Research Note: Expression of IGF-1 and IGF-1 Receptor Proteins in Skeletal Muscle Fiber Types in Chickens with Hepatic Fibrosis

We investigated the expression of insulin-like growth factor 1 (IGF-1) and IGF-1 type 1 receptor (IGF-1R) in skeletal muscle fiber types in chickens with hepatic fibrosis induced by bile duct ligation (BDL). Eleven hens, approximately 104 weeks old, were randomly assigned to BDL (n = 4) and sham surgery (SHAM; n = 7) groups. In BDL hens, histopathology revealed marked bile duct proliferation and liver fibrosis. The cross-sectional area (CSA) of myofibers from both the pectoralis (PCT) muscles significantly decreased in the BDL group compared with the SHAM group (P < 0.01). In contrast, the CSA of myofibers from the femorotibialis lateralis (FTL) muscle did not decrease in the BDL group. Type I fibers were large, round, and hypertrophic. Elongated type IIA and IIB fibers were also present. For IGF-1 immunostaining, the immunoreaction intensity was higher in the PCT in the BDL group than the SHAM group. Within the BDL group, type I fibers from FTL had a stronger immunoreaction intensity than the type II fibers. For IGF-1R immunostaining, the intensity of the immunoreactions was similar within the PCT in the BDL group compared with the SHAM group. For FTL, type I fibers had stronger reactions to IGF-1R than type II fibers in the BDL group. These results suggest that type I fibers express both IGF-1 and IGF-1R and become hypertrophic in chickens with hepatic fibrosis.

POS0197 SARS-CoV-2 INFECTION CAUSED THROMBOSIS IN THE LUPUS MODEL WITH ANTIPHOSPHOLIPID ANTIBODY, WHEREAS COVID-19 ASSOCIATED THROMBOSIS WAS IRRELEVANT IN PATIENTS WITH POSITIVE ANTIPHOSPHOLIPID ANTIBODY

Background

Thrombosis is a unique complication in coronavirus disease 2019 (COVID-19). We have reported that elevated ferritin and D-dimer on admission were the risk factors of thromboses by analyzing the patients sequentially admitted to our hospital due to COVID-19 (1). However, we have not analyzed thrombotic complications in the view of the antiphospholipid antibodies (aPLs), which are frequently detected in the COVID-19 patients.

Objectives

To elucidate the thrombogenic effects of aPLs in COVID-19, we analyzed the development of thrombosis in three lupus models after SARS-CoV-2 infection. Additionally, we evaluated the association of thrombotic events and the serum profile of aPLs in Japanese patients with COVID-19.

Methods

Three animal models of lupus (MRL-lpr/lpr, NZBxNZW F1 and NZW×BXSB F1) were evaluated in this study. NZW×BXSB F1 was also considered as a model of antiphospholipid syndrome (APS) since aPLs were detected with a high titer (2). Experimental SARS-CoV-2 infection was induced using mouse-passaged virus strain (3). The incidence of thromboses in the lungs and kidneys were identified by evaluating H&E staining and PTAH staining of paraffin-embedded sections. We have experienced 44 thrombotic events in 34 out of 594 patients admitted to our institute. As a non-thrombotic COVID-19, 68 patients were selected to make a 1 to 2 matched-pair based on the propensity score. In total 102 patients, seven types of aPLs (anti-cardiolipin (CL) IgG/IgM, anti-β2GP1 IgG/IgA/IgM, and anti-phosphatidyl serine/prothrombin complex (PS/PT) IgG/IgM) were measured using specific ELISA kits. The patients’ clinical characteristics and serological profile of aPLs were further evaluated.

Results

We identified the development of thromboses in the lungs or kidneys in 6 out of 12 (50%) NZW×BXSB F1 mice after the SARS-CoV-2 infection, whereas no thrombosis was observed in non-infected mice. Further, there was no thrombosis in the other lupus models (0%) after the infection. These findings might suggest the pathogenic role of aPLs under the SARS-CoV-2 infection.

Among our COVID-19 patients, 39 out of 102 (38%) were tested positive for one or more aPLs. The positive ratios of any aPLs were statistically indifferent between the patients with or without thrombosis; anti-CL IgG (8.8% vs 5.9%)/IgM (0% vs 5.9%), anti-β2GP1 IgG (21% vs 12%)/IgA (8.8% vs 15%)/IgM (0% vs 1.5%), and anti-PS/PT IgG (0% vs 2.9%)/IgM (12% vs 13%), respectively. In addition, their titers were relatively lower than those observed in APS patients. The patients’ characteristics and the prognosis of COVID-19 were comparable regardless of the detection of any aPLs. These findings suggested that COVID-19 associated aPLs were irrelevant to thrombotic complications.

Conclusion

Thromboses were induced after the infection of SARS-CoV-2 only in the APS model. However, aPLs detected in COVID-19 patients have little impact on the development of thrombosis. SARS-CoV-2 infection might have a high risk of thrombosis, especially in APS patients, as shown in the case report (4). The discrepancy of its thrombogenic effects of aPLs might be explained by the low titer of the antibody or the diversity of antibody epitope. Further analyses are required to clarify the mechanisms of aPLs production and the development of thrombosis in COVID-19.

References

[1]Oba S, et al. Arterial and Venous Thrombosis Complicated in COVID-19: A Retrospective Single-Center Analysis in Japan. Front Cardiovasc Med. 2021 Nov 19;8:767074.

[2]Hashimoto Y, et al. Anticardiolipin antibodies in NZW x BXSB F1 mice. A model of antiphospholipid syndrome. J Immunol. 1992 Aug 1;149(3):1063-8.

[3]Iwata-Yoshikawa N, et al. A lethal mouse model for evaluating vaccine-associated enhanced respiratory disease during SARS-CoV-2 infection. Sci Adv. 2022 Jan 7;8(1):eabh3827.

[4]Chidharla A, et al. A Case Report of COVID-Associated Catastrophic Antiphospholipid Syndrome Successfully Treated with Eculizumab. J Blood Med. 2021 Oct 30;12:929-933.

Disclosure of Interests

Seiya Oba: None declared, Tadashi Hosoya Speakers bureau: Janssen Pharmaceutical K.K.

Daiichi Sankyo Company, limited

Asahi Kasei Corporation

Ono pharmaceuticals

Eisai

Eli Lilly, Daisuke Kawata: None declared, Wenshi Lee: None declared, Mari Kamiya: None declared, Yoji Komiya: None declared, Hideyuki Iwai: None declared, Yuko Nukui: None declared, Shuji Tohda: None declared, Shinsuke Yasuda Speakers bureau: Abbvie,

Asahi Kasei Pharma,

Chugai Pharmaceutical,

Eisai, Eli Lilly,

GlaxoSmithKline,

Mitsubishi Tanabe Pharma,

Ono pharmaceutical,

Pfizer., Consultant of: ImmunoForge, Grant/research support from: Abbvie,

Asahi Kasei Pharma,

Chugai Pharmaceutical,

CSL Behring,

Eisai,

ImmunoForge,

Mitsubishi Tanabe Pharma,

Ono pharmaceutical

Deregulated expression of a novel component of TFTC/STAGA histone acetyltransferase complexes, rat SGF29, in hepatocellular carcinoma: possible implication for the oncogenic potential of c-Myc

c-Myc N-terminal conserved domains, MbI and MbII, are essential for c-Myc-mediated transformation and transactivation. These domains recruit the STAGA (SPT3-TAF9-GCN5-acetyltransferase) coactivator complex, but not TFTC (TATA-binding protein-free TAF-containing) to the target gene promoter. Although components of this complex are well conserved between yeast and mammals, four mammalian orthologs of yeast SPT8, SPT20, SGF11 and SGF29 remain to be identified. Here, we isolated a rat ortholog of yeast SGF29, a component of yeast SAGA (SPT-ADA-GCN5-acetyltransferase) complex. Both rat (r) SGF29 and c-myc mRNAs were overexpressed in five out of the eight tested rodent tumor cells. rSGF29 directly interacted with rADA3 and co-immunoprecipitated with two other TFTC/STAGA components, rGCN5 and rSPT3. rSGF29 was recruited to the c-Myc target gene promoters together with c-Myc, and it activated c-Myc target gene expressions. Downregulation of rSGF29 suppressed the expression of c-Myc target genes and inhibited anchorage-independent growth and tumorigenicity and lung metastasis of rat hepatoma K2 cells when injected into nude mice. These results show that rSGF29 is a novel component of TFTC/STAGA complexes and could be involved in the c-Myc-mediated malignant transformation.

Biophysical properties of stable microtubules in neurites revealed by optical techniques

We tested the stability of microtubules in the neurites of cultured dorsal root ganglion cells by dissolving the cytoplasmic membrane with detergent and exposing them to defined extracellular medium under the microscope. Smooth cytoplasmic filaments visualized after membrane removal were suggested to be microtubules by the preservation of all of the filaments in the presence but not in the absence of taxol. They were further confirmed to be microtubules by immunostaining with anti-tubulin antibody. Significant number of microtubules in the established neurites remained longer than 1 hour after membrane removal. To investigate their stabilization mechanism, we transected the exposed microtubules by laser microbeam irradiation and observed their length changes with video-enhanced microscopy. Microtubule fragments started to shorten on both sides of the transection site, more rapidly from the newly generated plus ends than from the minus ends. The maximal rate as well as the pattern of shortening correlated with the time of transection; microtubules transected later than 30 min after membrane removal shortened at rates less than 20 microm/min and typically with intermittent pauses, while the more labile microtubules included in the earlier transections shortened continuously at higher rates. Microtubules in neurites were thus stabilized by (1) stopping disassembly at local sites including the plus ends, and (2) slowing disassembly along the length. Observations of the course of disassembly also suggested the presence of specialized points along microtubules which is involved in anchoring microtubules to the substratum or transiently stopping disassembly.

Dose-dependent effect of an oral adsorbent, AST-120, in patients with early chronic renal failure

We evaluated the dose dependence of an oral adsorbent, AST-120, in 31 patients with early chronic renal failure (baseline serum creatinine: 1.2-3.0 mg/dl). Twenty-three patients were given AST-120 and eight patients were not. AST-120 was administered at three different maintenance doses, < 3.0 g, 3.0 g and 6.0 g/day, according to patients' ability to tolerate treatment. The treatment period was 12 months. The slope of the reciprocal of serum-creatinine concentration versus time was calculated to assess the progression of renal failure. This slope became significantly less steep after AST-120 treatment at 6.0 g/day, but did not change significantly at the other doses. These findings suggest that 6.0 g/day of AST-120 may delay the initiation of dialysis in patients with early chronic renal failure.

Movement of endoplasmic reticulum in the living axon is distinct from other membranous vesicles in its rate, form, and sensitivity to microtubule inhibitors

The endoplasmic reticulum (ER) is the major membranous component present throughout the axon. Although other membranous structures such as synaptic vesicles are known to move via fast axonal transport, the dynamics of ER in the axon still remains unknown. To study the dynamics of ER in the axon, we have directly visualized the movement of two ER-specific membrane proteins, the sarcoplasmic/endoplasmic reticulum calcium-ATPase and the inositol 1,4,5-trisphosphate receptor, both of which were tagged with green fluorescence protein (GFP) and expressed in cultured chick dorsal root ganglion neurons. In contrast to GFP-tagged synaptophysin that moved as vesicles at 1 microm/sec predominantly in the anterograde direction in the typical style of fast axonal transport, the two ER proteins did not move in a discrete vesicular form. Their movement determined by the fluorescence recovery after photobleaching technique was bi-directional, 10-fold slower (approximately 0.1 microm/sec), and temperature-sensitive. The rate of movement of ER was also sensitive to low doses of vinblastine and nocodazole that did not affect the rate of synaptophysin-GFP, further suggesting that it is also distinct from the well-documented movement of membranous vesicles in its relation with microtubules.

Neurofilaments of Klotho, the mutant mouse prematurely displaying symptoms resembling human aging

We reported previously that neurofilaments (NFs) of aged rats were highly packed in the axon and contained a smaller amount of NF-M as compared with those of young rats (Uchida et al. [1999] J. Neurosci. Res. 58:337-348). We studied NFs of the mutant mouse, named Klotho, which displays prematurely symptoms resembling human aging. The transport of axonal cytoskeletal proteins, including NFs, tubulin and actin, was decreased at the leading portion of the peak of transported proteins in Klotho during the process of premature aging. The nearest neighbor inter-NF distance in Klotho axons (35-39 nm) was shorter than that of the wild-type mouse (48-49 nm), indicating the packing of NFs in Klotho. The ratio of NF-M to NF-L was slightly decreased in cytoskeletons from the spinal cords of Klotho. These changes are similar, though not identical, to those observed in aged rats, and are the first evidence of age-related changes in the neurons of Klotho.

Morphological changes and recovery process in the tenotomized soleus muscles of the rat

Tenotomized soleus muscles of adult rats were analyzed morphologically and biochemically with special reference to the recovery process. Light microscopic observations of semi-thin sections showed that the characteristic central core lesion was most extensive at 1 week after tenotomy and began to diminish in extent at 2 weeks until no trace of lesion could be seen by 6th week, as confirmed by thin-section electron microscopy. Three phases of changes in the cross-sectional area of muscle fibers after tenotomy were demonstrated by morphometry: phase I designated as the initial increase up to the 3rd day, phase II as the progressive decrease until the 4th week, and phase III as the recovery to normal or even hypertrophy. In electron microscopy, the earliest alteration of myofibrils was recognized at 3 days after tenotomy. The Z discs showed a wavy or zigzag profile with frequent longitudinal splitting of myofibrils. From the 2nd week on, muscle fibers underwent a process of recovery, replacing the central core lesion with new myofibrils in which a reassembly of thick filaments into bundles of thin filaments took place, with Z discs being aligned adjacent to the peripheral complete myofibrils. In SDS-polyacrylamide gel electrophoresis, the molar ratio of myosin to actin diminished markedly as the central core lesion developed and gradually returned to normal with time, correlating well with the loss and subsequent reassembly of thick filaments.

Globular tail of myosin-V is bound to vamp/synaptobrevin

VAMP/synaptobrevin is one of a number of v-SNAREs involved in vesicular fusion events in neurons. In a previous report, VAMP was shown to form a complex with synaptophysin and myosin V, a motor protein based on the F-actin, and that myosin V was then released from the complex in a Ca(2+)-dependent manner. Here, we found that VAMP alone is bound to myosin V in a Ca(2+)-independent manner, and determined that the globular tail domain of myosin V is its binding site. The syntaxin-VAMP-myosin V formed in the presence of Ca(2+)/calmodulin (CaM). In the absence of CaM, only syntaxin-VAMP, or VAMP-myosin V complex was formed. Our results suggest that VAMP acts as a myosin V receptor on the vesicles and regulates formation of the complex.

Cilostazol prevents impairment of slow axonal transport in streptozotocin-diabetic rats

We studied the effects of cilostazol, an antiplatelet and vasodilating agent, on axonal transport patterns of cytoskeletal proteins in the motor fibers of sciatic nerve of streptozotocin-induced diabetic rats. Proteins labeled with L-[35S]methionine in 6-mm consecutive segments of the nerve were analyzed electrophoretically following fractionation into Triton-soluble and-insoluble subpopulations. Transport rates of proteins (particularly neurofilaments) in slow component a were reduced by 50% 2 weeks after labeling (4 weeks after streptozotocin). An apparent reduction of tubulin and actin was observed at later intervals after induction of diabetes. Actin transported in slow component b was also impaired, though to a lesser extent than in component a. Cilostazol prevented transport impairment of both slow components a and b without affecting hyperglycemia or reduction in body weight gain. These results suggest that in sciatic motor fibers early defects in slowly transported proteins are more marked in slow component a, and that impairment may be caused primarily by hemodynamic abnormalities.

Nitrous oxide, but not xenon, affects the signaling in the neuronal growth cone

1. Xenon (Xe) is an inert gas with the anesthetic property. To investigate whether Xe affects the neural network formation, the authors examined the biochemical characteristics of growth cones prepared from rat forebrains at different perinatal periods, in comparison with inhalation of N2O. 2. Fetal or neonatal rats were exposed to an atmosphere containing inhalational anesthetics (70% Xe or N2O) or the control atmosphere (30% O2 and 70% N2) for 6 h. After the exposure, isolated growth cone particles (IGCs) were prepared from their forebrains using a subcellular fractionation method. Protein composition, Ca2+-dependent protein phosphorylation, protein kinase C (PKC) activity, and degradation of PKC in the IGCs were compared among three groups. 3. No apparent change of protein composition in IGCs was observed by electrophoresis. Ca2+dependent phosphorylation of GAP-43 and MARCKS protein, and PKC activity in IGCs significantly decreased after exposure to N20. The degradation of PKC increased significantly after inhalation of N2O. 4. The authors concluded that Xe dose not change the above biochemical characteristic of the growth cones, suggesting that Xe is free from the teratogenic effect on the neuronal network formation and that Xe is a safe anesthetics for the perinatal neuronal development.

Multiple functional defects in peripheral autonomic organs in mice lacking muscarinic acetylcholine receptor gene for the M3 subtype

Muscarinic acetylcholine receptors consist of five distinct subtypes and have been important targets for drug development. In the periphery, muscarinic acetylcholine receptors mediate cholinergic signals to autonomic organs, but specific physiological functions of each subtype remain poorly elucidated. Here, we have constructed and analyzed mutant mice lacking the M(3) receptor and have demonstrated that this subtype plays key roles in salivary secretion, pupillary constriction, and bladder detrusor contractions. However, M(3)-mediated signals in digestive and reproductive organs are dispensable, likely because of redundant mechanisms through other muscarinic acetylcholine receptor subtypes or other mediators. In addition, we have found prominent urinary retention only in the male, which indicates a considerable sex difference in the micturition mechanism. Accordingly, this mutant mouse should provide a useful animal model for investigation of human diseases that are affected in the peripheral cholinergic functions.

Real-time observation of the disassembly of stable neuritic microtubules induced by laser transection: possible mechanisms of microtubule stabilization in neurites

By dissolving the membrane with detergent perfusion, we have shown that the established neurites of dorsal root ganglion cells cultured for more than 5 days contained microtubules which persisted outside the cell for a few minutes to more than 1h [Tashiro et al., 1997: J. Neurosci. Res. 50:81-93]. To investigate their stabilization mechanism, we transected the exposed microtubules by laser microbeam irradiation and observed their length changes with video-enhanced differential interference contrast microscopy. Microtubule fragments started to shorten on both sides of the transection site. more rapidly from the newly generated plus ends than from the minus ends. The maximal rate as well as the pattern of shortening correlated with the time of transection; microtubules transected later than 30 min after membrane removal shortened at rates less than 20 microm/min and typically with intermittent pauses, while the more labile microtubules included in the earlier transections shortened continuously at higher rates. Microtubules in neurites were thus stabilized by 1) stopping disassembly at local sites including the plus ends, and 2) slowing disassembly along the length. Transection also suggested the presence of specialized points along microtubules which are involved in anchoring microtubules to the substratum. Cell Motil. Cytoskeleton 42:87-100, 1999.

The mechanism of the neurotransmitter release in growth cones

The growth cone is considered the precursor of the presynaptic terminal. To elucidate the minimal molecular machinery required for exocytosis, we examined the characteristics of alpha-latrotoxin-induced exocytosis in growth cones. In isolated growth cones (IGC), neurotransmitters were released in a SNARE-dependent manner, but rab3A cycling was blocked. By supplying rabphilin, a rab3A acceptor found in low levels in IGC, the IGC obtained as high an exocytotic efficiency as adult synaptosomes, and the complete GDP-GTP conversion of rab3A occurred on growth cone vesicles (GCV). GCVs bound SNAREs but not NSF or alpha-SNAP; whereas in the rabphilin-supplied IGC, GCVs recruited both NSF and alpha-SNAP, to form the SNARE-NSF-SNAP complex. These results suggest that rab3A cycling is dependent upon the accumulation of rabphilin and is completed later than the SNARE mechanism, and that rabphilin is involved in determining the efficiency of exocytosis by modifying the SNARE mechanism.

Selective solubilization of high-molecular-mass neurofilament subunit during nerve regeneration

A reduction in neurofilament (NF) protein synthesis and changes in their phosphorylation state are observed during nerve regeneration. To investigate how such metabolic changes are involved in the reorganization of the axonal cytoskeleton, we studied the injury-induced changes in the solubility and axonal transport of NF proteins as well as their phosphorylation states in the rat sciatic nerve. In the control nerve, 15-25% of high-molecular-mass NF subunit (NF-H) was recovered in the 1% Triton-soluble fraction when fractionated in the presence of phosphatase inhibitors. After a complete loss of NF proteins distal to the injury site (70-75 mm from the spinal cord) 1 week after injury, NF-H detected in the regenerating sprouts at 2 weeks or later exhibited higher solubility (>50%) and lower C-terminal phosphorylation level than NF-H in the control nerve. Solubility increase was also apparent with L-[35S]methionine-labeled NF-H that was in transit in the proximal axon at the time of injury. The low-molecular-mass subunit remained in the insoluble fraction in both the normal and the regenerating nerves, indicating that selective solubilization of NF-H rather than total filament disassembly occurs during regeneration.

GABA(A) receptor in growth cones: the outline of GABA(A) receptor-dependent signaling in growth cones is applicable to a variety of alpha-subunit species

The growth cone is responsible for axonal elongation and pathfinding by responding to various modulators for neurite growth, including neurotransmitters. We demonstrated an outline of the gamma aminobutyric acid (GABA)(A)-dependent signaling in growth cones. Here, we examined the effects of the modulators of GABA(A) receptor on the signaling in growth cones. Phenobarbital or propofol, acting on beta-subunit, enhanced the [Cl(-)]infi change and [Ca(2+)](i) elevation by the GABA stimulation to isolated growth cones. Besides, propofol enhanced GABA-dependent phosphorylation of growth-associated protein of 43 kDa (GAP-43) by protein kinase C. In contrast, an alpha-subunit acting agent diazepam did not modulate any of the above signals. Next, we examined the effect of the developmental change of alpha-subunit on the outline of the GABA(A)-dependent signaling in growth cones. We also found that the amounts of several different alpha-subunit isoforms developmentally increased or decreased in growth cone membrane (GCM), but that the affinity and density of the [(3)H]diazepam binding sites were similar to those in adult synaptic membrane. Taken together, our results strongly suggest that each step of GABA(A)-dependent signaling in GCM is not modified by diazepam, indicating that the signaling pathway mediated by GABA(A) receptor in growth cones is applicable to any compositional change of alpha-subunit isoforms.

Relationship between erythropoietin and chronic heart failure in patients on chronic hemodialysis

In the present study, the relationship between the blood erythropoietin level and cardiac function was investigated in 15 patients on chronic hemodialysis who developed chronic heart failure. Another 45 patients without cardiac dysfunction were selected as a control group that was matched for gender, age, and the duration of dialysis. The erythropoietin level was 256.3 +/- 481.8 mU/ml in the heart failure group, which was significantly higher than that in the control group (17.0 +/- 10.0 mU/ml, P < 0.01). Eight of the 15 patients in the heart failure group maintained a hematocrit of more than 30% without receiving recombinant human erythropoietin therapy, whereas 29 of the 45 patients in the control group required erythropoietin. In the heart failure group, the erythropoietin level was significantly correlated with the levels of atrial natriuretic peptide and brain natriuretic peptide (P < 0.01). These results suggest that heart failure can increase the erythropoietin level in proportion to the severity of cardiac dysfunction, even in patients on long-term dialysis.

Effects of cilostazol, an antiplatelet agent, on axonal regeneration following nerve injury in diabetic rats

To evaluate the ability of cilostazol, an antiplatelet and vasodilating agent, to promote axonal regeneration in streptozotocin-induced diabetic rats, the time until beginning of regeneration (initial delay) and the axonal regeneration rate of the sciatic nerve were estimated using the pinch test, and ornithine decarboxylase activity was measured in dorsal root ganglia. At 5 weeks of diabetes, axonal regeneration rate remained unchanged but the initial delay was prolonged and ornithine decarboxylase induction was delayed in diabetic rats compared with those in normal rats. Cilostazol had little effect on these parameters in normal or diabetic rats. At 10 weeks of diabetes, diabetic rats showed both prolongation of initial delay and a decrease in axonal regeneration rate. Cilostazol markedly increased axonal regeneration rate in diabetic rats. Ornithine decarboxylase induction following nerve injury disappeared almost completely in diabetic rats but was maintained by cilostazol treatment. The effect of cilostazol in diabetic rats is thought to be mediated through its preventive effect on circulatory disorders. The active site of the drug appears to be early processes in nerve regeneration before ornithine decarboxylase induction. Further, the results suggest that the both axonal regeneration and this induction are sensitive to circulatory defects in diabetes.

Stimulation of L-type Ca2+ channel in growth cones activates two independent signaling pathways

Although growth cones respond to various modulators of neurite outgrowth, such as neurotrophins, neurotransmitters, and cell adhesion molecules, the signal-transducing mechanisms for these modulators in growth cones are unclear. Since recent studies have suggested that the signals of these modulators are mediated by Ca2+ influx through L-type voltage-sensitive Ca2+ channels (VSCCs) in the growth cone, we examined L-type VSCC-dependent signaling pathways, using isolated growth cones (IGCs) from developing rat forebrains. Binding assays revealed that L-type VSCC is enriched in growth cone membrane and gradually decreased in amount developmentally, while N-type VSCC has the opposite tendency. In intact IGCs, Bay K 8644 (BK, an L-type agonist) induced much more rapid elevation of [Ca2+]i than that in adult synaptosomes. Ca2+-dependent phosphorylation of GAP-43 and MARCKS protein by protein kinase C (PKC) was enhanced in the IGC by BK, resulting in the release of these proteins from the membrane, which is consistent with our recent report. In addition, the Ca2+-dependent degradation of brain spectrin (fodrin) by calpain was also enhanced by BK or GABA, consequently inducing the release of alpha-actinin from the membrane skeleton of the growth cones. The activities of PKC and calpain were not inhibited by inhibitors of the other, indicating that these reactions occur independently. Our results suggest that Ca2+ influx through L-type VSCCs activates two distinct signaling branches, probably in the different domains of the growth cone, i.e., Ca2+-dependent phosphorylation of GAP-43 and MARCKS protein, and Ca2+-dependent degradation of brain spectrin and the release of alpha-actinin by calpain.

Direct visualization and characterization of stable microtubules from the neurites of cultured dorsal root ganglion cells

We tested the stability of microtubules (MTs) in the neurites of cultured dorsal root ganglion cells by dissolving the cytoplasmic membrane with detergent and exposing them to defined extracellular medium under observation with a video-enhanced differential interference contrast (DIC) microscope. Smooth cytoplasmic filaments visualized after membrane removal were suggested to be MTs by the preservation of all of the filaments in the presence but not in the absence of taxol. They were further confirmed to be MTs by specific immunostaining with anti-tubulin antibody. A significant number of MTs in the established neurites of 6-day-old cultures remained longer than 10 min after membrane removal while MTs in the Schwann cell processes or in the distal regions of the growth cone-bearing neurites of 3-day-old cultures disappeared within 2 min. A population of very stable MTs persisting longer than 30 min was also found specifically in the 6-day-old cultures. Association with other structures or bundling seemed to stabilize the MTs to some degree. The most stable MTs, however, were not associated with some structure along the length but were mainly anchored at points, suggesting that specific point attachments may be another important mechanism operating in MT stabilization. The present method is thus capable of directly demonstrating the unusual stability of neuritic MTs, and provides a new system for further investigation on the mechanism of stabilization.

Increased solubility of high-molecular-mass neurofilament subunit by suppression of dephosphorylation: its relation to axonal transport

To investigate the role of phosphorylation in the turnover and transport of neurofilament (NF) proteins in vivo, we studied their solubility properties and axonal transport in the rat sciatic nerve using phosphatase inhibitors to minimize dephosphorylation during preparation. About 20% of the 200-kDa subunit (NF-H) in the axon was soluble in the 1% Triton-containing buffer under the present conditions, whereas this amount was less and more variable in the absence of phosphatase inhibitors. The 68-kDa subunit (NF-L) was exclusively insoluble and not affected by the inhibitors. Such selective solubilization of NF-H by phosphorylation differed significantly from the in vitro phosphorylation with cyclic AMP-dependent protein kinase, which resulted in NF disassembly. The carboxy-terminal phosphorylation state of NF-H probed with the phosphorylation-sensitive antibodies was also not directly related to solubility. The solubility of NF-H did not differ along the nerve. In contrast, the solubility of L-[35S]methionine-labeled, transported NF-H was lowest at the peak of radioactivity. Higher solubility at the leading edge, regardless of its location along the nerve, indicates that NF-H solubility is positively correlated with the rate of NF transport.

The soluble N-ethylmaleimide-sensitive factor attached protein receptor complex in growth cones: molecular aspects of the axon terminal development

Soluble N-ethylmaleimide-sensitive factor attached protein (SNAP) receptor (SNARE) mechanisms are thought to be involved in two important processes in axonal growth cones: (1) membrane expansion for axonal growth and (2) vesicular membrane fusion for mature synaptic transmission. We investigated the localization and interactions among the proteins involved in SNARE complex formation in isolated growth cone particles (GCP) from forebrain. We demonstrated that the SNARE complex is present in GCPs morphologically without synaptic vesicles (SVs) and associated with growth cone vesicles. However, the apparently SV-free GCP was lacking in the regulatory mechanisms inhibiting SNARE complex formation proposed in SV fusion, i.e., the association of synaptotagmin with the SNARE complex, and vesicle-associated membrane protein (VAMP)-synaptophysin complex formation. The core components of the SNARE complex (syntaxin, SNAP-25, and VAMP) accumulated for several days before postnatal day 7, when SVs first appeared, and preceded the accumulation of marker proteins such as synaptophysin, SV2, and V-ATPase. Our present results suggest that the SNARE mechanism for vesicular transmitter release is not fully functional in growth cones before the appearance of SVs, but the SNARE mechanism is working for membrane expansion in growth cones, which supports our recent report. We concluded that the regulation of the SNARE complex in growth cones is different from that in mature presynaptic terminals and that this switching may be one of the key steps in development from the growth cone to the presynaptic terminal.

Differential axonal transport of soluble and insoluble tau in the rat sciatic nerve

Axonal transport of microtubule-associated protein tau was studied in the motor fibers of the rat sciatic nerve 1-4 weeks after labeling of the spinal cord with [35S]methionine. As 60-70% of low molecular weight tau in this system was found to be insoluble in 1% Triton-containing buffer, labeled proteins in 6-mm consecutive nerve segments were first separated into Triton-soluble and insoluble fractions. Two-dimensional gel electrophoresis and immunoblotting with anti-tau antibody confirmed the presence of tau among labeled, transported proteins in both fractions. Isoform composition of labeled tau was similar to that of bulk axonal tau, the most acidic species with apparent molecular mass of 66 kDa being the major component. Transport profiles obtained by measuring radioactivities associated with this major isoform showed that soluble and insoluble tau were transported at different rates. Insoluble tau, which contained the majority of tau-associated radioactivity, was transported at 1.7 mm/day in slow component a (SCa), whereas soluble tau was transported faster, at 3 mm/day, corresponding to the rate of slow component b (SCb). Cotransport of insoluble tau with insoluble tubulin in SCa suggests its association with stable microtubules.

Signaling pathway downstream of GABAA receptor in the growth cone

The growth cone is responsible for axonal elongation and pathfinding by responding to various modulators for neurite growth, including neurotransmitters, although the sensor mechanisms are not fully understood. Among neurotransmitters, GABA is most likely to demonstrate activity in vivo because GABA and the GABAA receptor appear even in early stages of CNS development. We investigated the GABAA receptor-mediated signaling pathway in the growth cone using isolated growth cones (IGCs). Both the GABAA binding site and the benzodiazepine modulatory site were enriched in the growth cone membrane. In the intact IGC, GABA induced picrotoxin-sensitive Cl- flux (not influx but efflux) and increased the intracellular Ca2+ concentration in a picrotoxin- and verapamil-sensitive manner. Protein kinase C (PKC)-dependent phosphorylation of two proteins identified as GAP-43 and MARCKS protein was enhanced in the intact IGC stimulated by GABA, resulting in the release of MARCKS protein and GAP-43 from the membrane. Collectively, our results suggest the following scheme: activation of the functional GABAA receptor localized in the growth cone membrane-->Cl- efflux induction through the GABAA-associated Cl- channel-->Ca2+ influx through an L-type voltage-sensitive Ca2+ channel-->Ca(2+)-dependent phosphorylation of GAP-43 and MARCKS protein by PKC.