Adaptive mechanisms for high temperature
In the ST population, different mechanisms were involved in the
biological response to high temperature. Cells increase the baseline of
some proteins to adapt to high water temperature. RAB6A andDDX52 were found under high selection pressures in the ST
population. These genes are responsible for intracellular protein
transport, retrograde vesicle-mediated transport of Golgi apparatus to
endoplasmic reticulum, and ATP-dependent RNA helicase activity, a
response that reflects the protein turnover stimulated by heat shock
(Saadeldin et al., 2020).
Chronic thermal stress causes damaging effects on the cytoskeleton,
catalytic activities of enzymes, single- and double-stranded DNA breaks
and mutations, and inhibition of the DNA repair pathway, leading to
apoptosis and necrosis (Saadeldin et al., 2020). Genome-wide selective
sweep analysis of the ST population revealed that numerous genes
embedded in selected genome regions belonged predominantly to functional
categories related to paracellular permeability, including cell junction
(GO: 0030054), bicellular tight junction (GO: 0005923), apical junction
complex (GO: 0043296), cell–cell junction (GO: 0005911), and occluding
junction (GO: 0070160) (Supplementary Table S8), as well as
tight
junction pathway (KEGG: 04512), cell adhesion molecule pathway
(KEGG:04514), and its downstream pathway leukocyte transendothelial
migration (KEGG:04670). We identified several essential genes, includingLRAD4 , PAK3 , BCAS3 , MINK1 , PDLI4 ,RTN4RL1 , CLAUDIN (CLDN3 , CLDN4 , and CLDN6 ),CDH4 , NLGN3 , and JAM1 , within the genome regions
under selective sweeps. When exposed to heat stress, cells respond by
dramatically reorganizing various cytoskeletal networks, such as
microtubules, intermediate filaments, and actin microfilaments. TheLRDA4 gene serves as a negative regulator of TGF-beta signaling.
The TGF-beta signaling pathway is related to cytoskeleton organization
and involved in morphological changes associated with cellular responses
to chronic heat (Saadeldin et al., 2020). Therefore, LRAD4prohibits the expression of the TGF-beta signaling pathway to maintain
cell morphology during heat stress. The PAK3 , BCAS3 ,MINK1 , PDLI4 , and RTN4RL1 genes play an important
role in cytoskeleton regulation to ensure cytoskeletal integrity under
chronic heat shock. PAK3 plays a role in various signaling
pathways, including cytoskeleton regulation, cell migration, and cell
cycle regulation (Boda et al., 2004).
Cadherin, Claudin, and Jam1 are the key components of the tight
junctions of KEGG pathways. The tight junction maintains the integrity
of the epithelial cell layer that protects multicellular organisms under
external stress and stimuli (Anderson & Itallie, 2009). The tight
junction also serves as a physical barrier to prevent solutes and water
from passing freely through the paracellular space between epithelial
and endothelial cell sheets. Cell dehydration is an important cause of
high-temperature injury. Claudin4 , a member of the Claudin
family, was located in two significant GO terms (establishment of the
skin barrier, GO: 0061436; regulation of water loss via skin, GO:
0033561) that are related to water homeostasis.
Moreover, aside from negatively affecting the cytoskeleton and
cytomembrane integrity, high-temperature stress usually causes DNA
damage and apoptosis. We found that ATRX , RFC3 ,Npas2 , and RFA1 were under higher selection pressures in
the ST population than in the ZS and RS populations. These genes are
involved in DNA
repair or cellular response to DNA damage stimulus (Juhász, Elbakry,
Mathes, & Löbrich, 2018; Xia, Xiao, Gannon, & Li, 2010; Lisby, Barlow,
Burgess, & Rothstein, 2004). Four genes, namely, BNIP1 ,AIFM1 , AIFM3 , and Taok1 , were in the GO term
execution phase of apoptosis (GO: 0097194). Cells die as the execution
phase is completed. AIFM1 and AIFM3 , which are
apoptosis-inducing factors, trigger chromatin condensation and DNA
fragmentation in cells to induce programmed cell death.
Heat stress usually changes membrane permeability, thereby increasing
intracellular Na+ and Ca2+ (Park,
Han, Oh, & Kang, 2005). The TRPC4 channel is involved in various
physiological and pathophysiological processes, such as vascular smooth
muscles, endothelial functions, adiponectin regulation, and oxidative
stress. TRPC4 has an important role in regulating cytosolic
calcium ion concentration in mice and interferes with actin expression
(Yang et al., 2005). High temperature causes endoplasmic reticulum
physiological dysfunction and calcium homeostasis imbalance. High
temperature also induces unfolded and misfolded proteins to accumulate
in the endoplasmic reticulum in excess, thereby triggering the
endoplasmic reticulum stress response. Four genes, namely,DNAJB14 , AIFM1 , CREBRF , and Stc2 , are part
of the endoplasmic reticulum stress response. ER stress responses
include unfolded protein response, which is initially aimed at
compensating for damage but can eventually trigger cell death if
endoplasmic reticulum dysfunction is severe or prolonged. DNAJB14acts as a co-chaperone of HSPA8/Hsc70 in promoting protein
folding and trafficking, as well as in promoting unfolded proteins to
the endoplasmic reticulum-associated degradation pathway. AIFM1functions as an NADH oxidoreductase and a regulator of apoptosis. It
interacts with EIF3G , thereby inhibiting the EIF3 machinery and
protein synthesis. Moreover, it activates caspase-7 to amplify
apoptosis. Furthermore, DNAJB14 , CCT6 , CCT8 , andDNAJB13 are important molecular chaperones that assist in the
folding of proteins upon ATP hydrolysis, thereby preventing or
correcting misfolding caused by high temperature.
Thus, natural selection has substantially changed genetic variations in
a vast number of genes in the ST population related to the regulation of
the cytoskeleton, cell morphogenesis, ion transmembrane transport, DNA
repair, protein folding, and apoptosis. These processes may maintain the
integrity of cells and homeostasis, allowing the fitness of S.
japonica to increase under high-temperature stress.