Research report
Expression of p75NTR, trkB and trkC in nonmanipulated and axotomized motoneurons of aged rats

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Abstract

Several lines of evidence indicate that adult neurons remain dependent on neurotrophins and that changes in tissue expression of neurotrophins and/or their receptors may play a role in senile neurodegeneration. We have studied the expression of p75NTR, trkB and trkC, respectively, in lumbar motoneurons of young adult (2–3 months) and aged (30 months) rats subjected to sciatic transection using in situ hybridization and immunohistochemistry. Nonmanipulated age-matched animals were processed in parallel. In nonmanipulated aged rats, high levels of p75NTR could be seen in a number of motoneurons (10–15%), while in young adult animals no p75NTR could be detected. Seven days following sciatic axotomy, a conspicuous ipsilateral upregulation p75NTR was observed in young adult rats. Also in aged rats there was a marked ipsilateral increase in number of p75NTR expressing neurons (≈100%). In comparison to young adult rats, aged rats showed a decreased expression of both trkB (5/6 animals) and trkC (6/6 animals). Furthermore, in response to sciatic transection, 3 out of 5 aged rats did not show an increased expression of trkB. In aged rats, axotomy did not induce any significant change in trkC expression. In the young adult rats, we recorded a side-to-side effect with lower values ipsilaterally, however, it cannot be excluded that this difference was caused by an upregulation in the contralateral motoneurons. Oligonucleotide probes against BDNF and NT3 mRNA showed only very few faintly positive neurons in both age groups. Our results indicate that the pattern of regulatory changes of NT receptors in response to axotomy is different in aged and young adult rats. The lack of covariation between p75NTR and trkB and trkC regulation in aged rats indicates a changed role for p75NTR in senescent motoneurons.

Introduction

Among humans around 15% of the elderly population have significant motor handicaps apparently without any other concomitant neurological disorder [68]. Age related decline in muscle function has been considered to be due to `senile muscle atrophy' characterized by a decreased muscle fibre size, shifts in muscle fiber types and a loss of muscle fibres (reviewed in Refs. 35, 69). Muscle histochemical analysis in man and rat have shown changes characteristic for loss, or retraction, of motor axons, followed by a partial collateral re-innervation of the denervated fibres. This pattern of changes provides strong evidence that `senile muscle atrophy' is of neurogenic origin 1, 9, 19, 34, 37, 47, 54, 84, 98, 103. Loss of functions with advancing age has often been attributed to aging-related neuron death. However, in a number of more recent studies this notion has been challenged (e.g., Refs. 6, 25, 31, 50, 58, 80, 82, 83, 85, 93, 95, 101), and in the spinal cord of aged individuals only a small loss of motoneurons is seen (∼10–20%) 50, 52, 56, 57, 97. However, the aged nervous system is characterized by axonal aberrations, including axon dystrophy, demyelination/dysmyelination, degeneration and loss of axons 27, 53, 64, 66, 91, 92. Axon aberrations are frequent in both the ventral root and peripheral motor nerves 38, 64, and therefore spinal motoneurons may suffer from aging-related axon lesions. We have previously shown that lumbar motoneurons of aged rats show an upregulation of both calcitonin gene-related peptide (CGRP) and growth-associated protein GAP-43 [50]. Interestingly, lesioning of the motor axons in young adult rats induce similar changes in the expression of CGRP and GAP-43 72, 86, 94, indicating that the regulatory changes we see in aged motoneurons indeed may be due to axonal damage.

If the motoneurons connection to the peripheral target tissue is interrupted by axonal transection in adult rat, this will induce a transient changes in the expression of neurotrophin receptors (p75NTR and trkB) 13, 20, 28, 60, 61, 87. It has been well documented that motoneurons are strongly target-dependent in the pre-and early postnatal period and that target deprivation results in a massive loss of neurons due to a ceased access of one or several muscle-produced and retrogradely transported trophic factor(s) (reviewed in Refs. 71, 73, 81). In the adult rat, brain derived neurotrophic factor (BDNF), neurotrophin 3 (NT3) and neurotrophin 4 (NT4) are all present at low levels in the muscle tissue and neurotrophin receptors (p75NTR, trkB and trkC) mRNAs are expressed in adult motoneurons 10, 46, 76, 77and references cited above. It has been suggested based on experimental studies that, although motoneurons in the mature animal are not target-dependent, the continued expression of neurotrophins (NTs) may be important in the regeneration of lesioned axons 28, 78and possibly also for the integrity/phenotypic characteristics of the motoneurons (Refs. 26, 28; reviewed in Ref. [71]).

Recently, we have demonstrated that motoneurons in motor incapacitated aged rats express decreased levels of trkB mRNA [51]. In this study, we have examined the expression of p75NTR, trkB and trkC using ISH in aged and young adult rats subjected to mid-thigh sciatic transection. Nonmanipulated aged-matched animals were processed in parallel.

Section snippets

Eperimental animals, surgery and tissue collection

In this study, spinal cords from 11 aged (30 months old; b.wt. 250-400 g) and 12 young adult (2–3 month old; b.wt. 200–250 g) female Sprague–Dawley (SD; Bkl:SD) rats were used. For immunohistochemistry (IHC) and in situ hybridization (ISH). The rats were delivered at an age of 2 months and kept under standard barrier conditions (12 h daylight/12 h darkness) in standard cages, with free access to water and food pellets with reduced protein content (Lactamin, Vadstena, Sweden). Under these

p75NTR in motoneurons

p75NTR could not be detected in young adult motoneurons. In aged rats, on average, 2 neurons per section showed p75NTR mRNA, with the highest count in the animal with least symptoms and the lowest count in an animal with severe symptoms of incapacitation (Table 1A). In near-adjacent tissue sections from the same animals, IHC revealed that the increased levels of p75NTR mRNA translates to p75NTR protein (cf. Fig. 1A–C,D).

In response to sciatic lesioning, young adult rats showed a marked

Discussion

In this report, we show that the p75NTR receptor is upregulated in aged motoneurons and that a further upregulation can be induced by peripheral nerve transection. We also confirm and extend earlier findings that both trkB and trkC are downregulated with advancing age [51]; furthermore, we provide new evidence that no significant change in the expression pattern can be recorded after sciatic axotomy in a majority of the aged animals. Combined, it seems that in senescence, there is a

Acknowledgements

This study was supported by grants from the Swedish Medical Research Council (projects 10820 and 2887), Karolinska Institutet, L and H Ostermans fond för medicinsk forskning, Marcus och Amalia Wallenbergs Minnesfond, Konung Gustaf V:s och Drottning Victorias Stiftelse, Kapten A. Erikssons Stiftelse. We here acknowledge the generous gift of antiserum against p75NTR (Dr. G. Weskamp). The local ethical committee (i.e., Stockholms Norra Djurförsöksetiska Nämnd; project no.N263/95 and N90/97)

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