Nerve growth factor is expressed and stored in central neurons of adult zebrafish

Abstract

Nerve growth factor (NGF), a member of the neurotrophin family, was initially described as neuronal survival and growth factor, but successively has emerged as an active mediator in many essential functions in the central nervous system of mammals. NGF is synthesized as a precursor pro-NGF and is cleaved intracellularly into mature NGF. However, recent evidence demonstrates that pro-NGF is not a simple inactive precursor, but is also secreted outside the cells and can exert multiple roles. Despite the vast literature present in mammals, studies devoted to NGF in the brain of other vertebrate models are scarce. Zebrafish is a teleost fish widely known for developmental genetic studies and is well established as model for translational neuroscience research. Genomic organization of zebrafish and mouse NGF is highly similar, and zebrafish NGF protein has been reported in mature and two-precursors forms. To add further knowledge on neurotrophic factors in vertebrate brain models, we decided to determine the NGF mRNA and protein distribution in the adult zebrafish brain and to characterize the phenotype of NGF-positive cells. NGF mRNA was visualized by in situ hybridization on wholemount brains. NGF protein distribution was assessed on microtomic sections by using an antiserum against NGF, able to recognize pro-NGF in adult zebrafish brain as demonstrated also in previous studies. To characterize NGF-positive cells, anti-NGF was employed on microtomic slides of aromatase B transgenic zebrafish (where radial glial cells appeared fluorescent) and by means of double-immunolabeling against NGF/proliferative cell nuclear antigen (PCNA; proliferation marker) and NGF/microtube-associated protein2 (MAP2; neuronal marker). NGF mRNA and protein were widely distributed in the brain of adult zebrafish, and their pattern of distribution of positive perikaryal was overlapping, both in males and females, with few slight differences. Specifically, the immunoreactivity to the protein was observed in fibers over the entire encephalon. MAP2 immunoreactivity was present in the majority of NGF-positive cells, throughout the zebrafish brain. PCNA and aromatase B cells were not positive to NGF, but they were closely intermingled with NGF cells. In conclusion, our study demonstrated that mature neurons in the zebrafish brain express NGF mRNA and store pro-NGF.

The BDNF/TrkB Neurotrophin System in the Sensory Organs of Zebrafish

“…In teleost fish, the TrkB receptor is present as TrkB1 and TrkB2, due to a specific genome duplication, and the amino acid residues of its kinase domain are identical in zebrafish and mammals [ 29 , 30 , 31 ]. In zebrafish, NGF, NT3, and NT4 have been detected, respectively, in the brain and the cerebellum of adults and during development [ 32 , 33 , 34 ]. In particular, the distribution and functions of BDNF and TrkB in the brain have been widely studied [ 26 , 27 , 28 , 29 , 30 , 31 , 32 ].…”

“…In zebrafish, NGF, NT3, and NT4 have been detected, respectively, in the brain and the cerebellum of adults and during development [ 32 , 33 , 34 ]. In particular, the distribution and functions of BDNF and TrkB in the brain have been widely studied [ 26 , 27 , 28 , 29 , 30 , 31 , 32 ]. The investigation of BDNF functions, in earlier vertebrates than mammals, aims to provide additional and clearer information on BDNF physiological functions on the neurophysiology of the brain [ 35 ].…”

“…Zebrafish have become a prominent vertebrate model for studying diseases [ 36 , 37 ] since they have 82% orthologues of human disease-associated genes [ 37 ]. Recent evidence affirms that zebrafish represent a novel suitable model to study Alzheimer’s disease [ 38 ], Huntington’s disease [ 39 ], traumatic brain injury [ 32 , 40 ], and sensory organs’ diseases, especially blindness and deafness. Among factors involved in those pathologies is the BDNF/TrkB axis.…”

Identification and Expression of Neurotrophin-6 in the Brain of Nothobranchius furzeri: One More Piece in Neurotrophin Research

“…Remarkably, NT-6 mRNA is expressed in mature neurons, similar to other neurotrophins, such as BDNF, NGF, and NT-4 [50,51,52], which have been already documented in the brain of this model species. Most interestingly, neurotrophins display a peculiar neuronal expression also in zebrafish and the European eel [35,53,54,55]. These observations reinforce the hypothesis that the expression of neurotrophins in teleost fish is primarily linked to mature neurons.…”

Neurotrophins Time Point Intervention after Traumatic Brain Injury: From Zebrafish to Human

“…In detail for BDNF, NT4 increased quickly, similar to the receptor TrkB after TBI. Recent studies also described the expression pattern of NGF and its receptor TrkA in adult zebrafish brain under physiological condition and after TBI [119,120]. These studies evidenced that NGF is specifically expressed in mature neurons, and an interesting point is that NGF and its receptor TrkA increased after 7-10 days after TBI.…”