LRP5 mutant mice are unable to form a normal vascular capillary network in the deeper plexiform layers of the retina. Both immunostaining results of vascular endothelial cell-specific Tie2 and 3D reconstruction of GFP-positive endothelial cells indicate that clustering of vascular endothelial cells, instead of normal sprouting and migrating and/or anastomosing, is the cause of incomplete retinal vasculature in the deeper layers of LRP5 mutant retina. Thus, LRP5 signaling is essential for regulating the sprouting, migrating and/or anastomosing of endothelial cells in the deeper layers of the retina. Based on the expression of knockin LacZ gene under the control of endogenous LRP5 promoter, LRP5 is predominantly expressed in retinal Müller cells although its expression is also detected in retinal endothelial cells. It is unclear whether LRP5 in Müller cells, in endothelial cells or in both cell types mediates a signal to regulate the properties of endothelial cells.
Müller cells span the entire retina and their processes are in close contact with retinal blood vessels and interneurons. Müller cells produce factors capable of modulating vascular formation, vessel permeability, the integrity of blood-retinal barrier, and the activity and survival of other retinal neurons. Studies have shown that VEGF, a stimulator for angiogenesis, is expressed transiently by Müller cells in the retinal inner nuclear layer, closely preceding the formation of the deeper plexus of retinal vasculature 
. A recent study demonstrates that VEGF and its receptors VEGFR1 and VEGFR2 are expressed in Müller cells 
. Müller cells have also been suggested playing a role in inducing blood-retinal barrier properties in the deeper plexus 
. Changes in Müller cells have been detected in patients with type 1 diabetes 
. GFAP over-expression in Müller cells is an early marker of retinal injury in retinal ischemia 
. In the human retina during early diabetes, retinal Müller cells change from quiescent to an injury-associated phenotype and express high levels of GFAP 
. Many angiogenic and anti-angiogenic factors are produced by Müller cells. An increased level of VEGF and a decreased level of pigment epithelium-derived factor (PEDF) have been observed in cultured retinal Müller cells treated by high glucose 
, indicating that Müller cells may contribute to unbalanced levels of VEGF and PEDF in diabetic retinopathy.
The Müller cell-specific Slc38a5 shows significantly reduced transcription level in the LRP5 mutant retinas. Slc38a5 is a sodium coupled neutral amino acid transporter mainly responsible for the glutamine uptake in retinal Müller cells 
. Decreased expression of Slc38a5 mRNA is also reported in Norrin knockout mice 
. Similar to LRP5, Norrin is also predominantly expressed in the Müller cells 
. Interestingly, a previous study has shown that the loss of Müller cell specific glutamate transporter GLAST leads to reduced b-wave amplitude 
, suggesting the involvement of Müller cells in generating the b-wave of ERG. It is possible that a loss of LRP5 causes impaired Müller cells, thus leading to abnormal b-wave of ERG.
A recent study suggests that Norrin in the Müller cells activates FZD4 in endothelial cells 
. FZD4 is known to be important for the development of retinal vasculature including the vessels in the surface ganglion cell layer. LRP5 is a co-receptor for FDZ4. Similar to the loss of FDZ4, one can assume that a functional loss of LRP5 in endothelial cells will result in a defect of retinal vasculature. However, LRP5 knockout mice display a relatively normal vascular network in the ganglion cell layer. This difference in phenotype may be because LRP5 function is either dispensable or compensated by the presence of LRP6 in endothelial cells. It remains unclear why LRP5 is essential for the development of vasculature in the retinal deeper layers where LRP6 cannot compensate its function. Moreover, it is unknown whether co-presence of LRP5 and FDZ4 in the same cell is a prerequisite for LRP5 acting as a co-receptor of FDZ4.
We hypothesize that LRP5 mediates an essential signal in Müller cells to regulate retinal vasculature in the deeper plexus. The absence of LRP5 in Müller cells disrupts this signaling pathway, thus the endothelial cells do not receive the proper signals to form the vessel network in deeper retinal layers. This hypothesis may explain why only endothelial cells in the inner- and outer-plexiform layers rely on the LRP5-mediated signaling to sprout, form the lumen, branch and anastomose. LRP6 in endothelial cells cannot compensate the loss of LRP5 in Müller cells. This hypothesis also suggests the intricate interactions among Wnts, Norrin, LRP5/6, and FZD4 between endothelial cells and Müller cells. Future research will be needed to address the underlying molecular mechanism that precisely controls the patterning of retinal vessels in different layers and to understand how Müller cells provide guiding cues to the retinal vessel network in the deep layers of retina.