Perspectives of SLIT/ROBO signaling in placental angiogenesis

Abstract

A novel family of evolutionally conserved neuronal guidance cues, including ligands (i.e., Slit, netrin, epherin, and semaphorin) and their corresponding receptors (i.e., Robo, DCC/Unc5, Eph and plexin/ neuropilin), has been identified to play a crucial role in axon pathfinding and branching as well as neuronal cell migration. The presence of commonalities in both neural and vascular developments has led to some exciting discoveries recently, which have extended the functions of these systems to vascular formation (vasculogenesis) and development (angiogenesis). Some of these ligands and receptors have been found to be expressed in the vasculature and surrounding tissues in physiological and pathological conditions. It is postulated that they regulate the formation and integrity of blood vessels. In particular, it has been shown that the Slit/Robo pair plays a novel role in angiogenesis during tumorigenesis and vascular formation during embryogenesis. Herein we summarize briefly the characteristics of this family of neuronal guidance molecules and discuss the extra-neural expression and function of the Slit/Robo pair in angiogenesis in physiological and pathological settings. We report expression of Robo1 protein in capillary endothelium and co-expression of Slit2 and Robo1 proteins in syncytiotrophoblast in healthy term human placental villi. These cellular expression patterns implicate that the Slit/Robo signaling plays an autocrine and/or paracrine role in angiogenesis and trophoblast functions. We also speculate a possible role of this system in pathophysiological placental angiogenesis.

Fig. 1.

Domain structures of Slit and Robo. A. Primary structures of mammalian Slits contain leucine rich repeats (LRRs), EGF repeats, a laminin G domain, and a C-terminal cysteine-rich domain. B. Mammalian Robo proteins are constituted by immunoglobulin (Ig) motifs, fibronectin type III (FNIII) domains, a single transmembrane (TM) segment, and conserved cytoplasmic motifs (CC0-CC3).

Fig. 2.

Slit2 and Robol protein expression in healthy term human placental tissues. The placental tissues were collected from the University of California San Diego (UCSD) Medical Center Hospital with an approval from UCSD Institutional Review Board. The tissue segments were fixed with 3.7% paraformadehyde and then paraffin embedded. Sections (6-μm) were cut and used for immunohistochemically (Liao et al., 2005) localizing Slit2 and Robol proteins with anti-Slit2 monoclonal antibody (10 μg/ml) or anti-Robol monoclonal antibody (10 μg/ml) with the SuperPicture kit from Zymed Laboratories, Inc. (Invitrogen). Negative controls ran in parallel with anti-mouse IgG (10 μg/ml). Open arrowhead denotes syncytiotrophoblast and solid arrowhead denotes capillary endothelium.

Fig. 3.

Proposed paracrine/autocrine roles of Slit/Robo in trophoblast and capillary endothelial cells in the placenta. Trophoblast produced Slit2, possibly other Slits, regulates placental angiogenesis through binding to Robo1 and/or Robo4 on the endothelial cells, whereas they also regulate trophoblast functions via binding to Robo proteins.