Cilia, neural development and disease

Abstract

Neural development requires a series of cellular events starting with cell specification, proliferation, and migration. Subsequently, axons and dendrites project from the cell surface to form connections to other neurons, interneurons and glia. Anomalies in any one of these steps can lead to malformation or malfunction of the nervous system. Here we review the critical role the primary cilium plays in the fundamental steps of neurodevelopment. By highlighting human diseases caused by mutations in cilia-associated proteins, it is clear that cilia are essential to multiple neural processes. Furthermore, we explore whether additional aspects of cilia regulation, most notably post-translational modification of the tubulin scaffold in cilia, play underappreciated roles in neural development. Finally, we discuss whether cilia-associated proteins function outside the cilium in some aspects of neurodevelopment. These data underscore both the importance of cilia in the nervous system and some outstanding questions in the field.

Keywords: Axon; Cilia; Ciliopathy; Microtubule; Neurodevelopment.

Figure 1:. The cilium is required to transduce Shh signal and specify ventral neural tube cell fates.

A: Shh signaling components dynamically traffic within cilia. In the absence of Shh signal (A.1), receptor Ptch inhibits Smo leading to Gli repressor (GliR) production which represses target gene transcription. When Shh signal is present (A.2), it binds receptor Ptch and is endocytosed. Smo accumulates in the cilium and is activated leading to production of Gli activator (GliA) which, in turn turns on target genes. B: Ventral neural tube cell fates (floorplate -> progenitor 0 (p0)) are defined by their position relative to the notochord which secretes Shh ligand. Dorsal cell fates are specified by signals including BMPs and Wnts. C: Opposing Gli A and GliR gradients along the neural tube dorsal-ventral axis are determined via the amount and duration a given cell is exposed to Shh ligand. The GliA/GliR ratio induces the expression of a specific set of transcription factors that determine the distinct ventral fates.

Figure 2:. Shh is required for proliferation of granule neuron precursors in the cerebellum.

Cerebellar development occurs during embryonic and postnatal stages. Ciliated granule neuron precursor (GNP) cells (green) form the external granule layer (EGL) and respond to Shh produced by the Purkinje cells (purple) to proliferate. Once proliferation is complete, the GNPs migrate beneath the Purkinje cell layer forming the internal granule layer (IGL) where they mature to granule neurons (dark green). Cerebellar granule neurons are the most abundant neuron in the brain due to this proliferation step.

Figure 3:. Distinct cilia-dependent transcription response to Shh distinguishes axon pathfinding.

A: Contralateral (green) retinal ganglion cells (RGCs) are specified due to high Shh activity (purple gradient) and produce GliA that induces Shh mRNA for transport into the axon. Contralateral RGC axons project across the midline of the optic chiasm where they release Shh signal. B: Ipsilateral (red) RGCs are specified due to low Shh activity, resulting in GliA levels that turn on expression of Shh receptor, Boc. Boc is trafficked to the growth cone where it encounters Shh at the optic chiasm midline from the contralateral population. Boc interprets Shh as a repulsive cue so projects from the optic chiasm without crossing the midline.

Figure 4:. The ciliary axoneme and the axon are microtubule projections that share regulation via post-translational modifications of tubulin

In a neuron (bottom), microtubules project as a cilium (left) and an axon (right). These structures share many characteristics. The tubulin composing these microtubules can be post-translationally modified in several ways including being tyrosinated, detyrosinated, converted to the isoform delta2-tubulin, acetylated and deacetylated, and glutamylated/ deglutamylated. Kinesin motors are used for anterograde trafficking and dynein motors for retrograde trafficking in both structures. A: Glycylation is the only microtubule post-translational modification exclusive to cilia. Glycylation is important for regulating axoneme stability and length and is found in a gradient along the axoneme with most at the cilia base. B: The microtubule-based shaft of the axon ends at the growth cone which is composed of a microtubule and actin network. Newly formed microtubules are tyrosinated, which leads to enriched tyrosination near the growth cone. Tyrosinated microtubules can be detyrosinated and converted to a stable delta2-tubulin isoform which is very stable and found at the proximal axon base. Thus, tyrosination is critical for axon polarity and outgrowth.