The third vasculature gets attention.

Traditionally, researchers studying the vasculature examine either the biology of the large, arterial, high-pressure vessels or the resistance-regulating (arteriolar) or microvessels (capillaries), or less commonly, the venous, lowpressure vasculature. The lymphatic vasculature gets far less attention by the cardiovascular research community. This is despite the fact that the lymphatics play very important roles in the tissue clearance of macromolecules, in the removal of immigrated cells from tissues such as after inflammation, and in the interstitial fluid homeostasis. Nevertheless, comparably little is known about the growth and maintenance of lymphatics during adult life. In a laudable paper in this issue of Cardiovascular Research, Jin et al. describe experimental studies examining the postdevelopmental regulation of lymphangiogenesis bya novel regulator and validate in vitro findings in a lymphedema disease model in mice. These investigators developed the hypothesis that adrenomedullin promotes lymphangiogenesis after injury. This is suggested by a series of previous observations indicating that this peptide with known vasodilator functions (and diuresis induction due to renal vasodilation) is required for lymphangiogenesis during embryogenesis and is highly expressed in cancers with strong metastatic seeding into regional lymph nodes such as breast cancer. In their studies, Jin et al. use human lymphatic microvascular endothelial cells to demonstrate their proliferation and increased migration upon exposure to adrenomedullin as well as tube formation when grown on a matrix. Moreover, growth stimulation by adrenomedullin is shown convincingly to be cAMP-dependent and mediated through an MEK– ERK pathway. In mice with experimentally induced lymphatic interruption and lymph oedema of the tail, adrenomedullin increases lymphangiogenesis in the wound, leading to recovery of lymphatic integrity across the wound and relief of distal lymph oedema. Remarkably, these in vivo effects at the local wound site were induced with systemically administered recombinant adrenomedullin. Perhaps, it would have been further informative if the investigators would have included other lymphangiogenesisinducing peptides as comparators. In addition, examination of the effects of adrenomedullin on growth, migration, and network/tube formation of non-lymphatic microvascular endothelial cells would also have been interesting. There are other endothelial growth factors that have been shown to increase proliferation and migration in cultured lymphatic microvascular endothelial cells. These include VEGF-A, VEGF-C, and VEGF-D, each of which are products of different genes but have moderate homology. VEGF-A is not a lymphatic endothelium-specific factor, but VEGF-C and -D are specific for the lymphatics. Selectivity of action of these factors to induce microvascular versus lymphatic angiogenesis appears to be provided by the receptor, mainly VEGFR-3 on lymphatic endothelial cells. One intriguing possibility is that neuropilin-2 (Nrp-2), a receptor with roles in axon guidance, is also heavily expressed on lymphatic endothelial cells, suggesting a role in the guidance during lymphatic microvessel angiogenesis. Adrenomedullin is a pluripotent peptide with 52 amino acid residues and structural similarities to calcitonin gene-related peptide, CGRP. It is transcriptionally regulated, similar to VEGF, downstream of HIF-1. Due to its structural similarity with CGRP, adrenomedullin acts through the calcitonin receptor-like receptor (CALCLR) in a heterodimer with the receptor activity-modifying protein-2 or -3 (RAMP2 and -3), where RAMP2 induces the highest affinity. The CALCLR/RAMP2 receptor is required for murine embryonic vasculogenesis and lymphangiogenesis. CALCLR and RAMP2 are relatively enriched in lymphatic endothelial cells compared with HUVECs (which may explain semi-specific action of adrenomedullin as a lymphatic growth factor), and the former is transcriptionally upregulated by the venous and lymphatic endothelial cellspecific prospero-related homeobox-1 (Prox1) transcription factor. In venous endothelium, Prox1 regulates the transition of venous towards lymphatic endothelial cells, thus giving rise to sprouting of lymphatics from pre-formed veins during development, the current concept for developmental lymphangiogenesis. The present in vivo observations in the studies by Jin et al. show recovery of lymphatic continuation after interruption by wounding but leave open the origin of newly formed lymphatics: Longitudinal extension from the wound margin or formation of new lymph capillaries by sprouting from venoles? The opinions expressed in this article are not necessarily those of the Editors of Cardiovascular Research or of the European Society of Cardiology. *Corresponding author. Tel: þ1 310 222 3891; fax: þ1 310 782 1837. E-mail address: [email protected]