Summary Early onset epileptic encephalopathies (EOEE) are a debilitating spectrum of disorders associated with cognitive impairments. We present a clinical report of a KCNT2 mutation in an EOEE patient. The de novo heterozygous variant Phe240Leu SLICK was identified by exome sequencing and confirmed by Sanger sequencing. Phe240Leu rSlick and hSLICK channels were electrophysiologically heterologously characterized to reveal three significant alterations to channel function. First, [Cl−]i-sensitivity was reversed in Phe240Leu channels. Second, predominantly K+-selective WT channels were made to favor Na+ over K+ by Phe240Leu. Third and consequent to altered ion selectivity, Phe240Leu channels had larger inward conductance. Further, rSlick channels induced membrane hyperexcitability when expressed in primary neurons, resembling the cellular seizure phenotype. Taken together, our results confirm that Phe240Leu is a ‘change-of-function’ KCNT2 mutation, demonstrating unusual altered selectivity in KNa channels. These findings establish pathogenicity of the Phe240Leu KCNT2 mutation in the reported EOEE patient.

Progranulin (PGRN) is an evolutionarily conserved glycoprotein associated with several disease states, including neurodegeneration, cancer and autoimmune disorders. This protein has recently been implicated in the regulation of lysosome function, whereby PGRN may bind to and promote the maturation and activity of the aspartyl protease cathepsin D (proCTSD -inactive precursor, matCTSD -mature, enzymatically active form). As the full-length PGRN protein can be cleaved into smaller peptides, called granulins, we assessed the function of these granulin peptides in binding to proCTSD and stimulating matCTSD enzyme activity in vitro. Here, we report that fulllength PGRN and multi-granulin domain peptides bound to proCTSD with low-to submicromolar binding affinities. This binding promoted proCTSD destabilization, which was greater in magnitude for multi-granulin domain peptides than for full-length PGRN. Such destabilization correlated with enhanced matCTSD activity at acidic pH. The presence and function of multigranulin domain peptides has typically been overlooked in previous studies. This work provides the first in vitro quantification of their binding and activity on proCTSD. Our study highlights the significance of multi-granulin domain peptides in the regulation of proCTSD maturation and enzymatic activity and suggests that attention to PGRN processing will be essential for future understanding of the molecular mechanisms leading to neurodegenerative disease states with lossof-function mutations in PGRN.

Voltage‐dependent sodium (Nav) 1.8 channels regulate action potential generation in nociceptive neurons, identifying them as putative analgesic targets. Here, we show that Nav1.8 channel plasma membrane localization, retention, and stability occur through a direct interaction with the postsynaptic density‐95/discs large/zonula occludens‐l‐and WW domain‐containing scaffold protein called membrane‐associated guanylate kinase with inverted orientation (Magi)‐l. The neurophysiological roles of Magi‐1 are largely unknown, but we found that dorsal root ganglion (DRG)‐specific knockdown of Magi‐1 attenuated thermal nociception and acute inflammatory pain and produced deficits in Nav1.8 protein expression. A competing cell‐penetrating peptide mimetic derived from the Nav1.8 WW binding motif decreased sodium currents, reduced Nav1.8 protein expression, and produced hypoexcitability. Remarkably, a phosphorylated variant of the very same peptide caused an opposing increase in Nav1.8 surface expression and repetitive firing. Likewise, in vivo, the peptides produced diverging effects on nocifensive behavior. Additionally, we found that Magi‐1 bound to sequence like a calcium‐activated potassium channel sodium‐activated (Slack) potassium channels, demonstrating macrocomplexing with Nav1.8 channels. Taken together, these findings emphasize Magi‐1 as an essential scaffold for ion transport in DRG neurons and a central player in pain.—Pryce, K. D., Powell, R., Agwa, D., Evely, K. M., Sheehan, G. D., Nip, A., Tomasello, D. L., Gururaj, S., Bhattacharjee, A. Magi‐1 scaffolds Nav1.8 and Slack KNa channels in dorsal root ganglion neurons regulating excitability and pain. FASEB J. 33, 7315–7330 (2019). http://www.fasebj.org

The sodium-activated potassium (K) channel Kcnt1 (Slack) is abundantly expressed in nociceptor (pain-sensing) neurons of the dorsal root ganglion (DRG), where they transmit the large outward conductance and arbitrate membrane excitability. Slack channel expression at the DRG membrane is necessary for their characteristic firing accommodation during maintained stimulation, and reduced membrane channel density causes hyperexcitability. We have previously shown that in a pro-inflammatory state, a decrease in membrane channel expression leading to reduced Slack-mediated expression underlies DRG neuronal sensitization. An important component of the inflammatory milieu, PKA internalizes Slack channels from the DRG membrane, reduces , and produces DRG neuronal hyperexcitability when activated in cultured primary DRG neurons. Here, we show that this PKA-induced retrograde trafficking of Slack channels also occurs in intact spinal cord slices and that it is carried out by adaptor protein-2 (AP-2) via clathrin-mediated endocytosis. We provide mass spectrometric and biochemical evidence of an association of native neuronal AP-2 adaptor proteins with Slack channels, facilitated by a dileucine motif housed in the cytoplasmic Slack C terminus that binds AP-2. By creating a competitive peptide blocker of AP-2-Slack binding, we demonstrated that this interaction is essential for clathrin recruitment to the DRG membrane, Slack channel endocytosis, and DRG neuronal hyperexcitability after PKA activation. Together, these findings uncover AP-2 and clathrin as players in Slack channel regulation. Given the significant role of Slack in nociceptive neuronal excitability, the AP-2 clathrin-mediated endocytosis trafficking mechanism may enable targeting of peripheral and possibly, central neuronal sensitization.

p38 MAPK has long been understood as an inducible kinase under conditions of cellular stress, but there is now increasing evidence to support its role in the regulation of neuronal function. Several phosphorylation targets have been identified, an appreciable number of which are ion channels, implicating the possible involvement of p38 MAPK in neuronal excitability. The KNa channel Slack is an important protein to be studied as it is highly and ubiquitously expressed in DRG neurons and is important in the maintenance of their firing accommodation. We sought to examine if the Slack channel could be a substrate of p38 MAPK activity. First, we found that the Slack C-terminus contains two putative p38 MAPK phosphorylation sites that are highly conserved across species. Second, we show via electrophysiology experiments that KNa currents and further, Slack currents, are subject to tonic modulation by p38 MAPK. Third, biochemical approaches revealed that Slack channel regulation by p38 MAPK occurs through direct phosphorylation at the two putative sites of interaction, and mutating both sites prevented surface expression of Slack channels. Based on these results, we conclude that p38 MAPK is an obligate regulator of Slack channel function via the trafficking of channels into the membrane. The present study identifies Slack KNa channels as p38 MAPK substrates.