**1. Introduction**

*Nothobranchius furzeri* is a novel model organism for aging research [1–3] being its captive lifespan the shortest ever recorded for a vertebrate [4]. The life cycle of *N. furzeri*, indeed, is characterized by explosive growth [5] and rapid expression of aging phenotypes at behavioral, histological, and molecular levels [6–8]. Concerning the brain, *N. furzeri* displays typical aging hallmarks, including lipofuscin accumulation, age-dependent gliosis and rapid decay of adult neurogenesis [9]. The identification of specific genes under positive selection revealed potential candidates to explain the compressed lifespan of this fish. Several age-related genes, indeed, are under positive selection in *N. furzeri* and long-lived species, including humans, raising the intriguing hypothesis that the same gene could underlie evolution of both compressed and extended lifespan [10]. Remarkably, one of the variants in this fish granulin (W449 in the shorter-lived strain and C449 in the longer-lived strain) is within a motif that plays a key role in protein folding, and is mutated in human frontotemporal dementia [11]. The fish variant is predicted to generate functional consequences and is also found in wild fish, thus excluding its derivation from a spurious mutation arisen in the laboratory or from the bottleneck of a rare allele [10].

The assessment of the neurotrophin family came after the identification of the first two members: the nerve growth factor (NGF) [12] and the brain derived neurotrophic factor (BDNF) [13]. These members share stretches of highly homologous amino acid sequences [13], and both support the survival of cultured dorsal root ganglia neurons [14]. Afterwards, three more neurotrophins have been identified in vertebrate genomes: neurotrophin 3 (NT-3), neurotrophin 4 (NT-4) and the fast evolving neurotrophin-5 (NT-5) [15]. Neurotrophins are produced as pre-pro-peptides and undergo proteolytic cleavage before being secreted [16]. They exert many biological effects by their high affinity binding h to the specific tropomyosin-related kinase (Trk) or by a lower affinity interaction with the receptor p75NTR [17]. Specifically, NGF binds to TrkA, NT-3 to TrkC and, with lower affinity, to TrkA, while BDNF and NT-4 bind to TrkB [18]. In addition, p75 receptor can bind to unprocessed or mature neurotrophin and act as co-receptor of Trks [17]. In general, neurotrophins play a role in distinct, as well as partially overlapping, subsets of peripheral and central neurons. Further, individual neurons may also be responsive to more than one neurotrophin at a given time or at subsequent times during development [19]. According to the differential expression and cellular localization of their receptors, neurotrophins can elicit diverse cellular functions in different types of neurons and at different cellular loci [17,18]. Abnormalities associated with neurotrophins synthesis have been linked with neuropathies and neurodegenerative disorders, as well as age-associated cognitive decline.

The genome of teleost fishes contains homologs of the mammalian neurotrophins NGF, BDNF, NT-3 and NT-4 [20] but also a gene coding for one additional neurotrophin originally isolated and cloned in platyfish [21]: neurotrophin-6 (NT-6). The ortholog of this neurotrophic factor in *Danio rerio* [22] and *Cyprinus carpio* [23] was later described as neurotrophin-7 (NT-7). From the biochemical standpoint, NT-6 is featured by the presence of a 22 amino acid residue inserted between the second and third conserved cysteine containing domain. NT-6 promoted the survival of chick sympathetic and sensory dorsal root ganglion neurons, to the same extent of NGF, despite a lower specific activity [21]. Further molecular and phylogenetic studies have provided evidence that, in teleost fishes, NGF and NT-6 are paralogs and originated from duplication of an ancestral gene as consequence of the whole-genome duplication of teleost fishes [24].

Very few studies have been devoted to the role of this neurotrophin in fish, as well as to its expression and morphological distribution. Götz and coworkers [21] documented that NT-6 transcripts are significantly expressed during the embryonic development and adulthood of *Xiphophorus*, in brain, gill, liver and eye while a weak expression is displayed in heart, skin, spleen and skeletal muscles [21]. A very recent paper described NT-6 mRNA during zebrafish embryogenesis (from 12–96 h post fertilization) by whole-mount in situ hybridization [25]. Nittoli et al. reported that early transcript was detected at 16 hpf in two clusters of cells adjacent to the anterior and posterior of the inner ear primordium, and that its expression was lost from 48 h post fertilization onward [25].

In the present study, we investigated the age-related expression of NT-6 in the short-lived teleost, *N. furzeri*. Our findings contribute to: (i) better understand the evolutionary history of neurotrophins in vertebrates; (ii) elucidate their role in vertebrate brain; (iii) demonstrate that NT-6 is expressed in mature neurons of the adult brain; and (iv) document the stable age-associated changes of NT-6 over time.
