Animal medical genetics: a historical perspective on more than 50 years of research into genetic disorders of animals at Massey University.

Over the last 50 years, there have been major advances in knowledge and technology regarding genetic diseases, and the subsequent ability to control them in a cost-effective manner. This review traces these advances through research into genetic diseases of animals at Massey University (Palmerston North, NZ), and briefly discusses the disorders investigated during that time, with additional detail for disorders of major importance such as bovine α-mannosidosis, ovine ceroid-lipofuscinosis, canine mucopolysaccharidosis IIIA and feline hyperchylomicronaemia. The overall research has made a significant contribution to veterinary medicine, has provided new biological knowledge and advanced our understanding of similar disorders in human patients, including testing various specific therapies prior to human clinical trials.

Lysosomal dysfunction causes neurodegeneration in mucolipidosis II 'knock-in' mice.

Mucolipidosis II is a neurometabolic lysosomal trafficking disorder of infancy caused by loss of mannose 6-phosphate targeting signals on lysosomal proteins, leading to lysosomal dysfunction and accumulation of non-degraded material. However, the identity of storage material and mechanisms of neurodegeneration in mucolipidosis II are unknown. We have generated 'knock-in' mice with a common mucolipidosis II patient mutation that show growth retardation, progressive brain atrophy, skeletal abnormalities, elevated lysosomal enzyme activities in serum, lysosomal storage in fibroblasts and brain and premature death, closely mimicking the mucolipidosis II disease in humans. The examination of affected mouse brains at different ages by immunohistochemistry, ultrastructural analysis, immunoblotting and mass spectrometric analyses of glycans and anionic lipids revealed that the expression and proteolytic processing of distinct lysosomal proteins such as α-l-fucosidase, ß-hexosaminidase, α-mannosidase or Niemann-Pick C2 protein are more significantly impacted by the loss of mannose 6-phosphate residues than enzymes reaching lysosomes independently of this targeting mechanism. As a consequence, fucosylated N-glycans, GM2 and GM3 gangliosides, cholesterol and bis(monoacylglycero)phosphate accumulate progressively in the brain of mucolipidosis II mice. Prominent astrogliosis and the accumulation of organelles and storage material in focally swollen axons were observed in the cerebellum and were accompanied by a loss of Purkinje cells. Moreover, an increased neuronal level of the microtubule-associated protein 1 light chain 3 and the formation of p62-positive neuronal aggregates indicate an impairment of constitutive autophagy in the mucolipidosis II brain. Our findings demonstrate the essential role of mannose 6-phosphate for selected lysosomal proteins to maintain the capability for degradation of sequestered components in lysosomes and autophagolysosomes and prevent neurodegeneration. These lysosomal proteins might be a potential target for a valid therapeutic approach for mucolipidosis II disease.

Altered vitamin E status in Niemann-Pick type C disease.

Vitamin E (α-tocopherol) is the major lipid-soluble antioxidant in many species. Niemann-Pick type C (NPC) disease is a lysosomal storage disorder caused by mutations in the NPC1 or NPC2 gene, which regulates lipid transport through the endocytic pathway. NPC disease is characterized by massive intracellular accumulation of unesterified cholesterol and other lipids in lysosomal vesicles. We examined the roles that NPC1/2 proteins play in the intracellular trafficking of tocopherol. Reduction of NPC1 or NPC2 expression or function in cultured cells caused a marked lysosomal accumulation of vitamin E in cultured cells. In vivo, tocopherol significantly accumulated in murine Npc1-null and Npc2-null livers, Npc2-null cerebella, and Npc1-null cerebral cortices. Plasma tocopherol levels were within the normal range in Npc1-null and Npc2-null mice, and in plasma samples from human NPC patients. The binding affinity of tocopherol to the purified sterol-binding domain of NPC1 and to purified NPC2 was significantly weaker than that of cholesterol (measurements kindly performed by R. Infante, University of Texas Southwestern Medical Center, Dallas, TX). Taken together, our observations indicate that functionality of NPC1/2 proteins is necessary for proper bioavailability of vitamin E and that the NPC pathology might involve tissue-specific perturbations of vitamin E status.

Pathogenic cascades in lysosomal disease-Why so complex?

Lysosomal disease represents a large group of more than 50 clinically recognized conditions resulting from inborn errors of metabolism affecting the organelle known as the lysosome. The lysosome is an integral part of the larger endosomal/lysosomal system, and is closely allied with the ubiquitin-proteosomal and autophagosomal systems, which together comprise essential cell machinery for substrate degradation and recycling, homeostatic control, and signalling. More than two-thirds of lysosomal diseases affect the brain, with neurons appearing particularly vulnerable to lysosomal compromise and showing diverse consequences ranging from specific axonal and dendritic abnormalities to neuron death. While failure of lysosomal function characteristically leads to lysosomal storage, new studies argue that lysosomal diseases may also be appropriately viewed as 'states of deficiency' rather than simply overabundance (storage). Interference with signalling events and salvage processing normally controlled by the endosomal/lysosomal system may represent key mechanisms accounting for the inherent complexity of lysosomal disorders. Analysis of lysosomal disease pathogenesis provides a unique window through which to observe the importance of the greater lysosomal system for normal cell health.

Pathology of mucopolysaccharidosis IIIA in Huntaway dogs.

Dogs with mucopolysaccharidosis (MPS) IIIA were bred within an experimental colony. As part of characterizing them as a model for testing therapeutic strategies for the analogous disease of children, a pathologic study was undertaken. By histology, there were variably stained storage cytosomes within neurons, including many that stained for gangliosides. On ultrastructure examination, these cytosomes contained either moderately dense granular material, tentatively interpreted as precipitated glycosaminoglycan; a variety of multilaminar bodies, interpreted as being associated with secondary accumulation of gangliosides; or a mixture of both types. In the liver, storage vesicles also contained excess glycogen as a secondary storage product. In various tissues, there were large foamy macrophages. In the brain, many of these were in juxtaposition with neurons, and, on ultrastructure examination, they contained storage cytosomes similar to those in neurons. However, the neuron in association with such a macrophage frequently showed little such material.

Abnormal neuronal metabolism and storage in mucopolysaccharidosis type VI (Maroteaux-Lamy) disease.

Mucopolysaccharidosis (MPS) type VI, also known as Maroteaux-Lamy disease, is an inherited disorder of glycosaminoglycan catabolism caused by deficient activity of the lysosomal hydrolase, N-acetylgalactosamine 4-sulphatase (4S). A variety of prominent visceral and skeletal defects are characteristic, but primary neurological involvement has generally been considered absent. We report here that the feline model of MPS VI exhibits abnormal lysosomal storage in occasional neurones and glia distributed throughout the cerebral cortex. Abnormal lysosomal inclusions were pleiomorphic with some resembling zebra bodies and dense core inclusions typical of other MPS diseases or the membranous storage bodies characteristic of the gangliosidoses. Pyramidal neurones were shown to contain abnormal amounts of GM2 and GM3 gangliosides by immunocytochemical staining and unesterified cholesterol by histochemical (filipin) staining. Further, Golgi staining of pyramidal neurones revealed that some possessed ectopic axon hillock neurites and meganeurites similar to those described in Tay-Sachs and other neuronal storage diseases with ganglioside storage. Some animals evaluated in this study also received allogeneic bone marrow transplants, but no significant differences in neuronal storage were noted between treated and untreated individuals. These studies demonstrate that deficiency of 4S activity can lead to metabolic abnormalities in the neurones of central nervous system in cats, and that these changes may not be readily amenable to correction by bone marrow transplantation. Given the close pathological and biochemical similarities between feline and human MPS VI, it is conceivable that children with this disease have similar neuronal involvement.

Mitochondrial dysfunction in the neuronal ceroid-lipofuscinoses (Batten disease).

There are at least eight genetic entities known as the ceroid-lipofuscinoses in humans which share clinical and pathological features that have caused them to be grouped together under the eponym of Batten disease. They present pathologically as lysosomal storage diseases but are also characterised by severe neurodegeneration. Although the biochemical defects appear primarily centred on lysosomes and defects in proteolysis, the link between this and pathogenesis of neuronal death is poorly understood. The pathogenesis of neurodegeneration has been studied particularly in two animal models these being the English setter dog and the New Zealand Southhampshire sheep (OCL6). In these, and some of the human entities, there is evidence of mitochondrial dysfunction. This includes the accumulation of subunit c of ATP synthase as a component of storage material in at least six of eight genetic forms of the disease; structural abnormalities of mitochondria and selective loss of neurons in areas of the brain that are particularly metabolically active. Direct evidence of dysfunction comes from mitochondrial function tests in fibroblasts and, in animal models, isolated liver mitochondria. Supporting evidence of mitochondrial dysfunction was shown by disturbances in proportions of energy-rich phosphates in fibroblasts in some of these diseases. If these various defects were reflected in neurons, then it would support the hypothesis that neuron death was associated with energy-linked excitotoxicity.

New proteins from old diseases provide novel insights in cell biology.

The lysosomal disease concept was developed by Hers in 1963. At the time, few could have imagined the breadth and depth of knowledge about cell biology that these disorders would reveal. With a collective hindsight of nearly four decades, it is fair to say that we have learned more about the lysosomal system of cells through the study of these rare diseases than by any other means. Given the advancements of the past year, it is apparent that some of the most significant insights are yet to come, as we delineate the last remaining and most enigmatic of these diseases.

Critical role for glycosphingolipids in Niemann-Pick disease type C.

Niemann-Pick type C (NPC) disease is a cholesterol lipidosis caused by mutations in NPC1 and NPC2 gene loci. Most human cases are caused by defects in NPC1, as are the spontaneously occurring NPC diseases in mice and cats. NPC1 protein possesses a sterol-sensing domain and has been localized to vesicles that are believed to facilitate the recycling of unesterified cholesterol from late endosomes/lysosomes to the ER and Golgi. In addition to accumulating cholesterol, NPC1-deficient cells also accumulate gangliosides and other glycosphingolipids (GSLs), and neuropathological abnormalities in NPC disease closely resemble those seen in primary gangliosidoses. These findings led us to hypothesize that NPC1 may also function in GSL homeostasis. To evaluate this possibility, we treated murine and feline NPC models with N-butyldeoxynojirimycin (NB-DNJ), an inhibitor of glucosylceramide synthase, a pivotal enzyme in the early GSL synthetic pathway. Treated animals showed delayed onset of neurological dysfunction, increased average life span (in mice), and reduced ganglioside accumulation and accompanying neuropathological changes. These results are consistent with our hypothesis and with GSLs being centrally involved in the pathogenesis of NPC disease, and they suggest that drugs inhibiting GSL synthesis could have a similar ameliorating effect on the human disorder.

Neurons in Niemann-Pick disease type C accumulate gangliosides as well as unesterified cholesterol and undergo dendritic and axonal alterations.

Niemann-Pick disease type C (NPC) is a lethal neurologic storage disorder of children most often caused by a defect in the protein NPC1. To better understand the disease we thoroughly characterized the cellular and morphological alterations occurring in murine, feline, and human NPC. Using immunocytochemistry and filipin histochemistry we show that both gangliosides and unesterified cholesterol are differentially stored in neurons of the cerebral cortex, cerebellum, and hippocampus, as well as in liver. Double fluorescence labeling revealed that GM2 ganglioside and unesterified cholesterol were partially co-localized in vesicular structures, and triple fluorescence labeling utilizing a LAMP-1 antibody identified many of these organelles as part of the late endosomal/lysosomal pathway. These observations, coupled with the proposed role of NPC1 in intracellular cholesterol movement, suggest that GM3 and GM2 gangliosides as well as unesterified cholesterol may be retrogradely cleared from late endosomes/lysosomes by an NPC1-dependent mechanism. Cellular consequences of the NPC metabolic defect as shown by parvalbumin immunocytochemistry and rapid Golgi staining, respectively, revealed characteristic axonal spheroids on GABAergic neurons and ectopic dendritogenesis that followed a species-specific gradient of: mouse < feline < human. These studies suggest that the homeostatic regulation of gangliosides and cholesterol in neurons is mediated by NPC1 and that perturbations in this mechanism cause a complex neuronal storage disorder.

Gangliosides as modulators of dendritogenesis in normal and storage disease-affected pyramidal neurons.

Pyramidal cells initiate the formation of dendritic arbors in a prolific burst of neurite outgrowth during early cortical development. Although morphologically mature pyramidal neurons do not normally sprout additional primary dendrites, the discovery of ectopic dendritogenesis in neuronal storage diseases has revealed that these cells do retain this ability under appropriate stimulation. The capacity for renewal of dendritogenesis has been found to exhibit a species gradient with human > cat, dog, sheep > mouse. A consistent metabolic feature of ectopic dendrite-bearing pyramidal neurons is a heightened intracellular expression of GM2 ganglioside. Elevated expression of this same glycosphingolipid has also been found to correlate with normal dendritogenesis. Immature neurons in developing cat and ferret cortex exhibit high levels of GM2 ganglioside immunoreactivity coincident with normal dendritic sprouting and a similar relationship has now been shown for human cortical development. Ultrastructural studies of all three species revealed GM2 localized to vesicles in a manner consistent with Golgi synthesis and exocytic trafficking to the somatic-dendritic plasmalemma. We propose that GM2 ganglioside functions in glycosphingolipid-enriched microdomains (lipid rafts) in the plasmalemma to promote dendritic initiation through modulation of specific membrane proteins and/or their associated second messenger cascades.

A mouse model for mucopolysaccharidosis type III A (Sanfilippo syndrome).

Mucopolysaccharidosis type III A (MPS III A, Sanfilippo syndrome) is a rare, autosomal recessive, lysosomal storage disease characterized by accumulation of heparan sulfate secondary to defective function of the lysosomal enzyme heparan N- sulfatase (sulfamidase). Here we describe a spontaneous mouse mutant that replicates many of the features found in MPS III A in children. Brain sections revealed neurons with distended lysosomes filled with membranous and floccular materials with some having a classical zebra body morphology. Storage materials were also present in lysosomes of cells of many other tissues, and these often stained positively with periodic-acid Schiff reagent. Affected mice usually died at 7-10 months of age exhibiting a distended bladder and hepatosplenomegaly. Heparan sulfate isolated from urine and brain had nonreducing end glucosamine- N -sulfate residues that were digested with recombinant human sulfamidase. Enzyme assays of liver and brain extracts revealed a dramatic reduction in sulfamidase activity. Other lysosomal hydrolases that degrade heparan sulfate or other glycans and glycosaminoglycans were either normal, or were somewhat increased in specific activity. The MPS III A mouse provides an excellent model for evaluating pathogenic mechanisms of disease and for testing treatment strategies, including enzyme or cell replacement and gene therapy.

Ferret pyramidal cell dendritogenesis: changes in morphology and ganglioside expression during cortical development.

Pyramidal cell ontogenesis and basilar dendritic differentiation were evaluated concomitantly with ganglioside expression and distribution in ferret cerebral cortex. Layer V neurons began basilar dendritogenesis on postnatal day 1 (P1) with a peak in dendritic arborization occurring at P21. Layer II/III neurons, in contrast, were in early stages of basilar dendritic differentiation at P14, resulting in a complex dendritic arbor at P28. High performance thin-layer chromatography showed numerous changes in ganglioside expression during cortical development, including a decline of GM2 in the mature cortex. The temporal expression and cellular distribution of GM2, GD2, GM1, GD3, and GM3 gangliosides in developing cerebral cortex were determined by immunocytochemistry. GM2 immunoreactivity (IR) was most prominent in layer V neurons between P1 and P21 and in layer II/III neurons between P14 and P28 with staining diminishing to near absent levels in the adult. GM2-IR appeared as punctate structures within the somatodendritic domain and by electron microscopy was shown to be membrane-bound vesicles often in close proximity to the plasmalemma. Expression of GM2, but not of other gangliosides studied, followed two well-documented developmental neurogenic gradients: ventrolateral to dorsomedial and radial (inside-first outside-last). Onset of significant GD2 expression in layer II/III and V pyramidal cells was delayed until P14 and persisted in adult neocortex. GD3 was localized most prominently to glial-like cells, whereas GM1 was primarily localized to white matter. The close temporal and spatial concordance of GM2-IR in cortical pyramidal neurons undergoing dendritogenesis is consistent with its proposed role as a modulator of dendritic differentiation.

Ovine ceroid lipofuscinosis (OCL6): postulated mechanism of neurodegeneration.

It is proposed that ceroid lipofuscinosis in Southhampshire sheep (OCLSouthhampshire) be also known as OCL6 as it is syntenic with CLN6 of humans. Histopathological studies show a severe and progressive neurodegeneration of the cerebral cortex which sometimes appears to have a laminar pattern and which is accompanied by a severe midcortical astrocytosis. Other studies have shown that fibroblasts maintained in tissue culture have abnormal regulation of ATP synthase. If this was reflected in neurons, then selective neuron death is likely to be the result of energy-linked excitotoxicity of neurons receiving abundant glutamate input. Increased sensitivity of the NMDA receptor due to inefficient repolarization of the neuron membrane would allow increased cellular uptake of calcium, increased formation of free radicals, and neuron death. The general hypothesis, as developed for other chronic neurodegenerative diseases, is partly based on application of various drugs that block or mediate parts of the pathway involved. The same approach could be used to help test the hypothesis in OCL6 lambs and if successful some of the drugs might have therapeutic potential. As patterns of neurodegeneration are similar in various other forms of ceroid lipofuscinosis accumulating subunit c of mitochondrial ATP synthase, the model may have more general application than merely to CLN6.

GM2 ganglioside as a regulator of pyramidal neuron dendritogenesis.

One of the most profound events in the life of a neuron in the mammalian CNS is the development of a characteristic dendritic tree, yet little is understood about events controlling this process. Pyramidal neurons of the cerebral cortex are known to undergo a single explosive burst of dendritic sprouting immediately after completing migration to the cortical mantle, and following maturation there is no evidence that new, primary dendrites are initiated. Yet in one group of rare genetic diseases--Tay-Sachs disease and related neuronal storage disorders--cortical pyramidal neurons undergo a second period of dendritogenesis. New dendritic membrane is generated principally at the axon hillock and in time is covered with normal-appearing spines and synapses. In our studies of normal brain development and storage diseases we consistently find one feature in common in cortical pyramidal neurons undergoing active dendritogenesis: They exhibit dramatically increased expression of GM2 ganglioside localized to cytoplasmic vacuoles within neuronal perikarya and proximal dendrites. There is also evidence that the increase in GM2 precedes dendritic spouting, and that after dendritic maturation is complete (in normal brain) the GM2 levels in neurons become substantially reduced. These findings are consistent with GM2 ganglioside playing a pivotal role in the regulation of dendritogenesis in cortical pyramidal neurons.

Cellular pathology of lysosomal storage disorders.

Lysosomal storage disorders are rare, inborn errors of metabolism characterized by intralysosomal accumulation of unmetabolized compounds. The brain is commonly a central focus of the disease process and children and animals affected by these disorders often exhibit progressively severe neurological abnormalities. Although most storage diseases result from loss of activity of a single enzyme responsible for a single catabolic step in a single organelle, the lysosome, the overall features of the resulting disease belies this simple beginning. These are enormously complex disorders with metabolic and functional consequences that go far beyond the lysosome and impact both soma-dendritic and axonal domains of neurons in highly neuron type-specific ways. Cellular pathological changes include growth of ectopic dendrites and new synaptic connections and formation of enlargements in axons far distant from the lysosomal defect. Other storage diseases exhibit neuron death, also occurring in a cell-selective manner. The functional links between known molecular genetic and enzyme defects and changes in neuronal integrity remain largely unknown. Future studies on the biology of lysosomal storage diseases affecting the brain can be anticipated to provide insights not only into these pathogenic mechanisms, but also into the role of lysosomes and related organelles in normal neuron function.

Lysosomal storage diseases of animals: an essay in comparative pathology.

A wide variety of inherited lysosomal hydrolase deficiencies have been reported in animals and are characterized by accumulation of sphingolipids, glycolipids, oligosaccharides, or mucopolysaccharides within lysosomes. Inhibitors of a lysosomal hydrolase, e.g., swainsonine, may also induce storage disease. Another group of lysosomal storage diseases, the ceroid-lipofuscinoses, involve the accumulation of hydrophobic proteins, but their pathogenesis is unclear. Some of these diseases are of veterinary importance, and those caused by a hydrolase deficiency can be controlled by detection of heterozygotes through the gene dosage phenomenon or by molecular genetic techniques. Other of these diseases are important to biomedical research either as models of the analogous human disease and/or through their ability to help elucidate specific aspects of cell biology. Some of these models have been used to explore possible therapeutic strategies and to define their limitations and expectations.

GABAergic neuroaxonal dystrophy and other cytopathological alterations in feline Niemann-Pick disease type C.

Feline Niemann-Pick disease type C (NPC) is an autosomal recessive lysosomal storage disease which shares many of the clinical, biochemical and pathological features of the corresponding human disorder. Cytopathological alterations in distinct neuronal cell populations were investigated in this animal model to gain a better understanding of the pathogenesis of brain dysfunction. Golgi and immunocytochemical methods were employed to characterize the cell architectural changes occurring in neuronal somata, dendrites and axons at different stages of disease progression. Cortical pyramidal neurons in laminae II, III, and V exhibited various degrees of meganeurite and/or swollen axon hillock formation with or without ectopic dendritogenesis. Enlarged axon hillock regions with neuritic processes and spines were recognized early in the progression of feline NPC but were less prevalent in mid to late stages of the disease. Glutamic acid decarboxylase (GAD) immunocytochemistry demonstrated immunoreactive spheroids in numerous GABAergic axons in neocortex, subcortical areas, and cerebellum. Parvalbumin-immunoreactive axonal spheroid distribution in brain closely mirrored results from the GAD studies, whereas calbindin D-28k-immunoreactive spheroids were conspicuously absent in most cortical and subcortical areas examined. Purkinje cell axonal spheroid formation progressed in a distal to proximal direction, with eventual involvement of recurrent axon collaterals. Purkinje cell death and a concomitant decrease in the numbers of spheroids in the cerebellum were observed late in the disease course. Clinical neurological signs in feline NPC occur in parallel with neuronal structural alterations and suggest that GABAergic neuroaxonal dystrophy is a contributor to brain dysfunction in this disease.

Purification of feline lysosomal alpha-mannosidase, determination of its cDNA sequence and identification of a mutation causing alpha-mannosidosis in Persian cats.

alpha-Mannosidosis is a lysosomal storage disorder that is caused by the deficiency of lysosomal alpha-mannosidase. Feline alpha-mannosidosis is a well-characterized animal model used for studying pathological and therapeutic aspects of lysosomal storage disorders. We here report the purification of feline liver lysosomal alpha-mannosidase and determination of its cDNA sequence. The active enzyme consisted of three polypeptides, with molecular masses of 72, 41 and 12 kDa, joined by non-covalent forces. The cDNA sequence of feline lysosomal alpha-mannosidase was determined from reverse transcriptase PCR products obtained from skin fibroblast mRNA. The deduced amino acid sequence contained the N-terminal sequences of the 72 and 41 kDa peptides. This indicated that the enzyme is synthesized as a single-chain precursor with a putative signal peptide of 50 amino acids followed by a polypeptide chain of 957 amino acids, which is cleaved into the three polypeptides of the mature enzyme. The deduced amino acid sequence was 81.1 and 83.2% identical with the human and bovine lysosomal alpha-mannosidases sequences respectively. A 4 bp deletion was identified in an alpha-mannosidosis-affected Persian cat by DNA sequencing of reverse transcriptase PCR products. The deletion resulted in a frame shift from codon 583 and premature termination at codon 645. No lysosomal alpha-mannosidase activity could be detected in the liver of this cat. A domestic long-haired cat expressing a milder alpha-mannosidosis phenotype than the Persian cat had a lysosomal alpha-mannosidase activity of 2% of normal. This domestic long-haired cat did not possess the 4 bp deletion, proving molecular heterogeneity for feline alpha-mannosidosis.

Elevated GM2 ganglioside is associated with dendritic proliferation in normal developing neocortex.

Mature pyramidal neurons of cerebral cortex in several neuronal storage diseases elaborate ectopic dendrites. These dendrites appear specifically on pyramidal neurons containing elevated GM2 ganglioside and a variety of studies support the hypothesis that this ganglioside is responsible for inducing the new dendrite growth. To determine whether a similar association between GM2 ganglioside and dendrite growth occurs in normal neurons, we used an antibody to localize GM2 in developing cat neocortex. Our results show that GM2 ganglioside is elevated in normal cortical neurons during the period when dendritogenesis is occurring, but is greatly diminished in these cells after dendritic differentiation is complete. Elevations of GM2 occur in deep neurons earlier than in superficial ones, a sequence that corresponds closely to the inside-first, outside-last progression of cortical neuron differentiation. Ultrastructurally, GM2 immunoreactivity is found sequestered in vesicles with a distribution that coincides with sites of ganglioside synthesis and transport. The close association between elevated GM2 ganglioside and dendrite growth in cortical pyramidal neurons during normal development, coupled with a similar correlation between GM2 and ectopic dendritogenesis in neuronal storage diseases, support the view that this specific ganglioside plays a pivotal role in regulating dendritogenesis.