Haploinsufficiency for trkB and trkC receptors induces cell loss and accumulation of alpha-synuclein in the substantia nigra

TrkB but not trkC receptors are necessary for postnatal maintenance of hippocampal spines

Dendritic spines are major sites of excitatory synaptic transmission and changes in their densities have been linked to alterations in learning and memory. The neurotrophins brain-derived neurotrophic factor and neurotrophin-3 and their receptors, trkB and trkC, are thought to be involved in learning, memory and long-term potentiation (LTP). LTP is known to induce trkB and trkC gene expression as well as spinogenesis in the hippocampus. In the aging hippocampus, declines in trkB and trkC mRNA levels may underlie, at least in part, impairments in spatial memory and reductions in spine densities. To determine the significance of trkB and trkC for the maintenance of dendritic spines, we have analyzed Golgi-impregnated hippocampi of adult and aged mice heterozygous for trkB, trkC, or both along with respective wildtype littermates. Deletion of one allele of trkB, but not trkC, significantly reduces spine densities of CA1 pyramidal neurons in both adult and aged mice, as compared to age-matched controls. This indicates that trkB, but not trkC, receptors are necessary for the maintenance of hippocampal spines during postnatal life.

Genetic susceptibility to obsessive-compulsive hoarding: the contribution of neurotrophic tyrosine kinase receptor type 3 gene

Recent work suggests that neurotrophic factors may contribute to the genetic susceptibility to obsessive-compulsive disorder (OCD). Among other clinical dimensions, the presence of hoarding obsessions and compulsions has been shown to be correlated with a number of clinical and neuroimaging findings, as well as with a different pattern of genetic inheritance. We used a linkage disequilibrium (LD)-mapping approach to investigate whether neurotrophic tyrosine kinase receptor type 3 (NTRK3), the high-affinity receptor of neurotrophin 3 (NT-3), plays a role in increasing susceptibility to hoarding in OCD. We performed an association study of 52 tag single nucleotide polymorphisms (tagSNPs) covering the whole NTRK3 gene in a sample comprising 120 OCD patients and 342 controls. Single nucleotide polymorphism association and haplotype analysis were performed. Thirty-six of our patients (30%) exhibited significant hoarding obsessions and compulsions. A significant association of two SNPs in the 3' downstream region of NTRK3 gene and obsessive-compulsive hoarding was identified: rs1017412 [odds ratio (OR) = 2.16; P = 0.001] and rs7176429 (OR = 2.78; P = 0.0001), although only the latter remained significant after Bonferroni correction. Although the haplotype analysis did not show significant results, a more extended block of LD in the OCD hoarders with respect to the control group was observed, suggesting a lower haplotype diversity in these individuals. Our findings suggest that NTRK3 may contribute to the genetic susceptibility to hoarding in OCD and may constitute an interesting gene to focus on in studies of the genetic basis of obsessive-compulsive hoarding.

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.

Treatment of Parkinson's disease with trophic factors

Trophic factors are proteins that support and protect subpopulations of cells. A number have been reported to act on dopaminergic neurons in vitro and in vivo, making them potential therapeutic candidates for Parkinson's disease. All of these candidate factors protect dopaminergic neurons if given prior to, or with, selective neurotoxins. Fewer trophic factors, primarily glial-derived neurotrophic factor (GDNF) and its relative, neurturin (NRTN; also known as NTN), have been shown to restore function in damaged dopamine neurons after the acute effects of neurotoxins have subsided. A major barrier to clinical translation has been delivery. GDNF delivered by intracerebroventricular injection in patients was ineffective, probably because GDNF did not reach the target, the putamen, and intraputaminal infusion was ineffective, probably because of limited distribution within the putamen. A randomized clinical trial with gene therapy for NRTN is underway, in an attempt to overcome these problems with targeting and distribution. Other strategies are available to induce trophic effects in the CNS, but have not yet been the focus of human research. To date, clinical trials have focused on restoration of function (i.e., improvement of parkinsonism). Protection (i.e., slowing or halting disease progression and functional decline) might be a more robust effect of trophic agents. Laboratory research points to their effectiveness in protecting neurons and even restoring dopaminergic function after a monophasic neurotoxic insult. Utility for such compounds in patients with Parkinson's disease and ongoing loss of dopaminergic neurons remains to be proven.

Brain-derived neurotrophic factor expression in the substantia nigra does not change after lesions of dopaminergic neurons

Progressive and irreversible loss of specific neuronal cell populations is commonly seen in chronic neurodegenerative diseases such as Parkinson's disease (PD). Evidence is accumulating that apoptosis is a crucial cellular event responsible for the dysfunction and death of neurons in this disease. Thus, limiting apoptosis may prevent disease pathogenesis. Key to reducing apoptosis is the discovery of neuroprotective compounds that can be given to patients to minimize neuronal damage. In this manuscript, we reviewed the rationale of using an experimental strategy to provide neurotrophic support to injured neurons. Such rationale includes the increase of endogenous production of brain-derived neurotrophic factor (BDNF). BDNF is a potent inhibitor of apoptosis-mediated cell death and neurotoxin-induced degeneration of dopaminergic neurons. However, availability of BDNF may be reduced when dopaminergic neurons degenerate. Therefore, in this work, we have used several well-established neurotoxins for dopaminergic neurons, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, 6-OH-dopamine (6-OHDA), and the HIV protein gp120, to examine whether degeneration of nigrostriatal fibers alters BDNF expression. Our data show that these neurotoxins do not decrease the levels of BDNF in the substantia nigra, suggesting that up-regulation of BDNF synthesis by pharmacological means may be a viable therapy to slow down the progress of PD and other neurodegenerative diseases.

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.

Transcriptional dysregulation in a transgenic model of Parkinson disease

Alpha-synuclein has been implicated in Parkinson disease, yet the mechanism by which alpha-synuclein causes cell injury is not understood. Using a transgenic mouse model, we evaluated the effect of alpha-synuclein overexpression on gene expression in the substantia nigra. Nigral mRNA from wild type and alpha-synuclein transgenic mice was analyzed using Affymetrix gene arrays. At 3 months, before pathological changes are apparent, we observed modest alterations in gene expression. However, nearly 200 genes were altered in expression at 9 months, when degenerative changes are more apparent. Functional genomic analysis revealed that the genes altered at 9 months were predominantly involved in gene transcription. As in human Parkinson disease, gene expression changes in the transgenic model were also modulated by gender. These data demonstrate that alterations of gene expression are widespread in this animal model, and suggest that transcriptional dysregulation may be a disease mechanism that can be targeted therapeutically.

Absence of Ret signaling in mice causes progressive and late degeneration of the nigrostriatal system

Support of ageing neurons by endogenous neurotrophic factors such as glial cell line–derived neurotrophic factor (GDNF) and brain-derived neurotrophic factor (BDNF) may determine whether the neurons resist or succumb to neurodegeneration. GDNF has been tested in clinical trials for the treatment of Parkinson disease (PD), a common neurodegenerative disorder characterized by the loss of midbrain dopaminergic (DA) neurons. BDNF modulates nigrostriatal functions and rescues DA neurons in PD animal models. The physiological roles of GDNF and BDNF signaling in the adult nigrostriatal DA system are unknown. We generated mice with regionally selective ablations of the genes encoding the receptors for GDNF (Ret) and BDNF (TrkB). We find that Ret, but not TrkB, ablation causes progressive and adult-onset loss of DA neurons specifically in the substantia nigra pars compacta, degeneration of DA nerve terminals in striatum, and pronounced glial activation. These findings establish Ret as a critical regulator of long-term maintenance of the nigrostriatal DA system and suggest conditional Ret mutants as useful tools for gaining insights into the molecular mechanisms involved in the development of PD.

What does a neuron need to survive? Our body produces its own survival factors for neurons, so-called neurotrophic factors, which have additional roles in neuron differentiation, growth, and function. Declining production of a neurotrophic factor or impaired signal transduction in ageing neurons may contribute to pathological neurodegeneration in humans. Glial cell line–derived neurotrophic factor (GDNF) and brain-derived neurotrophic factor (BDNF) have been suggested as survival factors for midbrain dopaminergic neurons, a group of neurons primarily affected in Parkinson disease.

To investigate the physiological requirements for GDNF and BDNF to establish and maintain an important output pathway of these neurons—the nigrostriatal pathway—in the intact brain, we generated mutant mice with regionally selective ablations of the receptors for these survival factors, Ret (receptor of GDNF and related family members) or TrkB (BDNF receptor). Surprisingly, these mice survive to adulthood and show normal development and maturation of the nigrostriatal system. However, in ageing mice, ablation of Ret leads to a progressive and cell-type–specific loss of substantia nigra pars compacta neurons and their projections into the striatum. Our findings establish Ret and subsequent downstream effectors as critical regulators of long-term maintenance of the nigrostriatal system.

Ret, a receptor for glial cell line-derived neurotrophic factor, selectively regulates long-term maintenance of the nigrostriatal dopaminergic system.

How to make a mesodiencephalic dopaminergic neuron

Dopaminergic neurons located in the ventral mesodiencephalon are essential for the control of voluntary movement and the regulation of emotion, and are severely affected in neurodegenerative diseases such as Parkinson's disease. Recent advances in molecular biology and mouse genetics have helped to unravel the mechanisms involved in the development of mesodiencephalic dopaminergic (mdDA) neurons, including their specification, migration and differentiation, as well as the processes that govern axonal pathfinding and their specific patterns of connectivity and maintenance. Here, we follow the developmental path of these neurons with the goal of generating a molecular code that could be exploited in cell-replacement strategies to treat diseases such as Parkinson's disease.

RET is dispensable for maintenance of midbrain dopaminergic neurons in adult mice

Glial cell-line derived neurotrophic factor (GDNF)-mediated RET tyrosine kinase signaling is implicated in the survival of several PNS and CNS neuronal populations that are important in the pathogenesis of several disorders including Parkinson's disease and drug addiction. However, it has been difficult to study these processes and the physiological importance of this pathway in adult mice because of the neonatal lethality of Gdnf and Ret null mice. We report successful creation of RET conditional reporter mice to investigate postnatal physiologic roles of RET and monitor the fate of RET-expressing cell types. To delete RET specifically in dopaminergic neurons and determine the physiologic requirement of RET in the maintenance of substantia nigra compacta (SNC) and ventral tegmental area (VTA), we bred the RET conditional mice with mice that specifically express Cre from the dopamine transporter (Dat) locus. A detailed morphometric and biochemical analysis including dopaminergic neuron number and size in SNC and VTA, and fiber density in the striatum and nucleus accumbens, and dopamine levels indicate that RET is not required for providing global trophic support to midbrain dopaminergic neurons in adult mice. Furthermore, RET deficiency in these neurons does not cause major sensorimotor abnormalities. Hence our results support the idea that RET signaling is not critical for the normal physiology of the SNC and VTA in adult mice.

Neurotrophin receptor heterozygosity causes deficits in catecholaminergic innervation of amygdala and hippocampus in aged mice

We have recently shown that aged mice with haploinsufficiencies for the neurotrophin receptors trkB, trkC or both, trkB and trkC, display reduced cell numbers in the substantia nigra and in the dentate gyrus, but not in the amygdala. Moreover, both hippocampus and amygdala contain increased numbers of degenerated axonal fragments. Consistent with this observation and the expression of trkB and trkC by midbrain dopaminergic neurons, we show now that heterozygous deletion of the trkB or/and trkC receptor genes significantly reduces catecholaminergic, tyrosine hydroxylase (TH-) positive fiber densities in the hippocampus and amygdala mainly in aged (21-23 month old) mice. In the amygdala the phenotype was restricted to the lateral and basolateral nucleus of the amygdala. In adult (6 month old) mice, reductions in catecholaminergic fiber densities were only found in the hippocampal area CA3 and the dentate gyrus of heterozygous trkB and trkB/C mice. Our observations suggest that signaling through trkB and trkC neurotrophin receptors is important for the maintenance of the catecholaminergic innervation of two limbic key regions, the hippocampus and amygdala.

Shedding light into the role of BDNF in the pharmacotherapy of Parkinson's disease

Parkinson's disease (PD) is a chronic, neurodegenerative disease with a 1% incidence in the population over 55 years of age. Movement impairments represent undoubtedly the hallmark of the disorder; however, extensive evidence implicates cognitive deficits as concomitant peculiar features. Brain-derived neurotrophic factor (BDNF) colocalizes with dopamine neurons in the substantia nigra, where dopaminergic cell bodies are located, and it has recently garnered attention as a molecule crucial for cognition, a function that is also compromised in PD patients. Thus, due to its colocalization with dopaminergic neurons and its role in cognition, BDNF might possess a dual role in PD, both as a neuroprotective molecule, since its inhibition leads to loss of nigral dopaminergic neurons, and as a neuromodulator, as its enhanced expression ameliorates cognitive processes. In this review, we discuss the mechanism of action of established as well as novel drugs for PD with a particular emphasis to those interfering with BDNF biosynthesis.

Two distinct events, section compression and loss of particles (“lost caps”), contribute toz-axis distortion and bias in optical disector counting

Deformation of tissue sections in the z-axis can bias optical disector counting. When samples of particle densities are not representative for the entire tissue section, significant bias of estimated numbers can result. To assess the occurrence, prevalence, extent, sequence of events, and causes of z-axis distortion, the distribution of neuronal nucleoli in thick paraffin and vibratome sections was determined in chicken, rodent, and human brain tissues. When positions of neuronal nucleoli were measured in the z-axis, nucleoli were more frequent at the surfaces (bottom and top) of tissue sections than in the core. This nonlinear z-axis distribution was not lab-, equipment-, or investigator-specific, and was independent of age, fixation quality, coverslipping medium, or paraffin melting temperature, but in paraffin sections, was highly correlated with the tilt of the knife (cutting) angle. Manipulation of subsequent tissue processing steps revealed that two events contribute to z-axis distortion. Initially, a higher density of particles results at surfaces after sectioning, apparently due to section compression. Subsequently, particles can be lost to varying degrees from surfaces during floating or staining and dehydration, resulting in "lost caps." These results may explain different degrees of z-axis distortion between different types of sections and different labs, and reinforce the importance of checking z-axis distributions as a "quality control" prior to selection of guard zones in optical disector counting. Indirect approaches to assess section quality, such as resectioning in a perpendicular plane, yield additional artifacts, and should be replaced by a direct quantitative measurement of z-axis distribution of particles.