Axonopathies are a group of clinically diverse disorders characterized by the progressive degeneration of the axons of specific neurons. In hereditary spastic paraplegia (HSP), the axons of cortical motor neurons degenerate and cause a spastic movement disorder. HSP is linked to mutations in several loci known collectively as the spastic paraplegia genes (SPGs). We identified a heterozygous receptor accessory protein 1 (REEP1) exon 2 deletion in a patient suffering from the autosomal dominantly inherited HSP variant SPG31. We generated the corresponding mouse model to study the underlying cellular pathology. Mice with heterozygous deletion of exon 2 in Reep1 displayed a gait disorder closely resembling SPG31 in humans. Homozygous exon 2 deletion resulted in the complete loss of REEP1 and a more severe phenotype with earlier onset. At the molecular level, we demonstrated that REEP1 is a neuron-specific, membrane-binding, and membrane curvature-inducing protein that resides in the ER. We further show that Reep1 expression was prominent in cortical motor neurons. In REEP1-deficient mice, these neurons showed reduced complexity of the peripheral ER upon ultrastructural analysis. Our study connects proper neuronal ER architecture to long-term axon survival.

In guanosine diphosphate (GDP)-mannose pyrophosphorylase A (GMPPA), we identified a homozygous nonsense mutation that segregated with achalasia and alacrima, delayed developmental milestones, and gait abnormalities in a consanguineous Pakistani pedigree. Mutations in GMPPA were subsequently found in ten additional individuals from eight independent families affected by the combination of achalasia, alacrima, and neurological deficits. This autosomal-recessive disorder shows many similarities with triple A syndrome, which is characterized by achalasia, alacrima, and variable neurological deficits in combination with adrenal insufficiency. GMPPA is a largely uncharacterized homolog of GMPPB. GMPPB catalyzes the formation of GDP-mannose, which is an essential precursor of glycan moieties of glycoproteins and glycolipids and is associated with congenital and limb-girdle muscular dystrophies with hypoglycosylation of α-dystroglycan. Surprisingly, GDP-mannose pyrophosphorylase activity was unchanged and GDP-mannose levels were strongly increased in lymphoblasts of individuals with GMPPA mutations. This suggests that GMPPA might serve as a GMPPB regulatory subunit mediating feedback inhibition of GMPPB instead of displaying catalytic enzyme activity itself. Thus, a triple-A-like syndrome can be added to the growing list of congenital disorders of glycosylation, in which dysregulation rather than mere enzyme deficiency is the basal pathophysiological mechanism.

Distal nephron acid secretion is mediated by highly specialized type A intercalated cells (A-ICs), which contain vacuolar H-ATPase (V-type ATPase)-rich vesicles that fuse with the apical plasma membrane on demand. Intracellular bicarbonate generated by luminal H secretion is removed by the basolateral anion-exchanger AE1. Chronically reduced renal acid excretion in distal renal tubular acidosis (dRTA) may lead to nephrocalcinosis and renal failure. Studies in MDCK monolayers led to the proposal of a dominant-negative trafficking mechanism to explain AE1-associated dominant dRTA. To test this hypothesis , we generated an Ae1 R607H knockin mouse, which corresponds to the most common dominant dRTA mutation in human AE1, R589H. Compared with wild-type mice, heterozygous and homozygous R607H knockin mice displayed incomplete dRTA characterized by compensatory upregulation of the Na/HCO cotransporter NBCn1. Red blood cell Ae1-mediated anion-exchange activity and surface polypeptide expression did not change. Mutant mice expressed far less Ae1 in A-ICs, but basolateral targeting of the mutant protein was preserved. Notably, mutant mice also exhibited reduced expression of V-type ATPase and compromised targeting of this proton pump to the plasma membrane upon acid challenge. Accumulation of p62- and ubiquitin-positive material in A-ICs of knockin mice suggested a defect in the degradative pathway, which may explain the observed loss of A-ICs. R607H knockin did not affect type B intercalated cells. We propose that reduced basolateral anion-exchange activity in A-ICs inhibits trafficking and regulation of V-type ATPase, compromising luminal H secretion and possibly lysosomal acidification.

During the preparation of this manuscript, one of Kathrin N. Karle's affiliations was inadvertently omitted. The correct affiliation list is above. In addition, information was omitted from the Acknowledgments section. The correct Acknowledgments section is below. We thank M. Öhler, B. Schade, K. Schorr, A. Büschel, K. Stein, and R. Kaiser for their excellent technical assistance. We thank Nadine Breitkreutz for excellent technical analyses. We thank C. Kaether for valuable discussions and Laura McMillan for editing the final manuscript. This work was supported by grants from the Deutsche Forschungsgemeinschaft (DFG) to M.M. Kessels (KE685/3-1), B. Qualmann (QU116/6-1), C. Beetz (BE4069/1-1), T. Deufel and C.A. Hübner (DE307/8-1), and C.A. Hübner (HU800/6-1). The work of Kathrin N. Karle was supported by the German Ministry of Education and Research (BMBF) through funding to the mitoNET (01GM1113E).

Urofacial syndrome (UFS) describes the combination of urological problems and an inverted facial expression upon attempts to smile. Seventeen independent familial cases from different ethnicities have been described so far. Some of these have been linked to chromosome 10q. Very recently, homozygous loss-of-function mutations affecting the gene HPSE2 were identified in nine cases. Here, we describe a consanguineous UFS family from Pakistan with three of six siblings affected. We establish linkage to the chromosome 10q critical region and identify two non-synonymous HPSE2 variants. In silico analysis and screening of controls defines c.631T>C (p.Y211H) as a novel benign SNP and c.1628A>T (p.N543I) as the disease-causing mutation. Our study exemplifies the challenges in proper clinical diagnosis of UFS and, thereby, supports the hypothesis of the disease being under diagnosed. By identifying the first HPSE2 missense mutation it also provides a starting point for studies aimed at functionally understanding the unusual combination of symptoms as characterizing UFS.

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