A Cytosolic Sperm Protein Factor Mobilizes Ca2+ from Intracellular Stores by Activating Multiple Ca2+ Release Mechanisms Independently of Low Molecular Weight Messengers

Antony Galione,Keith T Jones,F Anthony Lai,Karl Swann

doi:10.1074/jbc.272.46.28901

Abstract

Ca2+ oscillations can be induced in mammalian eggs and somatic cells by microinjection of a cytosolic sperm protein factor. The nature of the sperm factor-induced Ca2+ signaling was investigated by adding sperm protein extracts to homogenates of sea urchin eggs, which contain multiple classes of Ca2+ release mechanisms. We show that the sperm factor mobilizes Ca2+ from non-mitochondrial Ca2+ stores in egg homogenates after a distinct latency. This latency is abolished by preincubation of sperm extracts with egg cytosol. The preincubation step is highly temperature-dependent and generates a high molecular weight, protein-based Ca2+-releasing agent that can also mobilize Ca2+ from purified egg microsomes. This Ca2+ release appears to be mediated via both inositol 1,4,5-trisphosphate and ryanodine receptors, since homologous desensitization of these two release mechanisms by their respective agonists inhibits further release by the sperm factor. However, sperm factor-induced Ca2+ release by these channels is independent of inositol 1,4, 5-trisphosphate or cADPR since antagonists of either of these two messengers did not block the Ca2+ release effected by the sperm factor. The sperm protein factor may cause Ca2+ release via an enzymatic step that generates a protein-based Ca2+-releasing agent.

Highlights

At fertilization in mammals and some marine invertebrates, the sperm activates the egg by inducing a series of Ca2ϩ oscillations involving release from intracellular stores [1, 2]
One hypothesis for signaling at fertilization proposes that sperm-egg fusion allows the diffusion of a soluble cytosolic sperm factor into the egg cytoplasm which leads to the activation of Ca2ϩ release [7, 8]
More, injecting cytosolic sperm extracts into mouse, hamster, bovine, human, and nemertean worm eggs cause Ca2ϩ oscillations similar to those seen at fertilization in each species [7, 11,12,13,14,15]

Summary

Introduction

At fertilization in mammals and some marine invertebrates, the sperm activates the egg by inducing a series of Ca2ϩ oscillations involving release from intracellular stores [1, 2]. Sperm factor extracts were incubated with sea urchin egg cytosol supernatant or microsomal fractions prepared on Percoll gradients as described above, in a 1:2 ratio, respectively, at 24 °C, except where stated otherwise.

Results

Conclusion