Alteration of fatty-acid-metabolizing enzymes affects mitochondrial form and function in hereditary spastic paraplegia

Abstract

Hereditary spastic paraplegia (HSP) is considered one of the most heterogeneous groups of neurological disorders, both clinically and genetically. The disease comprises pure and complex forms that clinically include slowly progressive lower-limb spasticity resulting from degeneration of the corticospinal tract. At least 48 loci accounting for these diseases have been mapped to date, and mutations have been identified in 22 genes, most of which play a role in intracellular trafficking. Here, we identified mutations in two functionally related genes (DDHD1 and CYP2U1) in individuals with autosomal-recessive forms of HSP by using either the classical positional cloning or a combination of whole-genome linkage mapping and next-generation sequencing. Interestingly, three subjects with CYP2U1 mutations presented with a thin corpus callosum, white-matter abnormalities, and/or calcification of the basal ganglia. These genes code for two enzymes involved in fatty-acid metabolism, and we have demonstrated in human cells that the HSP pathophysiology includes alteration of mitochondrial architecture and bioenergetics with increased oxidative stress. Our combined results focus attention on lipid metabolism as a critical HSP pathway with a deleterious impact on mitochondrial bioenergetic function.

Figure 1

Mutations in DDHD1 (A–C) Family trees and segregation analysis of the mutations identified in families FSP445 (A), THI26002 (B) and FSP375 (C). Squares represent males, circles represent females, diamonds indicate anonymized subjects, filled symbols represent affected individuals, and a double line indicates consanguinity. The following abbreviations are used: M, mutation; +, wild-type; and ^∗, sampled individuals. The electrophoregrams are shown in Figure S2. (D) A graphical representation of DDHD1 (RefSeq NM_030637.2) on chromosome 14 indicates the DDHD domain (gray boxes) and the location of the mutations (c.1249C>T [p.Gln417^∗], c.1766G>A [p.Arg589Gln; r.spl?], c.1874delT [p.Leu625^∗], and c.2438-1G>T [r.spl?]) (arrows). Black and white boxes represent coding and noncoding exons, respectively. The DDHD1 transcript is 5,603 bp long and is composed of 12 exons that encode an 872 amino acid protein. There are three known isoforms of DDHD1; isoforms b and c are longer than isoform a because they include an alternate in-frame coding exon (white box). The sequence contains a lipase consensus domain and also includes a putative coiled-coil-forming region and a DDHD domain between residues 611 and 858 of isoform a; the four conserved residues that can form a metal binding site are seen in phosphoesterase domains. This domain is found in retinal degeneration B proteins, as well as in a family of probable phospholipases.

Figure 2

CYP2U1 Mutations in SPG49-Affected Subjects (A) Segregation analysis of the c.947A>T mutation in two Saudi Arabian families. Their corresponding electrophoregrams are in Figure S2. (B) Conservation of the amino acids affected by missense variations with the use of ALAMUT software. (C–E) Pedigrees and segregation of mutations in AR-HSP-affected families HSP1363 (C), ITAP9 (D), and FSP544 (E). A T1-weighted sagittal cerebral MRI from individual HSP1363-IV.4 shows a thin corpus callosum (C, red arrow). A CT scan of individual FSP544-III.1 (E, top view) shows bilateral globus pallidus calcifications (E, green arrow), and a brain MRI (E, bottom view) shows white-matter abnormalities (E, blue arrow), including the “ear of the lynx” aspect of frontal horns of the lateral ventricles (E, orange arrow). (F) Schematic representation of CYP2U1 (coding exons are represented by blue boxes). The locations of the identified mutations (c.61_73del [p.Leu21Trpfs^∗19], c.784T>C [p.Cys262Arg], c.947A>T [p.Asp316Val], c.1139A>G [p.Glu380Gly], and c.1462C>T [p.Arg488Trp]) are shown with red arrows, and the cytochrome P450 domain is indicated by red boxes. The five exons of CYP2U1 (RefSeq NM_183075.2) cover 1,635 bp and encode a 544 amino acid protein with potential transmembrane domains and a heme binding site in the cytochrome P450 domain.

Figure 3

Mitochondrial Dysfunctions in Lymphoblasts from SPG28- and SPG49-Affected Individuals (A) Mitochondrial respiration rate in SPG28-affected subjects (FSP445-V.3 [SPG28-1] and FSP445-V.1 [SPG28-2] aged 44 and 42 years, respectively, at sampling), SPG49-affected subjects (FSP719-V.3 [SPG49-1] and FSP1015-IV.3 [SPG49-2] aged 18 and 21 years, respectively, at sampling), and three control lymphoblastoid cell lines (aged 34, 37, and 40 years at sampling). The routine respiration data are shown. These measurements were taken after trypan-blue staining and a count for the exclusion of the presence of massive cell death. (B) Total cellular (light gray bar) and mitochondrial (black bars) ATP content expressed as relative light unit (RLU) measured in SPG28, SPG49, and control lymphoblasts. (C) H2DCFDA fluorescence gives a measure of the concentration of cytosolic hydrogen peroxide in control cells (white bar) and in CYP2U1 (light gray bar) and DDHD1 (black bars) mutant cells. The p values comparing mutant cells to control cells are indicated above each histogram (Student’s t test). Error bars represent the SD.

Figure 4

Abnormal Structures in the Mitochondrial Network of SPG49-Affected Skin Fibroblasts Mitochondrial-network morphology was analyzed by confocal microscopy of skin fibroblasts obtained from control individuals (A) and from individual HSP1363-IV.1 (B). The images on the right are high magnifications (5× zoomed in) of the images on the left. The mitochondrial network was labeled by immunocytochemistry with specific antibodies directed against TOM20, an outer-membrane mitochondrial protein, and fluorescence confocal imaging on a Zeiss Vivatome microscope followed. Three-dimensional stacks were obtained, and the projection images are shown here. We observed two types of abnormal structures on the mitochondrial network of SPG49 cells: small “donut-like” vesicles (800 nm in diameter) suggestive of self-fused mitochondrial filaments (D) and larger circular subnetworks (5 μM in diameter) (C). Counting of these two types of abnormal structures was performed on 50 control cells and 50 cells from the tested subject (right panels). Mean values and p values (Student’s t test) are shown, and error bars represent the SD.

Figure 5

Schematic Representation of the Metabolic Connections between DDHD1 and CYP2U1 Enzymatic Activities The content of PA on the surface of mitochondria is known to regulate mitochondrial fusion; PA is generated by mitochondrial phospholipase D (MitoPLD) and is further degraded by lipin PA phosphatase. DDHD1 was previously identified as PA-phospholipase A1 (PLA1). The action of DDHD1 might regulate the content of PA on the surface of mitochondria and then be involved in mitochondrial fusion. The DDHD1 mutation causes reduced PA-PLA1 activity, and the resultant increased PA content on the surface of mitochondria might cause the impairment of mitochondrial fusion and lead to the dysfunction of mitochondria. In contrast, phosphatidylinositol (PI) serves as a substrate of DDHD1 to form 2-arachidonoyl lysophosphatidylinositol (LPI). 2-arachidonoyl LPI is known to act on GPR55, which is assumed to be a cannabinoid receptor. 2-arachidonoyl LPI might be hydrolyzed by lysophospholipase C into 2-arachidonoylglycerol, which is an endogenous agonist for cannabinoid receptors CB1 and CB2. Arachidonic acid can be released from PI through phospholipase A2, which includes PLA2G6 (iPLA2 and PNPLA9), and can also be generated from 2-arachidonoyl LPI by neuropathy target esterase (NTE, PNPLA6) given that this enzyme exhibits high lysophospholipase activity. Arachidonic acid is converted to various eicosanoids through the cyclooxygenase and lipoxygenase pathways. In addition, arachidonic acid is known to be the preferred substrate of CYP2U1 to form 19- or 20-HETE acids. Among these, 20-HETE acid is reported as a potent activator of the TRPV1 cation channel, which is a receptor of endocannabinoids, including anandamide and N-arachidonoyl dopamine. CYP2U1 can also metabolize esterified forms of arachidonic acid (EPA and DHA). These common arachidonic-acid metabolites can have effects on the endocannabinoid system through the CB1, GPR55, and TRPV1 receptors.