Functions of Vertebrate Ferlins

Abstract

Ferlins are multiple-C2-domain proteins involved in Ca2+-triggered membrane dynamics within the secretory, endocytic and lysosomal pathways. In bony vertebrates there are six ferlin genes encoding, in humans, dysferlin, otoferlin, myoferlin, Fer1L5 and 6 and the long noncoding RNA Fer1L4. Mutations in DYSF (dysferlin) can cause a range of muscle diseases with various clinical manifestations collectively known as dysferlinopathies, including limb-girdle muscular dystrophy type 2B (LGMD2B) and Miyoshi myopathy. A mutation in MYOF (myoferlin) was linked to a muscular dystrophy accompanied by cardiomyopathy. Mutations in OTOF (otoferlin) can be the cause of nonsyndromic deafness DFNB9. Dysregulated expression of any human ferlin may be associated with development of cancer. This review provides a detailed description of functions of the vertebrate ferlins with a focus on muscle ferlins and discusses the mechanisms leading to disease development.

Keywords: C2 domain; DFNB9; T-tubule system; calcium-sensor; dysferlin; dysferlinopathy; limb girdle muscular dystrophy type 2B (LGMD2B), membrane repair; muscular dystrophy; myoferlin; otoferlin.

Figure 1

Domain organization of MC2Ds proteins. MC2Ds protein superfamily includes at least twelve protein families: Munc13s (mammalian uncoordinated-13), Piccolo, RIM (Rab3-interacting molecule), SLPs (synaptotagmin-like proteins), DOC2s (double C2 domain proteins), ferlins, MCTPs (multiple C2 domain proteins with two transmembrane regions), extended synaptotagmins, PI3KC2s (phosphoinositide 3-kinases class II; a, accessory, c, catalytic domain), RASAL (Ras GTPase- activating-like protein) and copines. The striped domain pattern designates domains present not in all family members.

Figure 2

Domain organization of ferlins of bony vertebrates. Ferlin domain organization from selected species (Dr, Danio rerio; Mm, Mus musculus; Hs; Homo sapiens) using the genome browser Ensembl (Release 96 from April 2019) [36] was drawn according to SMART and Pfam [37,38]. The corresponding phylogenetic tree was produced using Clustal Omega multiple sequence alignment program using default parameters [39]. Translated proteins are from e!Ensembl. Zebrafish has 6 ferlin genes; fer1l5 is absent; however, two related otoferlin genes otofa and b are present. In the mouse, all 6 ferlin genes are present and encode proteins, whereas in humans, FER1L4 represents a pseudogene producing long noncoding RNA. Abbreviations: Chr, chromosome.

Figure 3

Phylogeny of ferlins in humans, mouse and zebrafish. The phylogenetic tree was produced using Clustal Omega multiple sequence alignment program using default parameters [39] and translated protein sequences from Figure 2. The branch length is indicative of the evolutionary distance between the sequences.

Figure 4

Mechanisms of dysferlin function in membrane repair. The model shows four possible and nonexclusive contributions of dysferlin to plasma membrane repair: (1) local formation of membranous patch or plug, triggered by calcium entry and supported, amongst others, by MG53 and annexins; (2) biogenesis and maintenance of the T-tubule system as a possible membrane reservoir; (3) cytoskeleton-dependent sorting of phosphatidylserine (PS) for the recruitment of macrophages, simultaneous contraction and subsequent sealing of the membrane wound and (4) exocytosis of lysosomes.

Figure 5

The network of dysferlin functions. Malfunctioning of one or several aspects contributes to the pathology of dysferlinopathies (red arrows). (1) Impaired sarcolemma repair leads to changes in Ca²⁺ homeostasis and in turn could be affected by intracellular Ca²⁺ compartmentalization and signaling. (2) The T-tubule system is necessary to maintain Ca²⁺ homeostasis and in turn could be affected by abnormalities in Ca²⁺ signaling. (3) Deficits in sarcolemma and T-tubule system repair may cause death of damaged myofibers and promote cycles of the muscle regeneration. Leakage of the muscle fibers contents may change properties of the regenerative niche. (4) Changes in Ca²⁺ compartmentalization and signaling in myofibers can result in dysregulation of, e.g., cytokines secretion and prolonged inflammatory responses. (5) Dysregulation of Ca²⁺ homeostasis may lead to myofibers death, which promotes cycles of muscle regeneration. (6) Sarcolemma repair may depend on the function of T-tubule system as a membrane reservoir and affect T-tubule system function via changes in Ca²⁺ homeostasis. (7) T-tubule system function may be affected by abnormalities in its structure arising during dysferlin-deficient muscle regeneration. (8) Malfunctioning of sarcolemma repair enhances leakage of damage-associated molecules, e.g., annexin A2, promoting inflammation. (9) Prolonged inflammation may result in incomplete cycles of regeneration and pro-inflammatory signaling may inhibit myogenesis.