Loss of Mtmr2 phosphatase in Schwann cells but not in motor neurons causes Charcot-Marie-Tooth type 4B1 neuropathy with myelin outfoldings.

Mutations in MTMR2, the myotubularin-related 2 gene, cause autosomal recessive Charcot-Marie-Tooth type 4B1 (CMT4B1). This disorder is characterized by childhood onset of weakness and sensory loss, severely decreased nerve conduction velocity, demyelination in the nerve with myelin outfoldings, and severe functional impairment of affected patients, mainly resulting from loss of myelinated fibers in the nerve. We recently generated Mtmr2-null(neo) mice, which show a dysmyelinating neuropathy with myelin outfoldings, thus reproducing human CMT4B1. Mtmr2 is detected in both Schwann cells and neurons, in which it interacts with discs large 1/synapse-associated protein 97 and neurofilament light chain, respectively. Here, we specifically ablated Mtmr2 in either Schwann cells or motor neurons. Disruption of Mtmr2 in Schwann cells produced a dysmyelinating phenotype very similar to that of the Mtmr2-null(neo) mouse. Disruption of Mtmr2 in motor neurons does not provoke myelin outfoldings nor axonal defects. We propose that loss of Mtmr2 in Schwann cells, but not in motor neurons, is both sufficient and necessary to cause CMT4B1 neuropathy. Thus, therapeutical approaches might be designed in the future to specifically deliver the Mtmr2 phospholipid phosphatase to Schwann cells in affected nerves.

Heparan sulfate proteoglycan induces the production of NO and TNF-alpha by murine microglia.

BACKGROUND: A common feature of Alzheimer's disease (AD) pathology is the abundance of activated microglia in neuritic plaques containing amyloid-beta protein (Abeta) and associated molecules including heparan sulfate proteoglycan (HSPG). Besides the role as pathological chaperone favouring amyloidogenesis, little is known about whether or not HSPG can induce microglial activation. Cultures of primary murine microglia were used to assess the effect of HSPG on production of proinflammatory molecules that are known to be present in neuritic plaques of AD. RESULTS: HSPG stimulated up-regulation of tumor necrosis factor-alpha (TNF-alpha), production of inducible nitric oxide synthase (iNOS) mRNA and accumulation of TNF-alpha protein and nitrite (NO2-) in a time- and concentration-dependent manner. The effects of HSPG were primarily due to the property of the protein core as indicated by the lack of microglial accumulation of TNF-alpha and NO2- in response to denaturated HSPG or heparan sulfate GAG chains (HS). CONCLUSION: These data demonstrate that HSPG may contribute to chronic microglial activation and neurodegeneration seen in neuritic plaques of AD.

Production of monocyte chemoattractant protein-1 in amyotrophic lateral sclerosis.

The presence of activated microglia in the spinal cord of amyotrophic lateral sclerosis (ALS) patients is usually accompanied by inflammatory biochemical changes, but these are largely unexplored. Monocyte chemoattractant protein-1 (MCP-1) is critical for recruitment of inflammatory cells of monocytic lineage after inflammation or injury to the central nervous system. MCP-1 concentrations were measured by an enzyme-linked immunosorbent assay in the cerebrospinal fluid (CSF) and the serum of 27 patients with ALS and 30 patients with noninflammatory neurological diseases. In ALS, circulating MCP-1 levels were significantly increased in the serum and particularly in the CSF. Immunoreactivity for MCP-1 in ALS spinal cord was detected mostly in astrocytes but also in microglia, neurons, and within the vasculature of the cord. Our findings suggest a role for MCP-1 as an important molecular mediator of the injury response in ALS.

Disruption of Mtmr2 produces CMT4B1-like neuropathy with myelin outfolding and impaired spermatogenesis.

Mutations in MTMR2, the myotubularin-related 2 gene, cause autosomal recessive Charcot-Marie-Tooth (CMT) type 4B1, a demyelinating neuropathy with myelin outfolding and azoospermia. MTMR2 encodes a ubiquitously expressed phosphatase whose preferred substrate is phosphatidylinositol (3,5)-biphosphate, a regulator of membrane homeostasis and vesicle transport. We generated Mtmr2-null mice, which develop progressive neuropathy characterized by myelin outfolding and recurrent loops, predominantly at paranodal myelin, and depletion of spermatids and spermatocytes from the seminiferous epithelium, which leads to azoospermia. Disruption of Mtmr2 in Schwann cells reproduces the myelin abnormalities. We also identified a novel physical interaction in Schwann cells, between Mtmr2 and discs large 1 (Dlg1)/synapse-associated protein 97, a scaffolding molecule that is enriched at the node/paranode region. Dlg1 homologues have been located in several types of cellular junctions and play roles in cell polarity and membrane addition. We propose that Schwann cell-autonomous loss of Mtmr2-Dlg1 interaction dysregulates membrane homeostasis in the paranodal region, thereby producing outfolding and recurrent loops of myelin.

Osteopontin: a novel axon-regulated Schwann cell gene.

Osteopontin (OPN) is a RGD-containing glycoprotein with cytokine-like, chemotactic, and pro-adhesive properties. During wound healing, OPN is abundantly expressed by infiltrating macrophages and has been implicated in posttraumatic tissue repair. To delineate a role in the regenerative response to axotomy we examined the expression of OPN in Wallerian degeneration of the sciatic nerve in rats. Unexpectedly, we found high constitutive expression of OPN by myelinating Schwann cells (SCs) in uninjured control nerves. OPN mRNA expression was confirmed in primary cultures of rat SCs. Upon axotomy, SC-expressed OPN in the degenerating distal nerve stump transiently increased during the first days after injury, but was continuously downregulated thereafter, reaching its minimum at Day 14. Macrophages invading axotomized nerves were OPN-negative. During late stages after axotomy, SC-OPN was reexpressed in regenerating but not permanently transected nerves. We also found OPN expression by myelinating SCs in human sural nerves with a dramatic reduction in severe axonal polyneuropathies. Taken together, our study identifies OPN as a novel Schwann cell gene regulated by axon-derived signals. The lack of OPN induction in infiltrating macrophages indicates fundamental differences in tissue repair between axonal injury in the peripheral nervous system and structural lesions in other organ systems.