Neuropathologic characterization of a canine model of mucopolysaccharidosis IIIB and additional studies in anti-inflammatory therapy and neuroinflammatory kinetics

Abstract

The mucopolysaccharidoses (MPSs) are a heterogenous group of lysosomal storage diseases that are designated MPS I through MPS IX (excluding MPS V and MPS VIII) that result in deficiencies in specific lysosomal enzymes that are responsible for the stepwise degradation of glycosaminoglycans (GAGs). Some of these MPSs demonstrate neuropathic presentations, which includes MPS I, II, III, and VII. The neuropathic forms of MPS are characterized by primary intralysosomal accumulation of heparan sulfate (HS) and secondary accumulation of gangliosides. Mucopolysaccharidosis III (MPS III, Sanfilippo Syndrome Type III) is characterized by 4 subtypes in humans (A-D), and fifth subtype in mice (E). MPS IIIB results from a deficiency in ?-N-acetylglucosamindase (Naglu) activity which results in primary accumulation of HS. There is currently no approved treatment for MPS IIIB, however, enzyme replacement therapy, gene therapy, and anti-inflammatory therapies are currently being examined. A knock-out mouse model and several naturally occurring animal models of MPS IIIB have been reported. To date, the mouse and canine models of MPS IIIB have been utilized for pathogenesis and therapeutic studies. The canine model is an ideal animal model because their larger size, easy of handling, outbred genetics, and longer lifespan. However, the temporospatial development of neuropathologic changes have not been reported for canine MPS IIIB. Here, we describe the development of neuropathologic changes in the brain, cervical spinal cord, and dorsal root ganglia of MPS IIIB dogs aged 2 to 26 months. Microscopic pathologic changes include early vacuolation of glial cells at 2 months of age, vacuolation of neurons at 10 months of age, and histochemical staining of storage inclusions as early as 2 months of age. Development of early neuroinflammation was demonstrated via quantitative immunohistochemical analysis for GFAP and Iba1 glial marker. Loss of Purkinje cells was observed as early as 10 months of age and was progressive at 18 and 26 months of age. Our results suggest that the canine model of MPS IIIB is a replicative model of human disease. Considering the similarities to human disease progression and marked neuroinflammation observed in the MPS IIIB affected animals, we conducted an anti-inflammatory therapeutic study. Pentosan Polysulfate (PPS) has been demonstrated to reduce inflammation and GAG excretion in the animal models of MPS and in human MPS I and II patients. Subcutaneous PPS injections (1.6 mg/kg human equivalent dose) were administered biweekly to MPS IIIB animals starting shortly after birth (4-11 days) for a total of 8 treatments/animal. MPS IIIB PPS treated animals (n=3) were compared to aged-matched unaffected (n=3) and untreated MPS IIIB (n=3) animals. Pathologic changes were examined in the brains and cervical spinal cord of all animals. The lysosomal storage compartment was assessed via LIMP2 immunolabeling. Neuroinflammation was assessed via GFAP and Iba1 immunolabeling and cytokine ELISAs. PPS treatment reduced LIMP2, GFAP, and Iba1 immunolabeling in select regions of interest. PPS treatment had a mild effect on TNF- ?, but no detectable effect on IL-1β or TGF-β. Overall, PPS treatment has a mild reductive effect on the size of the storage compartment and a prominent reductive effect on neuroinflammatory markers. Neuroinflammation is a commonly reported finding in the animal models of MPS III and we have previously reported microgliosis and astrocytosis in the canine model. Microglial activation in MPS IIIB is thought to occur through innate inflammatory pathways, such as the toll-like receptor (TLR)4 pathway. Activation of the TLR4 pathway as a result of HS oligosaccharide accumulation has been consistently demonstrated in unaffected and MPS affected mice. To determine the neuroinflammatory environment in the MPS IIIB canine brain, we utilized cytokine ELISAs and western blot analysis. Protein level expression of TNF- ?, IL-1, and TGF- β1 was increased in 18- and 20- month- old MPS IIIB animals compared to unaffected animals. Western blot analysis demonstrated increased protein expression of TLR4 and NFκB in MPS IIIB animals. Our results indicated activation of the TLR4 signaling pathway with production of neuroinflammatory mediators. Overall, these studies highlight the usability of this model for pathogenesis and therapeutic studies. In addition, these studies demonstrate the role neuroinflammation plays in MPS IIIB disease progression and how modulation of the immune response might be advantageous for the long-term management of MPS IIIB patients.