Timing as a mechanism of development and evolution in the cerebral cortex

One of the biggest mysteries in neurobiology concerns the mechanisms responsible for the diversification of the brain over different time scales i.e. during development and evolution. Subtle differences in the timing of biological processes during development, e.g. onset, offset, duration, speed and sequence, can trigger large changes in phenotypic outcomes. At the level of a single organism, altered timing of developmental events can lead to individual variability, as well as malformation and disease. At the level of phylogeny, there are known interspecies differences in the timing of developmental events, and this is thought to be an important factor that drives phenotypic variation across evolution, known as heterochrony. A particularly striking example of phenotypic variation is the evolution of human cognitive abilities, which has largely been attributed to the development of the mammalian-specific neocortex and its subsequent expansion in higher primates. Here, I review how the timing of different aspects of cortical development specifies developmental outcomes within species, including processes of cell proliferation and differentiation, neuronal migration and lamination, and axonal targeting and circuit maturation. Some examples of the ways that different processes might “keep time” in the cortex are explored, reviewing potential cell-intrinsic and -extrinsic mechanisms. Further, by combining this knowledge with known differences in timing across species, timing changes that may have occurred during evolution are identified, which perhaps drove the phylogenetic diversification of neocortical structure and function.

Cryo-EM structure of the human Kv3.1 channel reveals gating control by the cytoplasmic T1 domain

Kv3 channels have distinctive gating kinetics tailored for rapid repolarization in fast-spiking neurons. Malfunction of this process due to genetic variants in the KCNC1 gene causes severe epileptic disorders, yet the structural determinants for the unusual gating properties remain elusive. Here, we present cryo-EM structures of the human Kv3.1a channel, revealing a unique arrangement of the cytoplasmic T1 domain which facilitates interactions with C-terminal axonal targeting motif and key components of the gating machinery. Additional interactions between S1/S2 linker and turret domain strengthen the VSD/PD interface. Supported by MD simulations and electrophysiological and mutational analyses, we identify close communication between α6 helix of T1 domain, S4/S5 linker and S6T helix as responsible for the ultra-fast activation/deactivation and open state stabilisation that are unique to Kv3 channels. These findings provide fundamentally new insights into gating control and disease mechanisms and guide strategies for the design of pharmaceutical drugs targeting Kv3 channels.

Selective axonal translation of the mRNA isoform encoding prenylated Cdc42 supports axon growth

ABSTRACT The small Rho-family GTPase Cdc42 has long been known to have a role in cell motility and axon growth. The eukaryotic Ccd42 gene is alternatively spliced to generate mRNAs with two different 3′ untranslated regions (UTRs) that encode proteins with distinct C-termini. The C-termini of these Cdc42 proteins include CaaX and CCaX motifs for post-translational prenylation and palmitoylation, respectively. Palmitoyl-Cdc42 protein was previously shown to contribute to dendrite maturation, while the prenyl-Cdc42 protein contributes to axon specification and its mRNA was detected in neurites. Here, we show that the mRNA encoding prenyl-Cdc42 isoform preferentially localizes into PNS axons and this localization selectively increases in vivo during peripheral nervous system (PNS) axon regeneration. Functional studies indicate that prenyl-Cdc42 increases axon length in a manner that requires axonal targeting of its mRNA, which, in turn, needs an intact C-terminal CaaX motif that can drive prenylation of the encoded protein. In contrast, palmitoyl-Cdc42 has no effect on axon growth but selectively increases dendrite length. Together, these data show that alternative splicing of the Cdc42 gene product generates an axon growth promoting, locally synthesized prenyl-Cdc42 protein. This article has an associated First Person interview with one of the co-first authors of the paper.

AP-4 mediates vesicular transport of the 2-AG endocannabinoid producing enzyme DAGLB

The adaptor protein complex AP-4 mediates anterograde axonal transport and is essential for axon health. AP-4-deficient patients suffer from a severe neurological disorder, which encompasses neurodevelopmental and neurodegenerative features. While impaired autophagy has been suggested to account for axon degeneration in AP-4 deficiency, axon growth defects occur through an unknown mechanism. Here we use orthogonal proteomic and imaging methods to identify DAGLB (diacylglycerol lipase-beta) as a cargo of AP-4 vesicles. DAGLB is a key enzyme for the generation of 2-AG (2-arachidonoylglycerol), the most abundant endocannabinoid in brain. During normal development, DAGLB is targeted to the axon, where 2-AG signalling drives axonal growth. We show that DAGLB accumulates at the TGN of AP-4-deficient cells, including in iPSC-derived neurons from a patient with AP-4 deficiency syndrome. Our data thus support that AP-4 mediates axonal targeting of DAGLB, and we propose that axon growth defects in AP-4 deficiency may arise through spatial dysregulation of endocannabinoid signalling.

Temporal-specific roles of fragile X mental retardation protein in the development of the hindbrain auditory circuit

ABSTRACTFragile X mental retardation protein (FMRP) is an RNA-binding protein abundant in the nervous system. Functional loss of FMRP leads to sensory dysfunction and severe intellectual disabilities. In the auditory system, FMRP deficiency alters neuronal function and synaptic connectivity and results in perturbed processing of sound information. Nevertheless, roles of FMRP in embryonic development of the auditory hindbrain have not been identified. Here, we developed high-specificity approaches to genetically track and manipulate throughout development of the Atoh1+ neuronal cell type, which is highly conserved in vertebrates, in the cochlear nucleus of chicken embryos. We identified distinct FMRP-containing granules in the growing axons of Atoh1+ neurons and post-migrating NM cells. FMRP downregulation induced by CRISPR/Cas9 and shRNA techniques resulted in perturbed axonal pathfinding, delay in midline crossing, excess branching of neurites, and axonal targeting errors during the period of circuit development. Together, these results provide the first in vivo identification of FMRP localization and actions in developing axons of auditory neurons, and demonstrate the importance of investigating early embryonic alterations toward understanding the pathogenesis of neurodevelopmental disorders.

FGF receptors are required for proper axonal branch targeting in Drosophila

Proper axonal branch growth and targeting are essential for establishing a hard-wired neural circuit. Here, we examined the role of Fibroblast Growth Factor Receptors (FGFRs) in axonal arbor development using loss of function and overexpression genetic analyses within single neurons. We used the invariant synaptic connectivity patterns of Drosophila mechanosensory neurons with their innate cleaning reflex responses as readouts for errors in synaptic targeting and circuit function. FGFR loss of function resulted in a decrease in axonal branch number and lengths, and overexpression of FGFRs resulted in ectopic branches and increased lengths. FGFR mutants produced stereotyped axonal targeting errors. Both loss of function and overexpression of FGFRs within the mechanosensory neuron decreased the animal’s frequency of response to mechanosensory stimulation. Our results indicate that FGFRs promote axonal branch growth and proper branch targeting. Disrupting FGFRs results in miswiring and impaired neural circuit function.