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.