Modulation of Kv4.2 channel expression and gating by dipeptidyl peptidase 10 (DPP10)

Functionally active t1-t1 interfaces revealed by the accessibility of intracellular thiolate groups in kv4 channels

Gating of voltage-dependent K(+) channels involves movements of membrane-spanning regions that control the opening of the pore. Much less is known, however, about the contributions of large intracellular channel domains to the conformational changes that underlie gating. Here, we investigated the functional role of intracellular regions in Kv4 channels by probing relevant cysteines with thiol-specific reagents. We find that reagent application to the intracellular side of inside-out patches results in time-dependent irreversible inhibition of Kv4.1 and Kv4.3 currents. In the absence or presence of Kv4-specific auxiliary subunits, mutational and electrophysiological analyses showed that none of the 14 intracellular cysteines is essential for channel gating. C110, C131, and C132 in the intersubunit interface of the tetramerization domain (T1) are targets responsible for the irreversible inhibition by a methanethiosulfonate derivative (MTSET). This result is surprising because structural studies of Kv4-T1 crystals predicted protection of the targeted thiolate groups by constitutive high-affinity Zn(2+) coordination. Also, added Zn(2+) or a potent Zn(2+) chelator (TPEN) does not significantly modulate the accessibility of MTSET to C110, C131, or C132; and furthermore, when the three critical cysteines remained as possible targets, the MTSET modification rate of the activated state is approximately 200-fold faster than that of the resting state. Biochemical experiments confirmed the chemical modification of the intact alpha-subunit and the purified tetrameric T1 domain by MTS reagents. These results conclusively demonstrate that the T1--T1 interface of Kv4 channels is functionally active and dynamic, and that critical reactive thiolate groups in this interface may not be protected by Zn(2+) binding.

Trafficking of potassium channels

Recent progress in our understanding of the trafficking of potassium channels can be seen in particular when considering the Kv-type channels. To date, we have discovered that folding of the Kv1.3 T1 domain begins in the ribosomal exit tunnel, and that the cell surface expression of Kv4 channels is enhanced by the presence of two recently identified accessory subunits. Current advances are beginning to enable us to understand the Kv supermolecular complex containing these subunits in crystallographic detail. In addition, determinants that govern the dendritic or axonal targeting of Kv channels have also been identified. In terms of the bigger picture, the careful analysis of gene expression patterns in the brain paves the way for studying trafficking in a physiological context. Indeed, neuronal activity has recently been shown to fine-tune the localization of Kv2.1 channels in microdomains of the neuronal plasma membrane.

Dipeptidyl peptidase IV and related enzymes in cell biology and liver disorders

DP (dipeptidyl peptidase) IV is the archetypal member of its six-member gene family. Four members of this family, DPIV, FAP (fibroblast activation protein), DP8 and DP9, have a rare substrate specificity, hydrolysis of a prolyl bond two residues from the N-terminus. The ubiquitous DPIV glycoprotein has proved interesting in the fields of immunology, endocrinology, haematology and endothelial cell and cancer biology and DPIV has become a novel target for Type II diabetes therapy. The crystal structure shows that the soluble form of DPIV comprises two domains, an alpha/beta-hydrolase domain and an eight-blade beta-propeller domain. The propeller domain contains the ADA (adenosine deaminase) binding site, a dimerization site, antibody epitopes and two openings for substrate access to the internal active site. FAP is structurally very similar to DPIV, but FAP protein expression is largely confined to diseased and damaged tissue, notably the tissue remodelling interface in chronically injured liver. DPIV has a variety of peptide substrates, the best studied being GLP-1 (glucagon-like peptide-1), NPY (neuropeptide Y) and CXCL12. The DPIV family has roles in bone marrow mobilization. The functional interactions of DPIV and FAP with extracellular matrix confer roles for these proteins in cancer biology. DP8 and DP9 are widely distributed and indirectly implicated in immune function. The DPL (DP-like) glycoproteins that lack peptidase activity, DPL1 and DPL2, are brain-expressed potassium channel modulators. Thus the six members of the DPIV gene family exhibit diverse biological roles.

Molecular diversity and function of voltage-gated (Kv) potassium channels in epithelial cells

Voltage-gated K+ channels belonging to Kv1-9 subfamilies are widely expressed in excitable cells where they play an essential role in membrane hyperpolarization during an action potential and in the propagation of action potentials along the plasma membrane. Early patch clamp studies on epithelial cells revealed the presence of K+ currents with biophysical and pharmacologic properties characteristic of Kv channels expressed in excitable cells. More recently, molecular approaches including PCR and the availability of more selective antibodies directed against Kv alpha and auxiliary subunits, have demonstrated that epithelial cells from various organ systems, express a remarkable diversity Kv channel subunits. Unlike neurons and myocytes however, epithelial cells do not typically generate action potentials or exhibit dynamic changes in membrane potential necessary for activation of Kv alpha subunits. Moreover, the fact that many Kv channels expressed in epithelial cells exhibit inactivation suggest that their activities are relatively transient, making it difficult to ascribe a functional role for these channels in transepithelial electrolyte or nutrient transport. Other proposed functions have included (i) cell migration and wound healing, (ii) cell proliferation and cancer, (iii) apoptosis and (iv) O2 sensing. Certain Kv channels, particularly Kv1 and Kv2 subfamily members, have been shown to be involved in the proliferation of prostate, colon, lung and breast carcinomas. In some instances, a significant increase in Kv channel expression has been correlated with tumorogenesis suggesting the possibility of using these proteins as markers for transformation and perhaps reducing the rate of tumor growth by selectively inhibiting their functional activity.

Molecular physiology and modulation of somatodendritic A-type potassium channels

The somatodendritic subthreshold A-type K+ current (ISA) in nerve cells is a critical component of the ensemble of voltage-gated ionic currents that determine somatodendritic signal integration. The underlying K+ channel belongs to the Shal subfamily of voltage-gated K+ channels. Most Shal channels across the animal kingdom share a high degree of structural conservation, operate in the subthreshold range of membrane potentials, and exhibit relatively fast inactivation and recovery from inactivation. Mammalian Shal K+ channels (Kv4) undergo preferential closed-state inactivation with features that are generally inconsistent with the classical mechanisms of inactivation typical of Shaker K+ channels. Here, we review (1) the physiological and genetic properties of ISA, 2 the molecular mechanisms of Kv4 inactivation and its remodeling by a family of soluble calcium-binding proteins (KChIPs) and a membrane-bound dipeptidase-like protein (DPPX), and (3) the modulation of Kv4 channels by protein phosphorylation.