Long-term nucleus basalis cholinergic depletion induces attentional deficits and impacts cortical neurons and BDNF levels without affecting the NGF synthesis.

Basal forebrain cholinergic neurons (BFCNs) represent the main source of cholinergic innervation to the cortex and hippocampus and degenerate early in Alzheimer's disease (AD) progression. Phenotypic maintenance of BFCNs depends on levels of mature nerve growth factor (mNGF) and mature brain-derived neurotrophic factor (mBDNF), produced by target neurons and retrogradely transported to the cell body. Whether a reciprocal interaction where BFCN inputs impact neurotrophin availability and affect cortical neuronal markers remains unknown. To address our hypothesis, we immunolesioned the nucleus basalis (nb), a basal forebrain cholinergic nuclei projecting mainly to the cortex, by bilateral stereotaxic injection of 192-IgG-Saporin (the cytotoxin Saporin binds p75ntr receptors expressed exclusively by BFCNs) in 2.5-month-old Wistar rats. At 6 months post-lesion, Saporin-injected rats (SAP) showed an impairment in a modified version of the 5-Choice Serial Reaction Time Task (5-choice task). Postmortem analyses of the brain revealed a reduction of Choline Acetyltransferase-immunoreactive neurons compared to wild-type controls. A diminished number of cortical vesicular acetylcholine transporter-immunoreactive boutons was accompanied by a reduction in BDNF mRNA, mBDNF protein levels, markers of glutamatergic (vGluT1), and GABAergic (GAD65) neurons in the SAP-group compared to the controls. NGF mRNA, NGF precursor, and mNGF protein levels were not affected. Additionally, cholinergic markers correlated with the attentional deficit and BDNF levels. Our findings demonstrate that while cholinergic nb loss impairs cognition and reduces cortical neuron markers, it produces differential effects on neurotrophin availability, affecting BDNF but not NGF levels.

The human brain NGF metabolic pathway is impaired in the pre-clinical and clinical continuum of Alzheimers disease.

The NGF metabolic pathway entails the proteins that mature pro-nerve growth factor (proNGF) to NGF and those that degrade NGF. Basal forebrain cholinergic neurons require NGF for maintenance of cholinergic phenotype, are critical for cognition, and degenerate early in Alzheimer's disease (AD). In AD, NGF metabolism is altered, but it is not known whether this is an early phenomenon, nor how it relates to AD pathology and symptomology. We acquired dorsolateral/medial prefrontal cortex samples from individuals with Alzheimer's dementia, Mild Cognitive Impairment (MCI), or no cognitive impairment with high (HA-NCI) and low (LA-NCI) brain Aß from the Religious Orders Study. Cortical proNGF protein, but not mRNA, was higher in AD, MCI, and HA-NCI, while mature NGF was lower. Plasminogen protein was higher in MCI and AD brain tissue, with plasminogen mRNA not likewise elevated, suggesting diminished activation of the proNGF convertase, plasmin. The plasminogen activator tPA was lower in HA-NCI while neuroserpin, the CNS tPA inhibitor, was higher in AD and MCI cortical samples. Matrix metalloproteinase 9 (MMP9), which degrades NGF, was overactive in MCI and AD. Transcription of the MMP9 inhibitor TIMP1 was lower in HA-NCI. ProNGF levels correlated with plasminogen, neuroserpin, and VAChT while NGF correlated with MMP9 activity. In NCI, proNGF correlated with cerebral Aß and tau deposition and to cognitive performance. In summary, proNGF maturation is impaired in preclinical and clinical AD while mature NGF degradation is enhanced. These differences correlate with cognition, pathology, and cholinergic tone, and may suggest novel biomarkers and therapeutic targets.

A new role for matrix metalloproteinase-3 in the NGF metabolic pathway: Proteolysis of mature NGF and sex-specific differences in the continuum of Alzheimer's pathology.

Matrix metalloproteinase-3 (MMP-3) has been associated with risk of Alzheimer's disease (AD). In this study we introduce a novel role for MMP-3 in degrading nerve growth factor (NGF) in vivo and examine its mRNA and protein expression across the continuum of AD pathology. We provide evidence that MMP-3 participates in the degradation of mature NGF in vitro and in vivo and that it is secreted from the rat cerebral cortex in an activity-dependent manner. We show that cortical MMP-3 is upregulated in the McGill-R-Thy1-APP transgenic rat model of AD-like amyloidosis. A similar upregulation was found in AD and MCI brains as well as in cognitively normal individuals with elevated amyloid deposition. We also observed that frontal cortex MMP-3 protein levels are higher in males. MMP-3 protein correlated with more AD neuropathology, markers of NGF metabolism, and lower cognitive scores in males but not in females. These results suggest that MMP-3 upregulation in AD might contribute to NGF dysmetabolism, and therefore to cholinergic atrophy and cognitive deficits, in a sex-specific manner. MMP-3 should be further investigated as a biomarker candidate or as a therapeutic target in AD.

The NGF Metabolic Pathway: New Opportunities for Biomarker Research and Drug Target Discovery : NGF Pathway Biomarkers and Drug Targets.

Recent research has demonstrated that degeneration of the basal forebrain cholinergic system, far from being a mere downstream mediator of Alzheimer's disease (AD) symptoms, may play a disease-aggravating role in the continuum of AD pathology. The search for novel biomarkers of the cholinergic deficit in AD and novel therapeutic targets for the sustenance of the basal forebrain cholinergic system has therefore taken on more urgency. A novel model that explains the preferential vulnerability of basal forebrain cholinergic neurons in AD as the result of pathological alterations to nerve growth factor (NGF) metabolism offers an integrated investigative platform for the development of such biomarkers and therapeutics. By positing a reciprocal trophic interaction between the basal forebrain and its target tissues, this model can also explain the disease-modifying nature of the cholinergic deficit in AD and can incorporate other key factors in basal forebrain cholinergic degeneration, including NGF receptor changes and retrograde transport deficits in AD. This chapter will focus on the potential of NGF metabolic pathway biomarkers in AD as well as therapeutic targets to correct NGF metabolic deficits, aiding the development of novel pro-cholinergic therapeutics.

Nerve growth factor (NGF) pathway biomarkers in Down syndrome prior to and after the onset of clinical Alzheimer's disease: A paired CSF and plasma study.

BACKGROUND: The discovery that nerve growth factor (NGF) metabolism is altered in Down syndrome (DS) and Alzheimer's disease (AD) brains offered a framework for the identification of novel biomarkers signalling NGF deregulation in AD pathology. METHODS: We examined levels of NGF pathway proteins (proNGF, neuroserpin, tissue plasminogen activator [tPA], and metalloproteases [MMP]) in matched cerebrospinal fluid (CSF)/plasma samples from AD-symptomatic (DSAD) and AD-asymptomatic (aDS) individuals with DS, as well as controls (HC). RESULTS: ProNGF and MMP-3 were elevated while tPA was decreased in plasma from individuals with DS. CSF from individuals with DS showed elevated proNGF, neuroserpin, MMP-3, and MMP-9. ProNGF and MMP-9 in CSF differentiated DSAD from aDS (area under the curve = 0.86, 0.87). NGF pathway markers associated with CSF amyloid beta and tau and differed by sex. DISCUSSION: Brain NGF metabolism changes can be monitored in plasma and CSF, supporting relevance in AD pathology. These markers could assist staging, subtyping, or precision medicine for AD in DS.

Differential deregulation of NGF and BDNF neurotrophins in a transgenic rat model of Alzheimer's disease.

Evidence from human neuropathological studies indicates that the levels of the neurotrophins nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) are compromised in Alzheimer's disease. However, the causes and temporal (pathology-dependent) evolution of these alterations are not completely understood. To elucidate these issues, we investigated the McGill-R-Thy1-APP transgenic rat, which exhibits progressive intracellular and extracellular amyloid-beta (Aß) pathology and ensuing cognitive deficits. Neurochemical analyses revealed a differential dysregulation of NGF and BDNF transcripts and protein expression. While BDNF mRNA levels were significantly reduced at very early stages of amyloid pathology, before plaques appeared, there were no changes in NGF mRNA expression even at advanced stages. Paradoxically, the protein levels of the NGF precursor were increased. These changes in neurotrophin expression are identical to those seen during the progression of Alzheimer's disease. At advanced pathological stages, deficits in the protease cascade controlling the maturation and degradation of NGF were evident in McGill transgenic rats, in line with the paradoxical upregulation of proNGF, as seen in Alzheimer's disease, in the absence of changes in NGF mRNA. The compromise in NGF metabolism and BDNF levels was accompanied by downregulation of cortical cholinergic synapses; strengthening the evidence that neurotrophin dysregulation affects cholinergic synapses and synaptic plasticity. Our findings suggest a differential temporal deregulation of NGF and BDNF neurotrophins, whereby deficits in BDNF mRNA appear at early stages of intraneuronal Aß pathology, before alterations in NGF metabolism and cholinergic synapse loss manifest.

An inflammatory and trophic disconnect biomarker profile revealed in Down syndrome plasma: Relation to cognitive decline and longitudinal evaluation.

INTRODUCTION: Given that Alzheimer's pathology develops silently over decades in Down syndrome (DS), prognostic biomarkers of dementia are a major need. METHODS: We investigated the plasma levels of Aß, proNGF, tPA, neuroserpin, metallo-proteases and inflammatory molecules in 31 individuals with DS (with and without dementia) and in 31 healthy controls. We examined associations between biomarkers and cognitive decline. RESULTS: Aß40 and Aß42 were elevated in DS plasma compared to controls, even in DS individuals without dementia. Plasma Aß correlated with the rate of cognitive decline across 2 years. ProNGF, MMP-1, MMP-3, MMP-9 activity, TNF-α, IL-6, and IL-10 were higher in DS plasma, even at AD-asymptomatic stages. Declining plasma Aß42 and increasing proNGF levels correlated with cognitive decline. A combined measure of Aß and inflammatory molecules was a strong predictor of prospective cognitive deterioration. CONCLUSIONS: Our findings support the combination of plasma and cognitive assessments for the identification of DS individuals at risk of dementia.

The Brain NGF Metabolic Pathway in Health and in Alzheimer's Pathology.

Emerging research has re-emphasized the role of the cortical cholinergic system in the symptomology and progression of Alzheimer's disease (AD). Basal forebrain (BF) cholinergic nuclei depend on target-derived NGF for survival during development and for the maintenance of a classical cholinergic phenotype during adulthood. In AD, BF cholinergic neurons lose their cholinergic phenotype and function, suggesting an impairment in NGF-mediated trophic support. We propose that alterations to the enzymatic pathway that controls the maturation of proNGF to mature NGF and the latter's ulterior degradation underlie this pathological process. Indeed, the NGF metabolic pathway has been demonstrated to be impaired in AD and other amyloid pathologies, and pharmacological manipulation of NGF metabolism has consequences in vivo for both levels of proNGF/NGF and the phenotype of BF cholinergic neurons. The NGF pathway may also have potential as a biomarker of cognitive decline in AD, as its changes can predict future cognitive decline in patients with Down syndrome as they develop preclinical Alzheimer's pathology. New evidence suggests that the cholinergic system, and by extension NGF, may have a greater role in the progression of AD than previously realized, as changes to the BF precede and predict changes to the entorhinal cortex, as anticholinergic drugs increase odds of developing AD, and as the use of donepezil can reduce rates of hippocampal and cortical thinning. These findings suggest that new, more sophisticated cholinergic therapies should be capable of preserving the basal forebrain thus having profound positive effects as treatments for AD.

Identification and Preliminary Validation of a Plasma Profile Associated with Cognitive Decline in Dementia and At-Risk Individuals: A Retrospective Cohort Analysis.

Biomarker discovery is a major need for earlier dementia diagnosis. We evaluated a plasma signature of amyloid, metallo-proteinases (MMPs), and inflammatory markers in a cohort of at-risk individuals and individuals clinically diagnosed with probable Alzheimer's disease (pAD). Using multiplex arrays, we measured Aß40, Aß42, MMP-1, MMP-3, MMP-9, IFN-γ, TNF-α, IL-6, IL-8, and IL-10 in plasma from 107 individuals followed every 6 months for 3 years. Final diagnoses included: pAD (n = 28), mild cognitive impairment (MCI, n = 30), subjective memory impairment (SMI, n = 30), and asymptomatic (NCI, n = 19). Blood was drawn at final follow-up. We used linear and logistic regressions to examine biomarker associations with prior known decline on the Montreal Cognitive Assessment (MoCA) and the Cambridge Cognitive Examination (CAMCOG); as well disease progression by the time of blood-draw. We derived a biomarker composite from the individual markers, and tested its association with a clinical diagnosis of pAD. Lower Aß40 and Aß42 and higher IL-8, IL-10, and TNF-α were associated with greater cognitive decline per the MoCA and CAMCOG. MMP-3 was higher in SMI, MCI, and pAD than NCI. Whereas the other investigative molecules did not differ between groups, composite scores-created using MoCA/CAMCOG-based trends in Aß40, Aß42, MMP-1, MMP-3, IL-8, IL-10, and TNF-α- were associated with a final diagnosis of pAD (c-statistic 0.732 versus 0.602 for age-sex alone). Thus, plasma amyloid, MMP, and inflammatory biomarkers demonstrated differences in individuals with cognitive deterioration and/or progression to MCI/pAD. Our findings support studying these markers earlier in the continuum of probable AD as well as in specific dementias.

Ketamine modulates hippocampal neurogenesis and pro-inflammatory cytokines but not stressor induced neurochemical changes.

Considerable recent attention has focused on the rapid antidepressant effects observed in treatment resistant patients produced by the NMDA receptor antagonist, ketamine. Surprisingly, the effects of ketamine in the context of stressor exposure, as well as the consequences of its chronic use are unclear. Thus, we assessed the impact of acute and repeated ketamine treatment together with acute [restraint or lipopolysaccharide (LPS)] or chronic (unpredictable different psychogenic challenges) stressor exposure. Importantly, acute ketamine treatment did provoke an antidepressant-like effect in a forced swim test (FST) and this effect lasted for 8 days following repeated exposure to the drug. Although acute restraint and LPS individually provoked the expected elevation of plasma corticosterone and brain-region specific monoamine variations, ketamine had no influence on corticosterone and had, at best, sparse effects on the monoamine changes. Similarly, ketamine did not appreciably influence the stressor induced neurochemical and sucrose preference alterations, it did however, dose-dependently reverse the LPS induced elevation of the pro-inflammatory cytokines, interleukin-1ß (IL-1ß) and tumor necrosis factor-α (TNF-α). Likewise, repeated ketamine administration increased adult hippocampal neurogenesis. These data indicate that repeated ketamine administration had greater behavioral consequences than acute treatment and that the drug might be imparting antidepressant effects through its effects on neuroplasticity and inflammatory processes rather than the typical neurochemical/hormonal factors affected by stressors. This article is part of the Special Issue entitled 'Ionotropic glutamate receptors'.

Searching for new pharmacological targets for the treatment of Alzheimer's disease in Down syndrome.

Individuals with Down syndrome are at increased risk of developing Alzheimer's disease due to increase gene dosage resulting from chromosome 21 triplication. Although virtually all adults with Down syndrome will exhibit the major neuropathological hallmarks that define Alzheimer's disease, not all of them will develop the clinical symptoms associated with this disorder (i.e. dementia). Therefore, a good understanding of the pathophysiology of Alzheimer's disease in Down syndrome will be crucial for the identification of novel pharmacological targets to develop disease-modifying therapies for the benefit of Down syndrome individuals and for Alzheimer's sufferers alike. The study of biomarkers will also be essential for the development of better screening tools to identify dementia at its incipient stages. This review discusses the best-validated pharmacological targets for the treatment of cognitive impairment and Alzheimer's disease in Down syndrome. We further examine the relevance of newly discovered biological markers for earlier dementia diagnosis in this population.

The interactive effects of ketamine and magnesium upon depressive-like pathology.

Approximately one-third of patients with major depressive disorders (MDDs) are resistant to current treatment methods, and the majority of cases relapse at some point during therapy. This has resulted in novel treatments being adopted, including subanesthetic doses of ketamine, which affects aberrant neuroplastic circuits, glutamatergic signaling, and the production of brain-derived neurotrophic factor. Ketamine rapidly relieves depressive symptoms in treatment-resistant major depressive disorder patients with effects that last for up to 2 weeks even after a single administration. However, it is also a drug with an abusive potential and can have marked side effects. Hence, this study aimed at enhancing the antidepressant-like effects of ketamine (allowing for lower dosing regimens) by coadministering magnesium hydroaspartate (Mg(2+) normally affects the same receptors as ketamine) and also assessed whether an Mg(2+)-deficient diet would modify the impact of ketamine. It was found that a single 15 mg/kg dose of ketamine did indeed induce rapid antidepressant-like effects in the forced swim test but did not affect brain levels of the brain-derived neurotrophic factor. Contrary to our hypothesis, magnesium administration or deficiency did not influence the impact of ketamine on these outcomes. Thus, these data do not support the use of magnesium as an adjunct agent and instead suggest that further research involving other antidepressant and animal models is required to confirm the present findings.

Stressor-like effects of c-Jun N-terminal kinase (JNK) inhibition.

There is an urgent need for novel treatment strategies for stressor related disorders, particularly depression and anxiety disorders. Indeed, existing drug treatments are only clinically successful in a subset of patients and relapse is common. This likely stems from the fact that stressor disorders are heterogeneous with multiple biological pathways being affected. To this end, the present investigation sought to assess in mice the contribution of the c-Jun N terminal kinase (JNK) pathway to the behavioral, hormonal and neurochemical effects of an acute stressor. Indeed, although JNK has been shown to modulate glucocorticoid receptors in vitro, virtually nothing is known of the role for JNK in affecting stressor induced pathology. We presently found that the JNK antagonist, SP600125, (but not the p38 antagonist, SB203580) increased plasma corticosterone levels under resting conditions and in the context of an acute stressor (wet bedding + restraint). SP600125 also reduced exploration in an open field arena, but prevented the stressor induced increase in open arm exploration in an elevated plus maze. Finally, SP600125 affected noradrenergic activity in the central amygdala and locus coruleus under resting condition, but prevented the noradrenergic effects within the paraventricular nucleus of the hypothalamus that were induced by the acute stressor exposure. These data suggest inhibiting endogenous JNK can have stressor-like corticoid, behavioral and central monoamine effects under basal conditions, but can actually reverse some behavioral and neurochemical effects of an acute stressor. Thus, endogenous JNK appears to affect stress relevant processes in a context-dependent manner.