Sphingolipid metabolic disorders in Trembler mouse peripheral nerves in vivo result from an abnormal substrate supply

Myelin-associated glycoprotein and galactosylcerebroside expression in Schwann cells during myelination

The temporo-spatial expression profiles of the myelin-associated glycoprotein (MAG) and galactosylcerebrosides (Gal-CBs) were analysed during Schwann cell differentiation and myelination in Schwann cell cultures and in dorsal root ganglion/Schwann cell cocultures. The temporal expression profiles demonstrate that the mechanisms triggering and regulating MAG and Gal-CBs are different and independent. They also provide further support for a role of MAG, but not Gal-CBs, in the establishment of the Schwann cell/axon interaction and in the formation of the primary mesaxon, while both MAG and Gal-CBs may participate in the further maturation of the mesaxon. The spatial expression profiles demonstrate that although Gal-CBs are concentrated in the same structures (mesaxonal spirals, paranodes and Schmidt-Lanterman incisures) as MAG in both developing and mature sheaths, where it is therefore possible that the two might have complementary functions, they are also present in regions from which MAG is excluded.

Non-neuronal cells are not the limiting factor for the low axonal regeneration in C57BL/6J mice

Peripheral axonal regeneration was investigated in adult male mice of the C57BL/6J (C), BALB/cJ (B) and A/J (A) strains and in their F1 descendants using a predegenerated nerve transplantation model. Four types of transplants were performed: 1) isotransplants between animals of the C, B and A strains; 2) donors of the C strain and recipients of the C x B and C x A breeding; 3) donors of the B strain and recipients of the C x B breeding, and 4) donors of the A strain and recipients of the C x A breeding. Donors had the left sciatic nerve transected and two weeks later a segment of the distal stump was transplanted into the recipient. Four weeks after transplantation the regenerated nerves were used to determine the total number of regenerated myelinated fibers (TMF), diameter of myelinated fibers (FD) and myelin thickness (MT). The highest TMF values were obtained in the groups where C57BL/6J mice were the donors (C to F1 (C x B) = 4658 +/- 304; C to F1 (C x A) = 3899 +/- 198). Also, A/J grafts led to a significantly higher TMF (A to F1 (C x A) = 3933 +/- 565). Additionally, isotransplant experiments showed that when the nerve is previously degenerated, C57BL/6J mice display the largest number of myelinated fibers (C to C = 3136 +/- 287; B to B = 2759 +/- 170, and A to A = 2835 +/- 239). We also observed that when C57BL/6J was the graft donor, FD was the highest and MT did not differ significantly when compared with the other groups. These morphometric results reinforce the idea that Schwann cells and the nerve environment of C57BL/6J provide enough support to the regenerative process. In this respect, the present results support the hypothesis that the non-neuronal cells, mainly Schwann cells, present in the sciatic nerve of C57BL/6J mice are not the main limiting factor responsible for low axonal regeneration.

The peripheral myelin protein 22 and epithelial membrane protein family

The peripheral myelin protein 22 (PMP22) and the epithelial membrane proteins (EMP-1, -2, and -3) comprise a subfamily of small hydrophobic membrane proteins. The putative four-transmembrane domain structure as well as the genomic structure are highly conserved among family members. PMP22 and EMPs are expressed in many tissues, and functions in cell growth, differentiation, and apoptosis have been reported. EMP-1 is highly up-regulated during squamous differentiation and in certain tumors, and a role in tumorigenesis has been proposed. PMP22 is most highly expressed in peripheral nerves, where it is localized in the compact portion of myelin. It plays a crucial role in normal physiological and pathological processes in the peripheral nervous system. Progress in molecular genetics has revealed that genetic alterations in the PMP22 gene, including duplications, deletions, and point mutations, are responsible for several forms of hereditary peripheral neuropathies, including Charcot-Marie-Tooth disease type 1A (CMT1A), Dejerine-Sottas syndrome (DDS), and hereditary neuropathy with liability to pressure palsies (HNPP). The natural mouse mutants Trembler and Trembler-J contain a missense mutation in different hydrophobic domains of PMP22, resulting in demyelination and Schwann cell proliferation. Transgenic mice carrying many copies of the PMP22 gene and PMP22-null mice display a variety of defects in the initial steps of myelination and/or maintenance of myelination, whereas no pathological alterations are detected in other tissues normally expressing PMP22. Further characterization of the interactions of PMP22 and EMPs with other proteins as well as their regulation will provide additional insight into their normal physiological function and their roles in disease and possibly will result in the development of therapeutic tools.

Many facets of the peripheral myelin protein PMP22 in myelination and disease

Peripheral myelin protein 22 (PMP22) is a small, hydrophobic glycoprotein, which is most prominently expressed by Schwann cells as a component of compact myelin of the peripheral nervous system (PNS). Recent progress in molecular genetics revealed that mutations affecting the PMP22 gene including duplications, deletions, and point mutations are responsible for the most common forms of hereditary peripheral neuropathies including Charcot-Marie-Tooth disease type 1A (CMT1A), hereditary neuropathy with liability to pressure palsies (HNPP), and a subtype of Dejerine-Sottas Syndrome (DSS). Functionally, PMP22 is involved in correct myelination during development of peripheral nerves, the stability of myelin, and the maintenance of axons. While most of these functions relate to a role of PMP22 as a structural component of myelin, PMP22 has also been proposed as a regulator of Schwann cell proliferation and differentiation. In this review, we will discuss our current knowledge of PMP22 and its related proteins in the normal organism as well as in disease. In particular, we will focus on how the function of PMP22 and its regulation may be relevant to particular disease mechanisms.

Pathways of incorporation of fatty acid into glycerolipids of the murine peripheral nervous system in vivo: alterations in the dysmyelinating mutant trembler mouse

In vivo glycerolipid metabolism was studied in sciatic nerves of normal and Trembler mice. The results showed that two kinetically independent pathways were implicated in the labeling of diacylglycerophospholipids from [3H]palmitate: the Kennedy pathway and a 'direct acylation' pathway. In normal nerves, 45% of the glycerophospholipids were labeled, with a rate constant k3 = 3.9 x 10(-3) min-1, from phosphatidic acid and diacylglycerol intermediates, themselves formed with a rate constant of k1 = 0.24 min-1 from a free 3H-fatty acid pool, FFA1, that represents 45% of the total injected label. The remaining 55% of the glycerophospholipids were labeled from a kinetically distinct free 3H-fatty acid pool, FFA2, with a rate constant of k4 = 9.8 x 10(-2) min-1, via a process that does not implicate a detectably labeled metabolic intermediate ('direct acylation'). Glycerophospholipid labeling via the Kennedy pathway in the Trembler mouse sciatic nerves was reduced to 75% of the normal level, while labeling via the 'direct acylation' pathway was increased 1.4-fold. The values of the rate constants for free 3H-fatty acid utilisation (k1 and k4) were both increased about 2.5-fold, while that of glycerophospholipid formation from diacylglycerol (k3) was close to normal.