The organization of neurofilaments accumulated in perikaryon following aluminum administration: relationship between structure and phosphorylation of neurofilaments

Morphological and biochemical signs of age-related neurodegenerative changes in klotho mutant mice

Klotho mutant mice, defective in the klotho gene, develop multiple age-related disorders with very short lifespans. Introduction of the exogenous klotho gene into these mutant mice leads to an improvement in their phenotypes, while overexpression of this gene in wild-type mice significantly extends their lifespan. These observations suggest that the klotho gene/protein has an anti-aging function. Since there have been only a few reports with some disagreement about results on the CNS of the mutant mice, we tried to clarify whether the CNS neurons generate aging-like features, even in premature stages, using biochemical and morphological approaches. Results obtained from the mutant mice, when compared with wild-type mice, were as follows. Neurofilaments (NFs) were increased significantly in axons, with the subunit proteins showing a significant enhancement in phosphorylation or expression of NF-H or NF-L, respectively. Microtubules in Purkinje cell dendrites were closer to each other, and in the CNS tissue tubulin was unaltered, but microtubule-associated protein (MAP) 2 was significantly reduced in expression. Neuronal cellular organelles were morphologically disordered. Lysosomes, cathepsin D and light chain 3 of MAP1A/B (LC3) were augmented with the appearance of putative autophagy-related structures. Antiapoptotic Bcl-xL and proapoptotic Bax were reduced and enhanced, respectively, and mitogen-activated protein kinase was reduced. Synapse-related proteins and structures were decreased. Neuronal degeneration was evident in hippocampal pyramidal cells, and possibly in Purkinje cells. Astrocytic glial filaments and glial fibrillary acidic protein were increased in density and expression, respectively. Together, the CNS neuronal alterations in klotho mutant mice were quite similar to those found in aged animals, including even premature death, so this mouse should be a more appropriate animal model for CNS aging than those previously reported.

Abnormal expression of neurofilament proteins in dysmyelinating axons located in the central nervous system of jimpy mutant mice

Myelination in the peripheral nervous system is considered to increase the phosphorylation level of neurofilament proteins in the axon, resulting in an increase in axonal calibre. To understand the relationship between myelination and neurofilament proteins in axons, we examined jimpy mutant mice with a point mutation in the proteolipid protein gene and dysmyelination in the central nervous system. The jimpy mice exhibited a characteristic similarity in neurofilament nature to the myelin-deficient mice in the peripheral nervous system reported previously. The following novel results were obtained in the jimpy mice: dysmyelinated axons, in which the amount of non-phosphorylated neurofilament-H was drastically increased without a significant reduction of the phosphorylated form, compared with the control myelinated axons, did not suffer any decrease in their diameters. Expression levels of all neurofilament subunit proteins and their mRNAs were enhanced in the central nervous system tissue. Because the above biochemical data were obtained from the cytoskeletal fraction, at least some of the increased neurofilament-H and -M proteins appeared to be coassembled into neurofilaments but remained non-phosphorylated. Axonal neurofilaments of the jimpy were, probably due to this abnormal stoichiometry and phosphorylation state in neurofilaments, more compact and random in alignment with less prominent cross-bridges than those of the control, providing possible evidence for disturbing the axonal transport of other organelles. These results suggest that myelination regulates both the expression and phosphorylation of neurofilament proteins, and is essential for the cytoplasmic organization of myelinated axons.

Expression and polarized distribution of an inwardly rectifying K+ channel, Kir4.1, in rat retinal pigment epithelium

1. In the eye, different substances and ions including potassium (K+) are transported between neural retina and choroid via the subretinal space. Inwardly rectifying K+ channels (Kir) on the apical membrane of retinal pigment epithelial (RPE) cells are thought to play an essential role in K+ transport in the subretinal space. 2. Single-channel recordings from the apical membrane of RPE cells exhibited functional expression of a Kir channel with properties identical to those of Kir4.1, while recordings from the basolateral membrane showed no detectable Kir channel currents. 3. The expression of Kir4.1 mRNA in RPE cells was confirmed by RT-PCR analysis and in situ hybridization. Furthermore, using immunohistochemistry, we found that Kir4.1 was prominently expressed in RPE cells and localized specifically on the processes on their apical membrane. 4. Developmental studies revealed that expression of Kir4.1 started to appear 10 days or later after birth in RPE cells, in parallel with the maturation of retinal neuronal activity as represented by the a- and b-waves of the electroretinogram. 5. These data suggest that Kir4.1 is one of the Kir channels involved in RPE-mediated control of K+ ions in the subretinal space.

Expression of an inwardly rectifying K(+) channel, Kir4.1, in satellite cells of rat cochlear ganglia

Satellite cells are glial cells wrapped around somata of sensory and autonomic ganglion neurons. Neither their functional roles nor electrical properties have been fully clarified so far. Using immunohistochemistry, we found that inwardly rectifying K(+) channel subunit Kir4.1 (also called Kir1.2 or K(AB)-2) was expressed prominently in the satellite cells of cochlear ganglia. The Kir4.1 immunoreactivity was localized specifically at the myelin sheaths of satellite cells wrapping the somata of the ganglion neurons. Developmental expression of Kir4.1 in satellite cells paralleled development of the action potential in the auditory nerve. These results suggest that this channel in satellite cells may be responsible for the regulation of K(+) extruded from the ganglion neurons during excitation.

Carbon filaments direct the growth of postlesional plastic axons after spinal cord injury

The effect of implantation of carbon filaments and fetal tissues on the axonal regeneration following contusion injury in a rat model was investigated by in situ immunofluorescence. Female Sprague-Dawley rats were subjected to severe contusion injury to the spinal cord at T9-T10. All animals were divided into 5 groups (N = 5/group): normal controls. surgical controls, with carbon filament implants, with fetal tissue implants and with implants consisting of fetal tissue cocultured with carbon filaments. After a 10-week survival period, the astroglial response was assessed by immunoreactive glial fibrillary acidic protein and the neuro-axonal profile by immunoreactive phosphorylated and nonphosphorylated neurofilament proteins. The contusion injury resulted in: (a) dramatically increased immunoreactivity of glial fibrillary acidic protein indicating injury-associated reactive astrogliosis, (b) increase in immunoreactive phosphorylated neurofilament protein indicating upregulated phosphorylation of neurofilament protein, (c) with no change in the highly differentiated nonphosphorylated neurofilament protein which normally occur in the nonregenerating mature neurons. Implantation of fetal tissues alone following contusion injury did not show any appreciable change with regard to the immunoreactivities for the glial and neuronal markers studied, compared to the injury controls. However, the implantation of carbon filaments alone or together with fetal tissues directed the growth of glial fibrillary acidic protein-positive astroglia and phosphoneurofilament-positive neurites along the carbon fibers, with no effect on nonphosphoneurofilament protein. In conclusion, implantation of carbon filaments appears to be critical for facilitating the attachment of astroglia forming a substrate and scaffolding that can further support and direct the growth of postlesional plastic axons across the lesion. In addition, carbon filament prostheses in combination with fetal tissue implants provides an improved combinational approach to promote regrowth of injured neurons following injury.

Chronic administration of aluminum-fluoride or sodium-fluoride to rats in drinking water: alterations in neuronal and cerebrovascular integrity

This study describes alterations in the nervous system resulting from chronic administration of the fluoroaluminum complex (AlF3) or equivalent levels of fluoride (F) in the form of sodium-fluoride (NaF). Twenty seven adult male Long-Evans rats were administered one of three treatments for 52 weeks: the control group was administered double distilled deionized drinking water (ddw). The aluminum-treated group received ddw with 0.5 ppm AlF3 and the NaF group received ddw with 2.1 ppm NaF containing the equivalent amount of F as in the AlF3 ddw. Tissue aluminum (Al) levels of brain, liver and kidney were assessed with the Direct Current Plasma (DCP) technique and its distribution assessed with Morin histochemistry. Histological sections of brain were stained with hematoxylin & eosin (H&E), Cresyl violet, Bielschowsky silver stain, or immunohistochemically for beta-amyloid, amyloid A, and IgM. No differences were found between the body weights of rats in the different treatment groups although more rats died in the AlF3 group than in the control group. The Al levels in samples of brain and kidney were higher in both the AlF3 and NaF groups relative to controls. The effects of the two treatments on cerebrovascular and neuronal integrity were qualitatively and quantitatively different. These alterations were greater in animals in the AlF3 group than in the NaF group and greater in the NaF group than in controls.

Expression and clustered distribution of an inwardly rectifying potassium channel, KAB-2/Kir4.1, on mammalian retinal Müller cell membrane: their regulation by insulin and laminin signals

Inwardly rectifying potassium (K+) channels (Kir) in Müller cells, the dominant glial cells in the retina, are supposed to be responsible for the spatial buffering action of K+ ions. The molecular properties and subcellular localization of Müller cell Kir channels in rat and rabbit retinas were examined by using electrophysiological, molecular biological, and immunostaining techniques. Only a single population of Kir channel activity, the properties of which were identical to those of KAB-2/Kir4.1 expressed in HEK293T cells, could be recorded from endfoot to the distal portion of Müller cells. Consistently, Northern blot, in situ hybridization, and RT-PCR analyses indicated expression of Kir4. 1 in Müller cells per se. The Kir4.1 immunoreactivity was distributed in clusters throughout Müller cell membrane. The Kir4.1 expression in Müller cells disappeared promptly after culturing. When the dissociated Müller cells were cultured on laminin-coated dishes in the presence of insulin, Kir4.1 immunoreactivity was detected in a clustered manner on the cell membrane. Because insulin and laminin exist in the surrounding of Müller cells in the retina, these substances possibly may be physiological regulators of expression and distribution of Kir4.1 in Müller cells in vivo.

An ATP-dependent inwardly rectifying potassium channel, KAB-2 (Kir4. 1), in cochlear stria vascularis of inner ear: its specific subcellular localization and correlation with the formation of endocochlear potential

Cochlear endolymph has a highly positive potential of approximately +80 mV. This so-called endocochlear potential (EP) is essential for hearing. Although pivotal roles of K+ channels in the formation of EP have been suggested, the types and distribution of K+ channels in cochlea have not been characterized. Because EP was depressed by vascular perfusion of Ba2+, an inhibitor of inwardly rectifying K+ (Kir) channels, but not by either 4-aminopyridine or tetraethylammonium, we examined the expression of Kir channel subunits in cochlear stria vascularis, the tissue that is supposed to play the central role in the generation of positive EP. Of 11 members of the Kir channel family examined with reverse transcription-PCR, we could detect only expression of KAB-2 (Kir4.1) mRNA in stria vascularis. KAB-2 immunoreactivity was specifically localized at the basolateral membrane of marginal cells but not in either basal or intermediate cells. Developmental expression of KAB-2 in marginal cells paralleled formation of EP. Furthermore, deaf mutant mice (viable dominant spotting; WV/WV) expressed no KAB-2 in their marginal cells. These results suggest that KAB-2 in marginal cells may be critically involved in the generation of positive EP.

Preferential localization of annexin V to the axon terminal

To examine the participation of annexin V, a member of Ca(2+)-dependent phospholipid-binding proteins, in the process of synaptic vesicle exocytosis, rat central nervous tissue was analysed using biochemical and morphological techniques. By both fluorescence and confocal laser scanning microscopy, immunoreactivity for annexin V was predominantly localized around neuronal somata and dendrites, and the reactivity was mostly co-labeled with that for synaptophysin. The annexin V immunoreactivity was also detectable, but less intensely, in neuronal perikarya, glial cells and endothelial cells. Both immunoblot and immunoelectron microscopic analyses with intact tissues, synaptosomes and purified synaptic vesicles showed that annexin V was expressed in neurons, preferentially concentrated in axon terminals and associated with synaptic vesicles. Purified synaptic vesicles were relatively homogeneously distributed in the medium where Ca2+ was removed and thus the amount of annexin V was reduced drastically. The vesicles tended to be clustered in the fraction where endogenous annexin V is maintained, and the clusters were more conspicuous when purified human annexin V was added. Synaptic vesicles forming the clusters were not directly fused with each other but separated by a 10-15 nm gap that corresponded well with the size of single annexin V molecules. In axon terminals, globular structures 12-13 nm in diameter, similar in dimension to annexin V molecules, were distinctly found to be attached to the cytoplasmic surface of both vesicle membranes when the two vesicles were close to each other. These results suggest that annexin V belongs to the group of synaptic vesicle-associated proteins. Although its localization and significance in non-neuronal cells were not analysed here, at least in the axon terminal annexin V may participate in the cluster formation of synaptic vesicles by linking with the cytoplasmic surface of the vesicles in a Ca(2+)-dependent manner.