Mol. Cells 2014; 37(7): 503-510
Published online May 23, 2014
https://doi.org/10.14348/molcells.2014.0108
© The Korean Society for Molecular and Cellular Biology
Correspondence to : *Correspondence: suk-won.jin@yale.edu
Lymphatic vessels provide essential roles in maintaining fluid homeostasis and lipid absorption. Dysfunctions of the lymphatic vessels lead to debilitating pathological conditions, collectively known as lymphedema. In addition, lymphatic vessels are a critical moderator for the onset and progression of diverse human diseases including metastatic cancer and obesity. Despite their clinical importance, there is no currently effective pharmacological therapy to regulate functions of lymphatic vessels. Recent efforts to manipulate the Vascular Endothelial Growth Factor-C (VEGFC) pathway, which is arguably the most important signaling pathway regulating lymphatic endothelial cells, to alleviate lymphedema yielded largely mixed results, necessitating identification of new targetable signaling pathways for therapeutic intervention for lymphedema. Zebrafish, a relatively new model system to investigate lymphatic biology, appears to be an ideal model to identify novel therapeutic targets for lymphatic biology. In this review, we will provide an overview of our current understanding of the lymphatic vessels in vertebrates, and discuss zebrafish as a promising in vivo model to study lymphatic vessels.
Keywords lymphatic endothelial cell (LEC), lymphatic vessels, lymphedema, zebrafish
Since its first description in the 18th century, diverse physiological events have been attributed to lymphatic vessels (Alitalo, 2011; Kerjaschki, 2014). For instance, lymphatic vessels regulate interstitial fluid homeostasis, facilitate the absorption of lipids, and provide a conduit for immune cells as well as metastasizing tumors (Achen et al., 2005; Wang and Oliver, 2010). Dysfunctions of lymphatic vessels lead to the swelling of extremities in humans, collectively referred to as lymphedema (Connell et al., 2013; Murdaca et al., 2012). While primary lymphedema, caused by genetic mutations, is relatively rare, secondary lymphedema, which is induced by obstruction of lymphatic vessels, affects as many as 140?250 million people worldwide (Zuther and Norton, 2012). While substantial information on the development, morphogenesis, and maintenance of lymphatic vessels is available, how lymphatic vessels form during development remains poorly understood, partly due to the limitation of the model system of choice for lymphatic research. Zebrafish, which are amenable for time-lapse analysis and genetic manipulation (Gore et al., 2012; Wilkinson and van Eeden, 2014), have recently emerged as an important
Anatomical structures associated with the lymphatic system, particularly the lymph nodes, were first described as early as the era of Hippocrates (Chikly, 1997). However, it was not until 18th century that the ‘lymphatic system’ was discovered (Chikly, 1997; Kerjaschki, 2014). Like blood vessels, lymphatic vessels display a highly stereotypic hierarchy. Lymphatic capillaries absorb excessive interstitial fluid within tissue and transport it to the larger caliber lymphatic vessels such as lymphatic collecting vessels and lymphatic trunks. While lymphatic capillaries usually lack muscular layers, therefore relying on peristaltic movement to transport lymph, larger lymphatic vessels do possess contractile muscular layers to facilitate forward movement of lymph (Dejana et al., 2009; Pfeiffer et al., 2008). However, compared to blood vessels, the muscular layers encompassing lymphatic vessels are generally thinner, which generate weaker contractile force. Therefore, lymph fluid within the large caliber lymphatic vessels can move in a retrograde manner. To prevent backflow of lymph, large caliber lymphatic vessels have lymphatic valves alongside the lymphatic wall, similar to the valves found in the large veins (Bazigou and Makinen, 2013). Excessive interstitial fluid is returned to blood circulation
The basic building blocks of lymphatic vessels, lymphatic endothelial cells, are known to emerge from blood endothelial cells (BECs) during development (H?gerling et al., 2013; Yang et al., 2012). While lymphatic endothelial cells (LEC) share certain molecular characteristics with BECs, cellular morphology of LECs appears to be distinct from BECs: For instance, both LECs and BECs strongly express PECAM and VE-Cadherin (Oliver and Srinivasan, 2010). However, LECs form button-like discontinuous junctions, which make lymphatic vessels highly permeable to excessive interstitial fluid, macromolecules, as well as migrating leukocytes, while BECs tend to form more organized and compact cell junctions (Neufeld et al., 2014).
During development, LECs, similar to other cell types found in cardiovascular systems of vertebrates, are thought to arise within the mesoderm (Marcelo et al., 2013). In particular, lateral plate mesoderm (LPM), a subset of the mesodermal layer in embryos, has been implicated as the developmental source of LECs. LECs first emerge around embryonic week 6 to 7 in humans, E10 in mice, and about 2 days post-fertilization (dpf) in zebrafish, only after the formation of blood vessels (Bruy?re and No?l, 2010; K?chler et al., 2006; Neufeld et al., 2014; van der Putte, 1975). The majority of LECs appear to originate from veins during development (Fig. 1). At the turn of the 20th century, Florence Sabin elegantly demonstrated that LECs originate from veins, which was validated by recent lineage tracing and
The earliest event during lymphatic development in mouse is the polarized expression of Lymphatic Vessel Endothelial Hyaluronan Receptor 1 (Lyve1) around E9 within a subset of ECs within the cardinal vein, which is the first sign of lymphatic competency of ECs (Banerji et al., 1999; Prevo et al., 2001). While the cardinal vein appears to be the major source of LECs, recent reports suggest that a significant number of LECs may be derived from ECs within the intersegmental vessels (ISVs) at approximately E10.5 and E11 (Yang et al., 2012). At E11.5, mature jugular lymph sacs are formed. During the specification and differentiation of LECs, the expression of SOX18, a member of SRY-related HMG-box (SOX) family of transcription factors, and venous enriched transcriptional factor Coup-TFII appear to be instrumental (Fran?ois et al., 2008; Srinivasan et al., 2010): These two transcription factors cooperate to induce the expression of a lymphatic specific transcription factor, PROX1 (Wigle and Oliver, 1999; Wigle et al., 2002), which persists in LECs throughout their lifetime.
Although it is apparent that the PROX1 positive LECs within veins are the source of lymphatic vessels during development (Wigle et al., 2002), it is not clear how the separation between blood and lymphatic vessels occurs. Platelets have been implicated as the major driving force for initiating this process (Abtahian et al., 2003; Kim and Koh, 2010). By interacting with Podoplanin on the surface of LECs using CLEC-2 on their surface, platelets promote Syk/Spl76-mediated activation of Integrin signaling, which appears to be an essential step for the segregation of lymphatic vessels from the adjacent blood vessels. However, more recently, an alternative model has been proposed: Using high resolution
Similar to mouse, the progenitors for LECs appear to originate from the cardinal vein around 2 dpf in zebrafish (K?chler et al., 2006; Yaniv et al., 2006). Between 32 to 36 hpf, a group of angiogenic sprouts emerges from the cardinal vein, and eventually contribute to LECs. However, the exact anatomical locations of the progenitors for LECs remain largely unknown. The initial observation by Yaniv and colleagues indicate that LECs first migrate out of the parachodal vessel (Yaniv et al., 2006). However, this finding has been recently challenged by the fact that venous sprouts, which do not contribute to ISVs, may serve as a source for first group of LECs (Koltowska et al., 2013). Once specified, LECs rapidly migrate away from their original position and navigate along with arterial ISVs to make rudimentary lymphatic vessels (Yaniv et al., 2006). Around 3.5?4 dpf, the thoracic duct is clearly visible between the dorsal aorta and the cardinal vein (Kim and Kim, 2014; Yaniv et al., 2006), which expands and form lymph sacs at later stages (Okuda et al., 2012).
So far, a number of signaling pathways has been implicated in modulating lymphatic development (Fig. 2). Arguably, the most important signaling pathway for lymphatic development is Vascula Endothelial Growth Factor-C (VEGF-C) signaling. VEGF-C, which serves as a ligand for Vascular Endothelial Growth Factor Receptor 2 (VEGFR2/Flk1) and Vascular Endothelial Growth Factor Receptor 3 (VEGFR3/Flt4), was first identified as a protein with significant homology to VEGF-A and VEGF-B (Eichmann et al., 1998; Enholm et al., 2001; Jeltsch et al., 1997; Kukk et al., 1996). VEGF-C appears to be the main chemoattractant cue for migrating LECs (Karkkainen et al., 2004). In development, VEGF-C is highly expressed in the mesenchymal cells adjacent to developing lymphatic vessels, consistent with its proposed role. Targeted deletion of
While VEGF-C/VEGFR3 signaling functions as an on/off switch for lymphatic development, additional signaling pathways modulate and fine-tune lymphatic development. For instance, it has been shown that a number of G-protein Coupled Receptors (GPCRs) are critical for lymphatic development (Karpinich and Caron, 2014). To date, at least four GPCR ligand/receptor pairs have been implicated; Adrenomedullin/RAMP/CALCRL (Fritz-Six et al., 2008; Karpinich et al., 2013), Sphingosine-1-phosphoate/EDG1 (Aoyagi et al., 2012; Yoon et al., 2008), CXCL12/CXCR4 (Cha et al., 2012), and Apelin/APJ (Kim et al., 2013). While diverse ligand and extracellular stimuli such as light and shear stress can activate GPCRs (Venkatakrishnan et al., 2013), virtually all GPCRs require trimeric G-protein complexes, which consist of Gα, Gβ, and Gγ subunits, to mediate their signaling activity and elicit cellular responses (M?hle and Drost, 2012; Smrcka, 2013). Within G-protein complexes, the Gα subunit generally determines the signaling outcomes of GPCRs (Gilman, 1987; Kimple et al., 2011). We have recently identified Gα 13 as the main Gα subunit which functions as the essential downstream effector for diverse GPCRs to modulate lymphatic development (unpublished). GPCR signaling appears to be relayed by ERK1/2, AKT1/2 and ERK5 (Goldsmith and Dhanasekaran, 2007; Kidoya and Takakura, 2012; Nithianandarajah-Jones et al., 2012; Simon et al., 1991). Considering many targets of currently available pharmacological therapies are the components of GPCR signaling (DeWire and Violin, 2011; Dorsam and Gutkind, 2007; Smrcka, 2013), identification and understanding of the molecular and cellular basis of additional GPCRs and their contribution in lymphatic development and maintenance may help to develop effective translational applications for lymphatic disorders and associated diseases in the future.
While it is widely accepted that Notch signaling plays an instrumental role during angiogenesis, its function in lymphatic development remains unresolved because of conflicting evidence. For instance, lack of Notch signaling appears to be detrimental for lymphatic development; genetic deletion of Notch1 or inhibition of Notch activity by Dll4-Fc treatment led to lymphatic overgrowth and dilation of lymphatic vessels by promoting excessive sprouting of lymphatic vessels and increasing proliferation and survival of LECs (Murtomaki et al., 2013; Zheng et al., 2011), suggesting that Notch may negatively modulate lymphangiogenesis, similar to its role in angiogenesis. In contrast, Notch signaling appears to positively regulate lymphatic vessels in post-natal development (Niessen et al., 2011). Therefore, the function of Notch signaling during lymphatic development appears to be highly context-dependent. In addition to cell autonomous effects, Notch signaling is also critical for migration of lymphatic vessels. In zebrafish, attenuation of Dll4, Notch1, or Notch6 compromise the extension of lymphatic vessels alongside the arterial ISVs, suggesting that Notch signaling may mediate the interaction between arterial and lymphatic vessels to guide migrating lymphatic vessels (Geudens et al., 2010).
Recently, bone morphogenetic protein (BMP) signaling has been implicated in lymphatic development. While anti-angiogenic BMP9/Alk1 signaling promotes lymphatic development (Larriv?e et al., 2012; Levet et al., 2013), pro-angiogenic BMP2/Alk2/3 signaling inhibits lymphatic development (Dunworth et al., 2013; Kim et al., 2012; Wiley et al., 2011), suggesting distinct cellular responses toward BMP signaling in BECs and LECs. In addition, transforming growth factor β (TGFβ), a signaling ligand related to BMPs, appears to provide context-dependent regulation of lymphatic development. While TGFβ signaling can substantially decrease proliferation and migration of human dermal lymphatic microvascular endothelial cells and negatively modulate lymphangiogenesis
In addition to the aforementioned signaling pathways, a number of structural proteins and signaling mediators appear to provide essential regulation during morphogenesis of the lymphatic vessels. In humans, mutations in gap junction proteins including Connexin 37 (CX37/GJA4), 43 (CX43/GJA1) and 47 (CX47/GJC2), Integrin A9 (ITGA9), and components of RAS signaling are found to predispose the propensity to develop lymphedema [reviewed in (Brouillard et al., 2014)]. Connexin 37 (CX37/GJA4), 43 (CX43/GJA1) and 47 (CX47/GJC2) are required for the formation of lymphatic valve leaflets in conjunction with Calcineurin signaling (Meens et al., 2014). Similarly, Integrin A9 (ITGA9) has been also implicated in the formation of lymphatic valve leaflets (Bazigou et al., 2009).
Since the first description of the lymphatic vessels in zebrafish less than a decade ago (Yaniv et al., 2006), zebrafish have become an emerging model system for lymphatic research. Many key attributes make zebrafish an attractive model system for studying the lymphatic system. First, zebrafish embryos are externally fertilized and transparent which allows for easy access and manipulations. One of the key challenges in investigating the early phases of lymphatic development in the traditional mouse model is
Diverse tools to manipulate and visualize the lymphatic vessels in zebrafish contribute to its value as the new model system in lymphatic research.
To delineate the function of a specific locus during development in zebrafish, antisense Morpholino oligonucleotides (MOs) are widely used (Ekker, 2000). MOs are 20 to 25 base pair long nucleotides which bind to the target sequence on pre-mature or mature mRNA to block translation or splicing (Bill et al., 2009). The translation blocking-MOs target the area encompassing the initiation codon of the mature mRNA and inhibit translation, while splice blocking-MOs bind to the splicing donor within the intron or splicing receptor within the exon in pre-mature mRNAs to block proper mRNA splicing, which in turn introduce a premature stop codon to generate non-functional proteins. By targeting existing mRNAs, translation blocking-MOs can interfere with both maternal and zygotic transcripts. In contrast, splice blocking-MOs can only inhibit zygotic transcripts since the maternal mRNA are fully processed transcripts, therefore lacking exon/intron boundaries. MOs can be directly injected into zebrafish embryos at 1?2 cell stages. Similar to an allelic series in classical mutagenesis, a range of phenotypes can be created by titrating the amount of injected MO, which helps to define the function of a specific locus (Bill et al., 2009; Corey and Abrams, 2001; Heasman, 2002). Moreover, MOs can be combined with chemical antagonists/agonists, which allows for a small molecule screen in a sensitized background to identify modulators of lymphatic biology (Kim et al., 2013).
In addition, recent advances in targeted mutagenesis techniques allow fast and relatively easy generation of mutations affecting specific loci (Blackburn et al., 2013; Campbell et al., 2013). For instance, Zinc Finger Nucleases (ZFN) (Doyon et al., 2008; Meng et al., 2008), Transcription activator-like effector nucleases (TALEN) (Bedell et al., 2012), and more recently CRISPR/Cas9-based gene knockout (Hwang et al., 2013) strategies have been shown to be successful in generating mutations.
Lymphangiography (lymphography) is a traditional method to visualize the lymphatic vessels and diagnose abnormalities of the lymphatic system in the human body (Guermazi et al., 2003). In zebrafish, a modified version of lymphangiography can be performed to trace lymphatic structures. While injection of a radioactive contrasting agent followed by X-ray imaging is the standard procedure in humans (Guermazi et al., 2003), injection of fluorescently-labeled macromolecules such as Rhodamine dextran (e.g. 2 × 106 MW), is shown to be sufficient to visualize lymphatic vessels in zebrafish (Kim et al., 2013; Yaniv et al., 2006). In general, fluorescent spheres which are conjugated with high molecular weight particles, are injected subcutaneously into the tail region or directly into the thoracic duct (TD) in the trunk region of 3?5 dpf embryos or adults (Yaniv et al., 2006). In conjunction with chemical screening or MO-mediated knock-down, lymphangiography allows rapid detection of structural deviations in lymphatic vessels.
In addition, transgenic lines can be used to visualize various vascular structures in zebrafish (Kamei et al., 2010; Wilkinson and van Eeden, 2014) (Fig. 3). Transgenic lines which express fluorescent reporters under vascular specific promoters such as
While zebrafish is an attractive model to investigate lymphatic development, there are a number of pending issues to be addressed. For instance, it is possible that the molecular and cellular basis of lymphatic development and physiology in zebrafish may not be readily applicable to humans. Consistent with this idea, it has recently been suggested that zebrafish Prox1 is dispensable for lymphatic development (van Impel et al., 2014). By generating targeted mutagenesis, van Impel and colleague generated zebrafish
Mol. Cells 2014; 37(7): 503-510
Published online July 31, 2014 https://doi.org/10.14348/molcells.2014.0108
Copyright © The Korean Society for Molecular and Cellular Biology.
Jun-Dae Kim, and Suk-Won Jin*
Yale Cardiovascular Research Center, Section of Cardiovascular Medicine, Department of Internal Medicine, Yale University School of Medicine, New Haven, CT 06511, USA
Correspondence to:*Correspondence: suk-won.jin@yale.edu
Lymphatic vessels provide essential roles in maintaining fluid homeostasis and lipid absorption. Dysfunctions of the lymphatic vessels lead to debilitating pathological conditions, collectively known as lymphedema. In addition, lymphatic vessels are a critical moderator for the onset and progression of diverse human diseases including metastatic cancer and obesity. Despite their clinical importance, there is no currently effective pharmacological therapy to regulate functions of lymphatic vessels. Recent efforts to manipulate the Vascular Endothelial Growth Factor-C (VEGFC) pathway, which is arguably the most important signaling pathway regulating lymphatic endothelial cells, to alleviate lymphedema yielded largely mixed results, necessitating identification of new targetable signaling pathways for therapeutic intervention for lymphedema. Zebrafish, a relatively new model system to investigate lymphatic biology, appears to be an ideal model to identify novel therapeutic targets for lymphatic biology. In this review, we will provide an overview of our current understanding of the lymphatic vessels in vertebrates, and discuss zebrafish as a promising in vivo model to study lymphatic vessels.
Keywords: lymphatic endothelial cell (LEC), lymphatic vessels, lymphedema, zebrafish
Since its first description in the 18th century, diverse physiological events have been attributed to lymphatic vessels (Alitalo, 2011; Kerjaschki, 2014). For instance, lymphatic vessels regulate interstitial fluid homeostasis, facilitate the absorption of lipids, and provide a conduit for immune cells as well as metastasizing tumors (Achen et al., 2005; Wang and Oliver, 2010). Dysfunctions of lymphatic vessels lead to the swelling of extremities in humans, collectively referred to as lymphedema (Connell et al., 2013; Murdaca et al., 2012). While primary lymphedema, caused by genetic mutations, is relatively rare, secondary lymphedema, which is induced by obstruction of lymphatic vessels, affects as many as 140?250 million people worldwide (Zuther and Norton, 2012). While substantial information on the development, morphogenesis, and maintenance of lymphatic vessels is available, how lymphatic vessels form during development remains poorly understood, partly due to the limitation of the model system of choice for lymphatic research. Zebrafish, which are amenable for time-lapse analysis and genetic manipulation (Gore et al., 2012; Wilkinson and van Eeden, 2014), have recently emerged as an important
Anatomical structures associated with the lymphatic system, particularly the lymph nodes, were first described as early as the era of Hippocrates (Chikly, 1997). However, it was not until 18th century that the ‘lymphatic system’ was discovered (Chikly, 1997; Kerjaschki, 2014). Like blood vessels, lymphatic vessels display a highly stereotypic hierarchy. Lymphatic capillaries absorb excessive interstitial fluid within tissue and transport it to the larger caliber lymphatic vessels such as lymphatic collecting vessels and lymphatic trunks. While lymphatic capillaries usually lack muscular layers, therefore relying on peristaltic movement to transport lymph, larger lymphatic vessels do possess contractile muscular layers to facilitate forward movement of lymph (Dejana et al., 2009; Pfeiffer et al., 2008). However, compared to blood vessels, the muscular layers encompassing lymphatic vessels are generally thinner, which generate weaker contractile force. Therefore, lymph fluid within the large caliber lymphatic vessels can move in a retrograde manner. To prevent backflow of lymph, large caliber lymphatic vessels have lymphatic valves alongside the lymphatic wall, similar to the valves found in the large veins (Bazigou and Makinen, 2013). Excessive interstitial fluid is returned to blood circulation
The basic building blocks of lymphatic vessels, lymphatic endothelial cells, are known to emerge from blood endothelial cells (BECs) during development (H?gerling et al., 2013; Yang et al., 2012). While lymphatic endothelial cells (LEC) share certain molecular characteristics with BECs, cellular morphology of LECs appears to be distinct from BECs: For instance, both LECs and BECs strongly express PECAM and VE-Cadherin (Oliver and Srinivasan, 2010). However, LECs form button-like discontinuous junctions, which make lymphatic vessels highly permeable to excessive interstitial fluid, macromolecules, as well as migrating leukocytes, while BECs tend to form more organized and compact cell junctions (Neufeld et al., 2014).
During development, LECs, similar to other cell types found in cardiovascular systems of vertebrates, are thought to arise within the mesoderm (Marcelo et al., 2013). In particular, lateral plate mesoderm (LPM), a subset of the mesodermal layer in embryos, has been implicated as the developmental source of LECs. LECs first emerge around embryonic week 6 to 7 in humans, E10 in mice, and about 2 days post-fertilization (dpf) in zebrafish, only after the formation of blood vessels (Bruy?re and No?l, 2010; K?chler et al., 2006; Neufeld et al., 2014; van der Putte, 1975). The majority of LECs appear to originate from veins during development (Fig. 1). At the turn of the 20th century, Florence Sabin elegantly demonstrated that LECs originate from veins, which was validated by recent lineage tracing and
The earliest event during lymphatic development in mouse is the polarized expression of Lymphatic Vessel Endothelial Hyaluronan Receptor 1 (Lyve1) around E9 within a subset of ECs within the cardinal vein, which is the first sign of lymphatic competency of ECs (Banerji et al., 1999; Prevo et al., 2001). While the cardinal vein appears to be the major source of LECs, recent reports suggest that a significant number of LECs may be derived from ECs within the intersegmental vessels (ISVs) at approximately E10.5 and E11 (Yang et al., 2012). At E11.5, mature jugular lymph sacs are formed. During the specification and differentiation of LECs, the expression of SOX18, a member of SRY-related HMG-box (SOX) family of transcription factors, and venous enriched transcriptional factor Coup-TFII appear to be instrumental (Fran?ois et al., 2008; Srinivasan et al., 2010): These two transcription factors cooperate to induce the expression of a lymphatic specific transcription factor, PROX1 (Wigle and Oliver, 1999; Wigle et al., 2002), which persists in LECs throughout their lifetime.
Although it is apparent that the PROX1 positive LECs within veins are the source of lymphatic vessels during development (Wigle et al., 2002), it is not clear how the separation between blood and lymphatic vessels occurs. Platelets have been implicated as the major driving force for initiating this process (Abtahian et al., 2003; Kim and Koh, 2010). By interacting with Podoplanin on the surface of LECs using CLEC-2 on their surface, platelets promote Syk/Spl76-mediated activation of Integrin signaling, which appears to be an essential step for the segregation of lymphatic vessels from the adjacent blood vessels. However, more recently, an alternative model has been proposed: Using high resolution
Similar to mouse, the progenitors for LECs appear to originate from the cardinal vein around 2 dpf in zebrafish (K?chler et al., 2006; Yaniv et al., 2006). Between 32 to 36 hpf, a group of angiogenic sprouts emerges from the cardinal vein, and eventually contribute to LECs. However, the exact anatomical locations of the progenitors for LECs remain largely unknown. The initial observation by Yaniv and colleagues indicate that LECs first migrate out of the parachodal vessel (Yaniv et al., 2006). However, this finding has been recently challenged by the fact that venous sprouts, which do not contribute to ISVs, may serve as a source for first group of LECs (Koltowska et al., 2013). Once specified, LECs rapidly migrate away from their original position and navigate along with arterial ISVs to make rudimentary lymphatic vessels (Yaniv et al., 2006). Around 3.5?4 dpf, the thoracic duct is clearly visible between the dorsal aorta and the cardinal vein (Kim and Kim, 2014; Yaniv et al., 2006), which expands and form lymph sacs at later stages (Okuda et al., 2012).
So far, a number of signaling pathways has been implicated in modulating lymphatic development (Fig. 2). Arguably, the most important signaling pathway for lymphatic development is Vascula Endothelial Growth Factor-C (VEGF-C) signaling. VEGF-C, which serves as a ligand for Vascular Endothelial Growth Factor Receptor 2 (VEGFR2/Flk1) and Vascular Endothelial Growth Factor Receptor 3 (VEGFR3/Flt4), was first identified as a protein with significant homology to VEGF-A and VEGF-B (Eichmann et al., 1998; Enholm et al., 2001; Jeltsch et al., 1997; Kukk et al., 1996). VEGF-C appears to be the main chemoattractant cue for migrating LECs (Karkkainen et al., 2004). In development, VEGF-C is highly expressed in the mesenchymal cells adjacent to developing lymphatic vessels, consistent with its proposed role. Targeted deletion of
While VEGF-C/VEGFR3 signaling functions as an on/off switch for lymphatic development, additional signaling pathways modulate and fine-tune lymphatic development. For instance, it has been shown that a number of G-protein Coupled Receptors (GPCRs) are critical for lymphatic development (Karpinich and Caron, 2014). To date, at least four GPCR ligand/receptor pairs have been implicated; Adrenomedullin/RAMP/CALCRL (Fritz-Six et al., 2008; Karpinich et al., 2013), Sphingosine-1-phosphoate/EDG1 (Aoyagi et al., 2012; Yoon et al., 2008), CXCL12/CXCR4 (Cha et al., 2012), and Apelin/APJ (Kim et al., 2013). While diverse ligand and extracellular stimuli such as light and shear stress can activate GPCRs (Venkatakrishnan et al., 2013), virtually all GPCRs require trimeric G-protein complexes, which consist of Gα, Gβ, and Gγ subunits, to mediate their signaling activity and elicit cellular responses (M?hle and Drost, 2012; Smrcka, 2013). Within G-protein complexes, the Gα subunit generally determines the signaling outcomes of GPCRs (Gilman, 1987; Kimple et al., 2011). We have recently identified Gα 13 as the main Gα subunit which functions as the essential downstream effector for diverse GPCRs to modulate lymphatic development (unpublished). GPCR signaling appears to be relayed by ERK1/2, AKT1/2 and ERK5 (Goldsmith and Dhanasekaran, 2007; Kidoya and Takakura, 2012; Nithianandarajah-Jones et al., 2012; Simon et al., 1991). Considering many targets of currently available pharmacological therapies are the components of GPCR signaling (DeWire and Violin, 2011; Dorsam and Gutkind, 2007; Smrcka, 2013), identification and understanding of the molecular and cellular basis of additional GPCRs and their contribution in lymphatic development and maintenance may help to develop effective translational applications for lymphatic disorders and associated diseases in the future.
While it is widely accepted that Notch signaling plays an instrumental role during angiogenesis, its function in lymphatic development remains unresolved because of conflicting evidence. For instance, lack of Notch signaling appears to be detrimental for lymphatic development; genetic deletion of Notch1 or inhibition of Notch activity by Dll4-Fc treatment led to lymphatic overgrowth and dilation of lymphatic vessels by promoting excessive sprouting of lymphatic vessels and increasing proliferation and survival of LECs (Murtomaki et al., 2013; Zheng et al., 2011), suggesting that Notch may negatively modulate lymphangiogenesis, similar to its role in angiogenesis. In contrast, Notch signaling appears to positively regulate lymphatic vessels in post-natal development (Niessen et al., 2011). Therefore, the function of Notch signaling during lymphatic development appears to be highly context-dependent. In addition to cell autonomous effects, Notch signaling is also critical for migration of lymphatic vessels. In zebrafish, attenuation of Dll4, Notch1, or Notch6 compromise the extension of lymphatic vessels alongside the arterial ISVs, suggesting that Notch signaling may mediate the interaction between arterial and lymphatic vessels to guide migrating lymphatic vessels (Geudens et al., 2010).
Recently, bone morphogenetic protein (BMP) signaling has been implicated in lymphatic development. While anti-angiogenic BMP9/Alk1 signaling promotes lymphatic development (Larriv?e et al., 2012; Levet et al., 2013), pro-angiogenic BMP2/Alk2/3 signaling inhibits lymphatic development (Dunworth et al., 2013; Kim et al., 2012; Wiley et al., 2011), suggesting distinct cellular responses toward BMP signaling in BECs and LECs. In addition, transforming growth factor β (TGFβ), a signaling ligand related to BMPs, appears to provide context-dependent regulation of lymphatic development. While TGFβ signaling can substantially decrease proliferation and migration of human dermal lymphatic microvascular endothelial cells and negatively modulate lymphangiogenesis
In addition to the aforementioned signaling pathways, a number of structural proteins and signaling mediators appear to provide essential regulation during morphogenesis of the lymphatic vessels. In humans, mutations in gap junction proteins including Connexin 37 (CX37/GJA4), 43 (CX43/GJA1) and 47 (CX47/GJC2), Integrin A9 (ITGA9), and components of RAS signaling are found to predispose the propensity to develop lymphedema [reviewed in (Brouillard et al., 2014)]. Connexin 37 (CX37/GJA4), 43 (CX43/GJA1) and 47 (CX47/GJC2) are required for the formation of lymphatic valve leaflets in conjunction with Calcineurin signaling (Meens et al., 2014). Similarly, Integrin A9 (ITGA9) has been also implicated in the formation of lymphatic valve leaflets (Bazigou et al., 2009).
Since the first description of the lymphatic vessels in zebrafish less than a decade ago (Yaniv et al., 2006), zebrafish have become an emerging model system for lymphatic research. Many key attributes make zebrafish an attractive model system for studying the lymphatic system. First, zebrafish embryos are externally fertilized and transparent which allows for easy access and manipulations. One of the key challenges in investigating the early phases of lymphatic development in the traditional mouse model is
Diverse tools to manipulate and visualize the lymphatic vessels in zebrafish contribute to its value as the new model system in lymphatic research.
To delineate the function of a specific locus during development in zebrafish, antisense Morpholino oligonucleotides (MOs) are widely used (Ekker, 2000). MOs are 20 to 25 base pair long nucleotides which bind to the target sequence on pre-mature or mature mRNA to block translation or splicing (Bill et al., 2009). The translation blocking-MOs target the area encompassing the initiation codon of the mature mRNA and inhibit translation, while splice blocking-MOs bind to the splicing donor within the intron or splicing receptor within the exon in pre-mature mRNAs to block proper mRNA splicing, which in turn introduce a premature stop codon to generate non-functional proteins. By targeting existing mRNAs, translation blocking-MOs can interfere with both maternal and zygotic transcripts. In contrast, splice blocking-MOs can only inhibit zygotic transcripts since the maternal mRNA are fully processed transcripts, therefore lacking exon/intron boundaries. MOs can be directly injected into zebrafish embryos at 1?2 cell stages. Similar to an allelic series in classical mutagenesis, a range of phenotypes can be created by titrating the amount of injected MO, which helps to define the function of a specific locus (Bill et al., 2009; Corey and Abrams, 2001; Heasman, 2002). Moreover, MOs can be combined with chemical antagonists/agonists, which allows for a small molecule screen in a sensitized background to identify modulators of lymphatic biology (Kim et al., 2013).
In addition, recent advances in targeted mutagenesis techniques allow fast and relatively easy generation of mutations affecting specific loci (Blackburn et al., 2013; Campbell et al., 2013). For instance, Zinc Finger Nucleases (ZFN) (Doyon et al., 2008; Meng et al., 2008), Transcription activator-like effector nucleases (TALEN) (Bedell et al., 2012), and more recently CRISPR/Cas9-based gene knockout (Hwang et al., 2013) strategies have been shown to be successful in generating mutations.
Lymphangiography (lymphography) is a traditional method to visualize the lymphatic vessels and diagnose abnormalities of the lymphatic system in the human body (Guermazi et al., 2003). In zebrafish, a modified version of lymphangiography can be performed to trace lymphatic structures. While injection of a radioactive contrasting agent followed by X-ray imaging is the standard procedure in humans (Guermazi et al., 2003), injection of fluorescently-labeled macromolecules such as Rhodamine dextran (e.g. 2 × 106 MW), is shown to be sufficient to visualize lymphatic vessels in zebrafish (Kim et al., 2013; Yaniv et al., 2006). In general, fluorescent spheres which are conjugated with high molecular weight particles, are injected subcutaneously into the tail region or directly into the thoracic duct (TD) in the trunk region of 3?5 dpf embryos or adults (Yaniv et al., 2006). In conjunction with chemical screening or MO-mediated knock-down, lymphangiography allows rapid detection of structural deviations in lymphatic vessels.
In addition, transgenic lines can be used to visualize various vascular structures in zebrafish (Kamei et al., 2010; Wilkinson and van Eeden, 2014) (Fig. 3). Transgenic lines which express fluorescent reporters under vascular specific promoters such as
While zebrafish is an attractive model to investigate lymphatic development, there are a number of pending issues to be addressed. For instance, it is possible that the molecular and cellular basis of lymphatic development and physiology in zebrafish may not be readily applicable to humans. Consistent with this idea, it has recently been suggested that zebrafish Prox1 is dispensable for lymphatic development (van Impel et al., 2014). By generating targeted mutagenesis, van Impel and colleague generated zebrafish
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