Page 295 - Read Online

Thirugnanam et al. Vessel Plus 2020;4:26 I http://dx.doi.org/10.20517/2574-1209.2020.18 Page 9 of 16

SNRK in vasculogenesis
The initial loss of function studies in vivo for Snrk was performed in the vertebrate zebrafish model system.
Zebrafish offers various advantages as a model system including established genetics, loss and gain of
function technology, transparency of embryos, fluorescent transgenic reporter lines, ex vivo development
of embryos, and several others [72-74] . During embryonic development, angioblasts migrate from lateral
plate mesoderm to the midline, differentiate in arterial or venous endothelial cells and coalesce to form
cord-like structures which eventually mature to form the major axial blood vessels, namely dorsal aorta
[75]
(DA) and posterior cardinal vein (PCV) . The kinase function of SNRK was shown to be essential for
angioblast migration and is required for localization and maintenance of these cells in the lateral plate
[76]
mesoderm . SNRK also functions later (19-22 h post-fertilization) during angioblast differentiation to
arteries and veins in zebrafish where it is involved with Notch signaling-mediated arterial or venous (A/V)
specification. Studies in zebrafish reveals that within 2 h of the formation of angioblasts, angioblasts begin
their migration to the dorsal midline, and by 18 h post-fertilization, angioblasts coalesce at the midline to
form the two major axial vessels (DA & PCV) [77,78] . Thus, sufficient number of angioblasts are needed in the
embryo to facilitate this process. As described earlier, most kinases are counter balanced by phosphatases.
In the lateral plate mesoderm, dual-specific phosphatase 5 (DUSP5), a member of MAPK phosphatases
was identified as a vascular-specific gene in 2 independent microarray studies [74,79] . Subsequent analysis
shows that Dusp-5 phosphatase and SNRK kinase function together in maintaining angioblast populations
[20]
during embryonic vascular development . Thus, the signaling pathway and the targets that are regulated
by SNRK and DUSP5 are likely to reveal more understanding of the basic vasculogenesis process during
vertebrate organogenesis.

SNRK in angiogenesis
[80]
The main mediator of angiogenesis is the arrangement of ECs in tip and stalk cells . Tip cells containing
filopodia invade surrounding tissue leading to the path of neo-vessel formation. Vascular endothelial
[80]
growth factor and Notch signaling pathways are vital for tip cell differentiation . Most of our current
knowledge about the morphological processes and molecular regulation of angiogenesis are from the
developing zebrafish embryos or the vascularization of postnatal mouse retina [80,81] . In zebrafish, the
accessibility of embryos for live imaging provided unprecedented observations which have unraveled new
concepts in angiogenesis. Analysis of the zebrafish head and trunk vasculature reveals that SNRK controls
[20]
patterning of vessels in both locations . In retina, the segregation of vascular and avascular compartments
and the visualization of the progressive expansion of blood vessels from the center to periphery makes
it an attractive model to study tip versus stalk cell formation during angiogenesis. In this model,
endothelial SNRK was found to be essential for vascular patterning in that it controls vessel diameter and
[21]
vascularization area in the retina . This regulation is partially mediated through transcription factor
hypoxia-inducible factor-1a (HIF1α), which is induced under a physiological drop in oxygen concentration
below 60 mmHg, a condition referred to as hypoxia. The hypoxia state in tissue often induces angiogenesis.
Similarly, during both acute and chronic myocardial ischemia, angiogenesis is stimulated. Ischemia-driven
angiogenesis is primarily an adaptive physiological response to either an increase in tissue mass or elevated
oxygen consumption . Under ischemic condition, HIF1α is upregulated and was shown to interact
[82]
[21]
directly to SNRK promoter . This binding results in transcriptional upregulation of SNRK mRNA, which
[21]
then promotes angiogenesis by activating ITGB1 (β1 integrin)-mediated EC migration . Thus, SNRK
plays a vital function in developing vasculogenesis and ischemic angiogenesis. However, its exact role and
the underlying mechanisms associated with facilitating normal angiogenesis remains unknown.

Role of SNRK in disease
Inflammation and metabolic dysregulation are major contributing factors to disease. Because SNRK
participates in both processes, it is considered an important target for intervention. Based on SNRK’s
preponderance as a repressor of cellular functions, we consider SNRK as a checkpoint in cells. Thus,