Neonatal lethality of mouse A/J-7SM consomic strain is caused by an insertion mutation in the Dchs1 gene.

Homosomic mice of the A/J-7SM consomic mouse strain that introduced the entire chromosome 7 (Chr 7) of SM/J into the A/J strain exhibited neonatal lethality. We tentatively maintained segregating inbred strains (A/J-7ASM and A/J-7DSM) in which the central portion of Chr 7 was heterozygous for the A/J and SM/J strains, and the centromeric and telomeric sides of Chr 7 were homozygous for the SM/J strain, instead of the A/J-7SM strain. Based on the chromosomal constitution of Chr 7 in A/J-7ASM and A/J-7DSM mice, the causative gene for neonatal lethality in homosomic mice was suggested to be located within an approximately 1.620 Mb region between D7Mit125 (104.879 Mb) and D7Mit355 (106.499 Mb) on Chr 7. RT-PCR analysis revealed that homosomic mice lacked dachsous cadherin-related 1 (Dchs1), which is located within the D7Mit125 to D7Mit355 region and functions in the regulation of planar cell polarity. Screening for mutations in Dchs1 indicated that homosomic mice possessed an early transposable (ETn)-like sequence in intron 1 of Dchs1. Moreover, an allelism test between Dchs1 ETn-like-insertion alleles detected in homosomic mice and CRISPR/Cas9-induced Dchs1 deletion alleles revealed that Dchs1 is a causative gene for neonatal lethality in homosomic mice. Based on these results, we concluded that in the A/J-7SM strain, ETn-like elements were inserted into intron 1 of SM/J-derived Dchs1 during strain development, which dramatically reduced Dchs1 expression, thus resulting in neonatal lethality in homosomic mice. Additionally, it was suggested that the timing of lethality in Dchs1 mutant mice is influenced by the genetic background.

Amyloidogenic processing of amyloid ß protein precursor (APP) is enhanced in the brains of alcadein α-deficient mice.

Alzheimer's disease (AD) is a very common neurodegenerative disorder, chiefly caused by increased production of neurotoxic ß-amyloid (Aß) peptide generated from proteolytic cleavage of ß-amyloid protein precursor (APP). Except for familial AD arising from mutations in the APP and presenilin (PSEN) genes, the molecular mechanisms regulating the amyloidogenic processing of APP are largely unclear. Alcadein α/calsyntenin1 (ALCα/CLSTN1) is a neuronal type I transmembrane protein that forms a complex with APP, mediated by the neuronal adaptor protein X11-like (X11L or MINT2). Formation of the ALCα-X11L-APP tripartite complex suppresses Aß generation in vitro, and X11L-deficient mice exhibit enhanced amyloidogenic processing of endogenous APP. However, the role of ALCα in APP metabolism in vivo remains unclear. Here, by generating ALCα-deficient mice and using immunohistochemistry, immunoblotting, and co-immunoprecipitation analyses, we verified the role of ALCα in the suppression of amyloidogenic processing of endogenous APP in vivo We observed that ALCα deficiency attenuates the association of X11L with APP, significantly enhances amyloidogenic ß-site cleavage of APP, especially in endosomes, and increases the generation of endogenous Aß in the brain. Furthermore, we noted amyloid plaque formation in the brains of human APP-transgenic mice in an ALCα-deficient background. These results unveil a potential role of ALCα in protecting cerebral neurons from Aß-dependent pathogenicity in AD.

Constitutive cleavage of the single-pass transmembrane protein alcadeinα prevents aberrant peripheral retention of Kinesin-1.

Various membrane proteins are shed by proteinases, constitutively and/or when stimulated by external signals. While the physiological significance of external signal-induced cleavages has been intensely investigated, relatively little is known about the function of constitutive cleavages. Alcadeinα (Alcα; also called Calsyntenin-1) is an evolutionarily conserved type I single-pass transmembrane protein that binds to kinesin-1 light chain (KLC) to activate kinesin-1's transport of Alcα-containing vesicles. We found that Alcα was constitutively and efficiently cleaved to liberate its ectodomain into the extracellular space, and that full-length Alcα protein was rarely detected on the cell surface. The secretion efficiency of the ectodomain was unaltered by a mutation that both abolished Alcα's KLC-binding activity and attenuated its peripheral transport, suggesting that Alcα's cleavage occurred, at least partly, en route to the cell surface. We further demonstrated that uncleavable mutant Alcα proteins readily accumulated on the cell surface and induced aberrant peripheral recruitment of KLC1 and kinesin heavy chain. Our observations suggest that Alcα is efficiently processed in part to minimize the inappropriate peripheral retention of kinesin-1. This role might exemplify the functional relevance of the constitutive cleavage of single-pass transmembrane proteins.

Expression of sorting nexin 18 (SNX18) is dynamically regulated in developing spinal motor neurons.

The sorting nexin (SNX) family proteins, which contain a Phox homology (PX) domain, play crucial roles in regulating the intracellular membrane trafficking of the endocytic pathway. The proper coordination of this pathway is important for axonal elongation; however, little is known about the expression and intracellular dynamics of the SNX members during the formation of the nervous system. Here the authors found that SNX18, which belongs to the Src-homology-3-PX-Bin/Amphiphysin/Rvs domain-containing SNX subfamily, was specifically expressed in differentiating motor neurons in the chick and mouse embryonic spinal cord. The expression of SNX18 in embryonic spinal motor neurons was transient and was downregulated as the neurons matured. The authors further demonstrated that the localization of EGFP-SNX18 in growth cones was dynamically regulated and accumulated especially at areas in contact with permissive substrates. These findings collectively suggest that SNX18 may play an active role in axonal elongation.

IgSF molecule MDGA1 is involved in radial migration and positioning of a subset of cortical upper-layer neurons.

Mdga1, encoding a GPI-anchored immunoglobulin superfamily molecule containing an MAM domain, is expressed by a specific subset of neurons, including layer II/III projection neurons, in the mouse neocortex. To investigate the function of Mdga1 in corticogenesis, we generated Mdga1-deficient mice and backcrossed them to obtain a congenic background. Gross anatomy of the Mdga1-deficient brain at postnatal day (P) 14 showed no obvious phenotype. However, the migration of Mdga1-mutant neurons to the superficial cortical plate was clearly delayed. Most Mdga1-mutant neurons reached the lower portion of the upper cortical layer by embryonic day 18.5 and stayed there through P0. By P7, the location of the mutant cells was the same as wild-type. The location of Cux2-expressing upper-layer neurons in the cortical plate was largely unaffected. These observations indicated that Mdga1 is involved in the migration and positioning of a subset of cortical neurons and suggested that the radial migration of upper-layer neurons might be differentially regulated.