VEGFxxxb/VEGF ratio [26]. Additionally, the regulation of isoform specific transcription is regulated by an upstream open reading frame which acts as a cis-regulatory element that downregulates the expression of VEGF121 [27]. The regulation of VEGF expression is discussed below in more detail.

13.1.2VEGF Functions

VEGF has several physiological functions in the retina. It is important for the development of the retinal and choroidal vasculature. VEGF and its receptor VEGFR-2 are temporally and spatially correlated with normal development of ocular vasculature in humans [28]. The expression of VEGF in the developing retina is regulated by hypoxia [29]. Apart from vascular development, VEGF is a survival factor for endothelial cells [30] and is needed for the maintenance of the choroid [31]. It is also a survival factor for neuronal cells [32, 33] and protects the neurosensory retina [34] and the RPE [35]. Overexpression of VEGF in retinal ganglion cells protects axotomized neurons from delayed cell death in an ERK1/2 and Akt dependent manner [36]. Also, in ischemia, VEGF protects neuronal cells from apoptosis and delayed cell death [34]. Furthermore, VEGF is implicated in normal maintenance of the neuroretina [34], which is supported by the occurrence of significant neuronal apoptosis by adenoviral overexpression of soluble Flt1 (VEGF receptor) [37]. However, a blockage of VEGF by a soluble VEGF receptor for more than 7 months did not exhibit any anatomical or functional alterations [38]. Additionally, VEGF is involved in the chemotaxis of monocytes [39] which are involved in pathological neovascularization in the retina [40].

13.1.3Cells Secreting VEGF in the Retina

In the retina, a variety of cells have been described to produce VEGF, most notably the RPE, Müller cells, astrocytes, and pericytes. VEGF secreted by different cell types has different functions in the retina.

13.1.3.1Retinal Pigment Epithelium

RPE derived VEGF has been implicated to be involved in the development of the choroidal vasculature and photoreceptors, but not of the retinal vasculature [41, 42]. The importance of RPE derived VEGF can be seen in early embryonic development. Expression of RPE derived VEGF in development is not dependent on HIF-1a [41]. In the adult retina, RPE derived VEGF is important for the maintenance of the choroid and displays autocrine protective functions [31, 35]. RPE can be stimulated to increase VEGF production by a wide variety of factors,

e.g., TGF-b2, advanced glycation end products (AGE), or insults that induce the endoplasmic reticulum (ER) stress response (discussed in detail below).

13.1.3.2Müller Cells

Müller cell-derived VEGF does not seem to be involved in developmental regulation of the vasculature, as a conditional knockout of VEGF in Müller cells in mice did not exhibit any developmental defects in retinal or choroidal vasculature. Instead, the loss of Müller cell derived VEGF attenuates ischemia-induced depletion of tight-junction proteins and protection against ischemia-induced neovascularization [43]. Müller cell derived VEGF exerts protective effects on the photoreceptors, additional to autocrine protective functions [37]. VEGF secretion is kept on a low level in unstimulated Müller cells by N-methyl-D-aspartate (NMDA) receptor activation [44] but can be induced by hypoxia and hyperglycemia in an HIF-1a and internal ribosome entry site (IRES)/4E-BP1/2 dependent manner, respectively (discussed below) [45, 46].

13.1.3.3Astrocytes

Astrocytes express high levels of VEGF during development, predominantly VEGF164 (mouse equivalent to VEGF165) and to a lesser extent VEGF188 (mouse equivalent to VEGF189). Astrocytes have an important part in the development of the retinal vasculature, serving as a template for the developing vasculature, but a knockout of astrocyte-specific VEGF displays hardly any malformation of the vasculature. Instead, astrocyte-derived VEGF protects vessels from oxygen induced obliteration and collapse, so astrocyte-derived VEGF displays important stabilizing effects on the vascular network [47]. Astrocytes produce VEGF as a response to hypoxia, and this response has been shown to be important for pathological neovascularization [48].

13.1.3.4Pericytes

Pericytes produce VEGF in order to stabilize and promote the survival of endothelial cells [49]. VEGF secretion in pericytes can be enhanced by AGE [50] or hyperglycemia [51]. Pericyte production of VEGF is also induced by TGFb [49, 51]. In the diabetic retina, VEGF secretion is induced posttranscriptionally in pericytes via an mRNA stabilizing mechanism, which is Protein kinase C (PKC) dependent and induces an elevated binding of human RNA-binding protein (HuR) to VEGF mRNA (discussed in more detail below) [52]. Pericytes are also susceptible to VEGF signaling, as via a VEGFR-1 mediated pathway, elevated VEGF protein induces an ablation of pericytes from the retinal vasculature, resulting in vascular leakage [53].