Decreased TrkA gene expression in cholinergic neurons of the striatum and basal forebrain of patients with Alzheimer's disease

Cholinergic neurodegeneration and cholesterol metabolism dysregulation by constitutive p75NTR signaling in the p75exonIII-KO mice

Degeneration of basal forebrain cholinergic neurons (BFCNs) is a hallmark of Alzheimer's disease (AD). However, few mouse models of AD recapitulate the neurodegeneration of the cholinergic system. The p75 neurotrophin receptor, p75NTR, has been associated with the degeneration of BFCNs in AD. The senescence-accelerated mouse prone number 8 (SAMP8) is a well-accepted model of accelerated and pathological aging. To gain a better understanding of the role of p75NTR in the basal forebrain during aging, we generated a new mouse line, the SAMP8-p75exonIII-/-. Deletion of p75NTR in the SAMP8 background induces an increase in the number of BFCNs at birth, followed by a rapid decline during aging compared to the C57/BL6 background. This decrease in the number of BFCNs correlates with a worsening in the Y-maze memory test at 6 months in the SAMP8-p75exonIII-/-. We found that SAMP8-p75exonIII-/- and C57/BL6-p75exonIII-/- mice expressed constitutively a short isoform of p75NTR that correlates with an upregulation of the protein levels of SREBP2 and its targets, HMGCR and LDLR, in the BF of both SAMP8-p75exonIII-/- and C57/BL6-p75exonIII-/- mice. As the neurodegeneration of the cholinergic system and the dysregulation of cholesterol metabolism are implicated in AD, we postulate that the generated SAMP8-p75exonIII-/- mouse strain might constitute a good model to study long-term cholinergic neurodegeneration in the CNS. In addition, our results support the role of p75NTR signaling in cholesterol biosynthesis regulation.

Chronic basal forebrain activation improves spatial memory, boosts neurotrophin receptor expression, and lowers BACE1 and Aβ42 levels in the cerebral cortex in mice

The etiology of Alzheimer's dementia has been hypothesized in terms of basal forebrain cholinergic decline, and in terms of reflecting beta-amyloid neuropathology. To study these different biological elements, we activated the basal forebrain in 5xFAD Alzheimer's model mice and littermates. Mice received 5 months of 1 h per day intermittent stimulation of the basal forebrain, which includes cholinergic projections to the cortical mantle. Then, mice were behaviorally tested followed by tissue analysis. The 5xFAD mice performed worse in water-maze testing than littermates. Stimulated groups learned the water maze better than unstimulated groups. Stimulated groups had 2-3-fold increases in frontal cortex immunoblot measures of the neurotrophin receptors for nerve growth factor and brain-derived neurotrophic factor, and a more than 50% decrease in the expression of amyloid cleavage enzyme BACE1. Stimulation also led to lower Aβ42 in 5xFAD mice. These data support a causal relationship between basal forebrain activation and both neurotrophin activation and reduced Aβ42 generation and accumulation. The observation that basal forebrain activation suppresses Aβ42 accumulation, combined with the known high-affinity antagonism of nicotinic receptors by Aβ42, documents bidirectional antagonism between acetylcholine and Aβ42.

The Nerve Growth Factor Receptor (NGFR/p75NTR): A Major Player in Alzheimer's Disease

Alzheimer's disease (AD) represents the most prevalent type of dementia in elderly people, primarily characterized by brain accumulation of beta-amyloid (Aβ) peptides, derived from Amyloid Precursor Protein (APP), in the extracellular space (amyloid plaques) and intracellular deposits of the hyperphosphorylated form of the protein tau (p-tau; tangles or neurofibrillary aggregates). The Nerve growth factor receptor (NGFR/p75^NTR) represents a low-affinity receptor for all known mammalians neurotrophins (i.e., proNGF, NGF, BDNF, NT-3 e NT-4/5) and it is involved in pathways that determine both survival and death of neurons. Interestingly, also Aβ peptides can blind to NGFR/p75^NTR making it the "ideal" candidate in mediating Aβ-induced neuropathology. In addition to pathogenesis and neuropathology, several data indicated that NGFR/p75^NTR could play a key role in AD also from a genetic perspective. Other studies suggested that NGFR/p75^NTR could represent a good diagnostic tool, as well as a promising therapeutic target for AD. Here, we comprehensively summarize and review the current experimental evidence on this topic.

β-Hydroxybutyrate Ameliorates Aβ-Induced Downregulation of TrkA Expression by Inhibiting HDAC1/3 in SH-SY5Y Cells

Tyrosine kinase receptor A (TrkA) plays an important role in the protection of cholinergic neurons in Alzheimer's disease (AD). This study was designed to investigate whether β-hydroxybutyrate (BHB), an endogenous histone deacetylase (HDAC) inhibitor, upregulates the expression of TrkA by affecting histone acetylation in SH-SY5Y cells treated with amyloid β-protein (Aβ). The results showed that BHB ameliorated the reduction of cell vitality and downregulation of TrkA expression induced by Aβ. Furthermore, BHB inhibited the upregulation of HDAC1/2/3 expression and downregulation of histone acetylation (Ace-H3K9 and Ace-H4K12) levels in Aβ-treated cells. The expression of TrkA was upregulated in HDAC1- or 3-silenced SH-SY5Y cells. However, there was no significant difference in TrkA expression between the HDAC2 knockdown and control cells. In conclusion, this study demonstrates that BHB protects against Aβ-induced neurotoxicity in SH-SY5Y cells. The underlying mechanism of the effect may be associated with the upregulation of TrkA expression by inhibiting HDAC1/3.

Exploring the Pathogenesis of Alzheimer Disease in Basal Forebrain Cholinergic Neurons: Converging Insights From Alternative Hypotheses

Alzheimer disease (AD) represents an oncoming epidemic that without an effective treatment promises to exact extraordinary financial and emotional burdens (Apostolova, 2016). Studies of pathogenesis are essential for defining critical molecular and cellular events and for discovering therapies to prevent or mitigate their effects. Through studies of neuropathology, genetic and cellular, and molecular biology recent decades have provided many important insights. Several hypotheses have been suggested. Documentation in the 1980s of selective loss of cholinergic neurons of the basal forebrain, followed by clinical improvement in those treated with inhibitors of acetylycholinesterase, supported the "cholinergic hypothesis of age-related cognitive dysfunction" (Bartus et al., 1982). A second hypothesis, prompted by the selective loss of cholinergic neurons and the discovery of central nervous system (CNS) neurotrophic factors, including nerve growth factor (NGF), prompted the "deficient neurotrophic hypothesis" (Chen et al., 2018). The most persuasive hypothesis, the amyloid cascade hypothesis first proposed more than 25 years ago (Selkoe and Hardy, 2016), is supported by a wealth of observations. Genetic studies were exceptionally important, pointing to increased dose of the gene for the amyloid precursor protein (APP) in Down syndrome (DS) and a familial AD (FAD) due to duplication of APP and to mutations in APP and in the genes for Presenilin 1 and 2 (PSEN1, 2), which encode the γ-secretase enzyme that processes APP (Dorszewska et al., 2016). The "tau hypothesis" noted the prominence of tau-related pathology and its correlation with dementia (Kametani and Hasegawa, 2018). Recent interest in induction of microglial activation in the AD brain, as well as other manifestations of inflammation, supports the "inflammatory hypothesis" (Mcgeer et al., 2016). We place these findings in the context of the selective, but by no means unique, involvement of BFCNs and their trophic dependence on NGF signaling and speculate as to how pathogenesis in these neurons is initiated, amplified and ultimately results in their dysfunction and death. In so doing we attempt to show how the different hypotheses for AD may interact and reinforce one another. Finally, we address current attempts to prevent and/or treat AD in light of advances in understanding pathogenetic mechanisms and suggest that studies in the DS population may provide unique insights into AD pathogenesis and treatment.

NGF-dependent neurons and neurobiology of emotions and feelings: Lessons from congenital insensitivity to pain with anhidrosis

NGF is a well-studied neurotrophic factor, and TrkA is a receptor tyrosine kinase for NGF. The NGF-TrkA system supports the survival and maintenance of NGF-dependent neurons during development. Congenital insensitivity to pain with anhidrosis (CIPA) is an autosomal recessive genetic disorder due to loss-of-function mutations in the NTRK1 gene encoding TrkA. Individuals with CIPA lack NGF-dependent neurons, including NGF-dependent primary afferents and sympathetic postganglionic neurons, in otherwise intact systems. Thus, the pathophysiology of CIPA can provide intriguing findings to elucidate the unique functions that NGF-dependent neurons serve in humans, which might be difficult to evaluate in animal studies. Preceding studies have shown that the NGF-TrkA system plays critical roles in pain, itching and inflammation. This review focuses on the clinical and neurobiological aspects of CIPA and explains that NGF-dependent neurons in the peripheral nervous system play pivotal roles in interoception and homeostasis of our body, as well as in the stress response. Furthermore, these NGF-dependent neurons are likely requisite for neurobiological processes of 'emotions and feelings' in our species.

Cholinergic neuron gene expression differences captured by translational profiling in a mouse model of Alzheimer's disease

Cholinergic neurotransmission is impaired in Alzheimer's disease (AD), and loss of basal forebrain cholinergic neurons is a key component of disease pathogenicity and symptomatology. To explore the molecular basis of this cholinergic dysfunction, we paired translating ribosome affinity purification (TRAP) with RNA sequencing (TRAP-Seq) to identify the actively translating mRNAs in anterior forebrain cholinergic neurons in the TgCRND8 mouse model of AD. Bioinformatic analyses revealed the downregulation of 67 of 71 known cholinergic-related transcripts, consistent with cholinergic neuron dysfunction in TgCRND8 mice, as well as transcripts related to oxidative phosphorylation, neurotrophins, and ribosomal processing. Upregulated transcripts included those related to axon guidance, glutamatergic synapses and kinase activity and included AD-risk genes Sorl1 and Ptk2b. In contrast, the total transcriptome of the anterior forebrain showed upregulation in cytokine signaling, microglia, and immune system pathways, including Trem2, Tyrobp, and Inpp5d. Hence, TRAP-Seq clearly distinguished the differential gene expression alterations occurring in cholinergic neurons of TgCRND8 mice compared with wild-type littermates, providing novel candidate pathways to explore for therapeutic development in AD.

Development of Precision Small-Molecule Proneurotrophic Therapies for Neurodegenerative Diseases

Age-related neurodegenerative diseases, such as Alzheimer's disease, will represent one of the largest future burdens on worldwide healthcare systems due to the increasing proportion of elderly in our society. As deficiencies in neurotrophins are implicated in the pathogenesis of many age-related neurodegenerative disorders, it is reasonable to consider that global neurotrophin resistance may also become a major healthcare threat. Central nervous system networks are effectively maintained through aging by neuroprotective and neuroplasticity signaling mechanisms which are predominantly controlled by neurotrophin receptor signaling. Neurotrophin receptors are single pass receptor tyrosine kinases that form dimeric structures upon ligand binding to initiate cellular signaling events that control many protective and plasticity-related pathways. Declining functionality of the neurotrophin ligand-receptor system is considered one of the hallmarks of neuropathological aging. Therefore, it is imperative to develop effective therapeutic strategies to contend with this significant issue. While the therapeutic applications of cognate ligands for neurotrophin receptors are limited, the development of nonpeptidergic, small-molecule ligands can overcome these limitations, and productively regulate this important receptor system with beneficial effects. Using our advanced knowledge of the high-dimensionality complexity of receptor systems, the future generation of precision medicines targeting these systems will be an attainable goal.

Atrophy and structural covariance of the cholinergic basal forebrain in primary progressive aphasia

Primary progressive aphasia (PPA) is characterized by profound destruction of cortical language areas. Anatomical studies suggest an involvement of cholinergic basal forebrain (BF) in PPA syndromes, particularly in the area of the nucleus subputaminalis (NSP). Here we aimed to determine the pattern of atrophy and structural covariance as a proxy of structural connectivity of BF nuclei in PPA variants. We studied 62 prospectively recruited cases with the clinical diagnosis of PPA and 31 healthy older control participants from the cohort study of the German consortium for frontotemporal lobar degeneration (FTLD). We determined cortical and BF atrophy based on high-resolution magnetic resonance imaging (MRI) scans. Patterns of structural covariance of BF with cortical regions were determined using voxel-based partial least square analysis. We found significant atrophy of total BF and BF subregions in PPA patients compared with controls [F(1, 82) = 20.2, p < .001]. Atrophy was most pronounced in the NSP and the posterior BF, and most severe in the semantic variant and the nonfluent variant of PPA. Structural covariance analysis in healthy controls revealed associations of the BF nuclei, particularly the NSP, with left hemispheric predominant prefrontal, lateral temporal, and parietal cortical areas, including Broca's speech area (p < .001, permutation test). In contrast, the PPA patients showed preserved structural covariance of the BF nuclei mostly with right but not with left hemispheric cortical areas (p < .001, permutation test). Our findings agree with the neuroanatomically proposed involvement of the cholinergic BF, particularly the NSP, in PPA syndromes. We found a shift from a structural covariance of the BF with left hemispheric cortical areas in healthy aging towards right hemispheric cortical areas in PPA, possibly reflecting a consequence of the profound and early destruction of cortical language areas in PPA.

Bex3 Dimerization Regulates NGF-Dependent Neuronal Survival and Differentiation by Enhancing trkA Gene Transcription

The development of the nervous system is a temporally and spatially coordinated process that relies on the proper regulation of the genes involved. Neurotrophins and their receptors are directly responsible for the survival and differentiation of sensory and sympathetic neurons; however, it is not fully understood how genes encoding Trk neurotrophin receptors are regulated. Here, we show that rat Bex3 protein specifically regulates TrkA expression by acting at the trkA gene promoter level. Bex3 dimerization and shuttling to the nucleus regulate the transcription of the trkA promoter under basal conditions and also enhance nerve growth factor (NGF)-mediated trkA promoter activation. Moreover, qChIP assays indicate that Bex3 associates with the trkA promoter within a 150 bp sequence, immediately upstream from the transcription start site, which is sufficient to mediate the effects of Bex3. Consequently, the downregulation of Bex3 using shRNA increases neuronal apoptosis in NGF-dependent sensory neurons deprived of NGF and compromises PC12 cell differentiation in response to NGF. Our results support an important role for Bex3 in the regulation of TrkA expression and in NGF-mediated functions through modulation of the trkA promoter.

Small-molecule modulation of neurotrophin receptors: a strategy for the treatment of neurological disease

Neurotrophins and their receptors modulate multiple signalling pathways to regulate neuronal survival and to maintain axonal and dendritic networks and synaptic plasticity. Neurotrophins have potential for the treatment of neurological diseases. However, their therapeutic application has been limited owing to their poor plasma stability, restricted nervous system penetration and, importantly, the pleiotropic actions that derive from their concomitant binding to multiple receptors. One strategy to overcome these limitations is to target individual neurotrophin receptors — such as tropomyosin receptor kinase A (TRKA), TRKB, TRKC, the p75 neurotrophin receptor or sortilin — with small-molecule ligands. Such small molecules might also modulate various aspects of these signalling pathways in ways that are distinct from the programmes triggered by native neurotrophins. By departing from conventional neurotrophin signalling, these ligands might provide novel therapeutic options for a broad range of neurological indications.

Reactivation of atrophic neurons in Alzheimer's disease

Decreased metabolic rate may precede cognitive impairment in Alzheimer's disease (AD) and is thus an early occurring hallmark. Several observations in post-mortem brain indicate that activated neurons are better able to withstand aging and AD, a phenomenon paraphrased by us as 'use it or lose it'. Moreover, a number of pharmacological and nonpharmacological studies support the concept that activation of the brain has beneficial effects and may to a certain degree restore several aspects of cognition and other central functions. For instance, the circadian system may be restimulated in Alzheimer patients by exposing them to more light or transcutaneous nerve stimulation. A procedure has been developed to culture human post-mortem brain tissue that allows testing of the efficacy of putative stimulatory compounds such as neurotrophins.

TrkA gene ablation in basal forebrain results in dysfunction of the cholinergic circuitry

Dysfunction of basal forebrain cholinergic neurons (BFCNs) is an early pathological hallmark of Alzheimer's disease (AD). Numerous studies have indicated that nerve growth factor (NGF) supports survival and phenotypic differentiation of BFCNs. Consistent with a potential link to AD pathogenesis, TrkA, a NGF receptor, is expressed in cholinergic forebrain neuronal populations including those in BF and striatum, and is markedly reduced in individuals with mild cognitive impairment (MCI) without dementia and early-stage AD. To investigate the role of TrkA in the development, connectivity, and function of the BF cholinergic system and its contribution to AD pathology, we have generated a forebrain-specific conditional TrkA knock-out mouse line. Our findings show a key role for TrkA signaling in establishing the BF cholinergic circuitry through the ERK pathway, and demonstrate that the normal developmental increase of choline acetyltransferase expression becomes critically dependent on TrkA signaling before neuronal connections are established. Moreover, the anatomical and physiological deficits caused by lack of TrkA signaling in BFCNs have selective impact on cognitive activity. These data demonstrate that TrkA loss results in cholinergic BF dysfunction and cognitive decline that is reminiscent of MCI and early AD.

Upregulation of select rab GTPases in cholinergic basal forebrain neurons in mild cognitive impairment and Alzheimer's disease

Endocytic system dysfunction is one of the earliest disturbances that occur in Alzheimer's disease (AD), and may underlie the selective vulnerability of cholinergic basal forebrain (CBF) neurons during the progression of dementia. Herein we report that genes regulating early and late endosomes are selectively upregulated within CBF neurons in mild cognitive impairment (MCI) and AD. Specifically, upregulation of rab4, rab5, rab7, and rab27 was observed in CBF neurons microdissected from postmortem brains of individuals with MCI and AD compared to age-matched control subjects with no cognitive impairment (NCI). Upregulated expression of rab4, rab5, rab7, and rab27 correlated with antemortem measures of cognitive decline in individuals with MCI and AD. qPCR validated upregulation of these select rab GTPases within microdissected samples of the basal forebrain. Moreover, quantitative immunoblot analysis demonstrated upregulation of rab5 protein expression in the basal forebrain of subjects with MCI and AD. The elevation of rab4, rab5, and rab7 expression is consistent with our recent observations in CA1 pyramidal neurons in MCI and AD. These findings provide further support that endosomal pathology accelerates endocytosis and endosome recycling, which may promote aberrant endosomal signaling and neurodegeneration throughout the progression of AD.

Transcriptome profiling in neurodegenerative disease

Changes in gene expression and splicing patterns (that occur prior to the onset and during the progression of complex diseases) have become a major focus of neurodegenerative disease research. These signature patterns of gene expression provide clues about the mechanisms involved in the molecular pathogenesis of neurodegenerative disease and may facilitate the discovery of novel therapeutic drugs. With the development of array technologies and the very recent RNA-seq technique, our understanding of the pathogenesis of neurodegenerative disease is expanding exponentially. Here, we review the technologies involved in gene expression and splicing analysis and the related literature on three common neurodegenerative diseases: Alzheimer's disease, Parkinson's disease and Huntington's disease.

Neurotrophins regulate bone marrow stromal cell IL-6 expression through the MAPK pathway

BACKGROUND

The host's response to infection is characterized by altered levels of neurotrophins and an influx of inflammatory cells to sites of injured tissue. Progenitor cells that give rise to the differentiated cellular mediators of inflammation are derived from bone marrow progenitor cells where their development is regulated, in part, by cues from bone marrow stromal cells (BMSC). As such, alteration of BMSC function in response to elevated systemic mediators has the potential to alter their function in biologically relevant ways, including downstream alteration of cytokine production that influences hematopoietic development.

METHODOLOGY/PRINCIPAL FINDINGS

In the current study we investigated BMSC neurotrophin receptor expression by flow cytometric analysis to determine differences in expression as well as potential to respond to NGF or BDNF. Intracellular signaling subsequent to neurotrophin stimulation of BMSC was analyzed by western blot, microarray analysis, confocal microscopy and real-time PCR. Analysis of BMSC Interleukin-6 (IL-6) expression was completed using ELISA and real-time PCR.

CONCLUSION

BMSC established from different individuals had distinct expression profiles of the neurotrophin receptors, TrkA, TrkB, TrkC, and p75(NTR). These receptors were functional, demonstrated by an increase in Akt-phosphorylation following BMSC exposure to recombinant NGF or BDNF. Neurotrophin stimulation of BMSC resulted in increased IL-6 gene and protein expression which required activation of ERK and p38 MAPK signaling, but was not mediated by the NFkappaB pathway. BMSC response to neurotrophins, including the up-regulation of IL-6, may alter their support of hematopoiesis and regulate the availability of inflammatory cells for migration to sites of injury or infection. As such, these studies are relevant to the growing appreciation of the interplay between neurotropic mediators and the regulation of hematopoiesis.

Mixed lineage kinase phosphorylates transcription factor E47 and inhibits TrkB expression to link neuronal death and survival pathways

E47 is a basic helix-loop-helix transcription factor involved in neuronal differentiation and survival. We had previously shown that the basic helix-loop-helix protein E47 binds to E-box sequences within the promoter of the TrkB gene and activates its transcription. Proper expression of the TrkB receptor plays a key role in development and function of the vertebrate nervous system, and altered levels of TrkB have been associated with important human diseases. Here we show that E47 interacts with MLK2, a mixed lineage kinase (MLK) involved in JNK-mediated activation of programmed cell death. MLK2 enhances phosphorylation of the AD2 activation domain of E47 in vivo in a JNK-independent manner and phosphorylates in vitro defined serine and threonine residues within a loop-helix structure of AD2 that also contains a putative MLK docking site. Although these residues are essential for MLK2-mediated inactivation of E47, inhibition of MLKs by CEP11004 causes up-regulation of TrkB at a transcriptional level in cerebellar granule neurons and differentiating neuroblastoma cells. These findings allow us to propose a novel mechanism by which MLK regulates TrkB expression through phosphorylation of an activation domain of E47. This molecular link would explain why MLK inhibitors not only prevent activation of cell death processes but also enhance cell survival signaling as a key aspect of their neuroprotective potential.

Target identification for CNS diseases by transcriptional profiling

Gene expression changes in neuropsychiatric and neurodegenerative disorders, and gene responses to therapeutic drugs, provide new ways to identify central nervous system (CNS) targets for drug discovery. This review summarizes gene and pathway targets replicated in expression profiling of human postmortem brain, animal models, and cell culture studies. Analysis of isolated human neurons implicates targets for Alzheimer's disease and the cognitive decline associated with normal aging and mild cognitive impairment. In addition to tau, amyloid-beta precursor protein, and amyloid-beta peptides (Abeta), these targets include all three high-affinity neurotrophin receptors and the fibroblast growth factor (FGF) system, synapse markers, glutamate receptors (GluRs) and transporters, and dopamine (DA) receptors, particularly the D2 subtype. Gene-based candidates for Parkinson's disease (PD) include the ubiquitin-proteosome system, scavengers of reactive oxygen species, brain-derived neurotrophic factor (BDNF), its receptor, TrkB, and downstream target early growth response 1, Nurr-1, and signaling through protein kinase C and RAS pathways. Increasing variability and decreases in brain mRNA production from middle age to old age suggest that cognitive impairments during normal aging may be addressed by drugs that restore antioxidant, DNA repair, and synaptic functions including those of DA to levels of younger adults. Studies in schizophrenia identify robust decreases in genes for GABA function, including glutamic acid decarboxylase, HINT1, glutamate transport and GluRs, BDNF and TrkB, numerous 14-3-3 protein family members, and decreases in genes for CNS synaptic and metabolic functions, particularly glycolysis and ATP generation. Many of these metabolic genes are increased by insulin and muscarinic agonism, both of which are therapeutic in psychosis. Differential genomic signals are relatively sparse in bipolar disorder, but include deficiencies in the expression of 14-3-3 protein members, implicating these chaperone proteins and the neurotransmitter pathways they support as possible drug targets. Brains from persons with major depressive disorder reveal decreased expression for genes in glutamate transport and metabolism, neurotrophic signaling (eg, FGF, BDNF and VGF), and MAP kinase pathways. Increases in these pathways in the brains of animals exposed to electroconvulsive shock and antidepressant treatments identify neurotrophic and angiogenic growth factors and second messenger stimulation as therapeutic approaches for the treatment of depression.

ProNGF inhibits NGF-mediated TrkA activation in PC12 cells

Degeneration of cholinergic basal forebrain neurons (CBFN) is a hallmark in the pathology of Alzheimer's disease (AD). Critically depending upon the neurotrophic support through nerve growth factor (NGF), CBFN in the AD brain face elevated concentrations of the pro-form of NGF (proNGF) and suffer from an imbalance between TrkA and p75(NTR) expression. Research for the underlying mechanisms of CBFN death suggested a pro-apoptotic activity of proNGF. However, this finding could not be confirmed by all investigators and other studies even observed a neurotrophic function of proNGF. In the presence of these controversial findings we investigated the activity of proNGF in PC12 cells with specific emphasis on its neurotoxic versus neurotrophic action. In this study, we show that proNGF can mediate TrkA receptor signaling directly, yet in the manner of a partial agonist with a lower maximum activity than NGF. A pro-apoptotic activity of proNGF could not be confirmed in our cellular system. Interestingly and surprisingly, pre-incubation with proNGF at low, sub-active concentrations inhibited TrkA-mediated neurotrophic NGF signaling in PC12 cells. Our data support a novel hypothesis for the role of elevated proNGF levels in CBFN pathology in AD. Thus, proNGF can indirectly contribute to the slow neurodegeneration in AD by reducing NGF-mediated trophic support.

Down regulation of trk but not p75NTR gene expression in single cholinergic basal forebrain neurons mark the progression of Alzheimer's disease

Dysfunction of cholinergic basal forebrain (CBF) neurons of the nucleus basalis (NB) is a cardinal feature of Alzheimer's disease (AD) and correlates with cognitive decline. Survival of CBF neurons depends upon binding of nerve growth factor (NGF) with high-affinity (trkA) and low-affinity (p75(NTR)) neurotrophin receptors produced within CBF neurons. Since trkA and p75(NTR) protein levels are reduced within CBF neurons of people with mild cognitive impairment (MCI) and mild AD, trkA and/or p75(NTR) gene expression deficits may drive NB degeneration. Using single cell expression profiling methods coupled with custom-designed cDNA arrays and validation with real-time quantitative PCR (qPCR) and in situ hybridization, individual cholinergic NB neurons displayed a significant down regulation of trkA, trkB, and trkC expression during the progression of AD. An intermediate reduction was observed in MCI, with the greatest decrement in mild to moderate AD as compared to controls. Importantly, trk down regulation is associated with cognitive decline measured by the Global Cognitive Score (GCS) and the Mini-Mental State Examination (MMSE). In contrast, there is a lack of regulation of p75(NTR) expression. Thus, trk defects may be a molecular marker for the transition from no cognitive impairment (NCI) to MCI, and from MCI to frank AD.

Production of nerve growth factor by β-amyloid-stimulated astrocytes induces p75NTR-dependent tau hyperphosphorylation in cultured hippocampal neurons

Reactive astrocytes surround amyloid depositions and degenerating neurons in Alzheimer's disease (AD). It has been previously shown that beta-amyloid peptide induces inflammatory-like responses in astrocytes, leading to neuronal pathology. Reactive astrocytes up-regulate nerve growth factor (NGF), which can modulate neuronal survival by signaling through TrkA or p75 neurotrophin receptor (p75NTR). Here, we analyzed whether soluble Abeta peptide 25-35 (Abeta) stimulated astrocytic NGF expression, modulating the survival of cultured embryonic hippocampal neurons. Hippocampal astrocytes incubated with Abeta up-regulated NGF expression and release to the culture medium. Abeta-stimulated astrocytes increased tau phosphorylation and reduced the survival of cocultured hippocampal neurons. Neuronal death and tau phosphorylation were reproduced by conditioned media from Abeta-stimulated astrocytes and prevented by caspase inhibitors or blocking antibodies to NGF or p75NTR. Moreover, exogenous NGF was sufficient to induce tau hyperphosphorylation and death of hippocampal neurons, a phenomenon that was potentiated by a low steady-state concentration of nitric oxide. Our findings show that Abeta-activated astrocytes potently stimulate NGF secretion, which in turn causes the death of p75-expressing hippocampal neurons, through a mechanism regulated by nitric oxide. These results suggest a potential role for astrocyte-derived NGF in the progression of AD.

Therapeutic potential of neurotrophic factors in neurodegenerative diseases

There is a vast amount of evidence indicating that neurotrophic factors play a major role in the development, maintenance, and survival of neurons and neuron-supporting cells such as glia and oligodendrocytes. In addition, it is well known that alterations in levels of neurotrophic factors or their receptors can lead to neuronal death and contribute to the pathogenesis of neurodegenerative diseases such as Parkinson disease, Alzheimer disease, Huntington disease, amyotrophic lateral sclerosis, and also aging. Although various treatments alleviate the symptoms of neurodegenerative diseases, none of them prevent or halt the neurodegenerative process. The high potency of neurotrophic factors, as shown by many experimental studies, makes them a rational candidate co-therapeutic agent in neurodegenerative disease. However, in practice, their clinical use is limited because of difficulties in protein delivery and pharmacokinetics in the central nervous system. To overcome these disadvantages and to facilitate the development of drugs with improved pharmacotherapeutic profiles, research is underway on neurotrophic factors and their receptors, and the molecular mechanisms by which they work, together with the development of new technologies for their delivery into the brain.

Pontine cholinergic neurons depend on three neuroprotection systems to resist nitrosative stress

Brainstem cholinergic populations survive in neurodegenerative disease, while basal forebrain cholinergic neurons degenerate. We have postulated that variable resistance to oxidative stress may in part explain this. Rat primary cultures were used to study the effects of several nitrosative/oxidative stressors on brainstem (upper pons, containing pedunculopontine and lateraldorsal tegmental nuclei; BS) cholinergic neurons, comparing them with medial septal (MS), and striatal cholinergic neurons. BS cholinergic neurons were significantly more resistant to S-nitro-N-acetyl-d,l-penicillamine (SNAP), sodium nitroprusside (SNP), and hydrogen peroxide than were MS cholinergic neurons, which in turn were more resistant than striatal cholinergic neurons. Pharmacological analyses using specific inhibitors of neuroprotective systems also revealed differences between these three cholinergic populations with respect to their vulnerability to SNAP. Toxicity of SNAP to BS neurons was exacerbated by blocking NF-kappaB activation with SN50 or ERK1/2 activation by PD98059, or by inhibition of phosphoinositide-3 kinase (PI3K) activity by LY294002. In contrast, SNAP toxicity to MS neurons was augmented only by SN50, and SNAP toxicity to striatal cholinergic neurons was not increased by any of these three pharmacological agents. In neuron-enriched primary cultures, BS cholinergic neurons remained resistant to SNAP while MS cholinergic neurons remained vulnerable to this agent. Immunohistochemical experiments demonstrated nitric oxide (NO)-induced increases in nuclear levels of phospho-epitopes for ERK1/2 and Akt, and of the p65 subunit of NF-kappaB, within BS cholinergic neurons. These data indicate that the relative resistance of BS cholinergic neurons to toxic levels of nitric oxide involves three intrinsic neuroprotective pathways that control transcriptional and anti-apoptotic cellular functions.

Human cholinergic basal forebrain: chemoanatomy and neurologic dysfunction

The human cholinergic basal forebrain (CBF) is comprised of magnocellular hyperchromic neurons within the septal/diagonal band complex and nucleus basalis (NB) of Meynert. CBF neurons provide the major cholinergic innervation to the hippocampus, amygdala and neocortex. They play a role in cognition and attentional behaviors, and are dysfunctional in Alzheimer's disease (AD). The human CBF displays a continuum of large cells that contain various cholinergic markers, nerve growth factor (NGF) and its cognate receptors, calbindin, glutamate receptors, and the estrogen receptors, ERalpha and ERbeta. Admixed with these cholinergic neuronal phenotypes are smaller interneurons containing the m2 muscarinic acetylcholine receptor (mAChRs), NADPH-diaphorase, GABA, calcium binding proteins and several inhibitory neuropeptides including galanin (GAL), which is over expressed in AD. Studies using human autopsy material indicate an age-related dissociation of calbindin and the glutamate receptor GluR2 within CBF neurons, suggesting that these molecules act synergistically to induce excitotoxic cell death during aging, and possibly during AD. Choline acetyltrasnferease (ChAT) activity and CBF neuron number is preserved in the cholinergic basocortical system and up regulated in the septohippocampal system during prodromal as compared with end stage AD. In contrast, the number of CBF neurons containing NGF receptors is reduced early in the disease process suggesting a phenotypic silence and not a frank loss of neurons. In end stage AD, there is a selective reduction in trkA mRNA but not p75(NTR) in single CBF cells suggesting a neurotrophic defect throughout the progression of AD. These observations indicate the complexity of the chemoanatomy of the human CBF and suggest that multiple factors play different roles in its dysfunction in aging and AD.

Therapeutic strategies for Alzheimer disease: focus on neuronal reactivation of metabolically impaired neurons

Based on several lines of evidence, it has been hypothesized that decreased neuronal metabolic rate may precede cognitive impairment, contributing to neuronal atrophy as well as reduced neuronal function in Alzheimer disease (AD). Additionally, studies have shown that stimulation of neurons through different mechanisms may protect those cells from the deleterious effects of aging and AD, a phenomenon we paraphrased as "use it or lose it." Therefore, it is attractive to direct the development of therapeutic strategies toward stimulation of metabolic rate/neuronal activity to improve cognition and other symptoms in AD. A number of pharmacological and nonpharmacological approaches discussed here support the concept that stimulation of the brain has beneficial effects and may, to a certain degree, restore several aspects of cognition and other central functions. For instance, the circadian system, which controls the sleep/wake cycle, may be stimulated in AD patients by exposing them to more light or transcutaneous nerve stimulation. We will also discuss a procedure that has been developed to culture human postmortem brain tissue, which allows testing of the efficacy of putative stimulatory compounds.

Age-related sex differences in spatial learning and basal forebrain cholinergic neurons in F344 rats

Basal forebrain cholinergic neurons are important for spatial learning in rodents. Spatial learning ability is reportedly better in males than females, and declines with age. To examine the role of cholinergic function in sex- or age-related differences in spatial learning, we compared the size of basal forebrain cholinergic neurons (BFCN) of young and aged male and female Fischer 344 (F344) rats that had been trained in the Morris water maze. Young male and female rats were equally proficient in finding the platform during training trials, but probe tests revealed that young male rats had better knowledge of the platform's precise location. Impairments in spatial learning were observed in aged rats, and the advantage of males over females was lost. BFCN were significantly larger in young male than young female rats, and were correlated with spatial memory performance for both groups. BFCN were smaller in aged than young males; no change was seen between young and aged females. In the groups of aged rats the correlation between neuron size and spatial memory was lost. The present findings provide further evidence of a role for the basal forebrain cholinergic system in spatial learning, but reveal a complex interaction between sex, age and behavioral performance.

Neurotrophins and neurodegeneration

There is growing evidence that reduced neurotrophic support is a significant factor in the pathogenesis of neurodegenerative diseases such as Alzheimer's disease (AD), Parkinson's disease (PD) and amyotrophic lateral sclerosis (ALS). In this review we discuss the structure and functions of neurotrophins such as nerve growth factor, and the role of these proteins and their tyrosine kinase (Trk) receptors in the aetiology and therapy of such diseases. Neurotrophins regulate development and the maintenance of the vertebrate nervous system. In the mature nervous system they affect neuronal survival and also influence synaptic function and plasticity. The neurotrophins are able to bind to two different receptors: all bind to a common receptor p75NTR, and each also binds to one of a family of Trk receptors. By dimerization of the Trk receptors, and subsequent transphosphorylation of the intracellular kinase domain, signalling pathways are activated. We discuss here the structure and function of the neurotrophins and how they have been, or may be, used therapeutically in AD, PD, Huntington's diseases, ALS and peripheral neuropathy. Neurotrophins are central to many aspects of nervous system function. However they have not truly fulfilled their therapeutic potential in clinical trials because of the difficulties of protein delivery and pharmacokinetics in the nervous system. With the recent elucidation of the structure of the neurotrophins bound to their receptors it will now be possible, using a combination of in silico technology and novel screening techniques, to develop small molecule mimetics with much improved pharmacotherapeutic profiles.

Brain aging and Alzheimer's disease; use it or lose it

(1) Alzheimer's disease is a multifactorial disease in which age and APOE-epsilon 4 are important risk factors. (2) The neuropathological hallmarks of AD, i.e. amorphous plaques, neuritic plaques (NPs), pretangles, neurofibrillary tangles (NFT) and cell death are not part of a single pathogenetic cascade but may occur independently. (3) In brain areas where classical AD changes, i.e. NPs and NFTs, are present, such as the CA1 area of the hippocampus, the nucleus basalis of Meynert and the tuberomamillary nucleus, a decreased metabolic rate is found. The decreased metabolic rate appears not to be induced by the presence of pretangles, NFT or NPs. (4) Decreased metabolic rate may precede cognitive impairment and is thus an early occurring hallmark of AD, which, in principle, may be reversible. The observation that the administration of glucose or insulin enhances memory in AD patients also supports the view that AD has a metabolic basis. (5) Moreover, several observations in postmortem brain indicate that activated neurons are better able to withstand aging and AD, a phenomenon paraphrased by us as 'use it or lose it'. (6) It is, therefore, attractive to direct the development of therapeutic strategies towards restimulation of neuronal metabolic rate in order to improve cognition and other symptoms in AD. A number of pharmacological and non-pharmacological studies support the concept that activation of the brain has beneficial effects and may, to a certain degree, restore several aspects of cognition and other central functions. For instance, the circadian system may be restimulated in AD patients by exposing them to more light or transcutaneous nerve stimulation. A procedure has been developed to culture human postmortem brain tissue that allows testing of the efficacy of putative stimulatory compounds such as neurotrophins.

Gene expression profiles of cholinergic nucleus basalis neurons in Alzheimer's disease

Cholinergic neurons of the nucleus basalis (NB) are selectively vulnerable in Alzheimer's disease (AD), yet the molecular mechanisms associated with their dysfunction remain unknown. We used single cell RNA amplification and custom array technology to examine the expression of functional classes of mRNAs found in anterior NB neurons from normal aged and AD subjects. mRNAs encoding neurotrophin receptors, synaptic proteins, protein phosphatases, and amyloid-related proteins were evaluated. We found that trkB and trkC mRNAs were selectively down-regulated in NB neurons, whereas p75NTR mRNA levels remained stable in end stage AD. TrkA mRNA was reduced by approximately 28%, but did not reach statistical significance. There was a down-regulation of synaptophysin, synaptotagmin, and protein phosphatases PP1alpha and PP1beta mRNAs in AD. In contrast, we found a selective up-regulation of cathepsin D mRNA in NB neurons in AD brain. Thus, anterior NB neurons undergo selective alterations in gene expression in AD. These results may provide clues to the molecular pathogenesis of NB neuronal degeneration during AD.

The effects of chronic ethanol consumption on neurotrophins and their receptors in the rat hippocampus and basal forebrain

Damage to the basal forebrain frequently results in deficits in learning and memory. Mnenonic dysfunction also occurs following prolonged ethanol consumption in humans and in animal models of chronic ethanol intake, accompanied by specific abnormalities in synaptic transmission between the basal forebrain and hippocampus. The integrity of at least some of the reciprocal neuronal connections between these brain regions is influenced by target-derived neurotrophic factors. We used a semiquantitative reverse transcription polymerase chain reaction technique to measure the messenger RNA for neurotrophins BDNF and NGF, and for their receptors trkB, trkA, and the low affinity receptor, p75(NTR) in the hippocampus and basal forebrain of rats after 28 weeks of alcohol consumption without malnutrition. This chronic ethanol treatment (CET) resulted in a marked and selective reduction in basal forebrain trkA mRNA. Western blotting revealed a similar reduction of basal forebrain trkA protein. CET effects on basal forebrain trkA may reflect impaired NGF signaling that could compromise septohippocampal synaptic connections, cholinergic differentiation, and emergent functional abilities dependent on these properties.

Loss of basal forebrain P75(NTR) immunoreactivity in subjects with mild cognitive impairment and Alzheimer's disease

The long-held belief that degeneration of the cholinergic basal forebrain was central to Alzheimer's disease (AD) pathogenesis and occurred early in the disease process has been questioned recently. In this regard, changes in some cholinergic basal forebrain (CBF) markers (e.g. the high affinity trkA receptor) but not others (e.g., cortical choline acetyltransferase [ChAT] activity, the number of ChAT and vesicular acetylcholine transporter-immunoreactive neurons) suggest specific phenotypic changes, but not frank neuronal degeneration, early in the disease process. The present study examined the expression of the low affinity p75 neurotrophin receptor (p75(NTR)), an excellent marker of CBF neurons, in postmortem tissue derived from clinically well-characterized individuals who have been classified as having no cognitive impairment (NCI), mild cognitive impairment (MCI), and mild AD. Relative to NCI individuals, a significant and similar reduction in the number of nucleus basalis p75(NTR)-immunoreactive neurons was seen in individuals with MCI (38%) and mild AD (43%). The number of p75(NTR)-immunoreactive nucleus basalis neurons was significantly correlated with performance on the Mini-Mental State Exam, a Global Cognitive Test score, as well as some individual tests of working memory and attention. These data, together with previous reports, support the concept that phenotypic changes, but not frank neuronal degeneration, occur early in cognitive decline. Although there was no difference in p75(NTR) CBF cell reduction between MCI and AD, it remains to be determined whether these findings lend support to the hypothesis that MCI is a prodromal stage of AD.

The common neurotrophin receptor p75NTR enhances the ability of PC12 cells to resist oxidative stress by a trkA-dependent mechanism

Functional role(s) for the common neurotrophin receptor p75NTR in nerve growth factor (NGF) signaling have yet to be fully elucidated. Many studies have demonstrated that p75NTR can enhance nerve growth factor-induced survival mediated via the trkA receptor. In addition, newly identified pathways for p75NTR signaling have included distinct p75NTR-specific and trk-independent effects which generally appear to be pro-apoptotic. In the present study, we have examined the influence of p75NTR on NGF-mediated protective effects from hydrogen peroxide (H2O2)-induced apoptotic cell death of PC12 cells. Exposure of PC12 cells to H2O2 resulted in Caspase-3 activation and apoptosis. NGF protected PC12 cells against H2O2-mediated apoptosis in a dose-dependent manner and inhibited Caspase-3 activation. These effects of NGF required activation of both PI 3-kinase and MAP kinase signal pathways. When NGF binding to p75NTR was blocked by treating cells with either BDNF or PD90780, and where p75NTR expression was reduced by treating cells with antisense oligonucleotide to p75NTR, the protective effects of NGF were attenuated. Further, NGF had no effect on cell viability in PC12nn5 cells, which express only p75NTR. When trk-mediated signal transduction was blocked, leaving p75NTR signaling activated, PC12 cells were not more vulnerable to H2O2. These data suggest that p75NTR enhances the ability of PC12 cells to resist oxidative stress by a trkA-dependent mechanism, potentially by allosteric mechanisms. Further, potential trkA-independent and pro-apoptotic signaling of p75NTR does not contribute to apoptotic cell death of PC12 cells in a setting of oxidative insult.

Down-regulation of trkA mRNA within nucleus basalis neurons in individuals with mild cognitive impairment and Alzheimer's disease

Recent studies indicate that trkA expression is reduced in end-stage Alzheimer's disease (AD). However, understanding the neuropathologic correlates of early cognitive decline, as well as the changes that underlie the transition from nondemented mild cognitive impairment (MCI) to AD, are more critical neurobiological challenges. In these regards, the present study examined the expression of trkA mRNA in individuals diagnosed with MCI and AD from a cohort of people enrolled in a Religious Orders Study. Individuals with MCI and AD displayed significant reductions in trkA mRNA relative to aged-matched controls, indicating that alterations in trkA gene expression occur early in the disease process. The magnitude of change was similar in MCI and AD cases, suggesting that further loss of trkA mRNA is not necessarily associated with the transition of individuals from nondemented MCI to AD. The loss of trkA mRNA was not associated with education, apolipoprotein E allele status, gender, Braak score, global cognitive score or Mini-Mental Status Examination. In contrast, the loss of trkA mRNA in MCI and AD was significantly correlated with function on a variety of episodic memory tests.

Loss of nucleus basalis neurons containing trkA immunoreactivity in individuals with mild cognitive impairment and early Alzheimer's disease

Recent studies indicate that there is a marked reduction in trkA-containing nucleus basalis neurons in end-stage Alzheimer's disease (AD). We used unbiased stereological counting procedures to determine whether these changes extend to individuals with mild cognitive impairment (MCI) without dementia from a cohort of people enrolled in the Religious Orders Study. Thirty people (average age 84.7 years) came to autopsy. All individuals were cognitively tested within 12 months of death (average MMSE 24.2). Clinically, 9 had no cognitive impairment (NCI), 12 were categorized with MCI, and 9 had probable AD The average number of trkA-immunoreactive neurons in persons with NCI was 196, 632 +/- 12,093 (n = 9), for those with MCI it was 106,110 +/- 14,565, and for those with AD it was 86,978 +/- 12,141. Multiple comparisons showed that both those with MCI and those with AD had significant loss in the number of trkA-containing neurons compared to those with NCI (46% decrease for MCI, 56% for AD). An analysis of variance revealed that the total number of neurons containing trkA immunoreactivity was related to diagnostic classification (P < 0.001), with a significant reduction in AD and MCI compared to NCI but without a significant difference between MCI and AD. Cell density was similarly related to diagnostic classification (P < 0.001). There was a significant correlation with the Boston Naming Test and with a global score measure of cognitive function. The number of trkA-immunoreactive neurons was not correlated with MMSE, age at death, education, apolipoprotein E allele status, gender, or Braak score. These data indicate that alterations in the number of nucleus basalis neurons containing trkA immunoreactivity occurs early and are not accelerated from the transition from MCI to mild AD.

Neurotrophic factor strategies for the treatment of Alzheimer disease

Cholinergic neurons in the nucleus basalis of Meynert are reduced early in the course of Alzheimer disease, and the dysfunction of cholinergic neurons is believed to be primarily responsible for cognitive deficits in the disease. Nerve growth factor has a trophic effect on cholinergic neurons and therefore may have some beneficial effects on the cognitive impairment observed in patients with Alzheimer disease. Experimental studies demonstrated that a continuous infusion of nerve growth factor into the cerebroventricle prevents cholinergic neuron atrophy after axotomy or associated with normal aging and ameliorates cognition impairment in these animals. A clinical study in three patients with Alzheimer disease revealed, however, that a long-term intracerebroventricular infusion of nerve growth factor may have certain potentially beneficial effects, but the continuous intracerebroventricular route of administration is also associated with negative side effects that appear to outweigh the positive effects. Several other strategies have been suggested to provide neurotrophic support to cholinergic neurons. In this article, we review the neurotrophic factor strategies for the treatment of Alzheimer disease.

Nerve growth factor treatment in dementia

Millions of people are affected by Alzheimer disease. As longevity increases, so will the number of patients with dementia. This has led to an intense search for successful treatment strategies. One area of interest is neurotrophic factors. Brain development and neuronal maintenance, as well as protective efforts, are mediated by a large number of different neurotrophic factors acting on specific receptors. In neurodegenerative disorders, there may be a possibility of rescuing degenerating neurons and stimulating terminal outgrowth with use of neurotrophic factors. The first neurotrophic factor discovered was nerve growth factor (NGF). A wealth of animal studies have shown that cholinergic neurons are NGF sensitive and NGF dependent, which is especially interesting in cognitive disorders, in which central cholinergic projections are important for cognitive function. In Alzheimer disease, cholinergic neurons have been shown to degenerate. This suggests that NGF may be used to pharmacologically counteract cholinergic degeneration and/or induce terminal sprouting in Alzheimer disease. Data from animal studies, as well as from the author's recent clinical trial, in which NGF was infused to the lateral ventricle in patients with Alzheimer disease, will be presented. Effects of NGF on cognition, as well as issues regarding dosage, side effects, and alternative ways of administering NGF, will be discussed.

Expression of Trk isoforms in brain regions and in the striatum of patients with Alzheimer's disease

The TrkAII tyrosine kinase receptor differs from the TrkAI isoform by an insertion of six amino acids in the extracellular domain. We used RT-PCR to determine their respective distribution in rat and human brain. Only trkAII transcripts were detected in 12 rat brain regions, while both trkAI and trkAII transcripts were detected in the cerebellum and pituitary gland. In human, both trkAI and trkAII transcripts were detected in the frontal, temporal, and occipital cortex and thalamus, while only trkAI transcripts were detected in the hippocampus and cerebellum. In the caudate and putamen, trkAII transcripts were exclusively detected. Thereafter, we studied the expression of TrkA isoforms in the striatum of five patients with Alzheimer's disease (AD), four patients with non-AD dementia, seven patients with Parkinson's disease, and six paired nondemented elderly control individuals. In controls and non-AD patients, a constant expression of trkAII transcripts was detected within all striatum parts. In AD patients, a heterogeneous decrease in trkAII expression was observed in the caudate, putamen, and ventral striatum, resulting either in a drop of trkAII transcript levels or in a weak coamplification of trkAII and trkAI transcripts. The alteration of TrkAII gene expression paralleled those of choline acetyltransferase. Together with previous data, this suggests that the alteration of trk gene expression could contribute to a decrease in NGF binding sites and its protective effects on cholinergic neurons of AD patients.

Neurotrophic factors in Alzheimer's and Parkinson's disease brain

The biomedical literature on the subject of neurotrophic growth factors has expanded prodigiously. This essay reviews neurotrophic factors (NTF) and their receptors in Alzheimer's disease (AD) and Parkinson's disease (PD) brain and recent updates on receptor signaling. The hypotheses for specific NTF involvement in neurodegenerative diseases in human and as potential therapy are based mainly on experimental animal and in vitro models. There are wide gaps in information on regional synthesis and cell contents of NTFs and their receptors in human brain. Observations on AD brain indicate increases in NGF and decreases in BDNF in surviving neurons of hippocampus and certain neocortical regions and decreases in TrkA in cortex and nucleus basalis. In PD brain, the few data available indicate decreases in neuronal content of GDNF and bFGF in surviving substantia nigra dopaminergic neurons. There are very few data regarding age-dependent effects on NTFs and on their receptors in human brain. Since NTFs in neurons are subject to retrograde and, in at least some cases, to anterograde transport from and to target neurons, their effects may be related to synthesis in local or remote sites or to changes in axoplasmic transport. Also, certain NTFs and their receptors are found to be expressed in activated glia. Thus, comparative in situ data for transcription levels and protein contents for NTFs and their receptors in both sites of neuronal origin and termination in human brain are needed to understand their potential roles in treating human diseases.

Retinoic acid potentiated the protective effect of NGF against staurosporine-induced apoptosis in cultured chick neurons by increasing the trkA protein expression

Nerve growth factor (NGF) has already been shown to protect neurons and PC12 cells from cell death induced by different stimuli. When chick embryonic neurons were exposed to staurosporine (200 nM, 24 hr), the percentage of apoptotic neurons increased from 15% in controls to 80%, but the treatment with NGF alone did not show any neuroprotection. In the presence of retinoic acid (RA, 5 microM), however, NGF (20 pg/ml) reduced staurosporine-induced damage to 42% apoptotic neurons compared to 58% in the presence of RA (5 icroM) alone. TrkA protein expression in chick neurons was markedly reduced by staurosporine, but was found to be increased in the presence of RA and NGF compared with the treatment with staurosporine alone. The antiapoptotic effect caused by RA and NGF was abolished by the tyrosine kinase inhibitor K-252a, as well as by anti-trkA antibodies and anti-NGF antibodies suggesting that the increase in trkA protein expression contributed to its mechanism of action. In addition, RA-enhanced 2.6-fold the NGF secretion from cultured rat cortical astrocytes and conditioned medium of RA-treated astrocytes reduced the percentage of apoptotic chick neurons after a 24 hr-incubation with staurosporine in the same manner as the external addition of RA and NGF. Increasing the endogenous synthesis of growth factors as well as the expression of their receptors by small, blood-brain barrier-permeable drugs was suggested as a promising concept for neuroprotection.

Loss of cholinergic phenotype in basal forebrain coincides with cognitive decline in a mouse model of Down's syndrome

Mice with segmental trisomy of chromosome 16 (Ts65Dn) have been used as a model for Down's syndrome. These mice are born with a normal density of basal forebrain cholinergic neurons but, like patients with Down's syndrome, undergo a significant deterioration of these neurons later in life. The time course for this degeneration of cholinergic neurons has not been studied, nor is it known if it correlates with the progressive memory and learning deficits described. Ts65Dn mice that were 4, 6, 8, and 10 months old were sacrificed for evaluation of basal forebrain morphology. Separate groups of mice were tested on visual or spatial discrimination learning and reversal. We found no alterations in cholinergic markers in 4-month-old Ts65Dn mice, but thereafter a progressive decline in density of cholinergic neurons, as well as significant shrinkage of cell body size, was seen. A parallel loss of staining for the high-affinity nerve growth factor receptor, trkA, was observed at all time points, suggesting a biological mechanism for the cell loss involving this growth factor. Other than transient difficulty in learning the task requirements, there was no impairment of trisomic mice on visual discrimination learning and reversal, whereas spatial learning and reversal showed significant deficits, particularly in the mice over 6 months of age. Thus, the loss of ChAT-immunoreactive neurons in the basal forebrain was coupled with simultaneous deficits in behavioral flexibility on a spatial task occurring for the first time around 6 months of age. These findings suggest that the loss of cholinergic function and the simultaneous decrease in trkA immunoreactivity in basal forebrain may directly correlate with cognitive impairment in the Ts65Dn mouse

Cholinergic medial septum neurons do not degenerate in aged 129/Sv control or p75(NGFR)-/-mice

Cholinergic medial septum neurons express TrkA and p75 nerve growth factor receptor (p75(NGFR)) and interactions between TrkA and p75(NGFR) are necessary for high-affinity binding and signaling of nerve growth factor (NGF) through TrkA. In adult p75(NGFR)-deficient (-/-) mice, retrograde transport of NGF and other neurotrophins by these neurons is greatly reduced, however, these neurons maintain their cholinergic phenotype and size. Reduced transport of NGF has been proposed to play a role in Alzheimer's disease. Here, we investigated whether chronic and long-term absence of p75(NGFR) (and possibly reduced NGF transport and TrkA binding) would affect the cholinergic septohippocampal system during aging in mice. In young (6-8 months), middle aged (12-18 months), and aged (19-23 months) 129/Sv control mice the total number of choline acetyltransferase-positive medial septum neurons and the mean diameter and cross sectional area of the cholinergic cell bodies were similar. The cholinergic hippocampal innervation, as measured by the density of acetylcholinesterase-positive fibers in the outer molecular layer of the dentate gyrus was also similar across all ages. These parameters also did not change during aging in p75(NGFR) -/- mice and the number and size of the choline acetyltransferase-positive neurons and the cholinergic innervation density were largely similar as in control mice at all ages. These results suggest that p75(NGFR) does not play a major role in the maintenance of the number or morphology of the cholinergic basal forebrain neurons during aging of these mice. Alternatively, p75(NGFR) -/- mice may have developed compensatory mechanisms in response to the absence of p75(NGFR).

P75 neurotrophin receptor in the nucleus basalis of meynert in relation to age, sex, and Alzheimer's disease

In a previous study we showed that the staining of tyrosine kinase receptors (trks), which are high-affinity neurotrophin receptors (NTRs), is strongly diminished in the nucleus basalis of Meynert (NBM) of Alzheimer's disease (AD) patients, which may explain the lack of effect of NGF therapy in AD patients so far. Since the literature regarding the expression of low-affinity NTRs was rather controversial, the aim of the present study was to examine (i) possible changes in the staining of low-affinity NTRs, i.e., p75 in the human NBM, an area that is severely affected in AD; and (ii) alterations of these receptors in relation to risk factors for AD, e. g., age, sex, and menopause. Brain material of 31 controls and 30 AD patients was obtained at autopsy, embedded in paraffin, and stained immunocytochemically. Using an image analysis system, we quantified p75 immunoreactivity in both cell bodies and fibers at the level of the NBM. Our results showed a significant diminishment of p75 immunoreactivity in both cell bodies and fibers of NBM neurons in AD. We did not find any relationship between age or sex and the expression of p75 receptor in cell bodies. However, there was a clearly positive relationship between age and fiber staining in AD patients which suggests the occurrence of a p75 transport disorder as an early event in the process of AD. These observations and the earlier reported decreased staining of trk receptors show that degeneration of NBM neurons in AD is associated with a decreased neurotrophin responsiveness of NBM neurons in AD and that therapeutic strategies should be directed toward upregulation of receptors or facilitation of transport before an effect of neurotrophins in AD may be expected.