Immunohistochemical evidence for the presence of protein kinase C beta-subtype in rat motoneurons and myoneural junctions

Cellular localization of the atypical isoforms of protein kinase C (aPKCζ/PKMζ and aPKCλ/ι) on the neuromuscular synapse

Several classic and novel protein kinase C (PKC) isoforms are selectively distributed in specific cell types of the adult neuromuscular junction (NMJ), in the neuron, glia and muscle components, and are involved in many functions, including neurotransmission. Here, we investigate the presence in this paradigmatic synapse of atypical PKCs, full-length atypical PKC zeta (aPKCζ), its separated catalytic part (PKMζ) and atypical lambda-iota PKC (aPKCλ/ι). High resolution immunohistochemistry was performed using a pan-atypical PKC antibody. Our results show moderate immunolabeling on the three cells (presynaptic motor nerve terminal, teloglial Schwann cell and postsynaptic muscle cell) suggesting the complex involvement of atypical PKCs in synaptic function.

PKA, PKC, and AKAP localization in and around the neuromuscular junction

BACKGROUND

One mechanism that directs the action of the second messengers, cAMP and diacylglycerol, is the compartmentalization of protein kinase A (PKA) and protein kinase C (PKC). A-kinase anchoring proteins (AKAPs) can recruit both enzymes to specific subcellular locations via interactions with the various isoforms of each family of kinases. We found previously that a new class of AKAPs, dual-specific AKAPs, denoted D-AKAP1 and D-AKAP2, bind to RIalpha in addition to the RII subunits.

RESULTS

Immunohistochemistry and confocal microscopy were used here to determine that D-AKAP1 colocalizes with RIalpha at the postsynaptic membrane of the vertebrate neuromuscular junction (NMJ) and the adjacent muscle, but not in the presynaptic region. The labeling pattern for RIalpha and D-AKAP1 overlapped with mitochondrial staining in the muscle fibers, consistent with our previous work showing D-AKAP1 association with mitochondria in cultured cells. The immunoreactivity of D-AKAP2 was distinct from that of D-AKAP1. We also report here that even though the PKA type II subunits (RIIalpha and RIIbeta) are localized at the NMJ, their patterns are distinctive and differ from the other R and D-AKAP patterns examined. PKCbeta appeared to colocalize with the AKAP, gravin, at the postsynaptic membrane.

CONCLUSIONS

The kinases and AKAPs investigated have distinct patterns of colocalization, which suggest a complex arrangement of signaling micro-environments. Because the labeling patterns for RIalpha and D-AKAP 1 are similar in the muscle fibers and at the postsynaptic membrane, it may be that this AKAP anchors RIalpha in these regions. Likewise, gravin may be an anchor of PKCbeta at the NMJ.

Immunocytochemical distribution of Ca(2+)-independent protein kinase C subtypes (delta, epsilon, and zeta) in regenerating axonal growth cones of rat peripheral nerve

In the peripheral nerve, regenerating axonal sprouts usually emanate at nodes of Ranvier, and extend as growth cones along the inner surface of Schwann cells and/or through Schwann cell columns in the distal nerve segment. In order to elucidate the significance of Ca(2+)-independent protein kinase C in nerve regeneration, localizations of delta, epsilon and zeta subtypes were examined immunocytochemically in sprouts and growth cones of regenerating axons, as well as in normal intact nerves in the rat sciatic nerve. In normal nerves, intense immunoreactivities of delta, epsilon and zeta subtypes were present in axons of both myelinated and unmyelinated fibres. Subcellularly, the distribution of these subtypes in the axoplasm was patchy, and discontinuous in the axolemma and subaxolemmal peripheral zones of myelinated nerves. Some thin myelinated axons showed no immunoreactivity for epsilon subtype. Schwann cells of both myelinated and unmyelinated fibres had moderate immunoreactivities for each subtype. In areas of nerve regeneration, axonal sprouts at nodes of Ranvier, and growth cones extending along Schwann cell basal laminae, had intense immunoreactivities for delta, epsilon and zeta subtypes which are distributed diffusely throughout the axoplasm, and on the entire axolemma. In the sprouts, immunoreactivity for epsilon subtype was strong on the axolemma, but weak or almost absent in the axoplasm. These data, together with those of our previous study, indicate that Ca(2+)-independent protein kinase C subtypes (delta, epsilon and zeta) have basically the same distribution patterns as those of Ca(2+)-dependent subtypes in sprouts and growth cones of regenerating axons, as well as in normal intact axons; albeit epsilon subtype is somewhat different in distribution and intensity from delta and zeta subtypes. It is suggested that Ca(2+)-independent subtypes are involved in maintaining growth cone activities along with the Ca(2+)-dependent subtypes.

Characterization of two forms of protein kinase C alpha, with different substrate specificities, from skeletal muscle

We have investigated protein kinase C (PKC) in skeletal muscle cytosol and demonstrated the presence of two major activities. These did not correspond to different PKC isoenzymes but seemed to represent two species of PKC alpha as deduced by: elution during hydroxyapatite chromatography at KH2PO4 concentrations expected of PKC alpha; detection of the two species by three specific but unrelated anti-(PKC alpha) antibodies; immunodepletion of both activities with anti-(PKC alpha) antibody; and demonstration of identical requirements of both Ca2+ ions and lipid for activation. These species, termed PKC alpha 1 and PKC alpha 2, phosphorylated the modified conventional PKC pseudosubstrate peptide (19-31, Ser-25) equally well. Importantly, however, the activities differed in that PKC alpha 1 phosphorylated histone IIIS, and also peptides derived from the EGF receptor and glycogen synthase, to a much greater extent than did PKC alpha 2. Similarly, incubation of crude muscle extracts with either PKC alpha 1 or alpha 2 gave rise to different protein phosphorylation patterns. The involvement of proteolysis, dephosphorylation or oxidative modification in the interconversion of PKC alpha 1 and PKC alpha 2 during preparation was ruled out. Although some PKC-binding proteins were detected in overlay assays, their presence did not explain the anomalous PKC alpha 2 activity. The results suggest that a modification of PKC alpha in situ limits its substrate specificity, and indicate an additional level of control of the kinase that may be a site for modulation of PKC-mediated signal transduction.

Peripheral nerve regeneration

Peripheral nerve regeneration comprises the formation of axonal sprouts, their outgrowth as regenerating axons and the reinnervation of original targets. This review focuses on the morphological features of axonal sprouts at the node of Ranvier and their subsequent outgrowth guided by Schwann cells or by Schwann cell basal laminae. Adhesion molecules such as N-CAM, L1 and N-cadherin are involved in the axon-to-axon and axon-to-Schwann cell attachment, and it is suggested that integrins such as alpha 1 beta 1 and alpha 6 beta 1 mediate the attachment between axons and Schwann cell basal laminae. The presence of synaptic vesicle-associated proteins such as synaptophysin, synaptotagmin and synapsin I in the growth cones of regenerating axons indicates the possibility that exocytotic fusion of vesicles with the surface axolemma supplies the membranous components for the extension of regenerating axons. Almost all the subtypes of protein kinase C have been localized in growth cones both in vivo and in vitro. Protein kinase C and GAP-43 are implicated to be involved in at least some part of the adhesion of growth cones to the substrate and their growth activity. The significance of tyrosine kinase in growth cones is emphasized. Tyrosine kinase plays an important role in intracellular signal transduction of the growth of regenerating axons mediated by both nerve trophic factors and adhesion molecules. Growth factors such as NGF, BDNF, CNTF and bFGF are also discussed mainly in terms of the influence of Schwann cells on regenerating axons.

Distribution of protein kinase C (alpha, beta, gamma subtypes) in normal nerve fibers and in regenerating growth cones of the rat peripheral nervous system

The distribution of protein kinase C (alpha, beta, gamma subtypes) was studied using immunocytochemical techniques in normal nerve fibers and in regenerating sprouts (growth cones) from the nodes of Ranvier following crush injuries to the rat peripheral nervous system. In normal nerves, for each protein kinase C subtype, immunoreactivity was present in both myelinated and unmyelinated axons. In myelinated axons, immunoreactivity for all three subtypes was patchy in the axoplasm and diffuse in the subaxolemmal peripheral zones. No immunoreactivity was found in the microtubule and neurofilament (cytoskeletal) domain. In contrast, in unmyelinated axons, immunoreactivity was distributed diffusely in the axoplasm. Schwann cells of myelinated fibers exhibited protein kinase C immunoreactivity, but those of unmyelinated fibers did not. In regenerating nerves, early sprouts and growth cones extending through the crushed site along Schwann cell basal laminae exhibited intense immunoreactivity for all three subtypes. Immunoreactivity was distributed diffusely throughout the axoplasm of the regenerating sprouts (growth cones), in which microtubules and neurofilaments were very rare. Thus, the subcellular localization of the protein kinase C immunoreactivity in growth cones of early regenerating nerves differed from that of normal parent axons. These findings suggest that protein kinase C (alpha, beta and gamma subtypes), whose subcellular distribution becomes more extensive in regenerating axons, may have important functional roles in axonal sprouting and in the regulation of growth cone activity in the peripheral nervous system.

Developmental regulation of a protein kinase C isoform localized in the neuromuscular junction

Protein kinase C (PKC) is a family of protein serine/threonine kinases consisting of multiple isoforms whose distinct physiological roles within cells are unknown. The message encoding the nPKC theta isoform, a member of the novel calcium-independent class of PKCs, has recently been shown to be abundant in mouse skeletal muscle. The message for cPKC alpha, a calcium-dependent isoform, was also found to be highly expressed in this tissue. In an effort to distinguish between the physiological roles of these two isoforms of PKC in rat skeletal muscle, we examined their subcellular distribution, developmental expression and intracellular localization. We generated an isotype-specific antiserum directed against a peptide sequence unique to nPKC theta. This antiserum recognized a 79 kDa protein highly enriched in rat skeletal muscle, which is likely to be nPKC theta. cPKC alpha was also readily detectable in skeletal muscle, using another isotype-specific antibody, but it appeared to be ubiquitously expressed in all of the tissues we examined. Together these results suggest that nPKC theta, rather than cPKC alpha, is involved in physiological functions that are specific for skeletal muscle. The immunoreactivity for nPKC theta was highest in the membrane subcellular fraction compared to the cytosolic fraction of skeletal muscle. In contrast, cPKC alpha was found to be predominantly distributed in the cytosolic rather than the membrane fraction. nPKC theta appeared to be developmentally regulated postnatally in rat skeletal muscle, with a 4-fold increase in expression occurring exclusively in the membrane fraction during postnatal days 3 through 21. This time course coincides with the period in rat development associated with maturation of neuromuscular junctions. Expression of nPKC theta in rat spleen, another tissue expressing detectable levels of this isoform, was not found to be developmentally regulated during this time. cPKC alpha expression was found to increase slightly from postnatal days 3 through 11 and no developmental increase in expression of this isoform was observed in skeletal muscle during postnatal days 11 through 21. The intracellular localization of the PKC theta and alpha isoforms in rat skeletal muscle was examined by immunocytochemistry. nPKC theta was detected in association with the sarcolemma of skeletal muscle and was found to be localized in the neuromuscular junction. Enhanced staining for nPKC theta in the neuromuscular junction appeared as early as postnatal day 4 during development. Staining for nPKC theta in the neuromuscular junction persisted after prolonged denervation, suggesting that the enzyme is distributed postsynaptically.(ABSTRACT TRUNCATED AT 400 WORDS)

Localization of protein kinase C alpha, beta and gamma subspecies in sensory axon terminals of the rat muscle spindle

The localization of protein kinase C (PKC) alpha, beta and gamma subspecies in sensory axon terminals of muscle spindles in the plantar lumbrical muscles of rat was investigated by light and electron microscopic immunocytochemistry using monoclonal and polyclonal antibodies. Immunoreactivity for these subspecies was detected specifically in sensory axon terminals which wound spirally around the intrafusal muscle fibres of the muscle spindle. Immunostaining was found to be stronger with polyclonal than with monoclonal antibodies. By electron microscopy, immunoreactivity for alpha, beta and gamma subspecies was almost diffusely distributed in the cytoplasm of the axon terminal, and the overall pattern of distribution of immunoreactivity was similar for all three subspecies. In the cases of alpha and beta subspecies, some intensely immunostained regions were found in the cytoplasm, but no definite subcellular structures corresponding to such regions could be identified. Considering that PKC plays a crucial role in the regulation of ion channels, it is suggested that PKC might be involved in the control of mechanoelectric transduction in sensory axon terminals.

Protein kinase C-epsilon is a developmentally regulated, neuronal isoform in the chick embryo central nervous system

Protein kinase C (PKC) is expressed as many isoforms and in high quantities in the central nervous system (CNS), which suggests an important role for this enzyme in neuronal development and function. We used specific antibodies to investigate the expression of the known PKC isoforms in extracts from chick major CNS areas during embryogenesis, from day 3 (E3) of incubation to day 1 post-hatching (P1). PKC-epsilon was the predominant isoform and was expressed from E6 onward in all brain regions, except retina (E12 and on). PKC-alpha/beta and -zeta isoforms were expressed at lower levels prior to PKC-epsilon expression and throughout embryogenesis. No other isoforms were detected in neural tissue preparations. We then used neural culture systems derived from the chick CNS to study the expression of PKC isoforms in neuroblasts, cortical neurons, and cortical glial cells. Western blotting and immunostaining of neuroblast-enriched cultures, derived from E3 CNS, showed only the Ca(2+)-dependent PKC-alpha/beta to be present. Studies on neuronal cultures derived from E6 cerebral hemispheres revealed only the Ca(2+)-independent PKC-epsilon to be expressed in neurons, as predicted by the developmental studies on tissue homogenates. PKC-epsilon immunoreactivity was seen intracellularly in differentiating neurons, regardless of their neurotransmitter phenotypes, and it correlated well with the level of neuronal activity. Furthermore, PKC-alpha/beta immunoreactivity was verified on glia cells, as the glial lineage emerges in E15 cortical cultures. These data suggest that PKC-epsilon expression is associated with the final neuroblast division in neurons, and the correlation of PKC isoform expression and neural cell lineage is discussed.

Ultrastructural localization of protein kinase C beta-subspecies in the axon terminal of rat neuromuscular junction

Ultrastructural localization of protein kinase C (PKC) beta-subspecies in neuromuscular junctions of the rat lumbrical muscle was investigated by the immunoperoxidase and immunofluorescence methods. By light microscopy, PKC beta-like immunoreactivity (PKC beta-LIR) was found in the axon terminal expansions as well as in the preterminal axons. By confocal laser scanning microscopy, the staining for PKC beta-like immunoreactivity was more intense in the presynaptic regions just in contact with the acetylcholine receptor stained by FITC-alpha-bungarotoxin. By electron microscopy, PKC beta-like immunoreactivity was distributed non-uniformly in the terminal expansions. In the terminal expansions, PKC beta-like immunoreactivity was accumulated in the presynaptic regions in contact with the post-synaptic folds. This accumulation was approximately 0.1-0.2 microns in diameter, which comprised a part of the presynaptic plasma membrane and a group of synaptic vesicles adjacent to it. Weak immunoreactivity was also found diffusely in the axoplasmic matrix. The discrete presynaptic accumulation of PKC beta-subspecies may represent the strategical localization specialized for the effective regulation of neurotransmitter release.

Protein kinase C (alpha, beta, gamma) in Pacinian corpuscle

Immunocytochemical demonstration of protein kinase C (PKC) subspecies (alpha, beta, gamma) was carried out in Pacinian corpuscles of rat hind feet using monoclonal or polyclonal antibodies against each of these subspecies. The inner core cells and lamellae and the Schwann cell cytoplasm of the nerve fiber innervating the corpuscle were strongly positive for PKC alpha-immunoreactivity (IR). In contrast, the axon terminal and the outer core did not display any positive alpha-IR. Very weak PKC beta-IR was detected in the ultraterminal region of the axon terminal, while the trunk region showed no immunoreactivity. Very faint PKC beta-IR was found also in the lamellar cells located at the periphery of the inner core and the endoneurial fibroblasts in the intermediate layer. PKC gamma-IR was not detected in any part of the corpuscle. The strong PKC alpha-IR in the inner core and the presence or absence of PKC alpha-, beta-, and gamma-IR in the axon terminal are discussed from the point of view of the functional aspects of each part.

Colocalization of protein kinase C beta-subtype and calcitonin gene-related peptide in rat spinal cord

Colocalization of calcitonin gene-related peptide (CGRP) and protein kinase C beta-subtype (PKC-beta) like immunoreactivities (LI) and the subcellular localization of CGRP-LI were studied in the ventral horn of rat spinal cord. Ultrastructurally CGRP-LI was localized on the membranes of the Golgi-complexes, in multivesicular bodies and in vesicles adjacent to the Golgi-complex in motoneuron perikarya. The colocalization of PKC-beta and CGRP-LI was detected in most of the ventral horn motoneurons. However, few motoneurons were only PKC-beta-immunoreactive. These results suggest that PKC-beta may be involved in the regulation of CGRP release from motoric axon terminals.

Protein kinase C beta-subtype-immunoreactive motor and sensory nerves in rat muscle spindle

The localization of protein kinase C beta-subtype-like immunoreactivity (PKC-beta-LI) was studied in the muscle spindles of rat neck muscles. In the equatorial regions of muscle spindles PKC-beta-LI was detected in spiral sensory nerve endings surrounding intrafusal muscle fibers. In polar regions single PKC-beta-immunoreactive (IR) nerve fibers were found between intrafusal muscle fibers and some PKC-beta-IR motor nerve endings were seen on the surface of intrafusal fibers. These results suggest that PKC-beta may be involved in regulation of muscle contraction and tonus by modulating the sensitivity of afferent and efferent innervation of muscle spindles.

Protein kinase c-beta-like immunoreactivity in the developing and adult rat adrenal gland

Protein kinase c-beta-like immunoreactivity was studied in the adrenal gland of adult rats and at different pre- and postnatal stages of development (E17-P21) with an antibody specific to both the beta 1 and beta 2 subtypes of the kinase. In the adult rat adrenal gland, the immunoreactivity was seen in numerous nerve fibres in the adrenal medulla both in bundles and individually forming occasionally dense networks around chromaffin cell groups. Several protein kinase c-beta-immunoreactive fibres were also observed transversing the adrenal cortex towards the medulla. No remaining immunoreactive fibres two weeks after transection of the splanchnic nerve could be seen; nor was any immunoreactivity observed in the chromaffin cells of the adrenal medulla or in the cortical cells, but some faintly immunoreactive ganglion cells were detected in the adrenal medulla. The amount and distribution of protein kinase c-beta-like immunoreactivity in the fetal and developing adrenals was very similar to that seen in the adrenal glands of adult rats. On the basis of its localization, the beta-subtype of protein kinase c does not appear to be directly involved in the release of catecholamines from the adrenal medulla, but it might have a role in the regulation of neurotransmitter release from preganglionic cholinergic neurons.

The effects of a diacylglycerol kinase inhibitor at mammalian and amphibian neuromuscular junctions in vitro

Miniature endplate potentials were studied in mouse phrenic nerve-hemidiaphragm and frog pectoris-cutaneous neuromuscular preparations. The diacylglycerol kinase inhibitor R59022 increased the frequency of miniature endplate potentials in both preparations. The results indicate that endogenous diacylglycerol regulates acetylcholine release via activation of protein kinase C. Continuous monitoring of miniature endplate potential frequency in the frog preparation showed that higher doses of the inhibitor caused a large rise in release rate, which subsequently settled at a lower, though still augmented, level. It is suggested that a negative feedback may occur in response to high protein kinase C activity, possibly to inhibit diacylglycerol generation.