Novel truncated isoform of SK3 potassium channel is a potent dominant-negative regulator of SK currents: implications in schizophrenia

Abstract

The small-conductance calcium-activated K þ channel SK3 (SKCa3/KCNN3) regulates electrical excitability and neurotransmitter release in monoaminergic neurons, and has been implicated in schizophrenia, ataxia and anorexia nervosa. We have identified a novel SK3 transcript, SK3-1B that utilizes an alternative first exon (exon 1B), but is otherwise identical to SK3. SK3-1B, mRNA is widely distributed in human tissues and is present at 20-60% of SK3 in the brain. The SK3-1B protein lacks the N-terminus and first transmembrane segment, and begins eight residues upstream of the second transmembrane segment. When expressed alone, SK3-1B did not produce functional channels, but selectively suppressed endogenous SK3 currents in the pheochromocytoma cell line, PC12, in a dominant-negative fashion. This dominant inhibitory effect extended to other members of the SK subfamily, but not to voltage-gated K þ channels, and appears to be due to intracellular trapping of endogenous SK channels. The effect of SK3-1B expression is very similar to that produced by expression of the rare SK3 truncation allele, SK3-D, found in a patient with schizophrenia. Regulation of SK3 and SK3-1B levels may provide a potent mechanism to titrate neuronal firing rates and neurotransmitter release in monoaminergic neurons, and alterations in the relative abundance of these proteins could contribute to abnormal neuronal excitability, and to the pathogenesis of schizophrenia.

SK3-1C, a Dominant-negative Suppressor of SKCa and IKCa Channels

“…2C). The levels of SK3-1C in the trachea and salivary gland are higher than that reported for SK3, but in other non-neuronal tissues the levels of SK3-1C are comparable to that for SK3 (16). Thus, SK3-1C is natively expressed in peripheral human tissues but not brain.…”

“…Real-time Quantitative RT-PCR-Human total RNA master panel (Clontech) and total RNA isolated from human lymph node tissue and peripheral blood lymphocytes were used to profile the expression pattern of SK3, SK3-1C, and IKCa1 with methods previously described (16). Following DNase digestion, total RNA (2 g) was used as a template for first-strand cDNA synthesis using random hexamers (TaqMan TM reverse transcription reagents, Applied Biosystems).…”

“…BQ807391), porcine (BI341958), and bovine (CB467440) brain. When expressed in heterologous mammalian expression systems, both SK3-⌬ and SK3-1B suppress SK1, SK2, and SK3 channels (16,22). Such dominant-negative behavior in the brain should have consequences similar to pharmacological blockade of SK Ca channels, namely enhanced firing frequency and increased neurotransmitter release (3, 4, 7, 19 -21).…”

“…Recent quantitative RT-PCR studies have identified SK3 transcripts in other human tissues including the trachea, thyroid, spleen, and thymus (16). In the brain, SK3 is restricted to the midbrain monoaminergic neurons, basal ganglia, and limbic system, showing little overlap with SK1 and SK2 (7,13,14,16,17), and recent immunohistochemical studies have localized SK3 to presynaptic nerve termini (18). The SK3 channel serves as the intrinsic pacemaker in dopaminergic neurons and its pharmacological blockade leads to bursting action potentials and increased dopamine release (4, 19 -21).…”

“…The first, SK3-⌬, is a frameshift mutant identified in a patient with schizophrenia (22,24), and transcripts of the second, SK3-1B, have been detected in human (16), primate (accession no. BQ807391), porcine (BI341958), and bovine (CB467440) brain.…”