Molecular imaging of the association between serotonin degeneration and beta-amyloid deposition in mild cognitive impairment

BACKGROUND

Degeneration of the serotonin system has been observed in Alzheimer's disease (AD) and in mild cognitive impairment (MCI). In transgenic amyloid mouse models, serotonin degeneration is detected prior to widespread cortical beta-amyloid (Aβ) deposition, also suggesting that serotonin degeneration may be observed in preclinical AD.

METHODS

The differences in the distribution of serotonin degeneration (reflected by the loss of the serotonin transporter, 5-HTT) relative to Aβ deposition was measured with positron emission tomography in a group of individuals with MCI and a group of healthy older adults. A multi-modal partial least squares (mmPLS) algorithm was applied to identify the spatial covariance pattern between 5-HTT availability and Aβ deposition.

RESULTS

Forty-five individuals with MCI and 35 healthy older adults were studied, 22 and 27 of whom were included in the analyses who were "amyloid positive" and "amyloid negative", respectively. A pattern of lower cortical, subcortical and limbic 5-HTT availability and higher cortical Aβ deposition distinguished the MCI from the healthy older control participants. Greater expression of this pattern was correlated with greater deficits in memory and executive function in the MCI group, not in the control group.

CONCLUSION

A spatial covariance pattern of lower 5-HTT availability and Aβ deposition was observed to a greater extent in an MCI group relative to a control group and was associated with cognitive impairment in the MCI group. The results support the application of mmPLS to understand the neurochemical changes associated with Aβ deposition in the course of preclinical AD.

Positron emission tomography imaging of serotonin degeneration and beta-amyloid deposition in late-life depression evaluated with multi-modal partial least squares

Depression in late-life is associated with increased risk of cognitive decline and development of all-cause dementia. The neurobiology of late-life depression (LLD) may involve both neurochemical and neurodegenerative mechanisms that are common to depression and dementia. Transgenic amyloid mouse models show evidence of early degeneration of monoamine systems. Informed by these preclinical data, the hypotheses were tested that a spatial covariance pattern of higher beta-amyloid (Aβ) and lower serotonin transporter availability (5-HTT) in frontal, temporal, and parietal cortical regions would distinguish LLD patients from healthy controls and the expression of this pattern would be associated with greater depressive symptoms. Twenty un-medicated LLD patients who met DSM-V criteria for major depression and 20 healthy controls underwent PET imaging with radiotracers for Aβ ([11C]-PiB) and 5-HTT ([11C]-DASB). A voxel-based multi-modal partial least squares (mmPLS) algorithm was applied to the parametric PET images to determine the spatial covariance pattern between the two radiotracers. A spatial covariance pattern was identified, including higher Aβ in temporal, parietal and occipital cortices associated with lower 5-HTT in putamen, thalamus, amygdala, hippocampus and raphe nuclei (dorsal, medial and pontine), which distinguished LLD patients from controls. Greater expression of this pattern, reflected in summary 5-HTT/Aβ mmPLS subject scores, was associated with higher levels of depressive symptoms. The mmPLS method is a powerful approach to evaluate the synaptic changes associated with AD pathology. This spatial covariance pattern should be evaluated further to determine whether it represents a biological marker of antidepressant treatment response and/or cognitive decline in LLD patients.

Discovery and Preclinical Characterization of Usmarapride (SUVN-D4010): A Potent, Selective 5-HT4 Receptor Partial Agonist for the Treatment of Cognitive Deficits Associated with Alzheimer's Disease

A series of oxadiazole derivatives were synthesized and evaluated as 5-hydroxytryptamine-4 receptor (5-HT₄R) partial agonists for the treatment of cognitive deficits associated with Alzheimer's disease. Starting from a reported 5-HT₄R antagonist, a systematic structure-activity relationship was conducted, which led to the discovery of potent and selective 5-HT₄R partial agonist 1-isopropyl-3-{5-[1-(3-methoxypropyl) piperidin-4-yl]-[1,3,4]oxadiazol-2-yl}-1H-indazole oxalate (Usmarapride, 12l). It showed balanced physicochemical-pharmacokinetic properties with robust nonclinical efficacy in cognition models. It also showed disease-modifying potential, as it increased neuroprotective soluble amyloid precursor protein alpha levels, and dose-dependent target engagement and correlation of efficacy with oral exposures. Phase 1 clinical studies have been completed and projected efficacious concentration was achieved without any major safety concerns. Phase 2 enabling long-term safety studies have been completed with no concerns for further development.

Synthesis, Structure-Activity Relationships, and Preclinical Evaluation of Heteroaromatic Amides and 1,3,4-Oxadiazole Derivatives as 5-HT4 Receptor Partial Agonists

Alzheimer's disease (AD) is a neurodegenerative disorder that has a higher prevalence and incidence in people older than 60 years. The need for improved AD therapies is unmet as the current therapies are symptomatic with modest efficacy. Partial agonists of the 5-HT₄ receptor (5-HT₄R) offer both symptomatic and disease-modifying treatments as they shift amyloid-precursor-protein (APP) processing from the amyloidogenic pathway to the nonamyloidogenic pathway by activating the α-secretase enzyme. In addition, they also offer symptomatic treatment by increasing levels of the neurotransmitter acetylcholine in the brain. Because of this fascinating dual mechanism of action, several chemical scaffolds having 5-HT₄R pharmacophores were designed and evaluated. Most of the synthesized compounds showed potent in vitro affinities and in vivo efficacies. Upon analysis of focused structure-activity relationships, compound 4o was identified as a potent 5-HT₄R partial agonist with favorable ADME properties and good in vivo efficacy. GR-125487, a selective 5-HT₄R antagonist, attenuated the activity of compound 4o in the novel-object-recognition-test cognition model.

Co-modulation of an allosteric modulator of nicotinic receptor-cholinesterase inhibitor (galantamine) and a 5-HT4 receptor agonist (RS-67333): effect on scopolamine-induced memory deficit in the mouse

AIM

It is widely assumed that the upcoming therapeutics for Alzheimer's disease will require to act on more than one target to be effective. We investigated here whether a combination of the nicotinic receptor allosteric modulator/cholinesterase inhibitor galantamine can act synergistically with the type 4 serotonin receptor (5-HT4R) partial agonist, RS-67333, to counterbalance deficits in short- and long-term memory. To select sub-efficacious doses of both drugs, dose-response studies were first performed on the scopolamine-induced deficits of spontaneous alternation in the Y-maze task and of acquisition and retrieval processes in a passive avoidance task.

RESULT

For spontaneous alternation behavior, combination of 1 mg/kg galantamine and 0.5 mg/kg RS-67333 fully reversed the deficit. In the passive avoidance task, no sub-efficacious doses could be found in the retention paradigm, but a beneficial effect of the association has been demonstrated in the acquisition paradigm.

CONCLUSION

Mnesic effects of galantamine can be thus potentiated by activation of 5-HT4R. Such a combination treatment might (1) strengthen symptomatic relief, (2) attenuate adverse effects given the lower doses of each compound required, and (3) afford a disease-modifying effect given the known action of 5-HT4R on amyloidogenesis cascade.

Tailoring of membrane proteins by alternative splicing of pre-mRNA

Alternative splicing (AS) of RNA is a key mechanism for diversification of the eukaryotic proteome. In this process, different mRNA transcripts can be produced through altered excision and/or inclusion of exons during processing of the pre-mRNA molecule. Since its discovery, AS has been shown to play roles in protein structure, function, and localization. Dysregulation of this process can result in disease phenotypes. Moreover, AS pathways are promising therapeutic targets for a number of diseases. Integral membrane proteins (MPs) represent a class of proteins that may be particularly amenable to regulation by alternative splicing because of the distinctive topological restraints associated with their folding, structure, trafficking, and function. Here, we review the impact of AS on MP form and function and the roles of AS in MP-related disorders such as Alzheimer's disease.

Alzheimer culprits: cellular crossroads and interplay

Alzheimer's disease (AD) is the primary cause of dementia in the elderly and one of the major health problems worldwide. Since its first description by Alois Alzheimer in 1907, noticeable but insufficient scientific comprehension of this complex pathology has been achieved. All the research that has been pursued takes origin from the identification of the pathological hallmarks in the forms of amyloid-β (Aβ) deposits (plaques), and aggregated hyperphosphorylated tau protein filaments (named neurofibrillary tangles). Since this discovery, many hypotheses have been proposed to explain the origin of the pathology. The "amyloid cascade hypothesis" is the most accredited theory. The mechanism suggested to be one of the initial causes of AD is an imbalance between the production and the clearance of Aβ peptides. Therefore, Amyloid Precursor Protein (APP) synthesis, trafficking and metabolism producing either the toxic Aβ peptide via the amyloidogenic pathway or the sAPPα fragment via the non amyloidogenic pathway have become appealing subjects of study. Being able to reduce the formation of the toxic Aβ peptides is obviously an immediate approach in the trial to prevent AD. The following review summarizes the most relevant discoveries in the field of the last decades.

Identification of alternatively spliced multiple transcripts of 5-hydroxytryptamine receptor in mouse

5-Hydroxytryptamine receptors (HTRs) are coded by seventeen different genes in mouse. One of them is htr4 that codes for the HTR4 receptor, a G-protein coupled receptor containing seven transmembrane domains. In mouse, the gene is reported to contain 6 exons and 5 introns. Our present study reports the presence of four transcript variants of this gene encoding different N-termini. These transcripts are expressed in neuronal as well as non-neuronal tissues of mouse. We have identified five novel coding exons present at the 5' end of the gene which splice with the published internal exon in an alternative manner making a total of five transcripts, four new transcript variants (T1, T2l, T2s and T3) and one published earlier. All five transcripts encoding different N-termini were expressed in mouse brain. It was interesting to note the expression of only T3 transcript that was also detected in heart muscle and is the only htr4 transcript expressed in heart. For the first time a transcript of htr4 gene was detected in the heart of the mouse which might help us to make use of small laboratory animals to study HTR4 in heart. As this transcript is unique to the heart it can serve as potential therapeutic target for various cardiovascular disorders and dysregulation of heart rate, atrial contraction and atrial relaxation. These variants display heterogeneous properties in terms of the presence of signal peptide, acetylation, phosphorylation and glycosylation. Thus alternative splicing of htr4 producing heterogeneous N-termini increases the diversity of the receptor.

G protein-coupled receptors, cholinergic dysfunction, and Abeta toxicity in Alzheimer's disease

The beta-amyloid (Abeta) peptide is associated with the pathogenesis of Alzheimer's disease (AD). Evidence gathered over the last two decades suggests that the gradual accumulation of soluble and insoluble Abeta peptide species triggers a cascade of events that leads to the clinical manifestation of AD. Abeta accumulation has also been associated with the cholinergic dysfunction observed in AD, which is characterized by diminished acetylcholine release and impaired coupling of the muscarinic acetylcholine receptors (mAChRs) to heterotrimeric GTP-binding proteins (G proteins). Although the mechanism of Abeta-mediated toxicity is not clearly understood, evidence shows that Abeta accumulation has an effect on the oligomerization of the angiotensin II (AngII) AT(2) (angiotensin type 2) receptor and sequestration of the Galpha(q/11) family of G proteins. Sequestration of Galpha(q/11) results in dysfunctional coupling and signaling between M(1) mAChR and Galpha(q/11) and accompanies neurodegeneration, tau phosphorylation, and neuronal loss in an AD transgenic mouse model. Collectively, these results provide a putative link among Abeta toxicity, AT(2) receptor oligomerization, and disruption of the signaling pathway through M(1) mAChR and Galpha(q/11) and potentially contribute to our understanding of the cholinergic deficit observed in AD.

Alternative splicing and exon duplication generates 10 unique porcine 5-HT4 receptor splice variants including a functional homofusion variant

5-HT4 receptors are present in human and porcine atrial myocytes while they are absent from the hearts of small laboratory animals. The pig is therefore the only available nonprimate animal model in which to study cardiac 5-HT4 receptor function under physiological conditions. While several human splice variants of the 5-HT4 receptor have been described, the splicing behavior of this receptor in porcine tissue is currently unknown. Here we report on the identification of nine novel COOH-terminal splice variants of the porcine 5-HT4 receptor, which were named 5-HT4(b2, j, k, l, m, o, p, q, r). The internal h-variant was found in combination with several COOH-terminal exons. In addition, splice variants were found that comprised duplicated exons fused to the common region of the 5-HT4 receptor, thereby providing evidence for a duplication of the porcine HTR4 gene. One of these variants putatively encoded a nine transmembrane-spanning domain homofusion receptor, 5-HT4(9TM); also the other variants with a duplicated region might translate into functional, transcriptionally fused dimeric 5-HT4 receptor variants. The elucidation of the genomic context confirmed that the variants were not genomic artefacts but originated from alternative splicing. This was further corroborated by a functional analysis of the variants 5-HT4(a), 5-HT4(r), and 5-HT4(9TM). To our knowledge, our data are the first to report on a functional GPCR with more than seven predicted transmembrane domains. These findings urge for caution when interpreting data on 5-HT4 receptor-related pharmacology obtained in the pig; validation at the molecular level might be needed before extrapolating results to human.