Planar cell polarity signalling and mammalian neurulation

During early pregnancy, a crucial event in the developing embryo is the formation of the neural tube , which will later develop into the brain and spinal cord. Failure of the neural tube to form correctly leads to a group of birth defects called neural tube defects (NTDs), in which the brain and/or spinal cord of the fetus become irreversibly damaged, resulting in death before or shortly after birth, or handicap in surviving babies. Overall, NTDs occur in around 1 per 1,000 pregnancies although the rate varies and is significantly higher in some regions (e.g. Northern Ireland and Scotland). Worldwide, approximately 130,000 cases occur every year. The risk of NTDs depends on both inherited genetic factors and non-genetic factors such as diet, but the exact causes are not well understood. However, the risk of an affected pregnancy can be substantially reduced if the mother takes folic acid supplements before and during early pregnancy. Unfortunately, not all cases of NTDs are preventable by folic acid and it seems that to make further progress in prevention of NTDs will require a deeper understanding of the underlying causes of the defects (both genetic and environmental). Recent studies have revealed several genes, which when mutated in mice, prevent the initial step of neural tube closure, resulting in severe spina bifida. Significantly, these genes all seem to be involved in the same biochemical pathway, called the planar cell polarity pathway, which appears to be needed for co-ordinated movement of cells in the early embryo. In this project we will focus on these genes, to determine their specific function in the developing mouse embryo, and to understand how this is essential for the neural tube to form. Although these developmental studies will be carried out in mouse models, we have recently found that some humans with severe spina bifida also carry alterations in planar cell polarity genes. We plan to test whether these alterations (or mutations) are responsible for spina bifida in patients with NTDs, by testing the effect of the same mutation in mice. Identification of ?risk? genes will provide an opportunity to offer genetic counselling to couples who have affected pregnancies, and will be a key step towards developing new personalised therapies for prevention of NTDs.

Neurulation - formation of the neural tube - is a critical event in development of the central nervous system. Neural tube closure is initiated at the hindbrain/cervical boundary (Closure 1), after which closure occurs progressively and independently in brain and spine. Failure of any aspect of neurulation leads to neural tube defects (NTDs), a group of severe congenital malformations that are lethal at birth (e.g. anencephaly) or cause severe handicap in survivors (e.g. open spina bifida). The causes of human NTDs appear complex and multifactorial, involving both genetic and environmental factors, although major causative genes have yet to be identified. Determination of the underlying developmental mechanisms and identification of genetic risk factors would be an important step towards development of improved genetic counseling for NTDs and new preventive strategies.

Many mouse mutants develop NTDs, providing model systems for experimental analysis. A recent advance was the finding that mutations in genes of the planar cell polarity (PCP) signalling pathway cause the most severe NTD, craniorachischisis. In this condition, the hindbrain and entire spinal cord remain open as a result of failure of Closure 1. Mechanistically, PCP signalling seems to be required for correct regulation of cell movements that both narrow and lengthen the embryo during gastrulation, a process called convergent extension. The purpose of the present proposal is to build upon recent studies, by ourselves and others, that link PCP signalling with mouse NTDs. We will analyse in detail the role of PCP signalling in neurulation mechanisms, and investigate the possible role of PCP genes in causation of human NTDs. Specifically, we aim to:

1. Investigate the developmental link(s) between PCP signalling, convergent extension and initiation of neural tube closure. We will determine in which tissue PCP signalling is required, whether the function of PCP genes is cell autonomous or non-autonomous, and whether convergent extension also requires PCP independent cell-matrix interactions.

2. Determine the mechanisms by which PCP gene mutations predispose to defects of later spinal neurulation, that result in open spina bifida. These studies will address the possible role of convergent extension after initiation of neurulation.

3. Test putative mutations in PCP genes, that we have identified in human NTD cases, for functional effects on neural tube closure. This will be achieved by generation of equivalent mutations in knockin mice, and examination of knockin embryos for disturbance of convergent extension and neurulation.