Impairment of cytoskeletal protein transport due to aging or beta,beta'-iminodipropionitrile intoxication in the rat sciatic nerve

Positive changes in femoral nerve morphometry in older rats following aerobic training

The objective of the present study was to analyze alterations of the femoral nerve of aged rats subjected to aerobic training. Wistar rats (12-mo of age) were divided in to two groups: S group (sedentary) and T group (trained). The exercise protocol were 16 weeks long. The groups were sacrificed at 16 months. Ultrafine sections of the femoral nerve have been used. There was no change in the body weight between the groups. T group showed a significant increase in myelinated fiber area, axon diameter, myelin sheath thickness and myelin fiber number compared with sedentary controls. In exercised trained animals, histograms of the frequency distribution of myelinated axons according to their areas showed increased number of medium and large fibers in relation to small fibers, which decreased in number. Aerobic training animals, showed the distribution of myelinated fiber population according to their area being bimodal, with the distribution shifted to the right, indicating increased fiber area. The T group showed a percent damage of large myelinated fibers significantly lower compared to controls. No significant difference was observed between the groups for the g-ratio. The T group also showed a significant increase in the number of microtubules and neurofilaments in myelinated fibers, which was not observed in S group. In conclusion, aerobic training improves nerve structure without evidence of nerve damage and produces an attenuation on the modifications in femoral nerve that develop in old age.

Developmental downregulation of LIS1 expression limits axonal extension and allows axon pruning

The robust axonal growth and regenerative capacities of young neurons decrease substantially with age. This developmental downregulation of axonal growth may facilitate axonal pruning and neural circuit formation but limits functional recovery following nerve damage. While external factors influencing axonal growth have been extensively investigated, relatively little is known about the intrinsic molecular changes underlying the age-dependent reduction in regeneration capacity. We report that developmental downregulation of LIS1 is responsible for the decreased axonal extension capacity of mature dorsal root ganglion (DRG) neurons. In contrast, exogenous LIS1 expression or endogenous LIS1 augmentation by calpain inhibition restored axonal extension capacity in mature DRG neurons and facilitated regeneration of the damaged sciatic nerve. The insulator protein CTCF suppressed LIS1 expression in mature DRG neurons, and this reduction resulted in excessive accumulation of phosphoactivated GSK-3β at the axon tip, causing failure of the axonal extension. Conversely, sustained LIS1 expression inhibited developmental axon pruning in the mammillary body. Thus, LIS1 regulation may coordinate the balance between axonal growth and pruning during maturation of neuronal circuits.

Summary: Developmental downregulation of LIS1 coordinates the balance between axonal elongation and pruning, which is essential for proper neuronal circuit formation but limits nerve regeneration.

The age factor in axonal repair after spinal cord injury: A focus on neuron-intrinsic mechanisms

Age is an important consideration for recovery and repair after spinal cord injury. Spinal cord injury is increasingly affecting the middle-aged and aging populations. Despite rapid progress in research to promote axonal regeneration and repair, our understanding of how age can modulate this repair is rather limited. In this review, we discuss the literature supporting the notion of an age-dependent decline in axonal growth after central nervous system (CNS) injury. While both neuron-intrinsic and extrinsic factors are involved in the control of axon growth after injury, here we focus on possible intrinsic mechanisms for this age-dependent decline.

Axonal transport declines with age in two distinct phases separated by a period of relative stability

Axonal transport is critical for supplying newly synthesized proteins, organelles, mRNAs, and other cargoes from neuronal cell bodies into axons. Its impairment in many neurodegenerative conditions appears likely to contribute to pathogenesis. Axonal transport also declines during normal aging, but little is known about the timing of these changes, or about the effect of aging on specific cargoes in individual axons. This is important for understanding mechanisms of age-related axon loss and age-related axonal disorders. Here we use fluorescence live imaging of peripheral nerve and central nervous system tissue explants to investigate vesicular and mitochondrial axonal transport. Interestingly, we identify 2 distinct periods of change, 1 period during young adulthood and the other in old age, separated by a relatively stable plateau during most of adult life. We also find that after tibial nerve regeneration, even in old animals, neurons are able to support higher transport rates of each cargo for a prolonged period. Thus, the age-related decline in axonal transport is not an inevitable consequence of either aging neurons or an aging systemic milieu.

Motor axon regeneration and muscle reinnervation in young adult and aged animals

Injuries to peripheral nerves can cause paralysis and sensory disturbances, but such functional impairments are often short lived because of efficient regeneration of damaged axons. The time required for functional recovery, however, increases with advancing age (Verdú et al., 2000; Kawabuchi et al., 2011). Incomplete or delayed recovery after peripheral nerve damage is a major health concern in the aging population because it can severely restrict a person's mobility and independence. A variety of possible causes have been suggested to explain why nervous systems in aged individuals recover more slowly from nerve damage. Potential causes include age-related declines in the regenerative potential of peripheral axons and decreases in the supply or responsivity to trophic and/or tropic factors. However, there have been few direct analyses of age-related axon regeneration. Our aim here was to observe axons directly in young and old mice as they regenerate and ultimately reoccupy denervated neuromuscular synaptic sites to learn what changes in this process are age related. We find that damaged nerves in aged animals clear debris more slowly than nerves in young animals and that the greater number of obstructions regenerating axons encounter in the endoneurial tubes of old animals give rise to slower regeneration. Surprisingly, however, axons from aged animals regenerate quickly when not confronted by debris and reoccupy neuromuscular junction sites efficiently. These results imply that facilitating clearance of axon debris might be a good target for the treatment of nerve injury in the aged.

Molecular signaling how do axons die?

Axons depend critically on axonal transport both for supplying materials and for communicating with cell bodies. This chapter looks at each activity, asking what aspects are essential for axon survival. Axonal transport declines in neurodegenerative disorders, such as Alzheimer's disease, amyotrophic lateral sclerosis, and multiple sclerosis, and in normal ageing, but whether all cargoes are equally affected and what limits axon survival remains unclear. Cargoes can be differentially blocked in some disorders, either individually or in groups. Each missing protein cargo results in localized loss-of-function that can be partially modeled by disrupting the corresponding gene, sometimes with surprising results. The axonal response to losing specific proteins also depends on the rates of protein turnover and on whether the protein can be locally synthesized. Among cargoes with important axonal roles are components of the PI3 kinase, Mek/Erk, and Jnk signaling pathways, which help to communicate with cell bodies and to regulate axonal transport itself. Bidirectional trafficking of Bdnf, NT-3, and other neurotrophic factors contribute to intra- and intercellular signaling, affecting the axon's cellular environment and survival. Finally, several adhesion molecules and gangliosides are key determinants of axon survival, probably by mediating axon-glia interactions. Thus, failure of long-distance intracellular transport can deprive axons of one, few, or many cargoes. This can lead to axon degeneration either directly, through the absence of essential axonal proteins, or indirectly, through failures in communication with cell bodies and nonneuronal cells.

Morphological and biochemical changes of neurofilaments in aged rat sciatic nerve axons

We have made a detailed comparison of neurofilaments (NFs) in the axons of the sciatic nerves between young and aged rats. In young rats, NF density was similar between proximal and distal sciatic nerve, but it became higher in the proximal region of sciatic nerve of aged rats. In accordance with this morphological change, NF protein content decreased dramatically in the middle region of the sciatic nerves of aged rats. The ratio of NF-M to NF-H in aged rats was lower than that in young rats at the proximal region of sciatic nerves and further decreased in the distal region of sciatic nerve. We analyzed transcription and axonal transport of NF proteins in motor neurons in spinal cord which are the major constituents of sciatic nerve axons. Of the transcripts of the NF subunits, NF-M mRNA was particularly reduced in aged rats. Examination of slow axonal transport revealed that the transport rate for NF-M was slightly faster than that for NF-H in young rats, but slightly slower in aged rats. A decrease in both the synthesis and transport rate of NF-M with aging may contribute to the relative reduction in NF-M in the aged rat sciatic nerve. Although the relationship between NF packing and reduced NF-M is not clear at present, these changes in NFs may be associated with age-dependent axonal degeneration diseases.

Age-dependent changes in the midsized neurofilament subunit in sensory-motor systems of the cat brainstem: an immunocytochemical study

This study documents age-related changes in the immunoreactivity of the medium-molecular weight subunit of neurofilaments in sensory and motor neurons in the brainstem of the cat. In old age, there was a clear decrease in immunoreactivity in the following brainstem sensory and motor nuclei: sensory trigeminal, gracile, cuneate, and facial motor. Only a few neuronal perikarya and dendrites were labeled in these nuclei in old cats; moreover, when present, the labeling was weak. In contrast, in adult cats, these nuclei contained intensely stained neuronal perikarya and dendrites. In other sensory and motor nuclei of the brainstem, there was an obvious age-related increase in the immunoreactivity of the medium-molecular weight subunit of neurofilaments in the perikarya. Despite different patterns of age-related alterations in immunoreactivity within perikarya and dendrites in distinct brainstem regions, most sensory and motor axons in old cats were smaller than those in adult cats. A decrease in the medium-molecular weight neurofilament subunit in the dendrites may be the basis for the dendritic atrophy that has been shown to occur in sensory nuclei in old animals. The decrease in axonal size is likely to be one of the causes of the decrease in axonal conduction velocity, in these neurons, that was reported in our previous studies.

Motor neurons are rich in non-phosphorylated neurofilaments: cross-species comparison and alterations in ALS

The localization and distribution of non-phosphorylated neurofilaments (NP-NF) in the upper and lower motor neurons was investigated in the rat, the common marmoset, the rhesus monkey and man using the SMI-32 antibody. Within the spinal cord of all species studied, the most intense NP-NF immunoreactivity was observed within the ventral horn alpha-motor neurons. Concurrent staining for the cholinergic marker choline acetyltransferase (ChAT) demonstrated that virtually all of the ChAT-positive alpha-motor neurons contain NP-NF immunoreactivity. Although NP-NF staining was also observed in other neurons within the ventral and intermediate horns, these neurons were loosely scattered and contained a considerably lower staining intensity. The only other prominent NP-NF staining in the spinal cord occurred within the neurons of the dorsal nucleus of Clark and the intermediolateral cell column. Phosphorylated neurofilament (P-NF) immunoreactivity was found primarily in neuronal processes. Occasionally, a solitary motor neuron contained weak P-NF immunoreactivity. Within the brainstem, neurons in all cranial nerve motor nuclei contained intense NP-NF immunoreactivity. The distribution and apparent density of NP-NF immunoreactive neurons in these nuclei was virtually identical to that observed for neurons immunoreactive for ChAT. NP-NF immunoreactive neurons of relatively lower intensity were found in many other regions of the brainstem. All of the giant Betz cells of layer (L) V in the motor cortex contained dark NP-NF immunoreactivity. Within the spinal cord of amyotrophic lateral sclerosis (ALS) patients, both Nissl and NP-NF staining demonstrated the dramatic loss of alpha-motor neurons characteristic of this disorder. Some of the remaining motor neurons contained intense P-NF immunoreactivity. These observations suggest that NP-NF immunoreactivity is a good marker for motor neurons in health and disease and may be a useful tool for studies of motor neuron degeneration (MND).

The effects of cell passages on the cell morphology and the outcome of herpes simplex virus type 1 infection

Because cell cultures are essential in biological research which involves the analysis of virus morphogenesis, this study focused on examining the significance of cell passages. Human embryonic lung fibroblasts (MRC-5) at passage (P) 27 were seeded twice a week to P 32, P 40, and P 48, when just at confluence and then infected with herpes simplex virus type 1 (HSV-1). The structure of the non-virus-infected (MOCK) and HSV-1 infected cells, the amount of cellular infectious virus particles and the capability to express HSV-1 glycoproteins C (gC-1) and D (gD-1) were investigated by phase-contrast and immunofluorescence light microscopy, immunogold cryosection EM, plaque assays, immunoblots, and total protein assays. Modified cell structure including fragmentation of tubulin fibers were visible in MOCK from P 38 onwards. The quantity of vimentin remained unchanged while actin accumulated and beta-tubulin decreased in HSV-1 infected late P cells compared to early P cultures. Cells of high P counts contained significantly fewer infectious virus particles, very likely of lower virulence, and their expression of gC-1 and gD-1 were concordantly reduced. These observations indicate that the number of cell P must be considered in order to reproduce results of cell biology and viral morphogenesis. The MRC-5 cells ought not to be passaged more than ten times beyond P 27 in the laboratory.

Deprenyl induces the tyrosine hydroxylase enzyme in the rat dopaminergic nigrostriatal system

Chronic treatment of aged rats with deprenyl prevents age-induced protein oxidation in substantia nigra and protects tyrosine hydroxylase (TH) enzyme against inactivation [11]. With these precedents, we treated adult rats with deprenyl for 3 weeks in order to get further insight in the mechanism by which deprenyl exerts such actions. After completing the treatment, dopamine (DA) levels markedly increased in both striatum and substantia nigra while levels of the acid DA metabolites, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), decreased in the two brain areas, thus proving MAO-inhibiting properties of the treatment. We then studied the cellular expression of TH mRNA by in situ hybridization. Following treatment with deprenyl, levels of TH mRNA were significantly higher in individual dopaminergic nigral cell bodies than in those of control rats (+74%). Western blotting analysis of TH enzyme amount revealed a positive effect of the treatment in both the terminal field (+44%) and the cell body region (+31%). This correlation between TH mRNA and amount was also extended to TH enzyme activity in the two brain areas studied, which significantly increased in striatum (+57%) and substantia nigra (+35%) following deprenyl treatment. Taken together, our results clearly suggest a TH-inducing effect of deprenyl in the dopaminergic nigrostriatal system, which seems to be independent of its protective action against oxidative stress described previously. These results expand our knowledge about the beneficial effect of deprenyl in the therapy of Parkinson's disease.