Impaired protamination and sperm DNA damage in a Nellore bull with high percentages of morphological sperm defects in comparison to normospermic bulls

Abstract

The routine semen evaluation assessing sperm concentration, motility and morphology, does not identify subtle defects in sperm chromatin architecture. Bulls appear to have stable chromatin, with low levels of DNA fragmentation. However, the nature of fragmentation and its impact on fertility remain unclear and there are no detailed reports characterizing the DNA organization and damage in this species. The intensive genetic selection, the use of artificial insemination and in vitro embryo production associated to the cryopreservation process can contribute to the chromatin damage and highlights the importance of sperm DNA integrity for the success of these technologies. Frozen-thawed semen samples from three ejaculates from a Nellore bull showed high levels of morphological sperm abnormalities (55.8±5.1%), and were selected for complementary tests. Damage of acrosomal (76.9±8.9%) and plasma membranes (75.7±9.3%) as well as sperm DNA strand breaks (13.8±9.5%) and protamination deficiency (3.7±0.6%) were significantly higher compared to the values measured in the semen of five Nellore bulls with normospermia (24.3±3.3%; 24.5±6.1%; 0.6±0.5%; 0.4±0.6% for acrosome, plasma membrane, DNA breaks and protamine deficiency, respectively) (P<0.05). Motility and percentage of spermatozoa with low mitochondrial potential showed no differences between groups. This study shows how routine semen analyses (in this case morphology) may point to the length and complexity of sperm cell damage emphasizing the importance of sperm function testing.

Does sperm quality and DNA integrity differ in cryopreserved semen samples from young, adult, and aged Nellore bulls?

“…Similarly to our observation, Zubkova et al (2005) [ 4 ] evaluated protamination in young and old rats sperm and found that young animals showed the highest percentage of sperm with CMA3-positive staining compared to old animals. It is interesting to note that when compared to CMA3 staining levels reported in human sperm, bull CMA3 staining is markedly low [ 16 , 37 , 38 ]. Although, to our knowledge, there is no other study that corroborate our CMA3 readings in bull sperm, this observation may suggest that compared to human sperm the protamination level of bull sperm is rather high.…”

Cellular and Functional Physiopathology of Bull Sperm With Altered Sperm Freezability

“…In addition, freeze-thaw may impair the bonding of disulfide bridges in protamine, thus lead to increased DNA damage (71), protamine deficiency did not always result in DNA damage though (72). On the other hand, it is also intriguing that protamine deficiency was found to be low in bull compared to human (73). This seems to be the likely reason that bull sperm are more resilient against the detriments of freezing-thawing protocol (74).…”

Morphological defects, sperm DNA integrity, and protamination of bovine spermatozoa

“…(), and therefore, this measure is likely more comparable to the HCB parameter (mean 4.74%). In other studies with a smaller number of bulls, maximum values of between 2.0 and 3.7% (Simoes et al ., ; Rahman et al ., ; Carreira et al ., ) were obtained, based on methods using either single laser flow cytometry or microscopy. We can only speculate on the differences in levels of deprotamination between these studies and our results.…”

“…In one previous study, only two of 14 bulls showed a slightly higher percentage of CMA 3 ‐positive sperm cells (Simoes et al ., ) of 2.0%, compared to the mean of 1.1%. In another case study using the same technique, one bull sample was found to have 3.7% spermatozoa with deficient protamine (Carreira et al ., ). Rahman and co‐workers used a single spectra assay with minor modifications and found a decrease in protamine content in bulls exposed to a testicular heat insult in the form of scrotal insulation (Rahman et al ., ).…”

“…In a recent study of 40 Nellore bulls, a negative correlation between bull age, and protamine deficiency and DNA fragmentation was described, with the younger bulls also displaying increased morphological sperm abnormalities (Carreira et al ., ). Protamine deficiency is likely one of the contributing factors to DNA instability and damage in the bull; however, further investigations into the associations between sperm DNA integrity, protamine deficiency, and specific sperm morphology are warranted (Fortes et al ., ; Carreira et al ., ; Kipper et al ., ).…”