Pessimum range protein network regulation
The energy metabolism protein network associated with the pessimum range consists mainly of upregulated mitochondrial proteins, especially Cytochrome b-c1 complex subunit 7. Structural changes in the pessimum range include downregulation of ECM proteins including collagen subunits, and proteins forming the connection between the ECM and cell membrane, e.g., integrin-α. These proteins are highly responsive to the intensity of salinity stress rather than being uniformly regulated during hypersaline exposure. Structural proteins were a functional category with a high degree of non-linearity in regulation of mRNA and proteins in O. niloticus , further indicating that cell structure regulation is complex and likely fluctuates in response to internal and external signaling, especially around the critical threshold.
Tissue structure changes are expected, as O. mossambicus has dramatically reduced epithelial permeability as salinities increase above SW [53]. Ionocytes involved in osmoregulation in high salinity environments have unique apical crypts in comparison with other types of ionocytes [54], [55]. In high salinity, ionocytes form clusters [56], and develop a complex microtubule network along their basolateral membrane [57]. The formation of tubulin networks in response to salinity was first noted decades ago in Cyprinodon variegatus and Fundulous heteroclitus [57], [58]. This provides context for the highly downregulated uncharacterized protein we have identified as fucolectin-like. While the role of lectins is not fully understood in fish physiology [59], they likely play an important role in the regulation of microtubule networks in response to salinity. Binding sites have been identified on exposed fish gill epithelium which interact with the lectins wheat germ agglutinin (WGA), peanut lectin agglutinin (PNA), and concanacalvin A (ConA) [60]. WGA exposure stimulates Ca+ ion uptake in O. mossambicus in Ca+ deficient water and promote microtubule network formation [61]. WGA and PNA only react with FW specific ionocytes in O. mossambicus (WGA) [62] andOncorhynchus mykiss (PNA) [63]. The uncharacterized protein, along with rhamnose binding lectin which was also one of the most highly downregulated proteins in our data set, may be FW specific for O. mossambicus , and downregulation may help reduce Ca+uptake. This would help in osmoregulation and impact cell-cell adhesions through cadherin binding, which is also impacted by the high upregulation of δ-catenin 1.