Release of insulin produced by the choroid plexis is regulated by serotonergic signaling
Caio Henrique Mazucanti, Qing-Rong Liu, Doyle Lang, Nicholas Huang, Jennifer F. O’Connell, Simonetta Camandola, Josephine M. Egan
Caio Henrique Mazucanti, Qing-Rong Liu, Doyle Lang, Nicholas Huang, Jennifer F. O’Connell, Simonetta Camandola, Josephine M. Egan
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Research Article Endocrinology Neuroscience

Release of insulin produced by the choroid plexis is regulated by serotonergic signaling

Abstract

The choroid plexus (ChP) is a highly vascularized tissue found in the brain ventricles, with an apical epithelial cell layer surrounding fenestrated capillaries. It is responsible for the production of most of the cerebrospinal fluid (CSF) in the ventricular system, subarachnoid space, and central canal of the spinal cord, while also constituting the blood-CSF barrier (BCSFB). In addition, epithelial cells of the ChP (EChP) synthesize neurotrophic factors and other signaling molecules that are released into the CSF. Here, we show that insulin is produced in EChP of mice and humans, and its expression and release are regulated by serotonin. Insulin mRNA and immune-reactive protein, including C-peptide, are present in EChP, as detected by several experimental approaches, and appear in much higher levels than any other brain region. Moreover, insulin is produced in primary cultured mouse EChP, and its release, albeit Ca2+ sensitive, is not regulated by glucose. Instead, activation of the 5HT2C receptor by serotonin treatment led to activation of IP3-sensitive channels and Ca2+ mobilization from intracellular storage, leading to insulin secretion. In vivo depletion of brain serotonin in the dorsal raphe nucleus negatively affected insulin expression in the ChP, suggesting an endogenous modulation of ChP insulin by serotonin. Here, we show for the first time to our knowledge that insulin is produced by EChP in the brain, and its release is modulated at least by serotonin but not glucose.

Authors

Caio Henrique Mazucanti, Qing-Rong Liu, Doyle Lang, Nicholas Huang, Jennifer F. O’Connell, Simonetta Camandola, Josephine M. Egan

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Figure 5

CSF serotonin induces choroid plexus insulin secretion in a Ca2+-dependent way.

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CSF serotonin induces choroid plexus insulin secretion in a Ca2+-depende...
(A) Levels of insulin released in the culture media were increased by serotonin treatment, while IP3R antagonism had the opposite effect. (Control: mean = 0.02498, SEM = 0.001259; 5HT: mean = 0.03540, SEM = 0.001746; 2APB: mean = 0.01613, SEM = 0.001180; 5HT + 2APB: mean = 0.01775, SEM = 0.0008521; n = 6 independent cell cultures). One-way ANOVA with Tukey’s multiple comparisons test, ***P < 0.001; ****P < 0.0001. (B) Insulin gene expression was also positively affected by serotonin treatment (Ins1: control mean = 1.013, SEM = 0.07246, 5HT mean = 2.268, SEM = 0.3460, n = 6 independent cell cultures, P = 0.0053; Ins2: control mean = 1.006, SEM = 0.04640, 5HT mean = 1.307, SEM = 0.05719, n = 6 independent cell cultures, P = 0.0284). Student’s t test, **P < 0.01. (C) Levels of vesicle-associated proteins were also elevated by serotonin treatment (NSF: control = 1.0 ± 0.08828 SEM, 5HT = 2.272 ± 0.08034 SEM, n = 5 independent cell cultures, P < 0.0001; VAMP2: control = 1.0 ± 0.08333 SEM, 5HT= 1.310 ± 0.06804 SEM, n = 5 independent cell cultures, P = 0.0205; VAMP3: control = 1.0 ± 0.1422 SEM, 5HT = 1.785 ± 0.2997 SEM, n =5 independent cultures, P = 0.0455). Student’s t test, *P < 0.05; ****P < 0.0001. (D and E) Culture of EChP in the transwell system creates 2 isolated compartments, characterized by elevated transepithelial resistance, due to formation of tight junctions between the cells (ZO1, red). (F) 5HTR2C is localized on the apical membrane of the choroid plexus epithelium (green). Scale bar: 10 μm. (G) Serotonergic stimulation of the apical, but not basal, membrane increases insulin concentration in the culture media (control: mean = 9.633 pg/L, SEM = 0.6756; 5HT apical: mean = 15.63 pg/L, SEM = 0.2985; n = 6 independent cell cultures). One-way ANOVA, ****P < 0.0001. (H) Proposed mechanism of serotonergic regulation of insulin secretion in epithelial cells of the choroid plexus.