Abstract
Dipeptidyl peptidase-like protein 6 (DPP6) proteins co-assemble with Kv4 channel α-subunits and Kv channel-interacting proteins (KChIPs) to form channel protein complexes underlying neuronal somatodendritic A-type potassium current (ISA). DPP6 proteins are expressed as N-terminal variants (DPP6a, DPP6K, DPP6S, DPP6L) that result from alternative mRNA initiation and exhibit overlapping expression patterns. Here, we study the role DPP6 variants play in shaping the functional properties of ISA found in cerebellar granule (CG) cells using quantitative RT-PCR and voltage-clamp recordings of whole-cell currents from reconstituted channel complexes and native ISA channels. Differential expression of DPP6 variants was detected in rat CG cells, with DPP6K (41±3%)>DPP6a (33±3%)>>DPP6S (18±2%)>DPP6L (8±3%). To better understand how DPP6 variants shape native neuronal ISA, we focused on studying interactions between the two dominant variants, DPP6K and DPP6a. Although previous studies did not identify unique functional effects of DPP6K, we find that the unique N-terminus of DPP6K modulates the effects of KChIP proteins, slowing recovery and producing a negative shift in the steady-state inactivation curve. By contrast, DPP6a uses its distinct N-terminus to directly confer rapid N-type inactivation independently of KChIP3a. When DPP6a and DPP6K are co-expressed in ratios similar to those found in CG cells, their distinct effects compete in modulating channel function. The more rapid inactivation from DPP6a dominates during strong depolarization; however, DPP6K produces a negative shift in the steady-state inactivation curve and introduces a slow phase of recovery from inactivation. A direct comparison to the native CG cell ISA shows that these mixed effects are present in the native channels. Our results support the hypothesis that the precise expression and co-assembly of different auxiliary subunit variants are important factors in shaping the ISA functional properties in specific neuronal populations.
Highlights
The somatodendritic A-type potassium current (ISA) regulates neuronal excitability, firing frequency, action potential backpropagation, and synaptic plasticity [1,2,3,4]
Our results show that (1) DPP6K is the most common dipeptidyl peptidases 6 (DPP6) variant expressed in cerebellar granule (CG) cells, (2) DPP6K Nterminal domain regulates the slower phases of inactivation, recovery from inactivation, and steady-state inactivation of the ternary complex, and (3) The properties of the native ISA in CG cells are largely determined by the co-assembly of Kv4.2, KChIP3a, DPP6a, and DPP6K subunits into a heteromultimeric channel complex
Amplification curves from the cortex, cerebellum, and hippocampus showed that DPP6K and DPP6a are expressed at higher levels in cerebellum, whereas DPP6S and DPP6L are expressed at similar levels in these three brain regions (Fig. 1B)
Summary
The somatodendritic A-type potassium current (ISA) regulates neuronal excitability, firing frequency, action potential backpropagation, and synaptic plasticity [1,2,3,4]. Strong evidence for ISA channels being composed of a ternary complex of Kv4s, KChIPs, and DPLPs comes from coimmunoprecipitation and functional reconstitution experiments demonstrating that native-like functional properties requires coexpression of all three proteins [16,23]. In agreement with this model, the transcript and protein expression patterns of Kv4, KChIP, and DPLPs overlap in all regions of the brain, suggesting an obligatory ternary complex formation underlies ISA throughout the brain [24,25]
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