only mildly disabled at last follow-up (median, 8 months; range, 6-84 months).

INTERPRETATION

Caspr2 is an autoantigen of encephalitis and PNH previously attributed to VGKC antibodies. The occurrence of other autoantibodies may result in a complex syndrome that at presentation could be mistaken for a motor neuron disorder. Recognition of this disorder is important, because it responds to immunotherapy.

Dominant Cx26 mutants associated with hearing loss have dominant-negative effects on wild type Cx26

Mutations in GJB2, the gene encoding the human gap junction protein connexin26 (Cx26), cause either non-syndromic hearing loss or syndromes affecting both hearing and skin. We have investigated whether dominant Cx26 mutants can interact physically with wild type Cx26. HeLa cells stably expressing wild type Cx26 were transiently transfected to co-express nine individual dominant Cx26 mutants; six associated with non-syndromic hearing loss (W44C, W44S, R143Q, D179N, R184Q, and C202F) and three associated with hearing loss and palmoplantar keratoderma (G59A, R75Q, and R75W). All mutants co-localized and co-immunoprecipitated with wild type Cx26, indicating that they interact physically, likely by forming admixed heteromeric/heterotypic channels. Furthermore, all nine mutants inhibited the transfer of calcein in cells stably expressing Cx26, demonstrating that they each have dominant effects on wild type Cx26. Taken together, these results show that dominant-negative effects of these Cx26 mutants likely contribute to the pathogenesis of hearing loss.

Axonal pathology precedes demyelination in a mouse model of X-linked demyelinating/type I Charcot-Marie Tooth neuropathy

The X-linked demyelinating/type I Charcot-Marie-Tooth neuropathy (CMT1X) is an inherited peripheral neuropathy caused by mutations in GJB1, the gene that encodes the gap junction protein connexin32. Connexin32 is expressed by myelinating Schwann cells and forms gap junctions in noncompact myelin areas, but axonal involvement is more prominent in X-linked compared with other forms of demyelinating Charcot-Marie-Tooth disease. To clarify the cellular and molecular mechanisms of axonal pathology in CMT1X, we studied Gjb1-null mice at early stages (i.e. 2-4 months old) of the neuropathy, when there is minimal or no demyelination. The diameters of large myelinated axons were progressively reduced in Gjb1-null mice compared with those in wild-type littermates. Furthermore, neurofilaments were relatively more dephosphorylated and more densely packed starting at 2 months of age. Increased expression of β-amyloid precursor protein, a marker of axonal damage, was also detected in Gjb1-null nerves. Finally, fast axonal transport, assayed by sciatic nerve ligation experiments, was slower in distal axons of Gjb1-null versus wild-type animals with reduced accumulation of synaptic vesicle-associated proteins. These findings demonstrate that axonal abnormalities including impaired cytoskeletal organization and defects in axonal transport precede demyelination in this mouse model of CMT1X.

A patient with neurofibromatosis type 1 and Charcot-Marie-Tooth disease type 1B

We describe a patient with both neurofibromatosis type 1 and Charcot-Marie-Tooth disease type 1B. Although one might expect an overwhelming tumor burden due to the combination of these two disorders, the two mutations did not appear to interact.

Missense mutations in the copper transporter gene ATP7A cause X-linked distal hereditary motor neuropathy

Distal hereditary motor neuropathies comprise a clinically and genetically heterogeneous group of disorders. We recently mapped an X-linked form of this condition to chromosome Xq13.1-q21 in two large unrelated families. The region of genetic linkage included ATP7A, which encodes a copper-transporting P-type ATPase mutated in patients with Menkes disease, a severe infantile-onset neurodegenerative condition. We identified two unique ATP7A missense mutations (p.P1386S and p.T994I) in males with distal motor neuropathy in two families. These molecular alterations impact highly conserved amino acids in the carboxyl half of ATP7A and do not directly involve the copper transporter's known critical functional domains. Studies of p.P1386S revealed normal ATP7A mRNA and protein levels, a defect in ATP7A trafficking, and partial rescue of a S. cerevisiae copper transport knockout. Although ATP7A mutations are typically associated with severe Menkes disease or its milder allelic variant, occipital horn syndrome, we demonstrate here that certain missense mutations at this locus can cause a syndrome restricted to progressive distal motor neuropathy without overt signs of systemic copper deficiency. This previously unrecognized genotype-phenotype correlation suggests an important role of the ATP7A copper transporter in motor-neuron maintenance and function.

A novel recessive Nefl mutation causes a severe, early-onset axonal neuropathy

OBJECTIVE

To report the first cases of a homozygous recessive mutation in NEFL, the gene that encodes the light subunit of neurofilaments.

METHODS

Clinical and electrophysiologic data were evaluated, and a sural nerve biopsy from one affected child was examined by immunohistochemistry and electron microscopy. The ability of the mutant protein to form filaments was characterized in an established cell culture system.

RESULTS

Four of five siblings developed of a severe, progressive neuropathy beginning in early childhood. Serial nerve conduction studies showed progressively reduced amplitudes with age and pronounced slowing at all ages. Visual-evoked responses were slowed in three children, indicating that central nervous system axons were subclinically involved. All four affected children were homozygous for a nonsense mutation at glutamate 210 (E210X) in the NEFL gene; both parents were heterozygous carriers. A sural nerve biopsy from an affected patient showed markedly reduced numbers of myelinated axons; the remaining myelinated axons were small and lacked intermediate filaments. The E210X mutant protein did not form an intermediate filament network and did not interfere with the filament formation by wild-type human light subunit of neurofilaments in SW-13 vim(-) cells.

INTERPRETATION

This is the first demonstration of a recessive NEFL mutation, which appears to cause a simple loss of function, resulting in a severe, early-onset axonal neuropathy with unique features. These results confirm that neurofilaments are the main determinant of axonal caliber and conduction velocity, and demonstrate for the first time that neurofilaments are required for the maintenance of myelinated peripheral nervous system axons.

Connexin32 mutations cause loss of function in Schwann cells and oligodendrocytes leading to PNS and CNS myelination defects

The gap junction (GJ) protein connexin32 (Cx32) is expressed by myelinating Schwann cells and oligodendrocytes and is mutated in X-linked Charcot-Marie-Tooth disease. In addition to a demyelinating peripheral neuropathy, some Cx32 mutants are associated with transient or chronic CNS phenotypes. To investigate the molecular basis of these phenotypes, we generated transgenic mice expressing the T55I or the R75W mutation and an IRES-EGFP, driven by the mouse Cnp promoter. The transgene was expressed in oligodendrocytes throughout the CNS and in Schwann cells. Both the T55I and the R75W mutants were localized in the perinuclear cytoplasm, did not form GJ plaques, and did not alter the expression or localization of two other oligodendrocytic GJ proteins, Cx47 and Cx29, or the expression of Cx29 in Schwann cells. On wild type background, the expression of endogenous mCx32 was unaffected by the T55I mutant, but was partly impaired by R75W. Transgenic mice with the R75W mutation and all mutant animals with Gjb1-null background developed a progressive demyelinating peripheral neuropathy along with CNS myelination defects. These findings suggest that Cx32 mutations result in loss of function in myelinated cells without trans-dominant effects on other GJ proteins. Loss of Cx32 function alone in the CNS causes myelination defects.

Hereditary spastic paraplegia is a novel phenotype for GJA12/GJC2 mutations

Recessive mutations in GJA12/GJC2, the gene that encodes the gap junction protein connexin47 (Cx47), cause Pelizaeus-Merzbacher-like disease (PMLD), an early onset dysmyelinating disorder of the CNS, characterized by nystagmus, psychomotor delay, progressive spasticity and cerebellar signs. Here we describe three patients from one family with a novel recessively inherited mutation, 99C>G (predicted to cause an Ile>Met amino acid substitution; I33M) that causes a milder phenotype. All three had a late-onset, slowly progressive, complicated spastic paraplegia, with normal or near-normal psychomotor development, preserved walking capability through adulthood, and no nystagmus. MRI and MR spectroscopy imaging were consistent with a hypomyelinating leukoencephalopathy. The mutant protein forms gap junction plaques at cell borders similar to wild-type (WT) Cx47 in transfected cells, but fails to form functional homotypic channels in scrape-loading and dual whole-cell patch clamp assays. I33M forms overlapping gap junction plaques and functional channels with Cx43, however, I33M/Cx43 channels open only when a large voltage difference is applied to paired cells. These channels probably do not function under physiological conditions, suggesting that Cx47/Cx43 channels between astrocytes and oligodendrocytes are disrupted, similar to the loss-of-function endoplasmic reticulum-retained Cx47 mutants that cause PMLD. Thus, GJA12/GJC2 mutations can result in a milder phenotype than previously appreciated, but whether I33M retains a function of Cx47 not directly related to forming functional gap junction channels is not known.

Molecular mechanisms of inherited demyelinating neuropathies

The past 15 years have witnessed the identification of more than 25 genes responsible for inherited neuropathies in humans, many associated with primary alterations of the myelin sheath. A remarkable body of work in patients, as well as animal and cellular models, has defined the clinical and molecular genetics of these illnesses and shed light on how mutations in associated genes produce the heterogeneity of dysmyelinating and demyelinating phenotypes. Here, we review selected recent developments from work on the molecular mechanisms of these disorders and their implications for treatment strategies.

Gap junctions couple astrocytes and oligodendrocytes

In vertebrates, a family of related proteins called connexins form gap junctions (GJs), which are intercellular channels. In the central nervous system (CNS), GJs couple oligodendrocytes and astrocytes (O/A junctions) and adjacent astrocytes (A/A junctions), but not adjacent oligodendrocytes, forming a "glial syncytium." Oligodendrocytes and astrocytes each express different connexins. Mutations of these connexin genes demonstrate that the proper functioning of myelin and oligodendrocytes requires the expression of these connexins. The physiological function of O/A and A/A junctions, however, remains to be illuminated.

Human oligodendrocytes express Cx31.3: function and interactions with Cx32 mutants

Murine oligodendrocytes express the gap junction (GJ) proteins connexin32 (Cx32), Cx47, and Cx29. CNS phenotypes in patients with X-linked Charcot-Marie-Tooth disease may be caused by dominant effects of Cx32 mutations on other connexins. Here we examined the expression of Cx31.3 (the human ortholog of murine Cx29) in human brain and its relation to the other oligodendrocyte GJ proteins Cx32 and Cx47. Furthermore, we investigated in vitro whether Cx32 mutants with CNS manifestations affect the expression and function of Cx31.3. Cx31.3 was localized mostly in the gray matter along small myelinated fibers similar to Cx29 in rodent brain and was co-expressed with Cx32 in a subset of human oligodendrocytes. In HeLa cells Cx31.3 was localized at the cell membrane and appeared to form hemichannels but no GJs. Cx32 mutants with CNS manifestations were retained intracellularly, but did not alter the cellular localization or function of co-expressed Cx31.3. Thus, Cx31.3 shares many characteristics with its ortholog Cx29. Cx32 mutants with CNS phenotypes do not affect the trafficking or function of Cx31.3, and may have other toxic effects in oligodendrocytes.

Cx29 and Cx32, two connexins expressed by myelinating glia, do not interact and are functionally distinct

In rodents, oligodendrocytes and myelinating Schwann cells express connexin32 (Cx32) and Cx29, which have different localizations in the two cell types. We show here that, in contrast to Cx32, Cx29 does not form gap junction plaques or functional gap junctions in transfected cells. Furthermore, when expressed together, Cx29 and Cx32 are not colocalized and do not coimmunoprecipitate. To determine the structural basis of their divergent behavior, we generated a series of chimeric Cx32-Cx29 proteins by exchanging their intracellular loops and/or their C-terminal cytoplasmic tails. Although some chimerae reach the cell membrane, others appear to be largely localized intracellularly; none form gap junction plaques or functional gap junctions. Substituting the C-terminus or the intracellular loop and the C-terminus of Cx32 with those of Cx29 does not disrupt their colocalization or coimmunoprecipitation with Cx32. Substituting the C-terminus of Cx29 with that of Cx32 does not disrupt the coimmunoprecipitation or the colocalization with Cx29, whereas substituting both the intracellular loop and the C-terminus of Cx32 with those of Cx29 diminishes the coimmunoprecipitation with Cx29. Conversely, the Cx32 chimera that contains the intracellular loop of Cx29 coimmunoprecipitates with Cx29, indicating that the intracellular loop participates in Cx29-Cx29 interactions. These data indicate that homomeric interactions of Cx29 and especially Cx32 largely require other domains: the N-terminus, transmembrane domains, and extracellular loops. Substituting the intracellular loop and/or tail of Cx32 with those of Cx29 appears to prevent Cx32 from forming functional gap junctions.

Nectin-like proteins mediate axon Schwann cell interactions along the internode and are essential for myelination

Axon-glial interactions are critical for the induction of myelination and the domain organization of myelinated fibers. Although molecular complexes that mediate these interactions in the nodal region are known, their counterparts along the internode are poorly defined. We report that neurons and Schwann cells express distinct sets of nectin-like (Necl) proteins: axons highly express Necl-1 and -2, whereas Schwann cells express Necl-4 and lower amounts of Necl-2. These proteins are strikingly localized to the internode, where Necl-1 and -2 on the axon are directly apposed by Necl-4 on the Schwann cell; all three proteins are also enriched at Schmidt-Lanterman incisures. Binding experiments demonstrate that the Necl proteins preferentially mediate heterophilic rather than homophilic interactions. In particular, Necl-1 on axons binds specifically to Necl-4 on Schwann cells. Knockdown of Necl-4 by short hairpin RNA inhibits Schwann cell differentiation and subsequent myelination in cocultures. These results demonstrate a key role for Necl-4 in initiating peripheral nervous system myelination and implicate the Necl proteins as mediators of axo-glial interactions along the internode.

Two distinct heterotypic channels mediate gap junction coupling between astrocyte and oligodendrocyte connexins

Genetic diseases demonstrate that the normal function of CNS myelin depends on connexin32 (Cx32) and Cx47, gap junction (GJ) proteins expressed by oligodendrocytes. GJs couple oligodendrocytes and astrocytes (O/A channels) as well as astrocytes themselves (A/A channels). Because astrocytes express different connexins (Cx30 and Cx43), O/A channels must be heterotypic, whereas A/A channels may be homotypic or heterotypic. Using electrophysiological and immunocytochemical approaches, we found that Cx47/Cx43 and Cx32/Cx30 efficiently formed functional channels, but other potential heterotypic O/A and A/A pairs did not. These results suggest that Cx30/Cx30 and Cx43/Cx43 channels mediate A/A coupling, and Cx47/Cx43 and Cx32/Cx30 channels mediate O/A coupling. Furthermore, Cx47/Cx43 and Cx32/Cx30 channels have distinct macroscopic and single-channel properties and different dye permeabilities. Finally, Cx47 mutants that cause Pelizaeus-Merzbacher-like disease do not efficiently form functional channels with Cx43, indicating that disrupted Cx47/Cx43 channels cause this disease.

Cell expression of GDAP1 in the nervous system and pathogenesis of Charcot-Marie-Tooth type 4A disease

Mutations in the mitochondrial protein GDAP1 are the cause of Charcot-Marie-Tooth type 4A disease (CMT4A), a severe form of peripheral neuropathy associated with either demyelinating, axonal or intermediate pheno-types. GDAP1 is located in the outer mitochondrial membrane and it seems that may be related with the mitochondrial network dynamics. We are interested to define cell expression in the nervous system and the effect of mutations in mitochondrial morphology and pathogenesis of the disease. We investigated GDAP1 expression in the nervous system and dorsal root ganglia (DRG) neuron cultures. GDAP1 is expressed in motor and sensory neurons of the spinal cord and other large neurons such as cerebellar Purkinje neurons, hippocampal pyramidal neurons, mitral neurons of the olfactory bulb and cortical pyramidal neurons. The lack of GDAP1 staining in the white matter and nerve roots suggested that glial cells do not express GDAP1. In DRG cultures satellite cells and Schwann cells were GDAP1-negative. Overexpression of GDAP1-induced fragmentation of mitochondria suggesting a role of GDAP1 in the fission pathway of the mitochondrial dynamics. Missense mutations showed two different patterns: most of them induced mitochondrial fragmentation but the T157P mutation showed an aggregation pattern. Whereas null mutations of GDAP1 should be associated with loss of function of the protein, missense mutations may act through different pathogenic mechanisms including a dominant-negative effect, suggesting that different molecular mechanisms may underlay the pathogenesis of CMT4A.

Human connexin26 and connexin30 form functional heteromeric and heterotypic channels

Mutations in GJB2 and GJB6, the genes that encode the human gap junction proteins connexin26 (Cx26) and connexin30 (Cx30), respectively, cause hearing loss. Cx26 and Cx30 are both expressed in the cochlea, leading to the potential formation of heteromeric hemichannels and heterotypic gap junction channels. To investigate their interactions, we expressed human Cx26 and Cx30 individually or together in HeLa cells. When they were expressed together, Cx26 and Cx30 appeared to interact directly (by their colocalization in gap junction plaques, by coimmunoprecipitation, and by fluorescence resonance energy transfer). Scrape-loading cells that express either Cx26 or Cx30 demonstrated that Cx26 homotypic channels robustly transferred both cationic and anionic tracers, whereas Cx30 homotypic channels transferred cationic but not anionic tracers. Cells expressing both Cx26 and Cx30 also transferred both cationic and anionic tracers by scrape loading, and the rate of calcein (an anionic tracer) transfer was intermediate between their homotypic counterparts by fluorescence recovery after photobleaching. Fluorescence recovery after photobleaching also showed that Cx26 and Cx30 form functional heterotypic channels, allowing the transfer of calcein, which did not pass the homotypic Cx30 channels. Electrophysiological recordings of cell pairs expressing different combinations of Cx26 and/or Cx30 demonstrated unique gating properties of cell pairs expressing both Cx26 and Cx30. These results indicate that Cx26 and Cx30 form functional heteromeric and heterotypic channels, whose biophysical properties and permeabilities are different from their homotypic counterparts.

A central role for Necl4 (SynCAM4) in Schwann cell-axon interaction and myelination

Myelination in the peripheral nervous system requires close contact between Schwann cells and the axon, but the underlying molecular basis remains largely unknown. Here we show that cell adhesion molecules (CAMs) of the nectin-like (Necl, also known as SynCAM or Cadm) family mediate Schwann cell-axon interaction during myelination. Necl4 is the main Necl expressed by myelinating Schwann cells and is located along the internodes in direct apposition to Necl1, which is localized on axons. Necl4 serves as the glial binding partner for axonal Necl1, and the interaction between these two CAMs mediates Schwann cell adhesion. The disruption of the interaction between Necl1 and Necl4 by their soluble extracellular domains, or the expression of a dominant-negative Necl4 in Schwann cells, inhibits myelination. These results suggest that Necl proteins are important for mediating axon-glia contact during myelination in peripheral nerves.

CMT1X phenotypes represent loss of GJB1 gene function

OBJECTIVE

To investigate possible genotype-phenotype correlations and to evaluate the natural history of patients with Charcot-Marie-Tooth disease type 1X (CMT1X).

BACKGROUND

CMT1X is caused by over 260 distinct mutations in the gap junction beta 1 (GJB1) gene, located on the X chromosome, which encodes the gap junction protein connexin 32 (Cx32). The natural history of CMT1X is poorly understood, and it remains unknown whether particular mutations cause more severe neuropathies through abnormal gain-of-function mechanisms.

METHODS

We evaluated 73 male patients with CMT1X, who each have 1 of 28 different GJB1 mutations predicted to affect nearly all domains of Cx32. Disability was evaluated quantitatively by the CMT Neuropathy Score (CMTNS) as well as by the CMT Symptom Score (CMTSS) and the CMT Examination Score (CMTES), which are both based on the CMTNS. Patients were also evaluated by neurophysiology.

RESULTS

In all patients, disability increased with age, and the degree of disability was comparable with that observed in patients with a documented GJB1 deletion. Disability correlated with a loss of motor units as assessed by motor unit number estimates.

CONCLUSIONS

Taken together, these data suggest that most GJB1 mutations cause neuropathy by a loss of normal connexin 32 function. Therefore, treatment of male patients with Charcot-Marie-Tooth disease type 1X may prove amenable to gene replacement strategies.

Loss-of-function GJA12/Connexin47 mutations cause Pelizaeus-Merzbacher-like disease

Recessive mutations in GJA12/Cx47, the gene encoding the gap junction protein connexin47 (Cx47), cause Pelizaeus-Merzbacher-like disease (PMLD), which is characterized by severe CNS dysmyelination. Three missense PMLD mutations, P87S, Y269D and M283T, were expressed in communication-incompetent HeLa cells, and in each case the mutant proteins appeared to at least partially accumulate in the ER. Cells expressing each mutant did not pass Lucifer Yellow or neurobiotin in scrape loading assays, in contrast to robust transfer in cells expressing wild type Cx47. Dual whole-cell patch clamping of transfected Neuro2A cells demonstrated that none of the mutants formed functional channels, in contrast to wild type Cx47. Immunostaining sections of primate brains demonstrated that oligodendrocytes express Cx47, which is primarily localized to their cell bodies. Thus, the Cx47 mutants associated with PMLD likely disrupt the gap junction coupling between astrocytes and oligodendrocytes.