Raju K. Pullarkat

Proteomic Studies Identified a Single Nucleotide Polymorphism in Glyoxalase I as Autism Susceptibility Factor

Autism is a neurodevelopmental disability characterized by deficits in verbal communications, impairments in social interactions, and repetitive behaviors. Several studies have indicated strong involvement of multigenic components in the etiology of autism. Linkage analyses and candidate gene search approaches so far have not identified any reliable susceptibility genes. We are using a proteomic approach to identify protein abnormalities due to aberrant gene expression in autopsied autism brains. In four of eight autism brains, we have found an increase in polarity (more acidic) of glyoxalase I (Glo1) by two‐dimensional gel electrophoresis. To identify the molecular change resulting in the shift of Glo1 polarity, we undertook sequencing of GLO1 gene. Direct sequencing of GLO1 gene/mRNA in these brains, has identified a single nucleotide polymorphism (SNP), C419A. The SNP causes an Ala111Glu change in the protein sequence. Population genetics of GLO1 C419A SNP studied in autism (71 samples) and normal and neurological controls (49 samples) showed significantly higher frequency for the A419 (allele frequency 0.6 in autism and 0.4 in controls, one‐tailed Fishers test P < 0.0079). Biochemical measurements have revealed a 38% decrease in Glo1 enzyme activity in autism brains (one‐tailed t‐test P < 0.026). Western blot analysis has also shown accumulation of advanced glycation end products (AGEs) in autism brains. These data suggest that homozygosity for A419 GLO1 resulting in Glu111 is a predisposing factor in the etiology of autism.

Folic acid supplementation dysregulates gene expression in lymphoblastoid cells--implications in nutrition.

For over a decade, folic acid (FA) supplementation has been widely prescribed to pregnant women to prevent neural tube closure defects in newborns. Although neural tube closure occurs within the first trimester, high doses of FA are given throughout pregnancy, the physiological consequences of which are unknown. FA can cause epigenetic modification of the cytosine residues in the CpG dinucleotide, thereby affecting gene expression. Dysregulation of crucial gene expression during gestational development may have lifelong adverse effects or lead to neurodevelopmental defects, such as autism. We have investigated the effect of FA supplementation on gene expression in lymphoblastoid cells by whole-genome expression microarrays. The results showed that high FA caused dysregulation by ≥ four-fold up or down to more than 1000 genes, including many imprinted genes. The aberrant expression of three genes (FMR1, GPR37L1, TSSK3) was confirmed by Western blot analyses. The level of altered gene expression changed in an FA concentration-dependent manner. We found significant dysregulation in gene expression at concentrations as low as 15 ng/ml, a level that is lower than what has been achieved in the blood through FA fortification guidelines. We found evidence of aberrant promoter methylation in the CpG island of the TSSK3 gene. Excessive FA supplementation may require careful monitoring in women who are planning for, or are in the early stages of pregnancy. Aberrant expression of genes during early brain development may have an impact on behavioural characteristics.

Glyoxalase I polymorphism rs2736654 causing the Ala111Glu substitution modulates enzyme activity—implications for autism

Autism is a pervasive, heterogeneous, neurodevelopmental disability characterized by impairments in verbal communications, reciprocal social interactions, and restricted repetitive stereotyped behaviors. Evidence suggests the involvement of multiple genetic factors in the etiology of autism, and extensive genome‐wide association studies have revealed several candidate genes that bear single nucleotide polymorphisms (SNPs) in non‐coding and coding regions. We have shown that a non‐conservative, non‐synonymous SNP in the glyoxalase I gene, GLOI, may be an autism susceptibility factor. The GLOI rs2736654 SNP is a C→A change that causes an Ala111Glu change in the Glo1 enzyme. To identify the significance of the SNP, we have conducted functional assays for Glo1. We now present evidence that the presence of the A‐allele at rs2736654 results in reduced enzyme activity. Glo1 activity is decreased in lymphoblastoid cells that are homozygous for the A allele. The Glu‐isoform of Glo1 in these cells is hyperphosphorylated. Direct HPLC measurements of the glyoxalase I substrate, methylglyoxal (MG), show an increase in MG in these cells. Western blot analysis revealed elevated levels of the receptor for advanced glycation end products (RAGEs). We also show that MG is toxic to the developing neuronal cells. We suggest that accumulation of MG results in the formation of AGEs, which induce expression of the RAGE that during crucial neuronal development may be a factor in the pathology of autism. Autism Res 2011,4:262–270.

A novel assay for lysosomal pepstatin-insensitive proteinase and its application for the diagnosis of late-infantile neuronal ceroid lipofuscinosis

A highly sensitive assay for mammalian lysosomal pepstatin-insensitive proteinase (LPIP) is described using a synthetic peptide substrate coupled to aminotrifluoromethyl coumarin (AFC). LPIP is an endocarboxyl proteinase which has specific sequence requirements of Phe-Phe around the carboxyl terminal. This HPLC based assay can detect patients suffering from late-infantile neuronal ceroid lipofuscinosis (LINCL) and also heterozygote carriers in cultured lymphoid cells and skin fibroblasts. None of the patients analyzed had detectable enzyme activity confirming the defective gene product, while carriers had about 50% activity when compared with the normal controls. Neurological controls comprised of patients with other neurodegenerative disorders have LPIP activities similar to normal controls. LPIP activity is also detectable in amniocytes and chorionic villi. Thus the assay reported can also be used for prenatal diagnosis of LINCL.

Proteomic Approach for the Elucidation of Biological Defects in Autism

Proteomic-based approaches, which examine expressed proteins in tissues or cells, have great potential in the elucidation of biological defects in heterogeneous neurodevelopmental disorders such as autism. In this approach, tissue or cellular proteins from control and affected subjects are separated on two-dimensional (2-D) polyacrylamide gel electrophoresis, and those proteins that show marked changes in the concentration between control and affected subjects are identified by mass spectroscopy. This method has been successfully applied in the elucidation of the molecular biological defect in classic late-infantile neuronal ceroid lipofuscinosis (Sleat et al., 1997). Unlike the classical methods of genome-wide screening for chromosomal localization followed by positional cloning, the proteomic approach requires limited number of tissue samples and the study can be completed in a relatively short time. Currently, these methods are available for relatively abundant proteins and generally are not applicable for hydrophobic proteins because 2-D gel electrophoresis is not very effective in the analysis of hydrophobic proteins. The genetic defect results in either total loss of proteins or changes in molecular weight and/or isoelectric point will be detectable by the proteomic method. Because autism is a neurogenetic disorder, brain is the tissue of choice for proteomic study. For an oligogenic disorder such as autism, at least some of the aberrant (genes) proteins may be identified by this technology.

Brain lysosomal hydrolases in neuronal ceroid-lipofuscinoses

Although the neuronal ceroid-lipofuscinoses (NCLs) are often referred to as lysosomal storage disorders, information on brain lysosomal hydrolases in NCLs is not available. We have determined the specific activities of several acid hydrolases in postmortem brain gray matter of infantile (INCL), late infantile (LINCL), juvenile (JNCL), and adult (ANCL) forms of NCL, patients affected with other neurological disorders (ON), and normal controls. The specific activities of beta-hexosaminidase A and B were significantly high in JNCL gray matter, whereas in LINCL, the increase is significant only in beta-hexosaminidase compared to the controls. A significant increase in the activities of alpha-mannosidase, beta-glucuronidase, and acid phosphatase was also observed in LINCL and JNCL patients compared to the control values. beta-galactosidase activity was also found to be elevated in JNCL brains over the controls. In contrast, activities of beta-glucosidase and sialidase appeared to be lowered in INCL and LINCL. On the other hand, alpha-fucosidase, beta-mannosidase, and sulfatase were unaffected in NCLs brains. Thus, the present data indicate NCLs related abnormalities in some of the acid hydrolases in brain gray matter, which are primarily glycoproteins of lysosomal origin. These data in conjuction with the reported association of sphingolipid activator proteins (SAP) A and D and lysosomal glycoproteins with NCL storage bodies imply abberations in the glycoconjugate metabolism and lysosomal function.

4 Biochemistry of neuronal ceroid lipofuscinoses

Abstract This chapter summarizes the recent advances that have been made with respect to biochemical characterization of the neurodegenerative diseases collectively known as neuronal ceroid lipofuscinoses (NCL) or Batten disease. Genomic and proteomic approaches have presently identified eight different forms of NCL (namely, CLN1 through CLN8) based on mutations in specific genes. CLN1 and CLN2 are caused by mutations in genes that encodes lysosomal enzymes, palmitoyl protein thioesterase and pepstatin-insensitive proteinase, respectively. The protein involved in the etiology of CLN3 is a highly hydrophobic, presumably transmembrane protein. NCL are considered as lysosomal storage diseases because of the accumulation of autofluorescent inclusion bodies. The composition of inclusion bodies varies in different forms of the NCL. The major storage component in CLN2 is the subunit c of mitochondrial ATP synthase complex and its accumulation is the direct of lack of CLN2p in this disease. Mannose-6-phosphorylated glycoproteins accumulate in CLN3 and most likely their accumulation is the result of an intrinsic of the CLN3 protein. Significant levels of oligosaccharyl diphosphodolichol also accumulate in CLN3 and CLN2, whereas lysosomal sphingolipid activator proteins (saposins A and D) constitute major component of the storage material in CLN1. The issue of selective loss of neuronal and retinal cells in NCL still remains to be addressed. Identification of natural substrates for the various enzymes involved in NCL may help in the characterization of the cytotoxic factor(s) and also in designing rationale therapeutic interventions for these group of devastating diseases.

A Critical Tryptophan and Ca2+ in Activation and Catalysis of TPPI, the Enzyme Deficient in Classic Late-Infantile Neuronal Ceroid Lipofuscinosis

Background Tripeptidyl aminopeptidase I (TPPI) is a crucial lysosomal enzyme that is deficient in the fatal neurodegenerative disorder called classic late-infantile neuronal ceroid lipofuscinosis (LINCL). It is involved in the catabolism of proteins in the lysosomes. Recent X-ray crystallographic studies have provided insights into the structural/functional aspects of TPPI catalysis, and indicated presence of an octahedrally coordinated Ca2+. Methodology Purified precursor and mature TPPI were used to study inhibition by NBS and EDTA using biochemical and immunological approaches. Site-directed mutagenesis with confocal imaging technique identified a critical W residue in TPPI activity, and the processing of precursor into mature enzyme. Principal Findings NBS is a potent inhibitor of the purified TPPI. In mammalian TPPI, W542 is critical for tripeptidyl peptidase activity as well as autocatalysis. Transfection studies have indicated that mutants of the TPPI that harbor residues other than W at position 542 have delayed processing, and are retained in the ER rather than transported to lysosomes. EDTA inhibits the autocatalytic processing of the precursor TPPI. Conclusions/Significance We propose that W542 and Ca2+ are critical for maintaining the proper tertiary structure of the precursor proprotein as well as the mature TPPI. Additionally, Ca2+ is necessary for the autocatalytic processing of the precursor protein into the mature TPPI. We have identified NBS as a potent TPPI inhibitor, which led in delineating a critical role for W542 residue. Studies with such compounds will prove valuable in identifying the critical residues in the TPPI catalysis and its structure-function analysis.

Clinical, Pathologic, and Neurochemical Studies of an Unusual Case of Neuronal Storage Disease With Lamellar Cytoplasmic Inclusions: A New Genetic Disorder?

A child of first-cousin Puerto Rican parents had global developmental delay, failure to thrive, and hypotonia since early infancy. At 1½ years of age, she developed clinical and electrophysiologic evidence of progressive motor and sensory neuropathy At 2½ years, she developed visual impairment and optic atrophy followed by gradual involvement of the 7th, 9th, 10th, and 12th cranial nerves. Uncontrollable myoclonic seizures began at 4 years and she died at 6 years of age. Motor nerve conduction velocities were initially normal and later became markedly slowed. Sensory distal latency responses were absent. Lysosomal enzyme activities in leukocytes and fibroblasts were normal. Sural nerve and two muscle biopsies showed only nondiagnostic abnormalities. Electron microscopy of lymphocytes, skin, and fibroblasts showed cytoplasmic inclusions. Light microscopy of frontal cortex biopsy showed neuronal storage material staining positively with Luxol fast blue, and electron microscopy showed cytoplasmic membranous bodies in neurons, suggesting an accumulation of a ganglioside. At autopsy, all organs were small but otherwise normal and without abnormal storage cells in the liver, spleen, or bone marrow. Anterior spinal nerve roots showed loss of large myelinated axons. The brain was small and atrophic; cortical neurons showed widespread accumulation of storage material, most marked in the pyramidal cell layer of the hippocampus. Subcortical white matter was gliotic with loss of axons and myelin sheaths. In cortical gray matter there was a 35% elevation of total gangliosides, with a 16-fold increase in G M3, a three- to four-fold increase in GM2 gangliosides, and a 15-fold elevation of lactosyl ceramide. GM3 sialidase activity was normal in gray matter at 3.1 nmols/mg protein per hour and lactosyl ceraminidase I and II activities were 70% to 80% of normal. In white matter, total myelin was reduced by 50% but its composition was normal. Phospholipid distribution and sphingomyelin content were normal in gray matter, white matter, and in the liver. These biochemical findings were interpreted as nonspecific abnormalities. The nature of the neuronal storage substance remains to be determined. (J Child Neurol 1999;14:123-129).

Bovine Brain Gray and White Matter Exhibit Differential Protein Prenyl Transferase Activity

Protein farnesyl transferase and geranylgeranyl transferase-I activities were determined in gray and white matter from various regions of bovine brain. Farnesyl transferase activity was 3–8 times greater than geranylgeranyl transferase-I activity. However, farnesyl transferase activity was about 2 times greater in the white matter than in the gray matter in all regions of the brain. Mixing experiments indicated lack of farnesyl transferase activators in white matter. This difference in farnesyl transferase activity may be due to enzyme content and may have implications in brain cell function.