Lactocrine signaling is defined as transmission of bioactive factors from mother to offspring as a consequence of nursing. Lactocrine transmission of signaling molecules may be an evolutionarily conserved process through which bioactive factors necessary for support of neonatal development are delivered postnatally. Dependence on maternal resources for development in eutherian mammals extends into neonatal life for at least that period of time when nutrition is obtained solely from first milk (i.e., colostrum). Data for the pig (Sus scrofa domesticus) provide evidence of lactocrine mediated effects on development of the female reproductive tract and other somatic tissues. Porcine uterine gland development, an estrogen receptor-alpha (ESR1)-dependent process, begins within 2 d of birth [postnatal day (PND) 0]. A lactocrine-driven, ESR1-mediated process was proposed as a regulatory mechanism governing onset of uterine gland development and endometrial maturation in the neonatal pig. Gilts maintained in a lactocrine-null state for 2 d from birth by milk-replacer feeding displayed altered patterns of endometrial gene expression and retarded uterine gland development by PND 14. In lactocrine-null gilts, inhibition of endometrial and cervical ESR1 and vascular endothelial growth factor (VEGFA) expression observed on PND 2 persisted to PND 14, even after gilts were returned to nursing on PND 2. Collectively, data support a role for lactocrine signaling in regulation of critical neonatal developmental events. Maternal lactocrine programming of postnatal development may help to insure healthy developmental outcomes. A systems biology approach will be required to define and understand mechanistic dynamics of lactocrine signaling events that may ultimately connect genotype to phenotype and establish the parameters of reproductive potential.

The lactocrine hypothesis for maternal programming of neonatal development was proposed to describe a mechanism through which milk-borne bioactive factors, delivered from mother to nursing offspring, could affect development of tissues, including the uterus. Porcine uterine development, initiated before birth, is completed postnatally. However, age- and lactocrine-sensitive elements of the neonatal porcine uterine developmental program are undefined. Here, effects of age and nursing on the uterine transcriptome for 48 h from birth (Postnatal Day [PND] = 0) were identified using RNA sequencing (RNAseq). Uterine tissues were obtained from neonatal gilts (n = 4 per group) within 1 h of birth and before feeding (PND 0), or 48 h after nursing ad libitum (PND 2N) or feeding a commercial milk replacer (PND 2R). RNAseq analysis revealed differentially expressed genes (DEGs) associated with both age (PND 2N vs. PND 0; 3283 DEGs) and nursing on PND 2 (PND 2N vs PND 2R; 896 DEGs). Expression of selected uterine genes was validated using quantitative real-time PCR. Bioinformatic analyses revealed multiple biological processes enriched in response to both age and nursing, including cell adhesion, morphogenesis, and cell-cell signaling. Age-sensitive pathways also included estrogen receptor-alpha and hedgehog signaling cascades. Lactocrine-sensitive processes in nursed gilts included those involved in response to wounding, the plasminogen activator network and coagulation. Overall, RNAseq analysis revealed comprehensive age- and nursing-related transcriptomic differences in the neonatal porcine uterus and identified novel pathways and biological processes regulating uterine development.

Nursing supports neonatal porcine uterine and testicular development, however, lactocrine effects on cervical development are undefined. Studies were conducted to determine the effects of i) age and the imposition of the lactocrine-null state from birth (postnatal day 0 (PND0)) by milk replacer feeding on cervical histology; ii) imposition of the lactocrine-null state for 2 days from birth on cervical cell proliferation, as reflected by proliferating cell nuclear antigen immunostaining; and iii) a single feeding of colostrum or milk replacer, administered at birth, with or without oral IGF1, on cervical cell proliferation and phosphorylated AKT (pAKT) and B-cell lymphoma 2 (BCL2) protein levels at 12 h postnatal. Cervical crypt depth and height of luminal epithelium (LE) increased with age by PND14, when both responses were reduced in replacer-fed gilts. Cell proliferation was reduced in LE at PND2, and in crypt epithelium and stroma by PND14 in replacer-fed gilts. Returning replacer-fed gilts to nursing on PND2 did not rescue the cervical phenotype by PND14. A single feeding of colostrum, but not milk replacer, was sufficient to support cervical cell proliferation at 12 h postnatal. IGF1 supplementation induced cell proliferation in replacer-fed gilts, and increased cervical pAKT and BCL2 levels in colostrum-fed gilts and replacer-fed gilts at 12 h postnatal. Results indicate that age and nursing support porcine cervical development, support is initiated at first ingestion of colostrum, IGF1 may be lactocrine-active, and identification of lactocrine-active factors can be accomplished by 12 h postnatal using this bioassay system.

Nursing for 2 d from birth supports neonatal porcine uterine and cervical development. However, it is not clear how timing or duration of lactocrine signaling from birth (postnatal day = PND 0) affects development of neonatal female reproductive tract tissues. Therefore, studies were conducted to determine effects of age at first nursing and duration of nursing from birth on specific elements of the matrix metalloproteinase (MMP)/tissue inhibitor of metalloproteinase (TIMP) system in uterine and cervical tissues at PND 2. When nursing was initiated at 0 h or 30 min of age, targeted proteins, including proMMP9 and MMP9, were detected in uterine and cervical tissues on PND 2, as was uterine TIMP1. However, these proteins were undetectable when nursing was delayed for 12 h and when gilts were fed milk replacer for 48 h from birth. Increasing the duration of nursing from 30 min to 12 h from birth increased uterine (P < 0.05) and cervical (P < 0.001) MMP9 levels to those observed in gilts nursed for 48 h. Similarly, uterine TIMP1 levels increased with duration of nursing. Uterine MMP2 levels were detectable but unaffected by age at first nursing or duration of nursing from birth. Uterine MMP2 and MMP9 activities, monitored by zymography, reflected immunoblotting data. Results provide evidence for the utility of MMP9 and TIMP1 as markers of age- and lactocrine-sensitive porcine female reproductive tract development.

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