Mechanism of neurogenesis and neuro-regeneration in the adult teleost brain

Fish retain a remarkable potential of neuro-regeneration throughout life, whereas injury to neuronal system in the mammalian central nervous system (CNS) results in degeneration and loss of function. Hypothetically, the brain of teleost will regenerate after injury, however, the molecular mechanism underlying this phenomena as well as the reason for the lack of such regenerative capability in the mammalian CNS is still unknown. To address this issue, three main study objectives were placed, (i) to understand the mechanism involved in neuro-regeneration in teleosts brain (Chapter 2); (ii) to identify novel molecular factors involved in neuro-regeneration in the zebrafish brain (Chapter 3); and (iii) to investigate the potential of identified novel molecular factors in associating adult neurogenesis in the zebrafish brain (Chapter 4). In chapter 2, the study aimed to characterize the neuro-regenerative process by observing changes in apoptotic and proliferative activities in the brain of teleost, zebrafish (Danio rerio) and tilapia (Oreochromis niloticus). Morphological observations showed complete neuro-regeneration of the habenula region by 40 days and 60 days post-damage for zebrafish and tilapia respectively. Recovery of neuronal projections and axonal re-integration further proved complete neuro-regeneration in adult teleost. In chapter 3, the study aimed to identify the molecular factor(s) that play role in the early stage of regeneration, before first batch of newborn cells were significantly detected at the injury site, protein profiling through two-dimensional difference gel electrophoresis was done in the following chapter 3. Ten proteins were identified, which included cytoskeleton protein, transcription factors and binding proteins. Among these proteins identified, sprouty-related EVH1 domain-containing protein 2 (Spred-2) was chosen for further study as there is no study between Spred-2 and neurogenesis has been reported. Finally in chapter 4, the expression of spred-2 mRNA in the adult zebrafish brain was analyzed by in situ hybridization and their histological changes were examined during the brain injury. The zebrafish spred-2 mRNA containing cells were observed in most of the cell proliferative zones, which suggests the potential role of Spred-2 in the regulation of neurogenesis. On the other hand, histological study showed a decrease of spred-2 expression with a concurrent increase in apoptotic cells at the lesion site on the first two days after the injury. Damaged and dead neural cells release growth factors and neurotrophins which can down-regulate Spred-2 expression to unlock the activation of the ERK pathway. Proliferative and phosphorylated-ERK-immunoreactive cells significantly increased on day-4 post-lesion and co-expression of spred-2 mRNA in phosphorylated-ERK-immunoreactive cells were observed in neurogenic zones near the lesion site. These observations suggest that a decrease in spred-2 after injury activates the ERK pathway to stimulate cell proliferation in the adult zebrafish brain. Collectively, these studies showed the teleost CNS possesses excellent neuro-regenerative capability. The apoptotic neural cells could be the earliest source to induce cell proliferation. Differential protein profiling revealed that Spred-2 could be a novel molecule which can regulate neurogenesis in adult teleost CNS. High conservation of Spred-2 protein sequence in vertebrates suggests the potential role for Spred-2 in neurogenesis-regulation in vertebrates including mammals. Taken together, these results provide better cellular and molecular processes underlying adult neuro-regeneration of injured CNS