Online

Wysolmerski, Erin

Biology

2015

MS

Fibroblast growth factors (FGFs) are critical in many aspects of embryonic development and other cellular functions including apoptosis, cell adhesion, and proliferation. FGF8a, specifically, is known to initiate retinal ganglion cell (RGC) differentiation along with FGF3 early in retinal development (Martinez-Morales et al., 2005b). There has been little research into later roles for FGF8a in eye development. Here we show mRNA expression of fgf8a in the presumptive RGCs of 2 day-old zebrafish, past the time of RGC differentiation (28-48 hours)(Schmitt and Dowling, 1996). In addition, mRNA expression of putative receptor, FGFR1b, was localized outside the retina on the presumptive vasculature. Acerebellar (ace) mutants lacking FGF8a show mispatterned retinal vasculature and a lack of blood flow through the eye at 48 hpf. Further, we looked to see if this lack of blood flow had any effect on the developing neural retina. We found a significant reduction in the size of ace mutant eyes and also a reduction in total cell numbers in the retina starting at 48 hours post fertilization (hpf) suggesting a role for fgf8a in neurovascular signaling. The cause of the small eye phenotype was found to be due to a lack of proliferating cells and not an increase in cell death. We hypothesized if this phenotype was a result of a lack of blood flow to the retina. It has previously been reported that zebrafish survive and develop normally for 7 days without blood flow as the embryo receives nutrients by simple diffusion with its surroundings (Sehnert et al., 2002). To investigate the role that blood flow plays on the developing retina we utilized a silent heart mutant (sih) fish line, which lacks cardiac troponin t resulting in embryos without blood flow, as heart contractility does not initiate. To explore lack of blood flow to the retina as a cause for the observed ace mutant phenotype, sih mutant eye phenotypes were assessed. Retina cell counts from these embryos show a decreased eye diameter and a loss in total retina cell numbers due to lack of proliferation, phenocopying ace mutants. sih mutants also show a mis-patterning of their retinal vasculature with ectopic vessel branches similar to ace mutants. Our data support the small eye phenotype seen in both mutants is a result due to lack of proliferation. After morpholino knock down of the receptor, fgfr1b, we see mispatterend vasculature that phenocopies what we see in ace mutants. These finding led us to hypothesize that FGF8a, secreted by the RGCs, signals through its receptor, FGFR1b, on the retinal vasculature to promote cell growth and development. Further these data suggest that the retinal vasculature subsequently responds by secreting an unknown factor to support the proliferation and maintenance of the RGCs.