Uncovering the cellular pathways involved in Lowe syndrome

Lowe syndrome is an X-linked recessive disorder characterized by congenital cataracts, mental retardation, and kidney dysfunction. Mutations in the OCRLl gene lead to Lowe syndrome in all patients to date. Ocrl1 was previously identified as a PIP2 5-phosphatase localized to the trans-Golgi network (TGN). However, the mechanism through which ocrl1 leads to the Lowe syndrome phenotype is still unknown.; The aim of these studies was to define the role of OCRL1, and its paralog, INPP5B. The first goal was to study INPP5B in order to elucidate why murine Inpp5b compensates for ocrl1-deficiency while the human ortholog does not. Human tissues were found to express two INPP5B transcripts. The canonical human transcript lacks an 80-bp domain present in the murine transcript. However, a novel splice form of human INPP5B was found to contain this 80-bp region. Interestingly, the alternatively spliced form was expressed the least in tissues affected in Lowe syndrome. These results suggest that the 80-bp domain in murine INPP5B may be responsible for compensation of ocrl1-deficiency in mice. Additionally, expression of the alternatively spliced form of human INPP5B may contribute to the tissue-specificity of Lowe syndrome.; The second aim focused on identifying interacting proteins of ocrl1. Ocrl1 was found to interact with proteins involved in vesicle trafficking from the TGN. Using patient missense mutations, the ocrl1 binding sites of these proteins were mapped. The PIP2 5-phosphatase domain and C-terminal domain were both essential for the protein interactions. The results suggest that ocrl1 is involved in regulating vesicle formation and protein trafficking at the TGN.; The last aim focused on the RhoGAP-homology domain of ocrl1. This domain is evolutionarily conserved in both ocrl1 and Inpp5b but its function is unknown. In vitro GAP assays showed that ocrl1 had no RhoA-, Rac1, or Cdc42-GAP activity. The results suggest that the C-terminal domain does not function as a GAP but instead functions as a binding site for protein interactions.; The studies have given insight into the function of ocrl1 and its paralog, Inpp5b. Each aim contributed to the identification of pathways and protein targets that may be affected in Lowe syndrome.