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Achievement in IP3 - IP3 receptor signaling

IP3 is now considered to be an important messenger. IP3 was reported to release Ca2+ from non-mitochondrial stores but the mode of action was a mistery. IP3 molecule does not do anything without IP3 receptor and any kind of function of IP3 is found only when IP3 receptor exists. To understand the mechanism of the role of IP3 in cell signaling, it was essential to find IP3 receptor and characterize its property. Mikoshiba discovered the IP3 receptor and found that there are 3 types of isoforms in vertebrate. He discovered and characterized that IP3 receptor is a Ca2+ channel and that it is localized on the endoplasmic reticulum. He cloned cDNA and determined their entire sequences and solved 3-dimensional structure of regulatory domain and IP3 binding core by X-ray crystallographic analysis and whole IP3 receptor by cryo-electron microscopy. He found that the structure of regulatory site of the each types of isoform is different and that associated proteins are also different suggesting that each isoform has different functions. Therefore, it is IP3 receptor that determines the signaling pathway inside the cell and plays a role for multiple diversified cell signalings depending on which type of IP3 receptor IP3 binds. Therefore, determination of all the sequence of 3 types of IP3 receptor contributed to our understanding the physiology and pathophysiology which are evident from the ever increasing list of functions carried out by the combination of IP3 and each type of IP3 receptor, that is, IP3 - IP3 receptor signaling inside the cell. Dysregulation of each type of IP3 receptor is responsible for some of the major diseases in man and rodents such as fertilization abnormality, exocrine secretion deficiency, cardiac disease, taste abnormality and abnormal hair formation or osteoclast abnormality, and Alzheimer's disease and Huntington disease and ER stress-induced brain damage. Mikoshiba further developed inhibitors and succeeded to control the IP3 receptor function and also developed IP3 indicators and Ca2+ indicators to visualize the dynamic role of IP3 receptor in cell signaling which are widely used and contributed a great deal to the cell signaling field. (see more in datail at the bottom)

 

Activities in Developmental Neurobiology

    In order to elucidate the molecular basis of higher brain function, it is essential to know how the complicated structures in a brain are formed. We know that a series of critical developmental processes including fertilization, generation of the neuroectoderm, regional specification of the neural plate, arrangement of neural cells in a laminar fashion, and the formation and maturation of the neural circuits are involved. Our goal is to understand the mechanisms underlying those diverse processes in the nervous system by revealing the structure and function of the molecules that play important roles in development and in maturity. Ultimately, we would like to know how more complex cognitive processes recognition, learning and memory, and consciousness emerge from these foundational mechanisms.

Cell diversity in the cerebellum, which plays an important role in coordinated motor movement, is relatively limited. The neurons and glial cells of the cerebellum exhibit a unique developmental process that results in beautifully aligned cell layers with nicely arranged folia. Isolated from other parts of the brain, it is easy to study the developmental process of the cerebellum as well as its functional activities using an array of techniques from molecular and cellular biology, electrophysiology, histology and biophysics. Although the morphology and function of the cerebellum are unique, principles obtained from its study of are applicable to the whole brain.

Various cerebellar mutant mice show abnormalities in morphology and behavior. Comparing these mutant species to normal controls provides insight on the molecular events associated with development and function. Work in the Laboratory for Developmental Neurobiology, is based on the analyses of cerebellar mutant mice highlighted below:

  •  Reeler mutant mice show abnormal neuronal positioning and loss of folium formation. Study of reeler mutants yielded information about the molecular mechanisms of morphogenesis of the cerebellum. The lab also successfully generated an antibody for the reeler gene product as well as new mutant mouse species, yotari mice, which have a mutation in disabled-1 gene.
  • The identification of a new gene that was highly expressed in cerebellar granule cells, called Zic since it is a zinc finger protein enriched in the cerebellum. The Zic family participates in neural induction and regional specification.
  • Developmental work in the lab uses adenoviral vectors to analyze the compartmentalization of the cerebellar cortex.
  • From studies of mutant mice deficient in Purkinje cells and a mutant showing poor dendrite development and no spines, we discovered that a P400 protein, which is enriched in the Purkinje cell, is an IP3 receptor. The IP3 receptor plays a crucial role in development and in higher brain function.

Activitiy in IP3 receptor/Ca2+ signaling (2005-2009)

Our laboratory is focusing on the role of calcium (Ca2+) signaling in neurobiological activities. It is known that IP3 receptor (IP3R)(which we discoved and sequenced and found that it is a Ca2+ channel located at endoplasmic reticulum (Nature 1989)) is essential in regulation of Ca2+signaling and cell function. (1)IP3 receptor deficient mice and diseases We have generated mutant mice lacking each type of IP3R. IP3R2(-/-)IP3R3(-/-) mice (Science, 2005) showed similar symptom of Sjögren syndrome (dry mouth, dry eye), one of autoimmune diseases. We showed anti-IP3 receptor antibody in their sera (Mod. Rheumatol. 2007). Nasal tissues of the IP3R2(-/-)IP3R3(-/-) mice had severely degenerated, with pathological findings similar to those for rhinitis (nasal inflammation). IP3R2 and IP3R3 play critical roles in nasal mucus secretion, which is essential for tissue protection as well as odorant perception (E. J. Neurosci. 2008). Bone marrow-derived monocyte/macrophage precursor cells lacking IP3R2 did not exhibit Ca2+ oscillation or differentiation into multinuclear osteoclasts in response to recombinant receptor activator of NF-kappaB ligand/macrophage colony-stimulating factor stimulation (PNAS 2008). IP3R3(-/-) mice show abnormal behavioral and electrophysiological responses to sweet, umami, and bitter substances and IP3R3 associates with TRPM5 (J. Biol.Chem. 2007). (2) Discovery of New signaling pathways We developed genetically encoded fluorescent IP3 sensors and used them to monitor the spatiotemporal dynamics of cytosolic IP3 and Ca2+(J Cell Biol. 2006) and explained the mechanism of Ca2+ spike production in the presence of IP3. We discovered that IP3 releases not only Ca2+, but also releases IRBIT ( IP3 receptor binding protein released with inositol trisphosphate). Since IRBIT binds to the IP3 binding core in a phosphorylation dependent manner, IRBIT regulates IP3 induced Ca2+ release (Mol Cell 2006). In addition, IRBIT works as a third messenger to enhance pancreatic type Na, Bicarbonate co-transporter 1 (pNBC1) which regulates pH inside cells (PNAS 2006). The abnormality of pNBC1 shows glaucoma, cataracta, mental retardation, and low body weight. We recently found IRBIT binds and activates CFTR channel (J. Clin. Invest. 2008). Therefore, signaling pathway may be modified to be as follows: [signal  → IP3 → Ca2+ + IRBIT release]. Since IP3 R interacts with many molecules, IP3R works as a scaffold protein in addition to Ca2+ releasing channel. (3) New finding on the mechanism of axonal and dendritic outgrowth Neurite extension is important for the neural circuit formation. We found that Purkinje cells from IP3R1(-/-) mice exhibited abnormal dendritic morphology both in vivo and in cultured condition. We found that it is due to the absence of secretion of BDNF from parallel fibers with BDNF containing granules accumulated in the presynaptic terminals because of the absence of IP3R1 in granule cells (J. Neurosci. 2006). In addition, we here discovered that macropinocytosis, a clathrin-independent endocytosis, is important for neurite regulation (Mol Cell Neursci. 2009).

Read more in detailDiscovery of IP3 receptor

Rodbell Lecture at NIH (NIEHS)



©2007 Laboratory for Developmental Neurobiology, RIKEN BSI
Last update: 06/04/2013