The related neuronal endosomal proteins NEEP21 (Nsg1) and P19 (Nsg2) have divergent expression profiles in vivo

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Neurons are among the most morphologically complex cells in the body. This complexity manifests on two fronts. First, neurons are extremely large in size and extend axons and dendrites over long distances. A neuron's soma is roughly the size of an epithelial cell, and neuronal axons can extend up to 1 m in length in humans. Second, neurons have a highly polarized morphology with distinct functional domains, axons and dendrites, which are molecularly distinct. Many proteins are found in one domain but not the other (Barnes & Polleux, 2009). This complexity requires both that proteins be transported over long distances during development and throughout life, and also that proteins need to be accurately sorted to the correct location in this very large cell (Winckler, 2016). These special requirements for protein transport have resulted in many neuronal adaptations in terms of cytoskeleton and membrane transport in order to meet a neuron's specific needs (Yap & Winckler, 2012). Lastly, neurons are postmitotic and among the longest‐lived cells in the body. This long lifetime means that any problems due to mistrafficking or dysregulation of recycling and degradation have particularly devastating effects.
Many proteins are highly enriched or even specifically expressed in neurons. These include proteins that fundamentally underlie neuronal synaptic function, such as neurotransmitter receptors, but also cytoskeletal proteins and proteins regulating membrane transport. One such protein is Neuron Enriched Endosomal Protein of 21kDa (NEEP21/Nsg1), a small single‐pass transmembrane protein that is highly enriched in neurons (Ohnishi, Futamura, Kamino, & Nakamura, 2010; Sabéran‐Djoneidi et al., 1998). Interestingly, NEEP21 is restricted to the somatodendritic domain (Steiner et al., 2002; Yap et al., 2008).
NEEP21 has been shown to play a critical role in the trafficking and polarization of a variety of proteins including the axonal cell adhesion molecule L1/NgCAM, β‐APP, GluA2, and neurotensin receptors (Debaigt, Hirling, Steiner, Vincent, & Mazella, 2004; Norstrom, Zhang, Tanzi, & Sisodia, 2010; Steiner et al., 2002; Yap et al., 2008). When NEEP21 is knocked‐down in cultured neurons, L1/NgCAM becomes mislocalized to the somatodendritic region and to LAMP2‐positive endosomes (Yap et al., 2008). Missorting of cargo into LAMP2‐positive endosomes in the absence of NEEP21 is also observed for neurotensin receptor and for GluA2. This dependence of correct protein trafficking on neuronal proteins specific to certain domains of the neuron highlights the complexity of the endosomal sorting machinery in neurons.
NEEP21 belongs to a family of endosomal proteins including Calcyon (Caly) and P19 (Nsg2) (Muthusamy et al., 2009). NEEP21 and P19 show approximately 50% amino acid sequence homology to each other, and 30% to Calcyon. NEEP21 and P19 were both identified as being highly enriched in the brain and developmentally regulated (Sabéran‐Djoneidi et al., 1995). NEEP21 has been detected in rat brains at high levels up to P14, at which point the protein levels decline greatly (Steiner et al., 2002).
We recently showed that, surprisingly, NEEP21 and P19 were not expressed in all neurons cultured from embryonic rat hippocampus. Virtually, all CTIP2‐ and Satb2‐positive neurons in hippocampal cultures expressed both NEEP21 and P19 robustly. In contrast, both NEEP21 and P19 were expressed at very low levels in Prox1‐positive cells, which are likely derived from the dentate gyrus (Digilio et al., 2015). The differential expression found in cultured neurons thus prompted us to ask (a) if NEEP21 and P19 expression levels were similarly low in the dentate gyrus in vivo, (b) at what age NEEP21 and P19 expression disappeared, and (c) if NEEP21 and P19 expression profiles were always identical in vivo.
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