Cell-Type-Dependent Molecular Composition of the Axon Initial Segment

Laboratory of Cellular Neurophysiology, Institute of Experimental Medicine, Hungarian Academy of Sciences, 1083 Budapest, Hungary.
The Journal of Neuroscience : The Official Journal of the Society for Neuroscience (Impact Factor: 6.75). 01/2009; 28(53):14329-40. DOI: 10.1523/JNEUROSCI.4833-08.2008
Source: PubMed

ABSTRACT The exact site of initiation and shape of action potentials vary among different neuronal types. The reason for this variability is largely unknown, but the subunit composition, density and distribution of voltage-gated sodium (Nav) and potassium (Kv) channels within the axon initial segment (AIS) are likely to play a key role. Here, we asked how heterogeneous are the density and distribution of Nav and Kv channels within the AISs of a variety of excitatory and inhibitory neurons. Most of the studied cell types expressed a high density of Nav1.6, Kv1.1, and Kv1.2 subunits in their AIS, but the Nav1.1 subunit could only be detected in GABAergic interneurons. A proximo-distal gradient in the density of these subunits was observed within the AIS of certain nerve cells but not in others. For example, a gradual increase of the Nav1.6 subunit was observed in cortical layer 2/3 and hippocampal CA1 pyramidal cell (PC) AISs, whereas its density was rather uniform in layer 5 PC AISs. The Nav1.1 subunit was distributed evenly along the AIS of short-axon cells of the main olfactory bulb but was restricted to the proximal part of the AIS in cortical and cerebellar interneurons. Our results reveal a cell type-dependent expression of sodium and potassium channel subunits with varying densities along the proximo-distal axis of the AISs. This precise arrangement is likely to contribute to the diversity of firing properties observed among central neurons.

  • [Show abstract] [Hide abstract]
    ABSTRACT: Polarized distribution of signaling molecules to axons and dendrites facilitates directional information flow in complex vertebrate nervous systems. The topic we address here is when the key aspects of neuronal polarity evolved. All neurons have a central cell body with thin processes that extend from it to cover long distances, and they also all rely on voltage-gated ion channels to propagate signals along their length. The most familiar neurons, those in vertebrates, have additional cellular features that allow them to send directional signals efficiently. In these neurons, dendrites typically receive signals and axons send signals. It has been suggested that many of the distinct features of axons and dendrites, including the axon initial segment, are found only in vertebrates. However, it is now becoming clear that two key cytoskeletal features that underlie polarized sorting, a specialized region at the base of the axon and polarized microtubules, are found in invertebrate neurons as well. It thus seems likely that all bilaterians generate axons and dendrites in the same way. As a next step, it will be extremely interesting to determine whether the nerve nets of cnidarians and ctenophores also contain polarized neurons with true axons and dendrites, or whether polarity evolved in concert with the more centralized nervous systems found in bilaterians. © 2015. Published by The Company of Biologists Ltd.
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The Purkinje cell (PC) is among the most complex neurons in the brain and plays a critical role for cerebellar functioning. PCs operate as fast pacemakers modulated by synaptic inputs but can switch from simple spikes to complex bursts and, in some conditions, show bistability. In contrast to original works emphasizing dendritic Ca-dependent mechanisms, recent experiments have supported a primary role for axonal Na-dependent processing, which could effectively regulate spike generation and transmission to deep cerebellar nuclei (DCN). In order to account for the numerous ionic mechanisms involved (at present including Nav1.6, Cav2.1, Cav3.1, Cav3.2, Cav3.3, Kv1.1, Kv1.5, Kv3.3, Kv3.4, Kv4.3, KCa1.1, KCa2.2, KCa3.1, Kir2.x, HCN1), we have elaborated a multicompartmental model incorporating available knowledge on localization and gating of PC ionic channels. The axon, including initial segment (AIS) and Ranvier nodes (RNs), proved critical to obtain appropriate pacemaking and firing frequency modulation. Simple spikes initiated in the AIS and protracted discharges were stabilized in the soma through Na-dependent mechanisms, while somato-dendritic Ca channels contributed to sustain pacemaking and to generate complex bursting at high discharge regimes. Bistability occurred only following Na and Ca channel down-regulation. In addition, specific properties in RNs K currents were required to limit spike transmission frequency along the axon. The model showed how organized electroresponsive functions could emerge from the molecular complexity of PCs and showed that the axon is fundamental to complement ionic channel compartmentalization enabling action potential processing and transmission of specific spike patterns to DCN.
    Frontiers in Cellular Neuroscience 01/2015; 9(47). DOI:10.3389/fncel.2015.00047 · 4.18 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Epileptic encephalopathies are a phenotypically and genetically heterogeneous group of severe epilepsies accompanied by intellectual disability and other neurodevelopmental features –6. Using next-generation sequencing, we identified four different de novo mutations in KCNA2, encoding the potassium channel K V .2, in six isolated patients with epileptic encephalopathy (one mutation recurred three times independently). Four individuals presented with febrile and multiple afebrile, often focal seizure types, multifocal epileptiform discharges strongly activated by sleep, mild to moderate intellectual disability, delayed speech development and sometimes ataxia. Functional studies of the two mutations associated with this phenotype showed almost complete loss of function with a dominant-negative effect. Two further individuals presented with a different and more severe epileptic encephalopathy phenotype. They carried mutations inducing a drastic gain-of-function effect leading to permanently open channels. These results establish KCNA2 as a new gene involved in human neurodevelopmental disorders through two different mechanisms, predicting either hyperexcitability or electrical silencing of K V .2-expressing neurons. Many of the voltage-gated potassium channels (K V 1–K V 12) are expressed in the central nervous system (CNS), having an important role in neuronal excitability and neurotransmitter release 7. Mutations in potassium channel–encoding genes cause different neurological diseases, including benign familial neonatal seizures (KCNQ2, encoding K V 7.2; KCNQ3, encoding K V 7.3) 8–10 , neonatal epileptic encephalopathy (KCNQ2) 11,12 , episodic ataxia type 1 (EA1) (KCNA1, encoding K V 1.1) 13 and peripheral nerve hyperexcitability (KCNA1, KCNQ2) 13–15. In addition, antibodies against K V 1.1 or associated proteins such as contactin-associated protein 2 (CASPR2) or leucine-rich, glioma-inactivated 1 (LGI1) cause limbic encephalitis or neuromyotonia 16. Therefore, potassium channel genes represent interesting candidates for neurodevelopmental disorders. To identify mutations in presumed genetic forms of epilepsy, we designed a targeted resequencing panel 17 comprising 265 known and 220 candidate genes for epilepsy (Supplementary Table 1). Screening a pilot cohort of 33 patients, we identified mutations in known epilepsy-related genes in 16 cases 17. We evaluated the remaining 17 cases for mutations in candidate genes (Supplementary Note), which led to the detection of a heterozygous de novo mutation in KCNA2, c.1214C>T (encoding p.Pro405Leu), affecting the highly conserved pore domain of the voltage-gated potassium channel K V 1.2 (NM_004974, CCDS827). This mutation was not present in control databases (1000 Genomes Project, Exome Variant Server (EVS), dbSNP138 or the Exome Aggregation Consortium (ExAC) database). The affected female (patient 1) carrying this mutation had unre-markable early development until the onset of epilepsy at 17 months of age. The phenotype included febrile and afebrile alternating hemiclonic seizures and status epilepticus, reminiscent of Dravet syndrome. The electroencephalogram (EEG) showed multifocal spikes with marked activation during sleep. After seizure onset, ataxia and delay of psychomotor and language development became apparent. She had postnatal short stature, growth hormone deficiency and hypothyroidism. Seizures and ataxia responded poorly to antiepileptic drugs (topiramate, oxcarbazepine, valproic acid and bromide), including acetazolamide (known to be effective in EA1 caused by mutations in KCNA1; ref. 18). At last follow-up at 8 years of age, she had remained seizure free for the past 6 months without previous change of medication. We identified further KCNA2 mutations in several parallel studies (Supplementary Fig. 1). First, we performed whole-exome sequenc-ing in 86 parent-offspring trios with epileptic encephalopathy (31 with Dravet syndrome negative for mutations in SCN1A, 39 with myoclonic-atonic epilepsy (MAE) and 16 with electrical status epi-lepticus in slow-wave sleep (ESES)). Second, we performed panel sequencing (Supplementary Note) in 147 adults with a broad spectrum of epilepsy phenotypes associated with intellectual disability. Third, we performed whole-exome sequencing in an adult cohort of 10 independent trios with severe epilepsy and intellectual disability and whole-exome sequencing in another cohort of 12 independent, isolated index cases with early-onset ataxia and epilepsy. We identified six additional independent cases with previously unre-ported heterozygous KCNA2 variants (Table 1, Supplementary Fig. 2 and Supplementary Note). Patient 2 (initially classified as having MAE) carried the de novo mutation c.788T>C (encoding p.Ile263Thr). Patient 3 (intellectual disability with neonatal-onset focal epilepsy and cerebel-lar hypoplasia) carried the variant c.440G>A (encoding p.Arg147Lys), of unknown inheritance. We considered p.Arg147Lys to be a variant of unknown relevance because (i) it could not be confirmed as de novo, (ii) it was predicted to be benign using seven of nine prediction tools, (iii) a lysine occurs naturally at this position in Drosophila melanogaster and zebrafish, and (iv) the change did not show functional consequences (Supplementary Fig. 3, Supplementary Tables 1 and 2, and Supplementary Note). Patient 4 (initially classified as having Dravet
    Nature Genetics 03/2015; · 29.65 Impact Factor


Available from