Long-term adeno-associated viral vector-mediated expression of truncated TrkB in the adult rat facial nucleus results in motor neuron degeneration

Postnatal survival of phrenic motor neurons is promoted by BDNF/TrkB.FL signaling

The number of motor neurons (MNs) declines precipitously during the final trimester before birth. Thereafter, the number of MNs remains relatively stable, with their connections to skeletal muscle dependent on neurotrophins, including brain-derived neurotrophic factor (BDNF) signaling through its high-affinity full-length tropomyosin-related kinase receptor subtype B (TrkB.FL) receptor. As a genetic knockout of BDNF leads to extensive MN loss and postnatal death within 1-2 days after birth, we tested the hypothesis that postnatal inhibition of BDNF/TrkB.FL signaling is important for postnatal phrenic MN (PhMN) survival. In the present study, we used a 1NMPP1-sensitive TrkBF616A mutant mouse to evaluate the effects of inhibition of TrkB kinase activity on phrenic MN (PhMN) numbers and diaphragm muscle (DIAm) fiber cross-sectional area (CSA). Pups were exposed to 1NMPP1 or vehicle (DMSO) from birth to 21 days old (weaning) via the mother's ingestion in the drinking water. Following weaning, the right phrenic nerve was exposed in the neck and the proximal end dipped in a rhodamine solution to retrogradely label PhMNs. After 24 h, the cervical spinal cord and DIAm were excised. Labeled PhMNs were imaged using confocal microscopy, whereas DIAm strips were frozen at ∼1.5× resting length, cryosectioned, and stained with hematoxylin and eosin to assess CSA. We observed an ∼34% reduction in PhMN numbers and increased primary dendrite numbers in 1NMPP1-treated TrkBF616A mice. The distribution of PhMN size (somal surface area) DIAm fiber cross-sectional areas did not differ. We conclude that survival of PhMNs during early postnatal development is sensitive to BDNF/TrkB.FL signaling.NEW & NOTEWORTHY During early postnatal development, BDNF/TrkB signaling promotes PhMN survival. Inhibition of BDNF/TrkB signaling in early postnatal development does not impact PhMN size. Inhibition of BDNF/TrkB signaling in early postnatal development does not impact the number or CSA of DIAm fibers.

Chemogenetic inhibition of TrkB signalling reduces phrenic motor neuron survival and size

Brain derived neurotrophic factor (BDNF) signalling through its high-affinity tropomyosin receptor kinase B (TrkB) is known to have potent effects on motor neuron survival and morphology during development and in neurodegenerative diseases. Here, we employed a novel 1NMPP1 sensitive TrkBF616 rat model to evaluate the effect of 14 days inhibition of TrkB signalling on phrenic motor neurons (PhMNs). Adult female and male TrkBF616 rats were divided into 1NMPP1 or vehicle treated groups. Three days prior to treatment, PhMNs in both groups were initially labeled via intrapleural injection of Alexa-Fluor-647 cholera toxin B (CTB). After 11 days of treatment, retrograde axonal uptake/transport was assessed by secondary labeling of PhMNs by intrapleural injection of Alexa-Fluor-488 CTB. After 14 days of treatment, the spinal cord was excised 100 μm thick spinal sections containing PhMNs were imaged using two-channel confocal microscopy. TrkB inhibition reduced the total number of PhMNs by ∼16 %, reduced the mean PhMN somal surface areas by ∼25 %, impaired CTB uptake 2.5-fold and reduced the estimated PhMN dendritic surface area by ∼38 %. We conclude that inhibition of TrkB signalling alone in adult TrkBF616 rats is sufficient to lead to PhMN loss, morphological degeneration and deficits in retrograde axonal uptake/transport.

BDNF-dependent modulation of axonal transport is selectively impaired in ALS

Axonal transport ensures long-range delivery of essential cargoes between proximal and distal compartments, and is needed for neuronal development, function, and survival. Deficits in axonal transport have been detected at pre-symptomatic stages in the SOD1^G93A and TDP-43^M337V mouse models of amyotrophic lateral sclerosis (ALS), suggesting that impairments in this critical process are fundamental for disease pathogenesis. Strikingly, in ALS, fast motor neurons (FMNs) degenerate first whereas slow motor neurons (SMNs) are more resistant, and this is a currently unexplained phenomenon. The main aim of this investigation was to determine the effects of brain-derived neurotrophic factor (BDNF) on in vivo axonal transport in different α-motor neuron (MN) subtypes in wild-type (WT) and SOD1^G93A mice. We report that despite displaying similar basal transport speeds, stimulation of wild-type MNs with BDNF enhances in vivo trafficking of signalling endosomes specifically in FMNs. This BDNF-mediated enhancement of transport was also observed in primary ventral horn neuronal cultures. However, FMNs display selective impairment of axonal transport in vivo in symptomatic SOD1^G93A mice, and are refractory to BDNF stimulation, a phenotype that was also observed in primary embryonic SOD1^G93A neurons. Furthermore, symptomatic SOD1^G93A mice display upregulation of the classical non-pro-survival truncated TrkB and p75^NTR receptors in muscles, sciatic nerves, and Schwann cells. Altogether, these data indicate that cell- and non-cell autonomous BDNF signalling is impaired in SOD1^G93A MNs, thus identifying a new key deficit in ALS.

α2-Adrenergic Disruption of β Cell BDNF-TrkB Receptor Tyrosine Kinase Signaling

Adrenergic signaling is a well-known input into pancreatic islet function. Specifically, the insulin-secreting islet β cell expresses the Gi/o-linked α2-adrenergic receptor, which upon activation suppresses insulin secretion. The use of the adrenergic agonist epinephrine at micromolar doses may have supraphysiological effects. We found that pretreating β cells with micromolar concentrations of epinephrine differentially inhibited activation of receptor tyrosine kinases. We chose TrkB as an example because of its relative sensitivity to the effects of epinephrine and due to its potential regulatory role in the β cell. Our characterization of brain-derived neurotrophic factor (BDNF)-TrkB signaling in MIN6 β cells showed that TrkB is activated by BDNF as expected, leading to canonical TrkB autophosphorylation and subsequent downstream signaling, as well as chronic effects on β cell growth. Micromolar, but not nanomolar, concentrations of epinephrine blocked BDNF-induced TrkB autophosphorylation and downstream mitogen-activated protein kinase pathway activation, suggesting an inhibitory phenomenon at the receptor level. We determined epinephrine-mediated inhibition of TrkB activation to be Gi/o-dependent using pertussis toxin, arguing against an off-target effect of high-dose epinephrine. Published data suggested that inhibition of potassium channels or phosphoinositide-3-kinase signaling may abrogate the negative effects of epinephrine; however, these did not rescue TrkB signaling in our experiments. Taken together, these results show that (1) TrkB kinase signaling occurs in β cells and (2) use of epinephrine in studies of insulin secretion requires careful consideration of concentration-dependent effects. BDNF-TrkB signaling in β cells may underlie pro-survival or growth signaling and warrants further study.

Transduction of Craniofacial Motoneurons Following Intramuscular Injections of Canine Adenovirus Type-2 (CAV-2) in Rhesus Macaques

Reliable viral vector-mediated transgene expression in primate motoneurons would improve our ability to anatomically and physiologically interrogate motor systems. We therefore investigated the efficacy of replication defective, early region 1-deleted canine adenovirus type-2 (CAV-2) vectors for mediating transgene expression of fluorescent proteins into brainstem motoneurons following craniofacial intramuscular injections in four rhesus monkeys (Macaca mulatta). Vector injections were placed into surgically identified and isolated craniofacial muscles. After a 1- to 2-month survival time, animals were sacrificed and transgene expression was assessed with immunohistochemistry in the corresponding motoneuronal populations. We found that injections of CAV-2 into individual craniofacial muscles at doses in the range of ∼10¹⁰ to 10¹¹ physical particles/muscle resulted in robust motoneuronal transduction and expression of immunohistochemically identified fluorescent proteins across multiple animals. By using different titers in separate muscles, with the resulting transduction patterns tracked via fluorophore expression and labeled motoneuron location, we established qualitative dose-response relationships in two animals. In one animal that received an atypically high titer (5.7 × 10¹¹ total CAV-2 physical particles) distributed across numerous injection sites, no transduction was detected, likely due to a retaliatory immune response. We conclude that CAV-2 vectors show promise for genetic modification of primate motoneurons following craniofacial intramuscular injections. Our findings warrant focused attention toward the use of CAV-2 vectors to deliver opsins, DREADDs, and other molecular probes to improve genetics-based methods for primate research. Further work is required to optimize CAV-2 transduction parameters. CAV-2 vectors encoding proteins could provide a new, reliable route for modifying activity in targeted neuronal populations of the primate central nervous system.

Diverse distribution of tyrosine receptor kinase B isoforms in rat multiple tissues

Tyrosine receptor kinase B (TrkB), a receptor for brain-derived neurotrophic factor and neurotrophin-3, includes the alternatively spliced three isoforms specifically in rats. Each isoform, full length TrkB (TrkB FL), truncated TrkB type-1 (TrkB T1) and type-2 (TrkB T2), seems to mediate diverse cellular function. Some studies suggest that TrkB plays a key role in both neural and non-neural systems. In the present study, we examined mRNA and protein expression profile of three TrkB isoforms in normal adult rat multiple tissues. TrkB FL mRNA and protein were both highly expressed exclusively in brain. While TrkB T1 mRNA was highly expressed exclusively in brain, glycosylated TrkB T1 protein was expressed in brain and heart. TrkB T2 mRNA level in brain was the highest. In brain, TrkB FL mRNA expression was higher in cerebral cortex, but lower in brainstem. TrkB T1 mRNA expression was higher in hypothalamus, but lower in cerebellum. TrkB T2 mRNA expression was higher in cerebral cortex and cerebellum, but lower in brainstem. The present study for the first time clarified diverse distribution of three TrkB isoforms in rat multiple tissues and could serve as a useful resource for understanding the physiology and pathophysiology of mammals including human via comparing the expression pattern of TrkB isoforms.

Synapsins Are Downstream Players of the BDNF-Mediated Axonal Growth

Synapsins (Syns) are synaptic vesicle-associated phosphoproteins involved in neuronal development and neurotransmitter release. While Syns are implicated in the regulation of brain-derived neurotrophic factor (BDNF)-induced neurotransmitter release, their role in the BDNF developmental effects has not been fully elucidated. By using primary cortical neurons from Syn I knockout (KO) and Syn I/II/III KO mice, we studied the effects of BDNF and nerve growth factor (NGF) on axonal growth. While NGF had similar effects in all genotypes, BDNF induced significant differences in Syn KO axonal outgrowth compared to wild type (WT), an effect that was rescued by the re-expression of Syn I. Moreover, the significant increase of axonal branching induced by BDNF in WT neurons was not detectable in Syn KO neurons. The expression analysis of BDNF receptors in Syn KO neurons revealed a significant decrease of the full length TrkB receptor and an increase in the levels of the truncated TrkB.t1 isoform and p75^NTR associated with a marked reduction of the BDNF-induced MAPK/Erk activation. By using the Trk inhibitor K252a, we demonstrated that these differences in BDNF effects were dependent on a TrkB/p75^NTR imbalance. The data indicate that Syn I plays a pivotal role in the BDNF signal transduction during axonal growth.

Distinct transduction profiles in the CNS via three injection routes of AAV9 and the application to generation of a neurodegenerative mouse model

Using single-stranded adeno-associated virus serotype 9 (ssAAV9) vectors containing the neuron-specific synapsin-I promoter, we examined whether different administration routes (direct cerebellar cortical (DC), intrathecal (IT) and intravenous (IV) injections) could elicit specific transduction profiles in the CNS. The DC injection route robustly and exclusively transduced the whole cerebellum, whereas the IT injection route primarily transduced the cerebellar lobules 9 and 10 close to the injection site and the spinal cord. An IV injection in neonatal mice weakly and homogenously transduced broad CNS areas. In the cerebellar cortex, the DC and IT injection routes transduced all neuron types, whereas the IV injection route primarily transduced Purkinje cells. To verify the usefulness of this method, we generated a mouse model of spinocerebellar ataxia type 1 (SCA1). Mice that received a DC injection of the ssAAV9 vector expressing mutant ATXN1, a protein responsible for SCA1, showed the intranuclear aggregation of mutant ATXN1 in Purkinje cells, significant atrophy of the Purkinje cell dendrites and progressive motor deficits, which are characteristics of SCA1. Thus, ssAAV9-mediated transduction areas, levels, and cell types change depending on the route of injection. Moreover, this approach can be used for the generation of different mouse models of CNS/neurodegenerative diseases.

Expression of full-length and truncated trkB in human striatum and substantia nigra neurons: implications for Parkinson's disease

Brain derived neurotrophic factor (BDNF) is a potent mediator of cell survival and differentiation and can reverse neuronal injury associated with Parkinson's disease (PD). Tropomyosin receptor kinase B (trkB) is the high affinity receptor for BDNF. There are two major trkB isoforms, the full-length receptor (trkB.tk(+)) and the truncated receptor (trkB.t1), that mediate the diverse, region specific functions of BDNF. Both trkB isoforms are widely distributed throughout the brain, but the isoform specific distribution of trkB.t1 and trkB.tk(+) to human neurons is not well characterized. Therefore, we report the regional and neuronal distribution of trkB.tk(+) and trkB.t1 in the striatum and substantia nigra pars compacta (SNpc) of human autopsy tissues from control and PD cases. In both PD and control tissues, we found abundant, punctate distribution of trkB.tk(+) and trkB.t1 proteins in striatum and SNpc neurons. In PD, trkB.tk(+) is decreased in striatal neurites, increased in striatal somata, decreased in SNpc somata and dendrites, and increased in SNpc axons. TrkB.t1 is increased in striatal somata, decreased in striatal axons, and increased in SNpc distal dendrites. We believe changes in trkB isoform distribution and expression levels may be markers of pathology and affect the neuronal response to BDNF.

Differential regulation of BDNF, synaptic plasticity and sprouting in the hippocampal mossy fiber pathway of male and female rats

Many studies have described potent effects of BDNF, 17β-estradiol or androgen on hippocampal synapses and their plasticity. Far less information is available about the interactions between 17β-estradiol and BDNF in hippocampus, or interactions between androgen and BDNF in hippocampus. Here we review the regulation of BDNF in the mossy fiber pathway, a critical part of hippocampal circuitry. We discuss the emerging view that 17β-estradiol upregulates mossy fiber BDNF synthesis in the adult female rat, while testosterone exerts a tonic suppression of mossy fiber BDNF levels in the adult male rat. The consequences are interesting to consider: in females, increased excitability associated with high levels of BDNF in mossy fibers could improve normal functions of area CA3, such as the ability to perform pattern completion. However, memory retrieval may lead to anxiety if stressful events are recalled. Therefore, the actions of 17β-estradiol on the mossy fiber pathway in females may provide a potential explanation for the greater incidence of anxiety-related disorders and post-traumatic stress syndrome (PTSD) in women relative to men. In males, suppression of BDNF-dependent plasticity in the mossy fibers may be protective, but at the 'price' of reduced synaptic plasticity in CA3. This article is part of the Special Issue entitled 'BDNF Regulation of Synaptic Structure, Function, and Plasticity'.

Different roles of BDNF in nucleus accumbens core versus shell during the incubation of cue-induced cocaine craving and its long-term maintenance

Brain-derived neurotrophic factor (BDNF) contributes to diverse types of plasticity, including cocaine addiction. We investigated the role of BDNF in the rat nucleus accumbens (NAc) in the incubation of cocaine craving over 3 months of withdrawal from extended access cocaine self-administration. First, we confirmed by immunoblotting that BDNF levels are elevated after this cocaine regimen on withdrawal day 45 (WD45) and showed that BDNF mRNA levels are not altered. Next, we explored the time course of elevated BDNF expression using immunohistochemistry. Elevation of BDNF in the NAc core was detected on WD45 and further increased on WD90, whereas elevation in shell was not detected until WD90. Surface expression of activated tropomyosin receptor kinase B (TrkB) was also enhanced on WD90. Next, we used viral vectors to attenuate BDNF-TrkB signaling. Virus injection into the NAc core enhanced cue-induced cocaine seeking on WD1 compared with controls, whereas no effect was observed on WD30 or WD90. Attenuating BDNF-TrkB signaling in shell did not affect cocaine seeking on WD1 or WD45 but significantly decreased cocaine seeking on WD90. These results suggest that basal levels of BDNF transmission in the NAc core exert a suppressive effect on cocaine seeking in early withdrawal (WD1), whereas the late elevation of BDNF protein in NAc shell contributes to incubation in late withdrawal (WD90). Finally, BDNF protein levels in the NAc were significantly increased after ampakine treatment, supporting the novel hypothesis that the gradual increase of BDNF levels in NAc accompanying incubation could be caused by increased AMPAR transmission during withdrawal.

Deletion of the BDNF truncated receptor TrkB.T1 delays disease onset in a mouse model of amyotrophic lateral sclerosis

Brain Derived Neurotrophic Factor (BDNF) exerts strong pro-survival effects on developing and injured motoneurons. However, in clinical trials, BDNF has failed to benefit patients with amyotrophic lateral sclerosis (ALS). To date, the cause of this failure remains unclear. Motoneurons express the TrkB kinase receptor but also high levels of the truncated TrkB.T1 receptor isoform. Thus, we investigated whether the presence of this receptor may affect the response of diseased motoneurons to endogenous BDNF. We deleted TrkB.T1 in the hSOD1^G93A ALS mouse model and evaluated the impact of this mutation on motoneuron death, muscle weakness and disease progression. We found that TrkB.T1 deletion significantly slowed the onset of motor neuron degeneration. Moreover, it delayed the development of muscle weakness by 33 days. Although the life span of the animals was not affected we observed an overall improvement in the neurological score at the late stage of the disease. To investigate the effectiveness of strategies aimed at bypassing the TrkB.T1 limit to BDNF signaling we treated SOD1 mutant mice with the adenosine A2A receptor agonist CGS21680, which can activate motoneuron TrkB receptor signaling independent of neurotrophins. We found that CGS21680 treatment slowed the onset of motor neuron degeneration and muscle weakness similarly to TrkB.T1 removal. Together, our data provide evidence that endogenous TrkB.T1 limits motoneuron responsiveness to BDNF in vivo and suggest that new strategies such as Trk receptor transactivation may be used for therapeutic intervention in ALS or other neurodegenerative disorders.

Excitotoxicity downregulates TrkB.FL signaling and upregulates the neuroprotective truncated TrkB receptors in cultured hippocampal and striatal neurons

Brain-derived neurotrophic factor (BDNF) plays an important role in neuronal survival through activation of TrkB receptors. The trkB gene encodes a full-length receptor tyrosine kinase (TrkB.FL) and its truncated (T1/T2) isoforms. We investigated the changes in TrkB protein levels and signaling activity under excitotoxic conditions, which are characteristic of brain ischemia, traumatic brain injury, and neurodegenerative disorders. Excitotoxic stimulation of cultured rat hippocampal or striatal neurons downregulated TrkB.FL and upregulated a truncated form of the receptor (TrkB.T). Downregulation of TrkB.FL was mediated by calpains, whereas the increase in TrkB.T protein levels required transcription and translation activities. Downregulation of TrkB.FL receptors in hippocampal neurons correlated with a decrease in BDNF-induced activation of the Ras/ERK and PLCγ pathways. However, calpain inhibition, which prevents TrkB.FL degradation, did not preclude the decrease in signaling activity of these receptors. On the other hand, incubation with anisomycin, to prevent the upregulation of TrkB.T, protected to a large extent the TrkB.FL signaling activity, suggesting that truncated receptors may act as dominant-negatives. The upregulation of TrkB.T under excitotoxic conditions was correlated with an increase in BDNF-induced inhibition of RhoA, a mediator of excitotoxic neuronal death. BDNF fully protected hippocampal neurons transduced with TrkB.T when present during excitotoxic stimulation with glutamate, in contrast with the partial protection observed in cells overexpressing TrkB.FL or expressing GFP. These results indicate that BDNF protects hippocampal neurons by two distinct mechanisms: through the neurotrophic effects of TrkB.FL receptors and by activation of TrkB.T receptors coupled to inhibition of the excitotoxic signaling.

Truncated TrkB: beyond a dominant negative receptor

BDNF activates trkB receptors to regulate neuronal survival, differentiation, and proliferation. Mutations in the BDNF gene, altered BDNF expression, and altered trkB expression are associated with degenerative and psychiatric disorders. The full-length trkB receptor (trkB.tk(+)) undergoes autophosphorylation to activate intracellular signaling pathways. The truncated trkB receptor (trkB.t1) is abundantly expressed in the brain but lacks the catalytic tyrosine kinase domain. TrkB.t1 is a dominant-negative receptor that inhibits trkB.tk(+) signaling. While this is an important function of trkB.t1, it is only one of its many functions. TrkB.t1 sequesters and translocate BDNF, induces filopodia and neurite outgrowth, stimulates intracellular signaling cascades, regulates Rho GTPase signaling, and modifies cytoskeletal structures. TrkB.t1 is an active signaling molecule with regulatory effects on neurons and astrocytes.

Induction of adipose-derived stem cell into motoneuron-like cells using selegiline as preinducer

Cell therapy is one of the approaches taken to treatment of spinal cord disorders. In this study, adipose-derived stem cells (ADSCs) were induced to form motoneuron-like cells (MNLCs) using selegiline as preinducer, as well as Shh and all trans-retinoic acid (RA) as inducers. Selegiline was reported to induce the embryonic stem cells and bone marrow stromal cells into neuronal phenotype. ADSCs were evaluated using CD90, CD44, CD 49d, CD106, CD31, CD45, lipogenesis and osteogenesis. Dose response and time course studies were used in selecting the optimal concentration for selegiline using the percentage of viable cells (PVC) and percentages of immunoreactive cells (PIC) to nestin and neurofilament 68. Accordingly, such studies were used in selecting the optimal dose for RA using PVC and PIC to islet-1 and oligo-2. The expression of islet-1, oligo-2 and HLXB9 was evaluated using RT-PCR and immunocytochemistry. Real-time PCR was utilized in order to quantify the expression of islet-1, oligo-2 and HLXB9. ADSCs were immunoreactive to CD90, CD44 and CD 49d with consistent differentiation osteogenic and lipogenic cells. The optimal concentrations of selegiline and RA were 10⁻⁹ mM and 2 × 10⁻⁸ M, respectively. After two days, MNLCs showed high oligo-2 expression. MNLCs innervated myotubes; also, the release rate of synaptic vesicles using FM1-43 followed exponential decay model, and this rate in the induced MNLCs was approximately three times of that in the preinduced cells.

Mechanisms of synapse and dendrite maintenance and their disruption in psychiatric and neurodegenerative disorders

Emerging evidence indicates that once established, synapses and dendrites can be maintained for long periods, if not for the organism's entire lifetime. In contrast to the wealth of knowledge regarding axon, dendrite, and synapse development, we understand comparatively little about the cellular and molecular mechanisms that enable long-term synapse and dendrite maintenance. Here, we review how the actin cytoskeleton and its regulators, adhesion receptors, and scaffolding proteins mediate synapse and dendrite maintenance. We examine how these mechanisms are reinforced by trophic signals passed between the pre- and postsynaptic compartments. We also discuss how synapse and dendrite maintenance mechanisms are compromised in psychiatric and neurodegenerative disorders.

Analysis of gene expression in Parkinson's disease: possible involvement of neurotrophic support and axon guidance in dopaminergic cell death

Parkinson's disease (PD) is a neurodegenerative disorder characterized by the progressive loss of dopaminergic neurons in the substantia nigra. We have studied alterations in gene expression in the substantia nigra, the caudate nucleus and putamen of four PD patients and four matched controls using custom designed Agilent microarrays. To gain insight into changes in gene expression during early stages of dopaminergic neurodegeneration, we selectively investigated the relatively spared parts of the PD substantia nigra, and correlated gene expression changes with alterations in neuronal density. We identified changes in the expression of 287 transcripts in the substantia nigra, 16 transcripts in the caudate nucleus and four transcripts in the putamen. For selected transcripts, transcriptional alterations were confirmed with qPCR on a larger set of seven PD cases and seven matched controls. We detected concerted changes in functionally connected groups of genes. In the PD substantia nigra, we observed strong evidence for a reduction in neurotrophic support and alterations in axon guidance cues. As the changes occur in relatively spared parts of the PD substantia nigra, they suggest novel disease mechanisms involving neurotrophic support and axon guidance in early stages of cellular stress events, ultimately leading to dopaminergic cell death in PD.