Prokaryotic SPHINX sequences are conserved in mammalian brain and participate in neurodegeneration

ABSTRACT A new class of circular mammalian “SPHINX” DNAs, represented by the 1.8 and 2.4kb elements, were discovered in highly infectious cytoplasmic particles isolated from Creutzfeldt-Jakob Disease (CJD) and scrapie samples. These DNAs contained replication initiation sequences (REPs) with ∼70% homology to Acinetobacter phage REP segments. Antibodies against REP peptides from the 1.8kb DNA highlighted a 41kDa protein (spx) on Western blots, and in-situ studies revealed its tissue and cell-type specific expression, e.g., in pancreatic islet cells, keratinocytes and kidney tubules but not pancreatic exocrine cells, alveoli, and striated muscle. An intense spx signal in oocytes implicated maternal inheritance of the SPHINX 1.8 genome, a feature known only for bacterially derived mitochondrial DNA in mammals. To determine if spx concentrated at specific neurons and synapses, and also maintained a conserved pattern of architectural organization in mammals, we evaluated mouse, rat, hamster, Guinea Pig and human brains. Most outstanding was the cross-species concentration of spx in huge excitatory synapses of mossy fibers and small internal granule neuron synapses, the only excitatory neuron within the cerebellum. This synaptic localization was also demonstrable at the ultrastructural level. Vacuolar loss of these synaptic complexes, thinning of the internal granule cell layer, and pathological fibrillar spx accumulations within Purkinje neurons were obvious in CJD Guinea pig brains. In rats, these fibrillar changes marked hippocampal Pyramidal neurons and preceded prion protein misfolding. Spx may define different causes or processes of neurodegeneration. The evolutionary origin, persistence and modulation of SPHINX 1.8 opens an unexpected chapter in mammalian symbiosis.