The neurotrophins BDNF, NT-3, and NGF display distinct patterns of retrograde axonal transport in peripheral and central neurons

Models of neurological defects and defects in neurological models

The transition from research to patient following advances in transplantation research is likely to be disappointing unless it includes a better understanding of critically relevant characteristics of the neurological disorder and improvements in the animal models, particularly the behavioral features. The appropriateness of the model has less to do with the species than with how the species is used.

Serial changes in bladder, locomotion, and levels of neurotrophic factors in rats with spinal cord contusion

The aims of this study were to evaluate the evolution of the neurogenic bladder after spinal cord contusion and to correlate changes in bladder function with locomotor function and levels of neurotrophic factors. The MASCIS impactor was used to cause a mild contusion injury of the lower thoracic spinal cord of Sprague-Dawley rats. Rats were divided into four groups according to the length of time from injury to sacrifice, at 4, 14, 28, and 56 days after injury. Gait analysis was performed each week, and urodynamic study was performed just before sacrifice. Basso, Beattie, and Bresnahan (BBB) and coupling scores showed gradual recovery, as did the urinary voiding pattern and bladder volume; some parameters of micturition reached normal ranges. Brain-derived neurotrophic factor (BDNF) levels in the spinal cord, as detected by enzyme-linked immunosorbent assay, decreased with time, whereas neurotrophin-3 (NT-3) levels remained unchanged. The micturition pattern, bladder volume, and locomotor function continued to recover during the time of observation; BDNF levels in the spinal cord and bladder were inversely correlated with BBB scores and the restoration of bladder volume. We conclude that urodynamic changes in the bladder correlate with locomotion recovery but not with the levels of BDNF or NT-3 after modified mild contusion injury in rats.

Tri-partite complex for axonal transport drug delivery achieves pharmacological effect

BACKGROUND

Targeted delivery of pharmaceutical agents into selected populations of CNS (Central Nervous System) neurons is an extremely compelling goal. Currently, systemic methods are generally used for delivery of pain medications, anti-virals for treatment of dermatomal infections, anti-spasmodics, and neuroprotectants. Systemic side effects or undesirable effects on parts of the CNS that are not involved in the pathology limit efficacy and limit clinical utility for many classes of pharmaceuticals. Axonal transport from the periphery offers a possible selective route, but there has been little progress towards design of agents that can accomplish targeted delivery via this intraneural route. To achieve this goal, we developed a tripartite molecular construction concept involving an axonal transport facilitator molecule, a polymer linker, and a large number of drug molecules conjugated to the linker, then sought to evaluate its neurobiology and pharmacological behavior.

RESULTS

We developed chemical synthesis methodologies for assembling these tripartite complexes using a variety of axonal transport facilitators including nerve growth factor, wheat germ agglutinin, and synthetic facilitators derived from phage display work. Loading of up to 100 drug molecules per complex was achieved. Conjugation methods were used that allowed the drugs to be released in active form inside the cell body after transport. Intramuscular and intradermal injection proved effective for introducing pharmacologically effective doses into selected populations of CNS neurons. Pharmacological efficacy with gabapentin in a paw withdrawal latency model revealed a ten fold increase in half life and a 300 fold decrease in necessary dose relative to systemic administration for gabapentin when the drug was delivered by axonal transport using the tripartite vehicle.

CONCLUSION

Specific targeting of selected subpopulations of CNS neurons for drug delivery by axonal transport holds great promise. The data shown here provide a basic framework for the intraneural pharmacology of this tripartite complex. The pharmacologically efficacious drug delivery demonstrated here verify the fundamental feasibility of using axonal transport for targeted drug delivery.

Androgen regulates brain-derived neurotrophic factor in spinal motoneurons and their target musculature

Trophic factors maintain motoneuron morphology and function in adulthood. Brain-derived neurotrophic factor (BDNF) interacts with testosterone to maintain dendritic morphology of spinal motoneurons. In addition, testosterone regulates BDNF's receptor (trkB) in motoneurons innervating the quadriceps muscles as well as in motoneurons of the highly androgen-sensitive spinal nucleus of the bulbocavernosus (SNB). Given these interactive effects, we examined whether androgen might also regulate BDNF in quadriceps and SNB motoneurons and their corresponding target musculature. In both motoneuron populations, castration of males reduced BDNF immunolabeling, and this effect was prevented with testosterone replacement. ELISA for BDNF in the target musculature of quadriceps (vastus lateralis, VL) and SNB (bulbocavernosus, BC) motoneurons revealed that BDNF in the VL and BC muscles was also regulated by androgen. However, although castration significantly decreased BDNF concentration in the VL muscle, BDNF concentration in the BC muscle was significantly increased in castrates. Treatment of castrated males with testosterone maintained BDNF levels at those of intact males in both sets of muscles. Together, these results demonstrate that androgens regulate BDNF in both a sexually dimorphic, highly androgen-sensitive neuromuscular system as well as a more typical somatic neuromuscular system. Furthermore, in addition to the regulation of trkB, these studies provide another possible mechanism for the interactive effects of testosterone and BDNF on motoneuron morphology. More importantly, by examining both the motoneurons and the muscles they innervate, these results demonstrate that within a neural system, BDNF levels in different components are differentially affected by androgen manipulation.

Differential activity-dependent secretion of brain-derived neurotrophic factor from axon and dendrite

Brain-derived neurotrophic factor (BDNF) is essential for neuronal survival and differentiation during development and for synaptic function and plasticity in the mature brain. BDNF-containing vesicles are widely distributed and bidirectionally transported in neurons, and secreted BDNF can act on both presynaptic and postsynaptic cells. Activity-dependent BDNF secretion from neuronal cultures has been reported, but it remains unknown where the primary site of BDNF secretion is and whether neuronal activity can trigger BDNF secretion from axons and dendrites with equal efficacy. Using BDNF fused with pH-sensitive green fluorescent protein to visualize BDNF secretion, we found that BDNF-containing vesicles exhibited markedly different properties of activity-dependent exocytic fusion at the axon and dendrite of cultured hippocampal neurons. Brief spiking activity triggered a transient fusion pore opening, followed by immediate retrieval of vesicles without dilation of the fusion pore, resulting in very little BDNF secretion at the axon. On the contrary, the same brief spiking activity induced "full-collapse" vesicle fusion and substantial BDNF secretion at the dendrite. However, full vesicular fusion with BDNF secretion could occur at the axon when the neuron was stimulated by prolonged high-frequency activity, a condition neurons may encounter during epileptic discharge. Thus, activity-dependent axonal secretion of BDNF is highly restricted as a result of incomplete fusion of BDNF-containing vesicles, and normal neural activity induces BDNF secretion from dendrites, consistent with the BDNF function as a retrograde factor. Our study also revealed a novel mechanism by which differential exocytosis of BDNF-containing vesicles may regulate BDNF-TrkB signaling between connected neurons.

Proneurotrophin-3 is a neuronal apoptotic ligand: evidence for retrograde-directed cell killing

Although mature neurotrophins are well described trophic factors that elicit retrograde survival signaling, the precursor forms of neurotrophins (i.e., proneurotrophins) can function as high-affinity apoptotic ligands for selected neural populations. An outstanding question is whether target-derived proneurotrophins might affect neuronal survival/death decisions through a retrograde transport mechanism. Since neurotrophin-3 (NT-3) is highly expressed in non-neural tissues that receive peripheral innervation, we investigated the localized actions of its precursor (proNT-3) on sympathetic neurons in the present study. Pharmacological inhibition of intracellular furin proteinase activity in 293T cells resulted in proNT-3 release instead of mature NT-3, whereas membrane depolarization in cerebellar granule neurons stimulated endogenous proNT-3 secretion, suggesting that proNT-3 is an inducible bona fide ligand in the nervous system. Our data also indicate that recombinant proNT-3 induced sympathetic neuron death that is p75(NTR)- and sortilin-dependent, with hallmark features of apoptosis including JNK (c-Jun N-terminal kinase) activation and nuclear fragmentation. Using compartmentalized culture systems that segregate neuronal cell bodies from axons, proNT-3, acting within the distal axon compartment, elicited sympathetic neuron death and overrode the survival-promoting actions of NGF. Together, these results raise the intriguing possibility that dysregulation of proneurotrophin processing/release by innervated targets can be deleterious to the neurons projecting to these sites.

Overexpression of IGF-1 in muscle attenuates disease in a mouse model of spinal and bulbar muscular atrophy

Expansion of a polyglutamine tract in the androgen receptor (AR) causes spinal and bulbar muscular atrophy (SBMA). We previously showed that Akt-mediated phosphorylation of AR reduces ligand binding and attenuates the mutant AR toxicity. Here, we show that in culture insulin-like growth factor 1 (IGF-1) reduces AR aggregation and increases AR clearance via the ubiquitin-proteasome system through phosphorylation of AR by Akt. In vivo, SBMA transgenic mice overexpressing a muscle-specific isoform of IGF-1 selectively in skeletal muscle show evidence of increased Akt activation and AR phosphorylation and decreased AR aggregation. Augmentation of IGF-1/Akt signaling rescues behavioral and histopathological abnormalities, extends the life span, and reduces both muscle and spinal cord pathology of SBMA mice. This study establishes IGF-1/Akt-mediated inactivation of mutant AR as a strategy to counteract disease in vivo and demonstrates that skeletal muscle is a viable target tissue for therapeutic intervention in SBMA.

Role of Neurotrophin 3 in spinal neuroplasticity in rats subjected to cord transection

That neuroplasticity occurs in mammalian spinal cord is well known, though the underlying mechanism still awaits elucidation. This study evaluated the role of endogenous Neurotrophin-3 (NT-3) in the spinal neuroplasticity. Following cord transection at the junction between T9 and T10, the hindlimb locomotor functions of rats showed gradual but significant improvement from 7 to 28 days post-operation. Corresponding to this was a significant increase in the level of NT-3 in the cord segments caudal to injury site. Significantly, after NT-3-antibody administration, the spinal transected rats displayed poor hindlimb locomotor functions and a decrease in the number of neurons in spinal laminae VIII-IX. Whether NT-3-antibody was administered, corticospinal tract regeneration and somatosensory evoked potentials could not be detected. Our findings suggested that endogenous NT-3 could play an important role in spinal plasticity in adult spinal cords subjected to transection, possibly through a regulation of neuronal activity in the local circuitry.

Mesencephalic astrocyte-derived neurotrophic factor is neurorestorative in rat model of Parkinson's disease

Neurotrophic factors are promising candidates for the treatment of Parkinson's disease (PD). Mesencephalic astrocyte-derived neurotrophic factor (MANF) belongs to a novel evolutionarily conserved family of neurotrophic factors. We examined whether MANF has neuroprotective and neurorestorative effect in an experimental model of PD in rats. We also studied the distribution and transportation of intrastriatally injected MANF in the brain and compared it with glial cell line-derived neurotrophic factor (GDNF). Unilateral lesion of nigrostriatal dopaminergic system was induced by intrastriatal injection of 6-hydroxydopamine (6-OHDA). Amphetamine-induced turning behavior was monitored up to 12 weeks after the unilateral lesion. The local diffusion at the injection site and transportation profiles of intrastriatally injected MANF and GDNF were studied by immunohistochemical detection of the unlabeled growth factors as well as by autoradiographic and gamma counting detection of (125)I-labeled trophic factors. Intrastriatally injected MANF protected nigrostriatal dopaminergic nerves from 6-OHDA-induced degeneration as evaluated by counting tyrosine hydroxylase (TH)-positive cell bodies in the substantia nigra (SN) and TH-positive fibers in the striatum. More importantly, MANF also restored the function of the nigrostriatal dopaminergic system when administered either 6 h before or 4 weeks after 6-OHDA administration in the striatum. MANF was distributed throughout the striatum more readily than GDNF. The mechanism of MANF action differs from that of GDNF because intrastriatally injected (125)I-MANF was transported to the frontal cortex, whereas (125)I-GDNF was transported to the SN. Our results suggest that MANF is readily distributed throughout the striatum and has significant therapeutic potential for the treatment of PD.

Nerve growth factor in rheumatic diseases

OBJECTIVES

The nervous system modulates the immune response in many autoimmune syndromes by neurogenic inflammation. One of the pivotal mediators is nerve growth factor (NGF), which is known for its effects on neuronal survival and growth. There is considerable evidence that NGF acts as an important mediator of many immune responses. This article reviews the role of NGF in rheumatic diseases and strategies for potential therapeutic interventions.

METHODS

We conducted a database search using Medline and Medpilot. Eight hundred abstracts containing the keyword NGF and 1 of the following terms were reviewed: arthritis, neurogenic inflammation, rheumatoid arthritis, osteoarthritis, collagen arthritis, arteritis, psoriasis, psoriatic arthritis, Sjogren syndrome, systemic lupus erythematosus, gout, osteoporosis, lower back pain, lumbar disc herniation, nerve root compression, spondyloarthritis, spondylarthropathy, algoneurodystrophy, fibromyalgia, Kawasaki syndrome, polyarteritis nodosa, cytokine, vasculitis, pain, therapy, and antagonist. Articles were analyzed based on relevance and content. Most clinical trials and studies with human specimens were included. Studies with experimental animal models were selected if they contained relevant data.

RESULTS

NGF is overexpressed in many inflammatory and degenerative rheumatic diseases. Concentrations differ to some extent and sometimes even show contradictory results. NGF is found in serum, synovial fluid, and cerebrospinal fluid, and tissue specimens. NGF concentrations can be correlated with the extent of inflammation and/or clinical activity in many conditions. In rheumatoid arthritis, NGF levels are significantly higher as compared with osteoarthritis.

CONCLUSIONS

NGF is a significant mediator and modulator of inflammation. NGF sometimes shows detrimental and sometimes regenerative activity. These findings indicate potential therapeutic interventions using either NGF antagonists or recombinant NGF.

Muscle and bone plasticity after spinal cord injury: review of adaptations to disuse and to electrical muscle stimulation

The paralyzed musculoskeletal system retains a remarkable degree of plasticity after spinal cord injury (SCI). In response to reduced activity, muscle atrophies and shifts toward a fast-fatigable phenotype arising from numerous changes in histochemistry and metabolic enzymes. The loss of routine gravitational and muscular loads removes a critical stimulus for maintenance of bone mineral density (BMD), precipitating neurogenic osteoporosis in paralyzed limbs. The primary adaptations of bone to reduced use are demineralization of epiphyses and thinning of the diaphyseal cortical wall. Electrical stimulation of paralyzed muscle markedly reduces deleterious post-SCI adaptations. Recent studies demonstrate that physiological levels of electrically induced muscular loading hold promise for preventing post-SCI BMD decline. Rehabilitation specialists will be challenged to develop strategies to prevent or reverse musculoskeletal deterioration in anticipation of a future cure for SCI. Quantifying the precise dose of stress needed to efficiently induce a therapeutic effect on bone will be paramount to the advancement of rehabilitation strategies.

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.

Alteration of glial cell line-derived neurotrophic factor family receptor alpha-2 mRNA expression and its co-expression with neuronal nitric oxide synthase in pelvic ganglia following unilateral cavernous nerve injury

OBJECTIVES

The goal of this study was to determine the alterations of glial cell line-derived neurotrophic factor family receptor alpha-2 (GFRalpha2) mRNA expression in the major pelvic ganglia (MPG) and their relationship to the marker for the neural plasticity (growth-associated protein 43: GAP-43) and neuronal nitric oxide synthase (nNOS)-positive neurons following cavernous nerve injury.

METHODS

Cavernous nerves were transected unilaterally in 24 Sprague-Dawley rats aged 8 weeks. We used nine sham operated same animals as controls. Bilateral MPGs were harvested at 1, 3, and 6 months following nerve injury. The GFRalpha2 and GAP-43 mRNA expressions of the sham group and the injury group (3 months after surgery) were investigated by reverse transcription-polymerase chain reaction. We also investigated the expression profile of GFRalpha2 mRNA by in situ hybridization combined with nNOS immunostaining.

RESULTS

It was revealed semi-quantitatively that the GAP43 mRNA expression moderately increased in the intact MPG, and GFRalpha2 mRNA was maintained in the intact MPG but not in the injured one. A histological double-labeling study showed that the number of GFRalpha2 mRNA- and nNOS-positive neurons increased in the intact MPG and most GFRalpha2 mRNA expressions were colocalized with nNOS immunostaining.

CONCLUSIONS

The current study suggested that the GFRalpha2 mRNA alteration closely related to the nNOS expression following the cavernous nerve injury, which would be involved in the maintenance and recovery of erectile function.

A bi-directional carboxypeptidase E-driven transport mechanism controls BDNF vesicle homeostasis in hippocampal neurons

Anterograde transport of brain-derived neurotrophic factor (BDNF) vesicles from the soma to neurite terminals is necessary for activity-dependent secretion of BDNF to mediate synaptic plasticity, memory and learning, and retrograde BDNF transport back to the soma for recycling. In our study, overexpression of the cytoplasmic tail of the carboxypeptidase E (CPE) found in BDNF vesicles significantly reduced localization of BDNF in neurites of hippocampal neurons. Live-cell imaging showed that the velocity and distance of movement of fluorescent protein-tagged CPE- or BDNF-containing vesicles were reduced in both directions. In pulldown assays, the CPE tail interacted with dynactin along with kinesin-2 and kinesin-3, and cytoplasmic dynein. Competition assays using a CPE tail peptide verified specific interaction between the CPE tail and dynactin. Thus, the CPE cytoplasmic tail binds dynactin that recruits kinesins or dynein for driving bi-directional transport of BDNF vesicle to maintain vesicle homeostasis and secretion in hippocampal neurons.

Adenosine receptor A2A-R contributes to motoneuron survival by transactivating the tyrosine kinase receptor TrkB

Neurotrophins are potent survival factors for developing and injured neurons. However, they are not being used to treat neurodegenerative diseases because of difficulties in administration and numerous side effects that have been encountered in previous clinical trials. Their biological activities use Trk (tropomyosin-related kinase) transmembrane tyrosine kinases. Therefore, one alternative approach is to use transactivation pathways such as adenosine 2A receptor agonists, which can activate Trk receptor signaling independent of neurotrophin binding. However, the relevance in vivo and applicability of these transactivation events during neurodegenerative and injury conditions have never been extensively studied. Here we demonstrate that motoneuron survival after facial nerve lesioning is significantly enhanced by transactivation of Trk receptor tyrosine kinases by adenosine agonists. Moreover, survival of motoneurons directly required the activation of the BDNF receptor TrkB and an increase in Akt (AKT8 virus oncogene cellular homolog) activity. The ability of small molecules to activate a trophic response by using Trk signaling provides a unique mechanism to promote survival signals in motoneurons and suggests new strategies for using transactivation in neurodegenerative diseases.

A role for BDNF in cocaine reward and relapse

Brain-derived neurotrophic factor (BDNF) is important in regulating synaptic plasticity in the brain areas that process reward information. A new study reports that BDNF in the nucleus accumbens, a brain area critical for the rewarding effects of cocaine, promotes persistent cocaine-seeking behaviors and heightens relapse vulnerability.

Neurotrophic factors promote and enhance locomotor recovery in untrained spinalized cats

In spinal cats, locomotor recovery without rehabilitation is limited, but weight-bearing stepping returns with treadmill training. We studied whether neurotrophins administered to the injury site also restores locomotion in untrained spinal cats and whether combining both neurotrophins and training further improves recovery. Ordinary rat fibroblasts or a mixture of fibroblasts secreting brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3) (Fb-NTF) were grafted into T12 spinal transection sites. Cats with each type of transplant were divided into two groups: one receiving daily training and the other receiving no training. As expected, trained cats with/without neurotrophin-producing transplants could step on the treadmill. Untrained cats without neurotrophin-producing transplants could not locomote. However, untrained cats with neurotrophin-secreting transplants performed plantar weight-bearing stepping at speeds up to 0.8 m/s as early as 2 wk after transection. Locomotor capability and stance lengths in these animals were similar to those in animals receiving training alone, suggesting that administration of BDNF/NT-3 was equivalent to treadmill training in restoring locomotion in chronically spinalized cats. Cats receiving both interventions showed the greatest improvement in step length. Anatomical evaluation indicated that all transections were complete and that axons did not enter the cord caudal to the graft. Thus BDNF/NT-3 secreting fibroblasts were equivalent to training in their ability to engage the locomotor circuitry in chronic spinal cats. Furthermore, the rapid time-course of recovery and the absence of axonal growth through the transplants indicate that the restorative mechanisms were not related to supraspinal axonal growth. Finally, the results show that transplants beneficial in rodents are applicable to larger mammals.

Postnatal generation of neurons in the ventrobasal nucleus of the rat thalamus

Most CNS systems, including the trigeminal-somatosensory system, develop via a hierarchical order (from the periphery and up the neuraxis). We tested the hypothesis that development of the trigeminal system can proceed via a nonhierarchical mechanism (i.e., that neuronogenesis can occur postnatally). Preweanling rats were perfused, and brain sections were stained with cresyl violet or immunolabeled with NeuN (for neuronal counts), or processed for acetylcholinesterase (AChE) activity or p75 immunoreactivity [to identify boundaries of the ventrobasal nucleus (VB)]. Neuronal number decreased during the first postnatal week but increased 2.5-fold over the next 3 weeks. To determine whether this remarkable rise resulted from the generation of new neurons, preweanlings were given injections of bromodeoxyuridine (BrdU) on postnatal day 6 (P6) or P21. BrdU-positive VB cells were apparent on both days. Cumulative BrdU labeling showed that the cell cycle was 17.3 h on P6. Moreover, Ki-67, a protein elaborated throughout the cell cycle, was expressed by 25.8-29.3% of all VB cells on P6-P15, falling to 7.7% by P21. BrdU-positive VB cells coexpressed neuronal markers: NeuN, HuC/D, microtubule-associated protein 2, and a dextran placed in the somatosensory cortex. Note that postnatal neuronal generation was also evident in other thalamic nuclei (e.g., the lateral geniculate nucleus). Thus, the developing VB experiences two periods of neuronal generation. Prenatal neuronogenesis is part of hierarchical trigeminal-somatosensory development. Postnatal nonhierarchical neuronogenesis is intrathalamic and matches changes in neuromodulatory systems (exemplified by AChE activity and p75) and the arrival of corticothalamic afferents.

Effect of neurotrophin-3 on reinnervation of the larynx using the phrenic nerve transfer technique

Current techniques for reinnervation of the larynx following recurrent laryngeal nerve (RLN) injury are limited by synkinesis, which prevents functional recovery. Treatment with neurotrophins (NT) may enhance nerve regeneration and encourage more accurate reinnervation. This study presents the results of using the phrenic nerve transfer method, combined with NT-3 treatment, to selectively reinnervate the posterior cricoarytenoid (PCA) abductor muscle in a pig nerve injury model. RLN transection altered the phenotype and morphology of laryngeal muscles. In both the PCA and thyroarytenoid (TA) adductor muscle, fast type myosin heavy chain (MyHC) protein was decreased while slow type MyHC was increased. These changes were accompanied with a significant reduction in muscle fibre diameter. Following nerve repair there was a progressive normalization of MyHC phenotype and increased muscle fibre diameter in the PCA but not the TA muscle. This correlated with enhanced abductor function indicating the phrenic nerve accurately reinnervated the PCA muscle. Treatment with NT-3 significantly enhanced phrenic nerve regeneration but led to only a small increase in the number of reinnervated PCA muscle fibres and minimal effect on abductor muscle phenotype and morphology. Therefore, work exploring other growth factors, either alone or in combination with NT-3, is required.

Application of simple photobleaching microscopy techniques for the determination of the balance between anterograde and retrograde axonal transport

The directionality of axonal transport represents an important question in neurophysiological and neuropathological research. Various approaches such as videomicroscopy, radioisotopic and fluorescence-based techniques are used. Recently, a novel FRAP-based (fluorescent recovery after photobleaching) technique using synaptophysin-EGFP expression in primary neurons was applied, allowing reliable and sensitive evaluation of gross axonal transport changes using confocal live-imaging microscopy. Here, we describe a novel FLIP-based (fluorescence loss in photobleaching) approach using a synaptophysin-EGFP probe that allows the differential evaluation of the ante- and retrograde transport parameters. Furthermore, we improved the sensitivity of the probe by substituting EGFP with an ECFP/VenusYFP fusion FRET (fluorescence resonance energy transfer) pair. The use of this FRET couple improves the precision of axonal transport measurements by combining FLIP and FLAP (fluorescence localization after photobleaching) techniques and eliminating the need for pre-bleaching images and thus pixel shifts between various exposures, and by reducing the deleterious effect of photobleaching.

Translational considerations for CNS gene therapy

Gene transfer is being rigorously evaluated in the laboratory in the preparation for the development of clinical therapies. Many CNS diseases, which have proved more challenging to treat than peripheral disorders, are prime candidates for gene therapy. However, there are numerous considerations in the development of gene therapy, including delivery, maintenance of expression, transgene level regulation, toxicity of the viral vector system and safety of the gene product. The authors review these issues and discuss various approaches used in preclinical studies. Alzheimer's and Parkinson's disease are employed as models, in which much research has already been performed, to address disease-specific questions about gene therapy approaches.

Estrogen-BDNF interactions: implications for neurodegenerative diseases

Since its' discovery over 20 years ago, BDNF has been shown to play a key role in neuronal survival, in promoting neuronal regeneration following injury, regulating transmitter systems and attenuating neural-immune responses. Estrogen's actions in the young and mature brain, and its role in neurodegenerative diseases in many cases overlaps with those observed for BDNF. Reduced estrogen and BDNF are observed in patients with Parkinson's disease and Alzheimer's disease, while high estrogen levels are a risk factor for development of multiple sclerosis. Estrogen receptors, which transduce the actions of estrogen, colocalize to cells that express BDNF and its receptor trkB, and estrogen further regulates the expression of this neurotrophin system. This review describes the distribution of BDNF and trkB expressing cells in the forebrain, and the roles of estrogen and the BDNF-trkB neurotrophin system in Parkinson's disease, Alzheimer's disease and multiple sclerosis.

Axonal transport of BDNF precursor in primary sensory neurons

Recent studies have shown that the precursor of brain-derived neurotrophic factor (pro-BDNF) activates p75NTR with high affinity to induce apoptosis. Here we show that pro-BDNF is transported anterogradely and retrogradely in sensory neurons of adult rats. After a crush injury of sciatic nerves, dorsal roots or dorsal column in adult Sprague-Dawley rats, the immunoreactivity for pro-BDNF accumulated at both the proximal and distal segments. The accumulation reached a maximum at 24 h after injury. Western blot analysis also revealed pro-BDNF in sciatic nerve segments proximal and distal to the ligature and in the spinal cord. Biotinylated or Alexa-488-labelled pro-BDNF injected into sciatic nerve was internalized and transported both retrogradely and anterogradely within sensory neurons. These results demonstrate that pro-BDNF is anterogradely and retrogradely transported in sensory neurons, suggesting that endogenous pro-BDNF may be released and play important functions.

Cellular therapies in motor neuron diseases

Amyotrophic lateral sclerosis (ALS) and spinal muscular atrophy (SMA) are prototypical motor neuron diseases that result in progressive weakness as a result of motor neuron dysfunction and death. Though much work has been done in both diseases to identify the cellular mechanisms of motor neuron dysfunction, once motor neurons have died, one of potential therapies to restore function would be through the use of cellular transplantation. In this review, we discuss potential strategies whereby cellular therapies, including the use of stem cells, neural progenitors and cells engineered to secrete trophic factors, may be used in motor neuron diseases. We review pre-clinical data in rodents with each of these approaches and discuss advances and regulatory issues regarding the use of cellular therapies in human motor neuron diseases.

Immunohistochemical analysis of acid-sensing ion channel 2 expression in rat dorsal root ganglion and effects of axotomy

Several studies have suggested that acid-sensing ion channel 2 (ASIC2) plays a role in mechanoperception and acid sensing in the peripheral nervous system. We examined the expression and distribution of ASIC2 in the rat dorsal root ganglion, the co-localization of ASIC2 with tropomyosin-related kinase (trk) receptors, and the effects of axotomy on ASIC2 expression. ASIC2 immunoreactivity was observed in both neurons and satellite cells. ASIC2-positive neurons accounted for 16.5 +/- 2.4% of the total neurons in normal dorsal root ganglion. Most ASIC2-positive neurons were medium-to-large neurons and were labeled with neurofilament 200 kD (NF200). Within these neurons, ASIC2 was not evenly distributed throughout the cytoplasm, but rather was accumulated prominently in the cytoplasm adjacent to the axon hillock and axonal process. We next examined the co-localization of ASIC2 with trk receptors. trkA was expressed in few ASIC2-positive neurons, and trkB and trkC were observed in 85.2% and 53.4% of ASIC2-positive neurons, respectively, while only 6.9% of ASIC2-positive neurons were co-localized with trkC alone. Peripheral axotomy markedly reduced ASIC2 expression in the axotomized dorsal root ganglion neurons. On the other hand, intense ASIC2 staining was observed in satellite cells. These results show that ASIC2 is expressed in the distinct neurochemical population of sensory neurons as well as satellite cells, and that peripheral axotomy induced marked reductions in ASIC2 in neurons.

Receptor recycling mediates plasma membrane recovery of dopamine D1 receptors in dendrites and axons after agonist-induced endocytosis in primary cultures of striatal neurons

The pharmacological stimulation of G-protein-coupled receptor induces receptor internalization. Receptor's fate after the step of internalization remains poorly characterized despite its incidence on the neuronal responsiveness. In this context, we studied the dopamine (DA) D1 receptor (D1R) trafficking in a model of striatal neuronal culture that endogenously express the D1R. We first characterized by immunohistochemistry the spatial distribution of the compartments involved in the endocytic pathways and then the D1R trafficking in dendrites and axons. In dendrites, immunohistochemical analysis showed that acute stimulation by the D1R agonist SKF 82958 (1 microM) induces an internalization of D1R in early endosomes labeled with Alexa-488-conjugated transferrin. We show that, 20 min after removal of the agonist, the D1R immunolabeling pattern returns to the basal state in dendrites and in axons. Recovery was unaffected by cycloheximide (70 microM) but was prevented by monensin (100 microM) that inhibits endosomal acidification and receptor recycling. These data suggest that dendritic and axonal D1Rs are internalized after agonist stimulation and targeted to the recycling pathway demonstrating that the machinery involved in GPCR endocytosis and recycling is functional both in dendrites and in axons. Temporal characteristics observed for the recovery of D1R density to the basal state and those observed for the resensitization process strongly suggest that D1R recycling supports the receptor resensitization.

Tissue engineered nerve constructs: where do we stand?

Driven by enormous clinical need, interest in peripheral nerve regeneration has become a prime focus of research and area of growth within the field of tissue engineering. While using autologous donor nerves for bridging peripheral defects remains today's gold standard, it remains associated with high donor site morbidity and lack of full recovery. This dictates research towards the development of biomimetic constructs as alternatives. Based on current concepts, this review summarizes various approaches including different extracellular matrices, scaffolds, and growth factors that have been shown to promote migration and proliferation of Schwann cells. Since neither of these concepts in isolation is enough, although each is gaining increased interest to promote nerve regeneration, various combinations will need to be identified to strike a harmonious balance. Additional factors that must be incorporated into tissue engineered nerve constructs are also unknown and warrant further research efforts. It seems that future directions may allow us to determine the "missing link".

Immunohistochemical distribution of NGF, BDNF, NT-3, and NT-4 in adult rhesus monkey brains

Immunohistochemical distribution and cellular localization of neurotrophins was investigated in adult monkey brains using antisera against nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), and neurotrophin-4 (NT-4). Western blot analysis showed that each antibody specifically recognized appropriate bands of approximately 14.7 kDa, 14.2 kDa, 13.6 kDa, and 14.5 kDa, for NGF, BDNF, NT-3, and NT-4, respectively. These positions coincided with the molecular masses of the neurotrophins studied. Furthermore, sections exposed to primary antiserum preadsorbed with full-length NGF, BDNF, NT-3, and NT-4 exhibited no detectable immunoreactivity, demonstrating specificities of the antibodies against the tissues prepared from rhesus monkeys. The study provided a systematic report on the distribution of NGF, BDNF, NT-3, and NT-4 in the monkey brain. Varying intensity of immunostaining was observed in the somata and processes of a wide variety of neurons and glial cells in the cerebrum, cerebellum, hippocampus, and other regions of the brain. Neurons in some regions such as the cerebral cortex and the hippocampus, which stained for neurotrophins, also expressed neurotrophic factor mRNA. In some other brain regions, there was discrepancy of protein distribution and mRNA expression reported previously, indicating a retrograde or anterograde action mode of neurotrophins. Results of this study provide a morphological basis for the elucidation of the roles of NGF, BDNF, NT-3, and NT-4 in adult primate brains.

Correlation between the appearance of neuropeptides in the rat trigeminal ganglion and reinnervation of the healing root socket after tooth extraction

The neuropeptide substance P (SP) modulates bone metabolism. This study examined the temporal appearance of the neuropeptides SP and brain-derived nerve growth factor (BDNF) and their receptors (neurokinin-1 receptor (NK₁-R) and Trk B, respectively) in the rat trigeminal ganglion to investigate the role of neuropeptides in healing after tooth extraction. Rats were anesthetized and their upper right first molars were extracted; the rats were sacrificed 3 hours and 1–21 days after extraction. Their trigeminal ganglion and maxilla were removed, and cryosections were prepared and immunostained using specific antibodies against SP, BDNF, NK₁-R, and Trk B. In the tooth sockets after extraction, new bone and a few SP-­immunoreactive nerve fibers were first seen at day 7, and bone completely filled the sockets at day 21. In the trigeminal ganglion, the proportions of NK₁-R-, BDNF-, and Trk B-immuno­reactive neurons changed similarly, i.e., they initially decreased, increased rapidly to ­maximum levels by day 3, and then decreased gradually to control levels until 21 days. These findings suggest that the appearance of neuropeptides in the trigeminal ganglion, the reinnervation of SP-immunoreactive nerve fibers, and bone repair in the tooth socket during healing after extraction were correlated.

BDNF is expressed in skeletal muscle satellite cells and inhibits myogenic differentiation

In skeletal muscle, brain-derived neurotrophic factor (BDNF) has long been thought to serve as a retrograde trophic factor for innervating motor neurons throughout their lifespan. However, its localization in mature muscle fibers has remained elusive. Given the postulated roles of BDNF in skeletal muscle, we performed a series of complementary experiments aimed at defining the localization of BDNF and its transcripts in adult muscle. By reverse transcription-PCR, in situ hybridization, and immunofluorescence, we show that BDNF, along with the receptor p75NTR, is not expressed at significant levels within mature myofibers and that it does not accumulate preferentially within subsynaptic regions of neuromuscular junctions. Interestingly, expression of BDNF correlated with that of Pax3, a marker of muscle progenitor cells, in several different adult skeletal muscles. Additionally, BDNF was expressed in Pax7+ satellite cells where it colocalized with p75NTR. In complementary cell culture experiments, we detected high levels of BDNF and p75NTR in myoblasts. During myogenic differentiation, expression of BDNF became drastically reduced. Using small interfering RNA (siRNA) technology to knock down BDNF expression, we demonstrate enhanced myogenic differentiation of myoblasts. This accelerated rate of myogenic differentiation seen in myoblasts expressing BDNF siRNA was normalized by administration of recombinant BDNF. Collectively, these findings show that BDNF plays an important regulatory function during myogenic differentiation. In addition, the expression of BDNF in satellite cells is coherent with the notion that BDNF serves a key role in maintaining the population of muscle progenitors in adult muscle.

Brain-derived neurotrophic factor (BDNF) and food intake regulation: A minireview

Neurotrophins, and in particular BDNF, play important roles in proliferation, differentiation and survival of neurons during development, as well as in the synaptic activity and plasticity in many groups of mature neurons. Several lines of evidence suggest that BDNF and its high affinity receptor TrkB contribute to food intake and body weight control. In rodents, pharmacological treatments with BDNF induce reduction in food intake, whereas genetic models with an altered BDNF/TrkB signalling display hyperphagia and obesity. Genetic studies in humans have shown that mutations in the BDNF or TrkB genes may account for certain types of obesity or other forms of eating disorders. Since circulating levels of BDNF correlate with eating disorders in humans and peripheral BDNF treatments reduce hyperphagia and hyperglycaemia in obese diabetic rodents, an endocrine role of BDNF appears plausible and requires further investigation. A central anorectic action of BDNF has also been documented, with a primary focus on the hypothalamus and a more recent highlight on the brainstem integrator of energy homeostasis, the dorsal vagal complex. In this review, we will briefly present neurotrophins and their receptors and focus on experimental evidence which point out BDNF as a signalling component of food intake regulation, with a particular emphasis on the localization of the central anorectic action of BDNF.

Supraspinal brain-derived neurotrophic factor signaling: a novel mechanism for descending pain facilitation

In the adult mammalian brain, brain-derived neurotrophic factor (BDNF) is critically involved in long-term synaptic plasticity. Here, we show that supraspinal BDNF-tyrosine kinase receptor B (TrkB) signaling contributes to pain facilitation. We show that BDNF-containing neurons in the periaqueductal gray (PAG), the central structure for pain modulation, project to and release BDNF in the rostral ventromedial medulla (RVM), a relay between the PAG and spinal cord. BDNF in PAG and TrkB phosphorylation in RVM neurons are upregulated after inflammation. Intra-RVM sequestration of BDNF and knockdown of TrkB by RNA interference attenuate inflammatory pain. Microinjection of BDNF (10-100 fmol) into the RVM facilitates nociception, which is dependent on NMDA receptors (NMDARs). In vitro studies with RVM slices show that BDNF induces tyrosine phosphorylation of the NMDAR NR2A subunit in RVM via a signal transduction cascade involving IP(3), PKC, and Src. The supraspinal BDNF-TrkB signaling represents a previously unknown mechanism underlying the development of persistent pain. Our findings also caution that application of BDNF for recovery from CNS disorders could lead to undesirable central pain.

Nerve growth factor rapidly increases muscarinic tone in mouse medial septum/diagonal band of Broca

Nerve growth factor (NGF) has been implicated in maintaining and regulating normal functioning of the septohippocampal pathway. However, many aspects of its physiological actions and the underlying mechanisms await elucidation. In this study, we investigated the effect of acute NGF exposure on neurons in the mouse medial septum/diagonal band of Broca (MS/DB), focusing on the cholinergic neurons and the subpopulation of noncholinergic neurons that were identified to be putatively GABAergic. We report that MS/DB neurons in a thin slice preparation, when exposed to NGF via bath perfusion, rapidly and indiscriminately increased the rate of spontaneous firing in all MS/DB neurons. However, focal application of NGF to individual MS/DB neurons increased spontaneous firing in cholinergic, but not in the noncholinergic, subpopulation. The NGF-induced effect on cholinergic neurons was direct, requiring activation and signaling via TrkA receptors, which were immunohistochemically localized to the cholinergic neurons in the MS/DB. TrkA receptors were absent in putative GABAergic MS/DB neurons, and blockade of TrkA signaling in these and other noncholinergic neurons had no effect on their firing activity after exposure to NGF. Conversely, methyl scopolamine, blocked the increased firing activity of noncholinergic neurons during bath perfusion of NGF. We propose a cell type-specific mode of action for NGF in the MS/DB. The neurotrophin directly enhances cholinergic neuronal activity in the MS/DB through TrkA-mediated signaling, increasing acetylcholine release and, thus, muscarinic tone. This increase in muscarinic tone, in turn, results in heightened firing activity in noncholinergic MS/DB neurons.

BDNF heterozygous mice demonstrate age-related changes in striatal and nigral gene expression

TrkB receptors mediate the effects of BDNF on striatal medium spiny neurons and mesencephalic dopamine neurons. The effect of partial BDNF gene deletion on locomotor activity and the gene expression of these neurons was evaluated at 3, 12, and 24 months of age in BDNF heterozygous (BDNF(LacZ/neo+)) and wildtype mice. BDNF(LacZ/neo+) mice displayed less spontaneous horizontal activity than wildtypes at 3 and 24 months of age. Whereas striatal preproenkephalin and preprodynorphin mRNA and mesencephalic tyrosine hydroxylase mRNA levels were significantly lower at all ages in BDNF(LacZ/neo+) mice, GAD67 mRNA was only lower at 24 months. In contrast, BDNF(LacZ/neo+) mice expressed more trkB mRNA in the striatum at 3 months and less at 24 months of age than wildtypes. Total striatal cell number in the two genotypes was not different at 12 months of age, whereas Golgi staining revealed that the spine density on distal dendrites of medium spiny neurons was less in BDNF(LacZ/neo+) mice than in wildtypes at 24 months of age. These data indicate that endogenous BDNF is required to maintain the normal phenotype and functioning of striatal projection neurons and mesencephalic dopamine neurons and that exaggerated dysfunction of these neurons and a concomitant decline in locomotor behavior occurs during aging.

Ultrastructural evidence for a pre- and postsynaptic localization of full-length trkB receptors in substantia gelatinosa (lamina II) of rat and mouse spinal cord

Brain-derived neurotrophic factor (BDNF) exerts its trophic effects by acting on the high-affinity specific receptor trkB. BDNF also modulates synaptic transmission in several areas of the CNS, including the spinal cord dorsal horn, where it acts as a pain modulator by yet incompletely understood mechanisms. Spinal neurons are the main source of trkB in lamina II (substantia gelatinosa). Expression of this receptor in dorsal root ganglion (DRG) cells has been a matter of debate, whereas a subpopulation of DRG neurons bears trkA receptors and contains BDNF. By the use of two different trkB antibodies we observed that 7.7% and 10.8% of DRG neurons co-expressed BDNF + trkB but not trkA, respectively, in rat and mouse. Ultrastructurally, full-length trkB (fl-trkB) receptors were present at somato-dendritic membranes of lamina II neurons (rat: 66.8%; mouse: 73.8%) and at axon terminals (rat: 33.2%; mouse: 26.2%). In both species, about 90% of these terminals were identified as primary afferent fibres (PAFs) considering their morphology and/or neuropeptide content. All fl-trkB-immunopositive C boutons in type Ib glomeruli were immunoreactive for BDNF and, at individual glomeruli and axo-dendritic synapses, fl-trkB receptors were located in a mutually exclusive fashion at pre- or postsynaptic membranes. Thus, only a small fraction of fl-trkB-immunoreactive dendrites were postsynaptic to BDNF-immunopositive PAFs. This is the first ultrastructural description of fl-trkB localization at synapses between first- and second-order sensory neurons in lamina II, and suggests that BDNF may be released by fl-trkB-immunopositive PAFs to modulate nociceptive input in this lamina of dorsal horn.

Possible Role of Brain-Derived Neurotrophic Factor in the Pathogenesis of Coronary Artery Disease

Background— The neurotrophin (NT) family, including nerve growth factor NT-3 and brain-derived neurotrophic factor (BDNF), has a critical role in the survival, growth, maintenance, and death of central and peripheral neurons. NTs and their receptors are expressed in atherosclerotic lesions; however, their significance in cardiovascular disease remains unclear.

Methods and Results— To clarify the role of NTs in the pathogenesis of coronary artery disease, NT plasma levels in the aorta, coronary sinus, and peripheral veins of patients with unstable angina (n=38), stable effort angina (n=45), and non–coronary artery disease (n=24) were examined. In addition, regional expression of BDNF in coronary arteries was examined in autopsy cases and patients with angina pectoris by directional coronary atherectomy. The difference in BDNF levels, but not NT-3, between the coronary sinus and aorta was significantly greater in the unstable angina group compared with the stable effort angina and non–coronary artery disease groups. Immunohistochemical investigations demonstrated BDNF expression in the atheromatous intima and adventitia in atherosclerotic coronary arteries. BDNF expression was enhanced in macrophages and smooth muscle cells in atherosclerotic coronary arteries. Stimulation with recombinant BDNF significantly enhanced NAD(P)H oxidase activity and the generation of reactive oxygen species in cultured human coronary artery smooth muscle cells.

Conclusions— BDNF has an important role in atherogenesis and plaque instability via the activation of NAD(P)H oxidase.

Presence of neurotrophic factors in skeletal muscle correlates with survival of spinal cord motor neurons

To determine which combination of skeletal muscle-derived neurotrophic factors may be important for the survival of specific subpopulations of developing spinal cord motor neurons, we used Myf5 and MyoD (myogenic regulatory factors) knockouts, containing differentially committed myogenic precursor cells (MPCc) and immunohistochemistry against several muscle-secreted neurotrophic factors. At the peak of motor neuron cell death, skeletal muscle development is delayed in the back and body wall muscles of Myf5-/- embryos and in the limb muscles of MyoD-/- embryos. We hypothesized that, if the skeletal muscle was indeed an important source of survival factors for motor neurons, the back, the abdominal wall, and the forelimb MPCs of Myf5-/- or MyoD-/- embryos should produce at least some neurotrophic factors necessary for the survival of motor neurons. In this report, we demonstrate that (1) different MPCs lacking Myf5, MyoD, or Myf5/MyoD have different capabilities in providing factors potentially required for the survival of motor neurons and intramuscular nerve branching, (2) MPCs in double-mutant embryos do not contain neurotrophic factors in the absence of myogenic specification, and (3) different subpopulations of MPCs contain different combinations of neurotrophic factors potentially required for the survival of the specific subpopulations of innervating motor neurons.

The role of neurotrophins in the maintenance of the spinal cord motor neurons and the dorsal root ganglia proprioceptive sensory neurons

The aim of this study was to approach the question of neuronal dependence on neurotrophins during embryonic development in mice in a way other than gene targeting. We employed amyogenic mouse embryos and fetuses that develop without any skeletal myoblasts or skeletal muscle and consequently lose motor and proprioceptive neurons. We hypothesized that if, in spite of the complete inability to maintain motor and proprioceptive neurons, the remaining spinal and dorsal root ganglia tissues of amyogenic fetuses still contain any of the neurotrophins, that particular neurotrophin alone is not sufficient for the maintenance of motor and proprioceptive neurons. Moreover, if the remaining spinal and dorsal root ganglia tissues still contain any of the neurotrophins, that particular neurotrophin alone may be sufficient for the maintenance of the remaining neurons (i.e., mostly non-muscle- and a few muscle-innervating neurons). To test the role of the spinal cord and dorsal root ganglia tissues in the maintenance of its neurons, we performed immunohistochemistry employing double-mutant and control tissues and antibodies against neurotrophins and their receptors. Our data suggested that: (a) during the peak of motor neuron cell death, the spinal cord and dorsal root ganglia distribution of neurotrophins was not altered; (b) the distribution of BDNF, NT-4/5, TrkB and TrkC, and not NT-3, was necessary for the maintenance of the spinal cord motor neurons; (c) the distribution of BDNF, NT-4/5 and TrkC, and not NT-3 and Trk B, was necessary for the maintenance of the DRG proprioceptive neurons; (d) NT-3 was responsible for the maintenance of the remaining neurons and glia in the spinal cord and dorsal root ganglia (possibly via TrkB).

Direct, complex effects of estrogens on basal forebrain cholinergic neurons

Although controversial, estrogens remain one of the few agents purported to influence the incidence of Alzheimer's disease and one of their postulated mechanisms of action is their effects on basal forebrain cholinergic neurons. However, it is unclear whether the responses of cholinergic neurons to estrogens are direct or mediated via the retrograde influences of neurotrophins, known to be induced by estrogens in the hippocampus and neocortex. In the present study, we explore the issue of the primary site of action of estrogens by studying the regulation of expression of genes that characterize mature cholinergic neurons, i.e., choline acetyltransferase, trkA, and p75(NTR) in the medial septum and the nucleus basalis complex. In parallel, we study the hippocampal expression of NGF, BDNF, and NT-3, i.e., neurotrophins with known trophic roles on cholinergic neurons. Gene expression is studied by RT-PCR in ovariectomized female rats with and without estrogen supplementation within the physiological estradiol range and in rats with complete fimbria-fornix transactions treated with estrogen or vehicle. To clarify mechanisms of estrogen transduction in cholinergic neurons, we study the effects of estrogen treatment on fimbria-fornix-lesioned mice with genetic ablations of ER subtypes alpha and beta. The results of the present study suggest that, while estrogens do regulate BDNF expression in the hippocampus and neocortex, they also exert stimulatory non-trophic effects on basal forebrain cholinergic neurons, primarily on ChAT expression. Cholinergic neurons retain their ability to respond to estrogens after their complete separation from the hippocampus. The elimination of ERalpha alters significantly the phenotypic responsiveness of cholinergic neurons to estrogens, whereas elimination of ERbeta appears to have no effect. Our findings support the idea that estrogens directly enhance cholinergic neuron function and that ERalpha plays a significant role in transducing these regulatory effects.

Age-related alteration of neurturin receptor GFRa2 and nNOS in pelvic ganglia

Neurturin is a neurotrophic factor that is widely expressed in cavernous tissue and retrogradely transported to penis-projecting neurons via its receptor, glial cell line derived neurotrophic factor family receptor alpha-2 (GFRa2). To investigate the influence of aging on neural function on the penis, we examined the expression of GFRa2 mRNA in the major pelvic ganglion and its relationship to neuronal nitric oxide synthase (nNOS)- and tyrosine hydroxylase (TH)-positive neurons. GFRa2 and nNOS mRNA expression levels in RT-PCR showed age-related decreases in 1-, 3-, 6-, 12-, 18- and 24-month-old rats. In situ hybridization also revealed that the number of GFRa2-positive neurons in pelvic ganglia decreased with aging. A double-labeling study revealed the co-expression of GFRa2 and nNOS, which simultaneously decreased in old adult (24 months) and young castrated rats compared with young adult rats (3 months). These results suggest that aging and castration influence the numbers of nNOS- and GFRa2-positive neurons. Higher age might affect not only cavernous tissue but also the neural plasticity of the cavernous nerve related to erectile function.

Synaptic targeting of retrogradely transported trophic factors in motoneurons: comparison of glial cell line-derived neurotrophic factor, brain-derived neurotrophic factor, and cardiotrophin-1 with tetanus toxin

Glial cell line-derived neurotrophic factor (GDNF), brain-derived neurotrophic factor (BDNF), and cardiotrophin-1 (CT-1) are the most potent neurotrophic factors for motoneurons, but their fate after retrograde axonal transport is not known. Internalized trophic factors may be degraded, or they may be recycled and transferred to other neurons, similar to the known route of tetanus toxin. We tested whether neonatal rat hypoglossal motoneurons target retrogradely transported trophic factors to synaptic sites on their dendrites within the brainstem and subsequently transfer these trophins across the synaptic cleft to afferent synapses (transsynaptic transcytosis). Motoneurons retrogradely transport from the tongue radiolabeled GDNF, BDNF, and CT-1 as well as tetanus toxin. Quantitative autoradiographic electron microscopy showed that GDNF and BDNF were transported into motoneuron dendrites with labeling densities similar to those of tetanus toxin. Although tetanus toxin accumulated rapidly (within 8 h) at presynaptic sites, GDNF accumulated at synapses more slowly (within 15 h), and CT-1 never associated with synapses. Thus, some retrogradely transported neurotrophic factors are trafficked similarly but not identically to tetanus toxin. Both GDNF and BDNF accumulate at the external (limiting) membrane of multivesicular bodies within proximal dendrites. We conclude that tetanus toxin, GDNF, and BDNF are released from postsynaptic sites and are internalized by afferent presynaptic terminals, thus demonstrating transsynaptic transcytosis. CT-1, however, follows a strict degradation pathway after retrograde transport to the soma. Synaptic and transcytotic trafficking thus are restricted to particular neurotrophic factors such as GDNF and BDNF.

Neurotrophin-3 is a target-derived neurotrophic factor for penile erection-inducing neurons

The aim of this study was to determine whether the neurotrophins nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), and neurotrophin (NT)-3 could act as endogenous target-derived trophic factors for erection-inducing, i.e. penis-projecting major pelvic ganglion (MPG) neurons, and/or penile sensory neurons in adult rat. This was accomplished by studying the expression of NT mRNAs in the penis and their cognate receptors in the MPG and dorsal root ganglia (DRGs), and the retrograde axonal transport of radioiodinated NTs injected into the corpora cavernosa. Northern hybridization showed that NGF, BDNF, and NT-3 mRNAs are expressed in the shaft of the penis. In situ hybridization combined with usage of the retrograde tracer Fluoro-Gold showed that TrkC and p75 receptors are expressed in penis-projecting neurons of the MPG whereas the mRNAs for TrkA and TrkB receptors were undetectable. However, all the NT receptor mRNAs were expressed in penile sensory neurons of sacral level 1 (S1) DRG. (125)I-NT-3 injected into the shaft of the penis was retrogradely transported into the MPG and S1 DRG, whereas radioiodinated NGF and BDNF were transported specifically into the S1 DRG, thus confirming the existence of functional NT receptors in these penile neurons. In conclusion, these data suggest that NT-3 may act as a target-derived neurotrophic factor for both erection-inducing and penile sensory neurons, whereas NGF and BDNF may be more important for the sensory innervation of the penis.

The effect of endoneurial nerve growth factor on calcitonin gene-related peptide expression in primary sensory neurons

Recent findings indicate that calcitonin gene-related peptide (CGRP) is involved in neuropathic pain, this peptide being up-regulated in a small population of large- and medium-sized primary sensory neurons after peripheral nerve injury. In adult animals, the expression of CGRP is regulated by nerve growth factor (NGF). After nerve injury, NGF is up-regulated at the injury site for several weeks, and this up-regulation contributes to the onset of neuropathic pain. Using immunohistochemistry, we investigated the time course of the effect of an endoneurial injection of NGF on the expression of CGRP in primary sensory neurons. NGF increased the percentage of medium- to large-sized DRG neuron profiles expressing CGRP, did not modify the percentage of small-sized neurons expressing CGRP, and increased CGRP expression in the laminae III and IV of the dorsal horn. The effects of NGF were evident as soon as 1 day after endoneurial injection, and lasted for 5 days. Ten days after the injection of NGF, the patterns of CGRP expression in the DRG were normal, whereas a slight decrease in CGRP content was observed in the dorsal horn. The injection of vehicle did not produce any change on CGRP expression in primary sensory neurons. These results suggest that endoneurial NGF is responsible for the increase in CGRP expression in some large-sized neurons and their central processes observed after nerve injury in animal models of neuropathic pain. Our findings contribute to the understanding of the role of NGF in neuropathic pain.

BDNF function in adult synaptic plasticity: the synaptic consolidation hypothesis

Interest in BDNF as an activity-dependent modulator of neuronal structure and function in the adult brain has intensified in recent years. Localization of BDNF-TrkB to glutamate synapses makes this system attractive as a dynamic, activity-dependent regulator of excitatory transmission and plasticity. Despite individual breakthroughs, an integrated understanding of BDNF function in synaptic plasticity is lacking. Here, we attempt to distill current knowledge of the molecular mechanisms and function of BDNF in LTP. BDNF activates distinct mechanisms to regulate the induction, early maintenance, and late maintenance phases of LTP. Evidence from genetic and pharmacological approaches is reviewed and tabulated. The specific contribution of BDNF depends on the stimulus pattern used to induce LTP, which impacts the duration and perhaps the subcellular site of BDNF release. Particular attention is given to the role of BDNF as a trigger for protein synthesis-dependent late phase LTP--a process referred to as synaptic consolidation. Recent experiments suggest that BDNF activates synaptic consolidation through transcription and rapid dendritic trafficking of mRNA encoded by the immediate early gene, Arc. A model is proposed in which BDNF signaling at glutamate synapses drives the translation of newly transported (Arc) and locally stored (i.e., alphaCaMKII) mRNA in dendrites. In this model BDNF tags synapses for mRNA capture, while Arc translation defines a critical window for synaptic consolidation. The biochemical mechanisms by which BDNF regulates local translation are also discussed. Elucidation of these mechanisms should shed light on a range of adaptive brain responses including memory and mood resilience.

Treatment with trkC agonist antibodies delays disease progression in neuromuscular degeneration (nmd) mice

Spinal muscular atrophy with respiratory distress type 1 (SMARD1) is a fatal autosomal recessive disorder seen in infants. It is characterized by lower motor neuron degeneration, progressive muscle paralysis and respiratory failure, for which no effective treatment exists. The phenotype of neuromuscular degeneration (nmd) mice closely resembles the human SMARD1. The identification of the mutated mouse gene in nmd mice, Ighmbp2, led to the discovery of mutations of the homologous gene in humans with SMARD1. We have studied the nmd mouse model with in vivo electrophysiological techniques and evaluated the efficacy of Mab2256, a monoclonal antibody with agonist effect on the tyrosine kinase receptor C, trkC, on disease progression in nmd mice. Treatment with Mab2256 resulted in a significant but transient improvement of muscle strength in nmd mice, as well as normalization of the neuromuscular depression during high-frequency nerve stimulation. These results suggest the potential of using monoclonal agonist antibodies for neurotrophin receptors in lower motor neuron diseases such as SMARD1.