'Smelling' the cerebrospinal fluid: olfactory signaling molecules are expressed in and mediate chemosensory signaling from the choroid plexus

Olfactory Receptor OR2K2 Expression in Human Choroid Plexus as a Potential Marker in Early Sporadic Alzheimer's Disease

Epithelial cells comprising the choroid plexus (CP) form a crucial barrier between the blood and the cerebrospinal fluid, thereby assuming a central position in brain homeostasis and signaling. Mounting evidence suggests that the impairment of CP function may be a significant contributor to Alzheimer's disease (AD) pathogenesis. CP function relies on the expression of specific receptors, and the potential involvement of olfactory receptors (ORs) and taste receptors (TASRs) in chemical surveillance within the CP is being investigated. Previous studies have implicated ORs and TASRs in neurodegenerative disorders like AD, although the direct evidence of their expression in the human CP remains to be established. In this study, we conducted a transcriptomic analysis encompassing eleven ORs and TASRs in the CP, comparing samples from healthy age-matched controls to those from patients with AD spanning Braak stages I to VI. Among these receptors, a striking finding emerged-OR2K2 exhibited robust expression, with a statistically significant upregulation noted at Braak stage I. Surprisingly, at the protein level, OR2K2 showed a significant decrease in both Braak stage I and VI. Additionally, we identified CP epithelial cells as the source of OR2K2 expression, where it colocalized with autophagy markers LC3 and p62. We postulate that OR2K2 could be subjected to degradation by autophagy in the early stages of AD, triggering a compensatory mechanism that leads to increased OR2K2 mRNA transcription. This study uncovers a potential role for OR2K2 in AD pathogenesis, offering a novel perspective on the intricate dynamics at play in this neurodegenerative disorder.

Circadian rhythmicity of amyloid-beta-related molecules is disrupted in the choroid plexus of a female Alzheimer's disease mouse model

The choroid plexus (CP) is part of the blood-cerebrospinal fluid barrier (BCSFB) and was recently described as an important component of the circadian clock system. It is the principal source of cerebrospinal fluid (CSF) and responsible for the synthesis and secretion of various neuroprotective peptides including those involved in amyloid-β (Aβ) transport/degradation, contributing to Aβ homeostasis. Inadequate Aβ metabolic clearance and transport across the BCSFB have been associated with circadian dysfunctions in Alzheimer's disease (AD) patients. To investigate whether AD pathology influences Aβ scavengers circadian expression, we collected CP at different time points from an AD mouse model (APP/PS1) (female and male animals, aged 6- and 12-months-old) and analyzed their mRNA expression by Real-time RT-PCR. Only angiotensin-converting enzyme (Ace) expression in 6-month-old female wild-type mice and transthyretin (Ttr) expression in 12-month-old female wild-type mice presented significant rhythmicity. The circadian rhythmicity of Ace and Ttr, prompt us to analyze the involvement of circadian rhythm in Aβ uptake. A human CP papilloma (HIBCPP) cell line was incubated with Aβ-488 and uptake was evaluated at different time points using flow cytometry. Aβ uptake displayed circadian rhythmicity. Our results suggest that AD might affect Aβ scavengers rhythmicity and that Aβ clearance is a rhythmic process possibly regulated by the rhythmic expression of Aβ scavengers.

The choroid plexus: a missing link in our understanding of brain development and function

Studies of the choroid plexus lag behind those of the more widely known blood-brain barrier, despite a much longer history. This review has two overall aims. The first is to outline long-standing areas of research where there are unanswered questions, such as control of cerebrospinal fluid (CSF) secretion and blood flow. The second aim is to review research over the past 10 years where the focus has shifted to the idea that there are choroid plexuses located in each of the brain's ventricles that make specific contributions to brain development and function through molecules they generate for delivery via the CSF. These factors appear to be particularly important for aspects of normal brain growth. Most research carried out during the twentieth century dealt with the choroid plexus, a brain barrier interface making critical contributions to the composition and stability of the brain's internal environment throughout life. More recent research in the twenty-first century has shown the importance of choroid plexus-generated CSF in neurogenesis, influence of sex and other hormones on choroid plexus function, and choroid plexus involvement in circadian rhythms and sleep. The advancement of technologies to facilitate delivery of brain-specific therapies via the CSF to treat neurological disorders is a rapidly growing area of research. Conversely, understanding the basic mechanisms and implications of how maternal drug exposure during pregnancy impacts the developing brain represents another key area of research.

Deconstructing the functional neuroanatomy of the choroid plexus: an ontogenetic perspective for studying neurodevelopmental and neuropsychiatric disorders

The choroid plexus (CP) is a delicate and highly vascularized structure in the brain comprised of a dense network of fenestrated capillary loops that help in the synthesis, secretion and circulation of cerebrospinal fluid (CSF). This unique neuroanatomical structure is comprised of arachnoid villi stemming from frond-like surface projections-that protrude into the lumen of the four cerebral ventricles-providing a key source of nutrients to the brain parenchyma in addition to serving as a 'sink' for central nervous system metabolic waste. In fact, the functions of the CP are often described as being analogous to those of the liver and kidney. Beyond forming a barrier/interface between the blood and CSF compartments, the CP has been identified as a modulator of leukocyte trafficking, inflammation, cognition, circadian rhythm and the gut brain-axis. In recent years, advances in molecular biology techniques and neuroimaging along with the use of sophisticated animal models have played an integral role in shaping our understanding of how the CP-CSF system changes in relation to the maturation of neural circuits during critical periods of brain development. In this article we provide an ontogenetic perspective of the CP and review the experimental evidence implicating this structure in the pathophysiology of neurodevelopmental and neuropsychiatric disorders.

The role of circadian rhythm in choroid plexus functions

For several years, a great effort has been devoted to understand how circadian oscillations in physiological processes are determined by the circadian clock system. This system is composed by the master clock at the suprachiasmatic nucleus which sets the pace and tunes peripheral clocks in several organs. It was recently demonstrated that the choroid plexus epithelial cells that compose the blood-cerebrospinal fluid barrier hold a circadian clock which might control their multiple functions with implications for the maintenance of brain homeostasis. However, the choroid plexus activities regulated by its inner clock are still largely unknown. In this review, we propose that several choroid plexus functions might be regulated by the circadian clock, alike in other tissues. We provide evidences that the timing of cerebrospinal fluid secretion, clearance of amyloid-beta peptides and xenobiotics, and the barrier function of the blood-cerebrospinal fluid barrier are regulated by the circadian clock. These data, highlight that the circadian regulation of the blood-cerebrospinal fluid barrier must be taken into consideration for enhancing drug delivery to central nervous system disorders.

Limbic Expression of mRNA Coding for Chemoreceptors in Human Brain-Lessons from Brain Atlases

Animals strongly rely on chemical senses to uncover the outside world and adjust their behaviour. Chemical signals are perceived by facial sensitive chemosensors that can be clustered into three families, namely the gustatory (TASR), olfactory (OR, TAAR) and pheromonal (VNR, FPR) receptors. Over recent decades, chemoreceptors were identified in non-facial parts of the body, including the brain. In order to map chemoreceptors within the encephalon, we performed a study based on four brain atlases. The transcript expression of selected members of the three chemoreceptor families and their canonical partners was analysed in major areas of healthy and demented human brains. Genes encoding all studied chemoreceptors are transcribed in the central nervous system, particularly in the limbic system. RNA of their canonical transduction partners (G proteins, ion channels) are also observed in all studied brain areas, reinforcing the suggestion that cerebral chemoreceptors are functional. In addition, we noticed that: (i) bitterness-associated receptors display an enriched expression, (ii) the brain is equipped to sense trace amines and pheromonal cues and (iii) chemoreceptor RNA expression varies with age, but not dementia or brain trauma. Extensive studies are now required to further understand how the brain makes sense of endogenous chemicals.

Increased dopamine transmission and adult neurogenesis in trace amine-associated receptor 5 (TAAR5) knockout mice

Trace amine-associated receptors (TAARs) are a class of sensory G protein-coupled receptors that detect biogenic amines, products of decarboxylation of amino acids. The majority of TAARs (TAAR2-TAAR9) have been described mainly in the olfactory epithelium and considered to be olfactory receptors sensing innate odors. However, there is recent evidence that one of the members of this family, TAAR5, is expressed also in the limbic brain areas receiving projection from the olfactory system and involved in the regulation of emotions. In this study, we further characterized a mouse line lacking TAAR5 (TAAR5 knockout, TAAR5-KO mice) that express beta-galactosidase mapping TAAR5 expression. We found that in TAAR5-KO mice the number of dopamine neurons, the striatal levels of dopamine and its metabolites, as well as striatal levels of GDNF mRNA, are elevated indicating a potential increase in dopamine neuron proliferation. Furthermore, an analysis of TAAR5 beta-galactosidase expression revealed that TAAR5 is present in the major neurogenic areas of the brain such as the subventricular zone (SVZ), the subgranular zone (SGZ) and the less characterized potentially neurogenic zone surrounding the 3rd ventricle. Direct analysis of neurogenesis by using specific markers doublecortin (DCX) and proliferating cell nuclear antigen (PCNA) revealed at least 2-fold increase in the number of proliferating neurons in the SVZ and SGZ of TAAR5-KO mice, but no such markers were detected in mutant or control mice in the areas surrounding the 3rd ventricle. These observations indicate that TAAR5 involved not only in regulation of emotional status but also adult neurogenesis and dopamine transmission. Thus, future TAAR5 antagonists may exert not only antidepressant and/or anxiolytic action but may also provide new treatment opportunity for neurodegenerative disorders such as Parkinson's disease.

Cadaverine and Spermine Elicit Ca2+ Uptake in Human CP Cells via a Trace Amine-Associated Receptor 1 Dependent Pathway

The choroid plexus (CP) constitutes a barrier between the blood and the cerebrospinal fluid (CSF) which regulates the exchange of substances between these two fluids through mechanisms that are not completely understood. Polyamines as spermine, spermidine and putrescine are produced by all cells and are present in the CSF. Interestingly, their levels are altered in some neuronal disorders as Alzheimer's and Parkinson's diseases, thus increasing the interest in their signalling in the central nervous system (CNS). Cadaverine, on the other hand, is synthetized by the intestinal microbiome, suggesting that the presence of this bacterial metabolite in the CSF requires that it is up taken to the CNS across brain barriers. We knew that polyamines are detected by the olfactory signalling cascade operating at the CP, but the receptor involved had not been identified. The zebrafish TAAR13c was the only receptor known to bind a polyamine-cadaverine. Thus, we searched for a human receptor with homology to TAAR13c and found that some human TAARs including TAAR1 showed great homology. Then, we confirmed the expression of TAAR1 mRNA and protein in a human cell line of the CP, and in human CP samples. Calcium imaging assays after TAAR1 knockdown in these cells with a specific siRNA against TAAR1 showed a consistent reduction in the responses of these cells to cadaverine and spermidine, but not to spermine, suggesting that TAAR1 is activated by cadaverine and spermidine, but not spermine.

Bitter taste receptors profiling in the human blood-cerebrospinal fluid-barrier

The choroid plexus (CP) epithelial cells establish an important blood-brain interface, the blood-cerebrospinal fluid barrier (BCSFB), which constitutes a complementary gateway to the blood-brain-barrier for the entrance of several molecules into the central nervous system (CNS). However, the mechanisms that operate at the BCSFB to regulate the molecular traffic are still poorly understood. The taste signalling machinery, present in many extra-oral tissues, is involved in the chemical sensing of the composition of body fluids. We have identified this pathway in rat CP and hypothesised that it could also be present in the human BCSFB. In this study, we characterised the bitter taste receptors (TAS2Rs) expression profiling in human CP by combining data retrieved from available databases of the human CP transcriptome with its expression analysis in a human CP cell line and immunohistochemistry of human CP sections from men and women. TAS2R4, 5, 14 and 39 expression was confirmed in human CP tissue by immunohistochemistry and in HIBCPP cells by RT-PCR, immunofluorescence and Western blot. Moreover, the presence of downstream effector proteins GNAT3, PLCβ2 and TRPM5 was also detected in HIBCPP cells. Then, we demonstrated that HIBCPP cells respond to chloramphenicol via TAS2R39 and to quercetin via TAS2R14. Our findings support an active role of TAS2Rs at the human BCSFB, as surveyors of the bloodstream and CSF compositions. These findings open new avenues for studies on the uptake of relevant compounds for targeted therapies of the CNS.

The Rhythmicity of Clock Genes is Disrupted in the Choroid Plexus of the APP/PS1 Mouse Model of Alzheimer's Disease

BACKGROUND

The choroid plexus (CP), which constitutes the blood-cerebrospinal fluid barrier, was recently identified as an important component of the circadian clock system.

OBJECTIVE

The fact that circadian rhythm disruption is closely associated to Alzheimer's disease (AD) led us to investigate whether AD pathology can contribute to disturbances of the circadian clock in the CP.

METHODS

For this purpose, we evaluated the expression of core-clock genes at different time points, in 6- and 12-month-old female and male APP/PS1 mouse models of AD. In addition, we also assessed the effect of melatonin pre-treatment in vitro before amyloid-β stimulus in the daily pattern of brain and muscle Arnt-like protein 1 (Bmal1) expression.

RESULTS

Our results showed a dysregulation of circadian rhythmicity of Bmal1 expression in female and male APP/PS1 transgenic 12-month-old mice and of Period 2 (Per2) expression in male mice. In addition, a significant circadian pattern of Bmal1 was measured the intermittent melatonin pre-treatment group, showing that melatonin can reset the CP circadian clock.

CONCLUSION

These results demonstrated a connection between AD and the disruption of circadian rhythm in the CP, representing an attractive target for disease prevention and/or treatment.

Bitter taste signaling mediated by Tas2r144 is down-regulated by 17β-estradiol and progesterone in the rat choroid plexus

The blood-cerebrospinal fluid barrier is constituted by choroid plexus epithelial cells (CPEC) that regulate molecular trafficking between the blood and the cerebrospinal fluid. We hypothesize that taste receptors expressed in CPEC monitor the composition of these body fluids in a sex hormone dependent way. Thus, we compared the expression of taste related genes in the choroid plexus of sham and ovariectomized female rats, and then studied the effect of 17β-estradiol and progesterone in their expression and function. We found that the bitter receptors Tas2r109, Tas2r144, and the taste-related genes Plcb2 and Trpm5 were down-regulated by ovarian hormones in vivo and ex vivo with functional implications. Knocking-down Tas2r144 with a specific siRNA in a CPEC line (Z310) effectively reduced the Ca²⁺ response to the bitter compound denatonium benzoate, in a similar manner to female sex hormones alone, suggesting that female sex hormones downregulated the responses of CPEC to chemical stimuli by reducing Tas2r144.

Transcriptome sequencing of olfactory-related genes in olfactory transduction of large yellow croaker (Larimichthy crocea) in response to bile salts

Fish produce and release bile salts as chemical signalling substances that act as sensitive olfactory stimuli. To investigate how bile salts affect olfactory signal transduction in large yellow croaker (Larimichthy crocea), deep sequencing of olfactory epithelium was conducted to analyse olfactory-related genes in olfactory transduction. Sodium cholates (SAS) have typical bile salt chemical structures, hence we used four different concentrations of SAS to stimulate L. crocea, and the fish displayed a significant behavioural preference for 0.30% SAS. We then sequenced olfactory epithelium tissues, and identified 9938 unigenes that were significantly differentially expressed between SAS-stimulated and control groups, including 9055 up-regulated and 883 down-regulated unigenes. Subsequent Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analyses found eight categories linked to the olfactory transduction pathway that was highly enriched with some differentially expressed genes (DEGs), including the olfactory receptor (OR), Adenylate cyclase type 3 (ADCY3) and Calmodulin (CALM). Genes in these categories were analysed by RT-qPCR, which revealed aspects of the pathway transformation between odor detection, and recovery and adaptation. The results provide new insight into the effects of bile salt stimulation in olfactory molecular mechanisms in fishes, and expands our knowledge of olfactory transduction, and signal generation and decline.

Deep Sequencing of the Human Retinae Reveals the Expression of Odorant Receptors

Several studies have demonstrated that the expression of odorant receptors (ORs) occurs in various tissues. These findings have served as a basis for functional studies that demonstrate the potential of ORs as drug targets for a clinical application. To the best of our knowledge, this report describes the first evaluation of the mRNA expression of ORs and the localization of OR proteins in the human retina that set a stage for subsequent functional analyses. RNA-Sequencing datasets of three individual neural retinae were generated using Next-generation sequencing and were compared to previously published but reanalyzed datasets of the peripheral and the macular human retina and to reference tissues. The protein localization of several ORs was investigated by immunohistochemistry. The transcriptome analyses detected an average of 14 OR transcripts in the neural retina, of which OR6B3 is one of the most highly expressed ORs. Immunohistochemical stainings of retina sections localized OR2W3 to the photosensitive outer segment membranes of cones, whereas OR6B3 was found in various cell types. OR5P3 and OR10AD1 were detected at the base of the photoreceptor connecting cilium, and OR10AD1 was also localized to the nuclear envelope of all of the nuclei of the retina. The cell type-specific expression of the ORs in the retina suggests that there are unique biological functions for those receptors.

The choroid plexus as a sex hormone target: Functional implications

The choroid plexuses (CPs) are highly vascularized branched structures that protrude into the ventricles of the brain, and form a unique interface between the blood and the cerebrospinal fluid (CSF). In recent years, novel functions have been attributed to this tissue such as in immune and chemical surveillance of the central nervous system, brain development, adult neurogenesis and circadian rhythm regulation. Sex hormones (SH) are widely recognized as modulators in several neurodegenerative diseases, and there is evidence that estrogens and androgens regulate several fundamental biological functions in the CPs. Therefore, SH are likely to affect the composition of the CSF impacting on brain homeostasis. This review will look at implications of the CPs' sex-related specificities.

The choroid plexus in health and in disease: dialogues into and out of the brain

This article brings the choroid plexus into the context of health and disease. It is remarkable that the choroid plexus, composed by a monolayer of epithelial cells that lie in a highly vascularized stroma, floating within the brain ventricles, gets so little attention in major physiology and medicine text books and in the scientific literature in general. Consider that it is responsible for producing most of the about 150mL of cerebrospinal fluid that fills the brain ventricles and the subarachnoid space and surrounds the spinal cord in the adult human central nervous system, which is renewed approximately 2-3 times daily. As such, its activity influences brain metabolism and function, which will be addressed. Reflect that it contains an impressive number of receptors and transporters, both in the apical and basolateral sides of the epithelial cells, and as such is a key structure for the communication between the brain and the periphery. This will be highlighted in the context of neonatal jaundice, multiple sclerosis and Alzheimer's disease. Realize that the capillaries that irrigate the choroid plexus stroma do not possess tight junctions and that the blood flow to the choroid plexus is five times higher than that in the brain parenchyma, allowing for a rapid sensing system and delivery of molecules such as nutrients and metals as will be revised. Recognize that certain drugs reach the brain parenchyma solely through the choroid plexus epithelia, which has potential to be manipulated in diseases such as neonatal jaundice and Alzheimer's disease as will be discussed. Without further notice, it must be now clear that understanding the choroid plexus is necessary for comprehending the brain and how the brain is modulated and modulates all other systems, in health and in disease. This review article intends to address current knowledge on the choroid plexus, and to motivate the scientific community to consider it when studying normal brain physiology and diseases of the central nervous system. It will guide the reader through several aspects of the choroid plexus in normal physiology, in diseases characteristic of various periods of life (newborns-kernicterus, young adults-multiple sclerosis and the elder-Alzheimer's disease), and how sex-differences may relate to disease susceptibility.