Ankyrin-G coordinates assembly of the spectrin-based membrane skeleton, voltage-gated sodium channels, and L1 CAMs at Purkinje neuron initial segments

Abstract

The axon initial segment is an excitable membrane highly enriched in voltage-gated sodium channels that integrates neuronal inputs and initiates action potentials. This study identifies Nav1.6 as the voltage-gated sodium channel isoform at mature Purkinje neuron initial segments and reports an essential role for ankyrin-G in coordinating the physiological assembly of Nav1.6, βIV spectrin, and the L1 cell adhesion molecules (L1 CAMs) neurofascin and NrCAM at initial segments of cerebellar Purkinje neurons. Ankyrin-G and βIV spectrin appear at axon initial segments by postnatal day 2, whereas L1 CAMs and Nav1.6 are not fully assembled at continuous high density along axon initial segments until postnatal day 9. L1 CAMs and Nav1.6 therefore do not initiate protein assembly at initial segments. βIV spectrin, Nav1.6, and L1 CAMs are not clustered in adult Purkinje neuron initial segments of mice lacking cerebellar ankyrin-G. These results support the conclusion that ankyrin-G coordinates the physiological assembly of a protein complex containing transmembrane adhesion molecules, voltage-gated sodium channels, and the spectrin membrane skeleton at axon initial segments.

Reduced Sodium Current in Purkinje Neurons from NaV1.1 Mutant Mice: Implications for Ataxia in Severe Myoclonic Epilepsy in Infancy

“…1A3). These results indicate that Na V 1.1 and Na V 1.6 are the predominant sodium channels in the cell bodies and dendrites of Purkinje neurons, in agreement with previous work (Westenbroek et al, 1989;Gong et al, 1999;Caldwell et al, 2000;Jenkins and Bennett, 2001;Schaller and Caldwell, 2003). The observed distribution of Na V channels in P13-P14 cerebellar neurons in our experiments indicates that Na V 1.1 and Na V 1.6 channels conduct nearly all sodium current in Purkinje neurons in this age range.…”

“…However, Na V 1.1 channels are localized primarily in the cell bodies of Purkinje neurons (Westenbroek et al, 1989;Gong et al, 1999) (Fig. 1) (but see also Ogiwara et al, 2007), whereas Na V 1.6 channels are localized in cell bodies, dendrites, and axon initial segments Jenkins and Bennett, 2001;Schaller and Caldwell, 2003) (Fig. 1).…”

“…Na V 1.1, Na V 1.2, and Na V 1.6 channels are abundant in adult brain (Catterall, 2000;Goldin et al, 2000;Goldin, 2001;Trimmer and Rhodes, 2004). Na V 1.1 channels are primarily localized in cell bodies (Westenbroek et al, 1989(Westenbroek et al, , 1992, Na V 1.2 channels in unmyelinated axons and dendrites (Westenbroek et al, 1989(Westenbroek et al, , 1992Gong et al, 1999), and Na V 1.6 channels in myelinated axons and in dendrites Krzemien et al, 2000;Jenkins and Bennett, 2001). These channels participate in generation of both somatodendritic and axonal action potentials (Stuart and Sakmann, 1994;Johnston et al, 1996;Callaway and Ross, 1997;Raman and Bean, 1999b;Khaliq and Raman, 2006).…”

Voltage-Gated Sodium Channels and AnkyrinG Occupy a Different Postsynaptic Domain from Acetylcholine Receptors from an Early Stage of Neuromuscular Junction Maturation in Rats

“…During postnatal development of the AIS, ankyrinG often appears in advance of Na V 1s (Jenkins and Bennett, 2001). In mutant ankyrin-deficient neurons, there is an abnormal distribution of CAMs, ␤-spectrin, and Na V 1s and an inability to generate action potentials (Zhou et al, 1998;Jenkins and Bennett, 2001).…”

“…A clear decrease in the labeling of ␣-fodrin (Fig. 8C), a form of spectrin to which ankyrinG is likely to bind (Jenkins and Bennett, 2001), was observed in the region of high AChR density. In contrast, utrophin, a protein closely associated with the region of high AChR density in mammals Matsumura et al, 1992), occupied precisely the same domain as the AChRs (Fig.…”