Fibroblast growth factor-21 (FGF-21) is a recently discovered metabolic regulator. Here, we investigated the effects of FGF-21 in the pancreatic ␤-cell. In rat islets and INS-1E cells, FGF-21 activated extracellular signal-regulated kinase 1/2 and Akt signaling pathways. In islets isolated from healthy rats, FGF-21 increased insulin mRNA and protein levels but did not potentiate glucose-induced insulin secretion. Islets and INS-1E cells treated with FGF-21 were partially protected from glucolipotoxicity and cytokineinduced apoptosis. In islets isolated from diabetic rodents, FGF-21 treatment increased islet insulin content and glucose-induced insulin secretion. Short-term treatment of normal or db/db mice with FGF-21 lowered plasma levels of insulin and improved glucose clearance compared with vehicle after oral glucose tolerance testing. Constant infusion of FGF-21 for 8 weeks in db/db mice nearly normalized fed blood glucose levels and increased plasma insulin levels. Immunohistochemistry of pancreata from db/db mice showed a substantial increase in the intensity of insulin staining in islets from FGF-21-treated animals as well as a higher number of islets per pancreas section and of insulin-positive cells per islet compared with control. No effect of FGF-21 was observed on islet cell proliferation. In conclusion, preservation of ␤-cell function and survival by FGF-21 may contribute to the beneficial effects of this protein on glucose homeostasis observed in diabetic animals. Diabetes 55:2470 -2478, 2006 P ancreatic ␤-cell dysfunction is a central component of the pathogenesis of all forms of diabetes. Type 1 diabetes manifests from the autoimmune destruction of ␤-cells, whereas type 2 diabetes is characterized by reduced ␤-cell mass and marked functional defects, including impaired first-phase insulin secretion, increased proinsulin-to-insulin ratio, and elevated rate of ␤-cell apoptosis (1-3). The glucose-sensing and insulin-signaling pathways have been shown to play important roles in insulin secretion as well as ␤-cell growth and survival. For example, mice lacking insulin receptors, insulin receptor substrate-2, or Akt (protein kinase B) display marked defects in glucose sensing, insulin secretion, and ␤-cell mass (4 -6). The amount of secreted insulin is determined by the secretory activity of the ␤-cell and the total number of ␤-cells in the pancreas. Glucose plays an essential role in the control of secretory activity of ␤-cells. Metabolism of glucose leads to an increase in the ATP-to-ADP ratio, membrane depolarization, Ca 2ϩ influx, and stimulation of insulin secretion (7). ␤-Cell mass is governed by the balance between ␤-cell growth and ␤-cell death (apoptosis). Type 2 diabetic patients display a progressive loss of ␤-cells caused by an increased rate of ␤-cell apoptosis (8). However, the cause and mechanism(s) responsible for the increased apoptosis rate in type 2 diabetes are not well understood (9). Preventing ␤-cell death and increasing survival of the ␤-cell can be a valuable therapeutic approa...

Measurements of membrane capacitance were applied to dissect the cellular mechanisms underlying PKA-dependent and -independent stimulation of insulin secretion by cyclic AMP. Whereas the PKA-independent (Rp-cAMPS–insensitive) component correlated with a rapid increase in membrane capacitance of ∼80 fF that plateaued within ∼200 ms, the PKA-dependent component became prominent during depolarizations >450 ms. The PKA-dependent and -independent components of cAMP-stimulated exocytosis differed with regard to cAMP concentration dependence; the K d values were 6 and 29 μM for the PKA-dependent and -independent mechanisms, respectively. The ability of cAMP to elicit exocytosis independently of PKA activation was mimicked by the selective cAMP-GEFII agonist 8CPT-2Me-cAMP. Moreover, treatment of B-cells with antisense oligodeoxynucleotides against cAMP-GEFII resulted in partial (50%) suppression of PKA-independent exocytosis. Surprisingly, B-cells in islets isolated from SUR1-deficient mice (SUR1−/− mice) lacked the PKA-independent component of exocytosis. Measurements of insulin release in response to GLP-1 stimulation in isolated islets from SUR1−/− mice confirmed the complete loss of the PKA-independent component. This was not attributable to a reduced capacity of GLP-1 to elevate intracellular cAMP but instead associated with the inability of cAMP to stimulate influx of Cl− into the granules, a step important for granule priming. We conclude that the role of SUR1 in the B cell extends beyond being a subunit of the plasma membrane KATP-channel and that it also plays an unexpected but important role in the cAMP-dependent regulation of Ca2+-induced exocytosis.

The activity of dopaminergic (DA) substantia nigra (SN) neurons is essential for voluntary movement control. An intrinsic pacemaker in DA SN neurons generates their tonic spontaneous activity, which triggers dopamine release. We show here, by combining multiplex and quantitative real-time single-cell RT± PCR with slice patch±clamp electrophysiology, that an A-type potassium channel mediated by Kv4.3 and KChip3 subunits has a key role in pacemaker control. The number of active A-type potassium channels is not only tightly associated with the pacemaker frequency of individual DA SN neurons, but is also highly correlated with their number of Kv4.3L (long splice variant) and KChip3.1 (long splice variant) mRNA molecules. Consequently, the variation of Kv4a and Kv4b subunit transcript numbers is suf®-cient to explain the full spectrum of spontaneous pacemaker frequencies in identi®ed DA SN neurons. This linear coupling between Kv4a as well as Kv4b mRNA abundance, A-type channel density and pacemaker frequency suggests a surprisingly simple molecular mechanism for how DA SN neurons tune their variable ®ring rates by transcriptional control of ion channel genes. Keywords: Kv4/KChip/pacemaker activity/dopaminergic neurons/quantitative real-time TaqMan PCR Introduction Dopaminergic (DA) midbrain neurons are essential for important brain functions such as voluntary movement, working memory and reward (Kitai et al., 1999;Spanagel and Weiss, 1999;Goldman-Rakic et al., 2000). They are also closely involved in the aetiology of neuropsychiatric disorders including schizophrenia, drug abuse and Parkinson's disease (Spanagel and Weiss, 1999;Abi-Dargham et al., 2000;Obeso et al., 2000). Thus, it is of great interest to de®ne the molecular mechanisms that control electrical activity of DA midbrain neurons and consequently dopamine release. Best studied are the DA neurons in the substantia nigra (SN, A9) that release dopamine in their striatal target areas (Onn et al., 2000;Smith and Kieval, 2000). In vivo, these classical striatonigral DA neurons discharge in a pacemaker or irregular single spike mode and less frequently show burst activity (Grace and Bunney, 1984a,b;Kitai et al., 1999). In brain slice preparations, the regular pacemaker mode is retained even during inhibition of synaptic transmission (Grace and Onn, 1989;Lacey et al., 1989), indicating that the pacemaker activity of DA SN neurons is autonomously generated. Spontaneous electrical activity is believed to originate from intrinsic calcium-dependent oscillations of the membrane potential (Grace, 1991). The tuning of this basic oscillator is mediated by ion channels that operate in the subthreshold range and thus determine the frequency of this cellular pacemaker. Little is known about the molecular identity of the ion channels that control this neuronal pacemaker. Ever since the classic work of Connor and Stevens (1971a,b) in invertebrate neurons a general role for A-type potassium channels in frequency control has been assumed (Rudy, 1988;Grace, 1991;Coetzee et al.,...

A large variety of potassium channels is involved in regulating integration and transmission of electrical signals in the nervous system. Different types of neurons, therefore, require specific patterns of potassium channel subunits expression and specific regulation of subunit coassembly into heteromultimeric channels, as well as subunit-specific sorting and segregation. This was investigated by studying in detail the expression of six different alpha-subunits of voltage-gated potassium channels in the rat hippocampus, cerebellum, olfactory bulb and spinal cord, combining in situ hybridization and immunocytochemistry. Specific polyclonal antibodies were prepared for five alpha-subunits (Kv1.1, Kv1.2, Kv1.3 Kv1.4, Kv1.6) of the Shaker-related subfamily of rat Kv channels, which encode delayed-rectifier type and rapidly inactivating A-type potassium channels. Their distribution was compared to that of an A-type potassium channel (Kv3.4), belonging to the Shaw-related subfamily of rat Kv channels. Our results show that these Kv channel alpha-subunits are differentially expressed in rat brain neurons. We did not observe in various neurons a stereotypical distribution of Kv channel alpha-subunits to dendritic and axonal compartments, but a complex differential subcellular subunit distribution. The different Kv channel subunits are targeted either to presynaptic or to postsynaptic domains, depending on neuronal cell type. Thus, distinct combinations of Kv1 alpha-subunits are co-localized in different neurons. The implications of these findings are that both differential expression and assembly as well as subcellular targeting of Kv channel alpha-subunits may contribute to Kv channel diversity and thereby to presynaptic and postsynaptic membrane excitability.