Ischemia reperfusion (IR) injury is usually induced by high intraocular pressure (IOP) and is a pathological hallmark associated with sight-threatening diabetic retinopathy, glaucoma, and retinal artery occlusions (Minhas et al., 2016). One animal model of IR features high IOP-induced transient ischemia, followed by restoration of retinal blood flow (reperfusion), and has been widely used to investigate retinal ischemia-related diseases, and evaluates the therapeutic efficacy of candidate drugs (Vestergaard et al., 2019). Following reperfusion, a series of events occur in response to the development of retinal degeneration, including oxidative stress, retinal cell death, glutamate excitotoxicity, inflammatory responses, and retinal capillary impairment (Barzegar et al., 2019; Minhas et al., 2016; Mrugacz et al., 2021; Ren and Leveillard, 2022). To date, there are no clinically approved drugs for the treatment of retinal complications associated with IR injury (Barzegar et al., 2019).

PEDF is a 50 kDa, secreted glycoprotein expressed by a number of tissues, including the retina and retinal pigment epithelium (RPE) (Tombran-Tink et al., 1995). Administration of exogenous PEDF by gene transfer or injection of full-length protein provides a promising strategy to protect retinal neurons against detrimental IR conditions (Ogata et al., 2001; Stellmach et al., 2001; Takita et al., 2003). Knockout of the PEDF gene (SERPINF1) in mice causes glial cell activation, photoreceptor loss and vascular disturbances in the retina (Chen et al., 2022). PEDF also protects motor neurons against glutamate excitotoxicity, a potential drug target for treating ischemic stroke (Bilak et al., 1999; Shen et al., 2022). Various functional regions have been identified in PEDF that are responsible for interaction of PEDF with various cell surface molecules. For example, the 44-mer (Val78-Thr121) has been shown to facilitate PEDF binding with the PNPLA2 (patatin-like phospholipase domain-containing protein 2), a PEDF receptor (PEDF-R) (Kenealey et al., 2015). In addition, a lipase inhibitor, atglistatin has been shown to abolish the enzymatic activity of PEDF-R, leading to attenuate the cytoprotective effect of PEDF and the 44-mer on both retinal R28 cells and primary mouse hepatocytes cultured under serum deprivation condition (Bernardo-Colon et al., 2023; Kenealey et al., 2015; Shih et al., 2016).

Shorter peptides derived from PEDF have been proposed that are more practical for development as therapeutic agents than PEDF, due to cost of production and unexpected effects, potentially related to PEDF multifunctional activities (Ho et al., 2022b; Kenealey et al., 2015; Mirochnik et al., 2009). For example, a synthetic 7-mer peptide (Asp64-Ser70), derived from PEDF 34-mer, has been shown to display a stronger anti-angiogenic activity than the 34-mer (Ho et al., 2022b). In this study, a series of shorter peptides covering the PEDF 44-mer were synthesized. Our results revealed that the 6-mer showed a better cytoprotective activity than the 44-mer to protect cultured neuronal cells against cytotoxicity induced by glutamate and tBHP. The 6-mer also showed a higher neuroprotective ability than the 44-mer to relief IR-induced retinal cell apoptosis in experiment animals. The 6-mer effects were attenuated by atglistatin, strongly implying the underlying molecular mechanisms of the 6-mer involving in the activation of PEDF-R signaling.