CLN5 disease, late infantile variant phenotype neuronal ceroid lipofuscinosis, is a severe neurodegenerative disease caused by mutations in the CLN5 gene, which encodes a lysosomal protein of unknown function. Cln5-deficiency in mice leads to loss of thalamocortical neurons, and glial activation, but the underlying mechanisms are poorly understood. We have now studied the gene expression of Cln5 in the mouse brain and show that it increases gradually with age and differs between neurons and glia, with the highest expression in microglia. In Cln5(-/-) mice, we documented early and significant microglial activation that was already evident at 3 months of age. Loss of Cln5 also leads to defective myelination in vitro and in the developing mouse brain. This was accompanied by early alterations in serum lipid profiles, dysfunctional cellular metabolism and lipid transport in Cln5(-/-) mice. Taken together, these data provide significant new information about events associated with Cln5-deficiency, revealing altered myelination and disturbances in lipid metabolism, together with an early neuroimmune response.

The neuronal ceroid lipofuscinoses constitute the most common group of childhood neurodegenerative disorders. These devastating disorders still remain without effective treatment. The use of animal models has provided significant information about NCL pathogenesis, highlighting early glial activation and neuron loss in specific brain regions of affected animals. Here, we have characterized the timing and regional-specificity of the pathological events of CLN8 disease utilizing the Cln8 deficient mouse model, Cln8(mnd). We have studied the progression of neuron loss, astrocytosis and microglial activation from early to moderately symptomatic (1, 3 and 5 months) and late symptomatic (8 months) mice. In Cln8 deficiency, the somatosensory pathway comprising the thalamic ventral posterior nucleus (VPM/VPL) and the primary somatosensory cortex (S1BF) was found to be the most affected relay system. Scattered microglia that appeared partially activated were already present at 3 months of age, followed by astrocytosis and the loss of thalamic relay neurons at 5 months of age, with all these phenotypes and glial activation becoming more pronounced with disease progression. Reactive changes followed a similar pattern in the corresponding cortical target regions, but only moderate neuron loss was detected. Compared to the somatosensory system, in the visual thalamocortical pathway, neuron loss appeared relatively late in the disease, at 8 months. Neuron loss was preceded by glial activation in the dorsal lateral geniculate nucleus (LGNd) and in the primary visual cortex (V1). Taken together these data highlight the pathological targeting of the somatosensory thalamocortical pathway in Cln8 deficiency, in common with other forms of NCL. However, in contrast to other previously characterized NCL models, the Cln8(mnd) mouse shows relatively mild and late appearing pathology within the thalamocortical visual pathway.

Polycystic lipomembranous osteodysplasia with sclerosing leukoencephalopathy (PLOSL) is a recessively inherited disease characterized by early onset dementia associated with bone cysts. Our group has recently established the molecular background of PLOSL by identifying mutations in DAP12 and TREM2 genes. To understand how loss of function of the immune cell activating DAP12/TREM2 signaling complex leads to dementia and loss of myelin, we have analyzed here Dap12 and Trem2 expression in the mouse CNS. We show that Dap12 and Trem2 are expressed from embryonic stage to adulthood, and demonstrate a highly similar expression pattern. In addition, we identify microglial cells and oligodendrocytes as the major Dap12/Trem2-producing cells in the CNS and, consequently, as the predominant cell types involved in PLOSL pathogenesis. These findings provide a good starting point for the study of the molecular mechanisms of this inherited dementia and new evidence for the involvement of the immune system in neuronal degeneration.

SLC17A5 encodes a lysosomal membrane protein, sialin, which transports sialic acid from lysosomes. Mutations in sialin result in neurodegenerative sialic acid storage disorders, Salla disease (SD) and infantile sialic acid storage disease (ISSD). Here we analyzed sialin in mouse central nervous system (CNS) and primary cortical and hippocampal neurons and glia. In the CNS, sialin was predominantly expressed in neurons, especially in the proliferative zone of the prospective neocortex and the hippocampus in developing brain. In nonneuronal cells and primary glial cell cultures, mouse sialin was localized into lysosomes but interestingly, in primary neuronal cultures sialin was not targeted into lysosomes but rather revealed a punctate staining along the neuronal processes and was also seen in the plasma membrane. These data demonstrate a nonlysosomal localization of sialin in neurons and would imply a role for sialin in the secretory processes of neuronal cells.

Neuronal ceroid lipofuscinoses (NCLs) are recessively inherited neurodegenerative lysosomal storage disorders characterized by progressive motor and mental retardation, visual failure, and epileptic seizures. Finnish variant late infantile NCL (vLINCL(Fin)) is caused by mutations in the CLN5 gene. We have isolated the mouse Cln5 gene and analyzed its spatiotemporal expression in the central nervous system (CNS) by in situ hybridization and immunohistochemistry. Cln5 was expressed throughout the embryonic brain already at E15 and the expression steadily increased during development. Prominent expression was observed in cerebellar Purkinje cells, cerebral neurons, hippocampal pyramidal cells, and hippocampal interneurons. The expression pattern correlated with those CNS regions that get degenerated in CLN5 patients. In vitro expression of Cln5 in COS-1, HeLa, and neuronal cells further implied that mouse Cln5 is a soluble lysosomal glycoprotein, closely resembling human CLN5.

Infantile Neuronal Ceroid Lipofuscinosis (INCL) results from mutations in the palmitoyl protein thioesterase (PPT1, CLN1) gene and is characterized by dramatic death of cortical neurons. We generated Ppt1Deltaex4 mice by a targeted deletion of exon 4 of the mouse Ppt1 gene. Similar to the clinical phenotype, the homozygous mutants show loss of vision from the age of 8 weeks, seizures after 4 months and paralysis of hind limbs at the age of 5 months. Autopsy revealed a dramatic loss of brain mass and histopathology demonstrated accumulation of autofluorescent granular osmiophilic deposits (GRODS), both characteristic of INCL. At 6 months, the homozygous Ppt1Deltaex4 mice showed a prominent loss of GABAergic interneurons in several brain areas. The transcript profiles of wild-type and mutant mouse brains revealed that most prominent alterations involved parts of the immune response, implicating alterations similar to those of the aging brain and neurodegeneration. These findings make the Ppt1Deltaex4 mouse an interesting model for the inflammation-associated death of interneurons.

Infantile neuronal ceroid lipofuscinosis (INCL) is a severe neurodegenerative disorder of children, characterized by selective death of neocortical neurons. To understand early disease mechanisms in INCL, we have studied Ppt1(Deltaex4) knock-out mouse neurons in culture and acute brain slices. Global transcript profiling showed deregulation of key neuronal functions in knock-out mice including cholesterol metabolism, neuronal maturation, and calcium homeostasis. Cholesterol metabolism showed major changes; sterol biosynthesis was enhanced and steady-state amounts of sterols were altered at the cellular level. Changes were also present in early maturation of Ppt1(Deltaex4) neurons indicated by increased proliferative capacity of neuronal stem cells. Knock-out neurons presented unaltered electrophysiological properties suggesting uncompromised synaptic function in young animals. However, knock-out neurons exhibited more efficient recovery from glutamate-induced calcium transients, possibly indicating neuroprotective activation. This study established that the neuronal deregulation in INCL is linked to neuronal maturation, lipid metabolism and calcium homeostasis.