Control of Spontaneous Ca²⁺Transients Is Critical for Neuronal Maturation in the Developing Neocortex

Abstract

Neural activity plays roles in the later stages of development of cortical excitatory neurons, including dendritic and axonal arborization, remodeling, and synaptogenesis. However, its role in earlier stages, such as migration and dendritogenesis, is less clear. Here we investigated roles of neural activity in the maturation of cortical neurons, using calcium imaging and expression of prokaryotic voltage-gated sodium channel, NaChBac. Calcium imaging experiments showed that postmigratory neurons in layer II/III exhibited more frequent spontaneous calcium transients than migrating neurons. To test whether such an increase of neural activity may promote neuronal maturation, we elevated the activity of migrating neurons by NaChBac expression. Elevation of neural activity impeded migration, and induced premature branching of the leading process before neurons arrived at layer II/III. Many NaChBac-expressing neurons in deep cortical layers were not attached to radial glial fibers, suggesting that these neurons had stopped migration. Morphological and immunohistochemical analyses suggested that branched leading processes of NaChBac-expressing neurons differentiated into dendrites. Our results suggest that developmental control of spontaneous calcium transients is critical for maturation of cortical excitatory neurons in vivo: keeping cellular excitability low is important for migration, and increasing spontaneous neural activity may stop migration and promote dendrite formation.

Activity Regulates Cell Death within Cortical Interneurons through a Calcineurin-Dependent Mechanism

“…The pattern of NaChbac-induced firing, which reduced the degree of cell death in interneurons (Figures S4A and S4B) resemble early cortical network oscillations or giant depolarizing potentials (Allène et al, 2008; Khazipov et al, 2004) that occur when a cluster of action potentials result in intracellular calcium influx (Bando et al, 2016; Lin et al, 2010). Both with NaChbac and in vivo , these bursts occur at a frequency of about 1 mHz (Figure S4C), which has been shown previously to favor CaN activation (Dolmetsch et al, 1998).…”

“…In a complementary set of experiments, we increased the level of activity in these same populations of interneurons by using bacterial sodium channel (NaChbac), a bacterial voltage-gated sodium channel that decreases the threshold for firing (Bando et al, 2016; Lin et al, 2010). This channel was incorporated into an AAV-flex-P2A-mCherry vector (AAV-EF1:DIO:NaChbac-P2A-mCherry).…”

“…This latter study, like ours, discovered that, in vasointestinal peptide (VIP), interneurons, although they undergo apoptosis, survival is not regulated by activity. A growing body of literature indicates that calcium influx couples excitation to a variety of biological processes (Bando et al, 2016; Bean, 1989; Bonci et al, 1998; Brosenitsch and Katz, 2001; Mermelstein et al, 2000; Tsien et al, 1988; Wheeler et al, 2012; Yasuda et al, 2003), including transcription and alternative splicing (Ghosh and Greenberg, 1995; Greenberg et al, 1986; Iijima et al, 2011). We discovered that, with the exception of VIP cells, activity in interneurons directly modulates the activity of the phosphatase Calcineurin (CaN) and is sequentially required for maturation and survival.…”

Regulation of neurogenesis by calcium signaling

“…Ca 2+ influx through N-type Ca 2+ channels is believed to induce specific Ca 2+ fluctuations whose amplitude and frequency control the rate of cell migration [154]. However, in the neurogenic region of the subventricular zone, NSCs appear to require low levels of activity, possibly involving gap junctions, to maintain their migratory behavior [155,156]. As NSCs arrive at the sites where they are ultimately incorporated, they exhibit increased Ca 2+ signaling which arrests their migratory phenotype [156].…”

“…However, in the neurogenic region of the subventricular zone, NSCs appear to require low levels of activity, possibly involving gap junctions, to maintain their migratory behavior [155,156]. As NSCs arrive at the sites where they are ultimately incorporated, they exhibit increased Ca 2+ signaling which arrests their migratory phenotype [156]. Although more studies are needed to understand the basis of the various results, one possibility is that Ca 2+ both facilitates and inhibits migration based on its amplitude and kinetic signature.…”