Paired‐Related Homeodomain Proteins Prx1 and Prx2 are Expressed in Embryonic Pituitary Stem/Progenitor Cells and May Be Involved in the Early Stage of Pituitary Differentiation

Abstract

We recently cloned a paired-related homeodomain protein Prx2 as a novel factor in the pituitary. In the present study, we investigated the ontogenic profiles of Prx2 and another cognate Prx1 in the rat embryonic pituitary. Quantitative real-time polymerase chain reaction showed low expression of Prx2 and a marked increase of Prx1 on rat embryonic day (E)20.5. Immunohistochemical analyses using an antibody that recognises both proteins, with the aim of investigating their roles in pituitary organogenesis, demonstrated that PRXs first appear in the Rathke's pouch on E13.5 in the pituitary stem/progenitor cells expressing Prop1 and Sox2. After E16.5, the number of Prx-expressing cells was increased in both anterior and intermediate lobes. SOX2(+) stem/progenitor cells in the intermediate lobe started to produce PRXs, and PRX(+) /SOX2(+) /PROP1(+) -cells were present on the anterior side of the marginal cell layer and were scattered in the parenchyma of the anterior lobe. On the other hand, PRX(+) -cells negative for PROP1 and SOX2 were located in the anterior lobe. Analysis of the relationship with pituitary endocrine cells revealed that a part of PRX(+) /PROP1(-) /SOX2(-) -cells in the anterior lobe co-expressed all types of hormones. The proportion of co-localisation of PRXs and hormones was highest on the day each hormone first appeared. These data indicate that PRXs are produced in the pituitary progenitor cells and may play roles in the process of terminal differentiation during early pituitary organogenesis. An in vitro small interfering RNA-knockdown experiment in the pituitary-derived cell line, TtT/GF, revealed that PRX1 and PRX2 play roles in proliferation by different mechanisms because knockdown of Prx2, but not Prx1, induced the p21 expression. Furthermore, immunohistochemical analysis demonstrated that 76% of PRXs(+) cells were positive for a cell proliferation marker Ki67 in the E18.5 pituitary. This is the first report of the involvement of PRX1 and PRX2 in organogenesis of tissue originating from the ectoderm other than the mesoderm.

Pituitary stem cell regulation: who is pulling the strings?

“…Moreover, pairedrelated homeobox (Prrx) 1 and Prrx2, mainly known as mesenchymal markers and potentially involved in EMT (Ocaña et al 2012), are also upregulated in the adult pituitary stem cell population (SC-SP and SOX2 eGFP+ cell compartment) , Vankelecom 2010 (Table 1). PRRX1 is expressed in SOX2 + cells of the embryonic and adult (rat) pituitary and in a subset of all hormonal cell types in the adult pituitary (as analyzed at P30) (Susa et al 2012). In contrast, PRRX2 only emerges after birth from P30 in a subset of the SOX2 + cells .…”

“…In addition to the endocrine cells, the adenohypophysis contains nonhormonal cells including endothelial cells that form the sinusoidal capillary network, supportive mesenchymal cells and folliculostellate cells that have multiple functions (Allaerts & Vankelecom 2005, Susa et al 2012.…”

Pituitary Stem Cells: Quest for Hidden Functions

“…Finally, a battery of new candidate markers has emerged from recent studies (extensively reviewed in Vankelecom 2010 andChen 2014), including the retinoic acid-producing retinal aldehyde dehydrogenase 1 (Raldh1; Fujiwara et al 2007) and the retinoic acid receptor Rarb (Vankelecom 2010;Vankelecom and Chen 2014); the transcription factors PRRX1 and PRRX2 (expressed in proliferating RP progenitor cells until cell-cycle exit and start of differentiation; Susa et al 2012); the cell-adhesion molecule cadherin-18 (expressed in the MZ in a different pattern than CDH1; Chauvet et al 2009); the coxsackievirus and adenovirus receptor (CAR; Chen et al 2013); the Kr€ uppel-like factor 6 (KLF6; Ueharu et al 2014); and the juxtacrine signaling molecule ephrin-B2 (Yoshida et al 2015). A number of the pituitary stem cell markers identified may be involved in constructing and maintaining the 3D network, in particular the cadherins, ephrins and CAR.…”

Stem Cells in Neuroendocrinology

“…Finally, a battery of new candidate markers has emerged from recent studies (extensively reviewed in , including the retinoic acid-producing retinal aldehyde dehydrogenase 1 (Raldh1; Fujiwara et al 2007) and the retinoic acid receptor Rarb Vankelecom and Chen 2014); the transcription factors PRRX1 and PRRX2 (expressed in proliferating RP progenitor cells until cell-cycle exit and start of differentiation; Susa et al 2012); the cell-adhesion molecule cadherin-18 (expressed in the MZ in a different pattern than CDH1; Chauvet et al 2009); the coxsackievirus and adenovirus receptor (CAR; ; the Kr€ uppel-like factor 6 (KLF6; Ueharu et al 2014); and the juxtacrine signaling molecule ephrin-B2 . A number of the pituitary stem cell markers identified may be involved in constructing and maintaining the 3D network, in particular the cadherins, ephrins and CAR.…”