A neurofilament-associated kinase phosphorylates only a subset of sites in the tail of chicken midsize neurofilament protein

Four casein kinase I isoforms are differentially partitioned between nucleus and cytoplasm

The casein kinase I (CKI) family consists of at least seven vertebrate genes, some of which can be alternatively spliced. Previously, we have studied the four splice variants of the chicken CKIalpha gene. The four proteins differ only by the presence or absence of two peptides, a 28-amino-acid "L" insert in the catalytic domain and a 12-amino-acid "S" insert near the extreme C-terminus. Here cells were transfected with DNA encoding all four isoforms fused to the green fluorescent protein (GFP) and the localization of each protein was examined. We noted that the L insert includes the sequence PVGKRKR, which has the characteristics of a nuclear localization signal (NLS), and we show that the CKIalphaL and CKIalphaLS isoforms which contain this sequence are targeted to the nucleus, where a fraction becomes associated with nuclear speckles. In contrast the two isoforms lacking the L insert remain predominantly cytoplasmic. Mutation of the first lysine in the putative NLS to asparagine prevented the nuclear entry of GFP-CKIalphaL. Therefore different CKIalpha isoforms are targeted to different cellular compartments in a fashion modulated by alternate transcription and in these locations presumably phosphorylate and regulate different cellular substrates.

Identification of four alternatively spliced isoforms of chicken casein kinase I alpha that are all expressed in diverse cell types

Casein kinase I (CKI) is a family of widely expressed protein kinases. It is previously shown in mammalian tissues that CKIalpha exists as two or three alternatively spliced isoforms (Rowles et al.,1991; Zhang et al., 1996; Kuret et al., 1997). We now report that four alternatively spliced isoforms of CKIalpha are expressed in many chicken cells and tissues. A partial cDNA clone was isolated from a chicken brain library, using a probe derived from a bovine CKIalpha cDNA. The translated sequence of this clone was 100% identical to the bovine homolog containing the 'L' insert, with the addition of 12 amino acids just before the C terminus that had previously been reported in human and Xenopus CKIalpa. After completing the missing portion of the coding sequence by 5' RACE (rapid amplification of cDNA ends), full-length cDNA was PCR amplified from chicken brain cDNA, yielding four different products. These were cloned and sequenced and found to correspond to the four CKIalpha isoforms: CKIalpha, CKIalphaL, CKIalphaS and CKILalphaLS, where 'S' is the insert consisting of the 12 human/Xenopus C-terminal amino acids. Using reverse transcription and polymerase chain reaction (Rt-PCR), it was shown that the four isoforms are all expressed in neurons, fibroblasts and several tissues. This represents the first demonstration that four splice variants exist and are all expressed in a single type of cell.

The cytoskeleton and cell signaling: component localization and mechanical coupling

The three-dimensional intracellular network formed by the filamentous polymers comprising the cytoskeletal affects the way cells sense their extracellular environment and respond to stimuli. Because the cytoskeleton is viscoelastic, it provides a continuous mechanical coupling throughout the cell that changes as the cytoskeleton remodels. Such mechanical effects, based on network formation, can influence ion channel activity at the plasma membrane of cells and may conduct mechanical stresses from the cell membrane to internal organelles. As a result, both rapid responses such as changes in intracellular Ca2+ and slower responses such as gene transcription or the onset of apoptosis can be elicited or modulated by mechanical perturbations. In addition to mechanical features, the cytoskeleton also provides a large negatively charged surface on which many signaling molecules including protein and lipid kinases, phospholipases, and GTPases localize in response to activation of specific transmembrane receptors. The resulting spatial localization and concomitant change in enzymatic activity can alter the magnitude and limit the range of intracellular signaling events.

Mature sensory neuron-derived hybrid cell line expressing NGF-dependent neurite extension

We established a hybrid cell line which extended neurites in the presence of nerve growth factor by fusion between adult mouse sensory neurons and neuroblastoma C1300 cells using emetine and actinomycin D. The serine-phosphorylated 200 and 160 kDa neurofilament proteins were detected in the hybrid cells. In comparison, C1300 cells expressed both subunit of non-phosphorylated neurofilaments at serine residues.

Neurofilament phosphorylation and [125I]calmodulin binding by Ca2+/calmodulin-dependent protein kinase in the brain subcellular fractions of diisopropyl phosphorofluoridate (DFP)-treated hen

Diisopropyl phosphorofluoridate (DFP) produces organophosphorus ester-induced delayed neurotoxicity (OPIDN) in humans and sensitive animal species, e.g., adult chicken. The chickens were sacrificed 18 days after a single dose of DFP (1.7 mg/kg, s.c.), which produced severe ataxia or paralysis in 10-14 days. We studied Ca2+/calmodulin-dependent in vitro neurofilament phosphorylation by the brain subcellular fractions of control and DFP-treated hens. There was enhanced phosphorylation of all three NF subunits by the brain supernatant of treated hens. This was accompanied by enhanced autophosphorylation of both Ca2+/CaM-dependent protein kinase II (CaM-kinase II) subunits and increased calmodulin binding using either 125I-CaM or biotinylated calmodulin to only alpha subunit without concomitant increase in the amount of this enzyme. This enhanced phosphorylation of neurofilament subunits was completely and partially inhibited by mastoparan and KN-62, respectively. There was no alteration in the distribution of CaM-kinase II activity in treated hens and the activity was not related to its concentration in different subcellular fractions. The difference in 125I-CaM binding to CaM-kinase II alpha subunit in the brain supernatants of control and DFP-treated hens was not altered by its phosphorylation or dephosphorylation. The increased CaM-kinase II activity in the soluble fraction of DFP-treated hen brain may be involved in the aberrant phosphorylation of axonal neurofilaments, and thus play a role in OPIDN.