Expression and functions of cluster of differentiation 9 and 81 in rat mammary epithelial cells

Fluctuation of CD9/SOX2-positive cell populations during the turnover of GH- and TSH-producing cells in the adult anterior pituitary gland

The adenohypophysis is comprised of the anterior and intermediate lobes (AL and IL, respectively). Cluster of differentiation 9 (CD9)- and sex-determining region Y-box 2 (SOX2)-positive cells are stem/progenitor hormone-producing cells in the AL. They are located in the marginal cell layer (MCL) facing Rathke's cleft between the AL and IL (primary niche) and the parenchyma of the AL (secondary niche). We previously showed that, in rats, CD9/SOX2-positive cells in the IL side of the MCL (IL-side MCL) migrate to the AL side (AL-side MCL) and differentiate into prolactin-producing cells (PRL cells) in the AL parenchyma during pregnancy, lactation, and diethylstilbestrol treatment, all of which increase PRL cell turnover. This study examined the changes in CD9/SOX2-positive stem/progenitor cell niches and their proportions by manipulating the turnover of growth hormone (GH)- and thyroid-stimulating hormone (TSH)-producing cells (GH and TSH cells, respectively), which are Pit1 lineage cells, as well as PRL cells. After induction, the isolated CD9/SOX2-positive cells from the IL-side MCL formed spheres and differentiated into GH and TSH cells. We also observed an increased GH cell proportion upon treatment with GH-releasing hormone and recovery from continuous stress and an increased TSH cell proportion upon propylthiouracil treatment, concomitant with alterations in the proportion of CD9/SOX2-positive cells in the primary and secondary niches. These findings suggest that CD9/SOX2-positive cells have the potential to supply GH and TSH when an increase in GH and TSH cell populations is required in the adult pituitary gland.

Single-cell and spatial analyses reveal a tradeoff between murine mammary proliferation and lineage programs associated with endocrine cues

Although distinct epithelial cell types have been distinguished in glandular tissues such as the mammary gland, the extent of heterogeneity within each cell type and the degree of endocrine control of this diversity across development are incompletely understood. By combining mass cytometry and cyclic immunofluorescence, we define a rich array of murine mammary epithelial cell subtypes associated with puberty, the estrous cycle, and sex. These subtypes are differentially proliferative and spatially segregate distinctly in adult versus pubescent glands. Further, we identify systematic suppression of lineage programs at the protein and RNA levels as a common feature of mammary epithelial expansion during puberty, the estrous cycle, and gestation and uncover a pervasive enrichment of ribosomal protein genes in luminal cells elicited specifically during progesterone-dominant expansionary periods. Collectively, these data expand our knowledge of murine mammary epithelial heterogeneity and connect endocrine-driven epithelial expansion with lineage suppression.

The multiciliated cells in Rathke's cleft express CYP26A1 and respond to retinoic acid in the pituitary

The adenohypophysis consists of the anterior and intermediate lobes (AL and IL). The marginal cell layer (MCL), including the ventral region of the IL and the dorsal region of the AL lining the Rathke's cleft, acts as the primary stem/progenitor cell niches in adult adenohypophysis. The cells of the MCL on the IL side consisted of cluster of differentiation 9 (CD9)-positive stem/progenitor cells with or without motile cilia. However, any additional cellular properties of multiciliated CD9-positive cells are not known. The present study aimed to identify the character of the multiciliated cells in stem cell niche of the pituitary gland. We observed the fine structure of the multiciliated cells in the MCL of male Wistar rats at an early stage after birth and in adulthood (P60) using scanning electron microscopy. Since the previous study showed that the MCL cells of adult rats synthesize retinoic acid (RA), the present study determined whether the multiciliated cells are involved in RA regulation by the expression of retinal aldehyde dehydrogenase 1 (RALDH1) and CYP26A1, an enzyme synthesizing and degrading RA, respectively. Results showed that 96% of multiciliated cells in adult male rats expressed CYP26A1, while 60% expressed RALDH1. Furthermore, the isolated CD9-positive cells from the IL side MCL responded to RA and activated the degradation system of RA by increasing Cyp26a1 expression. These findings indicated that multiciliated cells are involved in RA metabolism in the MCL. Our observations provide novel insights regarding the stem cell niche of the adult pituitary.

CD9-positive cells in the intermediate lobe migrate into the anterior lobe to supply endocrine cells

The adenohypophysis is composed of the anterior and intermediate lobes (AL and IL), and secretes important hormones for growth, sexual development, metabolism, and reproduction. In the marginal cell layer (MCL) facing Rathke's cleft between the IL and AL, cluster of differentiation (CD) 9-, CD81-, S100β-, and SOX2-quadruple positive (CD9/CD81/S100β/SOX2-positive) cells in the adult IL are settled as tissue-resident stem/progenitor cells supplying hormone-producing cells to the AL. However, it is unclear how CD9/CD81/S100β/SOX2-positive cells in the IL-side MCL migrate into the AL across Rathke's cleft. In the present study, we performed chimeric pituitary tissue culture using S100β/GFP-transgenic rats and Wistar rats, and traced the footprint of S100β/GFP-expressing cells. We detected IL-side S100β/GFP-expressing cells in the AL tissue, demonstrating that these cells migrate from the IL to the AL. However, the cells failed to migrate in the opposite direction. Consistently, scanning electron microscopic analysis revealed well-developed cytoplasmic protrusions in the IL-side MCL, but not in the AL-side MCL, suggesting that IL-side CD9/CD81/S100β/SOX2-positive cells had higher migratory activity. We also searched for a specific marker for IL-side CD9/CD81/S100β/SOX2-positive cells and identified tetraspanin 1 (TSPAN1) from microarray analysis. Downregulation of Tspan1 by specific siRNA impaired cell migration and significantly reduced expression of snail family transcriptional repressor 2 (Slug), a marker of epithelial-mesenchymal transition (EMT). Therefore, CD9/CD81/S100β/SOX2-positive cells in the IL-side MCL can be stem/progenitor cells that provide stem/progenitor cells to the AL-side MCL via SLUG-mediated EMT and cell migration.

CD9-positive cells in the intermediate lobe of the pituitary gland are important supplier for prolactin-producing cells in the anterior lobe

A supply of hormone-producing cells from stem/progenitor cells is critical to sustain the endocrine activity of the pituitary gland. In the adenohypophysis composing the anterior and intermediate lobe (AL and IL, respectively), stem/progenitor cells expressing sex-determining region Y-box 2 (SOX2) and S100β are located in the marginal cell layer (MCL) facing Rathke's cleft (primary niche) and the parenchyma of the AL (secondary niche). Our previous studies using mice and rats indicated that the tetraspanin superfamily CD9 and CD81 are expressed in S100β/SOX2-positive cells of primary and secondary niches (named CD9/CD81/S100β/SOX2-positive cell), and the cells located in the AL-side niches exhibit plasticity and multipotency. However, it is unclear whether CD9/CD81/S100β/SOX2-positive cells in the IL-side primary niche are stem/progenitor cells for the AL or IL. Here, we successfully isolated pure CD9/CD81/S100β/SOX2-positive cells from the IL-side primary niche. They had a higher level of S100β and SOX2 mRNA and a greater pituisphere forming capacity than those of CD9/CD81/S100β/SOX2-positive cells isolated from the AL. They also had capacity to differentiate into all types of adenohypophyseal hormone-producing cells, concomitantly with the loss of CD9 expression. Loss of CD9 and CD81 function in CD9/CD81/S100β/SOX2-positive cells by siRNA treatment impaired prolactin cell differentiation. Consistently, in the pituitary gland of CD9/CD81 double knockout mice, dysgenesis of the MCL and a lower population of prolactin cells were observed. These results suggest that the CD9/CD81/S100β/SOX2-positive cells in the MCL of the IL-side are potential suppliers of adult core stem cells in the AL.

CD9, a tetraspanin target for cancer therapy?

In the present minireview, we intend to provide a brief history of the field of CD9 involvement in oncogenesis and in the metastatic process of cancer, considering its potential value as a tumor-associated antigenic target. Over the years, CD9 has been identified as a favorable prognostic marker or predictor of metastatic potential depending on the cancer type. To understand its implications in cancer beside its use as an antigenic biomarker, it is essential to know its physiological functions, including its molecular partners in a given cell system. Moreover, the discovery that CD9 is one of the most specific and broadly expressed markers of extracellular membrane vesicles, nanometer-sized entities that are released into extracellular space and various physiological body fluids and play a role in intercellular communication under physiological and pathological conditions, notably the establishment of cancer metastases, has added a new dimension to our knowledge of CD9 function in cancer. Here, we will discuss these issues as well as the possible cancer therapeutic implications of CD9, their limitations, and pitfalls.

Cluster of differentiation (CD) 9-positive mouse pituitary cells are adult stem/progenitor cells

SOX2-positive cells are stem/progenitor cells that supply hormone-producing cells; they are found in the anterior lobe of the rodent pituitary gland. However, they are likely composed of several subpopulations. In rats, a SOX2-positive cell populations can be distinguished by the presence of S100β. We identified the novel markers cluster of differentiation (CD) CD9 and CD81, members of the tetraspanin superfamily, for the identification of S100β/SOX2-positive cells. Recently, CD9/CD81 double-knockout mice were generated. Although they grew normally until 3 weeks after birth, they exhibited atrophy of the pituitary gland. These findings suggested that CD9/CD81/S100β/SOX2-positive cells in the mouse pituitary are adult stem/progenitor cells. To substantiate this hypothesis, we examined CD9 and CD81 expression in the adult and developing anterior lobe. Immunohistochemistry showed that CD9/CD81-positive cells began appearing from postnatal day 0 and settled in the stem cell niches (marginal cell layer and parenchyma) of the adult anterior lobe while expressing S100β. We next isolated CD9 -positive cells from the adult anterior lobe, using the anti-CD9 antibody for cell characterisation. The cells in culture formed free-floating three-dimensional clusters (pituispheres); moreover, induction into all types of hormone-producing cells was successful. Furthermore, reduction of CD9 and CD81 mRNAs by siRNAs inhibited cell proliferation. These findings indicate that CD9/CD81/S100β/SOX2-positive cells may play a role as adult stem/progenitor cells in SOX2-positive subpopulations, thus supplying hormone-producing cells in the postnatal anterior lobe. Furthermore, CD9 and CD81 are implicated in cell proliferation. The current findings provide novel insights into adult pituitary stem/progenitor cells.