Androgen receptor mRNA regulation in adult male and female hamster facial motoneurons: effects of axotomy and exogenous androgens

Alteration of protein folding and degradation in motor neuron diseases: Implications and protective functions of small heat shock proteins

Motor neuron diseases (MNDs) are neurodegenerative disorders that specifically affect the survival and function of upper and/or lower motor neurons. Since motor neurons are responsible for the control of voluntary muscular movement, MNDs are characterized by muscle spasticity, weakness and atrophy. Different susceptibility genes associated with an increased risk to develop MNDs have been reported and several mutated genes have been linked to hereditary forms of MNDs. However, most cases of MNDs occur in sporadic forms and very little is known on their causes. Interestingly, several molecular mechanisms seem to participate in the progression of both the inherited and sporadic forms of MNDs. These include cytoskeleton organization, mitochondrial functions, DNA repair and RNA synthesis/processing, vesicle trafficking, endolysosomal trafficking and fusion, as well as protein folding and protein degradation. In particular, accumulation of aggregate-prone proteins is a hallmark of MNDs, suggesting that the protein quality control system (molecular chaperones and the degradative systems: ubiquitin-proteasome-system and autophagy) are saturated or not sufficient to allow the clearance of these altered proteins. In this review we mainly focus on the MNDs associated with disturbances in protein folding and protein degradation and on the potential implication of a specific class of molecular chaperones, the small heat shock proteins (sHSPs/HSPBs), in motor neuron function and survival. How boosting of specific HSPBs may be a potential useful therapeutic approach in MNDs and how mutations in specific HSPBs can directly cause motor neuron degeneration is discussed.

Proteasomal and autophagic degradative activities in spinal and bulbar muscular atrophy

Spinal and bulbar muscular atrophy (SBMA or Kennedy's disease) is a fatal neurodegenerative disease characterized by the selective loss of motor neurons in the bulbar region of the brain and in the anterior horns of the spinal cord. The disease has been associated to an expansion of a CAG triplet repeat present in the first coding exon of the androgen receptor (AR) gene. SBMA was the first identified member of a large class of neurodegenerative diseases now known as CAG-related diseases, which includes Huntington's disease (HD), several types of spinocerebellar ataxia (SCAs), and dentatorubral and pallidoluysian atrophy (DRPLA). The expanded CAG tract is translated to an aberrantly long polyglutamine tract (ARpolyQ) in the N-terminal region of the AR protein. The elongated polyQ tract seems to confer a neurotoxic gain-of-function to the mutant AR, possibly via the generation of aberrant conformations (misfolding). Protein misfolding is thought to be a trigger of neurotoxicity, since it perturbs a wide variety of motor neuronal functions. The first event is the accumulation of the ARpolyQ into ubiquitinated aggregates in a ligand (testosterone) dependent manner. The mutant ARpolyQ also impairs proteasome functions. The autophagic pathway may be activated to compensate these aberrant events by clearing the mutant ARpolyQ from motor neuronal cells. This review illustrates the mechanisms at the basis of ARpolyQ degradation via the proteasomal and autophagic systems.

Neuroprotective actions of androgens on motoneurons

Androgens have a variety of protective and therapeutic effects in both the central and peripheral nervous systems. Here we review these effects as they related specifically to spinal and cranial motoneurons. Early in development, androgens are critical for the formation of important neuromuscular sex differences, decreasing the magnitude of normally occurring cell death in select motoneuron populations. Throughout the lifespan, androgens also protect against motoneuron death caused by axonal injury. Surviving motoneurons also display regressive changes to their neurites as a result of both direct axonal injury and loss of neighboring motoneurons. Androgen treatment enhances the ability of motoneurons to recover from these regressive changes and regenerate both axons and dendrites, restoring normal neuromuscular function. Androgens exert these protective effects by acting through a variety of molecular pathways. Recent work has begun to examine how androgen treatment can interact with other treatment strategies in promoting recovery from motoneuron injury.

Accelerating functional recovery after rat facial nerve injury: Effects of gonadal steroids and electrical stimulation

Objective

We investigated the combined effects of electrical stimulation and testosterone propionate on overall recovery time in rats with extracranial crush injuries to the facial nerve.

Study Design

Male rats underwent castration 3 to 5 days prior to right facial nerve crush injury and electrode implantation. Rats were randomly assigned to two groups: crush injury + testosterone or crush injury with electrical stimulation + testosterone. Recovery was assessed by daily subjective examination documenting vibrissae orientation/movement, semi-eye blink, and full eye blink.

Results

Milestones of early recovery were noted to be significantly earlier in the groups with electrical stimulation, with/without testosterone. The addition of testosterone to electrical stimulation showed significant earlier return of late recovery parameters and complete overall recovery.

Conclusion

Electrical stimulation may decrease cell death or promote sprouting to accelerate early recovery. Testosterone may affect the actual rate of axonal regeneration and produce acceleration in functional recovery. By targeting different stages of neural regeneration, the synergy of electrical stimulation and testosterone appears to have promise as a neurotherapeutic strategy for facial nerve injury.

Androgen regulation of axon growth and neurite extension in motoneurons

Androgens act on the CNS to affect motor function through interaction with a widespread distribution of intracellular androgen receptors (AR). This review highlights our work on androgens and process outgrowth in motoneurons, both in vitro and in vivo. The actions of androgens on motoneurons involve the generation of novel neuronal interactions that are mediated by the induction of androgen-dependent neurite or axonal outgrowth. Here, we summarize the experimental evidence for the androgenic regulation of the extension and regeneration of motoneuron neurites in vitro using cultured immortalized motoneurons, and axons in vivo using the hamster facial nerve crush paradigm. We place particular emphasis on the relevance of these effects to SBMA and peripheral nerve injuries.

Gonadal steroid hormones maintain the stress-induced acetylcholine release in the hippocampus: simultaneous measurements of the extracellular acetylcholine and serum corticosterone levels in the same subjects

To examine the role of gonadal steroid hormones in the stress responses of acetylcholine (ACh) levels in the hippocampus and serum corticosterone levels, we observed these parameters simultaneously in intact, gonadectomized, or gonadectomized steroid-primed rats. In both sexes of rats, neither gonadectomy nor the replacement of gonadal steroid hormone affected the baseline levels of ACh. However, gonadectomy severely attenuated the stress response of ACh, whereas the replacement of corresponding gonadal hormone successfully restored the response to intact levels. The gonadal hormones affected the serum corticosterone levels in a different manner; the testosterone replacement in orchidectomized rats suppressed the baseline and the stress response of corticosterone levels, whereas the 17beta-estradiol replacement in ovariectomized rats increased the levels. We further found that letrozole or flutamide administration in intact male rats attenuated the stress response of ACh. In addition, flutamide treatment increased the baseline levels of corticosterone, whereas letrozole treatment attenuated the stress response of corticosterone. Moreover, we found a low positive correlation between the ACh levels and corticosterone levels, depending on the presence of gonadal steroid hormone. We conclude that: 1) gonadal steroid hormones maintain the stress response of ACh levels in the hippocampus, 2) the gonadal steroid hormone independently regulates the stress response of ACh in the hippocampus and serum corticosterone, and 3) the sex-specific action of gonadal hormone on the cholinergic stress response may suggest a neonatal sexual differentiation of the septohippocampal cholinergic system in rats.

Androgen-induced neurite outgrowth is mediated by neuritin in motor neurones

In the brain, the spinal cord motor neurones express the highest levels of the androgen receptor (AR). Experimental data have suggested that neurite outgrowth in these neurones may be regulated by testosterone or its derivative 5alpha-dihydrotestosterone (DHT), formed by the 5alpha-reductase type 2 enzyme. In this study we have produced and characterized a model of immortalized motor neuronal cells expressing the mouse AR (mAR) [neuroblastoma-spinal cord (NSC) 34/mAR] and analysed the role of androgens in motor neurones. Androgens either activated or repressed several genes; one has been identified as the mouse neuritin, a protein responsible for neurite elongation. Real-time PCR analysis has shown that the neuritin gene is expressed in the basal condition in immortalized motor neurones and is selectively up-regulated by androgens in NSC34/mAR cells; the DHT effect is counteracted by the anti-androgen Casodex. Moreover, DHT induced neurite outgrowth in NSC34/mAR, while testosterone was less effective and its action was counteracted by the 5alpha-reductase type 2 enzyme inhibitor finasteride. Finally, the androgenic effect on neurite outgrowth was abolished by silencing neuritin with siRNA. Therefore, the trophic effects of androgens in motor neurones may be explained by the androgenic regulation of neuritin, a protein linked to neurone development, elongation and regeneration.

Androgen receptor messenger RNA and protein in adult rat sciatic nerve: implications for site of androgen action

Gonadal androgens exert a wide variety of effects on several neuromuscular systems, including controlling the developmental fate of motoneurons and neuromuscular synapses and promoting the growth of adult dendrites and axons. Paramount in understanding the molecular mechanisms behind androgen action is determining where androgen acts; does androgen act directly or indirectly on cells to change their fate and function? One step toward answering this question has been to determine which cells express androgen receptors (ARs). Motoneurons and skeletal muscles both have ARs and are, therefore, potential sites of androgen action. Recent evidence indicates that the sciatic nerve in rats also contains AR mRNA (Magnaghi et al. [1999] Brain Res. Mol. Brain Res. 70:36-44), although which cell type expresses ARs remains unanswered. In this study, we explored the question of which cell populations in the rat sciatic nerve express ARs. Using immunocytochemistry and reverse transcriptase-PCR, we confirmed the presence of AR protein and mRNA in sciatic nerve from adult rats and found a sex difference, favoring males, in the number of cell nuclei immunopositive for AR. This difference was not due to a sex difference in the overall number of cell nuclei. We also found a difference favoring males in AR mRNA, evidence also suggesting that AR expression is higher in males than in females. Results from double-immmunolabeling experiments in sciatic nerve from adult males suggest that, within the endoneurial compartment, endoneurial fibroblasts stain prominently for AR, with some endothelial cells also AR(+). Although Schwann cells showed light AR immunostaining, this staining is apparently nonspecific. We conclude that cells within peripheral nerve have ARs and may, therefore, mediate some of the effects of androgens on neuromuscular systems.

Neuroprotective effects of gonadal steroids on regenerating peripheral motoneurons

In this review, the neuroprotective actions of testosterone on three different populations of injured rat peripheral motoneurons, i.e. facial (FMN), spinal (SMN) and pudendal (PMN), will be discussed. We have extrapolated concepts from the neuroendocrine field regarding the trophic effects of gonadal steroids on target neural tissue to the nerve regeneration field. Exogenous administration of testosterone immediately after nerve injury impacts positively on functional recovery through actions mediated by the androgen receptor. The mechanism by which steroidal enhancement of the regenerative properties of injured motoneurons occurs may involve pre-existing androgen receptors, heat shock proteins, and modulation of the cellular stress response.