Is the choroid plexus an alternative source of prolactin to the central nervous system Impact on neurogenesis

Funder

Organizational Unit

Publications

The Choroid Plexus as a source and target of Prolactin in the brain

Publication . Brito, Ana Raquel Costa; Santos, Cecília Reis Alves; Gonçalves, Isabel Maria Theriaga Mendes Varanda

The choroid plexuses (CPs) are composed of a single layer of cuboid epithelial cells laying on highly irrigated connective tissue. In addition to being the main producer of cerebrospinal fluid (CSF) in the central nervous system, the CPs are also responsible for the production and secretion of peptides that modulate brain function. CP-derived peptides have been associated with several physiological functions, including inflammation and immune response, signaling, cell growth and cell proliferation, cell death, metabolism, and angiogenesis, among others. Proteins present in the CP secretome have been implicated in the modulation of neurogenesis during developmental stages and adult life. The identification of new neurogenic factors is highly relevant considering the increasing ageing of the population and the incidence of neurodegenerative disorders. Preliminary data from a rat CP microarrays study from our research group suggested local expression of several factors with the potential to promote neurogenesis that had never been associated with the CP before. One of the most relevant transcripts identified was prolactin. Amongst the numerous distinct biological functions attributed to prolactin, this hormone has been described as a neurogenic factor at specific stages of pregnancy and lactation in rodents. In addition, the presence of prolactin transcripts was higher in CPs collected from female rats than male rats, suggesting that sex hormones could modulate prolactin expression in the CP. The first aim of this thesis was to investigate if the rat CP could indeed be an alternative source of prolactin to the brain. In Chapter 4 we provide evidence that prolactin transcripts were present in pregnant rat CP, CP epithelial cells (CPEC) and in the rat immortalized CP cell line, Z310. Furthermore, a 63 kDa immunoreactive PRL was detected by Western blot in CP protein extracts as well as in culture medium supernatants after the incubation with rat pituitary and samples of rat cerebrospinal fluid and serum. Moreover, prolactin immunoreactive protein was present in both compartments of the blood-CSF barrier model which may indicate that CPEC can secrete prolactin not only through the apical membrane facing the CSF but also through the basal membrane facing the blood. To ascertain the possible influence of sex hormones in the production of prolactin in the rat CP as initially hypothesized, we performed 24-hour incubations of CP explants with different concentrations of estradiol, progesterone, or dihydrotestosterone. Nonetheless, neither of the hormones seem to modulate the levels of 63 kDa prolactin in the CP tissues, at least at the concentrations tested in this work, as described in Chapter 5. Besides being a source of prolactin, the CP is the brain structure with the higher expression of prolactin receptors (PRLR). The high expression of PRLR at the CP was initially associated with the existence of a receptor-mediated mechanism responsible for prolactin transport to the brain present. Nevertheless, considering recent evidence, it is now accepted that prolactin uptake is independent of its receptors. Information about the exact function of prolactin in the CP is still very scarce, especially in the postnatal stages. As so, another main goal of this thesis was to evaluate the effects of prolactin in the transcriptome of postnatal rat CP. In Chapter 6 we observed that prolactin exposure was associated with a reduction in neurogenesis-factor osteopontin, barrier protein claudin 5, and proliferation-related cyclin D1. On the other hand, prolactin incubation also led to an increase in pro-inflammatory interleukin 1 beta expression, suggesting that at a postnatal stage, prolactin exposure may increase the CP permeability, reduce the cellular proliferation and have a proinflammatory effect on the rat CP. Another goal of this thesis was to identify additional neurogenic factors secreted by the CP. In Chapter 7 we provide evidence that the rat CP may be a source of secreted frizzled-related protein 2 (SFRP2), previously described as a regulator of neural stem cells proliferation, differentiation and homeostasis. Notably, estradiol seems to modulate the levels of SFRP2 in the rat CP. In summary, the evidence reported throughout this work supports the relevance of the CP as a source of new peptides with the potential to modulate brain function, like prolactin and SFRP2. Further studies are necessary to understand the relevance of CP-derived prolactin and SFRP2 in brain function, especially in neurogenesis.

2023-02Doctoral thesis

Open access