Somatostatin replacement therapy for Alzheimer dementia

Defining early changes in Alzheimer's disease from RNA sequencing of brain regions differentially affected by pathology

Tau pathology in Alzheimer’s disease (AD) spreads in a predictable pattern that corresponds with disease symptoms and severity. At post-mortem there are cortical regions that range from mildly to severely affected by tau pathology and neuronal loss. A comparison of the molecular signatures of these differentially affected areas within cases and between cases and controls may allow the temporal modelling of disease progression. Here we used RNA sequencing to explore differential gene expression in the mildly affected primary visual cortex and moderately affected precuneus of ten age-, gender- and RNA quality-matched post-mortem brains from AD patients and healthy controls. The two regions in AD cases had similar transcriptomic signatures but there were broader abnormalities in the precuneus consistent with the greater tau load. Both regions were characterised by upregulation of immune-related genes such as those encoding triggering receptor expressed on myeloid cells 2 and membrane spanning 4-domains A6A and milder changes in insulin/IGF1 signalling. The precuneus in AD was also characterised by changes in vesicle secretion and downregulation of the interneuronal subtype marker, somatostatin. The ‘early’ AD transcriptome is characterised by perturbations in synaptic vesicle secretion on a background of neuroimmune dysfunction. In particular, the synaptic deficits that characterise AD may begin with the somatostatin division of inhibitory neurotransmission.

Somatostatin modulates insulin-degrading-enzyme metabolism: implications for the regulation of microglia activity in AD

The deposition of β-amyloid (Aβ) into senile plaques and the impairment of somatostatin-mediated neurotransmission are key pathological events in the onset of Alzheimer's disease (AD). Insulin-degrading-enzyme (IDE) is one of the main extracellular protease targeting Aβ, and thus it represents an interesting pharmacological target for AD therapy. We show that the active form of somatostatin-14 regulates IDE activity by affecting its expression and secretion in microglia cells. A similar effect can also be observed when adding octreotide. Following a previous observation where somatostatin directly interacts with IDE, here we demonstrate that somatostatin regulates Aβ catabolism by modulating IDE proteolytic activity in IDE gene-silencing experiments. As a whole, these data indicate the relevant role played by somatostatin and, potentially, by analogue octreotide, in preventing Aβ accumulation by partially restoring IDE activity.

Pharmacological Treatment of Alzheimer's Disease: Is it Progressing Adequately?

INTRODUCTION

Between 1993 and 2000 four acetylcholinesterase inhibitors were marketed as a symptomatic treatment for Alzheimer's disease (AD), as well as memantine in 2003. Current research is focused on finding drugs that favorably modify the course of the disease. However, their entrance into the market does not seem to be imminent.

RESEARCH DEVELOPMENT

The aim of AD research is to find substances that inhibit certain elements of the AD pathogenic chain (beta- and gamma-secretase inhibitors, alpha-secretase stimulants, beta-amyloid aggregability reducers or disaggregation and elimination inductors, as well as tau-hyperphosphorylation, glutamate excitotoxicity, oxidative stress and mitochondrial damage reducers, among other action mechanisms). Demonstrating a disease's retarding effect demands longer trials than those necessary to ascertain symptomatic improvement. Besides, a high number of patients (thousands of them) is necessary, all of which turns out to be difficult and costly. Furthermore, it would be necessary to count on diagnosis and progression markers in the disease's pre-clinical stage, markers for specific phenotypes, as well as high-selectivity molecules acting only where necessary. In order to compensate these difficulties, drugs acting on several defects of the pathogenic chain or showing both symptomatic and neuroprotective action simultaneously are being researched.

CONCLUSIONS

There are multiple molecules used in research to modify AD progression. Although it turns out to be difficult to obtain drugs with sufficient efficacy so that their marketing is approved, if they were achieved they would lead to a reduction of AD prevalence.

Reduced neuronal co-localisation of nardilysin and the putative alpha-secretases ADAM10 and ADAM17 in Alzheimer's disease and Down syndrome brains

The peptidase nardilysin is involved in degradation of neuropeptides and limited intracellular proteolysis. Recent reports point to an involvement of nardilysin in the pathophysiology of Alzheimer's disease. Nardilysin enhances the alpha-secretase activity of the disintegrin and metalloproteases (ADAMs) 10 and 17, thereby possibly contributing to reduced generation of amyloidogenic fragments from the amyloid precursor protein. A prerequisite for the alpha-secretase-stimulating effect of nardilysin on the activity of ADAMs in vivo is cellular co-expression of nardilysin with ADAM10 and/or ADAM17. We immunolocalised nardilysin, ADAM10, and ADAM17 in cortical regions of normal aged brain, in Alzheimer's disease, and in Down syndrome brains and counted the number of protease-expressing neurons. A considerable portion of neurons co-express nardilysin together with either ADAM10 or ADAM17. Compared to controls, in Alzheimer's disease and in Down syndrome brains there is a decreased cellular expression of all three antigens, and a reduction in the number of those neurons that co-express nardilysin with ADAM10 or with ADAM17. Our data are consistent with the notion that the proposed alpha-secretase-enhancing activity of nardilysin might play a role in human brain pathology.

Somatostatin and Alzheimer's disease

One of the most consistent neurochemical deficits in Alzheimer's disease is a reduction in cortical somatostatin concentrations. The probability of a predominant regulatory change is heightened by the finding that 90% of somatostatin positive nonpyramidal neurons are also positive for NADPH, and NADPH neurons are 'protected' in Alzheimer's disease and do not appear to be lost. The first evidence that somatostatin influences learning and memory processes in experimental animals was published more than a decade ago. These reports of somatostatin effects on cognitive functions in rats were later confirmed by several other studies. The somatostatin depleting substance cysteamine inhibited the learning and memory performance of rats in active and passive avoidance behavior tests. Post-mortem human studies suggest that although somatostatin concentration is reduced, the somatostatin receptors are less affected in the brain in Alzheimer's disease. These findings may be of importance for possible therapeutic approaches using somatostatin-receptor-influencing compounds.

Somatostatin receptors

In 1972, Brazeau et al. isolated somatostatin (somatotropin release-inhibiting factor, SRIF), a cyclic polypeptide with two biologically active isoforms (SRIF-14 and SRIF-28). This event prompted the successful quest for SRIF receptors. Then, nearly a quarter of a century later, it was announced that a neuropeptide, to be named cortistatin (CST), had been cloned, bearing strong resemblance to SRIF. Evidence of special CST receptors never emerged, however. CST rather competed with both SRIF isoforms for specific receptor binding. And binding to the known subtypes with affinities in the nanomolar range, it has therefore been acknowledged to be a third endogenous ligand at SRIF receptors. This review goes through mechanisms of signal transduction, pharmacology, and anatomical distribution of SRIF receptors. Structurally, SRIF receptors belong to the superfamily of G protein-coupled (GPC) receptors, sharing the characteristic seven-transmembrane-segment (STMS) topography. Years of intensive research have resulted in cloning of five receptor subtypes (sst(1)-sst(5)), one of which is represented by two splice variants (sst(2A) and sst(2B)). The individual subtypes, functionally coupled to the effectors of signal transduction, are differentially expressed throughout the mammalian organism, with corresponding differences in physiological impact. It is evident that receptor function, from a physiological point of view, cannot simply be reduced to the accumulated operations of individual receptors. Far from being isolated functional units, receptors co-operate. The total receptor apparatus of individual cell types is composed of different-ligand receptors (e.g. SRIF and non-SRIF receptors) and co-expressed receptor subtypes (e.g. sst(2) and sst(5) receptors) in characteristic proportions. In other words, levels of individual receptor subtypes are highly cell-specific and vary with the co-expression of different-ligand receptors. However, the question is how to quantify the relative contributions of individual receptor subtypes to the integration of transduced signals, ultimately the result of collective receptor activity. The generation of knock-out (KO) mice, intended as a means to define the contributions made by individual receptor subtypes, necessarily marks but an approximation. Furthermore, we must now take into account the stunning complexity of receptor co-operation indicated by the observation of receptor homo- and heterodimerisation, let alone oligomerisation. Theoretically, this phenomenon adds a novel series of functional megareceptors/super-receptors, with varied pharmacological profiles, to the catalogue of monomeric receptor subtypes isolated and cloned in the past. SRIF analogues include both peptides and non-peptides, receptor agonists and antagonists. Relatively long half lives, as compared to those of the endogenous ligands, have been paramount from the outset. Motivated by theoretical puzzles or the shortcomings of present-day diagnostics and therapy, investigators have also aimed to produce subtype-selective analogues. Several have become available.

In vivo internalization of the somatostatin sst2A receptor in rat brain: evidence for translocation of cell-surface receptors into the endosomal recycling pathway

To determine whether cellular compartmentalization of somatostatin receptors can be regulated in vivo, we examined the immunocytochemical distribution of the sst2A receptor (sst2AR) after stereotaxical injections of somatostatin analogs into the rat parietal cortex. Whereas CH-275, a sst1R agonist, failed to induce changes in the diffuse sst2AR immunostaining pattern characteristic of control animals, somatodendritic profiles displaying intracytoplasmic immunoreactive granules became apparent short-term after injection of either somatostatin or the sst2R agonist octreotide. Confocal microscopy revealed that 90% of sst2AR-immunoreactive endosome-like organelles displayed transferrin receptor immunoreactivity. At the electron microscopic level, the percentage of sst2AR immunoparticles dramatically decreased at the plasmalemma of perikarya and dendrites after octreotide injection. Conversely, it significantly increased in endosomes-like organelles. These results demonstrate that sst2ARs undergo, in vivo, rapid and massive internalization into the endocytic recycling compartment in response to acute agonist stimulation and provide important clues toward elucidating somatostatin receptor signaling in the mammalian brain.

Cellular biology of somatostatin receptors

Somatostatin, and the recently discovered neuropeptide cortistatin, exert their physiological actions via a family of six G protein-coupled receptors (sst1, sst2A, sst2B, sst3, sst4, sst5). Following the cloning of somatostatin receptors significant advances have been made in our understanding of their molecular, pharmacological and signaling properties although much progress remains to be done to define their physiological role in vivo. In this review, the present knowledge regarding neuroanatomical localization, signal transduction pathways, desensitization and internalization properties of somatostatin receptors is summarized. Evidence that somatostatin receptors can form homo- and heterodimers and can physically interact with members of the SSTRIP/Shank/ProSAP1/CortBP1 family is also discussed.

Neuropeptide changes in cortical and deep gray structures in Alzheimer's disease

The alteration of certain neuropeptide levels is a dramatic and consistent finding in the brains of AD patients. Levels of SS, which is normally present in high concentrations in cerebral cortex /75/, are consistently decreased in the neocortex, hippocampus and CSF of AD patients. In addition, decreased levels of SS correlate regionally with the distribution of neurofibrillary tangles in AD /47/. Most available evidence suggests that the subset of SS-containing neurons which lack NADPH diaphorase may be relatively vulnerable to degeneration in AD. CRF is another neuropeptide with frequently observed changes in AD. Levels of CRF, which is normally present in low concentrations in cortical structures /75/, are decreased in the neocortex and hippocampus of AD patients. However, levels of CRF in the CSF of AD patients are not consistently reduced, but this is likely a reflection of the relatively low levels of CRF normally present in cerebral cortex. Studies of deep gray structures in AD brains reveal elevated levels of GAL in the nucleus basalis. The ability of GAL to inhibit cholinergic neurotransmission has generated considerable interest, since degeneration of cholinergic neurons in the basal forebrain consistently occurs in AD. In addition, the presence of NADPH diaphorase in GAL-containing neurons may underlie the relative resistance of these neurons to degeneration. From the aforementioned studies, it appears that the neurons which are relatively resistant to neurodegeneration in AD contain NADPH diaphorase. It is hypothesized that the presence of NADPH diaphorase protects these neurons from neurotoxicity mediated by glutamate or nitric oxide. Although one recent study /147/ has reported an elevation of the microtubule-associated protein tau in the CSF of AD patients (and this could become a useful antemortem diagnostic tool for AD), no similar CSF abnormality has been found for any of the neuropeptides. Thus, the measurement of CSF neuropeptide levels presently remains unhelpful in the diagnosis and treatment of AD. Future research on neuropeptides and their potential roles in the pathogenesis, diagnosis, and treatment of AD will likely involve further development of pharmacological modulators of neuropeptide systems, together with the further study of brain neuropeptidases.

Changes in late auditory evoked potentials induced by growth hormone-releasing hormone (GHRH) but not somatostatin (SRIF) after peripheral administration in male controls

To investigate possible influences of growth hormone-releasing hormone (GHRH) and somatostatin (SRIF) on auditory perceptional processes, 12 subjects received either placebo (sodium chloride 0.9%), GHRH (50 micrograms), or SRIF (100 micrograms) on different days. Late auditory evoked potentials (AEP) were computed and further analyzed by using the brain electric source analysis (BESA) method. Reduced late AEP latencies were observed following GHRH administration. In contrast, SRIF had no significant effects on the AEP. The changes in late auditory processing seen after administration of GHRH were most likely induced by a direct central nervous action.

Metabolic effects of scopolamine and physostigmine in human brain measured by positron emission tomography

Two of the more consistent findings in Alzheimer's disease are depressions in frontal and temporoparietal glucose metabolism and a loss of cholinergic neurons in the nucleus basalis of Meynert. Nonetheless, cholinergic replacement strategies have had only minimal therapeutic successes. Whether this situation reflects the limited contribution of cholinergic deafferentation to the intellectual decline or the meager ability of the pharmaceuticals tested to exert their intended pharmacologic action remains unclear. To address this question, the distribution of cerebral abnormalities found in untreated Alzheimer patients, as revealed by positron emission tomography following 18F-fluorodeoxyglucose, were compared with the pattern of functional changes produced by drugs that block or stimulate cholinergic function. Physostigmine was administered to 6 Alzheimer patients to increase brain cholinergic neurotransmission. The anticholinergic scopolamine, given to normal volunteers, was administered to 6 age-matched controls. These data were compared to those obtained from the same subjects while receiving placebo. Amnestic doses of the anticholinergic, scopolamine increased glucose metabolism by up to 20% (p < 0.001) in all brain regions studied, except thalamus. This response contrasted with the metabolic reductions of up to 17% (p < 0.01), especially in parietal and frontal association cortices, occurring in unmedicated Alzheimer patients. Maximum tolerated doses of the anti-cholinesterase, physostigmine, rather than tending to normalize abnormalities in these patients, further reduced cerebral metabolism (p < 0.01) and increased metabolism in thalamus in a pattern inversely correlated (p < 0.001) with that produced by scopolamine. These results fail to support a cholinergic basis for the abnormal metabolic pattern in Alzheimer's disease.

Advances in the pharmacotherapy of Alzheimer's disease

The authors reviewed the literature on the agents proposed for the treatment of Alzheimer's disease (AD). Different classes of drugs have been tested for this indication including psychostimulants, anticoagulants, vasodilators, hyperbaric oxygen, hormones, nootropics, cholinomimetics, monoaminergics and neuropeptides without conclusive evidence of being beneficial for the treatment of this condition. Among the cholinomimetics recent research data seems to indicate that they might produce modest benefits in mild-to-moderate AD patients. Recently, other drugs have also been proposed including neurotrophic factors, phosphatidylserine, angiotension [corrected] converting enzyme (ACE) inhibitors, calcium channel blockers, acetyl-L-carnitine, xanthine derivatives, anti-inflammatory agents, aluminum chelate agents, and D-cycloserine. Of these new strategies few hold promise of more substantial benefits for AD, with the possibility of altering the course of the disease, but these drugs await confirmatory trials.

Regional cerebral glucose metabolism compared in rodents and humans

In order to compare regional brain glucose metabolism in rats and humans, this parameter was measured using Sokoloff's deoxyglucose method in rats, and positron emission tomography with magnetic resonance imaging in humans. An atlas of cerebral regions of interest common to both species was developed to facilitate the evaluation of the relationship in regional values. We found among the regions studied a significant positive correlation in their metabolic values (r = 0.72, P less than 0.001) and coefficients of variation (r = 0.59, P less than 0.01) suggesting that regional brain glucose consumption is comparable between rat and human. Results of this study support the view that rat and human brain may be phylogenetically linked functionally as well as anatomically.