Structural determinants of Trk receptor specificities using BDNF-based neurotrophin chimeras

Neurodegeneration and neuroregeneration in Chagas disease

Autonomic dysfunction plays a significant role in the development of chronic Chagas disease (CD). Destruction of cardiac parasympathetic ganglia can underlie arrhythmia and heart failure, while lesions of enteric neurons in the intestinal plexuses are a direct cause of aperistalsis and megasyndromes. Neuropathology is generated by acute infection when the parasite, though not directly damaging to neuronal cells, elicits immune reactions that can become cytotoxic, inducing oxidative stress and neurodegeneration. Anti-neuronal autoimmunity may further contribute to neuropathology. Much less clear is the mechanism of subsequent neuronal regeneration in patients that survive acute infection. Morphological and functional recovery of the peripheral neurons in these patients correlates with the absence of CD clinical symptoms, while persistent neuronal deficiency is observed for the symptomatic group. The discovery that Trypanosoma cruzi trans-sialidase can moonlight as a parasite-derived neurotrophic factor (PDNF) suggests that the parasite might influence the balance between neuronal degeneration and regeneration. PDNF functionally mimics mammalian neurotrophic factors in that it binds and activates neurotrophin Trk tyrosine kinase receptors, a mechanism which prevents neurodegeneration. PDNF binding to Trk receptors triggers PI3K/Akt/GSK-3β and MAPK/Erk/CREB signalling cascades which in neurons translates into resistance to oxidative and nutritional stress, and inhibition of apoptosis, whereas in the cytoplasm of infected cells, PDNF represents a substrate-activator of the host Akt kinase, enhancing host-cell survival until completion of the intracellular cycle of the parasite. Such dual activity of PDNF provides sustained activation of survival mechanisms which, while prolonging parasite persistence in host tissues, can underlie distinct outcomes of CD.

Cloning and expression of a novel neurotrophin, NT-7, from carp

Neurotrophins have been demonstrated to play important roles in the development and functioning of the nervous system. This family of proteins consists of four homologous members in mammals: NGF, BDNF, NT-3, and NT-4/5. A new member, called NT-6, was recently cloned from the platyfish Xiphophorus maculatus. This protein shares closer structural relationship to NGF than the other neurotrophins, but contains a characteristic insertion of 22 amino acids that constituted the heparin-binding domain. Here we report the cloning of a novel neurotrophin from the fish Cyprinus carpio (carp), which shared about 66% amino acid identity to Xiphophorus NGF and NT-6. The neurotrophin, designated NT-7, possesses structural characteristics common to all known neurotrophins, such as the presence of six conserved cysteine residues and the flanking conserved sequences. In addition, there is an insertion of 15 amino acids at the position corresponding to that observed for NT-6. The neurotrophic activity of NT-7 was demonstrated by its ability to promote neurite outgrowth and neuronal survival of chick dorsal root ganglia. Phosphorylation assay of various Trk receptors overexpressed in fibroblasts suggested that NT-7 could activate TrkA but not TrkB or TrkC. Northern blot analysis revealed that NT-7 was predominantly expressed in peripheral tissues, though weak expression was also detected in the brain. Like NT-6, this novel neurotrophin might represent yet another NGF-like neurotrophin in lower vertebrates.