*6.3. Transferrin*

TF is part of the TF superfamily, which also includes lactoferrin, melanotransferrin, and ovotransferrin, which are found in many species of both mammals and invertebrates. It consists in two lobes, each binding a Fe3<sup>+</sup> atom with a very high a ffinity (10<sup>22</sup> <sup>M</sup>−1). Its primary role is to maintain an environment devoid of free iron. TF synthesized by RPE, PR, and neuronal cells is found in the aqueous and vitreous humors [8,105]. By single-cell RNA sequencing of human neural retina, mRNA for TF was enriched in peripheral retina compared to fovea [188]. Its expression is amplified during inflammation or immunity to increase the bu ffering capacity of iron. In light-induced retinal degeneration, TF and TFR1 mRNA increased in retina immediately after light exposure and then decreased at basal level. One day after light exposure, TF was increased, whereas TFR1 was reduced compared to not illuminated mice [32]. TF has long been of therapeutic interest due to its antimicrobial capacity and the ubiquitous presence of TFR1 allowing penetration of the blood-brain barrier [189]. TF has also been used successfully in humans in iron metabolism pathologies and for its cytoprotective capacity [190].

Our laboratory is interested in the potential of TF for the treatment of retinal pathologies (Table 3). Our work has shown that administration of the iron-free form (apoTF) by intraperitoneal injections in rd10 mice, a model of retinitis pigmentosa, preserves PRs better compared to the use of other chelators or antioxidants [46,164]. Injected into the vitreous, TF is present throughout the neural retina (MGC) and is eliminated via its receptors by RPE and the choroid without any immunogenic or toxic e ffect on the retina [46,105]. Thus, TF administered in a model of light-induced degeneration, allows the restoration of iron homeostasis, decreases iron accumulation, reduces inflammation and apoptosis, and preserves PRs and visual function [43]. In an ex vivo model of retinal detachment, TF inhibits the degenerative processes activated by the iron excess by reducing necrosis, apoptosis, gliosis, and oxidative stress. In vivo, human TF constitutively expressed in transgenic mice (TG) reduces loss of cones, cleavage of caspase 3, an apoptosis e ffector, DNA breaks, and necrosis (Figure 3). In rats, TF injected at the time of the detachment, reduces retinal edema, cell death and preserves PRs. In addition to its ability to reduce the accumulation of iron in the retina following detachment, TF also acts on other cellular pathways, no doubt through its interaction with molecular partners which remain to be discovered [105].



**Model Experiment Physiopathology Administration Mode Therapeutic Action of Transferrin References** Retinal explant of mice Retinal detachment with iron exposure Retinas from TghTF Preservation of cones number and rod outer segments length. Lower necrosis Prevention of iron retinal accumulation [105] Retinal explant of rats Retinal detachment with iron exposure Addition of apo-human TF after iron exposure Preservation of rhodopsin expression level and cones number Lower necrosis and apoptosis Prevention of retinal iron accumulation [105] Subretinal injection of hyaluronic acid in mice Retinal detachment presenting iron accumulation in subretinal space TghTF mice Preservation of retinal histology, rods outer segments length and number of cones Diminution of retinal oedema and Müller glial cells activation Lower apoptosis and necrosis Regulation of pathways involved in biological functions [105] Subretinal injection of hyaluronic acid in rats Retinal detachment presenting iron accumulation in subretinal space Intravitreal injection of apo-hTF Preservation of retinal histology, rods outer segments length Diminution of retinal oedema [105]

**Table 3.** *Cont*.

**Legend Table 3:** ApoTF: transferrin without iron; HoloTF: transferrin binding iron; INL: inner nuclear layer; ONL: outer nuclear layer; PR: photoreceptors; TF: transferrin; TghTf: transgenic mice carrying the complete human transferrin gene.

**Figure 3.** Transferrin expression preserves the detached retina. **Legend Figure 3:** After retinal detachment (RD), photoreceptors died by apoptosis and necrosis. Transgenic mice (TG) expressing human transferrin (TF) were used to demonstrate the protective effects of TF. (**A**) Arrestin staining revealed cones in retinal sections of TG mice (arrows) after RD. Cone number was higher in TG compared with WT mice. (**B**) The ratio of cleaved/pro–caspase 3 protein level was lower in TG mice compared to WT mice after RD. (**C**) The number of nuclei positive apoptotic-DNA breaks, stained by TUNEL, was reduced in TG mice compared to WT mice. (**D**) Necrotic RIP kinase protein level was reduced in TG mice compared with WT mice. All values are represented as the mean ± SEM. Mann–Whitney test (*n* = 3–6), \* *p* ≤ 0.025. ONL: Outer nuclear layer. Scale bar, 100 μm. From [105]. Reprinted with permission from AAAS.
