Effects of axotomy and testosterone on androgen receptor mRNA expression in hamster facial motoneurons

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.

Androgen receptor function in motor neuron survival and degeneration

Polyglutamine repeat expansion in the androgen receptor is responsible for the motor neuron degeneration in X-linked spinal and bulbar muscular atrophy (SBMA; Kennedy's disease). This mutation, like the other polyglutamine repeat expansions, has proven to be toxic itself by a gain-of-function effect; however, a growing body of evidence indicates that loss of androgen receptor normal function simultaneously contributes to SBMA disease pathology, and, conversely, that normal androgen receptor signaling mediates important trophic effects upon motor neurons. This review considers the trophic requirements of motor neurons, focusing upon the role of known neurotrophic factors in motor neuron disease natural history, and the interactions of androgen receptor signaling pathways with motor neuron disease pathogenesis and progression. A thorough understanding of androgen receptor signaling in motor neurons should provide important inroads toward the development of effective treatments for a variety of devastating motor neuron diseases.

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.

Cellular localization of androgen and estrogen receptors in mouse-derived motoneuron hybrid cells and mouse facial motoneurons

The ability of gonadal steroid hormones to augment axonal regeneration after peripheral nerve injury has been well established in rat and hamster motoneuron systems, and provides a foundation for the use of these agents as neurotherapeutics. With the advent of mouse genetics and the availability of transgenic and knockout mice, the use of mice in studies of neuroprotection is growing. It has recently been demonstrated that both androgens and estrogens rescue motoneurons (MN) from injury in mouse-derived motoneuron hybrid cells in vitro and mouse facial motoneurons (FMN) in vivo (Tetzlaff et al. [2006] J Mol Neurosci 28:53-64). To elucidate the molecular mechanisms of these effects, the present study examined the cellular localization of androgen and estrogen receptors in mouse MN in vitro and in vivo. Immunoblotting and immunocytochemistry studies established the presence of androgen receptor (AR) and estrogen receptor alpha/beta in immortalized mouse motoneuron hybrid cells and AR and estrogen receptor alpha in mouse FMN.

Loss of endogenous androgen receptor protein accelerates motor neuron degeneration and accentuates androgen insensitivity in a mouse model of X-linked spinal and bulbar muscular atrophy

X-linked spinal and bulbar muscular atrophy (SBMA; Kennedy's disease) is a polyglutamine (polyQ) disease in which the affected males suffer progressive motor neuron degeneration accompanied by signs of androgen insensitivity, such as gynecomastia and reduced fertility. SBMA is caused by CAG repeat expansions in the androgen receptor (AR) gene resulting in the production of AR protein with an extended glutamine tract. SBMA is one of nine polyQ diseases in which polyQ expansion is believed to impart a toxic gain-of-function effect upon the mutant protein, and initiate a cascade of events that culminate in neurodegeneration. However, whether loss of a disease protein's normal function concomitantly contributes to the neurodegeneration remains unanswered. To address this, we examined the role of normal AR function in SBMA by crossing a highly representative AR YAC transgenic mouse model with 100 glutamines (AR100) and a corresponding control (AR20) onto an AR null (testicular feminization; Tfm) background. Absence of endogenous AR protein in AR100Tfm mice had profound effects upon neuromuscular and endocrine-reproductive features of this SBMA mouse model, as AR100Tfm mice displayed accelerated neurodegeneration and severe androgen insensitivity in comparison to AR100 littermates. Reduction in size and number of androgen-sensitive motor neurons in the spinal cord of AR100Tfm mice underscored the importance of AR action for neuronal health and survival. Promoter-reporter assays confirmed that AR transactivation competence diminishes in a polyQ length-dependent fashion. Our studies indicate that SBMA disease pathogenesis, both in the nervous system and the periphery, involves two simultaneous pathways: gain-of-function misfolded protein toxicity and loss of normal protein function.

Neuroregenerative and neuroprotective actions of neuroimmunophilin compounds in traumatic and inflammatory neuropathies

FK506 (tacrolimus, Prograf is an immunosuppressant drug that also has profound neuroregenerative and neuroprotective actions independent of its immunosuppressant activity. The separation of these properties has led to the development of non-immunosuppressant derivatives that retain the neurotrophic activity. This review focuses on the peripheral nerve actions of these compounds following mechanical injury (nerve crush or transection with graft repair) and in models of inflammatory neuropathies. Whereas FK506 may be indicative for the treatment of inflammatory neuropathies where its immunosuppressive action would be advantageous, non-immunosuppressant derivatives represent a new class of potential therapeutic agents for the treatment of human neurological conditions in general. Moreover, these studies have led to the discovery of a novel mechanism whereby these compounds activate intrinsic neuroregenerative and neuroprotective pathways in the neuron.

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.

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

Testosterone propionate (TP) administration at the time of facial nerve injury in the adult hamster augments the regenerative properties of the injured facial motoneurons (FMN), with the androgen receptor (AR) playing a key role in mediating the actions of TP on facial nerve regeneration. The purpose of the present study was to determine the effects of axotomy on AR mRNA expression in FMN. This was accomplished using in situ hybridization in conjunction with a (35)S-labeled AR riboprobe. Gonadally intact adult male and gonadectomized (gdx) adult female hamsters were subjected to a right facial nerve axotomy, with the left side serving as internal, unoperated control. Half the animals were subcutaneously implanted with a 10-mm TP Silastic capsule, and the other half were sham-implanted. An additional group of nonaxotomized, gonadally intact males was also included. Postaxotomy survival times were 1, 4, and 7 days. At 1 postoperative day 1, there were no effects of axotomy on AR mRNA levels. By postoperative days 4 and 7, axotomy caused a significant decrease in AR mRNA levels in FMN of gonadally intact males, relative to either the contralateral control FMN of the same animals or FMN from the group of gonadally intact males that were not subjected to facial nerve axotomy. There were no significant differences between AR mRNA levels in contralateral control FMN and FMN from the gonadally intact group of nonaxotomized males. TP administration at the time of axotomy had no effect on AR mRNA levels in either the axotomized or contrala(teral control FMN of gonadally intact males, relative to the nonaxotomized, gonadally intact male group. Corroborating our previous work, AR mRNA levels were reduced in the contralateral control FMN of gdx females, relative to the nonaxotomized, gonadally intact male group, with axotomy having no additional effects. The data are discussed in a mechanistic framework suggesting how TP acts to augment facial nerve regeneration.

Androgenic regulation of the central glia response following nerve damage

Current research on the effects of gonadal steroids on the brain and spinal cord indicates that these agents have profound trophic effects on many aspects of neuronal functioning, including cell survival, growth and metabolism, elaboration of processes, synaptogenesis, and neurotransmission (Jones et al., 1985; Luine, 1985; Nordeen et al., 1985; Matsumoto et al., 1988a,b; Gould et al., 1990). Since many of the aspects of normal neuronal functioning altered by gonadal steroids are affected by injury to the nervous system, we initiated a series of experiments designed to exploit the trophic capabilities of steroids as therapeutic agents in neuronal injury and repair (Kujawa et al., 1989, 1991; Kujawa and Jones, 1990). Three steroid-sensitive model systems were used for these studies: the hamster facial motoneuron, the rat sciatic motoneuron, and the hamster rubrospinal motoneuron. The results of our initial series of experiments suggest that androgens, and possibly estrogens, act either directly or indirectly on the injured motoneuron and enhance elements of the neuronal reparative response that are critical to successful recovery of function. Recently, we discovered that gonadal steroids may also modulate the central glia response to nerve damage. In this review, a summary of our data identifying a therapeutic role for androgens in enhancing the reparative response of motoneurons to injury is presented. This is followed by a discussion of the effects of androgens on the glial response to injury.