Deficiency of the Endocytic ProteinHip1Leads to DecreasedGdpd3expression, Low Phosphocholine, and Kypholordosis

Deficiency of huntingtin interacting protein 1 (Hip1) results in degenerative phenotypes. Here we generated aHip1deficiency allele where a floxed transcriptional stop-cassette and a humanHIP1cDNA were knocked-in to intron 1 of mouseHip1locus.CMV-Cre-mediated germline excision of the stop-cassette resulted in expression of HIP1 and rescue of theHip1knockout phenotype.Mxl-Cre--mediated excision led to HIP1 expression in spleen, kidney and liver, and also rescued the phenotype. In contrast,GFAP-Cre-mediatedHIP1expression in brain did not rescue the phenotype. Metabolomics and microarrays of severalHip1knockout tissues identified low phosphocholine (PC) levels and lowGlycerophosphodiester Phosphodiesterase Domain Containing 3 (Gdpd3) expression. Since Gdpd3 has lysophospholipase D activity that results in the formation of choline, a precursor of PC,Gdpd3downregulation could lead to the low PC levels. To test ifGdpd3contributes to the Hip1 deficiency phenotype, we generatedGdpd3knockout mice. Double knockout ofGdpd3andHip1worsened the Hip1 phenotype. This suggests that Gdpd3 compensates for Hip1 loss. More detailed knowledge of how Hip1 deficiency leads to low PC will improve our understanding of HIP1 in choline metabolism in normal and disease states.