*Review* **The Interplay between Peripherin 2 Complex Formation and Degenerative Retinal Diseases**

### **Lars Tebbe, Mashal Kakakhel, Mustafa S. Makia, Muayyad R. Al-Ubaidi \* and Muna I. Naash \***

Department of Biomedical Engineering, University of Houston, Houston, TX 77204, USA; ltebbe@Central.UH.EDU(L.T.);mkakakhe@central.uh.edu(M.K.);msmakia@Central.UH.EDU(M.S.M.)

 **\***Correspondence:malubaid@central.uh.edu (M.R.A.-U.);mnaash@central.uh.edu(M.I.N.);

Tel.: +1-713-743-1651 (M.I.N.)

Received: 4 February 2020; Accepted: 20 March 2020; Published: 24 March 2020

**Abstract:** Peripherin 2 (Prph2) is a photoreceptor-specific tetraspanin protein present in the outer segmen<sup>t</sup> (OS) rims of rod and cone photoreceptors. It shares many common features with other tetraspanins, including a large intradiscal loop which contains several cysteines. This loop enables Prph2 to associate with itself to form homo-oligomers or with its homologue, rod outer segmen<sup>t</sup> membrane protein 1 (Rom1) to form hetero-tetramers and hetero-octamers. Mutations in *PRPH2* cause a multitude of retinal diseases including autosomal dominant retinitis pigmentosa (RP) or cone dominant macular dystrophies. The importance of Prph2 for photoreceptor development, maintenance and function is underscored by the fact that its absence results in a failure to initialize OS formation in rods and formation of severely disorganized OS membranous structures in cones. Although the exact role of Rom1 has not been well studied, it has been concluded that it is not necessary for disc morphogenesis but is required for fine tuning OS disc size and structure. Pathogenic mutations in *PRPH2* often result in complex and multifactorial phenotypes, involving not just photoreceptors, as has historically been reasoned, but also secondary effects on the retinal pigment epithelium (RPE) and retinal/choroidal vasculature. The ability of Prph2 to form complexes was identified as a key requirement for the development and maintenance of OS structure and function. Studies using mouse models of pathogenic *Prph2* mutations established a connection between changes in complex formation and disease phenotypes. Although progress has been made in the development of therapeutic approaches for retinal diseases in general, the highly complex interplay of functions mediated by Prph2 and the precise regulation of these complexes made it difficult, thus far, to develop a suitable Prph2-specific therapy. Here we describe the latest results obtained in Prph2-associated research and how mouse models provided new insights into the pathogenesis of its related diseases. Furthermore, we give an overview on the current status of the development of therapeutic solutions.

**Keywords:** peripherin 2; retinal degeneration; retina; tetraspanin; photoreceptor
