Spatiotemporal expression in mouse brain of Kiaa2022, a gene disrupted in two patients with severe mental retardation.

We previously identified an inactivating disruption of the X-linked KIAA2022 gene by a chromosomal rearrangement in two male patients with severe mental retardation. In order to determine if KIAA2022 has a role during the development of the central nervous system, we have cloned its murine ortholog, Kiaa2022, determined its genomic structure and studied its expression during mouse development. We show that Kiaa2022 is preferentially expressed in the central nervous system and that the transcript is highly expressed in postmitotic neurons. The expression of Kiaa2022 is first detectable at E10.5 to reach a maximum at P3 where it is notably expressed in the hippocampus, the entorhinal cortex and strongly in the ventral premammillary nucleus. After P3, the expression of Kiaa2022 decreases and maintains very low levels thereafter. Our results show that Kiaa2022 is expressed in the developing brain and that it may play a role in postmitotic, maturing neurons.

Sensory defects in Necdin deficient mice result from a loss of sensory neurons correlated within an increase of developmental programmed cell death.

BACKGROUND: The human NECDIN gene is involved in a neurodevelopmental disorder, Prader-Willi syndrome (PWS). Previously we reported a mouse Necdin knock-out model with similar defects to PWS patients. Despite the putative roles attributed to Necdin, mainly from in vitro studies, its in vivo function remains unclear. In this study, we investigate sensory-motor behaviour in Necdin deficient mice. We reveal cellular defects and analyse their cause. RESULTS: We report sensory differences in Necdin deficient mice compared to wild type animals. These differences led us to investigate sensory neuron development in Necdin deficient mouse embryos. First, we describe the expression pattern of Necdin in developing DRGs and report a reduction of one-third in specified sensory neurons in dorsal roots ganglia and show that this neuronal loss is achieved by E13.5, when DRGs sensory neurons are specified. In parallel, we observed an increase of 41% in neuronal apoptosis during the wave of naturally occurring cell death at E12.5. Since it is assumed that Necdin is a P75NTR interactor, we looked at the P75NTR-expressing cell population in Necdin knock-out embryos. Unexpectedly, Necdin loss of function has no effect on p75NTR expressing neurons suggesting no direct genetic interaction between Necdin and P75NTR in this context. Although we exclude a role of Necdin in axonal outgrowth from spinal sensory neurons in early developmental stages; such a role could occur later in neuronal differentiation. Finally we also exclude an anti-proliferative role of Necdin in developing sensory neurons. CONCLUSION: Overall, our data show clearly that, in early development of the nervous system, Necdin is an anti-apoptotic or survival factor.

Expression of the Prader-Willi gene Necdin during mouse nervous system development correlates with neuronal differentiation and p75NTR expression.

The expression pattern of Necdin, a gene involved in the etiology of Prader-Willi syndrome and a member of the MAGE family of genes, is described during mouse nervous system development. Using RNA in situ hybridization, immunohistochemical staining, and colocalization with neuronal differentiation markers, we found that Necdin RNA and protein are expressed within post-mitotic neurons at all stages studied. From E10 to E12, Necdin is detected in all developing neurons, in both central and peripheral nervous system, with the highest expression levels in the diencephalon and the hindbrain. After E13, Necdin is expressed in specific structures of the nervous system, in particular the hypothalamus, the thalamus, and the pons, suggesting a specific developmental role therein. In addition, Necdin expression is also detected in non-neural tissues, such as the somites, the developing limb buds, the first branchial arches, the tong, and the axial muscles. Recently, Necdin and other MAGE proteins were found to interact in vitro with the intracellular domain of the p75NTR neurotrophin receptor, but this interaction has not been validated in vivo. We report here that the spatial and temporal expression of p75NTR is included in Necdin expression domain. These results are in agreement with Necdin proposed role on cell cycle arrest, inhibition of apoptosis and facilitation of neuronal differentiation in vitro, and with hypothalamic cellular deficiencies reported in mice with abrogation of the Necdin gene. Furthermore, they are also consistent with the putative role of Necdin in signaling events promoted by p75NTR during mouse nervous system development.

Necdin protects embryonic motoneurons from programmed cell death.

NECDIN belongs to the type II Melanoma Associated Antigen Gene Expression gene family and is located in the Prader-Willi Syndrome (PWS) critical region. Necdin-deficient mice develop symptoms of PWS, including a sensory and motor deficit. However, the mechanisms underlying the motor deficit remain elusive. Here, we show that the genetic ablation of Necdin, whose expression is restricted to post-mitotic neurons in the spinal cord during development, leads to a loss of 31% of specified motoneurons. The increased neuronal loss occurs during the period of naturally-occurring cell death and is not confined to specific pools of motoneurons. To better understand the role of Necdin during the period of programmed cell death of motoneurons we used embryonic spinal cord explants and primary motoneuron cultures from Necdin-deficient mice. Interestingly, while Necdin-deficient motoneurons present the same survival response to neurotrophic factors, we demonstrate that deletion of Necdin leads to an increased susceptibility of motoneurons to neurotrophic factor deprivation. We show that by neutralizing TNFα this increased susceptibility of Necdin-deficient motoneurons to trophic factor deprivation can be reduced to the normal level. We propose that Necdin is implicated through the TNF-receptor 1 pathway in the developmental death of motoneurons.