Roles of SAM and DDHD domains in mammalian intracellular phospholipase A1 KIAA0725p

Phosphatidic acid (PA)-preferring phospholipase A1 regulates mitochondrial dynamics

Recent studies have suggested that phosphatidic acid (PA), a cone-shaped phospholipid that can generate negative curvature of lipid membranes, participates in mitochondrial fusion. However, precise mechanisms underling the production and consumption of PA on the mitochondrial surface are not fully understood. Phosphatidic acid-preferring phospholipase A1 (PA-PLA1)/DDHD1 is the first identified intracellular phospholipase A1 and preferentially hydrolyzes PA in vitro. Its cellular and physiological functions have not been elucidated. In this study, we show that PA-PLA1 regulates mitochondrial dynamics. PA-PLA1, when ectopically expressed in HeLa cells, induced mitochondrial fragmentation, whereas its depletion caused mitochondrial elongation. The effects of PA-PLA1 on mitochondrial morphology appear to counteract those of MitoPLD, a mitochondrion-localized phospholipase D that produces PA from cardiolipin. Consistent with high levels of expression of PA-PLA1 in testis, PA-PLA1 knock-out mice have a defect in sperm formation. In PA-PLA1-deficient sperm, the mitochondrial structure is disorganized, and an abnormal gap structure exists between the middle and principal pieces. A flagellum is bent at that position, leading to a loss of motility. Our results suggest a possible mechanism of PA regulation of the mitochondrial membrane and demonstrate an in vivo function of PA-PLA1 in the organization of mitochondria during spermiogenesis.

A cascade of ER exit site assembly that is regulated by p125A and lipid signals

The inner and outer layers of COPII mediate cargo sorting and vesicle biogenesis. Sec16A and p125A (officially known as SEC23IP) proteins interact with both layers to control coat activity, yet the steps directing functional assembly at ER exit sites (ERES) remain undefined. By using temperature blocks, we find that Sec16A is spatially segregated from p125A-COPII-coated ERES prior to ER exit at a step that required p125A. p125A used lipid signals to control ERES assembly. Within p125A, we defined a C-terminal DDHD domain found in phospholipases and PI transfer proteins that recognized PA and phosphatidylinositol phosphates in vitro and was targeted to PI4P-rich membranes in cells. A conserved central SAM domain promoted self-assembly and selective lipid recognition by the DDHD domain. A basic cluster and a hydrophobic interface in the DDHD and SAM domains, respectively, were required for p125A-mediated functional ERES assembly. Lipid recognition by the SAM-DDHD module was used to stabilize membrane association and regulate the spatial segregation of COPII from Sec16A, nucleating the coat at ERES for ER exit.

TGL-mediated lipolysis in Manduca sexta fat body: possible roles for lipoamide-dehydrogenase (LipDH) and high-density lipophorin (HDLp)

Triglyceride-lipase (TGL) is a major fat body lipase in Manduca sexta. The knowledge of how TGL activity is regulated is very limited. A WWE domain, presumably involved in protein-protein interactions, has been previously identified in the N-terminal region of TGL. In this study, we searched for proteins partners that interact with the N-terminal region of TGL. Thirteen proteins were identified by mass spectrometry, and the interaction with four of these proteins was confirmed by immunoblot. The oxidoreductase lipoamide-dehydrogenase (LipDH) and the apolipoprotein components of the lipid transporter, HDLp, were among these proteins. LipDH is the common component of the mitochondrial α-keto acid dehydrogenase complexes whereas HDLp occurs in the hemolymph. However, subcellular fractionation demonstrated that these two proteins are relatively abundant in the soluble fraction of fat body adipocytes. The cofactor lipoate found in typical LipDH substrates was not detected in TGL. However, TGL proved to have critical thiol groups. Additional studies with inhibitors are consistent with the notion that LipDH acting as a diaphorase could preserve the activity of TGL by controlling the redox state of thiol groups. On the other hand, when TG hydrolase activity of TGL was assayed in the presence of HDLp, the production of diacylglycerol (DG) increased. TGL-HDLp interaction could drive the intracellular transport of DG. TGL may be directly involved in the lipoprotein assembly and loading with DG, a process that occurs in the fat body and is essential for insects to mobilize fatty acids. Overall the study suggests that TGL occurs as a multi-protein complex supported by interactions through the WWE domain.

Mutations in CYP2U1, DDHD2 and GBA2 genes are rare causes of complicated forms of hereditary spastic paraparesis

Complicated hereditary spastic paraplegias (HSP) are a heterogeneous group of HSP characterized by spasticity associated with a variable combination of neurologic and extra-neurologic signs and symptoms. Among them, HSP with thin corpus callosum and intellectual disability is a frequent subtype, often inherited as a recessive trait (ARHSP-TCC). Within this heterogeneous subgroup, SPG11 and SPG15 represent the most frequent subtypes. We analyzed the mutation frequency of three genes associated with early-onset forms of ARHSP with and without TCC, CYP2U1/SPG56, DDHD2/SPG54 and GBA2/SPG46, in a large population of selected complicated HSP patients by using a combined approach of traditional-based and amplicon-based high-throughput pooled-sequencing. Three families with mutations were identified, one for each of the genes analyzed. Novel homozygous mutations were identified in CYP2U1 (c.1A>C/p.Met1?) and in GBA2 (c.2048G>C/p.Gly683Arg), while the homozygous mutation found in DDHD2 (c.1978G>C/p.Asp660His) had been previously reported in a compound heterozygous state. The phenotypes associated with the CYP2U1 and DDHD2 mutations overlap the SPG56 and the SPG54 subtypes, respectively, with few differences. By contrast, the GBA2 mutated patients show phenotypes combining typical features of both the SPG46 subtype and the recessive ataxia form, with marked intrafamilial variability thereby expanding the spectrum of clinical entities associated with GBA2 mutations. Overall, each of three genes analyzed shows a low mutation frequency in a general population of complicated HSP (<1 % for either CYP2U1 or DDHD2 and approximately 2 % for GBA2). These findings underline once again the genetic heterogeneity of ARHSP-TCC and the clinical overlap between complicated HSP and the recessive ataxia syndromes.

Identification of pathogenic gene variants in small families with intellectually disabled siblings by exome sequencing

BACKGROUND

Intellectual disability (ID) is a common neurodevelopmental disorder affecting 1-3% of the general population. Mutations in more than 10% of all human genes are considered to be involved in this disorder, although the majority of these genes are still unknown.

OBJECTIVES

We investigated 19 small non-consanguineous families with two to five affected siblings in order to identify pathogenic gene variants in known, novel and potential ID candidate genes. Non-consanguineous families have been largely ignored in gene identification studies as small family size precludes prior mapping of the genetic defect.

METHODS AND RESULTS

Using exome sequencing, we identified pathogenic mutations in three genes, DDHD2, SLC6A8, and SLC9A6, of which the latter two have previously been implicated in X-linked ID phenotypes. In addition, we identified potentially pathogenic mutations in BCORL1 on the X-chromosome and in MCM3AP, PTPRT, SYNE1, and ZNF528 on autosomes.

CONCLUSIONS

We show that potentially pathogenic gene variants can be identified in small, non-consanguineous families with as few as two affected siblings, thus emphasising their value in the identification of syndromic and non-syndromic ID genes.

Hereditary spastic paraplegia: clinico-pathologic features and emerging molecular mechanisms

Hereditary spastic paraplegia (HSP) is a syndrome designation describing inherited disorders in which lower extremity weakness and spasticity are the predominant symptoms. There are more than 50 genetic types of HSP. HSP affects individuals of diverse ethnic groups with prevalence estimates ranging from 1.2 to 9.6 per 100,000. Symptoms may begin at any age. Gait impairment that begins after childhood usually worsens very slowly over many years. Gait impairment that begins in infancy and early childhood may not worsen significantly. Postmortem studies consistently identify degeneration of corticospinal tract axons (maximal in the thoracic spinal cord) and degeneration of fasciculus gracilis fibers (maximal in the cervico-medullary region). HSP syndromes thus appear to involve motor-sensory axon degeneration affecting predominantly (but not exclusively) the distal ends of long central nervous system (CNS) axons. In general, proteins encoded by HSP genes have diverse functions including (1) axon transport (e.g. SPG30/KIF1A, SPG10/KIF5A and possibly SPG4/Spastin); (2) endoplasmic reticulum morphology (e.g. SPG3A/Atlastin, SPG4/Spastin, SPG12/reticulon 2, and SPG31/REEP1, all of which interact); (3) mitochondrial function (e.g. SPG13/chaperonin 60/heat-shock protein 60, SPG7/paraplegin; and mitochondrial ATP6); (4) myelin formation (e.g. SPG2/Proteolipid protein and SPG42/Connexin 47); (5) protein folding and ER-stress response (SPG6/NIPA1, SPG8/K1AA0196 (Strumpellin), SGP17/BSCL2 (Seipin), "mutilating sensory neuropathy with spastic paraplegia" owing to CcT5 mutation and presumably SPG18/ERLIN2); (6) corticospinal tract and other neurodevelopment (e.g. SPG1/L1 cell adhesion molecule and SPG22/thyroid transporter MCT8); (7) fatty acid and phospholipid metabolism (e.g. SPG28/DDHD1, SPG35/FA2H, SPG39/NTE, SPG54/DDHD2, and SPG56/CYP2U1); and (8) endosome membrane trafficking and vesicle formation (e.g. SPG47/AP4B1, SPG48/KIAA0415, SPG50/AP4M1, SPG51/AP4E, SPG52/AP4S1, and VSPG53/VPS37A). The availability of animal models (including bovine, murine, zebrafish, Drosophila, and C. elegans) for many types of HSP permits exploration of disease mechanisms and potential treatments. This review highlights emerging concepts of this large group of clinically similar disorders.

Mutations in phospholipase DDHD2 cause autosomal recessive hereditary spastic paraplegia (SPG54)

Hereditary spastic paraplegias (HSP) are a genetically heterogeneous group of disorders characterized by a distal axonopathy of the corticospinal tract motor neurons leading to progressive lower limb spasticity and weakness. Intracellular membrane trafficking, mitochondrial dysfunction and myelin formation are key functions involved in HSP pathogenesis. Only recently defects in metabolism of complex lipids have been implicated in a number of HSP subtypes. Mutations in the 23 known autosomal recessive HSP genes explain less than half of autosomal recessive HSP cases. To identify novel autosomal recessive HSP disease genes, exome sequencing was performed in 79 index cases with autosomal recessive forms of HSP. Resulting variants were filtered and intersected between families to allow identification of new disease genes. We identified two deleterious mutations in the phospholipase DDHD2 gene in two families with complicated HSP. The phenotype is characterized by early onset of spastic paraplegia, mental retardation, short stature and dysgenesis of the corpus callosum. Phospholipase DDHD2 is involved in intracellular membrane trafficking at the golgi/ endoplasmic reticulum interface and has been shown to possess phospholipase A1 activity in vitro. Discovery of DDHD2 mutations in HSP might therefore provide a link between two key pathogenic themes in HSP: membrane trafficking and lipid metabolism.

A lysophospholipid acyltransferase antagonist, CI-976, creates novel membrane tubules marked by intracellular phospholipase A1 KIAA0725p

CI-976 is a lysophospholipid acyltransferase antagonist that is known to affect secretory and endocytic membrane-trafficking pathways likely by increasing the lysophospholipid content in membranes. Our previous study suggested that lysophospholipids formed through the action of an intracellular phospholipase A(1), KIAA0725p (also known as DDHD2 and iPLA(1)γ), may be important for the association of this enzyme with membranes. In this study, we examined the effect of CI-976 on the membrane association of KIAA0725p. While in HeLa cells KIAA0725p is localized in the Golgi and cytosol, in mouse embryonic fibroblasts (MEFs), it was found to be principally localized in the cytosol with some on post-endoplasmic reticulum compartments including the cis-Golgi. Treatment of MEFs with CI-976 induced the redistribution of KIAA0725p to membrane tubules, which were in vicinity to fragmented mitochondria. These tubules were not decorated with canonical organelle markers including Golgi proteins. A human KIAA0725p mutant, which exhibits decreased membrane-binding ability, was also redistributed to membrane structures upon CI-976 treatment. Our data suggest that the association of KIAA0725p with membranes is regulated by lipid metabolism, and that CI-976 may create unique membrane structures that can be marked by KIAA0725p.

Mutations in DDHD2, encoding an intracellular phospholipase A(1), cause a recessive form of complex hereditary spastic paraplegia

We report on four families affected by a clinical presentation of complex hereditary spastic paraplegia (HSP) due to recessive mutations in DDHD2, encoding one of the three mammalian intracellular phospholipases A(1) (iPLA(1)). The core phenotype of this HSP syndrome consists of very early-onset (<2 years) spastic paraplegia, intellectual disability, and a specific pattern of brain abnormalities on cerebral imaging. An essential role for DDHD2 in the human CNS, and perhaps more specifically in synaptic functioning, is supported by a reduced number of active zones at synaptic terminals in Ddhd-knockdown Drosophila models. All identified mutations affect the protein's DDHD domain, which is vital for its phospholipase activity. In line with the function of DDHD2 in lipid metabolism and its role in the CNS, an abnormal lipid peak indicating accumulation of lipids was detected with cerebral magnetic resonance spectroscopy, which provides an applicable diagnostic biomarker that can distinguish the DDHD2 phenotype from other complex HSP phenotypes. We show that mutations in DDHD2 cause a specific complex HSP subtype (SPG54), thereby linking a member of the PLA(1) family to human neurologic disease.