OR18-1 The Impact of Makorin Ring Finger Protein 3 (MKRN3) Overexpression in the Arcuate Nucleus of Postpubertal Female Mice on the HPG Axis

Background and Objective

Makorin Ring Finger Protein 3 (MKRN3), an inhibitor of the KISS1 and TAC3 promoters in the hypothalamic arcuate nucleus (ARC), is an important player in age of menarche and etiology of central precocious puberty. However, its potential role in delayed puberty and/or hypogonadism is not defined. Mkrn3 expression in the ARC declines prior to the onset of puberty and remains low in adulthood, supporting its role as an inhibitor of pubertal onset. Neonatal hypothalamic overexpression of Mkrn3 leads to delayed puberty in female mice; this study aimed to explore the effects of MKRN3 overexpression in the postpubertal mouse. We hypothesized that MKRN3 overexpression in the ARC of postpubertal female mice would result in suppression of the HPG axis, manifested as hypogonadotropic hypogonadism.

Methods and Results

Adult female C57BL/6 mice were stereotaxically injected into the ARC with a recombinant adeno-associated virus (rAAV-Mkrn3-IRES-EGFP;n=12) to overexpress Mkrn3, or with a control virus (rAAV-EGFP;n=9). Daily vaginal smears collected six weeks post-injection showed that rAAV-Mkrn3 injected mice spent significantly more time (62.7%) in diestrus compared to controls (41.3%;P<0.01). Mean body weight did not statistically differ between groups. Eight weeks after viral injection, gonad-intact females underwent stimulation tests while in diestrus. Tail vein blood samples were collected at baseline and 20, 40 and 60 minutes after stimulation. In response to the NK3R agonist, senktide, evaluated at two doses (5 nmol and 10 nmol ip, respectively), Mkrn3-overexpressing mice had a reduced peak (i.e., T20) LH response compared to controls (P<0.001 for both dosing paradigms). In contrast, in response to kisspeptin-10 (1 nmol ip), mice responded with similar peak serum LH levels. Subsequently, tests were repeated seven days after ovariectomy and estradiol capsule replacement (OVX+E2). Peak LH levels (T20) were again significantly lower in response to senktide (5 nmol ip) in the rAAV-Mkrn3 injected females (P<0.001). Peak serum LH levels (T20) were not different in response to kisspeptin-10 (1 nmol) or GnRH (1.25 ng/g body weight). Additionally, there were no significant differences in serum LH levels between groups 7 days after OVX or OVX + E2.

Conclusions

In this novel mouse model, developed to investigate the mechanism of action of MKRN3, an important neuroendocrine controller of puberty initiation, these findings suggest that MKRN3 can exert inhibitory effects on the reproductive axis in female mice beyond the juvenile period. In these mice, response to kisspeptin and GnRH was preserved but the response to senktide was blunted. These findings suggest that Mkrn3 may act in ARC kisspeptin neurons to inhibit kisspeptin synthesis or secretion and/or potentially the action of neurokinin B via its receptor. Modulation of MKRN3 expression may be a future therapeutic target for reproductive disorders.

Presentation: Monday, June 13, 2022 11:00 a.m. - 11:15 a.m.

Sex-specific pubertal and metabolic regulation of Kiss1 neurons via Nhlh2

Hypothalamic Kiss1 neurons control gonadotropin-releasing hormone release through the secretion of kisspeptin. Kiss1 neurons serve as a nodal center that conveys essential regulatory cues for the attainment and maintenance of reproductive function. Despite this critical role, the mechanisms that control kisspeptin synthesis and release remain largely unknown. Using Drop-Seq data from the arcuate nucleus of adult mice and in situ hybridization, we identified Nescient Helix-Loop-Helix 2 (Nhlh2), a transcription factor of the basic helix-loop-helix family, to be enriched in Kiss1 neurons. JASPAR analysis revealed several binding sites for NHLH2 in the Kiss1 and Tac2 (neurokinin B) 5' regulatory regions. In vitro luciferase assays evidenced a robust stimulatory action of NHLH2 on human KISS1 and TAC3 promoters. The recruitment of NHLH2 to the KISS1 and TAC3 promoters was further confirmed through chromatin immunoprecipitation. In vivo conditional ablation of Nhlh2 from Kiss1 neurons using Kiss1Cre:Nhlh2fl/fl mice induced a male-specific delay in puberty onset, in line with a decrease in arcuate Kiss1 expression. Females retained normal reproductive function albeit with irregular estrous cycles. Further analysis of male Kiss1Cre:Nhlh2fl/fl mice revealed higher susceptibility to metabolic challenges in the release of luteinizing hormone and impaired response to leptin. Overall, in Kiss1 neurons, Nhlh2 contributes to the metabolic regulation of kisspeptin and NKB synthesis and release, with implications for the timing of puberty onset and regulation of fertility in male mice.

SUN-100 Mice Lacking Paternally Expressed DLK1 Reach Puberty at a Lower Body Weight Than Littermate Controls

Body fat content along with a variety of genetic, environmental and psychosocial factors are responsible for the development and maintenance of reproductive function, especially in females. Epidemiologic studies indicate a relationship between increased body mass index and earlier puberty in girls. In contrast, a significant delay in puberty and menarche is seen in girls who are very physically active and have markedly diminished body fat. This link between reproduction and metabolism was reinforced with the recent report of loss-of-function mutations in the Delta-like homolog 1 (DLK1) gene in girls with central precocious puberty (CPP) and increased body fat. DLK1 is a paternally expressed gene located on chromosome 14q32.2 in a locus associated with Temple syndrome (TS), an imprinting disorder caused mainly by maternal parental disomy (mUPD). Dlk1 knockout mice display pre- and postnatal growth retardation, a phenotype that overlaps with human mUPD14. However, precocious puberty, a common finding associated with TS, was not carefully characterized in these mice. We used a Dlk1 deficient mouse model to determine the effects of Dlk1 on pubertal maturation. We confirmed by RT-qPCR that Dlk1 mRNA was undetectable in the mediobasal hypothalamus, where kisspeptin and other regulators of puberty are expressed, of Dlk^+/p- mice (which inherited the mutant allele from their father) whereas it was present in Dlk^+/+ mice. As reported previously, body weight was significantly lower in juvenile male and female Dlk^+/p- mice, compared to wild-type littermate controls. Interestingly, mutant and control female mice achieved vaginal opening, a marker of puberty onset, at a similar age (Dlk^+/p-: 29.8 ± 1.5 days, n=11 vs. Dlk^+/+: 29.1 ± 0.7 days, n=15, p=0.6) despite a considerably lower body weight in the Dlk1 deficient mice at the time of vaginal opening (Dlk^+/p-: 10.1 ± 0.8 g vs. Dlk^+/+: 14.3 ± 0.3 g, p<0.0001). Similarly, in the Dlk^+/p- males, preputial separation occurred at a lower body weight than in controls (Dlk^+/p-: 12.4 ± 0.3 g, n=9 vs. Dlk^+/+: 14.1 ± 0.2 g, n=19, p<0.0001). We hypothesize that the lack of Dlk1 at the hypothalamic level may be attenuating the effect of the low body weight on determining pubertal onset. These findings suggest that DLK1 is an important link between body weight and pubertal development in mice, as has been shown in humans.

OR08-3 The Role Of Neuronal Plasticity In The Timing Of Puberty Onset: Insights From A Mkrn3 Deficient Mouse Model

Makorin ring finger protein 3 (MKRN3) was identified as a regulator of puberty initiation with the report of loss-of-function mutations in association with central precocious puberty (CPP). MKRN3 is a maternally imprinted gene located on chromosome 15q11.2 in humans. In juvenile mice, Mkrn3 is highly expressed in the preoptic area and the arcuate nucleus of the hypothalamus, with a decrease in expression before puberty initiation. Together, these findings suggest an inhibitory role of MKRN3 on GnRH secretion during the prepubertal quiescent period. MKRN3 contains a RING finger motif and several zinc finger motifs, suggesting E3 ubiquitin ligase and RNA binding activities, respectively. To date, the mechanisms of action of MKRN3 in puberty initiation remain unknown. In this study, we used a Mkrn3 deficient mouse model to identify hypothalamic targets of MKRN3’s inhibitory effects. By Western blot and immunohistochemistry, we confirmed that Mkrn3 protein was undetectable in Mkrn3^+/- mice (which inherited the mutant allele from their father), but present in wildtype (Mkrn3^+/+) mice. Mkrn3 deletion was associated with accelerated puberty onset in female mice and a trend towards early puberty onset in male mice, as shown by an advanced day of female first estrus and a trend towards earlier age of male preputial separation in Mkrn3 deficient (Mkrn3^+/-) compared to Mkrn3^+/+animals. Hypothalamic mRNA levels of Gnrh1, Kiss1, Tac2 and Tac1, regulators of pubertal initiation, were measured by RT-qPCR in females at different ages across postnatal development (postnatal day (PND)10, PND15, PND20 and PND25). No significant difference was found in Gnrh1 and Kiss1 expression in the preoptic area, nor in Kiss1, Tac2 and Tac1 expression in the arcuate nucleus, between Mkrn3^+/+ and Mkrn3^+/- mice, at any of the postnatal day ages tested. Thus, to further investigate possible targets of MKRN3 action using an unbiased approach, we compared the transcriptome (via RNA-sequencing) and proteome (via Tandem Mass Tagging (TMT) method) of the arcuate nucleus of Mkrn3^+/+ and Mkrn3^+/- female mice at PND15. These analyses revealed differences in gene or protein expression between Mkrn3^+/+ and Mkrn3^+/- animals in factors involved in extracellular matrix organization, cell adhesion, and axon guidance pathways, which together control neuronal development and synaptic plasticity. These findings suggest that Mkrn3 may be important as a postnatal/prepubertal regulator of hypothalamic neuronal plasticity in mice. In humans, puberty has been shown to be a critical time window of neuronal plasticity. Thus, understanding the involvement of MKRN3 in these processes will provide new insights into pubertal disorders such as CPP and delayed puberty, and may lead to the development of new treatment strategies.