Codon modification of Tuba1a alters mRNA levels and causes a severe neurodevelopmental phenotype in mice

The tubulinopathies are an umbrella of rare diseases that result from mutations in tubulin genes and are frequently characterised by severe brain malformations. The characteristics of a given disease reflect the expression pattern of the transcript, the function of a given tubulin gene, and the role microtubules play in a particular cell type. Mouse models have proved to be valuable tools that have provided insight into the molecular and cellular mechanisms that underlie the disease state. In this manuscript we compare two Tuba1a mouse models, both of which express wild-type TUBA1A protein but employ different codon usage. We show that modification of the Tuba1a mRNA sequence results in homozygous lethality and a severe neurodevelopmental phenotype. This is associated with a decrease in the number of post-mitotic neurons, PAX6 positive progenitors, and an increase in the number of apoptotic cells. We attribute this to a decrease in the stability of the modified Tuba1a transcript, and the absence of compensation by the other neurogenic tubulins. Our findings highlight the importance of maintaining the wild-type coding sequence when engineering mouse lines and the impact of synonymous genetic variation.

The expression, localisation and interactome of pigeon CRY2

Cryptochromes (CRY) are highly conserved signalling molecules that regulate circadian rhythms and are candidate radical pair based magnetoreceptors. Birds have at least four cryptochromes (CRY1a, CRY1b, CRY2, and CRY4), but few studies have interrogated their function. Here we investigate the expression, localisation and interactome of clCRY2 in the pigeon retina. We report that clCRY2 has two distinct transcript variants, clCRY2a, and a previously unreported splice isoform, clCRY2b which is larger in size. We show that clCRY2a mRNA is expressed in all retinal layers and clCRY2b is enriched in the inner and outer nuclear layer. To define the localisation and interaction network of clCRY2 we generated and validated a monoclonal antibody that detects both clCRY2 isoforms. Immunohistochemical studies revealed that clCRY2a/b is present in all retinal layers and is enriched in the outer limiting membrane and outer plexiform layer. Proteomic analysis showed clCRY2a/b interacts with typical circadian molecules (PER2, CLOCK, ARTNL), cell junction proteins (CTNNA1, CTNNA2) and components associated with the microtubule motor dynein (DYNC1LI2, DCTN1, DCTN2, DCTN3) within the retina. Collectively these data show that clCRY2 is a component of the avian circadian clock and unexpectedly associates with the microtubule cytoskeleton.

A proteomic survey of microtubule-associated proteins in a R402H TUBA1A mutant mouse

Microtubules play a critical role in multiple aspects of neurodevelopment, including the generation, migration and differentiation of neurons. A recurrent mutation (R402H) in the α-tubulin gene TUBA1A is known to cause lissencephaly with cerebellar and striatal phenotypes. Previous work has shown that this mutation does not perturb the chaperone-mediated folding of tubulin heterodimers, which are able to assemble and incorporate into the microtubule lattice. To explore the molecular mechanisms that cause the disease state we generated a new conditional mouse line that recapitulates the R402H variant. We show that heterozygous mutants present with laminar phenotypes in the cortex and hippocampus, as well as a reduction in striatal size and cerebellar abnormalities. We demonstrate that homozygous expression of the R402H allele causes neuronal death and exacerbates a cell intrinsic defect in cortical neuronal migration. Microtubule sedimentation assays coupled with quantitative mass spectrometry demonstrated that the binding and/or levels of multiple microtubule associated proteins (MAPs) are perturbed by the R402H mutation including VAPB, REEP1, EZRIN, PRNP and DYNC1l1/2. Consistent with these data we show that the R402H mutation impairs dynein-mediated transport which is associated with a decoupling of the nucleus to the microtubule organising center. Our data support a model whereby the R402H variant is able to fold and incorporate into microtubules, but acts as a gain of function by perturbing the binding of MAPs.

Microtubules are polymers composed of tubulin proteins, which play an important role in the development of the human brain. Genetic mutations in tubulin genes are known to cause neurodevelopmental diseases, including lissencephaly which is characterised by an impairment in the migration of neurons. In this study we investigate how a common mutation (R402H) in TUBA1A causes lissencephaly by generating and characterising a mouse with the same variant. We show that affected animals recapitulate multiple aspects of the human disease; including laminar perturbations in the cortex and hippocampus, attributable to defects in neuronal migration at key developmental time points. To characterize the molecular implications of the R402H mutation we purified microtubules from the developing brain, and analysed the proteins present by mass spectrometry. This revealed that the binding of DYNC1I1/2 to microtubules is altered in mice with the R402H mutation. Our results provide insight into the molecular pathology underlying tubulin related disease states, and provide a foundation for the rational design of therapeutic interventions.

A high sensitivity ZENK monoclonal antibody to map neuronal activity in Aves

The transcription factor ZENK is an immediate early gene that has been employed as a surrogate marker to map neuronal activity in the brain. It has been used in a wide variety of species, however, commercially available antibodies have limited immunoreactivity in birds. To address this issue we generated a new mouse monoclonal antibody, 7B7-A3, raised against ZENK from the rock pigeon (Columba livia). We show that 7B7-A3 labels clZENK in both immunoblots and histological stainings with high sensitivity and selectivity for its target. Using a sound stimulation paradigm we demonstrate that 7B7-A3 can detect activity-dependent ZENK expression at key stations of the central auditory pathway of the pigeon. Finally, we compare staining efficiency across three avian species and confirm that 7B7-A3 is compatible with immunohistochemical detection of ZENK in the rock pigeon, zebra finch, and domestic chicken. Taken together, 7B7-A3 represents a useful tool for the avian neuroscience community to map functional activity in the brain.

Lidocaine is a nocebo treatment for trigeminally mediated magnetic orientation in birds

Even though previously described iron-containing structures in the upper beak of pigeons were almost certainly macrophages, not magnetosensitive neurons, behavioural and neurobiological evidence still supports the involvement of the ophthalmic branch of the trigeminal nerve (V1) in magnetoreception. In previous behavioural studies, inactivation of putative V1-associated magnetoreceptors involved either application of the surface anaesthetic lidocaine to the upper beak or sectioning of V1. Here, we compared the effects of lidocaine treatment, V1 ablations and sham ablations on magnetic field-driven neuronal activation in V1-recipient brain regions in European robins. V1 sectioning led to significantly fewer Egr-1-expressing neurons in the trigeminal brainstem than in the sham-ablated birds, whereas lidocaine treatment had no effect on neuronal activation. Furthermore, Prussian blue staining showed that nearly all iron-containing cells in the subepidermal layer of the upper beak are nucleated and are thus not part of the trigeminal nerve, and iron-containing cells appeared in highly variable numbers at inconsistent locations between individual robins and showed no systematic colocalization with a neuronal marker. Our data suggest that lidocaine treatment has been a nocebo to the birds and a placebo for the experimenters. Currently, the nature and location of any V1-associated magnetosensor remains elusive.

Subcellular analysis of pigeon hair cells implicates vesicular trafficking in cuticulosome formation and maintenance

Hair cells are specialized sensors located in the inner ear that enable the transduction of sound, motion, and gravity into neuronal impulses. In birds some hair cells contain an iron-rich organelle, the cuticulosome, that has been implicated in the magnetic sense. Here, we exploit histological, transcriptomic, and tomographic methods to investigate the development of cuticulosomes, as well as the molecular and subcellular architecture of cuticulosome positive hair cells. We show that this organelle forms rapidly after hatching in a process that involves vesicle fusion and nucleation of ferritin nanoparticles. We further report that transcripts involved in endocytosis, extracellular exosomes, and metal ion binding are differentially expressed in cuticulosome positive hair cells. These data suggest that the cuticulosome and the associated molecular machinery regulate the concentration of iron within the labyrinth of the inner ear, which might indirectly tune a magnetic sensor that relies on electromagnetic induction.

Mutations in the murine homologue of TUBB5 cause microcephaly by perturbing cell cycle progression and inducing p53-associated apoptosis

Microtubules play a crucial role in the generation, migration and differentiation of nascent neurons in the developing vertebrate brain. Mutations in the constituents of microtubules, the tubulins, are known to cause an array of neurological disorders, including lissencephaly, polymicrogyria and microcephaly. In this study we explore the genetic and cellular mechanisms that cause TUBB5-associated microcephaly by exploiting two new mouse models: a conditional E401K knock-in, and a conditional knockout animal. These mice present with profound microcephaly due to a loss of upper-layer neurons that correlates with massive apoptosis and upregulation of p53. This phenotype is associated with a delay in cell cycle progression and ectopic DNA elements in progenitors, which is dependent on the dosage of functional Tubb5. Strikingly, we report ectopic Sox2-positive progenitors and defects in spindle orientation in our knock-in mouse line, which are absent in knockout animals. This work sheds light on the functional repertoire of Tubb5, reveals that the E401K mutation acts by a complex mechanism, and demonstrates that the cellular pathology driving TUBB5-associated microcephaly is cell death.

High resolution anatomical mapping confirms the absence of a magnetic sense system in the rostral upper beak of pigeons

The cells that are responsible for detecting magnetic fields in animals remain undiscovered. Previous studies have proposed that pigeons employ a magnetic sense system that consists of six bilateral patches of magnetite containing dendrites located in the rostral subepidermis of the upper beak. We have challenged this hypothesis arguing that clusters of iron-rich cells in this region are macrophages, not magnetosensitive neurons. Here we present additional data in support of this conclusion. We have undertaken high resolution anatomical mapping of iron-rich cells in the rostral upper beak of pigeons, excluding the possibility that a conserved six-loci magnetic sense system exists. In addition we have extended our immunohistochemical studies to a second cohort of pigeons, confirming that iron rich cells in the upper beak are positive for MHCII and CD44, which are expressed by macrophages. We argue that it is important to critically assess conclusions that have been made in the past, while keeping an open mind as the search for the magnetoreceptor continues.

Discrete neocortical dynamics predict behavioral categorization of sounds

The ability to group stimuli into perceptual categories is essential for efficient interaction with the environment. Discrete dynamics that emerge in brain networks are believed to be the neuronal correlate of category formation. Observations of such dynamics have recently been made; however, it is still unresolved if they actually match perceptual categories. Using in vivo two-photon calcium imaging in the auditory cortex of mice, we show that local network activity evoked by sounds is constrained to few response modes. Transitions between response modes are characterized by an abrupt switch, indicating attractor-like, discrete dynamics. Moreover, we show that local cortical responses quantitatively predict discrimination performance and spontaneous categorization of sounds in behaving mice. Our results therefore demonstrate that local nonlinear dynamics in the auditory cortex generate spontaneous sound categories which can be selected for behavioral or perceptual decisions.