Myelination of regenerating sciatic nerve of the rat: lipid components and synthesis of myelin lipids

Time-Dependent Effect of Sciatic Nerve Injury on Rat Plasma Lipidome

Neuropathic pain is a condition affecting the quality of life of a substantial part of the population, but biomarkers and treatment options are still limited. While this type of pain is caused by nerve damage, in which lipids play key roles, lipidome alterations related to nerve injury remain poorly studied. Here, we assessed blood lipidome alterations in a common animal model, the rat sciatic nerve crush injury. We analyzed alterations in blood lipid abundances between seven rats with nerve injury (NI) and eight control (CL) rats in a time-course experiment. For these rats, abundances of 377 blood lipid species were assessed at three distinct time points: immediately after, two weeks, and five weeks post injury. Although we did not detect significant differences between NI and CL at the first two time points, 106 lipids were significantly altered in NI five weeks post injury. At this time point, we found increased levels of triglycerides (TGs) and lipids containing esterified palmitic acid (16:0) in the blood plasma of NI animals. Lipids containing arachidonic acid (20:4), by contrast, were significantly decreased after injury, aligning with the crucial role of arachidonic acid reported for NI. Taken together, these results indicate delayed systematic alterations in fatty acid metabolism after nerve injury, potentially reflecting nerve tissue restoration dynamics.

Lipid metabolism adaptations are reduced in human compared to murine Schwann cells following injury

Mammals differ in their regeneration potential after traumatic injury, which might be caused by species-specific regeneration programs. Here, we compared murine and human Schwann cell (SC) response to injury and developed an ex vivo injury model employing surgery-derived human sural nerves. Transcriptomic and lipid metabolism analysis of murine SCs following injury of sural nerves revealed down-regulation of lipogenic genes and regulator of lipid metabolism, including Pparg (peroxisome proliferator-activated receptor gamma) and S1P (sphingosine-1-phosphate). Human SCs failed to induce similar adaptations following ex vivo nerve injury. Pharmacological PPARg and S1P stimulation in mice resulted in up-regulation of lipid gene expression, suggesting a role in SCs switching towards a myelinating state. Altogether, our results suggest that murine SC switching towards a repair state is accompanied by transcriptome and lipidome adaptations, which are reduced in humans.

Seeding nerve sutures with minced nerve-graft (MINE-G): a simple method to improve nerve regeneration in rats

BACKGROUND

The aim of this study was to assess the effect of seeding the distal nerve suture with nerve fragments in rats.

METHODS

On 20 rats, a 15 mm sciatic nerve defect was reconstructed with a nerve autograft. In the Study Group (10 rats), a minced 1 mm nerve segment was seeded around the nerve suture. In the Control Group (10 rats), a nerve graft alone was used. At 4 and 12 weeks, a walking track analysis with open field test (WTA), hystomorphometry (number of myelinated fibers (n), fiber density (FD) and fiber area (FA) and soleus and gastrocnemius muscle weight ratios (MWR) were evaluated. The Student t-test was used for statistical analysis.

RESULTS

At 4 and 12 weeks the Study Group had a significantly higher n and FD (p = .043 and .033). The SMWR was significantly higher in the Study Group at 12 weeks (p = .0207).

CONCLUSIONS

Seeding the distal nerve suture with nerve fragments increases the number of myelinated fibers, the FD and the SMWR. The technique seems promising and deserves further investigation to clarify the mechanisms involved and its functional effects.

Deciphering the Lipid Architecture of the Rat Sciatic Nerve Using Imaging Mass Spectrometry

Knowledge on the normal structure and molecular composition of the peripheral nerves is essential to understand their pathophysiology and to select the regeneration strategies after injury. However, the precise lipid composition of the normal peripheral nerve is still poorly known. Here, we present the first study of distribution of individual lipids in the mature sciatic nerve of rats by imaging mass spectrometry. Both positive and negative ion modes were used to detect, identify and in situ map 166 molecular species of mainly glycerophospholipids, sphingomyelins, sulfatides, and diacyl and triacylglycerols. In parallel, lipid extracts were analyzed by LC-MS/MS to verify and complement the identification of lipids directly from the whole tissue. Three anatomical regions were clearly identified by its differential lipid composition: the nerve fibers, the connective tissue and the adipose tissue that surrounds the nerve. Unexpectedly, very little variety of phosphatidylcholine (PC) species was found, being by far PC 34:1 the most abundant species. Also, a rich composition on sulfatides was detected in fibers, probably due to the important role they play in the myelin cover around axons, as well as an abundance of storage lipids in the adipose and connective tissues. The database of lipids here presented for each region and for the whole sciatic nerve is a first step toward understanding the variety of the peripheral nerves' lipidome and its changes associated with different diseases and mechanical injuries.

The lipid sulfatide is a novel myelin-associated inhibitor of CNS axon outgrowth

CNS myelin is strongly inhibitory to growing axons and is thought to be a major contributor to CNS axon regenerative failure. Although a number of proteins present in myelin, including Nogo, MAG, and oligodendrocyte-myelin glycoprotein (OMgp), have been identified as myelin-associated inhibitors, studies of mice lacking these genes suggest that additional inhibitors present in CNS myelin remain to be identified. Here we have investigated the hypothesis that myelin lipids contribute to CNS regenerative failure. We identified sulfatide, a major constituent of CNS myelin, as a novel myelin-associated inhibitor of neurite outgrowth. Sulfatide, but not galactocerebroside or ceramide, strongly inhibited the neurite outgrowth of retinal ganglion cells (RGCs) when used as a purified lipid substrate. The mechanism involved in sulfatide-mediated inhibition may share features with other known inhibitors, because the Rho inhibitor C3 transferase lessened these effects. Myelin in which sulfatide was lacking or blocked using specific antibodies was significantly less inhibitory to RGC neurite outgrowth in vitro than was wild-type myelin, indicating that sulfatide is a major component of the inhibitory activity of CNS myelin. Mice unable to make sulfatide did not regenerate RGC axons more robustly after optic nerve crush than wild-type littermates under normal conditions but did exhibit a small but significant enhancement in the extent of zymosan-induced regeneration. These results demonstrate that specific lipids can powerfully inhibit axon growth, identify sulfatide as a novel myelin-associated axon growth inhibitor, and provide evidence that sulfatide inhibition contributes to axon regenerative failure in vivo.

Multiexponential T2 and magnetization transfer MRI of demyelination and remyelination in murine spinal cord

Identification of remyelination is important in the evaluation of potential treatments of demyelinating diseases such as multiple sclerosis. Local injection of lysolecithin into the brain or spinal cord provides a murine model of demyelination with spontaneous remyelination. The aim of this study was to determine if quantitative, multicomponent T(2) (qT(2)) analysis and magnetization transfer ratio (MTR), both indicative of myelin content, could detect changes in myelination, particularly remyelination, of the cervical spinal cord in mice treated with lysolecithin. We found that the myelin water fraction and geometric mean T(2) value of the intra/extracellular water significantly decreased at 14 days then returned to control levels by 28 days after injury, corresponding to clearance of myelin debris and remyelination which was shown by eriochrome cyanine and oil red O staining of histological sections. The MTR was significantly decreased 14 days after lysolecithin injection, and remained low over the time course studied. Evidence of demyelination shown by both qT(2) and MTR lagged behind the histological evidence of demyelination. Myelin water fraction increased with remyelination, however MTR remained lower after 28 days. The difference between qT(2) and MTR may identify early remyelination.

Myelin basic protein-mRNA used to monitor trimethyltin neurotoxicity in rats

Trimethyltin (TMT) is an alkyltin that targets neurons of the limbic system. A gene probe (i.e., mRNA) for myelin basic protein (MBP), a major component of central nervous system myelin, was used to monitor this toxic neuropathy in Sprague-Dawley rats. Animals were administered a single intraperitoneal injection of TMT-hydroxide at a neuropathic (8.0 mg/kg/body wt) or nonneuropathic (0.8 mg/kg/body wt) dose and sampled at 1, 3, or 7 days postexposure to correlate the progression of hippocampal neuropathology with probe (i.e., MBP-mRNA) levels. Microscopic examination of the brain showed only moderate but progressive damage over the 7-day postexposure period in animals treated with the neuropathic dose. Neuronal loss was first observed in the dendate gyrus and CA4 at 1 day postexposure, and progressed to the CA3c sector at 3 and 7 days postexposure. Elsewhere in the brain, minimal involvement of the entorhinal cortex neurons occurred 3 days postexposure and intensified by 7 days. No histological damage was seen at the nonneuropathic (0.8 mg/kg) dose. For gene probe analysis, the brain was divided into anterior and posterior halves. In rats treated with the neuropathic dose of TMT, the anterior brain showed progressive depressions of MBP-mRNA levels over the 1-, 3-, and 7-day postexposure period that correlated with increasing hippocampal neuropathology. The posterior brain showed no significant changes in MBP-mRNA levels with respect to that of controls over the same time period. At the nonneuropathic dose (0.8 mg/kg) a significant depression of MBP-mRNA levels occurred in the anterior brain at 7 days postexposure in the absence of overt histological damage.

Temporal changes in PO and MBP gene expression after crush-injury of the adult peripheral nerve

The crush-injured sciatic nerve provides a model to study Schwann cell regulation of myelin gene expression during the process of demyelination and remyelination. In order to investigate the possible transcriptional regulation of myelin gene expression, the quantity, quality and translational efficiency of PO (the major myelin glycoprotein) and MBP (the myelin basic proteins) coding messages were investigated as a function of time following crush-injury of the adult rat sciatic nerve. Northern blot analysis indicated that the size of the PO and MBP transcripts remain unchanged in the distal segments of crushed sciatic nerves at 1, 2, 4, 7, 10, 14 and 21 days after crush-injury. Dot-blot analysis showed a sharp drop in levels of PO and MBP coding transcripts 1 day after crush-injury with the lowest steady-state levels at 4-7 days. Message levels were found to increase after 7 days, the highest increase in levels of message was found to be between 10 and 14 days. The highest steady-state level of both transcripts was observed at 21 days. In vitro translation and immunoprecipitation of PO-translated products from various stages of crush-injury also indicated this trend. The pattern of gene expression of PO- and MBP-coding transcripts parallel each other and follow the pattern of demyelination and remyelination. The results are also consistent with our previous interpretation which suggests that PO and MBP gene expression is regulated at the level of transcription and that these two genes might be coordinately expressed. Western blot analysis of PO protein from these stages revealed a similar decrease and then increase in the levels of the protein.(ABSTRACT TRUNCATED AT 250 WORDS)

Age-related changes of RNA and lipid synthesis in vitro by retina and optic nerve of the rat

We examined the effects of age on RNA and lipid formation by whole retina and optic nerve in vitro. Male Wistar rats, aged 4, 12, and 24 mo, were used. From the results obtained the following conclusions may be drawn: 1. In assaying the lipid biosynthesis during aging, a striking difference between the retina and optic nerve clearly emerged; 2. In isolated retina, [3H]uridine incorporation into RNA was relatively constant at the three ages, whereas both [14C]palmitate and [3H]choline incorporation into lipids showed a substantial increase in rats at 24 mo of age compared with those at 4 mo; 3. In contrast, in the optic nerve of the oldest rats, compared with the youngest, a significant decrease of [14C]acetate and [14C]palmitate incorporation into acylglycerols, cerebrosides, and phospholipids was found. Each fatty acid precursor label was incorporated to a proportion that reflected the typical acyl group composition of individual lipids; 4. Following labeling of the optic nerve with [3H]choline, the specific radioactivity of choline-containing phospholipids was drastically decreased with increasing rat age; and 5. The incorporation of [2-3H]glycerol into optic nerve diacylglycerols, PtdEtn, and PtdIns declined with age, whereas no significant change took place in the incorporation into PtdCho. The results strongly support the concept that RNA metabolism of rat retina (most likely photoreceptor cell layer) is not altered during aging; on the contrary, phospholipid synthesis is stimulated in comparison with that of the optic nerve, for which a serious impairment was concomitantly observed. The physiological significance of these responses, and the mechanism by which retinal tissue is spared from the general age derangement of the nervous system, remain to be defined.

Gene expression in the presence or absence of myelin assembly

Regulation of myelin protein gene expression in the presence and absence of myelin assembly can be assessed using crushed or permanently transected adult sciatic nerves of rats. The P0 glycoprotein and the myelin basic protein (MBP) are the major myelin-specific proteins of the peripheral nervous system. The steady-state level of P0 and MBP messenger RNA was determined by dot-blot analysis of poly(A)+ RNA from crushed and transected nerves of rats at 35 days post operation. The rat P0-specific cDNA clone, pSN63c, and mouse MBP-specific cDNA clone, pHF43, were used as probes. The level and quality of the poly(A)+ RNA was assessed by in vitro translation and immunoprecipitation of the translation products with anti-chick P0 antibody. Comparison of the steady-state level of P0 and MBP transcripts and the level of anti-P0 immunoprecipitated translation products from RNA extracts of permanently transected, crushed, adult control and 21-day-old control rat nerves indicated that the level of P0 and MBP messages was significantly reduced in the permanently transected model, whereas it was restored to normal in the crushed sciatic nerve 35 days post injury. These results suggest that regulation of P0 and MBP gene expression most likely occurs at the transcriptional or post-transcriptional level in the two models of peripheral neuropathies. Northern blot analysis indicated the absence of differential splicing of the message in crushed or transected nerves. The experiments also indicate that these two important gene products required for myelin synthesis and assembly seem to be co-regulated. However, the data do not rule out the possibility that regulation of gene expression may also occur at the level of translation or post-translational processing.

Myelination process in the rat sciatic nerve during regeneration and development: molecular species composition and acyl group biosynthesis of choline-, ethanolamine-, and serine-glycerophospholipids of myelin fractions

The content of alkenyl-acyl, alkyl-acyl and diacyl types of the three major myelin glycerophospholipids such as PtdCho, PtdEtn and PtdSer was determined in myelin fractions prepared from sciatic nerve segments of rats at 12, 25 and 45 days after birth, and of adult rats (6-month-old) 90 days after crush injury. The biosynthesis and metabolic heterogeneity of lipid classes and types were also studied by incubation with [1-14C] acetate of nerve segments of young rats at different ages as well as crushed and sham-operated control nerve segments of adult rats. The analysis of composition and positional distribution in major individual molecular species extracted from light myelin and myelin-related fraction suggest that the metabolism of alkenyl-acyl-glycerophosphorylethanolamines and unsaturated species of PtdCho and PtdSer may not be regulated in the same manner during peripheral nerve myelination of developing rat and remyelination of regenerating nerve in the adult animal. The 14C-radioactivity incorporation into lipid classes and alkyl and acyl moieties of the three major phospholipids of sciatic nerve segments during the developmental period investigated revealed that Schwann cells were capable of synthesizing acyl-linked fatty acids in both myelin fractions at a decreasing rate and with different patterns during development. In regenerating sciatic nerve of adult animals the labeling of myelin lipid classes and types of remyelinating nerve segment distal to the crush site was markedly higher than that of sham-operated normal one; however, the magnitude and the pattern of the specific radioactivity never approached those observed during active myelination of the nerve in young animals.(ABSTRACT TRUNCATED AT 250 WORDS)