A choroid plexus apocrine secretion mechanism shapes CSF proteome and embryonic brain development

Background

Have you ever wondered how the developing brain filters environmental cues in the womb during pregnancy? The choroid plexus (ChP), a crucial brain structure producing cerebrospinal fluid (CSF), plays a vital role in this and has intrigued scientists for centuries, from the pioneering observations of anatomists like Thomas Willis in the 17th century [1] to contemporary neuroscience. Still today its complexity remains dazzling.

The choroid plexus is an epithelial structure rich in blood vessels, located within the brain’s ventricular system. Traditionally recognized as the primary site for bulk CSF fluid production, the ChP has garnered increasing attention from the scientific community due to its multifaceted role as both a sensory and dynamic barrier [2]. The ChP serves as a mediator, facilitating the integration of peripheral blood signals with secretory processes. It thereby aids in the maintenance of brain homeostasis and regulation of cell dynamics within the stem cell niche located in the ventricular zone (VZ), which is bathed by the CSF since early embryonic stages. During gestation, the ChP produces protein-rich cerebrospinal fluid (CSF) and matures prior to brain development [3, 4]. It has been shown that ChP dysfunction plays a significant role in the onset of neurodevelopmental disorders [5], however, the question remains what the specific mechanisms are that link an immature ChP to abnormalities in brain development.

In this preprinted work, the authors dissect the contribution of apocrine secretion by ChP epithelial cells to CSF composition. Apocrine secretion refers to the capacity of secretory cells to gather material at their apical ends, subsequently releasing it as extracellular vesicles that bud off from the cell itself. ChP epithelial cells release a variety of materials in the CSF, such as proteins, lipids, mRNA, and miRNA [6-9]. So how could embryonic ChP apocrine secretion physiologically regulate CSF signalling? What is its contribution to embryonic development? Thanks to the research conducted by Courtney and colleagues, we now have a deeper understanding of the mechanisms involved [10].

Key findings

ChP epithelial cell stimulation influences CSF composition through the activation of an apocrine secretion mechanism.

Epithelial secretion is mainly regulated by calcium signaling. For instance, ChP epithelial cells exhibit spontaneous, transient calcium (Ca²⁺) activity, which can be prolonged in response to stimuli such as serotonin (5-HT) [11]. Indeed, the serotonergic receptor 5HT2C is highly expressed by ChP epithelial cells [12]. Maternal administration of the 5HT2C agonist (WAY-161503) can activate ChP at relevant embryonic time points in mice, with apocrine secretion accounting for around 70% of embryonic ChP epithelial cells.

Following maternal WAY-161503 exposure, the authors of this study observed that 123 proteins were now enriched in the CSF, and among them were the Insulin-like growth factor 2 (Igf2) and Sonic hedgehog (Shh). Overall, the role of these proteins could be linked to a variety of biological functions (i.e., cell proliferation, differentiation, and migration, as well as neuronal functions). These changes in CSF composition did not remain confined to the CSF compartment, but provoked downstream alterations in nearby cells, the neural progenitors in the ventricular zone of the cerebral cortex.

CSF content guides neural identity and the pattern of neuronal migration.

The authors went on to investigate the impact of chronically overstimulating apocrine secretion on cerebral cortical development. They administered WAY-161503 subcutaneously to pregnant dams daily, from E12.5 to E16.5, and evaluated excitatory cortical neuron distribution in pups. Surprisingly, cortical excitatory neuronal populations were altered with a shift in the identity of a proportion of deep-layer neurons from Tbr1+ corticothalamic projection neurons to Satb2+ callosal projection neurons. The observed phenotype is likely due to the sensitivity of apical progenitors to CSF composition. These progenitors lie in the ventricular zone (VZ), are in direct contact with the CSF, and divide early to give rise to deep-layer cortical neurons [13, 14], the one mostly affected by chronic overstimulation of apocrine secretion. The observed alterations in the cytoarchitecture of the cerebral cortex were accompanied by several behavioral phenotypes in adult offspring, suggesting a long-lasting effect.

Numerous stimuli prompt an elevation in intracellular calcium levels which stands at the basis of Choroid Plexus (ChP) apocrine secretion.

Not only the activation of serotonergic receptors can activate ChP epithelial apocrine secretion. A variety of maternal stimuli were tested and among these were psychedelics (lysergic acid diethylamide – LSD), temperature-sensitive receptor activation, and maternal illness (e.g., Maternal immune activation – MIA) – they were all able to trigger apocrine secretion. Furthermore, in mice lacking the 5HT2C receptor, ChP apocrine secretion remained unaffected, maintaining the same baseline rate. The stimulation induced by 5HT2C activation correlated with a short elevation in intracellular calcium levels within ChP epithelial cells. The authors speculated whether calcium regulation might serve as a common mechanism triggering apocrine secretion, independently activated by different upstream receptors.

By examining the ChP Cell Atlas for the expression of receptors linked to calcium regulation in epithelial cells, the authors pinpointed Trpm3, a heat-activated transient receptor, as one of the most highly expressed candidate genes. Maternal administration of a Trpm3 agonist during pregnancy led to an increase in embryonic ChP apocrine structures and c-Fos expression, mirroring the downstream effects observed with serotoninergic stimulation. This association between the engagement of a temperature-sensitive receptor and potential modification of CSF composition led the authors to speculate that a similar interaction could play a role in determining long-term neurodevelopmental impairments resulting from hyperthermic episodes during pregnancy and other environmental factors.

Calpain is required for cytoskeletal remodeling and the assembly of the apocrine secretion machinery in ChP epithelial cells.

ChP explant culture experiments were used to identify the key players in apocrine secretion. Calpain, identified as a convergent pathway component for both the mechanisms used to activate ChP secretion, was demonstrated to be required for the formation of secretory structures at the apical side of ChP epithelial cells. Indeed, calpain inhibition, also in stimulating and responding ChP cells, resulted in a machinery assembly failure, confirming its essential role in cytoskeletal remodeling and secretory activity.

Overall, the authors illustrate the dynamic nature of the ChP as a barrier, capable of reacting to diverse environmental cues and adapting CSF composition accordingly (Figure 1). This ability allows the ChP to play a crucial role in regulating neural progenitor differentiation and pattern migration within the brain, thus establishing a connection between the developing ChP and the emergence of neurodevelopmental disorders.

Figure 1. Maternal administration of compounds during pregnancy triggers a domino effect within the embryonic Choroid Plexus (ChP) epithelial cells. This cascade results in the overstimulation of ChP epithelial cells, leading to the increase of intracellular calcium levels and subsequent enhanced apocrine secretion. Higher CSF protein concentration exerts an effect in modulating neurogenesis, thereby influencing early brain development. Inspired by Courtney et al., 2024.

Things we like

The ChP barrier is ideally positioned to receive and communicate CSF signals and therefore act as a gateway allowing environment–brain communication. Moreover, barrier malfunction— especially during critical windows of development—could be the key to understanding the aetiology of many neurodevelopmental disorders. In recent years, the Choroid Plexus has gained attention and different studies have shed light on its diverse functions in promoting brain health. There is a newfound understanding of apocrine secretion which reveals its significant impact on the onset of neurodevelopmental disorders. This represents a cornerstone in our journey toward elucidating the complex interplay between ChP function, CSF dynamics, and neurodevelopmental outcomes, and offers promising prospects for advancing our understanding and treatment of such disorders.

 

Questions for the authors

Do time-dependent fluctuations in CSF ion composition described during embryonic development correlate with changes in the basal rate of apocrine secretion?

How would you rule out any systemic effects resulting from maternal administration of different stimuli, either as an alternative or concurrent to direct stimulation of Choroid Plexus (ChP) epithelial cells, in influencing ChP apocrine secretion?

Can you elaborate on the similarities and differences observed in CSF composition and cortical layering after diverse maternal stimuli?

 

References

1. Herbowski L. The maze of the cerebrospinal fluid discovery. Anat Res Int. 2013;2013:596027. Epub 2014/01/08. doi: 10.1155/2013/596027. PubMed PMID: 24396600; PubMed Central PMCID: PMCPMC3874314.
2. Aburto MR, Cryan JF. Gastrointestinal and brain barriers: unlocking gates of communication across the microbiota-gut-brain axis. Nat Rev Gastroenterol Hepatol. 2024. Epub 2024/02/15. doi: 10.1038/s41575-023-00890-0. PubMed PMID: 38355758.
3. Lun MP, Monuki ES, Lehtinen MK. Development and functions of the choroid plexus–cerebrospinal fluid system. Nature Reviews Neuroscience. 2015;16(8):445-57. doi: 10.1038/nrn3921.
4. Fame RM, Lehtinen MK. Emergence and Developmental Roles of the Cerebrospinal Fluid System. Dev Cell. 2020;52(3):261-75. Epub 2020/02/13. doi: 10.1016/j.devcel.2020.01.027. PubMed PMID: 32049038.
5. Lehtinen MK, Bjornsson CS, Dymecki SM, Gilbertson RJ, Holtzman DM, Monuki ES. The choroid plexus and cerebrospinal fluid: emerging roles in development, disease, and therapy. J Neurosci. 2013;33(45):17553-9. Epub 2013/11/08. doi: 10.1523/JNEUROSCI.3258-13.2013. PubMed PMID: 24198345; PubMed Central PMCID: PMCPMC3818536.
6. Grapp M, Wrede A, Schweizer M, Huwel S, Galla HJ, Snaidero N, et al. Choroid plexus transcytosis and exosome shuttling deliver folate into brain parenchyma. Nat Commun. 2013;4:2123. Epub 2013/07/06. doi: 10.1038/ncomms3123. PubMed PMID: 23828504.
7. Tietje A, Maron KN, Wei Y, Feliciano DM. Cerebrospinal fluid extracellular vesicles undergo age dependent declines and contain known and novel non-coding RNAs. PLoS One. 2014;9(11):e113116. Epub 2014/11/25. doi: 10.1371/journal.pone.0113116. PubMed PMID: 25420022; PubMed Central PMCID: PMCPMC4242609.
8. Balusu S, Van Wonterghem E, De Rycke R, Raemdonck K, Stremersch S, Gevaert K, et al. Identification of a novel mechanism of blood-brain communication during peripheral inflammation via choroid plexus-derived extracellular vesicles. EMBO Mol Med. 2016;8(10):1162-83. Epub 2016/09/07. doi: 10.15252/emmm.201606271. PubMed PMID: 27596437; PubMed Central PMCID: PMCPMC5048366.
9. Thompson AG, Gray E, Mager I, Fischer R, Thezenas ML, Charles PD, et al. UFLC-Derived CSF Extracellular Vesicle Origin and Proteome. Proteomics. 2018;18(24):e1800257. Epub 2018/11/10. doi: 10.1002/pmic.201800257. PubMed PMID: 30411858.
10. Courtney Ye, Head JP, Yimer ED, Dani N, Shipley FB, Libermann TA, et al. 2024. doi: 10.1101/2024.01.08.574486.
11. Watson JA, Elliott AC, Brown PD. Serotonin elevates intracellular Ca2+ in rat choroid plexus epithelial cells by acting on 5-HT2C receptors. Cell Calcium. 1995;17(2):120-8. Epub 1995/02/01. doi: 10.1016/0143-4160(95)90081-0. PubMed PMID: 7736561.
12. Dani N, Herbst RH, McCabe C, Green GS, Kaiser K, Head JP, et al. A cellular and spatial map of the choroid plexus across brain ventricles and ages. Cell. 2021;184(11):3056-74 e21. Epub 2021/05/02. doi: 10.1016/j.cell.2021.04.003. PubMed PMID: 33932339; PubMed Central PMCID: PMCPMC8214809.
13. Tabata H, Kanatani S, Nakajima K. Differences of Migratory Behavior between Direct Progeny of Apical Progenitors and Basal Progenitors in the Developing Cerebral Cortex. Cerebral Cortex. 2009;19(9):2092-105. doi: 10.1093/cercor/bhn227.
14. Greig LC, Woodworth MB, Galazo MJ, Padmanabhan H, Macklis JD. Molecular logic of neocortical projection neuron specification, development and diversity. Nat Rev Neurosci. 2013;14(11):755-69. Epub 2013/10/10. doi: 10.1038/nrn3586. PubMed PMID: 24105342; PubMed Central PMCID: PMCPMC3876965.

 

 

 

 

Tags: brain development, choroid plexus, prelights peer support

Posted on: 12 March 2024 , updated on: 22 March 2024

doi: https://doi.org/10.1242/prelights.36776

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