Axonal transport of glycerophospholipids in regenerating sciatic nerve of the rat during aging

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.

Vesicular fast axonal transport rates in young and old rat axons

An isolated sciatic nerve preparation was used to measure the transport rates of more than 18,000 vesicles in 72 axons from young (3-4 months of age) and old (24-26 months of age) rats from two strains (Harlan Sprague-Dawley and Fisher-344). Average anterograde and retrograde vesicle transport rates were significantly slower in the older animals. The amount of slowing of anterograde vesicles was twice as great as the slowing of retrograde vesicles. Age-related slowing of vesicle transport was inversely proportional to vesicle speed, with the result that transport of the slowest and largest vesicles may essentially be blocked in older axons. One possible explanation for these data is that long-lived axonal cytoskeletal proteins are subject to age-related changes that impede vesicle transport.

Morphological correlates of physiological responses in partially denervated mouse muscle during aging

To evaluate the effect of partial denervation on age changes, morphological and physiological parameters were studied in young versus old extensor digitorum longus (EDL) muscles. The aims of this study were to elucidate synaptic maintenance in general and specifically to assess the adaptability of motor neurons from young and old animals to an enlarged field of innervation. Partial denervation was carried out in two groups of 30-40 g mice, aged within 6 or 24 months, by sectioning of the nerve supplying EDL muscle under methoxyfluorane anaesthesia. Sham operations were carried out on additional animals which served as controls. Five weeks' post-surgery, the safety factor of synaptic transmission was estimated by dividing the nerve-evoked twitch tension generated in 1 mM Ca2+ Krebs by that generated in 2.5 mM Ca2+ Krebs. Low Ca2+ Krebs caused a more pronounced decline in twitch tension in young control EDL muscles than old control. After partial denervation, young EDL twitch tensions were not significantly different while old were 20% of those in normal Krebs. Zinc Iodide Osmium (ZIO) stained nerve terminal parameters were significantly increased with aging. After partial denervation, nerve terminal areas were 46 and 39% larger in young and old mice, respectively, compared to their corresponding controls. Present data suggest that motor neurons of old mice are unable to develop and maintain an enlarged field of innervation.

Geometric response to nerve growth factor is preserved in aged rat sensory neurons: a single-neuron culture study

We previously established a single-neuron culture system to analyze the primary effects of neurotrophic factors and reported that NGF promoted neurite extension in young adult (4- to 6-month-old) rat dorsal root ganglion (DRG) neurons by promoting neurite arborization. In this study, we demonstrated that the effects of NGF on neurite regeneration in DRG neurons was well preserved in aged rats (20- to 24-month-old and 33-month-old). NGF did not increase the percent process-bearing neurons in aged rats, which indicated that neuronal survival was not promoted by NGF, but it significantly enhanced the number of branching points, total neurite length, and soma size in aged neurons. These effects of NGF on neurite geometry tended to be reduced to some extent in aged neurons and the initiation of neurite-outgrowth in aged neurons was also delayed as compared with young adult neurons. NGF-responsive subpopulation of neurons, found in the entire range of neuronal size, were preserved in aged rats. These findings indicate that NGF could play an important role in regeneration of injured DRG neurons of aged animals.

Myelinated fiber regeneration after sciatic nerve crush: morphometric observations in young adult and aging mice and the effects of macrophage suppression and conditioning lesions

To study myelinated nerve fiber regeneration during aging, the right sciatic nerves of 6- and 24-month-old mice were crushed at the sciatic notch. Two, 4, and 8 weeks later, both groups of mice were perfused. The sciatic nerves were processed so that the transverse sections of each nerve subsequently studied by light and electron microscopy included the entire posterior tibial fascicle 5 mm distal to the crush site. Two weeks after axotomy, fascicles of aging mice contained significantly fewer regenerated myelinated fibers than those of young adults. After 4 weeks, the difference in the number of myelinated fibers was less. However, measurements of myelinated fibers in fascicles of aging mice showed that areas of Schwann cell cytoplasm and myelin were significantly reduced at all intervals. In contrast, although axon diameters in aging mice were somewhat less 2 weeks after crushing, the difference decreased with time, suggesting that in nerves of aging mice, regenerative responses of Schwann cells were more affected than those of axons. Other experiments in young mice showed that myelinated fiber regeneration could be retarded by suppressing macrophage responses and was not significantly changed by conditioning lesions before crush injury.

Myelinated fiber regeneration after crush injury is retarded in sciatic nerves of aging mice

To compare nerve regeneration in young adult and aging mice, the right sciatic nerves of 6- and 24-month-old mice were crushed at the sciatic notch. Two weeks later, both groups of mice were perfused with an aldehyde solution, and, after additional fixation, the sciatic nerves were processed so that the transverse sections of each nerve subsequently studied by light and electron microscopy included the entire posterior tibial fascicle 5 mm distal to the crush site. The same level was sectioned in unoperated contralateral nerves; these nerves served as controls. Electron micrographs and the Bioquant Image Analysis System IV were used to measure areas of posterior tibial fascicles and count the number of myelinated axons, the number of unmyelinated axons, and their frequency in Schwann cell units. In aging mice, the total number of regenerating myelinated axons was significantly reduced, but totals of regenerating unmyelinated axons in aging and young adults did not differ significantly. In aging mice, the frequency of Schwann cells that contained a single unmyelinated axon was greater, suggesting that before myelination began, Schwann cell ensheathment of axons also was slowed. After axotomy by a crush injury, the area of the posterior tibial fascicle was less than that in young adults and the distal disintegration of myelin sheath remnants also appeared to be retarded. The results indicate that responses of neurons, axons, and Schwann cells could be important in slowing the regeneration of myelinated fibers found in sciatic nerves from aging mice.

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.

Effect of aging on the rate of axonal transport of choline-phosphoglycerides

The anterograde axonal transport of choline-phosphoglycerides was studied in sciatic nerve motoneurons of adult (3-month-old) and aged (24-month-old) rats. After the spinal cord injection of [2-3H]glycerol, choline-phosphoglycerides; the major phospholipid class was transported along the nerve. The axonal transport rate was determined by plotting the distance covered by the front of transported radioactivity as a function of the time employed. In aged animals the rate of the choline-phosphoglyceride anterograde axonal transport was about 68% lower than that of adults; furthermore, the rate slowed down along the nerve in the proximal-distal direction. This altered axonal transport mechanism might contribute to the degenerative processes observed in distal regions of peripheral nerve fibers of aged animals.