Understanding how a disease develops from a healthy to pathological condition is one of the challenges in human molecular genetics nowadays. Many genes have been identified as responsible for retinal dystrophies and some of them encode transcription factors that control crucial fate decisions, such as those occurring in the differentiation of specific retinal neuron types, or in photoreceptor precursors when deciding between the opposite rod-versus-cone fates. Some of these events are switch-on/switch-off processes that are tightly regulated to respond accurately to cell requirements. One of the most versatile –and reversible– means of protein activity regulation is post-translational modification by the conjugation of Ubiquitin and SUMO, as it has been shown for the retinal transcription factor Nr2E3. Our main hypothesis is that post-translation modification of key transcription and differentiation factors by ubiquitin and ubiquitin-like molecules (SUMO) regulate photoreceptor fate decisions. We aim to: 1) characterize the expression in the mouse retinas of the full catalogue of SUMO pathway enzymes plus the approximately 90 deubiquitinating enzymes (DUBs) encoded in the mammalian genome, by RT-PCR, Western blot and immunolocalization, generating an expression map of these genes in the mouse retina; and 2) explore the contribution of ubiquitin and SUMO in photoreceptor early transcriptional decisions by shotgun silencing screening of shRNA sublibraries against DUB and SUMO-related enzymes using a high-throughput platform. We expect that the results will not only shed light on how photoreceptor precursor cells decide their fate, in differentiation and apoptosis, but also in why disruption of these mechanisms is conducive to disease.

Protein quality control comprises basic pathways necessary for cellular homeostasis. Malfunctions in these quality control pathways are linked to malignancies, neurodegenerative diseases and metabolic syndromes. Most short-lived, abnormal, misfolded proteins are selectively recycled by the ubiquitin-proteasome system: proteins that need to be discarded are selectively ubiquitinated and the polyubiquitin chain is ultimately recognized by the proteasome for degradation. Post-translational modification of proteins by ubiquitin is accomplished through the concerted and hierarchical action of three types of ubiquitin ligases. Ubiquitination is reversible, and deubiquitination is accomplished by deubiquitinating enzymes (DUBs). One function of the ubiquitin proteasome system is to degrade misfolded proteins produced in the endoplasmic reticulum (ERAD). It is not well understood how ubiquitination of ERAD and non-ERAD substrates is regulated. Previous results from our group showed the relevance of the deubiquitinating enzyme Ubiquitin-Specific Protease 25 (USP25) in rescuing several protein components of the sarcomere (MyBPC1 among them), and now we are exploring its involvement in the regulation of human disease mutated proteins (such as CFTRdelta508; CD3delta; and the beta-amiloid protein) by ERAD. On the other hand, most ubiquitin pathway enzymes form part of multiple and labile protein complexes of proteins, which are post-translational modification platforms. We aim to characterize some ubiquitin ligase partners that interact with USP25.