Small heat shock proteins are necessary for heart migration and laterality determination in zebrafish

Abstract

Small heat shock proteins (sHsps) regulate cellular functions not only under stress, but also during normal development, when they are expressed in organ-specific patterns. Here we demonstrate that two small heat shock proteins expressed in embryonic zebrafish heart, hspb7 and hspb12, have roles in the development of left-right asymmetry. In zebrafish, laterality is determined by the motility of cilia in Kupffer’s vesicle (KV), where hspb7 is expressed; knockdown of hspb7 causes laterality defects by disrupting the motility of these cilia. In embryos with reduced hspb7, the axonemes of KV cilia have a 9+0 structure, while control embyros have a predominately 9+2 structure. Reduction of either hspb7 or hspb12 alters the expression pattern of genes that propagate the signals that establish left-right asymmetry: the nodal-related gene southpaw (spaw) in the lateral plate mesoderm, and its downstream targets pitx2, lefty1 and lefty2. Partial depletion of hspb7 causes concordant heart, brain and visceral laterality defects, indicating that loss of KV cilia motility leads causes coordinated but randomized laterality. Reducing hspb12 leads to similar alterations in the expression of downstream laterality genes, but at a lower penetrance. Simultaneous reduction of hspb7 and hspb12 randomizes heart, brain and visceral laterality, suggesting that these two genes have partially redundant functions in the establishment of left-right asymmetry. In addition, both hspb7 and hspb12 are expressed in the precardiac mesoderm and in the yolk syncytial layer, which supports the migration and fusion of mesodermal cardiac precursors. In embryos in which the reduction of hspb7 or hspb12 was limited to the yolk, migration defects predominated, suggesting that the yolk expression of these genes rather than heart expression is responsible for the migration defects.

HSPB7 is indispensable for heart development by modulating actin filament assembly

“…Mutations found within the HSPB7 gene are associated with heart disease (19)(20)(21)(22). In zebrafish, loss-of-function studies showed that HSPB7 is essential for left-right asymmetry and cardiac morphogenesis (23,24). However, the role of HSPB7 in mammalian heart is still unclear.…”

The BAG3-dependent and -independent roles of cardiac small heat shock proteins

“…Many intronic SNPs have been identified in the HSPB7-encoding gene and have been found to be highly associated with HF, DCM, and idiopathic DCM in human patients (117)(118)(119)(120)(121)(122), highlighting an important role for HSPB7 in maintaining cardiac function. Global depletion of Hspb7 in zebrafish disrupts normal cardiac morphogenesis (123,124), and the essential role of HSPB7 in cardiomyocytes is further supported by the discovery that the global and cKO of HSPB7 in mice results in embryonic lethality between embryonic days 11.5 and 12.5 (E11.5-12.5) (29). Using biochemical assays, our laboratory recently revealed that HSPB7 plays a critical role in directly modulating actin filament length by binding to monomeric actin and limiting its availability for polymerization (125).…”

Functional Evolution of Cardiac MicroRNAs in Heart Development and Functions

“…The small heat shock protein Hspb7 was chosen as an indicator for this purpose. Previous studies have reported that in 2dpf zebrafish larvae, hspb7 is mostly expressed in the heart (Marvin et al 2008), and hspb7 is colocalized with myl7, which is a cardiac-specific marker in the heart field at 22 somites (Lahvic et al 2013). Genetic loss of hspb7 leads to disrupted cardiac morphogenesis in zebrafish (Rosenfeld et al 2013), suggesting that Hspb7 plays a critical role in cardiac development.…”