Abnormal neuronal patterning occurs during early postnatal brain development of Scn1b -null mice and precedes hyperexcitability

Abstract

Voltage-gated Na + channel (VGSC) β1 subunits, encoded by SCN1B, are multifunctional channel modulators and cell adhesion molecules (CAMs). Mutations in SCN1B are associated with the genetic epilepsy with febrile seizures plus (GEFS+) spectrum disorders in humans, and Scn1b-null mice display severe spontaneous seizures and ataxia from postnatal day (P)10. The goal of this study was to determine changes in neuronal pathfinding during early postnatal brain development of Scn1b-null mice to test the hypothesis that these CAM-mediated roles of Scn1b may contribute to the development of hyperexcitability. c-Fos, a protein induced in response to seizure activity, was up-regulated in the Scn1b-null brain at P16 but not at P5. Consistent with this, epileptiform activity was observed in hippocampal and cortical slices prepared from the P16 but not from the P5-P7 Scn1b-null brain. On the basis of these results, we investigated neuronal pathfinding at P5. We observed disrupted fasciculation of parallel fibers in the P5 null cerebellum. Further, P5 null mice showed reduced neuron density in the dentate gyrus granule cell layer, increased proliferation of granule cell precursors in the hilus, and defective axonal extension and misorientation of somata and processes of inhibitory neurons in the dentate gyrus and CA1. Thus, Scn1b is critical for neuronal proliferation, migration, and pathfinding during the critical postnatal period of brain development. We propose that defective neuronal proliferation, migration, and pathfinding in response to Scn1b deletion may contribute to the development of hyperexcitability.β subunit | sodium channel | hippocampus N euronal voltage-gated Na + channels (VGSCs) are composed of one pore-forming α and two β subunits (1). β1 (encoded by SCN1B) modulates channel gating and cell surface expression (2). In addition, β1 is an Ig superfamily cell adhesion molecule (CAM) that participates in cell-cell and cell-matrix adhesion (3). The SCN1B splice variant β1B is a secreted CAM that is expressed predominantly during embryonic brain development (4). SCN1B mutations are associated with the spectrum of genetic epilepsy with febrile seizures plus (GEFS+) (OMIM 604233) epilepsies (5, 6). At the cellular level, SCN1B GEFS+ mutations cause a range of defects, including altered channel availability, disrupted adhesion (7), exclusion of β1 from the axon initial segment (AIS) of pyramidal neurons (8), reduced cell surface expression of β1, and intracellular retention of β1B (4, 9). In addition, a GEFS+ mutation in SCN1A decreases modulation of Na v 1.1 by β1 (10). Together, these results suggest a causal relationship between VGSC α-β1 interactions, cell-cell adhesion, and epilepsy.Scn1b-null mice display severe spontaneous seizures and ataxia from approximately postnatal day (P)10 (11). Scn1b deletion reduces resurgent Na + current (I Na ) in P14-P16 cerebellar granule neurons (CGNs) (12). In contrast, β1 has no effect on I Na in dissociated P14-P16 hippocampal neurons (9). However, CA3 action potentials have ...

The Role of Non-pore-Forming β Subunits in Physiology and Pathophysiology of Voltage-Gated Sodium Channels

“…Interestingly, β1 and β4 are proposed to play opposite roles in hippocampal neurons, such that β1 promotes channel inactivation, acting as a "brake" on excitability, whereas β4 promotes channel activation, acting as the "accelerator" (Aman et al 2009). This is consistent with findings that Scn1b null mice are spontaneously epileptic Brackenbury et al 2013).…”

“…In this same study, shRNA-mediated knockdown of Scn1b resulted in a reduction of Kv4-mediated current in neurons (Marionneau et al 2012). Scn1b null mice show prolonged action potentials and increased repetitive firing in cortical pyramidal neurons, supporting not only changes in sodium current, as shown in (Brackenbury et al 2013) but also a link between VGSC β1/β1B subunits and potassium channel activity (Marionneau et al 2012). These observations suggest a novel role for VGSC β1/β1B subunits in interacting with and modulating potassium channels, suggesting, perhaps, that these multifunctional proteins should be re-classified independently from the VGSC family.…”

“…Spontaneous behavioral seizures begin at approximately postnatal day 10 in Scn1b null mice . Importantly, developmental defects in the Scn1b null cerebellum and hippocampus precede the onset of seizures and are detectable at P5 (Brackenbury et al 2013). Taken together, these observations suggest that β1 and/or β1B function as CAMs in the developing CNS and that perturbation of Scn1b results in patterning defects which may set up a substrate for altered excitability.…”

“…Recordings from acutely isolated hippocampal pyramidal or bipolar neuronal soma found no measurable differences in sodium current properties in P10-18 Scn1b null neurons relative to wild type Patino et al 2009). However, recordings from Scn1b null brain slices from the same age range demonstrated hyperexcitability in the CA3 region of the hippocampus as well as epileptiform activity in the hippocampus and cortex (Patino et al 2009;Brackenbury et al 2013), suggesting that β1/β1B subunits in neuronal processes may be critical in regulating excitability. These excitability changes are proposed to contribute to the severe seizures observed in Scn1b null mice that are a model of DS Patino et al 2009).…”

“…However, in Scn1b null cerebellar sections, some of the axons traversing the ML deviated from this perpendicular trajectory, suggesting axon outgrowth and pathfinding deficits. In the developing hippocampus, Scn1b null mice display a series of developmental defects, including elevated levels of cellular proliferation in the hilus, ectopic Prox1-expressing dentate granule neurons in the hilus, decreased neuronal density in the dentate gyrus granule cell layer, increased thickness of the dentate gyrus granule cell layer, and abnormal axon outgrowth and pathfinding (Brackenbury et al 2013). Spontaneous behavioral seizures begin at approximately postnatal day 10 in Scn1b null mice .…”