The role of the new protein MRP in brain development

During the development of the central nervous system, immature precursor cells differentiate into neurons and extend long processes, axons and dendrites, to make contact with other neurons and to form the intricate network capable of information processing. Correct guidance of axons to the appropriate target cells is crucial for proper development. Axons are guided by signal molecules produced and secreted by surrounding cells. These signals are received at the tip of the extending axon, the growth cone, and are translated into rearrangements of the growth cone cytoskeleton consisting of microtubules and actin filaments, to bring about changes in the direction of axon extension. Proteins associated with microtubules and actin filaments and capable of crosslinking these two components of the growth cone cytoskeleton are believed to be important for the appropriate response of growth cones to extracellular signals. We have recently demonstrated that one such protein, MAP1B, is indeed crucial for axon guidance to ensure proper brain development. Searching existing nucleotide databases for related proteins we discovered novel genes and cDNAs encoding a novel and unique protein, termed MRP (MAP1B related protein), which so far has not been characterized in any species, Based on the relatedness of MRP to MAP1B and our detection of MRP expression in developmental-specific isoforms in the developing mouse brain I hypothesize that MRP is a novel microtubule-actin crosslinker with an essential function in brain development. I propose to test this hypothesis by cloning the MRP gene and cDNA, by generating and analysing MRP deficient mice, and by analysis of growth cone function of MRP deficient neurons in vitro by time lapse microscopy. In addition, I propose to elucidate the function of MRP by studying the influence of the protein and deletion mutants on cytoskeletal components in an array of cell biological and biochemical assays and by identifying cellular proteins interacting with MRP. These experiments will lead to the isolation and characterization of the last member of the MAP1B family of cytoskeletal crosslinkers involved in axon guidance. They will determine the specific role of MRP in brain development and provide insight into the function and mechanism of action of the shared and unique domains of MRP and its relatives. Moreover, the results of these studies are likely to enhance our knowledge of the mechanisms involved in the response of neuronal growth cones to extracellular signals to achieve correct wiring of the central and peripheral nervous system.

The cells of the human body contain a scaffold, called the cytoskeleton, which enables them to take on and preserve a certain shape. Nerve cells which extend long processes with which they connect to each other to form a network capable of information processing (the brain) are particularly dependent on their cytoskeleton. Alterations and defects in the cytoskeleton can lead to severe disorders such as mental retardation and Alzheimer`s disease. In addition, the cytoskeleton plays a role in the development of cancer cells and in chemotherapy. The building blocks of the cytoskeleton are special proteins which can connect to each other to from filaments inside the cells. Other proteins control in which parts of a cell the cytoskeleton is assembled or disassembled and they complete the cytoskeleton by connecting the filaments and stabilizing them. Some of these latter proteins are known. The aim of the present project was to discover additional such proteins and to determine their influence on the cytoskeleton. Hints that additional stabilizing proteins exist that have not yet been discovered were obtained when we checked the human genome database. At first we found a blueprint for the synthesis of such a protein. We then were able to prove that this blueprint is actually used by the cells to synthesize a novel protein. We unraveled details of the synthesis of this protein, obtained insights in some of its properties, and clarified which parts of the protein were necessary to dock onto cytoskeletal filaments. This novel protein takes part in building the cytoskeleton in nerve cells and other cells of the body and might play a role in the development of cancer cells. The results of our study expand our knowledge about the cytoskeleton and in the future might contribute to the better understanding of cellular mechanisms involved in the development of the nervous system and in the transformation of normal cells to cancer cells.