The presence of 17K Mr protein, a major specific substrate for kinase C, found in the triton-insoluble fraction of synaptosome prepared from rat brain

Excitable membranes and synaptic transmission: postsynaptic mechanisms. Localization of alpha-internexin in the postsynaptic density of the rat brain

The synaptic localization of alpha-internexin, a brain-specific intermediate filament protein, was investigated immunohistochemically in the rat brain. The specificity of the antibody used in this study was confirmed by Western blotting and the antibody specifically reacted with alpha-internexin in the neurofilament preparation and in the postsynaptic density (PSD) fraction. The alpha-internexin immunoreactivity was distributed in neurons, especially in the somata and dendrites, throughout the cerebral cortex. Immunoelectron microscopic examination showed the immunoreactivity in the PSD, while neurofilament M was not in the PSD. Thus alpha-internexin and neurofilament M are differentially localized in neuronal cells. Alpha-internexin content in the PSD fraction was relatively high even before the period of synaptogenesis and the content in the fraction was unchanged between young and adult rats (2-6 weeks old). These results suggest a role of alpha-internexin for early development and organization of the PSD.

ERK2-type mitogen-activated protein kinase (MAPK) and its substrates in postsynaptic density fractions from the rat brain

Mitogen-activated protein kinase (MAPK) and MAPK kinase (MAPKK) were detected by Western blotting in the synaptic fraction prepared from the rat brain. There were two bands immunoreactive to the anti-MAPK antiserum in the soluble, P2, synaptosome, and synaptic plasma membrane fractions. These immunoreactive bands possibly corresponded to extracellular signal-regulated kinase (ERK) 1 and 2 (Boulton et al., 1991b), respectively. Only ERK2 was detected in the postsynaptic density (PSD) fraction. We then surveyed MAPK substrates in the synaptic fractions using purified Xenopus MAPK (ERK2-type MAPK), and found a number of MAPK substrates unique to the PSD fraction. Thus, ERK2 is present in the synapse, especially at the postsynaptic site, and it may play a role(s) in synaptic function via the phosphorylation of synapse-specific substrates. Developmental changes in ERK2 also supported its role in the synapse.

Protein kinases involved in the expression of long-term potentiation

This review describes the protein kinases that are involved in long-term potentiation (LTP). The following items are described. 1. Ca2+/calmodulin-dependent protein kinase II (CaMKII) and protein kinase C (PKC) may play pivotal roles in the different phases of the expression of LTP. This involvement has been indicated mainly by using specific inhibitor of these kinases. The involvement of the CaMKII alpha-subunit was confirmed in mutant mice which are deficient in the gene for the subunit. 2. Involvement of persistently active protein kinases in the maintenance of LTP has been proposed and, since then, several studies have focused upon the persistent kinase. Both PKC and CaMKII are possible sources of the persistent kinase activities. 3. Protein kinases other than CaMKII or PKC (ex. protein kinase A, tyrosine kinases, mitogen-activated kinase) also play roles in the expression of LTP. 4. Finally, the importance of postsynaptic density as a device where complex chemical reactions related to neuronal signal transduction occur is discussed.

Characterization of protein kinase C activities in postsynaptic density fractions prepared from cerebral cortex, hippocampus, and cerebellum

Protein kinase C (PKC) activities, especially, substrates and PKC isozymes, associated with postsynaptic density (PSD) fractions isolated from rat cerebral cortex, hippocampus, or cerebellum were investigated. The 17k M(r) major substrate for PKC was associated with PSD fractions prepared from cerebral cortex and hippocampus, and several substrates including 18k M(r) protein were associated with PSD fraction isolated from cerebellum. The content of 17k M(r) substrate was extremely low in the PSD fraction prepared from cerebellum. PKCs-beta and gamma were associated with PSD fractions and PKC-alpha was virtually absent in the fraction prepared from the three different regions of the brain. All of PKCs-alpha, beta, and gamma were associated with synaptosome fractions. The 36k M(r) bands immunoreactive with anti-PKC-beta antibody, probably degradation products of native PKC-beta, were detected in both the PSD and synaptosome fractions from the three regions, and the ratio of the degradation fragments to native PKC molecule was higher in PSD fractions than in synaptosome fractions. The results suggest postsynaptic roles of PKCs-beta and gamma and involvement of proteolytic activation of PKC-beta in the postsynaptic signal processing.