Antioxidant Signaling Involving the Microtubule Motor KIF12 Is an Intracellular Target of Nutrition Excess in Beta Cells

SummaryBeta cell injury due to oxidative stress is a typical etiology of diabetes caused by nutritional excess, but its precise mechanism remains largely elusive. Here, we demonstrate that the microtubule motor KIF12 mediates an antioxidant cascade in beta cells as an intracellular target of excess fat intake or “lipotoxicity.” KIF12 knockout mice suffer from hypoinsulinemic glucose intolerance due to increased beta cell oxidative stress. Using this model, we identified an antioxidant signaling cascade involving KIF12 as a scaffold for the transcription factor Sp1. The stabilization of nascent Sp1 appeared to be essential for proper peroxisomal function by enhancing Hsc70 expression, and the pharmacological induction of Hsc70 expression with teprenone counteracted the oxidative stress. Because KIF12 is transcriptionally downregulated by chronic exposure to fatty acids, this antioxidant cascade involving KIF12 and Hsc70 is proposed to be a critical target of nutritional excess in beta cells in diabetes

Defects in Axonal Elongation and Neuronal Migration in Mice with Disrupted tau and map1b Genes

Tau and MAP1B are the main members of neuronal microtubule-associated proteins (MAPs), the functions of which have remained obscure because of a putative functional redundancy (Harada, A., K. Oguchi, S. Okabe, J. Kuno, S. Terada, T. Ohshima, R. SatoYoshitake, Y. Takei, T. Noda, and N. Hirokawa. 1994. Nature. 369:488–491; Takei, Y., S. Kondo, A. Harada, S. Inomata, T. Noda, and N. Hirokawa. 1997. J. Cell Biol. 137:1615–1626). To unmask the role of these proteins, we generated double-knockout mice with disrupted tau and map1b genes and compared their phenotypes with those of single-knockout mice. In the analysis of mice with a genetic background of predominantly C57Bl/6J, a hypoplastic commissural axon tract and disorganized neuronal layering were observed in the brains of the tau +/+map1b-/-mice. These phenotypes are markedly more severe in tau -/-map1b-/-double mutants, indicating that tau and MAP1B act in a synergistic fashion. Primary cultures of hippocampal neurons from tau -/-map1b-/-mice showed inhibited axonal elongation. In these cells, a generation of new axons via bundling of microtubules at the neck of the growth cones appeared to be disturbed. Cultured cerebellar neurons from tau -/-map1b-/-mice showed delayed neuronal migration concomitant with suppressed neurite elongation. These findings indicate the cooperative functions of tau and MAP1B in vivo in axonal elongation and neuronal migration as regulators of microtubule oganization

MAP2 is required for dendrite elongation, PKA anchoring in dendrites, and proper PKA signal transduction

Microtubule-associated protein 2 (MAP2) is a major component of cross-bridges between microtubules in dendrites, and is known to stabilize microtubules. MAP2 also has a binding domain for the regulatory subunit II of cAMP-dependent protein kinase (PKA). We found that there is reduction in microtubule density in dendrites and a reduction of dendritic length in MAP2-deficient mice. Moreover, there is a significant reduction of various subunits of PKA in dendrites and total amounts of various PKA subunits in hippocampal tissue and cultured neurons. In MAP2-deficient cultured neurons, the induction rate of phosphorylated CREB after forskolin stimulation was much lower than in wild-type neurons. Therefore, MAP2 is an anchoring protein of PKA in dendrites, whose loss leads to reduced amount of dendritic and total PKA and reduced activation of CREB

The Cytoskeletal Architecture of the Presynaptic Terminal and Molecular Structure of Synapsin 1

We have examined the cytoskeletal architecture and its relationship with synaptic vesicles in synapses by quick-freeze deep-etch electron microscopy (QF.DE). The main cytoskeletal elements in the presynaptic terminals (neuromuscular junction, electric organ, and cerebellar cortex) were actin filaments and microtubules. The actin filaments formed a network and frequently were associated closely with the presynaptic plasma membranes and active zones. Short, linking strands approximately 30 nm long were found between actin and synaptic vesicles, between microtubules and synaptic vesicles. Fine strands (30-60 nm) were also found between synaptic vesicles. Frequently spherical structures existed in the middle of the strands between synaptic vesicles. Another kind of strand (approximately 100 nm long, thinner than the actin filaments) between synaptic vesicles and plasma membranes was also observed. We have examined the molecular structure of synapsin 1 and its relationship with actin filaments, microtubules, and synaptic vesicles in vitro using the low angle rotary shadowing technique and QF.DE. The synapsin 1, approximately 47 nm long, was composed of a head (approximately 14 nm diam) and a tail (approximately 33 nm long), having a tadpole-like appearance. The high resolution provided by QF.DE revealed that a single synapsin 1 cross-linked actin filaments and linked actin filaments with synaptic vesicles, forming approximately 30-nm short strands. The head was on the actin and the tail was attached to the synaptic vesicle or actin filament. Microtubules were also cross-linked by a single synapsin 1, which also connected a microtubule to synaptic vesicles, forming approximately 30 nm strands. The spherical head was on the microtubules and the tail was attached to the synaptic vesicles or to microtubules. Synaptic vesicles incubated with synapsin 1 were linked with each other via fine short fibrils and frequently we identified spherical structures from which two or three fibril radiated and cross-linked synaptic vesicles. We have examined the localization of synapsin 1 using ultracryomicrotomy and colloidal gold-immunocytochemistry of anti-synapsin 1 IgG. Synapsin 1 was exclusively localized in the regions occupied by synaptic vesicles. Statistical analyses indicated that synapsin 1 is located mostly at least approximately 30 nm away from the presynaptic membrane. These data derived via three different approaches suggest that synapsin 1 could be a main element of short linkages between actin filaments and synaptic vesicles, and between microtubules and synaptic vesicles, and between synaptic vesicles in the nerve terminals.(ABSTRACT TRUNCATED AT 400 WORDS

Role of KIFC3 motor protein in Golgi positioning and integration

KIFC3, a microtubule (MT) minus end–directed kinesin superfamily protein, is expressed abundantly and is associated with the Golgi apparatus in adrenocortical cells. We report here that disruption of the kifC3 gene induced fragmentation of the Golgi apparatus when cholesterol was depleted. Analysis of the reassembly process of the Golgi apparatus revealed bidirectional movement of the Golgi fragments in both wild-type and kifC3−/− cells. However, we observed a markedly reduced inwardly directed motility of the Golgi fragments in cholesterol-depleted kifC3−/− cells compared with either cholesterol-depleted wild-type cells or cholesterol-replenished kifC3−/− cells. These results suggest that (a) under the cholesterol-depleted condition, reduced inwardly directed motility of the Golgi apparatus results in the observed Golgi scattering phenotype in kifC3−/− cells, and (b) cholesterol is necessary for the Golgi fragments to attain sufficient inwardly directed motility by MT minus end–directed motors other than KIFC3, such as dynein, in kifC3−/− cells. Furthermore, we showed that Golgi scattering was much more drastic in kifC3−/− cells than in wild-type cells to the exogenous dynamitin expression even in the presence of cholesterol. These results collectively demonstrate that KIFC3 plays a complementary role in Golgi positioning and integration with cytoplasmic dynein