Alternative splicing programming of axon formation

Abstract

Alternative pre-mRNA splicing generates multiple mRNA isoforms of different structures and functions from a single gene. While the prevalence of alternative splicing control is widely recognized, and the underlying regulatory mechanisms have long been studied, the physiological relevance and biological necessity for alternative splicing are only slowly being revealed. Significant inroads have been made in the brain, where alternative splicing regulation is particularly pervasive and conserved. Various aspects of brain development and function (from neurogenesis, neuronal migration, synaptogenesis, to the homeostasis of neuronal activity) involve alternative splicing regulation. Recent studies have begun to interrogate the possible role of alternative splicing in axon formation, a neuron-exclusive morphological and functional characteristic. We discuss how alternative splicing plays an instructive role in each step of axon formation. Converging genetic, molecular, and cellular evidence from studies of multiple alternative splicing regulators in different systems shows that a biological process as complicated and unique as axon formation requires highly coordinated and specific alternative splicing events. This article is categorized under: RNA Processing > Splicing Regulation/Alternative Splicing RNA in Disease and Development > RNA in Development.

Keywords: Axon guidance; CELF; NOVA; PTBP; RBFOX.

Figure 1.

Patterns of alternative splicing

Figure 2.

Different types of evidence to establish biological relevance of an alternative splicing event.

Figure 3.

Stages of axon formation

Figure 4.. Alternative splicing regulation of early axonogenesis

(a) Early axonogenesis (axonal growth and specification) is coordinated by a neuron-specific alternative splicing program. A combinatorial and coordinated alternative splicing regulatory network encompassing many RBPs and alternative splicing events accompanies the morphological and molecular changes of axon formation. Alternative splicing of Shtn1 produces two isoforms. SHTN1L promotes axonal growth. SHNT1S promotes axon specification. F.S. indicates axon fate specification. (b) The axonongenesis-associated splicing program including Shtn1 is coordinated by PTBP2. PTBP2 loss decreases SHTN1L and reduce axonal elongation. Meanwhile, SHTN1S is increased and some Ptbp2^−/− neurons can extend two TAU1+ axons.

Figure 5.. Alternative splicing regulation of axon guidance

(a) In the spinal cord, growing commissural axons of dorsal interneurons are first attracted to the ventral midline. After crossing the midline, the axons are repulsed away and never cross back. (b) NOVA regulate the isoform expression of DCC and ROBO. (c) Proposed isoform expression for wildtype (WT) neurons before and after crossing the midline. (d) Proposed isoform changes in the Nova^−/− before and after crossing the midline, and the associated phenotypes.

Figure 6.. Alternative splicing regulation of axon maturation

In wildtype (WT) neurons, ANKG expresses an isoform skipping exon 33 because of RBFOX repression. This isoform tethers ANKG to βIV spectrin and the periodic actin ring, thereby clustering sodium channels at the axon initial segment (AIS). In Rbfox knockout neurons, inclusion of exon 33 prevents ANKG from binding to βIV spectrin. As a result, AIS is not properly established and action potentials are impaired.