Germ cells are physically coupled to somatic support cells of the gonad during differentiation, but this coupling must be disrupted when they are mature, freeing them to participate in fertilization. In mammalian females, coupling occurs via specialized filopodia that project from the ovarian follicular granulosa cells to the oocyte. Here, we show that signaling through the epidermal growth factor receptor (EGFR) in the granulosa, which becomes activated at ovulation, uncouples the germ and somatic cells by triggering a massive and temporally synchronized retraction of the filopodia. Although EGFR signaling triggers meiotic maturation of the oocyte, filopodial retraction is independent of the germ cell state, being regulated solely within the somatic compartment, where it requires ERK-dependent calpain-mediated loss of filopodia-oocyte adhesion followed by Arp2/3-mediated filopodial shortening. By uncovering the mechanism regulating germ-soma uncoupling at ovulation, our results open a path to improving oocyte quality in human and animal reproduction.

Mitochondria play a fundamental role during development of the female germline. They are fragmented, round, and small. Despite these characteristics suggesting that they are inactive, there is accumulating evidence that mitochondrial dysfunctions are a major cause of infertility and generation of aneuploidies in humans. In addition, mitochondria and their own genomes (mitochondrial DNA-mtDNA) may become damaged with time, which might be one reason why aging leads to infertility. As a result, mitochondria have been proposed as an important target for evaluating oocyte and embryo quality, and developing treatments for female infertility. On the other hand, mutations in mtDNA may cause mitochondrial dysfunctions, leading to severe diseases that affect 1 in 4,300 people. Moreover, very low levels of mutated mtDNA seem to be present in every person worldwide. These may increase with time and associate with late-onset degenerative diseases such as Parkinson disease, Alzheimer disease, and common cancers. Mutations in mtDNA are transmitted down the maternal lineage, following a poorly understood pattern of inheritance. Recent findings have indicated existence in the female germline of a purifying filter against deleterious mtDNA variants. Although the underlying mechanism of this filter is largely unknown, it has been suggested to rely on autophagic degradation of dysfunctional mitochondria or selective replication/transmission of non-deleterious variants. Thus, understanding the mechanisms regulating mitochondrial inheritance is important both to improve diagnosis and develop therapeutic tools for preventing transmission of mtDNA-encoded diseases.

Mitochondrial function, largely regulated by the dynamics of this organelle, is inextricably linked to the oocyte health. In comparison with most somatic cells, mitochondria in oocytes are smaller and rounder in appearance, suggesting limited fusion. The functional implications of this distinct morphology, and how changes in the mitochondrial shape translate to mitochondrial function in oogenesis is little understood. We, therefore, asked whether the pro‐fusion proteins mitofusins 1 (MFN1) and 2 (MFN2) are required for the oocyte development. Here we show that oocyte‐specific deletion of Mfn1, but not Mfn2, prevents the oocyte growth and ovulation due to a block in folliculogenesis. We pinpoint the loss of oocyte growth and ovulation to impaired PI3K‐Akt signaling and disrupted oocyte‐somatic cell communication. In support, the double loss of Mfn1 and Mfn2 partially rescues the impaired PI3K‐Akt signaling and defects in oocyte development secondary to the single loss of Mfn1. Together, this work demonstrates that the mitochondrial function influences the cellular signaling during the oocyte development, and highlights the importance of distinct, nonredundant roles of MFN1 and MFN2 in oogenesis.

Oocyte mitochondria are increased in number, smaller, and rounder in appearance than mitochondria in somatic cells. Moreover, mitochondrial numbers and activity are narrowly tied with oocyte quality because of the key role of mitochondria to oocyte maturation. During oocyte maturation, mitochondria display great mobility and cluster at specific sites to meet the high energy demand. Conversely, oocyte mitochondria are not required during early oogenesis as coupling with granulosa cells is sufficient to support gamete's needs. In part, this might be explained by the importance of protecting mitochondria from oxidative damage that result in mutations in mitochondrial DNA (mtDNA). Considering mitochondria are transmitted exclusively by the mother, oocytes with mtDNA mutations may lead to diseases in offspring. Thus, to counterbalance mutation expansion, the oocyte has developed specific mechanisms to filter out deleterious mtDNA molecules. Herein, we discuss the role of mitochondria on oocyte developmental potential and recent evidence supporting a purifying filter against deleterious mtDNA mutations in oocytes.

1 Mitofusin 1 is required for the oocyte-granulosa cell communication that 1 regulates oogenesis 2 3 Thiago S SUMMARY 21Mitochondrial function, largely regulated by the dynamics of this organelle, is 22 inextricably linked to oocyte health. While the proteins that modulate 23 mitochondrial fusion, Mitofusin 1 (MFN1) and 2 (MFN2), are required for 24 embryogenesis, their role in oocyte development remains unclear. Here we 25show that the oocyte-specific deletion of Mfn1, but not Mfn2, results in a 26 complete loss of oocyte growth and ovulation due to a block in folliculogenesis 27 at the preantral-to-antral follicle transition. We pinpoint the loss of oocyte 28 ovulation to disrupted oocyte-somatic cell communication -Mfn1-null oocytes 29 are deficient for the production of the important somatic cell signaling factor 30 GDF9. Unexpectedly, the double loss of Mfn1 and Mfn2 mitigates the effects 31 on oocyte growth and ovulation, which is explained by a partial rescue of 32 oocyte-somatic cell communication and folliculogenesis. Together, this work 33 demonstrates that mitochondrial function influences communication of oocyte 34 with follicular somatic cells and suggests that the balanced expression of 35 modulators of mitochondrial dynamics is critical for proper oocyte development. 36Keywords: folliculogenesis, oocyte, mitochondria, mtDNA, fusion, mitofusin, 37 MFN1, MFN2, GDF9. 38 the rate of first polar body (PB1) extrusion, a readout for meiotic progression 130 to the metaphase-II stage. Only 3.1% of the oocytes that were ovulated by 131Mfn1&2-cKO mice contained PB1 ( Figure 1D), while 61.4% of them were 132 arrested at the GV stage. This finding was confirmed by in vitro maturation of 133 GV oocytes ( Figures 1E and 1F), indicating that Mfn1&2-cKO females were 134 infertile due to ovulation of unviable oocytes. Mating of superovulated 135Mfn1&2-cKO females with WT males also confirmed this as the ovulated 136 oocytes were not fertilized (data not show). The consequence of Mfn1 137

Offspring born to obese and diabetic mothers are prone to metabolic diseases, a phenotype that has been linked to mitochondrial dysfunction and endoplasmic reticulum [ER] stress in oocytes. In addition, metabolic diseases impact the architecture and function of mitochondria-ER contact sites [MERCs], changes which associate with mitofusin 2 [MFN2] repression in muscle, liver and hypothalamic neurons. MFN2 is a potent modulator of mitochondrial metabolism and insulin signaling, with a key role in mitochondrial dynamics and tethering with the ER. Here, we investigated whether offspring born to mice with MFN2-deficient oocytes are prone to obesity and diabetes. Deletion of Mfn2 in oocytes resulted in a profound transcriptomic change, with evidence of impaired mitochondrial and ER function. Moreover, offspring born to females with oocyte-specific deletion of Mfn2 presented increased weight gain and glucose intolerance. This abnormal phenotype was linked to decreased insulinemia and defective insulin signaling, but not mitochondrial and ER defects in offspring liver and skeletal muscle. In conclusion, this study suggests a link between disrupted mitochondrial/ER function in oocytes and increased risk of metabolic diseases in the progeny. Future studies should determine whether MERC architecture and function are altered in oocytes from obese females, which might contribute toward transgenerational transmission of metabolic diseases.