Initial formation of zebrafish brain ventricles occurs independently of circulation and requires thenagie okoandsnakehead/atp1a1a.1gene products

Abstract

The mechanisms by which the vertebrate brain develops its characteristic three-dimensional structure are poorly understood. The brain ventricles are a highly conserved system of cavities that form very early during brain morphogenesis and that are required for normal brain function. We have initiated a study of zebrafish brain ventricle development and show here that the neural tube expands into primary forebrain, midbrain and hindbrain ventricles rapidly, over a 4-hour window during mid-somitogenesis. Circulation is not required for initial ventricle formation, only for later expansion. Cell division rates in the neural tube surrounding the ventricles are higher than between ventricles and, consistently, cell division is required for normal ventricle development. Two zebrafish mutants that do not develop brain ventricles are snakehead and nagie oko. We show that snakehead is allelic to small heart, which has a mutation in the Na+K+ ATPase gene atp1a1a.1. The snakehead neural tube undergoes normal ventricle morphogenesis;however, the ventricles do not inflate, probably owing to impaired ion transport. By contrast, mutants in nagie oko, which was previously shown to encode a MAGUK family protein, fail to undergo ventricle morphogenesis. This correlates with an abnormal brain neuroepithelium, with no clear midline and disrupted junctional protein expression. This study defines three steps that are required for brain ventricle development and that occur independently of circulation: (1) morphogenesis of the neural tube, requiring nok function; (2) lumen inflation requiring atp1a1a.1function; and (3) localized cell proliferation. We suggest that mechanisms of brain ventricle development are conserved throughout the vertebrates.

Mitotic cell rounding and epithelial thinning regulate lumen growth and shape

“…We explored the cellular mechanisms responsible for this anisotropic growth. While the relevance of fluid movement in increasing the size of the cavity has been demonstrated in several experimental models 5,10,11 , with the involvement of paracellular transportation and epithelial chloride secretion 9,23 , its role in shaping the lumen has still not been evaluated. It is widely assumed that the increased intraluminal concentration of chloride (and other ions) promotes the movement of water from outside the organ into the lumen (by paracellular or transcellular transport), similar to what occurs in secretory epithelial organs 12,13 .…”

“…Lumen growth requires coordination between fluid ingress and increase of the luminal epithelial surface area. Fluids ingress has been shown to rely on a functional Na þ /K þ ATPase pump, the CFTR chloride channel and the formation of both paracellular pores and barriers by claudin proteins 5,[9][10][11] . The present view of the process indicates that fluid ingress is mediated by transepithelial fluid transport, in which the epithelium is considered as a pump driving fluid movement from outside of the organ into the lumen [12][13][14] .…”

A mutation in the zebrafish Na,K‐ATPase subunit atp1a1a.1 provides genetic evidence that the sodium potassium pump contributes to left‐right asymmetry downstream or in parallel to nodal flow

“…Molecular identification of the Na, K-ATPase subunit atp1a1a.1 as the gene affected in m883 for the first time provides genetic evidence for the previously postulated role for an ion pump during establishment of laterality in general. Previously, four other zebrafish alleles of atp1a1a.1 were published (Shu et al, 2003;Amsterdam et al, 2004;Yuan and Joseph, 2004;Lowery and Sive, 2005). While the alleles published by Shu and coworkers (heart and mind, had) and Yuan and coworkers (small heart) appear to be identical, the insertional mutation published by Amsterdam and coworkers has not been described in detail.…”

“…A search for candidate genes in this region (www.ZFIN.org) revealed that Chen and colleagues had mapped the mutation heart and mind (had la1 ) to this genomic region and shown that had affects the gene ATPase, Naϩ/Kϩ transporting, alpha 1a.1 polypeptide (atp1a1a.1) (Shu et al, 2003). More recently, other alleles of had have also been published: had hi3475 , a retroviral insertion (Amsterdam et al, 2004), had mw15b (Yuan and Joseph, 2004), and had to273a (Lowery and Sive, 2005), which initially had been described as snk to273a . In collaboration with Dr. Chen (UCLA), we performed complementation analysis of m883 and had la1 , which revealed non-complementation and demonstrated that m883 is an allele of had.…”

“…atp1a1a.1 has also been shown to be required for the inflation of the brain ventricle lumen (Lowery and Sive, 2005) Expression of atp1a1a.1…”