Phenotype analysis in neurological models of human disease

Palmitoyl-protein thioesterase-1 deficiency mediates the activation of the unfolded protein response and neuronal apoptosis in INCL

Numerous proteins undergo modification by palmitic acid (S-acylation) for their biological functions including signal transduction, vesicular transport and maintenance of cellular architecture. Although palmitoylation is an essential modification, these proteins must also undergo depalmitoylation for their degradation by lysosomal proteases. Palmitoyl-protein thioesterase-1 (PPT1), a lysosomal enzyme, cleaves thioester linkages in S-acylated proteins and removes palmitate residues facilitating the degradation of these proteins. Thus, inactivating mutations in the PPT1 gene cause infantile neuronal ceroid lipofuscinosis (INCL), a devastating neurodegenerative storage disorder of childhood. Although rapidly progressing brain atrophy is the most dramatic pathological manifestation of INCL, the molecular mechanism(s) remains unclear. Using PPT1-knockout (PPT1-KO) mice that mimic human INCL, we report here that the endoplasmic reticulum (ER) in the brain cells of these mice is structurally abnormal. Further, we demonstrate that the level of growth-associated protein-43 (GAP-43), a palmitoylated neuronal protein, is elevated in the brains of PPT1-KO mice. Moreover, forced expression of GAP-43 in PPT1-deficient cells results in the abnormal accumulation of this protein in the ER. Consistent with these results, we found evidence for the activation of unfolded protein response (UPR) marked by elevated levels of phosphorylated translation initiation factor, eIF2alpha, increased expression of chaperone proteins such as glucose-regulated protein-78 and activation of caspase-12, a cysteine proteinase in the ER, mediating caspase-3 activation and apoptosis. Our results, for the first time, link PPT1 deficiency with the activation of UPR, apoptosis and neurodegeneration in INCL and identify potential targets for therapeutic intervention in this uniformly fatal disease.

Disruption of PPT2 in mice causes an unusual lysosomal storage disorder with neurovisceral features

The palmitoyl protein thioesterase-2 (PPT2) gene encodes a lysosomal thioesterase homologous to PPT1, which is the enzyme defective in the human disorder called infantile neuronal ceroid lipofuscinosis. In this article, we report that PPT2 deficiency in mice causes an unusual form of neuronal ceroid lipofuscinosis with striking visceral manifestations. All PPT2-deficient mice displayed a neurodegenerative phenotype with spasticity and ataxia by 15 mo. The bone marrow was infiltrated by brightly autofluorescent macrophages and multinucleated giant cells, but interestingly, the macrophages did not have the typical appearance of foam cells commonly associated with other lysosomal storage diseases. Marked splenomegaly caused by extramedullary hematopoiesis was observed. The pancreas was grossly orange to brown as a result of massive storage of lipofuscin pigments in the exocrine (but not islet) cells. Electron microscopy showed that the storage material consisted of multilamellar membrane profiles ("zebra bodies"). In summary, PPT2 deficiency in mice manifests as a neurodegenerative disorder with visceral features. Although PPT2 deficiency has not been described in humans, manifestations would be predicted to include neurodegeneration with bone marrow histiocytosis, visceromegaly, brown pancreas, and linkage to chromosome 6p21.3 in affected families.

Disruption of PPT1 or PPT2 causes neuronal ceroid lipofuscinosis in knockout mice

PPT1 and PPT2 encode two lysosomal thioesterases that catalyze the hydrolysis of long chain fatty acyl CoAs. In addition to this function, PPT1 (palmitoyl-protein thioesterase 1) hydrolyzes fatty acids from modified cysteine residues in proteins that are undergoing degradation in the lysosome. PPT1 deficiency in humans causes a neurodegenerative disorder, infantile neuronal ceroid lipofuscinosis (also known as infantile Batten disease). In the current work, we engineered disruptions in the PPT1 and PPT2 genes to create "knockout" mice that were deficient in either enzyme. Both lines of mice were viable and fertile. However, both lines developed spasticity (a "clasping" phenotype) at a median age of 21 wk and 29 wk, respectively. Motor abnormalities progressed in the PPT1 knockout mice, leading to death by 10 mo of age. In contrast, the majority of PPT2 mice were alive at 12 mo. Myoclonic jerking and seizures were prominent in the PPT1 mice. Autofluorescent storage material was striking throughout the brains of both strains of mice. Neuronal loss and apoptosis were particularly prominent in PPT1-deficient brains. These studies provide a mouse model for infantile neuronal ceroid lipofuscinosis and further suggest that PPT2 serves a role in the brain that is not carried out by PPT1.

Behavioural phenotyping of mouse mutants

Behavioural phenotyping of mouse mutants is not a goal in itself but serves to characterise the behavioural effects of naturally occurring or experimentally induced mutations. Genetically engineered mouse mutants are valuable tools to elucidate the genetic control of behaviour and the interaction between genetic and environmental factors. However, a prerequisite for their use is the ability to assess different elements of behaviour. To this end, a battery of tests, which should be flexible enough to meet the needs of a particular study, should be used to characterise the behavioural phenotype. Detailed and extensive information about the effects of gene mutations is crucial for model building and model evaluation. Model building is an iterative process, switching between experimental data and theory formation. In order to facilitate this process and to allow comparison of results within and between laboratories, the standardisation of breeding, housing, and testing conditions is essential. The development and standardisation of sensitive, valid behavioural tests which are suited to phenotype mouse mutants is both a responsibility and a challenge to investigators of mouse behaviour.