TrkB and TrkC are differentially regulated by excitotoxicity during development of the basal ganglia

Neuronal survival in the brain: neuron type-specific mechanisms

Neurogenic regions of mammalian brain produce many more neurons that will eventually survive and reach a mature stage. Developmental cell death affects both embryonically produced immature neurons and those immature neurons that are generated in regions of adult neurogenesis. Removal of substantial numbers of neurons that are not yet completely integrated into the local circuits helps to ensure that maturation and homeostatic function of neuronal networks in the brain proceed correctly. External signals from brain microenvironment together with intrinsic signaling pathways determine whether a particular neuron will die. To accommodate this signaling, immature neurons in the brain express a number of transmembrane factors as well as intracellular signaling molecules that will regulate the cell survival/death decision, and many of these factors cease being expressed upon neuronal maturation. Furthermore, pro-survival factors and intracellular responses depend on the type of neuron and region of the brain. Thus, in addition to some common neuronal pro-survival signaling, different types of neurons possess a variety of 'neuron type-specific' pro-survival constituents that might help them to adapt for survival in a certain brain region. This review focuses on how immature neurons survive during normal and impaired brain development, both in the embryonic/neonatal brain and in brain regions associated with adult neurogenesis, and emphasizes neuron type-specific mechanisms that help to survive for various types of immature neurons. Importantly, we mainly focus on in vivo data to describe neuronal survival specifically in the brain, without extrapolating data obtained in the PNS or spinal cord, and thus emphasize the influence of the complex brain environment on neuronal survival during development.

Differential expression of neurotrophins in postnatal C57BL/6 mice striatum

Neurotrophin expression in early stages of development is crucial for brain assembly and function. In particular, postnatal expression of neurotrophins has not been well documented in the neostriatum and in general neurotrophins or their receptor mRNA's are normally reported, but not protein expression. In the present study, immunocytochemical expression of BDNF, NT-3 and NT-4/5 was characterized in striatal tissue of C57BL/6 mice at postnatal days 10^th (P10), 21^st (P21), 42^nd (P42) and 80^th (P80).

We found that the expression of BDNF diminished along the postnatal time course we evaluated, while staining for NT-4 increased up to age P42 and remained constant, thereafter in the cell's soma. In contrast, NT-3 was first expressed in the neostriatal bundles and later on, in neostriatal cell somas. These results provide information about differences in the spatial and temporal expression of each neurotrophin in the neostriatum during the first 80^th postnatal days.

RT-PCR procedures were also carried out to further determine whether protein levels of neurotrophins observed in the neostriatum were under control of gene expression. All neurotrophin mRNAs were expressed and only mRNA^BDNF was reduced during the postnatal evaluated days.

Differences in temporal expression of neurotrophins may be related to the heterochronic development of neostriatal cell populations, but also with the specificity of each neurotrophin modulating different neuronal targets.

Expression of trkB and trkC receptors and their ligands brain-derived neurotrophic factor and neurotrophin-3 in the murine amygdala

The neurotrophin brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3) and their cognate receptors, trkB and trkC, have a variety of physiological brain functions, ranging from cell survival to mechanisms involved in learning and memory and long-term potentiation (LTP). LTP can be induced in the cortex and hippocampus, as well as within the amygdala. However, the role of neurotrophins in amygdalar LTP is largely unknown. Expression patterns of BDNF and NT-3 and their cognate receptors in the adult mouse amygdala have not been analyzed in detail. We have therefore examined the expression of trkB, trkC, BDNF, and NT-3 mRNA and protein in different amygdalar nuclei as well as in the hippocampal areas CA1-CA3 and the dentate gyrus. The distribution pattern of trkB, trkC, BDNF, and NT-3 mRNA in the murine hippocampus is comparable to that seen in rats. Within most amygdalar nuclei, a moderate BDNF mRNA expression was found; however, BDNF mRNA was virtually absent from the central nucleus. No expression of NT-3 mRNA was found within the amygdala, but trkC mRNA-expressing cells were widely distributed within this brain region. trkB mRNA was strongly expressed in the amygdala. Because trkB is expressed in a full-length and a truncated form (the latter form is also expressed by nonneuronal cells), we also investigated the distribution of full-length trkB mRNA-expressing cells and could demonstrate that this version of trkB receptors is also widely expressed in the amygdala. These results can serve as a basis for studies elucidating the physiological roles of these receptors in the amygdala.

Adenosine A(2A) receptors are required for normal BDNF levels and BDNF-induced potentiation of synaptic transmission in the mouse hippocampus

Brain-derived neurotrophic factor (BDNF), a member of neurotrophin family, enhances synaptic transmission and regulates neuronal proliferation and survival. Both BDNF and its tyrosine kinase receptors (TrkB) are highly expressed in the hippocampus, where an interaction with adenosine A(2A) receptors (A(2A)Rs) has been recently reported. In the present paper, we evaluated the role of A(2A)Rs in mediating functional effects of BDNF in hippocampus using A(2A)R knock-out (KO) mice. In hippocampal slices from WT mice, application of BDNF (10 ng/mL) increased the slope of excitatory post-synaptic field potentials (fEPSPs), an index of synaptic facilitation. This increase of fEPSP slope was abolished by the selective A(2A) antagonist ZM 241385. Similarly, genetic deletion of the A(2A)Rs abolished BDNF-induced increase of the fEPSP slope in slices from A(2A)R KO mice The reduced functional ability of BDNF in A(2A)R KO mice was correlated with the reduction in hippocampal BDNF levels. In agreement, the pharmacological blockade of A(2)Rs by systemic ZM 241385 significantly reduced BDNF levels in the hippocampus of normal mice. These results indicate that the tonic activation of A(2A)Rs is required for BDNF-induced potentiation of synaptic transmission and for sustaining a normal BDNF tone in the hippocampus.

Adenosine A(2A) receptors modulate BDNF both in normal conditions and in experimental models of Huntington's disease

Brain-derived neurotrophic factor (BDNF), a member of the neurotrophin family, enhances synaptic transmission and regulates neuronal proliferation and survival. Functional interactions between adenosine A_2A receptors (A_2ARs) and BDNF have been recently reported. In this article, we report some recent findings from our group showing that A_2ARs regulate both BDNF functions and levels in the brain. Whereas BDNF (10 ng/ml) increased the slope of excitatory postsynaptic field potentials (fEPSPs) in hippocampal slices from wild-type (WT) mice, it was completely ineffective in slices taken from A_2AR knock-out (KO) mice. Furthermore, enzyme immunoassay studies showed a significant reduction in hippocampal BDNF levels in A_2AR KO vs. WT mice. Having found an even marked reduction in the striatum of A_2AR KO mice, and as both BDNF and A_2ARs have been implicated in the pathogenesis of Huntington’s disease (HD), an inherited striatal neurodegenerative disease, we then evaluated whether the pharmacological blockade of A_2ARs could influence striatal levels of BDNF in an experimental model of HD-like striatal degeneration (quinolinic acid-lesioned rats) and in a transgenic mice model of HD (R6/2 mice). In both QA-lesioned rats and early symptomatic R6/2 mice (8 weeks), the systemic administration of the A_2AR antagonist SCH58261 significantly reduced striatal BDNF levels. These results indicate that the presence and the tonic activation of A_2ARs are necessary to allow BDNF-induced potentiation of synaptic transmission and to sustain a normal BDNF tone. The possible functional consequences of reducing striatal BDNF levels in HD models need further investigation.

Mice heterozygous for neurotrophin-3 display enhanced vulnerability to excitotoxicity in the striatum through increased expression of N-methyl-D-aspartate receptors

The striatum is one of the brain areas most vulnerable to excitotoxicity, a lesion that can be prevented by neurotrophins. In the present study, intrastriatal injection of the N-methyl-d-aspartate receptor (NMDAR) agonist quinolinate (QUIN) was performed in mice heterozygous for neurotrophin-3 (NT3 +/-) or brain-derived neurotrophic factor (BDNF +/-) to analyze the role of endogenous neurotrophins on the regulation of striatal neurons susceptibility to excitotoxic injury. QUIN injection induced a decrease in dopamine- and cyclic AMP-regulated phosphoprotein of 32 kDa (DARPP-32) protein levels that was higher in NT-3 +/- than in BDNF+/- or wild type animals. This enhanced susceptibility was specific for enkephalin- and tachykinin-positive projection neurons, and also for parvalbumin-positive interneurons. However the excitotoxic damage in large interneurons was not modified in NT-3 +/- mice compared with wild type animals. This effect can be related to the regulation of NMDARs by endogenous NT-3. Thus, our results show that there is an age-dependent regulation of NMDAR subunits NR1 and NR2A, but not NR2B, in NT-3 +/- mice. The deficit of endogenous NT-3 induced a decrease in NR1 and NR2A subunits at postnatal day (P) 0 and P3 mice respectively, whereas an upregulation was observed in 12 week old NT-3 +/- mice. This differential effect was also observed after administration of exogenous NT-3. In primary striatal cultures, NT-3 treatment induced an enhancement in NR2A, but not NR2B, protein levels. However, intrastriatal grafting of NT-3 secreting-cells in adult wild type mice produced a down-regulation of NR2A subunit. In conclusion, NT-3 regulates the expression of NMDAR subunits modifying striatal neuronal properties that confers the differential vulnerability to excitotoxicity in projection neurons and interneurons in the striatum.

Reduced expression of the TrkB receptor in Huntington's disease mouse models and in human brain

Deficits of neurotrophic support caused by reduced levels of brain-derived neurotrophic factor (BDNF) have been implicated in the selective vulnerability of striatal neurones in Huntington's disease (HD). Therapeutic strategies based on BDNF administration have been proposed to slow or prevent the disease progression. However, the effectiveness of BDNF may depend on the proper expression of its receptor TrkB. In this study, we analysed the expression of TrkB in several HD models and in postmortem HD brains. We found a specific reduction of TrkB receptors in transgenic exon-1 and full-length knock-in HD mouse models and also in the motor cortex and caudate nucleus of HD brains. Our findings also demonstrated that continuous expression of mutant huntingtin is required to down-regulate TrkB levels. This was shown by findings in an inducible HD mouse model showing rescue of TrkB by turning off mutant huntingtin expression. Interestingly, the length of the polyglutamine tract in huntingtin appears to modulate the reduction of TrkB. Finally, to analyse the effect of BDNF in TrkB we compared TrkB expression in mutant huntingtin R6/1 and double mutant (R6/1 : BDNF+/-) mice. Similar TrkB expression was found in both transgenic mice suggesting that reduced TrkB is not a direct consequence of decreased BDNF. Therefore, taken together our findings identify TrkB as an additional component that potentially might contribute to the altered neurotrophic support in HD.

Developmental expression of neurotrophins and their receptors in postnatal rat ventral midbrain

Neurotrophins are a group of structurally related polypeptides that support the survival, differentiation, and maintenance of neuronal populations that express the appropriate high-affinity neurotrophin receptors. Two members of the neurotrophin family, brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3), have been shown to increase the survival of dopaminergic neurons from the ventral midbrain in vitro. Evidence suggests that ventral midbrain neurons might be able to derive support from these trophic factors in vivo through paracrine or autocrine interactions. Both BDNF and NT-3 mRNAs and their receptor mRNAs, trkB and trkC mRNAs, respectively, have been localized to the ventral mesencephalon. However, the relative expression levels of the neurotrophins and their receptor mRNAs throughout ontogeny and in adulthood have not been elucidated. In the present study, the postnatal developmental expression of BDNF, NT-3, trkB, and trkC mRNAs was analyzed via in situ hybridization to gain insight into the possible roles of these factors in vivo. We found that there was a developmental decline in the expression of BDNF and NT-3 mRNAs in the ventral mesencephalon. In contrast, no alterations in the expression of midbrain trkB or trkC mRNAs could be discerned. The present results suggest a role for BDNF and NT-3 in the earlier postnatal developmental events of responsive populations. The continued, albeit lower, expression of the neurotrophins in the ventral mesencephalon in adulthood also suggests a role for these factors in mature neuronal systems.

BDNF and NT-3 regulation of trkB and trkC mRNA levels in the developing chick spinal cord

In this study we investigated the effects of brain-derived neurotrophic factor (BDNF) and neurotrophin-3 on mRNA levels of TrkB and TrkC receptors. We used an expression system to supply developing chicks with the neurotrophic factor and then analysed the receptor mRNA levels at embryonic day 8 (E8), E10 and E15 using semi-quantitative RT-PCR. In control chicks, maximal expression levels of both receptors were observed at E10. Treatment with BDNF resulted in significant down-regulation of TrkB mRNA levels (P<0.05) at E10 but not E8 or E15. Treatment with NT-3 showed down-regulation of trkB levels at all developmental stages. TrkC mRNA levels were down-regulated at all developmental stages with NT-3 treatment and at E10 and E15 with BDNF treatment. For both receptors the down-regulation was greater in NT-3-treated chicks than those treated with BDNF. Thus, our data indicate that neurotrophin receptor mRNA levels in the spinal cord are regulated by neurotrophic factors during embryonic development.

Discordant changes in cortical TrkC mRNA and protein during the human lifespan

Neurotrophin-3 (NT-3) exerts its trophic effects in brain via tyrosine kinase receptor C (trkC) signaling. TrkC splice variants produce receptors with (full-length) and without (truncated) a tyrosine kinase domain. The relative abundance of trkC isoforms and the anatomical localization of trkC in the human prefrontal cortex (PFC) in relationship to development and maturation are currently unknown. We have examined the temporo-spatial expression of trkC protein and mRNA during the development of the human PFC. We have found two major isoforms, a full-length (150 kDa) and a truncated (50 kDa) form of the trkC protein in the human PFC. We report that the full-length form is expressed at low levels throughout development while the truncated form is expressed at moderate levels early in development and increases to reach mature levels by adolescence. In contrast, trkC mRNA levels are uniformly expressed throughout most of postnatal life, but decline in ageing. TrkC protein and mRNA are expressed in both pyramidal and non-pyramidal neurons; additionally, trkC protein is detected in glia and neuropil. Our results suggest that truncated trkC is prevalent in the human PFC and that neurons and glia may be responsive to NT-3 in the PFC throughout life.

Endogenous brain-derived neurotrophic factor protects dopaminergic nigral neurons against transneuronal degeneration induced by striatal excitotoxic injury

Injury to the central nervous system causes atrophy or death of connecting neurons and can modify the expression of neurotrophic factors. We observed transneuronal upregulation of brain-derived neurotrophic factor (BDNF) expression in the rat ipsilateral substantia nigra pars compacta after a striatal lesion induced by kainate. This effect is developmentally regulated because the enhancement of nigral BDNF expression was only observed when striatal lesion was performed on postnatal day (P) 15 and in adulthood, but not at P7. Interestingly, the lack of regulation of BDNF was coincident with the transynaptic degeneration of nigral neurons after striatal excitotoxic injury. Hence, the number of tyrosine hydroxylase-positive neurons in the substantia nigra pars compacta decreased when the lesion was performed at P7, but not at P15 or at P30. The analysis of the functional significance of this BDNF upregulation was done using trkB-IgG fusion proteins. After striatal injury, blockade of endogenous BDNF by trkB fusion proteins induced an atrophy of the dopaminergic neurons of the pars compacta. The injection of trkB-IgG fusion proteins did not modify the effects of kainate in the substantia nigra pars reticulata. Thus, our results show that BDNF exerts an autocrine/paracrine protective effect selectively on dopaminergic neurons against the loss of trophic support from the target striatum.

Neurotrophic factors in Huntington's disease

Huntington's disease is a neurodegenerative disorder characterized by the selective loss of striatal neurons and, to a lesser extent, cortical neurons. The neurodegenerative process is caused by the mutation of huntingtin gene. Recent studies have established a link between mutant huntingtin, excitotoxicity and neurotrophic factors. Neurotrophic factors prevent cell death in degenerative processes but they can also enhance growth and function of neurons that are affected in Huntington's disease. The endogenous regulation of the expression of neurotrophic factors and their receptors in the striatum and its connections can be important to protect striatal cells and maintains basal ganglia connectivity. The administration of exogenous neurotrophic factors, in animal models of Huntington's disease, has been used to characterize the trophic requirements of striatal and cortical neurons. Neurotrophins, glial cell line-derived neurotrophic factor family members and ciliary neurotrophic factor have shown a potent neuroprotective effects on different neuronal populations of the striatum. Furthermore, they are also useful to maintain the integrity of the corticostriatal pathway. Thus, these neurotrophic factors may be suitable for the development of a neuroprotective therapy for neurodegenerative disorders of the basal ganglia.