Neuropathogenesis of ovine neuronal ceroid lipofuscinosis

Abstract

The neuronal ceroid lipofuscinoses (NCLs, Batten disease) are inherited neurodegenerative diseases in children arid animals characterised by brain atrophy, progressive blindness and the accumulation of fluorescent lysosome derived storage bodies in cells. A CLN6 form in sheep has proven to be a valuable model of the human diseases. This thesis describes neuropathological changes that take place during postnatal brain development in affected sheep, including the phase beyond one year of age when clinical symptoms become apparent.

Lectin histochemistry and immunostaining revealed GF AP and MHC class II activated astrocytes, microglia and perivascular macrophages in defined brain regions at only 12 days of age. Progressive cortical atrophy was apparent from 12 days in the occipital lobe and from 6 months in the frontal lobe. Focal clusters of activated microglia were initially present in laminae II-IV of occipital and somatosensory cortical regions. Spreading activation and transformation of ramified microglia to brain macrophages coincided with the degeneration of different cortical layers and brain areas, foremost affected being regions associated with clinical symptoms, notably the visual and parieto-occipital cortices.

In contrast to glial activation, storage body fluorescence accumulated more uniformly within distinct regions of the cerebral cortex, and a steady increase was evident in subcortical brain structures throughout postnatal development. Storage bodies accumulated in neurons mainly but non-neuronal cells became a major repository as neurons disappeared. The cause of neurodegeneration and storage body accumulation may be separate manifestations of the genetic lesions. The early occurrence of astrocytic and microglial activation suggest that these cells may be important mediators in pathogenesis.

Immunostaining of different GABAergic neuron populations revealed a pronounced loss of parvalbumin containing intemeurons in the cerebral cortex. This loss occurred in parallel to the loss of pyramidal neurons in upper cortical layers and in line with the atrophy that developed at different times and rates in distinct cortical regions. A survey of the distribution and survival of specific neuron types defined by neurotransmitters and calcium binding proteins revealed that loss in the cerebral cortex was delayed in calbindin / somatostatin and calretinin containing neurons. Parvalbumin immunoreactive fast firing intemeurons were affected early on in the cerebral cortex and hippocampus, but they were spared within striatum and cerebellar cortex. This regional variability argues against a cell type specific neurodegeneration.

Increased detection of calretinin immunoreactive Cajal-Rezius cells was apparent in molecular layers of cerebral cortex and hippocampus from 12 months of age. A parallel increase in calretinin immunoreactive small granular cells was apparent in the subventricular zone of the lateral ventricles, the overlying white matter and the dentate gyrus. This occurred in parallel to a pronounced loss of myelin in the subventricular zone and is indicative of enhanced neurogenesis in brains of symptomatic sheep.

Layer II -III pyramidal neurons of the parieto-occipital cortex had smaller soma areas than they did in the normal sheep brain. Small somatic size of neocortical pyramidal neurons correlates with a reduction of dendrite branches in other neuropathies and mice with induced BDNF signalling defects in the forebrain. Experience-dependent modulation and protection of active synapses may be disturbed in the affected sheep. This is in line with the conclusion that neuronal connectivity and functional context appear to be important factors determining disease development.