Cytoarchitectural disruption of the superior colliculus and an enlarged acoustic startle response in the Tuba1a mutant mouse

Visual Function, Organization, and Development of the Mouse Superior Colliculus

The superior colliculus (SC) is the most prominent visual center in mice. Studies over the past decade have greatly advanced our understanding of the function, organization, and development of the mouse SC, which has rapidly become a popular model in vision research. These studies have described the diverse and cell-type-specific visual response properties in the mouse SC, revealed their laminar and topographic organizations, and linked the mouse SC and downstream pathways with visually guided behaviors. Here, we summarize these findings, compare them with the rich literature of SC studies in other species, and highlight important gaps and exciting future directions. Given its clear importance in mouse vision and the available modern neuroscience tools, the mouse SC holds great promise for understanding the cellular, circuit, and developmental mechanisms that underlie visual processing, sensorimotor transformation, and, ultimately, behavior.

Brain-specific knockin of the pathogenic Tubb5 E401K allele causes defects in motor coordination and prepulse inhibition

The generation, migration, and differentiation of neurons requires the functional integrity of the microtubule cytoskeleton. Mutations in the tubulin gene family are known to cause various neurological diseases including lissencephaly, ocular motor disorders, polymicrogyria and amyotrophic lateral sclerosis. We have previously reported that mutations in TUBB5 cause microcephaly that is accompanied by severe intellectual impairment and motor delay. Here we present the characterization of a Tubb5 mouse model that allows for the conditional expression of the pathogenic E401K mutation. Homozygous knockin animals exhibit a severe reduction in brain size and in body weight. These animals do not show any significant impairment in general activity, anxiety, or in the acoustic startle response, however, present with notable defects in motor coordination. When assessed on the static rod apparatus mice took longer to orient and often lost their balance completely. Interestingly, mutant animals also showed defects in prepulse inhibition, a phenotype associated with sensorimotor gating and considered an endophenotype for schizophrenia. This study provides insight into the behavioral consequences of tubulin gene mutations.

Neuronal migration disorders: Focus on the cytoskeleton and epilepsy

A wide spectrum of focal, regional, or diffuse structural brain abnormalities, collectively known as malformations of cortical development (MCDs), frequently manifest with intellectual disability (ID), epilepsy, and/or autistic spectrum disorder (ASD). As the acronym suggests, MCDs are perturbations of the normal architecture of the cerebral cortex and hippocampus. The pathogenesis of these disorders remains incompletely understood; however, one area that has provided important insights has been the study of neuronal migration. The amalgamation of human genetics and experimental studies in animal models has led to the recognition that common genetic causes of neurodevelopmental disorders, including many severe epilepsy syndromes, are due to mutations in genes regulating the migration of newly born post-mitotic neurons. Neuronal migration genes often, though not exclusively, code for proteins involved in the function of the cytoskeleton. Other cellular processes, such as cell division and axon/dendrite formation, which similarly depend on cytoskeletal functions, may also be affected. We focus here on how the susceptibility of the highly organized neocortex and hippocampus may be due to their laminar organization, which involves the tight regulation, both temporally and spatially, of gene expression, specialized progenitor cells, the migration of neurons over large distances and a birthdate-specific layering of neurons. Perturbations in neuronal migration result in abnormal lamination, neuronal differentiation defects, abnormal cellular morphology and circuit formation. Ultimately this results in disorganized excitatory and inhibitory activity leading to the symptoms observed in individuals with these disorders.

Microtubules and neurodevelopmental disease: the movers and the makers

The development of the mammalian cortex requires the generation, migration and differentiation of neurons. Each of these cellular events requires a dynamic microtubule cytoskeleton. Microtubules are required for interkinetic nuclear migration, the separation of chromatids in mitosis, nuclear translocation during migration and the outgrowth of neurites. Their importance is underlined by the finding that mutations in a host of microtubule associated proteins cause detrimental neurological disorders. More recently, the structural subunits of microtubules, the tubulin proteins, have been implicated in a spectrum of human diseases collectively known as the tubulinopathies. This chapter reviews the discovery of microtubules, the role they play in neurodevelopment, and catalogues the tubulin isoforms associated with neurodevelopmental disease. Our focus is on the molecular and cellular mechanisms that underlie the pathology of tubulin-associated diseases. Finally, we reflect on whether different tubulin genes have distinct intrinsic functions.

Mutations in the β-tubulin gene TUBB5 cause microcephaly with structural brain abnormalities

The formation of the mammalian cortex requires the generation, migration, and differentiation of neurons. The vital role that the microtubule cytoskeleton plays in these cellular processes is reflected by the discovery that mutations in various tubulin isotypes cause different neurodevelopmental diseases, including lissencephaly (TUBA1A), polymicrogyria (TUBA1A, TUBB2B, TUBB3), and an ocular motility disorder (TUBB3). Here, we show that Tubb5 is expressed in neurogenic progenitors in the mouse and that its depletion in vivo perturbs the cell cycle of progenitors and alters the position of migrating neurons. We report the occurrence of three microcephalic patients with structural brain abnormalities harboring de novo mutations in TUBB5 (M299V, V353I, and E401K). These mutant proteins, which affect the chaperone-dependent assembly of tubulin heterodimers in different ways, disrupt neurogenic division and/or migration in vivo. Our results provide insight into the functional repertoire of the tubulin gene family, specifically implicating TUBB5 in embryonic neurogenesis and microcephaly.

New insights into behaviour using mouse ENU mutagenesis

Identifying genes involved in behavioural disorders in man is a challenge as the cause is often multigenic and the phenotype is modulated by environmental cues. Mouse mutants are a valuable tool for identifying novel pathways underlying specific neurological phenotypes and exploring the influence both genetic and non-genetic factors. Many human variants causing behavioural disorders are not gene deletions but changes in levels of expression or activity of a gene product; consequently, large-scale mouse ENU mutagenesis has the advantage over the study of null mutants in that it generates a range of point mutations that frequently mirror the subtlety and heterogeneity of human genetic lesions. ENU mutants have provided novel and clinically relevant functional information on genes that influence many aspects of mammalian behaviour, from neuropsychiatric endophenotypes to circadian rhythms. This review will highlight some of the most important findings that have been made using this method in several key areas of neurological disease research.