Neuronal cyclin-dependent kinase-5 phosphorylation sites in neurofilament protein (NF-H) are dephosphorylated by protein phosphatase 2A.

Neurofilament (NF) protein [high molecular mass (NF-H)] is extensively phosphorylated in vivo. The phosphorylation occurs mainly in its characteristic KSP (Lys-Ser-Pro) repeat motifs. There are two major types of KSP motifs in the NF-H tail domain: KSPXKX and KSPXXX. Recent studies by two different laboratories have demonstrated the presence of a cdc2-like kinase [cyclin-dependent kinase-5 (cdk5)] in nervous tissue that selectively phosphorylates KSPXKX and XS/TXK motifs in NF-H and lysine-rich histone (H1). This article describes the identification of phosphatases dephosphorylating three different substrates: histone (H1), NF-H in a NF preparation, and a bacterially expressed C-terminal tail domain of NF-H, each containing KSPXKX repeats phosphorylated in vitro by cdk5. Among various phosphatases identified, protein phosphatase (PP) 2A from rabbit skeletal muscle appeared to be the most effective phosphatase in in vitro assays. Three phosphatase activity peaks--P1, P2, and P3--were partially purified from frozen rat spinal cord by ion exchange and size exclusion column chromatography and then characterized on the basis of biochemical, pharmacological, and immunochemical studies. One of the three peaks was identified as PP2A, whereas the others were mixtures of both PP2A and PP1. These three peaks could dephosphorylate cdk5-phosphorylated 32P-histone (H1), 32P-NF-H in the NF preparation, and 32P-NF-H tail fusion protein. These studies suggest the involvement of PP2A or a PP2A-like activity in the regulation of the phosphorylation state of KSPXKX motifs in NF-H.

Molecular characterization of a neuronal-specific protein that stimulates the activity of Cdk5.

Cyclin-dependent kinase, Cdk5, has been identified in neural tissue in connection with neurofilament and tau protein phosphorylation. This report describes the characterization of a 62-kDa protein that copurifies with Cdk5 from rat spinal cord homogenates. Dissociation of the protein from neural Cdk5 is concomitant with a reversible loss in kinase activity. Amino acid sequence information from tryptic peptide fragments was used to clone the complementary DNA from rat brain. A single full-length cDNA was characterized coding for a 67.5-kDa protein (p67). Exogenously expressed p67 stimulated Cdk5 kinase activity in vitro in a dose-dependent manner and when presented as an affinity matrix, selectively adsorbed Cdk5 from a cleared rat brain homogenate. In situ hybridization analysis of E18 rat embryos and adult rat brain demonstrated that p67 transcript expression is restricted to neural tissue. Immunohistochemical staining with an amino-terminal peptide-specific antibody further indicated that p67 is exclusively expressed in neurons. Localization in vivo and in cultured rat hippocampal neurons showed that p67 is highly enriched in axons. We propose that p67, by virtue of its regulation of Cdk5, participates in the dynamics of axonal architecture through the modulation of phosphorylation of cytoskeletal components.

Identification of endogenously phosphorylated KSP sites in the high-molecular-weight rat neurofilament protein.

The high-molecular-weight neurofilament protein (NF-H) is highly phosphorylated in vivo, with estimates as high as 16-51 mol of Pi/mol of protein. Most of the phosphorylation sites are thought to be located on Ser residues in multiple KSP repeats, in the carboxy-terminal tail region of the molecule. Because the extent and site-specific patterns of tail domain phosphorylation are believed to modulate neurofilament structure and function, it becomes essential to identify the endogenous sites of phosphorylation. In this study, we have used selective proteolytic cleavage procedures, Pi determinations, microsequencing, and mass-spectral analysis to determine the endogenously phosphorylated sites in the NF-H tail isolated from rat spinal cord. Twenty Ser residues in NF-H carboxy-terminal tail were analyzed; nine of these, all located in KSP repeats, were phosphorylated. No detectable phosphorylation could be identified in any of the 11 "non-KSP" Ser residues that were examined. KSPXKX, KSPXXX, and KSPXXK motifs were found to be phosphorylated. In addition, a 27-kDa KSP-rich domain, containing 43 virtually uninterrupted KSPXXX repeats, was isolated from the tail domain and found to contain between 30 and 35 mol of Pi/mol of protein. This domain appeared to be highly resistant to endoproteinase Glu-C digestion, although it contains a large number of glutamate residues. It could be proteolyzed, however, after dephosphorylation. This suggests that phosphorylation of the tail domain may contribute to neurofilament stability in vivo. A neuronal-derived protein kinase that specifically phosphorylates only KSPXKX motifs in neurofilaments has been reported. The presence of extensively phosphorylated KSPXXX repeats in NF-H in vivo suggests the existence of yet another, unidentified kinase(s) with specificity for KSPXXX motifs.

cdc2-like kinase from rat spinal cord specifically phosphorylates KSPXK motifs in neurofilament proteins: isolation and characterization.

A protein kinase that phosphorylates a specific KSP sequence [K(S/T)PXK], which is abundant in high molecular weight neurofilament (NF) proteins, was identified and isolated from rat spinal cord. Characterization of this enzyme activity revealed a close relationship with p34cdc2 kinase with respect to its molecular mass (32.5 kDa by SDS/PAGE) and substrate specificities. It could phosphorylate a synthetic peptide analog of the simian virus 40 large tumor antigen, reportedly a specific substrate for p34cdc2 kinase. Histone (H1) and peptide analogs of the KSP sequence present in the C-terminal end of rat and mouse neurofilament proteins were phosphorylated. This kinase did not phosphorylate alpha-casein and peptide substrates of other known second messenger-dependent or -independent kinases. Dephosphorylated rat NF protein NF-H was strongly phosphorylated by the purified enzyme; NF proteins NF-M and native NF-H, but not NF-L, were slightly phosphorylated. Studies on synthetic peptide analogs of KSP repeats with substitution of specific residues, known to be present in the C-terminal regions of NF-H, revealed a consensus sequence of X(S/T)PXK, characteristic of the p34cdc2 kinase substrate. On Western blots, the enzyme was immunoreactive with antibody against the C-terminal end of cdc2 kinase (mouse) and neuronal cdc2-like kinase from rat but not with an antibody against the conserved PSTAIRE region of the p34cdc2 kinase. The antibody against the C-terminal end of cdc2 kinase could immunoprecipitate (immunodeplete) the purified kinase activity. Since the adult nervous system is composed primarily of postmitotic cells, the present observations indicate a nonmitotic role for this cdc2-like kinase activity. The effective phosphorylation of NF-H by this kinase suggests a function in axonal structure.

Bovine neurofilament-enriched preparations contain kinase activity similar to casein kinase I--neurofilament phosphorylation by casein kinase I (CKI).

Neurofilament (NF)-enriched preparations from bovine spinal cord contain regulator-independent kinase activities that phosphorylate NF subunits as well as alpha-casein. CKI-7 (N-2-amino ethyl, 5-chloroisoquinoline, 8-sulfonamide), a specific inhibitor of casein kinase I (CKI), inhibits the phosphorylation of NF subunits in the neurofilament preparation. This inhibition occurs at a concentration range identical to concentrations where CKI-7 inhibits rabbit reticulocyte CKI phosphorylation of alpha-casein. Heparin, a specific inhibitor of casein kinase II (CKII), produced only 20% inhibition of 32P incorporation into NF subunits, and only at concentrations 5 to 10-fold higher than those required to inhibit CKII from reticulocytes. CKI from rabbit reticulocytes phosphorylated all three NF subunits (NF-H, NF-M and NF-L). Comparison of the tryptic phosphopeptide maps of NF-M, phosphorylated by the NF-associated kinase and CKI, indicates that the casein kinase I phosphorylates many of the peptides phosphorylated by the NF-associated kinase and this phosphorylation occurs at the carboxy terminal tail domain of the NF-M subunit. These studies suggest that the major independent kinase activity associated with NFs is CKI.

Casein kinases I and II from squid brain exhibit selective neurofilament phosphorylation.

In studies of the function of neurofilaments in the squid giant axon we showed that isolated neurofilament preparations from axoplasm are associated with high levels of casein kinase-like activity. To determine the role of these kinases in phosphorylation of neurofilament proteins, we isolated two kinases from squid brain which are also found in axoplasm, CK I and CK II. The CKI is similar to this axonal neurofilament-associated CKI-like kinase activity. CK I displayed a high specificity for the squid high molecular weight (NF220) and rat high molecular weight (NF-H) neurofilament proteins relative to alpha-casein, phosvitin, and middle (NF-M) and low (NF-L) rat neurofilament proteins. The brain CKII, with activity similar to that found in axoplasm, but not associated with neurofilaments, poorly phosphorylated NF220 and NF-H, while demonstrating similar affinities, relative to CK I, for NF-M, NF-L, alpha-casein, and phosvitin. The high affinity of squid neuronal CKI for squid NF220 and rat NF-H and its association with axonal neurofilaments suggest that this kinase may have a specific role in neurofilament phosphorylation essential for interaction with other cytoskeletal elements in the axon.

Distribution of Na+, K(+)-ATPase alpha-subunit isoforms in rat pituitary.

The distributions of alpha-subunit isoforms of the Na+,K(+)-ATPase in rat pituitary were determined by immunoblotting and immunohistochemistry. Immunoreactivity for all three forms is present in the neural lobe, whereas the anterior lobe contains only alpha 1 and alpha 2. Most areas of the intermediate lobe exhibit faint immunoreactivity for only alpha 1, but thin strands of cells which stain strongly for all three isoforms are also present in this lobe. The previously reported ouabain inhibitable Na+,K(+)-ATPase activity in the neural lobe is consistent with the presence of both alpha 2 and alpha 3 subunits.

Acute volume expansion in humans releases a factor which inhibits the vascular Na+-K+ pump.

The authors previously observed increased Na+-K+ pump inhibitory activity in the plasma of low renin essential hypertensive unselected with respect to renin status. The present experiments were done to determine the effect of acute volume expansion on plasma Na+-K+ pump inhibitory activity in normotensive subjects. The data show that acute volume expansion increases vascular Na+-K+ pump inhibitory activity in plasma, suggesting that the increased level of this inhibitory activity in low essential hypertensives is probably volume dependent. The unique feature of this study is the use of vascular tissue for the bioassay.

Effects of rat atrial extract on sodium transport and blood pressure in the rat.

Atrial cardiocytes contain specific atrial granules ( SAGs ) which are the storage site of atrial natriuretic factor (ANF). The purpose of the present study was to determine whether ANF produces natriuresis by inhibiting Na+-K+ pump activity and whether this factor is similar to the humoral sodium transport inhibiting factor ( HSTIF ) previously demonstrated in acutely volume expanded animals and humans as well as in experimental and human essential hypertension. Our results indicate that, in contrast to the HSTIF , ANF does not inhibit membrane Na+,K+-ATPase, vascular smooth muscle cell Na+-K+ pump activity, or sodium transport in the toad bladder. Intravenous infusion of ANF in the bilaterally nephrectomized, hexamethonium-treated rat produces only a small transient pressor response, probably due to potentiation of endogenous norepinephrine. These findings strongly suggest that the ANF is not the same as the HSTIF detected on acute volume expansion and in some forms of hypertension. They also suggest that the diuretic and natriuretic effects of ANF are due to mechanism(s) other than blood pressure elevation and inhibition of Na+-K+ pump activity.